Automatic transmission

An automatic transmission is an automobile gearbox that can change gear ratios automatically as the car or truck moves, thus freeing the driver from having to shift gears manually. (Similar but larger devices are also used for railroad locomotives.)

Related Topics:
Automobile - Gearbox - Gear ratio - Manually - Railroad - Locomotives

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Most cars sold in the United States since the 1950s have had automatic transmissions. This has, however, not been the case in Europe and much of the rest of the world. Automatic transmissions, particularly earlier ones, reduce fuel efficiency and power. Where fuel is expensive and, thus, engines generally smaller, these penalties were more burdensome. In recent years, automatic transmissions have significantly improved in their ability to support high fuel efficiency but manual transmissions are still generally more efficient. (This balance may finally shift with the introduction of practical continuously variable transmissions; see below.)

Related Topics:
United States - 1950s - Europe - Fuel efficiency - Manual transmission - Continuously variable transmission

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Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission.

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However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.

Related Topics:
Torque converter - Forklift - Lawn mower

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Recently manufacturers have begun to make continuously variable transmissions available. These designs can change the ratios over a range rather than between set gear ratios. Even though prototypes for CVT have been around for decades, it is just now reaching commercially practicability.

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