In a vehicle moving at high speed, airflow through the radiator cools the coolant, but at low speed or when the engine is idling, extra airflow comes from a fan.
Fans can be driven in different ways. More and more modern vehicles now use an electric fan. Air-conditioned cars often have extra fans.
Electric fans can be behind the radiator, in front, or both. This arrangement would be difficult with a belt-driven fan. Some fans can be driven from the crankshaft.
When an engine is mounted longitudinally, its fan is usually mounted on the water pump shaft. The drive belt then turns the water pump and fan. Some use a hydraulic link from the power steering system.
Fan blades can be rigid or flexible. Rigid blades tend to be noisy and use more energy. This noise can be reduced by using irregular spacing of the fan blades.
Some vehicles use a shroud to direct all of the air that the fan moves, through the radiator core. At high speeds, plenty of air is already flowing through the radiator. If the fan is always working at full speed, it’s a waste of energy. And since the engine drives the fan, it’s a waste of fuel too. What’s needed is some way to control the fan. A heat-sensitive switch in contact with the coolant can work like a thermostat, and turn the fan on and off according to coolant temperature.
Another way to alter the speed of the fan is with a viscous hub. This type of fan slips when it is cold, but as the engine heats up, it grips more and more.
