Combustion research currently aims to reduce emissions, whilst improving the fuel economy. Burning fuel in excess of air, or lean-burn combustion, is a promising alternative to conventional combustion, and can achieve these requirements simultaneously. However, lean-burn combustion poses new challenges, especially for internal combustion (IC) engines. Therefore, models used to predict such combustion have to be reliable, accurate and robust. In this work, the flamelet approach in the Reynolds-Averaged Navier- Stokes framework, is used to simulate flames relevant to spark-ignition IC engines. A central quantity in the current modelling approach is the scalar dissipation rate, which represents coupling between reaction and diffusion, as well as the flame front dynamics. In the first part of this thesis, the predictive ability of two reaction rate closures, viz. strained and unstrained flamelet models, are assessed through a series of experimental test cases. These cases are: spherically propagating methane- and hydrogen-air flames and combustion in a closed vessel. In addition to these models, simpler algebraic closures are also used for comparison. It is shown that the strained flamelet model can predict unconfined, spherically propagating methane-air flames reasonably well. By comparing spherical flame results with planar flames, under identical thermochemical and turbulence conditions, it is shown that the turbulent flame speed of spherical flames are 10 to 20% higher than that of planar flames, whilst the mean reaction rates are less influenced by the flame geometry. Growth of the flame brush thickness in unsteady spherical flames have been attributed to turbulent diffusion in past studies. However, the present analyses revealed that the dominant cause for this increase is the heat-release induced convective effects, which is a novel observation. Unlike methane-air flames, hydrogen-air flames have non-unity Lewis numbers. Hence, a novel two degrees of freedom approach, using two progress variables, is used to describe the thermochemistry of hydrogen-air flames. Again, it is shown that the strained flamelet model is able to predict the experimental flame growth for stoichiometric hydrogen-air flames. However, none of the models used in this work were able to predict lean hydrogen-air flames. This is because these flames are thermo-diffusively unstable and the current approach is inadequate to represent them. When combustion takes place inside a closed vessel, the compression of the end gases by the propagating flame causes the pressure to rise. This is more representative of real IC engines, where intermittent combustion takes place. The combustion models are implemented in a commercial computational fluid dynamics (CFD) code, STAR-CD, and it is shown that both strained and unstrained flamelet models are able to predict the experimental pressure rise in a closed vessel. In the final part of this work, a spark-ignition engine is simulated in STAR-CD using the flamelet model verified for simpler geometries. It is shown that this model, together with a skeletal mechanism for iso-octane, compares reasonably well with experimental cylinder pressure rise. Results obtained from this model are compared with two models available in STAR- CD. These models require some level of tuning to match the experiments, whereas the modelling approach used in this work does not involve any tunable parameters.

This thesis presents the results of an investigation of the differences in combustion between gasoline and natural gas in a spark-ignition engine. Combustion development is influenced by calorific value, specific heat, flame speed and the gaseous or liquid state of the fuel. Simple simulation programs were set up to investigate the effects of low flame speed and higher specific heat of the fuel-air mixture. Actual performance was measured on a single cylinder test engine using ionization probes as flame detectors and a pressure pick-up. The experimental results show that longer ignition delay and limited flame speed at high pressure and temperature are the main reasons for' the power loss of natural gas at high engine speed; this is in addition to the basic loss due to the replacement of air by gaseous fuel in the cylinder. From calculations, it was learned that specific heat and dissociation differences had little effect on power. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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Thesis (Ph.D.)--University of Melbourne, Dept. of Mechanical Engineering, 2009. / Typescript. Includes bibliographical references (leaves 231-245)

