passages around the valve seats when the valves are
o p e r a t i o n . To r e s t r i c t c o o l a n t c i r c u l a t i o n , a
thermostatically controlled valve, or thermostat, is
located in the head.
installed in the cylinder head water outlet. This valve
The passages of the water jacket are designed to
allows coolant to circulate freely only within the block
control circulation of coolant and provide proper
until the desired temperature is reached. This shortens
cooling throughout the engine. The pump forces
the warm up period. A bypass is used to direct the
c o o l a n t d i r e c t l y f r o m t h e l ow e r r a d i a t o r t a n k
water from the block back to the pump when the
connection into the forward portion of the cylinder
passage to the radiator is blocked by the closed
block. This type of circulation would, obviously, cool
thermostat (fig. 5-6).
the No. 1 cylinder first, causing the rear cylinders to
Thermostats are manufactured in several designs,
accept coolant progressively heated by the cylinders
the most common being the bellows and the pellet.
ahead. To prevent this condition, some blocks are
equipped with a water distribution tube (fig. 5-2) that
BELLOWS THERMOSTAT.--The BELLOWS
extends from front to rear of the block, with holes
thermostat (fig. 5-7) consists of a round bellows that
adjacent to (and directed at) the hottest parts of each
contains a small amount of a highly volatile liquid
cylinder. Some engines are equipped with ferrule type
(such as ether). The liquid creates a pressure when
water directors, which direct a jet of water toward the
heated. This pressure opens the valve as the engine
exhaust valve seats.
warms up and allows the coolant to circulate. Should
the bellows become punctured or ruptured, the
Thermostats
thermostat will remain in the open position.
Automatic control of the engine's temperature is
P E L L E T T H E R M O S TAT . -- T h e P E L L E T
necessary for efficient engine performance and
thermostat (fig. 5-8) uses the piston and spring
economical operation. If the engine is allowed to
pressure principle. A small mass of powder-like
operate at a low temperature, sludge buildup and
crystals is enclosed within a small sealed receptacle
excessive fuel consumption will occur. On the other
(pellet) from which a small piston rod extends to the
hand, overheating the engine or operating it above
valve. The crystals expand under heat and push the
normal temperature will result in burnt valves and
piston sufficiently to overcome spring tension. This
faulty lubrication. The latter usually causes early
opens the valve. Spring tension closes the thermostat
when the crystals contract. Operative failure of this
engine failure.
kind of thermostat is caused by impregnation of the
Since all engine parts are in a contracted state
pellet with coolant, causing it to remain in the open
when cold, the engine temperature should be brought
position.
to normal as quickly as possible. The water pump
starts coolant circulating the moment the engine is
OPERATION OF THE THERMOSTAT.--
started, which is undesirable for cold weather
Although a thermostat is designed to open at a specific
WAT E R
WAT E R
T H E R M O S TAT
PUMP
PUMP
OPENED
T H E R M O S TAT
CLOSED
ENGINE
ENGINE
FA N
FA N
E N G I N E WA R M - VA LV E
E N G I N E C O L D - VA LV E
O P E N E D B Y T H E R M O S TAT
C L O S E D B Y T H E R M O S TAT
A L L O W S WAT E R TO C I R C U L AT E
A L L O W S WAT E R TO C I R C U L AT E
THROUGH THE ENGINE
THROUGH THE ENGINE BUT
R A D I ATO R
A N D T H E R A D I ATO R .
R A D I ATO R
N O T T H E R A D I ATO R
Asf05006
Figure 5-6.--A thermostat in operation.
5-7
