Thermal Friction Characteristics of Engine Elements - Boundary Lubrication of Cam and Tappet

Abstract

As low-viscosity oil is used more widely to reduce friction losses in IC engines, concerns are growing about an increase in friction, wear and scuffing at the sliding surfaces on tribo-components. The authors conducted an analytical study on thermal friction characteristics of boundary lubrication based on the physical and chemical properties of interface materials and lubricants. The study was intended to investigate the effect of temperature on the function of oil additives as a friction modifier and/or anti-scuffing agent. Three low viscosity base oils, three extreme pressure and anti-wear agents and two organo-molybdenum friction modifiers were used to measure friction changes with a cam/follower test apparatus. The authors analyzed a relationship between friction and temperature in the running-in process of interfaces in consideration of the functional sensitivity of additives in low-viscosity oils. This analysis was focused on physicochemical properties on the basis that the oil viscosity, the adsorption of oil molecules, the chemical reaction of oil additives that forms a tribo-film low in shear stress, and the strength of interface materials are all under the influence of the exponential function of temperature, just like the Arrhenius’ equation. The results of the analysis found that the frictional shear stress changes depending on the intensity of involvement of surface temperature that affects oil viscosity, molecular adsorption / desorption and tribo-film formation in each running-in term.