An automatic gearbox uses a system that includes fluid coupling to transfer the engine's power to the transmission and a set of planetary gears that are locked or unlocked in different combinations to engage different gears. One of the most complicated components in modern vehicles, an automatic gearbox includes mechanical, hydraulic, electrical and computer systems.
The mechanical system
The mechanical system of an automatic gearbox contains a set of planetary gears controlled by a number of clutches and bands. A hydraulic system controls the clutches and bands themselves.
The gears in an automatic transmission are always engaged to the same gears. For example, in a simple planetary gear system with an outer ring gear, a central sun gear and two or more planet gears, the gears are always in contact with each other. If the sun gear is locked, rotating the ring gear would cause the planet gears to revolve in the same direction at a slower speed. However, by locking the planet gears, the ring gear's rotation would turn the sun gear in the opposite direction, providing a reverse gear.
Most gearboxes use two or more planetary sets, resulting in a larger number of possible gear combinations.
Controlling shift points
In newer cars, computers precisely control shift points using input from sensors in the engine and transmission to decide when to shift gears and how fast to engage them.
Non-computerized automatic systems use a governor, vacuum modulator and throttle cable to correctly detect when a shift is needed and change gears.