Experimental and Simulation Study on Reducing the Liquid Film and Improving the Performance of a Carbon-Neutral Methanol Engine

Methanol is a potential carbon-neutral fuel. It has a high latent heat of vaporization, making it difficult to achieve evaporation and mixing, and it is prone to forming a liquid film, which in turn affects engine performance. To reduce the liquid film and improve engine performance, this work investigates the influence mechanism of injection strategies on the generation of liquid films in the intake port and cylinder of an inline 6-cylinder port fuel injection (PFI) spark-ignition (SI) methanol engine and further explores the optimization scheme for improving engine performance. The results show that the end of injection (EOI) influences the methanol evaporation rate and the methanol–air mixing process, thereby determining the liquid film deposition, mixture distribution, and temperature distribution in the cylinder. As the EOI advances, the higher methanol evaporation rate during the intake process reduces the amount of methanol droplets and the deposition of a liquid film in the cylinder. The in-cylinder temperature is relatively high, while the mixture inhomogeneity slightly increases. As the EOI increases from 170 °CA to 360 °CA, the higher in-cylinder temperature and properly stratified mixture accelerate the early and middle stages of combustion, shorten the ignition delay, advance the center of combustion, and improve the brake thermal efficiency (BTE). However, further advancing the EOI results in the BTE remaining basically unchanged. Optimized injection timing can enhance the BTE by 1.4% to 2.4% under various load conditions. The increase in the EOI contributes to the reduction of HC emissions due to the weakening of the crevice effect with lower masses of methanol droplets and liquid film in the cylinder, while the increase in mixture inhomogeneity leads to an increase in CO emissions. In general, controlling the EOI at around 360 °CA can maintain relatively low CO emissions under various load conditions, while significantly reducing HC emissions by 71.2–76.4% and improving the BTE.