Automatic transmission

2006-11-27T15:05:41Z

130.191.152.21: /* Manually controlled automatic transmissions */

{{transmission types}}
An '''[[Automatic transmission|automatic]] transmission''' is an [[automobile]] [[gearbox]] that can change [[gear ratio]]s automatically as the car or truck moves, thus freeing the driver from having to shift gears [[manual transmission|manually]]. (Similar but larger devices are also used for railroad.)

Most cars sold in the United States since the 1950s have been equipped with an [[Automatic transmission|automatic]] transmission. This has, however, not been the case in Europe and much of the rest of the world. [[Automatic transmission|Automatic]] transmissions, particularly earlier ones, reduce [[fuel efficiency]] and power. Where fuel is expensive and, thus, engines generally smaller, these penalties are more burdensome. In recent years, [[Automatic transmission|automatic]] transmissions have significantly improved in their ability to support high fuel efficiency but [[manual transmission]]s are still generally more efficient. (This balance may finally shift with the introduction of practical [[continuously variable transmission]]s; see below.)

Most [[Automatic transmission|automatic]] transmissions have a set selection of possible gear ranges, often with a [[parking pawl]] feature that will lock the output shaft of the transmission.

However, some simple machines with limited speed ranges and/or fixed engine speeds only use a [[torque converter]] to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.

Recently manufacturers have begun to make '''[[continuously variable transmission]]s''' available. These designs can change the ratios over a range rather than between set gear ratios. Even though prototypes for CVT have been around for decades, it is just now reaching commercial practicability.

==Hydraulic [[Automatic transmission|automatic]] transmissions==
[[Image:Ford500autotrans.JPG|frame|right|The automatic transmission selector lever in a [[Ford Five Hundred]] car.]]
The predominant form of [[Automatic transmission|automatic]] transmission is hydraulically operated, using a fluid coupling or [[torque converter]] and a set of [[epicyclic gearing|planetary gearsets]] to provide a range of torque multiplication.

===Parts and operation===
A hydraulic [[Automatic transmission|automatic]] transmission consists of the following parts:
*''[[Fluid coupling]]'' or ''[[Torque converter]]'': A hydraulic device connecting the engine and the transmission. It takes the place of a mechanical [[clutch]], allowing the engine to remain running at rest without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.
*''Planetary gearset'': A compound planetary set whose bands and clutches are actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.
*''Valve body'': A hydraulic control center that receives pressurised fluid from a ''main pump'' operated by the fluid coupling/torque converter. The pressure coming from this pump is regulated used to run a network of spring-loaded valves, check balls and [[servo]] pistons. The valves use the pump pressure and the pressure from a [[centrifugal governor]] on the output side (as well as hydraulic signals from the range selector valves and the ''throttle valve'' or ''modulator'') to control which ratio is selected on the gearset; as the car and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. The hydraulic pressure controlled by these valves drives the various clutch and brake band actuators, thereby controlling the operation of the planetary gearset to select the optimum gear ratio for the current operating conditions. However, in many modern [[Automatic transmission|automatic]] transmissions, the valves are controlled by electro-mechanical servos which are controlled by the Engine Management System or a separate [[Transmission Control Unit|transmission controller]]. (See [[#History and improvements|History and improvements]] below.)

The multitude of parts, and the complex design of the valve body originally made hydraulic [[Automatic transmission|automatic]] transmissions much more complicated (and expensive) to build and repair than manual transmissions. In most cars (except US family, luxury, sport-utility vehicle, and minivan models) they have usually been extra-cost options for this reason. Mass manufacturing and decades of improvement have reduced this cost gap.

===History and improvements===
[[Oldsmobile]]'s [[1940]] models featured [[Hydra-Matic]] drive, the first mass-production fully [[Automatic transmission|automatic]] transmissions. Initially an Olds exclusive, Hydra-Matic had a [[fluid coupling]] (not a torque converter) and three planetary gearsets providing four speeds plus reverse. Hydra-Matic was subsequently adopted by [[Cadillac]] and [[Pontiac]], and was sold to various other automakers, including [[Bentley Motors Limited|Bentley]], [[Hudson Motor Car|Hudson]], [[Kaiser Motors|Kaiser]], [[Nash Motors|Nash]], and [[Rolls-Royce Limited|Rolls-Royce]]. From [[1950]] to [[1954]] [[Lincoln (automobile)|Lincoln]] cars were also available with GM Hydra-Matic. [[Mercedes-Benz]] subsequently devised a four-speed fluid coupling transmission that was similar in principle to Hydra-Matic, but did not share the same design.

