Even in the water cooled engine, air flow plays an important role. The movement of air across the fins of the radiator is as essential as the movement of coolant through the tubes of the radiator. Without air flow the car would certainly overheat, which is the reason the radiator is mounted in the front of the vehicle.
If your car were like a shark and constantly moving, you probably wouldn’t even need a fan. The ambient air flowing through the radiator as the car moves down the road would be enough to cool down the car’s coolant. But unlike the great white your car stops at lights and occasionally gets stuck in traffic. It’s then that the assist from the cooling fan becomes a necessity.
This assist is accomplished through the use of a belt driven or an electric cooling fan. The belt driven fan uses energy from the engine which is transferred to the fan by a pulley and belt assembly. Since the fan speed echoes the rpm’s of the engine this set up is especially efficient at higher rpms. Some applications use a fan clutch that allows the fan to disengage at higher speeds when the air flow provided by vehicle speed is enough to cool the engine. This same device engages when the car slows down in traffic and air flow is at a minimum.
The electric cooling fan operates like any other electrically powered accessory on the car. Its source of energy is the electrical system of the vehicle as supplied through the battery in conjunction with the alternator. An electric cooling fan can be thermostatically controlled or may be designed so it's turning any time the air conditioner is used. Sometimes the fan assembly is made up of two fans. One of the advantages of the electric fan is that the air can be either be pushed or sucked through the radiator depending on the logistical demands of mounting the fan. The belt driven fan, however, can only be mounted on the engine side of the radiator and always sucks air through it.
The shortcomings of both types of fans are obvious. The belt driven fan is only as good as the belt itself and the electric fan depends on the integrity of the motor that drives it and the energy needed as supplied from the electrical system. The belt driven fan also robs power from the engine, since it is always turning.
Of course most decisions about which fan type to use are made in the design phase of the manufacture process. If, however, you were re-doing anything from a 1956 Ford to a 2004 Toyota Matrix, you might be faced with the question: “How do I move the air around to best cool this car?”
The answer lies in the choice of either an electric or a belt driven cooling fan and how its advantages mesh with your car’s specific needs.