The effects of Dimethyl Ether(DME) injection timing or start of injection(SOI) in the range from -15° to -40° in crank angle at different internal exhaust gas recirculation(iEGR) rates on DME auto-ignition processes were investigated on a single cylinder optical engine using a high speed camera and thermodynamic analysis. The results show that at different iEGR rates

the advance of SOI leas to advanced crank angle of 50% fuel mass fraction burned(CA50)

shortened combustion duration and increased peak of heat release rate. However

at a certain SOI

CA50 delays

the combustion duration increases and the peak of heat release rate decreases. In the range of SOI between -15° and -25°

the combustion phase at the iEGR rate of 32% is later than that at the iEGR rate of 25%. At the SOI of -30° and -40°

the combustion phase at the iEGR rate of 32% is much advanced. The flame structure is related to the region of the DME auto-ignition. With the advance of SOI

the initial ignition point moves gradually away from the nozzle

and the combustion zones are more dispersed. Under different operating conditions

the relative flame intensity increases with the increase of the heat release rate

which indicates that the relative flame intensity can characterize the change of heat release rate level.