Automatic transmission

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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."[citation needed] 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.

History

Modern automatic transmissions can trace their origins to an early "horseless carriage" gearbox that was developed in 1904 by the Sturtevant brothers of Boston, Massachusetts. This unit had two forward speeds, the ratio change being brought about by flyweights that were driven by the engine. At higher engine speeds, high gear was engaged. As the vehicle slowed down and engine RPM decreased, the gearbox would shift back to low. Unfortunately, the metallurgy of the time wasn't up to the task, and owing to the abruptness of the gear change, the transmission would often fail without warning.

The next significant phase in the automatic transmission's development occurred in 1908 with the introduction of Henry Ford's remarkable Model T. The Model T, in addition to being cheap and reliable by the standards of the day, featured a simple, two speed plus reverse planetary transmission whose operation was manually controlled by the driver using pedals. The pedals actuated the transmission's friction elements (bands and clutches) to select the desired gear. In some respects, this type of transmission was less demanding of the driver's skills than the contemporary, unsynchronized manual transmission, but still required that the driver know when to make a shift, as well as how to get the car off to a smooth start.

The first automatic transmission using hydraulic fluid was developed in 1932 by two Brazilian engineers, José Braz Araripe and Fernando Lehly Lemos; Later, the prototype and the project were sold to General Motors, who introduced the technology in the 1940 Oldsmobile model as a "Hydra-Matic" transmission. However, an article published by the Wall Street Journal credits the German auto parts company ZF Friedrichshafen for the invention, which would have occurred shortly after World War I.

In 1934, both REO and General Motors developed semi-automatic transmissions that were less difficult to operate than a fully manual unit. These designs, however, continued to use a clutch to engage the engine with the transmission. The General Motors unit, dubbed the "Automatic Safety Transmission," was notable in that it employed a power-shifting planetary gearbox that was hydraulically controlled and was sensitive to road speed, anticipating future development.

Parallel to the development in the 1930s of an automatically shifting gearbox was Chrysler's work on adapting the fluid coupling to automotive use. Invented early in the 20th century, the fluid coupling was the answer to the question of how to avoid stalling the engine when the vehicle was stopped with the transmission in gear. Chrysler itself never used the fluid coupling with any of its automatic transmissions, but did use it in conjunction with a hybrid manual transmission called "Fluid Drive" (the similar Hy-Drive used a torque converter). These developments in automatic gearbox and fluid coupling technology eventually culminated in the introduction in 1939 of the General Motors Hydra-Matic, the world's first mass-produced automatic transmission.

Available as an option on 1940 Oldsmobiles and later Cadillacs, 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.

The Hydra-Matic was subsequently adopted by Cadillac and Pontiac, and was sold to various other automakers, including Bentley, Hudson, Kaiser, Nash, and Rolls-Royce. It also found use during World War II in some military vehicles. From 1950 to 1954, Lincoln cars were also available with the Hydra-Matic. Mercedes-Benz subsequently devised a four-speed fluid coupling transmission that was similar in principle to the Hydra-Matic, but of a different design.

Interestingly, the original Hydra-Matic incorporated two features which are widely emulated in today's transmissions. The Hydra-Matic's ratio spread through the four gears produced excellent "step-off" and acceleration in first, good spacing of intermediate gears, and the effect of an overdrive in fourth, by virtue of the low numerical rear axle ratio used in the vehicles of the time. In addition, in third and fourth gear, the fluid coupling only handled a portion of the engine's torque, resulting in a high degree of efficiency. In this respect, the transmission's behavior was similar to modern units incorporating a lock-up torque converter.

In 1956, GM introduced the "Jetaway" Hydra-Matic, which was different in design than the older model. Addressing the issue of shift quality, which was an ongoing problem with the original Hydra-Matic, the new transmission utilized two fluid couplings, the primary one that linked the transmission to the engine, and a secondary one that replaced the clutch assembly that controlled the forward gearset in the original. The result was much smoother shifting, especially from first to second gear, but with a loss in efficiency and an increase in complexity. Another innovation for this new style Hydra-Matic was the appearance of a Park position on the selector. The original Hydra-Matic, which continued in production until the mid-1960s, still used the reverse position for parking pawl engagement.

