How Does Automatic Transmission Work: An In-depth Explanation

Do you wonder how your automatic transmission vehicle shifts into the required gear while you do very little other than press your foot on the brake or the gas pedal?

That's what this article is all about – everything you need to know about this wonderful piece of engineering, the auto transmission. 

No exaggerations here, but as soon as you grasp how an automatic transmission works, you will be filled with awe at Alfred Horner Munro, who designed and invented it in the early 1920s. 

Built without computers like the automotive industry computerizes everything today, I’d say it was pure genius!

What Does a Transmission Do?

First, what is the purpose of transmissions? Why exactly do cars require a transmission?

When you study how the engine works, you’ll understand that the engine creates rotational power. For a car to move, that rotational power must be transferred to the tyres, and that’s what a drivetrain does.

In case you don’t know, your car’s transmission is a major part of that drivetrain. But here's another thing. 

A car engine must spin within a specified range of speed for it to function efficiently. Spinning lower than that range means that the car will not be able to move. Conversely, spinning way too fast could cause the engine to self-destruct. In other words, there must be a way to control the power transferred from the engine with clockwork precision. 

That’s where the transmission comes in.

It’s the function of a car transmission to ensure that the engine of your car spins optimally and that the wheels receive the appropriate levels of power needed to move or stop. 

The transmission is located right between the engine and the rest of the drivetrain. You can liken it to a power switchboard.

Types of Transmission

1. Manual Transmission

Manual transmissions accomplish their function by using gear ratios. Gears of different sizes are connected to each other, and this way, the level of power sent to the car can be raised without altering the engine’s rotational speed so much. 

You can control the gears that are engaged by pressing on the clutch and then shifting the gears as required.

2. Automatic Transmission

For automatic transmission, shifting gears is easier. All you have to do is step on the gas or press on the brakes. You can call it magic.

So, basically, the purpose of the transmission, whether manual or automatic, is to ensure that the engine rotates optimally, not going too fast or too slow, while creating appropriate power levels for your wheels to move and stop.

The only difference is that in automatic transmission cars, shifting gears is so much simpler and doesn’t involve a clutch.

Now that you have a basic idea regarding what a car transmission does, let’s look at the parts of an auto transmission to fully understand how it carries out its functions.

Parts of an Automatic Transmission System

Here are the main parts of an automatic transmission system:

Transmission Casing 

The casing houses all other parts of the transmission. It is made of aluminium and has the shape of a bell, and for this reason, it is sometimes called the bell casing. It does not only protect the transmission's moving gears, but in modern vehicles, it also contains sensors monitoring input and output rotational speeds. 

Torque Converter

Have you ever wondered why it is possible to activate the engine of a car but find it hard moving forward? The reason is that there is a disconnection in the flow of power from your car's engine to its transmission. 

The purpose of the disconnection is to give the engine time to run without supplying power to the entire drivetrain of the car. 

  • For a manual transmission, the clutch is used to disconnect the power from the engine to the drivetrain.
  • For an automatic transmission, where there is no clutch, you have the torque converter

The torque converter is between the transmission and the engine. It kind of resembles a doughnut positioned in the transmission casing's opening. 

Functions of a Torque Converter

The torque converter performs two functions –

  • It transfers the power created by the engine right to the input shaft of the transmission.
  • It multiplies the engine torque output.

The hydraulic power from the transmission fluid enables the torque converter to perform these vital functions. To understand this better, let’s see what parts make up a torque converter and how they work.

Components of the Torque Converter

The torque converter in most modern cars has four main parts. 

1. Pump 

The pump looks like a fan, made up of a couple of blades radiating from the core. It is fixed to the torque converter housing, which is attached to the engine’s flywheel. The pump spins in the same velocity as that of the crankshaft, forcing transmission fluid to the turbine from the centre. 

2. Turbine 

It is located right inside the torque converter and connected to the transmission's input shaft. It is not linked to the pump – it is at this point that the engine rotates at a speed which is different from the entire drivetrain. The transmission fluid from the pump spins the turbine blades, which in turn will send the fluid directly to the centre and then back to the pump.

3. Stator 

The stator, aka reactor, is located between the pump and the turbine. It resembles the propeller of an airplane and performs two basic functions:

  • Ensures that the transmission fluid is efficiently returned to the pump
  • Multiplies the torque produced by the engine to get the vehicle moving and sends less torque once the vehicle moves at the desired speed

The reactor's blades shift in such a way that each time the transmission fluid leaving the turbine touches the blades, that fluid is diverted in exactly the same direction as the rotation of the pump.

A one-way clutch connects the stator to a shaft fixed on the transmission, which means that the stator can only move in a single direction, ensuring a one-directional movement of the fluid from the turbine. 