The first torque converter [[Automatic transmission|automatic]], [[Buick]]'s [[Dynaflow]], was introduced for the [[1948]] model year. It was followed by [[Chevrolet]]'s [[Powerglide]] and Packard's [[Ultramatic]] for the [[1950]] model year. Each of these transmissions had only two forward speeds, relying on the torque converter for additional gear reduction.

In the early [[1950s]] [[Borg-Warner]] developed a series of three-speed torque converter automatics for [[Ford Motor Company]], [[Studebaker]], and several foreign and independent makes.

[[Chrysler Corporation|Chrysler]] was late in developing its own true [[Automatic transmission|automatic]], introducing the two-speed torque converter [[PowerFlite]] in [[1953]] and the three-speed [[TorqueFlite]] in [[1956]].

By the late [[1960s]] most of the fluid-coupling four-speeds and two-speed transmissions had disappeared in favor of three-speed units with torque converters. By the early [[1980s]] these were being supplemented and eventually replaced by [[overdrive (mechanics)|overdrive]]-equipped transmissions providing four or more forward speeds. Many transmissions also adopted the lock-up torque converter (a mechanical clutch locking the torque converter impeller and turbine together to eliminate slip at cruising speed) to improve fuel economy.

As the [[electronic control unit|engine computer]]s became more and more capable, even more of the valve body's functionality was offloaded to them. These transmissions, introduced in the late 1980s and early 1990s, remove almost all of the control logic from the valve body, and place it in into the engine computer. (Some manufacturers use a separate computer dedicated to the transmission but sharing information with the engine management computer.) In this case, [[solenoid]]s turned on and off by the computer control shift patterns and gear ratios, rather than the spring-loaded valves in the valve body. This allows for more precise control of [[shift point]]s and shift quality, and (on some newer cars) also allows semi-[[Automatic transmission|automatic]] control, where the driver tells the computer when to shift. The result is an impressive combination of efficiency and smoothness. Some computers even identify the driver's style and adapt to best suit it.

[[ZF Friedrichshafen AG]] and [[BMW]] were responsible for introducing the first five-speed [[Automatic transmission|automatic]] (the [[ZF 5HP18 transmission|ZF 5HP18]] in the 1992 [[BMW]] [[BMW E34|E34]] [[BMW 5-Series|5-Series]]) and the first six-speed (the [[ZF 6HP26 transmission|ZF 6HP26]] in the 2002 [[BMW]] [[BMW E65|E65]] [[BMW 7-Series|7-Series]]). [[Mercedes-Benz]]'s [[7G-TRONIC]] was the first seven-speed in 2003, with [[Toyota Motor Company]] introducing an 8-speed in 2007 on the [[Lexus LS]].

== [[Automatic transmission|Automatic]] Transmission Models ==
Some of the best known [[Automatic transmission|automatic]] transmission families include:
* [[List of GM transmissions|General Motors]] — [[Powerglide]], [[Turbo-Hydramatic]] 350 and 400, [[GM 4L60-E transmission|4L60-E]], [[GM 4L80-E transmission|4L80-E]]
* [[List of Ford transmissions|Ford]]: [[Cruise-O-Matic]], [[Ford C4 transmission|C4]], [[Ford C6 transmission|C6]], [[Ford AOD transmission|AOD/AODE]], [[Ford E4OD transmission|E4OD]], [[Ford ATX transmission|ATX]], [[Ford AXOD transmission|AXOD/AX4S/AX4N]]
* [[List of Chrysler transmissions|Chrysler]]: [[TorqueFlite]] 727 and 904, A500, A518, 45RFE, 545RFE
* [[BorgWarner]] (later [[Aisin AW]])
* [[List of ZF transmissions|ZF Friedrichshafen AG]]
* [[Allison Transmission]]
* [[Voith Turbo]]
* [[List of Aisin transmissions|Aisin AW]]; Aisin AW is a Japanese automotive parts supplier, known for its [[Automatic transmission|automatic]] transmissions and [[Navigation System|navigation]] systems
* [[List of Honda transmissions|Honda]]
* [[List of Jatco transmissions|Nissan/Jatco]]

[[Automatic transmission|Automatic]] transmission families are usually based on [[Ravigneaux]], [[Lepelletier]], or [[Simpson]] [[planetary gear]]sets. Each uses some arrangement of one or two central sun gears, and a ring gear, with differing arrangements of planet gears that surround the sun and mesh with the ring. An exception to this is the [[Hondamatic]] line from [[Honda]], which uses sliding gears on parallel axes like a manual transmission without any planetary gearsets. Although the Honda is quite different from all other automatics, it is also quite different from an automated manual transmission.

