Statistical Modeling of Plate Clearance Distribution for Wet Clutch Drag Analysis

Wet clutch packs are the key component for gear shifting in the step-ratio automatic transmission system. The clutch plates are coupled or de-coupled to alter gear ratios based on the driver’s actions and vehicle operating conditions. The frictional interfaces between clutch plates are lubricated with automatic transmission fluid (ATF) for both thermal and friction management. In a 10-speed transmission, there may be as many as 6 clutch packs. Under typical driving conditions, 2 to 3 clutch packs are open, shearing ATF and contributing to energy loss. There is an opportunity to improve fuel economy by reducing the associated viscous drag. An important factor that directly affects clutch drag is the clearance between rotating plates. The axial position of clutch plates changes continuously during operation. It is known in practice that not only the total clearance, but also its distribution between the plates affects the viscous drag. However, it is very difficult to measure the actual distribution for every clutch in a running transmission. Because of the limited theoretical understanding of plate movement, a fine tuning of clutch clearance design is often conducted based on trial and error during a vehicle’s development process. This article describes a statistical method for modeling the distribution of the plates in a clutch pack. The proposed method employs order statistics to represent dynamically-changing plate positions. A simulation study is conducted to investigate the effects of plate movement on open clutch drag in the slip region where ATF is present at the interface. It is shown that the model can predict the difference in drag torque with or without fixed clutch plates. The simulation results compare satisfactorily with experimental data. The model provides a mathematical insight into complex plate behaviors for open clutch drag torque analysis. The use of such a statistical model improves the fidelity of a drive cycle simulation for up-front examination of drivetrain efficiency, thus practitioners can take the impact of clearance into consideration when calibrating open clutch models.