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  A high pressure shock tube ‘‘HPST” has been designed and validated for the purpose of chemical kinetics studies at elevated pressures and temperatures. Using this facility, auto-ignition investigations are con- ducted for methane at 10, 20 and 40 bar and temperatures from 1400 K up to 2000 K. Three equivalence ratios (/ = 0.5, 1 and 2) for a methane/oxygen/argon mixture were studied in this paper under diluted conditions (argon \textgreater 90%). Experimental data showed good agreement with previous literature measure- ments and predictions from three different chemical kinetic models (Aramco Mech 1.3, USC Mech II and GRI Mech 3.0) commonly used in the literature, which confirms the quality of the experimental data obtained with the new high pressure shock tube and allows the expansion of the validation range of the mechanisms for ignition delay times to higher pressures. The investigations performed to explain the differences observed for ignition delay times predictions highlighted an important sensitivity of the USC mechanism predictions to equivalence ratio and an important sensitivity to CH3 +O2 reactions. Finally, the impact of pressure and equivalence ratio on ignition delay time is evaluated over a broad range of pressure (1–50 bar) and equivalence ratio (/ = 0.2–3).


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  This study examines condensation heat transfer in horizontal channels. Two separate condensation modules are tested using FC-72 as condensing fluid and water as coolant. The first module is dedicated to obtaining detailed heat transfer measurements of the condensing flow, and the second to video capture of the condensation film's interfacial behavior. Four dominant flow regimes are identified: smooth-annular, wavy-annular, stratified-wavy and stratified, whose boundaries show fair agreement with published flow regime maps. The film's interface is observed to feature an array of small ripples and relatively large waves, with the largest waves tending to merge into yet larger waves having greater liquid mass, amplitude and speed. This behavior is believed to influence condensation heat transfer, especially downstream. The local condensation heat transfer coefficient is highest near the inlet, where quality is near unity and the film thinnest, and decreases monotonically in the axial direction in response to the film thickening. This variation is very sensitive to the mass velocity of FC-72, and the heat transfer coefficient decreases sharply in the inlet region but this decrease slows significantly in the downstream region because of the combined effects of turbulence and interfacial waviness. The measured condensation heat transfer coefficient shows good agreement with a select number of correlations. ?? 2013 Elsevier Ltd. All rights reserved.


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  A reduced chemical kinetic mechanism for the oxidation of primary reference fuel (PRF) has been developed and applied to model internal combustion engines. Starting from an existing reduced reaction mechanism for n-heptane oxidation, a new reduced n-heptane mechanism was generated by including an additional five species and their relevant reactions, by updating the reaction rate constants of several reactions pertaining to oxidation of carbon monoxide and hydrogen, and by optimizing reaction rate constants of selected reactions. Using a similar approach, a reduced mechanism for iso-octane oxidation was built and combined with the n-heptane mechanism to form a PRF mechanism. The final version of the PRF mechanism consists of 41 species and 130 reactions. Validation of the present PRF mechanism was performed with measurements from shock tube tests, and HCCI and direct injection engine experiments available in the literature. The results show that the present PRF mechanism gives reliable performance for combustion predictions, as well as computational efficiency improvements for multidimensional CFD simulations. ?? 2008 The Combustion Institute.


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  The idea of using porous media in internal combustion engines is proposed recently for charge homogenization. In this paper, the idea of using metallic porous media instead of ceramic type has been introduced and the macroscopic characteristics of high pressure diesel fuel spray has been studied in interaction with this type of porous media. High speed schlieren imaging method is used to capture the spray development process and evaluate time-dependent characteristics of the spray. The Spray characteristics such as tip penetration and spray projected area has been compared with free spray case and spray interaction with metal PM. Several parameters such as PM pore density, thickness of PM, distance between PM and injector tip has been investigated under different injection and chamber pressures. The results show that interaction with PM could reduce the spray tip velocity and increase spray projected area which proportionally increase the fuel-air mixture homogenization inside the chamber. Post processing of the spray images also indicated that the spray pattern (before and after interaction with PM) strongly depends on the PM pore density and injection pressure.


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  The influence of internal nozzle flow characteristics over the evaporative spray development is studied experimentally for two different nozzle geometries and three different fuels. This is a continuation of previous work by the authors where non-evaporative isothermal spray development was studied experimentally for the same nozzle geometries and fuels. Current study reports macroscopic spray characteristics by imaging the liquid and vapor phases of the spray simultaneously using independent cameras and optical techniques. The liquid phase is captured by a fast-pulsed diffused back illumination setup, while the vapor phase is captured by a single-pass Schlieren setup with diaphragm. The nozzle geometries consist of a conical nozzle and a cylindrical nozzle with 8.6% larger outlet diameter when compared to the conical nozzle. Among the three fuels, two are pure components—n-heptane and n-dodecane—while the third consists of a three-component surrogate to better represent the physical and chemical properties of diesel fuel. For a fixed ambient density, the liquid penetration is controlled by ambient temperature while the vapor penetration is controlled by injection pressure. The cylindrical nozzle, in spite of higher mass flow rate and momentum flux, shows slower vapor spray tip penetration when compared to the conical nozzle. Also, the cylindrical nozzle consistently produced shorter liquid lengths. The vapor spray spreading angle is found to be inversely proportional to the spray tip penetration, largely influenced by the nozzle geometry and the ambient density. n-Heptane spray shows the shortest liquid lengths, followed by n-dodecane and finally the Surrogate. No significant difference in vapor penetration rates was found between fuels, confirming that the vapor spray is controlled by momentum, which is independent of fuel. This was not the case for the non-evaporative isothermal sprays previously studied by the authors. Liquid lengths show the expected responses to parametric variations of ambient temperature and density. Two empirical predictive models are presented and utilized to analyze the influence of fuel properties on the liquid length. The primary factor controlling the liquid length between fuels is found to be their volatility. Finally, the cylindrical nozzle exhibits larger line-of-sight contour fluctuations in both the liquid and vapor phases, which in turn contributes to the shorter liquid lengths and slower vapor penetration.


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  Experimental and modeling study of the oxidation of natural gas in a premixed flame, shock tube, and jet-stirred reactor


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  New experimental results have been obtained for the oxidation of a stoichiometric premixed synthetic natural gas flame at low pressure (10.6 kPa). Mole fraction profiles of stable and reactive species and temperature were measured using molecular beam/mass spectrometry, gas chromatography, and thermocouple techniques. A detailed reaction mechanism has been evaluated by comparison of computed and measured species mole fraction profiles. Good agreement was obtained for most molecular species (reactants, intermediate species, and final products). The proposed mechanism was also validated by modeling the laminar flame speeds of methane/air, ethane/air, and propane/air flames reported in the literature over a wide range of equivalence ratios. The model reproduces correctly the experimental values. The model was also used to simulate new experimental results obtained in a jet-stirred reactor (JSR) at atmospheric pressure and variable equivalence ratio (??=0.75:1.0:1.5) for the oxidation of methane-ethane mixtures representative of natural gas. The ignition delays of methane-oxygen-argon and methane-ethane-oxygen were modeled. Sensitivity analyses and reaction path analyses, based on species rates of reaction, were used to interpret the results, especially the effect of higher alkanes on the oxidation of natural gas. ?? 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.


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  Methane/ \textlessi\textgreatern\textless/i\textgreater -Butane Ignition Delay Measurements at High Pressure and Detailed Chemical Kinetic Simulations


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  Autoignition delay time measurements were recorded for blends of CH4/n-C4H10 in “air” at pressures of approximately 10, 16, 20, 25, and 30 atm from fuel-lean to fuel-rich conditions at two different fuel compositions, 90% CH4/10% n-C4H10 and 70% CH4/30% n-C4H10, and temperatures from 660 to 1330 K in both a rapid compression machine and a shock-tube facility. A detailed chemical kinetic model consisting of 1328 reactions involving 230 species was validated using the ignition delay data from this study. This mechanism has already been used to simulate previously published ignition delay times over a wide range of conditions. It was found that the model quantitatively reproduces the ignition delays from both rapid compression and reflected shock waves, accurately capturing reactivity as a function of the temperature, pressure, equivalence ratio, and fuel composition.


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  A distributed parameter model for the spray drying of multicomponent droplets with a crust formation


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  The new distributed parameter model is formulated for the purpose of the investigation of spray drying of multi-component solutions. The main feature of this model is the division of a droplet/wet particle into shells and formulating the mass and heat balances for each shell. In opposition to most similar numerical models, this one takes into account the crust formation during the second stage of drying. By means of this model, we investigate the conditions in which the segregation of the components in particle form takes place. We take into account the initial concentrations of the components in solution, its solubility and diffusion coefficients as model parameters for consideration of the different particle structures. © 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder.


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  Experimental study on diesel fuel injection characteristics under cold start conditions with single and split injection strategies


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  Studies on fuel flow characteristics under both room temperature (25 °C) and low temperature (-18 °C) were carried out under a wide range of injection pressures and back pressures using a long tube real-time fuel flow rate measuring instrument. Both single and split injection strategies were employed. Several modified correlations were proposed to analyze fuel injection characteristics. It was found that low fuel temperature caused longer injection delay, shorter injection duration, lower mass flow rate and less injected fuel mass. The injector discharge coefficient was affected more by fuel viscosity changes due to low temperature than by the geometric structure of the injector. Cold start conditions effectively limited the inception of cavitation as seen in the flow rate/pressure dependence and accelerated the transition of flow regime from cavitation to turbulent and laminar flow with the decrease of injection pressure. This contributed to increased chances for the existence of laminar flow and thus to reduction of the discharge coefficient. Besides, low fuel temperature significantly weakened the degree of interaction between individual parts of split injection and split mass flow rate shapes became much less continuous than those under room temperature. These findings are of great importance for engine cold start studies.


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  Numerical modelling of soot formation and oxidation using phenomenological soot modelling approach in a dual-fueled compression ignition engine


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  Improvement of diesel combustion with multiple injections at cold condition in a constant volume combustion chamber


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  A series of diesel spray combustion tests was carried out in a constant volume combustion chamber (CVCC) to investigate the effect of multiple injection strategies on cold startability. The experiments were performed under a simulated low temperature cold start condition. In-chamber pressure analysis and high speed flame imaging were conducted to compare the effectiveness of each injection strategy on cold startability. Spray targeting visualization was also performed to examine the wall impingement of injected fuel. The diesel fuel was injected into the CVCC with an injection pressure of 35ï¿½MPa. Multiple injection strategies with different amounts of pilot injection quantities were applied to improve the diesel combustion under simulated cold start ambient condition. The flame imaging and in-chamber pressure results showed that the multiple injection strategy provided better cold startability than the single injection condition. According to the pilot injection quantity, the peak of the flame luminosity and in-chamber pressure were gradually increased with larger pilot injection quantity cases. The peak of the variation rate of the in-chamber pressure with a multiple injection strategy was approximately 2 times higher than that of the single injection case. In terms of spray targeting imaging, the results indicated that the fuel impingement and flow to the piston crevice volume were increased with a larger amount of pilot2 injection. Therefore, the increment of the pilot1 injection quantity rather than the pilot2 injection quantity was suggested not only to improve cold startability, but also to reduce unburned hydrocarbon emissions under a cold start condition in a real engine.


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  SAE TECHNICAL PAPER SERIES Ultrafast X-ray Phase-Enhanced Microimaging for Visualizing Fuel Injection Process Ultrafast X-ray Phase-Enhanced Microimaging for Visualizing Fuel Injection Process


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  Study of the spray characteristics of a diesel surrogate for diesel engines under sub / supercritical states injected into atmospheric environment


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  Effects of radiation and compression on propagating spherical flames of methane / air mixtures near the lean flammability limit


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  An updated H 2/O 2 kinetic model based on that of Li et al. (Int J Chem Kinet 36, 2004, 566-575) is presented and tested against a wide range of combustion targets. The primary motivations of the model revision are to incorporate recent improvements in rate constant treatment and resolve discrepancies between experimental data and predictions using recently published kinetic models in dilute, high-pressure flames. Attempts are made to identify major remaining sources of uncertainties, in both the reaction rate parameters and the assumptions of the kinetic model, affecting predictions of relevant combustion behavior. With regard to model parameters, present uncertainties in the temperature and pressure dependence of rate constants for HO 2 formation and consumption reactions are demonstrated to substantially affect predictive capabilities at high-pressure, low-temperature conditions. With regard to model assumptions, calculations are performed to investigate several reactions/processes that have not received much attention previously. Results from ab initio calculations and modeling studies imply that inclusion of H + HO 2 = H 2O + O in the kinetic model might be warranted, though further studies are necessary to ascertain its role in combustion modeling. In addition, it appears that characterization of nonlinear bath-gas mixture rule behavior for H + O 2(+ M) = HO 2(+ M) in multicomponent bath gases might be necessary to predict high-pressure flame speeds within ∼15%. The updated model is tested against all of the previous validation targets considered by Li et al. as well as new targets from a number of recent studies. Special attention is devoted to establishing a context for evaluating model performance against experimental data by careful consideration of uncertainties in measurements, initial conditions, and physical model assumptions. For example, ignition delay times in shock tubes are shown to be sensitive to potential impurity effects, which have been suggested to accelerate early radical pool growth in shock tube speciation studies. In addition, speciation predictions in burner-stabilized flames are found to be more sensitive to uncertainties in experimental boundary conditions than to uncertainties in kinetics and transport. Predictions using the present model adequately reproduce previous validation targets and show substantially improved agreement against recent high-pressure flame speed and shock tube speciation measurements. Comparisons of predictions of several other kinetic models with the experimental data for nearly the entire validation set used here are also provided in the Supporting Information. Copyright © 2011 Wiley Periodicals, Inc.


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  Emission characteristics of high speed, dual fuel, compression ignition engine operating in a wide range of natural gas/diesel fuel proportions


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  In the effort to reduce pollutant emissions from diesel engines various solutions have been proposed, one of which is the use of natural gas as supplement to liquid diesel fuel, with these engines referred to as fumigated, dual fuel, compression ignition engines. One of the main purposes of using natural gas in dual fuel (liquid and gaseous one) combustion systems is to reduce particulate emissions and nitrogen oxides. Natural gas is a clean burning fuel; it possesses a relatively high auto-ignition temperature, which is a serious advantage over other gaseous fuels since then the compression ratio of most conventional direct injection (DI) diesel engines can be maintained high. In the present work, an experimental investigation has been conducted to examine the effects of the total air-fuel ratio on the efficiency and pollutant emissions of a high speed, compression ignition engine located at the authors' laboratory, where liquid diesel fuel is partially substituted by natural gas in various proportions, with the natural gas fumigated into the intake air. The experimental results disclose the effect of these parameters on brake thermal efficiency, exhaust gas temperature, nitric oxide, carbon monoxide, unburned hydrocarbons and soot emissions, with the beneficial effect of the presence of natural gas being revealed. Given that the experimental measurements cover a wide range of liquid diesel supplementary ratios without any appearance of knocking phenomena, the belief is strengthened that the findings of the present work can be very valuable if opted to apply this technology on existing DI diesel engines. © 2009 Elsevier Ltd. All rights reserved.


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  The growing interest in gaseous fuels (hydrogen and natural gas) for internal combustion engines calls for the development of computer models for simulation of gaseous fuel injection, air entrainment and the ensuing combustion. This paper introduces a new method for modeling the injection and air entrainment processes for gaseous fuels. The model uses a gaseous sphere injection methodology, similar to liquid droplet injection techniques used for liquid fuel injection. In this paper, the model concept is introduced and model results are compared with correctly- and under-expanded experimental data.


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  Multi-component vapor-liquid equilibrium model for LES of high-pressure fuel injection and application to ECN Spray A


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  We present and evaluate a two-phase model for Eulerian large-eddy simulations (LES) of liquid-fuel injection and mixing at high pressure. The model is based on cubic equations of state and vapor-liquid equilibrium calculations and can represent the coexistence of supercritical states and multi-component subcritical two-phase states via a homogeneous mixture approach. Well-resolved LES results for the Spray A benchmark case of the Engine Combustion Network (ECN) and three additional operating conditions are found to agree very well with available experimental data. We also address well-known numerical challenges of trans- and supercritical fluid mixing and compare a fully conservative formulation to a quasi-conservative formulation of the governing equations. Our results prove physical and numerical consistency of both methods on fine grids and demonstrate the effects of energy conservation errors associated with the quasi-conservative formulation on typical LES grids.


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  The need for using alternative fuel sources continues to grow as industry looks towards enhancing energy security and lowering emissions levels. In order to capture the potential of these megatrends, this study focuses on the relationship between ignition energy, thermal efficiency, and combustion stability of a 0.5 L single cylinder engine powered by compressed natural gas (CNG) at steady state operation. The goal of the experiment was to increase ignition energy at fixed lambda values to look for gains in thermal efficiency. Secondly, a lambda sweep was performed with criteria of maintaining a 4% COVIMEP by increasing the ignition energy until an appropriate threshold for stable combustion was found. The engine performance was measured with a combustion analysis system (CAS), to understand the effects of thermal efficiency and combustion stability (COVIMEP). Emissions of the engine were measured with an FTIR. The engine was instrumented with visualization equipment to capture real-time combustion images. The results show that increasing the ignition energy will increase the thermal efficiency of combustion by roughly 2-3%. In areas of lean combustion, the ignition energy level plays a pronounced role in the COVIMEP of the engine. At λ = 1.25, drastic improvement of COVIMEP was realized based on increasing the energy level. Additionally, the reduction of NOx, CO, and THC emissions is possible as they also are sensitive to the amount of ignition energy.


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  It is proved that for the 2-dimensional case with random shear flow of the G-equation model with strain term, the strain term reduces the front propagation. Also an improvement of the main result by Armstrong-Souganidis is provided.


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  Development of an optimized chemical kinetic mechanism for homogeneous charge compression ignition combustion of a fuel blend of \textlessi\textgreatern\textless/i\textgreater-heptane and natural gas using a genetic algorithm


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  Homogeneous charge compression ignition (HCCI) is a promising technique for advanced low-temperature combustion strategies that offers a high fuel conversion efficiency and low nitrogen oxide and soot emissions. One of the major problems associated with HCCI combustion engine application is the lack of direct control for combustion timing. A proposed solution for combustion timing control is to use a binary fuel blend in which two fuels with different autoignition characteristics are blended at various ratios on a cycle-by-cycle basis. Because dual-fuel diesel-natural-gas engines have already been used, a fuel blend of n-heptane (diesel-like fuel) and natural gas (mostly methane) is one of the best available options. The objective of this study is to optimize the chemical kinetic mechanisms available for n-heptane and natural gas to be used in a binary-fuel blend scenario. Using the genetic algorithm method, a combined mechanism was optimized and modelling results were verified against experimental results. The agreement between experimental and modelling results was found to be acceptable within the examined conditions. As a result, an optimized chemical kinetic mechanism for an n-heptane-natural-gas blend is presented.


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  Detailed measurements of near-nozzle spray formation are essential to better understand and predict the physical processes involved in diesel fuel atomisation. We used long-range microscopy to investigate the primary atomisation of diesel, biodiesel and kerosene fuels in the near-nozzle region, both at atmospheric and realistic engine conditions. High spatial and temporal resolutions allowed a detailed observation of the very emergence of fuel from the nozzle orifice. The fluid that first exited the nozzle resembled mushroom-like structures, as occasionally reported by other researchers for atmospheric conditions, with evidence of interfacial shearing instabilities and stagnation points. We captured the dynamics of this phenomenon using an ultra-fast framing camera with frame rates up to 5 million images per second, and identified these structures as residual fluid trapped in the orifice between injections. The residual fluid has an internal vortex ring motion which results in a slipstream effect that can propel a microscopic ligament of liquid fuel ahead. We showed that this mechanism is not limited to laboratory setups, and that it occurs for diesel fuels injected at engine-like conditions with production injectors. Our findings confirm that fuel can remain trapped in the injector holes after the end of injection. Although we could not measure the hydrocarbon content of the trapped vapourised fluid, we observed that its density was lower than that of the liquid fuel, but higher than that of the in-cylinder gas. We conclude that high-fidelity numerical models should not assume in their initial conditions that the sac and orifices of fuel injectors are filled with in-cylinder gas. Instead, our observations suggest that the nozzle holes should be considered partially filled with a dense fluid.


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  International Journal of Heat and Mass Transfer A heat transfer model for condensation accounting for non-equilibrium effects


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  Experimental investigation on spray-wall impingement characteristics of n-butanol/diesel blended fuels


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  n-Butanol is one of the most promising conventional fuel alternatives for diesel engines using advanced combustion modes such as homogeneous charge ignition and premixed charge compression ignition. However, effects associated with the structural design of these engines as well as early injection strategies still result unavoidable spray–wall impingement when using n-butanol/diesel blended fuels, which directly affects the combustion and emission characteristics of the diesel engines. The present work therefore investigated the spray–wall impingement characteristics of n-butanol/diesel blended fuels under various environmental conditions and when applying different injection strategies. The parameters that were varied included fuel temperature, environmental temperature and pressure, impingement distance, blending ratio, and the presence or absence of an oil film. The impingement spray radius (Rw) and height (Hw) were obtained experimentally so as to evaluate the impingement characteristics, and the effects of each parameter on Rwand Hwwere assessed. In addition, based on the momentum transition of the impinged spray, the variations in Rwand Hwwere attributed to four factors: the mass and momentum of the impinged spray; the spreading or splashing resistance of the spray, and the strength of air entrainment. The relative effects of each of these factors on Rwand Hwwere examined, and results showed that the spray impingement momentum had the greatest effect on Rw, followed by air entrainment, while the impact of the fuel viscosity and surface tension (or spreading resistance) were limited. However, the effects of spray impingement momentum and air entrainment on Hwwere less pronounced, while the viscosity and surface tension had a greater impact.


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  Effect of fuel injection pressure and injection timing on spray characteristics and particulate size-number distribution in a biodiesel fuelled common rail direct injection diesel engine


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  In this paper, effect of varying fuel injection pressures and injection timings on particulate size number distribution and spray characteristics was investigated in a single cylinder, common rail direct injection (CRDI) compression ignition (CI) engine fueled with Karanja biodiesel blends vis-à-vis baseline mineral diesel. The investigation results of spray tip penetration and spray area of biodiesel blends and diesel showed that higher fuel injection pressure result in a longer spray tip penetration and larger spray area than that at lower injection pressures at same elapsed time after the start of injection (SOI). In order to compare the effect of fuel injection parameters, 10, 20 and 50% Karanja biodiesel blends at 1500. rpm engine speed were compared with baseline data from mineral diesel. It was observed that average particulate size increased with retarding the SOI timings. Particulate number concentration was lowest for 10% biodiesel blend, which increased with further increase in biodiesel content in the blended test fuel. Addition of even very small quantity of biodiesel in the test fuel helped in reducing particulate emissions. © 2014 Elsevier Ltd.


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  Testing a small detailed chemical-kinetic mechanism for the combustion of hydrogen and carbon monoxide


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  A relatively small detailed mechanism has been developed for the combustion of various fuels, mainly hydrocarbons, in air or oxygen-inert mixtures. This mechanism has been tested previously for autoignition, premixed-flame burning velocities, and structures and extinction of diffusion flames and of partially premixed flames of many of these fuels. While submechanisms for hydrogen and carbon monoxide are essential components of this mechanism, thorough testing of the predictions of the mechanism for these simpler fuels has not been performed recently. Such testing is reported here and leads to modifications of rate parameters for a few of the most important elementary steps, as well as to deletion of one reaction and addition of another. ?? 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.


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  Despite great practical interest in how sprays emanate from fuel injectors, the near-nozzle region has remained a challenge for spray modelers. Recently, Eulerian models have shown promise in capturing the fast gas-liquid interactions in the near field. However, with the inclusion of compressibility, it can be difficult to maintain consistency between the hydrodynamic and thermodynamic variables. In order to resolve numerical inconsistencies that occur in segregated solutions of Eulerian spray model equations as well as to provide good scalability and stability, a new construction of a σ-Y model is introduced. This construction is built around an IMEX-RK3 algorithm which offers accuracy and efficiency. The new algorithm is compared to an existing implementation for speed and is validated against experimental measurements of spray evolution in order to test the accuracy. The predictions of the new construction are slightly more accurate and, when tested on 256 processors, are 34 times faster.


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  A multicomponent droplet evaporation model which discretizes the one-dimensional mass and temperature profiles inside a droplet with a finite volume method has been developed and implemented into a large-eddy simulation (LES) model for spray simulations. The LES and multicomponent models were used along with the KH-RT secondary droplet breakup model to simulate realistic fuel sprays in a closed vessel. The effect of various spray and ambient gas parameters on the liquid penetration length of different single component and multicomponent fuels was investigated. The numerical results indicate that the spray penetration length decreases non-linearly with increasing gas temperature or pressure and is less sensitive to changes in ambient gas conditions at higher temperatures or pressures. The spray models and LES were found to predict the experimental results for n-hexadecane and two multicomponent surrogate diesel fuels reasonably well. Copyright © 2013 SAE International.


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  Dynamics of the cavity and the surface film for impingements of single drops on liquid films of various thicknesses


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  This paper presents experimental and numerical investigations of single drop impacts onto liquid films of finite thickness. The dynamics of the drop impingement on liquid surface films, the shape of the cavity, the surface film dynamics and the residual film thickness are investigated and analysed. The shape of the penetrating cavity within the surface film is observed experimentally using a high-speed video system. Additionally, the thickness of the liquid film between the expanding, receding and retracting cavity and the solid wall is monitored in time using an optical sensor based on chromatic confocal imaging. The effects of various influencing parameters, such as the drop impingement velocity, liquid properties (surface tension and viscosity) and the initial liquid film thickness, on the time evolution of the cavity and film dynamics are investigated. Complementary to the experiments direct numerical simulations of the drop impacts and cavity expansion are performed using a volume-of-fluid free-surface capturing model in the framework of the finite volume numerical method. The numerical predictions of the film thickness dynamics agree well with the experiments for most phases of the impingement process. Finally, a scaling analysis of the residual film thickness between the cavity and the solid wall is performed for various impingement parameters. ?? 2010 Elsevier Inc.


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  The role of turbulence-chemistry interaction in autoignition and flame stabilization is investigated for spray flames at low temperature combustion (LTC) conditions by performing high-fidelity three-dimensional computational fluid dynamics (CFD) simulations. A recently developed Tabulated Flamelet Model (TFM) is coupled with a large eddy simulation (LES) framework and validated across a range of Engine Combustion Network (ECN) ambient temperature conditions for n-dodecane fuel. High resolution grids with 0.0625 mm minimum cell size and 25 million total cell count are implemented using adaptive mesh refinement over the spray and combustion regions. Simulations with these grids and multiple LES realizations, with a 103 species n-dodecane mechanism show good agreement with experimental data for all the ambient conditions investigated. This modeling approach with the computational cost advantage of tabulated chemistry is then extended towards understanding the auto-ignition and flame stabilization at an ambient temperature of 750 K. These low temperature conditions lead to substantially higher ignition delays and flame liftoff lengths, and significantly leaner combustion compared to conventional high temperature diesel combustion. These conditions also require the simulations to span significantly larger temporal and spatial dimensions thereby increasing the computational cost. The TFM approach is able to capture autoignition and flame liftoff length at the low temperature conditions. Significant differences with respect to mixing, species formation and flame stabilization are observed under low temperature compared to conventional diesel combustion. At higher ambient temperatures, formation of formaldehyde is observed in the rich region (phi \textgreater 1) followed by the formation of OH in the stoichiometric regions. Under low temperature conditions, formaldehyde is observed to form at leaner regions followed by the onset of OH formation in significantly lean regions of the flame. Qualitative differences between species formation and transient flame development for the high and low temperature conditions are presented. The two stage ignition process is further investigated by studying the species formation in mixture fraction space by solving 1D flamelet equations for different scalar dissipation rates and homogeneous reactor assumption. Results show that scalar dissipation causes these radicals to diffuse within the mixture fraction space. This significantly enhances ignition and plays a dominant role at such low temperature conditions which cannot be captured by the homogeneous reaction assumption based model.


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  Experimental characterization of closely coupled split isooctane sprays under flash boiling conditions


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  The characteristics of isooctane spray in the near field and far field were experimentally studied under various flash boiling conditions representing both low load and high load hot engine operation conditions. Closely coupled split injection strategy was also employed to study the influence of flash boiling on the primary breakup of split injections and the interaction between split injections. It was found that flash boiling considerably boosted the spray atomization, especially during the end of injection stage when a large amount of liquid fuel with low speed was observed. The interaction between split injections in liquid phase was significantly weakened under flash boiling condition due to the enhanced atomization but the interaction in gaseous phase was boosted because of the resultant quicker evaporation. The effect of dwell interval on the spray behavior under flash boiling condition was profound, causing significant variation for the delivered fuel mass and spray characteristics.


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  Multiphase flow phenomena relevant to spray combustion are reviewed, emphasizing the structure of the near-injector dense-spray region and the properties of secondary and primary breakup. Existing measurements of dense-spray structure are limited to round pressure-atomized sprays in still gases and show that the dispersed flow region is surprisingly dilute, that separated flow effects are significant because the flow is dilute and developing, and that atomization involves primary breakup at the liquid surface followed by secondary breakup, while effects of collisions are small. Available information about secondary breakup emphasizes breakup due to shock wave disturbances at large liquid/gas density ratios and shows that secondary breakup is a dominant feature of dense sprays that must be resolved as a function of time so that secondary breakup can be properly treated as a rate process. Finally, available information about primary breakup has been dominated by effects of disturbances in the injector passage; therefore, while some understanding of turbulent primary breakup has been achieved, more information about aerodynamic primary breakup is needed to address practical spray combustion processes. © 1995, All rights reserved.


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  Pilot injection strategy management of gasoline compression ignition (GCI) combustion in a multi-cylinder diesel engine


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  The present study focuses on the experimental investigation on the optimal pilot injection strategy under GCI combustion mode in a multi-cylinder heavy-duty diesel engine. Three experiments were conducted at a high-speed high-load operating point with different operating parameters and emission targets, namely engine-out NOx target, fuel injection pressure, and main injection timing. The engine-out NOx targets were set to 5.0 g/kWh and 1.5 g/kWh, and the gasoline injection pressures were set to 100 MPa and 140 MPa. These high and low values represent different requirements of SCR efficiency and practical capability of fuel supply system when addressing the hypothetical future tailpipe NOx limit of 0.02 g/hp-hr (0.027 g/kWh). The results show that the use of optimized pilot injection always achieves lower pressure rise rate and soot emissions than the single injection baseline. The pilot gasoline fuel with low injection pressure is more ignitable than that with high injection pressures, hence a distinct heat release spike usually occurs for a pilot injection. The optimal pilot mass should be increased for a higher fuel injection pressure because the pilot fuel stratification level decreases. A relatively late pilot timing is preferable for the early main injection timing. For the late main injection timing, however, a relatively early pilot timing with large pilot mass is preferable and brings about a distinct two stage high temperature heat release, which can reduce the fuel consumption and soot emissions simultaneously. The two stage split combustion process obtained by double injection with retard combustion phasing can be considered to be an important way to alleviate the requirement of GCI fuel system.


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  The Engineering Oa M Resource For Advancing Mobility A Discussion of Turbulent Flame Structure in Premixed Charges


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  A Simple New Analysis of Compressible Turbulent Two-Dimensional Skin Friction Under Arbitrary Conditions


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  Using a homogeneous equilibrium model for the study of the inner nozzle flow and cavitation pattern in convergent–divergent nozzles of diesel injectors


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  Development of multi-component diesel surrogate fuel models - Part I: Validation of reduced mechanisms of diesel fuel constituents in 0-D kinetic simulations


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  In the present work, development and validation of reduced chemical kinetic mechanisms for several different hydrocarbons are performed. These hydrocarbons are potential representative for practical diesel fuel constituents. n-Hexadecane (HXN), 2,2,4,4,6,8,8-heptamethylnonane (HMN), cyclohexane (CHX) and toluene are selected to represent straight-alkane, branched-alkane, cyclo-alkane and aromatic compounds in the diesel fuel. A five-stage chemical kinetic mechanism reduction scheme formulated in the previous work is applied to develop the reduced HMN and CHX models based on their respective detailed mechanisms. Alongside with the development of the reduced CHX model, a skeletal toluene sub-mechanism is constructed since the elementary reactions for toluene are subset of the detailed CHX mechanism. The final reduced HMN mechanism comprises 89 species with 319 elementary reactions, while the final reduced CHX mechanism which includes the toluene sub-mechanism consists of 80 species with 287 elementary reactions. Both reduced models are approximately 92% smaller than their respective detailed models in terms of total number of species and elementary reactions. Following that, both the newly developed fuel constituent reduced mechanisms, together with the formerly derived reduced HXN mechanism are comprehensively validated in zero-dimensional chemical kinetic simulations under a wide range of shock tube and jet-stirred reactor (JSR) conditions. Well agreement between the reduced and detailed mechanisms is achieved for ignition delay (ID) and species concentration predictions under both auto-ignition and JSR conditions, with a maximum relative error of 40%. In addition, the reduced models are further validated against the JSR experimental results for each diesel fuel constituents. The surrogate models are able to reasonably reproduce the experimental species concentration profiles in view of their simplified fuel chemistries. Deviations within one order of magnitude in the absolute values are recorded between the computations and measurements. Validation of these reduced models for each diesel fuel constituents in this work serves as a prerequisite for constructing a multi-component diesel surrogate fuel model. The compact yet accurate chemical models proposed here aid to reduce the chemistry size of the final multi-component diesel surrogate model such that it remains computationally efficient when it is incorporated with multi-dimensional CFD simulations. The reduced mechanism of each fuel constituent can also be used individually for other CFD applications.


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  Optical Investigations of the Vaporization Behaviors of Isooctane and an Optical , Non-fluorescing Multicomponent Fuel in a Spark Ignition Direct Injection Engine


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  This paper presents the performance results of a 5.9. kW stationary diesel engine which was converted into spark ignition mode and run on compressed natural gas (CNG), methane enriched biogas (Bio-CNG) and biogas produced from biomethanation of jatropha and pongamia oil seed cakes. The performance of the engine with 12.65 compression ratio was evaluated at 30??, 35?? and 40?? ignition advance of TDC. The maximum brake power produced by the engine was found at ignition advance of 35?? TDC for all the tested fuels. In comparison to diesel as original fuel, the power deteriorations of the engine was observed to be 31.8%, 35.6% and 46.3% on compressed natural gas, methane enriched biogas and raw biogas, respectively, due to its conversion from CI to SI mode. The methane enriched biogas showed almost similar engine performance as compared to compressed natural gas in terms of brake power output, specific gas consumption and thermal efficiency. ?? 2011 Elsevier Ltd.


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  Predicting Real Transportation Fuel Combustion Properties : Distinct Chemical Functionalities in Hydrocarbon Laminar Burning Velocities


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  Study of the spray characteristics of a diesel surrogate for diesel engines under sub/supercritical states injected into atmospheric environment


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  The spray characteristics of hydrocarbon fuel under superheated and supercritical conditions are quite different from the traditional liquid fuel injection. In this study, the macroscopic liquid phase and vapor phase spray geometry of n-heptane was investigated by Backlit Shadowgraph and Schlieren method to investigate the differences of spray pattern under various operation conditions such as fuel temperature, injection pressure and nozzle diameter. Furthermore, both the overall and near-field spray structures were analyzed to reveal the internal mechanism. During the experiments, the ambient environment was kept constant at atmospheric condition (25 °C, 0.1 MPa), the injection pressure was varied from 3 MPa to 5 MPa, always higher than the fuel critical pressure, and the fuel temperature was changed from 25 °C to 300 °C, spanning from liquid to supercritical state. Four different spray regimes have been identified based on the fuel temperature and spray geometry: liquid regime, flash boiling regime, near-critical regime and supercritical regime. Injection paths were analyzed on the basis of thermodynamic principle. The results showed that the fuel temperature has a minor effect on the macroscopic liquid phase and vapor phase characteristics below saturated-vapor temperature of 98 °C at the liquid regime. As the temperature is increased to about 200 °C, which is in the flash boiling regime, the local spray spreading angle of both the liquid and vapor phases increases, the downstream liquid phase of spray disappears gradually and high-intensity turbulence appears in the vapor phase image. At the near-critical regime where the fuel temperature is from 200 °C to the critical point (267 °C), the local spray angle of vapor phase begins to decrease and only a small area of liquid phase at the nozzle exit can be seen, while both the shock and flash boiling are visible. When the fuel temperature goes beyond the critical point locating in supercritical regime, the fuel spray is similar to a gas jet. The steady shock structure called Mach disk was observed and the fuel temperature has little influence on the Mach disk position and diameter in the supercritical regime. However, the fuel pressure and nozzle diameter significantly affect the Mach disk position and diameter.


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  Journal of Natural Gas Science and Engineering An experimental based ANN approach in mapping performance- emission characteristics of a diesel engine operating in dual-fuel mode with LPG


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  Spray combustion of biomass-based renewable diesel fuel using multiple injection strategy in a constant volume combustion chamber


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  Effect of a two-injection strategy associated with a pilot injection on the spray combustion process was investigated under conventional diesel combustion conditions (1000 K and 21% O2concentration) for a biomass-based renewable diesel fuel, i.e., biomass to liquid (BTL), and a regular No. 2 diesel in a constant volume combustion chamber using multiband flame measurement and two-color pyrometry. The spray combustion flame structure was visualized by using multiband flame measurement to show features of soot formation, high temperature and low temperature reactions, which can be characterized by the narrow-band emissions of radicals or intermediate species such as OH, HCHO, and CH. The objective of this study was to identify the details of multiple injection combustion, including a pilot and a main injection, and to provide further insights on how the two injections interact. For comparison, three injection strategies were considered for both fuels including a two-injection strategy (Case TI), single injection strategy A (Case SA), and single injection strategy B (Case SB). Multiband flame results show a strong interaction, indicated by OH emissions between the pilot injection and the main injection for Case TI while very weak connection is found for the narrow-band emissions acquired through filters with centerlines of 430 nm and 470 nm. A faster flame development is found for the main injection of Case TI compared to Cases SA and SB, which could be due to the high temperature environment and large air entrainment from the pilot injection. A lower soot level is observed for the BTL flame compared to the diesel flame for all three injection types. Case TI has a lower soot level compared to Cases SA and SB for the BTL fuel, while the diesel fuel maintains a similar soot level among all three injection strategies. Soot temperature of Case TI is lower for both fuels, especially for diesel. Based on these results, it is expected that the two-injection strategy could be effective in reducing soot and NOx(due to lower combustion temperature) simultaneously compared to either of the single injection strategies.


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  A model has been developed allowing to simulate the flow boiling process of a hydrocarbon feedstock. For the calculation of the different horizontal two-phase flow regimes that evolve during flow boiling, use is made of the Volume Of Fluid (VOF) model that uses a Piecewise Linear Interface Calculation (PLIC) method to reconstruct the interface between both phases in each computational cell. In-house developed codes calculate the mass and energy transfer phenomena occurring during this flow boiling process. As such, an existing CFD code is completed with a newly developed complete evaporation model. The developed model is used to simulate the flow boiling process of a hydrocarbon feedstock in the tubes of a convection section heat exchanger of a steam cracker. The simulation results show a succession of horizontal two-phase flow regimes in agreement with the literature. ?? 2008 Elsevier Ltd. All rights reserved.


