Innovative method of in-cylinder purification emission control in low-speed marine engines: Numerical study on ammonia/diesel high-pressure direct injection

With the stricter emission regulations and global carbon reduction targets, marine engine technology must transition towards low-carbon and zero-carbon alternatives. Although ammonia presents a significant CO₂-free advantage, it also poses substantial applied challenges due to high auto-ignition temperature, low laminar burning velocity, high latent heat of vaporization. For large-bore low-speed high pressure direct injection (HPDI) ammonia/diesel dual-fuel marine engines, there is still a lack of researches on the combustion characteristics and in-cylinder purification. In this paper, a constant volume combustion chamber model is developed to optimize the fuel injector arrangement based on heat release and emissions firstly, then, a three-dimensional numerical model is established based on this arrangement. The results show that owing to the high evaporation and mixing rates of ammonia, the late burning combustion duration in dual-fuel mode is much shorter compared to that in diesel mode. Decreasing the intensity of partial premixed combustion of ammonia significantly reduces NOx emissions but prolong the combustion duration. Diffusion combustion, which is associated with high-temperature regions, is more likely to occur in areas with an equivalence ratio approximately 1 during dual-fuel operation. An increase in the ammonia energy fraction will result in higher N₂O emissions; however, overall greenhouse gas emissions are expected to decrease. To mitigate the emissions of NOx and N₂O, a method involving the use of split diesel fuel is proposed. Results indicate that the reactions N₂O=N₂ + O, N₂O + H=N₂ + OH, and NO + H=N + OH can be enhanced by split diesel injection in conjunction with advanced ammonia injection. This leads to reductions in both NOx and N₂O emissions and facilitates the internal purification of the engine. Consequently, it has been found that the split diesel injection strategy exhibits the potential for highly efficient, ultra-low emission marine engine technology.