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Lubricating
Oil Contamination
Contamination or dilution of the
lubricating oil of biodiesel-fueled vehicles has
frequently been a concern of engine manufacturers.
The mechanism for the dilution is essentially the
same as for dilution with the heavier fractions of
diesel fuel. Low volatility fuel components, which
for biodiesel are essentially the entire fuel, are
slow to vaporize after injection into the cylinder.
Some of these low volatility compounds will be
deposited on the cylinder wall where they can be
swept down into the crankcase by the normal scraping
action of the piston's oil control rings. The two
key questions for lubricating oil contamination is
whether the amount of dilution is significant and
then whether the presence of the biodiesel, itself
an excellent lubricant, causes any deterioration in
the lubricant's performance.
Siekmann (1982) added known amounts
(5%, 10% and 20%) of methyl esters of soybean oil
and babassu oil, with iodine values of 128 and 17,
respectively, to lubricating oil. The iodine value
is a measure of the level of saturation of the oil.
High iodine values indicate large numbers of
carbon-carbon double bonds. The oil was tested in a
MacCoull apparatus which simulates engine bearing
working conditions. Test temperatures of 150°
C and 170° C were used for 8 hours. Results
showed an increase in viscosity and a decrease in
TBN (alkaline reserve). The TBN decreased 22% for
the 20% soybean oil methyl ester at 150°
C and decreased 46% at 170°
C. The changes were greater for greater
concentrations of methyl ester and strongly
increased for the less saturated feedstock (soybean
oil). Twenty-four hour tests with the MacCoull
apparatus showed that at 170°
C, 10% soybean methyl ester in the lubricating oil
cased the viscosity to increase from 60 cSt to 165
cSt and the TBN to decrease from 9.75 to 5.5.
Additional testing with engines and
vehicles (Seikmann et al., 1982) showed that the
MacCoull apparatus was a much more severe test than
an engine bench test, which in turn was more severe
than actual vehicle testing. Deliberate addition of
soybean methyl ester to the lubricating oil in a
bench test of an indirect injection (IDI) engine
showed similar increases in viscosity and decreases
in Total Base Number (TBN) as were found in the
MacCoull testing. However, when the IDI engine was
fueled with 100% soybean methyl ester, no evidence
of oil dilution was observed. Vehicle testing in
light-duty delivery vans showed that the viscosity
and TBN changed by only slight amounts even when 5%
soybean methyl ester was deliberately added to the
lubricating oil.
Siekmann and Pischinger (1983) used a Four Ball
Tester to evaluate the wear properties of mixtures
of lubricating oil and soybean ethyl esters. They
found that as the ethyl esters were added to new
lubricating oil, the measured wear levels decreased.
The optimum level of ethyl esters was 3% although
the initial value of the new oil was not reached
until a level of 38% ethyl esters was reached.
Strangely, when the test was conducted with used
oil, the addition of the soybean ethyl esters caused
an increase in wear level.
Blackburn et al. (1983) conducted
engine tests on direct injection engines and found
that the degree of ester contamination of the
crankcase oil was unacceptably high at approximately
0.2% of the fuel flow rate. During the oil service
interval there was a gradual decrease in the
lubricating oil viscosity caused by the increase in
the amount of the low viscosity fuel followed by an
oxidative thickening of the oil. Other
characteristics of the oil such as the TBN, Total
Acid Number (TAN), and insolubles stayed at
satisfactory levels although engine inspection
showed evidence of copper/lead bearing corrosion.
Wagner, Clark, and Schrock (1984)
tested a direct injection diesel engine with methyl,
ethyl and butyl esters of soybean oil. They observed
crankcase oil dilution which caused the oil
viscosity to decrease. Oil polymerization was not
observed to cause the viscosity to increase within
the normal oil service interval. Wear metal analysis
also did not show any deleterious consequences of
biodiesel use.
All of the studies described above
were conducted with engines typical of design
practice in the early 1980s. These engines would not
be expected to have the level of oil control that is
typical of post-1991 engines. Engines with modern
ring packages would be expected to have less fuel
dilution of the lubricating oil as the pathway from
the cylinder to the crankcase is more restricted.
However, Schafer (1997), in a more
recent study, reports similar observations to those
of Siekmann described above. More saturated methyl
esters produced from palm oil caused a slight
viscosity drop due simply to the dilution effect,
but more unsaturated methyl esters from rapeseed oil
showed a tendency for viscosity increase toward the
end of the 250 hour oil change interval. The actual
extent of fuel dilution was not quantified. Schafer
connected the polymerization of the unsaturated fuel
compounds to higher levels of cylinder head deposits
after 250 hours of continuous operation with soybean
methyl esters. Reductions in exhaust black smoke
levels with palm oil methyl esters caused soot
levels in the lubricating oil to be half of their
level with the baseline diesel fuel. The reduction
in the abrasive soot particles was confirmed by much
lower iron levels in the used oil. In spite of the
benefit from reduced soot levels, Schafer
recommended reduced oil change intervals with
biodiesel to eliminate the effects of fuel dilution.
Biodiesel consumers should contact their engine
manufacturer for information relating to recommended
oil change intervals when using biodiesel.
A recent study of the effect of biodiesel blends on
engine oil (6.) can be downloaded from the Technical
Papers section of this website (paper #10).
References
- Blackburn, J.H., R. Pinchin,
J.I.T. Nobre, B.A.L. Crichton, and H.W. Cruse,
(1983) "Performance of Lubricating Oils in
Vegetable Oil Ester-Fuelled Diesel Engines,"
Society of Automotive Engineers Technical Paper
No. 831355, SAE, Warrendale, PA.
- Siekmann, R.W., D. Blackman,
G.H. Pischinger, and L.D. Carvalho, (1982) "The
Influence of Lubricant Contamination by
Methylesters of Plant Oils on Oxidation
Stability and Life," Proc. of the Int'l
Conference on Plant and Vegetable Oils as Fuels,
ASAE, Fargo, ND, Aug. 2-4.
- Siekmann, R.W. and G.H.
Pischinger, (1983) "Evaluation of Lubricating
Oil Contaminated with Small Amounts of Soybean
Oil Ester in Comparison with Normal Diesel Oil
Operation," Vegetable oil as Fuel, Seminar III,
Agricultural Reviews and Manuals, ARM-NC-28,
Document No. A77.30:NC-28, October 19-20,
Peoria, Ill.
- Schafer, A., (1997)
"Vegetable Oil Fatty Acid Methyl Esters as
Alternative Diesel Fuels for Commercial Vehicle
Engines," Plant Oils as Fuels - Present State of
Science and Future Developments, Proceedings of
the Symposium held in Potsdam, Germany, Feb.
16-18, Ed. by N. Martini and J. Schell,
Springer, Berlin.
- Wagner, L.E., S.J. Clark, and
M.D. Schrock, (1984) "Effects of Soybean Oil
Esters on the Performance, Lubricating Oil, and
Wear of Diesel Engines," Society of Automotive
Engineers Technical Paper No. 841385, SAE,
Warrendale, PA.
- Schumacher, L.G., C.L.
Peterson, and J. Van Gerpen, "Engine Oil
Analysis of Diesel Engines Fueled with Biodiesel
Blends," ASAE Paper No. 01-6053, presented at
the American Society of Agricultural Engineers
2001 Annual Meeting, Sacramento, CA, July 2001.
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