Experimental study on the effects of two-stage direct water injection in-cylinder on combustion characteristics of high compression ratio lean-burn engine

Addressing the global energy crisis and environmental challenges, the development of efficient and clean internal combustion engine technology has become a critical research focus. A two-stage direct water injection in-cylinder (DWI) strategy was proposed to address the issues of knocking and combustion stability in high compression ratio (CR = 17) lean burn gasoline engines. The effects of no injection, single and two-stage DWI on the engine under stoichiometric (λ = 1.0) and lean-burn (λ = 1.4) conditions were also investigated experimentally. The experimental results show that under stoichiometric combustion conditions, the two-stage water injection can significantly suppress knocking by optimizing the second-stage water injection timing (60°CA BTDC), reducing the knocking intensity (KI) to 0.17 bar. Under lean-burn, the single water injection strategy demonstrates a better knocking suppression effect. The two-stage water injection strategy effectively controls the coefficient of variation of indicated mean effective pressure (COV_IMEP) below 2.57%, which is 0.4% lower than the single water injection strategy. In terms of thermal efficiency, the lean-burn combined with the water injection strategy increases the indicated thermal efficiency to 43.53%. Regarding emission characteristics, CO₂ emissions are mainly dominated by the λ value, and the water injection strategy has a minor impact. The water injection technology can significantly reduce NOx emissions. Under stoichiometric combustion conditions, two-stage water injection reduces NOx emissions by 45.4% (to 1305.87 ppm), and under lean-burn conditions, it further reduces them to 454.17 ppm. Notably, single water injection shows the best NOx control effect under lean-burn conditions, with the emission maintained at around 300 ppm.