A numerical study on fuel film and emissions formations during cold start in a diesel engine using an improved spray-wall impingement model

Understanding the physical process such as fuel film formation during cold start is essential to reduce engine emissions. In this study, an improved spray-wall impingement model was applied to the cold-start simulation of a diesel engine. The fuel film and emissions formations during cold start were studied in depth. The effects of injection strategies were investigated by the use of various designed injection schemes. The results showed that a certain amount of fuel film is deposited on the wall surface due to the poor spray evaporation caused by the adverse thermal atmosphere in the cylinder. The low combustion temperature, insufficient oxygen, and the deposited fuel film are the primary causes for high HC, CO and soot emissions. The NOx emissions are relatively low because of low combustion temperature and oxygen content. A double injection strategy can reduce the deposited and remaining fuel film by 13% due to improved mixing, which benefits the reduction of HC and CO emissions, while it has a negligible effect on NOx and soot emissions. The percentage of first injected fuel has little impact on the amount of the remaining fuel film and emissions in the studied range. Increasing engine speed can inhibit the formations of fuel film, HC, CO, and soot emissions, which is due to the improved fuel evaporation, better flow motion, and thermal environment in the cylinder. Double injection allows a smaller amount of injected fuel to obtain a stable ignition. Decreasing the injected fuel mass can suppress the formation of fuel film and improve mixing, thus reducing HC, CO, and soot emissions. This work expands the application scenarios of the newly developed spray-wall impingement model. The new model has been successfully applied to investigate the cold start process of diesel engines in detail. The results provide insights into the fuel film formation and its effect on emissions, which can guide the development of a cold-start emission control strategy.