In an automatic transmission, torque converter is the main device that transmits torque from the engine to the gear train. An open converter relies on fluid coupling to transmit torque, which causes energy loss and also causes engine to run at a higher speed. A single-plate locking or slipping clutch has been used between the pump and turbine of a torque converter so that the slip across the converter can be reduced to a level that increases the efficiency of the converter. This slip control, called Electronically Controlled Capacity Clutch (ECCC), also provides necessary damping to the driveline to ensure pleasant driveability similar to an open converter. New legislative and environmental pressures on car makers to improve fuel economy have resulted in aggressive use of ECCC in their automobiles. However, the controllability and heat capacity of the single-plate clutch limit the ability to engage the torque converter clutch (TCC) more aggressively (in earlier gears and lower vehicle speeds). At GM, an experimental multi-plate toque converter was designed, fabricated, and implemented in a RWD transmission. It was demonstrated in a 6-speed vehicle that driving pleaseability can be maintained with a more aggressive ECCC strategy. For this purpose, a control algorithm was developed to control the slip speed of the clutch and allow early ECCC starting in second gear. This paper describes in detail hardware implementation of the multi-plate clutch, development and implementation of the aggressive ECCC control strategy, and some vehicle test results.

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