Effects of injection strategy on combustion and emission of multiple low-carbon fuel compound injection engine

With the increasingly stringent global carbon emission control, low-carbon, zero-carbon and flexible fuels have become important in the future development of Internal Combustion Engines (ICEs). In order to make comprehensive use of the advantages of each fuel and avoid the disadvantages, this study proposes a multi-fuel engine that comprehensively utilizes gasoline, hydrogen and natural gas. In order to make comprehensive use of the advantages of different low carbon fuels, improve the fuel flexibility, as well as achieve good performance by regulating the combustion process with fuel blending, this study investigates a multi-fuel spark-ignition engine combining gasoline port fuel injection (PFI) with direct injection (DI) of hydrogen–natural gas blends. The results show that the direct injection strategy can greatly affect the stratification state of gaseous fuel in the cylinder and then improve the engine performance. Through integrated experimental and computational fluid dynamics (CFD) analysis at 1200 r⋅[jls-end-space/]min−1, 40% load and 20% gaseous fuel blending ratio, it is demonstrated that optimal combustion occurs at 120 °CA BTDC DI timing, where gaseous fuel stratification achieves localized richness near the spark plug while maintaining global homogeneity. Within the optimal injection timing, the combustion rate and cylinder peak-pressure increase, and both the torque and Brake Thermal Efficiency (BTE) are improved as well. As hydrogen blending in the gaseous fuel rises from 0% to 100%, maximum cylinder pressure increases by 18.65%, peak heat release rate rises by 23.01%, while torque and BTE gain 5.75% and 1.30%, respectively. This stems from hydrogen’s high reactivity, whose rapid combustion speed and flame temperature accelerate overall combustion, advancing the crank angle of peak cylinder pressure and increasing the maximum pressure rise rate, effectively shortening combustion duration. Hydrogen enrichment creates predictable emission shifts that NOx increases by 28.09% due to higher combustion temperatures promoting thermal NOx formation, while CO and HC emissions decrease through hydrogen’s high-temperature oxidation effects and reduced quenching distance.