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Page 9c

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

  1. 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.
  2. 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.
  3. 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.

  4. 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.

  5. 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.
     
  6. 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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