|
 The
modern automatic transmission is by far, the most
complicated mechanical component in today's
automobile. Automatic transmissions contain
mechanical systems, hydraulic systems, electrical
systems and computer controls, all working together
in perfect harmony which goes virtually unnoticed
until there is a problem.
What
is a transmission?
The transmission is a device that is connected to
the back of the engine and sends the power from the
engine to the drive wheels. An automobile engine
runs at its best at a certain RPM (Revolutions Per
Minute) range and it is the transmission's job to
make sure that the power is delivered to the wheels
while keeping the engine within that range. It does
this through various gear combinations. In first
gear, the engine turns much faster in relation to
the drive wheels, while in high gear the engine is
loafing even though the car may be going in excess
of 70 MPH. In addition to the various forward gears,
a transmission also has a neutral position which
disconnects the engine from the drive wheels, and
reverse, which causes the drive wheels to turn in
the opposite direction allowing you to back up.
Finally, there is the Park position. In this
position, a latch mechanism (not unlike a deadbolt
lock on a door) is inserted into a slot in the
output shaft to lock the drive wheels and keep them
from turning, thereby preventing the vehicle from
rolling.
There are
two basic types of automatic transmissions based on
whether the vehicle is rear wheel drive or front
wheel drive.
 On
a rear wheel drive car, the transmission is usually
mounted to the back of the engine and is located
under the hump in the center of the floorboard
alongside the gas pedal position. A drive shaft
connects the rear of the transmission to the final
drive which is located in the rear axle and is used
to send power to the rear wheels. Power flow on this
system is simple and straight forward going from the
engine, through the torque converter, then through
the transmission and drive shaft until it reaches
the final drive where it is split and sent to the
two rear wheels.
On a front wheel drive car, the transmission is
usually combined with the final drive to form what
is called a
transaxle. The engine on a front wheel drive car is
usually mounted sideways in the car with the
transaxle tucked under it on the side of the engine
facing the rear of the car. Front axles are
connected directly to the transaxle and provide
power to the front wheels. In this example, power
flows from the engine, through the torque converter
to a large chain that sends the power through a 180
degree turn to the transmission that is along side
the engine. From there, the power is routed through
the transmission to the final drive where it is
split and sent to the two front wheels through the
drive axles.
There are a number of other arrangements including
front drive vehicles where the engine is mounted
front to back instead of sideways and there are
other systems that drive all four wheels but the two
systems described here are by far the most popular.
A much less popular rear drive arrangement has the
transmission mounted directly to the final drive at
the rear and is connected by a drive shaft to the
torque converter which is still mounted on the
engine. This system is found on the new Corvette and
is used in order to balance the weight evenly
between the front and rear wheels for improved
performance and handling. Another rear drive system
mounts everything, the engine, transmission and
final drive in the rear. This rear engine
arrangement is popular on the Porsche. |
