Fuel reactivity stratification assisted jet ignition for low-speed two-stroke ammonia marine engine

Ammonia (NH3) has gained much attention as a carbon-free fuel due to its great potential to reduce carbon emissions in the shipping industry. However, ammonia is difficult to use directly in existing marine engines due to its low flame propagation speed and high auto-ignition temperature. In this work, a new combustion mode for low-pressure injection low-speed two-stroke ammonia marine engines: Reactivity Stratification Assisted Jet Ignition (RSAJI) is proposed and explored. The results of the study show that the RSAJI mode is very promising. Especially at low and medium loads, the peak pressure can be increased by 27.3%, and NH3 and N2O emissions in the exhaust gas can be reduced by 87.1% and 99.7%. Two key parameters: direct injection timing and ammonia substitution rate (ASR) determine the reactivity stratification in the main-chamber, which directly affects the jet ignition effect and turbulent flame speed. A thorough analysis of engine performance shows that the injection timing should be appropriately far from the Top Dead Center (75 degrees CA BTDC & 86.4 degrees CA BTDC). Further increases in ammonia substitution rates (ASR = 90%, 95%) will result in a reduction in power performance gains but will improve the indicated thermal efficiency (>50%).