LDV Measurements in an Engine with Square and Circular Piston Cups

Cycle-resolved LDV measurements of tangential and radial velocities were made in a ported engine within four piston cups. One cup was centered circular, one off-center circular, one centered square and one off-center square. The engine speed was 1200 rpm, the compression ratio 10.8, the squish area 75% and the TDC swirl ratio 4 for a pancake chamber. The velocity measurements were made at four depths in two axial sections. Near TDC in the centered circular cup, the profile of the ensemble-averaged tangential velocity tends to solid body with a swirl ratio of 12.5. In the centered square cup, the same velocity tends to solid-body profile along the short section and to top-hat profile along the outer part of the long section. The corresponding TDC swirl ratios are 11.3 and 5.5 due to mass conservation. The trends are similar but more complex in the off-center circular and square cups. In the centered circular cup, the swirl center is close to the cylinder axis near TDC. In the off-center circular cup, the swirl center tends to move from the cylinder axis to the cup axis after piston arrival at each plane; similarly for the square cups. The TDC angular momentum per unit mass is about 35% smaller in the square cups than in the circular ones. Near TDC in all the cups, the turbulence intensity is higher, anisotropic and non-homogeneous at the rims of the cups but relatively homogeneous and isotropic everywhere else. Near TDC the average turbulence intensity is some 25% to 35% higher in the square cups than in the circular ones. The fluctuation intensities are 1.5 to 3 times higher than the turbulence intensities in the central regions of the cups.