PSAA Curriculum
Unit
Physical Science Systems
Problem Area
Energy and Power Systems
Lubricating Oils: Viscosity and
Temperature
Lesson
Oil Comparison
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What is the difference in the pour rate?
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Why does a difference occur?
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What implications are there for the selection
of the proper viscosity rated oil?
Learning Objectives
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Define friction and explain the types of friction
that affect internal combustion engines.
Define viscosity as it relates to single and
multiviscosity lubricating oils.
Explain the effects of temperature on the
viscosity of single and multiviscosity oils.
Define synthetic oil and explain its
advantages and disadvantages compared to
petroleum-based oils.
Terms
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Adhesion
Cohesion
Flow rate
Friction
Lubricant
S.A.E.
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Sliding friction
Starting friction
Synthetic oil
Temperature
Viscosity
Viscosity index
What is friction and how does it affect
moving objects such as engines?
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The force that acts to resist the rotation of
objects that are in contact is friction.
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Friction can be desirable and even necessary
in some situations and clearly undesirable in
others.
What is friction and how does it affect
moving objects such as engines?
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In an internal combustion engine
friction leads to reduced power,
efficiency and wear on the engine
parts.
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Force of friction between two objects
depends upon the surface finish,
materials used and the pressure
forcing them together.
What is friction and how does it affect
moving objects such as engines?
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The material used affect the cohesion
(attraction of like substances) and adhesion
(attraction of unlike substances) between the
surfaces.
Starting friction is the resistance to movement
between two solids.
Sliding friction is the friction between two
moving objects.
What is viscosity and how does viscosity
relate to single and multiviscosity
lubricating oils?
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In an engine, moving parts are lubricated to
reduce wear, friction and heat.
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The role of a lubricant is to provide a thin
film of oil between moving parts that reduces
the cohesive forces between the two metallic
objects.
Viscosity
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Flow rate is an excellent test of viscosity
under different temperatures.
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Flow rate measures the amount of time a
given amount of oil takes to pass a certain
point at a given temperature.
Viscosity (cont.)
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Viscosity refers to the internal friction of a
fluid.
This friction is created by molecular attraction
or cohesion, which makes the fluid resistant
to flow.
A viscous fluid has a cohesive and sticky fluid
consistency.
Engine oils have to be viscous to be able to
adhere better to moving engine parts.
Viscosity (cont.)
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Multi viscosity oils contain polymers added
to a light base (5W, 10W, 20W) which prevent
the oil from thinning as much as increases in
temperature.
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A 20W50 oil is a 20 weight oil that will not thin
more than a 50 weight at high temperatures.
Viscosity (cont.)
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Higher viscosity
numbers usually mean
a thicker oil.
Lower viscosity means
thinner oil.
The weight given to oils
are numbers assigned
the S.A.E. (Society of
Automotive Engineers).
Viscosity (cont.)
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These numbers correspond to “real” viscosity
measured by several techniques.
Oils that fall into a certain range are
designated 5, 10, 20, 30, 40, 50 by the S.A.E.
The “w” means the oil meets specifications
for viscosity at 0 degrees Fahrenheit and is
suitable for winter use.
How do various temperatures affect the
viscosity of single viscosity and
multiviscosity lubricating oils?
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Viscosity index is an empirical number
indicating the rate of change in viscosity of an
oil within a given temperature range.
Higher numbers indicate a low change, lower
numbers indicate a relatively large change.
The higher the number, the better the oil
performs within a viscosity range.
Temperature affects…
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At cold temperatures the polymers in multi
viscosity oils coil up and allow the oil to flow
as the low numbers indicate.
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As the oil warms up the polymers begin to
unwind into long chains that prevent the oil
from thinning as much as it normally would.
What advantages and disadvantages do
synthetic oils have compared to naturally
occurring petroleum-based oils?
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Synthetic oil has been defined as any
lubricant that does not have a naturally
occurring mineral oil base-stock.
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True synthetic lubricants were defined as
those which do not rely on petro-chemicals to
be produced.
Synthetic Oils
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General characteristics for synthetic oils are
that far greater loads and temperatures can
be withstood than the equivalent mineral or
vegetable based lubricant.
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Synthetics have been developed as
lubricating oils for extreme conditions such as
truck engines and extra pressure gear trains.
Synthetic Oils
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Synthetics offer superior high temperature
oxidation resistance, high film strength, very
low tendency to form deposits, stable
viscosity base and low temperature flow
characteristics.
Synthetics are superior lubricants compared
to traditional petroleum oils.
The consumer must decide if the high cost is
justified in the application.
Review/Summary
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What is friction and how does it affect moving
objects such as engines?
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What is viscosity and how does viscosity
relate to single and multiviscosity lubricating
oils?
Review/Summary
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How do various temperatures affect the
viscosity of single viscosity and multiviscosity
lubricating oils?

What advantages and disadvantages do
synthetic oils have compared to naturally
occurring petroleum-based oils?