Study of Wedge-Actuated Continuously Variable Transmission

The mechanical efficiency of the current continuously variable transmission (CVT) suffers from high pump loss induced by a high-pressure system. A novel wedge mechanism is designed into the CVT clamp actuation system to generate the majority of clamp force mechanically. Therefore, the hydraulic system can operate at a low-pressure level most of the time, and the pump loss is greatly reduced to improve the CVT’s mechanical efficiency. Through dynamic analysis and design optimization, 90% of clamp force is contributed by the wedge mechanism and the rest of the 10% is generated by a conventional hydraulic system. The optimal design is validated through dynamic modeling using Siemens Virtual.Lab software by simulating the wedge clamp force generation, ratio change dynamics, and system response under tip-in conditions. After that, we built prototype components that target 70% of the clamp force contributed by the wedge mechanism and tested them on a transmission dynamometer. The testing results validated the design with reduced hydraulic pressure, the continuous variable unit (CVU) has above 96% peak efficiency and good ratio change capability. It maintains a stable ratio under different vehicle dynamics conditions, such as engine fire torque pulse, pothole, and engine braking. Fuel economy (FE) evaluation is performed under two scenarios with the Federal Test Procedure (FTP) driving cycle. Assuming 70% of pump loss reduction, the composite FE can be improved by 3.4%. More aggressively, assuming 90% of pump loss reduction, the FE improvement can be 4.4%. However, the major challenge of this concept for automotive application is the reverse gear function because the wedge mechanism can only work unidirectionally. A further consideration is needed such as moving the planetary gearset downstream of the CVU so that the wedge does not need to work bidirectionally.