As a result of these two design elements, torque is amplified right at the turbine. 

4. Torque Converter Clutch

As the transmission fluid travels from the pump to the torque converter turbine, there is a slight loss of power, causing the turbine to spin a little bit slower than the pump’s speed. 

It’s not an issue if the car begins to move because it is the difference in speed that helps the turbine to effectively supply increased torque to the vehicle's transmission. 

However, once the vehicle is in motion, this difference leads to some level of energy inefficiencies.

To prevent the loss of energy, torque converters today are equipped with a torque converter clutch, which is linked to the converter turbine. As soon as the car reaches a level of speed, the converter clutch is activated and makes the turbine and the pump spin at similar speeds. The converter clutch is controlled by a computer once it is engaged.

Let’s fuse everything together and analyse how the torque converter operates as you take your vehicle from a standstill up to a certain speed.

How a Torque Converter Works

As you switch your vehicle, it will go into idle mode. The pump spins in the same speed as your engine while sending the transmission fluid to the converter turbine. 

However, since the engine does not spin fast when the car is at a halt, the converter turbine will not spin fast, and this prevents it from sending torque to your vehicle’s transmission.

As you press your foot on the gas, the engine begins to spin faster. This results in the torque converter pump rotating faster too. With an increase in the speed of the pump, transmission fluid gets sent at a fast rate from the pump which spins the turbine even faster. 

The fluid is transferred to the stator by the blades of the turbine. At this point, the stator does not spin since the speed of the transmission fluid is not high enough yet.

Because of the way the stator’s blades are designed, the fluid can go right through them and gets diverted back in exactly the same direction of the pump’s spin. 

This mechanics lets the pump send fluid at a faster rate to the turbine, increasing fluid pressure. As fluid is returned, it reaches the turbine with increased torque, resulting in even more torque sent from the turbine to the transmission. At this point, your car begins to move forward.

This cycle keeps repeating itself as the speed of your car increases. As soon as you get to cruising speed, the blades of the reactor finally begin to spin since the transmission fluid has gotten to the required pressure. 

Once the reactor begins to spin, there is a reduction in torque because not much torque is required since your car is already moving. The converter clutch is engaged, and this results in the turbine spinning at exactly the same rate as the pump and the engine.

Hopefully, now, you understand the function of the torque converter in connecting or disconnecting the power generated by the engine to and from the transmission and how the torque sent to the transmission is multiplied to help the car move. 

Let's look next at the components that make up the automatic transmission to shift the gears automatically. We will be talking about planetary gears.

Planetary Gears

As the speed of your vehicle increases, less torque is required for your car to keep moving. The level of torque transmitted to the wheels of your car is regulated by the transmission through gear ratios. Lower gear ratio equals more torque supply, and higher gear ratio equals less torque supply.

For manual transmissions, you’ll be the one to carry out gear shift movements to alter gear ratios. For auto transmission, the increase and decrease of the gear ratios are automated, thanks to planetary gears.

Components of a Planetary Gear

1. The Ring Gear

This kind of gear resembles a ring, and its inner surface is comprised of angular-cut teeth. In an epicyclic gearbox, the ring gear is located in the outermost part. The ring gear's inner teeth constantly mesh with a planetary gear set at its outer part.

2. The Sun Gear

This gear also has angular-cut teeth. In an epicyclic gearbox, this gear is in the middle. It constantly meshes with planetary gears at its inner parts and is connected with an epicyclic gearbox's input shaft. 

3. The Planet Gears 

In between the ring gears and the sun gears, you will find planet gears. Their teeth constantly mesh with both the sun and ring gears at their inner and outer points, respectively.

The planet gears have their axis connected to a planet carrier that carries the epicyclic gearbox's output shaft. Planet gears can rotate on their axis while revolving between ring and sun gears, similar to how our solar system operates.

4. The Planet Carrier

The planet carrier helps in the final output transmission to an output shaft. It is attached to the planet gear's axis. Above the planet carrier, you’ll find the planet gears rotating. Also, the revolution of planetary gears results in the carrier's rotation.

5. The Clutch Band

Also referred to as the brake band, this device fixes the angular, sun, and planetary gears. The brake or the clutch in your vehicle controls this device. Just one set of planetary gears can give you a reverse drive and up to 5 levels of forward driving.

With inherent in-line shaft and cylindrical casing, planetary gears are the perfect replacement for standard pinion and gear reducers. They can be used in a wide variety of situations such as electric screwdrivers, power trains, and more. 

How Planetary Gears Work

Here are more details about the construction and mechanics of planetary systems to help you understand how they work.


