What device in an automatic transmission uses fluid to transmit power?

In an automatic transmission, a torque converter uses fluid to transmit power from the engine to the transmission.

It is a hydrodynamic coupling positioned between the engine and the automatic gear train. The engine spins an impeller, which accelerates transmission fluid; that moving fluid drives a turbine connected to the transmission input. A stator between them redirects the fluid and can help multiply torque when the vehicle is starting from rest.

This arrangement replaces the manually operated friction clutch used in most traditional manual cars. Because fluid can slip, the torque converter allows the engine to keep running while the vehicle is stopped in gear. Modern converters often include a lock-up clutch that mechanically connects the input and output at cruising speeds, reducing heat and efficiency losses.

A planetary gearset changes the available ratios, while clutch packs select and hold those gear elements. A CVT belt performs a different job in a continuously variable transmission. Those components may be inside an automatic transmission, but the fluid-powered coupling itself is the torque converter.