Experimental study of the impact of split injection strategy on combustion and thermal performance in an ultra-lean methanol free-piston engine generator

This study is the first to systematically investigate double injection strategy in a methanol-fueled, direct-injection Free-Piston Engine Generator (FPEG), aiming to improve the indicated thermal efficiency and combustion stability under ultra-lean conditions. A novel double injection strategy based on post-spark secondary injection is proposed and experimentally validated. The experimental includes three injection ratios, seven injection intervals, and a single injection baseline, totaling 22 test conditions. The effects of injection ratio and timing on combustion characteristics and performance are systematically investigated. Results show that under ultra-lean conditions (φ = 0.47), increasing the proportion of the second injection effectively enhances combustion intensity, improves indicated thermal efficiency, and reduces COV_IMEP. The post-spark double injection strategy enables efficient coupling between the second-stage fuel and the primary flame, significantly reinforcing the late-stage combustion. Among all tested conditions, D2I6 exhibits the best performance, with a 10.16 % increase in HRRmax, a 5.97 % improvement in indicated thermal efficiency, and a 21.88 % reduction in COV_IMEP compared to the single injection case. In contrast, the pre-spark double injection strategy facilitates the formation of a fuel-rich kernel near the spark plug, accelerating early flame development and shortening ECA0–10 and ECA10–90. The D3I2 condition achieves the best performance in this category, with an 8.13 % increase in peak heat release rate, an 8.21 % enhancement in indicated thermal efficiency, and a 25.82 % reduction in COV_IMEP.