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Revision as of 15:09, 19 July 2004
An automatic transmission is an automobile gearbox that can change gear ratios automatically as the car moves, thus freeing the driver from shifting gears manually.
Most cars sold in the United States since the 1950s have had automatic transmissions. This has, however, not been the case in Europe and much of the rest of the world. Automatic transmissions, particularly earlier ones, reduce fuel economy and power. Where fuel is expensive and, thus, engines generally smaller, these penalties were more burdensome. Automatic transmissions have significantly improved fuel efficiency in recent years.
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.
Hydraulic automatic transmissions
The predominant form of automatic transmission today is the hydraulic automatic transmission. This design uses hydraulic pressure to control a set of planetary gears using a series of clutches and bands.
Parts and operation
A hydraulic automatic transmission consists of the following parts:
- Torque converter This device fits between the engine and the transmission, providing a torque boost at low speeds and during acceleration, while leaving the two loosely coupled at rest; this allows the engine and transmission to be mated full-time without the need for a clutch, consequently the car has only two pedals. The torque converter provides an amount of continuously variable gearing between the fixed gear ratios.
- Planetary gearset The main gears in most hydraulic automatics are a compound planetary set. Gear ratios are selected by a system of bands and clutches, which are actuated by hydraulic servos controlled by the valve body.
- Valve body This component is the center of the system. The valve body receives pressurised fluid from a main pump connected to the transmission's input. The pressure coming from this pump is regulated and runs a network of spring-loaded valves, check balls and servo pistons. The valves use the pump pressure and the pressure from a 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 multitude of parts, and the complex design of the valve body make hydraulic automatic transmissions much more complicated (and expensive) to build and repair than manual transmissions. Except in cars for the USA market, they have usually been extra-cost options for this reason.
History and improvements
Oldsmobile's 1940 models featured Hydra-Matic drive, making this lineup the first vehicles with fully automatic transmission.
Hydra-Matic appeared as an Olds exclusive. It provided true clutchless driving with four forward speeds. Its fluid coupling between engine and transmission eliminated the clutch and its associated foot work. Olds made the Hydra-Matic available on all models for only an extra $57. In the early 50s, Olds produced its millionth Oldsmobile with automatic transmission, demonstrating Hydra-Matic's rapid rise in popularity.
The first hydraulic automatics were introduced by General Motors, Chrysler and Borg-Warner (who produced transmissions for Ford) in the early 1950s. These early models only provided 2 forward speeds, and were not able to handle much torque at first, but 3 speed models followed quickly.
In the 1970s and 1980s, the first big changes to hydraulic automatic transmission designs in years came. The first was the addition of an overdrive capability; fuel economy had become a big concern, and the addition of an overdrive helped increase fuel efficiency considerably on long cruises.
The second was the torque converter clutch or TCC. This concept first appeared in the mid-1980s with the advent of engine computers, and involved a solenoid-controlled clutch inside the torque converter, which would lock its input to its output when activated. The idea was to eliminate the drag or inefficiency caused by the fluid in the converter when operating at high speeds. The TCC was a replacement for the unwieldy dual input shaft system used on some older 4-speed automatics, notably Ford's AOD model.
As the engine computers became more and more capable, even more of the valve body's functionality was offloaded to them. The newest hydraulic automatics remove almost all of the control logic from the valve body, and place it in the hands of the engine computer. In this case, solenoids turned on and off by the computer control shift patterns and gear ratios, rather than the spring-loaded valves in the valve body. This allows for more precise control of shift points and shift quality, and (on some newer cars) also allows semi-automatic control, where the driver tells the computer when to shift.
Continuously variable transmissions
A different type of automatic transmission is the continuously variable transmission or CVT, which can smoothly alter its gear ratio by varying the distance between a pair of belt or chain-linked pulleys, wheels or cones.
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