Development of a Piston-Ring Lubrication Test-Rig and Investigation of Boundary Conditions for Modelling Lubricant Film Properties

A test-rig has been developed to simulate under idealised conditions the lubricating action between the piston-ring and the cylinder-liner in reciprocating engines. Complications arising in production engine piston-assemblies such as lubricant starvation, ring and piston dynamics, thermal and elastic deformations and blowby can thus be avoided so that the lubricant film characteristics are examined in isolation.

The lubricant film thickness and friction at the piston-ring/liner interface were simultaneously measured throughout the stroke as a function of speed and load and compared with the solution of the Reynolds equation for a range of boundary conditions. The examined conditions included the Swift-Stieber (Reynolds), the separation and limiting cases of the Floberg and the Coyne & Elrod boundary conditions using a numerically efficient general purpose program. For a Newtonian base-stock, the film thickness was found to be in close agreement with the numerical predictions for most of the cycle when the Swift-Stieber boundary condition was imposed at the cavitation boundary, except for the period soon after the dead-centres of reciprocation where the measured film thickness was lower than the predicted one. In addition, peaks in the measured hydrodynamic friction data correlated well with the shear rate calculated from the film thickness measurements, reaffirming the existence of thinner than predicted films after the flow reversals. This is in agreement with results reported in the literature where negative pressures have been measured after the reversals in the diverging part of the ring. A delayed cavitation mechanism during the growth of the cavitation region under transient conditions is thought to be responsible for the observed difference between measurements and predictions.