Fluid coupling

A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and industrial machine drives, where variable speed operation and/or controlled start-up without shock loading of the power transmission system is essential.
A fluid coupling is a sealed chamber containing two similarly shaped toroids facing components,impeller and turbine, immersed in fluid (usually oil). The driving impeller, often referred to as the pump or driving torus (the latter a General Motors automotive term), is rotated by the prime mover, which is typically an internal combustion engine or electric motor. The motion of the pump's radial chambers imparts a relatively complex centripetal motion to the fluid. Simplified, this is a centrifugal force that throws the oil outwards. The moving fluid reaches the casing and the enclosed shape forces the flow in the direction of the driven impeller, referred to as the turbine or driven torus (the latter also a General Motors term). Here, the Coriolis force reaction transfers the angular fluid momentum outward and across, applying torque to the turbine, thus causing it to rotate in the same direction as the pump. The fluid leaving the centre of the turbine returns to the pump, where the cycle repeats.
in automotive applications, the pump is connected to the flywheel of the engine (in fact, the coupling's enclosure may be part of the flywheel proper, and in this case is often referred to as a fluid flywheel), and thus is turned by the engine's crankshaft. The turbine is connected to the input shaft of the transmission. As engine speed increases while the transmission is in gear, torque is transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.