automatic

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

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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.

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

Conventionally, in order to select the mode, the driver would have to move a gear shift lever located on the steering column or on the floor next to him. 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. Some vehicles (like the Aston Martin DB9) 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).

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. A car should be allowed to come to a complete stop before setting transmission into park to prevent damage. Park is one of only two selections in which the car can be started. In some cars (notably those sold in the US), the driver must have the footbrake depressed before the transmission can be taken out of park.

Read more at Wikipedia.org