Impact of diesel post-injection strategy on combustion and emission characteristics of a high-speed ammonia/diesel dual direct-injection engine

As global demand for clean energy continues to rise, ammonia has gained increasing attention as a low-carbon energy carrier for internal combustion engines. Ammonia/diesel dual-fuel systems have emerged as a research focus due to their potential to significantly reduce greenhouse gas emissions. However, challenges including high levels of unburned ammonia emissions have hindered their widespread adoption. This study investigates the impact of diesel post-injection strategies on the combustion and emission characteristics of an ammonia/diesel dual direct-injection engine using a dedicated experimental platform. The results demonstrate that ammonia addition substantially reduces NO_X emissions while leading to increased CO emissions and ammonia slip. Under the diesel post-injection strategy, trailing ammonia spray undergoes more complete premixed combustion facilitated by flame entrainment from the post-injected diesel, effectively suppressing ammonia slip. Meanwhile, the combustion of post-injected diesel is unaffected by ammonia quenching, thereby promoting more complete oxidation and reducing CO emissions. This also helps reduce N₂O formation. However, in the ammonia-rich environment during post-injection combustion, localized high temperatures and sufficient oxygen promote thermal NO_X formation. Compared to the baseline condition without post-injection, the optimized strategy (DPIT = 8°CA ATDC, DPIR = 48 %) significantly reduces CO, and NH₃ emissions while maintaining or improving engine performance. Only a marginal increase in NO_X is observed. These findings confirm the technical feasibility of diesel post-injection as an effective strategy to enhance the adaptability of ammonia combustion and mitigate pollutant emissions in dual-fuel engines.