Laser Doppler Velocimetry (LDV) was used to make measurements of velocities and turbulence intensities at a location near the wedge-type combustion chamber center in a production SI engine under motored and firing conditions. The ensemble-averaged and conditionally-sampled data-processing techniques were employed to investigate the raw velocity data for motored and firing conditions respectively.
The turbulence field measured under firing conditions is different from the motored data only after the spark ignition begins. There exists a dramatic change in ensemble-averaged mean velocity in the direction of flame propagation due to the fast volume expansion of burned gas when the flame front pass through the measurement point. Initially, the flame moves away from the spark plug and toward the measurement point, pushing the unburned gas ahead of it. Passage of the flame induces a horizontal velocity in the opposite direction. After the flame passes the measurement point, a sharp increase in horizontal velocity occurs ahead of the flame as the burned gas expands into the chamber, pushing the unburned gas ahead of it. After the flame has passed the measurement point, the sharp increase in velocity is followed by a marked drop in velocity behind the flame, as the fresh mixture burned in the flame region expands compressing both the burned and unburned gases. Gradually the velocity increases again due to the angular momentum of the swirl.
Conditional sampling on the crank shaft angle of maximum pressure revealed that the engine cycles with smaller crank shaft angle of maximum pressure had higher burning rates and had mean velocities changing dramatically in the direction of flame propagation when the flame passed through the measurement point, suggesting a correlation between the turbulence intensity at time of ignition and the crank angle of maximum pressure. Furthermore, there exist different turbulence flow fields in the burned and unburned gas regions, suggesting different analytical techniques should be used to resolve the cyclic variations in the two regions.