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 converter for additional torque multiplication. In the early 1950s, BorgWarner developed a series of three-speed torque converter automatics for American Motors, 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. The latter was the first to utilize the Simpson compound planetary gearset.

General Motors produced multiple-turbine torque converters from 1954 to 1961. These included the Twin-Turbine Dynaflow and the triple-turbine Turboglide transmissions. The shifting took place in the torque converter, rather than through pressure valves and changes in planetary gear connections. Each turbine was connected to the drive shaft through a different gear train. These phased from one ratio to another according to demand, rather than shifting. The Turboglide actually had two speed ratios in reverse, with one of the turbines rotating backwards.

By the late 1960s, most of the fluid-coupling four-speed 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. 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 pump and turbine together to eliminate slip at cruising speed) to improve fuel economy.

As computerized engine control units (ECUs) became more capable, much of the logic built into the transmission's valve body was offloaded to the ECU. Some manufacturers use a separate computer dedicated to the transmission called a transmission control unit (TCU), also known as the transmission control module (TCM), which shares 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 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 introducing an eight-speed in 2007 on the Lexus LS 460. Derived from the 7G-Tronic, Mercedes-Benz unveiled a semi-automatic transmission with the torque converter replaced with a wet multi clutch called the AMG SPEEDSHIFT MCT. The 2014 Jeep Cherokee has the world's first nine-speed automatic transmission for a passenger vehicle to market.

Hydraulic automatic transmissions

The predominant form of automatic transmission is hydraulic machinery|hydraulically operated; using a fluid coupling or torque converter, and a set of epicyclic gearing|planetary gearsets to provide a range of gear ratios.

Hydraulic automatic transmissions consist of three major components:

Torque converter

A type of fluid coupling, hydraulically connecting the engine to the transmission. This takes the place of a friction clutch in a manual transmission. It transmits and decouples the engine power to the planetary gears, allowing the vehicle to come to stop with the engine still running without Stall (engine)|stalling.

A torque converter differs from a fluid coupling, in that it provides a variable amount of torque multiplication at low engine speeds, increasing breakaway acceleration. A fluid coupling works well when both the impeller and turbine are rotating at similar speeds, but it is very inefficient at initial acceleration, where rotational speeds are very different. This torque multiplication is accomplished with a third member in the coupling assembly known as the stator, which acts to modify the fluid flow depending on the relative rotational speeds of the impeller and turbine. The stator itself does not rotate, but its vanes are so shaped that when the impeller (which is driven by the engine) is rotating at a high speed and the turbine (which receives the transmitted power) is spinning at a low speed, the fluid flow hits the vanes of the turbine in a way that multiplies the torque being applied. This causes the turbine to begin spinning faster as the vehicle accelerates (ideally), and as the relative rotational speeds equalize, the torque multiplication diminishes. Once the impeller and turbine are rotating within 10% of each other's speed, the stator ceases to function and the torque converter acts as a simple fluid coupling.

Planetary gears train

Consisting of planetary gear sets as well as clutches and bands. These are the mechanical systems that provide the various gear ratios, altering the speed of rotation of the output shaft depending on which planetary gears are locked.

To effect gear changes, one of two types of clutches or bands are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate, thereby transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body (see below), their sequence controlled by the transmission's internal programming. Principally, a type of device known as a sprag or roller clutch is used for routine upshifts/downshifts. Operating much as a ratchet, it transmits torque only in one direction, free-wheeling or "overrunning" in the other. The advantage of this type of clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetaries, simply "taking up" the drivetrain load when actuated, and releasing automatically when the next gear's sprag clutch assumes the torque transfer. The bands come into play for manually selected gears, such as low range or reverse, and operate on the planetary drum's circumference. Bands are not applied when drive/overdrive range is selected, the torque being transmitted by the sprag clutches instead. Bands are used for braking; the GM Turbo-Hydramatics incorporated this.

Hydraulic controls

Uses special transmission fluid sent under pressure by an oil pump to control various clutches and bands modifying the speed of the output depending on the vehicle's running condition.

