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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 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 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

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 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

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 opposing, cone shaped drums in them (one on the input, one on the output side), which can be moved in and out, controled by the engine vacuum in the inlet manifold and engine RPM and centrifugal force acting on weights inside the conical drums. Moving the cones in and out has the effect of changing the driving and driven pulleys' diameter, thus changing the gear ratio. As 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 losses; however, their torque handling capability is limited by the strength of the belt or chain, and so CVTs have typically been limited to low powered cars and other light duty applications. Many small tractors for home and garden use have simple CVTs.

The first workable CVT, called VARIOMATIC, was designed and built by dutch 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. The CVT is also available in Honda and Mini Cooper cars.

Almost all motor scooters today are equipped with CVT.