How Does Auto Transmission Work: An In-depth Explanation


Mar 18th, 2019

How Does Auto Transmission Work: An In-depth Explanation

Today, a vast majority of car owners drive vehicles that have automatic transmissions. Does it intrigue you when you think about how your vehicle shifts into the required gear while you do very little other than put your foot down on the brake of your car or the gas pedal?

Well, in this article, we will be explaining to you everything you need to know about a very wonderful piece of engineering in the history of humans - the automatic transmission. No exaggeration here but as soon as you grasp how auto transmissions operate, you will be filled with admiration for the human race for inventing something as magnificent as this without computers.

The Purpose of Automatic Transmission

First, what is the purpose of transmissions? Why exactly do cars require a transmission?
When you study how the engine works, you realise that it creates rotational power. In order for the car to move, that rotational power has to be transferred to the tyres. This is basically what the drivetrain of a car does, and of course, the transmission is a component 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.

Therefore, there has to be a way in which the power produced by the engine is multiplied at the right time. There also has to be a way to reduce the level of power that is transferred from the engine with precise timing. This is where the transmission comes in.

Types of Transmission

What the transmission does is to make sure that the engine of your car spins optimally, that is, not too fast or slow. As it does this, it also ensures that the wheels receive the appropriate levels of power needed to move or stop. 

The transmission is located right between the engine and the whole of the drivetrain. You can liken it to your vehicle's power switchboard.

Manual Transmission

When it comes to manual transmissions, gear ratios are used to accomplish this. The manual transmission connects gears of different sizes with each other. By doing so, the level of power that is sent throughout the car can be raised without altering the rotational power speed level of your vehicle’s engine so much. For manual transmissions, the gears of the car that are engaged can be controlled by pressing down on your vehicle's clutch and then shifting the gears as required.

Automatic Transmission

For automatic transmission, you don't have to do any of that as superb engineering has made it easier. All you have to do is just step on the gas or press down on the brake pedals. You can call it magic.

So, basically, the purpose of the transmission of a car is to make sure that the engine rotates optimally, that is, not going excessively fast or slow while creating appropriate power levels for your wheels to move and stop.

Now that you have a basic idea regarding what a car’s transmission does, it is important that we consider the parts of an auto transmission which enable it to carry out its functions.

Parts of an Automatic Transmission

In this section, we will be considering the parts of a vehicle's automatic transmission which help it to function effectively.

Transmission Casing

In the transmission casing, you will find every component of the transmission. The casing has the shape of a bell, the reason that it is sometimes called the bell casing, and is made of aluminum. Apart from providing protection to the transmission's moving gears, the casing in modern vehicles is equipped with sensors which monitor the input rotational speed that is produced by the engine as well as the output rotational speed that is sent throughout the vehicle.

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 a clutch is not present, how is the disconnection done? Well, this is where the torque converter comes in. 

In a vehicle, you will find the torque converter located between the transmission and the engine. It kind of resembles a doughnut which is positioned in the transmission casing's opening. 

Functions of the Torque Converter

Basically, the torque converter performs two functions when it comes to torque transmission –
•It helps to transfer power that is created by the engine right to the input shaft of the transmission.
•It multiplies the engine torque output.
The torque converter is able to perform these vital functions because of the hydraulic power that is supplied by your vehicle’s transmission fluid which is contained in your vehicle’s transmission. To have a better understanding of this process, it is important to consider the components that make up a torque converter and how they work.

Components of the Torque Converter

In modern cars today, the torque converter consists of four main parts. 

1. Pump 

The pump looks like a fan and is made up of a couple of blades which radiates from its core. The pump is fixed to the house of the torque converter, and the housing is in itself attached to the flywheel of the engine. So, the pump spins with the same velocity as the crankshaft of the engine. Basically, the job of the pump is to pump transmission fluid to the turbine from the center.

2. Turbine

Right inside the house of the torque converter, you will find the turbine. It resembles a fan and is connected to the transmission's input shaft. Since it is not linked to the pump, this means that they do not necessarily move at the same speed. It is this particular point that gives the engine the ability to rotate at a speed which is different from the entire drivetrain.
It is the transmission fluid which is supplied by the pump that helps the turbine spin. The blades of the turbine have a design that ensures that any fluid received is sent directly to its center and then back to the pump.

