The influence of flash-boiling on spray-targeting and fuel film formation

The variety of fuels used in GDI engines is constantly increasing. This affects the combustion process because the spray formation and the mixture preparation is greatly influenced by the fuel properties. In particular, the reduction of the saturation temperature is promoting the so-called flash-boiling effect, when the bulk liquid or a component of the fuel mixture becomes superheated during the injection into the low-pressure combustion chamber.This study reveals the enormous influence of flash-boiling on the spray/wall interaction and the consequent deposition of fuel wall films. This is of major importance because the formation of wall films must be avoided, since they are the main route to soot particle emissions.In order to increase the understanding of the wall film formation the spray/wall interaction is investigated under conditions representative of a homogeneously charged gasoline engine. Gasoline and iso-octane are injected using a modern six-hole nozzle. Emphasis is placed on the influence of the initial fuel temperature. Using high-speed imaging and infrared thermography, the spray propagation, the spray-targeting and the evaporation duration of the wall films are analysed.It is found that increasing the fuel temperature does not lead to a constant reduction of the wall film problem. Rather, the flash-boiling-induced 'contraction' of the spray leads to an accumulation of the wall film mass in a small area. This delays the evaporation process to a great extent and can lead to increased soot particle emissions. From the results, a critical value of the degree of superheat is identified and the findings transferred to various single component fuels, in order to avoid undesired fuel film accumulation.