Time-Resolved Fuel Film Thickness Measurement for Direct Injection SI Engines Using Refractive Index Matching 2011-01-1215
The fuel film thickness resulting from fuel spray impingement on a flat transparent window was characterized in a high pressure high temperature cell for various thermodynamic conditions, injection pressures, injection durations, fuel types and injector technologies by Refractive Index Matching technique. The ambient conditions at injection timing were similar to that of a direct injection spark ignition engine at Top Dead Center, with the distance between the injector tip and the impinging surface set to 10 mm. The spray axis was set normal to the rough transparent window surface at ambient temperatures of 453 K, 573 K and 673 K, and ambient densities of 5.0 kg/m₃, 6.0 kg/m₃ and 6.5 kg/m₃. Injection pressures of 100 and 200 bar were investigated. Three injector technologies were studied: piezo-electric, multi-holes and swirl types. Two fuels, iso-octane and model gasoline, were tested. The choice of the transparent window surface roughness and the development of an optimized calibration methodology allowed submicron measurements of the fuel film thickness. Spatial distributions as well as time-resolved evolution of the fuel film thickness were obtained during the evaporation process. The results show that the fuel film thickness strongly depends on the ambient temperature, but is not significantly affected by the ambient density in the operating range investigated. The injection pressure and the injection duration were found to have a significant effect. Depending on the conditions, different fuel film structures were observed on the piston surface, ranging from discrete fuel pockets to continuous fuel film. Additional fuel film visualizations by Laser-Induced Fluorescence were compared qualitatively with the RIM images. The RIM images were consistent with the LIF images in terms of film structure and evaporation dynamic confirming the results. Two kinds of evaporation processes were identified, in particular when using different types of injector technology. A strong correlation was found between the liquid fuel film structure (discrete pockets or continuous film) and the evaporation dynamic (fast of slow evaporation rate, respectively). A conceptual model was proposed to explain this result. It was based on the ratio of contour length to film surface. It was proposed that faster evaporation rates occur at the edges of the film characterized by smaller thicknesses, and that as a consequence film structures characterized by high contour length to film surface ratio have higher evaporation rates. Finally it was proposed that the discrete pockets film structure leading to fast evaporation rate is due to the string structure of the piezo-electric injector sprays.