Numerical Study On The Combustion Control Strategy Of Gasoline Compression Ignition(GCI) At Low Loads

Gasoline Compression Ignition(GCI)has been regarded as one of the novel combustion modes to achieve clean and high efficiency combustion in a broad range of load.At low loads,influenced by the poor auto-ignition characteristic of gasoline and the low in-cylinder pressure/temperature,GCI combustion is often confronted with combustion stability problem and is very sensitive to the boundary conditions,which therefore is one of the hotspots in the IC engine research community.It has been demonstrated that that internal EGR introduced by Variable Valve Actuation(VVA)could effectively improve combustion at low loads,however,deteriorated emissions are also observed as the load increases with internal EGR,especially for the soot emission.Therefore,in the current work,numerical study has been conducted to extend the low load operation and to optimize the combustion and emissions by means of EGR and injection strategy optimization at low loads.Previous experiments showed that stable combustion could only be achieved at load higher than 4.6 bar IMEP with 2.2bar intake pressure without internal EGR.Therefore,numerical study has been firstly conducted to study the effects of internal EGR via VVA,injection timing and boost pressure on combustion at low loads,then measures and strategies for the low load extension in GCI mode has been explored.The current simulation results indicate that,auto-ignition and combustion could be greatly improved by introducing internal EGR through double exhaust valve opening valve strategy,wherein the high temperature thermal effects of the internal EGR plays a leading role under this circumstance.Meanwhile,boost pressure and injection timing optimization were also found to be helpful to improve the in-cylinder compression pressure and oxygen concentration and mixture stratification,thus improved combustion can be obtained as well.The overall results show that 97% combustion efficiency could be achieved at 2.3bar IMEP condition,thus the low load operation in GCI mode can be effectively extended through the combination of internal EGR,intake boost and injection strategy optimization.Numerical study was then conducted to investigate the coupled control strategy to improve the GCI combustion and emissions at various low load conditions through injection and EGR strategy optimization.The results show that double injection strategy could improve the combustion efficiency at 3.2bar IMEP load condition.This can be attributed to the reason that the double injection can improve the overlap area between the high temperature and high equivalence regions through mixture distribution optimization.The combustion efficiency could be improved by 6.5% via double injection strategy with optimized timing and ratio at this condition,which highlights the importance of in-cylinder temperature and mixture distributions on GCI combustion.In addition,although internal EGR was found to be helpful for combustion and emissions improvement at low loads,however,soot emission becomes the major concern as the load increases.It was found that external EGR should be adopted in addition to internal EGR to suppress the soot and NOx emissions at 4.5 bar IMEP condition,mainly due to its ability to reduce the temperature and to prolong the ignition delay for better mixing process.Meanwhile,double injection strategy should also be used to further reduce the locally over-rich and high temperature regions.Therefore,high efficiency and low soot/NOx GCI combustion can be achieved by the coupling control of injection strategy and combination of internal/external EGR at low loads.