How Does an Automatic Transmission Work?

Written by dan ferrell
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How Does an Automatic Transmission Work?
(8-Speed Automatic Transmission Cutmodel on Lexus IS F - Photo courtesy of Wikimedia Commons)

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If you could take a peek at the engine compartment of each one of all those cars around you on the road, you will be surprised at the variety of automatic transmission designs. Yet, their goal is one and the same: transmit engine turning power to the wheels.

Transmitting Turning Power

It all starts in a place between the engine and the transmission with a component called torque converter, a large doughnut shaped clutch, hidden inside the bell housing-the front part of the transmission bolted to the engine. The converter is actually a pump and turbine mechanism assembled together with a series of huge vain passages designed to create a strong centrifugal force using the oil going through them.

Porsche automatic transmission torque converter - Photo courtesy of Wikimedia Commons
Porsche automatic transmission torque converter - Photo courtesy of Wikimedia Commons

Once you start the engine, the torque converter begins to rotate at engine speed and waits for you to shift gears. When you go from Park to Drive, the converter turbine uses a shaft to engage a series of gear sets located inside their own drum cases. These gear sets are called planetary gearsets because their varied size gears mesh together in a planetary system-like configuration to rotate and provide different torque speeds.

A series of clutch disks pressed together somewhere around the middle of the transmission-depending on design-provide a mechanism to engage one or more gearsets together. At the same time, bands around the gearset drums help to stop and free those same gears to give you a smooth transition from low to higher speeds and vice versa.

As you drive down your quiet neighbourhood, turn onto Main Street and reach the highway accelerating and decelerating, stopping and merging with traffic, hundreds of commands travel to different parts of the transmission. These commands come from a box called valve body attached to the underside of your transmission. This is the command centre of the transmission hydraulic system.

When you act on the accelerator, a valve called the governor in the valve body-with the help of other valves, sensors and oil pressure-actuates gears, clutches and bands to increase, decrease or maintain speed. The valve body also provides a mechanism to let you engage different speed ranges manually-1, 2 or D-and change direction to pull out of a parking spot-R.

Through the years, the automatic transmission has become more sophisticated with the introduction of electronic computer control, as they grow into smaller, smoother and more reliable designs. But its goal remains the same: transmit your speed desire to the wheels to take you to work, the local supermarket or a miles away vacation and back to home.

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