How It Works & Why It's Critical
When you consider that the typical six-cylinder engine has something in the neighborhood of 6,000 fiery explosions inside it every minute when operating at typical cruising speed, it's a wonder engines hold together at all. With combustion temperatures registering in the neighborhood of a couple thousand degrees, its obvious that something has to be done to manage the tremendous heat if there's going to be any hope of keeping metal parts from melting—and getting to Grandma's house without walking.
Where does all that heat go? Roughly half of it goes out the exhaust system, where, from the engine's standpoint, it's out of the picture. The rest of it is absorbed by the engine: soaked up by its pistons, cylinder heads, valves and many other costly bits. None of these components can take unlimited heat. They've got to be continuously cooled; otherwise, they'll weaken and quickly and catastrophically fail. So as fast as combustion puts heat into these components, it's up to the engine's cooling system to take it out and dump it somewhere else.
Early automotive engines relied on the cooling effect of the air around the engine to keep heat in check. That worked well when engines made a fraction of the power and heat of modern engines. Even worse, for aerodynamic reasons, modern high-output engines are tightly packed in crowded engine compartments where airflow is restricted. Liquid cooling is the answer. By endlessly pumping comparatively cool liquid from a radiator to the hot engine, then back to the radiator, the engine's heat can be transferred to the air passing through the radiator.
The components that do this are relatively straightforward. The radiator is appropriately sized to be able to dissipate the maximum amount of heat the engine is going to produce, in the hottest weather conditions the vehicle is likely to be used in. The radiator is connected to the engine with a series of pipes and hoses. The engine turns the water pump, which circulates the water through the system anytime the engine is running. Water is a good conductor of heat, so even very small passageways through the engine can carry away a great deal of heat energy. By placing these passageways in the hottest parts of the engine surrounding the combustion chambers, critical components can be kept in a safe temperature zone.
At the other temperature extreme, the cooling system effectively shuts down automatically to speed warm-up. A thermostat is placed in the coolant loop where it can divert a portion of the coolant flow away from the radiator and directly back to the engine. This way, when the engine is starting up cold, it can reach operating temperature quickly without the cooling effect of the radiator. As operating temperature approaches, the thermostat gradually opens and the radiator goes to work keeping everything in the thermal sweet spot.
To help move cooling air through the radiator when the vehicle is traveling slowly, an electric or engine-driven fan helps out. The whole cooling system is sealed with a cap that traps building pressure inside as the coolant warms. Water boils at a higher temperature when it's under pressure, which explains how you might see a temperature well above 212-degrees F on a calibrated coolant temperature gauge, and still be far from boiling over. If the coolant DOES reach the boiling point, the excess is squirted into a catch tank, where it waits patiently until the engine cools enough to siphon it back into the cooling system.
The coolant system also has another job: transferring engine heat into the car's interior in cold weather. The heater core is essentially a small radiator plumbed into the cooling system that's positioned inside the vehicle's climate control system. So instead of radiating its heat to the outside atmosphere like the main radiator, it transfers its heat to the inside air. By mixing the hot air flowing from the heater core with unheated air, the temperature you feel through the vents can be adjusted. So you get a nice comfortable 72-degrees, which is a bit more pleasant than the 2,000-degree furnace-blast your pistons are feeling. Not to worry, your cooling system is looking out for them.