The Role of Cavitation Inside High Pressure GDI Injector

Today, improvement of engine efficiency and reduction of exhaust emission are mandatory for internal combustion engine designers. Direct fuel injection systems are one of the main instruments to reach these goals but, in general, they need a very fine tuning to obtain the best match between fuel injection system characteristics and engine fluid dynamic behaviour. About gasoline direct injection engines, many experimental works on direct fuel injection systems have provided evidence of cavitation formation inside the injectors under typical operating conditions. The presence of cavitation inside injector nozzles could be associated with both positive and negative aspects. Experimental evidences have underlined as cavitating structures have a beneficial effect on spray characteristics in terms of droplet break-up and air/fuel mixture formation and a detrimental effect on surface erosion inside injectors. In particular, surface erosion is due to the collapse of vapour bubble cluster near the internal injector walls: when it happens, pressure waves are generated and a very severe instant load on the internal injector surfaces could be recorded. For these reasons, today the analysis of cavitation evolution inside injectors remains one of the key point to address the designers toward the optimization of geometry nozzle configuration in terms of spray pattern and characteristics, and injector durability. This paper presents a detailed Computational Fluid Dynamic (CFD) analysis of the flow conditions inside a real Gasoline Direct Injector (GDI) under real operating conditions. The aim of these analysis was the evaluation of cavitation evolution inside the injector and, in particular, inside each injector nozzle in order to underline their different fluid dynamic behaviour. Moreover, an evaluation about cavitation erosion risk was performed by the definition of specific parameters. All the CFD simulations were ran considering a three phases flow: liquid fuel, fuel vapour generated by cavitation, and air. In particular, air was considered in two ways: as the fluid filling the injection ambient and as dispersed phase inside the continuous liquid phase. From the cavitation damaging point of view, the influence of dispersed air plays a fundamental role because in some cases the presence of a dispersed compressible gas (air) inside the main liquid phase could promote a sort of “protection” from mechanic cavitation erosion.