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How does an automatic transmission work?

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Quick answer

An automatic transmission links the engine to the wheels through a torque converter, a fluid coupling that lets the car stop in gear. Behind it, planetary gearsets produce each gear ratio when hydraulic clutches hold particular gears still. A computer reads speed and throttle position and engages the clutches to shift automatically.

An automatic transmission changes gear by itself, using a fluid coupling called a torque converter and sets of planetary gears that clutches and brakes lock in different combinations. A control unit reads road speed and accelerator position, then picks the ratio and shifts without the driver touching a clutch pedal.

Step by step

  1. The engine turns the torque converter's impeller, which pumps fluid against the turbine and drives the gearbox input shaft.
  2. In first gear, clutches hold one part of a planetary gearset so the output turns much slower than the engine.
  3. As road speed rises, the control unit signals the valve body to send fluid pressure to a different clutch pack.
  4. One clutch releases while the next engages, changing which gear member is held and giving a taller ratio smoothly.
  5. At steady cruising speed, a lock-up clutch joins the impeller and turbine directly so no power is lost to slip.

Key things to understand

  1. The engine spins an impeller inside the torque converter, flinging transmission fluid against a turbine connected to the gearbox.
  2. This fluid link lets the car idle in gear at a standstill, and a stator multiplies torque when pulling away.
  3. Planetary gearsets, each with a sun gear, planet gears and a ring gear, give different ratios depending on which part is held.
  4. Hydraulic clutches and bands lock or free those gear parts, and a valve body routes pressurised fluid to engage them.
  5. An electronic control unit uses speed, load and throttle position to time each shift, then a lock-up clutch removes slip when cruising.

Labelled diagram

  1. Torque converter

    Impeller, turbine and stator pass engine power through moving transmission fluid.

  2. Planetary gearset

    Sun, planet and ring gears give different ratios when one member is held.

  3. Clutch packs

    Hydraulically squeezed plates lock chosen gear members to select each ratio.

  4. Valve body

    A maze of channels and solenoids directs fluid pressure to the right clutch.

  5. Control unit

    Reads speed, throttle and load to decide the moment for every gear change.

Power path through a conventional automatic gearbox

Real-world examples

  • Budget hatchbacks in India, such as the Maruti Suzuki Celerio, popularised AMT gearboxes for city traffic.
  • Most full-size pickup trucks and SUVs in the United States use torque-converter automatics with eight or more gears.
  • Many hybrid cars use an electronically controlled planetary gearset that blends engine and motor power without stepped gears.
  • City buses commonly use heavy-duty automatics so drivers can pull away smoothly at every stop.

Common mistake

✗ Myth: Shifting to Neutral at traffic lights saves fuel and protects an automatic gearbox.

✓ Fact: Modern automatics are built to sit in Drive with the brake held, and any fuel saving from Neutral is tiny. Frequent shuffling between Drive and Neutral only adds wear to the clutches inside.

People also ask

How does an automatic car change gears without a clutch?
It does have clutches, just not a pedal for one. Inside the gearbox, several hydraulic clutch packs lock parts of planetary gearsets. A control unit decides when to shift and sends oil pressure to release one clutch and apply another, swapping the gear ratio in a fraction of a second.
What is a torque converter in an automatic transmission?
A torque converter is a sealed, fluid-filled coupling between the engine and gearbox. An engine-driven impeller throws oil at a turbine that turns the gearbox, while a stator redirects returning oil to multiply torque at low speed. It lets the car stop in gear without stalling the engine.
Are automatic cars less fuel efficient than manual?
Older automatics with four or five gears and a slipping torque converter often used more fuel than manuals. Modern units with many gears, lock-up clutches and smart shift logic, or CVTs and dual-clutch designs, frequently match or beat manuals on official fuel economy tests.
Why does an automatic car creep forward when I release the brake?
The engine keeps spinning the torque converter's impeller even at idle, and that moving fluid pushes gently on the turbine. The small torque passed through is enough to roll the car slowly unless the brake holds it.
What does Park actually do?
Selecting Park drops a metal pawl into a notched wheel on the transmission's output shaft, mechanically locking the driven wheels. It is not a brake, so on steep slopes drivers should also use the parking brake to take load off the pawl.
Is a CVT the same as an automatic transmission?
A CVT is a type of automatic, but it has no fixed gears. It uses a steel belt or chain running between two cone-shaped pulleys that change diameter, giving a smooth, continuous range of ratios instead of distinct steps.
What is an AMT gearbox?
An automated manual transmission is an ordinary manual gearbox and clutch fitted with electric or hydraulic actuators. A computer presses the clutch and moves the gear lever for the driver, which is cheaper than a torque-converter automatic but can feel jerkier during shifts.

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