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Tappet/bore friction and torque at the camshaft were measured for a direct acting bucket tappet using a cam/tappet friction apparatus. Tappet/bore and cam/tappet friction torque and friction coefficient as a function of cam angle were derived from those measurements. The results showed that, for the particular geometry tested, tappet/bore friction torque accounted for about 13% of the total cam/tappet/bore friction torque at 250 cam rpm. This fraction decreased with increasing speed. Tappet bore friction was greatest at about ± 40 degrees of cam angle, where side loads on the tappet bore were highest. In contrast, earlier results for a center pivot rocker arm design showed tappet bore friction to be negligible.

The influence of friction reducing oil additives was assessed as- part of a broader study of valve train friction. Experiments were performed using a motored valve train to measure overall oil effects. Experiments were carried out using a cam/tappet tribometer to evaluate the detailed behavior of the cam/tappet contact. Both sliding and roller tappets were employed in the study the results illustrate the potential for the reduction of frictional losses in sliding cam/tappet contacts through lubricant optimization.