Fluid-motion and turbulence phenomena important to fuel-spray dynamics, mixing, and combustion have been examined in a motored direct-injection engine using photon-correlation laser-Doppler velocimetry. Operating at 11:1 compression ratio, the engine incorporated a centrally located, cylindrical piston bowl and an off-axis, directed intake port. This study concentrates on the tangential (swirl) and radial (squish) velocity components during the compression stroke and the early portion of the expansion stroke. Measurements were made at low engine speeds (primarily 600 r/min) along two orthogonal radii at four depths within the piston-bowl's TDC volume.
The mean flow pattern in diametral planes can be described as off-axis swirl with a moving center of rotation that varies with both depth and engine speed. Peak swirl velocities occur about 20 deg BTDC of compression. As the piston passes through TDC, squish deforms the swirl profiles and, near the bowl rim, produces a nearly reversible radial flow into and out of the bowl. Intake-generated velocity fluctuations decay slowly until just before TDC, when the fluctuation intensity shows a sharp peak consistent with production of turbulence near the bowl entrance.