How Automatic Transmission Works

An automatic transmission works by engaging the clutch when required and disengaging it when not needed. The act of shifting gears in an automatic transmission is called, “the throws.” There are two types of gearshifts: 1) mechanical (or smooth shift); 2) electronic (or torque converter).

The four-speed manual transmission was the only option available for sale in North America until 1989, when an auto maker introduced its first automatic. The car got rid of a clutch pedal and replaced it with gears that changed speed by themselves,.

The “how automatic transmission works animation” is a video that explains what an automatic transmission does.

Welcome back to Gearhead 101, a series aimed at educating automotive newbies on the fundamentals of vehicle operation.

If you’ve been following Gearhead 101, you already know how a vehicle engine works, how it distributes power to the drivetrain, and how a manual gearbox works as a power switchboard between the engine and the powertrain.

However, most people nowadays (at least in the United States) drive automobiles with automatic gearboxes. Have you ever wondered how your automobile can automatically shift into the proper gear without you having to do anything except push the gas or brake pedals?

Hold on to your butts, then. The automatic gearbox is going to take you through one of the most astounding feats of mechanical (and fluid) engineering in human history.

(I’m not exaggerating when I say that once you grasp how automatic transmissions operate, you’ll be amazed that humans invented them without computers.)

Time for a Retrospective: What is the Purpose of a Transmission?

Let’s take a brief look at why cars require transmissions in the first place before we delve into the details of how an automatic gearbox works.

The engine in your automobile generates rotational power, as we described in our primer on how a car engine works. We must transmit the rotational power to the wheels in order to propel the automobile. The car’s powertrain, of which the gearbox is a component, accomplishes this.

But there’s a catch: in order to run effectively, an engine can only spin at a specific speed. You won’t be able to get the automobile running from a stop if it spins too low; if it spins too rapidly, the engine will self-destruct.

What we need is a technique to increase the engine’s power when it’s required (starting from a stop, climbing a hill, etc.) while reducing the amount of power transmitted when it’s not (going downhill, going really fast, slamming on the brakes).

This is where the transmission begins.

No matter what condition you’re in, the gearbox guarantees that your engine spins at an ideal pace (neither too slow nor too fast) while simultaneously giving the proper amount of power to your wheels to drive and stop the automobile. It lies between the engine and the rest of the drivetrain and functions as the vehicle’s power switchboard.

We discussed how manual gearboxes do this using gear ratios before. You can improve the amount of power transmitted to the remainder of the automobile by connecting various sized gears together without increasing the rotational speed of the engine all that much. If you don’t understand what gear ratios are, I suggest watching the video we posted last time before continuing; nothing else will make sense until you do.


By squeezing the clutch and moving the gears into position, you can control which ratios are engaged in a manual gearbox.

Brilliant engineering decides whether gear is engaged in an automatic gearbox without you having to do anything but hit the gas or brake pedals. It’s automotive enchantment.

Automatic Transmission Components

Automatic transmission diagram illustration.

So, by now, you should have a basic grasp of what a gearbox does: it guarantees that your engine spins at an appropriate pace (neither too slow nor too fast), while also supplying the proper amount of power to your wheels to drive and stop the automobile in any condition.

Let’s look at the components that make this possible in the case of an automatic transmission:

Casing for transmission

Automatic transmission casing illustration.

All of the transmission’s components are housed in a transmission case. Because it resembles a bell, it’s sometimes referred to as a “bell case.” Aluminum is often used for transmission casings. The bell case on contemporary automobiles includes several sensors that detect input rotating speed from the engine and output rotational speed to the rest of the car, in addition to protecting all of the transmission’s working gears.

Converter of Torque

Have you ever wondered why you can start your automobile but it won’t drive forward? That’s because the engine’s power is removed from the transmission. The engine may continue to operate even if the rest of the car’s powertrain isn’t receiving any power because of this disconnect. By pushing in the clutch on a manual gearbox, you disconnect power from the engine to the drivetrain.

However, in an automatic gearbox without a clutch, how do you separate power from the engine to the rest of the drivetrain?

Of course, a torque converter is required.

This is where the automatic transmission’s dark magic starts (we haven’t even discussed planetary gears yet).

