Combustion and emission characteristics of an ammonia diesel dual-fuel medium-speed marine engine under high-load conditions

Ammonia, a zero-carbon fuel producing no CO₂ during combustion, is recognized by the IMO as key for future zero-emission shipping. Ammonia engines are thus pivotal for maritime decarbonization. However, ammonia’s inherent characteristics—high ignition energy, slow flame speed, and narrow flammability limits—present challenges for marine engine application. While research on ammonia engine combustion and emissions exists, most focus on small-bore high-speed engines, with limited data on large-bore engines under high AER and load. This study investigates a 270 mm bore marine medium-speed engine. Using port-injected ammonia ignited by direct-injected diesel, the effects of AER and λ on combustion and emissions were systematically explored at high load (IMEP = 18.6 bar). Results show stable combustion up to 86 % AER. Increasing AER raised peak cylinder pressure, temperature, and pressure rise rate, while thermal efficiency exhibited a non-linear trend, overall exceeding diesel mode efficiency. Higher AER increased total NH₃ emission but decreased its escape rate (defined as the ratio of NH₃ emission value to the supplied ammonia mass flow rate), NOx emissions first decreased, then increased, and N₂O decreased continuously. Over the λ range tested, its impact on peak pressure and pressure rise rate was limited, but higher λ significantly lowered peak temperature and thermal efficiency at low λ. Increasing λ raised NH₃ emissions and escape rate, NO_x emissions first increased then decreased, while N₂O rose continuously. Crucially, higher λ effectively reduced CO₂ and equivalent CO₂ emissions.