Design of capacitance probes for oil film thickness measurements between the piston ring and liner in internal combustion engines

Measurement of the capacitance formed between the piston ring and a probe m ounted in the cylinder liner provides an accurate means of determining the oil film thickness provided that the region between the ring and probe is f looded with oil and the dielectric constant of the oil is known. All aspect s of the design, construction, installation of ca pacitance probes, and ana lysis of the resulting measurements are reviewed in this paper Biases intro duced due to the fringing of the electric field curvature of the ring face profile, roughness of the the ring profile, and the tilt angle of the ring face are analyzed, and correction algorithms are proposed. Errors associate d with the proposed algorithms are gauged through comparisons to finite dif ference solutions. Shielding the sensing electrode is found to eliminate fr inging effects and also stray capacitance which can affect the signal. A re ctangular probe design with a high aspect ratio is suggested as nit optimum . The small axis of the probe provides high spatial resolution, while the l onger length, which is in the circumferential direction, provides a suffici ent surface area to ensure sufficient signal strength. A design procedure w hich allows for the sizing of probe dimensions for a given level of allowab le error and capacitance measuring circuitry is developed.