Optimization strategies for reducing NH3 emissions under low-load conditions in a large-bore ammonia-diesel dual-fuel medium-speed marine engine

Ammonia is considered as a crucial alternative fuel for maritime decarbonization due to its carbon-free combustion. However, the application of ammonia faces challenges such as difficult ignition, slow flame propagation, and a narrow flammable range in compression ignition engines, leading to a prominent issue of unburned NH3 emissions, especially under low-load conditions. This study focuses on a large-bore ammonia-diesel medium-speed engine for marine applications. Through multi-cylinder engine (MCE) and single cylinder engine (SCE) experiments, the formation mechanism of unburned NH3 emissions and combustion optimization strategies under low-load, high-speed conditions are systematically investigated. MCE test results show that under 25% load and 750 r/min conditions, unburned NH3 emissions can reach 42.01 g/kWh. Based on this, refined control studies on intake air temperature (Tair_in), excess air coefficient (λ), and diesel pilot-main injection parameters, including pilot injection ratio (PQR), pilot injection timing (SOI_Dp), injection pressure (PCR), were conducted on a SCE. Results show that increasing Tair_in to 343 K and reducing λ to 1.64 improve in-cylinder thermal conditions, reducing NH3 emissions by ∼30%. Furthermore, an optimized injection strategy (PQR of 30%, SOI_Dp at −40 °CA ATDC, PCR of 1200 bar) achieved a synergistic reduction of ∼80% in NH3 emissions. This study demonstrates the effectiveness of the synergistic “thermal-atmosphere-injection” strategy in suppressing unburned NH3 emissions under low-load conditions. It is important to note that NOx and N2O are also significant emission metrics, but unburned NH3 is the predominant issue under low-load conditions, which represents the primary bottleneck for current engineering applications. Hence, this study prioritizes its investigation. By clarifying the dominant controlling factors of unburned NH3, this work lays a theoretical and experimental foundation for future multi-objective synergistic emission reduction (including NOx and N2O), and provides an engineering pathway for clean combustion control in large-bore ammonia-diesel medium-speed marine engines.