Editing Automatic transmission

{{Short description|Type of motor vehicle transmission}}
[[File:Automatic transmission cut.jpg|thumb|right|Cutaway view of a 2007 [[Toyota A transmission#AA80EValvetrain|Toyota AA80E]] hydraulic automatic]]
[[File:Ford Escort 1.9 1992-autotrans.jpg|thumb|right|Typical gear selector for an automatic transmission]]

An '''automatic transmission''' ('''AT''') or '''automatic gearbox''' is a multi-speed [[transmission (mechanics)|transmission]] used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions.

The 1904 Sturtevant "horseless carriage gearbox" is often considered to be the first true automatic transmission.<ref>{{cite web |url=https://www.autorepairsanantonio.com/40-automatic-transmission-history |title=Automatic Transmissions- A Brief History |website=AutoServiceExperts |year=2003|url-status=live|archive-url=https://web.archive.org/web/20200418231514/https://www.autorepairsanantonio.com/40-automatic-transmission-history |archive-date=2020-04-18}}</ref><ref name=hearst>{{cite magazine|url=https://www.caranddriver.com/research/a31884931/what-is-an-automatic-car/ |title=What is an Automatic Car?|author=Hearst Autos Research|magazine=Car and Driver |date=13 April 2020<!--from page source-->|url-status=live|archive-url=https://web.archive.org/web/20200417081714/https://www.caranddriver.com/research/a31884931/what-is-an-automatic-car/ |archive-date=2020-04-17}}</ref>{{Failed verification|date=March 2024|reason=Source 1 does not mention the Sturtevant transmission. Source 2 cites source 1.}} The first mass-produced automatic transmission is the General Motors ''[[Hydramatic]]'' two-speed hydraulic automatic, which was introduced in 1939.

Automatic transmissions are especially prevalent in vehicular [[drivetrain]]s, particularly those subject to intense mechanical acceleration and frequent idle/transient operating conditions; commonly commercial/passenger/utility vehicles, such as [[bus]]es and [[waste collection vehicle]]s.

==Prevalence==
Vehicles with internal combustion engines, unlike electric vehicles, require the engine to operate in a narrow range of rates of rotation, requiring a gearbox, operated manually or automatically, to drive the wheels over a wide range of speeds.

Globally, 43% of new cars produced in 2015 were manual transmissions, falling to 37% by 2020.<ref name="Statista"/> Automatic transmissions have long been prevalent in the United States, but only started to become common in Europe much later. In Europe in 1997, only 10–12% of cars had automatic transmissions.<ref>{{cite web |url=https://gearsmagazine.com/magazine/automatic-trends-in-europe-automatic-transmissions-on-the-rise/ |title=Gears Magazine - Automatic Trends in Europe: Automatic Transmissions on the Rise}}</ref>

In 1957 over 80% of new cars in the United States had automatic transmissions.<ref name="Statista">{{cite web |url=https://www.statista.com/statistics/204123/transmission-type-market-share-in-automobile-production-worldwide/ |title=Transmission type market share worldwide 2025}}</ref> Automatic transmissions have been standard in large cars since at least 1974.<ref>{{cite web |url=https://www.scaruffi.com/politics/cars.html |title=Timeline of the Automobile Industry}}</ref> By 2020 only 2.4% of new cars had manual transmissions.<ref>{{cite web |title=The Stick Shift Index: Do People Still Drive Stick Shift Cars in America? |url=https://www.carmax.com/articles/stick-shift-index |website=Carmax |access-date=4 April 2023}}</ref> Historically, automatic transmissions were less efficient, but lower fuel prices in the US made this less of a problem than in Europe.<ref>{{cite web |url=https://garagedreams.net/car-facts/why-are-automatic-cars-so-popular-in-the-united-states |title=Why Are Automatic Cars So Popular in the United States? |date=4 November 2022}}</ref>

In the United Kingdom, a majority of new cars have had automatic transmissions since 2020. Several manufacturers including Mercedes and Volvo no longer sell cars with manual transmissions. The growing prevalence of automatic transmissions is attributed to the increasing number of electric and hybrid cars, and the ease of integrating it with safety systems such as [[Collision avoidance system|Autonomous Emergency Braking]].<ref>{{cite news| last=Griffiths |first=Hugo |title=Automatics for the people: less than a third of new cars can be ordered with a manual gearbox [in the UK]| publisher=carwow| date=19 October 2022 |url=https://www.carwow.co.uk/news/6414/automatic-gearboxes-become-the-norm}}</ref><ref>{{cite news |last1=Grose |first1=Thomas K. |title=U.K. Motorists Shift Gears to Automatic Vehicles |url=https://www.usnews.com/news/best-countries/articles/2021-06-09/british-motorists-move-away-from-manual-transmission-vehicles |access-date=4 April 2023 |work=U.S. News & World Report |date=9 June 2021}}</ref>

