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Modern formulation in a wide variety of lubricants including engine oils and transmission fluids is designed to control friction through film-forming tribochemical reactions induced by the functional additives mixtures. Although many cases on the synergistic or antagonistic effects of additives on friction have been reported, their mechanisms are poorly understood. This study focused on the influences of metallic detergents on tribochemical reactions. We examined the mechanical properties of detergent-containing lubricants confined at a single-asperity contact and their contributions to tribochemical phenomena. We found that detergents enlarged the confinement space required for generating repulsive force and shear resistance. This means that these detergents provide steric effects under nanoconfinement at interfacial contacts.

To evaluate the influence of FAME, which has poor oxidation stability, on engine oil performance, an engine test was conducted under large volumes of fuel dilution by post-injection. The test showed that detergent consumption and polymerization of FAME were accelerated in engine oil, causing a severe deterioration in piston cleanliness and sludge protection performance of engine oil.

The energy conserving by engine oils has been required from the viewpoint of the environmental issue. The fuel efficiency of passenger car engine oils has been improved by adding friction modifiers. However, engine oils containing friction modifiers can not be applied to 4-stroke motorcycles. Because motorcycles normally have a wet clutch system inside the crankcase and such engine oils can decrease the clutch capacity.1) ∼ 2) Therefore, it is important for motorcycle engines to investigate additives, which can increase friction coefficient on paper-based friction materials. In this study, friction coefficients of engine oils formulated with different additives such as dispersants and detergents were evaluated with a reciprocating friction tester. Several types of polybutenyl succinimides, sulfonates, phenates and salicylates were used as dispersants and detergents.