Design of Capacitance Probes for Oil Film Thickness Measurements Between the Piston Ring and Linear in Internal Combustion Engines

Abstract

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 and the dielectric constant of the oil is known. All aspects of the design, construction, installation of capacitance probes, and analysis of the resulting measurements are reviewed in this paper. Biases introduced due to the fringing of the electric field, curvature of the ring face profile, roughness of the ring profile, and the tilt angle of the ring face are analyzed, and correction algorithms are proposed. Errors associated with the proposed algorithms are gauged through comparisons to finite difference solutions. Shielding the sensing electrode is found to eliminate fringing effects and also stray capacitance which can affect the signal. 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 sufficient signal strength. A design procedure which allows for the sizing of probe dimensions for a given level of allowable error and capacitance measuring circuitry is developed.