Not to be confused with the impeller inside the torque converter, the pump is typically a gear pump mounted between the torque converter and the planetary gearset. It draws transmission fluid from a sump and pressurizes it, which is needed for transmission components to operate. The input for the 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 and there is enough transmission fluid, but the disadvantage is that when the engine is not running, no oil pressure is available to operate the main components of the transmission, and is thus impossible to push-start a vehicle equipped with an automatic transmission. Early automatic transmissions also had a rear pump for towing purposes, ensuring the lubrication of the rear-end components.

The governor is connected to the output shaft and regulates the hydraulic pressure depending on the vehicle speed. The engine load is monitored either by a throttle cable or a vacuum modulator. The valve body is the hydraulic control center that receives pressurized fluid from the 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 vehicle 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 electronic engine control unit (ECU) or a separate transmission control unit.

The hydraulic & lubricating oil, called automatic transmission fluid (ATF), provides lubrication, corrosion prevention, and a hydraulic medium to convey mechanical power (for the operation of the transmission). 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.

In some modern cars, computers use sensors on the engine to detect throttle position, vehicle speed, engine speed, engine load, etc. to control the exact shift point. The computer transmits the information via solenoids that redirect the fluid the appropriate clutch or servo to control shifting.

Continuously variable transmissions

A fundamentally different type of automatic transmission is the continuously variable transmission, or CVT, which can smoothly and steplessly 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. Some early forms, such as the Hall system (which dates back to 1896<ref>Hydraulic Variable Transmission Mechanism, John William Hall, GB Patents, No 7479(1896), No 22406(1901), No 442(1903) and No 4148(1907). 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 Continuously variable transmission (IVT). These concepts provide zero and reverse gear ratios.

E-CVT

Some 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 motor-generators.

Dual-clutch transmissions

A dual-clutch transmission, or DCT (sometimes referred to as a twin-clutch transmission or double-clutch transmission), is a modern type of semi-automatic transmission and electrohydraulic manual transmission. It uses two separate clutches for odd and even gear sets. It can fundamentally be described as two separate manual transmissions (with their respective clutches) contained within one housing, and working as one unit. They are usually operated in a fully automatic mode, and many also have the ability to allow the driver to manually shift gears in semi-automatic mode, albeit still using the transmission's electro-hydraulics.

Automatic transmission modes

Conventionally, in order to select the transmission operating mode, the driver moves a selection lever located either on the steering column or on the floor (as with a manual on the floor, except that automatic selectors on the floor do not move in the same type of pattern as manual levers do). In order to select modes, or to manually select specific gear ratios, the driver must push a button in (called the shift-lock button) or pull the handle (only on column mounted shifters) out. Some vehicles 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). Previously, quadrant-selected automatic transmissions often used a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to driver error causing unintentional gear selection, as well as the danger of having a selector (when worn) jump into reverse from low gear during engine braking maneuvers.

Depending on the model and make of the transmission, these controls can take several forms. However most include the following:

Park (P)

This selection mechanically locks the output shaft of transmission, restricting the vehicle from moving in any direction. A parking pawl prevents the transmission from rotating, and therefore the vehicle from moving. However, the vehicle's non-driven wheels are still free to rotate, and the driven wheels may still rotate individually (because of the differential). For this reason, it is recommended to use the hand brake (parking brake) because this actually locks (in most cases) the wheels and prevents them from moving. It is typical of front-wheel-drive vehicles for the parking brake to lock the rear (non-driving) wheels, so use of both the parking brake and the transmission park lock provides the greatest security against unintended movement on slopes. This also increases the life of the transmission and the park pin mechanism, because parking on an incline with the transmission in park without the parking brake engaged will cause undue stress on the parking pin, and may even prevent the pin from releasing. A hand brake should also prevent the car from moving if a worn selector accidentally drops into reverse gear while idling.

A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage. Usually, Park (P) is one of only two selections in which the car's engine can be started, the other being Neutral (N). This is typically achieved via a normally open inhibitor switch (sometimes called a "neutral safety switch") wired in series with the starter motor engagement circuit, which is closed when P or N is selected, completing the circuit (when the key is turned to the start position). In many modern cars and trucks, the driver must have the foot brake applied before the transmission can be taken out of park. The Park position is omitted on buses/coaches (and some road tractors) with automatic transmission (on which a parking pawl is not practical), which must instead be placed in neutral with the air-operated parking brakes set.

