Measurement of Gas Velocity in a Combustion Chamber of a Small-Sized, Two-Stroke S.I. Engine by LDA : 2nd Report, Evaluation of Turbulence Characteristics by Stationary Time-Averaged Analysis

The intensity and structure of turbulence in a flame front may be changed by expansion flow due to combustion. Therefore the velocity and turbulence in a combustion chamber of a spark ignition engine are measured by mans of a laser Doppler anemometer with fiber-optic pick-up under firing conditions. The engine is operated at a speed of 2500 rpm with propane-air mixtures. Data acquisition window width affects the turbulence intensity (u') and integral time scale of turbulence (Lt) of the analysis. In the experiment, a short (0.67 ms) and long (4 ms) window are selected and compared with each other. In the combustion period, twice the intensity of u' and an increase of 40 % in Lt are observed in the case of short window analysis. A stronger u' and larger Lt are observed in the long window analysis and it is shown that the Lt expands laterally during the compression stroke and vertically during the expansion stroke.