An experimental study of spray mixing in a direct injection engine

Abstract

Abstract

The effects of injection timing, injector type, injection pressure and engine flowfield on fuel spray mixing in a direct injection engine were investigated using planar laser-induced fluorescence. The fluorescence images had sufficient resolution and quality to permit, for the first time in an engine, the calculation of the scalar dissipation rate. The probability density function of the scalar dissipation scaled by the mean showed excellent agreement with turbulent jet and shear layer data for late injection conditions, indicating that the same fundamental mixing process existed in the different flows. The effect of shot noise limited such comparisons for the more homogeneous early injection conditions. A dual-metric method was developed to characterize the degree of mixedness. The two metrics employed were the spatial variation, which describes the homogeneity of the scalar population, and the mean scalar dissipation, which describes the average magnitude of local scalar gradients and represents the rate of fine-scale mixing. Using this method, it was found that the presence of a strong bulk flowfield dominated the mixing rate in the test engine, while injector characteristics showed lesser effects. The data set averaged results of the two metrics for a wide range of conditions were found to define a single, unique curve that was accurately described by a quadratic relationship. This curve defines the path that turbulent mixing follows from an initial segregated state to the fully mixed limit.