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  Characterization of Multi-hole Spray and Mixing of Ethanol and Gasoline Fuels under DI Engine Conditions


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  Because of their robustness and cost performance, multi-hole gasoline injectors are being adopted as the direct injection (DI) fuel injector of choice as vehicle manufacturers look for ways to reduce fuel consumption without sacrificing power and emission performance. To realize the full benefits of direct injection, the resulting spray needs to be well targeted, atomized, and appropriately mixed with charge air for the desirable fuel vapor concentration distributions in the combustion chamber. Ethanol and ethanol-gasoline blends synergistically improve the turbo-charged DI gasoline performance, especially in down-sized, down-sped and variable-valve-train engine architecture. This paper presents the spray imaging results from two multi-hole DI gasoline injectors with different design, fueled with pure ethanol (E100) or gasoline (E0), under homogeneous and stratified- charge conditions that represent typical engine operating points. Both high-speed Schlieren and Mie scattering results are presented for tests in pressurized chamber and optical accessible engine. Multidimensional Computation Fluid Dynamics (CFD) model for predicting DI multi-hole ethanol spray behaviors in optical engine is also discussed. The effects of injection timing on the bulk flow motion and fuel- air mixing, in terms of tumble ratios, turbulence, and fuel wall film behaviors are also discussed.


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  Ultra-high speed imaging study of the diesel spray close to the injector tip at the initial opening stage with single injection


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  This study focused on the primary breakup of spray at the initial injector opening stage at injection to atmospheric conditions. A real-time mass flow rate instrument was first used to study the injection characteristics at the injector opening stage. A long distance microscope together with an ultrahigh speed CCD camera was then employed to study the primary breakup of diesel spray by photography technique with the help of backlighting. The mechanism of the formation of the mushroom shaped spray head was discussed and the laminar flow regime was expected to be the main reason for the resultant shape. During the initial spray stage, the mushroom spray head penetrated almost linearly with respect to time. The quantification of the fuel mass/spray area ratio suggested that the dispersion of the spray is greatly improved within a very short time, which is assumed to be caused by the further opening of injector.


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  Optical study of spray-wall impingement impact on early-injection gasoline partially premixed combustion at low engine load


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  Spray-wall impingement caused by early fuel injection for gasoline partially premixed combustion (PPC) can lead to low combustion efficiency and a significant rise of UHC emissions. But the influence of spray-wall impingement on the in-cylinder combustion process is not well understood. In this study, multiple optical diagnostics were applied to investigate the ignition, flame development and UHC formation of gasoline PPC with early single fuel-injection in a light-duty optical engine under low engine load. Natural combustion luminosity images and emission spectra were obtained. Planar laser-induced fluorescence (PLIF) of the fuel-tracer and formaldehyde were used to explore the fuel/air mixing and UHC formation in PPC, respectively. The results indicated that there was a fuel-injection time window (about −30° to −60° ATDC in the present study), during which the spray-impingement led to a decrease in combustion efficiency. The fuel-trapping effect in the squish region and piston crevice was shown to be the main reason because it prevented the fuel/air mixture from entering the combustion chamber. Two typical fuel injection timings of −35° (PPC-35) and −60° (PPC-60) were chosen for further study. For both cases, ignition sites first emerged in the fuel-rich regions and then the flames developed to the fuel-lean regions. The formaldehyde PLIF images revealed distinct flame front in the flame development process. For the PPC-35 case, residual formaldehyde persisted in the fuel-lean regions late during the power stroke and might become a source of UHC emissions. When misfire happened, the combustion chamber was filled with formaldehyde. For the PPC-60 case, the flame development was composed of initial flame front propagation and following sequential auto-ignition, and the flame expansion speed of the initial flame front propagation was much higher than that in SI (spark ignition) or SACI (spark assisted compression ignition) combustion. When the injection timing was further advanced (earlier than −60°), the impact of spray-wall impingement on PPC was reduced because of more time being available for fuel premixing.


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  Development of in-cylinder spray targeting, plume penetration and atomization of the gasoline direct-injection (GDi) multi-hole injector is a critical component of combustion developments, especially in the context of the engine downsizing and turbo-charging trend that has been adopted in order to achieve the European target CO2, US CAFE, and concomitant stringent emissions standards. Significant R&D efforts are directed towards the optimization of injector nozzle designs in order to improve spray characteristics. Development of accurate predictive models is desired to understand the impact of nozzle design parameters as well as the underlying physical fluid dynamic mechanisms resulting in the injector spray characteristics. This publication reports Large Eddy Simulation (LES) analyses of GDi single-hole skew-angled nozzles, with β=30° skew (bend) angle and different nozzle geometries. The objective is to extend previous works to include the effect of nozzle-hole length over diameter ratio (l/d) and fuel injection pressure on spray skew angle, spray plume cone angle and primary breakup length. The authors provide an LES analyses of a single hole nozzle representative of the a purpose-designed GDi multi-hole injector seat. The simulations are confirmed by comparison with the spray near-field breakup structure obtained through optical shadowgraphy and phase-contrast X-ray imaging techniques. The jet morphology indicates turbulent structures and primary atomization in the immediate vicinity of the nozzle exit. Nozzle l/d reduction produced a notable increase of plume cone angle and spray trajectory deviation (from geometric nozzle axis). The analyses provide insight into the fluid flow within the nozzle generating the observed spray plume changes. Overall, the comparison of LES and spray imaging data shows good predictive model capability with respect to jet breakup morphology, plume trajectory and plume angle. Copyright © 2014 SAE International.


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  Large Eddy simulation of a syngas jet flame: Effects of preferential diffusion and turbulence-chemistry interaction


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  A large eddy simulation of the turbulent syngas non-premixed jet flame of Sandia ETH/Zurich B is conducted using an in-house version of FireFOAM, a fire simulation solver within OpenFOAM. Combustion is modeled using (i) the newly extended eddy dissipation concept for the large eddy simulation published by the authors' group, (ii) the 74-step CO-H2-O2 mechanism and (iii) the relatively simple tabulated chemistry approach. The effects of the nonunity Lewis number and thermal diffusion (=Soret diffusion) are considered in the mass fraction and enthalpy equations. Systematic validation and model sensitivity studies have been conducted against published experiments of the turbulent syngas diffusion flame from the international workshop on measurement & computation of turbulent flames (TNF workshop). The predictions were in very good agreement with the relevant experimental data. The axial position of peak H2O moved toward the nozzle direction owing to the different diffusion coefficients of H2. In the radial direction, the effect of thermal diffusion was observed at x/d \textless 20, whereas those of the nonunity Lewis number and difference in chemistry were found at x/d \textless 40. After considering the effects of the nonunity Lewis number and thermal diffusion, as well as the detailed reaction mechanisms, the results were slightly better than those obtained under previous numerical conditions.


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  Detailed chemical kinetic reaction mechanisms for primary reference fuels for diesel cetane number and spark-ignition octane number


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  A detailed chemical kinetic reaction mechanism is developed for primary reference fuel mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethyl nonane for diesel cetane ratings. The mechanisms are constructed using existing rules for reaction pathways and rate expressions developed previously for the primary reference fuels for gasoline octane ratings, n-heptane and iso-octane. These reaction mechanisms are validated by comparisons between computed and experimental results for shock tube ignition and for oxidation under jet-stirred reactor conditions. The combined kinetic reaction mechanism contains the submechanisms for the primary reference fuels for diesel cetane ratings and submechanisms for the primary reference fuels for gasoline octane ratings, all in one integrated large kinetic reaction mechanism. Representative applications of this mechanism to several test problems are presented, describing fuel/air autoignition variations with changes in fuel cetane and octane numbers, and others describing fuel combustion in a jet-stirred reactor environment with the fuel varying from pure 2,2,4,4,6,8,8-heptamethyl nonane (cetane number of 15) to pure n-hexadecane (cetane number of 100). ?? 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.


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  The present work reports a one-dimensional model to predict the liquid length and spray penetration of diesel sprays when using blends of single-component fuels. A high-pressure liquid-vapour equilibrium has been implemented by means of fugacity coefficients, together with the hypothesis of a real-gas mixture to calculate the partial enthalpy of each component. The model has been validated in a first step by means of an experimental study using binary blends of n-decane and n-hexadecane, where the temporal evolution of the liquid length and vapor spray penetration have been measured. Results show that the model predicts adequately the spray penetration for the different fuels at various conditions. A six-component fuel has also been investigated. Results indicate that this fuel has a very similar evaporative behavior to n-hexadecane, which is confirmed by both experiments and model predictions.


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  A Transport Equation Residual Model Incorporating Refined G-Equation and Detailed Chemical Kinetics Combustion Models


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  combustion models has been developed and implemented in the ERC KIVA-3V release2 code for Gasoline Direct Injection (GDI) engine simulations for better predictions of flame propagation. In the transport equation residual model a fictitious species concept is introduced to account for the residual gases in the cylinder, which have a great effect on the laminar flame speed. The residual gases include CO2, H2O and N2 remaining from the previous engine cycle or introduced using EGR. This pseudo species is described by a transport equation. The transport equation residual model differentiates between CO2 and H2O from the previous engine cycle or EGR and that which is from the combustion products of the current engine cycle. The refined G-equation and detailed chemical kinetics include revision of a PRF chemistry mechanism, and introduction of a Damkohler criterion which determines whether the G-equation model or chemical kinetics should be used for assessing the combustion processes in flame-containing cells. Validation of the revised PRF mechanism shows that the calculated ignition delay matches shock tube data very well. The Damkohler criterion is based on a comparison between a laminar flame propagation time scale and the chemical kinetics time scale, and the results from implementation of the Damkohler model range between the G-equation model and pure chemistry, depending on the conditions. The integrated model was used to simulate the combustion process in a Gasoline Turbocharged Direct Injection (GTDI) engine, and the same set of combustion model parameters for both high load and low load were used. For both high load and low load operating conditions, good agreement with experimental in-cylinder pressure, heat release rates and Mass Fraction Burned (MFB) data was obtained.


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  Comparison of turbulent flame speeds from complete averaging and the G- equation Comparison of turbulent and the G-equation flame speeds from complete averaging


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  Characterization of the Near-Field Spray and Internal Flow of Single-Hole and Multi-Hole Sac Nozzles using Phase Contrast X-Ray Imaging and CFD


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  It is well know that the internal flow field and nozzle geometry affected the spray behavior, but without high-speed microscopic visualization, it is difficult to characterize the spray structure in details. Single-hole Diesel injectors have been used in fundamental spray research, while most directinjection engines use multi-hole nozzle to tailor to the combustion chamber geometry. Recent engine trends also use smaller orifice and higher injection pressure. This paper discussed the quasi-steady near-nozzle Diesel spray structures of an axisymmetric single-hole nozzle and a symmetric twohole nozzle configuration, with a nominal nozzle size of 130 Î¼m, and an attempt to correlate the observed structure to the internal flow structure using computational fluid dynamic (CFD) simulation. The test conditions include variation ofinjection pressure from 30 to 100 MPa, using both Diesel and Biodiesel fuels, under atmospheric condition. The imaging technique utilizes a 150 pico-second synchrotron-based Ultrafast Phase-contrast X-ray in order to freeze-capture the fast moving jet and achieve excellent spatial resolution in order to compare the wavy jet structure. Both nozzle holes originate from a sac of identical geometry, but different flow structure inside the nozzle cause significant difference in the observed flow structure near the nozzle exit. The ultrafast fast images revealed unique surface and internal morphology of the fuel sprays that can be identified. The two-hole nozzle produces much more unstable jet structure under same injection conditions. The early wavelength developed in thejet is measured to be on the order of 30âˆ¼80 Î¼m, with the frequency range of 5 to 10 MHz, depending on the injection conditions up to 60 MPa injection pressure when the wavelength analysis is still feasible. The differences between the nozzle configurations are investigated using CFD simulation. The results show that the three-dimensional fluid flow entering the two-hole nozzle generates stronger streamline curvature and stream-wise counter-rotational vortices which are by default absent in the axisymmetric single-hole nozzle. It also produces thicker shear layer and higher turbulence. The interactions of downwash entrance flow with turbulence potentially enhance the instability and produce wider spray cone angles.


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  A detailed chemical kinetic mechanism has been developed to describe the oxidation of small hydrocarbon and oxygenated hydrocarbon species. The reactivity of these small fuels and intermediates is of critical importance in understanding and accurately describing the combustion characteristics, such as ignition delay time, flame speed, and emissions of practical fuels. The chosen rate expressions have been assembled through critical evaluation of the literature, with minimum optimization performed. The mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactor, shock tube, jet-stirred reactor, and flame studies. The current mechanism contains accurate kinetic descriptions for saturated and unsaturated hydrocarbons, namely methane, ethane, ethylene, and acetylene, and oxygenated species; formaldehyde, methanol, acetaldehyde, and ethanol.


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  Nozzle internal flow and spray primary breakup with the application of closely coupled split injection strategy


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  Nozzle internal flow dictates the spray breakup, fragmentation and combustible mixture preparation. The dynamics of nozzle internal flow and the closely followed primary breakup under various injection pressures were studied by employing high-speed microscopic imaging technique. The effects of interaction between split injection by varying dwell interval and distribution of energizing duration between split injections on the nozzle internal flow were also investigated by using multiple injection strategy. It was found that the initial in-nozzle condition and injection pressure governs the spray tip morphology and breakup process through the strength of interaction between liquid fuel and air bubbles. Throttling effect introduced a large cloud of cavitation vapor bubbles at the needle seat during the initial stage and when stage when effective injection pressure is low. During steady flow stage, high pressure suppressed the initiation of cavitation through throttling but enhanced the cavitation generation through flow redirection. When multiple injection strategy is employed, cavitation was overall weakened, especially for flow redirection at the nozzle inlet due to reduced effective injection pressure. The injected fuel mass was reduced, and breakup quality deteriorated significantly, especially under low injection pressure. It is therefore unwise to use closely coupled split injection strategy under low injection pressure. This studied is believed to be useful for injection control and related modeling.


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  Investigation of Diesel and CNG Combustion in a Dual Fuel Regime and as an Enabler to Achieve RCCI Combustion


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  The advantages of applying Compressed Natural Gas (CNG) as a fuel for internal combustion engines are well known. In addition to a significant operating cost savings due to a lower fuel price relative to diesel, there is an opportunity to reduce the engine's emissions. With CNG combustion, some emissions, such as Particulate Matter (PM) and Carbon Dioxide (CO2), are inherently reduced relative to diesel fueled engines due to the nature of the combustion and the molecular makeup of the fuel. However, it is important to consider the impact on all emissions, including Total Hydrocarbons (THC) and Carbon Monoxide (CO), which can increase with the use of CNG. Nitrogen Oxides (NOx) emission is often reported to decrease with the use of CNG, but the ability to realize this benefit is significantly impacted by the control strategy and calibration applied. FEV has investigated the emissions and performance impact of operating a heavy-duty diesel engine with CNG in a dual fuel mode. The CNG was introduced via injectors mounted to an inlet pipe located upstream of the intake manifold. The fumigation approach included a mixer to improve the distribution of gas prior to delivery to the cylinder. The initial investigations sought to determine how the performance of a heavy-duty diesel engine would be affected by the introduction of CNG. For this effort there was no change to the base engine calibration, and the ability to maximize substitution of diesel with CNG was investigated. It was observed that the ability to maximize substitution of diesel with CNG across the operating map was limited by extremely high THC levels, combustion instability and limitations in peak cylinder pressure and exhaust gas temperature. With the application of a simplified engine calibration with a single diesel injection and Exhaust Gas Recirculation (EGR), timing adjustments allowed higher CNG substitution levels in several areas of the operating map. A further increase in gas substitution along with higher fuel conversion efficiency, improved combustion stability and even lower emissions could be achieved through Reactivity Controlled Compression Ignition (RCCI) combustion. This approach required a unique injection strategy along with a careful balance of EGR rates and boost pressure. Under this combustion regime it was possible to observe a simultaneous reduction of NOx and PM emissions, approaching engine-out emission levels that could avoid, or significantly minimize, aftertreatment of these species. With the desire to quickly apply CNG systems to existing diesel engine architecture in an effort to reap the benefit in fuel cost savings, manufacturers and system developers must be careful to understand the full impact on the engine's performance and emissions. Tests conducted as part of this investigation have revealed that an un-optimized approach to CNG introduction can lead to extreme THC emissions that mostly consist of Methane (CH4). In addition, the maximum gas substitution level is significantly limited in most regions of the engine operating map. Thus, the ability to specifically tune the calibration for operation with CNG is essential to achieving the maximum benefit in fuel cost savings and emission control. Copyright © 2014 SAE International.


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  A reduced mechanism for predicting the ignition timing of a fuel blend of natural-gas and n-heptane in HCCI engine


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  Improvements to Combustion Models for Modeling Spark-Ignition Engines Using the G -equation and Detailed Chemical Kinetics COMBUSTION MODELS


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  Experimental study of the injection conditions influence over n-dodecane and diesel sprays with two ECN single-hole nozzles. Part II: Reactive atmosphere


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  The second part of this experimental analysis, presented in this paper, seeks to go deep on the characterization of the Spray C and Spray D nozzles from the Engine Combustion Network, investigating the penetration of fuel spray at reacting conditions alongside characteristic parameters of combustion such as ignition delay and lift-off length. Both ECN mono-orifice injectors have similar nozzle flow capacity but different conicity degrees and corner sharpness, being Spray C more susceptible to cavitate. Schlieren imaging technique was employed to quantitatively measure reactive penetration and ignition delay, while lift-off length was identified through OH∗chemiluminescence. As in the inert part of this research, n-dodecane and commercial diesel were selected for the tests, thereby the effect of the fuel properties in the measured parameters was analyzed. Also, once again the concept of R-parameter, defined as the penetration derivative respect to the square root of time was calculated to delve into the penetration behavior. The experiments were performed in a constant pressure-flow facility able to reproduce engine-like thermodynamic conditions. Results revealed that R-parameter evolution can be divided in four stages: an inert zone, a ‘bump', a ‘valley' part and a quasi-steady one that overlaps the previous inert part. Those stages are highly governed by ambient temperature and oxygen concentration. Nozzle geometry and fuel properties demonstrated to have a noteworthy influence on all measured parameters.


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  Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methane–air mixture


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  In this study, the chemical, thermal and diffusion effects of H2 and CO addition on the characteristics of methane laminar flame are examined numerically by using the CHEMKIN 2.0 code with a modified GRI-Mech 3.0 mechanism. The results reveal that a better agreement between the measured and predicted laminar flame speed is obtained by using the modified mechanism. The effect of H2 addition to flame speed is mainly due to chemical effect at lean and stoichiometric conditions. However, with the addition of CO, around 75% of the increase in laminar flame speed is due to thermal effect. Soret diffusion of H have an obviously effect on laminar flame speed, lowing it around the peak segment. Furthermore, the combined Soret effect of H and H2 basically accounts for the total Soret diffusion effect. With the addition of H2 and CO, the lean flammability limit is extended to a leaner mixture at atmospheric pressure. A linear correlation between the laminar flame speed and the relative amount of H2 was found in tertiary CH4/CO/H2 mixtures. However, this linear correlation is undermined when CO mole fraction is over than 45%.


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  To provide more insight into the laminar burning velocity reduction of diluted syngas premixed flames, the laminar burning velocities of diluted syngas premixed (H2/CO/N2/air and H2/CO/CO2/air) laminar spherical flames were systematically and thoroughly investigated experimentally via Schlieren technology. In the present investigation, the volume fraction of H2 was varied from 30% to 70%, the volume fraction of CO was varied from 30% to 70%, the volume fraction of CO2 was varied from 0% to 60%, and the equivalence ratio was varied from 0.7 to 1.0. In the present investigation, the effects of equivalence ratio, hydrogen fraction, and dilution fraction on the laminar burning velocities were studied. Compared to N2 dilution, the laminar burning velocity reduction with CO2 dilution was analysed through the thermal, transport, and chemical effects. In addition, the contributions of third-body and direction effects were also studied.


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  AbstractIn the current work, a series-hybrid vehicle has been constructed that utilizes a dual-fuel, Reactivity Controlled Compression Ignition (RCCI) engine. The vehicle is a 2009 Saturn Vue chassis and a 1.9L turbo-diesel engine converted to operate with low temperature RCCI combustion. The engine is coupled to a 90 kW AC motor, acting as an electrical generator to charge a 14.1 kW-hr lithium-ion traction battery pack, which powers the rear wheels by a 75 kW drive motor.Full vehicle testing was conducted on chassis dynamometers at the Vehicle Emissions Research Laboratory at Ford Motor Company and at the Vehicle Research Laboratory at Oak Ridge National Laboratory. For this work, the US Environmental Protection Agency Highway Fuel Economy Test was performed using commercially available gasoline and ultra-low sulfur diesel. Fuel economy and emissions data were recorded over the specified test cycle and calculated based on the fuel properties and the high-voltage battery energy usage.The results were used to provide estimates of fuel economy and emissions performance with comparisons to a commercially available hybrid vehicle. The estimates also provide guidelines for system improvements to further increase the vehicle performance.


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  Analysis of diesel spray dynamics using a compressible Eulerian/VOF/LES model and microscopic shadowgraphy


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  This paper presents numerical and experimental analysis of diesel engine spray dynamics in the region very close to the nozzle exit. Diesel fuel is injected through a single solid cone injector with sharp-edged nozzle inlet. Numerical investigations are conducted in an Eulerian framework by applying a Volume of Fluid interface capturing technique integrated with Large-Eddy Simulation turbulence modelling. Cavitation is modelled, by allowing liquid fuel to flash to gas at the fuel vapor pressure. The computational domain and settings mimic the experimental injector internal geometry and experimental operating conditions. In-nozzle disturbances are qualitatively well modelled by implementing the no-slip condition at the injector walls as well as cavitation and compressibility effects for each phase. A mesh dependency study is conducted with four different grid resolutions. Data are presented around the start of penetration (SOP) and up to the time when shock waves at the gas-liquid interface are well developed, the quasi-steady stage of injection. At SOP, an umbrella-shaped leading edge is captured in both the numerical and experimental studies however only the experimental images demonstrated a semi-transparent cloud of air-fuel mixture at the leading edge. A previously undescribed toroidal starting vortex near the nozzle exit is captured experimentally and numerically. Development of cavitation, down to the end of nozzle hole leads to the detachment of liquid from the nozzle hole walls and subsequently the diminution of boundary layer effects and thus reduced in-nozzle turbulence, and increased liquid jet velocity.


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  Experimental assessment of various methods of determination of laminar flame speed in experiments with expanding spherical flames with positive Markstein lengths


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  In this work, we simulated the convective section of Unit I of Meliti Power Plant (330 MWel) in Florina, Greece, using the macro heat exchanger model (MHEM). As boundary conditions, previous temperature and velocity field data have been used, referring to the exit surface of combustion chamber. The MHEM approximates the pressure losses, using the porous media model, and the heat transfer, using the number of transfer units (NTU) model. The results have been validated against standard operating data, provided by the plant manufacturer. The working fluid outlet temperature for each heat exchanger, the total heat transfer, and the temperature distribution throughout the whole convective section have been calculated, showing good agreement with the respective data, under full operational load. Further, a parametric investigation of the level has been conducted, in order to validate the applied boundary conditions. Overall, we evidence that the MHEM can be a quite effective alternative for heat exchanger simulations. ?? 2013 Elsevier Ltd. All rights reserved.


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  Effect of diesel injection strategies on natural gas/diesel RCCI combustion characteristics in a light duty diesel engine


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  Reactivity controlled compression ignition (RCCI) combustion mode is an attractive combustion strategy due to its potential in satisfying the strict emission standards. In this study, the effects of direct injection (DI) strategies on the combustion and emission characteristics of a modified light duty RCCI engine, fueled with natural gas (NG) and diesel were numerically investigated. In this way, Converge CFD code employing a detail chemical kinetics mechanism was used for 3D simulation of combustion process and emissions prediction. NG with higher octane number (ON) is mixed with air through intake port, while diesel fuel with lower ON is directly injected into the combustion chamber during compression stroke by means of split injection strategy. The effects of several parameters, including the premixed ratio (PR) of NG, diesel fuel fraction in first and second injection pulses, first and second start of injection timing (SOI1 and 2), injection pressure and the spray angle on the engine performance and emission characteristics are investigated. The results indicate that these parameters have significant effects on the light duty RCCI engine performance and engine out emissions. Also, it was demonstrated that by decreasing the first injection pressure from 450 to 300 bar, the gross indicated efficiency increases by 5% and CA50 is retarded by 4 CAD. Moreover, by reducing the spray angle from 144° to 100°, the gross indicated efficiency decreases by 4% and CA50 is advanced by 6 CAD. The results showed that reduction in NOx emission is achievable, while controlling HC and CO emissions, by means of increasing the NG fraction, advancing the SOI1, increasing the fuel fraction in first DI injection with lower injection pressure and employing a wider injector spray angle.


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  CFD Modeling of Spark Ignited Gasoline Engines- Part 1 : Modeling the Engine under Motored and Premixed-Charge Combustion Mode Experimental Apparatus : SI Engine


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  When oxygen reacts with pyro graphite at high flow' raües and at pressures of about 0.2 atm, the reaction rate increases steeply with temperature from 1050"C to 1700'C; above ühis temperature the rate ceases to rise and the reaction curve becomes nearly horizontal. Since much faster rates of reaction are observed with other forms of carbon under otherwise idenüical reaction conditions, the pyro graphite rates must be genuine chemical rates not, substantially influenced by gas phase mass transfer. In the past the reacüions of oxygen at very low pressures with carbon filaments have shown that the reaction curve under üheso conditions reaches a maximum aü a temperature of about ll00"C, which is 600'C less than the temperaüure at which the maximum rato wiüh pyro graphite at a pressure of 0.2 atm was observed. The theory of Blyholder, Binford and Ey'ring, slightly modified, is shown to account satis- facúorily for this difference between lhe low and high pressure results. The reacüions of reactor quality graphite with oxygen have also been studied. The reaction curves reach a maxirnum at about 1600'C in a manner very similar to those ofpyro graphite, and the rate then falls approciably between 1600'C and 2000'C. The rates in this case are considerably higher than with pyro graphite, and they are certainly parüly mass-transfer controlled: but the similariúy in shape with the pyro graphite results suggests that, alühough the absolute values of ühe rates may be in need of considerable correct'ion for mass transfer, the curves for reactor graphite aro in broad ouüline of a shape determined by chemical factors.


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  With the increase in the diesel injection pressure in modern diesel engines, the velocity of the diesel spray increases accordingly, and shock waves are induced when the spray is supersonic. To better understand the effect of shock waves on the spray development, the evolution of the high-pressure diesel spray and shock wave under different diesel injection pressures and ambient pressure has been investigated using the Schlieren imaging method. The results show that the shock wave affects the development of the spray and the mixing effect between the spray and surrounding air. The shock wave promotes development of the spray tip penetration and enhances the gas entrainment effect. However, the shock wave has an inhibitory effect on the spray development in the radial direction. In addition, the shock wave angle determines the range of influence on the ambient air, which affects atomization of the spray. The shock wave angle increases with the decrease in the spray front Mach number. An empirical correlation of the shock wave angle and spray front Mach number was proposed in this paper. The calculated results are in good agreement with the experimental data. In addition, a diagram of the shock wave generation and classification in a diesel engine is proposed.


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  Turbulent Flame speed and thickness : phenomenology , evaluation , and application in multi-dimensional simulations


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  A model for the laminar flame speed of binary fuel blends and its application to methane/hydrogen mixtures


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  Recent studies have demonstrated promising performance of adding hydrogen to methane in internal combustion engines and substantial attention has been devoted to binary fuel blends. Due to the strong nonlinearity of chemical reaction process, the laminar ﬂame speed of binary fuel blends cannot be obtained from linear combination of the laminar ﬂame speed of each individual fuel constituent. In this study, theoretical analysis is conducted for a planar premixed ﬂame of binary fuel blends and a model for the laminar ﬂame speed is developed. The model shows that the laminar ﬂame speed of binary fuel blends depends on the square of the laminar ﬂame speed of each individual fuel component. This model can predict the laminar ﬂame speed of binary fuel blends when three laminar ﬂame speeds are available: two for each individual fuel component and the third one for the fuel blends at one selected blending ratio. The performance of this model as well as models reported in the literature is assessed for methane/hydrogen mixtures. It is demonstrated that good agreements with calculations or measurements can be achieved by the present model prediction. Moreover, it is found that the present model also works for other binary fuel blends besides methane/hydrogen.


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  Enhancement of a laminar premixed methane/oxygen/nitrogen flame speed using femtosecond-laser-induced plasma


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  Correlations for the Laminar Flame Speed, Adiabatic Flame Temperature and Ignition Delay Time for Methane, Ethanol and n-Decane


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  Extension of PREMIER combustion operation range using split micro pilot fuel injection in a dual fuel natural gas compression ignition engine: A performance-based and visual investigation


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  The effects of split pilot fuel injection on engine performance and PREmixed Mixture Ignition in the End Gas Region (PREMIER) combustion characteristics were investigated in a single-cylinder dual-fuel natural gas engine ignited with diesel fuel. In particular, the effect of second spray timing on combustion mode was examined. PREMIER combustion was observed in a wider range of operating conditions with split injection strategy compared to single injection strategy. We determined that it was possible to both decelerate heat release and suppress knocking to PREMIER combustion, and accelerate heat release and promote normal combustion to PREMIER combustion, with suitable second injection timing. The maximum of thermal efﬁciency of PREMIER combustion operation with split injection was close to the results obtained with knocking operation. In-cylinder images showed that split injection strategy advances or retards the progress of combustion by controlling the size and rate of growth of ﬂame kernels, depending on the timing of the second injection. The combustion progress is earlier when the pilot fuel delivered during the ﬁrst injection autoignites during the second injection. Kernel growth and the ﬁnal size were adversely affected when the second injection was initiated after pilot fuel autoignition.


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  Extension of PREMIER combustion operation range using split micro pilot fuel injection in a dual fuel natural gas compression ignition engine


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  Numerical Study on the Combustion Process of n-heptane Spray Flame in Methane Environment Using Large Eddy Simulation


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  Large eddy simulation is applied for the combustion process of n-heptane spray ﬂame in methane environment. The inﬂuence of methane (CH4) concentration on the auto-ignition process and ﬂame structures are discussed in detail. By increasing the CH4 concentration in the ambient gas, auto-ignition is delayed obviously at a low initial temperature because of the delayed ﬁrst-stage ignition. While at a high temperature, the auto-ignition time is less inﬂuenced by the addition of CH4. However, after the addition of CH4, the concentration of the mixture where the high-temperature ignition is initialed becomes much richer and it is independent of the initial temperatures. In addition, the chemical explosive mode analysis (CEMA) method shows that the mixture in high-temperature regions is non-explosive without CH4 addition, and the abrupt transition between the explosive and non-explosive mixture at the tip of the reacting spray is observed by adding CH4 in the ambient gas, indicating that a ﬂame front is developing downstream. This ﬂame front exists for a long time at low CH4 concentration. The eigenvalue of the Jacobian matrix for the CH4/air mixture in the ambient gas is increased signiﬁcantly with the addition of CH4, indicating that the combustion process is dominated by auto-ignition. Finally, results show that the development of the ﬂame is accelerated by the addition of CH4 in the ambient gas.


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  Mechanism of hydrocarbon reduction using multiple injection in a natural gas fuelled/micro-pilot diesel ignition engine


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  Research has shown that a large amount of natural gas (NG) is unburned at light loads in an NG fuelled/micro-pilot diesel compression ignition engine. A mechanism of unburned hydrocarbon (HC) reduction using multiple injections of micro-pilot diesel has been proposed in this paper. Multidimensional computations were carried out for a dual-fuel engine based on a modified CAT3401 engine configuration. The computations show that a split injection with a small percentage (e.g. 30 per cent of diesel in the second injection pulse) can significantly reduce HC, CO and NOxemissions. Based on parametric studies to optimize the timing of both of the injection pulses, HC emissions could be reduced by 90 per cent, with a reduction in CO emissions of 50 per cent and NOx emissions of 70 per cent in comparison to a singleinjection pulse-base case configuration.


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  Effect of the diesel injection timing and the pilot quantity on the combustion characteristics and the fine-particle emissions in a micro-diesel pilot-ignited natural-gas engine


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  Natural gas is a potential alternative to conventional liquid fuels for utilization in automotive internal combustion engines. Its utilization can significantly reduce the levels of nitrogen oxide and particulate matter emissions from dual-fuel diesel/ natural-gas engines. However, this reduction in the particulate matter emissions may induce an increase in the quantity of particles of smaller size, which are harmful to human health. The engine parameters, namely the diesel injection timing and the pilot quantity directly, affect the homogeneity of the fuel–air mixture, the spatial distribution of initial ignition centres and the quantity of initial ignition centres. In this paper, the effects of these two engine parameters on the combustion and the particle emission characteristics of micro-diesel pilot-ignited natural-gas fuel are investigated. The combustion parameters were calculated from the cylinder pressure data. An electrical low-pressure impactor was employed to measure the particle number concentration and particle size distribution. Experimental results indicate that the ignition delay period is prolonged and the particle number concentration progressively increases with advance of the injection timing, while the particle mass concentration undergoes a steady decrease and then a rapid increase. On the other hand, increasing the pilot diesel quantity can shorten the ignition delay period and enhance the burning rate. Meanwhile, the particle mass concentration increases evidently but the particle number concentration decreases unexpectedly. It should be noted that the total number of fine particles is sensitive to advancing injection and decreasing pilot mass, especially above the 20° crank angle before top dead centre injection timing and below the basic pilot mass.


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  The simultaneous reduction of fuel consumption and pollutant emissions, namely NOx and soot, is the predominant goal in modern engine development. In this context, low temperature combustion concepts are believed to be the most promising approaches to resolve the above mentioned conflict of goals. Disadvantageously these combustion concepts show high peak pressures or high rates of pressure rise due to early ignition and high reaction rates especially at high loads. Furthermore, there are still challenges in controlling combustion phasing. In this context using a small amount of pilot diesel injected directly into the combustion chamber to ignite a highly diluted gasoline air mixture can overcome the aforementioned difficulties. As the gasoline does not ignite without the diesel, the pilot injection timing can be used to control combustion phasing. By increasing dilution even high loads with low rates of pressure rise and without knocking are possible. This paper shows the results of experimental investigations carried out on a heavy duty boosted single cylinder diesel engine. Based on the indicated cylinder pressure, the combustion process is characterized by performing knock analyses as well as thermodynamic analyses. Furthermore, an optically accessible engine has been set up to investigate both the diesel injection and the combustion process by means of digital high speed imaging. Together with the thermodynamic analyses the results of these optical investigations make up the base for the presented theoretical model of this combined diesel-gasoline combustion process. To show the load potential of this Dual-Fuel-CAI concept, the engine was operated at 2100 1/min with an IMEP of 19 bar. NOx emissions did not exceed 0.027 g/kWh.


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  https://asmedigitalcollection.asme.org/gasturbinespower/article/doi/10.1115/1.4005997/455691/Controlling-Gasoline-Low-Temperature-Combustion-by


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  The medium-speed dual-fuel engine has become popular in the marine industry for its advantages of fulfilling the stringent emission regulations and relative affordability of natural gas. In such engines, the ignition process importantly influences the subsequent combustion processes and engine performance. This work developed a phenomenological micro-pilot ignition model with a minimal number of tuning parameters aiming to improve the understanding of the ignition event and enable better control of the dual-fuel engine. The model comprises of a spray and a chemistry submodel to accurately capture the interaction between the direct injection of a small amount of diesel fuel (called micro-pilot) and a two-stage ignition of the diesel fuel mixed with the surrounding reactive charge in relatively low temperature. A 1D transient spray model is adapted to reproduce the micropilot spray characteristics by assuming a realistic trapezoidal fuel injection profile and the varying discharge coefficient during the transient spray period. The chemical reactions are modeled with a 0D transient flamelet approach based on an opposed flow reactor. The model is validated using three sets of experimental data, namely ECN Spray A (constant volume chamber), RCEM with optical accessibility, and finally, medium-speed dual-fuel engine. Quantitatively good predictions of the spray formation, ignition delay, and ignition location over broad conditions ranging from the conventional diesel ignition to the micro-pilot ignition in the dual-fuel engine are demonstrated. Finally, the developed model is used to explore the characteristics of micro-pilot ignition under conditions relevant to the medium-speed dual-fuel engines.


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  Numerical investigation of the impact of gas composition on the combustion process in a dual-fuel compression-ignition engine


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  This study discusses the model of operation of a dual-fuel compression-ignition engine, powered by gaseous fuel with an initial dose of diesel fuel as the ignition inhibitor. The study used a zerodimensional multiphase mathematical model of a dual-fuel engine to simulate the impact of enhancing Natural Gas (NG) with other gases on the combustion process. The model simulated the thermodynamic parameters of the gas mixture in the cylinder of a dual-fuel (NG/Diesel), turbocharged, four cylinder CRDI (Common-Rail Direct Injection) engine. The tests discussed herein were conducted for steady state engine operation, for partial load and constant consumption of gaseous fuel. In the discussed tests, carbon dioxide and higher hydrocarbons (ethane and propane) were used as additions to NG.


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  The use of calculation software for pedagogical purposes in degree courses is a central tool for addressing disciplinary knowledge in engineering careers. Particularly, open source and free license tools generally have lower associated costs than their propriety equivalents while offering a similarly rich set of functionalities. This document presents a software tool called guiEditor designed to create script applications based on graphical interfaces for GNU Octave. guiEditor is a visual editor, which incorporates different functionalities aimed at allowing the development of graphical applications based on basic programming knowledge. Within the development environment it incorporates, in addition to the typical controls of the graphical interfaces, new controls that extend the script language used. In addition, as an example of the development environment potential, two applications created with guiEditor are presented. The assessment made by the students about a third application, called ltitool, which has been used in degree courses to address Process Control topics, is also presented.


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  In reciprocating internal combustion engines, the Otto cycle indicates the best thermal efficiency under a given compression ratio. To achieve an ideal Otto cycle, combustion must take place instantaneously at top dead center, but in fact, this is impossible. Meanwhile, if we allow slower piston motion around top dead center, combustion will be promoted at that period; then both the in-cylinder pressure and degree of constant volume will increase, leading to higher thermal efficiency. In order to verify this hypothesis, an engine with slower piston motion around top dead center, using an ideal constant volume combustion engine, was built and tested. As anticipated, the degree of constant volume increased. However, thermal efficiency was not improved, due to increased heat loss. Accordingly more experiments, which achieved a slower piston motion around top dead center by adopting a larger ratio between the connecting-rod length and the crank radius, were carried out using direct injection stratified charge combustion, which allows selective reduction of heat loss. High thermal efficiency was attained, as expected. On the other hand, an engine with a faster piston motion around top dead center, created by decreasing the ratio between connecting-rod length and crank radius, attained high thermal efficiency with quick burn premixed spark ignition combustion.


