Oil Film Thickness Measurement


A top marine engine manufacturer commissioned MTI Instruments to design and manufacture a non-contact sensor to measure the microscopic oil film layer between the engine cylinder wall and the piston ring.¬†Engine testing is normally performed through a range of low RPM’s to high RPM’s (400- 6000) with piston linear speeds approaching 500 inches/second and at various torques.

A measuring system with high frequency response and small sensor size was essential to keep engine machining to a minimum while not altering the engine operating characteristics. Accumeasure™ capacitive sensors measure the minuscule capacitance change due to the oil thickness variation and convert this change into a thickness measurement.

A custom probe was designed to be significantly smaller than the piston ring width to allow it to obtain dynamic measurement data as the piston ring passes the sensor face. High temperature epoxy and materials were used in the threaded probe fabrication to enable it to operate at temperatures as high as 285 C (545 F) and to withstand high pressure.

Due to the high frequency response required for the application, MTI engineers then designed a custom amplifier filter, eliminating signal overshoot with minimal phase delay in the electronics (Bessel filter).

MTI Accumeasure TM sensors can also be used in other applications such as determining the ratio of oil/air present in an engine passage. The Accumeasure TM equipment provides a distinct voltage output, depending upon whether oil, air or a mixture of both (foam) is present. The system works equally well in transmission research to determine the amount of cavitations in the fluid. Any foaming of the fluid indicates air is present in the mixture and will cause a noticeable change in the output signal. This type of data proves invaluable in determining the effect of mixing various additives into transmission fluids and the resultant knowledge helps researchers design a superior, longer lasting product.