Potential of green ammonia as a carbon-free fuel: Integrated hydrogen-ammonia-dme multi-fuel strategies for road transport IC engines

Green ammonia is emerging as a promising carbon-free energy carrier for internal combustion (IC) engines. This review summarizes recent advances in sustainable ammonia production and its application as an alternative fuel. Production pathways include renewable-powered Haber-Bosch, electrochemical synthesis, plasma-assisted processes, and biological methods. Beyond fertilizers, ammonia is gaining interest in transportation, power generation, refrigeration, and energy storage. As a fuel, it offers high hydrogen density (108 g H 2 L −1 ), carbon-free combustion, and easier storage than hydrogen. In dual-fuel engines, ammonia can replace up to 95% of diesel energy, reducing CO 2 emissions by 15-89%. However, high substitution ratios may increase NOx, N 2 O, and NH 3 slip due to ignition delay and incomplete combustion. Efficiency gains of 4-13% are achievable through optimized injection strategies, while 20% hydrogen enrichment and higher injection pressures can reduce unburned NH 3 by up to 28%. Further research is needed in advanced combustion, after-treatment, and techno-economic assessment.