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Understanding Automotive Cooling Systems

By Tom Benford

Your car's cooling system is really quite remarkable, but most people don't understand how - and how well - it works. The engine in your car runs best at a fairly high temperature. When the engine is cold, components wear out faster, the engine is less efficient and it emits more pollution. So the important role of the cooling system is to permit the engine to heat up as quickly as possible, and then to keep it running at a constant temperature.

To better understand how the cooling system works, it's a good idea to look at the overall operation of the engine. Inside a running engine fuel is constantly burning. A lot of the heat from this combustion goes right out the exhaust system, but some of it soaks into the engine, heating it up. The engine runs best when its coolant is about 180-220° Fahrenheit (depending on the year of your car). At this temperature:

There are two types of cooling systems found on cars: liquid-cooled and air-cooled; but since most cars are liquid-cooled, we'll focus on that system exclusively here.

How it works in a nutshell:

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. The cooling system involves a lot of plumbing. Let's start at the pump and work our way through the system in more detail.

Basically, the pump sends the fluid into the engine block, where it makes its way through passages in the engine around the cylinders. Then it returns through the cylinder head of the engine. The thermostat is located where the fluid leaves the engine. The plumbing around the thermostat sends the fluid back to the pump directly if the thermostat is closed. If it is open, the fluid goes through the radiator first and then back to the pump. There's also a separate circuit for the heating system. This circuit takes fluid from the cylinder head and passes it through a heater core and then back to the pump.

cooling system diagram

And on cars with automatic transmissions, there is normally also a separate circuit for cooling the transmission fluid built into the radiator. The oil from the transmission is pumped by the transmission through a second heat exchanger inside the radiator.


Cars have to operate in a wide variety of temperatures, from well below freezing to well over 100° F (38° C), so the fluid used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat.

Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines. The fluid that most cars use is a mixture of water and ethylene glycol, also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly, as you can see on this chart:

water temperature chart

The temperature of the coolant can sometimes reach 250° to 275° F (121° to 135° C). Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point. The cooling system uses pressure to further raise the boiling point of the coolant. Just as the boiling temperature of water is higher in a pressure cooker, the boiling temperature of coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45° F (25° C) so the coolant can withstand the high temperatures. Antifreeze also contains additives to resist corrosion.

Water Pump

The water pump is a simple centrifugal pump driven by a belt connected to the crankshaft of the engine. The pump circulates fluid whenever the engine is running. The water pump uses centrifugal force to send fluid to the outside while it spins, causing fluid to be drawn from the center continuously. The inlet to the pump is located near the center so that fluid returning from the radiator hits the pump vanes. The pump vanes then fling the fluid to the outside of the pump, where it can enter the engine.

The fluid leaving the pump flows first through the engine block and cylinder head, then into the radiator and finally back to the pump.

The walls of the cylinder are quite thin and the engine block is mostly hollow.
The walls of the cylinder are quite thin and the engine block is mostly hollow.

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