When you travel down to your nearest drive-thru to stuff your fat face with burgers, you're using one of the most amazing pieces of modern technology seen on this planet since the cattle prod: your car's automatic transmission. This non-technical guide will explain how it works.

If you have ever driven a car with an automatic transmission, you will know that there are two main differences between an automatic transmission and manual transmission:

1. There is no clutch pedal in an automatic transmission car
2. There is no gear shift in an automatic transmission car
3. There is no point in an automatic transmission car

Both the automatic transmission and the manual transmission accomplish exactly the same thing, and are identical in every respect, except that they do it in totally different ways. By pure chance it turns out that the way an automatic transmission does it is absolutely amazing!

In this needlessly complex and over-extended article we'll slog our way through an automatic transmission inch by shit wrenchingly tedious inch. We'll start with the key to the whole system: planetary gearsets. Then if you're still breathing we'll see how the transmission is put together, learn how the controls work and discuss some of the intricacies involved in controlling a transmission.

Some Basics

Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds to drive the car in any direction you want, as long as it is forwards or backwards.

Without a transmission, cars would be limited to one gear ratio, and that ratio would allow the car to travel at 80 mph, making the gear ratio similar to tenth gear in most manual transmission cars.

Actual footage of Ford's first auto gear being tested.

You've probably never tried driving a manual transmission car using only tenth gear. If you did, you'd quickly find out that it didn't have tenth gear, so at high speeds the engine would be on fire and the car would be stationary. A car like this would wear out very quickly and would be nearly undriveable in the sense that it would never drive anywhere. Nonetheless Volvo developed such cars in the 70s. Many agree that they are some of the most reliable stationary forms of transport available, with a near 100% safety record.

The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft (fig. b) to achieve the various gear ratios, while in an automatic transmission the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible. We will discuss this later.

Planetary Gearsets

Let us take apart and look inside an automatic transmission. You will find an amazing assortment of parts in a fairly small space. Among other things you see:

• An extremely ingenious planetary gearset
• A set of co-synchronous bands that lock parts of a gearset
• A set of 3 anti-synchronous wet-plate crutchless facia plates to lock other parts of the gearset
• A loose screw
• An incredibly odd hydraulic control system that controls the clutches and bands and crutches
• Sticky fluid
• A large pump to move the fluid around

Cut-sliced elevation view of a gearbox.

The center of attention is the planetary gearset. About the size of an antelope, this single part creates all the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gearset do its shit.

This versatile piece of gearing appears in many places. You may recognize it from an electric chair. But that's not all. An automatic transmission contains two complete planetary gearsets folded together into one component. See my book 'How Gear Ratios Can Bore You to Death' for an introduction to planetary gearsets.

Any planetary gearset has three main components:

• the sun gear
• the planet gears and the planet gears' touch plate
• the ring a ding dong gear.

Each of these components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset. Let's take a look at a single planetary gearset. Hang in there.

The Simplest Planetary Gearset

Animation of the different gear ratios related to automatic transmissions in reverse. Click on the buttons on the left in the table above the right hand side.

In its simplest form — Input Output Stationary — the calculation of gear ratio is as follows:

A Sun (S) Planet Carrier (C) Ring (R) 1 + R/S 3.4:1/when R<S/4_6.3498+S:9876543

B Planet Carrier (C) Ring (R) Sun (S) 1 / (1 + S/R) 0.71:1 (R*54/^9.7-0.0065)

C Sun (S) Ring (R) Planet Carrier (C) R/S = 1

It is obvious at a glace from the above formulae that locking any two of the three components together will lock up the whole device at a 0:1 gear reduction. Notice that the first gear ratio listed above is a reduction and an increase at the same time — or put another way — the output speed is slower than the input speed but could be faster at the same time. The second is an overdrive — or put another way — the output speed is faster than the input speed but only when the input speed is 5x the overall output speed, if it was originally less than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission, and I'm losing the will to live.

The Hillman Ratio (1949): The first car with automatic transmission.

So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With just two of these gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs to enable us to bring the Earth to the brink of environmental catastrophe and collect the shopping from the local Wal-Mart!