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Definition of Properties of Engine Oil
Kinetic Viscosity
Kinetic viscosity is a basic physical property of engine
oil and is considered as the most important aspect
when evaluating oil.
Contamination of oil by blow-by gas and deterioration
of oil by its natural aging increase the kinetic viscosity
and degrade the performance of viscosity, which will
cause the deposition of sludge inside the engine and
oil filter clogging. Contamination of oil by fuel and
sheared molecules of viscosity index improver in oil
decrease the viscosity and degrade the performance
of viscosity, which will cause insufficient lubrication
and friction/wear of engine parts.
Base Number
Base number shows the ability to neutralize acids
such as organic acid due to engine oil oxidation, or
sulfurous or sulfuric acid due to the sulfur content of
fuel.
Because base number indicates the amount of disper-
sant detergent in oil, it can be used to estimate con-
sumption of basic dispersant detergent. The ability to
disperse sludge declines as dispersant detergent is
used up.
Acid Number
The acid number in oil increases as the organic acid is
being derived by the engine oil oxidation, or sulfurous
acid or sulfuric acid derived by the combustion of sul-
fur content of fuel, or the oil becomes contaminated
with imperfect combustion products.
An increase in the acid number will result in corrosion
or wear of the inner parts of the engine (such as cylin-
der liners or metal) due to sulfur content, and piston
ring seizure due to sludge.
Water Content
Water in oil promotes corrosion/wear, and decreases
lubricity in sliding parts.
Flash Point
The flash point is lowered by contamination with fuels.
Flash point is measured to check the dilution of fuel.
The dilution of fuel reduces oil film, and causes insuffi-
cient lubrication that will cause friction or wear of
engine parts.
Insoluble
Insoluble includes acid products of engine oil, imper-
fect combustion products, sludge or soot, metal abra-
sive particles and dust. Insoluble is an indication of
degradation/contamination of oil.
Dispersant detergent, which is an additive in engine
oil, absorbs sludge particles, and disperses them as
fine particles in oil. Total insoluble density and
remaining dispersibility can be obtained by measuring
insoluble and coagulated insoluble (chemical speciali-
ties which stop action of disperse detergent and col-
lect the sludge dispersed in oil) to understand engine
oil contamination level, and thereby, piston ring sei-
zure or premature wear can be prevented before it
occurs.
Chapter 5 ENGINE OIL
5-5
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