engine power, creates destructive heat and rapid wear of
other less critical engine parts that also need a constant
parts. The greater the friction present between moving
supply of oil, such as the camshaft, valve stems, rocker
parts, the greater the energy required to overcome that
arms, and timing chains.
friction. The Increase In energy adds to the amount of
heat generated, causing moving parts that are deprived
8-3. Oil as a Coolant. Engine oil circulated throughout
of oil to melt, fuse, and seize after a very short period of
the engine also serves to remove heat from the friction
The effectiveness of a modern
points. The oil circulates through the engine and drains
lubrication system makes possible the use of friction-type
to the sump. The heat picked up by the oil while It Is
bearings (para 19-7) In an engine. Friction between the
circulated Is removed by an airflow around the outside of
pistons and the cylinder walls Is severe, making effective
the sump. In some Instances where the sump Is not
lubrication of this area Imperative. Lubrication of the
exposed to a flow of air, It Is necessary to add an oil
connecting rod and main bearings is crucial because of
cooling unit that transfers the heat from the oil to the
the heavy loads that are placed on them. There are
engine cooling system.
Section II. ENGINE OILS
crankcase. This dirt, combined with foreign matter
8-4. General. Mineral oil Is used in most internal
entering through the crankcase breather pipe, mixes with
combustion engines. Engine oils generally are classified
the oil, and when forced into the bearings, greatly
according to their performance qualilties and their
Water, one of the products of
combustion, will seep by the piston rings as steam and
condense In the crankcase. The water In the crankcase
a. How Oil Lubricates (Fig. 8-2).
then will emulsify with the oil to form a thick sludge.
Products of fuel combustion will mix with the oil as they
(1) Every moving part of the engine is
enter the crankcase through blowby (para 3-6). The oil,
designed to have a specific clearance between it and its
when mixed with the contaminants, loses its lubricating
bearing. As oil is fed to the bearing it forms a film,
qualities and becomes acidic. Engine oil must be
preventing the rotating part from actually touching the
changed periodically to prevent contaminated oil from
allowing excessive wear and causing etching of bearings.
Oil contamination is controlled In the following ways.
(2) As the part rotates, the film of oil acts as a
series of rollers. Because the moving parts do not
Control engine temperature; a hotter
actually touch each other, friction Is reduced greatly.
running engine burns its fuel more completely and
evaporates the water produced within it before any
(3) It is Important that sufficient clearance be
appreciable oil contamination occurs.
allowed between the part and the bearing. Otherwise the
film might be too thin. This would allow contact between
(2) The use of oil filters removes particulate
the parts, causing the bearing to wear or burn up.
matter from the oil before it reaches the bearings,
(4) It also is Important that the clearance not be
too large between rotating parts and their bearings. This
(3) An adequate crankcase ventilation system
Is true particularly with heavily loaded bearings like those
will purge the crankcase of blowby fumes effectively
found on the connecting rods. The heavy loads could
before a large amount of contaminants can mix with the
then cause the oil film to be squeezed out, resulting In
(4) The use of air intake filters trap foreign
b. Oil Contamination (Fig. 8-3). Oil does not wear
material and keep it from entering the engine.
out but it does become contaminated. When foreign
matter enters through the air Intake, some of it will pass
c. Oil Dilution (Fig. 8-3). Engine oil thins out when
by the piston rings and enter the
mixed with gasoline, causing a dramatic