A Valve Train Friction and Lubrication Analysis Model and Its Application in a Cam/Tappet Wear Study

Wear of valve train components has increasingly become a problem in engine durability and reliability these days, due to the many design changes to meet the requirements of emission legislation and high performance of automotive engines. To minimize friction and the possibility of severe wear at the cam/tappet interface, the analysis at the design stage of tribological behavior of the cam/tappet pair is important and has become an important feature in valve train design. This paper describes the development of a valve train friction and lubrication analysis model and its application in a cam/tappet wear study.

The model is based on established technology including kinematic and dynamic analyses, prediction of Hertzian stress of both line and elliptical contacts, the elastohydrodynamic lubrication (EHL) theory a mixed-friction model which separately predicts hydrodynamic and boundary friction and estimation of the average surface temperature using the flash temperature concept. Part of the results of a push-rod type valve train and its validation against experimental measurements of a motoring test have been reported. Further the tribological performance of this valve train with a designed cam profile and with an assumed worn-cam profile were studied. From this study it has been found that the surface temperature of the cam/tappet has an important effect on cam wear at high engine speeds. The predicted results show that once abnormal cam wear occurs, it will intend to spread towards cam nose. The developed computer model has proven to be a very useful tool in assessing and enhancing the performance of a given valve train design.