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The development of cross-flow single overhead camshaft designs of engines led to the introduction of pivoted cam followers with pads that were subjected to uni-directional rolling/sliding under heavy contact loads. Such components were prone to wear failure by a mechanism involving severe surface roughening. The initiating wear mechanism was eventually shown to be a form of “mild” wear and the Archard wear equation was used successfully to model the pattern of wear seen on cams and followers. The use of rigs to assess the wear performance of different lubricants has hitherto been a very poor predictor of engine performance, because of the complex interaction of materials, kinematics and forces in real engines. As a result, most automotive lubricant development relies on engine testing, which is expensive and time-consuming. Also, the complexities of the engine environment make it difficult to obtain much scientific insight into the tribological processes involved.

A piston ring-pack lubrication model has been developed which takes into account both lubricant viscosity/temperature and viscosity/shear rate variations. In addition, lubricant starvation of the upper piston rings, due to restriction of the oil supply by the lower rings, has been included. Inputs to the model include piston ring profiles (measured using Talysurf profilometry) and gas pressure distributions throughout the ring-pack. The latter were calculated using the (known) combustion chamber pressure diagram at the relevant engine operating conditions. The model was validated by comparing predicted oil film thicknesses with those measured using a laser-induced fluorescence technique on a Caterpillar-1Y73 single-cylinder diesel engine. The engine was run at a range of speeds with two different, fully formulated, multigrade lubricants, and the oil film thickness under each of the piston rings was measured.

Piston ring wear, bearing wear and fuel consumption have been measured in European engines run under fully warmed-up conditions at steady speed and load. The engines were lubricated with motor oils which were formulated from a range of types and concentrations of VI improvers and base oil blends. The viscosities, viscosity/temperature relationships and viscosity/shear-rate characteristics of these formulations varied widely. The results for all the oils tested showed a poor correlation with their low shear-rate viscosities measured at 210°F or 150°C but a good correlation with their viscosities measured at shear rates of 105 to 106 sec-1.