Investigation of performance and emissions characteristics in a two-stroke ammonia/diesel dual-fuel marine engine utilizing a high-pressure injection strategy

Cofiring ammonia and diesel is considered a feasible strategy for applying NH₃ as the alternative fuel in marine engines. In this study, a numerical model was developed based on a large two-stroke ammonia/diesel marine engine operating in high-pressure dual-fuel (HPDF) mode. Several control strategies, including the injection strategy, ammonia substitution rate (ASR), exhaust gas recirculation (EGR) and fuel start of injection (SOI), were employed to investigate the performance and emission characteristics of the engine. The results demonstrate that the centralized nozzle configuration promotes superior heat and mass transfer between NH₃ and diesel, thereby resulting in enhanced combustion performance and lower emissions of unburned NH₃. As the ammonia substitution ratio rises, the pressure, temperature, and emissions of NO and CO₂ decreased. Meanwhile, the increased NH₃ content can also cause an increase in the unburned NH₃ emission and an extended fuel ignition delay. Optimizing SOI significantly improves performance; NH₃ SOI at 358 °CA and diesel SOI at 356 °CA yield better combustion and lower NOx, unburned NH₃, and soot. Meanwhile, the Exhaust Gas Re-circulation (EGR) technology was employed to further control NOx emissions. The results demonstrated that an excessive EGR rate could cause deteriorated combustion performance and a considerable rise in unburned NH₃, soot, and N₂O emissions. When the EGR is 30 %, the NOx emission is 1.56 g/kWh, which meets the Tier III emission limit. It is anticipated that this study can provide some suggestions for the optimal operating conditions of ammonia/diesel dual-fuel engines in HPDF mode.