Torque converter

A torque converter is a type of fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine, to a rotating driven load. The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. It is usually located between the engine's flexplate and the transmission.

The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. This is a feature beyond that of the simple fluid coupling, which can match speeds but only provides the same torque, thus less power.

Some of these devices are also equipped with a temporary locking mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.

By far the most common form of torque converter in automobile transmissions is the hydrokinetic device described in this article. There are also hydrostatic systems which are widely used in small machines such as compact excavators.

There are also mechanical designs for continuously variable transmissions and these also have the ability to multiply torque. They include the pendulum-based Constantinesco torque converter, the Lambert friction gearing disk drive transmission and the Variomatic with expanding pulleys and a belt drive.

A fluid coupling is a two element drive that is incapable of multiplying torque, while a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque.

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Wikipedia