Technology explained

How does the torque converter actually work?

In classic automatic transmissions, such as those produced by automotive supplier ZF, a torque converter ensures smooth power transmission.

(Bild: ZF)

Even if the competition comes from two sides: the torque converter still plays an important role in cars. But how does it actually work?

The combustion engine is bad at starting off. Because it first has to build up its power through speed, it cannot drive the wheels directly from the start, unlike an electric motor. In order to enable it to start up at idling speed and then to achieve a smooth power connection, it is first separated from the rest of the drivetrain. Either via a clutch - or, in the case of classic automatic transmissions, via the torque converter.

The task of the torque converter is therefore basically the same as that of the clutch: It separates and connects the engine and transmission. However, instead of friction linings, it uses oil to transmit power. The viscous fluid is set in motion on the engine side by a pump wheel in the torque converter housing. This flow hits a fixed guide wheel and is directed to a turbine wheel, which absorbs the power and transfers it to the transmission.

From there, it is sent to the wheels and the vehicle slowly starts to move. As soon as the pump and turbine wheel or the engine and transmission are turning at the same speed, the torque converter lock-up clutch is engaged.

Automatic torque converter has advantages
The torque converter lock-up clutch is necessary because power transmission via a fluid is always inefficient - too much power is lost in the flow. However, this minor disadvantage is offset by a number of advantages, above all the increase in torque due to hydrodynamic effects which, unlike with the friction clutch, do not generate any mechanical load. The wear-free technology is therefore particularly popular for trailer operation. There is also a slight increase in comfort, as the hydraulic oil in the torque converter buffers the engine vibrations somewhat. The creep effect is also often perceived as pleasant: as the engine and transmission are never completely separated in "D" mode, the vehicle always moves slowly, even without accelerating. This can be an advantage when carefully maneuvering or parking.

In recent years, the torque converter has benefited greatly from a general trend towards automatic transmissions; the manual gearbox is disappearing. However, the torque converter automatic has increasing competition: on the one hand, the electric car, which does not need any special starting technology because the engine immediately builds up power. And from the other side in the form of the dual-clutch transmission, which also changes gears automatically, but works with electrically actuated clutches instead of a torque converter. Nevertheless, this low-wear technology should still have a future, especially in commercial vehicles. And probably also for hybrid drives in the medium term.