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  The conversion of heavy-duty CI engines to natural gas (NG) SI operation have the potential to increase the use of NG in the transportation sector in the United States. More, the increased turbulence of a bowl-in-piston combustion chamber can increase the ﬂame speed under more eﬃcient lean conditions. The main objective of this study was to investigate if a 3D G-equation-based RANS simulation (i.e., reasonable computational costs and running times) can predict the eﬃciency and emissions of such converted engine, for various NG compositions and operating conditions. The model was validated with experimental data from a single-cylinder CI research engine that replaced the fuel injector with a spark plug and fumigated NG inside the intake manifold using a lowpressure gas injector. Using a unique set of model tuning parameters, the model was able to qualitatively predict the eﬀect of NG composition on engine performance and emissions over a range of operating conditions that changed spark timing, equivalence ratio, and engine speed. The model also captured the double-peak heat release rate seen at advanced spark timing in the experiments. The results showed that a lower methane number (MN) increased peak pressure and indicated mean eﬀective pressure. Higher H/C ratio advanced combustion phasing. More, higher MN lowered nitrogen oxides but increased unburned hydrocarbons emissions. However, while a lower MN increased carbon monoxide (CO) production during the combustion process, there was no clear trend for engine-out CO emissions. Overall, the predicted gas composition eﬀects on engine eﬃciency and emissions were relatively small, at least for the range of operating conditions investigated here. However, the results suggest that the 3D CFD model described here is suitable for combustion phenomena analysis such the ﬂame behavior in a bowl-in-piston combustion chamber.


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  Prediction of autoignition in a lifted methane/air flame using an unsteady flamelet/progress variable model


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  An unsteady ﬂamelet/progress variable (UFPV) model has been developed for the prediction of autoignition in turbulent lifted ﬂames. The model is a consistent extension to the steady ﬂamelet/progress variable (SFPV) approach, and employs an unsteady ﬂamelet formulation to describe the transient evolution of all thermochemical quantities during the ﬂame ignition process. In this UFPV model, all thermochemical quantities are parameterized by mixture fraction, reaction progress parameter, and stoichiometric scalar dissipation rate, eliminating the explicit dependence on a ﬂamelet time scale. An a priori study is performed to analyze critical modeling assumptions that are associated with the population of the ﬂamelet state space.


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  Effect of turbulence–chemistry interactions on chemical pathways for turbulent hydrogen–air premixed flames


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  Ñ Combustion Modeling with the G-Equation Ñ Numerical investigations concerning the turbulent flame front propagation in Gasoline Direct Injection (GDI) engines were made by implementing a flamelet model in the CFD code Fire. The advantage of this combustion model is the decoupling of the chemistry from the turbulent flow. For this purpose the combustion chamber has to be divided into a burned and an unburned area, which is realized by transporting a scalar field (G-Equation). The reference value defines the present averaged flame position. The complete reaction kinetics is calculated interactively with the CFD code in a one dimensional Representative Interactive Flamelet (RIF) code. This combustion model was verified by simulating a 2.0 l-2 V gasoline engine with homogeneous combustion where a parameter study was conducted to check the flamelet model for plausibility. Finally, the potential of this combustion model was investigated by simulating a hypothetical 2.0 l-4 V GDI engine.


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  With the increasing concern regarding diesel engine emissions, including nitrogen oxides (NOX) and particulate matter (PM), and the rising of energy demand as well, the utilization of alternative fuels in diesel engine has been found to be an attractive solution. Natural gas is a very promising and highly attractive fuel because of its domestic availability, widespread distribution infrastructure, low cost, and clean-burning qualities to be used as a transportation fuel. Natural gas/diesel dual fuel is an operation mode in which natural gas is introduced into the intake air upstream of the manifold and then ignited by the direct injected diesel in the cylinder. The aim of this paper is to identify the potential use of natural gas/diesel dual fuel on diesel engine. In this literature review, the combustion, emission and performance characteristics of natural gas/diesel dual fuel combustionmode published mainly in scientiﬁc journals have been collected and critically analyzed. A wide range of natural gas mass ratio which represents the mass fraction of natural gas in the total fuel and different types of engines were involved. It has been found that dual fuel mode has a lower compression pressure and a longer ignition delay compared with normal diesel mode. The application of dual fuel mode signiﬁcantly decreases the NOX, carbon dioxide (CO2) and PM emissions. However, the hydrocarbon (HC) and carbon monoxide (CO) emissions may increase by several times or even more than 100 times in comparison to normal diesel combustion. And there appears a trade-off relationship between NOX and HC emissions with dual fuel mode. The engine power is decreased up to 2.1% at dual fuel mode, but the power loss can be reduced or recovered by changing some of the operating parameters. The brake thermal efﬁciency (BTE) of dual fuel mode is lower at low and intermediate loads, while under high engine load conditions it is similar or a little higher when compared with normal diesel mode, and the maximum increase is about 3%. The COVIMEP seems to be generally higher than normal diesel mode and it decreases with the increasing engine load.


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  Modeling turbulence–chemistry interaction in lean premixed hydrogen flames with a strained flamelet model


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  With increasing interest to reduce the dependency on gasoline and diesel, alternative energy source like compressed natural gas (CNG) is a viable option for internal combustion engines. Spark-ignited (SI) CNG engine is the simplest way to utilize CNG in engines, but direct injection (DI) Diesel-CNG dual-fuel engine is known to offer improvement in combustion efficiency and reduction in exhaust gases. Dual-fuel engine has characteristics similar to both SI engine and diesel engine which makes the combustion process more complex. This paper reports the computational fluid dynamics simulation of both DI dual-fuel compression ignition (CI) and SI CNG engines. In diesel-CNG dual-fuel engine simulations and comparison to experiments, attention was on ignition delay, transition from auto-ignition to flame propagation and heat released from the combustion of diesel and gaseous fuel, as well as relevant pollutants emissions. Injection timing for diesel pilot was changed in a test matrix and the most efficient and least pollutant producing cases were selected and simulation results were correlated to the experimental data for those cases. The possible end-gas auto-ignition-like processes under certain test conditions like PREmixed Mixture Ignition in the End-gas Region (PREMIER) were also demonstrated. SI CNG lean burn engine tests and simulations were directed to check the ignitability, combustion stability, heat release rate for different spark energy values. The results show that a combined approach of using both chemical kinetics and G-equation formulations in RANS is capable of capturing most of the physical-chemical processes in both the dual-fuel CI and SI engine combustions.


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  An Experimental and Computational Investigation of Water Condensation inside the Tubes of an Automotive Compact Charge Air Cooler


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  To address the need of increasing fuel economy requirements, automotive Original Equipment Manufacturers (OEMs) are increasing the number of turbocharged engines in their powertrain line-ups. The turbine-driven technology uses a forced induction device, which increases engine performance by increasing the density of the air charge being drawn into the cylinder. Denser air allows more fuel to be introduced into the combustion chamber, thus increasing engine performance. During the inlet air compression process, the air is heated to temperatures that can result in preignition resulting and reduced engine functionality. The introduction of the charge air cooler (CAC) is therefore, necessary to extract heat created during the compression process. The present research describes the physics and develops the optimized simulation method that defines the process and gives insight into the development of CACs. The present research develops a 3-D computational model using ANSYS® Fluent of the CAC internal flow with condensate and validates the predictions of the 3-D model using measurements from experimental data. Finally, the research presents a correlation that provides an approach for designing heat exchangers for practical applications that encounter moisture in the intake air stream. The overall benefit presented is an experimentally validated simulation methodology to evaluate and design CACs that function outside the condensate formation zone during vehicle operation modes.


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  http://www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Catalogs/Hotwire_Catalog_2980465.pdf


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  http://www.dantecdynamics.com/docs/products-and-services/general-literature/Hot-wire_catalog_238.pdf


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  A model for the laminar flame speed of binary fuel blends and its application to methane / hydrogen mixtures


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  Shock-tube study of the ignition of methane / ethane / hydrogen mixtures with hydrogen contents from 0 % to 100 % at different pressures


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  Numerical Study and Parameter Optimization on a Diesel - Natural Gas Dual Fuel Engine Chemical Effects of NG on Diesel Ignition


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  Coupling of in situ adaptive tabulation and dynamic adaptive chemistry: An effective method for solving combustion in engine simulations


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  Using detailed mechanisms to include chemical kinetics in computational fluid dynamics simulations is required for many combustion applications, yet the resulting computational cost is often extremely prohibitive. In order to reduce the resources dedicated to this stage, we investigated the coupling of the dynamic adaptive chemistry (DAC) reduction scheme with the in situ adaptive tabulation (ISAT) algorithm. This paper describes the tabulation of dynamic adaptive chemistry (TDAC) method which takes advantage of both ISAT and DAC to reduce the impact of the mesh and the oxidation mechanism on the computational cost, particularly for unsteady applications like internal combustion engines. In the context of homogeneous charge compression ignition (HCCI), we performed simulations on simplified 2D cases using various n-heptane mechanisms and on a real case mesh using a detailed 857-species iso-octane mechanism. Compared to the direct integration of the combustion reactions, results are in very good agreements and a speed-up factor above 300 is obtained. This is significantly better than what was reported for ISAT and DAC which illustrates the synergy of the two methods. In addition, an experimental validation has also been performed with low load HCCI data. Accordingly, the TDAC method is a significant improvement for the computation of the combustion chemistry in engine simulations and allows the use of detailed mechanisms with practical case meshes in simulations that are inconceivable using direct integration. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.


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  Modeling and 3D-Simulation of the Kinetic Effects in the Post-Flame Region of Turbulent Premixed Flames Based on the G-Equation Approach


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  Influence of steam dilution on the ignition of hydrogen , syngas and natural gas blends at elevated pressures


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  Numerical simulation of non-premixed turbulent combustion using the eddy dissipation concept and comparing with the steady laminar flamelet model


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  Numerical simulations of the Sandia flame CHNa and the Sydney bluff-body stabilized flame HM1E are reported and the results are compared to available experimental data. The numerical method is based on compressible URANS formulations which were implemented recently in the OpenFOAM toolbox. In this study, the calculations are carried out using the conventional compressible URANS approach and a standard k- turbulence model. The Eddy Dissipation Concept with a detailed chemistry approach is used for the turbulence-chemistry interaction. The syngas (CO/H2) chemistry diluted by 30 % nitrogen in the Sandia flame CHNa and CH4/H2 combustion in the Sydney flame HM1E are described by the full GRI-3.0 mechanism. A robust implicit Runge-Kutta method (RADAU5) is used for integrating stiff ordinary differential equations to calculate the reaction rates. The radiation is treated by the P1-approximation model. Both target flames are predicted with the Steady Laminar Flamelet model using the commercial code ANSYS FLUENT as well. In general, there is good agreement between present simulations and measurements for both flames, which indicates that the proposed numerical method is suitable for this type of combustion, provides acceptable accuracy and is ready for further combustion application development.


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  The effects of CO addition on the autoignition of H 2 , CH 4 and CH 4 / H 2 fuels at high pressure in an RCM


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  Effect of turbulent mixing on the end gas auto-ignition of n-heptane/air mixtures under IC engine-relevant conditions


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  The effect of turbulence on the end-gas auto-ignition (AI) of stoichiometric n-heptane/air mixtures under engine relevant conditions is numerically studied through a combined methodology of Large Eddy Simulation (LES) and one-dimensional stand-alone Linear Eddy Model (LEM). Similar end-gas auto-ignition process in super rapid compression machine experiments is first been qualitatively reproduced through LES. To further investigate the effect of small-scale turbulence-chemistry interaction on the end-gas auto-ignition in the near-wall region which consists of a core region and a boundary layer, 1-D LEM simulations are performed by extracting the thermal-chemical and turbulent parameters at top dead center from LES. The parametric study covers a range of representative fluctuation velocity values under engine-like conditions at two initial temperatures of 700 K and 900 K, which are below and within the Negative Temperature Coefficient regime respectively. It is generally found that, increased turbulence intensity delays AI formation and also reduces the kernel size at the onset of AI. At the initial temperature of 700 K, intense turbulence can change the combustion mode of the end-gas following the initial spontaneous auto-ignition in the core region. Diffusive - reactive flame structures are observed and the kernel expansion is governed by turbulent flame propagation. At a high initial temperature of 900 K, the formation of “cold” AI kernels is inhibited with increased turbulence intensity. A budget analysis reveals that the expansion processes of nascent kernels mainly are spontaneous ignitions and turbulence may enhance the local scalar mixing. However, turbulence won't change the nature of auto-ignition propagation wave driven dominantly by exothermicity during the end-gas combustion process.


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  An investigation of in situ adaptive tabulation for premixed and nonpremixed combustion engine simulations with primary reference fuel mechanism


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  The object of this work is to enhance the ISAT performance for transient combustion simulations for both premixed and non-premixed combustion process. In this work, dynamic pruning (DP) strategy for not-frequently-accessed tabulated points has been employed. For DP approach, the action considered when the table is full is to delete the most recently used leaf (at the tail of most recently used leaf) and replace it by the new entry in ISAT. The performances of ISAT with DP are presented firstly both in a homogenous charged compression ignition (HCCI) engine and a diesel engine. In present study, we investigate the sensitivity of ignition and composition inhomogeneity to heat loss/chemistry model for HCCI combustion. Composition inhomogeneity is governed by heat loss, turbulence diffusion and chemical reaction. Meanwhile, the effect of composition and temperature inhomogeneity on chemistry acceleration with ISAT-DP is studied in detail. It is shown that, ISAT-DP method can effectively reduce computational cost and achieve a large speedup factor of around 2 compared with original ISAT method. Memory size in ISAT has a tight influence on computational efficiency. And an overall speed up factor of around 4 for diesel engine is also achieved.


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  An experimental and numerical study on diesel injection split of a natural gas/diesel dual-fuel engine at a low engine load


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  A numerical investigation on methane combustion and emissions from a natural gas-diesel dual fuel engine using CFD model


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  Natural gas (NG)-diesel dual fuel engines have been criticized for their high emissions of unburned methane. The past research on methane emissions from dual fuel engines has focused on the measurement of methane concentration in exhaust gases. The development of approaches capable of minimizing methane emissions requests the detailed spatial distribution of methane in-cylinder during the combustion and post combustion processes. However, it is difficult to experimentally measure the spatial distribution of methane in-cylinder. This research presents a numerical study on the combustion process of a NG-diesel dual fuel engine using the computational fluids dynamics (CFD) model CONVERGE coupled with a reduced primary reference fuel (PRF) mechanism. The model was validated against the heat release process and the emissions of nitrogen oxide, methane and carbon monoxide measured in a single cylinder dual fuel engine. The validated CFD model was applied to investigate the combustion of methane and n-heptane and the spatial distribution of methane in the dual fuel engine. This is most likely the first attempt to visualize the spatial distribution of methane in dual fuel engines using CFD. The objective of this study is to numerically simulate the methane combustion process, especially the methane present outside the pilot spray, quantify the methane combustion in each combustion stage, and visualize the spatial methane distribution in cylinder. The results showed that the momentum produced by the pilot fuel injection and combustion pushed the combustion products of pilot fuel and methane within the pilot spray plume toward the unburned methane-air mixture. Such a movement enhanced the mixing of the hot combustion products and the relatively cold unburned methane-air mixture during the main combustion process and dominated the combustion of methane presented outside the pilot fuel spray plume. Based on the simulation results at a low load condition (4.05bar), the main combustion process consumed 43–53% of the methane fumigated into the intake mixture. The post-combustion oxidation process consumed 17–29% of the intake methane, which was 36.2–51.8% methane that survived the main combustion process. In comparison, 27–35% methane emitted the engine without participating the combustion process. The unburned methane at exhaust valve opening was mainly observed at the center of the cylinder. In comparison, the contribution of the crevice and boundary layer around the cylinder liner to methane emissions was relatively small. The slip of methane through the dual fuel engines was due to the fact that the premixed mixture was too lean to support the propagation of the turbulent flame initiated by the pilot fuel and the lack of pilot fuel vapor reaching the center of the combustion chamber because of the geometric limitations of the fuel injection system and the reduced mass of pilot fuel injected into the cylinder. The approaches aiming to enhance the combustion of methane and minimize methane emissions from dual fuel engines should focus on those capable of increasing the volume of pilot fuel vapor formed after injected into the cylinder.


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  Direct numerical simulation of homogeneous turbulence in combination with premixed combustion at low Mach number modelled by the


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  Large-eddy simulation study on cycle-to-cycle variation of knocking combustion in a spark-ignition engine


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  The cycle-to-cycle variation in the knock intensity is commonly encountered under abnormal combustion conditions. The severity of these abnormal combustion events can vary significantly, and the efficiency of engines at high loads is limited in practice by heavy knocking phenomena. Since, a thorough analysis of such recurrent but non-cyclic phenomena via experiments alone becomes highly cumbersome, in the present work, a multi-cycle large-eddy simulation study was performed to quantitatively predict cyclic variability in the combustion process and cyclic knock intensity variability in a direct injection spark-ignition engine. To account for the turbulence-chemistry interaction effects on flame propagation, the G-equation combustion model was used. Detailed chemistry was solved outside the flame front with a toluene primary reference fuel skeletal kinetic mechanism. For both the mild knock and heavy knock conditions, the numerical results were validated against experimental measurements. Based on the simulation results, a correlation analysis was performed considering combustion phasing, peak cylinder pressure and maximum amplitude of pressure oscillation. Furthermore, a detailed three-dimensional spatial analysis illustrated the evolution of auto-ignition kernel development and propagation of pressure waves during knocking combustion for three typical cycles with different knock intensities. It was found that an early occurrence of auto-ignition in the end gas was prone to high knock intensity. Although multiple auto-ignition kernels were observed in different cycles, the degree of coupling between chemical heat release and pressure waves varied, thereby leading to different maximum amplitude of pressure oscillation values.


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  There is signiﬁcant interest in using hydrogen and natural gas for enhancing the performance of diesel engines. We report herein a numerical investigation on the ignition of n-C7H16/H2 and n-C7H16/CH4 fuel blends. The CHEMKIN 4.1 software is used to model ignition in a closed homogenous reactor under conditions relevant to diesel/HCCI engines. Three reaction mechanisms used are (i) NIST mechanism involving 203 species and 1463 reactions, (ii) Dryer mechanism with 116 species and 754 reactions, and (iii) a reduced mechanism (Chalmers) with 42 species and 168 reactions. The parameters include pressures of 30 atm and 55 atm, equivalence ratios of ɸ ¼ 0.5, 1.0 and 2.0, temperature range of 800e1400 K, and mole fractions of H2 or CH4 in the blend between 0 and 100%. For n-C7H16/air mixtures, the Chalmers mechanism not only provides closer agreement with measurements compared to the other two mechanisms, but also reproduces the negative temperature coefﬁcient regime. Consequently, this mechanism is used to characterize the effects of H2 or CH4 on the ignition of n-C7H16. Results indicate that H2 or CH4 addition has a relatively small effect on the ignition of n-C7H16/air mixtures, while the n-C7H16 addition even in small amount modiﬁes the ignition of H2/air and CH4/air mixtures signiﬁcantly. The n-C7H16 addition decreases and increases the ignition delays of H2/air mixtures at low and high temperatures, respectively, while its addition to CH4/air mixtures decreases ignition delays at all temperatures. The sensitivity analysis indicates that ignition characteristics of these fuel blends are dominated by the pyrolysis/oxidation chemistry of n-heptane, with heptyl (C7H16-2) and hydoxyl (OH) radicals being the two most important species.


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  A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.


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  Twenty Eighth International Symposium on Combustion, University of Edinburgh, Edinburgh, Scotland (GB), 07/30/2000--08/04/2000


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  Experimental Analysis of Diesel-Ignited Methane Dual-Fuel Low-Temperature Combustion in a Single-Cylinder Diesel Engine


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  This paper focuses on the effect of diesel injection timing, intake boost pressure, and diesel injection pressure on diesel-methane dual-fuel combustion performed in a single-cylinder research engine. The engine was operated at a constant speed of 1,500 revolutions per minute (rpm) while percentage of methane energy substitution (PES) and load were maintained at 80% and 5.1 bar net indicated mean effective pressure (IMEP), respectively. The start of injection (SOI) of diesel was varied from 250 crank angle degrees (CAD) to 350 CAD while keeping the injection pressure at 500 bar and intake boost pressure at 1.5 bar. Advancing SOI from 330° to 300° reduced indicated specific NOx (ISNOx) emissions from 11.9 g=kW · h to less than 0.02 g=kW · h; further advancement of SOI did not yield any significant ISNOx reduction. Net indicated fuel conversion efficiency (IFCE) increases from 29.4% at 350 CAD SOI to 40.5% at 300 CAD SOI. Combustion efficiency trends are consistent with unburned hydrocarbon (HC) and carbon monoxide (CO) emission trends. Moreover, smoke emissions were lower than 0.1 filter smoke number (FSN) for all SOIs. A diesel injection pressure sweep from 200 to 1,300 bar at 300 CAD SOI showed that very low injection pressures lead to apparently more heterogeneous combustion and higher ISNOx, indicated specific CO (ISCO), and indicated specific HC (ISHC) emissions, whereas smoke, IFCE, and combustion efficiency remained unaffected. An injection pressure of approximately 500 bar appeared to be optimal for early SOIs. Finally, an intake boost pressure sweep from 1.1 to 1.8 bar at 300 CAD SOI and 500 bar injection pressure showed that ISNOx and smoke remained fairly low at all conditions (ISNOx < 0.15 g=kW · h; smoke < 0.1 FSN); however, increasing boost pressure resulted in an increase in both ISHC and ISCO emissions while combustion efficiency and IFCE were reduced. DOI: 10.1061/(ASCE)EY.1943-7897.0000235. © 2014 American Society of Civil Engineers.


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  Parametric investigation of natural gas port injection and diesel pilot injection on the combustion and emissions of a turbocharged common rail dual-fuel engine at low load


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  In this paper, a common rail diesel research engine was converted to operate in dual-fuel mode and extensive experiments were conducted to investigate the effects of natural gas injection timing on the combustion and emissions performance under different pilot injection pressure and timing at low load conditions. The presented results include the cylinder pressure, heat release rate (HRR), ignition delay, combustion duration and brake thermal efﬁciency, as well as CO, HC and NOx emissions at different natural gas injection timing under pilot injection pressure (46 and 72 MPa) and pilot injection timing (À8° and À17° ATDC) operation conditions at low load (BMEP = 0.24 MPa). The results indicated that retarded natural gas injection timing can achieve a stratiﬁed-like air–fuel mixture in cylinder under the different pilot injection conditions, which provided a method to improve the combustion performance and exhaust emissions at low load. Moreover, under higher pilot injection pressure (72 MPa) conditions, better combustion performance, such as shorter ignition delay and combustion duration, higher brake thermal efﬁciency, were achieved; however, the exhaust emissions signiﬁcantly increased compared with those under lower pilot injection pressure (46 MPa). On the other hand, under the advanced pilot injection timing (À17° ATDC), the combustion performance was radically better, THC and CO emissions were lower but the NOx emissions were signiﬁcantly higher compared with those under the regular pilot injection timing (À8° ATDC). This is attributed to faster ﬂame propagation speed, better combustion phasing and higher volumetric efﬁciency. Consequently, employing appropriate natural gas injection timing accompanied with reasonable pilot injection parameters is critical to further improve combustion performance and exhaust emissions of a dual-fuel engine at low loads.


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  Yang et al. - 2014 - Parametric investigation of natural gas port injection and diesel pilot injection on the combustion and emission(2).pdf


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  1. Introduction. Like the proverbial elephant, turbulent combustion exhibits so many different aspects that two investigators, chosen at random from the set of all investigators examining the subject, will with high probability disagree entirely about its general characteristics. For example, few will know the full definitions of all of the terms “k-ϵ modelling,” “strange attractor,” “Favre average,” “conserved scalar,” “Kolmogorov scale,” “functional differentiation,” “coherent structure,” “large-eddy Damköhler number,” “random vortex,” “spectral transfer,” “intermittency” and “age theory.” Almost all of the investigators will consider at least one of the items in this sequence to be practically irrelevant to turbulent combustion. Yet, for each of the items there likely exist investigators who would identify that item as the central aspect of the subject. Turbulent combustion has now become so multifaceted that no one has the background necessary to give a complete and impartial review of it. Therefore the reader must prepare himself for an incomplete and somewhat biased review. Fifteen years ago, in the introduction to a review on laminar combustion [1], it was remarked that there had been little progress in the theory of turbulent combustion, and it was predicted that the future rate of progress would not be much greater than that of the immediate past. The truth of that prediction was short-lived (just as with most predictions in turbulent combustion). Research in the subject has mushroomed in-numerous diverse directions in recent years, and a few of the avenues have led fruitfully to genuine discoveries of lasting value. The intent of the present article is to attempt to highlight the most promising avenues and to identify some of the pitfalls along the many seductively ruinous paths.


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  The recent advent of highly effective drilling and extraction technologies has decreased the price of natural gas and renewed interest in its use for transportation. Of particular interest is the conversion of dedicated diesel engines to operate on dual-fuel with natural gas injected into the intake manifold. Dual-fuel systems with natural gas injected into the intake manifold replace a significant portion of diesel fuel energy with natural gas (generally 50% or more by energy content), and produce lower operating costs than diesel-only operation. Diesel-natural gas engines have a high compression ratio and a homogeneous mixture of natural gas and air in the cylinder end gases. These conditions are very favorable for knock at high loads. In the present study, knock prediction concepts that utilize a single step Arrhenius function for diesel-natural gas dual-fuel engines are evaluated. A heavy duty diesel engine with the capability of running both natural gas and diesel is operated at points where knock occurs and the cylinder pressure traces are recorded. Constants of the Arrhenius function for dual-fuel operation are determined empirically based on the cylinder pressure and temperature histories. Several methods for knock prediction and coefficient fitting are evaluated, and their accuracy is compared. Results indicate that Arrhenius integral methods can determine if a cycle will knock with accuracy up to 72% using coefficients similar to those for methane.


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  Development of a Flame Propagation Model for Dual-Fuel Partially Premixed Compression Ignition Engines


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  The limitations of an existing characteristic–time combustion (CTC) model are explored and a new combustion model is developed and applied to simulate combustion in dual-fuel engines in which the premixed natural gas is ignited by the combustion ﬂame initiated by a diesel spray. The model consists of a diesel auto-ignition model and a ﬂame propagation model. A G-equation-based model previously developed to simulate SI engine combustion was incorporated with an auto-ignition model to simulate the ﬂame propagation process in partially premixed environments. The computer code is based on the KIVA-3V code and consists of updated sub-models to simulate more accurately the fuel spray atomization, auto-ignition, combustion, and emissions processes. Modiﬁcations were made to implement the level set G-equation approach and to track the location of the ﬂame as a function of the turbulent ﬂame speed, ﬂame curvature, ﬂow velocity, and the movement of the computational mesh in the engine environment. Good agreement with engine experiments was obtained by using the present model.


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  The autoignition and combustion behaviour of an n-heptane spray in a premixed methane/air charge was investigated. A Rapid Compression Expansion Machine (RCEM) with a free ﬂoating piston was employed to reach engine relevant conditions at Start of Injection (SOI) of the micro n-heptane pilot. The methane content in the ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber; the ambient equivalence ratio for the methane content ranged from 0.0 to 0.99. Filtered OH⁄ chemiluminescence images of the combustion were taken with a UV intensiﬁed camera at a rate of 20 kHz through the optical access in the piston and schlieren imaging was performed at 20 kHz for the detection of density gradients arising from pilot injection, ignition and ﬂame propagation in the unburned mixture. Filtered photomultiplier signals of the total emitted light for OH⁄, CH⁄ and CÃ2 radicals as well as the pressure signal were simultaneously recorded and the effect of methane content, charge temperature and ambient oxygen concentration on ignition locations, ignition timing and combustion behaviour was analysed. It was found that increasing methane contents in the ambient mixture considerably prolongs the ignition delays of the pilot spray due to inhibiting effects of the premixed methane on the reactions leading to high temperature ignition triggered by n-heptane. Longer ignition delays due to lower ambient temperatures or dilution of the ambient oxidizer cause higher heat release during the combustion of the pilot spray, since more methane/air is entrained and mixed with the pilot spray and oxidized simultaneously. This behaviour is reversed with increasing ignition delays arising from increased methane addition. There, lower heat release rates during pilot combustion were observed due to decreased reactivity in the n-heptane spray area. Comparison of different strategies with respect to the determination of high temperature ignition delay highlighted shortcomings as well as the potential in the usage of spectrally ﬁltered chemiluminescence data, since changes in the spectrum of the emitted chemiluminescence are expected to change over time in the case of pilot ignition due to the transition between combustion modes. An assessment of the methane content inﬂuence on the ignition reactions by means of homogeneous, adiabatic Perfectly Stirred Reactor (PSR) simulations with a suitable reaction mechanism is further presented which allows for separation of inﬂuences and supports the experimental ﬁndings.


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  A transient, one dimensional premixed laminar reaction front is used as a model problem to further understand the physical processes inﬂuencing reaction front propagation during the various stages of spark-assisted compression ignition (SACI) combustion for both constant and variable domain pressures. This approach is consistent with the wrinkled laminar ﬂame representation of turbulent, spark ignited engine combustion. With the proper choice of timescales and pressure rise rate, it applies to the interaction of the ﬂame with auto-igniting end-gas in a typical automotive engine. Under the conditions simulated by a transient ﬂame code, the reaction front begins as a deﬂagration, propagating into an end-gas with an initially negligible level of reaction progress. The diffusive–reactive nature of the front is maintained until signiﬁcant levels of end-gas reaction progress, where the burning velocity depends upon the degree of pre-reaction. At the time of the end-gas maximum chemical power, the maximum temperature gradient and peak rate of heat conduction within the front diminish to the point where combustion becomes chemically controlled. Although signiﬁcant increases in burning velocity are observed at the onset of chemically controlled combustion within the front, the end-gas is within one front time from the completion of combustion. As a result, no more than one front thickness is consumed by the apparent propagation of the spontaneous ignition front.


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  This paper presents an analysis of the cycle-to-cycle combustion variation as reﬂected in the combustion pressure data of a single cylinder, naturally aspirated, four stroke, Ricardo E6 engine converted to run as dual fuel engine on diesel and gaseous fuel of LPG or methane. A measuring set-up consisting of a piezo-electric pressure transducer with charge ampliﬁer and fast data acquisition card installed on an IBM microcomputer was used to gather the data of up to 1200 consecutive combustion cycles of the cylinder under various combination of engine operating and design parameters. These parameters included type of gaseous fuel, engine load, compression ratio, pilot fuel injection timing, pilot fuel mass, and engine speed. The data for each operating conditions were analyzed for the maximum pressure, the maximum rate of pressure rise—representing the combustion noise, and indicated mean effective pressure. The cycle-to-cycle variation is expressed as the mean value, standard deviation, and coefﬁcient of variation of these three parameters. It was found that the type of gaseous fuel and engine operating and design parameters affected the combustion noise and its cyclic variation and these effects have been presented.


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  Combustion noise, knock and ignition limits data are measured and presented for a dual fuel engine running on dual fuels of Diesel and three gaseous fuels separately. The gaseous fuels used are liqueﬁed petroleum gas, pure methane and compressed natural gas mixture. The maximum pressure rise rate during combustion is presented as a measure of combustion noise, and the knocking and ignition limits are presented as torque output at the onset of knocking and ignition failure. Experimental investigation on the dual fuel engine revealed the noise generated from combustion, knocking and ignition limits for all gases at diﬀerent design and operating conditions. A Ricardo E6 Diesel version engine is converted to run on dual fuel of Diesel and the tested gaseous fuel and is used throughout the work. The engine is fully computerized, and the cylinder pressure data, crank angle data and engine operating variables are stored in a PC for oﬀ line analysis. The eﬀects of engine speeds, loads, pilot injection angle, pilot fuel quantity and compression ratio on combustion noise, knocking torque, thermal eﬃciency and maximum pressure are examined for the dual engine running on the three gaseous fuels separately. The combustion noise, knocking and ignition limits are found to relate to the type of gaseous fuels and to the engine design and operating parameters.


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  Combustion pressure rise rate and thermal eﬃciency data are measured and presented for a dual fuel engine running on a dual fuel of Diesel and compressed natural gas and utilizing exhaust gas recirculation (EGR). The maximum pressure rise rate during combustion is presented as a measure of combustion noise. The experimental investigation on the dual fuel engine revealed the noise generated from combustion and the thermal eﬃciency at diﬀerent EGR ratios. A Ricardo E6 Diesel version engine is converted to run on a dual fuel of Diesel and compressed natural gas and having an exhaust gas recycling system is used throughout the work. The engine is fully computerized, and the cylinder pressure data and crank angle data are stored in a PC for oﬄine analysis. The eﬀects of EGR ratio, engine speeds, loads, temperature of recycled exhaust gases, intake charge pressure and engine compression ratio on combustion noise and thermal eﬃciency are examined for the dual fuel engine. The combustion noise and thermal eﬃciency of the dual fuel engine are found to be aﬀected when EGR is used in the dual fuel engine.


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  Combustion pressure data are measured and presented for a dual fuel engine running on dual fuel of diesel and compressed natural gas, and compared to the diesel engine case. The maximum pressure rise rate during combustion is presented as a measure of combustion noise. Experimental investigation on diesel and dual fuel engines revealed the noise generated from combustion in both cases. A Ricardo E6 diesel version engine is converted to run on dual fuel of diesel and compressed natural gas and is used throughout the work. The engine is fully computerized and the cylinder pressure data, crank angle data are stored in a PC for off-line analysis. The effect of engine speeds, loads, pilot injection angle, and pilot fuel quantity on combustion noise is examined for both diesel and dual engine. Maximum pressure rise rate and some samples of ensemble averaged pressure–crank angle data are presented in the present work. The combustion noise, generally, is found to increase for the dual fuel engine case as compared to the diesel engine case. © 2000 Elsevier Science Ltd. All rights reserved.


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  Impact of pilot diesel ignition mode on combustion and emissions characteristics of a diesel/natural gas dual fuel heavy-duty engine


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  The brake thermal efﬁciency and exhaust emission issues are still not fully-resolved to diesel/natural gas dual fuel engines. To better understand the effect of pilot diesel ignition mode on combustion and emissions characteristics of dual fuel engines, a detailed study concerned with diesel injection timing was conducted. The testing work was operated on a 6-cylinder turbocharged intercooler diesel/natural gas dual fuel heavy-duty engine at light load operations, and diesel injection timing was controlled over a very wide range. The investigated results show that the diesel injection timing (Tinj) has an obvious effect on pilot diesel ignition mode. A signiﬁcant advancing Tinj leads to pilot diesel ignition mode differs from traditional diesel engine compression ignition mode in the sense that it does not occur at a speciﬁc place in the spray, which is a two-stage autoignition mode. With advancing Tinj, engine combustion and emissions characteristics, including cylinder pressure, cylinder temperature, heat release rate, start of combustion (SOC), ignition delay, combustion duration, crank angle of 50% heat release (CA50), nitrogen oxides (NOx) and total hydrocarbon (THC), show completely different variation trends in different ignition modes. Overall, higher thermal efﬁciency and lower emissions can be achieved simultaneously in two-stage autoignition mode. Satisfactory results can be obtained with higher brake thermal efﬁciency (35%), lower NOx (60 ppm) and THC (0.4%) emissions, when Tinj is 42.5 °CA BTDC.


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  This paper presents a detailed experimental analysis of diesel–ignited propane dual fuel combustion on a 12.9-l, six-cylinder, production heavy-duty diesel engine. Gaseous propane was fumigated upstream of the turbocharger air inlet and ignited using direct injection of diesel sprays. Results are presented for brake mean effective pressures (BMEP) from 5 to 20 bar and different percent energy substituted (PES) by propane at a constant engine speed of 1500 rpm. The effect of propane PES on apparent heat release rates, combustion phasing and duration, fuel conversion and combustion efﬁciencies, and engine-out emissions of oxides of nitrogen (NOx), smoke, carbon monoxide (CO), and total unburned hydrocarbons (HC) were investigated. Exhaust particle number concentrations and size distributions were also quantiﬁed for diesel–ignited propane combustion. With stock engine parameters, the maximum propane PES was limited to 86%, 60%, 33%, and 25% at 5, 10, 15, and 20 bar BMEPs, respectively, either by high maximum pressure rise rates (MPRR) or by excessive HC and CO emissions. With increasing PES, while fuel conversion efﬁciencies increased slightly at high BMEPs or decreased at low BMEPs, combustion efﬁciencies uniformly decreased. Also, with increasing PES, NOx and smoke emissions were generally decreased but these reductions were accompanied by higher HC and CO emissions. Exhaust particle number concentrations decreased with increasing PES at low loads but showed the opposite trends at higher loads. At 10 bar BMEP, by adopting a different fueling strategy, the maximum possible propane PES was extended to 80%. Finally, a limited diesel injection timing study was performed to identify the optimal operating conditions for the best efﬁciency-emissions-MPRR tradeoffs.


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  A numerical investigation on combustion and emission characteristics of a dual fuel engine at part load condition


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  Dual fuel engines are more attractive due to lower emission levels in comparison with conventional diesel engines particularly at full loads. But it is required to study dual fuel combustion process with more details at part loads due to the poor performance and high CO and UHC emissions at these conditions. In the present study, numerical modeling of OM-355 dual fuel (injection of diesel pilot fuel to premixed mixture of air and methane) engine has been performed by using KIVA-3V code at part and full loads. Sub-models of the code were modiﬁed to simulate the fuel spray atomization, combustion and pollutants emissions processes, accurately. Results indicate that in-cylinder pressure, heat release rate and exhaust emissions predictions are in good agreement with experiments at all loads. Results show that a lean premixed natural gas mixture is ignited slowly. The slow progress of combustion process at part load, leads the heat release to be drawn more toward the expansion stroke which causes incomplete combustion, and consequently high amounts of UHC and CO will be emitted. It is found that at part loads, areas that are inﬂuenced by diesel diffusion ﬂames are ignited and premixed natural gas ﬂame could not be propagated properly. Hence development of diesel diffusion ﬂame is required to burn lean natural gas mixture. But at full load, in addition to the diesel diffusion ﬂames, premixed natural gas ﬂame could be propagated suitably. Also, at part load because of low gas temperature in the environment of diesel spray and low diesel fuel temperature, diesel liquid droplets evaporate lately which are far from injector nozzles. Hence, it causes diesel diffusion ﬂame from spray of each injector nozzles to be developed distinctly. It can be deduced that the ﬂame structure is affected by operating conditions. Finally the effect of increasing the diesel fuel quantity on improving methane combustion is studied. The studied strategy could help to improving natural gas combustion due to enlarge the size of diesel combustion region.


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  Natural gas (NG)-diesel dual fuel engines have been criticized for their high emissions of unburned methane. The past research on methane emissions from dual fuel engines has focused on the measurement of methane concentration in exhaust gases. The development of approaches capable of minimizing methane emissions requests the detailed spatial distribution of methane in-cylinder during the combustion and post combustion processes. However, it is diﬃcult to experimentally measure the spatial distribution of methane in-cylinder.


