Small Engines
Cooling Systems
Principles of Engine Cooling
The average temperature of burned gases in the cylinder is about 3600°F (1982°C). Keeping an engine clean,
cool, and properly lubricated will extend its service life. About 1/3 of the heat is carried away by the cooling
system. The exhaust system carries away 1/3 of the heat, and what remains is used to produce engine power.
The small engine you work with in the shop uses an air cooling system. Various parts surrounding the aircooled engine direct the flow of air.
Oil also cools both air-cooled and water-cooled engines.
The crankshaft, piston, camshaft, and some other internal engine parts are directly dependent on motor oil for
cooling. Each of these parts has a temperature limit that cannot be exceeded. Oil absorbs heat from these
components and transfers it back to the crankcase.
How Air Cooling Works
The heat of combustion (rapid burning and expansion of gas)
travels from the cylinder through the cylinder walls by
conduction. Conduction is the transfer of heat through a solid
material. When the heat reaches the outer surface of the
cylinder, air forced over the surface carries it away by
convection. Convection is the transfer of heat thru air. The
flywheel generates airflow on an air-cooled engine. The flywheel
has fins, which blow air around the cylinder wall and cooling fins.
Sheet metal parts called baffles direct the airflow around the
engine. Cooling fins increase the surface area around the
outside of the cylinder. The greater the surface area in contact
with cool air, the more rapidly heat can be carried away.
Dirt, grass, and other materials that become lodged in the
cooling fins around the cylinder will tend to insulate the cylinder,
causing hot spots and engine overheating. Keeping the engine
clean actually helps keep it cool!
The engine cover (fan cover) is also part of an air-cooled engine.
It helps redirect airflow from the flywheel back on to the cooling
fins on the cylinder.
TURN OVER>>>
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How Water Cooling Works
Some engines are water cooled because they are used in or
around a water source. Water is an excellent medium for
cooling engines. It is inexpensive, readily available, and absorbs
heat well. Water is 4 times as effective than air for engine
cooling. Cooling fins are required on air-cooled engines but not
on water-cooled engines. Cylinders of liquid-cooled engines are
surrounded by coolant passages called water jackets. A small
pump keeps water circulating through the jackets. The water
absorbs the heat of combustion and carries it away from the
engine. In cold weather, antifreeze solutions are added to the
water to prevent it from freezing. In some water-cooled engines,
the temperature of the cooling water is controlled by a
thermostat. The thermostat stops circulation of coolant until the
engine reaches a specific operating temperature.
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