Enhancement of combustion performance in ammonia-diesel dual-fuel engines with the maximum ammonia blending ratio of 70% through staged high-pressure injection strategy

Replacing traditional fossil fuels with carbon-free alternatives has become a crucial emission reduction strategy. Research on ammonia-diesel dual-fuel internal combustion engines is increasing. Nevertheless, studies focusing on the regulation of in-cylinder pressure traces to improve the combustion performance in ammonia-diesel dual-fuel engines are still scarce. This study therefore proposes a staged high-pressure injection strategy and numerically analyzes how its parameters affect combustion and emission characteristics. The results indicate that this strategy effectively improves diesel ignition efficiency and enables rapid, complete ammonia combustion. At high ammonia energy ratios of 60% and 70%, it significantly enhances combustion compared to direct blending. Furthermore, advancing the first injection timing resolves ignition delay, while adjusting the first injection quantity controls the pressure at top dead center. Shortening the second injection duration enhances the mixing of diesel spray with residual ammonia, promoting ammonia combustion and increasing peak cylinder pressure. For the engine model in this study, under a 70% ammonia energy ratio, the adjusted setting injects 40% of the diesel from −12 °CA to −9.48 °CA and the remaining 60% from −2 °CA to 1.02 °CA. Compared to the rated condition, this results in only a 0.23 °CA ignition delay, a 0.03 MPa lower top dead center pressure, and a 0.08 MPa lower peak pressure.