Dynamic structure of direct-injection gasoline engine sprays: air flow and density effects

The characteristics of the hollow-cane fuel spray produced by a centrally- located pressure-swirl atomizer have been investigated in a constant-volume pressure chamber and in a motored single-cylinder direct-injection spark-ignition (DISI) research engine. The aim is to describe the effect of elevated chamber pressure and in-cylinder bulk air motion on the spray development process. In addition to ambient conditions, chamber pressures of 5 and 7 bar absolute, corresponding to air densities of 5.82 and 8.14kg/m3, respectively, at atmospheric temperature (300 ?), were investigated as these conditions are representative of the range of in-cylinder pressure and densities corresponding to "early" through 'late" injection strategies. Spray dynamics in a constant-volume chamber, under near-quiescent flow conditions, are shown to be representative of in-cylinder sprays and therefore provide appropriate means for evaluating the relative effect of the intake air flow and in-cylinder density on the fuel spray development process. A wide range of operating conditions of a four-valve DISI engine with a centrally located pressure-swirl atomizer were examined at engine speeds of 700 and 1500 rpm and for four start-of-injection (SOI) timings of 90', 180', 270', and 300' after top-dead-center (aTDC) of intake. The results revealed that fuel spray impingement on the flat piston occurred only with injection at 300' ATDC, and that larger droplets are produced by pressure-swirl atomizers operating at higher gas pressures which suggests that achieving consistent late injection strategies for low-load operation with spray-guided systems using such injector designs may be an insurmountable problem