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The thermostat's main job is to allow the engine to heat up quickly, and
then to keep the engine at a constant temperature. It does this by regulating
the amount of water that goes through the radiator. At low temperatures, the
outlet to the radiator is completely blocked -- all of the coolant is
recirculated back through the engine.
Once the temperature of the coolant rises to between 180 and 195 F (82 - 91
C), the thermostat starts to open, allowing fluid to flow through the radiator.
By the time the coolant reaches 200 to 218 F (93 - 103 C), the thermostat is
open all the way.
If you ever have the chance to test one, a thermostat is an amazing thing to
watch because what it does seems impossible. You can put one in a pot of
boiling water on the stove. As it heats up, its valve opens about an inch,
apparently by magic! If you'd like to try this yourself, go to a car parts
store and buy one for a couple of bucks.
The secret of the thermostat lies in the small cylinder located on the
engine-side of the device. This cylinder is filled with a wax that begins to
melt at around 180 F (different thermostats open at different temperatures, but
180 F is a common one). A rod connected to the valve presses into this wax.
When the wax melts, it expands significantly, pushing the rod out of the
cylinder and opening the valve. If you have read How Thermometers Work
done the experiment with the bottle and the straw, you have seen this process
in action -- the wax just expands a good bit more because it is changing from a
solid to a liquid in addition to expanding from the heat.
This same technique is used in automatic openers for greenhouse vents and
skylights. In these devices, the wax melts at a lower temperature.
Like the thermostat, the cooling fan has to be controlled so that it
allows the engine to maintain a constant temperature.
Front-wheel drive cars have electric fans
engine is usually mounted transversely, meaning the output of the engine points
toward the side of the car. The fans are controlled either with a thermostatic
switch or by the engine computer, and they turn on when the temperature of the
coolant goes above a set point. They turn back off when the temperature drops
below that point.
Rear-wheel drive cars with longitudinal engines usually have engine-driven
. These fans have a thermostatically controlled viscous clutch
. This clutch is
positioned at the hub of the fan, in the airflow coming through the radiator.
This special viscous clutch is much like the viscous coupling
sometimes found in all-wheel