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Kevin ClemensNotes From the Road

Shifting Gears
By Kevin Clemens

Many industry analysts predicted that the introduction of the automatic transmission, more than 50 years ago, would mark the beginning of the end for the manual transmission. After all, who would want to do all the work of shifting gears if a hydraulically powered automatic transmission could do it for you?

Well, it turned out that those early automatics ate up considerable amounts of power and a small, but important, segment of drivers still insisted on a manual transmission for better fuel economy, higher levels of performance, plus greater driver involvement. But engineers kept working on the automatic transmission, refining its performance and improving its efficiency.
During the 1980s and '90s, the marriage of computer control to the automatic's hydraulics resulted in significant improvements. Suddenly, the automatic was a valid choice, even for cars with extremely high performance or drivers looking for good fuel economy.
Just What Does a Manual Do?
Manual transmissions are pretty simple - their gears are located along parallel shafts inside the transmission housing. Power flows when gears are meshed. During gear changes, or when the car is stationary and the engine is idling, a clutch is used to interrupt the flow of power from the engine to the transmission.
When it is time to shift, the driver slides one gear forward or back until it engages a different gear on the other shaft. Gear speeds can be synchronized to aid in their engagement. But for an automatic transmission, a whole new type of gear system was required.
How Does an Automatic Work?
Planetary Gears
The heart of an automatic transmission is its planetary gear set. A planetary gear set consists of a sun gear on a shaft in the center, an outer ring gear with internally facing gear teeth, and planet gears that engage between the sun gear and the ring gear. Each of these three 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 spin 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. It sounds complicated, but over the years it has been perfected to work smoothly and seamlessly.
Instead of a Clutch
The function of a clutch in an automatic transmission is handled by a torque converter. This device consists of an impeller driven by the engine and a turbine connected to the transmission, facing each other and separated by a fraction of an inch. (Think of two pinwheels placed face to face.) Between these two parts is a light oil automatic transmission fluid (ATF). While the engine is idling, the impeller turns slowly, and the force created by the motion of the ATF is not enough to move the car.
As engine speed is increased, however, greater force is transmitted through the ATF to the turbine, and power flows into the transmission. The faster the engine revs, the faster the impeller spins, imparting more force to the turbine. Depending upon the configuration of the impeller and turbine, the torque converter can also multiply the torque from the engine.
Built-in Inefficiencies
Hydraulically-operated automatic transmissions became quite sophisticated in their ability to choose and shift gears. But there were two problems; the torque converter used up a lot of energy as the ATF slipped past the turbine, and most early automatics only had two or three forward gears. All of this wasn't too critical when fuel was cheap and most American cars had large-displacement V8 engines, but it became a real problem in 1973 during the first fuel crisis. For a time, the automatic transmission seemed doomed, but several innovations overcame these problems.
First, a clutch was added inside the torque converter to lock the impeller to the turbine when the car reached a certain speed. By locking the torque converter at higher speeds, no energy was wasted in torque converter slippage.
To solve the problem of limited gear ratios, additional planetary gear sets were fitted in a line, so that the output from one became the input for the next, multiplying the number of gear ratios available. Instead of two or three forward speeds, typical automatics now had four, with the top gear being an overdrive for better highway fuel economy. In addition, the hydraulic/mechanical automatics gained electronic controls to more precisely select the right gear at the right time, and to further smooth out the shifting. These transmission computers also controlled the lockup clutches in the torque converter, allowing this action to take place even at very low speeds. All of this greatly increased the efficiency of the automatic transmission.

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