Made available in DSpace on 2015-03-03T11:52:40Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-06-13Bitstream added on 2015-03-03T12:06:10Z : No. of bitstreams: 1 000801083.pdf: 1280442 bytes, checksum: 6dcfc6fb20131fddca0a32bff15754cf (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Nestre trabalho é desenvolvida uma modelagem matemática para os ciclos irreversíveis Otto e Diesel. Os ciclos são analisados entre dois reservatórios com taxa de capacidade térmica infinita, com os processos de troca de calor ocorrendo em trocadores de calor entre o fluido de trabalho e os reservatórios térmicos. As irreversibilidades são decorrentes dos processos de troca de calor ocorrendo em tempo finito, da taxa de perda de calor do reservatório de alta temperatura para o reservatório de baixa temperatura e dos processos de compressão e expansão não-isoentrópicas. São utilizados três critérios de otimização: função ecológica, coeficiente ecológico de desempenho e potência máxima de saída. Estas funções são otimizadas com relação à temperatura de entrada no processo de adição de calor. São analisados as otimizações ecológicas e então comparadas com a potência máxima. Os resultados são apresentados através das curvas de potência e critério ecológico, eficiência térmica ecológico e taxa de geração de entropia e critério ecológico. São analisados os comportamentos de potência líquida, eficiência térmica e taxa de geração de entropia otimizadas ecologicamente através dos quais são avaliadas as influências de alguns parâmetros nos seus comportamentos. Por fim, são analisadas as razões entre a potência otimizada por critérios ecológicos e a potência máxima, eficiência térmica otimizada por critérios ecológicos e a eficiência térmica na condição de potência máxima, a taxa de geração de entropia otimizada por critérios ecológicos e a taxa de geração de entropia na condição de potência máxima. A análise dos resultados comprova que as otimizações ecológicas apresentam o melhor compromisso entre potência líquida e o ambiente. Os resultados poderão ser utilizados como critério relevante no aperfeiçoamento de projetos dos motores de combustão interna / In this work is developed a mathematical model for the irreversible Otto and Diesel cycles. The cycle is analyzed between two reservois with infinite thermal capacitance, where the processes of heat exchange occuring in the heat exchangers between the working fluid and the thermal reservoir at constant temperatures. The irreversibilities follow from the heat exchange processes occurring in finite time, the loss of heat from the hot source to the cold source and the noisentropic compression and expansion processes. Three optimization criteria are used: ecological function, ecological coefficient of performance and maximum power output. These functions are optimized with respect to the inlet temperature of heat addition process. Ecological optimizations are analyzed and compared to maximum power. The results are presented through the power and ecological creteria, thermal efficiency and ecological criteria and entropy generation rate and ecological criteria curves. The results are presented though the power curves and ecological criteria, thermal efficiency and ecological and entropy generation rate and ecological criteria. Analyzes the behavior of power, efficiency and rate of entropy generation ecologically optimized through which they are evaluated the influences of some parameters on their behavior. Finally, we analyze the ratio between ecological criteria for optimum power and maximum power, optimized thermal efficiency by ecological criteria and the maximum power efficiency, the ratio between the entropy generation rate optimized for ecological criteria and entropy generation rate of maximum power. The results show that the ecological optimizations present the best compromisse between power and environment. The results can be used as an important criterion in developing projects of internal combustion engines

In this thesis Direct Numerical Simulation (DNS) is used to investigate the development of turbulent premixed flame kernels during the early stages of growth typical of the period following spark ignition. Two distinct aspects of this phase are considered: the interaction of the expanding kernel with a field of decaying turbulence, and the chemical and thermo-diffusive response of the flame for different fresh-gas compositions. In the first part of the study, three-dimensional, repeated simulations with single-step chemistry are used to generate ensemble statistics of global flame growth. The surface-conditioned mean fluid-velocity magnitude is found to vary significantly across different isosurfaces of the reaction progress variable, and this is shown to lead to a bias in the distribution of the Surface Density Function (SDF) around the developing flame. Two-dimensional simulations in an extended domain indicate that this effect translates into a similar directional bias in the Flame Surface Density (FSD) at later stages in the kernel development. Properties of the fresh gas turbulence decay are assessed from an independent, non-reacting simulation database. In the second part of this study, two-dimensional simulations with a detailed 68-step reaction mechanism are used to investigate the thermo-diffusive response of pure methane-air, and hydrogen-enriched methane-air flames. The changes in local and global behaviour due to the different laminar flame characteristics, and the response of the flames to strain and curvature are examined at different equivalence ratios and turbulence intensities. Mechanisms leading to flame quenching are discussed and the effect of mean flame curvature is assessed through comparison with an equivalent planar flame. The effects of hydrogen addition are found to be particularly pronounced in flame kernels due to the higher positive stretch rates and reduced thermo-diffusive stability of hydrogen-enriched flames.

Current stoichiometric spark-ignited engine technologies require costly catalytic converters for reductions in tailpipe emissions. Load control is achieved by using a throttle, which is a leading contributor to reductions in efficiency. Spark-ignited lean burn natural gas engines have been proven to be more efficient and emit fewer pollutants than their stoichiometric counterparts. Load reduction in these engines can be achieved by regulating the air/fuel ratio of the intake charge thereby reducing the efficiency penalties inherent to throttling. Partially stratified charge (PSC) can provide further reductions in emissions and improvements in efficiency by extending the lean limit of operation. PSC is achieved by the ignition of a small quantity of natural gas in the vicinity of the spark plug. This creates an easily ignitable mixture at the spark plug electrodes, thereby providing a high energy ignition source for the ultra-lean bulk charge. Stratified charge engine operation using direct injection (DI) has been proposed as a method of bridging the throttleless load reduction gap between idle and ultra-lean conditions. A previous study was conducted to determine if PSC can provide a high-energy ignition source in a direct injected stratified charge engine. Difficulties with igniting the PSC injections in an air-only bulk charge were encountered. This study focuses on a fundamental Schlieren image-based analysis of PSC combustion. Natural gas was injected through a modified spark plug located in an optically accessible combustion bomb. The relationships between PSC injection timing, fuel supply pressure and spark timing were investigated. Spark timing is defined as the duration between commanded start of injection and the time of spark. As the fuel supply pressure was increased, the minimum spark timing that lead to successful combustion also increased. The largest spark timing window that led to successful combustion was determined to be 80 ms wide at an injection fuel supply pressure of 300 psi. The amount of unburned natural gas increased with increasing spark timing. A cold flow study of the PSC injection system was also conducted. The PSC injection solenoid was found to have a consistent average injection delay of 1.95 ms. The slope of the linear response region of observed injection duration to commanded injection duration was 8.4. Due to plenum effects, the average observed injection duration of the entire PSC system was an order of magnitude longer than the commanded injection duration and was found to vary significantly with fuel supply pressure. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Mehdi Namazian. / Ph.D.

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