The Torque Converter in Operation
The torque converter has three phases or stages of operation: the stall phase, the acceleration phase, and the coupling phase.
During the stall phase, the transmission is in gear, and the turbine is held stationary by the brakes/drivetrain. The impeller/pump is spinning at engine speed while the turbine is held stationary.
During acceleration, torque multiplication occurs, the impeller is spinning faster than the turbine. The stator's one-way clutch to locks; the curved vanes redirect transmission fluid in the same direction as crankshaft rotation. This redirected flow assists the impeller resulting in torque multiplication. This phase occurs when the vehicle is accelerating from the stoplight and continues up until cruising speeds.
At cruising speeds, the coupling phase occurs; the turbine speed is around 90% of impeller speed.
During the coupling phase, the lockup clutch engages. The mechanical connection cools the transmission fluid and increases efficiency.
Many modern automatic transmissions partially engage the torque converter clutch in all forward gears except first. A pressure control solenoid varies the pressure to the clutch, improving performance and fuel mileage.