Statistical Modeling of Plate Clearance Distribution for Wet Clutch Drag Analysis

Paper #:
  • 2017-01-2446

  • 2017-10-08
Wet clutch packs are the key component for gear shifting in the step-ratio automatic transmission system. They are coupled or de-coupled to alter gear ratios based on driver’s demand 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 any driving conditions, 2 to 3 clutch packs are typically open, shearing ATF and contributing to energy loss. There is an opportunity to improve fuel economy by reducing this viscous drag. One main factor that directly affects clutch drag is the clearance between rotating plates. The axial position of clutch plates changes continually at every instance. It is empirically known that not only the total clearance, but also its distribution between the plates affects the viscous drag. However, it is impractical to measure the actual distribution for every clutch in a running transmission. Because of the limited theoretical understanding of plate movement, clutch clearance design is often conducted based on trial and error during a vehicle development process. This article describes a statistical method for modeling the distribution of the plates in a clutch pack. The proposed method employs the 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 a slip region where ATF is present at the interface. It is shown that the model can quantify the difference in drag torque with or without fixed clutch plates, as compared to 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 drain-train efficiency.
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