Which type of automatic transmission relies on planetary gears?

A torque-converter automatic transmission relies on planetary gears. In the conventional hydraulic automatic, planetary gearsets provide the selectable forward ratios, while a torque converter connects the engine to the transmission.

A planetary set contains a sun gear, planet gears mounted on a carrier and a ring gear. By holding one member, driving another and taking power from the third, the transmission can create different ratios, direct drive or reverse. Hydraulic pressure, friction clutches and brake bands—or electronically controlled clutch packs in newer units—select the required arrangement.

The answer is sometimes confused with a continuously variable transmission. A CVT usually uses variable-diameter pulleys and a belt or chain rather than stepped planetary gearsets. A dual-clutch transmission can use conventional parallel shafts and gear pairs, while an automated manual is essentially a manual gearbox operated by actuators.

The torque converter itself is not the gearset. It is a fluid coupling that allows the engine to keep running while the car is stopped and can multiply torque during launch. The planetary gearsets perform the ratio-changing work.