Calibration of Laser Fluorescence Measurements of Lubricant Film Thickness in Engines

A laser fluorescent diagnostic method was employed to measure lubricant film thickness on the cylinder wall/piston interface of two engines. The system output signal was calibrated using lubricant samples of known thickness, and by comparison of a known piston ring profile to measured lubricant film contours. Agreement of the results of the two calibration methods was within 5%. A relative calibration was performed with three oils having different additive packages, and with an oil contaminated through use in a commercially operated engine. The calibration coefficients for the oils, relating output voltage to film thickness, varied within a factor up to two, depending on lubricant type and age.

The laser fluorescent apparatus was installed for use with a single cylinder test version of the Cummins VT-903 diesel engine. An optical passage was created through the block and cylinder wall using a quartz window. Preliminary results in the motored engine indicate a lubricant film thickness of approximately two microns, at the upper portion of the cylinder wall, on the minor-thrust face.