SPRAY CHARACTERISTICS OF AN AIR-ASSISTED FUEL INJECTOR FOR 2-STROKE DIRECT-INJECTION GASOLINE-ENGINES

Abstract

An air-assisted fuel injector was constructed for application to a two-stroke direct-injection engine, and experimental studies were performed to investigate the operating and spray characteristics of this injector. Schlieren photography was employed to observe the shape and penetration of the spray. The effects of the nozzle deflector and injected air-fuel ratio (ALR) on spray characteristics were studied. In addition, we investigated whether there were large droplets resulting from fuel dripping at the beginning and end of injection. Except for an small ALR range, very few big droplets over similar to 100 mu m in diameter were produced. Also, fuel was ejected from the poppet exit not in the form of ligaments or sheets but rather in the form of finely atomized droplets. The sizes and velocities of droplets were measured by a TY imaging system. For an ALR below 1, the mean drop size increased rapidly with decreasing ALR. However, when it was greater than 2, the reduction in mean drop size was less noticeable with increasing ALR. Sauter mean diameter (SMD) turned out to he under 12 mu m for an ALR greater than 1. In addition, the size distribution of droplets showed almost linear characteristics when displayed in square-root/normal probability plots. The MMD/SMD ratio was shown to scatter between 1.1 and 1.2. Velocities and sizes of droplets were measured simultaneously and compared with the criteria of the bag breakup model and boundary-layer stripping model. Since droplet velocities were too low relative to droplet sizes, it turned out that there was almost no possibility for secondary breakup.