Heat Exchanger Basics

• Where cars are concerned, heat exchangers come in four basic varieties: water-to-air (such as the radiator), air-to-water (some types of intercoolers) and air-to-air (other types of intercoolers). Heat exchangers work by temperature differential: Heat from the substance to be cooled migrates to the cooling medium -- air, in the case of a radiator -- as long as the cooling medium doesn't get saturated by the heat.
  
  

Radiator Cooling

• If air remains stagnant around the radiator, it'll quickly heat up and match the temperature of the fluid inside. This is why radiators need a constant flow of air, either by a fan at low vehicle speeds or by natural air pressure coming in through the grille at highway speeds. Low outside temperatures and high barometric pressures (greater air density) can increase that radiator's cooling capacity; high outside temperatures and low pressure (as at high altitude) can reduce the radiator's ability to cool the engine.