Bulk Cylinder Flowfield Effects on Mixing in DISI Engines

Valve deactivation followed by multiple compression-expansion strokes was employed to remove intake-generated turbulence from the bulk gas in an internal combustion engine. The result was a nearly quiescent flowfield that retains the same time-varying geometry and, to a first approximation, thermodynamic conditions as a standard engine. Mass loss, and more significantly heat loss were found to contribute to a reduction in the peak cylinder pressure in the cycle following two compression-expansion strokes. The reduction of the turbulence was verified both computationally and by performing premixed combustion studies.

Mixing studies of both liquid spray jets and gaseous jets were performed. Laser-induced fluorescence images of high spatial resolution and signal-to-noise ratio were acquired, allowing the calculation of the two in-plane components of the scalar dissipation rate. The effect of in-cylinder vaporization was found to be relatively small, but with valve-deactivated operation the effect of vaporization was found to be more significant due to the decrease in bulk gas temperature. There was a very significant decrease in the level of homogeneity of gas jets into a quiescent valve-deactivated engine flow as compared to the same gas jet into a standard in-cylinder case, which leads to the major finding of this study: the bulk in-cylinder flowfield has a controlling effect on the mixing of gasoline direct-injection spray jets.