Measurements of Flows During Scavenging in a Two-Stroke Engine 910671

This paper introduces a new measurement technique into the field of engine diagnostics. LlPA (Laser lnduced Photochemical Anemometry) is used to measure velocities and velocity gradients over a chosen plane in a motored two-stroke engine during scavenging. The LlPA technique consists of tracking a phosphorescing grid which is created by laser lines directed into the flow. A 308nm pulsed excimer laser beam is divided into 15 lines which are sent through the appropriate optics to create the grid. The grid energizes the seed chemical that is premixed in the carrier gas. The seed chemical used consists of a mixture of phosphorescent gases with nitrogen as the carrier. The results are not bias by out of plane motions, or the inertia or the charge of particles. In each plane forty-four simultaneous points of data are taken with an approximate grid mesh size of 3mm × 3mm. These measurements are taken over thirty consecutive cycles. Each measurement is taken by photographing two separate grids. The first grid is non-deformed, and the second (deformed) grid is photographed 0.1 4 milliseconds later. Each intersection of the grid lines is considered a data point, and because the deformations are on the order of a grid mesh size there is no ambiguity in tracking data points from one photo to the next. By measuring the distance and direction each intersection travels and by knowing the time delay between each photograph, the two velocity components in the grid plane, the turbulence intensities, the Reynolds stress, and the vorticity are calculated.
A loop scavenged, crankcase charged 125cc Kawasaki single cylinder engine was modified to allow optical access. Images were taken of grids formed in planes parallel to the piston head near bottom dead center. Using a gated intensified CID camera, the data frames were recorded on 112 inch video tape. Averages over the area of interest and over the ensemble of two-dimensional maps were used to look at mixing, cyclic variability, and general flow phenomena.


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