Application of enhanced combustion strategies and optimized ignition timing for achieving high thermal efficiency and low N₂O emissions of marine ammonia engine at full-load condition

To meet the requirement of reducing greenhouse gas (GHG) emissions, the application of carbon-free fuel ammonia in marine engines has gained importance. However, the use of ammonia as fuel leads to low thermal efficiency and high emissions of pollutants in engines. Increasing the rate of combustion of the fuel mixture in the engine helps to solve this problem. Therefore, the influence of hydrogen volume fraction (X_H2) and oxygen volume fraction (X_O2) in the main chamber, via numerical simulations, on the combustion and emission characteristics of a marine ammonia engine featuring a pre-chamber. Further analysis was conducted via adjustments in the start of ignition (SOI) to optimize both engine performance and emissions. The results showed that the increase of both X_H2 and X_O2 contributed to the improvement of indicated thermal efficiency (ITE) and the reduction of N₂O emissions. However, this is usually accompanied by higher NO_x emissions, especially in the case of high X_O2. In addition, adjusting the SOI resulted in the engine ITE is greater than 47.6% in each case and reduces GHG emissions by about 80% (<40 ppm N₂O). Finally, chemical kinetic analysis showed that oxygen-enriched or hydrogen-enriched conditions did not change the main reaction pathway.