Normally-Engaged Dual-Piston Clutch for Engine Stop-Start Application

For the conventional 6 speed automatic transmission with engine stop-start powertrain, an electrically-driven auxiliary pump is implemented to maintain the transmission line pressure as required to lock-up the CB1234 clutch during engine auto-stop conditions. Upon releasing the brake pedal, the transmission engages into first gear with the objective to accelerate the vehicle in a responsive manner. In this study, a novel normally-engaged dual-piston clutch concept is designed to keep the CB1234 clutch locked-up during engine auto-stop conditions with the intention to eliminate the auxiliary pump without compromising vehicle performance. This dual piston clutch concept requires a relatively low line pressure to release the normally-engaged clutch when needed, thus, minimizing the hydraulic pumping work. To explore the functionality of this concept under a wide-open-throttle (WOT) auto-start transition, modeling and simulation of the normally-engaged dual-piston clutch is completed. A component-level model of the dual-piston clutch and preloaded Belleville spring is developed in the AMESim environment. The model is refined and validated by comparing the simulated results with spin rig performance data. Important dynamic features, including the CB1234 clutch pressure and release pressure profiles are predicted and explained. The developed component model is integrated into a vehicle model and validated by comparing simulation and vehicle test data under auto-start conditions. Using the validated model, a parametric study is conducted to examine the effect of the hydraulic orifice size in the release piston fluid passage. The study identified the appropriate orifice size requirements, illustrating that the normally engaged dual-piston clutch has potential to lock-up the CB1234 clutch under wide-open-throttle auto-start condition.