The Cooling System

The engine runs best when its coolant is about 200 degrees Fahrenheit (93 degrees Celsius). A lot of the heat from the combustion of an engine goes right out the exhaust system, but some of it soaks into the engine, heating it up. Most of the energy in the gasoline (perhaps 70%) is converted into heat, and it is the job of the cooling system to take care of that heat. There are two types of cooling systems found on cars: liquid-cooled and air-cooled.

Liquid Cooling
The cooling system on liquid-cooled cars circulates a fluid through pipes and passageways in the engine. As this liquid passes through the hot engine it absorbs heat, cooling the engine. After the fluid leaves the engine, it passes through a heat exchanger, or radiator, which transfers the heat from the fluid to the air blowing through the exchanger.

Air Cooling
Some older cars, and very few modern cars, are air-cooled. Instead of circulating fluid through the engine, the engine block is covered in aluminum fins that conduct the heat away from the cylinder. A powerful fan forces air over these fins, which cools the engine by transferring the heat to the air.
In a few cars the engine is air-cooled instead Air-cooling makes the engine lighter but hotter, generally decreasing engine life and overall performance. Since most cars are liquid-cooled, we will focus on that system.

The Engine
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 very long, it can seize. When this happens, the metal has actually gotten hot enough for the piston to weld itself to the cylinder. This usually means the complete destruction of the engine.

One interesting way to reduce the demands on the cooling system is to reduce the amount of heat that is 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. Ceramic is a poor conductor of heat, so less heat is conducted through to the metal and more passes out of the exhaust.

Water Pump
The water pump is either driven by the fan belt or the timing belt on some newer vehicles. The pump moves coolant from the radiator, through the engine, and back into the radiator. The pump has a shaft with a pulley on one end and a pump rotor on the other end. When the pulley is spun by a belt, the rotor moves the coolant.

Radiator
The radiator is a type of heat exchanger with a series of thin channels where the coolant flows through and it cooled by air flowing through it. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.
Radiators usually have a tank on each side, and inside the tank is a transmission cooler. In the picture above, you can see the inlet and outlet where the oil from the transmission enters the 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.

Thermostat
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.

The thermostat controls the flow of coolant through the engine. When your engine is cold, it actually operates with less efficiency. So until the engine warms up to it's ideal temperature, the thermostat keeps coolant from flowing. 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.
Your thermostat can fail in either an open or closed condition. When it fails in the open position, your engine may actually run too cold and you will have bad gas mileage. If the thermostat fails in the closed position, your engine will overheat since no coolant will be flowing through the engine.

Pressure Cap
The radiator cap actually increases the boiling point of your coolant by about 45 F (25 C). How does this simple cap do this? The same way a pressure cooker increases the boiling temperature of water. The cap is actually a pressure release valve, and on cars it is usually set to 15 psi. 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.

Hoses
The hoses carry the coolant from the radiator to the waterpump and from the engine to the radiator. Hoses are generally made of rubber and can deteriorate with time. Hose should be flexible and not dry rotted (check for tiny cracks in the hose).

Fan
Like the thermostat, the cooling fan has to be controlled so that it allows the engine to maintain a constant temperature.
Front-wheel drive cars have electric fans because the engine is usually mounted transversely, meaning the output of the engine points toward the side of the car. 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. Rear-wheel drive cars with longitudinal engines usually have engine-driven cooling fans.

Coolant
Coolant is generally a mix of water and ethylene glycol. Many times called Anti-Freeze, coolant serves many purposes. As it's common name implies, coolant prevents freezing, but it also provides lubrication for the waterpump, increases boiling point of water, and keeps rust and scale from forming in your cooling system. Coolant must be changed on a regular basis, check your owners manual for the recommended schedule. PH level is critical to keeping your coolant from becoming a metal eating liquid. When you check your coolant for freeze point, also check out the PH level and make sure your coolant has not turned acidic.

Common Problems
Let's look at the common problems cars have with the cooling system.

Broken hose. Hoses and tubing wear out and leak coolant fluid.
Broken fan belt. The water pump is driven by the engine through a belt. If the belt breaks the water pump can not turn and coolant will not be circulated through the engine. This will also lead to engine overheating.
Damaged radiator cap. The radiator cap is designed to hold a certain pressure in the coolant system. Most caps hold 8 - 12 PSI. This pressure raises the point in which the coolant will boil and maintains a stable system. If your cap does not hold pressure, then the car could overheat on hot days since the system never becomes pressurized.
Water pump failure. Most commonly you will hear a screeching noise and will be able to see coolant leaking from the front of the pump or under the car. Early signs are small spots of coolant under the car after being parked overnight and a strong coolant odor while driving.
Head gasket... have large amounts of white smoke flowing out of your exhaust? Could be a head gasket. The head gasket seals the cylinder head to the engine block and also seals the coolant passages. When this gasket fails coolant can enter the cylinder and it will be turned to vapor as the engine fires. Head gaskets most often fail after the engine has experienced an overheating situation. When super hot, the cylinder head can warp and allow the gasket to fail.

Preventive Maintenance:

Check all belts and hoses regularly.
Look out for coolant leaks underneath the car
Change your coolant every 2 - 3 years depending on the manufacturers recommendations.
Inspect your radiator cap for deterioration of the rubber seal.
Have your coolant system flushed every 5 years. It gets all the corrosion which has built up out of the system.

WARNING: Never open your engine radiator when the engine is hot. The pressure in the cooling system can cause hot coolant to splash out and burn you