Analysis And Experimental Research On The Dynamics Of Hydraulic System Of CVTs

Continuously Variable Transmission(CVT)is a most optimized automatic transmission presently,which can continuously adjust the transmission ratio according to driving conditions to ensure that the engine always works in an economical area.However,the CVT would transmit power under the friction between the pulley and pushbelt,and drive and control the variator through the hydraulic system,finally resulting in a low transmission efficiency,only about 70%(NEDC cycle),which is significantly lower than that of the manual transmission and other automatic transmission.Decreasing the pulley pressure could decrease the pump pressure correspondingly and lead to improve the transmission efficiency,but at the same time,it would cause the shifting and slipping between pulley and pushbelt and invalid transmission.To avoid this,the hydraulic system has to be able to respond faster as a result of the reduced safety.First of all,the CVT especially the structure and working principle of its hydraulic system are introduced in details,and the influencing factors of transmission efficiency are explored from various aspects.Based on the existing research results of the industry,the hydraulic system response problem would be researched based on the clamping dynamics,and the basic properties of the hydraulic system,the mechanical and hydraulic stiffness,are considered as the research objects,additionally some influencing factors,such as oil temperature,pressure,air content,ratio,pulley position and running speed,are selected as the actual factors to do the definite research.Secondly,based on the existing AMESim simulation model of another CVT,combining with the experimental lay out of the target research object,the AMESim simulation model of the hydraulic system of the object CVT is established after modified and improved.Simulations only about oil temperature,pressure,air content and ratio are carried out with the control variable method.Comparing the simulation results,the change of mechanical and hydraulic stiffness and the extent of influence of different factors on the pressure response are concluded,and the relevant possible reasons are analyzed.Then the plan of bench test and the corresponding test rig structure are analyzed and designed,and all the bench tests are carried out according to the plan.Tests only about oil temperature,pressure,ratio,pulley position and running speed are conducted with the control variable method,to investigate the influence of the selected factors on mechanical and hydraulic stiffness and the influencing extent on pressure response on actual occasions.Moreover,according to the test results,the relevant possible reasons are analyzed.Finally,comparing the simulation results with the measurement results,the relationship and the influencing extent between the selected factors and the hydraulic pressure response are obtained after being comprehensively analyzed,and the effectiveness of the simulation model is verified.The research done in this paper effectively analyzes the influence and its extent of hydraulic stiffness and mechanical stiffness on the pressure response of the hydraulic system,providing not only an effective direction guidance and theoretical reference for further investigation improving the working efficiency of the CVT,but also data information and methods for other similar researched about hydraulic systems.