3. Stator

The stator is also known as the reactor. It is located between the pump and the turbine. The stator resembles the propeller of an airplane and performs two basic functions which are –

•To ensure that the transmission fluid that is returned to the pump is sent back efficiently.
•It multiplies the torque that is produced by the engine in order to get the vehicle moving. Also, it is responsible for sending less torque as soon as the vehicle moves at a good rate.

The stator is capable of these functions thanks to innovative engineering. Firstly, the reactor's blades shift in such a way that each time the transmission fluid which leaves the turbine comes in contact with the blades, a diversion of that fluid occurs in exactly the same direction with the rotation of the pump.

A one-way clutch is used to connect the stator to a shaft that is fixed on the transmission. What this implies is that the starter has the ability to only move in a single direction. This is the key factor that ensures the one-directional movement of the fluid which comes from the turbine. The stator only spins at the moment where the turbine's fluid speed gets to a particular level.

It is these two elements of the stator's design that helps the pump function effectively and produce enhanced fluid pressure. In turn, the torque is amplified right at the turbine. Since the turbine is connected to the transmission, the transmission can receive more torque.

4. Torque Converter Clutch

Governed by the principles of fluid dynamics, the movement of the transmission fluid from the pump directly to the torque converter turbine results in loss of power. This causes the turbine to spin at a speed which is a little bit slower than the speed of the pump. If the car is begins to move, this is not an issue 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.

In order to prevent the loss of energy, many 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.

With this new learning about the parts of the torque converter, let us fuse everything together and analyse how the torque converter operates as you take your vehicle from a standstill up to a certain speed.

How the Torque Converter Works

As you switch your vehicle, it will go into idle mode. The pump spins with a similar speed as your engine while sending the transmission fluid to the converter turbine. However, due to the fact that 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.

However, due to how the stator’s blades are designed, the fluid is able to go right through them, and the fluid is diverted back in exactly the same direction that the pump spins in. This gives the pump the ability to send fluid at a faster rate to the turbine, thereby, building increased fluid pressure. As the fluid is returned, it reaches the turbine with increased torque. This results in the turbine delivering even more torque to your vehicle's 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. This is because, at this level, not much torque is required to get the car moving since your car already moves at a good rate. Then 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 that is generated by the engine to and from the transmission and how the torque that is sent to the transmission is multiplied in order to help the car move. Let's look next at the components that make up the automatic transmission which help your vehicle to shift automatically. We will be considering 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 which is 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 in order to alter gear ratios. However, for auto transmission, the increase and decrease of the gear ratios are automated. This is possible due to the presence of planetary gears.

Components of a Planetary Gear

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.

The Sun Gear

This gear, also, has angular-cut teeth. In an epicyclic gearbox, this gear is located in the middle. This gear constantly meshes with planetary gears at its inner parts. It is connected with an epicyclic gearbox's input shaft. In order to get varying outputs, some gears are used.

The Planet Gears

In between the ring gears and the sun gears, you will find planet gears. Its 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 have the ability to rotate on their axis while revolving between ring and sun gears and this is similar to how our solar system operates.

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, planetary gears revolution results in the carrier's rotation.

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 is used to control this device. Just one set of planetary gears can give you a reverse drive and up to 5 levels of forward driving. This is dependent on the particular gear set component that is being moved or held stationary.

With an inherent in-line shaft and cylindrical casing, planetary gears are seen as 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. In order to really understand how they work, certain details need to be understood.

Let’s consider how planetary systems are constructed and their mechanics. This will help you see some factors that are not really obvious and the role that they play.


In very basic planetary gearing, you will find three gear sets that have varying levels of freedom. It rotates around the axis revolving around sun gears that spin in place. On the outside, a ring gear which is fixed 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 a 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.

Increasing Reduction

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. In order 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 planetary systems that are compound, the reductions that they give are very much higher. 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 representation of multiplied individual ratios.

