ID 231

Experimental characterization of tip wetting in GDI injectors

 

Abstract:

In gasoline direct injection engines with homogeneous and stoichiometric combustion strategies, particulate emissions are almost exclusively caused by liquid fuel attached to combustion chambers walls. When the flame reaches these films, high temperatures and lack of oxygen can lead to formation of soot deposits on the affected surfaces. Fortunately, wetting of piston, spark plug, cylinder liner and intake valves can be minimized by adjusting the injection timing and strategy and optimizing the targeting of the spray.

Injector tip wetting – i.e. liquid fuel remaining on the tip of the injector after the end of injection – is a further source of particulate emissions. The underlying fluid dynamics are poorly understood and, while it is known that changes in injector design can lead to significant improvements in particulate emissions, clear correlations have only been found for some injector design parameters.

In order to improve understanding of the relevant mechanisms and to develop models that can be implemented in 3D CFD codes, a comprehensive and detailed experimental database is needed. To this end, tip wetting is visualized using high-speed cameras and quantified using laser-induced fluorescence. For the latter experiments, results obtained with commercial gasoline and different combinations of surrogate fuels and tracers are compared. The clearest results have been obtained with the tracer 1-methylnaphthalene. With regards to operating conditions, increased injection pressure is consistently found to decrease tip wetting and particulate emissions and tip wetting is most pronounced when flash-boiling of the spray occurs.