How Does an Automatic Transmission Work


Automatic transmission: how it works

Advantages and disadvantages of the automatic transmission

Practical aspects of the automatic transmission

An automatic transmission is a transmission very commonly used in automobiles. It frees the driver from the management of gear shifting and the clutch. Let’s take a closer look at how it works, the advantages and disadvantages, and the practical aspects of the automatic transmission.

How does an automatic transmission work?

An automatic transmission is made up of several components comparable to a manual transmission, but with a different technology that uses the following elements:

A hydrodynamic torque converter: this is the equivalent of the clutch in a manual transmission. This converter, consisting of a turbine, pump, and stator, allows the engine to run while the car is stationary and the transmission is not running.

A gearbox consisting of one or more planetary gears: each planetary gear set is a set of gears, each element of which is associated with a transmission shaft:

  • the planetary gear and the planetary shaft;
  • the planetary gears and the planetary carrier shaft;
  • the ring gear and the ring gear shaft.

Hydraulic control is associated with an electronic management system that controls the various gear ratios’ engagement. This hydraulic control acts according to several parameters:

  • the speed of the vehicle;
  • the position of the accelerator pedal;
  • the mode selected by the driver if he has the choice.

Like all transmissions used in automobiles, the automatic transmission must perform several functions:

  • allow the car to park while keeping the engine running;
  • adapt the engine speed to the speed of the vehicle.

As its name implies, the automatic transmission must perform these operations without the assistance of the driver. This means that the driver does not need to operate a clutch or shift the transmission gears. The driver can concentrate on accelerating, braking, and steering the vehicle.

Good to know: Other types of transmissions, such as robotic transmissions or CVTs, allow the driver to take care of the transmission’s operation.

Advantages and Disadvantages of Automatic Transmission

The automatic transmission has apparent advantages:

  • Ease of use for the driver. He doesn’t need to operate any clutch or shift lever.
  • Shifting gears with a quality automatic transmission is smooth and goes unnoticeable.
  • A potential reduction in fuel consumption thanks to an optimization of the engine speed.
  • Little maintenance, as there is no clutch to change.

But the automatic gearbox also has some weaknesses, factual or of a more personal nature:

  • An automatic gearbox is more massive than its manual equivalent.
  • Therefore, the hydrodynamic torque converter results in over-consumption compared to a mechanical clutch.

Good to know: these advantages and disadvantages explain why the automatic transmission is much better established in a country like Canada, where cars are already large and heavy, and where driving comfort seems to outweigh driving sensations.

Practical aspects of the automatic transmission

For the driver, the use of the transmission is greatly simplified when it is automatic.

Easy Control

An automatic transmission generally offers a limited number of controls for the driver: no clutch pedal since there is no clutch., which means more comfortable driving.

Practical gear shifting

The driver can place the transmission lever in several positions:

  • The Parking position locks the transmission and acts as a handbrake when the vehicle is parked.
  • The Neutral position corresponds to neutral and allows the transmission to turn freely without driving the wheels.
  • The Drive position enables the car to move forward.
  • The Reverse position, which is reverse.

In some cases, the transmission lever gives access to other driving modes. For example, the Sport mode corresponds to a more nervous electronic gear shift management than in the Drive mode.

A gear selector, which allows the driver to choose the gear he or she wants.

Good to know: even if the driver has a choice of gears, the electronic transmission management system will take over if it feels that the engine speed becomes too low or too high to avoid stalling or overrevving.


The automatic transmission is more complicated than a simple manual transmission and can be costly to repair if it fails.

In practice, however, automatic transmissions are reliable and more economical to maintain than a manual transmission because they do not have a clutch that needs to be changed regularly.

Learn more about transmissions with

    •  Maintenance of the automatic transmission requires the right choice of transmission oil.
    •  Find out everything you need to know about changing your gearbox.

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