==Efficiency==
The efficiency (power output as a percentage of input) of conventional automatic transmissions ranges from 86 to 94%. Manual transmissions are more fuel efficient than all but the newest automatic transmissions due to their inherently low parasitic losses (typically of about 4%) in addition to being cheaper to make, lighter, better performing, and of simpler mechanical design. [[Fuel economy in automobiles|Fuel economy]] worsens with lower efficiency. However, manual transmissions have the disadvantage of requiring the driver to operate the [[clutch]] and change gear whenever required.<ref name="NAP">{{cite book |title=Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles |publisher=National Academies Press |publication-place=Washington, D.C. |date=28 September 2015 |isbn=978-0-309-37388-3 |chapter-url=https://nap.nationalacademies.org/read/21744/chapter/7|chapter=5: Transmissions|doi=10.17226/21744}}</ref>

Real-world tests reported in 2022 found that in typical driving manual transmissions achieved 2 to 5% better fuel economy than automatics, increasing to 20% with an expert driver. Some laboratory tests show automatics in a better light due to the tests using a prescribed shifting pattern for manuals not always optimized for economy. However, on long highway journeys manual transmissions require maintaining a very specific cruising speed to optimise economy, making automatics preferable.<ref>{{cite web |last=Tamayo |first=Niky |title=Have automatic transmissions dethroned manuals as the fuel-economy kings? |publisher=Topgear Philippines| date=30 May 2022 |url=https://www.topgear.com.ph/features/feature-articles/manual-vs-automatic-fuel-economy-2022-a2584-20220530}}</ref>

==Hydraulic automatic transmission==
{{more citations needed section|date=July 2017}}

===Design===
[[File:ZF Stufenautomatgetriebe 8HP70.jpg|thumb|Cutaway view of a [[ZF 8HP transmission]]: [[torque converter]] on the left, planetary gearsets in the centre, control mechanisms at the bottom]]

The most common design of automatic transmissions is the hydraulic automatic, which typically uses planetary gearsets that are operated using hydraulics.<ref>{{cite web |title=How Automatic Transmissions Work |url=https://auto.howstuffworks.com/automatic-transmission.htm |website=www.howstuffworks.com |access-date=22 November 2020 |language=en |date=29 November 2000}}</ref><ref>{{cite journal |title=What Makes Automatic Transmissions Automatic |journal=Popular Mechanics |date=February 1955 |pages=169–173 |url=https://books.google.com/books?id=390DAAAAMBAJ&q=true&pg=RA1-PA169 |access-date=22 November 2020 |publisher=Hearst Magazines |language=en}}</ref> The transmission is connected to the engine via a torque converter (or a [[fluid coupling]] prior to the 1960s), instead of the [[Clutch#Friction clutches|friction clutch]] used by most [[manual transmission]]s.<ref name="awnc">{{cite web|title=How does the AT work?|url=http://www.aw-nc.com/products/drivetrain/structure/index.html|website=AW North Carolina|access-date=6 October 2014|url-status=dead|archive-url=https://web.archive.org/web/20141006084301/http://www.aw-nc.com/products/drivetrain/structure/index.html|archive-date=6 October 2014}}</ref>

====Gearsets and shifting mechanism====
A hydraulic automatic transmission uses planetary gearsets instead of the manual transmission's design of gears lined up along input, output and intermediate shafts. To change gears, the hydraulic automatic uses a combination of internal clutches, friction bands or brake packs. These devices are used to lock certain gears, thus setting which gear ratio is in use at a given time.

A [[sprag clutch]] (a ratchet-like device which can [[freewheel]] and transmits torque in only one direction) is often used for routine gear shifts. The advantage of a sprag clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetary gearsets, simply "taking up" the drivetrain load when actuated, and releasing automatically when the next gear's sprag clutch assumes the torque transfer.

The friction bands are often used for manually selected gears (such as low range or reverse) and operate on the planetary drum's circumference. Bands are not applied when the drive/overdrive range is selected, the torque being transmitted by the sprag clutches instead.

====Hydraulic controls====
The aforementioned friction bands and clutches are controlled using [[automatic transmission fluid]] (ATF), which is pressurized by a pump and then directed to the appropriate bands/clutches to obtain the required gear ratio.<ref name="awnc"/> The ATF provides lubrication, corrosion prevention, and a hydraulic medium to transmit the power required to operate the transmission. Made from petroleum with various refinements and additives, ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.

The ''main pump'' which pressurises the ATF is typically a gear pump mounted between the torque converter and the planetary gear set. The input for the main pump is connected to the torque converter housing, which in turn is bolted to the engine's flexplate, so the pump provides pressure whenever the engine is running. A disadvantage of this arrangement is that there is no oil pressure to operate the transmission when the engine is not running, therefore it is not possible to [[push start]] a vehicle equipped with an automatic transmission with no rear pump (aside from several automatics built prior to 1970, which also included a rear pump for towing and push-starting purposes). The pressure of the ATF is regulated by a ''governor'' connected to the output shaft, which varies the pressure depending on the vehicle speed.

The ''valve body'' inside the transmission is responsible for directing hydraulic pressure to the appropriate bands and clutches. It receives pressurized fluid from the main pump and consists of several spring-loaded valves, check balls, and [[servomechanism|servo]] pistons. In older automatic transmissions, the valves use the pump pressure and the pressure from a [[centrifugal governor]] on the output side (as well as other inputs, such as throttle position or the driver locking out the higher gears) to control which ratio is selected. As the vehicle and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. In more recent automatic transmissions, the valves are controlled by [[solenoids]]. These solenoids are computer-controlled, with the gear selection decided by a dedicated [[transmission control unit]] (TCU) or sometimes this function is integrated into the [[engine control unit]] (ECU). Modern designs have replaced the centrifugal governor with an electronic speed sensor that is used as an input to the TCU or ECU. Modern transmissions also factor in the amount of load on an engine at any given time, which is determined from either the [[throttle]] position or the amount of intake manifold vacuum.

