A detailed multi-zone thermodynamic simulation has been developed for the direct-injection (DI) diesel engine combustion process. For the purpose of predicting heterogeneous type combustion systems, the model explores the formation of preignition radicals, start of combustion, and eventual heat release. These mechanisms are described based on the current understanding and knowledge of the diesel engine combustion acquired through advanced laser-based diagnostics. Six zones are developed to take into account the surrounding bulk gas, liquid-and vapor-phase fuel, pre-ignition mixing, fuel-rich combustion products as well as the diffusion flame combustion products. A three-step phenomenological soot model and a nitric oxide emission model are applied based on where and when each of these reactions mainly occurs within the diesel fuel jet evolution process. Caton, for his excellent guidance, patience and support throughout the course of this research. I would never have been able to finish my dissertation without his guidance. I would like to thank Dr. Jacobs, who is always willing to help and give his best suggestions. My sincerest appreciation goes to Dr. Petersen and Dr. Bowersox for their support of my research. Many thanks to Josh Bittle, Hoseok Song, Jiafeng Song in the Advanced Engine Research Lab for the experimental data used in the model comparisons and suggestions on my research. I would also like to thank Junnian Zheng who helped me learn CHEMKIN. My research would not have been possible without their help. I also want to extend my gratitude to my friends and colleagues and the department faculty and staff for making my time at Texas A&M University a great experience. Finally, my profoundest love goes to my mother and father for their amazing love and encouragement with their best wishes. Your unwavering love and support have made me who I am today. v
Cylinder-to-cylinder variation in a multi-cylinder diesel engine was found to increase substantially when transitioning to a low-temperature combustion mode. This study was started to investigate the potential influence this effect could have on the emissions levels. Initial testing showed an imbalance in the fuel distribution that prompted this article to focus on data from before and after swapping two injectors under both conventional and low-temperature combustion modes. A significant improvement is observed in cylinder variation based both on visual heat release inspection and on mean effective pressure variation. This is likely a result of a changing combination of exhaust gas recirculation and fuel distribution such that less cylinder-to-cylinder variation is present (e.g. high dilution and low fuel, switched to low dilution and low fuel).Interestingly, despite the reduced cylinder-to-cylinder variation, the results show that the emissions levels are actually not affected. Despite the lack of influence on emissions results, the cylinder-to-cylinder variation in low-temperature combustion modes is still a critical factor that could impact its ability to be implemented in a commercial setting. Further cylinder balancing was attempted and achieved by introducing small (microsecond) adjustments to each cylinder start of injection and injection duration. The balancing is effective, but due to exhaust gas recirculation imbalance, a single adjustment setting does not apply to both conventional and low-temperature combustion modes. Additionally, day-to-day ambient conditions also negate the effectiveness. This supports the idea that some type of consumer-based real-time automatic balancing system may be needed in the future.
A three-step phenomenological soot model and a nitric oxide emission model have been developed by applying the current understanding of conceptual models for direct-injection diesel engine combustion processes. The three-step soot model incorporates the physical processes of fuel pyrolysis, soot inception and soot oxidation. The nitric oxide model is governed by the Zeldovich (thermal) mechanism and N2O intermediate mechanism. With the local information provided by the previously developed multi-zone thermodynamic diesel engine combustion model, the emission models can be successfully applied via specific detection concerning where and when each of these reactions mainly occur within the diesel fuel jet evolution process. The simulation was completed for a 4.5-L, inline four-cylinder diesel engine. The results demonstrated that this method, which incorporates emission models into the developed multi-zone diesel engine combustion model, has the potential to qualitatively predict the effects of various engine parameters on the engine-out soot and nitric oxide emissions. The results showed that advanced injection timing and higher injection pressure lead to the increase of nitric oxide concentration because of not only the increased residence time but also the higher entrainment rate of fresh gas into combustion products. Soot formed in-cylinder decreases with increasing injection pressure and advanced start of injection timing mainly due to the extension of the lift-off length and lower local equivalence ratio. In spite of decreased ambient oxygen concentration, the extended lift-off length and the reduced combustion temperature contribute to the reduction of soot formation under heavy exhaust gas recirculation levels.
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