Numerical study on the wall-impinging diesel spray mixture formation, ignition, and combustion characteristics in the cylinder under cold-start conditions of a diesel engine

The combustion of wall-impinging diesel spray is a typical phenomenon in a diesel engine due to the limited in-cylinder space, especially for small and medium-sized diesel engines under the cold-start condition. To investigate the wall-impinging diesel spray combustion process at low-temperature and low-speed conditions, the mixture formation, ignition, and combustion characteristics of the fuel were quantitatively analyzed using a computational fluid dynamics model. It was discovered that the diesel spray was guided by the piston surface and the limited in-cylinder space, resulting in more vapor-phase fuel flowing into the region between the two spray paths, Thus, high-temperature ignition (HTI) occurred in this region. During a cold-start, in-cylinder HTI also occurred in the region between the two spay paths with different injection timings. The flame then propagated first to the region with the higher equivalence ratio near the piston surface and finally to the region with the lower equivalence ratio in the center of the combustion chamber. Because an earlier injection timing resulted in both a leaner mixture in the cylinder at top dead center and the transition period from the low-temperature reactions to the high-temperature reactions, there was a limited ability to adjust the HTI and thermal power conversion by controlling the injection timing. The optimal control of in-cylinder combustion during the cold start of a diesel engine depends on the HTI, the center of combustion, and the deposited fuel mass. The optimal of injection timing during a cold start of the diesel engine in this study was -13 degrees CA ATDC.