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The reason the coolant goes into the radiator is to allow air to pass
through it and cool the coolant. When you are driving fast enough,
the air rushes through the grille of the car and passes through the
radiator core. If you aren't driving fast enough to push air through
the radiator, then the fan will pull the air through.
The fan improves cooling when you are driving at slow speeds, or if
the engine is idling. It is usually mounted on the water pump shaft,
and is turned by the same belt that drives the water pump and the
alternator, although it can be mounted as an independent unit. Most
independently mounted fans are electric.
Belt Powered Fans
The fan's activity is not always necessary, and it takes power from
the engine to spin. For this reason a thermostatic control, or fan
clutch, is often used to reduce drive torque when it isn't needed
(variable-speed fan). A different type of fan uses centrifugal force
to move its flexible plastic blades, by flattening them when the
engine rpm is high (flexible-blade fan). The less angle the blades
have, the less power they use. The idea of these units is to save
horsepower and reduce the noise the fan makes.
A fan can have from four to six blades to suck the air through the
radiator. Often the radiator has a shroud for the fan to keep it from
recirculating the same hot air that has collected behind the
radiator. Many fans have irregularly spaced blades to reduce resonant
noise.
Electric Fans
Front-wheel drive engines mounted transversely usually use electric
fans to cool the engine. The radiator is located in the usual place,
but an electric motor drives the fan. A thermostatic switch is used
to turn the fan on and off at predetermined temperature settings,
which it senses. The exception to this is air conditioning. If you
turn on the air conditioner, you bypass the thermostatic switch, and
the fan runs continuously. If you turn off the air conditioner, the
thermostatic switch is re-activated, and goes back to turning the fan
on and off, according to its instructions. Many cars have one
electric fan for normal cooling and a separate one just for when the
air conditioner is on.
There are some really nice features about the electric fan. The
nicest feature is that you don't have to keep an eye on the
treacherous old fan belt -- there isn't one, so you don't have to
worry about its health and fitness. It's also quieter, and less of a
power drain on the engine. They also help your engine by continuing
to cool it after it's turned off.
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