Investigations On The Influence Of Stratified Flame And DME Micro-flame On Gasoline High-dilution Combustion

High dilution low temperature combustion such as SI-CAI can reach high efficiency and low NOx simultaneously.However,due to its high sensitivity of in-cylinder states,it faces serious cyclic variation and heat release controlling problems under certain operation conditions,which limits its further application.To overcome the challenges,in this paper,stratified flame ignition（SFI）combustion and dimethyl ether（DME）micro-flame ignition（MFI）combustion is investigated,based on a single cylinder gasoline engine equipped with variable valve actuation system.Aiming at high efficiency,low emissions and better combustion stability.SFI combustion was investigated firstly,The influence of injection strategies on the regulation ability of spark timing,and the characteristics of each stage of SI-CAI combustion was studied.The results indicate that,the effect of spark timing on combustion phasing can be enhanced with proper in-cylinder fuel stratification at low load condition.In medium-high load condition,the average heat release speed during the flame propagation in SI-CAI can be controlled by direct injection（DI）ratio and the start of injection（SOI）.Compared to homogeneous charge combustion,the indicated efficiency of SFI increases by 7.4%,and the coefficient of variation of IMEP(COV_IMEP)decreases by 53%.However,SFI still suffers from high NOx emission and large cyclic variation of maximum pressure rise rate.Then,MFI combustion was investigated to overcome the problems of SFI.The influence of spark timing,DME injection strategy and dilution conditions on combustion was investigated,and the comparison between SFI and MFI was performed.The results indicate that,the heat release of MFI could be divided into three stages:auto-ignition,flame propagation,and auto-ignition,the initial heat release rate of MFI is higher than that of SFI,which is beneficial in enhancing the control of combustion.Compared to homogeneous charge combustion,the indicated efficiency of MFI increases by 12.4%,NOx emission decreases by 70%and the COV_IMEP is less than 2%.On the other hand,the cyclic variation of the maximum pressure rise rate decreased remarkably as well.In summary,SFI could improve the thermal efficiency and stability of SI-CAI combustion by optimizing the characteristics of heat release rate,but it is still insufficient in controlling the maximum pressure rise rate.MFI shows significant advantages in the control of combustion phasing and regulating the maximum pressure rise rate.On the other hand,MFI is able to obtain higher thermal efficiency and lower emissions simultaneously.