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Vehicles need a transmission to transfer the power from the engine to the drive shaft and the differential to let the wheels turn. The transmission varies the torque, the speed and the direction by changing the transmission ratios and enables the car to start with a high torque. An automatic gearbox, or automatic transmission system, is a gearbox which, after switching on the gear, does not require manual switching. The obvious advantage of an automatic transmission to the driver is the lack of a clutch pedal and manual shift pattern in normal driving.
The most popular form found in automobiles is the hydraulic automatic transmission. Besides the traditional hydraulic automatic transmissions, 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.