Characterization of the lubrication regimes and quantification of the frictional losses are very important factors for the design of durable IC engines with improved fuel economy. Therefore, the current work has focused on the development of a tribology test rig that allows for the direct measurement of the instantaneous piston-assembly friction force under motoring conditions. The test rig was used to examine the effects of oil viscosity and engine speed on both the lubrication regimes and the friction force of the piston-assembly. Furthermore, the experimental data served to generate Stribeck curves for the coefficient of friction at different points in the cycle. Tear-down experiments were conducted to assess the friction contribution of each component in the piston-assembly. The results demonstrated that the magnitude of the friction force decreases with increasing oil grade under both boundary and mixed lubrication regimes. However, it tends to increase with increasing oil viscosity under a hydrodynamic lubrication regime. Moreover, the engine speed above which the hydrodynamic lubrication regime becomes prevalent at the midpoint of the stroke tends to decrease with increasing oil viscosity.

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