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The effect of lubricant viscosity on the temperature and thickness of oil film on a piston ring in a diesel engine was analyzed by using unsteady state thermohydrodynamic lubrication analysis, i.e., Reynolds equation and an unsteady state two-dimensional energy equation with heat generated from viscous dissipation. The oil film viscosity was then estimated by using the mean oil film temperature and the shear rate for multigrade oils. Since the viscosity for multigrade oils is affected by both the oil film temperature and shear rate, the viscosity becomes lower as the shear rate between the ring and liner becomes higher. Under low load conditions, the viscosity decreases due to temperature rise and shear rate, while under higher load conditions, the decrease in viscosity, is attributed only to the shear rate. The oil film thickness between the ring and liner decreases with a decrease of the oil viscosity. The oil film thickness calculated by using the viscosity estimated by both the shear rate and the oil film temperature gave the smallest values. For multigrade oils, the viscosity estimation method using both the mean oil film temperature and shear rate is the most suitable one to predict the oil film thickness. Moreover, the heat transfer at ring and liner surfaces was examined.

This paper describes an analysis of oil film thickness on a piston ring of a diesel engine. The analysis of the oil film thickness has been performed by using Reynolds equation and unsteady, two-dimensional energy equation with heat generated from viscous dissipation. The mean oil film temperature was determined from the calculation of the temperature distribution in the oil field which was calculated using the energy equation. The oil film viscosity was then estimated using the mean oil film temperature. The effect of oil film temperature on the oil film thickness of a piston ring was examined. This model has been verified with published experimental results. Moreover, the heat flow at ring and liner surfaces was examined. Results show that the oil film thickness could be calculated using the viscosity estimated from the mean oil film temperature. The calculated values generally agree with the measured values. For higher engine speed conditions, the maximum values of the calculated oil film thickness are larger than the measured values.

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