Study On The Influence Of Internal And External EGR Coupling Control On Gasoline Compression Ignition Combustion And Emissions

In recent years,with the declining global oil storage and the deep awakening of people’s awareness of energy conservation and environmental protection,traditional spark ignition gasoline engines can no longer meet increasingly stringent emission regulations.More and more researchers have turned their attention to new combustion modes in order to expect engines to be more efficient and clean.Homogeneous Charge Compression Ignition(HCCI)is a new combustion mode with high efficiency and low emissions,which has great research value and development potential.This combustion mode injects new vitality into traditional gasoline engines.However,the combustion mode of gasoline homogeneous charge compression ignition still has problems such as difficulty in achieving compression ignition,difficulty in controlling ignition point,rough operation under heavy load,and excessive NOx emissions.In response to the above issues,this article uses a combination of experimental and simulation techniques to explore the impact of different internal exhaust gas recirculation(internal EGR)rates and different coupling of internal and external EGR rates on gasoline compression ignition combustion,emissions,and load.By controlling the internal and external EGR rates,the combustion process of gasoline compression ignition is optimized,and the combustion phase is flexibly controlled while achieving compression ignition,Improve the thermal efficiency and the upper limit of the average indicated pressure(IMEP)of compression ignition,and reduce its emissions and pressure rise rate.In this study,a ZR180 single cylinder diesel engine was used as a test prototype.Instead of injecting gasoline into the intake port,a variable valve timing(VVT)system and an electronically controlled external EGR system were installed to achieve compression ignition of gasoline and control of internal and external EGR rates.Using this as a test platform,the internal EGR rate was controlled by individually changing the exhaust valve closing phase strategy(EVC strategy),and the external EGR rate was controlled by changing the opening degree of the external EGR valve.The variation laws of combustion,emissions,and load under different internal EGR rates and different internal and external EGR rate coupling were analyzed.The coupling range of internal and external EGR rates that can achieve compression ignition under 1000 r/min,full throttle,lambda=1.05,and EVC strategy is determined.It was found that when external EGR was not added,the IMEP and indicated thermal efficiency first increased and then decreased as the internal EGR rate increased.An external EGR that increased by 7.5% at high load operating points could increase the IMEP upper limit by about 5.3%,increase the thermal efficiency by about 0.8%,and reduce the fuel consumption rate by about 2.6%.At the same time,the NOx emission amplification significantly decreased.Introducing external EGR at low load operating points could cause engine misfire.In this paper,a three-dimensional CONVERGE simulation model of the aforementioned gasoline compression ignition prototype was established to simulate and calculate the gasoline compression ignition combustion process with different internal and external EGR rates coupled under the EVC strategy.The temperature distribution and gas component distribution in the cylinder during the compression ignition process were analyzed in detail,and the variation rules of combustion,emissions,and load during the test were explained.The differences in the effect of internal and external EGR on the compression ignition combustion process and the reasons for the differences were analyzed.It was found that the heating effect of external low-temperature exhaust gas was weak and the heat capacity effect was strong,making its combustion inhibition effect more significant.By individually changing the exhaust valve lift(EVL)strategy to control the internal EGR rate,the combustion and emission laws of different coupling internal and external EGR rates under this strategy were studied,and compared with the combustion and emission laws under the EVC strategy.It was found that the IMEP and thermal efficiency of this strategy at high load operating conditions were lower than those under the EVC strategy,and the incomplete combustion phenomenon was more severe at low load operating conditions,After adding external EGR,the improvement effect of IMEP and thermal efficiency at high load operating points is poor,and small load operating points are prone to misfire.