An automatic transmission of a vehicle commonly includes a torque converter for coupling the transmission to the vehicle's engine. The torque converter provides a fluid coupling which allows the engine to spin somewhat independently of the transmission so that the transmission can slow or stop without stalling the engine. Beneficially, the torque converter also provides a magnification of torque from the engine to the transmission when accelerating from a halt. However, at higher vehicle speeds where the speed of the transmission catches up to the speed of the engine the fluid coupling is less efficient than a rigid coupling between the transmission and the engine, and results in increased fuel consumption.
It has been proposed to provide a torque converter with a lockup clutch which provides a selectively operable direct connection between the engine and the transmission to eliminate slippage and to improve fuel efficiency. The lockup clutch has a driving part which is driven by the engine and a driven part which is in direct connection with the transmission. The driving part and the driven part are able to be brought selectively into frictional engagement so as to provide the direct driving connection between the engine and the transmission, and are able to be brought selectively out of frictional engagement when the direct connection is not desired, for example when the vehicle is at a standstill.
The applicant has determined that by providing more accurate control of the lockup clutch better fuel economy may be achieved.