This is how automatic transmissions work (2023)

This is how automatic transmissions work (1)

If you have ever driven a car with an automatic transmission, then you know that there are two major differences between an automatic transmission and an automatic transmissionmanual transmission:

  1. There is no clutch pedal in a car with an automatic transmission.
  2. In a car with an automatic transmission, there is no gear shift. Once you put the gear inhandle, everything else is automatic.

Both the automatic transmission (plus itstorque converter) and amanual transmission(with hiscoupling) achieve exactly the same thing, but they do it in completely different ways. It turns out the way an automatic transmission does this is absolutely amazing!


In this article we will go through an automatic transmission. Let's start with the key to the overall system: planetary gears. Then we look at how the gearbox is put together, learn how the controls work and discuss some of the intricacies involved in controlling a gearbox.



  1. Purpose of an automatic transmission
  2. planetary gear
  3. planetary gear ratios
  4. compound planetary gear
  5. First course
  6. second course
  7. third gear
  8. exaggerate
  9. Turning back
  10. Clutches and bands in an automatic transmission
  11. If you put the car in the parking lot
  12. Automatic transmission: hydraulics, pumps and regulators
  13. Automatic transmission: valves and modulators
  14. electronically controlled transmission

Purpose of an automatic transmission

This is how automatic transmissions work (2)

As with a manual transmission, the main task of the automatic transmission is toMotor-to operate in its narrow speed range while providing a wide range of output speeds.

Without a gearbox, cars would be limited to onetransmission ratio, and this ratio would have to be chosen to allow the car to travel at the desired top speed. If you want a top speed of 80 mph then the gear ratio would be similar to third gear in most manual transmission cars.


Chances are you've never tried driving a manual transmission car in third gear only. If you did that, you'd quickly find that you were almost out of throttle on startup, and at high speeds the engine would roar near the redline. Such a car would wear out very quickly and be almost undriveable.

So the transmission uses gears to use the engine more effectively.torque, and to keep the engine running at a reasonable speed. When towing or transporting heavy objects, your vehicle's transmission can become so hot that the transmission oil burns. To protect the transmission from serious damage, drivers who tow should equip vehicles with ittransmission cooler.

The main difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different setstransmissionto the output shaft to achieve the different gear ratios, while in an automatic transmission the same set of gears produces all the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission.

Let's look at how the planetary gear works.


planetary gear

This is how automatic transmissions work (3)

If you disassemble an automatic transmission and look inside, you will find a multitude of parts in a relatively small space. Among other things you will see:

  • An ingenious planetary assembly
  • A set of bands for locking parts of a gear set.
  • A set of three wet multi-plate clutches used to lock other parts of the gear set
  • An incredibly weird hydraulic system that controls the clutches and bands.
  • A large gear pump to move the transmission oil

is the focusplanetary connection. About the size of a cantaloupe, this part creates all the different gear ratios that the gearbox can create. Everything else in the transmission is there to helpplanetary connectiondo your thing This amazing piece of gear has already been featured on HowStuffWorks. You might recognize it by theelectronic screwdriverArticle. An automatic transmission contains two complete planetary gear sets folded together into a single component. RegardHow gear ratios workfor an introduction to planetary gears.


Every planetary gear set has three main components:

  1. Issun gear
  2. Isplanetary gearand the planetary gearsTransporter
  3. Isjagged crown

Each of these three components can be the input, the output, or held stationary. Choosing which part plays which role determines the gear ratio for the gear set. Let's take a look at a single planetary gear set.


planetary gear ratios

One of the planetary gear sets in our transmission has a 72 tooth ring gear and a 30 tooth ring gear. With this gear set we can achieve many different gear ratios.

