Silp Ratio Control Of V-belt CVT In Non-stable Conditions

Metal V-belt CVT(continuously variable transmission(CVT) has the following advantages :simple structure, small volume, light weight, continuous variation ratio, comfort driving and so on, the market possession rate has been continuously improving in recent years. Theoretically speaking, CVT can remain engine speed and torque in the vicinity of the optimum economic curve. However, due to its transmission efficiency is lower than the hydraulic automatic transmissions, fuel efficiency with CVT is not significantly improved. More and more studies show that CVT transmission efficiency is related to the slip ratio between CVT belt and pulley.Therefore, in order to make the transmission efficiency of CVT the highest, the slip rate should be kept at an optimal value. In order to guarantee the stability under various operating conditions of CVT, traditional clamping force control method usually adds the safety factor to clamping force of secondary pulley to make the slip rate is less than the optimal slip ratio. This paper proposes a kind of control strategy that is aimed at control the target slip rate and makes the slip rate correction under unstable conditions in order to improve the stability and transmission efficiency of CVT.First of all, this paper analyzes the structure, the transmission ratio and thrust force of CVT and compares double pressure hydraulic control circuit with single pressure hydraulic control circuit. And then determine the main factors that cause the loss of CVT efficiency, mainly including the friction loss of the transmission mechanism and the overflow loss of the hydraulic system.Then, slip ratio is introduced. By measuring the position of the current primary pulley and speed of primary and secondary pulley, the current slip ratio can be measured. The relationship between the slip ratio and the transmission efficiency is measured by the test, and then the optimal slip ratio curve is obtained under the different speed ratio.Furthermore, this paper analyzes the relationship between the actual optimal slip ratio and the theoretical optimal slip ratio. The results show that the slip ratio under unstable conditions should be different from that of the bench test. This paper establishes recognition method of vehicle driving cycle, different driving condition determines slip rate correction coefficient and then determines the actual optimal slip ratio.Finally, this paper proposes clamping force control method under unstable conditions and establishes the simulation model of the vehicle with CVT by using Simulink and AMESim software. This paper also simulates the clamping force of unstable conditions in different cases, such as the acceleration, deceleration, uphill, downhill. And then compares them with the traditional clamping force control method. Experimental results show that this method increases CVT stability, improves the efficiency of the CVT, and verifies the feasibility of the clamping force control method.