The multitude of parts, along with the complex design of the valve body, originally made hydraulic automatic transmissions much more expensive and time-consuming to build and repair than manual transmissions; however mass-production and developments over time have reduced this cost gap.

====Torque converter====
{{main|Torque converter}}
[[File:Torque-converter-cutbox-model.jpg|thumb|Torque converter – cutaway view]]
To provide coupling and decoupling of the engine, a modern automatic transmission uses a torque converter instead of the [[clutch|friction clutch]] used in a manual transmission.<ref name="awnc"/><ref name="tracy">{{cite web|last1=Tracy|first1=David|title=This is how an automatic transmission works|url=http://jalopnik.com/this-is-how-an-automatic-transmission-works-517581894|website=Jalopnik|publisher=Gawker Media|access-date=6 October 2014}}</ref>

===History===
{{globalize|section|United States|date=November 2020}}

====1904–1939: Predecessors to the hydraulic automatic====
The 1904 Sturtevant "horseless carriage gearbox" is often considered to be the first automatic transmission for motor vehicles.<ref>{{cite web |title=The American Automobile 1861-1929 Chapter 2 |url=http://www.earlyamericanautomobiles.com/americanautomobiles2.htm |website=www.earlyamericanautomobiles.com |access-date=1 November 2020 |archive-date=22 February 2020 |archive-url=https://web.archive.org/web/20200222192744/http://www.earlyamericanautomobiles.com/americanautomobiles2.htm |url-status=dead}}</ref><ref>{{cite web |title=The Pros and Cons of Automatic Transmissions |url=https://stevesimports.com/blog/the-pros-and-cons-of-automatic-transmissions/ |website=www.stevesimports.com |access-date=1 November 2020 |language=en |date=30 September 2019}}</ref> At higher engine speeds, high gear was engaged. As the vehicle slowed down and engine speed decreased, the gearbox would shift back to low. However, the transmission was prone to sudden failure, due to the transmission being unable to withstand forces from the abrupt gear changes.

The adoption of planetary gearsets was a significant advance towards the modern automatic transmission. One of the first transmissions to use this design was the manual transmission fitted to the 1901–1904 [[Wilson-Pilcher]] automobile.<ref name="Automotor">{{cite news |title=The Wilson-Pilcher Petrol Cars |work=The Automotor Journal |date=16 April 1904 |pages=463–468, also pp. 492–496, and pp. 519–521}}</ref> This transmission was built in the United Kingdom and used two epicyclic gears to provide four gear ratios. A foot clutch was used for standing starts, gear selection was using a hand lever, helical gears were used (to reduce noise) and the gears used a constant-mesh design. A planetary gearset was also used in the 1908 [[Ford Model T]], which was fitted with a two-speed manual transmission (without helical gears).

An early patent for the automatic transmission was granted to Canadian inventor Alfred Horner Munro of Regina in 1923.<ref>{{cite web |last1=Government of Canada |title=Canadian Patent Database |url=http://brevets-patents.ic.gc.ca/opic-cipo/cpd/eng/patent/235757/summary.html |website=www.brevets-patents.ic.gc.ca |access-date=22 November 2020 |language=en |date=15 June 2015}}</ref> 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.<ref>{{cite web|last1=Warwick|first1=Alan|title=Who Invented the Automatic Gearbox|date=9 April 2013 |url=http://www.nwtgearboxes.co.uk/who-invented-the-automatic-gearbox|publisher=North West Transmissions|access-date=11 October 2014}}</ref>

In 1923, a patent was approved in the United States describing the operation of a transmission where the manual shifting of gears and manual operation of a clutch was eliminated. This patent was submitted by Henry R. Hoffman from Chicago and was titled: ''Automatic Gear Shift and Speed Control''. The patent described the workings of such a transmission as "...having a series of clutches disposed intermediate the engine shaft and the differential shaft and in which the clutches are arranged to selectively engage and drive the differential shaft dependent upon the speed at which the differential shaft rotates". However, it would be over a decade later until automatic transmissions were produced in significant quantities. In the meantime, several European and British manufacturers would use [[preselector gearbox]]es, a form of manual transmission which removed the reliance on the driver's skill to achieve smooth gear shifts.

