Simulation Investigation On Optimization Of Combustion Process In A Heavy-duty Diesel Engine

Due to the pressures of energy shortages and environmental protection, researchers are continually striving to improve the thermal efficiency and to reduce the pollutant emissions of engines. As one of the crucial parameters that could reflect the combustion process, heat release characteristic greatly influences the thermal efficiency, mechanical load and emissions of diesel engine. The optimization of combustion process is one of the effective approaches to realize high efficiency and clean combustion.In this paper, numerical investigation was conducted to investigate the effects of combustion parameters on diesel engine's performance and emissions. Simulation models of one-stage and modified two-stage turbocharging HD diesel engine were built. Two important combustion parameters, CA50 timing and combustion duration (CA10-CA90), were taken to represent the combustion heat release characteristics. The results show that, there exist optimal CA50 timing and combustion duration which could realize the highest thermal efficiency under different operating conditions. The optimal CA50 is retarded away from TDC and combustion duration is also prolonged as the engine load increases and engine speed decreases. However, there are differences between the actual and ideal combustion processes which caused by the restriction of engine thermal and mechanical loads, as well as emissions, thus inevitably result in combustion loss. Further research shows that, the combustion loss is mainly caused by the extended combustion duration under low load conditions. Therefore as the load increases, combustion loss also increases. The effect of CA50 on thermal efficiency is much more notable than combustion duration, while for pollutant emissions, the influence of combustion duration is more prominent. For diesel engine with EGR system, reducing pumping losses is also an effective measure to improve the thermal efficiency of diesel engine.Based on these results, two simulation models were built to investigate the effects of pipeline layouts of high pressure EGR systems on performance and emissions of a heavy duty diesel engine. Simulation results show that the high pressure EGR system with separated pipeline layout, which could make better use of pulsing energy of exhaust gas, can enhance the ability of EGR introduction (increasing the intake-exhaust pressure difference), especially under low speed, high load conditions for the reason that exhaust gas is difficult to introduced under such operating conditions, and reduce the pumping losses, therefore much better trade-off between NOx and fuel economy can be achieved; When two-stage turbocharging system is applied, the differences between the two pipeline layout high pressure EGR systems become quite small; The fuel economy can be further improved by optimizing the EGR pipe geometric dimensions of a separated high pressure EGR two-stage boost system.