How an Automatic Transmission Works

Step 1

Why a car needs a transmission

The engine and the wheels want different things. The transmission sits between them and translates one into the other. An automatic transmission does this by itself, with no clutch pedal.

Enginespins fast, but pulls weakly

Wheels needforce when starting, speed on the highway

Step 2

The planetary gear set — the heart of an automatic

Inside a classic automatic there aren't ordinary gears on separate shafts, but planetary gear sets. Each looks like a solar system and has three main parts:

Sun gearThe gear in the center of the set.

Planets and carrierSmall gears around the sun gear. The carrier holds their axles and turns along with them.

Ring gearA large ring with teeth on the inside. The planet gears roll along it.

Step 3

The main rule

The engine turns one part, we hold one part still, and the third sends the rotation to the wheels.

turned by the engine
held still
to the wheels

The 3D lesson is color-coded the same way. Example: the engine turns the sun gear, and the ring gear is held. The planet gears roll along the stationary ring gear and drag the carrier with them. The carrier turns 2.67 times slower than the sun gear — and with 2.67 times more torque. That's first gear: a heavy car pulls away easily.

Step 4

Change what we hold — the gear changes

In the lesson's model, the sun gear has 39 teeth, the planet gear 13, and the ring gear 65. Here's what you get:

Gear	Held	Engine turns	To the wheels	Ratio
1	ring gear	sun gear	carrier — slow, strong	2.67
2	sun gear	ring gear	carrier — faster	1.6
3	nothing: a clutch locks the sun gear to the ring gear	whole set	carrier — same as engine	1.0
R	carrier	sun gear	ring gear — in reverse	1.67

The ratio shows how many times slower the output turns than the engine. Real automatics have two to four such sets, which is why they have four to ten gears — the principle is the same.

Step 5

What holds the parts: brakes and clutches

The parts are held not mechanically but by oil. The transmission pump builds pressure, the oil flows through the valve body to the pistons, and they squeeze a brake or clutch. The control unit decides what to engage — based on speed and throttle position.

BrakeA steel band around a drum, or a stack of discs pressed against the case. It stops a part.

ClutchA stack of discs between two parts. It locks them together — they turn as one.

Step 6

The torque converter: why the car doesn't stall at a stop

Instead of a clutch, an automatic has a torque converter: two "fans" in oil. The engine turns the impeller, which pushes oil onto the turbine, and the turbine spins the transmission input shaft. There is no rigid link.

Stopped, foot on the brakeOil slips between the impeller and turbine — the engine runs, the car stands still.
Press the gasThe oil hits the turbine harder — it speeds up, and the car pulls away.
Cruising steadilyThe lockup clutch engages: the impeller and turbine turn together, with no losses.

Shifter

Modes P, R, N, D

PParkNo rotation goes anywhere, and the output shaft is locked by a parking pawl.

RReverseWe hold the carrier — the ring gear turns the opposite way.

NNeutralNothing is held. The wheels are free, and the car can be pushed.

DDriveThe transmission shifts gears by itself as you speed up.

Repair books for automatic and other transmissionsguides for specific transmissions — with teardown steps and diagrams Full machine fluid exchange in an automatic transmissionChangan UNI-T Full machine fluid exchange in an automatic transmissionToyota Alphard How to check the automatic transmission fluid levelToyota Hilux Surf No reverse in an automatic: diagnosisNissan Presage

3D models: Turbo Hydramatic 400 — Jorma Rysky, Planetengetriebe — majme (Sketchfab, CC BY 4.0), simplified for the site.