Experimental study on the performance and optimization of pilot diesel-ignited high-pressure direct-injection methanol combustion at low loads

The pilot diesel-ignited high-pressure direct-injection (HPDI) methanol combustion mode has attracted considerable attention due to its the high thermal efficiency and ultra-low emissions with a high methanol substitution rate, but suffers from combustion instability and incomplete combustion under low loads. In this study, the impact of methanol substitution rate (MSR), intake condition, and injection parameters on combustion characteristics and emissions of the pilot diesel-ignited HPDI methanol combustion under low loads was explored, and an optimization path was proposed. The results indicated that reducing the intake pressure and the methanol injection pressure can reduce CO and NOx emissions, with minimal impact on combustion stability. Increasing the intake temperature and reducing the diesel injection pressure can reduce CO emissions and coefficient of variation (COV), but may lead to higher NOx emissions. An optimal diesel injection timing can reduce CO emissions and improve combustion stability. Through the optimization path of "delay diesel injection timing - increase intake temperature - reduce intake pressure - reduce methanol injection pressure - reduce diesel injection pressure," the optimization of the pilot diesel-ignited HPDI methanol engine was achieved under low loads, resulting in an efficient and stable combustion with a MSR of 95.8 % and an ITE of 51.34 %.