The first automatic transmission using hydraulic fluid was developed in 1932 by two Brazilian engineers, José Braz Araripe and Fernando Lehly Lemos.<ref>{{Cite web|date=2019-08-30|title=Top 10 All-Time Auto Inventions|url=https://www.inventorsdigest.com/articles/top-10-all-time-auto-inventions/|access-date=2021-03-29|website=Inventors Digest|language=en-US}}</ref><ref name="hearst"/>

The evolution towards mass-produced automatic transmissions continued with the 1933–1935 [[REO Motor Car Company]] ''Self-Shifter'' semi-automatic transmission,<ref name="hagerty.com">{{cite web |title=6 of the worst automatic transmissions ever |url=https://www.hagerty.com/media/maintenance-and-tech/6-of-the-worst-automatic-transmissions-ever/ |website=www.hagerty.com |access-date=9 August 2020 |date=8 August 2019}}</ref> which automatically shifted between two forward gears in the "Forward" mode (or between two shorter gear ratios in the "Emergency low" mode). Driver involvement was still required during normal driving, since standing starts required the driver to use the clutch pedal.<ref>{{cite web|url=http://auto.howstuffworks.com/reo-cars1.htm|title=How Reo Cars Work|date=15 June 2007|access-date=24 October 2017}}</ref> This was followed in 1937 by the [[Oldsmobile]] ''Automatic Safety Transmission''. Similar in operation to the REO ''Self-Shifter'', the ''Automatic Safety Transmission'' shifted automatically between the two gear ratios available in the "Low" and "High" ranges and the clutch pedal was required for standing starts. It used a planetary gearset.<ref>{{cite web |title=The Oldsmobile automatic safety transmission operated by the short lever shown on the steering column |url=https://digitalcollections.nypl.org/items/510d47db-bb52-a3d9-e040-e00a18064a99 |website=www.nypl.org |access-date=9 August 2020 |language=en}}</ref><ref>{{cite web |title=No clutch to press! No gears to shift! |url=https://www.autonews.com/article/19970818/ANA/708180702/no-clutch-to-press-no-gears-to-shift |website=www.autonews.com |access-date=9 August 2020 |language=en |date=18 August 1997}}</ref><ref>{{cite web |title=How Oldsmobile Cars Work |url=https://auto.howstuffworks.com/oldsmobile-cars1.htm |website=www.howstuffworks.com |access-date=9 August 2020 |language=en |date=20 June 2007 |archive-date=20 September 2020 |archive-url=https://web.archive.org/web/20200920073229/https://auto.howstuffworks.com/oldsmobile-cars1.htm |url-status=dead}}</ref> The Chrysler ''[[Fluid Drive]]'', introduced in 1939, was an optional addition to manual transmissions where a fluid coupling (similar to a torque-convertor, but without the torque multiplication) was added, to avoid the need to operate a manual clutch.<ref>{{cite web |title=The Fluid Drive torque converter |url=https://www.allpar.com/mopar/fluidrive.html |website=www.allpar.com |access-date=3 November 2020}}</ref><ref>{{cite web |title=Video: An Introduction to Chrysler Fluid Drive |url=https://www.macsmotorcitygarage.com/video-an-introduction-to-chrysler-fluid-drive/ |website=www.macsmotorcitygarage.com |access-date=3 November 2020 |date=24 October 2017}}</ref>

====1939–1964: Early hydraulic automatics====
The General Motors [[Hydra-Matic]] became the first mass-produced automatic transmission following its introduction in 1939 (1940 model year). Available as an option in cars such as the [[Oldsmobile Series 60#1939–1940|Oldsmobile Series 60]] and [[Cadillac Sixty Special#1938–1941|Cadillac Sixty Special]], the Hydra-Matic combined a [[fluid coupling]] with three [[hydraulics|hydraulically controlled]] planetary gearsets to produce four forward speeds plus reverse. The transmission was sensitive to engine throttle position and road speed, producing fully automatic up- and down-shifting that varied according to operating conditions. Features of the Hydra-Matic included a wide spread of ratios (allowing both good acceleration in first gear and cruising at low engine speed in top gear) and the fluid coupling handling only a portion of the engine's torque in the top two gears (increasing fuel economy in those gears, similar to a [[Lock-up clutch|lock-up]] torque converter). Use of the Hydra-Matic spread to other General Motors brands and then to other manufacturers starting 1948 including [[Hudson Motor Car Company|Hudson]], [[Lincoln Motor Company|Lincoln]], [[Kaiser Motors|Kaiser]], [[Nash Motors|Nash]], [[Holden]] (Australia), as well as [[Rolls-Royce Motor Cars|Rolls-Royce]] and [[Bentley]] licensing production in the UK and providing the transmission to [[Jensen Motors]], [[Armstrong Siddeley]] and other UK manufacturers.<ref>{{cite web | url=https://ateupwithmotor.com/terms-technology-definitions/rolls-royce-automatic-gearbox | title=Don't Call It Hydra-Matic: The Rolls-Royce and Bentley Automatic Gearbox < Ate up with Motor | date=4 September 2022 }}</ref> During World War II, the Hydra-Matic was used in some military vehicles.

The first automatic transmission to use a torque converter (instead of a fluid coupling) was the Buick [[Dynaflow]], which was introduced for the 1948 model year. In normal driving, the Dynaflow used only the top gear, relying on the torque multiplication of the torque convertor at lower speeds. The Dynaflow was followed by the Packard [[Ultramatic]] in mid-1949 and the Chevrolet [[Powerglide]] for the 1950 model year. Each of these transmissions had only two forward speeds, relying on the converter for additional torque multiplication. In the early 1950s, [[BorgWarner]] developed a series of three-speed torque converter automatics for car manufacturers such as American Motors, Ford and Studebaker. [[Chrysler]] was late in developing its own true automatic, introducing the two-speed torque converter [[PowerFlite]] in 1953, and the three-speed [[TorqueFlite]] in 1956. The latter was the first to utilize the Simpson compound planetary gearset.

