Automotive Automatic Transmission

Automatic Transmission

An automatic transmission is an automobile gearbox that can change gear ratios automatically as the car or truck moves, thus freeing the driver from having to shift gears manually. (Similar but larger devices are also used for railroad locomotives.)

Most cars sold in the United States since the 1950s have been equipped with an automatic transmission. This has, however, not been the case in Europe and much of the rest of the world. Automatic transmissions, particularly earlier ones, reduce fuel efficiency and power. Where fuel is expensive and, thus, engines generally smaller, these penalties are more burdensome. In recent years, automatic transmissions have significantly improved in their ability to support high fuel efficiency but manual transmissions are still generally more efficient. (This balance may finally shift with the introduction of practical continuously variable transmissions; see below.)

Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission.

However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.

Recently manufacturers have begun to make continuously variable transmissions commonly available (earlier models such as the Subaru Justy did not popularize CVT). These designs can change the ratios over a range rather than between set gear ratios. Even though CVTs have been used for decades in a few vehicles (e.g. DAF saloons and the Volvo 340 series that succeeded them, and later the Subaru Justy), the technology has recently gained greater acceptance among manufacturers and customers.

Automatic transmission modes

In order to select the mode, the driver must move a gear shift lever which can be located on the steering column or on the floor next to the driver. In order to select gears/modes the driver must push a button in (called the shift lock button) or pull the handle (only on column mounted shifters) out.

Automatic Transmissions have various modes depending on the model and make of the transmission. Some of the common modes are:

Park (P) – This selection mechanically locks the transmission, restricting the car from moving in any direction. A pin prevents the transmission from moving forward (although wheels, depending on the drive train, can still spin freely), it is recommended to use the hand brake (or emergency brake) because this actually locks the wheels and prevents them from moving, and increases the life of the transmission and the park mechanism. In order for the car to be moved out of park, the driver must depress the brake fully, same goes for putting it into park. The driver also must come to a complete stop before putting it into park to prevent damage to the transmission. This is only one of two selections in which the car can be started.

Reverse (R) – This puts the car into the reverse gear, giving the ability for the car to back up. In order for the driver to select reverse they must come to a complete stop, and push the shift lock button in and select reverse. Not coming to a complete stop can cause severe damage to the transmission.

Neutral (N)– This disconnects the transmission from the wheels so the car can move freely under its own weight. This is the only other selection in which the car can be started.

Drive (D)– This allows the car to move forward and accelerate through a range of gears. The number of gears a transmission has depends on the model, but they can commonly range from 3, 4 (the most common), 5, 6 (found in VW/Audi Direct Shift Gearbox), and 8 in the new model of Lexus cars.

* D4 – In Honda and Acura automatics this mode is used commonly for highway use (as stated in the manual) and uses all 4 forward gears.
* D3 – This is also found in Honda and Acura automatics and only uses the first 3 gears and according to the manual it is used for stop & go traffic such as city driving.
* + - and M – This is the manual selection of gears for automatics with Tiptronic. The driver can shift up and down at their will.

Second (2 or S) – This mode limits the transmission to the first two gears, or more commonly locks the transmission in second gear. 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.

First (1 or L) – This mode locks the transmission in first gear only. It will not accelerate through any gear range. This, like second, can be used during the winter season, or towing.

Some cars when put into D will automatically lock the doors or turn on the daytime running lights.

For More Information, Please Visit: http://en.wikipedia.org/wiki/Automatic_transmission