Automatic Transmission

Images for Automatic Transmission

 automatic gearbox

 automatic gearbox

 automatic gearbox

 automatic gearbox

An automatic (also called automatic gearbox, or auto transmission) is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually. Most automatic transmissions have a defined set of gear ranges, often with a parking pawl feature that locks the output shaft of the transmission.

Besides automatics, there are also other types of automated transmissions such as continuous variable transmissions (CVTs) 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, automatic transmissions differ significantly in internal operation and driver's feel from semi-automatics and CVTs. An automatic uses a torque converter instead of clutch to manage the connection between the transmission gearing and the engine. In contrast, a CVT uses a belt or other torque transmission schema 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.

 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. 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), and continuous variable transmissions (CVTs) (such as Lineartronic).

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 most automatic selectors on the floor don't move in the same type of pattern as a manual lever; most automatic levers only move vertically). 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). Prior to this, 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.

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, although the vehicle's non-driven roadwheels may still rotate freely. For this reason, it is recommended to use the hand brake (or parking brake) because this actually locks (in most cases) the rear wheels and prevents them from moving. 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. An efficiently adjusted hand brake should also prevent the car from moving if a worn selector accidentally drops into reverse gear during early morning fast-idle engine warm-upsIt should be noted that locking the transmission output shaft using park does not positively lock the driving wheels. If one driving wheel has little vertical load it will tend to slip, and will rotate in the opposite direction to the more heavily loaded non-slipping wheel.  

Reverse (R)

 This engages reverse gear within the transmission, permitting the vehicle to be driven backwards. In order for the driver to select reverse in modern transmissions, they must come to a complete stop, push the shift lock button in (or pull the shift lever forward in the case of a column shifter) and select reverse. Not coming to a complete stop can cause severe damage to the transmission.

Neutral/No gear (N)

  This disengages all gear trains within the transmission, effectively disconnecting the transmission from the driven roadwheels, so the vehicle is able to move freely under its own weight and gain momentum without the motive force from the engine (engine braking). This is the only other selection in which the vehicle's engine can be started.

Drive (D)

This position allows the transmission to engage the full range of available forward gear trains, and therefore 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 probably the most common offering in cars from 2010 in cars like Toyota Camry V6 models, the newer GM cars and trucks, Ford cars and trucks. 

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.

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.

Manumatic

Manumatic is a portmanteau of the words manual and automatic, that applies to a class of automotive transmissions.

Manumatic refers to an automatic transmission that allows convenient driver control of gear selection. This type of transmission was introduced in the 1990s. For most of automotive history, automatic transmissions already allowed some control of gear selection using the console or column shifter. Manumatics enhanced this feature by providing either steering wheel mounted paddle shifters or a modified shift lever for more convenient operation. Different car manufacturers have been using a variety of labels for their manumatic transmissions, such as 'Tiptronic', 'Geartronic', 'Touchshift', 'Sporttronic', 'clutchless-manual' and others.

A manumatic differs from a semi-automatic transmission in its method of power transfer from the engine to the transmission. A manumatic uses a torque converter, like a traditional automatic transmission, while a semi-automatic transmission uses a clutch (or multiple clutches), like a traditional manual transmission. Therefore, a semi-automatic transmission offers a more direct connection between the engine and wheels than a manumatic and is preferred in high performance driving applications. A manumatic is often preferred for street use because its fluid coupling makes it easier for the transmission to consistently perform smooth shifts. Some manumatic and semi-automatic transmissions allow the driver to have full control of gear selection, while many will intervene by shifting automatically at the low end and/or high end of the engine's operating range, depending on throttle position. Manumatics and most semi-automatic transmissions also provide the option of operating in the same manner as a conventional automatic transmission by allowing the transmission's computer to select gear changes.