Automatic transmission
An automatic transmission or automatic gearbox is a multi-speed transmission used in motor vehicles that does not require any input from the driver other than throttle position to change forward gears under normal driving conditions.
The 1904 Sturtevant "horseless carriage gearbox" is often considered to be the first true automatic transmission. The first mass-produced automatic transmission is the General Motors Hydramatic four-speed hydraulic automatic, which was introduced in 1939.
Automatic transmissions are especially prevalent in vehicular drivetrains, particularly those subject to intense mechanical acceleration and frequent idle/transient operating conditions; commonly commercial/passenger/utility vehicles, such as buses and waste collection vehicles.
Prevalence
Vehicles with internal combustion engines, unlike electric vehicles, require the engine to operate in a narrow range of rates of rotation, requiring a gearbox, operated manually or automatically, to drive the wheels over a wide range of speeds.Globally, 43% of new cars produced in 2015 were manual transmissions, falling to 37% by 2020. Automatic transmissions have long been prevalent in the United States, but only started to become common in Europe much later. In Europe in 1997, only 10–12% of cars had automatic transmissions.
In 1957 over 80% of new cars in the United States had automatic transmissions. Automatic transmissions have been standard in large cars since at least 1974. By 2020 only 2.4% of new cars had manual transmissions. Historically, automatic transmissions were less efficient, but lower fuel prices in the US made this less of a problem than in Europe.
In the United Kingdom, a majority of new cars have had automatic transmissions since 2020. Several manufacturers including Mercedes and Volvo no longer sell cars with manual transmissions. 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.
Efficiency
The efficiency of conventional automatic transmissions ranges from 86 to 94%. Manual transmissions are more fuel efficient than all but the newest automatic transmissions due to their inherently low parasitic losses in addition to being cheaper to make, lighter, better performing, and of simpler mechanical design. Fuel economy worsens with lower efficiency. However, manual transmissions require the driver to operate the clutch and change gear whenever required.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. Some laboratory tests show automatics in a better light due to the tests using a prescribed shifting pattern for manuals not always optimized for economy. However, on long highway journeys manual transmissions require maintaining a very specific cruising speed to optimise economy, making automatics preferable.
Hydraulic automatic transmission
Design
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, 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 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 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, which is pressurized by a pump and then directed to the appropriate bands/clutches to obtain the required gear ratio. 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.The main pump which pressurises the ATF is typically a gear pump mounted between the torque converter and the planetary gear set. The input for the main pump is connected to the torque converter housing, which in turn is bolted to the engine's flexplate, so the pump provides pressure whenever the engine is running. A disadvantage of this arrangement is that there is no oil pressure to operate the transmission when the engine is not running, therefore it is not possible to push start a vehicle equipped with an automatic transmission with no rear pump. The pressure of the ATF is regulated by a governor connected to the output shaft, which varies the pressure depending on the vehicle speed.
The valve body inside the transmission is responsible for directing hydraulic pressure to the appropriate bands and clutches. It receives pressurized fluid from the main pump and consists of several spring-loaded valves, check balls, and servo pistons. In older automatic transmissions, the valves use the pump pressure and the pressure from a centrifugal governor on the output side to control which ratio is selected. As the vehicle and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. In more recent automatic transmissions, the valves are controlled by solenoids. These solenoids are computer-controlled, with the gear selection decided by a dedicated transmission control unit or sometimes this function is integrated into the engine control unit. Modern designs have replaced the centrifugal governor with an electronic speed sensor that is used as an input to the TCU or ECU. Modern transmissions also factor in the amount of load on an engine at any given time, which is determined from either the throttle position or the amount of intake manifold vacuum.
The multitude of parts, along with the complex design of the valve body, originally made hydraulic automatic transmissions much more expensive and time-consuming to build and repair than manual transmissions; however mass-production and developments over time have reduced this cost gap.
Torque converter
To provide coupling and decoupling of the engine, a modern automatic transmission uses a torque converter instead of the friction clutch used in a manual transmission.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 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.
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.
The evolution towards mass-produced automatic transmissions continued with the 1933–1935 REO Motor Car Company Self-Shifter semi-automatic transmission, which automatically shifted between two forward gears in the "Forward" 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 was added, to avoid the need to operate a manual clutch.