Study on Combustion and Emission Characteristics of Marine Ammonia-Diesel Dual-Fuel Engines Under High Ammonia Substitution Ratios

Ammonia is regarded as a promising carbon-free fuel for the green and low-carbon transition of the shipping industry, yet its high ignition energy requirement and slow flame propagation make high ammonia substitution ratios (AER) challenging for ammonia/diesel dual-fuel marine engines. This study numerically investigates the combustion and emission characteristics of a marine ammonia-diesel dual-fuel engine under high AER. A computational model of the MAN L23/30H four-stroke marine engine was developed in CONVERGE, coupling the RANS equations with the RNG k-ε turbulence model, the KH-RT spray breakup model, and the SAGE detailed-chemistry combustion solver, and was validated against experimental in-cylinder pressure data. Combustion and emission behaviors under AER of 50–90% at 100% and 50% loads, together with the effects of the ammonia premixed equivalence ratio at the 90% substitution level, were systematically analyzed. The results show that dual-fuel combustion proceeds through diesel pilot auto-ignition followed by ammonia flame propagation, and that increasing AER extends the ignition delay and first elevates and then reduces peak cylinder pressure. At 100% load, raising AER from 50% to 70% improves both combustion efficiency and indicated thermal efficiency, whereas at 90% AER and especially at 50% load, the drastically reduced pilot diesel quantity severely deteriorates combustion. An optimal ammonia premixed equivalence ratio of approximately 0.5 was identified for thermal efficiency, beyond which combustion phasing is markedly delayed and misfire eventually occurs. Emissions analysis reveals non-monotonic NO and NO2 formation governed by the reduction effects of ammonia-derived radicals, higher N2O formation under low-temperature, low-load conditions, and substantially increased NOx and unburned NH3 emissions at 90% AER. The novelty of this work lies in systematically quantifying the combustion and emission behavior of a marine ammonia/diesel dual-fuel engine over a wide high-AER range (50–90%) under both full and partial loads, identifying the optimal ammonia premixed equivalence ratio (~0.5), and mechanistically interpreting the non-monotonic nitrogen-emission trends, thereby providing theoretical guidance for the combustion organization and optimization of ammonia/diesel dual-fuel marine engines under high substitution ratios.