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The requirements for emission control, lower fuel consumption and higher engine output have changed the engine valve train system to 4-valve/cylinder and higher cam lift designs, and these changes make the cam/tappet lubrication conditions more severe than before. Under such a working condition, there is a high possibility that cam/tappet surface damages such as scuffing, pitting and wear may occur. Among the damages, the wear of cam/tappet is the most difficult to predict since the wear mechanism still remains unclear. To understand the lubrication condition and therefore, the wear mechanism at the cam/tappet contact, friction was measured with a newly developed apparatus. Measurement results showed that the lubrication condition between cam and tappet is predominantly in the mixed and boundary lubrication conditions.

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.