HYDRAULIC AUTOMATIC TRANSMISSIONS

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

Parts and operation

A hydraulic automatic transmission consists of the following parts: 

Torque converter: A type of fluid coupling, hydraulically connecting the engine to the transmission. It takes the place of a mechanical clutch, allowing the transmission to stay in gear and the engine to remain running while the vehicle is stationary, without 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. This is accomplished with a third member in the coupling assembly known as the stator, and by altering the shapes of the vanes inside the coupling in such a way as to curve the fluid's path into the stator. 

The stator captures the kinetic energy of the transmission fluid, in effect using the leftover force of it to enhance torque multiplication.

Pump not to be confused with the impeller inside the torque converter, 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 flywheel, so the pump provides pressure whenever the engine is running and there is enough transmission fluid.

Planetary gearset: A compound epicyclic planetary gearset, whose bands and clutches are actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.
Clutches and bands: 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.[citation needed].
Valve body: 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 (TCU). (See History and improvements below.)
Hydraulic & lubricating oil: called automatic transmission fluid (ATF), this component of the transmission 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.

AUTOMATIC TRANSMISSION - OTHER MODES

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 gears.

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 Torqueflite 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 aChrysler, 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 asPorsche's Tiptronic. 

The M feature can also be found in Chrysler and General Motors products such as the Dodge Magnum, Journey, and Pontiac G6, as well as Toyota's Camry, Corolla, Fortuner, Previa and Innova. 

Mitsubishi and some Audi models (Audi TT), meanwhile 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 Peugeot 206. 

Meanwhile, the driver can shift up and down at will by toggling the (console mounted) shift lever like 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 Europe 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 wheelspin on snow or ice. 

On GM cars, this was D2 in the 1950s, and is Second Gear Start after 1990.

On Ford, Kia, and Honda automatics, this feature can be accessed by moving the gear selector to 2 to start, then taking your foot off the accelerator while selecting D once the car is moving.

Brake (B)

A mode selectable on some Toyota models. In non-hybrid cars, this mode lets the engine do compression braking, also known as engine braking, typically when encountering a steep downhill. 

Instead of engaging the brakes, the engine in a non-hybrid car switches to a lower gear and slows down the spinning tires.

The engine holds the car back, instead of the brakes slowing it down. 

GM called this "HR" ("hill retarder") and "GR" ("grade retarder") in the 1950s. For hybrid cars, this mode converts the electric motor into a generator for the battery. 

It is not the same as downshifting in a non-hybrid car, but it has the same effect in slowing the car without using the brakes.

AUTOMATIC TRANSMISSION - MODE 1 or LOW (FIRST)

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.

AUTOMATIC TRANSMISSION - MODE 2 or S(SECOND)

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 the winter time. 

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.

AUTOMATIC TRANSMISSION - MODE 3(THIRD)

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 RPM range is reached in order to prevent engine damage. 


This gear is also recommended while towing a caravan.

AUTOMATIC TRANSMISSION MODE - Overdrive (D, OD, or a boxed [D])

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 (56–72 km/h).


Under hard acceleration or below 35–45 mph (56–72 km/h), the transmission will automatically downshift.


Vehicles with this option should be driven in this mode unless circumstances require a lower gear.

AUTOMATIC TRANSMISSION - DRIVE(D)

This position allows the transmission to engage the full range of available forward gear trains, 


allows the vehicle to move forward and accelerate through its range of gears. 


The number of gear ratios a transmission has depends on the model, but they initially ranged from three (predominant before the 1990s), to four and five speeds (losing popularity to six-speed autos, though still favored by Chrysler and Honda/Acura)


Six-speed automatic transmissions are now probably the most common offering Toyota Camry V6 models, the Chevrolet Malibu LTZ, Corvette, GM trucks, Pontiac G8, Ford Falcon BF 2005-2007 and Falcon FG 2008 - current in Australia with 6 speed ZF, and most newer model Ford/Lincoln/Mercury vehicles).


seven-speed autos are becoming available (found in Mercedes 7G gearbox, Infiniti, VW Group DSG, as are eight-speed autos in the newer models of Lexus, BMW ZF 8HP and VW, cars (VW Touareg).