Automatic Transmissions (AT)

Overview for AT

ATs are composed of a hydrodynamic torque converter and a mechanical gearbox, which realizes the gear ratios via complex planetary gearsets, both operated via the transmission control unit.

The hydrodynamic torque converter enables smooth vehicles starts like the clutch in AT/AMT systems, but it also has two unique features: the torque multiplication and the creeping. Torque multiplication means that the torque converter does not only transmit the engine torque, but under specific operating conditions, it can significantly magnify the input torque. This enables particular high drive torque during vehicle starts.

The torque converter cannot completely disconnect the engine from the gearbox, which means, it is never completely open. Even when disengaged, there is a noticeable amount of torque reaching the gearbox. This is utilized to move the vehicle very slowly, even without touching the accelerator pedal and is referred as creeping. This is an inherent feature of an AT, and is beneficial for precise maneuvering (you can stop the vehicle any time by pressing the brake pedal).

The mechanical part of an AT is made up of planetary gearsets instead of simple gearwheel pairs. The shifting elements of the planetary gearsets are capable of operating under drive torque, so the tractive force on the vehicle wheels is not interrupted during gearshifts. Unlike for DCTs, the capability for powershifting is not limited to sequential (one-by-one) gearshifting, it is available for gear jumps as well.

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