Combustion characteristics of a direct-injection natural gas engine under various fuel injection timings

The combustion characteristics of a direct-injection natural gas engine under various fuel injection timings were investigated. The results showed that fuel injection timing had a large influence on the engine performance, combustion and emissions and these influences became largely in the case of late injection. Over-late injection would supply insufficient time for the fuel–air mixing of the late part of the injected fuel, bringing poor quality of mixture formation and subsequently resulting in the slow combustion rate, the long combustion duration and high HC concentration. However, early injection gave a slight influence on both engine combustion and emissions. There existed an optimum fuel injection timing where the maximum cylinder pressure, the maximum rate of pressure rise and the maximum rate of heat release would get their highest values along with the shortest combustion durations, the shortest heat release duration and more concentrated heat release process closing to the top-dead-centre while maintaining the low level of HC and CO emissions.