Effects of equivalence ratio and pilot fuel mass on ignition/extinction and pressure oscillation in a methane/diesel engine with pre-chamber

Abstract

For reducing emissions and saving energy, pilot-ignited methane/diesel engines are attracting considerable attention as an alternative to traditional diesel engines. However, this dual-fuel engine suffers from poor ignition characteristics and knock occurrence under specific operating conditions. Thus, the present study comprehensively aims to address the effect of the CH4 equivalence ratio and pilot fuel amount on the ignition/extinction process, in-cylinder pressure oscillation, and engine performance in a low-speed dual-fuel engine with pre-chamber. The results show that the fuel-mixture ignition delay is prolonged with a decrease in the CH4 equivalence ratio, but it decreases as the pilot fuel amount increases. However, beyond a certain amount, an additional increase in pilot fuel will not affect the methane combustion, indicated engine power, and emissions. It was also found that extinction occurs with a CH4 equivalence ratio of 0.3, because the reaction of the low-temperature intermediate species CH2O becomes almost stagnant. The increase in CH4 equivalence ratio can improve the engine power, but it also induces pressure oscillation and promotes nitrogen oxide production. Moreover, the pressure oscillation or maximum amplitude of pressure oscillation (MAPO) increases with the increase in pilot fuel amount, although it has no significant change when the pilot fuel proportion is beyond 1.5%. The present study can provide a theoretical basis for improving the combustion stability of dual-fuel engines under low engine load and avoiding the risk of knock under moderate to high engine loads.