In 1956, the General Motors Hydra-Matic (which still used a fluid coupling) was redesigned based around the use of two fluid couplings to provide smoother shifts. This transmission was called the ''Controlled Coupling Hydra-Matic'', or "Jetaway" transmission. The original Hydra-Matic remained in production until the mid-1960s at GM, with the licensed Rolls-Royce Automatic transmission soldiering on until 1978 on the [[Rolls-Royce Phantom VI]]. In 1964, General Motors released a new transmission, the [[Turbo Hydramatic]], a three-speed transmission which used a torque convertor. The Turbo Hydramatic was among the first to have the basic gear selections (''park'', ''reverse'', ''neutral'', ''drive'', ''low'') which became the standard gear selection used for several decades.

====1965–present: increased ratio count and electronics====
By the late 1960s, most of the fluid-coupling two-speed and four-speed transmissions had disappeared in favor of three-speed units with torque converters. Also around this time, [[whale oil]] was removed from the automatic transmission fluid.<ref>{{cite web|url=http://www.machinerylubrication.com/article_detail.asp?articleid=392 |title=Almost Burned |website=www.machinerylubrication.com |archive-url=https://web.archive.org/web/20070927082224/http://www.machinerylubrication.com/article_detail.asp?articleid=392 |archive-date=27 September 2007}}</ref> During the 1980s, automatic transmissions with four gear ratios became increasingly common,<ref>{{cite web |title=U.S. Patent 5370589 |url=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.htm&r=1&p=1&f=G&l=50&d=PTXT&S1=5370589.PN.&OS=pn%2F5370589&RS=PN%2F5370589 |website=www.patft.uspto.gov |archive-url=https://web.archive.org/web/20130514181727/http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.htm&r=1&p=1&f=G&l=50&d=PTXT&S1=5370589.PN.&OS=pn%2F5370589&RS=PN%2F5370589 |archive-date=14 May 2013 |url-status=dead}}</ref> and many were equipped with lock-up torque convertors in order to improve fuel economy.

[[Electronics]] began to be more commonly used to control the transmission, replacing mechanical control methods such as spring-loaded valves in the valve body. Most systems use [[solenoid]]s which are controlled by either the [[engine control unit]], or a separate [[transmission control unit]]. This allows for more precise control of shift points, shift quality, lower shift times and manual control.

The first five-speed automatic was the [[ZF_5HP_transmission|ZF 5HP18 transmission]], debuting in 1991 on various [[BMW]] models. The first six-speed automatic was the [[ZF 6HP26 transmission]], which debuted in the 2002 [[BMW 7 Series (E65)]]. The first seven-speed automatic was the [[Mercedes-Benz 7G-Tronic transmission]], which debuted a year later. In 2007, the first eight-speed transmission to reach production was the [[Toyota A transmission#AA80E|Toyota AA80E transmission]]. The first nine-speed and ten-speed transmissions were the 2013 [[ZF 9HP transmission]] and 2017 ''Toyota Direct Shift-10A'' (used in the [[Lexus LC]]) respectively.

===Gear selectors===
{{main|Gear stick #Automatic transmission}}
The gear selector is the input by which the driver selects the operating mode of an automatic transmission.<ref>{{cite book|title=Automatic transmissions and transaxles|author=Thomas W Birch|publisher=Pearson Education|place=Upper Saddle River, N.J.|year=2012|isbn=9780132622271}}</ref> Traditionally the gear selector is located between the two front seats or on the steering column, however electronic rotary dials and push-buttons have also been occasionally used since the 1980s, as well as push buttons having been used in the 1950s and 1960s by [[Rambler (automobile)]], [[Edsel]], and most famously, by [[Chrysler]]. A few automobiles employed a lever on the instrument panel, such as the 1955 Chrysler Corporation cars, and notably, the Corvair.

====P–R–N–D–L positions====
Most cars use a "P–R–N–D–L" layout for the gear selector, which consists of the following positions:<ref>{{cite web |url=https://www.ecfr.gov/current/title-49/subtitle-B/chapter-V/part-571/subpart-B/section-571.102 |title=Standard No. 102; Transmission shift position sequence |website=Code of Federal Regulations |at=Title 49 § 571.102 |access-date=August 16, 2023}}</ref>

