Combustion process and PM emission characteristics in a stratified DISI engine under low load condition

An experimental study was carried out to investigate the combustion process and particulate matter (PM) emission characteristics in a spray guided direct injection spark ignition (DISI) engine under lean-stratified operation. In-cylinder pressure analysis, PM emission measurement, and visualization of the in-cylinder combustion were applied.Heat release rate during stratified combustion showed 2-staged combustion characteristics. The main combustion was preceded with fast combustion speed and high peak of heat release rate. Combustion process after the main combustion showed longer combustion duration and relatively lower intensity of heat release rate. Propagation of non-luminous blue flame was observed during the early stage of the main combustion process. Mixing controlled combustion with luminous sooting flame development was observed during the late stage of the main combustion. Lastly, non-luminous blue flame was dominant during the secondary combustion process, which was interpreted as the combustion of the unburned lean-homogeneous mixture.PM emission characteristics were investigated under various injection timing conditions. The number and total weight of PM emission were increased as the injection timing was retarded. These trends were coincident with the trend of sooting flame luminosity during the combustion process with variation of injection timing. PM emission and mixing controlled combustion were found to have mainly originated from the inhomogeneous gas phase distribution of the stratified mixture.