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  Homogeneous charge compression ignition (HCCI) engine uses a relatively new mode of combustion technology. In principle, there is no spark plug or injector to assist the combustion, and the combustion auto-ignites in multiple spots once the mixture has reached its chemical activation energy. The challenges in developing HCCI engines are the difﬁculties in: controlling the auto-ignition of the mixture and the heat release rate at high load operations, achieving a cold start, meeting emission standards and controlling knock. At low engine speeds, early auto-ignition can occur, possibly leading to knocking, while late auto-ignition at high engine speeds will make HCCI susceptible to misﬁre. Hydrogen greatly reduces emissions levels but with reduced power. However, when hydrogen is combined with diesel in dual-fuel mode, low NOx, CO and particulate matter (PM) emissions levels can be achieved, and engine efﬁciency can be increased by 13–16%. Numerical methods are commonly used to predict HCCI engines0 performance (i.e. emissions levels, brake thermal efﬁciency and combustion phasing), which is costeffective compared to solely relying on experimentation. The multi-zone method promises better simulation results compared to the single-zone model by combining detailed chemical kinetics with simpliﬁed 3D modeling so that turbulence and inhomogeneity in the mixture are considered; good agreement between simulations and experiments have been achieved. Speciﬁc strategies used in the experimental method (e.g. fuel additives, inlet air heating, inlet air pressurizing, exhaust gas recirculation (EGR) and injection methods), and numerical method (e.g. single-zone and multi-zone models, mixing model, turbulence model and multi-dimensional model), and other issues associated with HCCI engines are discussed in this paper.


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  Characterization of particle number and mass size distributions from a small compression ignition engine operating in diesel/methane dual fuel mode


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  The use of gas fuels in compression ignition engines through dual fuel technology represents a promising way to reach a good compromise among sustainable development, energy conservation and environmental preservation. Dual fuel is an operation mode in which the gas, the primary fuel, is mixed with air in the intake manifold before entering into the cylinder. The air–fuel mixture is then ignited by a small amount of liquid fuel directly injected in the cylinder towards the end of the compression stroke. Methane is particularly attractive because of its potential to reduce particulate emissions that are likely the most critical issue of diesel engine exhaust. The present work deals with an experimental activity carried out on a compression ignition engine modiﬁed to run in diesel/methane dual fuel mode. The engine is a three-cylinder, 1.0 L of displacement, equipped with a common rail injection system. Experiments were carried out at different engine speeds and loads. For each engine speed, a ﬁxed part of the total energy was supplied by the diesel combustion and the remaining by varying the concentration of methane in the intake. Thermodynamics analysis of the combustion process was performed by in-cylinder pressure signal. Particulate matter emissions were measured at the exhaust by commercial instruments. The sizing and the counting of the particles were performed in the diameter range 5.6–560 nm. It was found out that the effect of diesel/methane combustion on particle formation is strongly affected by the operating condition. In particular, it arose that DF combustion emits lower amount of particles whose size with respect to diesel fuel is related to the different combustion evolution.


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  Direct Numerical Simulations of premixed methane flame initiation by pilot n-heptane spray autoignition


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  Performance and emissions of a heavy-duty diesel engine fuelled with diesel and LNG (liquid natural gas)


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  This paper presents the effects of liquid natural gas on a heavy-duty diesel engine. Natural gas was used as primary fuel, while a pilot amount of diesel was used as an ignition source. The amount of each fuel was adjusted to obtain comparable brake torque and power output from the dual engine operation, while no knocking was observed. The engine performance and emissions from the diesel and dual fuel engine tests were conducted over the engine speed range between 1100 and 1900 rpm. The engine performance included torque, power, speciﬁc fuel consumption (SFC), volume efﬁciency, and thermal efﬁciency. The emissions tested were total hydrocarbon (THC), nitrogen oxides (NOx), carbon dioxide (CO2) and carbon monoxide (CO) emissions. The results showed that the maximum portion of natural gas in the dual fuel engine operation was up to 77.90% at 1300 rpm. Compared to the diesel operation, the dual fuel operation showed less speciﬁc fuel consumption, thermal efﬁciency, and volumetric efﬁciency. The emissions of THC and CO from the dual fuel engine operation were higher, while the emissions of NOx and CO2 were lower. Ó 2013 Elsevier Ltd. All rights reserved.


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  Improvement of dual-fuel biodiesel-producer gas engine performance acting on biodiesel injection parameters and strategy


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  Dual-fuel biodiesel-producer gas combustion has shown potential in reducing nitric oxides and particulate emission levels compared to only diesel operation; however, engine overall eﬃciency is slightly penalized, while the main drawbacks are represented by the higher levels of total hydrocarbons and carbon monoxide emissions. In this work, the improvements in the combustion development deriving from the splitting of the liquid fuel injection at low loads have been assessed using a 0.51 L single-cylinder research diesel engine equipped with a high pressure common rail injection system and operated in dual-fuel mode. In this case, a synthetic producer gas was used as inducted gaseous fuel, while biodiesel was used as pilot fuel.


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  One of the main challenges associated with homogeneous charge compression ignition (HCCI) combustion engine application is the lack of direct control on ignition timing. One of the solutions to this problem is mixing two fuels with various properties at a variety of ratios on a cycle-by-cycle basis. In the current study, a reduced mechanism for a fuel blend of natural-gas and n-heptane is proposed. The approach is validated for the prediction of ignition timing in the HCCI combustion engine. A single-zone combustion model is used to simulate the HCCI engine. A two-stage reduction process is used to produce two reduced mechanisms of existing semi-detailed GRI-Mech. 3.0 mechanism that contains 53 species and 325 reactions and Golovichev’s mechanism consisting of 57 species and 290 reactions for natural gas and n-heptane fuels, respectively. Firstly, the unimportant species and related reactions are identiﬁed by employing the directed relation graph with error propagation (DRGEP) reduction method and then, to extend reduction, the principal component analysis (PCA) method is utilized. To evaluate the validity of the reduced mechanism, representative engine combustion parameters such as peak pressure, maximum heat release, and CA50 are used. The reduced mechanism of GRI-Mech. 3.0 mechanism, containing 19 species and 39 reactions, and the reduced mechanism of Golovichev’s mechanism, consisting of 40 species and 95 reactions, provide good prediction for the mentioned parameters in comparison with those of detailed mechanisms. The combination of the generated reduced mechanisms is used to develop a reaction mechanism for a fuel blend of natural-gas/n-heptane. Then, the genetic algorithm is used for optimization of reaction rate constants in the newly generated mechanism. Simulation results agree well with the experimental results under various operating conditions, while maintaining small errors (less than 2° CA) for the mentioned engine combustion parameter. The proposed mechanism, which includes 41 species and 109 reactions, and two generated reduced mechanisms (for natural gas and n-heptane) are available as Supporting Information for this article.


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  Multidimensional CFD simulation of syngas combustion in a micro-pilot-ignited dual-fuel engine using a constructed chemical kinetics mechanism


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  A multidimensional computational ﬂuid dynamics (CFD) simulation of a constructed syngas chemical kinetic mechanism was performed to evaluate the combustion of syngas in a supercharged dual-fuel engine for various syngas initial compositions under lean conditions. The modelled results were validated by comparing predictions against corresponding experimental data for a supercharged dual-fuel engine. The predicted and measured in-cylinder pressure, temperature, and rate of heat release (ROHR) data were in good agreement. The effect of the hydrogen peroxide chain-propagation reaction on the progress of combustion under supercharged conditions was examined for different types of syngas using various initial H2 concentrations. The effect of the main syngas kinetic mechanism reactions on the combustion progress was analysed in terms of their contribution to the total heat of the reaction. The best results compared with experimental data were obtained in the range of equivalence ratios below about 0.8 for all types of syngas considered in this paper. As the equivalence ratio increased above 0.8, the results deviated from the experiment data. The spatial distribution of the in-cylinder temperature and OH* within this equivalence-ratio range showed the completeness of the combustion. The present CFD model captured the overall combustion process well and could be further developed into a useful tool for syngas-engine combustion simulations.


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  The use of natural gas as a partial supplement for liquid diesel fuel is a very promising solution for reducing pollutant emissions, particularly nitrogen oxides (NOx) and particulate matters (PM), from conventional diesel engines. In most applications of this technique, natural gas is inducted or injected in the intake manifold to mix uniformly with air, and the homogenous natural gas–air mixture is then introduced to the cylinder as a result of the engine suction.


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  A Simulation Model for the Combustion Process of Natural Gas Engines with Pilot Diesel Fuel as an Ignition Source


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  Experimental and Theoretical Analysis of the Combustion and Pollutants Formation Mechanisms in Dual Fuel DI Diesel Engines


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  In the effort to reduce pollutant emissions from diesel engines various solutions have been proposed, one of which is the use of natural gas as supplement to liquid diesel fuel, with these engines referred to as fumigated, dual fuel, compression ignition engines. One of the main purposes of using natural gas in dual fuel (liquid and gaseous one) combustion systems is to reduce particulate emissions and nitrogen oxides. Natural gas is a clean burning fuel; it possesses a relatively high auto-ignition temperature, which is a serious advantage over other gaseous fuels since then the compression ratio of most conventional direct injection (DI) diesel engines can be maintained high. In the present work, an experimental investigation has been conducted to examine the effects of the total air–fuel ratio on the efﬁciency and pollutant emissions of a high speed, compression ignition engine located at the authors’ laboratory, where liquid diesel fuel is partially substituted by natural gas in various proportions, with the natural gas fumigated into the intake air. The experimental results disclose the effect of these parameters on brake thermal efﬁciency, exhaust gas temperature, nitric oxide, carbon monoxide, unburned hydrocarbons and soot emissions, with the beneﬁcial effect of the presence of natural gas being revealed. Given that the experimental measurements cover a wide range of liquid diesel supplementary ratios without any appearance of knocking phenomena, the belief is strengthened that the ﬁndings of the present work can be very valuable if opted to apply this technology on existing DI diesel engines.


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  One of the major problems associated with HCCI combustion engine application is lack of direct control for combustion timing. A proposed solution for combustion timing control is using a binary fuel blend in which two fuels with different auto-ignition characteristics are blended at various ratios on a cycle-bycycle basis.


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  Experimental study of the effects of natural gas injection timing on the combustion performance and emissions of a turbocharged common rail dual-fuel engine


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  Natural gas combustion with pilot ignition has been considered to be one of the most promising ways to utilize natural gas in existing diesel engine without serious engine modiﬁcation and it has been widely researched all over the world. In this study, three experiments of different loads (BMEP 0.240 MPa, 0.480 MPa and 0.767 MPa) were performed on a 2.8 L four-cylinder, natural gas manifold injection dual-fuel engine to investigate the effects of natural gas injection timing on engine combustion performance and emissions. The pilot injection parameters (pilot injection timing and pressure) and natural gas injection pressure remain constant at a speed of 1600 rpm in the experiment. The cylinder pressure, HRR, CoVimep, ﬂame development duration, CA50 and brake thermal efﬁciency were analyzed. The results indicated that under low and part engine loads, the ﬂame development duration and CA50 can be reduced by properly retarding natural gas injection timing, while the CoVimep increased with retarded natural gas injection timing. As a result, the brake thermal efﬁciency is increased and the combustion stability slightly deteriorates. Meanwhile, under low and part engine loads, PM emissions in the dual-fuel engine is much lower than that in conventional diesel engines, furthermore, at high load, the PM emissions are near zero. CO and HC emissions are reduced with retarded natural gas injection timing under low and part loads, however, NOx emissions are slightly increased. Under high load, the ﬂame development duration and CA50 are obviously prolonged with retarded natural gas injection timing companied with a deterioration of brake thermal efﬁciency. CO and HC emissions are not signiﬁcantly varied with retarded natural gas injection timing under high load, except that NOx emissions decreased slightly.


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  Theoretical study of the effects of pilot fuel quantity and its injection timing on the performance and emissions of a dual fuel diesel engine


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  Various solutions have been proposed for improving the combustion process of conventional diesel engines and reducing the exhaust emissions without making serious modiﬁcations on the engine, one of which is the use of natural gas as a supplement for the conventional diesel fuel, the so called dual fuel natural gas diesel engines. The most common type of these is referred to as the pilot ignited natural gas diesel engine (PINGDE). Here, the primary fuel is natural gas that controls the engine power output, while the pilot diesel fuel injected near the end of the compression stroke auto-ignites and creates ignition sources for the surrounding gaseous fuel mixture to be burned. Previous research studies have shown that the main disadvantage of this dual fuel combustion is its negative impact on engine eﬃciency compared to the normal diesel operation, while carbon monoxide emissions are also increased. The pilot diesel fuel quantity and injection advance inﬂuence signiﬁcantly the combustion mechanism. Then, in order to examine the eﬀect of these two parameters on the performance and emissions, a comprehensive two-zone phenomenological model is employed and applied on a high-speed, pilot ignited, natural gas diesel engine located at the authors’ laboratory. According to the results, the simultaneously increase of the pilot fuel quantity accompanied with an increase of its injection timing results to an improvement of the engine eﬃciency (increase) and of the emitted CO emissions (decrease) while it has a negative eﬀect (increase) of NO emissions.


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  Effects of pilot fuel quantity on the emissions characteristics of a CNG/diesel dual fuel engine with optimized pilot injection timing


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  For CNG/diesel dual fuel engines, the effects of pilot fuel quantity and injection timing are noticeable and signiﬁcant. In this study, the emission characteristics of a CNG–diesel dual fuel engine with different pilot diesel fuel quantity and optimized pilot injection timing were investigated. The CO emission levels under dual fuel mode are considerably higher than that under normal diesel operation modes even at high load, which indicated that there exist some ﬂame extinction regions. Dual fuel mode reduces NOx emissions by 30% averagely in comparison to diesel mode. That is because most of the fuel is burned under lean premixed conditions which result in lower local temperature. The unburned HC emissions under dual-fuel mode are obviously higher than that of the normal diesel mode, especially at low to medium loads. And around 90% of the THC emissions were unburned methane, which means the ﬂame does not propagate throughout the charge. THC emissions reduce signiﬁcantly with the increase of the pilot diesel quantity. Thanks to the premixed nature of the combustion mode and the methane molecular structure, the PM emission is reduced obviously under dual fueling condition. The PM emission is increased with the increase of the pilot fuel quantity.


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  Integration of a Continuous Multi-Component Fuel Evaporation Model with an Improved G-Equation Combustion and Detailed Chemical Kinetics Model with Application to GDI Engines


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  Effects of natural gas injection timing and split pilot fuel injection strategy on the combustion performance and emissions in a dual-fuel engine fueled with diesel and natural gas


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  Considering the drawbacks of a diesel/natural gas dual-fuel combustion at low load usually is relative to undesirable natural gas-air-diesel mixture distributions in-cylinder and natural gas supply method and pilot diesel injection strategy are believed to play a critical role in the process of mixture formation in the cylinder. So an experimental study has been conducted to explore the eﬀects of natural gas injection timing and split pilot injection strategy on the combustion performance and emissions characteristics in a diesel/natural gas dual-fuel engine.


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  Performance and emissions of a dual-fuel pilot diesel ignition engine operating on various premixed fuels


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  A multi-dimensional computational ﬂuid dynamics (CFD) model coupled with chemical kinetics mechanisms was applied to investigate the effect of various premixed fuels and equivalence ratios on the combustion, performance, and emissions characteristics of a dual-fuel indirect injection (IDI) pilot diesel ignition engine. The diesel fuel is supplied via indirect injection into the cylinder prior to the end of the compression stroke. Various premixed fuels were inducted into the engine through the intake manifold. The results showed that the dual-fuel case using hydrogen/diesel has a steeper pressure rise rate, higher peak heat release rate (PHRR), more advanced ignition timing, and shorter ignition delay compared to the natural gas/diesel and methanol/diesel dual-fuel cases. For leaner mixtures (UP < 0.32), the hydrogen/diesel case has a higher indicated mean effective pressure (IMEP) and indicated thermal efﬁciency (ITE); however, the methanol/diesel case has the maximum IMEP and ITE for richer mixtures (UP > 0.32). For instance, with an equivalence ratio of 0.35, the ITE is 56.24% and 60.85% for hydrogen/diesel and methanol/diesel dual-fuel cases, respectively. For an equivalence ratio of 0.15, the natural gas/diesel simulation exhibits partial burn combustion and thus results in a negative IMEP. At equivalence ratios of 0.15, 0.2, and 0.25, the methanol/diesel case experiences misﬁring phenomenon which consequently deteriorates the engine performance considerably. As for the engine-out emissions, the hydrogen/diesel results display carbon monoxide (CO) free combustion relative to natural gas/diesel and methanol/diesel engines; however, considerable amount of nitrogen oxides (NOx) emissions are produced at an equivalence ratio of 0.35 which exceeds the Euro 6 NOx limit. Due to the larger area exposed to high temperature regions and the higher content of oxygen with increased methanol, soot and CO emissions are signiﬁcantly reduced for richer premixed methanol mixtures. According to these ﬁndings, a dual-fuel engine operating on methanol and diesel performs better at rich conditions, whereas the performance of hydrogen and diesel is superior to that of natural gas/diesel and methanol/diesel mixtures at lean conditions.


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  Natural gas/diesel dual-fuel combustion is currently one of the most promising LTC strategies for the next generation of heavy-duty engines. While this concept is not new and it has been deliberated lengthily in the past two decades, several uncertainties still exist. A major shortcoming of this concept is associated with its low thermal eﬃciency and high level of unburned methane and CO emissions under low engine load conditions. The present paper reports an experimental and numerical study on the eﬀect of diﬀerent injection strategies (single and two pulses injection of pilot diesel fuel) on the combustion performance and emissions of a heavy duty natural gas/diesel dual-fuel engine at 25% engine load. The results of single diesel injection mode showed that advancing diesel injection timing from 10 to 30 °BTDC reduced unburned methane and CO emissions by 62% and 61% and increased thermal eﬃciency by 6%; however, NOx emissions increased by 74%. In order to achieve NOx – CH4 and NOx – CO trade-oﬀ and increased thermal eﬃciency at low load conditions, the eﬀect of split injection strategy was experimentally and numerically examined. The results of split injection mode revealed that split injection strategy considerably increases the in-cylinder peak pressure compared to that of single injection (10 °BTDC). The results showed also that the heat release produced by the ﬁrst injection of diesel fuel considerably increased the in-cylinder charge temperature before the start of the second injection. The ﬂame zone of the split injection mode is markedly higher than that of the single injection due to larger heat release produced during the ﬁrst injection which promotes the combustion of the second one. When the ﬁrst injection timing is close to the second injection timing, the MPRR of split injection mode is higher than that of single injection (10 °BTDC). However, further advancing of the ﬁrst injection timing continuously decreased the MPRR. OH radical analysis showed that for advanced ﬁrst injection timings (38–50 °BTDC), the overall growth rate of OH radical becomes slower and its distribution is narrower as indicated by the wider non-reactive blue zones compared with those observed at a late ﬁrst injection timing in the initial stages of combustion. However, OH radicals gradually grow during last stages of combustion in the expansion stroke, indicating that a more premixed combustion takes place in these cases. For very advanced ﬁrst injection timing of 55 °BTDC, the OH distribution is similar to that of the single injection mode with lower OH intensity at initial stages of combustion and they barely grow during the late expansion stroke. At this condition, the ignition of premixed mixture is mainly controlled by the second diesel fuel injection. The trade-oﬀ between NOx – CH4 and NOx – CO is achieved when applying split injection. Compared to single injection (10 °BTDC), the ﬁrst injection timing of 50 °BTDC decreased unburned methane and CO emissions by 60% and 63%, respectively, and increased the thermal efﬁciency by 8.9%. However, NOx emissions were maintained at the same level as single injection mode (10 °BTDC).


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  For engine knock prediction, surrogate fuels are often composed of iso-octane and n-heptane since they are the components of the Primary Reference Fuel (PRF). By deﬁnition, a PRF has no octane sensitivity (S = RON-MON). However, for a commercial gasoline fuel holds RON > MON and therefor S > 0. More complex surrogates are Toluene Reference Fuels (TRF) and Ethanol containing Toluene Reference Fuels (ETRF). In this work, the impact of the surrogate formulation on the prediction of ﬂame propagation and auto-ignition in the unburnt gases are investigated. The surrogates are composed such that the Research Octane Number is the same. The auto-ignition events ahead of the ﬂame front are predicted using 3D CFD and a combustion model based on the ETRF mechanism by Seidel (2017). The strength of the auto-ignition is determined using the detonation diagram by Bradley and co-workers (2002, 2003). Applying the diﬀerent surrogates, ignition kernels of diﬀerent size and reactivity are predicted. The results indicate a dependency on the local temperature history and the low temperature chemistry of the fuel species. The comparison of homogenous constant volume reactor and transient simulations show that the analysis of ignition delay time and octane rating solely from homogenous simulations is not suﬃcient if the knock tendency of a surrogate in engine simulations needs to be characterized.


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  Physicochemical effects of varying fuel composition on knock characteristics of natural gas mixtures


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  The physicochemical origins of how changes in fuel composition affect autoignition of the end gas, leading to engine knock, are analyzed for a natural gas engine. Experiments in a lean-burn, high-speed medium-BMEP gas engine are performed using a reference natural gas with systematically varied fractions of admixed ethane, propane and hydrogen. Thermodynamic analysis of the measured non-knocking pressure histories shows that, in addition to the expected changes arising from changes in the heat capacity of the mixture, changes in the combustion duration relative to the compression cycle (the combustion ‘‘phasing’’) caused by variations in burning velocity dominate the effects of fuel composition on the temperature (and pressure) of the end gas. Thus, despite the increase in the heat capacity of the fuel–air mixture with addition of ethane and propane, the change in combustion phasing is actually seen to increase the maximum end-gas temperature slightly for these fuel components. By the same token, the substantial change in combustion duration upon hydrogen addition strongly increases the end-gas temperature, beyond that caused by the decrease in mixture heat capacity. The impact of these variations in in-cylinder conditions on the knock tendency of the fuel have been assessed using autoignition delay times computed using SENKIN and a detailed chemical mechanism for the end gas under the conditions extant in the engine. The results show that the ignition-promoting effect of hydrogen is mainly the result of the increase in end-gas temperature and pressure, while addition of ethane and propane promotes ignition primarily by changing the chemical autoignition behavior of the fuel itself. Comparison of the computed end-gas autoignition delay time, based on the complete measured pressure history of each gas, with the measured Knock-Limited Spark Timing shows that the computed delay time accurately reﬂects the measured knock tendency of the fuels.


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  Ignition-promoting effect of NO2 on methane, ethane and methane/ethane mixtures in a rapid compression machine


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  Autoignition delay times of stoichiometric methane, ethane and methane/ethane mixtures doped with 100 and 270 ppm of NO2 have been measured in a RCM in the temperature range 900–1050 K and pressures from 25 to 50 bar. The measurements show that addition of NO2 to CH4/O2/N2/Ar and CH4/C2H6/ O2/N2/Ar mixtures results in a signiﬁcant reduction in the autoignition delay time and that the ignitionpromoting eﬀect of NO2 increases substantially with increasing temperature, from $20% to more than a factor of two over the range of temperature studied. Addition of NO2 to C2H6/O2/N2/Ar mixtures results in only a modest reduction in ignition delay time over the range of pressure and temperature measured. Computations with an updated chemical mechanism show good agreement with the measurements for undoped methane, but overpredict the delay times for undoped ethane and underestimate the eﬀects of replacing 10% methane by ethane. For NO2-containing mixtures, the model predicts the observed trend in decreasing delay time with increasing NO2 fraction. However, the computations tend to overestimate the eﬀect of NO2 addition on ignition, particularly for C2H6 mixtures. Analysis of the reaction mechanism for the eﬀects of NO2 addition to methane mixtures indicates that the ignition-promoting eﬀect of NO2 is related to the appearance of new conversion channels for CH3 and CH3OO, i.e., NO2 + CH3 ? NO + CH3O and NO + CH3OO ? NO2 + CH3O, generation of chain-initiating OH radicals through NO/ NO2 interconversion, i.e., NO2 + H ? NO + OH and NO + HO2 ? NO2 + OH, and to the direct initiation step CH4 + NO2 ? CH3 + HNO2. Analyses further show that the formation of CH3NO2 via CH3 + NO2(+M) M CH3NO2(+M) essentially inactivates NO2. This reaction limits the promoting eﬀect of NO2 at lower temperatures and higher pressures, where stabilization of CH3NO2 is favored, explaining the experimentally observed trends.


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  The effects of CO addition on the autoignition of H2, CH4 and CH4/H2 fuels at high pressure in an RCM


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  Autoignition delay times of stoichiometric and fuel-lean (u = 0.5) H2, H2/CO, CH4, CH4/CO, CH4/H2 and CH4/CO/H2 mixtures have been measured in an Rapid Compression Machine at pressures ranging from 20 to 80 bar and in the temperature range 900–1100 K. The effects of CO addition on the ignition of H2, to 50% CO in the fuel, and CH4, to 20% CO in the fuel, are observed to be negligible both experimentally and computationally for the conditions studied here. The addition of syngas to methane results in ignition behavior that resembles an equivalent methane/hydrogen fuel mixture with the same hydrogen fraction. In contrast to results previous presented in the literature, we thus observe no inhibiting effect from CO for the conditions in our experiments.


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  Gasoline-direct-injection (GDI) engines have been adopted increasingly by the automotive industry in the recent years due to their performance, effects on the environment, and customers' demand on advanced technology. However, the knowledge of detailed combustion process in such engines is still not thoroughly analyzed and understood. With optically accessible engines (OAE) and advanced measuring techniques, such as high-speed digital imaging, the incylinder combustion process is made available directly to researchers. The present study primarily focuses on the effects of different parameters of engine control on the combustion process, such as fuel types, valve deactivation, ignition timing, spark energy, injection timing, air-fuel ratio, and exhaust gas recirculation. Three engine heads of a 2.0L GDI engine are used with modification to acquire different optical access. Pure (E0) and ethanol-blended (E85) gasoline are used, and the engine speed and coolant temperature are kept low to imitate start-up situations in the normal driving condition. The visualization study is realized with two highspeed digital cameras, a color one and a monochrome one with an intensifier, to capture the combustion images.


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  Effect of Hydrogen Concentration on Engine Performance, Exhaust Emissions and Operation Range of PREMIER Combustion in a Dual Fuel Gas Engine Using Methane-Hydrogen Mixtures


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  A single cylinder, supercharged dual fuel gas engine with micro-pilot fuel injection is operated using methane only and methane-hydrogen mixtures. Methane only experiments were performed at various equivalence ratios and equivalence ratio of 0.56 is decided as the optimum operating condition based on engine performance, exhaust emissions and operation stability. Methane-hydrogen experiments were performed at equivalence ratio of 0.56 and 2.6 kJ/cycle energy supply rate. Results show that indicated mean effective pressure is maintained regardless of hydrogen content of the gaseous fuel while thermal efficiency is improved and presence of hydrogen reduces cyclic variations. Increasing the fraction of hydrogen in the fuel mixture replaces hydrocarbon fuels and reduces carbon monoxide and hydrocarbon emissions. Mixtures with higher hydrogen content undergo faster heat release from flame propagation, approach knocking limit faster and are less knock resistant. 40% methane – 60% hydrogen mixture is prone to premature autoignition and superknocking, and is the critical concentration limit for methane-hydrogen mixtures.


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  Numerical simulation of syngas combustion with a multi-spark ignition system in a diesel engine adapted to work at the Otto cycle


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  This work focuses on the construction of a 2D dynamic model, taking into consideration the turbulent ﬂux combustion reactions of syngas inside a combustion chamber and its displacement through the cylinder of a diesel engine model OM 447 LA converted to Otto cycle operation. The engine has a multi-spark ignition system. The geometry of both the chamber and cylinder is symmetric to a radius of 0.064 m and to a length of 0.17595 m. The simulation is carried out on only half of the system, with a premixed supply of the syngas and air. The supply temperature of the mixture is 336 K. The supply relation air/syngas ratio is 1.1:1, and the supply pressure of the mixture is 1 bar. The gaseous phase is modeled as a multi-component mixture comprised of carbon monoxide (CO), hydrogen (H2), methane (CH4), nitrogen (N2), carbon dioxide (CO2) and oxygen (O2). The study describes a Computational Fluid Dynamic (CFD) numerical model, in which the conservation of matter, motion and energy equations are solved; in addition, sub-models are used to represent the turbulence intensity and the multiple reactions. The model predicts the proﬁles of syngas speed, temperature, chemical composition, pressure, and turbulence intensity for the gases when the working parameters and the supply characteristics are modiﬁed (air–syngas ratio, initial temperature of the mixture, initial pressure, compression ratio, and engine speed). The equation formulation is elliptic staggered. The result is a simple nonlinear map that resolves combustion time sequences using the commercial code CFD in PHOENICS.


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  The ignition, combustion and flame structure of carbon monoxide/hydrogen mixtures. Note 1: Detailed kinetic modeling of syngas combustion also in presence of nitrogen compounds


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  The kinetic characterization of the H2/CO system is of interest right now due mainly to its role in sustainable combustion processes. The aim of this paper is to revise and validate a detailed kinetic model of hydrogen and carbon monoxide mixture combustion with particular focus not only on NOx formation but also on interactions with nitrogen species. Model predictions and experimental measurements are discussed and compared across a wide range of operating conditions. This study moves from the detailed analysis of species proﬁles in syngas oxidation in ﬂow reactor and laminar premixed ﬂames to global combustion properties (ignition delay times and laminar ﬂame speeds) by referring to a large set of literature data. According to recent literature, the validation of the kinetic scheme conﬁrmed there was a need to slightly modify the kinetic parameters of two relevant CO2 formation reactions (CO + OH = CO2 + H and CO + O + M = CO2 + M) and of reaction HONO + OH = NO2 + H2O.


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  The possibility of turbocharging into a natural gas homogeneous charge compression ignition (HCCI) engine is experimentally investigated. Experiments are performed using a naturally aspirated engine ﬁtted with an external supercharger and a butterﬂy valve for back pressure control to simulate a turbocharger. The results indicate that the thermal efﬁciency can be improved by raising the engine compression ratio and lowering the boost pressure. At an engine compression ratio of 21 and turbocharging pressure of 0.19 MPa, the brake thermal efﬁciency reaches 43%, with NOx emissions of only 10 ppm or less. Finally, the performance of the engine ﬁtted with a newly developed turbocharger is demonstrated. As a result, 43.3% brake thermal efﬁciency, 0.98 MPa brake mean effective pressure, and 13.8 ppm NOx emission have been realized. This value shows the possibility that a power generating efﬁciency of 40% at a power output of 50 kW could be achieved when applied to combined heat and power (CHP), even allowing for energy losses in the generator and the power inverter. For practical use, the ignition timing control, operation control, including how to start or load input and ensuring durability, remain to be investigated. To clarify these issues, we have initiated the endurance test for a newly developed 25 kW HCCI package, although the engine is not supercharged.


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  Dual Fuel Reaction Mechanism 2.0 including NOx Formation and Laminar Flame Speed Calculations Using Methane/Propane/n-Heptane Fuel Blends


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  This study presents the further development of the TU Wien dual fuel mechanism, which was optimized for simulating ignition and combustion in a rapid compression expansion machine (RCEM) in dual fuel mode using diesel and natural gas at pressures higher than 60 bar at the start of injection. The mechanism is based on the Complete San Diego mechanism with n-heptane extension and was attuned to the RCEM measurements to achieve high agreement between experiments and simulation. This resulted in a speciﬁc application area. To obtain a mechanism for a wider parameter range, the Arrhenius parameter changes performed were analyzed and updated. Furthermore, the San Diego nitrogen sub-mechanism was added to consider NOx formation. The ignition delay time-reducing eﬀect of propane addition to methane was closely examined and improved. To investigate the propagation of the ﬂame front, the laminar ﬂame speed of methane–air mixtures was simulated and compared with measured values from literature. Deviations at stoichiometric and fuel-rich conditions were found and by further mechanism optimization reduced signiﬁcantly. To be able to justify the parameter changes performed, the resulting reaction rate coeﬃcients were compared with data from the National Institute of Standards and Technology chemical kinetics database.


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  Laminar flame speed, Markstein length, and cellular instability for spherically propagating methane/ethylene–air premixed flames


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  Dilution gas and hydrogen enrichment on the laminar flame speed and flame structure of the methane/air mixture


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  In this study, the mechanisms of the chemical, thermal and dilution eﬀects of the CO2, H2O, N2 and EGR quantitatively analyzed on the laminar ﬂame speed, laminar ﬂame structure and key radicals proﬁles of the premixed methane/air mixture. Moreover, the artiﬁcial species of the FCO2, FH2O, FN2 and FEGR were used to separate the combined chemical and physical eﬀects. Furthermore, the impacts of the hydrogen enrichment coupled with the EGR on the laminar ﬂame speed, the laminar ﬂame structure and key radicals proﬁles of the premixed methane/air mixture was also studied in detailed. The results indicated that the chemical eﬀect of the CO2 dilution gas produced the greatest impacts on the laminar ﬂame speed, adiabatic combustion temperature and key radicals formation of the methane/air, and followed by the H2O vapor, EGR and N2. In addition, the dilution limitation of the CO2 in the methane/air was smallest, followed by the H2O vapor, EGR and N2. Moreover, the thermal eﬀect of the CO2 in the methane/air was strongest due to its highest speciﬁc heat capacity, followed by the H2O vapor, EGR and N2. The laminar ﬂame speed and adiabatic combustion temperature of the methane/hydrogen/air increased with increasing the hydrogen. Furthermore, the lean-burn limitation of the methane/air was extended with the increase of the hydrogen. The radical pool, such as H, O, OH, accelerated the chain branching reactions and the chain propagation reactions, and thereby increasing the eﬀect of the chemical ampliﬁer during the combustion of the premixed methane/air mixture.


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  Effects of Carbon Dioxide on Laminar Burning Speed and Flame Instability of Methane/Air and Propane/Air Mixtures: A Literature Review


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  This article reviews the impact of carbon dioxide (CO2) on laminar burning speed and stability of the flame for methane (CH4) and propane (C3H8) combustion with air. Mixtures of CH4 and CO2, also known as biogas, are considered as low-cost alternative fuels. Biogas is widely used in various industrial and residential applications. Mixtures of C3H8 and CO2 are considered as alternative refrigerants with low global warming potential and low flammability. Laminar burning speeds were reported using different experimental methods. Laminar burning speeds have also been numerically calculated by one-dimensional steady code using three chemical kinetic mechanisms. Results depict a decrease in equilibrium flame temperature and laminar burning speed of both CH4/air and C3H8/air mixtures with the existence of CO2 in those mixtures. The maximum laminar burning speeds of neat CH4 and C3H8 are observed to be in the vicinity of an equivalence ratio of 1.1; however, as the percentage of CO2 increases the maximum laminar burning speeds shift toward stoichiometric mixtures. Carbon dioxide also increases flame thickness and suppresses flame instability by the combination of thermal-diffusive and hydrodynamic effects.


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  Effects of Carbon Dioxide on Laminar Burning Speed and Flame Instability of Methane/Air and Propane/Air Mixtures


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  The chemical effect of CO <sub>2</sub> on the methane laminar flame speed in O <sub>2</sub> /CO <sub>2</sub> atmosphere


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  This paper studied in detail the chemical effects of CO2 on methane laminar flame speed in O2/CO2 atmosphere both experimentally and numerically. Experimentally, flames were established in the counterflow configuration at atmosphere pressure with inlet temperature (Tu) of 293K and the methane laminar flame speed (S ) in O2/CO2 atmosphere was measured with the help of PIV technique. Numerically, A comprehensive comparison between the flame speeds in O2/CO2 atmosphere and O2/N2 atmosphere with an identical mole fraction of O2 is established by introducing the artificial materials U, V, W, X, Y, Z to gradually isolate the effects of chemical properties and specific heat, mass diffusivity, radiation, thermal conductivity and density. A detailed sensitivity analysis is conducted for C1-C3 so that a overall effect of the elementary reactions is given. Then combined with the path analysis especially for the target elementary reactions, the ways of how they influence the methane flame speed can be obtained. The results showed that the reasons of the decline in speed in O2/CO2 atmosphere were mainly chemical properties and specific heat of CO2. The chemical effects of CO2 on the methane laminar flame speed led to a tremendous reduction in the reaction rate of CO+OH<=>CO2+H in O2/CO2 atmospheres as compared to those in O2/N2 atmospheres. Among the 3 reactions including CO+OH<=>CO2+H that are both significant in methane flame speed sensitivity and path analysis, high concentration in CO2 reduced the pathways of the positive ones and increased those of the negative ones.


