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An automatic transmission is much easier to drive than a manual
transmission, because you don't have to use a clutch pedal or
gearshift lever. An automatic transmission does the work all by
itself. The first automatic transmission appeared in 1939.
Automatic transmissions automatically change to higher and lower
gears with changes in the car's speed and the load on the engine.
These transmissions are also aware of how far down you have pushed
the gas pedal, and shift accordingly.
The system is operated by transmission fluid pressure; shift valves
control the gear changes. A "governor" controls the shifting of the
gears. It's linked to the output shaft and throttle valve and
controls the transmission fluid supply, at different pressures, to
the shift valve. Here's how it works: the output shaft turns the
governor. The faster the car goes, the faster the governor turns. Oil
is sent from the pump to the shift valves by centrifugal force from
the governor. The shift valves move out, and send the transmission
fluid to the gear shifting mechanisms in the transmission. When you
slow down, the valves move in, and send the transmission fluid in the
opposite direction. This action changes the gears.
By routing the pressure to the clutches and brake bands, the
different gears are selected.
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