Automatic transmission
The growing prevalence of automatic transmissions is attributed to the increasing number of electric and hybrid cars, and the ease of integrating it with safety systems such as Autonomous Emergency Braking.
[10] Real-world tests reported in 2022 found that in typical driving manual transmissions achieved 2 to 5% better fuel economy than automatics, increasing to 20% with an expert driver.
However, on long highway journeys manual transmissions require maintaining a very specific cruising speed to optimise economy, making automatics preferable.
The friction bands are often used for manually selected gears (such as low range or reverse) and operate on the planetary drum's circumference.
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.
[14][15] The ATF provides lubrication, corrosion prevention, and a hydraulic medium to transmit the power required to operate the transmission.
Made from petroleum with various refinements and additives, ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.
It receives pressurized fluid from the main pump and consists of several spring-loaded valves, check balls, and servo pistons.
An early patent for the automatic transmission was granted to Canadian inventor Alfred Horner Munro of Regina in 1923.
[20] 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 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.
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.
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.
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 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.
[31] During the 1980s, automatic transmissions with four gear ratios became increasingly common,[32] and many were equipped with lock-up torque convertors in order to improve fuel economy.
[33] 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.
In descending order of the highest gear available: Many modern transmissions include modes to adjust the shift logic to prefer either power or fuel economy.
"Economy" (also called "Eco" or "Comfort") modes cause gear shifts to occur at lower engine speeds to reduce fuel consumption.
The flexibility of a CVT with suitable control may allow the engine to operate at a constant angular velocity while the vehicle moves at varying speeds.
[38][39] In most car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.
The first DCT to reach production was the Easidrive automatic transmission introduced on the 1961 Hillman Minx mid-size car.
[43] These devices were intended to reduce the difficulty of operating conventional unsynchronised manual transmissions ("crash gearboxes") that were commonly used at the time, especially in stop-start driving.
[47][48][49] Both systems used hydraulic actuators and electrical solenoids, and a designated transmission control unit (TCU) for the clutch and shifting, plus steering wheel-mounted paddle shifters, if the driver wanted to change gear manually.
Modern fully automatic AMTs, such as Selespeed and Easytronic, have now been largely superseded and replaced by the increasingly widespread dual-clutch transmission design.