* '''Park''' (''P''): This position disengages the transmission from the engine (as with the ''neutral'' position), and a [[parking pawl]] mechanically locks the output shaft of the transmission. This prevents the driven wheels from rotating to prevent the vehicle from moving. The use of the [[hand brake]] (''parking brake'') is also recommended when parking on slopes, since this provides greater protection from the vehicle moving. The ''park'' position is omitted on buses/coaches/tractors, which must instead be placed in ''neutral'' with the air-operated parking brakes set. Some early passenger car automatics, such as the pre-1960 [[Chrysler]] cars and the [[Corvair Powerglide]], did not have the ''park'' feature at all. These cars were started in ''neutral'' and required the driver to apply a parking brake when parked. The original [[Hydra-Matic]] from GM instead engaged a parking pawl when placed in ''reverse'' with the engine off, thus dispensing with a ''park'' position until the adoption of the Controlled Coupling Hydra-Matic in 1956.<br>The ''park'' position usually includes a lockout function (such as a button on the side of the gear selector or requiring that the brake pedal be pressed) which prevents the transmission from being accidentally shifted from ''park'' into other gear selector positions. Many cars also prevent the engine from being started when the selector is in any position other than ''park'' or ''neutral'' (often in combination with requiring the brake pedal to be pressed).
* '''Reverse''' (''R''): This position engages ''reverse'' gear, so that the vehicle drives in a backwards direction.<ref>{{cite web|url=https://www.merriam-webster.com/dictionary/reverse%20gear |title=Reverse Gear |publisher=Merriam-webster.com |access-date=2018-06-24}}</ref> It also operates the reversing lights and on some vehicles can activate other functions including [[parking sensor]]s, [[backup camera]]s and reversing beepers (to warn pedestrians).<br>Some modern transmissions have a mechanism that will prevent shifting into the ''reverse'' position when the vehicle is moving forward, often using a switch on the brake pedal or electronic transmission controls that monitor the vehicle speed.
* '''Neutral''' (''N''): This position disengages the transmission from the engine, allowing the vehicle to move regardless of the engine's speed. Prolonged movement of the vehicle in ''neutral'' with the engine off at significant speeds ("coasting") can damage some automatic transmissions, since the lubrication pump is often powered by the input side of the transmission and is therefore not running when the transmission is in ''neutral''. The vehicle may be started in ''neutral'' as well as ''park''.
* '''Drive''' (''D''): This position is the normal mode for driving forwards. It allows the transmission to engage the full range of available forward gear ratios.
*'''Low''' (''L''): This position provides for engine braking on steep hills. It also provides for a lower gear ratio for starting out when heavily loaded.

Some automatic transmissions, especially by [[General Motors]] from 1940 to 1964, used a layout with ''reverse'' as the bottom position (e.g. N–D–L–R or P–N–D–L–R).<ref>{{cite news |url=https://www.thestar.com/autos/2016/11/12/prndl-why-mess-with-it.html |title=PRNDL — why mess with it? |work=Toronto Star |first=Jim |last=Kenzie |date=November 12, 2016 |access-date=August 16, 2023}}</ref>{{failed verification |date=August 2023}}

====Other positions and modes====
[[File:Aero Star MP37F Allison Shift selector and Parking brake lever.jpg|thumb|right|Push-button gear selector in a 2010–2014 [[Mitsubishi Fuso Aero Star]] bus]]
Many transmissions also include positions to restrict the gear selection to the lower gears and engages the [[engine braking|engine brake]]. These positions are often labelled "L" (low gear), "S" (second gear) or the number of the highest gear used in that position (eg 3, 2 or 1). If these positions are engaged at a time when it would result in excessive engine speed, many modern transmissions disregard the selector position and remain in the higher gear.

In descending order of the highest gear available:
* '''3''': Restricts the transmission to the lowest three gear ratios. In a 4-speed automatic transmission, this is often used to prevent the car shifting into the [[overdrive (mechanics)|overdrive]] ratio. In some cars,{{which|date=November 2020}} the position labelled "D" performs this function, while another position labelled "OD" or a boxed "[D]" allows all gears to be used.
* '''2''' (also labelled "S"): Restricts the transmission to the lowest two gear ratios. In some cars, it is also used to accelerate from standstill in 2nd gear instead of 1st, for situations of reduced traction (such as snow or gravel). This function is sometimes called "winter mode", labelled "W".
* '''1''' (also labelled "L"): Restricts the transmission to 1st gear only, also known as a "low gear". This is useful when a large torque is required at the wheels (for example, when accelerating up a steep incline); however use at higher speeds can run the engine at an excessive speed, risking overheating or damage.

Many modern transmissions include modes to adjust the shift logic to prefer either [[Engine power|power]] or [[Fuel economy in automobiles|fuel economy]]. "Sport" (also called "Power" or "Performance") modes cause gear shifts to occur at higher engine speeds, allowing higher acceleration. "Economy" (also called "Eco" or "Comfort") modes cause gear shifts to occur at lower engine speeds to reduce fuel consumption.

====Manual controls====
{{main|Manumatic}}
[[File:Paddle Shifter - 2013 BMW X5 xdrive 35i (9709131265).jpg|thumb|Paddle shifter (labelled "+") in a 2013 [[BMW X5]] ]]
Since the 1990s, systems to manually request a specific gear or an upshift/downshift have become more common. These ''manumatic'' transmissions offer the driver greater control over the gear selection that the traditional modes to restrict the transmission to the lower gears.

Use of the manumatic functions are typically achieved either via paddles located beside the steering column, or "+" and "-" controls on the gear selector. Some cars offer drivers both methods to request a manual gear selection.
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==Continuously variable transmission (CVT)==
{{Main|Continuously variable transmission}}
[[File:GearBoxRotRotVar.gif|thumb|Operating principle for a pulley-based CVT]]
A ''continuously variable transmission'' (CVT) can change seamlessly through a continuous (infinite) range of gear ratios, compared with other automatic transmissions that provide a limited number of gear ratios in fixed steps. The flexibility of a CVT with suitable control may allow the engine to operate at a constant angular velocity while the vehicle moves at varying speeds.

