Co-combustion of ammonia and hydrogen in spark ignition engines-State-of-the-art and challenges

To limit the negative impact on the environment, increasingly stringent regulations are being introduced regarding the reduction of pollutant emissions. In automotive applications, reciprocating internal combustion engines (ICE) are under immense pressure due to emissions. Society aims to move away from fossil fuels. However, it is acknowledged that ICE will continue to play a significant role in electricity generation systems and certain transportation sectors. To achieve zero emissions or drastically reduce emissions, alternative carbon-free fuels are being pursued. Hydrogen (H2) is a natural substitute for fossil fuels, and recently, ammonia (NH3) has been considered an intriguing fuel. Ammonia is easier to transport and store compared to hydrogen. NH3 is a good hydrogen carrier with significant potential for use in ICE. However, ammonia combustion in such applications exhibits challenges related to its low burning rate. Here, hydrogen emerges as a high reactivity fuel, used as a combustion enhancer for NH3. Hydrogen-assisted ammonia combustion shows great promise for future of ICE. This paper presents a summary of research findings on the NH3/H2 combustion process in a spark-ignition ICE. Concepts of dual-fuel NH3/H2 engine operation, the influence of fuel proportions on cylinder pressure profiles, heat release, combustion stages, and engine emissions aspects are discussed. The directions of development of SI piston engines powered by NH3/H2 are given.