Numerical study on the combustion and emission characteristics of an ammonia/diesel dual direct injection engine under low-load conditions

Ammonia has attracted significant attention for its potential to achieve zero carbon emissions in internal combustion engines. However, the fumigating ammonia/diesel dual-fuel mode suffers from low combustion efficiency and high unburned ammonia emissions under low-load conditions. Ammonia/diesel dual direct injection holds the key to high-efficiency ammonia engines. However, its potential in increasing ammonia energy ratio and combustion efficiency has not been fully investigated under low engine loads; meanwhile, detailed formation kinetics of key species, such as NH₂, NO, and N₂O, in ammonia diffusion combustion need to be analyzed. In this study, the ammonia/diesel dual direct-injection combustion mode was implemented under low engine loads; ammonia is directly injected to promote diffusion combustion. The results show that the ammonia/diesel dual direct-injection combustion mode significantly enhances combustion performance and reduces ammonia slip. When ammonia is injected at the compression top dead center, unburned ammonia emissions are substantially reduced by 72% compared to the premixed ammonia case. The advantage of ammonia direct-injection combustion lies in its relatively small spatial distribution range that overlaps with the high-temperature combustion zone of diesel combustion, providing favorable thermal conditions for ammonia oxidation. When ammonia is injected after high-temperature reactions, the local temperature and OH concentration in the cylinder increase, promoting ammonia decomposition into NH₂ and combustion. Finally, we explored the effects of the spatial relationship between the ammonia spray and the diesel spray on ammonia combustion under low-load operating conditions. The ammonia combustion performance is improved when there is a spatial separation between the ammonia and diesel fuel sprays under low-load conditions. This is due to the endothermic evaporation of the liquid ammonia, which can lead to diesel flame quenching and the consumption of OH radicals in the diesel flames when NH₃ decomposes into NH₂.