SPRAY STRUCTURE FROM DOUBLE FUEL INJECTION IN MULTIHOLE INJECTORS FOR GASOLINE DIRECT-INJECTION ENGINES

Abstract

The spray characteristics of double fuel injection in multihole injectors for direct-injection gasoline engines have been evaluated in a constant-volume chamber using iso-octane as fuel. Measurements of droplets' mean and rms velocity and their diameters were obtained using a 2D phase Doppler anemometer (PDA) at injection pressures up to 120 bar, atmospheric chamber pressures, and ambient temperatures up to 115° C Complementary spray visualization made use of a pulsed light, a CCD camera, and a high-speed video camera synchronized with the injection process. Spray images with double injection revealed that there are delay times in needle opening and closing on the order of 0.6 ms and 0.3 ms, respectively, and that dwell times less than 0.5 ms introduced a prespray prior to the second injection event, while for dwell times more than 0.5 ms, the images showed that the overall jet-spray structure in the cases of the first and second injection event remained the same as those of single injection. PDA results near the injector exit quantified the relatively strong prespray at 0.3 ms dwell time and revealed that its strength reduced with increasing dwell time so that at 1 ms there was no sign of the prespray. The effect of ambient chamber temperature on the spray velocity for double injection was found to be significant at temperatures above 90° C; results at higher temperatures, 115° C, showed a consistent reduction in the mean droplets' velocity up to 10% within the core of the first and second sprays, with a corresponding increase in the rms velocity fluctuations due to the increased vaporization rates and the droplets' momentum loss. The droplet size distribution of the double-injection spray was, in general, similar to that of single injection except during the dwell time when the droplets arithmetic mean diameter with prespray was found to be smaller than those without the prespray because of the different type of droplets and the much higher data rates in the case of the prespray.