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Automatic Transmission

Contributed By: Spencer C.

The Basics of how an automatic transmission works

An automatic transmission is an incredibly complex device and so in this article, I will touch briefly only on the most basic operation of the device. However, before starting, it is important to familiarize yourself with the vocabulary to the hardware found in an automatic trans. Within, there are three main sets of gears: the sun gear, the plant gears and the planets' carrier, and the ring gear. The sun gear is located in the center and is typically the part connected to the engine via the torque converter. The planets and their carrier can be locked and unlocked from the output shaft as can the ring gear. This compound gear set is locked and unlocked through an extremely complicated hydraulic system which won't even be expound on. It is also important to know that there are a series of clutches involved connected to the hydraulic system that lock these gears to offer different ratios Function.

In the introduction, we familiarized ourselves with the compound gear set and its sister system of clutches. Take some time to look at the flash video located here {} and try the buttons on the left side of the table. Notice the rotation of the planetary gears (the output) when you lock the ring gear. Notice how much faster the ring (the output) spins when you lock the sun gear. Lastly, notice how, when the ring carrier car is locked, the output (the ring) rotates slowly in reverse when the sun is the input. Inside of an automatic transmission, there are typically two or three planetary gears sets around a single sun gear to provide more gears than just the three offered in one. The way the gears are engaged with the clutches is controlled by the current gear shifter settings. Take a look at this {} flash animation and familiarize yourself with the different outputs available(note the torque converter in the animation from the torque converter article). Note that this transmission utilizes two planetary gear sets to transmit engine power. Also note the clutches that engage the ring gears in both when the shifter is in park and reverse.


The control of an automatic transmission is done by a set of devices, known collectively as the controller. It takes as input the current speed of the car and the position of the accelerator pedal to determine the best gear for the car. The controller also contains special safety features that prevent the car from going into park and reverse while the car is still moving, as well as preventing shifting to a gear selected by the shifter that would potentially damage the transmission at the current speed. For example, if you tried to shift to first gear at freeway speeds, the controller would wait until you slowed to the appropriate speeds until shifting.

Also, the controller makes sure that you get the best mechanical advantage for the situation. Suppose you put the metal to the pedal; the controller would delay gear shifts and remain in the lower gears much longer to achieve the best torque conditions. If you were to accelerate very slowly, however, you could shift very rapidly through all of the gears. In essence, you can still control the shifting within an automatic transmission. Most systems now days utilize computers and fuzzy logic to learn the way the user accelerates, and by doing so can choose a better gear. In other words, it optimizes timing to how you drive; you train your transmission.


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