In this very basic liquid-cooling system, a coolant is stored in a radiator, and in the engine. As the engine heats up, a natural circulation starts, as coolant rises through the engine block by convection. It passes through the top hose, and into the radiator. Inside the radiator, heat is removed from the coolant as it falls from the top to the bottom. When it reaches the bottom, it returns to the engine through the lower radiator hose.
This process is called thermo-siphon. It was common in older cars which had low-powered engines and high, narrow radiators.
In modern cars, the engines are more powerful, and radiators are low and wide, and a thermo-siphon process couldn’t move the coolant quickly enough.
Instead, a water pump forces it through passages called waterjackets in the engine block. It collects heat by conduction, and becomes hot itself. Heated coolant then returns to the radiator for cooling.
And the cycle is repeated. Heat is removed from the engine, and dispersed. Preventing overheating is one function of the cooling system. It also helps the engine reach its best operating temperature as soon as possible. Every engine has a temperature at which it operates best. Below this temperature, ignition and combustion can be difficult.
Most engine wear occurs during this warm-up period. And most pollution too.
One function of the thermostat is to shorten the warming-up period. It operates according to coolant temperature. When coolant is cold, it is closed. When a cold engine starts, coolant circulates within the engine block and cylinder head and through a coolant bypass to the water pump inlet. It can’t get to the radiator. As the engine warms up, the coolant trapped in the engine gets hotter and hotter. This starts to open the thermostat, allowing hot coolant to flow to the radiator.