==Continuously variable transmissions==
{{main|continuously variable transmission}}

A different type of [[Automatic transmission|automatic]] transmission is the ''continuously variable transmission'' or ''CVT'', which can smoothly alter its [[gear ratio]] by varying the diameter of a pair of [[belt (mechanical)|belt]] or [[roller chain|chain]]-linked [[pulley]]s, [[wheel]]s or [[cone (mechanical)|cone]]s. Some continuously variable transmissions use a [[hydrostatic drive]] consisting of a [[variable displacement pump]] and a hydraulic motor to transmit power without gears. CVT designs are usually as fuel efficient as manual transmissions in city driving, but early designs lose efficiency as engine speed increases.

A slightly different approach to CVT is the concept of ''[[toroidal CVT]]'' or ''[[Continuously variable transmission#infinitely variable transmission|IVT]]'' (from infinitely variable transmission). These concepts provide zero and reverse gear ratios.

Some current [[Hybrid Cars|hybrid vehicles]], notably those of [[Toyota]], [[Lexus]] and [[Ford Motor Company]], have an "electronically-controlled CVT" ([[E-CVT]]). In this system, the transmission has fixed gears, but the ratio of wheel-speed to engine-speed can be continuously varied by controlling the speed of the third input to a [[differential]] using an [[electric motor]]-[[generator]].

==Manually controlled [[Automatic transmission|automatic]] transmissions==
Most [[Automatic transmission|automatic]] transmissions offer the driver a certain amount of manual control over the transmission's shifts (beyond the obvious selection of forward, reverse, or neutral). These are offered for yuppies who drive BMW 325s who are incapable of learning to shift a real manual gearbox. Those controls take several forms:
* ''Throttle kickdown'': Most [[Automatic transmission|automatic]] transmissions include a switch on the throttle linkage that will force the transmission to downshift into the next lower ratio if the throttle is fully engaged. The switch generally only functions up to a certain road speed, so as to prevent a downshift that would overrev the engine. Some transmissions also had a part-throttle kickdown, obviating the need to "floorboard" the throttle to downshift.
* ''Low gear ranges'': Many transmissions have switches or selector positions that allow the driver to limit the maximum ratio that the transmission may engage. On older transmissions, this was accomplished by a mechanical lockout in the transmission valve body preventing an upshift until the lockout was disengaged; on computer- controlled transmissions, the same effect is accomplished electronically. The transmission can still upshift and downshift automatically between the remaining ratios: for example, in the ''3'' range, a transmission could shift from first to second to third, but not into fourth or higher ratios. Some transmissions will still upshift automatically into the higher ratio if the engine reaches its maximum permissible speed in the selected range.
* ''Manual controls'': Some transmissions have a mode in which the driver has full control of ratio changes (either by moving the selector or through the use of buttons or paddles), completely overriding the hydraulic controller. Such control is particularly useful in cornering, to avoid unwanted upshifts or downshifts that could compromise the vehicle's balance or traction. "Manumatic" shifters, first popularized by [[Porsche]] in the [[1990s]] under the [[trade name]] [[Tiptronic]], have become a popular option on [[sports cars]] and other performance vehicles. With the near-universal prevalence of electronically controlled transmissions, they are comparatively simple and inexpensive, requiring only software changes and the provision of the actual manual controls for the driver. The amount of true manual control provided is highly variable: some systems will override the driver's selections under certain conditions, generally in the interest of preventing engine damage.

Some [[Automatic transmission|automatic]] transmissions modified or designed specifically for [[drag racing]] may also incorporate a transmission brake, or "trans-brake," as part of a manual valve body. Activated by electrical solenoid control, a trans-brake simultaneously engages the first and reverse gears, locking the transmission and preventing the input shaft from turning. This allows the driver of the car to raise the engine rpm against the resistance of the torque converter, then launch the car by simply releasing the trans-brake switch.

==External links==
*[http://auto.howstuffworks.com/[[Automatic transmission|automatic]]-transmission.htm How [[Automatic transmission|Automatic]] Transmissions Work] on [[HowStuffWorks]]
*[http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US5370589&F=0| US5370589 Lepelletier's concept is shown on this patent]
*[http://www.toomonline.com/gearboxlinks.htm Randolph Toom webpage] — a survey of current [[Automatic transmission|automatic]] transmissions
*[http://www.torotrak.com Articles related to IVT]
*[http://www.allisontransmission.com Allison Transmission Website]
*[http://www.powertraincontrolsolutions.com PCS [[Automatic transmission|Automatic]] Transmission Controller Website]

[[Category:Automotive transmission technologies]]
[[Category:Auto parts]]

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Automatic transmission