An experimental and theoretical study of piston ring lubrication

Abstract

The work presented in this thesis is concerned with developing a greater understanding of piston ring lubrication and consists of both experimental and theoretical components. It begins with a review of piston ring and ring pack lubrication studies, describing how the understanding of the operation of piston rings has progressed since the late 19th Century.
A well-established method of analysing the hydrodynamic lubrication of a single piston ring, utilizing coordinate data to describe the ring face profile, is then presented. Calculation of mixed/boundary friction, when the lubricant film thickness falls below a pre-determined level, is included. The predictions of this program are then compared to results obtained by other workers to validate its operation.
This analysis is then developed to investigate the influence of the circumferential variation in piston ring face profile on the lateral motion of the piston ring centre. The complex relationship between variation in ring face profile, angular ring gap position and bore shape is then examined. These features are shown to have a significant effect on the calculation of ring centre locus, oil transport and lubricant film thicknesses. The model is then applied to predict the influence of the distorted bore shape of a cylinder from an experimental engine apparatus.
Following this, the development, testing, calibration and performance of a capacitance based measurement system capable of recording piston ring face profiles on a single pass, is described.
This system was used with twenty-one transducers, of novel design, to allow the axial and circumferential distribution of lubricant film thickness to be monitored on an engine liner. This liner was fitted in an engine apparatus which had the additional facility of being able to measure the instantaneous friction between the piston assembly and cylinder liner using a "floating liner" technique.
Experimental measurements of instantaneous friction force and the circumferential distribution of lubricant film thickness are then compared to theoretical predictions for a range of operating conditions. The comparisons reveal a reasonable level of agreement between measurements with consistent overall trends.
The effect of varying bore distortion is also examined theoretically. It is shown that an increase in the severity of bore distortion does not necessarily result in an increase in net oil transport.