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  Laminar Burning Velocity of Methane/Air Mixtures and Flame Propagation Speed Close to the Chamber Wall


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  AbstarINac+t Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Experimental studiesbVoenolitahReelcahmericnhaer&bIunnrnovinatgionv,e2l9o1ciAtvyen(uLeBDVre)yfaonusdDtahneiefl,la7m85e20pLriompaayg, Fartiaonncespeed close to the wall of prEexmpiexreimdcDeménpetaatrhlteasmntueend-taieSiryssftoèlnmametsheÉesnleaartgméatiinqpaurreessbesuturErnneivniorgofnv1nelmboaecnrittay-nI(dMLTaBmAVtbl)aineatninqtduetet,hm4erpufeelraAamltfureerdep,Kroafopstralegrp,ao4toi4o3rn0a0snNpdaenertidecsh,cFlmorasinexcteuorethse, hwaavlel obfeepnrecmonixdeudctmedetahnadnea-naairlyfzleadm. eTshaet ma eptrheasnseu-raeiromf 1ixtbuarre aLnBdVamanbdiefnltamteemppreorpataugraet,iofnorspeoeodr arnedsrtiucdhiemd iixntuarecso,nhstaavnet bveoelunmceoncdoumcbteudstaionnd cahnamlyzbedr .wTithhe amcreythliacnwe-ianidromwisxtuusriengLBthVe sahnaddfolwamgreaphrotpeacghantiqounesapnededaahrieghst-usdpieeeddicnaamceoran.sTtahnet vrraeeAogcclabouoisrnmrtddrseeatddccltioitimemraabgtguueesrstseioaadnrreaectaahn.naaamTllyhybzzeeeerdrdweusuiustshilitnnsaggcaraaryelMMicinaawttllqaaiunbbidtssoeccwrrgiisppottoudsaainnnaddggbrbtehooeetthmhshetthhnaeetdoswswyyisstgthtereammapvhaannitleddacbhttlhhneeeiqlMMiuteeaartatallntaaudbbreaccoohfddoiegrehhLh-aasBvpvVeeebbdteheceaeanntmsvvheaaorllaiwidd. saaTttheietddes adduDggeserciaciensrirentegernasaihcatsstoiidlnnuhiisgtgefeefarwewtgairnaitietutsghnhretenpmpesrrdetieewstasss-tosisuaourup.knrr.sseeTTfahiirhnrnoeeeccmrrcreeoetaahsmssuseueemllbt..tsousFFniaolsoldhryrrieonttahgwhidneedsterfhfqecllastauostmmiertt.deeheTeipgphnrfroeolosotahpepdmeaasggyealiasagtpttetreiirreomooaenntpsumasrsregepepnqaeeautteesiiddorwoenllniiihtestthittpglloeehafevlliditinahttveiedelreraeasambctttmurluoeerrseaeetnlsithehestfeasafwrsesaachblbttimuiecevreheoeennsastrfffoeoololiruurunetnneLitoudadBnrrnsflfVoyeofdrroetvrthxvhaacdrleltoeiipudcdsrtgaahehttcoaiilosowtohinnnssegebbhiuuttteohstatet uthssaeinlewgs.aalDlduwieffheteroreenthsteosmecthe-aunopgs.ceTdilhlcaeltiirmoenasutselatscreosnhpdorietwisoentnhsta,atdntudhebtfuolialadminceogpmrreobnpionavagataitoitoinonnopfsopmliecueiedltsi,dpelhecerapethadesneemsomaanlemdnoainsst utlhcinehefaurstluycroeemxccpoerupeldst scidloeoncsreaeantsode, thhpeeraotwlionanglglionwfghtthehereeuinsnovbemusrtemnoeesdnctmirleliatxuttriuonrnpese, raiorcedo.pmrebsinenat,iodnueoftoraadicaotimvebihneaatitolnososftomtuhletipwlaellp,hfelanmome ecnuarvsautcuhreaasncdomflapmreessitorentcahn.d hTeahteinmgaionfstchoepeunobf uthrinsepdapmeirxitsutroe,asasecsosmthbeinfeaatsioibniloitfy roafduisaitnivgethheehaetaltodssemtoanthde–wouatldl,ooflratmemepceurravtuarteurfuenacntidonflfaomr heesattrdetecmha.nd ©P©P©Pfbreeeoeuee22e2nrrri00r0elo---11dc1rrrveee77ai7anvvvstTTgTtiiii.eeeoshhhwwwnTeeethhAAuusAuaecnnnuutuedddttdtnvhheeheiaaorrorsorrtrrrrrriyrsseeosei...sscssiPpPptPpnwouououonnbbnbbefssollslriiiiiietsAsbbsbhhhhiiildlleleveciiieddttdatovyyylnbbebaooosyyldyoftfferEEpEttut,hhhellclssleesdetoeeeiooovcvv(rranisriigegegehterraarapadnnLnlLeLliiirtotzztizidddnwoiii.n.n.nd,ggLgiaiccncsnoootbdemmmornmtmmmyepiii(tttdPottteeieolaoeeertgteoouoy,fffg.dIaIINTNNel)ehFF,FprUUUew).eBBBaTs--w-11o1eu111ase..teshdtiemraasstceaentchaaersioeesrrs(otluro,dwyo,.bmtTahienedeidudmihst,eraihctitgdhies)macanondndstivhstareeldueeodsifswt6rei6rc5et Kceoymwopradrse:dexwpietrhimreensutalltslafmroinmarabudrynninagmvieclohceiatyt;dfleammaenpdrompaogdaetilo,nprspeveeioducslolysedteovwealollp; emdetahnadnev/aailridmaitxetdurbesy. the authors.


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  Laminar flame speed correlations for methane, ethane, propane and their mixtures, and natural gas and gasoline for spark-ignition engine simulations


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  The laminar flame speed plays an important role in spark-ignition engines, as well as in many other combustion applications, such as in designing burners and predicting explosions. For this reason, it has been object of extensive research. Analytical correlations that allow it to be calculated have been developed and are used in engine simulations. They are usually preferred to detailed chemical kinetic models for saving computational time. Therefore, an accurate as possible formulation for such expressions is needed for successful simulations. However, many previous empirical correlations have been based on a limited set of experimental measurements, which have been often carried out over a limited range of operating conditions. Thus, it can result in low accuracy and usability. In this study, measurements of laminar flame speeds obtained by several workers are collected, compared and critically analyzed with the aim to develop more accurate empirical correlations for laminar flame speeds as a function of equivalence ratio and unburned mixture temperature and pressure over a wide range of operating conditions, namely f = 0:6À1:7, pu = 1À50 atm and Tu = 298À800 K. The purpose is to provide simple and workable expressions for modeling the laminar flame speed of practical fuels used in spark-ignition engines. Pure compounds, such as methane and propane and binary mixtures of methane/ethane and methane/propane, as well as more complex fuels including natural gas and gasoline, are considered. A comparison with available empirical correlations in the literature is also provided.


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  Effects of pressure and preheating temperature on the laminar flame speed of methane/air and acetone/air mixtures


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  Laminar ﬂame speeds of acetone/air mixtures have been investigated at elevated temperature and high pressure conditions in regards to the importance of this intermediate oxygenated molecule in hydrocarbon oxidation. Experiments were performed in an elevated pressure combustion vessel speciﬁcally designed around a concept of a premixed Bunsen ﬂame. In a ﬁrst step, measurements of the laminar ﬂame speed of gaseous CH4/air mixtures were performed with OHÅ chemiluminescence and OH-PLIF methodologies and compared with literature data in order to validate the experimental setup. The effects of the preheating temperature (373–523 K), pressure (0.1–1.0 MPa) and equivalence ratio (0.6–1.3) on the laminar ﬂame speed of acetone/air mixtures were then examined. The experiments were ﬁnally complemented and compared with numerical simulations conducted with Cosilab commercial software package using published detailed chemical kinetic mechanisms for proposing a correlation relationship of the acetone/air laminar ﬂame speed with pressure, temperature and equivalence ratio.


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  Comparative study on the laminar flame speed enhancement of methane with ethane and ethylene addition


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  A systematic analysis of the ﬂame speed enhancement eﬀects of ethane and ethylene addition to methane was performed. Flame speeds of outwardly propagating spherical ﬂames were measured in a high-pressure, cylindrical ﬂame speed vessel using schlieren photography. It was ensured that the measured data were outside the ignition- and chamber-conﬁnement-aﬀected radii. The unstretched, unburnt ﬂame speeds were obtained using nonlinear extrapolation methods from the experimentally determined stretched ﬂame speeds. The AramcoMech 1.3 kinetics mechanism was validated against ﬂame speeds of ethane, ethylene, and acetylene over a wide range of pressures and equivalence ratios. Good agreement between the measured data and those reported in the literature was seen. The model agreed closely with the measurements at fuel-lean conditions, but a slight over prediction was observed for the fuel-rich cases. Flame speeds of 80/20 and 60/40 (by volume) CH4/C2Hx mixtures in air and in air with excess nitrogen were measured over a wide range of fuel concentrations. In all cases, the addition of ethylene increased the ﬂame speed more than did an equivalent addition of ethane. The Arrhenius eﬀect (predominantly kinetic eﬀect) was the principal driver for ﬂame speed enhancement of methane with ethane/ethylene addition. Sensitivity analysis revealed that thermal and diﬀusive pathways were equally contributing to the ﬂame speed enhancement of CH4/C2H6 mixtures, but the latter played a comparatively minor role in the C2H4-based blends.


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  Experimental study of the effect of CF3I addition on the ignition delay time and laminar flame speed of methane, ethylene, and propane


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  Since no suitable replacement has been found for Halon 1301 (CF3Br), the approach for the next generation of ﬁre suppressants has been to identify diverse agents or blends of agents for speciﬁc applications. Halon 13001 (CF3I) is a possible candidate for unoccupied areas. However, there are very few fundamental data to help in validating its detailed kinetics mechanism. In this study, ignition delay time and laminar ﬂame speed measurements of the eﬀects of CF3I on CH4, C2H4, and C3H8 have been investigated for the ﬁrst time. The ignition delay times were obtained in a shock tube with mixtures of fuel/O2/CF3I highly diluted in 98% argon (vol.), and the ﬂame speeds were measured using an expanding, spherical ﬂame. Results from the new experiments show a signiﬁcant decrease in the laminar ﬂame speed for small additions of suppressant. The eﬀects on the ignition delay time are strongly dependent on the hydrocarbon: the ignition delay time of CH4 is signiﬁcantly decreased by CF3I addition, while a signiﬁcant increase in the ignition delay time was observed for the lowest temperatures investigated with C2H4. Ignition delay times for C3H8 were mostly unchanged, except for the lowest temperatures (below 1400 K) where a small decrease in the reactivity was observed. Compared to recent results obtained with CF3Br, CF3I showed a smaller suppressant eﬀect under the conditions investigated. A detailed kinetics mechanism assembled from a C1–C3 mechanism from NUI Galway and a CF3I sub-mechanism from NIST predicts well the results obtained in the shock tube with CH4 and C3H8. The C2H4 data with CF3I are however poorly predicted, pointing at some deﬁciencies in the model for C2H4/CF3I interactions.


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  An experimental study on spherically expanding flames propagation of methane- air mixtures was conducted at constant pressure to measure unstretched laminar flame speeds, laminar burning velocity and flame stretch. The mixtures were ignited at equivalence ratios of 0.7, 0.9 and 1.0, under ambient pressure and temperature. It was found that the unstretched laminar burning velocity increased with the equivalence ratio. The flame propagation speed showed different trends at different equivalence ratio for tested mixtures.


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  The present paper reports on the assessment of different turbulence-chemistry interaction closures for modeling turbulent combustion in the Moderate and Intense Low oxygen Dilution (MILD) combustion regime. 2D RANS simulations have been carried out to model flames issuing from two burners i.e. Delft-Jet-in-Hot-Coflow (DJHC) burner and Adelaide JHC burner which imitate the MILD combustion. In order to model these flames, two different approaches of turbulence-chemistry interaction models, i.e. Eddy Dissipation Concept (EDC) and PDF based models, are considered; while in the PDF based modeling, two different variants are invoked to understand the applicability of the PDF based models in the MILD regime: one is based on presumed shape PDF (i.e. steady flamelet (SF) model) approach and the other one is transported PDF approach. For transported PDF method, two different solution approaches namely, Lagrangian solution method (LPDF) and Multi-Environment Eulerian PDF (MEPDF) model are considered. A comprehensive study has been carried out by comparing the results obtained from these different models. For the DJHC burner, the computations are carried out for a jet speed corresponding to Reynolds numbers of Re = 4100, whereas the Adelaide JHC burner computations are performed for a jet speed corresponding to Reynolds number of Re = 10000. The effects of molecular diffusion on the flame characteristics are also studied by using different micro-mixing models. In the case of DJHC burner, it has been observed that the mean axial velocity and the turbulent kinetic energy profiles are in good agreement with the measurements. However, the temperature profiles are over-predicted in the downstream region by both EDC and the PDF based models. In the context of Adelaide JHC burner, the profiles of the temperature and the mass fraction of major species (CH4, H2, O2, H2O, CO2) obtained using LPDF approach are in better agreement with the measurements compared to those obtained using EDC model; although, both the solution approaches fail to capture CO and OH radical profiles.


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  Numerical Analysis of Pollutant Formation in Direct- Injection Spark-Ignition Engines by Incorporating the G- Equation with a Flamelet Library


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  Laminar flame speeds of H 2 / CO with CO 2 dilution at normal and elevated pressures and temperatures


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  Numerical Study on Evaporation Characteristics of Single Urea-Water Solution ( UWS ) Droplet and Variation of Evaporation and Wall-Interaction Characteristics of UWS Spray with Cell Density in SCR Mixing Chamber


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  Three-dimensional numerical simulations of the particle loading effect on gas flow features for low pressure cold spray applications


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  This paper reports numerical investigations to develop a high quality simulation model for the Cold Spray (CS) process as part of the project, Supersonic Spray Advanced Modelling (SSAM). Cold spray is a process in which unmelted solid particles are accelerated through a De Laval nozzle toward a target surface and deposited, which avoids or minimises the detrimental effects that arise from melting. The gas flow injected allows the particles to achieve velocities up to 1000 m/s until they impact onto a substrate, bonding with plastic deformation. Analytical and numerical tools were used in the recent years to study the most important parameters of the fluid dynamic of the process. These parameters are often overestimated or non-validated, as many authors tend to neglect inter-phase couplings. The interactions between the gas flow features and particulate phase are important for the present multiphase flows, and many of the connected phenomena lack physical comprehension. For this purpose, a detailed 3D model for RANS simulations has been realised and the gas and particle dynamics in the nozzle and jet were analysed. Advancements for future computational methods are derived from this study, in order to build a comprehensive and coherent numerical model for cold spray applications.


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  Multi-Dimensional Modeling of Direct-Injection Diesel Spray Liquid Length and Flame Lift-off Length using CFD and Parallel Detailed Chemistry


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  Recent measurements by Siebers et al. have shown that the flame of a high pressure Diesel spray stabilizes under quiescent conditions at a location downstream of the fuel injector. The effects of various ambient and injection parameters on the flame “lift-off” length have been investigated under typical Diesel conditions in a constant-volume combustion vessel. In the present study, the experiments of Siebers et al. have been modeled using a modified version of the KIVA-3V engine simulation code. Fuel injection and spray breakup are modeled using the KH-RT model that accounts for liquid surface instabilities due to the Kelvin-Helmholtz and Rayleigh-Taylor mechanisms. Combustion is simulated using Convergent Thinking's recently developed detailed transient chemistry solver (SAGE) that allows for any number of chemical species and reactions to be modeled. While detailed chemistry is believed to be an accurate methodology for modeling Diesel combustion, in the past the extensive run times rendered it impractical. To expedite the calculations, SAGE has been implemented into KIVA using the Message-Passing Interface (MPI). This implementation allows for the chemical reactions to be simulated in parallel on multiple CPUs. An n-heptane mechanism was used to model Diesel fuel ignition and combustion. The improved KIVA-3V code was used to simulate the spray-bomb cases over a wide range of injection pressures, nozzle hole sizes, ambient temperatures, and ambient densities. In general, excellent agreement was obtained between the measurements and simulation results for liquid length and flame lift-off length.To further validate the chemical kinetics model, simulations of a heavy-duty direct injection Diesel engine were conducted. The results indicate that the detailed chemistry model is able to accurately predict ignition delay and cylinder-averaged pressure for a range of start of injection timings.


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  In this study, a 3D CFD model for the formation of NOx and N2O in a lignite fired 1.2 MWth CFB pilot plant is developed. The decoupled approach (decoupled from combustion simulation) is tested for the minimization of computational cost. As combustion simulation is prerequired, this was achieved through a simplified 3-D CFD combustion model. The developed model is then applied to the pilot-scale 1.2 MWth CFB plant and validated against experimental data. As concerns the NO x-N2O model, an extensive literature review is also carried out for the incorporation of the appropriate reactions network and respective reaction rates expressions. Results show that homogenous reactions are favoured on the lower section of the bed, due to the abundance of fuel devolatilization products. On the other hand, on the upper section, heterogeneous reactions govern nitric oxide formation/reduction. It is found that for the lignite examined in this work, HCN is released in negligible amounts during char combustion. The proposed and validated CFD model for NO x and N2O, is capable of examining the effect of different operational parameters and coal properties on the overall nitric oxides emissions from a CFB combustor, with low computational cost and without the additional expenses for pilot-scale experiments. ?? 2013 Elsevier Ltd. All rights reserved.


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  Study about the link between injection strategy and knock onset in an optically accessible multi-cylinder GDI engine


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  The effects of different injection strategies on the mixture formation and combustion processes in a gasoline direct injection (GDI) engine are investigated. A synergic experimental and numerical analysis is realized on an optically accessible multi-cylinder engine under single and double injections, these last delivering the same amount of gasoline in two equal parts, the first during intake, the second during compression. In-cylinder pressure cycles and high temporal and spatial resolution combustion images are collected to highlight the possible occurrence of abnormal phenomena. AVL FIRE simulations are realized to characterize into detail the turbulent reacting flow and to evaluate the influence of mixture formation on flame propagation and the possible formation of zones in the end-gas region with conditions leading to self-ignition. Pollutants formation is also analysed. The flame propagation characteristics are indeed found different for the various injection strategies: for single injection, a faster spread occurs in the opposite side with respect to the injector position and towards the exhaust valves, while under split injection the flame front preferentially moves towards the intake valves. Computational fluid dynamics results give the relevant support to understand these peculiarities as related to in-cylinder equivalence ratio distribution and turbulent kinetic energy. In-chamber knock visualization is related to the numerical results of a simple self-ignition chemical model active in the end-gas zone, not yet reached by the main flame front. Split injection is shown to lead to advantages in terms of reduction of cyclic variability and tendency to knock, hence in increasing energy efficiency and in determining an overall reduction of undesired products of incomplete combustion.


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  The aim of this paper is to investigate the emission and performance characteristics of a commercial diesel engine (Deutz FL8 413F) being operated on natural gas with pilot diesel ignition. A computer program has been developed to model the experimental data usinga chemical kinetic reaction mechanism of the Gas–Diesel (dual-fuel) combustion. A detailed chemical kinetic reaction mechanisms of natural gas and NOx were used to predict the main combustion characteristics (temperature, pressure and species concentrations) under the conditions of this study. The followingsections include a description of the experimental facilities, discussion of numerical simulation and engine test results. The performance in terms of accuracy of the networks is assessed by comparison with the experiments. A reasonably good prediction of performance and emission was obtained by computation covering the whole range of the engine operating conditions. It can be summarized that the results of this study are satisfactory. © 2001 Éditions scientifiques et médicales Elsevier SAS dual-fuel


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  Numerical study of the impact on high-pressure and evaporating spray behavior of nozzle cavitation at typical diesel engine conditions


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  In order to more accurately reproduce diesel sprays a strategy including measurement of nozzle inlet pressure at the realistic diesel injection condition, modeling of nozzle cavitating flow and detailed coupling of nozzle exit flow and spray was presented, moreover, the validity of this strategy was firstly verified against the quantitative spray data obtained by planar laser induced exciplex fluorescence (PLIEF) technique. Based on the above strategy, the effect of cavitation phenomenon on spray formation at the typical diesel engine condition was further evaluated. The final numerical results mainly clarified that the contribution of cavitation phenomena to primary breakup is quite appreciable, and subsequently the evolution of the high-pressure and evaporating diesel spray structure greatly changes as cavitation occurs inside fuel injection nozzles. Moreover, evaluating the effects of cavitation phenomena on realistic diesel spray cannot be only confined to primary breakup or near-nozzle field.


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  Experimental study on microscopic and macroscopic characteristics of diesel spray with split injection


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  The microscopic and macroscopic characteristics of diesel spray with split injection strategy were systematically investigated by employing ultra-high speed imaging in the near-field, high speed imaging and Phase Doppler Particle Analyzer (PDPA) techniques. It was found that for single injection there were four stages for the development of spray penetration, largely depending on injection pressure and ambient pressure. With split injection strategy, the "wake driving force" caused by higher speed of the wake of the first injection than that of periphery gas tended to distort the second split injection, whereas the 'air driving force' caused by the first split injection tended to make the second plume symmetric. Furthermore, the second split injection showed lower macroscopic penetration and spray area during the initial stage but higher macroscopic penetration and spray area at the later stage than the first injection. In addition, higher injection pressure lead to better dispersion and smaller droplets. The strong collision (both primary collision and secondary collision) caused larger droplets for split injection than for single injection. Lower effective injection pressure due to lower injector opening for split injection strategy was believed to be partly responsible for the larger droplet sizes.


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  Measurements of the auto-ignition of n -heptane / toluene mixtures using a rapid compression machine


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  Annals of Nuclear Energy Evaluation of mechanistic wall condensation models for horizontal heat exchanger in PAFS ( Passive Auxiliary Feedwater System )


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  CFD Modeling of Spark Ignited Gasoline Engines- Part 2 : Modeling the Engine in Direct Injection Mode along with Spray Validation


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  CFD simulations of diesel spray tip penetration with multiple injections and with engine compression ratios up to 100:1


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  A diesel spray tip penetration was simulated with CFD by varying the compression ratio from the conventional compression ratio of a diesel engine, 18:1, to an ultra-high compression ratio of 100:1. The gas densities of the spray chambers for the compression ratios of 18:1 and 100:1 were 17.97 and 74.8 kg/m3, respectively. The spray tip penetration was CFD simulated by adopting both the 'Gas jet spray model', and 'Normal gas jet profile model with breakup length formula' (NGJBL model) into the KIVA-3V code. The spray tip penetration was CFD simulated for various spray patterns, such as single injection, pilot injection and split injection. The CFD simulation of the spray tip penetration with an 18:1 compression ratio was compared with a previous study with identical experimental conditions and showed that the 'Gas jet spray model' over predicted. The CFD simulation results with the 'KIVA-3V Standard spray model' and the NGJBL model were generally in good agreement with the experimental data. The spray tip penetration was CFD simulated with the NGJBL model with step-by-step increased compression ratio. The spray tip penetration rapidly decreased for the range of compression ratio of 18:1-45:1 as the compression ratio increases and the spray tip penetration did not change much with increasing the compression ratios in the range of 60:1-100:1. The CFD simulation results of the spray tip penetration, with respect to varying compression ratios, were generally in agreement with previous experimental studies. © 2013 Elsevier Ltd. All rights reserved.


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  Ignition and kinetic modeling of methane and ethane fuel blends with oxygen : A design of experiments approach


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  Simultaneous imaging of fuel, OH, and three component velocity fields in high pressure, liquid fueled, swirl stabilized flames at 5kHz


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  journal homepage: www.elsevier.com/locate/combustflame Simultaneous imaging of fuel, OH, and three component velocity fields in high pressure, liquid fueled, swirl stabilized flames at 5 kHz R a b s t r a c t This paper describes implementation of simultaneous, high speed (5 kHz) stereo PIV, OH and fuel-PLIF in a pressurized, liquid fueled, swirl stabilized flame. The experiments were performed to characterize the flow field, qualitative heat release and fuel spray distributions, and flame dynamics. Acquiring high speed OH-PLIF in pressurized, liquid fuel systems is difficult due to the strong overlap of the fuel's absorption and emission spectra with the OH fluorescence spectrum. To overcome difficulties associated with the overlap, the OH and fuel fluorescence signals were partially separated by using two cameras with differing spectral filters and data acquisition timing. Upon data reduction, regions containing fuel, OH and a mixture of fuel and OH are identified. Instantaneous and time-averaged results are discussed showing the flow field, flame position and dynamics, and spray distribution from the fuel signal for two multi-component liquid fuels, at two inlet temperatures and three pressures. These results are used to infer several important observations on coupled flow and flame physics. Specifically, the flame is "M-shaped" at higher preheat temperature and higher fuel/air ratio, as opposed to no visible reaction on the inside of the annular fuel/air jet at low temperature and fuel/air ratio conditions. While such fundamentally different flame topologies in gaseous, premixed flames are well known, these results show that there are also different families of flame shapes and heat release distributions in spray flames. In addition, the flame position with respect to the flow is different for the liquid-fueled flame than for gaseous, premixed flames-in premixed flames with this geometry, the flame lies in the low velocity shear layer separating the reactants and the recirculating products. In contrast, the flame location is controlled by the spray location in this spray flame, as opposed to the shear layer. For example, reactions are observed near the nozzle outlet in the core of the high velocity annular jet, something which would not be observed in the premixed flame configuration. Also of interest is the near invariance of the key flow features-such as jet core trajectory or shear layer locations-to the operating condition changes for this study, even as the spray penetration and distribution, and flame position change appreciably.


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  This study explores condensation of FC-72 in horizontal tubes. Using high-speed video motion analysis, dominant condensation flow regimes are identified for different combination of mass velocities of FC-72 and cooling water. Additionally, detailed heat transfer measurements are used to explore both axial and circumferential variations of the condensation heat transfer coefficient. Four different regimes are identified: stratified, stratified-wavy, wavy-annular with gravity influence, and wavy-annular without gravity influence. In the latter regime, which is achieved at high FC-72 mass velocities, annular film transport is dominated by vapor shear with negligible gravity effects. Using different types of regime maps, prior relations for transitions between regimes are assessed, and new, more accurate transition relations developed. The heat transfer coefficient is shown to be highest near the inlet, where quality is near unity and the film thinnest, and decreases gradually along the condensation length because of axial thickening of the liquid film. This study also explores the predictive capabilities of prior heat transfer correlations and a control-volume-based annular flow model. The experimental data of both the local and average condensation heat transfer coefficients show fair to good agreement with predictions of prior and popular correlations. But superior predictions in both trend and magnitude are achieved with the annular flow model.


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  A new edition of the bestseller on convection heat transfer A revised edition of the industry classic, "Convection Heat Transfer, Fourth Edition, " chronicles how the field of heat transfer has grown and prospered over the last two decades. This new edition is more accessible, while not sacrificing its thorough treatment of the most up-to-date information on current research and applications in the field. One of the foremost leaders in the field, Adrian Bejan has pioneered and taught many of the methods and practices commonly used in the industry today. He continues this book's long-standing role as an inspiring, optimal study tool by providing: Coverage of how convection affects performance, and how convective flows can be configured so that performance is enhanced How convective configurations have been evolving, from the flat plates, smooth pipes, and single-dimension fins of the earlier editions to new populations of configurations: tapered ducts, plates with multiscale features, dendritic fins, duct and plate assemblies (packages) for heat transfer density and compactness, etc. New, updated, and enhanced examples and problems that reflect the author's research and advances in the field since the last edition A solutions manual Complete with hundreds of informative and original illustrations, "Convection Heat Transfer, Fourth Edition" is the most comprehensive and approachable text for students in schools of mechanical engineering.


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  https://books.google.com/books?id=9yC91-gpU8sC{\&}pgis=1{\%}5Cnhttp://doi.wiley.com/10.1002/9781118671627


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  Two techniques were used to study non-evaporating diesel sprays from common rail injectors which were equipped with twin-hole and single-hole nozzles for comparison. To characterise the sprays, high speed optical imaging and X-ray radiography were used. The former was performed at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) at Monash University, while the latter was performed at the 7-BM beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The optical imaging made use of high temporal, high spatial resolution spray recordings on a digital camera from which peripheral parameters in the initial injection phase were investigated based on edge detection. The X-ray radiography was used to explore quantitative mass distributions, which were measured on a point-wise basis at roughly similar sampling rate. Three twin-hole nozzles of different subtended angles and a single-hole nozzle were investigated at injection pressure of 1000 bar in environments of 20 bar back pressure. Evidence of strong cavitation was found for all nozzles examined with their CDranging from 0.62 to 0.69. Penetration of the twin-hole nozzles was found to lag the single-hole nozzle, before the sprays merged. Switching in hole dominance was observed from one twin-hole nozzle, and this was accompanied by greater instability in mass flow during the transient opening phase of the injectors.


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  Detection of common carotid artery stenosis using duplex ultrasonography: A validation study with computed tomographic angiography


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  Background: Severe stenosis of the common carotid artery (CCA), while uncommon, is associated with increased risk of transient ischemic attack and stroke. To date, no validated duplex ultrasound criteria have been established for grading the severity of CCA stenosis. The goal of this study was to use receiver-operating curve (ROC) analysis with computed tomographic angiography as the reference standard to establish duplex ultrasound criteria for diagnosing ???50% CCA stenosis. Methods: The study cohort included 64 patients (42 men, 22 women) with a mean age of 65 ?? 12 years (range, 16-89 years) who had CCA peak systolic velocity (PSV) ???150 cm/sec and underwent computed tomographic angiography (CTA) of the cervical and intracerebral vessels within 1 month of the duplex examination. One study was excluded because the CTA was technically inadequate, whereas another was excluded because the patient underwent bilateral CCA stenting. The CCA ipsilateral to any of the following was excluded from the analysis: innominate artery occlusion (n = 1), previous stenting of the ICA or CCA (n = 7), carotid endarterectomy (n = 1), or carotid-to-carotid bypass (n = 1). Thus, the data set included 62 patients and 115 vessels. Bland-Altman analysis was used to examine the agreement between two measures of luminal reduction measured by CTA: percent diameter stenosis and percent area stenosis. Receiver operating characteristic (ROC) analysis was used to determine optimal PSV and EDV thresholds for diagnosing ???50% CCA stenosis. Results: Severity of CCA stenosis was \textless50% in 76 vessels, 50%-59% in eight, 60%-69% in eight, 70%-79% in nine, 80%-89% in three, 90%-99% in five, and occluded in six. Duplex ultrasonography identified six of six (100%) patients with 100% CCA occlusion by CTA. Bland-Altman analysis showed poor agreement between percent stenosis determined by vessel diameter compared with percent stenosis determined by reduction in lumen area. Therefore, subsequent analysis was performed using percent stenosis by area. ROC analysis of different PSV thresholds for detecting stenosis ???50% showed that \textgreater182 cm/sec was the most accurate with a sensitivity of 64% and specificity of 88% (P \textless .0001). Sensitivity, specificity, and accuracy of carotid duplex were higher when the stenosis was located in the mid or distal aspects of the CCA (sensitivity 76%, specificity 89%, area under curve 0.84, P \textless .001) than in the intrathoracic and proximal segment of the artery (P = NS). ROC analysis of different EDV thresholds for detecting CCA stenosis ???50% showed that \textgreater30 cm/sec was the most accurate with a sensitivity of 54% and a specificity of 74% (P \textless .0239). Conclusions: Duplex ultrasonography is highly sensitive, specific, and accurate for detecting CCA lesions in the mid and distal CCA. Use of peak systolic velocity may lead to improved detection of CCA disease and initiation of appropriate therapy to reduce the risk of stroke. ?? 2010 Society for Vascular Surgery.


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  Detailed assessment of diesel spray atomization models using visible and X-ray extinction measurements


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  The physical mechanisms characterizing the breakup of a diesel spray into droplets are still unknown. This gap in knowledge has largely been due to the challenges of directly imaging this process or quantitatively measuring the outcomes of spray breakup, such as droplet size. Recent x-ray measurements by Argonne National Laboratory, utilized in this work, provide needed information about the spatial evolution of droplet sizes in selected regions of the spray under a range of injection pressures (50–150 MPa) and ambient densities (7.6–22.8 kg/m3) relevant for diesel operating conditions. Ultra-small angle x-ray scattering (USAXS) measurements performed at the Advanced Photon Source are presented, which quantify Sauter mean diameters (SMD) within optically thick regions of the spray that are inaccessible by conventional droplet sizing measurement techniques, namely in the near-nozzle region, along the spray centerline, and within the core of the spray. To quantify droplet sizes along the periphery of the spray, a complementary technique is proposed and introduced, which leverages the ratio of path-integrated x-ray and visible laser extinction (SAMR) measurements to quantify SMD. The SAMR and USAXS measurements are then utilized to evaluate current spray models used for engine computational fluid dynamic (CFD) simulations. We explore the ability of a carefully calibrated spray model, premised on aerodynamic wave growth theory, to capture the experimentally observed trends of SMD throughout the spray. The spray structure is best predicted with an aerodynamic primary and secondary breakup process that is represented with a slower time constant and larger formed droplet size than conventionally recommended for diesel spray models. Additionally, spray model predictions suggest that droplet collisions may not influence the resultant droplet size distribution along the spray centerline in downstream regions of the spray.


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  This research investigates high pressure diesel fuel injection into the combustion chamber by performing computational simulations using the Euler-Eulerian multiphase approach. Six diesel-like conditions were simulated for which the liquid fuel jet was injected into a pressurised inert environment (100% N2) through a 205 μm nozzle hole. The analysis was focused on the liquid jet and vapour penetration, describing spatial and temporal spray evolution. For this purpose, an Eulerian multiphase model was implemented, variations of the sub-model coefficients were performed, and their impact on the spray formation was investigated. The final set of sub-model coefficients was applied to all operating points. Several simulations of high pressure diesel injections (50, 80, and 120 MPa) combined with different chamber pressures (5.4 and 7.2 MPa) were carried out and results were compared to the experimental data. The predicted results share a similar spray cloud shape for all conditions with the different vapour and liquid penetration length. The liquid penetration is shortened with the increase in chamber pressure, whilst the vapour penetration is more pronounced by elevating the injection pressure. Finally, the results showed good agreement when compared to the measured data, and yielded the correct trends for both the liquid and vapour penetrations under different operating conditions.


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  This paper provides an analysis of high-pressure phenomena and its potential effects on the fundamental physics of fuel injection in Diesel engines. In particular, we focus on conditions when cylinder pressures exceed the thermodynamic critical pressure of the injected fuel and describe the major differences that occur in the jet dynamics compared to that described by classical spray theory. To facilitate the analysis, we present a detailed model framework based on the Large Eddy Simulation (LES) technique that is designed to account for key high-pressure phenomena. This framework is then used to perform a thermodynamic analysis of the flow. We focus on the experiments being conducted in the high-pressure combustion vessel at Sandia National Laboratories using n-heptane as a reference fuel. The calculations are performed by rigorously treating the experimental geometry and operating conditions, with detailed treatment of relevant thermophysical mixture properties. Results demonstrate that n-heptane enters the chamber as a compressed liquid, not a spray, and is heated at supercritical pressure. Further analysis suggests the classical view of spray atomization as an appropriate model at these particular conditions is questionable. Instead, nonideal real-fluid behavior must be taken into account using a multicomponent formulation that applies to arbitrary hydrocarbon mixtures at high-pressure supercritical conditions. Copyright © 2012 SAE International.


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  The performance and emissions of a single-cylinder natural gas fueled engine using a pilot ignition strategy have been investigated. Small diesel pilots (2-3% on an energy basis), when used to ignite homogeneous natural gas-air mixtures, are shown to possess the potential for reduced NO x emissions while maintaining good engine performance. The effects of pilot injection timing, intake charge pressure, and charge temperature on engine performance and emissions with natural gas fueling were studied. With appropriate control of the above variables, it was shown that full-load engine-out brake specific NO x emissions could be reduced to the range of 0.07-0.10 g/kWh from the baseline diesel (with mechanical fuel injection) value of 10.5 g/kWh. For this NO x reduction, the decrease in fuel conversion efficiency from the baseline diesel value was approximately one to two percentage points. Total unburned hydrocarbon (HC) emissions and carbon monoxide (CO) emissions were higher with natural gas operation. The nature of combustion under these conditions was analyzed using heat release schedules predicted from measured cylinder pressure data. The importance of pilot injection timing and inlet conditions on the stability of engine operation and knock are also discussed.


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  Experimental study on spray characteristics of long-chain alcohol-diesel fuels in a constant volume chamber


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  This study aims at study effects of blending long-chain alcohol (n-butanol or n-pentanol) with diesel on the spray characteristics and compare the differences of those spray characteristics between n-butanol-diesel blends and n-pentanol-diesel blends. Results showed that with the addition of long-chain alcohols, the spray tip penetration decreased, the spray cone angle increased and the spray area increased, while with the increasing proportions of long-chain alcohols in blended fuels, the spray area decreased whereas the peak tip velocity increased. Under the same blended ratios, n-butanol-diesel blends showed longer tip penetration, larger cone angle, larger spray area and lower peak tip velocity compared to n-pentanol-diesel blends in the same injection conditions. Furthermore, from the comparison between spray morphology of each tested fuel with each other, it can be inferred that n-butanol-diesel blends have largest spray areas and thinnest spray boundaries, followed by n-pentanol-diesel blends and then pure diesel fuel.


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  Numerical Investigation of Soot Emission in Direct-Injection Spark-Ignition Engines Using a Detailed Soot Model Framework


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  ScienceDirect Ignition delay time measurements behind reflected shock-waves for a representative coal-derived syngas with and without NH 3 and H 2 S impurities


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  Global and Local Response of Premixed Flames during Flame- Vortex Interactions under Distinct Configurations


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  Effects of Lewis number and ignition energy on the determination of laminar flame speed using propagating spherical flames


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  The trajectories of outwardly propagating spherical flames initiated by an external energy deposition are studied theoretically, numerically, and experimentally by using hydrogen/air mixtures. Emphasis is placed on how to accurately determine the laminar flame speeds experimentally from the time history of the flame fronts for mixtures with different Lewis numbers and ignition energies. The results show that there is a critical flame radius only above which is the linear and non-linear extrapolation for flame speeds valid. It is found that the critical radius depends strongly on the Lewis number. At large Lewis numbers, the critical radius is larger than the minimum flame radius used in the experimental measurements, leading to invalid flame speed extrapolation. The results also show that there is a maximum Karlovitz number beyond which propagating spherical flame does not exist. The maximum Karlovitz number decreases dramatically with the increase of Lewis number. Furthermore, the results show that the ignition energy has a significant impact on the flame trajectories. It is found that the unsteady flame transition causes a flame speed reverse phenomenon near the maximum Karlovitz number with different ignition energies. The occurrence of flame speed reverse greatly narrows the experimental data range for flame speed extrapolation. The strong dependence of flame trajectory on ignition energy and the existence of the flame speed reverse phenomenon are also confirmed by experimental results.


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  Effects of Nozzle Diameter on Diesel Spray Flames: A numerical study using an Eulerian Stochastic Field Method


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  The present numerical study aims to assess the performance of an Eulerian Stochastic Field (ESF) model in simulating spray flames produced by three fuel injectors with different nozzle diameters of 100 μm, 180 μm and 363 μm. A comparison to the measurements shows that although the simulated ignition delay times are consistently overestimated, the relative differences remain below 28%. Furthermore, the change of the averaged pressure rise with respect to the variation of nozzle diameter is captured by the model. The simulated flame lift-off lengths also agree with the measurements, with a maximum relative difference of 13%. The spray flame produced by a larger nozzle diameter has a fuel-richer premixed core region despite the longer lift-of length, indicating that the higher fueling rate used with the larger nozzle diameter is a more dominating factor than the lift-off length is in influencing the air entrainment into the upstream of the spray flames. In addition, the simulated normalised flame lengths are found to decrease when the nozzle diameters increase. These predictions are in good qualitative agreement with the experimental observation. This work proves that the ESF model can serve as an important tool for the simulation of spray flames in marine diesel engines, where fuel injectors with different nozzle diameters are applied for pilot and main injections.


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  Simplified fuel cracking process in reduced mechanism development: PRF – PAH kinetic models for combustion and soot prediction


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  In this paper, a special technique for visualizing the first 1.5 mm of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behavior in the near-nozzle field. For this purpose, a Diesel axi-symmetric nozzle has been analyzed. Firstly, the nozzle has been geometrically and hydraulically characterized. Mass flow measurements at stationary conditions have allowed the detection of the pressure conditions for mass flow choking, usually related with cavitation inception in the literature. Nevertheless, with the objective to get a deeper knowledge of cavitation phenomenon, near-nozzle field visualization technique has been used to detect cavitation bubbles injected in a chamber pressurized with liquid fuel. Using backlight illumination, the differences in terms of density and refractive index have allowed the distinction between vapour and liquid fuel phases. From these visualization results, two important conclusions can be established: on the one hand, cavitation bubbles have been detected at the nozzle exit for pressure drop conditions at which mass flow was not choked yet. On the other hand, it could be seen that the jet formed by cavitation bubbles spread as pressure drop conditions became stronger. Finally, spray visualization in a nitrogen pressurized chamber has been developed at stationary conditions. In order to analyze cavitation influence on spray characteristics, pressure drop has been modified near the values at which cavitation bubbles have been detected out of the nozzle. Two different test strategies have been used for this purpose: fixing injection pressure, which implied a change in chamber density for each test point, or fixing chamber pressure. Both kinds of measurements revealed a noticeable increment of spray cone angle and spray contour irregularities related with the presence of cavitation bubbles at the orifice outlet. This fact can be assumed as an indicator of atomization improvement induced by the collapse of cavitation bubbles at the nozzle exit. © 2009 Elsevier Ltd. All rights reserved.