CVTs are used in [[car]]s, [[tractor]]s, [[Side-by-side (vehicle)|UTVs]], [[motor scooter]]s, [[snowmobile]]s, and [[earthmoving equipment]].

The most common type of CVT uses two pulleys connected by a [[Belt drive|belt]] or [[Chain drive|chain]], however, several other designs have also been used.
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==Dual-clutch transmission (DCT)==
{{Main|Dual-clutch transmission}}
[[File:Dual-clutch transmission.svg|thumb|right|Schematic of a DCT]]
A ''dual-clutch transmission'' (DCT, sometimes referred to as a ''twin-clutch transmission'', or ''double-clutch transmission'') uses two separate [[clutch]]es for odd and even [[gear train|gear sets]].<ref name=AMS>{{cite journal|title=Powertrain — transmissions: Shift in power to the gearbox|url=http://www.unofficialbmw.com/images/SHIFT1.pdf|journal=[[Auto, Motor und Sport|AMS]]|publisher=UnofficialBMW.com|date=September–October 2003|access-date=31 October 2009|url-status=live|archive-url=https://web.archive.org/web/20110717164509/http://www.unofficialbmw.com/images/SHIFT1.pdf|archive-date=17 July 2011}}</ref> The design is often similar to two separate [[manual transmission]]s with their respective clutches contained within one housing, and working as one unit.<ref name="AEI">{{cite journal|title=Automatic-shifting dual-clutch transmissions are poised to grab share from traditional transmissions thanks to their combination of efficiency and convenience|url=http://www.dctfacts.com/documents/1-117-6-20.pdf|journal=AEI-online.org|publisher=DCTfacts.com|date=June 2009|access-date=31 October 2009|url-status=dead|archive-url=https://web.archive.org/web/20111007173150/http://www.dctfacts.com/documents/1-117-6-20.pdf|archive-date=7 October 2011}}</ref><ref name="PorschePDK_Glossary">{{cite web |title=Porsche Doppelkupplung (PDK) |url=http://www.porsche.com/microsite/technology/default.aspx?pool=uk&ShowSingleTechterm=PTPDopKuGe&Category=&Model=&SearchedString=&SelectedVariant=|publisher=Porsche.com |access-date=31 October 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081205135604/http://www.porsche.com/microsite/technology/default.aspx?pool=uk&ShowSingleTechterm=PTPDopKuGe&Category=&Model=&SearchedString=&SelectedVariant=|archive-date=5 December 2008}}</ref> In most car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.

The first DCT to reach production was the ''Easidrive'' automatic transmission introduced on the 1961 ''[[Hillman Minx]]'' mid-size car. This was followed by various eastern European tractors through the 1970s (using manual operation via a single clutch pedal), then the ''[[Porsche 962|Porsche 962 C]]'' racing car in 1985. The first DCT of the modern era was used in the 2003 ''Volkswagen Golf R32''. Since the late 2000s, DCTs have become increasingly widespread, and have supplanted hydraulic automatic transmissions in various models of cars.
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==Automated manual transmission (AMT)==
{{Main|Automated manual transmission}}
''Automated manual transmission (AMT)'', sometimes referred to as a ''clutchless manual'', is a type of multi-speed [[automobile]] [[transmission (mechanics)|transmission system]] that is closely based on the mechanical design of a conventional [[manual transmission]], and automates either the clutch system, the gear shifting, or both simultaneously, requiring partial, or no driver input or involvement.<ref name="NAP"/><ref>{{cite journal |title=Research on Accurate Engagement Control of Electrohydraulic Clutch Actuator |url=https://www.sae.org/publications/technical-papers/content/2005-01-1787/ |website=www.sae.org |series=SAE Technical Paper Series |access-date=26 July 2020 |language=en |date=11 April 2005|doi=10.4271/2005-01-1787 |last1=Yulong |first1=Lei |last2=Hua |first2=Tian |volume=1|url-access=subscription }}</ref><ref>{{cite web |title=Transmission Actuation |url=https://www.fte-automotive.com/fileadmin/user_upload/Valeo_Getriebe_EN_web.pdf |website=www.fte-automotive.com |access-date=26 July 2020 |archive-date=3 November 2022 |archive-url=https://web.archive.org/web/20221103071423/https://www.fte-automotive.com/fileadmin/user_upload/Valeo_Getriebe_EN_web.pdf |url-status=dead}}</ref><ref>{{Cite web|url=https://www.repairsmith.com/i/blog/manual-vs-automatic-transmission-shift-know-about/|title=Manual vs Automatic Transmission: A Shift to Know About {{pipe}} RepairSmith Blog|date=May 6, 2020}}</ref>

Earlier versions of these transmissions that are [[Semi-automatic transmission|semi-automatic]] in operation, such as ''[[Autostick#Volkswagen Autostick|Autostick]]'', control only the [[clutch]] system [[Automation|automatically]] — and use different forms of actuation (usually via an [[actuator]] or [[Servomechanism|servo]]) to automate the clutch, but still require the driver's input and full control to manually actuate gear changes by hand. Modern versions of these systems that are fully automatic in operation, such as ''[[Selespeed]]'' and ''[[Easytronic]]'', require no driver input over gear changes or clutch operation. Semi-automatic versions require only partial driver input (i.e., the driver must change gears manually), while fully automatic versions require no manual driver input, whatsoever ([[Transmission control unit|TCU]] or [[Electronic control unit|ECU]] operates both the clutch system and gear shifts automatically).

