How Automatic Transmissions Work

Automobile technology has come a long way since the invention and development of the first automobiles over 100 years ago. Even if you're a fan of the dependable and traditional manual transmissions, you will find automatic transmissions to be very simple and just as reliable. If you're about to take driving lessons in a newer car, you will likely be learning how to use an automatic transmission. And even though automatic cars are extremely easy to drive, it's still nice to know what goes on behind the scenes when you are driving your vehicle.

In the article below, we'll talk about how modern automatic gearboxes work, contributing to the usefulness and ease of automatic transmission. Read on to find out how your car works hard while you enjoy your driving experiences!

Gear Sets and Torque Converters

Nowadays, automatic gearboxes have planetary gears. This is a set of gears that consists of:

  • The Sun Gear, which is the central gear with an outer ring and internal teeth
  • The Planet Gears, which are smaller gears that rotate between the sun and its ring

It is easy to see where the gears got their name. The speed of your vehicle is increased when the sun gear is locked and the planet carrier moves the planet gears. Likewise, your speed is reduced when the ring gear is in the lock position and the sun gear is the one being driven through the gearbox. When there is no power being transmitted to the sun gear and the planet carrier is locked, that is how your car achieves a reverse position.

The drive train of the transmission system works in tandem with this gearbox set, helping to deliver the right amount of torque and speed to your wheels accordingly. While the driver would typically have to do this on their own with the previous manual transmission systems, now it is all done for them in a swift and easy manner.

Torque Converter

The torque converter is a fluid drive that acts like the clutch, and it uses hydraulic pressure to move itself. It is a highly effective replacement for the manual clutch, and works quite easily to convert between torque and speed.

The way it works is very specific, though. The converter essentially comes with three main components for its function:

  1. The impeller, which is bolted to a flywheel
  2. The turbine, which is directly connected to the input shaft of the gearbox
  3. The central reactor, which is a one-way clutch that acts as a central reactor between the impeller and the turbine

Let's take a couple of examples to understand this further. When the engine speed is increased, which is what happens when you step down on the gas pedal, making your car go faster, the centrifugal force created by the hydraulic fluid is released by the impeller, which transmits torque to the turbine.

Then, the central reactor converts this torque back to the impeller when the car is reduced to lower speeds, lending much more torque to the driver. Have you ever noticed that the car is more difficult to control when you go at extremely high speeds? This is because the less torque you have, the less power there is in your car, which is what allows you to safely control the speed and direction of your vehicle. If you go really fast, you may notice that the car almost feels more lightweight, and it takes a lot longer to brake to a halting position.

If you start to speed up, the torque dependency decreases, which sends the central reactor into a free spin. The torque converter will then connect to the gearbox, acting as the bridge between the gearbox and the engine.

Locking the planetary gears to the planet carrier can help you achieve the same effect that the hydraulic fluid of the torque converter has when it is subjected to centrifugal forces.

The Epicyclic Gearing Process

Epicyclic gearing is the term for the type of gearing process that the automatic transmission goes through when the driver increases or decreases their speed in their vehicle. Since it is all done behind the scenes, drivers are much less aware of the different gear settings that the car goes into when compared to the gear settings that must be selected by hand during manual transmission.

The epicyclic gear drives are described as follows:

  • Drive one - Also known as first gear, this is the gear that is engaged when the forward-drive clutch turns the first ring gear. The planet gears carry to the sun gear, using a brake band so that the second ring gear reduces the speed of the engine. This is the lowest forward speed of the car.
  • Drive two - In second gear, there is an intermediate ratio. The clutch is engaged, and the planet gears revolve around the sun gear, which uses one but not all of the speed reductions. This is a medium amount of speed for a car, or perhaps the average amount of speed you use when driving on a highway or a long street.
  • Drive three - Also known as top gear, this is the gear setting that helps you achieve the fastest speed possible. The reverse clutch is engaged along with the forward-drive clutch, which means the planet gears cannot turn. The carrier shaft carries the output, which turns the speed into the direct drive function, lending itself to make the car much faster.

Of course, there is also a reverse function, which is a gear setting marked with R on your transmission selector. When in reverse, the forward-drive clutch is not in use. The reverse clutch is engaged instead, making the sun gear drive forward. One of the planet carriers is braked while the second ring gear goes in the opposite direction. This makes for a safe and effective reverse. While it is possible to reverse at higher speeds, it is recommended to remain in a lower speed as it is more difficult to control the vehicle in a reverse position.