I know it should be in the Technical Forum but I know it won't get the viewing that would be helpful to many people.
Almost from the beginning of the automobile, there have been manual transmissions. Unlike electric motors or steam engines that produce their maximum torque when they are stalled at zero-rpm, internal combustion engines must be turning over in order to produce any usable torque. So the first problem faced by early car designers was to allow an engine to idle when a car was stopped and then to progressively engage drive as the engine speeded up from idle. The solution that they came up with was to allow the driver to control this transition through a foot-operated clutch that could be slipped as the throttle was used to increase engine speed.
Because these internal combustion engines only create usable torque over a fairly narrow range of engine speeds, a set of selectable gear ratios was also needed to further multiply the torque and provide constant acceleration. The clutch came in handy here too because the driver could disengage drive momentarily and change drive ratios by changing gears in the transmission. Although clutch work and gear changing required some skill, most drivers learned to cope with the situation, which was considered an integral part of driving.
In the late 1930s, however, engineers began to experiment with transmissions and clutches that would shift themselves. The first step was the torque converter. This device consists of an impeller driven by the engine and a turbine connected to the transmission. Between these two parts is a working fluid, usually a light oil. When the impeller turns slowly, while the engine is idling, the force created by the motion of the oil is insufficient to move the car forward.
As engine speed is increased, the force is transmitted through the fluid to the turbine and power flows into the transmission. The faster the impeller spins, the more force is imparted to the turbine. Torque converters can also be used to multiply the torque from the engine depending upon the configuration of the impeller and turbine. By the early 1950s some companies, such as Chrysler, offered a torque converter with their manual transmissions to allow clutchless starts from rest. This made driving easier, but more was coming.
The next step to automatic gear changing would require a complete redesign of the transmission. Manual transmissions are pretty simple in that their gears are located along parallel shafts inside the transmission housing. Power flows when gears are meshed and it is straightforward enough to slide one gear forward or back until it engages the gear on another shaft. Gear speeds can be synchronized to aid in their engagement. But for an automatic transmission, a whole new type of gear would be required.
Planetary gear systems had been used for many years for precision instruments and for systems that needed a variety of ratios within a compact package. A planetary set consists of a sun gear on a shaft in the center, an outer most ring gear with internally facing gear teeth and planet gears that engage between the sun gear and the ring gear on their own carrier. Each of these thee elements can act as an input or an output gear, or may be held stationary, allowing the other gears to move around it. Because the three gear elements are all located around the same axis, friction bands can be used to limit the motion of one of the elements while allowing the other two to transmit torque. In addition, the engagement of different elements can take place without interrupting power flow.
Consider the planetary gear set (shown in several images here). The outer ring gear B, sun gear A and planet gears with their carrier C are all indicated. If we drive the sun gear A with the engine and lock the outer ring gear B, we can transmit power from the planet gear carrier at a very high ratio, as we might use for first gear. Next, if we lock the ring gear B and apply power through the planetary gear C, we transmit power to the sun gear A. Other ratios can be found by powering one gear and locking another. Reverse rotation of the ring gear B results if we lock the planetary gear C and power the sun gear A.
It sounds complicated, but using various hydraulically operated brake bands and actuators, the use of this planetary gear system became the basis for the automatic transmission of today.
Hydraulically operated transmissions became quite sophisticated in their ability to choose gears. But there were two problems. The torque converter used up a lot of energy as the fluid slipped past the turbine. This wasn't too critical when fuel was cheap, but became a real problem in 1973 during the first fuel crises.
Also, because automatics were usually used with lazy American V8 engines, the lack of more than two or three gear ratios was more than made up for by the huge amounts of torque produced by the large displacement engines. This changed too when big V8s were made obsolete by their massive thirst for fuel. The automatic seemed doomed, but several innovations made it possible to still have fuel-efficient cars that shift themselves.
First of all, friction clutches were added to torque converters to lock the impeller to the turbine when the car reaches a certain speed. This lockup could be done hydraulically, or more recently electronically by a command from the transmissions computerized controller. By locking the torque converter after starting from rest no energy was wasted in slippage at higher speeds. To solve the problem of limited gear ratios, several planetary gear sets are placed in a line so that the output from one becomes the input for the next, multiplying the number of gear ratios available. These ratios are now also chosen by an electronic computer instead of by a hydro-mechanical system, although many transmissions still use hydraulic pressure to actuate the bands, which lock the elements of the planetary gear set.
The use of computer control also allows different shift patterns to be developed for different driving situations. Trailer towing, for example requires different gear considerations that sport driving on a twisty highway. Some systems use driver control to choose between these modes while others automatically sense the driving conditions and choose the best programming.
The quality of the shift between gears can also be improved by interfacing the transmission and engine controllers together. Power from the engine is interrupted for an instant when the transmission is shifting between elements of the planetary gear set. This softens the shift and makes it nearly imperceptible.
The biggest irony about automatic transmissions is that they may soon make the manual transmissions beloved by auto enthusiasts obsolete. The problem is that the driver of a manual transmission equipped car shifts when he or she feels like it, which may not be optimum for fuel economy and exhaust emissions. An automatic transmission can be programmed to optimize both of these requirements and can monitor engine performance to keep emissions to a minimum and efficiency at a maximum.
The latest automatics are so smooth and efficient that the old arguments against them are falling apart. Soon, the only reason to keep the old manual gearbox will be a sense of nostalgia.
Cross-section of transaxle with automatic transmission showing orientation of internal parts. - Bentley Manual
