Measurement of oil film thickness between piston ring and liner using strain gauge

Purpose - The aim of this paper is to develop a technique to measure the oil film thickness between piston ring and liner throughout the stroke, without impairing the surface properties of the piston ring and liner. Mechanical properties of the piston ring, like ring stiffness, are also not altered. Effect of variation in bore on the movement of piston ring can be studied with the proposed technique. Design/methodology/approach - The gap Hmin between the cylinder liner and the piston ring is formed due to the hydrodynamic pressure generated by the presence of oil film between piston ring and liner. This gap can be inferred by measuring the movement of the inner surface of piston ring with reference to a sensor mounted on the piston at a fixed distance from the piston ring. The piston ring is connected to the sensor through reasonably rigid member. The underlying assumption here is that there is no elastic deformation of the piston ring due to the hydrodynamic pressure. The fundamental sensor to measure oil film thickness used in this setup is a set of strain gauges. Findings - It is possible to measure oil film thickness by the proposed arrangement for the entire stroke without changing the surface properties. Mechanical properties of the piston ring, like ring tension, are not affected. The results possibly provide the correct picture of the piston ring movement throughout the stroke. The measurement at near zero speed can give information on the movement of the piston ring due to hydrodynamic action and to the variation in the bore. The measurement is not affected by engine vibrations. The proposed technique can be helpful in validating the theoretical models proposed in the literature. Originality/value - The measurement is possible only in unfired condition. However, this attempt can be considered as the basis to measure OFT in fired condition with necessary improvements. It is not feasible to measure quantity of lubricant/extent of lubricant on leading or trailing edge of piston. Effect of temperature on the oil film thickness cannot be studied as the engine is not fired. It is assumed that the piston ring does not pass through elasto-hydrodynamic lubrication regime. Debris/worn out particles in the oil may affect the indicated oil film thickness at local points.