A complete one-dimensional mixed lubrication model has been developed to predict oil film thickness and friction of the piston ring-pack. An average flow model and a roughness contact model are used to consider the effects of surface roughness on both hydrodynamic and boundary lubrication. Effects of shear-thinning and liner temperature on lubricant viscosity are included. An inlet condition is applied by considering the unsteady wetting location at the leading edge of the ring. A ‘film non-separation’ exit condition is proposed to replace Reynolds exit condition when the oil squeezing becomes dominant. Three lubrication modes are considered in the model, namely, pure hydrodynamic, mixed, and pure boundary lubrication. All of these considerations are crucial for studying the oil transport, asperity contact, and friction especially in the top dead center (TDC) region where the oil control ring cannot reach.
The model is applied to a single-cylinder diesel engine since its low-speed and high cylinder pressure characteristics create severe lubrication conditions for the ring-pack. The model predictions on oil film thickness, friction, asperity contact, and oil transport on the liner are discussed. One topic addressed is the top ring's ability to carry extra oil to the region above TDC of the oil control ring during the compression stroke due to the raised cylinder pressure. Surface roughness is found to have an important impact on this oil transport mechanism. Subsequently the effects of shear-thinning, liner temperature, and surface roughness on ring/liner asperity contact and friction power loss are studied.
The model is also applied to a single-cylinder gasoline engine on which extensive oil film thickness measurements were made using a Laser Induced Fluorescence (LIF) technique. A preliminary comparison shows good agreement between model predictions and the measurements for three different oils. The shear-thinning effect of multigrade oils is observed both in the model predictions and measurements.