This is how automatic transmissions work (4)

When two of the three components are locked together, the entire device is locked with a 1:1 gear reduction. Note that the first gear ratio listed above is areduction-- The output speed is slower than the input speed. The second is aexaggerate-- the output speed is faster than the input speed. The last one is a reduction again, but the output direction is reversed. There are several other gear ratios that can be obtained from this planetary gear set, but these are the ones relevant to our automatic transmission. You can see them in the animation below:

So this gear set can produce all these different gear ratios without having to engage or disengage other gears. With two of these gear sets in a row we can get the four forward gears and one reverse gear that our gearbox needs. We will put the two sets of gears together in the next section.


compound planetary gear

This automatic transmission uses a set of gears, calledcompound planetary gear, which looks like a single planetary gear set, but actually behaves like two combined planetary gear sets. It has a ring gear, which is always the output of the gearbox, but it has two sun gears and two planetary gear sets.

Let's look at some of the parts:


This is how automatic transmissions work (5)

The figure below shows the planets in the satellite carrier. Note that the planet on the right is lower than the planet on the left. The right planet does not interfere with the crown, it interferes with the other planets. Only the planet on the left intervenes in the crown.

This is how automatic transmissions work (6)

Below you can see the inside of the planet carrier. The shorter gears only mesh with the smaller sun gear. The largest planets mesh with the largest sun wheel and with the smallest planets.

This is how automatic transmissions work (7)

The animation below shows how all parts are connected in a transfer.


First course

In first gear, the smaller sun gear is driven clockwise by the turbinetorque converter. The satellite carrier tries to rotate counterclockwise, but the unidirectional carrier keeps it stationary.coupling(which only allows clockwise rotation) and the crown rotates the output. The small gear has 30 teeth and the ring gear has 72, so the gear ratio is:

Ratio = -R/S = -72/30 = -2.4:1


So the rotation is negative 2.4:1 which means the exit direction would beoppositethe input address. But the starting address is really thatevenas an input address: This is where the trick with the two planetary sets comes into play. The first set of planets mates with the second set, and the second set rotates the crown; this combination reverses the direction. You can see that this would also cause the larger sun gear to rotate; However, since the clutch is disengaged, the larger sun gear is free to rotate in the opposite direction of the turbine (counterclockwise).

second course

This gearbox does a very good thing when it comes to getting the gear ratio needed for second gear. It acts like two planetary gear sets that are connected to each other via a common planetary carrier.

The first stage of the planetary carrier actually uses the larger sun gear as a ring gear. So the first stage consists of the sun (the smallest sun gear), the planet carrier and the ring gear (the largest sun gear).


The entrance is the small sun wheel; The ring gear (large sun gear) is held stationary by the band and the output is the planet carrier. For this phase with the sun as input, the satellite carrier as output and the fixed crown, the formula is:

1 + R/E = 1 + 36/30 = 2.2:1

With each revolution of the small sun gear, the planet carrier rotates 2.2 times. In the second stage, the planetary carrier acts as the input for the second set of planetary gears, the largest sun gear (which is held stationary) as the sun, and the ring gear as the output, so the gear ratio is:

1 / (1 + S/R) = 1 / (1 + 36/72) = 0,67:1

To get the total second gear ratio, we multiply the first stage by the second, 2.2 x 0.67 to get a 1.47:1 ratio.


third gear

Most automatic transmissions have a 1:1 ratio in third gear. You will remember from the previous section that we only need to put two of the three parts of the planetary gear together to get a 1:1 output. With the arrangement in this gear set, it's even easier: we just have to engage the clutches that connect each of the sun gears to the turbine.

If both sun gears rotate in the same direction, the planetary gears will lock because they can only rotate in opposite directions. This locks the ring gear to the planets and causes everything to rotate as a unit, creating a 1:1 ratio.



By definition, an overdrive has a faster output speed than its input speed. It's an increase in speed, the opposite of a decrease. In this transmission, the overdrive achieves two things at the same time. when you readHow torque converters work, you learned about a lock-up torque converter. To improve efficiency, some cars have a mechanism that locks the torque converter, so engine power goes directly to the gearbox.

In this transmission, when overdrive is engaged, a shaft attached to the torque converter housing (which is bolted to the flywheel) is clutch-connected to the planet carrier. The small sun gear spins freely and the larger sun gear is carried by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our diagram once more, this time with the planet carrier as the input, the fixed sun gear and the ring gear as the output.


