The automatic transmission is one of the most complex things in today's cars. It combines several technologies into one purpose, pulling together mechanical, hydraulic, electrical and computer-controlled systems into one unit that does nothing but make shifting gears automatic and behind the scenes for most drivers. Auto trannies have become so common that many drivers in the U.S. no longer know how to use a standard transmission at all.
The auto trans is made up of 5 basic components. These components all work together to create the automatically shifting transmission that makes driving so easy today.
The planetary gears (or gear sets) are the primary mechanical component of the automatic transmission. These provide the gear ratios that convert the engine's rotations per minute (RPM) into higher or lower speeds for the wheels that move the car. They also upgrade or downgrade the amount of torque (power) applied to those wheels to provide smoother movement.
The planetary gear set will have three parts: the sun gear (main gear coming from the engine), the ring gear (an outer gear that "rings" it all) and planetary gears for changing how power is transferred from the central sun gear to the outer ring gear.
Combinations of these gears create the forward and reverse gears of the vehicle.
This is the clutch in the clutchless transmission. Sound funny? Well, it's automated and the driver does not have to push a pedal or pull a lever to make it happen, thus making it (for the driver) clutchless. What the torque converter does is allow the engine to continue running while the vehicle is stopped by keeping everything moving, but not connected to the drive train moving the car.
Normally, for the engine to stop, all gears must be disconnected and the car must be in "neutral." This is dangerous in an automatic as this may mean the vehicle presses against the brakes while stopped, causing undue wear and potential starting hazards.
The torque converter "clutches" everything away and transfers the power of the engine through a turbine.
The hydraulic system is a maze of tubes and channels through which hydraulic fluid (either transmission fluid or motor oil) pass in order to affect different aspects of the transmission's operation. Some will be diverted as a power source to affect shifting, some will lubricate moving parts, others will open or close valves to allow different of the transmission to stay in synch.
When opened, this looks like a maze made by termites in a tree. The various openings facilitate the movement of hydraulic fluid to various valves which open and close either by electronic control or when the hydraulic pressure is at a specific level. This ultimately controls the shifting of gears. The valve body is the brains of the auto trans.
Computer Controls, Governor, Etc.
On older vehicles, the entire process of the automatic trans is controlled through engine vacuum via a vacuum modulator. In more modern vehicles, it's electronically controlled through a computer. In both cases, stimulation and data from the engine cause the controller (or governor) to elicit shifting in the transmission from one gear to another.