This paper presents a study on the effect of injection rate on the performance and emissions of a direct-injection gasoline engine operated with seven kinds of DI injection and a port-injection. The injection pressures were set at 0.3 and 7 MPa for port injection and DI operation modes respectively. The spray characteristics of DI injectors were obtained by measuring the SMD, droplet concentration and spray angle. The engine was run at a constant speed and load of 1000 rpm and 0.22 MPa of BMEP. The BSFC reduction of DI from port injection was 13% at most in which 4% was derived from the decrease in pumping loss and the remaining 9% was due to the improvement of the cycle efficiency. As for the injection rate effect in DI operation, the BSFC takes the minimum at a value of the injection rate where the maximum difference reached to 7%. It was revealed that this is due to the variation of the combustion efficiency and then corresponds to the behavior of droplet concentration. Soot emission correlated well with the SMD. Thus the performance and emissions were mostly explained by the spray characteristics.

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