Valvetrain Friction Reduction through Thin Film Coatings and Polishing

In a direct-acting mechanical bucket tappet–type valvetrain, the cam and tappet contact is responsible for about 85% of the total valvetrain frictional losses. Because this contact operates primarily in a mixed lubrication regime, it offers an opportunity for friction reduction through surface engineering. The friction reduction potential of thin Mn-phosphate coating, diamond-like carbon coating, and polishing on the bucket surface was explored using a motored valvetrain rig equipped with 3.5L V6 engine head. The durability of tappets and cam lobes was also evaluated using a different motored valvetrain rig consisting of a single lobe and a single tappet. The polished buckets demonstrated substantial friction benefit over current production buckets at all speeds investigated. The diamond-like carbon coated buckets did not show any additional friction reduction benefit. The wear data demonstrated much less wear with polished buckets and also for cam lobes when in contact with polished buckets compared to current production buckets and cam lobes. The composition of antiwear surface films on polished buckets was found to be similar to that on current production buckets.