Torque Converters

A torque converter is a hydraulic fluid coupling that is used to transmit power from one or more engines or motors to a driveshaft or other output shaft. more...

It takes the place of a mechanical clutch, and, within certain operating speed ranges, multiplies input torque, providing the equivalent of a reduction gear. Torque converters are commonly found in automotive automatic transmissions, but are also used in marine propulsion systems and various industrial machine tools.

Function

Basic fluid-coupling elements

A torque converter, like any fluid coupling, is a sealed chamber filled with hydraulic fluid (typically light oil) and containing a pump (or impeller) driven by the engine(s) and a turbine connected to an output shaft.

The impeller is a toroid disc connected to the engine's crankshaft (or output shaft of the motor or other power source). When the engine or power source is operating it turns the impeller at the same speed. The rotation of the radial chambers on the inner surface of the pump imparts a centrifugal radial flow to the fluid in the converter, which causes hydraulic fluid to strike the outer edges of the turbine. The radial chambers on the surface of the turbine transmit the angular momentum of the fluid centripetally, reversing its direction and exerting a twisting force torque on the turbine disc that causes it to rotate in the same direction as the impeller. The fluid exits the center of the turbine and returns to the impeller to begin the cycle again.

Because some of the kinetic energy imparted to the fluid is lost to friction (raising the temperature of the fluid rarther than causing motion within it), the turbine always slips (rotates slower than the impeller), particularly at very low speeds. If the speed of the impeller is very low, such as at idle speed for an automobile engine, the torque exerted on the turbine output shaft will not be enough to overcome the shaft's inertia, allowing the shaft to remain stationary without stalling the engine and eliminating the need for de-clutching.

Read more at Wikipedia.org