Laser-induced fluorescence (LIF) measurement device has been employed for measuring oil film thickness of piston and piston rings in an internal combustion engine. Reducing the size of the measurement apparatus is believed to expand the application of the LIF method. Thus, an experimental small-scale LIF measurement device has been developed. A blue laser diode was used for the light source, and a photo diode was used for the fluorescence light detection. Including such elements as lenses and optical filters, the width, length, and thickness of the device are 30, 30, and 15 mm, respectively. A drive circuit for the blue laser diode was constructed to stabilize the laser light intensity, and an amplification circuit was made for the photo diode to improve the light detection sensitivity as well as the signal frequency response. The results of the preliminary tests determined sufficient sensitivity, linearity, and frequency response required for the oil film thickness measurement. Subsequently, the device was tested on a diesel truck air compressor, and the piston ring oil film thickness was measured. The measurement results were compared with the results obtained with a conventional LIF measurement device consisting of a He-Cd laser unit and a photomultiplier. In accordance with the experimental results, the small LIF measurement device was determined to have an allowable capability for the oil film thickness measurement in an internal combustion engine.

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