The engine block and cylinder head have many passageways cast or machined in them to allow for fluid flow. These passageways direct the coolant to the most critical areas of the engine.
Temperatures in the combustion chamber of the engine can reach 4,500° F (2,500° C), so cooling the area around the cylinders is critical. Areas around the exhaust valves are especially crucial, and almost all of the space inside the cylinder head around the valves that is not needed for structure is filled with coolant. If the engine goes without cooling for too long, it can seize. When this happens, the metal has actually gotten hot enough for the piston to weld itself to the cylinder which usually results in the complete destruction of the engine.
One way to reduce the demands on the cooling system is to reduce the amount of heat transferred from the combustion chamber to the metal parts of the engine. Some engines do this by coating the inside of the top of the cylinder head with a thin layer of ceramic. Since ceramic is a poor conductor of heat, less heat is conducted through to the metal and more passes out of the exhaust.
The head of the engine also has large coolant passageways.
As noted earlier, a radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan. Some cars (e.g., Corvettes) are equipped with aluminum radiators, since they have a higher thermal coefficient than brass or copper units. These radiators are made by brazing thin aluminum fins to flattened aluminum tubes. The coolant flows from the inlet to the outlet through many tubes mounted in a parallel arrangement. The fins conduct the heat from the tubes and transfer it to the air flowing through the radiator.
The tubes sometimes have a type of fin inserted into them called a turbulator, which increases the turbulence of the fluid flowing through the tubes. If the fluid flowed very smoothly through the tubes, only the fluid actually touching the tubes would be cooled directly. The amount of heat transferred to the tubes from the fluid running through them depends on the difference in temperature between the tube and the fluid touching it. So if the fluid that is in contact with the tube cools down quickly, less heat will be transferred. By creating turbulence inside the tube, all of the fluid mixes together, keeping the temperature of the fluid touching the tubes up so that more heat can be extracted, and all of the fluid inside the tube is used effectively.
Radiators usually have a tank on each side, and inside the tank is a transmission cooler. The transmission cooler is like a radiator within a radiator, except instead of exchanging heat with the air, the oil exchanges heat with the coolant in the radiator.
The radiator cap actually increases the boiling point of your coolant by about 45° F (25° C). The cap is actually a pressure release valve, and on cars it is usually set to 15psi. The boiling point of water increases when the water is placed under pressure. When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled.
The thermostat's main job is to allow the engine to heat up quickly, and then to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked — all of the coolant is recirculated back through the engine.
Once the temperature of the coolant rises to between 180° and 195° F (82°-91° C), the thermostat starts to open, allowing fluid to flow through the radiator. By the time the coolant reaches 200° to 218° F (93°-103° C), the thermostat is open all the way.
Like the thermostat, the cooling fan has to be controlled so that it allows the engine to maintain a constant temperature. Later model cars have electric fans. The fans are controlled either with a thermostatic switch or by the engine computer, and they turn on when the temperature of the coolant goes above a set point. They turn back off when the temperature drops below that point.
Older cars have engine-driven cooling fans. These fans have a thermostatically-controlled viscous clutch positioned at the hub of the fan, in the airflow coming through the radiator.
You may have heard that if you car is overheating you should open all the windows and run the heater with the fan going at full blast. That's good advice, because the heating system is actually a secondary cooling system that mirrors the main cooling system on your car.
The heater core, which is located in or under the dashboard, is really a small radiator. The heater fan blows air through the heater core and into the passenger compartment of your car.
The heater core draws its hot coolant from the cylinder head and returns it to the pump — so the heater works regardless of whether the thermostat is open or closed.
That pretty much sums it up!