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TM 9-8000
vehicles and equipment because no radiator or hoses
9-1. Need for Cooling. All internal combustion engines
are required.  Air cooling generally will not be used
are equipped with some type of cooling system because
wherever water cooling is practical. This is because air-
of the high temperatures they generate during operation.
cooled engines do not run at even temperatures and
High temperatures are necessary to generate the high
require extensive use of aluminum to dissipate heat.
gas pressures that act on the head of the piston. Power
cannot be produced efficiently without high temperatures.
c.  Other Sources of Engine Cooling.  There are
However, it is not possible to use all of the heat of
other sources of heat dissipation for the engine in
combustion without harmful results. The temperature in
addition to the cooling system.
the combustion chamber during the burning of the fuel is
(1) The exhaust system dissipates as much, if
well above the melting point of iron. Therefore, if nothing
not more, heat than the cooling system, although that is
is done to cool the engine during operation, valves will
not its purpose.
burn and warp, lubricating oil will break down, and
(2) The engine oil, as stated in paragraph 81,
bearings and pistons will overheat, resulting in engine
removes heat from the engine and dissipates it to the air
from the sump.
(3) The fuel provides some engine cooling
9-2. Cooling Mediums.
through vaporization.
(4) A measurable amount of heat is dissipated to
a.  Liquid.  Liquid is the most popular coolant in
the  air  through  radiation  from
the  engine.
automotive use.  A liquid cooling system provides the
most positive cooling and is best for maintaining an even
engine temperature.
b.  Air.  Air cooling is most practical for small
vehicle motion. It should be noted that the downward
9-3. Flow of Coolant (Fig. 9-1). A simple liquid-cooled
flow of coolant through the radiator creates what is
cooling system consists of a radiator, coolant pump,
known as a thermosiphon action.  This simply means
piping, fan, thermostat, and a system of jackets and
that as the coolant is heated in the jackets of the engine,
passages in the cylinder head and cylinder block through
it expands. As it expands, it becomes less dense and
which the coolant circulates.
Some engines are
therefore lighter. This causes it to flow out of the top
equipped with a water distribution tube inside the cooling
outlet of the engine and into the top tank of the radiator.
passages that directs additional coolant to the points
As the coolant is cooled in the radiator, it again becomes
where the temperatures are highest.  Cooling of the
more dense and heavier.  This causes the coolant to
engine parts is accomplished by keeping the coolant
settle to the bottom tank of the radiator. The heating in
circulating and in contact with the metal surfaces to be
the engine and the cooling in the radiator therefore
cooled. The pump draws the coolant from the bottom of
creates a natural circulation that aids the water pump.
the radiator, forces it through the jackets and passages,
The earliest automotive vehicles relied on thermosiphon
and ejects it into the upper tank on the top of the
action and used no water pump.
radiator. The coolant then passes through a set of tubes
to the bottom of the radiator from which the cooling cycle
9-4. Engine WaterJackets (Fig. 9-1).
begins again. The radiator is situated in front of a fan
that is driven either by the water pump or an electric
motor. The fan ensures an airflow through the radiator at
a.  The water passages in the cylinder block and
cylinder head form the engine water jacket.


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