In recent years, ammonia (NH₃) has attracted growing interest as a carbon-neutral fuel, especially in the marine sector where internal combustion engines dominate. However, ammonia combustion presents challenges such as low flame speed, high ignition energy, and emissions of NOx, N₂O, and unburned NH₃. While various combustion strategies have been proposed for large engines, limited research addresses ammonia combustion in marine engines under low-load conditions.

This study investigates ammonia/diesel dual-fuel combustion in a marine engine at 25% load using numerical simulations. It focuses on the effects of injector positioning and ammonia injection timing on engine performance and emissions. Results show that placing the NH₃ injector 60 degrees from the diesel injector increases the swirl ratio by 32.52% and lowers the specific fuel oil consumption (SFOC) to 170 g/kWh, slightly better than the diesel baseline of 172 g/kWh. Emissions of N₂O and unburned NH₃ were nearly eliminated, while NOx and the equivalent greenhouse gas emissions dropped by 77% and 90%, respectively. Timing analysis further showed that synchronising the ammonia start of injection (SOI-NH₃) with diesel at -4° CA ATDC improved thermal efficiency by 3.76%. Conversely, a longer dwell time between injections led to incomplete combustion and higher N₂O and unburned NH₃ emissions. These findings provide key insights into optimising ammonia injector placement and timing, offering a foundation for designing cleaner, more efficient ammonia-fueled two-stroke marine engines.