This paper discusses the effects of lubricant viscosity on the friction characteristics of engine bearing and cam/tappet which are the typical moving parts of an engine and operate in different lubrication regimes. Based on the measured crankshaft temperatures, we calculated the friction coefficient of the engine bearing according to Sommerfeld number by a simple heat equilibrium equation. The oil film thicknesses between cam and tappet were measured in a motored cylinder head which had a direct acting type overhead camshaft. The boundary and viscous friction components were estimated separately according to a parameter defined as the ratio of the central oil film thickness to the composite surface roughness. These two friction components were added to calculate the friction coefficient. Finally, the motoring friction torque was measured and compared with the estimated friction coefficient.
The friction of engine bearings, which operate in hydrodynamic lubrication regime, increases as the lubricant viscosity increases. This increase in lubricant viscosity results in the increase of the crankshaft temperatures and friction torque. On the other hand, the friction of the cam/tappet contacts, which operate in both the boundary and elastohydrodynamic lubrication regimes, is reduced by increasing the viscosity and engine speed. The cause of this contrary phenomenon in the cam/tappet contacts is that the boundary friction component decreases due to the thicker oil film thickness.