Cooling System – Keeping The Engine Cool

The radiator usually has two narrow tanks, joined by a honeycombed metal matrix (sometimes called a core). The coolant (a mixture of water and antifreeze) flows from one tank, through the matrix to the other tank. On some cars with automatic transmission, a transmission fluid cooler is built into the radiator. The radiator relies on the flow of air through the matrix produced by the car’s forward motion, supplemented by the cooling fan when necessary, to cool the coolant inside.

The water pump is driven by the engine’s timing belt or chain, or by an auxiliary drivebelt. Most water pumps consist of a rotating impeller inside a housing.

The cooling fan draws cool air over the radiator when the speed of the car is too low, or the air temperature is too high, to give enough cooling.

The cooling fan is usually electrically-operated, although some older cars may have a belt-driven fan. A few cars have a viscous fan attached to one of the engine pulleys – a viscous fluid coupling causes the fan to turn as the engine heats up.

The thermostat is normally located inside a housing on the engine. Its job is to allow the engine to warm up quickly by restricting the flow of coolant to the radiator when cold, and also to regulate the normal operating temperature of the engine.

The system uses a pressure cap, either on the expansion tank or on the radiator. The pressure cap effectively pressurises the cooling system as the temperature rises, which raises the boiling point of the coolant. It acts as a safety valve by venting steam or hot coolant if the pressure rises above a certuain level. The pressure cap also acts as a vacuum relief valve to stop a vacuum forming in the system as it cools.

The expansion tank allows room for the coolant to expand as it heats up. Any air or gas bubbles which form in the coolant return to the expansion tank, and are released in the air space above the coolant.