Evaluation of GDI spray structures under flash boiling and strong collapse conditions with diverse fuels

Abstract

GDI (Gasoline Direct Injection) sprays are a matter of continuous research due to their potential for higher thermal efficiency and power output. Injections in GDI engines tend to happen at relatively high temperatures, which, combined with volatile fuels, are a perfect environment for flashing sprays, which are present in recently developed GDI engine passenger cars. Phenomena like flash boiling and especially spray collapse are still being investigated due to their complexity and impact on the engine performance. In the present research, the impact of different levels of flash boiling in the sprays is analyzed. Four surrogate mono-component fuels were tested with different volatile properties. Two optical techniques were carried out simultaneously: light extinction images (DBI) and frontal MIE scattering. The spray width was used to represent the radial expansion and study its relation with the spray collapse due to flash boiling. The degree of collapse was quantified by means of the instantaneous spray width variation for which three levels of collapse were identified. A novel MIE frontal approach was employed to study the spray surface evolution and the collapsing time by monitoring the nozzle tip clouding (opaquing) by the fuel. Results showed an increment of the width with the ratio of saturation to ambient pressure (Rp), as well as a strong correlation between the spray surface and the Rp at each fuel temperature when the spray is stabilized. Based on the MIE technique, three collapse instants were identified throughout the injection time depending on the Rp.