Research on destructive knock combustion mechanism of heavy-duty diesel engine at low temperatures

Abstract

ABSTRACT To clarify the mechanism of destructive knock combustion and piston ablation in heavy-duty diesel engines operating at low temperatures, CONVERGE simulation and wall temperature measurement were performed. The results show that as the initial temperature (intake valve closing instant) decreases from 380K to 307K, the spray wet-wall ratio increases from 12.8% to 22.2%. The fuel film evaporates slowly at low temperature before the hot flame, so the ignition is seriously delayed until the fuel injection ends, and the free-jet autoignition shifts to wall-attached combustion. However, the fuel film evaporation reverses after hot flame, and the maximum evaporation rate at 307K is about 54 times that at 380K. The wall temperature rises sharply to 350-450°C during wall-attached combustion. As the initial temperature decreases, the maximum pressure rise rate increases from 0.9MPa/°CA to 8.3MPa/°CA, and the pressure waves repeatedly oscillate along the end-center-end path in the cylinder. The highest pressure at the monitoring point is nearly an order of magnitude higher than the average, and the oscillation amplitude is as high as 6-10MPa, which far exceeds the failure threshold. The piston whose strength has been greatly reduced after thermal shock quickly undergoes ablation damage under strong pressure shock.