Automobile Cooling Systems

An gasoline-powered automobile engine operates in a modified Otto cycle which may produce temperatures in the neighborhood of 2500°C. Approximately 70% of the energy from fuel combustion is converted into heat. This requires a high capacity cooling system to prevent the overheating of the engine. Most current automobile engines are water-cooled. Water has the advantage of a high heat capacity, but the disadvantages of a relatively high freezing point and relatively low boiling point for the purpose of auto cooling. Typically a mixture of ethylene glycol and water is used in a pressurized cooling system to transfer heat from the engine block.

The effect of ethylene glycol on the freezing and boiling points is shown in the table.

The advantage of the lowered freezing point is obvious, but the raised boiling point is also significant and makes the antifreeze solution important in summer as well. The above boiling points are at atmospheric presure, but a higher maximum operating temperature can be achieved by pressurizing the cooling system. It is typical to have a spring-loaded pressure cap on the cooling system which permits the pressure to rise to about 14 to 15 lb/in² above atmosphere (i.e., an absolute pressure of about two atmospheres). For the coolant mixture, this can raise the boiling point another 25°C or 45°F.