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The purpose of this paper is to present a measurement technique wherein the film thickness is measured in unfired condition for entire stroke length but without impairing the original condition of piston ring, liner and lubricant, i.e. non-invasively. Film thickness is measured at different speeds up to 500 rpm. The measurements are initially carried out at near zero speed followed by speeds mentioned above. Measurement highlights the combined effect of variation of bore diameter and ring face profile on the film thickness.

The film thickness is measured with the help of a set of strain gauges. Four strain gauges are mounted on a sufficiently elastic steel strip which is mounted in a simply supported condition. This assembly of strain gauge is mounted on small rectangular bracket. A cutout is made in the piston to accommodate the bracket. A pin bearing a slot of size sufficient enough to accommodate the piston ring on one side is fixed between the piston ring and the strain gauge assembly. This ensures the transfer of the movement of the piston ring on to the strain gauge. The deflection of the strain gauge is pre-calibrated against a sufficiently accurate dial gauge. Hence any radial movement of the piston ring is sensed by the strain gauge assembly. A data logger unit is connected to the strain gauge output to log the data at every crank angle. A rotary encoder is connected to the crank shaft, to have the correlation of the strain gauge output with the crank angle.

The technique is capable of measuring oil film thickness for entire stroke at low speeds in unfired engines. The effect of variation in bore diameter on the oil film thickness is significant and hence such measurement can enlighten the path for research to reduce friction. The experimental results of the oil film thickness are in good agreement with predicted values, particularly in the forward stroke (BDC to TDC).

The methodology is not suitable for fired engines as on date but can be taken up as a future work with necessary modifications. It does not take into consideration the effect of elasto-hydrodynamic lubrication.

It can be used to measure OFT between piston ring and liner in unfired engines and reciprocating compressors also.

It can help to indentify the areas of research so that the friction between piston ring and liner can be reduced thus increasing efficiency of the engine and reducing fuel consumption and emissions.

The work presented is a part of PhD work under progress at S V National Institute of Technology, Surat, India. The setup is in the college premises and the experiments are conducted on the same.

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.

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.

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.

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.