automatic transmission

 

An automatic transmission (commonly abbreviated as "AT") is an automobile gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually (similar but larger devices are also used for railroad locomotives).

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 CVTs have been used for decades in two-wheeled scooters and in a few cars (e.g. DAF saloons and the Volvo 340 series that succeeded them, and later the Subaru Justy), the technology has recently gained greater acceptance among manufacturers and customers.

Comparison with manual transmission

Main article: Comparison of manual and automatic transmissions

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. In most Asian markets, automatic transmission became very popular from the 1990s. Automatic transmission provides lower fuel efficiency and power than manual transmission, though the gap is significantly narrowed with modern designs. Automatic transmission is easier for drivers, especially beginners. In some jurisdictions, drivers passing their driving test in an automatic-transmission vehicle will not be licensed to drive a manual transmission vehicle.

Automatic transmission modes

Transmission types
Manual Sequential manual Non-synchronous Automatic Tiptronic Semi-automatic Twin-clutch gearbox Saxomat
Continuously-variable Variomatic Multitronic
Bicycle gearing Derailleur gears Hub gears

Conventionally, in order to select the mode, the driver would have to move a gear shift lever located on the steering column or on the floor next to him/her. In order to select gears/modes the driver must push a button in (called the shift lock button) or pull the handle (only on column mounted shifters) out. Some vehicles (like the Aston Martin DB9) position selector buttons for each mode on the cockpit instead, freeing up space on the central console. Vehicles conforming to US Government standards must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise).

Automatic Transmissions have various modes depending on the model and make of the transmission. Some of the common modes are:

Park (P) – This selection mechanically locks the transmission, restricting the car from moving in any direction. A parking pawl prevents the transmission from moving forward (although wheels, depending on the drive train, can still spin freely), it is recommended to use the hand brake (or emergency brake) because this actually locks the wheels and prevents them from moving, and increases the life of the transmission and the park mechanism. A car should be allowed to come to a complete stop before setting transmission into park to prevent damage. Park is one of only two selections in which the car can be started. In some cars (notably those sold in the US), the driver must have the footbrake depressed before the transmission can be taken out of park. The Park position is omitted on buses/coaches with automatic transmission, which must be placed in neutral with the parking brakes set.

Reverse (R) – This puts the car into the reverse gear, giving the ability for the car to drive backwards. In order for the driver to select reverse they must come to a complete stop, and push the shift lock button in and select reverse. Not coming to a complete stop can cause severe damage to the transmission. Many modern automatic gearboxes have a safety mechanism in place, which does to some extent prevent (but doesn't completely avoid) inadvertently putting the car in reverse when the vehicle is moving. This mechanism usually consists of a moveable physical barrier on either side of the Reverse position, and is electronically linked to the brake pedal, which needs to be pressed in order to allow putting the car in reverse. Some electronic transmissions prevent/delay engagement of reverse gear altogether when the car is moving.

Neutral/No gear (N)– This disconnects the transmission from the wheels so the car can move freely under its own weight. This is the only other selection in which the car can be started.

Drive (D)– This allows the car to move forward and accelerate through a range of gears. The number of gears a transmission has depends on the model, but they can commonly range from 3, 4 (the most common), 5, 6 (found in VW/Audi Direct Shift Gearbox), 7 (found in Mercedes 7G gearbox) and 8 in the new model of Lexus cars. Some cars when put into D will automatically lock the doors or turn on the Daytime Running Lamps.

As well as the above modes, there are also other modes dependant on the manufacturer and model. Some examples include;

• D4 – In Honda and Acura automatics this mode is used commonly for highway use (as stated in the manual) and uses all 4 forward gears.
• D3 – This is also found in Honda and Acura automatics and only uses the first 3 gears and according to the manual it is used for stop & go traffic such as city driving.
• + − and M – This is the manual selection of gears for automatics, such as Porsche's Tiptronic. The driver can shift up and down at their will, like in a semi-automatic transmission. This mode may be engaged either through a selector/position or by actually changing gear (e.g. tipping the gear-down paddle).

OverDrive ([D], OD, or a boxed D) - This mode is used in some transmissions (including late 1980s Chevrolet) 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 in these cars engaged under steady speeds or low acceleration at 45mph (72 kph); it would automatically come on at 65 mph (104 kph) under hard acceleration.

Second (2 or S) – This mode limits the transmission to the first two gears, or more commonly locks the transmission in second gear. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in the winter time.

First (1 or L) – This mode locks the transmission in first gear only. It will not accelerate through any gear range. This, like second, can be used during the winter season, or towing.

Hydraulic automatic transmissions

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.)
• Hydraulic & Lubricating Oil: called Automatic Transmission Fluid, or ATF, this component of the transmission provides lubrication, corrosion prevention, and a hydraulic medium to convey mechanical power. Primarily made from refined petroleum and processed to provide properties that promote smooth power transmission and increase service life, the ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.

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 Packard's Ultramatic in mid-1949 and Chevrolet's Powerglide 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 American Motors Corporation, Ford Motor Company, Studebaker, and several other manufacturers in the US and other countries. 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. Also around this time, whale oil was removed from automatic transmission fluid.^[1] 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 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 460.

Automatic transmission models

Some of the best known automatic transmission families include:

• General Motors — Powerglide, Turbo-Hydramatic 350 and 400, 4L60-E, 4L80-E, Holden Trimatic
• 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
• 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
• Volkswagen - 01M, DSG (Direct Shift Gearbox)
• Drivetrain Systems International (DSI) - M93, M97 and M74 4speeds, M78 and M79 6Speeds

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 have a part-throttle kickdown, eliminating the need to "floorboard" the throttle to downshift.
• Mode Selection: Allows the driver to choose between preset shifting programs. For example, Economy mode saves fuel by upshifting at lower speeds, while Sport mode (aka Power or Performance) delays shifting for maximum acceleration. The modes also change how the computer responds to throttle input.
• 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.
• Second Gear Takeoff: Some older automatics, particularly those fitted behind larger capacity engines (and given tyre technology of the time) when 2 was manually selected, would take off in 2nd gear instead of first, and then not shift into a higher gear until returned to D. This was ostensibly to reduce torque multiplication when proceeding forward from a standstill in conditions where traction was limited - snow or ice covered roads for example. This can also be accomplished on the aforementioned Tiptronic style transmissions by selecting manual mode and up shifting whilst stopped.

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.

See also

• AMC and Jeep transmissions
• Hydraulics

External links

• How Automatic Transmissions Work on HowStuffWorks
• How To Drive a Car with Automatic Transmission
• US5370589 Lepelletier's concept is shown on this patent
• Randolph Toom webpage — a survey of current automatic transmissions
• Articles related to IVT
• Allison Transmission Website
• PCS Automatic Transmission Controller Website
• Automatic Transmission Controllers
• Automatic Transmission Repair Advice
• Units that shaped the evolution of automatic transmission

References

1. ^ Almost Burned

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