Pilot injection strategies for methane slip reduction in methane-diesel dual-fuel engines

The objective of this study is to investigate the effects of key factors related to pilot injection on the performance and exhaust emission characteristics of a Dual Fuel (DF) engine. To this end, combustion and exhaust data were obtained under variations in pilot injection timing, injection duration, injection pressure, and multi-stage injection strategies. The results showed that as the pilot injection timing was advanced, Indicated Mean Effective Pressure (IMEP) increased. However, when the pilot injection was advanced beyond Before Top Dead Center (BTDC) 16°, IMEP no longer increased, and heat release characteristics caused by end-gas auto-ignition appeared. As the injection duration increased, IMEP increased linearly. When the injection pressure rose above 1000 bar, fuel supply and spray penetration increased, and unlike the case of increased injection duration where only the pilot fuel quantity rose, end-gas auto-ignition occurred. In terms of exhaust emission characteristics, as the pilot injection timing was advanced, the injection duration lengthened, and the injection pressure increased, unburned methane emissions decreased while nitrogen oxide emissions increased. Among the multi-stage pilot injection strategies, the Pre 2nd Pilot showed the best performance, achieving the shortest combustion duration, highest IMEP, lowest Coefficient of Variation (COV) of IMEP, and a significant reduction in unburned methane, thereby recording the lowest greenhouse gas emissions. Although Nitrogen oxides (NOₓ) emissions increased due to higher combustion temperature and pressure, they remained relatively low compared to other conditions with similar greenhouse gas levels.