Operation - TM-750-2540012

the tank to operate properly. Most tanks are equipped with

1-12.

Thermostat

an overflow pipe and drain cock.

a. General. Full-length engine water jackets, large

c. Operation. Either expansion of the coolant when

efficient radiator cores, and rapid coolant circulation in the

it is heated or steam pressure due to boiling will force

system provide the extra cooling required for engine

coolant into the overflow tank. Boiling may occur during

operation under heavy load in hot weather. However,

operation, but it happens more often after the engine is

under lighter engine load in cool weather, the same

stopped. When the engine cools down, pressure in the

amount of cooling would remove too much heat from the

system drops below the pressure of air outside and any

engine. Therefore, the amount of heat removed from the

coolant held in the overflow tank is forced back into the

engine must be controlled for different operating conditions

radiator. The overflow tank prevents loss of coolant from

and air temperatures. This is done by the thermostat

boiling during periods of severe vehicle operation.

which regulates engine temperature by automatically

However, if the overflow from the radiator is so great that

controlling the amount of coolant flowing through the

the tank is filled, coolant will be lost through the tank

radiator core.

overflow.

Construction

(1) Bellows-type The thermostat (fig. 1-7) consists

of a valve and a heat-operated unit which moves the valve.

This type of thermostat-operating unit contains a special

liquid designed to boil at a certain temperature. When that

temperature Is reached, the boiling liquid creates gas

pressure which expands the bellows and opens the

thermostat valve. When the liquid cools and condenses,

pressure is reduced, allowing the bellows to contract and

close the valve.

(2) Coil spring-Type In this type of thermostat (fig 1-

8), the valve is operated by a bimetallic coil which depends

for its operation upon the difference of coefficients of

expansion of the two metals. The coil expands and opens

the valve when heated above a certain temperature. As

Figure 1-6. Radiator overflow tank.

the coil cools down, it contracts and closes the valve

CLOSED

Figure 1-7. Bellows-type thermostat - cutaway view.

1-6