Piston Ring / Cylinder Bore Friction Under Flooded and Starved Lubrication Using Fresh and Aged Engine Oils

The friction reducing capability of engine oils in the piston ring/cylinder bore contact was investigated under fully-flooded and starved lubrication conditions at 100° C using a laboratory piston ring/cylinder bore friction rig. The rig is designed to acquire instantaneous transient measurements of applied loads and friction forces at the ring/bore interface in reciprocating motion over a 50.8 mm stroke. The effects of increasing load and speed on the friction coefficient have been compared with new and used engine oils of different viscosity that were formulated with and without friction modifying additives. Test results with fully formulated engine oils containing molybdenum dithiocarbamate (MoDTC) show that friction is always lower than that obtained with non-friction modified oils but in regions of persistent starvation the coefficient of friction can increase significantly, approaching levels equivalent to fully-flooded non-friction modified formulations. The laboratory rig was able to distinguish multistage differences in the rate of deterioration of friction reducing capabilities during starvation among several MoDTC-containing oils. This indicates that low friction retention is dependent on complex oil additive interactions that differ with each oil formulation.