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Increasing pressure for reduced fuel consumption, particularly in Japan and Europe, has motivated the development of various engine concepts that improve part-load efficiency. One of these concepts is Gasoline Direct Injection, or GDI. By injecting fuel under high pressure directly into the combustion chamber later in the engine cycle and using various piston geometry and combustion chamber airflow techniques, a locally stratified, globally lean mixture can be used to obtain part load power with greater efficiency. Emissions concerns, however, particularly hydrocarbons from unburned mixture and liquid films on the piston and cylinder walls, are a major concern. The purpose of this project was to observe the interaction of a GDI fuel spray with the moving piston crown in an optical access engine, describe the interaction qualitatively, devise a means to measure the residual fuel film on the piston quantitatively, and compare the results with a Computational Fluid Dynamics (CFD) simulation. CFD was also used extensively to aid in the design of the experimental apparatus, particularly in selecting the spray and injection parameters that would allow stratified charge operation in the simple geometry available in the optical engine. The CFD simulations predicted that within the operating window for stratified charge operation, between 1% and 4% of the injected fuel would remain on the piston as a liquid film. The most influential parameter for the quantity of residual fuel film was the piston temperature. The experimental results support the CFD simulations qualitatively, but the amount of fuel film remaining on the piston is under-predicted. High-speed video footage shows a vigorous spray impingement on the piston crown resulting in vapor production. The primary GDI combustion process, a roughly premixed flame, is followed by a turbulent diffusion flame or "pool fire" in the region of the liquid film once the piston is warm enough to produce vapor at a high enough rate.

Piston Fuel Film Observations in an Optical Access GDI Engine