A simulative study of flex-fuel methanol-ammonia blends: Extended cold start performance and emission analysis in spark-ignition engines

The transportation industry is facing increasing pressure to reduce greenhouse gas emissions, driving growing interest in alternative fuels for marine engines. This study investigates the performance, combustion characteristics, and emission profiles of methanol-ammonia dual-fuel blends in a turbocharged V-8 engine under varying thermal conditions, with particular emphasis on cold-start operation. A comprehensive numerical model was validated against experimental data for pure methanol combustion, demonstrating strong agreement in predicting in-cylinder pressure, heat release rates, exhaust temperatures, and emissions. The validated model was then applied to analyze methanol-ammonia fuel blends ranging from M100 (pure methanol) to M25 (25% methanol, 75% ammonia by energy content) across intake temperatures from 278 to 298 K. Under cold-start scenarios, ammonia-rich blends significantly reduced CO₂ emissions by up to 80%. They lowered NO_x emissions by 90% at M25 compared to pure methanol, primarily due to richer combustion mixtures and lower cylinder temperatures. However, these blends exhibited reduced engine performance, with indicated mean effective pressure dropping by 14%–20% and indicated efficiency by 7.5%–26%, alongside up to 34% increase in brake-specific fuel consumption. CO emissions increased notably at high ammonia ratios, attributed to incomplete combustion under cold intake conditions. Contour analyses revealed that methanol-rich blends sustain better performance and shorter combustion delays during cold starts, while ammonia’s slow-burning properties extended combustion duration notably beyond M75 blends.