A numerical investigation of injection pressure effects on wall-impinging ignition at low-temperatures for heavy-duty diesel engine

Abstract

The effect of injection pressure on impinging ignition at low temperatures is not systematic or even confusing. This work is to reveal the impinging ignition mechanism by the numerical method to further improve the success rate and thermal efficiency at cold-start conditions in a diesel engine. In which, n-dodecane was used as the diesel surrogate, and the intermediate reaction radicals, equivalence ratio, and temperature were studied to analyze chemical reaction processes and local burn conditions. The simulation well represents the experiments. The results show that increasing injection pressure with the same mass leads to a faster fuel spreading and makes the mixture region wider and leaner for the impinging spray simulations, and the spray-wall impingement can instantly change the direction of the fuel movement and make fuel more dispersed. Under low-temperature cold-wall conditions, the physical ignition conditions are gradually destroyed due to the diffusion during longer ignition delay and finally result in misfire at higher injection pressure. In the misfire case, there is a generation of CH2O, but no obvious generation of OH. It proves that the low-temperature reactions occurred but transition into high-temperature reaction was stopped even the local temperature has reached the decomposition threshold (1000 K) of H2O2. However, lengthening injection duration can guarantee the success of ignition at higher injection pressures. It indicates that concentration is a key parameter for triggering the high-temperature reaction, and which can be made up by extending the injection duration. This research proves and explains why the lower injection pressure is helpful to the combustion in the cold start process from the mechanism level, and also has important implications for the design of fuel injection strategy in diesel engine cold start.