A kinetic investigation of the critical ambient temperature for auto-ignition of the diesel jet under cold start conditions

Investigations of the high-pressure spray misfire under low temperature conditions is of great significance for the cold startability improvement of the diesel engine. In this study, the CFD model coupled with kinetic chemistry mechanism was used to investigate the critical ambient temperature (Ta) for successful ignition of the n-dodecane spray. Results show that two essential steps are needed for a successful two-stage ignition of the spray: the first one is the propagating of the cool flame from fuel-lean regions to fuel-rich regions; the second one is the excitation of the high temperature combustion (HTC) reactions at fuel-rich regions. Both the 1st and 2nd stage ignition delays (ID) increase as Ta decreases. The critical Ta for the auto-ignition of the n-dodecane spray at present conditions is 745 K, below which the local equivalence ratio of the first kernels decrease sharply and the 2nd stage pre-ignition ID performs a sharp rise. The main cause of the misfire at low temperature ambient is the low reaction rate of the HTC reactions due to the overmixing of the spray at the end of the injection. Long injection duration is helpful to reduce the critical Ta and increase the cold start ability of the diesel engine.