The Impact of Pilot Diesel Injection Strategies on the Combustion and Emission Characteristics of Diesel–Natural Gas Dual-Fuel Medium-Speed Marine Engines Based on Large-Eddy Simulation

AbstractWith the tightening of emission regulations for marine engines, it has become increasingly important to explore efficient and clean combustion strategies in diesel–natural gas dual–fuel marine engines. This study, for the first time, compared and analyzed the effects of single and split injection strategies on the combustion process in a marine medium-speed dual-fuel engine with a natural gas substitution rate of 93.3%. Optimal strategies were compared for single injection [Case A: start of injection (SOI)=−15° crank angle (CA) after top dead center (ATDC)] and split injection (Case B: SOI1=−60°CA ATDC, SOI2=−10°CA ATDC). The analysis of Cases A and B revealed that employing a split pilot diesel injection strategy enhances engine thermal efficiency—Case B had a 0.77% increase in efficiency compared with Case A. More importantly, the split injection strategy proved to be more effective in reducing emissions; compared with Case A, Case B had a 36.3% and 49.4% decrease in NOx and CH4 emissions, respectively. A deeper examination of the combustion process in Case B revealed that the reactivity in specific cylinder regions was enhanced, thereby increasing the flame propagation speed in those areas. Moreover, the strategy reduced the proportion of pilot diesel diffusion combustion, leading to lower high temperatures and aiding in the reduction of NOx formation. These findings provide a technological reference for improving energy conversion efficiency and facilitating cleaner combustion in future applications of low-carbon fuels in dual-fuel marine engines.