Combustion and emission characteristics of ammonia-diesel marine high pressure direct injection low-speed dual-fuel engine

In order to effectively reduce greenhouse gas emissions and decrease reliance on conventional fossil fuels, ammonia has emerged as a leading zero-carbon alternative fuel for marine engines. Utilizing test data from a marine dual-fuel engine with a high compression ratio of 36.9, we simulated the combustion process of a large-bore, high pressure direct injection two-stroke marine dual fuel engine. This study investigates the effects of the ammonia-energy ratio (AER) and ammonia injection timing (AIT) on combustion and emission characteristics. The results indicate that ammonia can be combusted stably at a compression ratio of 36.9. The AER increases, the in-cylinder pressure shows a trend of "rising first and then decreasing". Additionally, the heat release rate curve broadens, afterburning intensifies, and the duration of combustion is extended. Consequently, the indicated thermal efficiency (ITE) decreases from 54.84 to 42.2%, while carbon dioxide (CO₂) emissions are reduced from 472.5 g/(kW·h) to 52.5 g/(kW·h), representing a reduction of 88.9%. AER63% represents the optimal operating condition for the engine, allowing it to achieve high power density without surpassing the maximum explosive pressure (Pmax). Additionally, the nitrogen oxides (NO_x) emissions comply with Tier III emission limits. The advancement of AIT resulted in heat release advancement, increased isovolumicity, higher in-cylinder pressures, and advancement of CA10, CA50 and CA90, but prolonged combustion duration CA10-90. The advancement of AIT resulted in higher Pmax, ITE and the indicated mean effective pressure (IMEP). During the change of AIT from 4 °CA before top dead center (BTDC) to 4 °CA after top dead center (ATDC), ITE increased from 39.7 to 56.3%, a rise of 16.6%. Additionally, the IMEP increased from 1.89 MPa to 2.09 MPa. For every 1 °CA advance in AIT, Pmax increased by 0.72 MPa. The change in AIT had no effect on CO₂ emissions. However, when AIT was delayed, the reduction in NO_x was more significant than the reduction in nitrous oxide (N₂O). Specifically, NO_x was reduced by 0.15 g/(kW·h), while the equivalent CO₂ emissions from N₂O decreased by 1.31 g/(kW·h) for every 1°CA of AIT delay.