Fluid coupling

On a manual transmission, there is a mechanical connection between the engine and transmission through the clutch. On an automatic transmission a fluid coupling provides a viscous fluid to connect the engine output and the transmission. It transfers engine torque to the transmission input shaft through the use of two units with vanes (called a " torus") operating very close together in a bath of oil. The engine drives one torus causing it to throw oil outward and into the other torus which then begins to turn the transmission input shaft. A fluid coupling cannot increase torque above that produced by the crankshaft. Buick's Dynaflow is an example of this kind of coupling.