Between the engine and the gearbox lies the torque converter. It’s a donut-shaped object that sits within the transmission’s bell case’s large aperture. In terms of torque transmission, it has two basic functions:

  1. Transfers power from the engine to the input shaft of the gearbox.
  2. Multiplies the torque output of the engine.

The hydraulic power produced by the transmission fluid within your transmission allows it to fulfill these two duties.

To comprehend how this works, we must first comprehend how the various components of a torque converter function.

Torque Converter Components

Illustration of parts of torque converter for automatic transmission.

In most contemporary automobiles, a torque converter consists of four basic components: 1) the pump, 2) the stator, 3) the turbine, and 4) the torque converter clutch.

1. Get a pump (aka impeller). The pump resembles a fan. Its core is surrounded by a swarm of blades. The torque converter housing is fastened directly to the engine’s flywheel, and the pump is mounted directly to it. As a result, the pump spins at the same rate as the crankshaft of the engine. (This is important to know when we go through how the torque converter works.) The transmission fluid is “pumped” outwards from the center to the…


Turbine number two. The turbine is housed within the converter. It resembles a fan, much like the pump. The turbine is directly connected to the transmission’s input shaft. Because it is not attached to the pump, it may travel at a different pace. This is a crucial aspect. This is what permits the engine to rotate at a faster rate than the rest of the drivetrain.

The transmission fluid that is delivered from the pump allows the turbine to spin. The blades of the turbine are intended to direct the fluid it receives towards the center of the turbine and then back to the pump.

3. The Stator (aka Reactor). Between the pump and the turbine is the stator. Is there a pattern here? It resembles a fan blade or an aircraft propeller. The stator does two tasks: 1) it more effectively returns transmission fluid from the turbine to the pump, and 2) it doubles torque from the engine to assist in getting the vehicle started, but then transmits less torque after the car is running at a decent speed.

It does this via some ingenious engineering. First, the reactor’s blades are built such that when transmission fluid from the turbine strikes the stator’s blades, it is redirected in the same direction as the pump’s spin.

Second, a one-way clutch connects the stator to a fixed shaft on the gearbox. The stator can only travel in one direction as a result of this. This guarantees that the turbine’s fluid is channeled in a single direction. When the fluid speed from the turbine reaches a specific level, the stator will begin to spin.

The stator’s two design components make the pump’s job simpler and create higher fluid pressure. As a result, the turbine generates more torque, and since the turbine is linked to the gearbox, more torque can be transferred to the transmission and the rest of the automobile. Whew. 

4. Clutch for the torque converter As the transmission fluid travels from the pump to the turbine, power is lost due to fluid dynamics. The turbine spins at a little slower rate than the pump as a consequence of this. When the automobile is first starting underway, this isn’t an issue (in fact, the difference in speed permits the turbine to give more torque to the gearbox), but after it’s cruising, the discrepancy causes some energy inefficiencies.

Most current torque converters incorporate a torque convertor clutch attached to the turbine to prevent this energy loss. The torque converter clutch engages when the automobile reaches a specific speed (typically 45-50 mph), causing the turbine to spin at the same speed as the pump. When the converter clutch is engaged, it is controlled by a computer.

That concludes the torque converter’s components.

Inner part of torque converter clutch.

Let’s put it all together and see how the torque converter reacts when you accelerate from a halt to cruising speed:


When you start the automobile, it sits idle. The pump is rotating at the same speed as the engine and transferring transmission fluid to the turbine, but the turbine isn’t spinning fast enough to impart torque to the gearbox since the engine isn’t spinning very quickly at a dead stop.

You take a step forward and press the gas pedal. The engine spins quicker as a result, and the torque converter pump spins faster as well. The transmission fluid is travelling fast enough from the pump to start spinning the turbine faster because the pump is rotating quicker. The fluid is sent to the stator by the turbine blades. Because the transmission fluid speed isn’t high enough, the stator isn’t rotating yet.

However, the stator’s blades are designed in such a way that when the fluid passes through them, it is diverted back to the pump in the same direction as the pump is spinning. This permits the pump to return the fluid to the turbine at a faster rate, resulting in increased fluid pressure. The fluid returns with additional torque to the turbine, leading the turbine to produce more torque to the gearbox. The automobile begins to go ahead.