Reverse (R)

This engages reverse gear within the transmission, permitting the vehicle to be driven backward, and operates a switch to turn on the white backup lights for improved visibility (the switch may also activate a beeper on delivery trucks or other large vehicles to audibly warn other drivers and nearby pedestrians of the driver's reverse movement). To select reverse in most transmissions, the driver must come to a complete stop, depress the shift-lock button (or move the shift lever toward the driver in a column shifter, or move the shifter sideways along a notched channel in a console shifter) and select reverse. The driver should avoid engaging reverse while the vehicle is moving forwards, and likewise avoid engaging any forward gear while traveling backwards. On transmissions with a torque converter, doing so at very low speed (walking pace) is not harmful, but causes unnecessary wear on clutches and bands, and a sudden deceleration that not only is uncomfortable, but also uncontrollable since the brakes and the throttle contribute in the same direction. This sudden acceleration, or jerk, can still be felt when engaging the gear at standstill, but the driver normally suppresses this by holding the brakes. Traveling slowly in the right direction while engaging the gear minimizes the jerk further, which is actually beneficial to the wearing parts of the transmission. Electronically controlled transmissions may behave differently, as engaging a gear at speed is essentially undefined behavior. Some modern transmissions have a safety mechanism that will resist putting the car in reverse when the vehicle is moving forward; such a mechanism may consist of a solenoid-controlled physical barrier on either side of the reverse position, electronically engaged by a switch on the brake pedal, so that the brake pedal needs to be depressed in order to allow the selection of reverse. Some electronic transmissions prevent or delay engagement of reverse gear altogether while the car is moving.

Some shifters with a shift button allow the driver to freely move the shifter from R to N or D without actually depressing the button. However, the driver cannot shift back to R without depressing the shift button, to prevent accidental shifting which could damage the transmission, especially at high speeds.

Neutral / No gear (N)

This disengages all gear trains within the transmission, effectively disconnecting the transmission from the driven wheels, allowing the vehicle to coast freely under its own weight and gain momentum without the motive force from the engine. Coasting in idle down long grades (where law permits) should be avoided, though, with many transmission designs as the transmission's lubrication pump is commonly driven by the input (engine) side, which may not provide sufficient fluid flow at engine idle speed for high-speed travel. Similarly, emergency towing (with the driven wheels of the disabled and non-running vehicle on the ground) with an automatic transmission in neutral is not permitted by the manufacturer for many vehicles. Manufacturers understand emergency situations and list limitations of towing a vehicle in neutral (usually not to exceed 55 mph and 50 miles). This is the only other selection in which the vehicle's engine may be started.

Drive (D)

This position allows the transmission to engage the full range of available forward gear ratios, allowing the vehicle to move forward and accelerate through its range of gears. The number of gear ratios within the transmission depends on the model, but three was initially predominant. In the 1990s four and five speeds became common. Six-speed automatic transmissions were probably the most common offering in cars and trucks from about 2010, and were still common (especially in older or less expensive models of vehicles) in 2017. However, seven-speed automatics had become available in some high-performance production luxury cars (found in Mercedes-Benz|Mercedes 7G-Tronic gearbox, Infiniti), as are eight-speed autos in models from 2006 introduced by Aisin Seiki Co. in Lexus, ZF, Hyundai Motor Company and General Motors. From 2013 are available nine speeds transmissions produced by ZF and Mercedes 9G. In the 2017 model year Ford and General Motors introduced a 10-speed transmission.

Overdrive ('D', 'OD', or a boxed [D] or the absence of an illuminated 'O/D OFF')

This mode is used in some transmissions 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 (Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 35-45 mph. Under hard acceleration or below 35-45 mph, the transmission will automatically downshift. Other vehicles with this selector (for example light trucks) will not only disable up-shift to the overdrive gear, but keep the remaining gears available for use of engine braking. Drivers should verify the behavior of this switch and consider the benefits of reduced friction brake use when city driving where speeds typically do not necessitate the overdrive gear.

