At gasoline direct injection, light-duty engines operated with homogeneous, stoichiometric combustion mode, particulate emissions are mainly formed in diffusion flames that result from prior fuel wall wetting. Besides the piston, liner, and intake valves, the injector tip acts as a main particulate source when fuel is adhered to it during an injection. Hence, this injector tip fuel wetting process and influences on this process need to be analyzed and understood to reduce engine-out particulate emissions.
The present work analyzes the injector tip wetting process in an experimental way with a high-speed and high-resolution measurement system at an optically accessible pressure chamber. The performed measurements reveal that injector tip wetting can occur during the complete injection event by different mechanisms. Large spray cone angles at start and at end of injection or distortions of the spray result in direct contact of the fuel spray with the step-hole wall. Additionally, fuel accumulates during an injection in the step-hole volume and discharges onto the injector tip surface subsequently. Furthermore, a poor primary breakup at end of injection can lead to an additional fuel wetting amount.
It is shown that the occurrence and the extent of these mechanisms and the fuel wetting amount highly depend on the boundary conditions during injection. Variations of the fuel temperature, the encountered ambient pressure, and the fuel pressure are performed to point out the influences of these boundary conditions. In addition, injector design parameter show an impact on the fuel wetting process and the fuel wetting amount. This is demonstrated by a variation of the step-hole geometry and by an increase of the total needle lift.
