Feb 262019
 

If you’ve ever driven an automatic car and wondered how the engine works the gears for you, or why it doesn’t stall the moment you take your foot off the accelerator, we are here to enlighten you.

Unlike a manual car, an automatic transmission doesn’t have a clutch pedal, which means no gear shift either. Once you move the ‘gear’ box or paddle into drive mode, all you have to do is control the speed and the brakes, everything else is managed for you – hence the automatic moniker.

So if an automatic car provides the same overall output as a manual car, what is the difference between them?

Actually, quite a lot.

In fact, the way an automatic transmission works is in direct contrast to how a manual transmission works – but with the exact same end result – your car moving.

Let’s start at the beginning with the fundamentals of what makes an automatic transmission work, and go from there.

What is a transmission

Automatic transmission. Image credit: Art of Manliness.

The transmission is essential to driving your car, without it, you would be confined to a solitary gear ratio. Putting that into perspective: if you wanted to drive the car at say, 70mph, the corresponding solitary gear would probably be third gear in a manual car.

Now, if you’ve ever tried to accelerate from stationary in third gear, you’ll appreciate the delicacy of the situation; and if you’ve ever driven a car in third gear at 70mph, you’ll know it screams in protest.

The transmission therefore is the power switchboard of the car. It includes gears, which allow you to take maximum advantage of the engine’s torque, so you can go from zero to hero, seamlessly.

To do that with a manual transmission, you lock and unlock the various gears to the output shaft by pressing your foot on the clutch and moving the gears into the required place, to utilise all the gear ratios.

In an automatic transmission, this process is done for you, through a device known as the planetary gear set.

The secret to automatic transmission: the planetary gear set

Planetary gear set. Image credit: Instructables.

About the size of a small melon, the planetary gear set – a little piece of engineering ingenuity – is responsible for creating all of the different gear ratios required to get you smoothly out of the start gates and up to full speed. Everything else in an automatic transmission is merely there to facilitate the planetary gear set to do its thing.

There are three parts to the planetary gear set (so named because of these components):

  1. The sun gear
  2. The planet gears
  3. The ring gear

Each of these gears can be the input, the output or remain in stationary; deciding which combination they go in, determines the gear ratio for the gear set.

Image credit: How Stuff Works 2018

The different inputs are decided upon by connecting and disconnecting a series of clutches and steel brake bands inside the automatic transmission. The brake bands themselves are controlled by a hydraulic system, and whether the clutch engages or not is driven automatically by a combination of mechanical, hydraulic and electrical magic*.

*OK, not necessarily magic, but it’s easier to watch this video than read a wall of text on the subject.

Gear ratio decision for an automatic transmission

The gear ratio for an automatic transmission is therefore determined by which segments of the planetary gear system are moving or not. And the movement of the various segments is decided upon, depending on which clutch or band is engaged.

And that, dear readers, is a brief overview of how an automatic transmission works.

Having gone through all that, allow me to blow your mind with a quick fact: engineers developed the above wizardry in 1932 – before computers were invented.


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