88 Auto Engine Performance and Driveability
Oil Pump
The engine oil pump develops oil pressure and flow
to circulate oil throughout the lubrication system. Usually,
it is driven by a gear on the camshaft or crankshaft. Most
gear-driven oil pumps are mounted near the bottom of the
engine and connect to the camshaft drive gear through
a shaft. Some oil pumps are installed in the engine front
cover and are directly driven by the engine crankshaft. The
oil pump speed varies with engine speed, since the pump
is driven by the engine.
Engine oil pumps are always constant-displacement
types. A constant-displacement pump delivers the same
amount of oil with each revolution. The faster the pump
revolves, the more oil it delivers. Figure 5-27 illustrates the
external-gear and rotor designs of oil pump. The external-
gear design is most common; however, more and more
engines are equipped with rotor pumps.
When the pump gears rotate, the gear teeth unmesh in
the inlet area. This creates a low-pressure area that draws
oil from the oil pan or oil tank. The oil is carried around
the housing in the spaces between the gears and housing.
When the gear teeth mesh at the outlet, a high-pressure
area is created. The oil is squeezed out of the discharge
port. The process is then repeated.
Pressure Regulator
The oil pump has enough capacity to deliver sufficient
oil pressure and flow at idle speeds. At higher engine speeds,
the pump will produce too much oil pressure and flow.
This may rupture seals or filter elements, affect hydraulic
lifter operation, or cause oil burning.
Oil pump output is controlled by a pressure regulator.
The pressure regulator consists of a valve that is held closed
by a spring, Figure 5-28. Oil pressure from the pump pushes
against the valve on the opposite side from the spring. When
the oil pressure reaches a certain point, the spring is com-
pressed and the valve opens. This dumps excess oil into the
sump, which is at a lower (near atmospheric) pressure. As a
result, the oil pressure is regulated. Engine oil pressure is usu-
ally regulated to about 35–45 psi (240–310 kPa). Minimum
pressure at idle should be 15–20 psi (105–140 kPa).
Oil Filter
The oil filter removes small particles and contami-
nants from the engine oil. It consists of a stamped metal
housing containing, typically, a pleated paper filter ele-
ment. Other types of elements may be used. The filter is
always installed on the outlet side of the oil pump. Oil
under pressure flows into the filter, through the filter ele-
ment, and out of the filter.
The oil filter contains an internal bypass. This allows
oil to flow past the filter if the element becomes clogged.
However, the oil is not filtered when this happens. Some
oil filters have an anti-drainback valve. This valve closes to
keep oil from draining out of the filter when the engine is
not operated for long periods.
The oil filter is replaced during an oil change as part
of normal engine maintenance. A typical oil filter, such as
the ones shown in Figure 5-29, can be removed from the
engine by unscrewing it from the mounting pad. In some
engine designs, the oil filter is mounted on the firewall or
Figure 5-27. The oil pump may be of the external gear or rotor
design. Both designs have a positive displacement.
Outlet
Inlet
Inlet
Gear
Gear
External gear pump
Outlet
Rotor pump
Internal
gear
External gear
Figure 5-28. The pressure regulator controls the maximum
oil pressure. Normal oil pressure will not unseat the valve.
However, high oil pressure forces the valve open, allowing oil
to flow to the pan.
Oil pressure acting on the ball
compresses the spring
High oil
pressure
Oil flows to the pan
Seat
Ball
Spring holding the ball
against the seat
Normal
oil
pressure