Ratio = 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

So the output rotates once every two-thirds of a motor revolution. If the engine is rotating at 2000 revolutions per minute (rpm), the output speed is 3000 rpm. This allows the cars to cruise at highway speeds while keeping the engine revs nice and slow.


Turning back

Reverse gear is very similar to first gear except that instead of the small sun gear being driven by the torque converter turbine, the larger sun gear is driven and the small sun gear freewheels in the opposite direction. The planet carrier is attached to the housing with the reversing band. So, according to our equations on the last page, we have:

With this transmission, the reverse ratio is therefore slightly smaller than the first gear.


transmission ratio

This transmission has four forward gears and one reverse gear. Let's summarize the gear ratios, inputs and outputs:

This is how automatic transmissions work (8)

After reading these sections you are probably wondering how to connect and disconnect the various inputs. It does this through a series of clutches and bands within the transmission. In the next section we will see how they work.


Clutches and bands in an automatic transmission

In the last section we discussed how the gearbox creates each gear ratio. For example, when we talked about overdrive, we said:

In this transmission, when overdrive is engaged, a shaft attached to the torque converter housing (which is bolted to the flywheel) is clutch-connected to the planet carrier. The small sun gear spins freely and the larger sun gear is carried by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing.

In order for the transmission to run at full speed, many things have to be connected and disconnected via clutches and bands. The planet carrier is connected to the torque converter housing via a clutch. The small sun is separated from the turbine by a clutch so that it can rotate freely. The large sun gear is fixed to the case so it cannot rotate. Each gear change triggers a series of events like this, activating and deactivating various clutches and bands. Let's look at a band.


There are two bands in this transmission. Belts in a transmission are literally bands of steel that wrap around parts of the gear train and connect to the case. They are actuated by hydraulic cylinders in the gearbox housing.

This is how automatic transmissions work (9)

In the picture above you can see one of the bands on the gear case. The gear train is removed. The metal rod is connected to the piston that drives the belt.

This is how automatic transmissions work (10)

Above you can see the two pistons that drive the belts. Hydraulic pressure applied to the cylinder through a series of valves causes the pistons to push the bands and lock that part of the gear train in the housing.

Isclawsin transmission they are somewhat more complex. There are four clutches in this transmission. Each clutch is actuated by pressurized hydraulic fluid entering a piston within the clutch. The springs ensure that the clutch opens when the pressure drops. Below you can see the clutch drum and piston. Check out the rubber seal on the piston - this is one of the components that gets replaced when your transmission is overhauled.

This is how automatic transmissions work (11)

The figure below shows the alternating layers of clutch friction material and steel plates. The friction material is grooved on the inside where it engages one of the gears. The steel plate is knurled on the outside where it attaches to the clutch housing. These clutch plates are also replaced when the transmission is modified.

This is how automatic transmissions work (12)

Pressure for the clutches is supplied through passages in the shafts. The hydraulic system controls which clutches and bands are engaged at any given time.

If you put the car in the parking lot

It may seem like a simple matter to lock the gear and prevent it from turning, but there are actually some complex requirements for this mechanism. First, you need to be able to release it when the car is on a hill (the car's weight is resting on the mechanism). Second, you need to be able to engage the mechanism even if the lever doesn't line up with the gear. Third, something has to prevent the lever from popping out once it's engaged.

The mechanism that does all of this is pretty cool. First, let's look at some of the parts.

This is how automatic transmissions work (13)

The parking brake mechanism engages the teeth on the exit to keep the car stationary. This is the part of the transmission that connects to the driveshaft. So if that part can't rotate, the car can't move.

This is how automatic transmissions work (14)

Above you can see the parking mechanism protruding into the case where the gears are located. Note that it has tapered sides. This helps release the parking brake when parking on a hill - the weight of the car will help shift the parking mechanism due to the angle of the cone.

This is how automatic transmissions work (15)

This dipstick is connected to a cable that is operated by your car's shifter.

