Stratified premixed combustion optimization of a natural gas/biodiesel dual direct injection engine

The natural gas (NG) stratified premixed combustion mode (NSPC) demonstrates significant potential for improving thermal efficiency and lowering emissions when compared to the NG mixing-limited combustion mode (NMLC). In this study, the NSPC for a NG/biodiesel dual direct injection engine was optimized through numerical simulation. The initial evaluation of biodiesel as the pilot fuel reveals its capacity to reduce soot and CO emissions while simultaneously increasing thermal efficiency in comparison with pure diesel. The subsequent examination delved into the effects of the injection timing of biodiesel and natural gas on mixture distribution, combustion, and emissions characteristics. The findings indicates that a well-balanced combination of moderate injection timing of natural gas and slightly delayed injection timing of biodiesel results in significantly lowered CO and soot emissions while maintaining high thermal efficiency, but with a trade-off of increased NOx emissions. The engine performance was further refined through the integrated application of exhaust gas recirculation (EGR) and pilot fuel pre-injection. Achieving a high thermal efficiency of 42.5%, minimal CO emissions at 1.5 g/kWh, soot emissions below 1 × 10−4 g/kWh, and NOx emissions less than 2 g/kWh, is achieved by configuring the start of injection for natural gas and biodiesel at −56°CA ATDC and −10°CA ATDC, respectively, with a biodiesel pre-injection ratio of 30%, and an EGR rate of 36%.