Effects of Ultra-High Injection Pressure and Flash Boiling Onset on GDI Sprays Morphology

Ultra-high injection pressures, as well as flash-boiling occurrence, are among the most important research fields recently explored for improving Gasoline Direct Injection (GDI) engine performance. Both of them play a key role in the enhancement of the air/fuel mixing process, in the reduction of tailpipe pollutant emissions, as well as in the investigation of new combustion concepts. Injector manufacturers are even more producing devices with ultra-high injection pressures capable of working with flashing sprays. Flash-boiling of fuel sprays occurs when a super-heated fuel is discharged into an environment whose pressure is lower than the saturation pressure of the fuel and can dramatically alter spray formation due to complex two-phase flow effects and rapid droplet vaporization. In GDI engines, typically, it occurs during the injection process when high fuel temperatures make its saturation pressures higher than the in-cylinder one. Flash boiling significantly affects the spray structure and fuel-air mixture formation with, potentially, if spray collapse is avoided (with the consequent risk of spray-wall impingement), positive consequences for the engine performance and pollutant emissions. So, this work proposes a combined experimental and numerical characterization of the spray issued by a multi-hole device to highlight the combined role of the injection pressure (up to 700 bar) and the flash boiling occurrence on the spray morphology. Experimental observations of the spray were performed using the Mie-scattering technique. Collected data allowed to evaluate the capabilities of an Eulerian-Lagrangian code in reproducing the injection processes. CFD models for flashing and non-flashing conditions were developed featuring specific breakup and vaporization models suitable for flashing sprays. The numerical model achieves a pretty good level of agreement with the experimental data, and, in particular, it highlights the importance of injection pressure in avoiding spray collapse.