Piston ring lubrication and friction in an engine

Abstract

This work is concerned with the lubrication between a one-ring-piston assembly and a cylinder in an engine. A special apparatus, with a stationary piston and a reciprocating cylinder was used, by means of which the variation of the friction on the piston assembly during the cycle was recorded. The variable factors governing the friction could be independently controlled. Thermocouples were installed at various positions in the piston, the ring and the cylinder, providing a method of determining the mean temperature of the oil between the piston assembly and the cylinder. The tests included an Investigation of the effects of viscosity, speed, and pressure on piston friction force. Results of these tests show that full film lubrication prevails, when the viscosity of the oil is comparatively high. At low viscosities and under pressure, a breakdown of the oil film occurs in the rig at B.D.C. (corresponding to T.D.C. in an actual engine). A theoretical analysis of the lubrication and friction between a piston-ring and a cylinder is proposed for a parallel faced ring. The oil film thickness under the ring is calculated (on the assumption that the piston clearance is empty) and is found to be in the order of .0003", under zero differential pressure conditions. A correlation of this oil film thickness is carried out by heat transfer calculations, which give results of the same order. Under low viscosity (30 cp) and high pressure (250 lb/sq.in.) conditions, the ring oil film thickness is found to be in the order of .0001".