Engine Oil Rheology and Tribology
Engine Oil Rheology and Tribology
SAE SP-97/1303SAEWarrendale, USA199783 pp.ISBN 0-7680-0072-6 (soft covers)
Keywords: Engine oil, Rheology, Society of Automotive Engineers, Tribology
This publication by the Rheology and Tribology Section of the American Society of Automotive Engineers,Inc., aims "to provide automotive engineers and lubrication scientists with insights into ... areas of tribology and rheology which are critical to the success of emerging automobile technologies".
New engine oils are being designed to meet the requirements of the International Lubricants Standardisation and Approval Committee ILSAC GF-3 and the American Petroleum Institute API SK performance categories to be implemented around the year 2000. These specifications include improved fuel economy, lower emissions, enhanced durability and extended drain intervals. The SAE seeks to provide an open forum where problems can be aired and solutions proposed.
The eight papers cover traction coefficient measurement, oil film thickness measurement in both journal bearings and the piston ring-cylinder wall zone, the pressure-viscosity and viscoelastic behaviour of lubricating oils and results from the ROCLE fuel lubricity test. There are also papers on the performance of oil soluble molybdenum compounds and the ball rust test as an alternative to the 11D engine corrosion test.
Traction
The paper from Japan reports on the measurement of volume viscosity by ultrasonic absorption and of traction coefficients on a four roller traction machine at 3.14m/s average sliding speed and 1Gpa mean Hertzian pressure. The five representative synthetic fluids measured were a fatty acid, an ester, polybutene, the base oil of"Santotrac" traction fluid and an aromatic fluid comprising five benzene compounds identified only as 5P4E. High volume viscosity delays the build up of pressure in a fluid subjected to compression, resulting in a delay before the fluid exhibits its high pressure properties. This has the effect on a fluid of reducing the rate with which the shear (dynamic) viscosity and shear modulus of the fluid increase under the effect of high pressure, such as is present in elastohydrodynamic lubrication. This reduction reduces the measured traction coefficient. The traction coefficient is the traction force F (resulting from the shear stress in an oil film under pressure) divided by the maximum possible value W.W is the coefficient fo friction times the normal load on the surface. Volume viscosities measured were between 1.3 (for 5P4E) and 2.7 (for polybutene)times the shear viscosity.
The most striking effect of volume viscosity was to prevent the expected correlation between shear viscosity at high pressure and traction coefficient. 5P4E with the highest viscosity pressure coefficient of 37.3Gpa-1 and the highest shear viscosity of 390mPas was found to have a lower traction coefficient (0.072 compared with 0.098) than the ester cyclohexyl cyclohexaneccarboxylate with corresponding values of 25.2Gpa-1 and 8mPas. The cause of this was the higher volume viscosity (510mPas compared with 20mPas) of the 5P4E with its bulky molecular structure.
The Santotrac base fluid 2,4-dicyclohexyl-2-methyl-pentane had a traction coefficient only marginally higher than that of the ester even though its shear viscosity under 1Gpa pressure of 1.7 × 1011Pas calculated from the Barus equation was two orders of magnitude greater.
Lubricant viscoelasticity
Work on the viscoelastic properties of lubricants containing viscosity improving polymers using an oscillating rheometer was reported in a paper from Imperial College, London. An equation relating the first normal stress behaviour, to its shear viscosity is suggested. Some viscoelastic behaviour of base oils without polymer additives was detected.
Ongoing work on the effects of non-Newtonian behaviour on the performance of multigrade lubricants in plain journal bearings at Shell Research, De Monfort University and the University of Wales was presented in two papers. It confirmed that half speed whirl could occur in a dynamically loaded bearing and that piezo viscosity effects can have a stabilising effect even under full film conditions. At high eccentricities pressure thickening had a much stronger influence on load carrying capacity than either shear thinning or temperature thinning.
