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Of late direct injection engines are replacing carbureted and port-injected engines due to their high thermal efficiency and fuel economy. One of the reasons for the increased fuel economy is the ultra-lean mixture with which the engine operates under low loads. Under the low load conditions, the air-fuel ratio of the mixture near the spark plug is near stoichiometric values while the overall mixture is lean, which is called stratified mixture. In order to achieve this, proper air motion during the late stages of compression is a must. Quality of the mixture depends on the time of injection as well as the type of fuel injector and mixture preparation concept used. Engines employing air-guided mixture preparation are considered as the second-generation engines. The air-guided mixture preparation method uses either swirl or tumble in preparing the mixture.

This Paper presents the results of experimental work conducted on a four-stroke spark ignited engine to reduce the emissions and to enhance the performance using semi direct electronic fuel injection. A comparative study was carried out with the base engine for finding out the effect of semi direct injection. The injector is mounted on the inlet manifold in such a way that the fuel is injected on the back of the inlet valve. The in-cylinder pressures were recorded for 1000 continuous cycles using a piezo-electric pressure pickup and PC based data acquisition system. The results show that the stratified mixture formation achieved by semi-direct injection results in improved engine performance and reduced emission & cycle-to-cycle variations.