Nozzle Tip Wetting in GDI Injector at Flash-boiling Conditions

The flash-boiling spray has become a timely topic of seeking the high mixture quality and clean combustion of the gasoline-direction-injection (GDI) engines. The flash-boiling spray, which has the enlarged individual spray cone angles and enhanced plume-plume interactions, are likely to favor the fuel-film formation, otherwise known as the nozzle tip wetting. Recently, the nozzle tip wetting has caused widespread concern because it is considered to be an essential source of particle emissions in GDI engines. In this study, an experimental investigation was proposed to clarify the possible influence of flash-boiling spray on nozzle tip wetting by taking advantage of the X-ray phase-contrast imaging (XPCI) technique. Due to the high brilliance, high energy, and low emittance of X-ray, the fuel flow inside the metal nozzle and spray characteristics at the nozzle exit was observed simultaneously, and results were discussed quantitatively to understand the underlying mechanism. It is found that the post-injected fuel caused by the needle bouncing is one primary source that results in tip wetting at non-flash-boiling conditions. Rising fuel temperature at non-flash-boiling conditions would not significantly affect spray dynamics and tip wetting. At flash-boiling conditions, tip wetting rises quickly during fuel injection, and a maximum appears under transitional flash-boiling conditions. This phenomenon can be understood by the enhanced droplet collision against the nozzle wall and boosted droplet evaporation under flash-boiling.