The effect of lubricant viscosity on the temperature and thickness in oil film on a piston ring in a diesel engine was analyzed by using unsteady state thermohydrodynamic lubrication analysis, that is Reynolds equation and an unsteady state two-dimensional (2-D) 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 multi grade oils. The shear rate between the ring and liner becomes higher, so that the viscosity for the multi grade oil is affected by the oil film temperature and shear rate, and the viscosity becomes lower. Under low temperature condition, the viscosity becomes lower due to viscous heating and shear rate and under higher temperature condition, the viscosity affected by the shear rate becomes lower. The oil film thickness between the ring and liner decreases with decrease of the oil viscosity, and it is the thinnest that the oil film thickness is calculated by using the viscosity estimated by both the shear rate and the oil film temperature. Moreover, the heat transfer at ring and liner surfaces was examined.

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