As your automobile accelerates, this cycle repeats again. The transmission fluid achieves a pressure that causes the reactor blades to finally rotate when you reach cruising speed. Torque is lessened while the reactor spins. Because the automobile is traveling at a decent rate, you don’t need much torque to propel it at this point. When the torque converter clutch is engaged, the turbine spins at the same rate as the pump and engine.


So, the torque converter permits or stops power from the engine from being communicated to the gearbox, and also multiplies torque to the transmission to start the automobile moving from a standstill. It’s time to look at the transmission components that enable the automobile to shift automatically.

Planetary Mechanics

Planetary gear set of automatic transmission.

As your vehicle accelerates, less torque is required to keep it moving. Gear ratios allow transmissions to raise or decrease the amount of torque transmitted to the car’s wheels. More torque is provided with a lower gear ratio. The less torque supplied, the greater the gear ratio.

To change the gear ratios on a manual gearbox, you must move your gear shift.

Gear ratios rise and decrease automatically with an automatic gearbox. And this is made possible by the clever design of a planetary gear.

A planetary gear is made up of three parts:

  1. A hat and sunglasses. Located at the planetary gear set’s middle.
  2. Planet gears and pinions, as well as their carrier. Three or four smaller gears that round the sun gear and are always in contact with it. The carrier mounts and supports the planet gears (or pinions). Each planet gear rotates on its own shaft, which is attached to the carrier. Planet gears rotate in addition to orbiting the sun gear.
  3. This is the ring gear. The outer gear, the ring gear, contains internal teeth. The ring gear is in continuous mesh with the planet gears and surrounds the remainder of the gear set.

Reverse drive and five stages of forward drive may be achieved with a single planetary gear set. It all depends on whether one of the three gear set components is moving or stationary.


Let’s look at it in action, with the various components operating as input gear (the gear that generates power), output gear (the gear that receives power), or being kept motionless.

Sun Gear is the input gear, Planetary Carrier is the output gear, and Ring Gear is the holding gear.

Gear Reduction with Sun Gear Input.

The solar gear is the input gear in this case. The ring gear is stationary. The planetary gears will spin on their own carrier shafts and wander around the inside of the ring gear in the opposite direction as the sun gear, with the ring gear kept in place. The carrier rotates in the same direction as the sun gear as a result of this. As a result, the carrier becomes the output gear.

The input gear (in this example, the sun gear) rotates faster than the output gear in this design, resulting in a low gear ratio (the planet carrier). However, the planet carrier generates much more torque than the solar gear can handle.

When the automobile is just getting started, this setup would be employed.

Ring Gear: input gear / Planetary Carrier: output gear / Sun Gear: maintained stationary

Internal view of an automatic transmission gear.

The sun gear remains fixed in this case, but the ring gear becomes the input gear (delivering power to the gear system). The spinning planet gears will wander around the sun gear and carry the planet carrier with them since the sun gear is being held.

The planet carrier is an output gear that travels in the same direction as the ring gear.

This setup produces a slightly greater gear ratio than the first. However, the input gear (ring gear) continues to spin faster than the output gear (the planetary carrier). The planetary gear then delivers extra torque, or power, to the remainder of the powertrain as a consequence. This setup is most usually in use when your automobile accelerates from a standstill or while traveling up a hill.

Sun Gear is the input gear, the Planetary Carrier is the output gear, and the Ring Gear is the input gear.

Planetary carrier of automatic transmission gear.

Both the sun gear and the ring gear serve as input gears in this instance. That is, they are both spinning in the same direction and at the same speed. The planetary gears do not rotate on their respective shafts as a result of this. Why? If the input members are the ring gear and the sun gear, the ring gear’s internal teeth will attempt to spin the planetary gears in one direction, while the sun gear’s exterior teeth will try to drive them in the other way. As a result, they become immovable. The whole device (sun gear, planetary carrier, and ring gear) spins at the same rate and transfers the same amount of power. Direct drive occurs when the input and output transmit the same amount of torque.

When you’re travelling at 45-50 mph, this arrangement comes into play.

The Sun Gear is kept fixed; the Planetary Carrier is the input gear; and the Ring Gear is the output gear.


Overdrive with sun gear held.

The solar gear is kept fixed in this case, and the planetary carrier serves as the input gear for delivering power to the gear system. The output gear is now the ring gear.

