chap21 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 21, the reader should
be able to:

•
•
•

Prepare for Engine Repair (A1) ASE certification test

content area “D” (Lubrication and Cooling Systems
Diagnosis and Repair).
Explain engine oil ratings.
Describe how an oil pump and engine lubrication
work.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 21, the reader should

be able to:

•
•

Discuss how and when to change the oil and filter.
Explain how to inspect an oil pump for wear.

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Engine oil is the lifeblood of any engine. The purposes of

engine oil include the following:

1.
2.
3.
4.

Lubricating all moving parts to prevent wear
Helping to cool the engine
Helping to seal piston rings
Cleaning, and holding dirt in suspension in the oil
until it can be drained from the engine
5. Neutralizing acids that are formed as the result of
the combustion process
6. Reducing friction
7. Preventing rust and corrosion
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KEY TERMS:
boundary lubrication • hydrodynamic lubrication
longitudinal header • oil gallery
positive displacement pump • pressure regulating valve
viscosity • windage tray

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LUBRICATION PRINCIPLES
Lubrication between two moving surfaces results from an oil
film that separates the surfaces and supports the load.

Figure 21–1 Oil molecules cling to metal
surfaces but easily slide against each other.

Although oil does not compress,
it does leak out around the oil
clearance between the shaft and
the bearing. In some cases, the oil
film is thick enough to keep the
surfaces from seizing, but can
allow some contact to occur. This
is called boundary lubrication.

The specified oil viscosity and oil clearances must be adhered to
during service to help prevent boundary lubrication and wear from
occurring, usually when under heavy load at low speed.
Continued
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The movement of the shaft helps prevent contact with the bearing. If
oil were put on a flat surface and a heavy block were pushed across
the surface, the block would slide more easily than if it were pushed
across a dry surface.
The reason is that a wedge-shaped oil film is built up between the
moving block and the surface, as illustrated here. This action is called
hydrodynamic lubrication and depends on force applied to how fast
the speed between the objects and the thickness of the oil.

Figure 21–2
Wedge-shaped
oil film developed
below a moving
block.

Continued
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Thickness of oil is called the viscosity and is defined as the
ability of the oil to resist flow. High-viscosity oil is thick and
low-viscosity oil is thin. The prefix hydro- refers to liquids, as in
hydraulics, and dynamic refers to moving materials.
Hydrodynamic lubrication takes
over as the shaft rotates in the
bearing to produce a wedgeshaped film curved around the
bearing. This film supports the

bearing and reduces turning
effort to a minimum.
Figure 21–3 Wedge-shaped oil film curved
around a bearing journal.

Most bearing wear occurs during the initial start-up and continues
until a hydrodynamic film is established.
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ENGINE LUBRICATION SYSTEMS
The primary function of the engine lubrication system is to

maintain a positive and continuous oil supply to the bearings.
Engine oil pressure must be high enough to get oil to the
bearings with enough force to cause oil flow required for proper
cooling.
The normal engine oil pressure range is from 10 to 60 psi (200 to
400 kPa) (10 psi per 1000 engine rpm).
Hydrodynamic film pressures developed in high-pressure areas
of the engine bearings may be over 1,000 psi (6,900 kPa).
Relatively low engine oil pressures obviously could not support
these high bearing loads without hydrodynamic lubrication.
Continued
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OIL PUMPS
All production automobile
engines have a full-pressure
oil system. Oil is drawn from
the bottom of the oil pan and
is forced into the lubrication
system under pressure.
In most engines that use a
distributor, the distributor
drive gear meshes with a
gear on the camshaft, as
shown here.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 21–4 An oil pump driven by the camshaft.

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The oil pump is driven from the end of the distributor shaft, often
with a hexagon-shaped shaft. Some engines have a short shaft gear
that meshes with the cam gear to drive both distributor and oil
pump. With these methods, the pump turns at one half engine speed.

Figure 21–5 The operation of a rotor- type oil pump.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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In other engines, the oil pump is driven by the crankshaft, in a setup
similar to that of an automatic transmission pump, so it turns at the
same speed as the crankshaft. Examples of a crankshaft-driven oil
pump are shown here.

Figure 21–6 A typical oil pump mounted in
the front cover of the engine that is driven
by the crankshaft.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 21–7 Geroter-type oil pump driven
by the crankshaft.

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Most automotive engines use one of two types of oil pumps:
gear or rotor. All oil pumps are called positive displacement
pumps, and each rotation of the pump delivers the same
volume of oil; thus, everything that enters must exit.
The gear-type oil pump consists of two spur gears in a closefitting housing—one gear is driven while the other idles. As the
gear teeth come out of mesh, they tend to leave a space, which
is filled by oil drawn through the pump inlet.
When the pump is pumping, oil is carried around the outside
of each gear in the space between the gear teeth and the
housing, as shown in Figure 21–8.

Continued
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As the teeth mesh in the center, oil is forced from the teeth into an oil
passage, producing oil pressure. The rotor-type oil pump consists
essentially of a lobe-shape gear meshing with a lobed rotor.

The center lobed section is
driven and the outer section
idles. As the lobes separate,
oil is drawn in just as it is
drawn into gear-type pumps.
As the pump rotates, it carries
oil around and between the
lobes. As the lobes mesh, they
force oil from between them
under pressure in the same
manner as a gear-type pump.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 21–8 In a gear-type oil pump, the oil
flows through the pump around the outside
of each gear. This is an example of a positive
displacement pump, wherein everything
entering the pump must leave the pump.

