Ammonia as a carbon-free fuel for internal combustion engines: A review of combustion fundamentals, blending strategies, and emission control technologies

Ammonia represents a highly promising zero-carbon fuel for internal combustion engines (ICEs). However, current reviews on ammonia-fueled ICEs are fragmented, limited to single blending systems or narrow operating conditions, and lack systematic integration of combustion kinetics and synergies between advanced combustion and aftertreatment technologies. This review presents a comprehensive, comparative analysis of ammonia fuel covering physicochemical properties, laminar combustion, in-cylinder control, and exhaust aftertreatment. It horizontally compares chemical kinetic mechanisms of three typical blends: NH 3 /H 2 (radical pool multiplication), NH 3 /CH 4 (thermal-kinetic coupling with C–N interaction), and NH 3 /CH 3 OH (chemically bound oxygen regulation). A synergistic framework is established to couple blending, in-cylinder organization, and aftertreatment for simultaneous reduction of nitrogen oxides (NO x ), nitrous oxide (N 2 O), and unburned ammonia. The three blends are quantified and ranked for engineering applications. Coupling effects of high-pressure direct injection, the Miller cycle, and blending on efficiency and stability were revealed. This review fills key literature gaps and provides theoretical and engineering support for developing high-efficiency, low-emission ammonia ICEs.