Coupling Optimization Research Of Crankshaft And Bearings Based On Elastohydrodynamics Lubrication

In the situation of energy conservation and emission reduction, strengtheningdegree of modern internal combustion engine is constantly improving. In order toadapt to high speed, high power and the complex working conditions, higherreliability requirement of internal combustion engine is put forward in thedevelopment of new products.Engine crankshaft is a high-speed operation component which bears alternatingload, and is responsible for the power output. The designing rationality of crankshaftand its bearings affects the friction loss and service life of the engine. An efficient andstandardized positive development needs to optimize the crankshaft and bearings as awhole, so that the dynamic strength and lubrication can all obtain excellentcomprehensive properties.However, in the traditional structure design of internal combustion engine, onone hand, experience is often dominant and the calculation is based on simplifiedmodels, which may lead to parts failure because of considering the actual factorsinsufficiency. So this method has not been able to meet the needs of modernizationpositive development. On the other hand, conservative structure scheme is often takenwhich is not advantageous for lightweight design and the economy improving.Besides, designing crankshaft and bearings respectively, alone with the lubricationperformance or alone with the structural strength as an optimization object, lackscomprehensive consideration of crankshaft bearing system in dynamics, tribology andelastic mechanics, which might increase the product development’s risk.In this paper, basing on the development demand of a2.7L commercial vehiclediesel engine, in order to make the crankshaft and bearings have good reliability andlubricating properties, lubrication calculation is performed and the influence factorsstudy is made for the crankshaft and bearings system. Such theories and techniquesare applied as finite element modeling, modal reduction technique, flexible multibody dynamics and elastohydrodynamic lubrication theory. A virtual prototype modelof engine is created in the AVL Power_unit. The final calculation results mainlyinclude the minimum oil film thickness, oil film pressure and its distribution, frictionloss etc. Analysis of the bearing clearance, oil hole position, groove width, thecentrifugal moment balance rate and other factors’ influence on lubricatingperformance is made. At the same time, fatigue strength of crankshaft is analyzedbased on dynamic calculation’s load. The results show that the initially designedmodel of main bearings has poor lubrication condition while the conrod bearings canmeet the requirements of lubrication, and the minimum fatigue safety factor ofcrankshaft is1.11. Therefore, an overall optimization study based on an approximatemodel for crankshaft and bearings is taken. There’re two optimization objects in theapproximate model, that is, minimum total friction loss of bearings and minimumcrankshaft’s weight. And this model is subject to two constraint conditions. One is thatthe minimum film thickness must be larger than0.6μ m. Other one is that thecrankshaft fatigue safety factor has to be greater than1.3. Through optimization, thetotal friction loss of bearings reduces by27.4%. Finally, through the modal test and bending fatigue test of crankshaft, the accuracy of finite element model is verified andthe strength reliability of crankshaft is proved to be up to the mustard.