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. When towing
or hauling heavy objects, your vehicle's transmission can get hot
enough to burn up the transmission fluid. In order to protect the
transmission from serious damage, drivers who tow should buy vehicles
equipped with transmission coolers.
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.
Let's take a look at how the planetary gearset works.