Most automatic transmissions include some means of forcing a downshift (Throttle kickdown) into the lowest possible gear ratio if the throttle pedal is fully depressed. In many older designs, kickdown is accomplished by mechanically actuating a valve inside the transmission. Most modern designs use a solenoid-operated valve that is triggered by a switch on the throttle linkage or by the engine control unit (ECU) in response to an abrupt increase in engine power.

Mode selection allows the driver to choose between preset shifting programs. For example, Economy mode saves fuel by upshifting at lower engine speeds, while Sport mode (aka "Power" or "Performance") delays upshifting for maximum acceleration. Some transmission units also have Winter mode, where higher gear ratios are chosen to keep revs as low as possible while on slippery surfaces. The modes also change how the computer responds to throttle input.

Conventionally, automatic transmissions have 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 by firmware. 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.

Third (3)

This mode limits the transmission to the first three gear ratios, or sometimes locks the transmission in third gear. This can be used to climb or going down hill. Some vehicles will automatically shift up out of third gear in this mode if a certain revolutions per minute (RPM) range is reached in order to prevent engine damage. This gear is also recommended while towing a trailer.

Second (2 or S)

This mode limits the transmission to the first two gear ratios, or locks the transmission in second gear on Ford, Kia, and Honda models. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in winter. It is usually recommended to use second gear for starting on snow and ice, and use of this position enables this with an automatic transmission. Some vehicles will automatically shift up out of second gear in this mode if a certain RPM range is reached in order to prevent engine damage.

Although traditionally considered second gear, there are other names used. Chrysler models with a three-speed automatic since the late 1980s have called this gear 3 while using the traditional names for Drive and Low. Oldsmobile has called second gear as the 'Super' range — which was first used on their 4-speed Hydramatic transmissions, although the use of this term continued until the early 1980s when GM's Turbo Hydramatic automatic transmissions were standardized by all of their divisions years after the 4-speed Hydramatic was discontinued.

Some automatics, particularly those fitted to larger capacity or high torque engines, either when "2" is manually selected, or by engaging a winter mode, will start off in second gear instead of first, and then not shift into a higher gear until returned to "D." Also note that as with most American automatic transmissions, selecting "2" using the selection lever will not tell the transmission to be in only 2nd gear; rather, it will simply limit the transmission to 2nd gear after prolonging the duration of 1st gear through higher speeds than normal operation. The 2000–2002 Lincoln LS V8 (the five-speed automatic without manumatic capabilities, as opposed to the optional sport package w/ manu-matic 5-speed) started in 2nd gear during most starts both in winter and other seasons by selecting the "D5" transmission selection notch in the shiftgate (for fuel savings), whereas "D4" would always start in 1st gear. This is done to reduce torque multiplication when proceeding forward from a standstill in conditions where traction was limited — on snow- or ice-covered roads, for example.

First (1 or L [Low])

This mode locks the transmission in first gear only. In older vehicles, it will not change to any other gear range. Some vehicles will automatically shift up out of first gear in this mode if a certain RPM range is reached in order to prevent engine damage. This, like second, can be used during the winter season, for towing, or for downhill driving to increase the engine braking effect. The "Austin Mini" automatic transmission is different in this respect - This mode locks the transmission in first gear, but the gearbox has a freewheel on the overrun. Closing the throttle after acceleration results in the vehicle continuing at the same speed and only slowing down due to friction and wind resistance. During this time, the engine RPM will drop back to idle until the throttle is pressed again. What this means is that in "First", engine braking is not available and "2" is the lowest gear that should be used whilst descending hills. The Mini's (and the 1100/1300's) 4-speed Automatic transmission was unusual in that it allowed manual selection of all forward gears, enabling the driver to "take off" from a standstill in any of the four ratios. It also provided no "Park" position.

Manual controls

Some transmissions have a mode in which the driver has full control of ratios change (either by moving the selector, or through the use of buttons or paddles), completely overriding the automated function of 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. Since these gearboxes also have a throttle kickdown switch, it is impossible to fully exploit the engine power at low to medium engine speeds.

Manufacturer-specific modes

As well as the above modes there are other modes, dependent on the manufacturer and model. Some examples include:

D5

In Hondas and Acuras equipped with five-speed automatic transmissions, this mode is used commonly for highway use (as stated in the manual), and uses all five forward gear ratios.