This is how automatic transmissions work (16)

When the shift lever is placed in the park position, the rod pushes the spring against the small cone bushing. When the park mechanism is oriented to fall into one of the notches in the output gear section, the cone bushing pushes the park mechanism down. When the mechanism is aligned at one of the highest points of the exit, the spring will push the cone bushing, but the lever will not engage until the car rolls a little and the teeth are properly aligned. That's why your car sometimes jerks a bit after you park it and release the brake pedal; You will need to roll a little to align the teeth so the parking mechanism can engage.

Once the car is parked safely, the jack holds the lever down to keep the car from rolling out of park when on a hill.

Automatic transmission: hydraulics, pumps and regulators


The automatic transmission in your car has to fulfill numerous tasks. You may not realize how many different ways it works. For example, these are some of the characteristics of an automatic transmission:

  • When the car is in overdrive (in a four-speed transmission), the transmission automatically selects a gear based on vehicle speed and accelerator pedal position.
  • When accelerating smoothly, shifts are performed at lower speeds than when accelerating at full throttle.
  • When you press the accelerator pedal, the transmission downshifts to the next lower gear.
  • If you move the gear selector to a lower gear, the transmission will downshift unless the car is speeding for that gear. If the car is speeding, it will wait for you to slow down and then downshift.
  • When you shift the transmission into second gear, it never shifts out of second gear or up, even at a complete stop, unless you move the shift lever.

You've probably seen something like this before. It's really the brains of the automatic transmission that manages all of these functions and more. The passages you can see direct fluid to all the different components of the transmission. Vias cast into the metal are an efficient way to channel liquids; Without them, many hoses would be needed to connect the various parts of the transmission. First we will discuss the key components of the hydraulic system; then we will see how they work together.

This is how automatic transmissions work (17)

The Bomb

Automatic transmissions have a proper pump, calledgear pump. The pump is usually located on the gearbox cover. It draws fluid from a sump at the bottom of the transmission and feeds it to the hydraulic system. It also feeds themtransmission coolerand thetorque converter.

This is how automatic transmissions work (18)

The internal gear of the pump meshes with the torque converter housing so that it rotates at the same speed as the engine. The outer gear is turned by the inner gear, and as the gears turn, fluid is drawn from the sump on one side of the crescent and pushed into the hydraulics on the other side.

The governor

IsGovernorIt's a smart valve that tells the transmission how fast the car is going. It is connected to the output, the faster the car moves, the faster the controller rotates. Inside the regulator is a spring-loaded valve that opens in proportion to the speed of rotation of the regulator: the faster the regulator turns, the wider the valve opens. The pump fluid is fed to the governor through the output shaft.

The faster the car drives, the more the control valve opens and the more fluid pressure it lets through.

This is how automatic transmissions work (19)

Automatic transmission: valves and modulators

This is how automatic transmissions work (20)

In order to shift properly, the automatic transmission needs to know how hard the engine is working. There are two different ways to do this. Some cars have a simple wire connection connected to athrottlewhen transferring. The harder the accelerator pedal is pressed, the more pressure is exerted on the throttle valve. Other cars use avacuum modulatorapply pressure to the butterfly valve. The modulator senses boost pressure, which increases when the engine is under a higher load.

IsHand valveThis is where the shift lever snaps into place. Depending on the gear selected, the manual valve feeds hydraulic circuits that lock certain gears. For example, when the shifter is in third gear, it feeds a circuit that prevents overdrive from engaging.

switching valvesapply hydraulic pressure to the clutches and bands to engage each gear. The transmission valve body contains multiple shift valves. The shift valve determines when to shift from one gear to the next. For example, the 1-to-2 shift valve determines when to shift from 1st to 2nd gear. The switching valve is pressurized on one side with regulator fluid and on the other side with the throttle valve. The pump supplies them with fluid, and they direct that fluid into one of two circuits to control which gear the car is in.

The shift valve delays a shift when the car accelerates rapidly. When the car accelerates smoothly, it shifts to a lower gear. Let's discuss what happens when the car accelerates smoothly.

