Characterization and Modeling of Wet Clutch Actuator for High-Fidelity Propulsion System Simulations

Innovations in mobility are built upon a management of complex interactions between sub-systems and components. A need for CAE tools that are capable of system simulations is well recognized, as evidenced by a growing number of commercial packages. However impressive they are, the predictability of such simulations still rests on the representation of the base components. Among them, a wet clutch actuator continues to play a critical role in the next generation propulsion systems. It converts hydraulic pressure to mechanical force to control torque transmitted through a clutch pack. The actuator is typically modeled as a hydraulic piston opposed by a mechanical spring. Because the piston slides over a seal, some models have a framework to account for seal friction. However, there are few contributions to the literature that describe the effects of seals on clutch actuator behaviors. In a routine simulation, a spring constant is commonly tuned to match vehicle data, assuming that it captures the effects of seal friction. The validity of this approach is not well established. This article describes the characterization and empirical modeling of a wet clutch actuator. The effect of seal friction is examined in detail during stroking and de-stroking. It is found that the seal friction is highly non-linear and directional. It introduces a significant error in clutch applied force calculation unless seal friction is explicitly accounted for. Propulsion system simulations are conducted to demonstrate the significant impact of seal friction on clutch operation and the quality of simulations. A framework of a new actuator model is proposed to represent seal friction based on empirical observations of its complex behaviors.