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  Numerical simulation of the transient heat transfer during nucleate boiling of refrigerant HFE-7100 ´ rique du transfert de chaleur transitoire lors Simulation nume ´ bullition nucle ´ e ´ e du frigorige ` ne HFE-7100 de l ' e


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  The low-temperature auto-ignition of fuels is a complex process, occurring in multiple stages with distinct chemical processes governing each stage. The conversion from alkyl radical to chain branching products, which occurs through successive O2 additions followed by thermal decomposition of the products, is at the core of the auto-ignition process. Our detailed understanding of this central process continues to evolve, with recent theoretical kinetics studies providing a particularly comprehensive description of the radical oxidation process for propane. In this study, we employ this improved description in a detailed numerical and analytical exploration of the first-stage ignition delay for low-temperature auto-ignition of propane, which may be considered as a prototype for larger alkane fuels. The traditional first-stage of ignition can be divided into two stages (stage-1A and stage-1B). During stage-1A, the concentration of radicals grows exponentially, and both OH and HO2 are important in the consumption of the fuel and generation of alkyl radicals. Stage-1A ends when the concentration of HO2 is sufficiently high that the chain-terminating bimolecular reaction HO2 + HO2 becomes competitive with other HO2 reactions including HO2 + fuel, thus slowing the HO2 concentration rise such that it is no longer a key contributor to fuel consumption. During stage-1B, increasing temperature and growing side reactions with secondary chemistry reduce the positive feedback and the concentrations of ketohydroperoxide species stop growing exponentially. The end of this stage is associated with the maximum in ketohydroperoxide, after which it is depleted. We present simple analytical approximations for the time it takes to complete these two sub-stages. These expressions clarify which rate constants control first-stage ignition, and they quantify how the ignition is influenced by mixture composition, temperature and pressure. The analysis is also extended to longer alkane fuels and is shown to provide fairly reliable predictions of the first-stage ignition delay.


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  With increasing interest to reduce the dependency on gasoline and diesel, alternative energy source like compressed natural gas (CNG) is a viable option for internal combustion engines. Spark-ignited (SI) CNG engine is the simplest way to utilize CNG in engines, but direct injection (DI) Diesel-CNG dual-fuel engine is known to offer improvement in combustion efficiency and reduction in exhaust gases. Dual-fuel engine has characteristics similar to both SI engine and diesel engine which makes the combustion process more complex. This paper reports the computational fluid dynamics simulation of both DI dual-fuel compression ignition (CI) and SI CNG engines. In diesel-CNG dual-fuel engine simulations and comparison to experiments, attention was on ignition delay, transition from auto-ignition to flame propagation and heat released from the combustion of diesel and gaseous fuel, as well as relevant pollutants emissions. Injection timing for diesel pilot was changed in a test matrix and the most efficient and least pollutant producing cases were selected and simulation results were correlated to the experimental data for those cases. The possible end-gas auto-ignition-like processes under certain test conditions like PREmixed Mixture Ignition in the End-gas Region (PREMIER) were also demonstrated. SI CNG lean burn engine tests and simulations were directed to check the ignitability, combustion stability, heat release rate for different spark energy values. The results show that a combined approach of using both chemical kinetics and G-equation formulations in RANS is capable of capturing most of the physical-chemical processes in both the dual-fuel CI and SI engine combustions


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  This paper presents an analysis of the cycle-to-cycle combustion variation as reflected in the combustion pressure data of a single cylinder, naturally aspirated, four stroke, Ricardo E6 engine converted to run as dual fuel engine on diesel and gaseous fuel of LPG or methane. A measuring set-up consisting of a piezo-electric pressure transducer with charge amplifier and fast data acquisition card installed on an IBM microcomputer was used to gather the data of up to 1200 consecutive combustion cycles of the cylinder under various combination of engine operating and design parameters. These parameters included type of gaseous fuel, engine load, compression ratio, pilot fuel injection timing, pilot fuel mass, and engine speed. The data for each operating conditions were analyzed for the maximum pressure, the maximum rate of pressure rise - representing the combustion noise, and indicated mean effective pressure. The cycle-to-cycle variation is expressed as the mean value, standard deviation, and coefficient of variation of these three parameters. It was found that the type of gaseous fuel and engine operating and design parameters affected the combustion noise and its cyclic variation and these effects have been presented. ?? 2004 Elsevier Ltd. All rights reserved.


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  Multicomponent Liquid and Vapor Fuel Distribution Measurements in the Cylinder of a Port-Injected , Spark-Ignition Engine


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  Correlation of Air Fuel Ratio with Ionization Signal Metrics in a Multicylinder Spark Ignited Engine METHODS


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  Advances in rapid compression machine studies of low- and intermediate-temperature autoignition phenomena


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  Rapid compression machines (RCMs) are widely used to acquire experimental insights into fuel autoignition and pollutant formation chemistry, especially at conditions relevant to current and future combustion technologies. RCM studies emphasize important experimental regimes, characterized by low-to intermediate-temperatures (600À1200 K) and moderate to high pressures (5À80 bar). At these conditions, which are directly relevant to modern combustion schemes including low temperature combustion (LTC) for internal combustion engines and dry low emissions (DLE) for gas turbine engines, combustion chemistry exhibits complex and experimentally challenging behaviors such as the chemistry attributed to cool flame behavior and the negative temperature coefficient regime. Challenges for studying this regime include that experimental observations can be more sensitive to coupled physical-chemical processes leading to phenomena such as mixed deflagrative/autoignitive combustion. Experimental strategies which leverage the strengths of RCMs have been developed in recent years to make RCMs particularly well suited for elucidating LTC and DLE chemistry, as well as convolved physical-chemical processes. Specifically, this work presents a review of experimental and computational efforts applying RCMs to study autoignition phenomena, and the insights gained through these efforts. A brief history of RCM development is presented towards the steady improvement in design, characterization, instrumentation and data analysis. Novel experimental approaches and measurement techniques, coordinated with computational methods are described which have expanded the utility of RCMs beyond empirical studies of explosion limits to increasingly detailed understanding of autoignition chemistry and the role of physical-chemical interactions. Fundamental insight into the autoignition chemistry of specific fuels is described, demonstrating the extent of knowledge of low-temperature chemistry derived from RCM studies, from simple hydrocarbons to multi-component blends and full-boiling range fuels. Emerging needs and further opportunities are suggested, including investigations of under-explored fuels and the implementation of increasingly higher fidelity diagnostics.


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  Biodiesel fuels form an important constituent of the renewable energy resources landscape. These fuels are produced through transesterification of fatty acids with methanol or ethanol to yield long-chain methyl or ethyl esters of 16-18 carbon atoms, frequently with one or more CC double bonds. The CC double bonds are expected to play a major role in the combustion chemistry of these biodiesel fuels, especially in the area of soot emission. To better understand the effect of these double bonds on the sooting properties of esters, we have measured the sooting tendencies of twenty C4to C7unsaturated esters and report their Yield Sooting Indices (YSI). The CC double bond was found to have a significant effect on the sooting tendency. In most cases, the unsaturated ester was more sooting than its saturated counterpart, although the increase in sooting tendency was strongly influenced by other structural features of the compound. Esters with the CC double bond nearest the carbonyl group had lower increases in the sooting tendency from the parent saturated ester than compounds where the CC double bond was further along the carbon backbone. Unsaturated methyl esters had larger increases in YSI (compared to their parent saturated ester) than unsaturated ethyl or propyl esters. The strong dependence of the sooting tendencies of these esters on their chemical structure indicates the complexity of the chemical processes involved, and is an important area for further studies of these compounds.


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  Mechanistic Study of Ignition Characteristics of Diesel / Methanol and Diesel / Methane Dual Fuel Engine


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  AbstractDeveloping an improved understanding of transient mixing and combustion processes inherent in diesel injection is an important element in the design of advanced engines. This paper provides a detailed analysis of these processes using an idealized benchmark configuration designed to facilitate precise comparisons between different models and numerical methods. The computational domain is similar to the Engine Combustion Network (www.sandia.gov/ECN) Spray-A injector with n-dodecane as the fuel. Quantified idealizations are made in the treatment of boundary conditions to eliminate ambiguities and unknowns associated with the actual injector(s) used in the experiment. These ambiguities hinder comparisons aimed at understanding the accuracy of different models and the coupled effects of potential numerical errors. Prior to understanding the impact of injector imperfections on their performance, it is first necessary to understand how models perform in a well-controlled environment with well-defined boundary conditions. Here, we focus on the latter while accurately matching the operating conditions used in the experiments. Relevant high-pressure phenomena are treated with real-fluid thermodynamics and transport for multicomponent mixtures. A highly resolved calculation is performed to study both scalar-mixing and initiation of combustion. Results are assembled into a database that can facilitate one-to-one comparisons between codes. The combined set of results is analyzed to provide physical insights related to localized broadband transient mixing and combustion processes that are typically not available from experiments.


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  Comparative study on chemical kinetic schemes for dual-fuel combustion of n -dodecane / methane blends


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  Detailed Chemical Kinetic Modelling of Surrogate Fuels for Gasoline and Application to an HCCI Engine


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  SCHR ODINGER MODELS FOR REALIZABLE BOSE – EINSTEIN CONDENSATION IN d + 1 DIMENSIONS ( d = 1 , 2 , 3 ) coupling constant c \textgreater 0


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  Particle formation by flame spray pyrolysis is investigated theoretically and compared with particle size distribution data of ZrO2 production at 100-300 g/h. Spray droplet size distributions as well as droplet and gas velocities are measured by phase Doppler anemometry along the axis and radius of the solid-cone-type spray. Multicomponent droplet combustion is simulated, accounting for droplet polydispersity, transport, and evaporation. The dynamics of the product particle size distributions are investigated, accounting for coagulation and sintering by population balance equations. The effect of the initial droplet size and liquid feed rate on the evolution of the product particle size and polydispersity is analyzed.


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  Natural gas/diesel dual-fuel combustion is currently one of the most promising LTC strategies for the next generation of heavy-duty engines. While this concept is not new and it has been deliberated lengthily in the past two decades, several uncertainties still exist. A major shortcoming of this concept is associated with its low thermal efficiency and high level of unburned methane and CO emissions under low engine load conditions. The present paper reports an experimental and numerical study on the effect of different injection strategies (single and two pulses injection of pilot diesel fuel) on the combustion performance and emissions of a heavy duty natural gas/diesel dual-fuel engine at 25% engine load. The results of single diesel injection mode showed that advancing diesel injection timing from 10 to 30 °BTDC reduced unburned methane and CO emissions by 62% and 61% and increased thermal efficiency by 6%; however, NOxemissions increased by 74%. In order to achieve NOx– CH4and NOx– CO trade-off and increased thermal efficiency at low load conditions, the effect of split injection strategy was experimentally and numerically examined. The results of split injection mode revealed that split injection strategy considerably increases the in-cylinder peak pressure compared to that of single injection (10 °BTDC). The results showed also that the heat release produced by the first injection of diesel fuel considerably increased the in-cylinder charge temperature before the start of the second injection. The flame zone of the split injection mode is markedly higher than that of the single injection due to larger heat release produced during the first injection which promotes the combustion of the second one. When the first injection timing is close to the second injection timing, the MPRR of split injection mode is higher than that of single injection (10 °BTDC). However, further advancing of the first injection timing continuously decreased the MPRR. OH radical analysis showed that for advanced first injection timings (38–50 °BTDC), the overall growth rate of OH radical becomes slower and its distribution is narrower as indicated by the wider non-reactive blue zones compared with those observed at a late first injection timing in the initial stages of combustion. However, OH radicals gradually grow during last stages of combustion in the expansion stroke, indicating that a more premixed combustion takes place in these cases. For very advanced first injection timing of 55 °BTDC, the OH distribution is similar to that of the single injection mode with lower OH intensity at initial stages of combustion and they barely grow during the late expansion stroke. At this condition, the ignition of premixed mixture is mainly controlled by the second diesel fuel injection. The trade-off between NOx– CH4and NOx– CO is achieved when applying split injection. Compared to single injection (10 °BTDC), the first injection timing of 50 °BTDC decreased unburned methane and CO emissions by 60% and 63%, respectively, and increased the thermal efficiency by 8.9%. However, NOxemissions were maintained at the same level as single injection mode (10 °BTDC).


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  Computer tomography based structure characterization of expanded intumescent coatings for fire protection


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  Systematic Computational Study on the Unimolecular Reactions of Alkylperoxy ( RO 2 ), Hydroperoxyalkyl ( QOOH ), and Hydroperoxyalkylperoxy ( O 2 QOOH ) Radicals


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  Modification and Extension of a Standard Volume of Fluid Solver For Simulating Boiling Heat Transfer


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  An earlier paper has shown the ability to predict the phasing of knock onset in a gasoline PFI engine using a simple ignition delay equation for an appropriate surrogate fuel made up of toluene and PRF (TPRF). The applicability of this approach is confirmed in this paper in a different engine using five different fuels of differing RON, sensitivity, and composition - including ethanol blends. An Arrhenius type equation with a pressure correction for ignition delay can be found from interpolation of previously published data for any gasoline if its RON and sensitivity are known. Then, if the pressure and temperature in the unburned gas can be estimated or measured, the Livengood-Wu integral can be estimated as a function of crank angle to predict the occurrence of knock. Experiments in a single cylinder DISI engine over a wide operating range confirm that this simple approach can predict knock very accurately. The data presented should enable engineers to study knock or other auto-ignition phenomena e.g. in premixed compression ignition (PCI) engines without explicit chemical kinetic calculations.


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  Fluid-structure interaction analysis of turbulent pulsatile flow within a layered aortic wall as related to aortic dissection


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  Turbulent pulsatile flow and wall mechanics were studied numerically in an axisymmetric three-layered wall model of a descending aorta. The transport equations were solved using the finite element formulation based on the Galerkin method of weighted residuals. A fully-coupled fluid-structure interaction (FSI) analysis was utilized in this investigation. We calculated Von Mises wall stress, streamlines and fluid pressure contours. The findings of this study show that peak wall stress and maximum shear stress are highest in the media layer. The difference in the elastic properties of contiguous layers of the wall of the aorta probably determines the occurrence of dissection in the media layer. Moreover, the presence of aortic intramural hematoma is found to have a significant effect on the peak wall stress acting on the inner layer. ?? 2009 Elsevier Ltd. All rights reserved.


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  In the last years, laser beam drilling became in- creasingly important for many technical applications as it allows the contactless production of high quality drill holes. So far, mainly short laser pulses are of industrial relevance, as they offer a good compromise between precision and ef- ficiency and combine high ablation efficiency with low ther- mal damage of the workpiece. Laser beam drilling in this pulse length range is still a highly thermal process. There are two ablation mechanisms: evaporation and melt expulsion. In order to achieve high quality processing results, a basic process understanding is absolutely necessary. Yet, process observations in laser beam drilling suffer from both the short time scales and the restricted accessibility of the interaction zone. Numerical simulations offer the possibility to acquire additional knowledge of the process as they allow a direct look into the drill hole during the ablation process. In this contribution, a numerical finite volume multi-phase simu- lation model for laser beam drilling with short laser pulses shall be presented. The model is applied for a basic study of the ablation process with µs and ns laser pulses. The ob- tained results show good qualitative correspondence with experimental data.


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  This article puts forward the quadrature method of moments (QMoM) for modeling droplet composition during the spray vaporization process. This method is implemented for solving the Continuous Thermodynamic Model (CTM) of multi-component droplet vaporization, an advantageous alternative to the classical Discrete Component Model (DCM) when the droplet is formed of a great number of components. The CTM approach consists in modeling the droplet's composition using a probability density function (PDF). This method was first tried out for vaporizing droplets by Hallett, who assumed a Gamma-function for the PDF. However, Harstadt et al. underlined some problems in the case of vapor condensation on the droplet surface, since the Gamma-PDF model presumes the PDF's mathematical form. The QMoM which does not require this hypothesis is studied in this article, according to Lage's research dealing with QMoM application to phase equilibria. The numerical features of QMoM are investigated in detail, and then the method is implemented for the difficult test case of vapor condensation. The results are analyzed to illustrate the application of QMoM to multi-component droplet vaporization modeling and to provide a better understanding of the QMoM main advantages and limitations. ?? 2009 Elsevier Ltd. All rights reserved.


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  The Development of an Electronic Control Unit for a High Pressure Common Rail Diesel / Natural Gas Dual- Fuel Engine


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  Experimental and numerical analysis of the performance and exhaust gas emissions of a biogas / n-heptane fueled HCCI engine


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  Experimental study on the diesel and biodiesel spray characteristics emerging from equilateral triangular orifice under real diesel engine operation conditions


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  The triangular spray has a potential to accelerate air-fuel mixing rate and undergo axis-switching during the injection process, which both are beneficial for improving the atomization quality. This paper presents an experimental investigation on the macro-spray characteristics and the mechanism of axis-switching with diesel and biodiesel emerging from equilateral triangular orifice of diesel nozzle, by using shadowgraph technique, under various injection pressures (50 MPa, 70 MPa, 90 MPa) and backpressures (1 MPa, 2 MPa, 3 MPa). Two CCD cameras were set to capture the macro spray images at the minor and major view planes simultaneously. Macro spray characteristics values were processed from the spray images in different view planes by MATLAB. The results indicated that the spray width of biodiesel was larger than that of diesel at all conditions from all view planes. In addition, both diesel and biodiesel emerging from equilateral triangular orifice underwent axis-switching during the spray progress even under the typical diesel engine operation conditions. Moreover, the difference of equilateral triangular spray widths between minor plane and major plane decreases as the backpressure increases for both fuels, it is expected that the increasing of backpressure enhances the aerodynamic effects, resulting in inhibiting the spray axis-switching. Furthermore, the difference of diesel spray widths between the major plane and minor plane was much higher than that of biodiesel. Because the higher surface tension and viscosity of biodiesel can cause smaller surface-instabilities and also resist the spray deformation, which are conductive to suppress the biodiesel spray axis-switching, and thus ultimately lead to narrowing the difference of biodiesel spray widths between minor plane and major plane. Finally, the spray tip penetration of biodiesel was consistent longer than that of diesel, while the spray cone angle of biodiesel was smaller than that of diesel, and also there was an interaction between major and minor view planes for diesel spray cone angle values at the backpressure of 1 MPa, but it didn't happen for biodiesel during the whole injection process. The present results indicated that the diesel spray was much easier to undergo axis-switching. The higher probability of axis-switching, larger spray cone angle and spray width of diesel demonstrated better air-fuel mixing quality than biodiesel.


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  We describe a number of algorithms to perform basic geometric operations on ellipsoids in n spatial dimensions, for n ¸ 1. These algorithms are im- plemented in ELL LIB, a library of Fortran subroutines. With E, E1 and E2 being given ellipsoids, and p a given point, the tasks considered include: determine the point in E which is closest to p or furthest from p; grow or shrink E so that its boundary intersects p; project E onto a given a±ne space; determine a separating hyperplane between E1 and E2; determine an ellipsoid (of small volume) which covers E1 and E2.


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  A multidimensional computational fluid dynamics (CFD) simulation of a constructed syngas chemical kinetic mechanism was performed to evaluate the combustion of syngas in a supercharged dual-fuel engine for various syngas initial compositions under lean conditions. The modelled results were validated by comparing predictions against corresponding experimental data for a supercharged dual-fuel engine. The predicted and measured in-cylinder pressure, temperature, and rate of heat release (ROHR) data were in good agreement. The effect of the hydrogen peroxide chain-propagation reaction on the progress of combustion under supercharged conditions was examined for different types of syngas using various initial H2 concentrations. The effect of the main syngas kinetic mechanism reactions on the combustion progress was analysed in terms of their contribution to the total heat of the reaction. The best results compared with experimental data were obtained in the range of equivalence ratios below about 0.8 for all types of syngas considered in this paper. As the equivalence ratio increased above 0.8, the results deviated from the experiment data. The spatial distribution of the in-cylinder temperature and OH within this equivalence-ratio range showed the completeness of the combustion. The present CFD model captured the overall combustion process well and could be further developed into a useful tool for syngas-engine combustion simulations. ?? 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


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  E ff ects of natural gas injection timing and split pilot fuel injection strategy on the combustion performance and emissions in a dual-fuel engine fueled with diesel and natural gas


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  This paper considers the kinetic pathways of hydrogen oxidation in turbulent, premixed H2–air flames. It assesses the relative roles of different reaction steps in H2 oxidation relative to laminar flames, and the degree to which turbulence–chemistry interactions alters the well understood oxidation pathway that exist in laminar flames. This is done by analyzing the turbulent, lean (ϕ = 0.4), H2–air flame DNS database from Aspden et al. [17]. The relative roles of dominant reaction steps in heat release and radical formation/consumption are analyzed at different Karlovitz numbers and compared with laminar stretched flame calculations from counterflow flames and perfectly stirred reactors. It is found that both the progress variable conditioned and spatially integrated contributions of the dominant reactions remain qualitatively similar between a highly turbulent and a laminar unstretched flame. Larger changes, up to a factor of about two, occur in the relative roles of reactions with secondary influences on heat release and radical production/consumption. These results suggest that the kinetic routes through which H2 is oxidized remain essentially constant between laminar, unstretched flames and high Karlovitz number flames.


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  Natural Gas for High Load Dual-Fuel Reactivity Controlled Compression Ignition in Heavy-Duty Engines


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  Development of a Neuro Genetic Algorithm Based Virtual Sensing Platform for the Simultaneous Prediction of NOx, Opacity and BSFC in a Diesel Engine Operated in Dual Fuel Mode with Hydrogen under Varying EGR Conditions


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  Development and Validation of a Reduced Chemical Kinetic Mechanism for Computational Fluid Dynamics Simulations of Natural Gas / Diesel Dual-Fuel Engines


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  This paper is concerned with the development of a first-principle theoretical method to explain the dependence of contact angle on the size of liquid droplets on smooth solid substrates. It has been shown that the contact angle for axisymmetric droplets is size-dependent and, consequently, cannot satisfy the original Young equation. However, this contact angle can be theoretically deduced for any droplet material and mass as a function of the asymptotic contact angle (the limiting value of the contact angle as droplet size decreases). The proposed approach is based on the fact that the two major forces acting on a droplet, that is, the surface force and the gravity force, are independent of each other. It has been demonstrated that for sessile droplets on smooth surfaces, the contact angle can be uniquely determined for given droplet mass (or volume) and liquid/solid/gas properties. ?? 2005 Elsevier B.V. All rights reserved.


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  The impingement of fuel spray on the wall in direct-injection spark-ignition (DISI) engines affects the fuel-air mixture formation, combustion and exhaust emissions. A detailed understanding of the fuel spray and impingement is required to reduce the undesirable products of combustion whilst maintaining good fuel economy. In this study, the droplet size was measured by particle image analysis (PIA). The microscopic characteristics of impinging spray were obtained using ultra-high-speed imaging. By changing the injection and ambient pressures, the influences of breakup and coalescence on the droplet behavior were investigated. Before impingement, the region near the center of the spray has larger droplets size and lower droplet number density than the edge, which suggests that spray breakup and atomization is poor in the center region. After impingement, the droplet size decreases along the distance from the wall under low ambient pressure. However, large droplets appear in the region far from the wall under high ambient pressure, indicating the existence of a coalescence effect on the droplets.


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  The key challenge in making chemical mechanism development predictive is being able to accurately estimate any possible rate coefficient k(T) even if there are no experimental data. Reaction Mechanism Generator (RMG) is an open-source software project that can build detailed kinetic models for chemical reacting systems (http://rmg.sourceforge.net). It uses a database of rules to propose elementary chemical reactions and to estimate the necessary thermochemical and kinetic parameters. We are modifying the algorithm used to estimate kinetic data to make the estimated reaction rates more reliable and easier to document in cases where they are estimated from sparse data. We present a brief overview of RMG, a discussion of the kinetics estimation options, an explanation of the chosen algorithm, and an assessment of its performance.


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  A comprehensive literature search and a series of shock-tube experiments were undertaken to determine the ignition delay times for mixtures of methane with ethane, propane or butane, and for a typical natural gas fuel. Ignition delay experiments were conducted at equivalence ratios of 0.45–1.25 for temperatures 1300–2000 K and pressures 3–15 atm. The combined data were used to develop general correlations for predicting the ignition delays of binary methane-hydrocarbon mixtures and multicomponent natural gas mixtures in terms of temperature and the initial fuel and oxygen concentrations. For natural gas, the ignition delay was correlated by the empirical expression t = 1.77 × 10−14 exp (18693/T)[O2]−1.05[CH4]0.66[HC]−0.39 in which concentrations are expressed in molecules per cubic cm and the [HC] factor represents the total molar concentration of all non-methane hydrocarbons. Chemical kinetic modeling was also performed to define the ignition mechanisms, and to permit extrapolation of data and prediction of the effects of using vitiated air for combustor testing.


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  Freely propagating laminar methane/air flames were modeled for spatially stratified equivalence ratio conditions at atmospheric pressure. The GRI 2.11 mechanism was used with a modified version of the Lawrence Livermore National Laboratory HCT code to describe the reaction kinetics. Results showed that the laminar flame speed was strongly affected by the equivalence ratio gradient and by the burned gas composition and temperature. Production of molecular hydrogen from the original fuel and its transport to the reaction zone as well as heat transfer from the burned to the fresh gases are key factors in under- standing the influence of stratification on laminar flame speed. Combinations of different mixture strati- fication conditions can either enhance or reduce the flame velocity when compared with homogeneous mixtures. Mixture stratification is also responsible for higher flame resistance to extinction both on the lean and rich sides of the equivalence ratio. The importance of heat and mass transfer on the observed results implies that their extrapolation to high pressure and to turbulent systems must be made with care.


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  Assessing the predictions of a NO x kinetic mechanism on recent hydrogen and syngas experimental data


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  Extension of PREMIER combustion operation range using split micro pilot fuel injection in a dual fuel natural gas compression ignition engine : A performance-based and visual investigation


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  Theoretical and Computational Fluid Dynamics Transient Pressure Effects in the Evolution Equation for Premixed Flame Fronts


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  A Triangulated Lagrangian Ignition Kernel Model with Detailed Kinetics for Modeling Spark Ignition with the G-Equation-Part I: Geometric Aspects


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  © 2018 SAE International. All Rights Reserved. Modeling ignition kernel development in spark ignition engines is crucial to capturing the sources of cyclic variability, both with RANS and LES simulations. Appropriate kernel modeling must ensure that energy transfer from the electrodes to the gas phase has the correct timing, rate and locations, until the flame surface is large enough to be represented on the mesh by the G-Equation level-set method. However, in most kernel models, geometric details driving kernel growth are missing: either because it is described as Lagrangian particles, or because its development is simplified, i.e., down to multiple spherical flames. This paper covers the geometric aspects of kernel development, which makes up the core of a Triangulated Lagrangian Ignition Kernel model. One (or multiple, if it restrikes) spark channel is initialized as a one-dimensional Lagrangian particle thread. Each channel particle is advected as a Lagrangian tracker plus a turbulent dispersion term, with least-squares field reconstruction to compensate for the lesser mesh resolution. The 1D thread discretization is dynamically updated to stick to the user's resolution request; plus, particles falling into the wall boundary layer are flagged and deactivated, such that actual spark channel can effectively translate along the electrode's surface. Energy transfer to the gas phase is accounted for via direct chemical kinetics integration at each channel particle, added with a time-varying energy source term. Flame kernels can develop at any particle locations: they are initialized as a triangulated icosahedron, and evolved (and merged) as a unique particle-based triangulation, with dynamically enforced resolution, employing the G-Equation formulation directly at the kernel level. Preliminary kernel development tests show that the triangulated geometry implementation allows to capture sub-grid-scale features of the early flame kernel development, which cannot be achieved with the former DPIK model.


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  Crank-Angle Resolved Modeling of Fuel Injection , Combustion and Emission Formation for Engine Optimization and Calibration on Real-Time Systems


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  Numerical analysis of the combustion and emission characteristics of diesel engines with multiple injection strategies using a modified 2-D flamelet model


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  The multiple injection strategy has been widely used in diesel engines to reduce engine noise, NOx and soot formation. Fuel injection developments such as the common-rail and piezo-actuator system provide more precise control of the injection quantity and time under higher injection pressures. As various injection strategies become accessible, it is important to understand the interaction of each fuel stream and following combustion process under the multiple injection strategy. To investigate these complex processes quantitatively, numerical analysis using CFD is a good alternative to overcome the limitation of experiments. A modified 2-D flamelet model is further developed from previous work to model multi-fuel streams with higher accuracy. The model was validated under various engine operating conditions and captures the combustion and emissions characteristics as well as several parametric variations. The model is expected to be used to suggest advanced injection strategies in engine development processes.


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  A popular contemporary approach in predicting enhanced flame speeds in premixed turbulent combustion involves averaging or closure theories for the G-equation involving both large-scale flows and small-scale turbulence. The G-equation is a Hamilton-Jacobi equation involving advection by an incompressible velocity field and nonlinear dependence on the laminar flame speed; this G-equation has been derived from the complete Navier-Stokes equations under the tacit assumptions that the velocity field varies on only the integral scale and that the ratio of the flame thickness to this integral scale is small. Thus there is a potential source of error in using the averaged G-equation with' turbulent velocities varying on length scales smaller than the integral scale in predicting enhanced flame speeds. Here these issues are discussed ip the simplest context involving velocity fields varying on two scales where a complete theory of nonlinear averaging for predicting enhanced flame speeds without any ad hoc approximations has been developed recently by the authors. The predictions for enhanced flame speeds of this complete averaging theory versus the averaging approach utilizing the G-equation are compared here in the simplest context involving a constant mean flow and a small-scale steady periodic flow where both theories can be solved exactly through analytical formulas. The results of this comparison are summarized briefly as follows: The predictions of enhanced flame speeds through the averaged G-equation always underestimate those computed by complete averaging. Nevertheless, when the transverse component of the mean flow relative to the shear is less than one in magnitude, the agreement between the two approaches is excellent. However, when the transverse component of the mean flow relative to the shear exceeds one in magnitude, the predictions of the enhanced flame speed by the averaged G-equation significantly underestimate those computed through complete nonlinear averaging, and in some cases, by more than an order of magnitude.


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  Controlled autoignition (CAI) engines ideally operate at very lean stoichiometries to achieve low NOx emissions. But at high loads, when combustion approaches stoichiometric, they become noisy and severe engine knock develops. A possible cause is the development of amplifying pressure waves near the hot spots that inevitably occur in the autoigniting gas. This paper presents the results from numerical solutions at realistic engine conditions of the detailed chemical kinetic equations with acoustic wave propagation. Those calculations that involve hot spots must include a spatial dimension. Because of this, they are much more time-consuming than for the homogeneous case. A model system of mixtures of 0.5 H2-0.5 CO with air for equivalence ratios, ϕ, between 0.45 and 1.0 has been used at engine-like temperatures and pressures. These calculations investigate the behaviour for various values of ϕ, hot spot size and temperature elevation. They demonstrate that strong pressure spikes and developing detonations occur when the spatial gradient of temperature is such that the autoignition moves into the unburnt mixture at approximately the speed of sound, so that the chemical and acoustic waves reinforce each other. It is also shown that this resonance, in itself, is not sufficient for a developing detonation to occur and it is also necessary for the mixture to be sufficiently reactive. The criterion for this in terms of the homogeneous chemical kinetics is developed. It is postulated that the conditions for the resonance will occur somewhere in the cylinder for most mixtures but only for ϕ close to stoichiometric do strong pressure waves develop, leading to noisy and possibly damaging combustion. Copyright © 2002 Society of Automotive Engineers, Inc.


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  Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures


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  Ignition delays of lean mixtures of methane-hydrogen with various hydrogen volumetric contents were experimentally studied in a shock tube together with modeling analysis. Results show that the ignition behavior of the methane-hydrogen mixture depending on pressure resembles that of methane for hydrogen fraction less than 40%, with the ignition delays decreasing with increasing pressure. For the hydrogen fraction equal 60%, a negligible promoted effect of pressure on the ignition of the methane-hydrogen mixture is exhibited. For hydrogen fractions equal or greater than 80%, however, the ignition response resembles that of hydrogen in that the ignition delay exhibits a complex dependence on pressure and two-step transition in the global activation energy. Compared with calculated values using four available mechanisms, the NUI Galway mechanism yielded the closest agreement, and was adopted in the sensitivity analysis of the ignition kinetics. The sensitivity analysis well explained the experimental results which the ignition delay decreases with increasing temperature regardless of whether methane (typical fuel 80%CH 4/20%H 2) or hydrogen (typical fuel 20%CH 4/80%H 2) dominates the ignition process. Rate of production analysis shows that the promoted effect of the hydrogen on the oxidation of the methane is mainly due to the concentrations of the free radicals such as H, O and OH increase with increasing hydrogen fraction, and lead to the total reaction rate is enhanced. Consumption of methane is mainly through these reactions in which the active free radicals participate. ?? 2011 The Combustion Institute.


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  Laminar flame speeds and ignition delay times of methane-air mixtures at elevated temperatures and pressures


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  Measurements on laminar flame speeds and ignition delay times of methane/air mixtures at elevated pressures and temperatures were carried out in a constant volume bomb and shock tube. The performances of GRI Mech 3.0, USC Mech II, and Aramco Mech 1.3 mechanisms were also evaluated from the data obtained. Results showed that the measured laminar flame speeds from the constant volume bomb by the linear method are slightly higher than those from the counter flow flame at rich mixtures and lower at lean mixtures. At rich mixtures, the laminar flame speeds with linear method are higher than that with non-linear method. The available mechanisms give slight overprediction to the constant volume bomb data at lean mixtures, and large underprediction at rich mixtures at elevated temperatures and pressures. Overall reaction order decreases and then increases with the rising of pressure from 0.1 to 10.0 MPa because of the chain reaction mechanism. For the ignition delay times, the three mechanisms are in good accordance with the experimental data of lean and stoichiometric mixtures at atmospheric pressure, while the discrepancy between calculation and measurement is increased at elevated pressures. These mechanisms seem to lack good sensitivity to the rich mixtures, especially at elevated pressures. Correlations for laminar burning velocities and ignition delay time of methane-air mixtures are provided.


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  Detailed Simulations of Stratified Ignition and Combustion Processes in a Spray-Guided Gasoline Engine using the SparkCIMM / G-Equation Modeling Framework


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  Measurement of the laminar burning velocity using the confined and unconfined spherical flame methods – A comparative analysis


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  There is significant interest in using hydrogen and natural gas for enhancing the performance of diesel engines. We report herein a numerical investigation on the ignition of n-C7H16/H2 and n-C7H16/CH4 fuel blends. The CHEMKIN 4.1 software is used to model ignition in a closed homogenous reactor under conditions relevant to diesel/HCCI engines. Three reaction mechanisms used are (i) NIST mechanism involving 203 species and 1463 reactions, (ii) Dryer mechanism with 116 species and 754 reactions, and (iii) a reduced mechanism (Chalmers) with 42 species and 168 reactions. The parameters include pressures of 30 atm and 55 atm, equivalence ratios of ??= 0.5, 1.0 and 2.0, temperature range of 800-1400 K, and mole fractions of H2 or CH4 in the blend between 0 and 100%. For n-C7H16/air mixtures, the Chalmers mechanism not only provides closer agreement with measurements compared to the other two mechanisms, but also reproduces the negative temperature coefficient regime. Consequently, this mechanism is used to characterize the effects of H2 or CH4 on the ignition of n-C7H16. Results indicate that H2 or CH 4 addition has a relatively small effect on the ignition of n-C 7H16/air mixtures, while the n-C7H16 addition even in small amount modifies the ignition of H2/air and CH4/air mixtures significantly. The n-C7H16 addition decreases and increases the ignition delays of H2/air mixtures at low and high temperatures, respectively, while its addition to CH4/air mixtures decreases ignition delays at all temperatures. The sensitivity analysis indicates that ignition characteristics of these fuel blends are dominated by the pyrolysis/oxidation chemistry of n-heptane, with heptyl (C7H16-2) and hydoxyl (OH) radicals being the two most important species. ?? 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


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  Premixed turbulent combustion in the corrugated flamelets regime has been addressed by modeling of a G-equation containing a gas-expansion term, the form of which was derived by generalizing results of Sivashinsky along lines initiated by Frankel. Previous analyses by the first author of the mean, variance, and two-point correlation spectrum of G, as well as their relation to the turbulent burning velocity, were extended to include the influences of gas expansion. It is shown that this effect has the same form as kinematic restoration but acts in the opposite direction. Because of this similarity in form, no new modeling approximations are required. Onaverage, kinematic restoration dominates gas expansion. In physical terms, this means that the energy-containing eddies at the integral scale are strong enough to suppress gas- dynamic instabilities at smaller scales. Gas expansion lessens the smoothing effect of kinematic restoration, thereby increasing the variance ofGand the turbulent flame brush thickness. It also increases the turbulent burning velocity by a factor that increases with increasing ratio of unburned to burned gas density and with decreasing ratio of turbulence intensity to laminar burning velocity. The effect on the burning velocity, however, is not large, remaining less than 40% even as the turbulence intensity begins to decrease below the laminar burning velocity. Introduction


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  For engine knock prediction, surrogate fuels are often composed of iso-octane and n-heptane since they are the components of the Primary Reference Fuel (PRF). By definition, a PRF has no octane sensitivity (S = RON-MON). However, for a commercial gasoline fuel holds RON \textgreater MON and therefor S \textgreater 0. More complex surrogates are Toluene Reference Fuels (TRF) and Ethanol containing Toluene Reference Fuels (ETRF). In this work, the impact of the surrogate formulation on the prediction of flame propagation and auto-ignition in the unburnt gases are investigated. The surrogates are composed such that the Research Octane Number is the same. The auto-ignition events ahead of the flame front are predicted using 3D CFD and a combustion model based on the ETRF mechanism by Seidel (2017). The strength of the auto-ignition is determined using the detonation diagram by Bradley and co-workers (2002, 2003). Applying the different surrogates, ignition kernels of different size and reactivity are predicted. The results indicate a dependency on the local temperature history and the low temperature chemistry of the fuel species. The comparison of homogenous constant volume reactor and transient simulations show that the analysis of ignition delay time and octane rating solely from homogenous simulations is not sufficient if the knock tendency of a surrogate in engine simulations needs to be characterized.


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  Chemiluminescence imaging has been applied to a parametric investigation of diesel autoignition. Time-resolved images of the natural light emission were made in an optically accessible DI diesel engine of the heavy-duty size class using an intensified CCD video camera. Measurements were obtained at a base operating condition, corresponding to a motored TDC temperature and density of 992 K and 16.6 kg/m3, and for TDC temperatures and densities above and below these values. Data were taken with a 42.5 cetane number blend of the diesel reference fuels for all conditions, and measurements were also made with no. 2 diesel fuel (D2) at the base condition. For each condition, temporal sequences of images were acquired from the time of first detectable chemiluminescence up through fully sooting combustion, and the images were analyzed to obtain quantitative measurements of the average emission intensity. These data are presented and discussed with respect to the liquid-phase fuel penetration and the apparent heat release rates. In addition, a spectrometer equipped with an intensified OMA was used to record both chemiluminescence and sooting combustion spectra.The data show diesel autoignition to be a progressive process that occurs simultaneously over the downstream region of all the fuel jets. The first chemiluminescence is detected shortly after the start of injection for all the conditions studied, and it grows brighter and shifts downstream as the jet penetrates across the chamber, until soot luminosity dominates the emission. Although this process is qualitatively the same for all conditions, the rate at which it proceeds varies greatly with TDC temperature and density. The temporal evolution of the natural emission is similar for the reference and D2 fuels. The spectral data show that the chemiluminescence arises mainly from formaldehyde and CH emission, with no OH emission detected. In contrast, the sooting-combustion spectrum (taken after the diffusion flame has formed) shows OH and “gray-body” soot emission.


