Swirl, Squish and Turbulence in Stratified-Charge Engines: Laser-Velocimetry Measurements and Implications for Combustion

Laser-Doppler velocimetry has been used to investigate the effects of piston-bowl geometry (cylindrical and reentrant) and intake-swirl ratio (4.5 and 6.5) on the structure and evolution of the turbulent flow field in a motored engine (compression ratio: 10.6, speed: 600 r/min). High-shear regions and associated turbulence production are observed just inside the bowl entrance around TDC of compression. Before TDC, these regions are created in both geometries by the opposing effects of swirl and squish. As the piston passes through TDC and the bulk squish flow reverses, the high-shear, turbulence-producing region inside the rim of the cylindrical bowl disappears, but it persists within the reentrant bowl as a direct consequence of the geometry. These and other experimental results (e.g., swirl-pattern centering) are discussed with respect to specific aspects of combustion in direct-injection stratified-charge engines, including cyclic variation in the air-fuel ratio at the ignition site and effects of squish-generated turbulence on light-load hydrocarbon and heavy-load smoke emissions.