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For reducing the exhaust emissions and improving the ignition characteristics, the effect of pilot injection was investigated experimentally in a turbocharged direct injection diesel engine. The pilot injection quantity was varied by changing the seat diameter of the Dodge plunger installed in the newly developed pilot injector while the separation period between the beginning of pilot injection and that of main injection was fixed at a short interval in the present experiment. The pilot injection effect on combustion was compared with the case of normal injection in two fuel oils with the cetane indexes of 55 and 40 respectively.

The effect of water in the suction air and in the emulsified fuel on NO formation was analyzed experimentally and theoretically from the point of view of changes in specific heat and the entrained air rate of the burned gas in the combustion chamber. The specific heat was calculated accurately through the chemical equilibrium composition analysis considering the absolute humidity of the suction air, the amount of water added in the emulsified fuel as well as the residual gas fraction. In order to estimate the NO formation rate, the burned gas temperature which is dominated by the excess air ratio and the specific heat of the burned gas was calculated by using the two-zone combustion model. The effect of absolute humidity of the suction air on NO formation is so large that a change in absolute humidity of 0.01 kg/kg results in about 20% reduction of NOx. This large reduction is based on only about 1% change in specific heat of the burned zone gas.