Effects of different combustion modes on the thermal efficiency and emissions of a diesel pilot-ignited natural gas engine under low-medium loads

Research on dual-fuel (DF) engines has become increasingly important as engine manufacturers seek to reduce carbon dioxide emissions. There are significant advantages of using diesel pilot-ignited natural gas engines as DF engines. However, different combustion modes exist due to variations in the formation of the mixture. This research used a simulation model and numerical simulations to explore the combustion characteristics of high-pressure direct injection (HPDI), partially premixed compression ignition (PPCI), and double pilot injection premixed compression ignition (DPPCI) combustion modes under a low-medium load. The results revealed that the DPPCI combustion mode provides higher gross indicated thermal efficiency and more acceptable total hydrocarbon (THC) emission levels than the other modes. Due to its relatively good performance, an experimental study was conducted on the DPPCI mode engine to evaluate the impact of the diesel dual-injection strategy on the combustion process. In the DPPCI mode, a delay in the second pilot ignition injection time increased THC emissions (a maximum value of 4.27g/(kW center dot h)), decreased the emission of nitrogen oxides (a maximum value of 7.64 g/(kW center dot h)), increased and then subsequently decreased the gross indicated thermal efficiency values, which reached 50.4% under low-medium loads.