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Chapter 5 ENGINE OIL
Definition of properties of engine oil
Viscosity
Viscosity is a basic physical property of engine oil and
is considered as the most important aspect when eval-
uating oil.
Contamination of oil by blow-by gas and deterioration
of oil by its natural aging increase the 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 viscos-
ity, which will cause insufficient lubrication and friction/
wear of engine parts.
Total base number
Total base number (TBN) shows the ability to neutral-
ize acids such as organic acid due to engine oil oxida-
tion, or sulfurous or sulfuric acid due to the sulfur
content of fuel.
Because TBN indicates the amount of dispersant
detergent in oil, it can be used to estimate consump-
tion of basic dispersant detergent. The ability to dis-
perse sludge declines as dispersant detergent is used
up.
Total acid number
The total 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 combus-
tion of sulfur content of fuel, or the oil becomes con-
taminated with imperfect combustion products.
An increase in the total acid number will result in cor-
rosion or wear of the inner parts of the engine (such
as cylinder liners or metal) due to sulfur content, and
piston ring seizure due to sludge.
5-4
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 remain-
ing dispersibility can be obtained by measuring insolu-
ble and coagulated insoluble (using chemical
specialities to stop action of disperse detergent and to
collect the sludge dispersed in oil) by which piston ring
seizure or premature wear can be prevented before it
occurs.
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