Engine Oil Performance Requirements; Engine Oil Deterioration Mechanisms - Mitsubishi S4S-DT Operation & Maintenance Manual

Chapter 5 ENGINE OIL
Engine oil performance
requirements
Engine oil requires the following performances.
Excellent dispersion performance (the ability of oil to
disperse sludge in the oil) at high temperature that
prevents engine oil deterioration due to sludge accu-
mulation and soot contamination.
Excellent acid-neutralizing performance that prevents
oxidative degradation due to fuel sulfur content.
Excellent high temperature oxidation stability that en-
dures continuous operation under prolonged high-
load.
Sufficient viscosity concentration to maintain the cold
start performance, and lubrication performance at
high temperature.
Good rust and corrosion resistance to water.
Good foam resistance to prevent the lubricating qual-
ity from lowering due to oxidation.
Engine oil deterioration
mechanisms
Engine oil deteriorates due to natural deterioration
and due to the contamination. The natural deteriora-
tion of oil has two primary causes; one is the degrada-
tion caused by oxidation reaction or thermo
decomposition of base oil and additives, and the other
is the degradation in the performance due to con-
sumption of additives during use.
Contaminants such as fuel and combustion products
(soot, water vapor or oxidation products) that intrude
into oil have critical influence on oil quality. Soot ad-
heres to the oil film of cylinder wall, and is scraped off
the cylinder wall by the piston rin g. Such soot increas-
es the rate of insoluble substances in the engine oil
and can cause the wear of piston rings and cylinder
walls.
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Abrasion powder in the engine oil also accelerates
deterioration as it can catalyze oxidation reaction.
Dust and dirt entered from outside deteriorate the en-
gine oil as well. Contamination and deterioration pro-
cess accelerates with operation time.
Deterioration products and contaminants in the en-
gine oil, if it is a small amount, are harmless as they
can be dispersed in oil. However, if it is a large
amount, they become harmful. Since such products
and contaminants flow out of the oil pan and start to
accumulate inside the piston and in the oil system,
they eventually lead to serious problems such as pis-
ton ring sticking and bearing scuffing.
Sulfur content in fuel is burned and transformed into
sulfurous acid gas and sulfuric gas that cause corro-
sive wear of cylinders and piston rings. A detergent
additive in the engine oil neutralizes them into harm-
less substances. As the detergent additive is con-
sumed in its role of neutralizing, the engine oil total
base value decreases. A decrease in the total base
value indicates a corresponding decrease in soot dis-
persion ability. As a result, deposits on the pistons in-
crease.
Due to oxygen in the air, oil temperature rise under
high-load continuous operation causes oxidation deg-
radation. As oxidation degradation accelerates, oxi-
dative products are polymerized. The polymerized
oxidative products cause the oil viscosity to increase,
which leads to the generation of sludge and varnish.
As a result, problems such as lubrication failure and
piston ring sticking occur. Also acid substances gen-
erated by oxidation can cause problems like main
bearing corrosion.