Spray shape and atomization quality of an outward-opening piezo gasoline di injector
Artikel i vetenskaplig tidskrift, 2007
The spray formation and consequent atomization of an outward opening piezo-electric gasoline DI injector have been experimentally investigated in a constant pressure spray chamber. The sizes and velocities of the droplets and the resulting spray shape were evaluated, under different boundary conditions, using Planar Mie scattering and Planar Laser-induced Fluorescence (PLIF) in combination with Phase Doppler Anemometry (PDA) analyses and high-speed video photography. The use of piezo-electric actuation for gasoline DI injectors provides an additional means to control the atomization and spray shape that is not available with solenoid-driven injectors such as swirling and multi-hole type injectors. For instance, with piezo injectors up to four injections per cycle are possible, and the fuel flow rate can be controlled by adjusting needle lift. The captured high-speed video images show that a hollow-cone spray forms as the fuel exits the outward-opening nozzle. Shortly after the start of injection, the momentum exchange with the surrounding air creates a recirculation zone at the leading edge of the spray. The images also show that the position and size of this recirculation zone depends on the chamber back pressure and that it too can be controlled by the injection timing. In addition, the PDA analysis shows that the creation of the recirculation zone, with a fuel pressure of 20 MPa, results in good atomization, slow-moving droplets, and helps guide fuel to the spark plug in stratified charge operation. The effects of flash boiling on the injector's sprays were also investigated. The results indicate that flash boiling affects the spray shape less than when multi-hole injectors are used. Furthermore, it has been shown that the use of multiple injections per cycle can decrease the spray penetration and provide an extended window of ignitable mixture at the spark plug.