Effect of dispersant concentration with friction modifiers and anti-wear additives on the tribofilm composition and boundary friction

To extend drain intervals and improve efficiency, new engine oils with increased dispersant concentration and reduced viscosity are required. Low viscosity engine oils can increase the prevalence of boundary friction at low temperature and increase its severity at higher temperatures. As a result, combinations of organic and inorganic friction modifiers (FM) will be used to reduce boundary friction across a range of temperatures, also preventing damage to vehicle catalysts. This paper presents an experimental case study of such new generation of fully-formulated engine lubricants with varying concentrations of polyisobutylene succinimide dispersant, organic and inorganic FM. Representative conditions pertaining to those encountered at the top dead centre reversal of the piston compression ring-cylinder liner contact are created, and the generated friction measured through use of a sliding-strip tribometry. Subsequently, X-Ray Photoelectron Spectroscopy (XPS) is used to determine the composition of the formed surface tribofilms in order to explain the observed frictional characteristics. The key interactions and frictional behaviour of the dispersant and friction modifiers are highlighted across a range of operating temperatures.