86 Auto Engine Performance and Driveability
Lubrication is the most obvious job of engine oil.
Oil reduces friction by forming a layer between moving
parts. All engine parts, no matter how finely machined,
have microscopic high spots. As one part moves across
another, these spots will contact each other. As a result,
the parts begin to wear and overheat from friction. The
oil separates the moving parts, preventing the high spots
from touching. The oil acts like a set of microscopic ball
bearings, allowing the parts to slide against each other.
Friction is greatly reduced.
Cooling is one of the less obvious jobs of engine
oil. It is difficult to remove heat from the piston heads.
They cannot be cooled by the cooling system, yet the
pistons are among the hottest parts in the engine. They
will overheat and melt if the heat is not removed. Even
a slightly overheated piston can develop a localized hot
spot that will cause detonation. Oil splashed under the
piston head absorbs heat from the piston. Oil splash also
removes heat from the cylinder walls, rods, crankshaft,
and other engine parts that cannot be directly cooled by
the engine cooling system. The oil itself is cooled by the
cooling system and by direct heat transfer when it is in
the oil pan.
Sealing is another task of engine oil. A thin film of oil
between the piston rings and cylinder wall seals in pressure.
If this oil is not present, pressure leakage (loss of compres-
sion) will prevent the engine from running.
For some moving parts, such as connecting rod journals
and rod bearings, engine oil provides shock absorption. The
oil cushions the shock when the rod changes direction at
high speeds. This extends bearing life and prevents engine
knocking. Other moving parts, such as the piston skirts and
cam lobes, are also cushioned by oil.
Cleaning is also an important job of engine oil. Engines
constantly collect impurities. Unburned gasoline, carbon
from the combustion process, and water vapor get into the
crankcase. These impurities can form sludge and varnish
deposits in an engine. The deposits can cause the engine
to overheat, burn oil, and prematurely wear out. Engine
oil contains detergents that prevent formation of sludge
by picking up impurities and holding them in suspension.
When the oil circulates through the oil filter, the impurities
are trapped.
The American Petroleum Institute (API) classifies oil
according to various factors that affect oil’s ability to prevent
friction and deposits in an engine. The API service classifi-
cation is printed on the oil container, Figure 5-24. The API
Figure 5-23. Automotive engines use a combination of pressurized and splashed oil to lubricate engine components. The lubrication
system also serves to cool the internal engine components. Note that this engine also uses oil pressure to control a variable timing
system. (Ford)
Housing asy-
engine variable timing
Housing asy-
engine variable
timing LH
Housing asy-
engine variable timing
Block asy-
cylinder
Adapter asy-
oil filter
Oil filter
Camshaft
sprocket
bolt
Oil pan asy
Oil pump-
screen & cover asy
From head
Back to
head
To chain
tensioner
Metering
orifice
Filter
Retard side
of variable
camshaft
timing
Tensioner-
engine
timing chain
Advance side
of variable
camshaft
timing
Pump asy-
oil less screen
& cover
Valve
body asy
Oil to front
of camshaft
timing
Variable camshaft timing
auxiliary view
Variable timing asy RH
auxiliary view
Oil to rear
of camshaft
timing rotor
Triggerwheel
Engine
variable
camshaft
timing asy
From head
Head-cylinder
Engine variable
camshaft timing asy
Tensioner-
engine timing
chain LH