automatic transmission
An automatic transmission (commonly "AT" or "Auto") is an automobile gearbox that can change gear ratios automatically as the vehicle moves, freeing the driver from having to shift gears manually. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment.
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. Continuously variable transmissions (CVTs) can change the ratios over a range rather than between set gear ratios. CVTs have been used for decades in two-wheeled scooters but have seen limited use in a few automobile models. Recently, however, CVT technology has gained greater acceptance among manufacturers and customers, especially in Nissan automobiles.
Some machines with limited speed ranges or fixed engine speeds, such as some forklift trucks and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.
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