Direct Calibration of LIF Measurements of the Oil Film Thickness Using the Capacitance Technique

A direct calibration has been performed on laser-induced fluorescence measurements of the oil film in a single cylinder air-cooled research engine by simultaneously measuring the minimum oil film thickness by the capacitance technique. At the minimum oil film thickness the capacitance technique provides an accurate measure of the ring-wall distance, and this value is used as a reference for the photomultiplier voltage, giving a calibration coefficient. This calibration coefficient directly accounts for the effect of temperature on the fluorescent properties of the constituents of the oil which are photoactive. The inability to accurately know the temperature of the oil has limited the utility of off-engine calibration techniques. Data are presented for the engine under motoring conditions at speeds from 800 - 2400 rpm and under varying throttle positions. The resulting calibrated oil film measurements are compared to the statically measured ring profile, to oil film measurements which were calibrated by using the ring profile fitting technique, and to the ring profile obtained from the capacitance data. Under certain conditions significant differences existed between the directly calibrated oil film measurements and those obtained by calibrating with the ring profile fitting, indicating that the true piston ring profile observed under dynamic operation of the engine may be significantly different from that measured statically off the engine. This was confirmed by the data from the capacitance probes. In these situations the lack of a direct calibration can lead to erroneous results.