In-Vehicle Characterization of Wet Clutch Engagement Behaviors in Automatic Transmission Systems 2018-01-0395
A new generation of a planetary-gear-based automatic transmission system is designed with an increasing number of ratio steps. It requires synchronous operation of one or more wet clutches, to achieve a complex shift event. A missed synchronization results in drive torque disturbance which may be perceived by vehicle occupants as an undesirable shift shock. Accurate knowledge of clutch behaviors in an actual vehicle environment is indispensable for achieving precise clutch controls and reducing shift calibration effort. Wet clutches are routinely evaluated on an industry-standard SAE#2 tester during the clutch design process. While it is a valuable tool for screening relative frictional behaviors, clutch engagement data from a SAE#2 tester do not correlate well with vehicle shift behaviors due to the limited reproducibility of realistic slip, actuator force profiles, and lubrication conditions. Advanced clutch testers with programmable slip and force controller are available for replicating torque phase and inertia phase of shifting. However, it remains a challenge to substantiate bench test data in the absence of actual clutch behaviors observed in a vehicle. This article describes the in-vehicle characterization of wet clutch engagement torque, with a focus on temperature effects during gear ratio changing. Clutch torques are accurately identified based on transmission torque measurements at input and output shafts. The analysis shows that the relationships between actuator force and clutch torque are highly non-linear with respect to transmission oil temperature, demonstrating the importance of in-vehicle clutch characterization for robust shift control. Furthermore, a squeeze film analysis is conducted to provide physical insight into clutch behaviors during torque transfer phase of shifting, where Coulomb’s linear friction model is not applicable.