Figure 1-48.-Thermostat operation.
from the block to the pump when the passage to the
radiator is blocked by the closed thermostat (fig. 1-48).
Some stationary engines and large trucks are
equipped with shutters that supplement the action of the
thermostat in providing a faster warmup and in
maintaining proper operating temperatures. When the
engine coolant is below a predetermined temperature,
between 185°F to 195°F, the shutters, located in front of
the radiator, remain closed and restrict the flow of air
through the radiator. Then, as the coolant reaches proper
temperature, the shutters start to open.
The simplest type of cooling is the air-cooled, or
direct method, in which the heat is drawn off by moving
air in direct contact with the engine. The rate of the
cooling is dependent upon the area exposed to the
cooling air, the heat conductivity of the metal used, the
volume of the metal or its size in cross section, the
amount of air flowing over the heated surfaces, and the
difference in temperature between the exposed metal
The overflow tank serves as a receptacle for coolant
forced out of the radiator overflow pipe and provides for
its return to the system. As the engine cools, the
balancing of pressures causes the coolant to siphon back
into the radiator.
Cooling systems using an overflow tank are known
as closed cooling systems (fig. 1-49). Coolant is usually
added to this system through the overflow tank that is
marked for proper coolant level. NEVER remove the
radiator cap located on the radiator unless you are
positive the system is cold. If there is any pressure in the
radiator, it will spray you with hot steam and coolant.
Use extreme caution when performing operators
maintenance on a closed cooling system.
Some engines use an expansion tank in their cooling
system (fig. 1-50). The tank is mounted in series with
the upper radiator hose and is used to supply extra room
for coolant expansion and generally takes the place of
the upper radiator tank. The pressure cap and the
overflow line are also mounted on the expansion tank.
Figure 1-49.-Closed cooling system.