Toward highly-efficient combustion of ammonia–hydrogen engine: Prechamber turbulent jet ignition

Ammonia (NH3) energy has emerged as a new international strategy to mitigate climate change and energy crises because ammonia–hydrogen fuel can effectively overcome the barriers in hydrogen energy development. However, improvement of the combustion efficiency of ammonia fuel is a significant obstacle to this development. Therefore, this paper comprehensively reviews the potential and capability of ammonia as a carbon-free fuel in internal combustion engines (ICEs). This paper covers the key and representative developments in the areas of pure ammonia and ammonia–hydrogen combustion characteristics, ammonia spray, NH3-dissociated hydrogen process, and ammonia–hydrogen engines. Thereafter, we investigate the advantages of TJI in ICEs in terms of parameters such as the lean limit, knock limit, engine performance, and emissions. Overall, the prechamber TJI offers promising advantages of multipoint ignitions and fast jet-flame velocity to resolve the current limitations of ammonia combustion. The application of prechamber TJI on the ammonia engine is presented with the proposal of a novel concept of reactivity-controlled TJI for ammonia engines. Ultimately, the future challenges involving the hybrid system of ammonia engine, highly-efficient ammonia engine, and post-treatment SCR for NOx emission and unburnt NH3 are presented. This review entails useful insights for the future design of the ammonia–hydrogen engine.