Combustion and emission control of hydrogen–ammonia blends in internal combustion engines

• Comprehensive review of NH 3 -H 2 combustion in ICEs with 110+ references. • Quantitative synthesis of engine performance and emissions from 30+ experimental studies. • Visualized research gap map (Fig. 3) with TRL 1–9 assessment across five strategic priorities. • Analysis of NH 3 slip and N 2 O emissions with mitigation strategies. • Future research directions including smart injection and advanced after-treatment. The global increase in energy demand has driven widespread reliance on combustible fuels, which are essential in both residential and industrial applications. However, this dependency on hydrocarbons has resulted in significant environmental and health risks due to the emissions they produce. In response to these challenges, hydrogen (H 2 ) and ammonia (NH 3 ) have emerged as promising renewable and carbon–neutral energy carriers, offering a potential solution to mitigate CO 2 emissions in transportation. For long-range applications, such as marine diesel engines, traditional alternatives like battery-electric, hybrid, or fuel cell solutions are often prohibitively expensive and unable to provide sufficient torque output. The low auto-ignition temperature of ammonia necessitates the use of highly reactive fuels, such as hydrogen, to facilitate its combustion process. Ammonia as a marine fuel offers several advantages, including the elimination of sulfur oxides (SOx) and particulate matter (PM) emissions. However, recent studies highlight significant challenges associated with burning hydrogen–ammonia blends, particularly related to the ignition delays of hydrogen and ammonia. These delays can result in incomplete combustion and the formation of higher levels of nitrogen oxides (NOx), including NO, NO 2 , and N 2 O, which exacerbate environmental concerns. This review provides a comprehensive examination of the physicochemical properties of ammonia and hydrogen as potential fuels for internal combustion engines (ICEs). It discusses various blending methods and fuel injection techniques, and highlights the critical knowledge gaps that must be addressed to further advance the development of H 2 -NH 3 blended fuels for practical applications. The review aims to guide future research and innovation in this promising field by identifying the key obstacles and potential solutions to enhance the combustion efficiency and reduce the emissions of H 2 -NH 3 fuel blends.