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Coolant, also referred to as anti-freeze or antifreeze, is a liquid that assists in preventing engines from overheating. It is comprised of ethylene glycol, mixed with water. It circulates through the engine, where the engine heat is absorbed, to the radiator where the heat is dispersed and the liquid cools.

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Coolant aids in both the prevention of engines overheating as well as freezing in cold weather. The coolant transfers heat away from the engine, is cooled, and returns to absorb more heat.

Coolant also maintains proper temperature levels, and brings a cold engine to an optimal operating temperature as quickly as possible.


Coolant is generally comprised of a mixture of 50% water, 50% ethylene glycol.


Ethylene glycol has a freezing point of around 9 degrees Fahrenheit(-12 Celsius). When combined with water(at 50/50), the mixture's freezing point is reduced to -35 degrees Fahrenheit(-37 Celsius). The boiling point is raised to 223 degrees Fahrenheit(106 C).

Under Pressure

Cooling systems that pressurize the mixture raise its boiling point further. These systems use a pressure cap that keeps the cooling system under a defined pressure. If the pressure in the system exceed the pressure the cap was designed for, the cap rises and allows excess air or coolant to escape. When the pressure of the system drops, the cap seals the system again.

On older cars, a coolant recovery tank was used to keep coolant levels constant in the event some coolant had to be let out of the system at some point.

Flow in a Engine

The coolant circulates through the engine and radiator by means of a water pump.

While Cold

When the engine starts up, it is cold and not operating at an efficient temperature. In order to raise the engine temperature as quickly as possible, coolant flow is restricted by the thermostat. The thermostat keeps the coolant in the engine until the temperature rises to a point where it opens, around 180-195 degrees Fahrenheit.

When Warm

As the thermostat opens, coolant begins to flow into the radiator. The radiator consists of a coiled pipe covered by fins that allow air to pass over it. The air flow can be either from a fan or the movement of the vehicle. As the coolant travels through the pipe, the air passing through the fins cool it. This lower temperature coolant then flows back into the engine where it is heated again, allowing hot coolant into the radiator. This cycle is continuous while the engine is running.

This article was last edited on December 8th, 2010 at 2:18 PM
Category: Cooling