Measurement of dynamic lubricating oil film thickness between piston ring and liner in a motored engine

The interface between the piston rings and cylinder liner plays an important role in total frictional losses and mechanical wear of internal combustion engines and is increasingly coming under scrutiny as legislated particulate emission standards are becoming more and more stringent. The capacitance method is used for measurement of minimum oil film thickness at the piston ring–liner interface in the present investigations. Measurement of capacitance formed between the piston ring and a probe mounted flush with the liner provides an accurate measurement of oil film thickness provided that the region between the probe and liner is flooded with lubricating oil whose dielectric constant is known. This paper presents detailed design of sensor, instrumentation and measurement of lubricating oil film thickness using capacitive micro-sensor. The present investigation is carried out in a motored engine in order to validate the sensor and instrumentation and it can be directly employed in a firing engine also.The oil film thickness was measured at different speeds at three different locations, i.e. close to TDC, mid stroke position and close to BDC position and the results are accordingly presented in this paper. Lubricating oil film thickness is found to vary between 0.2 and 8 μm in the motored engine. At a particular position, lubricating oil film thickness varies significantly in upward and downward stroke of the engine due to reversal in direction of piston tilt.