PRIN. OF INTERNAL COMBUSTION ENGINES - OD1619 LESSON 2/TASK 2
remove heat from the friction points. The oil circulates through the engine and
drains to the sump. The heat picked up by the oil while it is circulated is
reduced by an airflow around the outside of the sump. In some instances, where the
sump is not exposed to a flow of air, it is necessary to add an oil cooling unit
generally are classified according to their performance qualities and their
(1) How Oil Lubricates (figure 49 on the following page).
(a) Every moving part of the engine is designed to have a specific clearance
from its adjacent surface. As oil is fed to the surface it forms a film,
preventing the moving part from actually touching the surface.
(b) As a part rotates, the film of oil acts as a series of rollers. Because
the moving parts do not actually touch each other, friction is reduced greatly.
(c) It is important that sufficient clearance be allowed between the part and
the bearing; otherwise the film might be too thin. This would allow contact
between the parts, causing the bearing to wear or burn up.
(d) It also is important that the clearance not be too large between rotating
parts and their bearings. This is true particularly with heavily loaded bearings
like those found on the connecting rods. The heavy loads could then cause the oil
film to be squeezed out, resulting in bearing failure.
(2) Oil Contamination (figure 50 on page 62). Oil does not wear out, but it does
become contaminated. When foreign matter enters through the air intake, some of it
will pass by the piston rings and enter the crankcase. This dirt, combined with
foreign matter entering through the crankcase