Calibration of the laser-induced fluorescence technique applied to oil film thickness measurements on a diesel engine cylinder liner by using capacitance gauges

The calibration of the laser induced fluorescence (LIF) technique applied to oil film measurements on a modified Cummins L-10 diesel engine cylinder liner was carried out by using optical fibers and capacitance probes installed in the liner. The errors associated with the LIF calibration method were investigated and the calibration procedure was applied to a real engine system.; Optical fibers with a core diameter of 50 microns and capacitance gauges having a diameter of 300 microns were installed in the cylinder liner at the same location. The LIF and capacitance measurements were simultaneously recorded. The minimum separation distance between liner and top compression ring, measured when the ring passed in front of the probes, was used to calibrate the LIF voltage signal measured by the optical fiber. The calibration procedure was carried out during each engine cycle. The capacitance data were calibrated by using the parallel plate formula and then correcting it for errors associated with electrostatic field distortions at the edge of the probe and real ring geometry. The LIF calibration coefficient was obtained by comparing the minimum separation distance under the top ring, as calculated from the capacitance data, to the LIF voltage signal at the same location under the ring.; The results showed that the calibration can be performed without assuming the ring profile seen by the liner installed probes, as required by other techniques. This is a major advantage as the ring rotation in the groove is generally unknown and can affect the accuracy of the previously used calibration methods. The new calibration method was applied to a real engine system. The effect of engine speed, intake pressure and fueling rate variation on oil film thickness on the liner was investigated. The oil film thickness under the top ring increased with increasing engine speed and decreased with increasing fueling rates. No consistent results were obtained for different intake boost pressures. The oil film shape under the ring was not well characterized at the 1/6th crank angle sample rate available for this study. The sampling rates of at least 1/20th crank angle are recommended for future work.