Longman Dictionary of Contemporary English
Wiktionary
n. (context automotive English) A mechanical transmission which shifts gears automatically in response to speed and/or load, rather than requiring the operator to do so manually; an automatic gearbox.
WordNet
n. a transmission that automatically changes the gears according to the speed of the car [syn: automatic drive]
Wikipedia
An automatic transmission, also called auto, self-shifting transmission, n-speed automatic (where n is its number of forward gear ratios), or AT, is a type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually. Like other transmission systems on vehicles, it allows an internal combustion engine, best suited to run at a relatively high rotational speed, to provide a range of speed and torque outputs necessary for vehicular travel. The number of forward gear ratios is often expressed for manual transmissions as well (e.g., 6-speed manual).
The most popular form found in automobiles is the hydraulic automatic transmission. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment. This system uses a fluid coupling in place of a friction clutch, and accomplishes gear changes by hydraulically locking and unlocking a system of planetary gears. These systems have a defined set of gear ranges, often with a parking pawl that locks the output shaft of the transmission to keep the vehicle from rolling either forward or backward. Some machines with limited speed ranges or fixed engine speeds, such as some forklifts and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.
Besides the traditional hydraulic automatic transmissions, there are also other types of automated transmissions, such as a continuously variable transmission (CVT) and semi-automatic transmissions, that free the driver from having to shift gears manually, by using the transmission's computer to change gear, if for example the driver were redlining the engine. Despite superficial similarity to other transmissions, traditional automatic transmissions differ significantly in internal operation and driver's feel from semi-automatics and CVTs. In contrast to conventional automatic transmissions, a CVT uses a belt or other torque transmission scheme to allow an "infinite" number of gear ratios instead of a fixed number of gear ratios. A semi-automatic retains a clutch like a manual transmission, but controls the clutch through electrohydraulic means. The ability to shift gears manually, often via paddle shifters, can also be found on certain automated transmissions ( manumatics such as Tiptronic), semi-automatics (BMW SMG, VW Group DSG), and CVTs (such as Lineartronic).
The obvious advantage of an automatic transmission to the driver is the lack of a clutch pedal and manual shift pattern in normal driving. This allows the driver to operate the car with as few as two limbs (possibly using assist devices to position controls within reach of usable limbs), allowing amputees and other disabled individuals to drive. The lack of manual shifting also reduces the attention and workload required inside the cabin, such as monitoring the tachometer and taking a hand off the wheel to move the shifter, allowing the driver to ideally keep both hands on the wheel at all times and to focus more on the road. Control of the car at low speeds is often easier with an automatic than a manual, due to a side effect of the clutchless fluid-coupling design called "creep" that causes the car to want to move while in a driving gear, even at idle. The primary disadvantage of the most popular hydraulic designs is reduced mechanical efficiency of the power transfer between engine and drivetrain, due to the fluid coupling connecting the engine to the gearbox. This can result in lower power/torque ratings for automatics compared to manuals with the same engine specs, as well as reduced fuel efficiency in city driving as the engine must maintain idle against the resistance of the fluid coupling. Advances in transmission and coupler design have narrowed this gap considerably, but clutch-based transmissions (manual or semi-automatic) are still preferred in sport-tuned trim levels of various production cars, as well as in many auto racing leagues.
The automatic transmission was invented in 1921 by Alfred Horner Munro of Regina, Saskatchewan, Canada, and patented under Canadian patent CA 235757 in 1923. (Munro obtained UK patent GB215669 215,669 for his invention in 1924 and US patent 1,613,525 on 4 January 1927). 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. The first automatic transmission using hydraulic fluid may have been developed in 1932 by two Brazilian engineers, José Braz Araripe and Fernando Lehly Lemos; subsequently the prototype and plans were sold to General Motors who introduced it in the 1940 Oldsmobile as the "Hydra-Matic" transmission. They were incorporated into GM-built tanks during World War II and, after the war, GM marketed them as being "battle-tested." However, a Wall Street Journal article credits ZF Friedrichshafen with the invention, occurring shortly after World War I. ZF's origins were in manufacturing gears for airship engines beginning in 1915; the company was founded by Ferdinand von Zeppelin.
Usage examples of "automatic transmission".
It's a multipurpose 4x4 wheeled vehicle with automatic transmission, power steering, and a Detroit Diesel 150-hp diesel V-8, air-cooled engine.
They had a first-class apartment now, and they rode back and forth to work in a 1940 Oldsmobile that had an automatic transmission.
I must order you to set in motion your automatic transmission recording circuit and replay the earlier portions of the Leader's speech.
Afterwards she reconstructed the sequence: Russell had yanked her shin sideways so that her foot slipped off the brake and onto the accelerator, then thrown the automatic transmission into reverse, then leaned hard on her knee.
Anyone monitoring his automatic transmission to the Company would read it as a long series of entries on the literature of the Socialist movement in Britain, guaranteed to send them channel-surfing on to monitor some other operative's more interesting datafeed.
He checked the automatic transmission fluid, saw it was all right.
For this reason, an automatic transmission costs more, for there it is the tree itself that decides, in accordance with its own Sovereign Equilibrium.
It was an eight-cylinder job with a sports package that included automatic transmission with a floor shift, bucket seats, side mirrors, white-wall tires—.
It was an eight-cylinder job with a sports package that included automatic transmission with a floor shift, bucket seats, side mirrors, white-wall tires - and a wickedly sparkling black paint job that contrasted nicely with the crimson racing stripes over the trunk and hood.
It was a strangely mechanical sound, one that made Bob think of an automatic transmission low on fluid.
The survey shuttle had been easy to find, as its recognition beam had been left on automatic transmission, as per standard procedure.
Because the van had an automatic transmission, a driver could leave the gear shift in drive, leap out of the van, and know that the van would follow the direction in which it had been aimed: in this case, down the lane toward the mansion.
Probably he'd used too high a gear or worse, an automatic transmission, either of which would put too great a stress on the motor.
He parked it where he would still be parking twenty-three years later, although by then he would have traded up to a Dodge Ram with the automatic transmission and the four-wheel drive.
He revved up the engine and shifted the automatic transmission into sow.