Thermal Friction Characteristics of Engine Elements

— Application of Rough Interface Contact Logistic Curve for Cam and Tappet

Abstract

The ICEs are facing tribological issues from marine cylinder oil, vehicle engine LOC and sludge contamination due to fuels and lubricants. It is desirable in fuel economy to apply lubricants which form the tribofilm low in shear stress that suppresses the increase in mixed and boundary friction under the lubrication with engine oils low in viscosity. As described in previous works, their property to form the tribofilm depends on the adsorption rate of oil molecules and the tribochemical reaction rate of additives. Both of these rates change exponentially with temperature as defined in Arrhenius law. The friction data are those measured under the running-in when surface roughness Rq decreases over time. In analyses the oil-film thickness is predicted under the specific relationship between the film thickness ratio of EHL oil-film thickness to Rq and the load bearing ratio of surface asperity contact. Each coverage ratio of the four contact areas; hydrodynamic oil-film, adsorbed film of oil molecules, tribofilm and true metal-metal contact, is determined in accordance with logistic curves for rough interfaces. The temperature rise due to the shear stress of oil-film or metal-metal contact is iteratively calculated. The shear stress for the metal interface is referred to the change of Vickers hardness with temperature. It is assumed that the surface temperature and the shear stress are same in each contact area of adsorbed oil molecules and tribofilm. As an analysis result, it has been found that the surface temperature deeply involves in the friction reduction effect of oil additives.