Supercharging as the method of increasing the output of diesel engines has a long history. Recently, because the potential for lower exhaust emissions for a given power output, supercharging has been considered as a method to reach increasingly strict emissions standards. Some research investigating the effects of supercharging has shown favorable results in terms of emissions(e.g. *). Also some fundamental studies have examined the effect of ambient pressures on the characteristics of spray and ignition in constant volume combustion borb. However, for further improvement of combustion when utilizing supercharging, more detailed information inside of the combustion chamber is needed about the effects of supercharging on fuel spray and combustion. In order to gather this information, it is necessary to observe the processes within the combustion chamber of a supercharged engine. From these observations, an analysis can be made based on both the real processes and the changes in engine perform” and emissions.Optical access to the Diesel combustion chamber has been achieved since the mid 1930's. High speed direct combustion photography and shadowgraph, when applied to Diesel engines, can give insight into spray penetration and breakup, ignition phenomena, local A/F ratios, and flame spreading rates. More detailed information such as flame motion and flame temperature can also be available by means of the cross correlation method and the two-color method.In an effort to gain insight on the effects of supercharging on fuel spray and combustion, a study has been performed on an optically accessible DI diesel engine. Through the use of a combustion chamber visualization system and photograph processing; changes in fuel spray, ignition and combustion due to supercharging have been revealed. Flame motion and temperature have been measured for a range of boost pressures while other parameters were held constant.Based on the results of combustion analysis, the effects of supercharging on the combustion and the methods related to combustion improvement have been discussed. In addition, the changes of combustion characteristics were shown to correlate well with charges in engine performance and emissions.