The mechanism that locks an automatic transmission’s torque converter directly to the engine to eliminate slip is the lock-up clutch.
A conventional torque converter transfers power through transmission fluid between an impeller, turbine, and stator. That fluid coupling allows some speed difference, or slip, which is useful during takeoff but wastes energy and creates heat once the vehicle is cruising.
When operating conditions permit, the lock-up clutch mechanically links the converter’s impeller and turbine. Power then travels through a solid mechanical connection rather than mainly through circulating fluid, improving efficiency and reducing heat. It does not normally remain engaged during every driving condition; the transmission controller may release it during launch, low-speed maneuvering, braking, or sudden acceleration.
Packard’s Ultramatic, introduced in 1949, was an early automotive application of converter lock-up. The feature later became widespread, especially from the late 1970s onward as manufacturers pursued better fuel economy. A common mix-up is confusing this clutch with the stator’s one-way clutch: the stator clutch controls stator motion, while the lock-up clutch removes converter slip.