Development of a Low Loss Clutch for CVT Reverse Function 2019-01-0774
Continuously variable transmissions (CVT) provide superior fuel economy by enabling internal combustion engines to operate at their “sweet spots”. However, there is still potential to improve CVT system’s mechanical efficiency, and further enhance vehicle-level fuel economy. In the past, extensive research work has focused on the core continuously variator unit (CVU) that includes pulleys and a belt or chain. Another thread of research has centered on optimization of CVT clamping force control to reduce hydraulic system loss. Nonetheless, to the best of our knowledge, very little research has looked into the planetary gear sets and clutches that enable the CVT system to switch between forward, neutral and reverse gears. The state-of-the-art reverse clutch usually consists of multiple friction and steel plates, and is normally open during all forward driving maneuvers. The relative speed between friction and steel plates is identical to turbine speed, which generate spin loss. We believe there is an opportunity to improve the CVT system mechanical efficiency by replacing the reverse plate clutch with a low loss clutch, for example a binary clutch. At first, we studied the theoretical spin loss associated with plate clutches. Secondly, we identified the most challenging shifting maneuver using the lever diagram analogy. Thirdly, we proposed a control strategy to address the most critical rolling garage drive-to-reverse shift, and conducted one-dimensional simulation using an AMESim model to validate our strategy by comparing our simulation result with available vehicle data. At last, we obtained spin loss data from dynamometer testing to evaluate the potential fuel economy benefit.