# Automatic Transmission

> A conventional automatic replaces the clutch pedal with a fluid torque converter and swaps gears by clamping parts of planetary gearsets with hydraulic clutches. See how it pulls away smoothly, multiplies torque and shifts without interrupting drive.

- Level: Intermediate · about 15 minutes · Transmission & Clutch
- Lesson page: https://maxrecursion.github.io/AutoVerse/topics/automatic-transmission

## What you will learn

- Explain how a torque converter couples the engine to the gearbox and multiplies torque
- Describe how a planetary gearset produces different ratios by holding different members
- Explain how clutches and brakes inside the gearbox select a gear
- Compare torque-converter automatics with dual-clutch and CVT designs

## The idea, in plain language

A car's engine has to keep turning even when the car is stopped, so something must let the wheels stand still while the engine idles. In a manual car that job belongs to the clutch pedal. In a traditional automatic it is done by a torque converter: two fan-like wheels facing each other in a sealed case full of oil. The engine spins one of them, which flings oil at the other and makes it turn. At idle the push is weak enough for the brakes to hold the car; press the accelerator and the push becomes strong enough to move it.

Instead of sliding gears in and out of mesh like a manual box, an automatic uses compact planetary gearsets — a central sun gear, small planet gears around it and an outer ring gear. Holding one part still and driving another gives a particular ratio. Hydraulic clutches and brakes inside the gearbox grab different parts of the gearsets to choose each gear.

A control unit reads road speed, accelerator position and engine load, then decides when to change gear. Because one clutch is released while the next is applied, drive to the wheels continues through the shift, which is why a good automatic feels seamless.

## The engineering explanation

The torque converter has three elements: the impeller (pump), bolted to the engine flexplate; the turbine, splined to the gearbox input shaft; and the stator, mounted on a one-way clutch. At high slip — pulling away — fluid leaving the turbine is redirected by the stationary stator back into the impeller in a helpful direction, multiplying torque; a stall torque ratio of roughly 2:1 is typical. As turbine speed approaches impeller speed the stator freewheels and the unit behaves as a fluid coupling. A lock-up clutch then joins impeller and turbine mechanically to eliminate slip losses at cruise, and modern units may lock up as early as second gear, often with controlled micro-slip to damp vibration.

A simple planetary set obeys the Willis relation (S + R) ω_c = S ω_s + R ω_r, where S and R are sun and ring tooth counts. With the ring held, sun as input and carrier as output, the reduction is 1 + R/S; holding the sun and driving the ring gives a smaller reduction; locking any two members together gives 1:1; holding the carrier reverses rotation. Compound arrangements (Simpson, Ravigneaux, Lepelletier) combine sets so that five or six shift elements provide six to ten forward ratios.

The valve body routes line pressure to the clutch packs and band brakes. A transmission control unit commands pressure-control solenoids, modulating apply and release pressures so that the off-going element releases precisely as the on-coming element takes up torque (a clutch-to-clutch shift), while the engine ECU briefly trims torque to smooth the change. Shift maps balance economy, performance and comfort; adaptive logic learns clutch wear. Alternatives include dual-clutch transmissions (two automated dry or wet clutches feeding odd and even gear shafts, pre-selecting the next gear) and CVTs (a steel push-belt or chain running between variable-width pulleys for continuously variable ratio).

## Step by step

1. **Fluid coupling at idle.** The engine spins the impeller, which flings oil at the turbine. At idle this push is gentle enough for the brakes to hold the car. *In engineering terms:* Transmitted torque rises roughly with the square of impeller speed, so drag at idle is small but grows quickly as engine speed increases.
2. **Torque multiplication from rest.** When pulling away, a third wheel (the stator) redirects returning oil to give extra push. *In engineering terms:* With the turbine slow, the stator is held by its one-way clutch and redirects flow, giving a torque ratio up to about 2:1 at stall.
3. **Planetary gears make the ratios.** Holding one part of a planetary gearset and driving another produces a lower or higher gear. *In engineering terms:* (S + R) ω_c = S ω_s + R ω_r. Ring held, sun input, carrier output gives a reduction of 1 + R/S; two members locked gives direct drive.
4. **Clutches and brakes pick the gear.** Oil-pressure-operated clutches grab or release parts of the gearsets to choose each gear. *In engineering terms:* Each gear is defined by a combination of applied clutches (member to member) and brakes (member to case), typically two or three elements at a time.
5. **The computer times the shift.** A control unit decides when to change gear and swaps clutches so smoothly that drive is never cut. *In engineering terms:* Clutch-to-clutch shifts overlap the off-going and on-coming elements under closed-loop pressure control, with engine torque reduction during the inertia phase.
6. **Lock up for efficiency.** At steady speed a clutch locks the converter solid so no energy is lost in the oil. *In engineering terms:* The lock-up clutch removes converter slip losses; controlled slip at low speed isolates firing pulses from the driveline.

