The effect of DME injection timing on the compression ignition and flame development of lean methane-air mixture was studied on a single cylinder optical engine equipped with port methane injection and in-cylinder DME direct injection. Results show that in the case of DME direct injection，the combustion process in the cylinder is mainly controlled by DME injection timing. When the DME injection timing is in the range of 40° to 60° BTDC

the heat release rate curve shows a unimodal distribution and a bimodal distribution in the range of 15° to 30° BTDC. At this time

the in-cylinder combustion of DME/methane/air mixture shows the characteristics of self ignition and flame propagation. Moreover

the DME injection timing also influences the location of ignition point. With the delay of DME injection timing

the distribution of ignition zone is more concentrated. Compared with port methane injection with spark ignition

under compression ignition conditions

the DME direct injection can significantly increase the flame propagation speed of methane air mixture in the early stage of combustion

thus accelerating the combustion process.