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The structure of turbulent flames was examined in a square-piston engine. Submicron-sized smoke particles were added to the homogeneous propane/air mixture. The Mie-scattering of these particles allowed to visualize two-dimensional flame contours. A copper vapour laser generated pulsed laser sheets at a frequency up to 12 kHz. The flame contours were evaluated with digital image processing methods: fractal analysis, complex Fourier transform procedure and statistical flame contour analysis. The following flame parameters have been deduced applying these methods: Fourier factor, combustion zone thickness, mean radius of curvature, fractal dimension, wrinkling factor, flame-let crossing frequency, integral length scale and flame front thickness.

2-dimensional time resolved (200 frames/s) flow field measurements have been made in a transparent SI square piston engine using a movie version of particle image velocimetry (PIV). To this end the beam of a copper vapor laser was formed into a light sheet and was double pulsed with a pulse separation of 50 μs at a repetition rate of 200 Hz. A rotating drum camera was used to record the Mie-scattered signals from seeding particles. The circumferential velocity of the drum of the camera causes an image shifting of the two exposures taken with a double pulse. By proper adaption of drum and engine speed, a series of up to 70 double pulsed images per individual engine cycle may be recorded on film. This film data may be evaluated uniquely with respect to both magnitude and direction of individual flow vectors in the flow field.

A cycle-resolved two-dimensional flame visualization technique using Mie-scattering from submicron sized smoke particles added to the homogeneous charge mixture of a spark-ignition engine has been developed. This diagnostic technique was applied to a square piston engine with four windows. Pulsed laser sheets were generated by a copper vapor laser at a frequencies of 6 kHz. The light scattered by the smoke particles was collected by a drum camera on high sensitivity photographic film. The flame contours were analyzed using fractal analysis and complex Fourier transform procedures. Radii of curvature, wrinkling factors and turbulent burning velocities have been determined at different engine speeds and equivalence ratios.