Effects of injection timing on combustion and mixture formation of a methanol direct injection engine equipped with a small volume passive pre-chamber

Abstract

The pursuit of high efficiency and cleanliness has always been the focus of engine research. As one of the most promising low-carbon renewable alternative fuel, methanol has gained increasing attention in engine applications. Inspired by the advantages of gasoline direct injection (GDI) in fuel economy, methanol direct injection (MDI) engine has also become a research hot spot. Similar to GDI, the mixture distribution is crucial for MDI engines, especially in the late injection stratified operation mode, an unreasonable mixture distribution of rich and lean regions will result in slow and incomplete combustion, or even flame quenching. Due to the characteristics of multi-point ignition and the high ignition energy, pre-chamber jet flame ignition has the potential to effectively enhance combustion. In this paper, the effect of injection timing on combustion characteristics in a methanol direct injection engine equipped with a small volume pre-chamber are studied experimentally. The mixture formation processes in both the main-chamber and the pre-chamber at different injection timings were also explored through simulations. The research findings indicated that the engine exhibits optimal power performance at the injection timing of − 270 °CA ATDC (after top dead center), primarily attributed to the enhanced volumetric efficiency and improved mixture homogeneity. However, the mixture concentration in the pre-chamber is leaner, resulting in the longest ignition delay period. When the injection timing is located on the middle state of the intake stroke, specifically − 210 °CA ATDC, the ignition delay period is the shortest, indicating that the mixture concentration and temperature in the pre-chamber are conducive to the flame kernel formation and flame propagation. In addition, the volumetric efficiency of this injection case is maximized due to the combined effects of spray-wall interaction, spray-air interaction, and the flow field. When injection occurs on compression stroke, the mixture concentration in the pre-chamber is higher; however, there exists a significant and discontinuous concentration gradient in the main-chamber, which limits the enhancement of the flame propagation speed and the combustion efficiency, especially for − 120 °CA ATDC injection case, the engine exhibits longest combustion duration and the lowest IMEP. The conclusions can provide theoretical basis and empirical data support for enhancing the combustion of methanol direct injection engines equipped with passive pre-chambers.