Effects of Turbulence on Mixture Stratification in a Small-Bore Utility Engine 2012-32-0005
The current work investigates the in-cylinder mixing of a fluorescent tracer species inducted into the engine through a small-diameter tube mounted along the inner port wall and the remaining inlet stream in a small-bore utility engine. Planar laser-induced fluorescence (PLIF) measurements were acquired on a single plane, parallel to and approximately 4 mm below the cylinder head deck, throughout the intake and compression strokes. The data were analyzed to qualitatively and quantitatively describe the evolution of the mixture stratification. The highest degree of stratification in the mean field was observed at a timing of 90 crank angle (CA) degrees after top dead center (aTDC) of the intake stroke, which corresponds closely to the point of maximum intake valve lift (105 CA degrees aTDC). After peak valve lift, the distinct flow structures in the field of view maintained a relatively high intensity, while the surrounding regions became increasingly mixed, until the mean flow evolved into a mostly homogeneous scalar field at bottom dead center of the intake stroke (180 CA degrees aTDC). Probability distribution functions of the corrected and normalized intensity data were used to quantitatively compare the effects of valve style, port orientation, port geometry, and engine speed. Shrouded valve cases exhibited faster mixing than the non-shrouded valve cases. Intake port orientation had a large influence on the observed stratification patterns and PDFs at peak valve lift, but those differences were not apparent at 180 CA degrees aTDC. Port geometry did not show any significant differences in the mixture stratification. The mixing rate was slightly different at higher engine speeds. In the individual-cycle images, finer-scale eddies were apparent at 1800 rpm versus 600 rpm, likely due to the higher turbulence intensity and higher turbulent Reynolds number at faster mean piston speed.