| A Continuously Variable Transmission(CVT)offers a continuum of gear ratios between desired limits,which consequently enhances the fuel economy and dynamic performance of a vehicle by better matching the engine operation conditions to variable driving scenarios.As a kind of ideal transmission system,the structure and layout of CVT are simple.In addition,power transmission of CVT will not be interrupted during the speed ratio changing process.At present,CVT has been widely used in automotive transmission system.Consequently,CVT has broad developing prospects.The rocker-pin joint chain type CVT as a kind of CVT is different from the metal belt CVT because of rocker-pin joint chain,the key transmission part.In this paper,the research object is on the certain prototype of chain type CVT,which is designed for a mini MPV.On the basis of illustration about its structure and principles,verification and optimization be done in function and performance.The road test shows that vehicle longitudinal vibration appeared frequently when accelerating in CVT high speed ratio condition.Speed ratio abnormal fluctuation phenomenon is found by checking the signals of sensors.The test bench research of the chain type CVT shows that the transmitting torque impact speed ratio in the case of high speed ratio: the speed ratio decreases with increase of CVT transmitting torque.Furthermore,it is proved that the tension of the chain has large influence on the length of chain through chain tensile test.While the variation of the chain's length lead to the variation of the pulley wheel working radius,which causes fluctuation of the speed ratio.The speed ratio variation characteristics of different structures are studied from testing aspect.Then the dynamic characteristic of CVT during acceleration is studied from theoretically aspect.With the application of bi-mass model of CVT,the relationship between the speed ratio changing rate and vehicle dynamic characteristic is obtained from the analysis of the transient acceleration process of CVT.Combining with the structure of the transmission,mechanism of speed ratio fluctuation is analyzed.Vehicle dynamic model coupling the engine characteristic with variation characteristic of transmission is built and used to simulate vehicle response that caused by the speed ratio variation characteristic of existing structure.The simulation result reproduces phenomenon where speed ratio and engine speed fluctuate,which shows that the speed ratio variation features of the chain type CVT indeed cause the fluctuation of speed ratio and vehicle speed and proves the validity of vehicle model and theoretical analysis of speed ratio fluctuation.Put experiment data into this model to test the optimized transmission structure,and compare the result with original result.The comparison proves that the optimized structure can resolve the speed fluctuation problem that occurs during vehicle acceleration.In the end the full vehicle test verifies the above estimation and analysis of speed ratio fluctuation problem and shows the speed ratio fluctuation problem and longitudinal vibration problem that occur during vehicle acceleration are completely resolved. |
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