Effect of ammonia reforming on combustion and emission characteristics of a 4-valve engine with an active pre-chamber

Ammonia (NH₃), as a hydrogen carrier and carbon-free fuel, offers an attractive opportunity for engines to achieve carbon neutrality. Turbulent jet ignition (TJI) combined with ammonia reforming shows the great capacity in ammonia-fueled engines. In this study, the effects of reforming strategy in an ammonia-fueled TJI are numerically studied, addressing the reforming ratio and reforming region. The results show that when only using reformate in the pre-chamber, the promoting effect of jet flame is more effective on the initial combustion phase. There are still very high NH₃ emissions due to the low reactivity in the main chamber. Further using reformate both in the pre-chamber and the main chamber, all the combustion stages (ST-CA10, CA10-50, CA50-90) can be shortened almost linearly with the increase of reforming ratio. Besides, the unburned NH₃ can be reduced to an acceptable level when the reforming ratio reaches 200 ‰ (hydrogen energy ratio of 18.50 %). The main reason is that the jet-induced strong flow field is coincident with the whole combustion stage. Further increasing the reforming ratio (pure hydrogen) in the pre-chamber, a high combustion efficiency and acceptable NH₃ emission can be achieved at a low hydrogen energy ratio (7.08 %). However, knocking combustion will happen at high reforming ratio with a low knock intensity. The results can provide some guidance for making the best-promoting benefit of the limited hydrogen in ammonia TJI engines with different reforming strategies.