## Key concepts

- **Torque converter**: A fluid coupling with a stator that lets the engine idle in gear and multiplies torque when the turbine is slower than the impeller.
- **Planetary (epicyclic) gearset**: Sun, planets on a carrier, and ring gear in constant mesh; holding or driving different members gives different ratios. (`(S + R) ω_c = S ω_s + R ω_r`)
- **Planetary reduction (ring held)**: Driving the sun with the ring held and taking output from the carrier gives this reduction ratio. (`i = 1 + R / S`)
- **Lock-up clutch**: A friction clutch inside the converter that joins engine and gearbox directly to eliminate slip losses.

## Common misconceptions

- **Myth:** Automatics are always less efficient than manuals. **Reality:** Early automatics lost energy in the converter, but lock-up clutches, more ratios and better control mean many modern automatics match or beat a manual on economy.
- **Myth:** Selecting neutral at traffic lights saves the gearbox. **Reality:** Automatics are designed to sit in drive with the brake applied; frequent shuffling between N and D can add wear to the engagement clutches.
- **Myth:** A CVT and a torque-converter automatic work the same way. **Reality:** A CVT varies its ratio continuously with pulleys and a belt or chain; a conventional automatic steps between fixed planetary ratios.

## In the real world

- Creep: an automatic in drive edges forward when you lift off the brake because the converter transmits a little torque at idle.
- Pressing the accelerator firmly triggers a kick-down to a lower gear, putting the engine nearer its power peak.
- Engine speed briefly rises above road speed when pulling away hard: that is converter slip before the turbine catches up.

## Common questions

### How does an automatic transmission change gears?

It uses planetary gearsets, which give different ratios depending on which of their parts is held still or driven. Hydraulically or electronically controlled clutches and bands lock those parts in different combinations, and the control unit chooses the combination from road speed, throttle and load.

### What does a torque converter do?

It replaces the clutch pedal. Fluid flung by an engine-driven impeller turns a turbine connected to the gearbox, so the car can sit still in gear with the engine running, and at low speeds the converter multiplies torque. A lock-up clutch joins the two halves at cruising speed to avoid slip.

### What is the difference between an automatic, a CVT and a dual-clutch gearbox?

A conventional automatic uses a torque converter and planetary gears with fixed ratios. A CVT varies its ratio continuously, typically with a belt or chain between two variable pulleys. A dual-clutch gearbox is an automated manual with one clutch for odd gears and one for even gears, so it can preselect the next gear.

## The story behind it

### Hermann Föttinger (1905)

Combined a pump and a turbine in one housing, the ancestor of the torque converter. While chief designer at the AG Vulcan Stettin shipyard from 1904, Föttinger developed a fluid coupling that combined a pump and a turbine in a single unit. His 1905 patents covered both fluid couplings and torque converters, and the source says its further development produced the automatic car transmission. He filed more than 100 patent applications in his career.

Sources: [Wikipedia: Hermann Föttinger](https://en.wikipedia.org/wiki/Hermann_F%C3%B6ttinger)

## Related lessons

- [Torque and Power](https://maxrecursion.github.io/AutoVerse/topics/torque-and-power)
- [The Differential](https://maxrecursion.github.io/AutoVerse/topics/differential)
- [Drivetrain Layouts](https://maxrecursion.github.io/AutoVerse/topics/drivetrain-layouts)
- [Mechanical Sympathy](https://maxrecursion.github.io/AutoVerse/topics/mechanical-sympathy)

## Further reading

- [How automatic gearboxes work](https://www.howacarworks.com/basics/how-automatic-gearboxes-work) — How a Car Works

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Original AutoVerse lesson (AutoVerse original content). Simulation values in AutoVerse labs are illustrative. Source page: https://maxrecursion.github.io/AutoVerse/topics/automatic-transmission