An Attempt to Calibrate the Laser Induced Fluorescence (LIF) Signal used for Oil Film Thickness (OFT) Measurements in Simulating Test Rigs

The issue of laser induced fluorescence calibration for engine and simulating rigs was studied using both a static calibration rig and a dynamic method involving a modified piston-ring specimen. The technique used to measure oil film thickness was laser induced fluorescence through optical fibres. Previous experimental studies have been conducted in test rigs and engines to obtain in-situ results showing the uncertainties lying within every applied method. In this study a comparison of two proposed methods is performed ? bench calibration (micrometer based) and dynamic for a single piston-ring test rig application. Engine in-situ calibration was not an option so that uncertainties that follow the direct measurements in the piston cylinder engine assembly and the difficulty in determining the exact properties of the lubricant at the ring-pack, which simultaneously affect fluorescence (e.g. local temperature at point of measurement, lubricant degradation) could be avoided. A temperature parametric study of the dynamic calibration coefficient is also presented and a comparison between results from capacitance transducer minimum oil film thickness measurements and the LIF measurements. The discussion that follows the results, tries to enlighten the reasons behind inconsistencies found between the proposed methods.