Modern automated manual transmissions (AMT) have their roots and origins in older [[clutchless manual]] transmissions that began to appear on mass-production automobiles in the early 1930s and 1940s, prior to the introduction of hydraulic automatic transmissions. These systems were designed to reduce the amount of clutch or gear shifter usage required by the driver.<ref>{{Cite web|url=https://www.caranddriver.com/research/a31862769/how-does-an-automatic-transmission-work/|title=How Does an Automatic Transmission Work?|first=Hearst Autos|last=Research|date=April 13, 2020|website=Car and Driver}}</ref> These devices were intended to reduce the difficulty of operating conventional [[Non-synchronous transmission|unsynchronised manual transmissions]] ("crash gearboxes") that were commonly used at the time, especially in stop-start driving. An early example of this transmission was introduced with the [[Hudson Commodore]] in 1942, called ''Drive-Master''. This unit was an early [[semi-automatic transmission]], based on the design of a conventional manual transmission, which used a [[Servomechanism|servo]]-controlled [[vacuum]]-operated clutch system, with three different gear shifting modes, at the touch of a button; manual shifting and manual clutch operation (fully manual), manual shifting with automated clutch operation (semi-automatic), and automatic shifting with automatic clutch operation (fully automatic).<ref>{{Cite web |url=https://auto.howstuffworks.com/1941-1947-hudson3.htm |title=1942 Hudson {{pipe}} HowStuffWorks |access-date=2020-11-25 |archive-date=2020-08-12 |archive-url=https://web.archive.org/web/20200812072455/https://auto.howstuffworks.com/1941-1947-hudson3.htm |url-status=dead}}</ref><ref>{{cite web |url=http://hudsonterraplane.com/tech/other/DrivemasterServiceInformation.pdf |title=Hudson Drive-Master Service Information |publisher=Hudson Motor Car Company |location=Detroit, Michigan |access-date=9 July 2021 |url-status=live|archive-url=https://web.archive.org/web/20160319034651/http://www.hudsonterraplane.com/tech/other/DrivemasterServiceInformation.pdf |archive-date=2016-03-19}}</ref><ref>{{Cite web |url=https://auto.howstuffworks.com/1950-hudson-commodore.htm |title=1950 Hudson Commodore {{pipe}} HowStuffWorks |access-date=2020-11-25 |archive-date=2020-10-23 |archive-url=https://web.archive.org/web/20201023002530/https://auto.howstuffworks.com/1950-hudson-commodore.htm |url-status=dead}}</ref> Another early example of this transmission system was introduced in the 1955 [[Citroën DS]], which used a 4-speed ''BVH'' transmission. This semi-automatic transmission used an automated clutch, which was [[Hydraulic actuator|actuated using hydraulics]]. Gear selection also used [[Hydraulic machinery|hydraulics]], however, the gear ratio needs to be manually selected by the driver. This system was nicknamed ''Citro-Matic'' in the U.S.

The first modern AMTs were introduced by [[BMW]] and [[Ferrari]] in 1997, with their ''SMG'' and ''F1'' transmissions, respectively.<ref>{{Cite web|url=https://magazine.ferrari.com/en/cars/2020/04/24/news/the-ferrari-automated-gearbox-from-f1-to-ferrari-roma-79244/|title=Gearing Up|website=magazine.ferrari.com}}</ref><ref>{{Cite web|url=https://europe.autonews.com/article/19970804/ANE/708040768/ferrari-launches-f1-style-gearbox-on-f355|title=FERRARI LAUNCHES F1-STYLE GEARBOX ON F355|date=August 4, 1997|website=Automotive News Europe}}</ref><ref>{{Cite web|url=https://europe.autonews.com/article/19970609/ANE/706090799/magneti-marelli-transmission-from-f1|title=MAGNETI MARELLI: TRANSMISSION FROM F1|date=June 9, 1997|website=Automotive News Europe}}</ref> Both systems used [[hydraulic actuator]]s and [[Solenoid valve|electrical solenoids]], and a designated [[transmission control unit]] (TCU) for the clutch and shifting, plus steering wheel-mounted paddle shifters, if the driver wanted to change gear manually.

Modern fully automatic AMTs, such as ''[[Selespeed]]'' and ''[[Easytronic]]'', have now been largely superseded and replaced by the increasingly widespread [[dual-clutch transmission]] design.

==See also==
{{transmission types}}
*[[:Category:Automobile transmissions]]
*[[:Category:Automatic transmission tradenames]]
*[[Motor vehicle]]
*[[Park by wire]]
*[[Shift by wire]]
*[[Torque converter]]

==References==
{{Reflist}}

{{Powertrain}}
{{Authority control}}

{{DEFAULTSORT:Automatic Transmission}}
[[Category:Automotive transmission technologies]]
[[Category:Automobile transmissions]]
[[Category:Mechanical power control]]