Paper 11: The Use of a Motored Engine to Study Piston-Ring Wear and Engine Friction

Abstract
The paper describes a study of the conditions which lead to mechanical, or attritive, wear of a top piston- ring. A motored single-cylinder engine was used, and piston-ring wear measurements were made using a radioactive tracer technique. By connecting the inlet and exhaust ports together and pressurizing the system with nitrogen, peak cylinder pressures could be matched to those in a firing engine and corrosive wear minimized. Ring belt oil viscosities were matched to those in service either by using low-viscosity oils or by restricting the cooling. Wear was caused, under steady-speed conditions, by increasing the peak pressure or the cylinder-liner temperature. However, even under the most severe conditions, the wear rate eventually became negligible with the liner surface becoming highly polished. Under comparable conditions in a fired engine a steady rate of wear was observed. It was found that large variations in piston-ring wear could occur without any detectable change in the motoring torque. Torque was linearly related to the peak cylinder pressure. The component of the motoring torque attributable to the piston assembly friction varied as the square root of the ring belt oil viscosity. The results suggest that, in a fired engine, hydrodynamic lubrication should be attainable throughout the stroke if roughening of the surfaces by the effects of combustion products could be prevented.

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