A tribological study of the design and performance of automotive cams

Analytical methods to enable the evaluation of important lubrication operational parameters at the contact between any cam and follower mechanism (excluding valve trains incorporating rolling element followers or hydraulic lash adjusters) have been collated, critically assessed and developed. A robust and user friendly computer program, which incorporated these methods, was written in order that the tribological conditions existing at the cam/follower interface of any type of valve train in common use in today's internal combustion engines could be studied. The output from the program included graphical displays of frictional torque, minimum lubricant film thickness and Hertzian stress around the cam cycle. Such studies were performed on a cam and flat faced follower system, a cam and centrally pivoted follower system, a cam and end pivoted follower system and a desmodromic system (comprising a conventional cam and centrally pivoted. system and a desmodromic cam and end pivoted follower system). The computer program also allowed parametric studies to be carried out on valve train mechanisms. Parametric studies of three different valve trains, including the valve trains from the Rover 2300 and the Ford 2.0 litre Pinto engines, have been presented, the results being presented in graphical and tabular form. The loadings, orbits, and power losses associated with the camshaft bearings of the Ford 2.0 litre Pinto engine were evaluated using existing dynamically loaded bearing analysis techniques. The total frictional power loss predicted for the three camshaft bearings was found to be equal to approximately one fifth of that calculated for all of the cam/follower interfaces throughout the operational speed range of the engine. An experimental single valve desmodromic valve train apparatus was designed and commissioned to test the accuracy of the valve train lubrication analysis computer program. The apparatus allowed studies to be made of the running-in of valve trains operating at lubricant temperatures of 40C, 60C and. 80C, by applying the electrical resistivity technique. Analytical models used to predict which cam/follower pair was in control of the valve at any point around the cam cycle were tested using an electrical continuity technique and were found to show good agreement with practice. Good agreement was also found between the theoretically predicted and measured torque and power required to drive the valve train.