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Automatic transmission

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. Like other transmission systems on vehicles, it allows an internal combustion engine, best suited to run at a relatively high rotational speed, to provide a range of speed and torque outputs necessary for vehicular travel.

The most popular form found in automobiles is the hydraulic automatic transmission. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment. This system uses a fluid coupling in place of a friction clutch.

Besides traditional 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.

Automatic transmission modes

Most automatic transmissions offer the driver a certain amount of manual control over the transmission’s shifts. Depending on the model and make of the transmission, these controls can take several forms. However most include the following:

  • Park (P) : This selection mechanically locks the output shaft of transmission, restricting the vehicle from moving in any direction. A parking pawl prevents the transmission from rotating, and therefore the vehicle from moving. However, it should be noted that the vehicle’s non-driven wheels are still free to rotate, and the driven wheels may still rotate individually (because of the differential). For this reason, it is recommended to use the hand brake (parking brake) because this actually locks (in most cases) the wheels and prevents them from moving.
  • Reverse (R) This engages reverse gear within the transmission, permitting the vehicle to be driven backward, and operates a switch to turn on the white backup lights for improved visibility.
  • Neutral / No gear (N) This disengages all gear trains within the transmission, effectively disconnecting the transmission from the driven wheels, allowing the vehicle to coast freely under its own weight and gain momentum without the motive force from the engine
  • Drive (D) This position allows the transmission to engage the full range of available forward gear ratios, allowing the vehicle to move forward and accelerate through its range of gears. The number of gear ratios within the transmission depends on the model, but they initially ranged from three (predominant before the 1990s), to four and five speeds (losing popularity to six-speed autos). Six-speed automatic transmissions are probably the most common offering in cars and trucks from 2010 in car makers as Toyota, GM and Ford. However, seven-speed automatics are becoming available in some high-performance production luxury cars (found in Mercedes 7G gearbox, Infiniti ), as are eight-speed autos in models from 2006 introduced by Aisin Seiki Co. in Lexus, ZF and Hyundai Motor Company. From 2013 are available nine speeds transmissions produced by ZF and Mercedes 9G.
  • Overdrive (‘D’, ‘OD’, or a boxed [D] or the absence of an illuminated ‘O/D OFF’) This mode is used in some transmissions to allow early computer-controlled transmissions to engage the automatic overdrive. In these transmissions, Drive (D) locks the automatic overdrive off, but is identical otherwise. OD (Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 35–45 mph (56–72 km/h).
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