Effects of Composition and Surface Finish of Silicon Nitride Tappet Inserts on Valvetrain Friction

Abstract In order to build more fuel efficient engines, new materials and lubricant formulations are being sought to reduce frictional losses. The valvetrain contributes about 6-10% of the total frictional losses in an engine. To reduce valvetrain frictional losses, polished silicon nitride tappet inserts were evaluated for their friction reduction potential. Silicon nitrides obtained from three sources were polished using three different processes: the supplier's conventional diamond polishing technique and two non-diamond polishing techniques-“Ford Finish” and chemo-mechanical polishing. The valvetrain friction torque was measured in a laboratory apparatus using a single cam lobe rotating against a direct acting mechanical bucket tappet with production engine hardware. The friction torque values obtained with surfaces prepared by all three processes differed significantly although their initial centerline average surface roughnesses were similar. All three silicon nitride surfaces prepared by “Ford Finish” showed lower friction torque than the production steel surface when an engine oil containing no friction modifier was used. With a low friction oil containing a friction reducing additive, friction torque values were significantly lower and polished silicon nitride surfaces did not offer additional friction reduction benefit relative to the production steel surface. A simple calculation showed a maximum of about 0.5 % fuel economy benefit can be gained due to polishing with the engine oil without friction modifier but very little, if any, with low friction oil. However, silicon nitride inserts may be useful for weight reduction and increased durability.