Science Fair Project Encyclopedia
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, see here.)
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 efficiency and power. Where fuel is expensive and, thus, engines generally smaller, these penalties were 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.)
Learning to drive a car with manual transmission is harder, but many people find it more pleasurable after that initial learning stage.
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 use conical adjustable sheaves to change the ratios over a range rather than between set gear ratios. Even though this design has been around in prototype form for decades, it is just now reaching commercially practicability.
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 brake 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 a small amount of continuously variable power transmission 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 brake bands and clutches, which are actuated by hydraulic servos controlled by the valve body.
- Valve body This component is the control 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 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 microchip. (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.
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.
The second was the torque converter clutch (TCC or Lockup torque converter). This concept first appeared in the early 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 an effort to improve fuel economy with a relatively minor modification to the transmission. (A related development was the dual-input (one shaft mechanically driven and one hydraulically driven) in the Ford AOD transmission.)
As the engine computers became more and more capable, even more of the valve body's functionality was offloaded to them. These transmissions, introduced in the late 1980s and early 1990s, remove almost all of the control logic from the valve body, and place it in into the engine computer. (Some manufacturers use a separate computer dedicated to the transmission but sharing information with the engine management 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.
Automatic Transmission Models
Some of the best known automatic transmission families include:
- General Motors: Powerglide, Turbo-Hydramatic 350 and 400, 700R4, 200-4R
- Ford: Cruise-o-matic, FMX, C4, C6, AOD, E4OD, ATX, AXOD
- Chrysler: Torqueflite 727 and 904
Automatic transmission families are usually based on Ravigneaux , Lepelletier , or Simpson planetary gearsets. Each uses some arrangement of one or two central sun gears, and a ring gear, with differing arrangements of planet gears that surround the sun and mesh with the ring.
Lepelletier's concept is shown on patent US5370589|.
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 diameter of a pair of belt or chain-linked pulleys, wheels or cones . Some continuously variable transmissions use a hydrostatic drive consisting of a variable displacement pump and a hydraulic motor to transmit power without gears. CVT designs are usually as fuel efficient as manual transmissions in city driving, worse in highway driving, and are more expensive to buy and maintain. The expense reflects the difficulty of manufacturing a reliable product.
A slightly different approach to CVT is the concept of toroidal CVT or IVT (from infinitely variable transmission). These concepts provide zero and reverse gear ratios.
Some articles related to IVT can be found at http://www.torotrak.com
Manual Valve Body automatic transmissions
Some automatic transmissions have a "manual" valve body. These transmissions are generally limited to sports cars or race cars. A "manual" valve body allows the driver to shift gears at will, similar to a manual transmission, but with no clutch.
Some manual valve bodies are also capable of "semi-manual function". In this case the transmission can function as a standard automatic OR it may be shifted as a manual transmission.
With the advent of computer controls, it has become much simpler to build a hybrid system that allows fully automatic operation, fully manual operation, or a mix of the two. This is now a popular feature on sport cars from various manufacturers.
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