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To explore the measures that can be used to improve the fuel economy of internal combustion engines, we investigated how friction at the piston ring-cylinder liner interface is influenced by the boundary lubrication performance of engine oils. We formulated several engine oils with varying boundary lubrication performance and tested them for ring-liner friction by using a floating liner friction tester. We used friction modifiers (FMs) to modify the boundary lubrication performance of engine oils. We found that ring-liner friction is well correlated with the friction coefficients in boundary lubrication regimes when measured by a laboratory friction tester. We also found that the impact of the boundary lubrication performance of engine oils was emphasized in low viscosity engine oils. It makes it possible for improved boundary lubrication performance to inhibit or overcome the viscosity reduction-induced increase of friction energy.

Friction loss at the piston ring-cylinder liner interface in an internal combustion engine strongly affects the fuel economy of automobiles. However, the relationships between viscosity characteristics of engine oils and friction at ring-liner interface are not well understood. In this study, we experimentally measured ring-liner friction using a floating liner method with various formulations of engine oils. Two types of engine oils were tested: Non-Newtonian oils that contain polymer additive viscosity modifiers (VMs) and Newtonian VM-free oils. We first tested VM-free oils with different base oil viscosities and found that the dominant friction energy mechanism changed from hydrodynamic lubrication to mixed lubrication as engine oil viscosity or piston speed were decreased. Friction energy reached a minimum at this transition point.