Call number: LD2668 .T4 1984 G666 / Master of Science

Le fonctionnement en régime très pauvre ou avec forts taux d'EGR des moteurs à allumage commandé (MAC) permet de réduire efficacement les émissions de CO2 et de Nox ; cependant ces stratégies se heurtent à l'augmentation des variabilités cycliques. Ces dernières sont principalement dues à la phase d'allumage qui devient critique de dilution. Le modèle ECFM-LES actuellement utilisé à IFPEn, basé sur la notion de densité de surface de flamme, est insuffisant pour décrire l'allumage dans ces conditions critiques. Dans ces travaux, l'approche TF-LES est adoptée, l'allumage étant alors décrit par un emballement cinétique des réactions chimiques lors d'une élévations locale de la température. Ces travaux définissent et évaluent une stratégie de simulation pour TF-LES en configuration moteur, qui permette une prédiction fine des allumages critiques et de la propagation turbulente de la flamme, afin de décrire le cycle moteur complet.Dans une première partie, des DNS d'allumages turbulents ont été réalisées, en modélisant la phase d'allumage par un dépôt d'énergie thermique (Lacaze et al., (2009)). Les calculs ont simulé les expériences d'allumage de Cardin et al. (2013), dans lesquelles l'énergie minimum d'allumage (MIE) d'un mélange mtéhane-air a été mesuré, pour différentes richesses pauvres et sous différentes intensités turbulentes. L'objectif principal des simulations a été de déterminer les paramètres numériques et physiques du modèle permettant de reproduire les allumages de l'expérience. Deux types de schémas cinétiques ont été évalués : un schéma simplifié et un schéma analytique (ARC), ce dernier reproduisant et les délais d'auto-allumage et la vitesse de flamme laminaire. Les résultats ont permis de définir des critères d'allumage et de mettre en évidence les différentes prédiction d'allumage avec les deux types de schémas cinétiques. Les résultats ont été également démontré que l'approche choisie permettait de prédire les bons niveaux d'énergie pour les allumages laminaires et à faible nombres de Kalovitz (Ka<10). Aux plus hauts nombres de Karlovitz, il a été montré que le modèle ED était insuffisant pour prédire les énergie d'allumage et qu'une description plus fine du dépôt d'énergie est nécessaire.Dans la seconde partie des travaux, un modèle de plissement dynamique (Wang et al., 2012) a été étudié, afin de décrire le développement hors-équilibre de la flamme dans la phase de propagation turbulente. Des études sur des flammes sphériques laminaires ont d'abord été menées. Ensuite, les premiers tests de configuration moteur ayant révélé des incompatibilités du modèle, des modifications ont été proposées. Le modèle de plissement dynamique modifié a été finalement évalué sur la configuration moteur ICAMDAC. Les résultats obtenus ont été comparés aux résultats obtenus par Robert et al. (2015) avec le modèle ECFM-LES, qui utilise une équation de transport de densité de surface de flamme décrivant le plissement hors-équilibre de la flamme. Les résultats obtenus avec le plissement dynamique sont en très bon accord avec ceux du modèles ECFM-LES, démontrant ainsi la capacité du modèle dynamique à prédire des valeurs de plissement hors-équilibre. D'autre part, le modèle dynamique s'ajustant automatiquement aux conditions de turbulence de l'écoulement, nul besoin n'est d'ajuster la constante de modélisation en fonction du régime moteur, comme c'est le cas pour l'équation de transport de la densité de surface de flamme. / The use of lean equivalence ratios or high EGR rates in spark ignition engines (SIE) enables to optimize CO2 and NOx emissions; however too important dilution rates leads to increased cycle-to-cycle variability. These latter are mostly due to the ignition phase, which becomes critical when dilution rates are important and requires high ignition energy. The ECFM-LES model currently used in IFPEN, which is based on the flame surface density concept, is not sufficient to describe ignition in these critical conditions. The TF-LES approach was chosen in this study, principally because it directly resolved chemistry and can thus model ignition via a local raise of the temperature. The present work defines and evaluates a simulation strategy for TF-LES in SIE configurations, that enables a fine prediction of critical ignitions and of the turbulent flame propagation.In the first part, DNS of turbulent ignition were performed. The ignition phase was modeled using a thermal energy deposit (ED model, Lacaze et al.). Simulations reproduced the ignition experiments of Cardin et al. who determined the minimum ignition energy (MIE) of lean premixed methane/air mixtures, for different turbulence characteristics. The main purpose of the study was to determine the numerical and physical model parameters, which enable to reproduce Cardin et al. experiments. Two types of kinetic schemes were evaluated: a simplified kinetic scheme and an analytical kinetic scheme (ARC), that can predict both the auto-ignition delays and the laminar flame speed, while keeping affordable CPU times. Results analysis enabled to define ignition criteria and to highlight the differences in terms of ignition prediction using the two kinetic schemes. Results also demonstrated that the chosen approach could recover correct levels of ignition energy for laminar and low Karlovitz number cases (Ka<10). For higher Karlovitz number cases, the ED model was found to be insufficient to predict the ignition and a finer description of the energy deposit is required.In the second part, a dynamic wrinkling model (Wang et al., 2012) was studied to describe the out-of-equilibrium behavior of the flame during the propagation phase. Studies on laminar spherical flames were first performed, to assess the laminar degeneration of the model. Then, as first tests in an engine configuration have revealed incompatibilities of the model, modifications were proposed. The modified dynamic model was finally tested in the ICAMDAC engine configuration. Results of the simulations were compared against previous results of Robert et al. obtained with the ECFM-LES model using a transport equation for the flame surface density that can describe the out-of-equilibrium wrinkling of the flame. Results obtained with the dynamic model are in very good agreement with the ones of Robert et al., thus demonstrating the ability of the dynamic model to predict out-of-equilibrium values in the engine configuration. Besides, the dynamic model self-adapts to the turbulence conditions, hence does not require any model parameter adjustment, as is it the case for models based on the flame surface density transport equation.

