Measurement of the capacitance formed between the piston ring and a probe mounted in the cylinder liner provides an accurate means of determining the oil film thickness, provided that the region between the ring and probe is flooded with oil andthe dielectric constant of the oil is known. All aspects of the design, construction, installation and analysis of the resulting measurements are reviewed in this paper. Biases introduced due to the fringing of the electric field, curvature of the ringface profile, roughness of the ring profile, and the tilt angle of the ring face are analyzed, and correction algorithms are proposed. Shielding the sensing electrode is found to eliminate fringing effects and also stray capacitance which can affect thesignal. A rectangular probe design with a high aspect ratio is suggested as an optimum. The small axis of the probe provides high spatial resolution, while the longer length, which is in the circumferential direction, provides a sufficient surface area to ensure strong signals. A design procedure is developed which allows sizing of the probe dimensions for a given level of allowable error and capacitance measuring circuitry.
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