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

The characteristics of natural-gas direct-injection combustion under various fuel injection timings were studied by using a rapid compression machine. Results show that natural-gas direct injection can result in combustion that is much faster than homogeneous combustion while shortening the time interval between injection timing and ignition timing can markedly decrease the combustion duration. Unburned hydrocarbon would increase over a wide range of equivalence ratios, shortening the time interval between injection timing and ignition timing can decrease the value to that of homogeneous-mixture combustion. The NO_X level is high but the CO level is low over a wide range of equivalence ratios and is little affected by fuel injection timing. High values of pressure rise due to combustion can be realized and it is insensitive to the variation in fuel injection timing. High combustion efficiency can be achieved, which is also independent of injection timing.