Abstract
A lock-up clutch control system for modern automatic transmissions is an effective way to improve fuel economy and dynamic performance. The major problem of a lock-up clutch is that undesired torque fluctuation and booming noise reduced the controllability especially at low vehicle speed. Also, frequent engagement and disengagement of a lock-up clutch reduces the life times of clutch friction material and components. Therefore, it is important to operate lock-up clutch in optimal conditions such as converter mode, lock-up mode, and slip control mode. In this paper, we focused on the slip control of a lock-up clutch to understand dynamic behavior. Because modeling of nonlinear dynamics of a lock-up clutch and a damper system is highly difficult, commonly nonlinear properties of the systems are obtained from experimental data. We employ the LuGre friction model for the dynamic friction model of a lock-up clutch, which describes the friction phenomena reasonably well. The vehicle dynamic model, including engine, torque converter, transmission, vehicle and tire, was implemented on co-simulation. In order to investigate the fuel consumption of a vehicle with slip controlled lock-up clutch system, the backward-facing simulation was investigated using lock-up clutch map with slip control schedule. To avoid undesired transient vibrations such as slip shudder and surge, we implemented a feedback controller to follow target slip ratio as well as to obtain improved responses. Consequently, the result of simulation was compared to real driving cycle experiments to validate the reliability of the simulator. Results of the comparison suggest that the slip controlled lock-up clutch system improves both fuel efficiency and dynamic performance of a vehicle.
Keywords: Lock-up clutch, Slip control, Torque converter, Fuel economy, Engine load ratio