Advanced Bench Test Methodology for Generating Wet Clutch Torque Transfer Functions for Enhanced Drivability Simulations 2019-01-2340

A wet clutch continues to play a critical role for step-ratio automatic transmissions and finds new utilities in hybrid and electrified propulsion systems. A torque transfer function is often employed in practice for sophisticated clutch slip controls. It provides a simple, yet practical framework to represent clutch torque as a function of actuator force. An accurate transfer function is also increasingly desired in today's vehicle design process to enable upfront assessment of clutch controls through simulations. The most common approach is based on Coulomb's linear friction model, where the coefficients are adaptively identified based on vehicle data. However, it is generally difficult to tune Coulomb's model for hydrodynamic behaviors even if the reference vehicle data are available. It also remains a challenge to produce in-vehicle clutch behaviors on a component test bench to determine realistic transfer function before prototype vehicles are built. SAE#2 test procedure is the industry standard for evaluating clutch frictional behaviors. It is a viable tool for durability assessment, but not designed to characterize hydrodynamic behaviors for clutch controls. This research focuses on the development of a methodology to generate realistic clutch transfer functions using an advanced engagement bench tester. The test stand is equipped with programmable slip and force controllers to replicate both torque phase and inertia phase of gear shifting. It accommodates a clutch module, not only the clutch pack, to reproduce actual in-vehicle lubrication conditions. The clutch behaviors are characterized for various combinations of operating conditions. The bench test data are compared with SAE#2 data to highlight the sensitivity of hydrodynamic behaviors to force and slip profiles. A regression technique is utilized to represent clutch behaviors as a transfer function in non-linear forms using data from the advanced tester. Shift simulations are conducted to demonstrate the value of realistic transfer functions to enable upfront drivability assessment for control development.