Water-cooled engine blocks and cylinder heads have interconnected coolant passages therethrough. At the top of the cylinder head, all channels converge to a single outlet.
A pump, driven by a pulley and belt from the crankshaft, drives hot coolant out of the engine to the radiator, which is a form of heat exchanger.
Unwanted heat is passed from the radiator into the air stream, and the cooled liquid then returns to an inlet at the bottom of the block and flows back into the channels again.
Typically, the pump sends coolant up through the engine and down through the radiator, expanding with hot water, becoming lighter and rising above the cold water when heated. Its natural tendency is to flow upwards and the pump helps to circulate.
The radiator is linked to the engine by rubber hoses, and has a top and bottom tank connected by a core a bank of many fine tubes.
The tube passes through a hole in a stack of thin metal fins such that the core has a very large surface area and can quickly lose heat from the cooler air passing through it.
In older cars, the pipes run vertically, but modern, low-end cars have cross-flow radiators, from one side to the other.
In an engine at its normal operating temperature, the coolant is only slightly below the normal boiling point.
The risk of boiling is avoided by increasing the pressure in the system, which increases the boiling point.
Additional pressure is limited by the radiator cap, which has a pressure valve inside. When the pressure is too high, the valve is opened and the coolant flows out through the overflow pipe.
In this type of cooling system, if the engine is very hot, the coolant is continuously lost slightly. The system needs to be added from time to time.
Later cars have a sealed system in which any overflow goes into an expansion tank, from which it is sucked back into the engine when the remaining liquid cools.