Editing Automatic transmission (section)

===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>