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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?
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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.
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