Ammonia Integration in Internal Combustion Engines: A Review of Current Status, Challenges, and Future Directions

The urgent drive to decarbonize the transportation sector has positioned ammonia as a carbon-free fuel with high potential in dual-fuel and enrichment strategies for internal combustion engines (ICE). This review consolidates recent findings on ammonia–diesel/biodiesel, ammonia–gasoline, and ammonia–hydrogen blends, with a focus on their performance, emissions, and safety implications. In compression-ignition (CI) engines, partial ammonia substitution in the range of 10–40% ammonia energy share (AES) increases in-cylinder pressure and heat release rate by 5–15%, improves brake thermal efficiency (BTE) by 4–12%, and reduces brake-specific fuel consumption (BSFC) by 5–10%. Carbon-based emissions are substantially reduced, with CO₂ decreases of up to 80%, CO reductions approaching 98%, and HC reductions of up to 80%, although NO_x emissions exhibit wide variability. In spark-ignition (SI) engines, ammonia–gasoline blends yield modest BTE gains (≤20–30% ammonia energy ratio, AER) but higher BSFC, while reducing CO₂ by up to 60%, CO by up to 50%, and HC by up to 90%. Challenges remain with unburned NH₃ slip and elevated NO_x. Hydrogen and oxygen enrichment enhance reactivity, with small H₂ fractions (≤5%) markedly improving ignition stability and O₂ enrichment up to 25% increasing flame velocity by 2.5 times and BTE by ∼3%, albeit with higher NO_x emissions. Safety considerations─ammonia toxicity, hydrogen flammability, material embrittlement, and high-pressure operation─necessitate robust mitigation strategies, including advanced leak detection, resilient materials, optimized injection strategies, and aftertreatment systems (selective catalytic reduction (SCR), oxidation catalysts, plasma reactors). With appropriate controls and design innovations, ammonia and its blends can enable efficient, stable, and low-carbon propulsion, contributing to the decarbonization of sustainable transportation.