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A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the prime mover to the automatic gear train, which then drives the load. It is thus usually located between the engine's flexplate and the transmission. The equivalent device in a manual transmission is the mechanical clutch.
A torque converter serves to increase transmitted torque when the output rotational speed is low. In the fluid coupling embodiment, it uses a fluid, driven by the vanes of an input impeller, and directed through the vanes of a fixed stator, to drive an output turbine in such a manner that torque on the output is increased when the output shaft is rotating more slowly than the input shaft, thus providing the equivalent of an adaptive reduction gear. This is a feature beyond what a simple fluid coupling provides, which can match rotational speed but does not multiply torque. Fluid-coupling–based torque converters also typically include a lock-up function to rigidly couple input and output and avoid the efficiency losses associated with transmitting torque by fluid flow when operating conditions permit.