Application of Empirical Asperity Contact Model to High Fidelity Wet Clutch System Simulations 2019-01-1301
Wet clutches are complex hydrodynamic devices used in both conventional and electrified drivetrain systems. They couple or de-couple powertrain components for applications such as automatic shifting, engine disconnect and on-demand AWD. Clutch engagement behaviors vary greatly, depending on design parameters and operating conditions. Because of their direct impact on vehicle drivability and fuel economy, a predictive CAE model is desired for enabling analytical design verification processes. During engagement, a wet clutch transmits torque through viscous shear and asperity contact. A conventional Coulomb’s model, which is routinely utilized in shift simulations, is inadequate to capture non-linear hydrodynamic effects for higher fidelity analysis. Extensive research has been conducted over the years to derive hydrodynamic torque transfer models based on 1D squeeze film or 3D CFD. They are typically coupled with an elastic asperity contact model for mechanical torque transfer. However, the recent advancement reveals no significant asperity deformation at the frictional surface and establishes a new empirical asperity contact model. This paper describes the integration of empirical asperity contact model with CFD for developing a high-fidelity wet clutch engagement model. The asperity model is examined in detail for four friction materials to highlight complex contact mechanisms. The simulations using the integrated CFD clutch model demonstrate the importance of using the right asperity contact model. The simulation results are validated against bench test data that is designed to replicate actual clutch lubrication conditions in a vehicle. They provide a breakdown of hydrodynamic and mechanical torques, enabling numerical examination of clutch engagement behaviors for varying conditions.