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An automatic transmission (also called automatic gearbox) is a type of motor vehicle

transmission that can automatically change gear ratios as the vehicle moves, freeing

the driver from having to shift gears manually. Most automatic transmissions have a

defined set of gear ranges, often with a parking pawl feature that locks the output

shaft of the transmission stroke face to keep the vehicle from rolling either forward or

backward. Similar but larger devices are also used for heavy-duty commercial and

industrial vehicles and equipment. Some machines with limited speed ranges or fixed

engine speeds, such as some forklifts and lawn mowers, only use a torque converter to

provide a variable gearing of the engine to the wheels. Besides automatics, there are

also other types of automated transmissions such as a continuously variable

transmission (CVT) and semi-automatic transmissions, that free the driver from

having to shift gears manually, by using the transmission’s computer to change gear, if

for example the driver were redlining the engine. Despite superficial similarity to

other transmissions, automatic transmissions differ significantly in internal operation

and driver’s feel from semi-automatics and CVTs. An automatic uses a torque

converter instead of a clutch to manage the connection between the transmission

gearing and the engine.

In contrast, a CVT uses a belt or other torque transmission scheme to allow an

“infinite” number of gear ratios instead of a fixed number of gear ratios. A semi-

automatic retains a clutch like a manual transmission, but controls the clutch through

electrohydraulic means. A conventional manual transmission is frequently the base

equipment in a car, with the option being an automated transmission such as a

conventional automatic, semi-automatic, or CVT. The ability to shift gears manually,

often via paddle shifters, can also be found on certain automated transmissions

(manumatics such as Tiptronic), semi-automatics (BMW SMG), and CVTs (such as

Lineartronic). The first automatic transmission was invented in 1921 by Alfred Horner

Munro of Regina, Saskatchewan, Canada, and patented under Canadian patent CA

235757 in 1923. (Munro obtained UK patent GB215669 215,669 for his invention in

1924 and US patent 1,613,525 on 4 January 1927). Being a steam engineer, Munro

designed his device to use compressed air rather than hydraulic fluid, and so it lacked

power and never found commercial application. The first automatic transmissions

using hydraulic fluid were developed by General Motors during the 1930s and

introduced in the 1940 Oldsmobile as the “Hydra-Matic” transmission. They were

incorporated into GM-built tanks during World War II and, after the war, GM

marketed them as being “battle-tested”.

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