Most people are familiar with the difference between a manual and automatic transmission. A manual transmission has an extra pedal for operating the clutch, which allows for drivers to switch gears based on how fast he or she wants to go. That pedal doesn’t exist in an automatic car; once you’re in drive, you can operate your car at any speed using only the gas pedal.
Similarly to that of a manual transmission, the automatic transmission’s purpose is to allow the engine to operate within its narrow range of optimum rpm speeds as often as possible while simultaneously allowing the rpm of the car’s wheels to vary on a wider scale.
Without a multi-gear system, a car made to travel 80 miles per hour would have little to no accelerating power at 0-40 miles per hour and would come close to exploding doing 80.
In order to avoid this issue, automatic transmissions use a planetary gearset to create different gear ratios between the car’s engine and transmission.
The planetary gearset is about the size of a coconut and creates all of the different gear ratios that the transmission can produce. An automatic transmission actually contains two complete planetary gearsets folded together into one component.
Planetary gearsets are composed of a sun gear, planet gears and the planet gears’ carrier, and the ring gear. The gear ratio for the gearset is determined by which of these components acts as the input, acts as the output, and remains stationary.
For full automatic transmission capabilities, two planetary gearsets are lined up in a row. One is smaller than the other.
In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. A torque converter is a method of fluid coupling that allows the engine to spin independently of the transmission. It is necessary that it be able to do this so that a car can roll to a stop without killing the engine.
It’s because of torque converters that your car requires you to brake at stoplights; the pressure of the torque passed through the torque converter is what makes your car slowly roll forward when you engage neither the acceleration nor the brakes. When the light turns green and you accelerate forward, you are effectively telling the engine to pump more fluid into the torque converter, causing torque to be transmitted to the wheels.
Back to first gear: the smaller sun gear is rotating clockwise, which would make the planet carrier spin counterclockwise if it weren’t held still by a one-way clutch. The ring gear turns the output, and because the small gear has 30 teeth while the ring gear has 37, the gear ration becomes -R/S = -72/30 = -2.4:1. This means the output direction would be opposite the input direction if there weren’t two sets of planets. When this -2.4:1 ratio engages the second set of planets, the direction is reversed again, meaning the car will start to roll forward.