The Effect of Spark Location on Combustion in a Variable-Swirl Engine

Measurements are presented showing the effect of swirl level and spark location on burn duration in a homogeneous-charge engine. Laser shadowgraph photographs of the flame structure were used to help interpret the observed results. As expected, without swirl the burn duration was a direct function of flame travel distance, such that central ignition was optimal. When swirl was introduced, off-axis ignition was aided by flame-holder effects that enhanced the flame speed in the circumferential direction. However, only for the highest swirl level studied (swirl number = 8.3) was the burn rate increased by moving the ignition point toward the cylinder wall. For lower swirl levels, central ignition was still preferable.