Thermal Elastohydrodynamic Lubrication Analysis And Low Friction Design Of The Cam/Tappet Conjunction Of Automobile Engines

In recent years,the lack of resources and the polluted air make energy conservation and environmental protection become a hot issue of concern.The large amount of automobiles have a serious problem of energy consumption and pollution emission.It is of great significance to reduce the friction loss of each friction pair in automobile engines.As one of the key friction pairs in the engine,the cam/tappet conjunction accounts for 13% of the friction loss of the whole engine.And this paper studies the change of the lubrication performance and the distribution of the friction coefficient in one cycle of the cam-tappet pair.For the low friction design,this paper discusses the influence of different spring parameters and the volume of sodium fillers on the friction loss of the cam-tappet pair.The effects of different coatings on the cam-tappet are also investigated.Finally,the dynamics of the cam-tappet system during start up is investigated.And the friction-reducing effect of the coatings during the start up process is analyzed.Firstly,the thermal elastohydrodynamic lubrication(TEHL)model of the cam-tappet pair is established by considering the squeezing effect and the heat.The Reynolds equation,the elastic deformation equation,the load balance equation and the energy equations are solved jointly to attain the lubrication results of cam-tappet.The results show that the change trend of the minimum oil film thickness is similar to that of the entrainment velocity,and the distribution of the maximum film pressure and the maximum film temperature has similarity with the contact pressure curve.In addition,the change trend of the friction coefficient curve is similar to the maximum film pressure.On the basis of the established TEHL model,the effects of the spring parameters of the cam-tappet system on its lubrication and tribology are numerically simulated.The results show that the smaller the spring stiffness and the preload force,the smaller the coefficient of friction and the friction loss.The influence of the different volumes of sodium filler on the friction loss is negligible.Then,a TEHL numerical model is established to simulate the influence of the mechanical properties and thermal properties on the lubrication and friction coefficient of the cam-tappet pair.The effects of the coating thickness,the ambient temperature and coating position on friction pair are also studied.The results show that the "low thermal inertia" coatings and the "soft" coatings can significantly reduce the cam-tappet friction coefficient,and "high thermal inertia" coatings and "hard" coatings increase the friction coefficient of the cam/tappet conjunction,and their effects will increase with the thickness of the coating.The influence of coating of both the cam and tappet on friction loss is greater than the effect of coating of the tappet only,which is greater than the effect of coating of the cam only.Finally,the lubrication performance of the cam-tappet pair during cold start and warm start is investigated.At the same time,the influence of the coating on the cam-tappet conjunction during the start up is also studied.The results show that the curves of the minimum film thickness,the maximum film temperature and the friction power loss show a tendency to increase periodically,the friction coefficient curve shows a cyclical decrease.The friction coefficient during cold start up is much larger than that of the warm start.The “low thermal inertia” coating can greatly reduce the friction coefficient and friction power during the start up process.