Boiling During Engine Operation

Figure 2-24. Expansion of water and antifreeze compound

solution caused by heat.

Evaporation. Liquid can change to a gas by

evaporation as well as by boiling. Boiling takes place

within the liquid, and evaporation occurs on the surface.

The rate of evaporation is controlled by a number of

conditions which include the temperature of the liquid and

Figure 2-23. Effect of increased pressure in cooling

the amount of surface exposed to the air.

Water

system on boiling points of water and antifreeze compound

evaporates at all temperatures, but contrary to common

solutions.

opinion, the loss of water from the cooling system by

evaporation is negligible as compared to other causes of

d. Boiling During Engine Operation. Violent boiling

coolant loss, especially in systems equipped with pressure

of coolant effects the operation of the cooling system in

caps. Evaporation of slight water leakage outside the

several ways. The formation of steam in the water jacket

cooling system may be so rapid that the leakage will be

results in steam pockets which present the coolant from

dried up as fast as it is formed, and there will be no

coming in contact with the metal. This interferes with

moisture at the leak, especially when the system is hot.

transfer of heat to the coolant. Pressure developing from

steam formation In the heater jacket forces coolant out the

2-29.

Antifreeze Protection

overflow pipe. Continued operation of the engine with

boiling coolant can run the cooling system almost

a. Freezing Damage.

When water freezes at +

completely dry.

32F, it forms solid ice and expands approximately 9

e. After-boil (fig 2-24) Following long, hard

percent In volume. This expansion takes place with a

operation, boiling may occur after the engine is shut off,

terrific force. If water is allowed to freeze in the cooling

even though the coolant has not boiling during operation.

system, the force of the expansion will crack the engine

This after-boil is caused by the rapid rise of coolant

water jacket (fig 2-25) and cause serious damage to the

temperature in the heater jacket, sometimes as much as

radiator and other parts. For this reason, the vehicle must

20 F or more. The temperature rise Is due to the fact that

never be exposed to freezing temperatures without

the coolant Is still absorbing heat produced in the engine

antifreeze protection in the system.

during operation and cannot get rid of It with circulation

and air flow stopped. After-boil occurs less frequently and

results in less overflow loss when the boiling point of the

coolant is comparatively high. After-boil losses of coolant

can be prevented by use of radiator pressure caps and

overflow tanks.

2-22