How does a radiator carry engine heat into the passing air?

A car engine releases energy from fuel, but only some becomes useful mechanical work. Hot gases also transfer energy to engine parts. Without adequate heat removal, their temperatures could rise beyond useful limits. In a liquid-cooled engine, circulating coolant links hot metal inside the engine to air outside the vehicle.

A pump moves coolant through passages around hot engine regions. Energy transfers from the metal into the liquid, which carries it onward. In the radiator, coolant flows through many narrow tubes connected to thin metal fins. Heat passes through the tube walls and fins into the surrounding air. The fins increase the area available for transfer; they do not manufacture cold.

Air must keep moving past that surface. Vehicle motion can supply airflow, and a fan can help when conditions require it. Warm air leaving the radiator carries energy away, while cooler incoming air can accept more. For a given coolant flow, the heat-transfer rate is related to the mass passing each second, its specific heat capacity and its temperature decrease through the radiator.

Too much cooling during warm-up is also undesirable. A thermostat changes the coolant route as temperature rises, allowing more flow through the radiator when needed. Many arrangements provide a bypass route while radiator flow is restricted. This regulates temperature around an operating range rather than trying to make the engine as cold as possible.

Coolant returns to the engine and repeats its journey. Its mixture helps control freezing, boiling and corrosion, while system pressure also raises the boiling temperature. That pressure does not replace the need to transfer heat into the air. A useful distinction emerges: the coolant circulates in a loop, but the energy has a through-route from fuel, through engine and cooling surfaces, to the surroundings.