Split Injection has a potential to reduce simulataneously both NOx and soot emissions from a D.I. Diesel engine. But the mechanisms of simultaneous reduction of both NOx and soot emissions have not been clarified. To figure out the mechanism of change in emissions characteristics of the D.I. Diesel engine with the Split injection, an experimental study was conducted by using electrically controlled common rail injection system and a high pressure bomb. One of the sprays from a multi hole nozzle was injected into a two-dimensional model combustion chamber. The spray was observed by schliren photography with a high-speed video camera. In this report, effects of an injection mass ratio between the 1st and 2nd injection stages are described. The spray for the 2nd injection has larger tip penetration thanthose of the 1st or single injection spray. Especially, when the injection mass ratio is 50-50% and the injection interval is the shortest, the 2nd spray gains the largest spray tip velocity.
