ASSESSMENT OF FRICTION FOR CAM-ROLLER FOLLOWER VALVE TRAIN SYSTEM SUBJECTED TO MIXED NON-NEWTONIAN REGIME OF LUBRICATION

The tribology of cam-roller follower conjunction is highly dependent on the engine type and working conditions. The interface experiences transient conditions due to variations in contact geometry and kinematics, as well as loading. These lead to instantaneous and capricious behavior of the lubricant through the contact, which determines the regime of lubrication. The resulting frictional characteristics are affected by the shear of the lubricant film and the interaction of rough surfaces themselves. Thus, specific analysis is required for any intended new engine configuration. Therefore, a tribo-dynamic model, combining valve train dynamics, contact kinematics and tribological analysis is required. An important issue is to develop a simple yet reliable and representative model to address the above mentioned pertinent issues. This would make for rapid scenario-building simulations which are critical in industrial design time-scales. The current model has been developed in response to the above mentioned requirements. A multi-body dynamic model for the valve train system based on the key design parameters is developed and integrated with an EHL tribological model for the cam-follower contact. To keep the model simple and easy to use and to avoid time-consuming computations, the analytical EHL model makes use of Grubin's oil film thickness equation. Viscous and boundary contributions to friction are obtained as these account for the losses which adversely affect the engine fuel efficiency.