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  Thin reaction zone and distributed reaction zone regimes in turbulent premixed methane/air flames: Scalar distributions and correlations


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  A series of premixed turbulent methane/air jet flames in the thin reaction zone (TRZ) and distributed reaction zone (DRZ) regimes were studied using simultaneous three-scalar high-resolution imaging measurements, including HCO/OH/CH2O, CH/OH/CH2O, T/OH/CH2O and T/CH/OH/. These scalar fields offer a possibility of revisiting the structures of turbulent premixed flames in different combustion regimes. In particular, CH2O provides a measure of the preheat zone, CH/HCO a measure of the inner layer of the reaction zone, and OH a measure of the oxidation zone. Scalar correlations are analyzed on both single-shot and statistical basis, and resolvable correlated structures of ∼100 µm between scalars are captured. With increasing turbulence intensity, it is shown that the preheat zone and the inner layer of the reaction zone become gradually broadened/distributed, and the correlation between HCO and [OH]LIF×[CH2O]LIFdecreases. A transition from the TRZ regime to the DRZ regime is found around Karlovitz number of 70–100. The physical and chemical effects on the broadening of the flame are investigated. In the TRZ regime the inner layer marker CH and HCO remains thin in general although occasional local broadening of CH/HCO could be observed. Furthermore, there is a significant probability of finding CH and HCO at rather low temperatures even in the TRZ regime. In the DRZ regime, the broadening of CH and HCO are shown to be mainly a result of local reactions facilitated by rapid turbulent transport of radicals and intermediate reactants in the upstream of the reaction paths. Differential diffusion is expected to have an important effect in the DRZ regime, as H radicals seemingly play a more important role than OH radicals.


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  An SMLD Joint PDF Model for Turbulent Non-Premixed Combustion Using the Flamelet Progress-Variable Approach


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  This paper provides an improved flamelet/progress variable (FPV) model for the simulation of turbulent combustion, employing the statistically most likely distribution (SMLD) approach for the joint probability density function (PDF) of the mixture fraction, Ζ, and of the progress parameter, Λ. Steady-state FPV models are built presuming the functional shape of the joint PDF of Ζ and Λ in order to evaluate Favre-averages of thermodynamic quantities. The mixture fraction is widely assumed to behave as a passive scalar with a mono-modal behaviour modelled by a β-distribution. Moreover, under the hypothesis that Ζ and Λ are statistically independent, the joint PDF coincides with the product of the two marginal PDFs. In this work we discuss these two constitutive hypotheses. The proposed model evaluates the most probable joint distribution of Ζ and Λ, relaxing some crucial assumption on their statistical behaviour. This provides a more general model in the context of FPV approach and an effective tool to verify the adequateness of widely used hypotheses. The model is validated versus experimental data of well-known test cases, namely, the Sandia flames. The results are also compared with those obtained by the standard FPV approach, analysing the role of the PDF functional form on turbulent combustion simulations.


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  Implementation and improvement of ISAT in HCCI engine multidimensional modeling with detailed chemical kinetics


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  In situ adaptive tabulation (ISAT) has been implemented into homogeneous charge compression ignition (HCCI) engine multidimensional modeling with detailed chemical kinetics, and the performance of ISAT was discussed. The results showed that ISAT could reduce the computational time remarkably, and control the global error efficiently. Taking into account of the characteristics of chemical reactions during HCCI engine combustion process, an enhanced approach, the partial ISAT (PaISAT) is presented, and it can significantly improve the accuracy and speed-up factor. Moreover, the required memory of ISAT is reduced due to adoption of dynamic trimming technique.


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  Chemistry acceleration with tabulated dynamic adaptive chemistry in a realistic engine with a primary reference fuel


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  Detailed kinetic reaction mechanisms are necessary for accurate prediction of combustion characteristics such as ignition and emissions in realistic engines. However, the calculation of chemically reacting flows with detailed chemistry is computational expensive due to the large number of species and reactions involved. In this study, a combined approach of dynamic adaptive chemistry (DAC) and in situ adaptive tabulation (ISAT) for efficient chemistry calculations has been implemented into a three-dimensional flow solver to simulate a homogenous charged compression ignition (HCCI) engine with a primary reference fuel (PRF). In the combined method, ISAT speeds up the chemistry calculation by reducing the number of integrations of ordinary differential equations (ODEs) governing chemical kinetics through tabulating and re-using the ODE solutions. At the meantime, DAC accelerates the necessary ODE integrations via the use of locally valid reduced mechanisms, which are obtained using the direct relation graph (DRG) method. The study shows that ISAT-DAC can achieve a speedup factor of about three with accurate prediction of composition and heat release even for the pressure-varying transient engine simulation with significant composition inhomogeneity resulting from wall heat loss. A detailed analysis reveals that the combined method effectively reduces the computational cost through taking advantage of the respective acceleration characteristics of DAC and ISAT at different combustion stages. In the low temperature combustion stage between about 650 K and 1000 K, even though the reduction in the mechanism size and consequently the ODE integration of the chemical kinetics by DAC is not significant, the combined method can still reach a speed-up factor of more than 100 due to the fact that the tabulated entries can effectively be reused. For the high temperature region, even though the tabulated entries cannot be reused due to the rapid change of the pressure and large composition inhomogeneity resulting from the active combustion and heat loss, DAC can effectively reduce the size of the needed ODE integrations by freezing a significant number of unimportant species and reactions. The study further quantifies the effect of composition inhomogeneity on the computation efficiency in detailed chemistry calculations in realistic engine simulations. For the case considered, the temperature inhomogeneity due to the wall heat loss obviously increases at around top dead center (TDC), reaching a level of 350 K difference in temperature within the combustion chamber. Consequently, the overall computational efficiency in chemistry calculation by the combined method has been reduced by 40% compared to the case without heat loss.


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  Effects of in-Cylinder Bulk Flow and Methane Supply Strategies on Charge Stratification , Combustion and Emissions of a Dual-Fuel DI Diesel Engine


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  The physicochemical origins of how changes in fuel composition affect autoignition of the end gas, leading to engine knock, are analyzed for a natural gas engine. Experiments in a lean-burn, high-speed medium-BMEP gas engine are performed using a reference natural gas with systematically varied fractions of admixed ethane, propane and hydrogen. Thermodynamic analysis of the measured non-knocking pressure histories shows that, in addition to the expected changes arising from changes in the heat capacity of the mixture, changes in the combustion duration relative to the compression cycle (the combustion "phasing") caused by variations in burning velocity dominate the effects of fuel composition on the temperature (and pressure) of the end gas. Thus, despite the increase in the heat capacity of the fuel-air mixture with addition of ethane and propane, the change in combustion phasing is actually seen to increase the maximum end-gas temperature slightly for these fuel components. By the same token, the substantial change in combustion duration upon hydrogen addition strongly increases the end-gas temperature, beyond that caused by the decrease in mixture heat capacity. The impact of these variations in in-cylinder conditions on the knock tendency of the fuel have been assessed using autoignition delay times computed using SENKIN and a detailed chemical mechanism for the end gas under the conditions extant in the engine. The results show that the ignition-promoting effect of hydrogen is mainly the result of the increase in end-gas temperature and pressure, while addition of ethane and propane promotes ignition primarily by changing the chemical autoignition behavior of the fuel itself. Comparison of the computed end-gas autoignition delay time, based on the complete measured pressure history of each gas, with the measured Knock-Limited Spark Timing shows that the computed delay time accurately reflects the measured knock tendency of the fuels. © 2014 The Combustion Institute.


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  Numerical simulation of flames is a growing field bringing important improvements to our understanding of combustion. The main issues and related closures of turbulent combustion modeling are reviewed. Combustion problems involve strong coupling between chemistry, transport and fluid dynamics. The basic properties of laminar flames are first presented along with the major tools developed for modeling turbulent combustion. The links between the available closures are illuminated from a generic description of modeling tools. Then, examples of numerical models for mean burning rates are discussed for premixed turbulent combustion. The use of direct numerical simulation (DNS) as a research instrument is illustrated for turbulent transport occurring in premixed combustion, gradient and counter-gradient modeling of turbulent fluxes is addressed. Finally, a review of the models for non-premixed turbulent flames is given.


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  Modeling Knock in Spark-Ignition Engines Using a G-equation Combustion Model Incorporating Detailed Chemical Kinetics


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  In this paper, knock in a Ford single cylinder directinjection spark-ignition (DISI) engine was modeled and investigated using the KIVA-3V code with a G-equation combustion model coupled with detailed chemical kinetics. The deflagrative turbulent flame propagation was described by the G-equation combustion model. A 22-species, 42-reaction iso-octane (iC8H18) mechanism was adopted to model the auto-ignition process of the gasoline/air/residual-gas mixture ahead of the flame front. The iso-octane mechanism was originally validated by ignition delay tests in a rapid compression machine. In this study, the mechanism was tested by comparing the simulated ignition delay time in a constant volume mesh with the values measured in a shock tube under different initial temperature, pressure and equivalence ratio conditions, and acceptable agreements were obtained. The mechanism was further validated by modeling a gasoline homogeneous charge compression ignition (HCCI) engine at both low and high engine speeds. The G-equation combustion model was validated on the Ford DISI engine with spark advance and intake manifold pressure sweeps. Based on the model validation, knocking combustion under boost and globally stoichiometric operating conditions was simulated. Finally, knock mitigation strategies using cooled EGR and/or two-stage mixing were assessed based on the numerical analysis.


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  Ignition-promoting effect of NO 2 on methane , ethane and methane / ethane mixtures in a rapid compression machine


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  A continuous multi-component fuel evaporation model has been integrated with an improved G-equation combustion and detailed chemical kinetics model. The integrated code has been successfully used to simulate a gasoline direct injection engine. In the multi-component fuel model, the theory of continuous thermodynamics is used to model the properties and composition of multicomponent fuels such as gasoline. In the improved Gequation combustion model a flamelet approach based on the G-equation is used that considers multicomponent fuel effects. To precisely calculate the local and instantaneous residual which has a great effect on the laminar flame speed, a transport equation residual model is used. A Damkohler number criterion is used to determine the combustion mode in flame containing cells. To consider the change of local fuel vapor mixture composition, a PRF adaptive method is proposed that formulates a relationship between the fuel vapor mixture PRF number (or Octane number) and the fuel vapor mixture composition based on the mean molecular weight and variance of the fuel vapor mixture composition in each cell. The laminar flame speed has been updated to consider multi-component fuel effects, as a function of pressure, temperature, equivalence ratio, residual, and fuel vapor mixture PRF number. To model the chemistry process in the unburned region in front of the flame and in burned regions behind the flame, a recently developed PRF mechanism is used to describe the multi-component fuel mixture. Simulations of the vaporization of a single droplet with a single-component fuel (iso-octane) are compared with multi-component fuel cases. The vaporization of a spray of multi-component fuel injected into a chamber is simulated for both normal and flash-boiling conditions. The new PRF mechanism is validated against shock tube data. In addition, gasoline direct injection engine combustion is simulated at different manifold absolute pressures, different end-offuel injection timings, and different spark ignition timings. The simulated in-cylinder pressures and heat release rates are compared with experiment data and good agreements are found.


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  Most of the spark ignition engine cycle simulations use turbulent burning models which require a knowledge of laminar burning velocity of the fuel-air mixture as a function of mixture strength, unburned mixture temperature and pressure. In this work burning velocity data of different alternative spark ignition engine fuels obtained by various workers have been compared and critically evaluated. Empirical and semi-empirical correlations, suitable for cycle simulation studies, are presented for laminar burning velocity as a function of mixture strength, unburned mixture temperature, pressure, and residual gas fraction. Fuels considered include ethanoi, methanoi, alcohol/water blends, isooctane/alcohol blends, propane and isooctane. Experimental data obtained by the present author constitute the major part of the data used in correlations. Published data of other workers and the predictions of theoretical thermo-kinetic models have also been considered in correlations. IN THE SIMULATION of the spark ignition engine cycles, modelling of the turbulent flame propagation presents particular problems [1].* Generally, turbulent flame propagation models are based on laminar burning velocity, turbulence intensity, and one or more scale parameters of turbulence [1-5], ^or a given fuel, laminar burning velocity is a function of the mixture strength, unburned mixture temperature and pressure. However, for engine combustion applications, mass fraction of the residual gases in the fresh mixture should also be taken into account. ^Numbers In brackets designate references at end of paper. The burning velocity, which is also called flame velocity or normal combustion velocity, is the velocity at which unburned fuel-air gas mixture moves through the combustion wave in the direction normal to the wave surface. Although, this definition of the flame velocity is straightforward, the experimental determination of it presents some complications. Different measurement and data evaluation techniques have long been known to produce burning velocity data with significant deviations. Detailed critical reviews of various measurement and data evaluation techniques can be found in [6,7]. It was suggested that closed vessel explosion techniques accompanied by density corrections can give good possibilities for the accurate measurement of burning velocity [b,7]. Although thermokinetic combustion models have recently been quite successful for combustion predictions of simple hydrocarbon fuels [8-11], uncertainties in the rates of the elementary reactions [12], and in transport properties of intermediate oxidation species [13] prevent, at the moment, the use of kinetic models for practical fuels in engine cycle simulations.


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  Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450. K and absolute pressures from 1 to 8. bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure. © 2013 The Combustion Institute.


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  ScienceDirect Effects of hydrogen concentration on premixed laminar flames of hydrogen e methane e air


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  Reduction and validation of a chemical kinetic mechanism including necessity analysis and investigation of CH 4 / C 3 H 8 oxidation at pressures up to 120 bar using a rapid compression machine


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  Comparison and Sensitivity Analysis of Turbulent Flame Speed Closures in the RANS G-Equation Context for Two Distinct Engines


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  Laminar burning speed measurement and flame instability study of H 2 / CO / air mixtures at high temperatures and pressures using a novel multi-shell model


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  The thickness of the instantaneous flamelets in a turbulent flame brush on a weak-swirl burner burning in the thin reaction zones regime has been analysed experimentally, theoretically, and numerically. The experimental flame thickness has been measured correlating two simultaneous Rayleigh images and one OH-image from two closely spaced cross sections in the flame. It appears that the low temperature edge of the flame is thickened by turbulent eddies but that these structures cannot penetrate far enough into the flame front to distort the inner layer for the moderate Karlovitz numbers used. The flame front based on the temperature gradient at the inner layer becomes thinner for lean flames and thicker for rich methane-air flames. This has been explained theoretically and numerically by studying the influence of flame stretch and preferential diffusion on the flame thickness. It appears that the flame front thickness at the inner layer (and mass burning rate) is not influenced by turbulent mixing processes, and it seems that eddies of the size of the inner layer have to be used to change this picture. Experiments closer to the boundary of the broken reaction zones regime have to confirm this in the future.


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  Approximations for Burning Velocities and Markstein Numbers for Lean Hydrocarbon and Methanol Flames


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  Experimental study and modeling of shock tube ignition delay times for hydrogen – oxygen – argon mixtures at low temperatures


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  Application of a New Turbulent Flame Speed Combustion Model on Burn Rate Simulation of Spark Ignition Engines


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  A systematic analysis of the flame speed enhancement effects of ethane and ethylene addition to methane was performed. Flame speeds of outwardly propagating spherical flames were measured in a high-pressure, cylindrical flame speed vessel using schlieren photography. It was ensured that the measured data were outside the ignition- and chamber-confinement-affected radii. The unstretched, unburnt flame speeds were obtained using nonlinear extrapolation methods from the experimentally determined stretched flame speeds. The AramcoMech 1.3 kinetics mechanism was validated against flame speeds of ethane, ethylene, and acetylene over a wide range of pressures and equivalence ratios. Good agreement between the measured data and those reported in the literature was seen. The model agreed closely with the measurements at fuel-lean conditions, but a slight over prediction was observed for the fuel-rich cases. Flame speeds of 80/20 and 60/40 (by volume) CH4/C2Hx mixtures in air and in air with excess nitrogen were measured over a wide range of fuel concentrations. In all cases, the addition of ethylene increased the flame speed more than did an equivalent addition of ethane. The Arrhenius effect (predominantly kinetic effect) was the principal driver for flame speed enhancement of methane with ethane/ethylene addition. Sensitivity analysis revealed that thermal and diffusive pathways were equally contributing to the flame speed enhancement of CH4/C2H6 mixtures, but the latter played a comparatively minor role in the C2H4-based blends.


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  The comparative and combined effects of hydrogen addition on the laminar burning velocities of methane and its blends with ethane and propane


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  Laminar burning velocities of hydrocarbon blends of relevance to natural gas combustion, with addition of 0, 10, 35 and 50% hydrogen, were measured using the heat flux method. Hydrocarbon blends were methane (80%)/ethane (20%), methane (80%)/propane (20%) and methane (80%)/ethane (10%)/propane (10%), and in addition experiments were performed using pure methane as a fuel. For the first time it was shown experimentally that hydrogen promotes laminar burning velocity of blends with heavier hydrocarbons to a smaller extent than the well-studied effect on methane. Measurements show that enrichment by hydrogen results in 20–40% lower increase in laminar burning velocity for hydrocarbon blends compared to pure methane, depending on stoichiometry. Modeling points at the importance of increasing concentrations of OH, O and H radicals in H2 enriched flames. At lean conditions increase in H atom concentration is of particular importance. The results are rationalized based on asymptotic flame theory analysis.


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  In situ adaptive tabulation (ISAT) for combustion chemistry in a network of perfectly stirred reactors (PSRs) for the investigation of soot formation and growth


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  This paper presents an efficient computational implementation of the in situ adaptive tabulation (ISAT) approach (Pope, 1997) for combustion chemistry in a network of perfectly stirred reactors (PSRs) for the investigation of soot formation and growth. This study, for the first time, extends the thermochemical composition vector to contain the soot moments, using the method of moments with interpolative closure (MOMIC) as a soot model with six concentration moments. A series of PSR calculations is carried out using the direct integration (DI) and ISAT approaches. Assessment of the accuracy of ISAT approach is conducted through direct comparisons with DI calculations. Moreover, complimentary cumulative distribution function (CCDF), sensitivity of ISAT calculations with respect to the absolute error tolerance values and speedup are analyzed for two different test cases of ethylene–air using two different chemical kinetic mechanisms. A maximum speedup of 50 × was achieved for an error tolerance of 10−4.


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  ScienceDirect Shock tube and kinetic study of C 2 H 6 / H 2 / O 2 / Ar mixtures at elevated pressures


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  OpenFOAM Computational Fluid Dynamic Simulations of Two-Phase Flow and Mass Transfer in an Advanced-Flow Reactor


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  ISSN: 0888-5885
  Issue: 26
  Pages: 6649–6659
  Publication Title: Industrial & Engineering Chemistry Research
  Volume: 54
  DOI: 10.1021/acs.iecr.5b00480


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  Experimental and modeling study on auto-ignition characteristics of methane / hydrogen blends under engine relevant pressure


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  Exhaust gas recirculation effects on flame structure and laminar burning speeds of H 2 / CO / air flames at high pressures and temperatures


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  Phenomenology of methane flame propagation into compositionally stratified , gradually richer mixtures


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  Laminar flame speed and Markstein length of syngas at normal and elevated pressures and temperatures


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  The kinetic characterization of the H2 / CO system is of interest right now due mainly to its role in sustainable combustion processes. The aim of this paper is to revise and validate a detailed kinetic model of hydrogen and carbon monoxide mixture combustion with particular focus not only on NOx formation but also on interactions with nitrogen species. Model predictions and experimental measurements are discussed and compared across a wide range of operating conditions. This study moves from the detailed analysis of species profiles in syngas oxidation in flow reactor and laminar premixed flames to global combustion properties (ignition delay times and laminar flame speeds) by referring to a large set of literature data. According to recent literature, the validation of the kinetic scheme confirmed there was a need to slightly modify the kinetic parameters of two relevant CO2 formation reactions (CO + OH = CO2 + H and CO + O + M = CO2 + M) and of reaction HONO + OH = NO2 + H2 O. ?? 2007 International Association for Hydrogen Energy.


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  An ignition delay and kinetic modeling study of methane , dimethyl ether , and their mixtures at high pressures


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  A G -equation Combustion Model Incorporating Detailed Chemical Kinetics for PFI / DI SI Engine Simulations G -equation description of turbulent


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  The Computation of Constrained and Unconstrained Equilibrium Compositions of Ideal Gas Mixtures using Gibbs Function Continuation


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  We consider the computation of the chemical-equilibrium state of an ideal gas mixture of given elemental composition, with and without linear constraints on the species present. While computer programs to solve these problems have been available for over thirty years, it has been found that they are not always successful in determining the chemical equilibrium state when constraints are present. In this paper the constrained equilibrium problem is examined in detail. It is shown that the specified constraints can be replaced by a (generally simpler) set of reduced constraints. For fixed temperature and pressure, the equilibrium composition is that which minimizes the Gibbs function subject to these constraints. The solution to this minimization problem is known in terms of Lagrange multipliers (or constraint potentials), which are determined as the solution to a set of non-linear equations. It is shown that Newton's method can fail to obtain the solution because the iteration matrix can be singular. The new method of Gibbs function continu- ation is introduced which is guaranteed to obtain the solution. Based on this, a practical algorithm is presented which has been implemented and tested in a Fortran code. It is claimed that this algorithm is assured to determine the (constrained or unconstrained) equilibrium state for all well-posed problems.


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  Autoignition characteristics of n-heptane/air, gasoline/air, and ternary surrogate/air mixtures were studied behind reflected shock waves in a high-pressure, low-temperature regime similar to that found in homogeneous charge compression ignition (HCCI) engine cycles. The range of experiments covered combustion of fuel in air for lean, stoichiometric, and rich mixtures (??=0.5, 1.0, 2.0), two pressure ranges (15-25 and 45-60 atm), temperatures from 850 to 1280 K, and exhaust gas recirculation (EGR) loadings of (0, 20, and 30%). The ignition delay time measurements in n-heptane are in good agreement with the shock tube study of Fieweger et al. (Proc. Combust. Inst. 25 (1994) 1579-1585) and support the observation of a pronounced, low-temperature, NTC region. Strong agreement was seen between ignition delay time measurements for RD387 gasoline and surrogate (63% iso-octane/20% toluene/17% n-heptane by liquid volume) over the full range of experimental conditions studied. Ignition delay time measurements under fuel-lean (??=0.5) mixture conditions were longer than with ??=1.0 mixtures at both the low- (15-25 atm) and high- (45-60 atm) pressure conditions. Ignition delay times in fuel-rich (??=2.0) mixtures for both gasoline and surrogate were indistinguishable in the low-pressure (15-25 atm) range, but were clearly shorter at high-pressures (45-60 atm). EGR loading affected the ignition delay times similarly for both gasoline and surrogate, with clear trends indicating an increase in ignition delay time with increased EGR loading. This data set should provide benchmark targets for detailed mechanism validation and refinement under HCCI conditions. ?? 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.


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  A transient, one dimensional premixed laminar reaction front is used as a model problem to further understand the physical processes influencing reaction front propagation during the various stages of spark-assisted compression ignition (SACI) combustion for both constant and variable domain pressures. This approach is consistent with the wrinkled laminar flame representation of turbulent, spark ignited engine combustion. With the proper choice of timescales and pressure rise rate, it applies to the interaction of the flame with auto-igniting end-gas in a typical automotive engine. Under the conditions simulated by a transient flame code, the reaction front begins as a deflagration, propagating into an end-gas with an initially negligible level of reaction progress. The diffusive-reactive nature of the front is maintained until significant levels of end-gas reaction progress, where the burning velocity depends upon the degree of pre-reaction. At the time of the end-gas maximum chemical power, the maximum temperature gradient and peak rate of heat conduction within the front diminish to the point where combustion becomes chemically controlled. Although significant increases in burning velocity are observed at the onset of chemically controlled combustion within the front, the end-gas is within one front time from the completion of combustion. As a result, no more than one front thickness is consumed by the apparent propagation of the spontaneous ignition front. © 2012 The Combustion Institute.


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  Experimental study on the explosion characteristics of methane / air mixtures with hydrogen addition


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  Improved combustion submodels for modelling gasoline engines with the level set G equation and detailed chemical kinetics


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  Ignition delay times , laminar flame speeds , and mechanism validation for natural gas / hydrogen blends at elevated pressures


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  MODELLING THE COMBUSTION IN A DUAL FUEL HCCI ENGINE Investigation of knock , compression ratio , equivalence ratio and timing in a Homogeneous Charge Compression Ignition ( HCCI ) engine with natural gas and diesel fuels using modelling and simulation Hos


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  Premixed flames are widely utilized in spark ignition engines for automobiles and in advanced gas turbine engine systems for power generation. In the present study, the fundamental properties of premixed flames such as initiation, propagation, and extinction are systematically investigated using asymptotic theoretical analysis, detailed numerical simulations, and/or experimental measurements. A general theory on spherical flame initiation and propagation and an accurate numerical algorithm for adaptive simulation of unsteady reactive flow are first developed. The general theory describes different flame regimes and transitions among them. This could be utilized to study effects of radiative heat loss, ignition power, and preferential diffusion on spherical flame initiation, propagation, and extinction. The numerical solver is thoroughly tested and validated. It is shown to be able to accurately and efficiently model propagating flames with detailed chemical mechanisms. With the help of the general theory and the numerical solver, the controlling factor for spherical flame initiation and how it relates to the minimum ignition energy are then investigated. It is found that there exists a critical flame radius controlling the spherical flame initiation and that the minimum ignition energy is proportional to the cube of the critical flame radius. Moreover, results show that the preferential diffusion between heat and mass (Lewis number) plays an important role in the spherical flame initiation. Both the critical flame radius and the minimum ignition energy are found to increase with the Lewis number. The general theory and the numerical solver are also employed to study the accuracy of laminar flame speed measurements utilizing propagating spherical flames. Different effects such as ignition, unsteadiness, compression, and stretch are found to decrease the accuracy of these measurements. New flame speed determination methods to obtain more accurate flame speeds by correcting for these effects are developed. For example, both the Compression-Corrected Flame Speed (CCFS) and the Stretch-Corrected Flame Speed (SCFS) are demonstrated not only to significantly improve the accuracy of the flame speed measurements but also to greatly extend the parameter range of experimental conditions. The effects of radiation on flame propagation, extinction, and flammability limits are also investigated. Opposite trends for the change of flame propagation speed and flame extinction/flammability limits are found for the outwardly and inwardly propagating spherical fames. A functional expression for the Markstein length is found in terms of the Lewis number and the radiative heat loss. Moreover, the spectrally dependent radiation absorption effect on propagating spherical flames is measured by using CO2-diluted CH4/O2/He mixtures at normal and elevated pressures. Radiation absorption is found to increase the flame speed and extend the flammability limit. In addition, the combined effects of flame curvature, radiation, and stretch on flame extinction are revealed via premixed tubular flames. It is found that the coupling between radiation and flame curvature leads to multiple flame bifurcations and extinction limits. Finally, the effects of kinetic and transport coupling on ignition and flame propagation are investigated by using dimethyl ether (DME) blended methane/air mixtures. It is shown that the addition of a small amount of DME to methane leads to a significant decrease in the ignition time, while the flame speed of DME/CH4/air mixture is only linearly proportional to the DME fraction.


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  On the accuracy of laminar flame speeds measured from outwardly propagating spherical flames : Methane / air at normal temperature and pressure


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  ASYMPTOTIC GROWTH RATES AND STRONG BENDING OF Downloaded 01 / 29 / 17 to 35 . 16 . 120 . 93 . Redistribution subject to SIAM license or copyright ; see http://www.siam.org/journals/ojsa.php unburned fluid ሬԦ ሺݔሻ flame front burned fluid


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  A unified model for the prediction of laminar flame transfer functions : comparisons between conical and V-flame dynamics


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  Lean Hydrogen Flame Deflagration Velocities and Flammability Limits Reacting Navier-Stokes Conservation Equations


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  Knock in Dual-Fuel Diesel Combustion with an E85 Ethanol/Gasoline Blend by Multi-dimensional Simulation


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  Hierarchical and comparative kinetic modeling of laminar fl ame speeds of hydrocarbon and oxygenated fuels


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  The knock study of high compression ratio SI engine fueled with methanol in combination with different EGR rates


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  Methanol, with a high octane index, is considered to be one of the most ideal fuels for high compression ratio engines. In this paper, the SAGE models combined with methanol chemical reaction kinetics (consisting of 46-species and 247-elementary reactions) are used to study knock combustion. Knock combustion is simulated in a high compression ratio SI methanol engine with different EGR rates. The results show that the introduction of EGR into the combustion chamber can reduce the knock intensity and delay the knock onset time. Without EGR, the intake side of the combustion chamber is prone to knocking mostly. As the EGR rate increases, the exhaust side of the combustion chamber is the most likely to knocking. OH radicals can be considered an indicator of the temperature, and H2O2 can be regarded as an indicator of the propagating flame front. OH marks the knock phase duration and HCO marks the onset of knocking. The reaction intensities of CH2O, H2O2, CO and OH species are higher than other species during knocking combustion. The greater the reaction rate is, the greater the knock intensity will be.


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  The laminar flame speeds of methane-oxygen-nitrogen mixtures as functions of the flame temperature Tad and system pressure p have been experimentally determined by using the counterflow, twin-flame technique. These data are then compared with numerically-calculated values obtained by using an independently validated kinetic scheme. Results show that the overall reaction order n and activation energy Ea are far from being constants, that n decreases with increasing p and decreasing Tad whereas Ea increases with increasing p, and that n can actually assume negative values; the last result implies that the mass burning rate of some weakly burning flames may decrease with increasing pressure. By further identifying the crucial reaction steps through sensitivity analysis, the present results are interpreted on the basis of the influence of chain branching-termination mechanisms on the overall reaction rate.


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  Determination , correlation , and mechanistic interpretation of effects of hydrogen addition on laminar flame speeds of hydrocarbon – air mixtures


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  Experimental and numerical study on lean premixed methane – hydrogen – air flames at elevated pressures and temperatures


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  Investigation into the Effect of Flame Propagation in the Gasoline Compression Ignition by Coupling G-Equation and Reduced Chemical Kinetics Combustion Model


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  ScienceDirect Uncertainty in stretch extrapolation of laminar flame speed from expanding spherical flames


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  http://dx.doi.org/10.1016/j.combustflame.2016.06.016 http://tcg.mae.cornell.edu/pubs/Pope{\_}FDA{\_}08.pdf http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-33645450533{\&}partnerID=40{\&}rel=R8.2.0 https://doi.org/10.1080/713665229 http://dx.doi.org/10.1016/


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  Simplified Formulations for Turbulent Combustion of Hydrogen Reacting Navier-Stokes Conservation Equations


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  ScienceDirect Kinetic analysis of H 2 addition effect on the laminar flame parameters of the C1 e C4 n-alkane-air mixtures : From one step overall assumption to detailed reaction mechanism


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  Effects of hydrogen addition on the propagation of spherical methane / air flames : A computational study


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  Study on the potential of BML-approach and G -equation concept-based models for predicting swirling partially premixed combustion systems : URANS computations


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  Cell formation effects on the burning speeds and flame front area of synthetic gas at high pressures and temperatures


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  The thesis illustrates the application of computational fluid dynamics$\backslash$n(CFD) to turbulent reactive two-phase flows in piston engines.$\backslash$n$\backslash$n$\backslash$nThe focus of the thesis lies on numerical simulations of spray combustion$\backslash$nphenomena with an emphasis on the modeling of turbulence/chemistry$\backslash$ninteraction effects using a detailed chemistry approach. The turbulence/chemistry$\backslash$ninteraction model accounts for the effects of turbulent micro-mixing$\backslash$non the chemical reaction rates. The models have been implemented$\backslash$nin the {$\backslash$bf KIVA3-V} code and successfully applied to spray combustion$\backslash$nanalysis in a constant volume and a DI Diesel engine. The limitations$\backslash$nand difficulties of representing the spray in a Lagrangian fashion$\backslash$nare also adressed.$\backslash$n$\backslash$n$\backslash$nThree different liquid fuels have been used in the simulations: n-heptane,$\backslash$nmethanol and dimethyl ether (DME). Detailed and reduced chemical$\backslash$nmechanisms have been developed and validated for all these fuels$\backslash$nand reasonable agreement between experimental data and numerical$\backslash$nsimulations has been obtained.


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  On the extraction of laminar flame speed and Markstein length from outwardly propagating spherical flames


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  Numerical investigation of the effect split injection scheme on exergy terms in an IDI (indirect injection) diesel engine by three dimensional modeling


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  The investigations suggest that it is possible to control the injected masses between two subsequent injections in split injection schemes by modern injection systems. It offers the opportunity to optimize the injection system. During this investigation, therefore, the exergy analysis is carried out under various two-step injection schemes in which the percentage of total fuel injected in the first pulse is changed from 75% to 100%, and the value of dwelling time duration is 20°CA (crank angle) for all cases. The calculations were performed on a Lister 8.1 IDI (indirect injection) diesel engine at full load operation. The energy analysis for these schemes was performed during a closed cycle by utilizing a three-dimensional CFD (computational fluid dynamics) code. The cylinder pressure results of the baseline engine were compared with the corresponding experimental data which showed a good agreement. For the exergy analysis, an in-house computational code was developed, which uses the results of the energy analysis at various cases. With crank angle position and the percentage of total fuel injected in the first pulse under different injection schemes, various rates of exergy and cumulative exergy components are identified and calculated separately. The results showed that the values of work exergy and exergy efficiency increase by 11.1% when the mass ratio injected in first pulse is changed from 75% to 100%.


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  The use of schlieren imaging at high acquisition rate has been adopted as a standard optical technique for the analysis of vaporizing diesel sprays under engine-like conditions. A single-pass schlieren arrangement is typically used for the study of axially drilled single-orifice nozzles, as vessels with multiple optical accesses regularly allow line of sight visualization. Contrarily, for multi-spray nozzles, measurements are commonly performed through a single optical access, in which case a double-pass arrangement is employed. As a consequence, the light beams pass through the test section twice, increasing the optical sensitivity of the schlieren setup. However, the influence this has on the macroscopic spray characteristics is still unclear. The scope of this study is to analyze the differences in vapor phase penetration and spreading angle measured for the same injection event, through high-speed imaging, for both single and double-pass schlieren configurations. Experiments were carried out with a three hole nozzle with a nominal orifice diameter of 90 μm, named Spray B from the Engine Combustion Network, using commercially available diesel fuel and in non-reactive conditions. The impact of different injection pressures, chamber temperatures and densities on the spray captured by each setup was assessed. On the results, vapor phase penetration and spreading angle followed the expected trend found in the literature, for the different boundary conditions tested. Comparing the optical setups, vapor phase penetration and spreading angle results obtained with the double-pass arrangement were marginally higher than those from the single-pass. The deviation was observed throughout all tested conditions. For spray tip penetration, although the discrepancy was approximately constant for different injection pressures and chamber temperature, it increased with increasing density. These results highlight the importance of a proper understanding regarding the limitations of optical diagnostics, in particular for results used in calibration of computational models.


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  A parallel computing Eulerian/Lagrangian multi-scale coupling procedure for diesel spray simulation is presented. Early breakup of the diesel jet is captured by using a compressible Volume of Fluid (VOF) method. In regions where the phase interface can no longer be sufficiently resolved, separated and small scale liquid structures are described by a Lagrangian Parcel Tracking (LPT) approach, in conjunction with secondary breakup modelling and a turbulence stochastic dispersion model. The coupling of these two descriptions utilises a Region Coupling Method and an efficiently parallelised droplet identification and extraction procedure. This approach enables run-time VOF-LPT field coupling and filters small-scale liquid structures that are suitable candidates for Eulerian-liquid-structure/Lagrangian droplet conversion, preserving their position, mass and momentum. The coupling procedure is initially applied to model the atomisation of a simple liquid jet and the results are compared with that of a statistical coupling approach to demonstrate the performance of the developed coupling procedure. Its application is then extended to simulate a real diesel spray from a nozzle with a sharp entrance. Coupling in-nozzle phenomena such as flow separation, flow detachment and turbulence to the primary and secondary spray atomisation, provides a tool for the prediction of complex spray dynamics.


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  Various solutions have been proposed for improving the combustion process of conventional diesel engines and reducing the exhaust emissions without making serious modifications on the engine, one of which is the use of natural gas as a supplement for the conventional diesel fuel, the so called dual fuel natural gas diesel engines. The most common type of these is referred to as the pilot ignited natural gas diesel engine (PINGDE). Here, the primary fuel is natural gas that controls the engine power output, while the pilot diesel fuel injected near the end of the compression stroke auto-ignites and creates ignition sources for the surrounding gaseous fuel mixture to be burned. Previous research studies have shown that the main disadvantage of this dual fuel combustion is its negative impact on engine efficiency compared to the normal diesel operation, while carbon monoxide emissions are also increased. The pilot diesel fuel quantity and injection advance influence significantly the combustion mechanism. Then, in order to examine the effect of these two parameters on the performance and emissions, a comprehensive two-zone phenomenological model is employed and applied on a high-speed, pilot ignited, natural gas diesel engine located at the authors' laboratory. According to the results, the simultaneously increase of the pilot fuel quantity accompanied with an increase of its injection timing results to an improvement of the engine efficiency (increase) and of the emitted CO emissions (decrease) while it has a negative effect (increase) of NO emissions. © 2007 Elsevier Ltd. All rights reserved.


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  Development of multi-component surrogates of diesel from indirect coal liquefaction for spray analysis


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  The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame ?Sandia flame D?. The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel. ©


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  The split cycle engine is a new reciprocating internal combustion engine with a potential of a radical efficiency improvement. In this engine, the compression and combustion-expansion processes occur in different cylinders. In the compression cylinder, the charge air is compressed through a quasi-isothermal process by direct cooling of the air. The high pressure air is then heated in a recuperator using the waste heat of exhaust gas before induction to the combustion cylinder. The combustion process occurs during the expansion stroke, in a quasi-isobaric process. In this paper, a fundamental theoretical cycle analysis and one-dimensional engine simulation of the split cycle engine was undertaken. The results show that the thermal efficiency (η) is mainly decided by the CR (compression ratio) and ER (expansion ratio), the regeneration effectiveness (σ), and the temperature rising ratio (N). Based on the above analysis, a system optimization of the engine was conducted. The results showed that by increasing CR from 23 to 25, the combustion and recuperation processes could be improved. By increasing the expansion ratio to 26, the heat losses during the gas exchange stroke were further reduced. Furthermore, the coolant temperatures of the compression and expansion chambers can be controlled separately to reduce the wall heat transfer losses. Compared to a conventional engine, a 21% total efficiency improvement was achieved when the split cycle was applied. It was concluded that through the system optimization, a total thermal efficiency of 53% can be achieved on split cycle engine.


