Radiator Pressure Cap

d. Core Function In the radiator core, a large

(2) Vacuum-valves The vacuum valve opens only

amount of heat is rapidly transferred from the cool ant into

when pressure within the cooling system drops below

the air. Through the water tubes, the flow of coolant is

outside air pressure as the engine cools down. Higher

divided into many small streams which causes a small

outside pressure then forces the valves open, which allows

amount of cooling liquid to be exposed to a comparatively

air to enter the system by way of the overflow pipe. When

large cooling surface in the tubes. This results in rapid

pressure inside and outside again becomes approximately

flow of heat from the coolant to the tubes and air fins Heat

the same the vacuum valve closes. This automatic action

is carried away from the tubes and fins by the air moving

of the vacuum valve prevents collapse of hose and other

through the core

thin-walled parts of the cooling system without internal

support.

1-10.

Radiator Pressure Cap

(Fig 1-5)

Operation of Pressure System

a. General. The radiator pressure cap contains two

(1) When a system is equipped with a pressure cap

spring-loaded normally closed valves. The larger valve is

it is generally referred to as a pressure-cooling system.

called the pressure valve and the smaller one is called the

The radiator used in such a system is especially designed

vacuum valve. A shoulder in the radiator filler neck

to withstand extra pressure. Tightness of all connections

provides a seat for the bottom of the cap assembly and a

and joints is particularly important since pressure naturally

gasket on this seat prevents leakage between the cap and

aggravates any existing leakage. An airtight cooling

the filler neck. By closing off the overflow pipe opening,

system is necessary to obtain the benefits of pressure

the pressure cap prevents overflow loss of coolant during

cooling.

normal operation. It also allows a certain amount of

pressure to develop within the system which raises the

(2) Except under the heaviest driving conditions or in

boiling point of the coolant and permits the engine to

extremely hot weather, most military engines can be

operate at higher temperatures without coolant overflow

operated without pressure in the systems. However, in

from boiling.

some vehicles normal operating temperature is always

above the boiling point of the coolant. Proper functioning

of the pressure-cooling system is absolutely necessary to

avoid large overflow losses of coolant from boiling, even

under average operating conditions.

1-11.

Radiator Overflow Tank

(Fig 1-6)

a. General. Radiator overflow tanks, sometimes

called surge tanks or expansion tanks, are standard

equipment for some vehicles. They are Installed on other

vehicles as special equipment for operation in hot or dry

country. The overflow tank serves as a receptacle for

coolant overflowing from the radiator and provides for its

return to the system. Thus, the overflow tank conserves

coolant and reduces the need for frequent filling of the

radiator.

Figure 1-5. Radiator pressure cap - cross-sectional view.

b. Construction.

Overflow tanks may vary in

b. Function

capacity from 2 quarts to a gallon or more. They are

(1) Pressure valve The pressure valve acts as a

usually mounted fairly high with reference to the cooling

safety valve to relieve extra pressure within the system. In

system. The bottom of the tank is connected to the

the majority of transport vehicles, the pressure valves

radiator overflow pipe through a metal tube which is

open at about 4 pounds per square inch. When the valve

usually connected by short pieces of flexible hose. When

is forced open, it allows steam and coolant to escape

the cooling system is equipped with an overflow tank, the

through the overflow pipe until the pressure drops below

pressure cap is placed on the tank rather than on the

its opening point. Four pounds pressure in the cooling

radiator, and a plain cap is used on the radiator. This cap

system will prevent overflow loss from boiling of water until

arrangement leaves the overflow pipe open to the tank.

a coolant temperature of about 225F (at sea level) is

The plain cap on the radiator must always be pressure-

reached.

Some vehicles have valves with opening

tight to permit

pressures as high as 17 pounds This pressure raises the

boiling point of water to about 255F.

1-5