How does an automatic transmission work?

Many modern day vehicles have an automatic transmission. Despite their multitude on our roads, most drivers are completely unaware of how they work. Have you ever stopped to consider how your car shifts gears automatically or why the engine does not die when you come to a stop?

Well, if you have, then this blog is for you. And while we don’t promise to give you a deep understanding of this complex system, we pledge to simplify it to point where you have some idea of what’s going on in a traditional, torque converter-based system.


Bell housing


To get a better understanding of how an automatic transmission works, the best place to start is the bell housing, which contains a torque converter as opposed to a clutch found in manual vehicles.

The bell housing is the place where the transmission connects to the engine. Inside the transmission is the planetary gear sets, which are responsible for providing different gear ratios.

The torque converter and the planetary gear sets are the two main components of an automatic transmission – so let’s look at them a bit closer.

Torque converter


You know how you have a clutch on a manual transmission? A torque converter basically takes its place in an automatic transmission.

The major parts of a torque converter are the impeller, turbine, stator and the lock up clutch. So how do all these parts work? Well, the impeller is part of the torque converter that connects directly to the engine. It drives the turbine, which in turn is connected to the transmission input shaft where it provides torque.

Transmission fluid flows in a loop between the impeller and turbine. Between these two components sits the stator, which redirects the fluid as it returns from the turbine to the impeller. This minimalises churning loses and increases torque output.

By providing you with more torque, it assists you with accelerating or when you’re vehicle is stationary (the reason your car never stalls).

Planetary gear set


The torque converter clarifies how the engines send power to the transmission, but the planetary gear sets explain how an automatic transmission changes gears.

The planetary gear set is really the heart of an automatic transmission. This one part creates all the possible gear ratios that the transmission can produce.

All planetary gear sets have three main components, which are:

  • The sun gear
  • The planet gears (also the planet gear’s carriers) and
  • The ring gear


As you can probably work out, the planet gears rotate around the sun gear, while the planet carrier connects the planet gears. The ring gear is on the outside and meshes with the planet gears.

By using the clutches and brakes, it alters the input and output of the system, which changes the overall gear ratio. All three parts can be the input, output or remain stationary. Depending on what role each part plays, it will determine the gear ratio for the gear set. That is how an automatic transmission knows when to change gears.

As you can see, automatic transmissions aren’t some kind of black magic – it’s just a set of parts working together. We hope this has illuminated your understanding of automatic transmissions and helped to foster an appreciation of how ingenious this system is. We think they are great, but we may be biased as we work with them every day.