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The pump is sized so that it will maintain a pressure of at least 10
psi (70 kPa) in the oil gallery when the engine is hot and idling.
Pressure will increase about 10 psi for each 1000 rpm as the engine
speed increases, because the engine driven pump also rotates faster.
NOTE: The oil pump is the only engine component using unfiltered oil.

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OIL PRESSURE REGULATION
In engines with a full-pressure lubricating system, maximum
pressure is limited with a relief valve (sometimes called the
pressure regulating valve) located at the outlet of the pump. It
controls pressure by bleeding off oil to the inlet side of the pump.

The relief valve spring tension
determines the maximum oil

pressure.
If a pressure relief valve
is not used, the engine oil
pressure will continue to
increase as the engine
speed increases.
Continued

Figure 21–9 Oil pressure relief valves are spring
loaded. The stronger the spring tension, the
higher the oil pressure.

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Maximum pressure is usually limited to the lowest pressure that
will deliver enough lubricating oil to all engine parts that need
to be lubricated.

3 to 6 gallons per minute are required to lubricate the engine.
The oil pump is large enough to provide pressure at low engine
speeds and small enough so cavitation won’t occur at high speed.
Cavitation occurs when the pump tries to pull oil faster than it can
flow from the pan to the pickup. When it cannot get enough oil, it
will pull air. This puts air pockets or cavities in the oil stream. A
pump is cavitating when it is pulling air or vapors.
NOTE: Sheet-metal covers over the pickup screen prevent cavitation. Oil
is trapped under the cover, helping prevent the oil pump from drawing in
air, especially during sudden stops or during rapid acceleration.
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After the oil leaves the pump, it is delivered to the oil filter and then
to the moving parts through drilled oil passages. See Figure 21–10.
It needs no pressure after it reaches the parts that are to be
lubricated. The oil film between the parts is developed and
maintained by hydrodynamic lubrication.
Excessive oil pressure requires more horsepower and provides no
better lubrication than the minimum effective pressure.

Continued
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Figure 21–10 A typical engine design that uses both pressure and splash lubrication. Oil travels
under pressure through the galleries (passages) to reach the top of the engine. Other parts are
lubricated as the oil flows back down into the oil pan or is splashed onto parts.

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FACTORS AFFECTING OIL PRESSURE
Oil pressure can only be produced when the oil pump has a capacity
larger than all the “leaks” in the engine. The leaks are the clearances
at end points of the lubrication system. end points are at the edges of
bearings, the rocker arms, the connecting rod spit holes, etc.

These clearances are designed into the engine and necessary for its
proper operation. As parts wear and clearance becomes greater,
more oil will leak out. The oil pump capacity must be great enough
to supply extra oil for these leaks.
If the leaks are greater than the pump capacity, engine oil
pressure is low. As the engine speed increases, the pump capacity
increases and the pump tries to force more oil out of the leaks.
This causes the pressure to rise until it reaches the regulated
maximum pressure.
Continued
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The viscosity of the engine oil affects both the pump capacity
and the oil leakage. Thin oil or oil of very low viscosity slips past
the edges of the pump and flows freely from the leaks.
Hot oil has a low viscosity, and therefore, a hot engine often has
low oil pressure. Cold oil is more viscous (thicker) than hot oil.
This results in higher pressures, even with the cold engine idling.
High oil pressure occurs with a cold engine, because the oil relief
valve must open further to release excess oil than is necessary
with a hot engine. This larger opening increases the spring
compression force, which in turn increases the oil pressure.
Putting higher-viscosity oil in an engine will raise engine oil
pressure to the setting of the relief valve at a lower engine
speed.

Continued

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OIL PUMP CHECKS
The cover is removed to check condition of the oil pump. The
gears and housing are examined for scoring. If gears and
housing are heavily scored, the entire pump should be replaced.
If they are lightly scored, the clearances should be measured.
These include the space between the gears and housing, the
space between the teeth of the two gears, and space between the
side of the gear and pump cover. A feeler gauge is often used to
make these measurements.
Gauging plastic can be used to measure the space between the
side of the gears and the cover. The oil pump should be replaced
when excessive clearance or scoring is found. See Figure 21–11.
Continued

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Figure 21–11
(a) A visual inspection indicated that this pump cover was worn. (b) An embedded particle of
something was found on one of the gears, making this pump worthless except for scrap metal.
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NOTE: The oil pump is the “garbage pit” of the entire engine. Any and all

debris is often forced through the gears and housing of an oil pump.

Figure 21–12 (b) If debris gets into an oil
pump, the drive or distributor shaft can twist
and/or break. When this occurs, the engine will
lose all oil pressure.

Figure 21–12 (a) The oil pump is the only part in
an engine that gets unfiltered engine oil. The oil
is drawn up from the bottom of the oil pan and
is pressurized before flowing to the oil filter.

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OIL PASSAGES IN THE BLOCK
From the filter, oil goes through a drilled hole that intersects with a
drilled main oil gallery or longitudinal header. This is a long hole
drilled from the front of the block to the back. Inline engines use one
oil gallery; V-type engines may use two or three galleries.

In some engines, oil goes to the cam bearings first, and then to the
main bearings. It is important that the oil holes in the bearings
match with the drilled passages in the bearing saddles so that the
bearing can be properly lubricated.

Over a long period of use, bearings will wear. This wear causes
excess clearance. The excess clearance will allow too much oil to
leak from the side of the bearing. When this happens, there will be
little or no oil left for bearings located farther downstream in the
lubricating system. This is a major cause of bearing failure.
Continued
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