Oil film thickness
A method of direct measurement of the film thickness of oil films between piston rings and the cylinder wall is described in a paper from the University of Wisconsin. The method consisted of a direct calibration of a laser induced fluoroescence signal by simultaneously measuring the oil film thickness with a small capacitance transducer at the same axial location as the fibre optic used for measuring the helium-cadmium induced fluoreoescence. The tests were run on a lawnmower engine with a bore of 87 mm run at speeds up to 2,400 rpm. Graphs of the variation of voltage and film thickness crank angle were presented.
Fuel lubricity
A paper from the University of Saskatchewan reported ROCLE (crossed axis roller on cylinder lubricity evaluator) test on a range of fuels. The test applies a stress of 0.9Gpa and a speed of 1.17m/s for one minute on Falex test races and tapered rollers. It gives lubricity numbers (LN). Lower sulphur diesel fuel and Kerosene had LN values around 0.6, high sulphur diesel achieved 1.0 while two bio-esters performed better with values of 1.4 (Canola methyl ester) and 1.7 for an edible Canola oil. Canola is a type of rape seed. Small amounts of bio-ester added to winter diesel fuel increased its performance to above the pass level of LN = 1.
The ROCLE is the Falex alternative to the various other fuel lubricity tests in use, but no correlation data were available to be included in the paper. The other tests listed were the high frequency reciprocating rig (HFRR), the Southwest lubricity wear test(SLWT) and the ball on cylinder lubricity evaluator (BOCLE). The SLWT test uses the BOCLE equipment.
Friction reducing additives
Work at Ford Motor Company has investigated the effect of other additives and the type of base oil on the performance of molybdenum dialklydithiocarbamate (Mo(dtc)2 ) and zinc dialkyldithiophosphate (Zn(dtp)2) as friction reducing and antioxidant additives in engine oil. The base oils were hexadecane (with and without a hindered phenol anti-oxidant), 4cSt polyalphaolefin, very high viscosity index mineral oil (VI 120, negligible sulphur and 5 per cent aromatics) and two solvent refined Group 1 base oils with viscosities of 30cSt. Samples were degraded in reactors at 160ºC. Friction was tested in a PCS Ball-on-flat high frequency reciprocating rig. The overall conclusion was that formulating engine oils for long lasting friction reduction requires "careful balancing of additives and consideration of base oil effects".
All papers in the publication were of a high standard and made valuable contributions to better understanding of engine tribology.
Effect of volume viscosity on traction coefficient
Narihiko Yoshimura and Noboru Umemoto, Tonen Corp.
Tsunamitsu Nakahara, Tokyo Institute of Technology.
An approach to determining non-linear lubricant viscoelastic properties
C. Arcoumanis and P. Ostovar, Imperial College of Science, Technology and Medicine.
Direct calibration of LIF measurements of the oil film thickness using the capacitance technique
M.J. Stiyer and J.B. Ghandhi, University of Wisconsin-Madison.
Base oil effects on friction reducing capabilities of molybdenum dialkyldithiocarbamate containing engine oils
Milton D. Johnson, Ronald K. Ensen, and Stefan Korcek, Ford Motor Co.
Development of the ball rust test-A laboratory test replacing the sequence 11D engine test
Changsoo Kim, General Motors R&D Center.
Cheng C. Kuo and David M. Marchand, Ethyl Corp.
The Rocle Test for diesel and bio-diesel fuel lubricity
Rob M.C. Galbraith and P. Barry Hertz, University of Saskatchewan.
On the importance of non-Newtonian effects in journal bearing lubrication: a numerical approach
K. Walters, University of Wales.
T.W. Bates, R.C. Coy, and B.P. Williamson, Shell Research and Technology Centre.
The effect of lubricant rheology on the performance of dynamically loaded journal bearings
L.E. Scales, Shell Research and Technology Centre.
A.R. Davies and D.Rh. Gwynllyw, University of Wales.
X.K. Li, De Montfort University.
T.N. Phillips, University of Wales.
B.P. Williamson, Shell Research and Technology Centre.
