the shutter opening. A stationary engine uses a
SHUTTERSTAT (long thermostatic valve) connected
to the engine cooling system with hoses or pipes that
allow the coolant to circulate through the valve. The
temperature of the coolant, when it reaches a
predetermined temperature, causes the valve to expand
extending a rod which through linkage forces the
shutters open. Trucks, equipped with an air brake, use a
smaller thermostatic valve that actuates an air valve.
This air valve allows pressure from the air tank to enter
the air cylinder attached to the shutter operating
mechanism, forcing the shutters open.
Expansion (Recovery) Tank
Many cooling systems have a separate coolant
reservoir or expansion tank, also called the recovery
tank. It is partly filled with coolant and is connected to
the overflow tube from the radiator filler neck. The
coolant in the engine expands, as the engine heats up.
Instead of dripping out of the overflow tube onto the
ground and being lost out of the system completely, the
coolant flows into the expansion tank.
When the engine cools, a vacuum is created in the
The vacuum siphons some of the
coolant back into the radiator from the expansion tank.
In effect, a cooling system with an expansion tank is a
closed cooling system (fig. 6-13). Coolant can flow
back and forth between the radiator and the expansion
tank. This occurs as the coolant expands and contracts
from the heating and cooling. Under normal conditions,
no coolant is lost. Coolant is added in this system
through the expansion tank that is marked for proper
coolant level. NEVER remove the 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
whenever you work around a closed cooling system.
An advantage to the use of an expansion tank is that
it eliminates almost all air bubbles from the cooling
system. Coolant without bubbles absorbs heat better.
Although the coolant level in the expansion tank goes
up and down, the radiator and cooling system are kept
full. This results in maximum cooling efficiency.
Figure 6-13.Closed cooling system.