D4

This mode is also found in Honda and Acura four or five-speed automatics, and only uses the first four gear ratios. According to the manual, it is used for stop-and-go traffic, such as city driving.

D3 or 3

This mode is found in Honda, Acura, Volkswagen and Pontiac four-speed automatics and only uses the first three gear ratios. According to the manual, it is used for stop-and-go traffic, such as city driving.

D2 and D1

These modes are found on older Ford transmissions (C6, etc.). In D1, all three gears are used, whereas in D2 the car starts in second gear and upshifts to third.

S or Sport

This is commonly described as Sport mode. It operates in an identical manner as "D" mode, except that the upshifts change much higher up the engine's rev range. This has the effect on maximising all the available engine output, and therefore enhances the performance of the vehicle, particularly during acceleration. This mode will also downchange much higher up the rev range compared to "D" mode, maximising the effects of engine braking. This mode will have a detrimental effect on fuel economy. Hyundai has a Norm/Power switch next to the gearshift for this purpose on the Tiburon.

Some early GMs equipped with HYDRA-MATIC transmissions used (S) to indicate Second gear, being the same as the 2 position on a Chrysler, shifting between only first and second gears. This would have been recommended for use on steep grades, or slippery roads like dirt, or ice, and limited to speeds under 40 mph. (L) was used in some early GMs to indicate (L)ow gear, being the same as the 2 position on a Chrysler, locking the transmission into first gear. This would have been recommended for use on steep grades, or slippery roads like dirt, or ice, and limited to speeds under 15 mph.

+ −, and M

This is for the Manual mode selection of gears in certain automatics, such as Porsche and Honda's Tiptronic and BMW and Kia's Steptronic. The M feature can also be found in vehicles such as the Dodge Magnum and Journey; Pontiac G6; Mazda3, Mazda6, and Mazda CX-7; Toyota Camry, Toyota Corolla, Toyota Fortuner, Toyota Previa and Toyota Innova; Kia Forte (K3/Cerato), Kia Optima (K5), Kia Cadenza (K7) and Kia K9 (Quoris). Mitsubishi Montero Sport / Pajero sport and some Audi models (Audi TT) do not have the M, and instead have the + and -, which is separated from the rest of the shift modes; the same is true for some Peugeot products like the Peugeot 206. Meanwhile, the driver can shift up and down at will by toggling the (console mounted) shift lever similar to a semi-automatic transmission. This mode may be engaged either through a selector/position or by actually changing the gears (e.g., tipping the gear-down paddles mounted near the driver's fingers on the steering wheel).

Winter (W)

In some Volvo, Mercedes-Benz, BMW and General Motors models, a winter mode can be engaged so that second gear is selected instead of first when pulling away from stationary, to reduce the likelihood of loss of traction due to wheel spin on snow or ice. On GM cars, this was D2 in the 1950s, Second Gear Start after 1990, and Snow/Ice mode in the 2010s. On Ford, Kia, and Honda automatics, this feature can be accessed by moving the gear selector to 2 to start, then taking one's foot off the accelerator while selecting D once the vehicle is moving.

Brake (B)

A mode selectable on some Toyota models, as well as electric cars from several manufacturers. It can be used to decelerate, or maintain speed going downhill, without using the conventional brakes. In non-hybrid cars, B mode selects a lower gear to increase engine braking. GM called this "HR" ("hill retarder") and "GR" ("grade retarder") in the 1950s. In hybrids such as the Toyota Prius, which have a fixed gear ratio, B mode slows the car in part by increasing engine air intake, which enhances engine braking. In electric cars such as the Nissan Leaf and Mitsubishi i-MiEV, B mode increases the level of regenerative braking when the accelerator pedal is released.

Some automatic transmissions modified or designed specifically for drag racing may also incorporate a transbrake as part of a manual valve body. Activated by electrical solenoid control, a transbrake 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 transbrake switch.

Comparison with manual transmission

Most cars sold in North America since the 1950s have been available with an automatic transmission, based on the fact that the three major American car manufacturers had started using automatics. Conversely, in Europe a manual gearbox is standard, with only 20% of drivers opting for an automatic gearbox compared with the United States.In some Asian markets and in Australia, automatic transmissions have become very popular since the 1980s.

