Effect of Swirl Rate on Mixture Formation in a Spark Ignition Engine Based on Laser 2-D Visualization Techniques

The mixture distribution and in-cylinder flow field inside the combustion chamber of a spark ignition engine with a swirl control intake system were measured by a pair of laser two-dimensional visualization techniques. The planer-laser-induced exciplex fluorescence technique was used to visualize the in-cylinder mixture formation by obtaining spectrally separated fluorescence images of liquid and vapor phase fuel distributions. The particle image velocimetry (PIV) was used to obtain the images of in-cylinder flow field. Experiments were carried out under various swirling conditions (from high [Rs=3.8] to low [Rs=0.4] swirl rates) to clarify the effect of swirl rate on mixture formation during the intake and compression strokes. Under the high swirling condition, fuel vapor was spread and rotated along the cylinder wall by the swirling flow during the compression stroke. Consequently, the vapor was distributed around the cylinder wall in a doughnut-like shape in the final stage of the compression stroke, with a poor mixture region at the center of the combustion chamber. Under the low swirling condition, a high concentration of fuel vapor was distributed at the exhaust valve side because of the fuel impinging on the cylinder wall on that side and because the fuel mixture traveled in the vertical vortex flow. Combustion flame propagation and engine performance were also measured under the same swirling conditions. With increased swirl rate, the fuel evaporation rate increased and this led to increased combustion stability and reduced HC emissions.