Most automatic transmissions have a set selection of possible gear ranges, often with a parking brake feature that will lock the output shaft of the transmission.
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2 Continuously variable 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.
A hydraulic automatic transmission consists of the following parts:
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 gas mileage 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 it 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 (or CVTs) operate on the theory of pulleys; a large pulley connected to a smaller pulley with a belt or chain will operate in the same manner as a large gear meshing with a smaller gear. CVTs have two cone-shaped pulleys in them (one on the input, one on the output), which can be moved in and out. Moving the cones has the effect of making the driving and driven pulleys change size, thus changing the gear ratio; since there are no predefined steps in the system, it's referred to as continuously variable.
CVTs have much smoother operation than hydraulic automatics, are simpler to build and repair, and provide better fuel economy by avoiding torque converter slippage; however, their torque handling capability is limited by the strength of the belt or chain inside them, and so CVTs have typically been limited to small cars and other light-duty applications. Many small tractors meant for home and garden use have simple CVTs in them as well.
The first workable CVT, called VARIOMATIC, was designed and built by Huib van Doorne, co-founder of DAF, in the late 1950s, specifically to produce an automatic transmission for a small, affordable car. The first DAF car using van Doorne's CVT was produced in 1958. Van Doorne's patents were later sold to Volvo along with DAF's car business.
CVT transmissions have been refined over the years and are much improved from their origins. Possibly the largest vehicle currently sold with a CVT is the Nissan Murano, a mid-size sport utility vehicle with a V6 engine sold in the North American market.Hydraulic automatic transmissions
Parts and operation
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. They have usually been extra-cost options on most cars for this reason.History and improvements
Continuously variable transmissions