Various single and split injection schemes are studied to provide a better understanding of fuel distribution during cold starting in DI diesel engines. Improved spray-wall interaction, fuel film and multicomponent vaporization models are used to analyze the combustion processes.
Better combustion characteristics are obtained for the split injection schemes than with a single injection. An analysis of the fuel impingement processes identifies the mechanisms involved in producing the differences in vaporization and combustion of the fuel. A greater amount of splashing occurred for the split injections compared to a single injection. This behavior is attributed to the decreased film thickness (less dissipation of impingement energy), the decreased impingement area (obtained by increasing the impingement Weber number), and most importantly, the reduced frequency of drop impingement. In comparison, the single injection scheme resulted in an impingement time between drops (i.e., drop frequency) that was considerably smaller than the time required for splashing. Thus, subsequent impinging drops interfered with the splashing process, resulting in a greater quantity of adhered fuel on the walls. The increased splashing for the split injection schemes reduced the overall droplet size, which further aided vaporization and combustion. The partitioning of the injected fuel and the dwell between injection pulses have shown to provide better combustion characteristics for cold starting.
One common approach to improve startability is to increase the amount of fuel injected. This type of strategy was also modeled. However, the results indicate that better combustion characteristics may be achieved through the optimization of the injection scheme as an alternative to overfueling.