The characteristics of fluid motion in the piston bowl of a small-bore, high compression ratio engine were investigated. In-cylinder velocities were measured using laser Doppler velocimetry (LDV), hot-wire anemometry (HWA), and the flow was observed using high-speed flow visualization. A reentrant piston bowl and two cylindrical piston bowls were studied at two different levels of intake-generated swirl to investigate the effect swirl and squish flow had on the bulk air motion and turbulence in the piston bowl. An elliptical piston bowl was studied to determine if an unstable flow could be produced in the bowl, which could be additional source of turbulence production.
Laser-Doppler velocimetry measurements of the tangential and radial velocity components in the cylindrical and reentrant piston bowls showed that the level of intake-generated swirl played an important role in determining the ensemble-mean flow pattern and the turbulence distribution. The kinetic energy stored in the swirl flow was the primary source for turbulent kinetic energy later in the cycle. Some of this kinetic energy was converted to turbulence prior to TDC in the region of high velocity gradients near the lip of the piston bowl.
The elliptical piston bowl created a complex flow pattern that resulted in the complete destruction of the large-scale swirl flow and in a greater generation of turbulence than was seen in the cylindrical piston bowls. The large-scale swirl flow broke into smaller-scale vortices within the piston bowl near TDC. The production of small-scale turbulent kinetic energy occurred over a longer period of time, and lasted later into the expansion stroke than was observed in the cylindrical piston bowls. These results were consistent with an unstable elliptical swirling flow, and indicated that this may provide an additional mechanism for turbulence production in the piston bowls of internal combustion engines.