Design :
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. The transmission is connected to the engine via a torque converter (or a
fluid coupling prior to the 1960s), instead of the
friction clutch used by most
manual transmissions.
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.
History 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. 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. 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. 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. 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 gearboxes, 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. 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. 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. 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.
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 and
Cadillac Sixty Special, the Hydra-Matic combined a
fluid coupling with three
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 torque converter). Use of the Hydra-Matic spread to other General Motors brands and then to other manufacturers starting 1948 including
Hudson,
Lincoln,
Kaiser,
Nash,
Holden (Australia), as well as
Rolls-Royce and
Bentley licensing production in the UK and providing the transmission to
Jensen Motors,
Armstrong Siddeley and other UK manufacturers. 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. During the 1980s, automatic transmissions with four gear ratios became increasingly common, 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
solenoids 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
Jatco 5R01, introduced in 1989 by Nissan in the
Cedric and
Gloria, and was electronically controlled. 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 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 The gear selector is the input by which the driver selects the operating mode of an automatic transmission. 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" (also simply called "PRNDL") layout for the gear selector, which consists of the following positions: •
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.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. It also operates the reversing lights and on some vehicles can activate other functions including
parking sensors,
backup cameras and reversing beepers (to warn pedestrians).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).
Other positions and modes bus Many transmissions also include positions to restrict the gear selection to the lower gears and engages the
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 ratio. In some cars, 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
power or
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 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. ==Continuously variable transmission (CVT)==