Dynamic EHL Film Thickness in Cam and Follower Contacts of Various Valve Lifts 2000-01-1789
The resistance of driving forces in engine mainly comes from contact behaviors of many components such as piston skirt, piston rings, big-end or main bearings and other components of valve train system. The contact between cam and follower has one of the severest tribological behaviors in these components due to cyclic fluctuation of loads and sudden change of relative contacting speed. In order to verify the characteristics of frictional resistance and wear in these components, the investigation to find the minimum film thickness of these tribological components should be performed preferentially. This verification gives useful clue for better performance and endurance life of the whole engine system. In this investigation, the most important input parameters are the applied loads and the relative contacting velocities during the cycle. These parameters can be obtained from the simulation of kinematics and dynamics of valve train system. In this work, the valve train system of push-rod type which is in wide use for diesel engines are studied for the calculation of applied loads and relative contacting velocities between cam and follower. The follower model in our work is of flat shape, which provides basic concepts of contact behaviors between cam and follower and is applicable to various curvatured followers.
The contact geometry between cam and follower is very non-conformal that results in concentrated fluid film pressure and very high-stress on the contact region. Therefore, it is necessary to include the elastic deformation of contact region in the computation of lubricant film thickness, which is called elastohydrodynamic lubrication (EHL) in tribology. The conventional analysis for the numerical computation of fluid film thickness with elastic deformation of contact region is performed by Newton-Raphson method for its fast convergence characteristics. However, high fluctuation and sudden change of loads and velocities occurring in the contact between cam and follower frequently make it impossible for Newton-Rahpson method to get both converged and stable solutions. In particular, this method cannot provide converged solution under the condition of high load above 1.0 GPa which frequently occurs in cam and follower contact. Multigird multi-level method for the solver of non-linear partial differential equation including solid deformation is preferred to Newton-Rahpson method for better convergence and stability and is applied in our research.
Many researches about the contacts between cam and follower have investigated EHL film thickness either without dynamic loading effect or only with curve fitting formula such as Dowson-Hamrock's, because including squeeze film effect makes it hard to obtain convergence and stability. Therefore, inaccurate information about minimum film thickness without dynamic loading condition causes inappropriate design of valve train system and wrong selection of cam and follower materials. In our work, we developed computation tools both for kinematics and dynamics of valve train system of push-rod type and for fluid film thickness with elastic deformation on the basis of dynamic loading condition with multigrid multi-level method. The results of minimum film thickness with the respects of both static and dynamic loading conditions are compared for the contact of flat follower with various cam lift profiles during the entire cycle.