In very basic planetary gearing, you will find three gear sets with varying levels of freedom. It rotates around the axis revolving around sun gears that spin in place. On the outside, a fixed ring gear binds the planets. The planet is grouped with the sun gears and the ring gears in such a way that torque is carried in a straight line.

For simple planetary gear setups, the sun gear rotates at a high speed via input power. The planets mesh with the ring and sun gears which all orbit as they rotate. Planets are fixed to individual rotating members called the cage or the carrier. The revolution of the planet carrier supplies high torque output and low speed.

It is not always necessary to have fixed components. For differential systems, you will find that every member rotates. This helps to accommodate one output which is driven by double inputs as well as one input driving double outputs.

Compounded Reductions

While circling the sun gear, planet gears get lots of teeth engaged. This helps them keep up with various driver turns for every revolution of the output shaft. To carry out reduction between conventional pinions and gear, a very small pinion will be meshed with a sizable gear.

Basically, planetary gears give reductions up to 10:1. For compound planetary systems, the resulting reductions are much higher. 

  • Serial arrangement - Increasing or decreasing speed can be done in certain ways like connecting planetary stages serially. The first stage gives a rotational output which is connected to the next stage's input, and the final reduction is a result of multiplied individual ratios.
  • Hybrid arrangement – Standard gear reducers are introduced in the planetary train. It is a simple alternative to the series configuration and is preferred as a way of reducing the speed of the input which could be extremely high for certain planetary units. It also creates an input-output offset. 

Taking the Torque

At different points, planetary gears mesh with sun and ring gears. This engages more teeth to drive the load, and so, planetary gears need gears that are smaller but larger in number than conventional pinion-gear reduction. 

One consideration that is not so obvious is that in equally spaced multiple planets, the input and output shaft bearings don’t need to bear the radial load resulting from tangential gears because the reactions are cancelled out. 

Also, since the bearings are not acted upon by such forces, the likelihood of distortion occurring to the outer casing is very much reduced.

The presence of more planets will lead to the rise in torsional rigidity as well as load capacity. The higher the load division, the less the chances of gear teeth deflecting and wearing out. 

It implies that a considerably large load could possibly be driven in planetary gear units that are relatively small and streamlined. 

Beyond spur gears, you could find helical gears for load capacity. With helical planetary gears, there are axial reactions so there is no cancellation with multiple planets. This way, the bearings are not responsible for thrust load in any way.

Wear & Tear

Inline planetary systems can evenly distribute the load among all its components, and the economic result is proof of this distribution. If all components are of the same quality, the potential weak link would be the bearings providing support for each planetary gear.

There is very little space here, so unlike in conventional gear and pinion reducers with plenty of space to contain larger bearings, planet bearings happen to be small. 

Also, the effect of cancelling that comes with multiple planets with respect to radial loads is only applicable along the central shaft. As a matter of fact, the planet bearing's radial loads are responsible for turning the carrier.

High speeds and heavy planet gears cause cyclic and thermal fatigue and could also lead to centrifugal forces.

Planet Balancing

In real-life scenarios, the planets do not really take up a load that is perfectly balanced. A planet could be radially closer to the sun axis or end up being farther away than others. It is also possible that the carrier rotation's axis could be a bit off. 

In an automatic transmission, you would find multiple planetary gear sets. These gear sets work in unison to produce different gear ratios. Since the gears constantly mesh in planetary systems, you do not have to engage or disengage gears when shifting. This is totally different from manual transmission.

We’ve broken down all the essential parts into bits and how they work, so let’s now head straight to how an auto transmission works.

How an Auto Transmission Works

Now it’s pretty clear that automatic transmissions are made up of many different parts, designed and engineered to ensure that you can smoothly start and drive your car.

Now, let’s look at the bigger picture and try to understand how power flows in automatic transmissions.

First, the pump of the torque converter receives power from the engine. The pump then sends this power straight to the turbine through the transmission fluid. As the fluid gets to the turbine, it is returned to the pump through the stator.

What the stator does is increase transmission fluid power, thus increasing the power transmitted to the turbine and creating a rotation of vortex power within the torque converter.

The central shaft that links to the transmission is also connected to the torque turbine. So, when the turbine spins, it also turns the shaft, resulting in the transmission of power to the first set of planetary gears.

The planetary gear system will move or remain stationary, depending on which components are mobile or static. The arrangement of the planetary gear solely determines the gear ratio or level of power that the automatic transmission transmits to the drivetrain.


An automatic transmission makes your driving experience seamless and smooth. For one, you do not have to bother with a clutch pedal which is found in a manual transmission vehicle and absent in an automatic. 

Also, you do not have to operate any gear shift. The only thing that you need to do is take your vehicle from park to drive. Once done, the auto transmission takes care of the rest.

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