A second option involves the introduction of standard gear reducers in the planetary train. This kind of configuration is referred to as a hybrid and is preferred in certain cases as a very simple alternative to a series of planetary stages. It is, also, preferred as a way of reducing the speed of the input which could be extremely high for handling by certain planetary units. Also, it creates an input-output offset. In order to get the right angle sometimes, bevel gears could be fixed to inline planetary systems.

Taking the Torque

At different points, planetary gears mesh with sun and ring gears. This engages more teeth in order to drive the load. Therefore, planetary gears need gears that are smaller but larger in number when compared to the conventional pinion-gear reduction. One consideration that is not so obvious is that when it comes to multiple planets that have equal spacing, the input shaft bearings and the output shaft bearings do not have to bear the radial load that results from tangential gears. This is because the reactions are canceled out. Additionally, 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. Although planets primarily come in three kinds, it is possible to find more or less. Additionally, equal spacing existing between multiple planets is quite common.

Beyond spur gears, you could find helical gears for load capacity. With helical planetary gears, axial reactions exist, and there is no cancellation with multiple planets just as is experienced in reactions of tangential and separating gears. This way, the bearings are not responsible for thrust load in any way.

Wear & Tear

With respect to the length of life and depreciation, inline planetary systems have the ability to evenly distribute the load among all of the prominent components, and the economic result is proof of this distribution. In a case where all components happen to be of similar quality if a weak link is to be pointed out potentially, it probably will be the bearings which provide support for each planetary gear.

There is very little space here, so unlike in conventional gear and pinion reducers which have lots of space to contain larger bearings, planet bearings happen to be small. Keep in mind, also, that 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.

Cyclic fatigue and thermal fatigue in the bearings could be enhanced due to limitations in load distribution coupled with the ability of planet gears to spin extremely fast. Also, high speeds and heavy planet gears lead to centrifugal forces that could create an additional burden.  We are not in any way implying that planet bearings lack the ability to outlive certain components. Also, precision bearings that are of a high grade when combined with high tolerance gears that are of a low grade do not in any way signify a ceteris paribus arrangement at all.

Planet Balancing

In real-life scenarios, the planets do not really take up a load that is perfectly balanced. A planet could one way or another 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. Today, with a slow but continuous reduction in manufacturing precision coupled with an increase in planet production, there is a higher tendency that the imbalance will continue to increase.

There are times when imbalance produces an effect that is quite small and can be accepted by the operation. It is even possible for the planets to wear-in and, then, begin to get the load engaged in a more even manner gradually. However, there are designs which are extremely sensitive to the slightest of imbalances. These designs need components and assemblies that have very high precision. In a case like this, accuracy in pointing out the accurate locations around the sun gear’s axis for the planet pins is very essential.

There are other ways to enhance the balancing of planet loads. One of such ways is to use floating sub-assemblies to provide very little radial movement for the planet carrier and the sun. This gives components room to shift quite a bit, and this helps to even out the loading. 


The noise situation for planetary systems is not any worse. In fact, it is a lot better than what you get as standard gear and pinion reduces. When you have smaller gears, it results in the production of a pitch line velocity that is very low when compared to a pinion and gear set. But when you have lots of planet teeth which are identical engaging each other with a similar frequency as the revolution of the input shaft, it creates noise, especially, when it occurs at a very high level of speed. 

What complicates this situation more is that the teeth meshing usually occurs in circular motions. In times like this, making use of a spur gear which is made of very high quality is good. However, helical gears which carry out tooth engagement gradually instead of instantaneously are usually more preferable for operations like this.

One other way of reducing noise is to design the system in a manner that enables the planets to mesh in a way that supplies a cancelling effect when meshing is done out of phase. It is also a great idea to dampen the system since it helps to discourage resonance.


When planetary gear trains run continuously at considerably high levels of speed, they will generate heat. In basic gear and pinion systems, huge bearing and surface area is required for the load, and this leads to a very high heat sink. Heat dissipation rates can be greatly limited by the planetary unit compactness. In cases like this, extra measures are usually applied. 

The heat exchanger may be used to circulate the lubricant, or you could introduce a cooling fan. For a continuous operation, there is far less opportunity for the system to cool itself than if it ran at intervals. If cooling is not sufficient enough, the allowance for speed may have to be reduced. As was stated earlier, another alternative is to make use of certain speed reducers which will be connected before the planetary reducer.