Vehicles equipped with automatic transmissions are not as complex to drive. Consequently, in some jurisdictions, drivers who have passed their driving test in a vehicle with an automatic transmission will not be licensed to drive a manual transmission vehicle. Conversely, a manual license will allow the driver to drive vehicles with either an automatic or manual transmission. Countries in which such driving license restrictions are applied include some states in Australia, Austria, Belgium, Belize, China, Croatia, Denmark, Dominican Republic, Estonia, Finland, France, Germany, Hong Kong, Hungary, India, Indonesia, Ireland, Israel, Japan, Latvia, Lebanon, Lithuania, Macau, Malaysia, Mauritius, the Netherlands, New Zealand (restricted license only), Norway, Philippines, Poland, Portugal, Qatar, Romania, Russia (as of April 2014), Saudi Arabia (as of March 2012), Singapore, Slovenia, South Africa, South Korea, Spain, Sweden, Switzerland, Taiwan, Trinidad and Tobago, United Arab Emirates and the United Kingdom.

A conventional manual transmission is frequently the base equipment in a car, with the option being an automated transmission such as a conventional automatic, semi-automatic, or CVT.

Effects on vehicle control

Cornering

Unexpected gear changes can affect the attitude of a vehicle in marginal conditions.

Maintaining constant speed

Torque converters and CVT transmissions make changes in vehicle speed less apparent by the engine noise, as they decouple the engine speed from vehicle speed.

Lockup torque converters that engage and disengage at certain speeds can make these speeds unstable — the transmission wastes less power above the speed at which the torque converter locks up, usually causing more power to the wheels for the same throttle input.

Controlling wheelspin

Torque converters respond quickly to loss of traction (torque) by an increased speed of the driving wheels for the same engine speed. Thus, under most conditions, where the static friction is higher than the kinetic friction, the engine speed must be brought down to counteract wheelspin when it has occurred, requiring a stronger or quicker throttle reduction by the driver than with a manual transmission, making wheelspin harder to control. This is most apparent in driving conditions with much higher static friction than kinetic, such as packed hard snow (that turns to ice by friction work), or snow on top of ice.

Climbing steep slippery slopes

In situations where a driver with a manual transmission can't afford a gearshift, in fear of losing too much speed to reach a hilltop, automatic transmissions are at a great advantage — whereas driving a manual car depends on finding a gear that is not too low to enter the bottom of the hill at the necessary speed, but not too high to stall the engine at the top of the hill, sometimes an impossible task, this is not an issue with automatic transmissions, not just because gearshifts are quick, but they typically maintain some power on the driving wheels during the gear change.

Energy efficiency

Earlier hydraulic automatic transmissions were almost always less energy efficient than manual transmissions due mainly to viscous and pumping losses, both in the torque converter and the hydraulic actuators. 21% is the loss on a 3 speed Chrysler Torqueflite compared to a modern GM 6L80 automatic. A relatively small amount of energy is required to pressurise the hydraulic control system, which uses fluid pressure to determine the correct shifting patterns and operate the various automatic clutch mechanisms. However, with technological developments some modern Continuously variable transmission are more fuel efficient than their manual counterparts and modern 8-speed automatics are within 5% as efficient as a manual gearbox.

Manual transmissions use a mechanical clutch to transmit torque, rather than a torque converter, thus avoiding the primary source of loss in an automatic transmission. Manual transmissions also avoid the power requirement of the hydraulic control system, by relying on the human muscle power of the vehicle operator to disengage the clutch and actuate the gear levers, and the mental power of the operator to make appropriate gear ratio selections. Thus the manual transmission requires very little engine power to function, with the main power consumption due to drag from the gear train being immersed in the lubricating oil of the gearbox.

The on-road acceleration of an automatic transmission can occasionally exceed that of an otherwise identical vehicle equipped with a manual transmission in turbocharged diesel applications. Turbo-boost is normally lost between gear changes in a manual whereas in an automatic the accelerator pedal can remain fully depressed. This however, is still largely dependent upon the number and optimal spacing of gear ratios for each unit, and whether or not the elimination of spooldown/accelerator lift off represent a significant enough gain to counter the slightly higher power consumption of the automatic transmission itself.

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References

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