The planetary gears are forced to wander around the held sun gear while the planet carrier spins, which accelerates the ring gear. The ring gear rotates more than one full revolution in the same direction for every complete rotation of the planet carrier. This is a high gear ratio, which means that the output speed is higher but the torque is lower. “Overdrive” is another term for this combination.

When you’re traveling at 60+ mph on the interstate, you’ll be in this position.

There are frequently many planetary gear sets in an automated gearbox. They collaborate to generate a variety of gear ratios.

Because the gears of a planetary gear system are always in mesh, gear changes may be done without engaging or disengaging gears, as in a manual gearbox.

But, in order to achieve such variable gear ratios, how does an automated transmission determine which elements of the planetary gear system should operate as the input gear, output gear, or be kept stationary?

Brake bands and clutches are used within the gearbox.

Clutches and Brake Bands

Metal brake bands with organic friction substance are used. The brake bands may be tightened to keep the ring or sun gear motionless, or they can be relaxed to allow the ring or sun gear to rotate. A hydraulic system controls whether a brake band tightens or loosens.

Brake bands and clutch plates of automatic transmission.

A planetary gear system’s many components are additionally connected by a series of clutches. Automatic transmission clutches are made up of many metal and friction discs, which is why they’re frequently called “multi disc clutch assemblies.” The clutch is activated when the discs are forced together. A clutch may turn a planetary gear portion into an input gear or keep it immobile. It all relies on how the planetary gear is linked. Mechanical, hydraulic, and electrical design all have a role in whether or not a clutch engages. And it all occurs on its own.

The complexities of how the numerous clutches interact to retain and drive various components are now rather extensive. It’s too complex to put into words. It’s easiest to grasp visually. I strongly advise you to watch this video, which will lead you through the process:


The Workings of an Automatic Transmission

An automatic transmission, as you can see, has a lot of moving components. It combines mechanical, hydraulic, and electrical engineering to provide a seamless transition from zero to highway cruising speed.

So let’s take a look at the power flow in an automatic transmission from a broad picture perspective.

The torque converter’s pump receives power from the engine.

The torque converter’s turbine receives power from the pump through transmission fluid.

The transmission fluid is returned to the pump through the stator by the turbine.


The stator multiplies the transmission fluid’s power, enabling the pump to return more energy to the turbine. Inside the torque converter, a vortex power rotation is formed.

The turbine is linked to the transmission by a central shaft. The shaft rotates while the turbine spins, transferring power to the transmission’s first planetary gear set.

The torque converter’s power will either force the sun gear, planetary carrier, or ring gear of the planetary gear system to move or remain stationary, depending on whether multiple disc clutch or brake band is engaged in the gearbox.

The gear ratio is determined by which sections of the planetary gear system are moving and which are not. The amount of power the transmission provides to the remainder of the drive train is determined by the planetary gear configuration (sun gear serving as input, planetary carrier functioning as output, ring gear stationary — see above).

In general, that’s how an automatic transmission works. Things are regulated and modified by sensors and valves, but that’s the essence of it.

It’s something that’s simpler to grasp when you see it. I strongly advise you to view the video below. The background we went through will help you comprehend it a lot better:


So, what did I say? The automatic gearbox is just great.

You’ll have a fair notion of what’s going on beneath the hood now as you travel down the highway and feel the automobile swap gears.



The “automatic transmission repair” is a process that uses an oil pump to force the engine’s output shaft to rotate at the speed of the input shaft. The result of this is that power is transferred from the engine to the wheels without any mechanical connection between them.

Frequently Asked Questions

How does an automatic transmission shift?

A: An automatic transmission shifts by sending power the whole time it is in gear. This will cause your car to do all of its work while you are driving, allowing for a more energy efficient drive.

How does an automatic car work?

A: The car has a long list of tasks that it is programmed to accomplish. These tasks can range from steering and accelerating to braking and turning, with each task being assigned different features such as headlights or the horn.

Does automatic transmission have clutch?

A: Automatic transmissions do not have a clutch. They use a torque converter to transmit power from the engine to the wheels based on how much pressure is being applied in each gear. Clutch refers to something that stops or slows down an engine by pushing against it, like your cars brakes

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