As the speed of the car increases, the pressure on the regulator increases. This pushes the shift valve until the first gear circuit closes and the second gear circuit opens. Since the car accelerates slightly, the throttle does not put much pressure on the shift valve.

When the car accelerates quickly, the throttle puts more pressure on the shift valve. This means that the governor pressure must be higher (and therefore vehicle speed must be higher) before the shift valve moves far enough to engage second gear.

Each switching valve responds to a specific pressure range; So when the car speeds up, the 2-3 shift valve takes over because the governor pressure is high enough to activate that valve.

electronically controlled transmission

This is how automatic transmissions work (21)

Electronically controlled transmissions, found in some newer cars, still use hydraulics to drive the clutches and belts, but each hydraulic circuit is controlled by an electric motor.Solenoid. This simplifies the transfer piping and allows for more advanced control schemes.

In the last section we looked at some of the control strategies used by mechanically controlled transmissions. Electronically controlled transmissions have even more sophisticated control schemes. In addition to monitoring vehicle speed and throttle position, the transmission controller can monitor engine speed, whether the brake pedal is depressed and even theAntiblockiersystem.

Using this information and an advanced control strategy based on fuzzy logic, a control system programming method that uses human-like thinking, electronically controlled transmissions can:

  • Automatically downshifts when going downhill to control speed and reduce brake wear
  • Shift up when braking on slippery surfaces to reduce the braking torque applied by the engine
  • Disable upshifting when entering a corner on a winding road

Let's talk about the last feature: preventing upshifts when cornering on a winding road. Suppose you are driving on a winding, uphill mountain road. When driving on straight stretches of road, the transmission shifts into second gear to give you enough acceleration and power to climb hills. When approaching a corner, slow down, ease off the accelerator and possibly apply the brakes. Most transmissions will shift into third gear or even overdrive when you take your foot off the accelerator. Then, as you accelerate out of the corner, downshift again. But if you were driving a manual transmission car, you would probably keep the car in the same gear all the time. Some automatic transmissions with advanced control systems can recognize this situation after taking a few corners and "learn" not to upshift again.

For more information on automatic transmissions and related topics, see the links below.

Originally published: November 29, 2000

Frequently asked questions about the automatic transmission

What are the main differences between an automatic transmission and a manual transmission?

There are two main differences between an automatic transmission and a manual transmission. Firstly, there is no clutch pedal in a car with an automatic transmission. Second, in a car with an automatic transmission, there is no gear shift. So once you've engaged the gear, everything else is automatic.

What is the main task of an automatic transmission?

The primary job of an automatic transmission is to keep the engine running within its narrow speed range while providing a wide range of output speeds.

When did the automatic transmission become standard?

Aftersixth, automatic transmissions became the American standard in the late 1980s.

How does an automatic transmission work?

According to, "[an] automatic transmission uses sensors to determine when to shift gears and uses internal oil pressure to shift them."

What is in an automatic transmission?

The inside of an automatic transmission has a planetary gear set, a band set, a set of three wet multi-plate clutches, a hydraulic system, and a large gear pump.

much more information

related posts

  • How manual transmissions work
  • How torque converters work
  • how gears work
  • How gear ratios work
  • This is how clutches work
  • This is how car cooling systems work
  • How car engines work

More great links

Top Articles
Latest Posts
Article information

Author: Lilliana Bartoletti

Last Updated: 12/19/2022

Views: 5621

Rating: 4.2 / 5 (53 voted)

Reviews: 84% of readers found this page helpful

Author information

Name: Lilliana Bartoletti

Birthday: 1999-11-18

Address: 58866 Tricia Spurs, North Melvinberg, HI 91346-3774

Phone: +50616620367928

Job: Real-Estate Liaison

Hobby: Graffiti, Astronomy, Handball, Magic, Origami, Fashion, Foreign language learning

Introduction: My name is Lilliana Bartoletti, I am a adventurous, pleasant, shiny, beautiful, handsome, zealous, tasty person who loves writing and wants to share my knowledge and understanding with you.