Film Thickness Measurement of the Cylinder/Liner Contact

Measurements of Piston Ring Oil Film Thickness

The oil film thickness between a piston ring and cylinder liner is a critical parameter for the operation and design of an internal combustion engine. There are many capacitance and optical based methods to measure this thin film thickness; however, they all need to penetrate through the cylinder to acquire the measurement. In this project, a non-invasive ultrasonic method has been applied on a motored test engine.

An ultrasonic sensor is glued to the wet side of the cylinder. When the sensor is excited, an ultrasonic pulse propagates through the cylinder and a proportion of the pulse is reflected back from the liner-ring oil film layer. The proportion of an ultrasonic pulse that is reflected from the layer, known as reflection coefficient, varies with the acoustic properties of the materials and oil film thickness. The ultrasonic reflection pulses are captured and recorded as the rings pass over the transducer location. The oil film thickness is calculated from the proportion of the pulse reflected and the acoustic properties of the oil and piston liner materials. This method is portable, non-invasive and is currently being extended to fired automotive engines.


  1. R. S. Dwyer-Joyce, D. A. Green, P. Harper, R. Lewis, S. Balakrishnan, P. D. King, H. Rahnejat and S. Howell-Smith,“The measurement of Liner-Piston Skirt Oil Film Thickness by an Ultrasonic Means”, SAE Technical Paper Series 2006-01-0648, SAE 2006 World Congress, Detroit.