LDA Characterization of Gas Flow in a Combustion Chamber of a Four-Stroke S.I. Engine

The swirl and axial components of gas velocity in the disk type combustion chamber of a fired and motored spark ignition engine were measured using a fiber-optic laser Doppler Anemometer (LDA). The engine was operated at a speed of 1500 rpm, a volumetric efficiency of ηυ = 0.5, an equivalence ratio ϕ = 1.4 and with an ignition timing of θ_ig = 30°BTDC (MBT condition).

The gas velocities at 9 points on a diameter at mid-height of the combustion chamber were processed by the cycle-resolved frequency discrimination method. The bulk (mean) velocity was determined by frequency components lower than a cut-off frequency of 667Hz. The flame propagation pattern was detected by ionization probes set at 17 points on the piston.

The results indicate significant differences in flow characteristics between motored and fired conditions during the combustion period. At the fired operation the maximum value of turbulence intensity is 30% higher, the integral spatial scale and microscale are 60% and 40% larger respectively, the swirl ratio is 50% lower and the swirl center is 40% eccentrically shifted on the radius, comparing with the motored operation.