Why fans are using in vehicle?

When the vehicle is going at high speed with a light load, the natural draft of air passing through the radiator may be sufficient for cooling the engine, but when the vehicle is moving under a heavy load and at a slow speed for instance driving uphill, the natural draft is certainly insufficient to produce the desired cooling. This explains why a fan is a necessary part of the engine cooling system.

            It is mounted behind the radiator on the same shaft on which the water pump is mounted. It is driven by a V-belt from the crankshaft pulley. It is generally made of sheet metal, but these days moulded plastic materials, for example, nylon and polypropylene are also being used for making fans.

For efficient and economical running, it required that the fan must give adequate air flow at all the conditions of vehicle load and speed. More flow than the minimum necessary for effecting cooling at any particular time is simply uneconomical. Thus the commonly used method of running the fan at one constant speed ratio with the engine is not desirable. 

 For example, the fan is designed to give adequate airflow at low vehicle speeds. Say, when going uphill when the airflow due to vehicle speed is tiny, obviously the air drought at high vehicle speed will be much more than the desired when the airflow due to vehicle speed itself is quite high. Thus the fan will be unnecessarily consuming engine power, which has been estimated as much as 5 percent of the engine. This is clearly waste and must be avoided.