Analysis of mixture formation process in a two-stroke boosted uniflow scavenged direct injection gasoline engine

Abstract

The 2-stroke engine has the great potential for aggressive engine downsizing and downspeeding because of its double firing frequency. For a given torque, it is characterized with a lower mean effective pressure and lower peak in-cylinder pressure than a 4-stroke counterpart. In order to explore the potential of 2-stroke cycle whilst avoiding the drawbacks of conventional ported 2-stroke engines, a novel 2-stroke Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine was proposed and designed. In order to achieve the stable lean-burn combustion in the BUSDIG engine, the mixture preparation, especially the fuel stratification around the spark plug, should be accurately controlled. As the angled intake scavenge ports produce strong swirl flow motion and complex transfer between the swirl and tumble flows in the 2-stroke BUSDIG engine, the interaction between the in-cylinder flow motions and the direct injection and its impact on the charge preparation in the BUSDIG engine are investigated in this study by three dimensional (3D) computational fluid dynamics (CFD) simulations. Both the single injection and split injections are applied and their impact on the mixture formation process is investigated. The start of injection (SOI) timing and split injection ratio are adjusted accordingly to optimize the charge preparation for each injection strategy. The results show that the strong interaction between the fuel injection and in-cylinder flow motions dominates the mixture preparation in the BUSDIG engine. Compared to the single injection, the split injection shows less impact on the large scale flow motions. Good fuel stratification around the spark plug was obtained by the late SOI timings at 300/320 °CA with the equal amount in each injection. However, when a higher tumble flow motion is produced by the 8 scavenge ports design, the better fuel charge stratification can be achieved with the later single injection at SOI of 320 °CA.