Significant advancements have been made in recent years in the development of combustion system for spark-ignition direct-injection engine (SIDI) engine, which have resulted in fuel economy saving, low exhaust emission and a significant power advantage under homogeneous fuel operation, compared to equivalent PFI (Port Fuel Injection) engines. Key challenge for small-displacement SIDI engine, which has short path lengths between the injector and piston and is therefore prone to increase wall wetting, is minimizing or eliminating the amount of wall wetting to reduce smoke emission. A side-injection system also requires sufficient spray penetration to fully transport fuel to the centrally mounted spark plug at the desired injection timing event. The previous paper “Experimental Study of Smoke Emission on Small-Displacement Spark-Ignition Direct-Injection Engine (SAE Paper No.
2006-32-0105)” is a part of an investigation to reduce smoke emissions through experimental study including visualization work evaluating injector spray pattern, injection timing and spark timing. It was found that the smoke formation is related to the wall wetting amount at the start of combustion. The wall wetting amount at the start of combustion depends mainly on injection timing, spray drop size, piston temperature and spark timing. Thus, in this study, three types of injector are discussed to clarify the impact of the fuel spray characteristics on smoke emissions: swirl injector, fan (slit) injector and multi-hole injector. The multi-hole injector can reduce smoke emissions because of enhanced vaporization, resulting in a shortened liquid length. This is attributable to increased air entrainment available because the spray does not collapse under elevated cylinder pressures typical of late injection.