Automatic Transmission, How it works?

The operation of an automatic transmission is explained here with help of animation. Allison-1000 transmission model, which has 6 speed and reverse is used for this purpose. The video starts with an explanation of planetary gear set. Just by engaging few clutch packs different output speed can be achieved in automatic transmission. A brief introduction of the torque converter is also given here.

Here is the 2D version of the video. In my opinion this video is more helpful than the 3D video to understand the technology.

[IamTiper]

You know those videos where someone explains something complicated, in a simple way, and suddenly you see how easy it is? This is not one of them.

[TheDutchGuy]

Me: trying to understand
Narrator: *Adds another clutchpack and planetary gear set*
Me: Blue screen of death

[RayDragonfly]

*For anyone struggling to understand:*

_First watch the video and try to understand where parts are and how they interact and come back to this comment if you have questions._

(I'll be referring to each part as P1, P2, P3, P4, and P5. C1 acts on P1, C2 acts on P2, C3 acts on P3, C4 acts on P4, and C5 acts on P5)

P1 is the Hub and case
P2 is the P4 planet carrier and clutch module
P3 is the first set of gears (ring gear, planet gears, and sun gear) and the P4 planet carrier
P4 is the second set of gears (ring gear, planet gears, and sun gear) and the P5 planet carrier
P5 is the third set of gears (ring gear, planet gears, and sun gear) and the output shaft

In this type of automatic transmission the clutch packs are always in contact with whatever they're meant to engage with they're just allowed to spin freely.
This creates friction between any parts in contact with the clutch packs which is why manual transmission systems are generally more fuel efficient.
When the clutch packs are engaged they are no longer able to spin freely to induce a lot more friction in turn stopping what they're engaged to.

For example:
The C1 clutch is connected to the hub (gear) on the end of the input shaft and the case on the intermediate shaft.
When disengaged the C1 clutch will be able to spin freely without imparting much of any force on the intermediate shaft.
When engaged the C1 clutch increases friction with the hub to essentially extend the hub to the case and apply force to the intermediate shaft.
This lets the input shaft turn the intermediate shaft directly as long as the C1 clutch is engaged.

*For the first, seconds, third, and fourth gear:*

The C1 clutch pack engages to turn the intermediate shaft as explained above. The intermediate shaft is only connected to P4 and P5 sun gears so C1 only affects P4 and P5.

*First gear:*

First gear uses P1 and P5

C1 and C5 engage in first gear.
C1 engages because the intermediate shaft is only turned when C1 is engaged.
This means that the sun gear in P5 is only turned when C1 is engaged.

A problem arises however, if we only engage C1 we'll also be turning the sun gear in P4 which can turn the P5 planet carrier/ring gear.

To combat this C5 is also engaged.
This "connects" the P5 ring gear to the transmission case.
The transmission case is stationary so when C1 engages it stops the ring gear from being able to turn.

So C1 engages to supply force to the P4 and P5 sun gear.

C5 engages to stop the P4 sun gear from turning the P5 ring gear.

*Second gear:*

Second gear uses P1 and P4.

C1 and C4 engage in second gear.
C1 engages because the intermediate shaft is only turned when C1 is engaged.
This means that the sun gears in P4 and P5 are turned only when C1 is engaged.

When the P5 sun gear turns, it only turns the P5 planet gears.
When the P4 sun gear turns, it turns the P4 planet gears however these are connected to the P5 planet carrier.
This means that the P4 planet gears increase the speed the P5 ring gear.
This leads to the planet gears turning more quickly than in first gear.
This increases the output shaft turn speed.

A problem arises when we don't engage C4.
The P3 sun gear isn't connected to the intermediate shaft but instead connected to the input shaft.
This means that the P3 sun gear turns at any point the engine is supplying force.
The P3 planet gears turn the P4 planet carrier at the same rate as the input shaft, this would be 1:1 input:output which would require too much force from the engine.
This would result in the engine stalling (Results in a jump from first gear to fourth gear)
To combat this, the C4 clutch is engaged.
The C4 clutch "connects" the P4 ring gear to the gear set case.
This means that C4 stops P3 from supplying force to the output shaft.

So C1 engages to supply force to the P4 and P5 sun gears.
C4 engages to stop the P3 sun gear from turning the P4 planet carrier (prevents a jump to fourth gear).

*Third gear:*

Third gear uses P1 and P3.

C1 and C3 engage in third gear.
C1 engages to turn P4 and P5 sun gears.

The P3 sun gear connects directly to the clutch module which is in turn connected to the input shaft.
This means that the P3 sun gear is turning as long as the engine is supplying force.

A problem arises when C3 isn't engaged.
The P3 ring gear is only connected to C3 and the P3 ring gears.
When the P3 sun gear turns while C3 isn't engaged it would turn the P4 planet carrier.
This means that they will enter fourth gear resulting in engine strain/stalling.
When C3 is engaged, the P3 ring gear remains stationary slowing the P4 ring gear.
This means that the force supplied to the output shaft is decreased.

So C1 engages to supply force to the P4 and P5 sun gears.
C3 engages to reduce torque required from the engine (prevents a jump to fourth gear).

