Studies were performed on the spray and combustion characteristics of an air-assisted fuel injector which was designed for application to 2-stroke direct injection engine. Firstly, 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. Effects from the nozzle deflector and injected air-fuel ratio(ALR) 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 $\sim 100 \mu{m}$ 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 TV imaging system. This imaging system has a capability to measure very small drops(calibrated down to 3.9$\mu$m in diameter) by developing a reliable calibration procedure and an image processing algorithm for automatic sizing and counting of the particle images. 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. SMD turned out to be 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 simultaneously measured 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. Secondly, visual study on the combustion of the spray from the air-assisted fuel injector was performed using a single-shot optical engine driven by a rapid compression and expansion machine(RCEM). In these study, the effects of ignition method, intake swirl ratio, spark timing, injection timing, and injected fuel and air mass were investigated. It was found out that three strike ignition method exhibited more stable ignitability compared with single strike one. The distortion of the spray toward the swirl direction was severe and the higher the swirl ratio, the faster the buming rate. The interval between the spark timing and injection timing determined the dispersion of the droplet cloud and so had a effect on the combustion. Therefore optimum timings were necessary for the best performance. The injected fuel and air mass also had a effect on the droplet dispersion by affecting the mean drop size of the spray. Lastly, computation of the spray from the air-assisted fuel injector was performed using KIVA-II code. In this study, the effects of the conditions in the poppet exit were investigated.