Planetary gearing has varying speed ranges which are mostly dependent on the application. Most times, speed rating is usually influenced by gear drive size since pitch lines that have a higher velocity could result in an increase in heat which exceeds any form of cooling effect. Of course, you will find small planetary drives which have a speed that numbers in thousands of rpm.

Scenario Considerations

This time, let us consider situations where the various components that make up the planetary gear perform different functions by acting as input gear, an output gear, or staying stationary. Note that the input gear is responsible for power generation and the output gear is responsible for receiving the power.

1st Scenario: input (sun gear), output (planetary carrier), stationary (ring gear)

Here the role of the input gear is performed by the sun gear. Also, the ring gear stays stationary. As the sun gear moves, the ring gear stays fixed, and this causes the rotation of the planetary gears and their walking in the ring gear's interior in a direction that is opposite to that of the sun gear. This way, it results in the rotation of the carrier in a direction that is similar to the sun gear, thereby, causing the carrier to perform the role of the output gear.

What this does is to create a gear ratio that is low implying that the sun gear which serves as the input gear rotates in a way that is significantly faster than the planet carrier which performs the role of the output gear. Mostly, this kind of configuration comes into play at the point where the car gets started.

2nd Scenario: input (ring gear), output (planetary carrier), stationary (sun gear)

Here, the sun gear performs the role of staying stationary. The input gear role is played by the ring gear which does the job of supplying power to the entire gearing system. Due to the fact that the sun gear stays stationary, the planet gears that spin will be made to take along the planetary carrier while walking round the sun gear. The direction of movement of the planet carrier is the same as that of the ring gear. The planet carrier here performs the role of the output gear.

What this does is to create a gear ratio that is somewhat higher than in the first scenario. This time, the ring gear which performs the role of the input gear still spins at a very fast rate that is higher than the planetary carrier which performs the role of the output gear. Ultimately, this causes the planetary gear to supply more torque to the entire drivetrain. As your vehicle begins to increase in speed from being at a halt, this is the configuration that most likely is playing. Also, this configuration would take effect during an uphill drive.

3rd Scenario: input (sun gear), output (planetary carrier), input (ring gear)

In this scenario, we have two input gears which are the sun and the ring gears. What this means is that these two gears spin with the same level of speed and are doing so in a similar direction. 

What this does is that it makes the planetary gears unable to spin on the singular shafts. What is the reason for this you might ask? When the ring gear and the sun gear perform the role of the input gear, the ring gear's internal teeth will make an attempt to get the planetary gears rotating in a particular direction. At the same time, the sun gear's external teeth will attempt to drive the planetary gears in a direction that is opposite to that of the ring gear. This keeps them locked into place.

At this point, both the sun gear, the ring gear, and the planetary carrier all move unanimously with the same level of speed and produce an equal level of power. A direct drive occurs when the input gear and the output gear transmit an equal level of torque. If you are driving at about 45 to 50 mph, this configuration would most likely be active.

4th Scenario: input (planetary carrier), output (ring gear), stationary (sun gear)

This time around, we look at a situation where the role of the stationary gear is performed by the sun gear. The planetary carrier performs the role of the input gear which is responsible for supplying power while the output gear role is performed by the ring gear.
With the rotation of the planetary carrier, the stationary gear which is the sun gear will have the planetary gears walking around it, and this causes the ring gear to be driven faster. As the planetary carrier completes a single rotation, it leads to the ring gear rotating above a complete revolution in exactly the same direction.  

Basically, this gear ratio is high, and it leads to the supply of higher output speed while creating less torque. We refer to this as overdrive. Assuming you were cruising at about 60 mph and above, this configuration would most likely be active.

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 for changes to be made. This is totally different from the manual transmission.

You could be wondering how the auto transmission has the ability to tell what components of the system should perform the roles of input, output, and stationery gears in order to produce gear ratios that vary. Well, this is where brake bands and clutches that are found inside the transmission come into play. Let us talk about them.

Brake Bands & Clutches

What exactly are brake bands?

