A fan clutch is a device used to keep an engine at optimal temperature by controlling the speed of the cooling fan. Cars stay cool by circulating fluid from the engine through the radiator. Air blows into the radiator as the car moves, cooling the fluid, which then cools the engine. The cooling fan sits behind the radiator. When the radiator gets too hot, the fan clutch speeds the fan up to draw more air in past the radiator.
The fan clutch is a sealed box with two plates attached to axles. One axle is attached by a belt drive to the crankshaft. It takes power from the engine and uses it to spin the plate. The other plate is attached to the the fan. The plates do not actually touch each other like they do in a friction clutch. Instead, they are surrounded by a fluid which couples them together.
When the plate attached to the axle spins, it causes the fluid to swirl around. This fluid tugs on the other plate, making it spin and turn the fan. The fluid doesn't pull very strongly, so normally the fan turns pretty slowly, using only a little power. As the fluid in the fan clutch heats up, however, it thickens, becoming viscous. As it gets thicker, pulls more strongly on the plate attached to the fan. This makes the fan spin faster to deal with the increasing heat.
