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CATEGORIES (articles) > Transmission > Automatic Gearboxes > Automatic transmission explained

Automatic transmission explained

Transmission types
  • Tiptronic
  • Twin-clutch Gearbox
Derailleur gears
Hub gears

An automatic transmission is an automobile gearbox that can change gear ratios automatically as the car or truck moves, thus freeing the driver from having to shift gears manually. (Similar but larger devices are also used for railroad locomotives.)

Most cars sold in the United States since the 1950s have been equipped with an automatic transmission. This has, however, not been the case in Europe and much of the rest of the world. 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 transmissions have significantly improved in their ability to support high fuel efficiency but manual transmissions are still generally more efficient. (This balance may finally shift with the introduction of practical continuously variable transmissions; see below.)

Most 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 transmissions commonly available (earlier models such as the Subaru Justy did not popularize CVT). 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 transmissions

The automatic transmission selector lever in a Ford Five Hundred car.

The predominant form of automatic transmission is hydraulically operated, using a fluid coupling or torque converter and a set of planetary gearsets to provide a range of torque multiplication.

Parts and operation

A hydraulic 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: 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 and 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 transmissions, the valves are controlled by electro-mechanical servos which are controlled by the Engine Management System or a separate transmission controller. (See History and improvements below.)
The multitude of parts, along with the complex design of the valve body, originally made hydraulic 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 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, Hudson, Kaiser, Nash, and Rolls-Royce. From 1950 to 1954 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, 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 was late in developing its own true 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-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 engine computers 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, solenoids 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 points, shift quality, lower shift times and (on some newer cars) semi-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 (the ZF 5HP18 in the 1992 BMW E34 5-Series) and the first six-speed (the ZF 6HP26 in the 2002 BMW E65 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 Models

Some of the best known automatic transmission families include:

  • General Motors — Powerglide, Turbo-Hydramatic 350 and 400, 4L60-E, 4L80-E
  • Ford: Cruise-O-Matic, C4, C6, AOD/AODE, E4OD, ATX, AXOD/AX4S/AX4N
  • Chrysler: TorqueFlite 727 and 904, A500, A518, 45RFE, 545RFE
  • BorgWarner (later Aisin AW)
  • ZF Friedrichshafen AG
  • Allison Transmission
  • Voith Turbo
  • Aisin AW; Aisin AW is a Japanese automotive parts supplier, known for its automatic transmissions and navigation systems
  • Honda
  • Nissan/Jatco
Automatic transmission families are usually based on Ravigneaux, Lepelletier, or Simpson planetary gearsets. 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 article: continuously variable transmission

A different type of 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 or chain-linked pulleys, wheels or cones. 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 IVT (from infinitely variable transmission). These concepts provide zero and reverse gear ratios.

Some current 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 transmissions

Most 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). Those controls take several forms:

  • Throttle kickdown: Most 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 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.

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