An experimental investigation was performed on a small direct injection (DI) diesel engine equipped with a common-rail injection system to reduce exhaust emissions through HCCI (homogenous charge compression ignition) combustion. Recently, strict environmental standard requirements call for both lower fuel consumption and reduced emissions that could not be achieved by conventional diesel combustion. In this work experimental investigations to achieve simultaneous reduction of NOx and soot by combustion of more diluted fuel/air mixture before the start of ignition were carried out. To realize this fundamental concept, the experimental conditions including injection timing and EGR rate are varied with the different engine configurations. For reducing the deposition of early injected fuel, spray angle of injector is reduced to 60° and piston head shape also modified to fit with the new injector and to reduce the compression ratio to 15:1 for expanding the ignition delay to form diluted mixture before the ignition. Experimental results show that reduced spray angle with modified piston head allow very low NOx and soot emission level while maintaining the high IMEP of diesel combustion.

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