Radiators are used for cooling internal combustion engines.

They operate by passing a liquid coolant through the engine block, where it is heated, then through the radiator itself where it loses this heat to the atmosphere. This coolant is usually water-based, but may also be oil. It is usual for the coolant flow to be pumped.

To cool down the engine, coolant heated from flowing through the engine is fed into the header of the radiator via the inlet and then cools down as it circulates through the tubes to the opposite header and cold coolant exits back into the engine via the outlet, and the cycle is repeated. As it circulates through the tubes, the coolant transfers its heat to the tubes which, in turn, transfer the heat to the fins that are lodged between each row of tubes. The fins then radiate the heat transferred by the tubes to the surrounding air, hence the term radiator.

Radiators are often paired with a fan that blows air through the radiator. Air is an important part of the heat transfer process because it takes the heat away from the radiator. Air heats up relatively quickly and that in order for the radiator to continue to transfer heat to the surrounding air effectively, the heated air must continuously be replaced by cool air so that the heat transfer process can continue. That’s why the radiator is located behind the grill at the front end of the car. As the car moves, air flows through the radiator and continuously "takes" away heat. However, when the car is not moving or when natural airflow is insufficient, a fan pointed directly at the core forces more air through it and more heat is transferred.