What is an Automatic Transmission?

The transmission is a group of parts that transmits power from a vehicle's engine to the drive axle, which in turn makes the wheels go around. If you do not use a clutch to switch gears, your vehicle has an automatic transmission. If you use a clutch, your transmission is manual. Automatic transmissions contain an oil-like fluid to cool and lubricate the mechanism. The level and quality of this fluid should be checked regularly (see your vehicle owner's manual) and replaced if needed. If your transmission performance seems good, the fluid is clear and red, and you have not had to add fluid, leave your transmission alone.

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. Without a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars.

You've probably never tried driving a manual transmission car using only third gear. If you did, you'd quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undriveable. So the transmission uses gears to make more effective use of the engine's torque, and to keep the engine operating at an appropriate speed.

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 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 in an automatic transmission. When you take apart and look inside an automatic transmission, you find a huge assortment of parts in a fairly small space. Among other things, you see:

* An ingenious planetary gearset
* A set of bands to lock parts of a gearset
* A set of three wet-plate clutches to lock other parts of the gearset
* An incredibly odd hydraulic system that controls the clutches and bands
* A large gear pump to move transmission fluid around

The center of attention is the planetary gearset. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gearset do its thing. An automatic transmission contains two complete planetary gearsets folded together into one component. Any planetary gearset has three main components:

* The sun gear
* The planet gears and the planet gears' carrier
* The ring gear

Each of these three 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. One of the planetary gearsets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset. Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction.

	INPUT	OUTPUT	STATIONARY	CALCULATION	GEAR RATIO
1	Sun	Planet Carrier	Ring	1 + R / S	3.4:1
2	Planet Carrier	Ring	Sun	1 / (1+ S / R)	0.71:1
3	Sun	Ring	Planet Carrier	- R / S	-2.4:1

Notice that the first gear ratio listed above is a reduction -- the output speed is slower than the input speed. The second is an overdrive -- the output speed is faster 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. This one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With two of these gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs.