Investigation of Fuel Injector Tip Wetting Using Laser-induced Fluorescence under Flash Boiling Conditions

Abstract

Tip wetting due to the needle descent and the rapid pressure drop in the nozzle has been identified as an important problem in gasoline direct injection (GDI) engines. The deposited film cannot fully evaporate before the onset of combustion and when the flame reaches the injector, high temperature and the lack of oxygen will lead to significant particulate emissions. Attempts have been made to reduce the tip wetting phenomenon by re-designing the nozzle geometry parameters. However, existing research is still limited to indirect measurements by analyzing the relationship between PN emissions and operating conditions and optical methods visualizing the results are fairly limited. Besides, flash boiling sprays have been considered a promising technology for GDI-related applications, but their performance on tip wetting remains controversial. Although it is expected that improvements in vaporization could reduce the deposition, research also shows that the spray-tip interaction could be enhanced by excessive plume width. To clarify the underlying mechanism, investigations of tip wetting phenomena under flash boiling conditions were conducted in a constant volume chamber by controlling the ambient pressure and fuel temperature. Laser-induced fluorescence (LIF) method was adopted and 266nm Nd: YAG laser was used to excite the commercial gasoline. The intensified CCD camera equipped with a microscope lens was applied to capture the fluorescence. The characteristics of tip wetting (film area and time-resolved evolution) were thoroughly analyzed, and the influence of flash boiling on tip wetting was evaluated.