Oil Thickening
in Light Duty Diesel Engines |
Richard Oliver |
From time to time we encounter
instances of excessive oil thickening in light duty diesel engines. While
the factors that contribute to this are many and varied, the obvious culprit
in the minds of most motorists is the oil itself, which is, in reality, the
least likely cause of the condition.
High performance engine oil
marketed by major reputable oil companies has been developed to the point
that in heavy duty diesel highway engines, 40,000kms between service intervals
is not unusual and the oil is still in reasonably good condition. The exact
same lubricant, usually a SAE 15W/40 meeting at least API CF4/SG specification,
may not go to 5,000kms in light duty engines without significant thickening
and deterioration.
The major difference in oil
performance is due not only to differences in engine design, but mostly due
to operating condition. A diesel engine does not achieve combustion efficiency
until it reaches operating temperature, which usually takes around 15 to
20 minutes of running. During this warm up period, incomplete combustion
deposits excessive quantities of carbon and soot on the cylinder walls where
it is collected and absorbed by the dispersant additive in the oil. Frequent
stops and starts also increase the moisture contamination of the engine.
As most light duty engines
are not spectacular performers, most people tend to drive them hard through
the gears, which leads to overfuelling and further contamination of the oil.
The combustion by products from petrol engines are volatile and can be driven
off once the engine has run at full operating temperature for a period of
time. However, the combustion by products from a diesel engine cannot be
driven off by the engines temperature, and as such are totally
accumulative.
The build up of combustion
residue eventually leads to the oil becoming very thick at cold start up,
in turn not flowing through the engine correctly thus contributing to increased
wear rates. The excessive moisture levels combined with the soot and carbon
in the oil produces sludge deposits especially in the valve cover.
The way to head off these problems
is to ensure a good quality branded name SA 15W/40 of at least API CF4/SG
specification is used, and the oil is changed every 5,000kms and filter at
10,000km or sooner.
(Courtesy Australian Institure
of Automotive Engineers.)
The oil that we use in
our workshop is API SLCS specification SA 15W/50 which is 3 grades
better than that listed above and is a semi-synthetic oil. |