Numerical Investigation of Combustion Mechanisms and Emission Trade-Offs in a Two-Stroke Direct-Injection Ammonia/Diesel Dual-Fuel Marine Engine

Sammanfattning

This study investigates the combustion and emission characteristics of a two-stroke, direct-injection ammonia/diesel dual-fuel compression-ignition engine under low-load conditions relevant to large marine engines. Although ammonia is carbon-free and can reduce direct CO2 emissions, its low reactivity, strong evaporation cooling, and nitrogen chemistry may cause incomplete combustion, NH3 slip, and N2O formation. A closed-cycle CFD framework was therefore applied using an Eulerian–Lagrangian spray formulation, a modified gRNG k-ε turbulence model, detailed spray sub-models, and a 69-species/389-reaction ammonia/n-heptane kinetic mechanism. One diesel-only case and three ammonia/diesel cases with ammonia injection durations of 3 ms, 4 ms, and 5 ms were simulated, corresponding to ammonia energy ratios up to 49.4%. The model was assessed against experimental data and reference simulations, followed by analyses of grid independence, engine performance, emissions, combustion processes, and swirl ratio sensitivity. The simulations reproduce the main combustion and emission trends with acceptable accuracy. Increasing ammonia injection duration and ammonia energy ratio increases the second-stage heat release from ammonia oxidation and raises indicated mean effective pressure from 1.84 bar to 3.70 bar. Carbon-related emissions decrease per energy input, but incomplete ammonia oxidation increases NH3 slip and N2O emission, causing total greenhouse-gas-equivalent emissions to rise from 79.0 g/MJ to 192.9 g/MJ. In-cylinder analysis shows that ammonia combustion depends on its engulfment by the diesel flame, while colder or weakly connected hot regions promote unburned ammonia and N2O. The swirl ratio analysis indicates that moderate swirl improves spray–flame interaction, whereas excessive swirl over-disperses the fuel, resulting in incomplete combustion. Among the tested cases, a swirl ratio of 15 gives the best overall emission balance. This study concludes that cleaner ammonia/diesel operation requires not only a higher ammonia energy ratio, but careful control of spray interaction, turbulent mixing, ignition structure, and nitrogen chemistry.