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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.



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 available. These designs can change the ratios over a range rather than between set gear ratios. Even though prototypes for CVT have been around for decades, it is just now reaching commercial practicability.


Parts and operation

A hydraulic automatic transmission consists of the following parts:

  • Fluid coupling or Torque converter: A hydraulic device connecting the engine and the transmission. It takes the place of a mechanical clutch, allowing the engine to remain running at rest without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.
  •  Planetary gearset: A compound planetary set whose bands and clutches are actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.
  • Valve body: A hydraulic control center that receives pressurised fluid from a main pump operated by the fluid coupling/torque converter. The pressure coming from this pump is regulated 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 car 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 Engine Management System or a separate transmission controller. (See History and improvements below.)

The multitude of parts, and 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.


All text of this article available under the terms of the GNU Free Documentation License (see Copyrights for details).

  
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