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Recent approval of emission standards for small utility engines by the California Air Resources Board suggested that substantial reductions in emissions from small utility engines will soon be required. Although the 1994 standards can be met by simple engine modifications, the 1999 standards may require the use of emission control technologies such as catalytic converters because they are more stringent. In this research catalytic control of individual hydrocarbons such as methane, ethylene, benzene, and toluene were evaluated. A platinum coated catalyst treated emissions from a 107cc, four-cycle gasoline engine loaded with a 1.4KW portable generator. Determination of emissions was performed at three different load levels: 0%, 50% and 92% of the engine rated load. Among the four hydrocarbons, toluene was oxidized as much as 60%, and benzene 40%, whereas ethylene remained unaffected by the catalyst. Also, a 5% to 10% methane oxidation occurred in one trial. / Master of Science

The motivation for this work stems from the enacting of stricter emissions requirements for the mid 1990's by the California Air Resources Board. It is foreseen that these requirements will favor the use of alcohol fuels in quantities comparable to the present usage of gasoline and diesel in order to reduce emissions of carbon monoxides (CO) and nitrogen oxides (NOx). The use of alcohol fuels at this level will substantially increase the amount of aldehyde emissions. This poses a problem in that aldehydes are odorants, components of photochemical smog, and volatile aldehydes are eye and respiratory tract irritants; therefore, it is only a matter of time before they too are strictly regulated. This thesis focuses on a systematic analysis of aldehyde emissions from alcohol fuels with respect to compression ratio. Compression ratio has been selected as the primary variable for this study, because alcohol-fueled vehicles are usually modified to have higher compression ratios than their gasoline-fueled counterparts in order to take advantage of alcohols' higher octane rating. The investigation is being conducted using a single-cylinder variable-compression ratio Waukesha-CFR engine. The aldehyde emissions are measured for various fuel alcohol percentages at different compression ratios and MBT timing. The effects on conventional vehicle emissions (Le. NOx, CO, unburned hydrocarbons) are also being measured so that tradeoffs between conventional emissions and aldehyde emissions can be determined. The goal of this research was to locate any trends between alcohol fuels and compression ratios which will allow for an optimization of these parameters to minimize aldehyde emissions. It was desired that this be achieved without sacrificing engine performance or increasing other regulated emissions. The variance of compression ratio was found to affect the pollutant formation process via its effects on temperature. The increasing expansion ratio, which accompanies increasing compression ratio, resulted in lower post .. expansion burned-gas temperatures. Temperature's influence on the rate of reactions was found to be the driving force in the formation of most of the pollutants. The experiment showed a definitive reduction in CO emissions with the use of alcohol fuels. The results also indicated an inherent tradeoff between NOx and formaldehyde emissions. / Master of Science