Insight on Early Spray Formation Process of a High-Pressure Swirl Injector for DISI Engines 2003-01-1809

An early formation process of the spray, which was injected by a high-pressure swirl-type injector that is widely used in direct injection spark ignition (DISI) gasoline engines, was investigated through image analyzing techniques. The sprays were illuminated both by an Nd:YAG laser light sheet for getting the spray tomograms and by a tungsten lamp for getting the scattered back light shadow images of the sprays. The sprays were imaged by using a high-resolution CCD camera and a high-speed digital imaging system. The early development aspects of the spray were investigated in detail through the measurement of the tip penetration, cone angle and width of the early spray. At the start of injection, the liquid column emerges first, and it forms the “pre-swirl spray” without the swirl component. Following the liquid column, the liquid sheet emerges, however its radial velocity component is weak to form the complete hollow-cone spray. This spray changes into the “weak-swirl spray”. Consecutively, the liquid sheet of which the radial velocity component is enough to form the hollow-cone spray emerges, and it forms the “main-swirl spray”. During the early injection period, the initial cone angle of the spray changes dramatically with time from the start of injection (SOI). With emerging the liquid sheet, the initial cone angle increases steeply due to the liquid sheet expanding. After a short time (0.19 ms from SOI in this study), the spray cone angle remains constant with time from SOI, regardless of ambient pressure. The main body angle decreases with time due to the pressure difference between the inner and the outer region of the spray, and it remains constant after 0.8 ms from SOI. Based on the evolution of the spray cone angle with time, the spray development stages are classified into the following five stages: (I) liquid column emerging stage; (II) liquid column expanding stage; (III) liquid sheet emerging and expanding stage; (IV) development of three types of sprays stage; and (V) fully developed hollow-cone spray stage.