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The drive train serves two functions: it transmits power from the
engine to the drive wheels, and it varies the amount of torque.
"Power" is the rate or speed at which work is performed. "Torque" is
turning or twisting force. Multiple ratio gearboxes are necessary
because the engine delivers its maximum power at certain speeds, or
RPM (Rotations Per Minute). In order to use the same engine RPM's at
different road speeds, it is necessary to change the "Gear Ratio"
between the engine and the drive wheels. Just like a bicycle, the car
has to switch gears in order to move at a wide range of speeds.
Unlike your bicycle, the car's drivetrain also has to allow you to
back up. (Well, you could push it backwards if you ate your Wheaties)
There are actually two sets of gears in the drive train; the
transmission and the differential. The transmission allows the gear
ratio to be adjusted, and the differential lets the drive wheels turn
at different speeds.
Manual transmissions usually have four or five speeds, and often have
"overdrive", which means that the output shaft can turn faster than
the input shaft for fuel economy on the highway. Some use an electric
clutch and a switch that controls whether the overdrive is engaged or
not. An interesting development on a few cars is the "clutchless"
manual transmission, which uses a stick shift and an automatic
electric clutch. Speed and position sensors, mini computers, and
throttle controls keep the engine from over-revving when the driver
shifts gears. As with many automotive "inventions", this is an old
idea which may now reach feasibility due to the computer revolution.
Automatic transmissions commonly use three forward gears to blend
speed and torque. In the case of a three-speed transmission, first
gear delivers maximum torque and minimum speed for starting. Second
gear offers medium torque and speed for acceleration and hill
climbing. Third gear allows maximum speed with minimum torque for
highway travel. A reverse gear permits backward movement.
A transmission is a speed and power changing device installed at some
point between the engine and driving wheels of a vehicle. It provides
a means for changing the ratio between engine RPM (Revolutions Per
Minute) and driving wheel RPM to best meet each particular driving
situation.
Some types of drive train layouts use a "Transaxle", which is
simply a combination of the transmission and the differential. These
are usually found on front wheel drive cars, but are also used on
mid- and rear-engine cars. Some exotic cars have their engine in the
front, and a transaxle in the rear of the car for better weight
balance.
Torque is derived from power. The amount of torque obtainable from a
source of power is proportional to the distance from the center of
rotation at which it is applied. It is logical, then, that if we have
a shaft (in this case, the crankshaft) rotating at any given speed,
we can put gears of different sizes on the shaft and obtain different
results. If we put a large gear on the shaft, we will get more speed
and less power at the rim than with a small gear. If we place another
shaft parallel to our driving shaft and install gears on it in line
with those on the driving shaft, we can obtain almost any desired
combination of speed or power within the limits of the engine's
ability. That is exactly what an automobile transmission does by
means of gears and other devices.
There are two types of transmissions; manual and automatic. If you
have a manual transmission, you have to shift the gears yourself,
usually with a stick located on your console and the clutch pedal.
If you have an automatic transmission, the mechanism changes without
any help from you. This is accomplished through a system that works
by oil pressure. Each shift of the gears is controlled by a shift
valve; the gears shift change depending on speed, the road, and load
conditions.
Another basic component of all drive trains is some form of a clutch.
it allows the engine to continue rotating while the gears and wheels
are stationary. Automatic transmission cars use a "torque converter"
in lieu of a clutch.
From the back of the engine to where the rubber meets the road, the
drivetrain encompasses one of the most complicated systems of your
car. Some people say looking at a transmission "makes their brain
hurt".
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