Brake bands are manufactured using metal which is lined with organic friction material. These bands have the ability to tighten in order to hold certain gears in a stationary position (ring and sun gears) and can also loosen in order to give them the allowance that is needed for them to spin. The ability of a brake band to tighten or loosen is mostly under the control of the hydraulic system.

The components that you find in a planetary gearing system are connected by the clutches. In automatic transmissions, the elements that comprise the transmission clutches are multiple metals as well as friction discs. The clutch engages when these discs come together. 

A clutch performs the role of making the planetary gear part function as an input gear. Also, it can make this same gear parts function as a stationary gear. Basically, this process is influenced by the connection that exists between the planetary gear and the clutch. It is the fusion of mechanical design, hydraulic design, as well as, the electrical design that determines if a clutch will engage or not. The best part is that all of this is automated.

Epicyclic Gearbox Working Principle

In this section, we will talk about how the epicyclic gearbox or a planetary gear system works.

Basically, the epicyclic gearbox works on the principle that gear fixing is carried out to obtain the level of torque or the production of speed that is required. Fixing the sun gear, planet gears, or ring gears leads to changes in gear ratios that range from high torque levels to high levels of speed. Now, we will consider how gear ratios are gotten -

1st gear ratio
The first gear ratio is responsible for giving the vehicle high torque ratios, enabling your car to move from a dead state. This ratio is gotten when you fix the ring gear. This results in the rotation of the planet carrier via the power that is transmitted to the car’s sun gear.

2nd gear ratio
The second gear ratio is responsible for giving your car high-speed ratios, enabling your car to achieve greater speed as you drive. In order to obtain these ratios, what you need to do is fix the sun gear. This will result in the planet carrier becoming the driven component while the ring gear will serve as the driving component. This helps the achievement of high-speed ratios as you drive.

Reverse gear ratio
It is the duty of the reverse gear to ensure that the output shaft works in a reverse direction. This eventually leads to your car moving in a reverse direction. In order to obtain this gear, what you have to do is to fix your planet gear carrier. This will cause the ring gear to become the driven member while the sun gear will serve as the driver component.  

Keep in mind that in order to achieve more speed ratios or higher torque ratios, the number of planet gears as well as sun gears in the epicyclic gearbox need to be increased.

Now, with all of these essential parts and how they work broken down into bits, let us head straight into deciphering how an auto transmission works.

How an Auto Transmission Works

From everything that has been discussed so far, it is clear that there are lots of components which are contained in an auto transmission. An automatic gearbox combines mechanical engineering, fluid engineering, as well as electrical engineering to ensure that you are able to move smoothly from the idle state to the state where you cruise smoothly at high speed on the highway.

Now, we will take a look at the bigger picture 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 of the torque converter through the transmission fluid. As the fluid gets to the turbine, it is returned to the pump through the aid of the stator.

What the stator does is to increase transmission fluid power which helps the pump to transmit increased power to the turbine. This actually creates a rotation of vortex power within the torque converter.

A central shaft which has a linkage with the transmission is connected to the torque turbine. The spinning of the turbine causes the shaft to spin, and this results in the first set of planetary gears receiving power.

Based on the multiple disk clutch which is engaged or even the brake band that gets engaged, the power that is generated by the torque converter will result in the gears in the planetary gear system moving or staying stationary.

This is where the gear ratio gets to be determined. Based on what components in the planetary system that are mobile or static, the gear ratio will be determined. The kind of arrangement that you have for your planetary gear will be the sole determinant of the level of power which the automatic transmission transmits throughout the entire drive train of the vehicle.

In broad terms, this is exactly the mode of operation of an automatic transmission.


It is important to know that auto gearboxes and manual transmissions are built for the same purpose. However, if you have had the opportunity to drive both types, you realise that they operate differently. As a driver, you will notice that an automatic transmission makes your driving experience seamless and smooth. For one, you do not have to battle with a clutch pedal which is found in a manual transmission vehicle. Also, you do not have to operate any gear shift. The only thing that is needed is to take your vehicle from park to drive. Once done, the auto transmission takes care of the rest.

Did you enjoy this in-depth look at how a transmission works? You should check out What Parts Make Up a Car Engine? and Engine Capacity Explained!

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