The Influence of Lubricant Rheology on Friction in the Piston Ring-Pack

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. Good agreement was found between measured and predicted oil film thicknesses, although lubricant shear thinning effects had to be included to obtain satisfactory agreement for the top compression ring. Following validation, the lubrication model was used to predict frictional losses in a diesel engine, for a range of lubricant rheological properties.