Investigation on injector positioning and timing for optimum performance and emission reduction on a marine two stroke engine converted to ammonia/diesel

Ammonia is a promising carbon-free fuel for marine decarbonization; however, its low flame speed, high ignition energy requirement, and NOx/N₂O/NH₃ emissions challenge its application in large engines. In ammonia/diesel dual-fuel operation, pilot diesel injection is essential for ignition control, but its low energy contribution (<5%) limits its influence on ammonia-governed combustion processes such as mixing and flame propagation. This study numerically investigates ammonia injector positioning and timing in a two-stroke marine engine converted to ammonia/diesel high-pressure direct injection (HPDF) at 75% load. The results showed that; positioning the ammonia injector 60° from the diesel injector increased the in-cylinder swirl ratio by 32.52% and reduced ISFOC to 161.6 g/kWh (164.5 g/kWh in diesel mode), while nearly eliminating N₂O and unburned NH₃ and reducing NOx by 77%. Synchronizing diesel and ammonia injection at 356°CAD further increased thermal efficiency by 3.76% and reduced ISFOC and NOx by 3.63% and 71%, respectively. Increasing injection dwell time prolonged ignition delay and combustion duration due to ammonia evaporation cooling effect.