*Fourth gear:*

Fourth gear uses P1 and P2.

C1 and C2 engage in fourth gear.
C1 engages to turn the P4 and P5 sun gears.

P2 is connected to the clutch module and the P1 planet carrier
When C2 is engaged, P2 turns at the same rate as the input shaft.
This results in the ring gear and sun gear of P5 turning at the same rate.
This means that the input and output shaft turn at the same speed.

If C2 is not engaged you would remain in fourth gear but it requires more force to turn the output shaft which is less efficient.
This is because P3 is connected to the input shaft.

So C1 engages to supply force to the P5 sun gear.
C2 engages to supply direct force to the P5 ring gear.

*Fifth gear:*

Fifth gear uses C2 and C3.

C2 engages to turn the P4 planet carrier.

C3 engages to stop the P3 ring gear.
With C1 disengaged the intermediate shaft can rotate independently of the input shaft.
When C3 is engaged it prevents entering fourth gear and supplies the P4 ring gear with force.
This results in a greater turn rate at the output shaft.

So C2 engages to supply force to the P4 planet carrier.
C3 engages to supply force to the P4 planet carrier and prevent fourth gear.

*Sixth gear:*

Sixth gear uses C2 and C4.

C2 engages to turn the P4 planet carrier

C4 engages to stop the P4 ring gear.
With C1 disengaged the intermediate shaft can rotate independently of the input shaft.
When C4 is engaged it prevents entering fourth gear and supplies the P4 planet carrier with force.
This results in a greater turn rate at the output shaft than fifth gear because it requires more torque to move the input shaft.

So C2 engages to supply force to the P4 planet carrier.
C4 engages to stop the P4 ring gear and prevent fourth gear.

*Reverse gear:*

Reverse gear uses C3 and C5.

Force is supplied to the intermediate shaft through P3.

C3 is used to stop the P3 ring gear to prevent counter rotation from P4/friction.
This allows the P3 planet gears to revolve around the P3 sun gear.
This results in the P4 ring gear turning in the same direction of the input shaft.

The P4 ring gear will cause the P4 planet gears to rotate in the same direction.
The P4 sun gear will rotate in the opposite direction of the planet gears.
This results in the intermediate shaft having opposite rotation to the input shaft.


C5 is used to stop the P4 ring gear to prevent counter rotation from the output shaft/friction.
This allows the P5 planet gears to revolve around the P5 sun gear.
The P5 sun gear rotates in the opposite way to the input shaft.
The P5 planet gears revolve around the P5 sun gear in the same direction it rotates.
This supplies a force to the output shaft that is opposite the force supplied to the input shaft.

So C3 allows P3 to rotate the P3 planet carrier.
C5 allows the P5 planet gears to revolve around the P5 sun gear.

If you have any questions feel free to ask in the replies!

[je]

Would you now explain something simple, like the quantum phase shifting of subatomic particles?

[SurajSharmaFourKnob]

After this I went and read some rocketscience to relax

[koorite.]

These comments are awesome! I just finished a transmission class in college and didn't fully understand how an automatic transmission works, so I thought I would watch a video on it, and now I still dont understand.

[danielevans]

Some kid in my freshman engineering class made a full working miniature Lego car. Working electric engine, complete with working Lego power windows, and a full complete lego automatic transmission. Took him about 2 and a half years to complete.

The guy was absolutely insane.

[LoveofTech]

Well, you did an okay job explaining the "transmission" part. What about the "automatic" part?

[corazzinatanner]

I've watched and rewatched this, the only thing that makes sense is why transmissions are so damn expensive

[nivedlaxsea]

“This is the simplest form of an automatic transmission” *nervous laughter*

[SDCustoms]

Thank God for engineers... as weird as they are, I'm extremely grateful for the knowledge snd skill they possess to make things like this possible. This stuff is mind boggling.

[Chevifier]

Well now I love manual transmissions even more. This left me with more questions than answers.😂

[godbwithme]

After 68 times of watching; I think i finally understand 2nd gear.

[wildtrex]

When they say "it's not Rocket science", they really mean to say "it's not an automatic Transmission".

[danishraza]

I didn't have the idea that it happens all after simply pressing my accelerator. One of the brilliant inventions of engineering.

[chasbodaniels]

Both of my ex-brothers in law have their own automatic xmission repair shops, and neither one finished 11th grade.
As someone else commented, their brains must work differently than most of us.

[Caleb_FC]

you know i think i’ll just let my car do its thing

[deraldmadson]

I was a physics major in college and there was definitely some stuff where I just accepted it and said well the numbers work out. This is one of those times.

[kwunmeichan]

For your replaying and replaying:
1:02 Ring gear stops, sun gear moves; 1:15 All move
2:43 First gear
3:26, 3:44, 3:54 Second gear
4:29, 4:53 Fourth gear
5:20 Sixth gear
5:43, 5:55, 6:19 Fifth gear
6:10 Third gear
6:31 Reverse gear

[estieFa]

Thank you for helping me to understand transmissions less than I ever imagined possible.