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  Revisit of Diesel Reference Fuel ( n-Heptane ) Mechanism Applied to Multidimensional Diesel Ignition and Combustion Simulations


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  The quantification and reduction of the uncertainties in the extrapolation process in laminar flame speed measurements were studied using expanding spherical flames under positive Markstein length (Lb\textgreater 0) conditions. The experimental and computational results were first compared showing their differences. The performance of three extrapolation formulas was then examined under a wide range of experimental conditions using various extrapolation ranges and pressures. It is found that the extrapolation uncertainty contains two sources of error, namely model error and random error. The individual effects of the upper and lower bounds of the extrapolation range under various Lb conditions were studied and the ratio of Lb and the flame radius range is found to be the controlling parameter of the model error. Small value of |Lb/Rf| allows the neglect of the model error by increasing the upper or the lower bound of the flame radius range. A new empirical parameter, Lb/Rf,new, was defined according to the experimental results, while it is recognized that the random error is mainly affected by the number of points for extrapolation and that at least 30 points should be used to remove the random error.


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  In the present investigation the Spray A reference configuration defined in the framework of the Engine Combustion Network (ECN) has been modeled by means of an Unsteady Flamelet Model (USFM) including detailed parametric studies to evaluate the impact of ambient temperature, oxygen concentration and density. The study focuses on the analysis of the spray ignition delay, the flame lift-off length and the internal structure of the spray and flame according to the experimental information nowadays available for validating the results provided by the model. Promising results are obtained for the nominal case and also for the parametric variations (temperature, oxygen\ldots) in terms of liquid and vapor penetration, ignition delay (ID) and lift-off length (LOL). The model permits to predict the ID and the LOL which constitute two parameters of key importance for describing the characteristics of transient reacting sprays. Valuable insight on the details of the combustion process is obtained from the analysis of formaldehyde (CH2O), acetylene (C2H2) and hydroxide (OH) species in spatial coordinates and also in the so-called ϕ-T maps. Important differences arise in the inner structure of the flame in the quasi-steady regime, which is closely linked to soot formation, when varying the ambient boundary conditions. Additionally, the auto-ignition process is investigated in order to describe in detail the spatial onset and propagation of combustion. Results confirm the impact of the ambient conditions on the regions of the spray where start of combustion takes place, so the relation between the local scalar dissipation rate and mixture fraction variance is also discussed. This investigation provides an insight of the potential of the USFM combustion model to describe the physical and chemical processes involved in transient spray combustion.


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  This study is concerned with the data-driven analysis of predictive parameters associated with regime transition for ignition and detonation. To this end, we introduce machine learning as a technique to analyze the predictive capability of these parameters. Machine learning enables the critical evaluation of combustion regimes from disparate sources and allows for a generalized comparison of the parameters. The parameter set is composed of features that have been found to be effective in classifying ignition regimes and are supposed to have a universality in predicting the regularity of the ignition process. Three different configurations are examined: weak ignition shock tubes, mild ignition in rapid compression machines, and the regularity of cellular detonations. All configurations are demonstrated to show a strong sensitivity to the heat deposition time of the chemical reaction. Additionally, detonations are found to be primarily sensitive to the heat release and show that an increased chemical sensitivity has a regularizing effect when plotted against the heat release rate. The data are found to be well classified with only one or two parameters, indicating that a universal, governing parameter is plausible.


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  The eccentric needle motion has been regarded as an important cause of in-nozzle flow disturbances and hole-to-hole spray variations of modern fuel injectors. However, the experimental investigations on this subject have rarely been reported due to great difficulties in the direct measurements of eccentric needle motion and the flow characteristics of individual holes in an injection nozzle. In this study, the axial and eccentric needle motion and the near-nozzle spray dynamics of 3-hole, 5-hole and 8-hole diesel injection nozzles were investigated in varied injection pressure conditions using an X-ray phase-contrast imaging technique. The eccentric needle motion and near-nozzle spray axial velocity showed temporal oscillations during the injection with different oscillation frequencies. The frequencies of these two oscillations were independent to the nozzle hole number and injection pressure. To discuss the mutual dependency of these two oscillations, the time traces of the spray axial velocity were measured for individual holes. The results showed that the oscillation phase, frequency and amplitude of the spray axial velocity were almost identical for all holes, which indicated that the sac pressure variation rather than hole-to-hole flow variation can be the primary cause of the spray axial velocity oscillation by the eccentric needle motion. The adequacy of this consideration was thoroughly discussed using a mass balance model in the nozzle sac that predicted the sac pressure variation during an injection event.


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  Experimental and modeling study of laminar flame speed and non-premixed counterflow ignition of n -heptane


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  Wall shear stress variations and unsteadiness of pulsatile blood-like flows in 90-degree bifurcations


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  Complex and slow interaction of different mechanical and biochemical processes in hemodynamics is believed to govern atherogenesis. Over the last decades studies have shown that fluid mechanical factors such as the Wall Shear Stress (WSS) and WSS gradients can play an important role in the pathological changes of the endothelium. This study provides further indications that the effects of fluid mechanical aspects are correlated with the diseased regions of the larger arteries. Unsteady high temporal WSS gradients (TWSSG), a function of the shear-thinning property of the non-Newtonian viscosity, move with the separation bubble. Red Blood Cell (RBC) dilution due to the secondary flows determines the magnitudes of the WSS and TWSSG. The results indicate that the focal nature of the TWSSG may have implications on the response of the endothelium. © 2013 Elsevier Ltd.


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  Abstract Atomization involves complex physical processes and gas-liquid interaction. Primary atomization on diesel spray is not well understood due to the difficulties to perform experimental measurements in the near nozzle field. Hence computational fluid dynamics (CFD) has been used as a key element to understand and improve diesel spray. A recent new code for incompressible multiphase flow with adaptive octree mesh refinement has been used to perform simulations of atomization at low injection pressure conditions. The multiphase flow strategy to manage different flows is the volume of fluid (VOF) method. The adaptive mesh allows to locally refine the mesh at each time step where a better resolution is needed to capture important gradients instead of using a static mesh with a fixed and high number of cells which, in turn, would lead to an unaffordable computational cost. Even with this approach, the cell number is very high to achieve a Direct Numerical Simulation (DNS) at reasonable computational cost. To reduce the computational cost, an idea has been explored, the possibility of setting a maximum number of cells of the domain. Following this idea, the code has been tested with different configurations to understand their effects on numerical stability, the change in different spray parameters and the benefits achieved in terms of execution time. The outcomes have been validated against a theoretical model.


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  Numerical Study on Evaporation of Spherical Droplets Impinging on the Wall Using Volume of Fluid ( VOF ) Model


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  Split injection in a homogeneous stratified gasoline direct injection engine for high combustion efficiency and low pollutants emission


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  The effects of splitting the injection event in a GDI (gasoline direct injection) engine operating with a HOS (homogeneous stratified) lean charge are analysed through experimental and numerical techniques. Injection is assumed as divided in two parts, each delivering the same gasoline amount, the first occurring during intake, the second during compression. The work is initially focused on the experimental characterization of the engine under study for the collection of data concerning the in-chamber combustion development. Beside measurements of in–cylinder pressure, UV chemiluminescence is applied to follow the OH radicals formation from spark ignition up to the late combustion phase, thanks to the optical accessibility to the combustion chamber. The collected data serve to the validation of a properly formulated three-dimensional (3D) CFDs model for the simulation of the whole engine working cycle. The definition of the control strategy leading to the greatest combustion efficiency and lowest pollutants emission is made in two steps through numerical optimization: the 3D CFD model is first run to build a low number of samples to be used within a Gaussian RSM (response surface method) to reconstruct, in the DOE (design of experiments) space, the integral of the in–cylinder pressure over volume in the closed valve period; in the second step, the coupling between the 3D engine model and the Simplex algorithm is performed in a restricted DOE subdomain defined according to the first step analysis. The assessed methodology highlights the synchronization of both spark ignition and split injection leading to the highest power output and the lowest pollutants release. The experimental verification of the numerical findings is finally carried out.


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  Effects of pilot fuel quantity on the emissions characteristics of a CNG / diesel dual fuel engine with optimized pilot injection timing


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  Numerical Investigation of Spark Ignition Events in Lean and Dilute Methane / Air Mixtures Using a Detailed Energy Deposition Model


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  Coupled DEM–CFD simulation of drying wood chips in a rotary drum – Baffle design and model reduction


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  A numerical investigation of the evaporation process of a liquid droplet impinging onto a hot substrate


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  A numerical investigation of the evaporation process of n-heptane and water liquid droplets impinging onto a hot substrate is presented. Three different temperatures are investigated, covering flow regimes below and above Leidenfrost temperature. The Navier-Stokes equations expressing the flow distribution of the liquid and gas phases, coupled with the Volume of Fluid Method (VOF) for tracking the liquid-gas interface, are solved numerically using the finite volume methodology. Both two-dimensional axisymmetric and fully three-dimensional domains are utilized. An evaporation model coupled with the VOF methodology predicts the vapor blanket height between the evaporating droplet and the substrate, for cases with substrate temperature above the Leidenfrost point, and the formation of vapor bubbles in the region of nucleate boiling regime. The results are compared with available experimental data indicating the outcome of the impingement and the droplet shape during the impingement process, while additional information for the droplet evaporation rate and the temperature and vapor concentration fields is provided by the computational model. ?? 2006 Elsevier Ltd. All rights reserved.


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  Experimental and numerical investigations of primary atomization in a high-pressure diesel jet are presented. Information on flow processes and structures inside and near nozzle exit are described at early and quasi-steady stages of injection. The numerical method is based on the Volume Of Fluid (VOF) phase-fraction interface capturing technique, in an Eulerian framework. The influence of grid resolution, convection interpolation scheme and temporal integration scheme on the modeling of jet physics are investigated. The present flow setup includes in-nozzle disturbances with the no-slip condition at the walls. All experimental operating conditions are replicated in the numerical models. The early stage liquid jet leading edge demonstrates an umbrella-shaped structure in the numerical results which is in qualitative agreement with experimental imaging. Data obtained provide insight into the flow behavior in the dense region including commencement of fragmentation and early spray angle formation. Experimental images show a cloud of air-fuel mixture at the early stage of injection. The existence of ingested air inside the injector after needle closure could be the source of the observed deviation between experimental and numerical results. The results show that the jet breakup rate and liquid core length increase in cases with higher grid resolutions. The early spray angle from the numerical results at the quasi-steady stage, shows good agreement with experimental data.


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  Study of auto-ignition and extinction characteristics of diesel blended with oxygenates in laminar opposed non-premixed flames


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  Experimental study on the effect of nozzle hole-to-hole angle on the near-field spray of diesel injector using fast X-ray phase-contrast imaging


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  Fuel atomization and vaporization process play a critical role in determining the engine combustion and emission. The primary near-nozzle breakup is the vital link between the fuel emerging from the nozzle and the fully atomized spray. In this study, the near-nozzle spray characteristics of diesel injectors with different umbrella angle (UA) were investigated using fast X-ray phase-contrast imaging and quantitative image processing. A classic ‘dumbbell' profile of spray width (SW) composed of three stages: opening stage, semi-steady stage and closing stage. The SW peaks of two-hole injectors were more than twice of that of single-hole injector at the opening and closing stages, corresponding to the hollow-cone spray. This indicated the vortex flow was formed with the increase of the UA. The higher injection pressure had little influence on the SW while led to earlier breakup closer to the nozzle at lower needle lift. Significant fuel effect on the SW at higher needle lift was found. However, this effect could be neglect at lower needle lift due to the leading role of internal flow and cavitation on the near-field spray characteristics. In addition, the morphology-based breakup process was observed, which highlighted the important effect of internal flow on the spray development. The possibility of using hollow-cone spray in diesel injector was also discussed.


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  Effect of engine parameters and type of gaseous fuel on the performance of dual-fuel gas diesel engines-A critical review


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  Petroleum resources are finite and, therefore, search for their alternative non-petroleum fuels for internal combustion engines is continuing all over the world. Moreover gases emitted by petroleum fuel driven vehicles have an adverse effect on the environment and human health. There is universal acceptance of the need to reduce such emissions. Towards this, scientists have proposed various solutions for diesel engines, one of which is the use of gaseous fuels as a supplement for liquid diesel fuel. These engines, which use conventional diesel fuel and gaseous fuel, are referred to as 'dual-fuel engines'. Natural gas and bio-derived gas appear more attractive alternative fuels for dual-fuel engines in view of their friendly environmental nature. In the gas-fumigated dual-fuel engine, the primary fuel is mixed outside the cylinder before it is inducted into the cylinder. A pilot quantity of liquid fuel is injected towards the end of the compression stroke to initiate combustion. When considering a gaseous fuel for use in existing diesel engines, a number of issues which include, the effects of engine operating and design parameters, and type of gaseous fuel, on the performance of the dual-fuel engines, are important. This paper reviews the research on above issues carried out by various scientists in different diesel engines. This paper touches upon performance, combustion and emission characteristics of dual-fuel engines which use natural gas, biogas, producer gas, methane, liquefied petroleum gas, propane, etc. as gaseous fuel. It reveals that 'dual-fuel concept' is a promising technique for controlling both NOx and soot emissions even on existing diesel engine. But, HC, CO emissions and 'bsfc' are higher for part load gas diesel engine operations. Thermal efficiency of dual-fuel engines improve either with increased engine speed, or with advanced injection timings, or with increased amount of pilot fuel. The ignition characteristics of the gaseous fuels need more research for a long-term use in a dual-fuel engine. It is found that, the selection of engine operating and design parameters play a vital role in minimizing the performance divergences between an existing diesel engine and a 'gas diesel engine'. ?? 2008 Elsevier Ltd. All rights reserved.


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  Characterization of Low Load Ethanol Dual-Fuel Combustion using Single and Split Diesel Injections on a Heavy-Duty Engine


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  Effects of the injection strategy on the mixture formation and combustion characteristics in a DISI (direct injection spark ignition) optical engine


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  The present study investigates the effect of the injection strategy on the mixture quality and the consequential combustion characteristics. The injection strategies considered in the present study were divided into two parts: single injection (with a wide range of injection timings during the intake stroke) and double injection (with a short second injection during the compression stroke). The in-cylinder pressure and high-speed flame images were acquired to evaluate the combustion characteristics, and KIVA simulation was conducted to observe the in-cylinder flow and the fuel spray spreading procedure. For the single injection, retarding the injection timing caused an increase in the combustion speed and in-cylinder pressure. In this case, the major factor that increased the in-cylinder pressure was the turbulence intensity; the later injection produced a stronger turbulence at the spark timing, which caused a higher in-cylinder pressure. Meanwhile, the flame propagation direction was mainly affected by the equivalence ratio distribution. The flame speed toward the fuel-lean region was significantly slower than toward the fuel-rich region. The double injection strategy utilized the additional turbulence induced by the spray motion to increase the engine power.


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  A CFD study of water condensation inside the tubes of an automotive compact charge air cooler using large eddy simulation approach


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  Copyright © 2016 by ASME. The usage of compressed air generated by supercharger or turbocharger by automotive Original Equipment Manufacturers (OEM) is growing with the aim to increase engine performance by increasing the density of the air charge being drawn into the cylinder. Denser air coupled with more fuel pulled into the combustion chamber results in increased engine performance. The inlet air is heated during compression which can cause pre-ignition, which leads to reduced engine functionality. The charge air cooler (CAC) is a heat exchanger introduced to extract heat created during the compression process. Previous research developed a 3-D Computational Fluid Dynamics (CFD) model using the k-epsilon turbulent model with near wall treatment to resolve turbulence in the small channels of the CAC. [1] The present research uses a refined computational scheme with a Large Eddy Simulation (LES) model to solve local data as a function of time and location and correlates the result to the experimental measurements, as well as compare to the k-epsilon approach. Using LES resulted in the ability to correlate any portion of the experimental data and take a closer look at local heat transfer between the outside surface of the tube and the cooling air. Large Eddy Simulation for heat transfer gave more information required for design of CACs which is difficult to collect for various operating conditions by experiment. The overall benefit presented is a validated simulation methodology that predicts condensation, which is then used to evaluate and design CACs that function outside the condensate formation zone during various vehicle operation modes.


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  Spray ignition and flame stabilisation in the frame of diesel-like combustion conditions combine fundamental and complex physical and chemical processes. In this work, a numerical investigation has been performed to evaluate the potential of integrating detailed chemistry into CFD calculations, in order to improve predictions and gain more insight in involved processes. This work has been carried out using the capabilities of OpenFOAM??code, which provides an opensource framework for 3D-CFD simulations, including an ODE solver for solving chemical kinetics. As a general methodology, this study is based on simulating free n-heptane sprays injected into a constant volume vessel, corresponding to the conditions of the experimental database provided by Sandia National Laboratories. Calculations results have been compared to experiments, evaluating the effect of a wide range of ambient conditions on spray ignition and combustion characteristics. Specifically, this research checks the performance of some relevant n-heptane oxidation mechanisms found in the literature, with different degree of complexity, for modelling the chemical history of the fuel. The results of this investigation show the relative influence of chemical mechanism on spray/flame structure in terms of ignition delay and also ignition and flame stabilisation sites. The comprehensive mechanism performs generally better than more simplified chemistry models. However, its accuracy is also compromised for modelling advanced diesel-like combustion concepts based on injecting the spray into a low oxygen concentration environment. ?? 2010 Elsevier Ltd.


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  A model for high-pressure vaporization of droplets of complex liquid mixtures using continuous thermodynamics


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  This paper presents a comprehensive model for the transient high-pressure vaporization process of droplets of complex liquid mixtures with large number of components in which the mixture composition, the mixture properties, and the vapor-liquid equilibrium (VLE) are described by using the theory of continuous thermodynamics. Transport equations, which are general for the moments and independent of the distribution functions, are derived for the semi-continuous systems of both gas and liquid phases. A general treatment of the VLE is conducted which can be applied with any cubic equation of state (EOS). Relations for the properties of the continuous species are formulated. The model was further applied to calculate the sub- and super-critical vaporization processes of droplets of a representative petroleum fuel mixture- diesel fuel. The results show that the liquid mixture droplet exhibits an intrinsic transient vaporization behaviour regardless of whether the pressure is sub- or super-critical. The regression rate of the liquid mixture droplet is reduced significantly during the late vaporization period. The comparison with the results of a single-component substitute fuel case emphasizes the importance of considering the multi-component nature of practical mixture fuel and the critical vaporization effects in practical applications. This paper provides a practical means for more realistically describing the high-pressure vaporization of practical fuels. ?? 2002 Elsevier Science Ltd. All rights reserved.


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  Extinction Characteristics of Isolate n-Alkane Fuel Droplets During Low Temperature Cool Flame Burning


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  Direct numerical simulations of ignition of a lean n-heptane/air mixture with temperature inhomogeneities at constant volume: Parametric study


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  The effect of thermal stratification on the ignition of a lean homogeneous n-heptane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 58-species reduced kinetic mechanism developed for very lean mixtures from the detailed LLNL mechanism (H.J. Curran et al., Combust. Flame 129 (2002) 253-280). Two-dimensional DNS are performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields. The influence of variations in the initial temperature field, imposed by changing the mean and variance of temperature, and the ratio of turbulence to ignition delay timescale on multi-stage ignition of a lean n-heptane/air mixture is studied. In general, the mean heat release rate increases more slowly with increasing thermal stratification regardless of the mean initial temperature. Ignition delay decreases with increasing thermal stratification for high mean initial temperature relative to the negative temperature coefficient (NTC) regime. It is, however, increased with increasing thermal fluctuations for relatively low mean initial temperature resulting from a longer overall ignition delay of the mixture. Displacement speed and Damköhler number analyses reveal that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. Chemical explosive mode analysis (CEMA) also verifies that mixing counterbalances chemical explosion at the reaction fronts for cases with large temperature fluctuation. It is also found that if the ratio of turbulence to ignition delay timescale is short, resultant diminished scalar fluctuations cause the overall ignition to occur by spontaneous ignition. However, the overall effect of turbulence is small compared to the effect of thermal stratification. These results suggest that the critical degree of thermal stratification for smooth operation of homogeneous charge compression-ignition (HCCI) engines depends on both the mean and fluctuations in initial temperature which should be considered in controlling ignition timing and preventing an overly rapid increase in pressure in HCCI combustion. © 2011 The Combustion Institute.


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  Modeling Ignition and Combustion in Spark- Ignition Engines Using a Level Set Method Reprinted From : Modeling of SI Engines


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  Experimental investigation of atomization and combustion characteristics of high-pressure pulse sprays


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  This study investigated the effect of fuel injection pressure (20-80 MPa) on atomization and combustion characteristics using a stationary combustor with an intermittent supply of liquid fuel. In the atomization experiments, spray tip penetration and spray width increased, and the Sauter mean diameter of droplets decreased with an increase in the fuel injection pressure. In the combustion experiments, the OH emission intensity and combustion gas temperature upstream of the furnace increased with the fuel injection pressure, whereas nitrogen oxides emissions were reduced. The results demonstrate that high-pressure pulse spray combustion can reduce NOXemissions and enhance the combustion load by an increase in the fuel injection pressure.


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  An experimental study of real multi-hole Diesel nozzles is performed under current DI Diesel engines operating conditions. The aim of the investigation is to study the influence of orifice geometry on the flow at the nozzle exit and to analyse its effect on the spray in evaporative conditions. Special attention is taken in the study of the influence of cavitation on the orifice internal flow and spray development. The spray liquid-phase fuel penetration has been characterized. The visualization was made in a wide optical access engine for different operating conditions. From the measurements, the dependencies of nozzle geometry, injection conditions and ambient conditions on liquid-phase length were studied and analyzed. A model for liquid-phase fuel penetration in diesel sprays based on nozzle flow parameters has been proposed and validated. © 2007 Elsevier Ltd. All rights reserved.


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  Numerical simulations of non-reacting and reacting diesel and biodiesel sprays in a constant-volume chamber are presented in this paper and some of the results compared to measured data. The conditions of the study include the effects of chamber air dilution with inert gases and varying chamber temperature. n-Heptane is used as the surrogate fuel for diesel fuel and a tri-component fuel mixture consisting of n-heptane, methyl decanoate, and methyl 9-decenoate is used as the surrogate fuel for biodiesel. The computed and measured liquid-phase penetrations in the non-reacting sprays agree within 30% for the range of conditions considered. The vapour penetrations agree within 10%. For the reacting simulations, the turbulence/chemistry interaction is modelled using an unsteady flamelet generated manifold (UFGM) sub-model. As reported in prior work, the UFGM model predicts the flame lift-off height well. The soot model employed is a semi-empirical two-equation model that accounts for nucleation of soot precursors, surface growth via acetylene addition and PAH condensation, coagulation, and oxidation. NO is modelled using the kinetics from a sub-mechanism of GRI-Mech 3.0. The measured trends in soot distribution in the sprays are predicted well although there are significant differences in actual soot volume fractions.


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  Voltage , and Energy Deposition Characteristics of Spark Ignition Systems Reprinted From : SI Combustion and Direct Injection SI Engine Technology


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  File Size: 153K View All Collapse References Lilly, L. R. C., ed., Diesel Engine Reference Book, Butterworths, 1985.Karim, G. A., %u201CThe Dual Fuel Engine,%u201D published in Automotive Engine Alternatives, ed. by Evans Robert L., Plenum Press, New York/London, 1987, pp. 83-104.Karim, G. A., Ito, K., Abraham, M. and Jensen, L., %u201CAn Examination of the Role of Formaldehyde in the Ignition Processes of a Dual Fuel Engine,%u201D 1991, SAE Paper No. 912367; also published in %u201CGaseous Fuels for Engines%u201D (SP-888) and Transactions of SAE.Karim, G. A. and Zhaoda, Yan, %u201CModelling of Autoignition and Knock in a Compression Ignition Engine of the Dual Fuel Type,%u201D Proc. of the Institution of Mechanical Engineering, published in Computers in Engine Technology, September, 1991, C430/035, pp. 141-148.Wierzba, P., Karim, G. A. and Wierzba, I., %u201CAn Analytical Examination of the Combustion of a Turbulent Fuel Jet in an Environment Containing Premixed Fuel or a Diluent and Air,%u201D, Proc. 15th ASME/ETCE Conference, Houston, Texas, 1992, published in PD-Vol. 39, Fossil Fuel Combustion, ed. by Ruiz Roberto, pp. 53-59.Karim, G. A. and Khan, M. O., %u201CExamination of Effective Rates of Combustion Heat Release in a Dual-Fuel Engine,%u201D J. of Mech. Eng. Science, February, 1968, 10, pp. 13-23.Karim, G. A. and Liu, Zhigang, %u201CA Predictive Model for Knock in Dual Fuel Engines,%u201D, 1992, SAE Paper No. 921550; also published in %u201CNatural Gas: Fuels and Fueling%u201D (SP-927) and Transactions of SAE.Klimstra, J., %u201CPerformance of Lean Natural Gas-Fueled Engines - On Specific Fuel Consumption, Power Capacity and Emissions,%u201D SAE Paper No. 901495 published in SAE SP-832, 1990.Glassman, I., Combustion, 2nd Edition, Academic Press, New York, 1987.Product Status: In Stock See papers presented at International Fuels & Lubricants Meeting & Exposition, October 1993, Philadelphia, PA, USA, Session: International Fuels & Lubricants Meeting & Exposition Purchase more technical papers and save! With TechSelect, you decide what SAE Technical Papers you need, when you need them, and how much you want to pay. Learn more \textgreater


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  Extended modeling of decelerating turbulent jets for diesel spray's penetration after end-of-injection


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  In order to meet the growing demand for reduction of pollutant emissions and improvement in thermal efficiency, advanced combustion modes, such as low-temperature combustion (LTC), premixed charge compression ignition (PCCI), have been adopted in modern diesel engines. Compared to conventional diesel combustion, the ignition delay becomes longer in advanced combustion modes. The longer ignition delay usually results in the fuel injection ending earlier than ignition. Therefore, the spray propagation after end-of-injection (EOI) plays a significant role in diesel combustion process, and it is needed to estimate the spray evolution and mixture formation after EOI. For this purpose, a simple and analytical diesel spray model including the spray evolution after the EOI is developed in this study. To develop the model, the theoretical analysis on the decelerating process of the turbulent jet tip is performed based on the integral momentum flux and mass flow rate of the injected fluid over the cross-sectional area at the jet tip. It is observed that the decrease of mass flow rate of the injected fluid over the tip cross-sectional area causes reduction of the momentum flux over the tip cross-sectional area and the turbulent jet tip decelerating. Then the mass flow rate of injected fluid over the tip cross-sectional area is formulated, furthermore the analytical equation of the turbulent jet tip penetration during decelerating state is derived, and the correctness of the developed analytical equation has been proved theoretically. Finally, the calculation is extended to diesel spray penetration, and the calculated results are validated against the one-dimensional discrete model and the experimental data from Engine Combustion Network (ECN) respectively.


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  Observations of soot optical property characteristics using high-speed , multiple wavelength , extinction imaging in heavy-duty diesel sprays


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  Reduced chemical kinetic mechanisms for the oxidation of representative surrogate components of a typical multi-component automotive fuel have been developed and applied to model internal combustion engines. Starting from an existing reduced mechanism for primary reference fuel (PRF) oxidation, further improvement was made by including additional reactions and by optimizing reaction rate constants of selected reactions. Using a similar approach to that used to develop the reduced PRF mechanism, reduced mechanisms for the oxidation of n-tetradecane, toluene, cyclohexane, dimethyl ether (DME), ethanol, and methyl butanoate (MB) were built and combined with the PRF mechanism to form a multi-surrogate fuel chemistry (MultiChem) mechanism. The final version of the MultiChem mechanism consists of 113 species and 487 reactions. Validation of the present MultiChem mechanism was performed with ignition delay time measurements from shock tube tests and predictions by comprehensive mechanisms available in the literature.A combustion model was developed to simulate engine combustion with multi-component fuels using the present MultiChem mechanism, and the model was applied to simulate HCCI and DI engine combustion. The results show that the present multi-component combustion model gives reliable performance for combustion predictions, as well as computational efficiency improvements through the use of reduced mechanism for multi-dimensional CFD simulations. ?? 2010 The Combustion Institute.


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  Quenching , Ignition , Flame Propagation , and Extinction in Hot- Spots at Elevated Temperature and Pressure


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  Numerical Simulation of Combustion of Single Component and Multicomponent Unsupported and Fiber-supported Droplets in


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  A generalization of the classical Hertz-Knudsen and Schrage laws for the evaporation mass and energy fluxes at a liquid-vapor interface is derived from kinetic theory and a simple model for a velocity dependent condensation coefficient. These expressions, as well as the classical laws and simple phenomenological expressions, are then considered for the simulation of recent experiments [Phys. Rev. E 59, 419 (1999)]]. It is shown that mean condensation and evaporation coefficients in the mass flow influence the results only if they are small compared to unity and that the expression for evaporation mass flow determines the temperature of the liquid. Moreover, it is shown that the expression for evaporation energy flow plays the leading role in determining the interface temperature jump, which can be obtained in good agreement with the experiment from the generalized kinetic theory model and phenomenological approaches, but not from the classical kinetic-theory-based Hertz-Knudsen and Schrage laws. Analytical estimates show that the interface temperature jump depends strongly on the temperature gradient of the vapor just in front of the interface, which explains why much larger temperature jumps are observed in spherical geometry and the experiments as compared to planar settings.


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  Experimental Optimization of a Small Bore Natural Gas-Diesel Dual Fuel Engine with Direct Fuel Injection


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  Potential of a jet-induced shock wave to influence an upstream droplet cloud in compression-ignition engines using multiple injection strategies


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  High-pressure diesel-fuel sprays have been shown to emit shock wave under certain conditions, while the advanced injection strategy used in internal-combustion engines involve multiple injections taking place within a short time frame. Experimental study of double injection has shown in two instances that the first fuel-spray cloud could be accelerated before the arrival of the second fuel spray. Herein, it is hypothesized that a shock wave emitted from an injection interacts with the fuel-spray cloud of a preceding injection and is responsible of a push-away mechanism on the first droplet cloud reported on in the literature. In this context, the shock waves emitted by fuel-spray jets with a commercial 5-hole diesel injector injecting into a pressure vessel were characterized with schlieren visualizations and dynamic pressure measurements taken with single- and double-injection strategies. The experimental results confirm the shock presence based on schlieren measurements. The measured shock conditions show a different shock topology from most shock-tube experiments as the expansion wave closely followed the shock front, resulting in a thin shocked region and a short duration of the droplet exposition to the post-shock gas conditions. Experimental measurements were then used as initial conditions in a 1D multiphase simulation model allowing simulation of the shock-wave interaction with the droplet cloud under engine-related conditions. The model was used to conduct a parametric study on the droplet-cloud characteristics and showed that, as the cloud density increased, the shock intensity and droplet-induced velocity decreased. Finally, the model was used to illustrate that the push-away interaction mechanism could be explained by the shock wave-fuel-droplet-cloud interaction.


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  Development of a reduced n-dodecane-PAH mechanism and its application for n-dodecane soot predictions


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  A reduced n-dodecane-polycyclic aromatic hydrocarbon (PAH) mechanism is presented for modeling the combustion and soot formation processes of n-dodecane and compared with the optical studies conducted in the constant volume vessel from the engine combustion network (ECN). The proposed reduced n-dodecane-PAH mechanism consists of 100 species and 432 reactions, and it is also validated with available ignition delays and species concentration profiles from shock tubes and jet stirred reactors (JSR). The fuel-air mixing process of non-reacting n-dodecane sprays was validated with available liquid and vapor penetration, and fuel mixture fraction distribution data. The proposed mechanism was coupled with a multi-step soot model to predict the combustion and soot formation processes. The effects of ambient temperature, density, oxygen concentration, injection pressure and also reaction mechanism on soot formation are analyzed and discussed. The results show that the simulations capture the important characteristics of the soot formation process. Increasing injection pressure and ambient oxygen concentration helps soot reduction by either enhancing the mixing process to suppress the soot formation rate, or by increasing the available oxygen to accelerate the soot oxidation rate, while higher ambient temperature and density have negative effects on soot formation. In addition, comparisons between results with the proposed n-dodecane mechanism and a reduced n-heptane mechanism show that the general combustion characteristics are quite similar. However, the n-heptane mechanism tends to predict longer ignition delays and lift-off lengths under spray combustion conditions, although the predicted ignition delay of n-heptane is very close to or even shorter than n-dodecane under homogeneous conditions. This observation shows that although the simulations provide reasonable combustion and soot predictions under a wide range of operating conditions, more effort is still needed to refine chemical kinetic mechanisms. © 2014 Elsevier Ltd. All rights reserved.


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  An advanced combustion investigation system was established in this paper. Based on this system, the multiple-injection spray development under various operating conditions was presented. The experiment result shows that the effect of ambient pressure and injection pressure on multiple injection spray are very conspicuous, especially the effect of injection fluctuation on multiple injections.


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  Over the past 30 years, the Eddy Dissipation Concept (EDC) has been widely applied in the industry for the numerical simulations of turbulent combustion problems. The success of the EDC is mainly due to its ability to incorporate detailed chemical mechanisms at an affordable computational cost compared to some other models. Detailed kinetic schemes are necessary in order to capture turbulent flames where there is strong coupling between the turbulence and chemical kinetics. Such flames are found in Moderate and Intense Low-oxygen Dilution (MILD) combustion, where chemical time scales are increased compared with conventional combustion, mainly because of slower reactions (due to the dilution of reactants). Recent modelling studies have highlighted limitations of the standard EDC model when applied to the simulation of MILD systems, noticeably a significant overestimation of temperature levels. Modifications of the model coefficients were proposed to account for the specific features of MILD combustion, i.e. an extension of the reaction region and the reduction of maximum temperatures. The purpose of the present paper is to provide functional expressions showing the dependency of the EDC coefficients on dimensionless flow parameters such as the Reynolds and Damköhler numbers, taking into account the specific features of the MILD combustion regime, where the presence of hot diluent and its influence on the flow and mixing fields impacts on the reaction rate and thermal field. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet in Hot Co-flow (JHC) burner at different co-flow compositions (3%, 6% and 9% O2 mass fraction) and fuel-jet Reynolds numbers (5000, 10,000 and 20,000). Results show promising improvement with respect to the standard EDC formulation, especially at diluted conditions and medium to low Reynolds numbers.


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  Combustion , performance , regulated and unregulated emissions of a diesel engine with hydrogen addition


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  Simultaneous imaging of diesel spray atomisation and evaporation processes in a single-cylinder CR diesel engine


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  In direct injection diesel engines, combustion and formation of pollutants are directly influenced by the spatial and temporal distributions of the injected fuel. In this study mixture formation during the pre-combustion phase of a diesel engine was investigated using the laser-induced exciplex fluorescence (LIEF) technique. The main purpose of this investigation was to develop an experimental setup capable of providing the full-field view of both liquid and vapour phases of evaporating diesel sprays during the fuel injection process inside the combustion chamber of a diesel engine with optical access. An expanded laser beam was employed for full combustion chamber visualisation. In this study two model fuels were tested; one consisted of 89% decane, 10% α-methyl-naphthalene and 1% TMPD and the other 88% decane, 10% α-methyl-naphthalene and 2% TMPD. The spray atomisation and evaporation processes during the pre-combustion phase of a diesel engine were measured at an injection pressure of 1200. bar and the engine speed of 1500. rpm. The results demonstrated the capability of the full-field LIEF technique in simultaneous imaging of liquid fraction and fuel vapour distribution during high pressure fuel injection process. It also highlighted the effect of dopant concentration on the fluorescence intensity of liquid and vapour signals. The exciplex system containing 1% TMPD produced better visualisation of the liquid phase, though the crosstalk in the vapour phase precluded accurate detection of the vapour phase signal. In contrast, the exciplex system containing 2% TMPD resulted in satisfactory visualisation of the vapour phase; however the intensity of the liquid phase was compromised as a result. This was presumed to be mainly due to the spectral shift of the exciplex species and/or TMPD decomposition at elevated temperatures and pressures. © 2013 Elsevier Inc.


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  Study of the effect of ambient vapour concentration on the spray structure of an evaporating n-hexane spray


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  The characteristics of fuel spray is a widely investigated area of research. From these studies the effect of parameters like injection pressure, ambient pressure, temperature, ambient density are well known. Ambient vapour concentration play a crucial role in droplet evaporation. So spray structure can get influenced with ambient vapour concentration through the heating and evaporation process. The present paper deals with this subject. To study the effect of ambient concentration alone, it is important to conduct the experiments at same thermodynamic conditions. This paper presents a methodology in which the spray characteristics can be studied at different ambient concentration of the vapour while maintaining the same ambient pressure, temperature and density. The effect of vapour concentration on spray is presented in terms of spray penetration, cone angle, projected spray area and liquid length for a n-hexane spray. The effects of heating and evaporation of the spray are discussed in the context of available correlation for liquid length.


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  Macroscopic and microscopic characterization of diesel spray under room temperature and low temperature with split injection


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  The diesel spray morphology characteristics in the near-field and far-field were studied under both room temperature (RT, 25 °C) and low temperature (LT, - 2 °C) with high speed imaging technique. The microscopic characteristics, namely droplet velocity and droplet size were then investigated with a Phase Doppler Particle Analyzer (PDPA) under the same conditions. Single injection and split-injection strategies were employed. It was found that the increased viscosity under LT caused less injected fuel and obviously poorer dispersion thus smaller spray area and slower penetration compared with RT. Split-injection strategy significantly varied spray characteristics under LT while the raised injection pressure greatly narrowed the difference between split injections caused by this cutting-edge injection strategy. The higher surface tension under LT enabled the droplets to keep stable and retain the spheroidal shape. Consequently, higher droplet velocity and larger size were detected under LT. In addition, strong collision for split injection strategy resulted in larger droplets compared with single injection and LT further deteriorated the poor dispersion.


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  Effect of Cooled Egr on Modified Light Duty Diesel Engine for Combustion, Performance and Emissions Under High Pressure Split Injection Strategies


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  The multi-environment probability density function approach together with a tabulated chemistry model has been developed to numerically simulate spray combustion processes under Diesel-like combustion conditions. In this study, the joint composition PDF is approximated using the multi-environment PDF, consisting of a combination of weights and weighted scalars in both chemical composition space and physical space. To account for the auto-ignition process and subsequent flame propagation, a flamelet generated manifold is generated by utilizing steady and unsteady flamelet solutions. The spray dynamics are represented by the state-of-art physical models in context with the Eulerian–Lagrangian approach. The RANS-based computations are made for the n-dodecane non-reacting and reacting spray jets. In terms of spray and vapor penetration, the mean and rms mixture fraction, ignition delay, and lift-off height, the numerical results are in reasonably good agreement with the available experimental data of the Engine Combustion Network. Based on the numerical results, detailed discussions are made for the effects of the ambient conditions on the structures and characteristics of high-pressure Diesel-fueled spray jet flames.


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  Ignition , Combustion and Exhaust Emission Characteristics of Micro-pilot Ignited Dual-fuel Engine Operated under PREMIER Combustion Mode


