Study on the combustion and emission characteristics of an ethanol-gasoline fueled free piston engine generator based on optimized in-cylinder fuel-air mixing

To overcome the thermophysical challenges of ethanol-gasoline blends, such as low volatility and high latent heat of vaporization, in Direct-Injection Free Piston Engine Generator (FPEG), this study systematically investigates methods to improve in-cylinder mixture quality and subsequent engine performance. Using a numerical simulation method validated with experimental boundary conditions, this paper explores a twofold approach on an FPEG running E30 fuel: optimizing the “thermal environment” by controlling intake temperature and refining the “fuel delivery” through a two-stage injection strategy. The results indicate that increasing the intake temperature to an optimal 313 K significantly enhances mixture homogeneity and reduces soot, while temperatures above this yield diminishing returns. Compared to a single injection, the pre-injection strategy markedly improves the indicated thermal efficiency and drastically cuts soot emissions by promoting a more uniform premixed charge, though at the cost of slightly higher NOx ​emissions. Increasing the pre-injection ratio boosts peak pressure and thermal efficiency while monotonically decreasing soot, but it also elevates in-cylinder temperatures, leading to more NOx. The pre-main injection interval is identified as a critical parameter for balancing performance and emissions; a short interval (15° ECA) maximizes thermal efficiency, showing a 15.90% relative improvement over single injection, whereas a longer interval (30° ECA) is more effective for simultaneous soot and NOx reduction. These findings provide a crucial theoretical basis for the development of cleaner and more efficient FPEGs using alternative blended fuels.