What Oil Additives Do
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How the Chemist Has Assisted the Oil• Industry in the Development of Lubricants Having Improved Characteristics
THE lower the viscosity of an oil, the greater may be the consumption, and special piston-ring layouts will have to be developed to deal with the problem. As friction is reduced by the use of low-viscosity oils, not only does fuel consumption improve, but the maximum power allilable is also increased.
These are some of the findings of Mr. A. Towle, M.Sc. (Eng.), technical' director of Artglarnol, Ltd., Derby, who read a paper dealing with the use of additives in lubricating oils to the Automobile Division of the Institution of Mechanical Engineers, in London, on Tuesday. Experience of oils of extremely low viscosity tended to confirm his opinion • that, for anything but extremely light service, they should be heavily treated with additives, otherwise excessive crankshaft and bearing wear would occur. He believed that a compromise would be struck and that in Britain, S.A.E. IOW oils would come into general use during the winter and oils between an S.A.E. lOW and 20 for summer use. For tropical conditions, S.A.E. 30 would probably be suitable.
Functions of an Oil A lubricating oil had first to prevent metal-to-metal contact of the moving • surfaces, and, secondly, to act as a cooling medium. Oils in the higher viscosity range might be expected to meet the first requirement better than low-viscosity oils, whereas the latter would be more effective as a coolant.
There was no known chemical laboratory test which could be used to determine whether a lubricant would be satisfactory in service. The only way of proving its performance was by full-scale engine tests under carefully controlled conditions in a laboratory, or by service trials.
On the subject of additives, Mr. Towle said it was logical to consider them as being any substance which. when added to a mineral oil, improved one or more of its qualities. Colloidal graphite, molybdenum disulphide, pour-point depressants, viscosity-index improvers, oxidation inhibiters, detergents, heavy-duty additives, extremepressure additives, anti-foam agents and rust inhibitors were included.
Load-carrying capacity tests made by Mr.' Towle had failed to show any improvement through the presence of colloidal graphite. Molybdenum disulphide had been used in oils for applications where the bearing pressures were extremely high and, added to grease, a test in a Falex machine indicated a 50-per cent, increase in loadcarrying capacity.
Pour-point depressants depressed the pour-point of the finished lubricant below that of the base oil, which, particularly if derived from a paraffinic type of wax-bearing crude, might have a much higher pour point than that acceptable in service.
Tests carried out in several American laboratories had shown that light neutral oils treated with heavy polymersviscosity-index improvers—behaved like oils of lower viscosity in those parts of the engine where the rate of shear of the oil was high, such as the bearings and cylinder walls.
Behind the rings, however, the shear rate was low, and it was claimed that the oil conveyed to the combustion chamber corresponded with the measured viscosity of the oils. Such oils were, therefore, said to combine the advantages of easy starting and reduced fuel consumption associated with low-viscosity oils, with the lower oil consumption afforded by higherviscosity lubricants.
Anglamol, Ltd., had made tests on the Motor Industry Research Association's proving ground, the results in general bearing out the American claims. The extremeeconomies recorded by some American observers were not, however, reproduced.
Complex Action •
Under high-temperature operations, oils may oxidize to form oil-insoluble carbonaceous compounds, which might be deposited on the pistons, or might form oil-soluble resins and acids. Oxidation inhibitors provided protection against oxidation, but their action was complex.
The function of a _detergent was to maintain in suspension in the oil burned or partially burned products of combustion, as well as oxidation products of the lubricating oil. Detergents had always been associated with cleaner engines, reduced ring sticking and greater freedom from harmful carbonaceous deposits. .
Heavy-duty additives imparted to the oil properties of .detergency, resistance. to oxidation and high resistance to wear. Four official grades of heavy-duty lubricating oil had been standardized. These, in ascending order of detergency, were known as 2-104B, which was now. obsolete, MIL-L-2104A: and Supplement 1 and Series 2.
All were of American origin, but the British Ministry of Supply had a specification known as DEF 2101 A, Which corresponded in quality level to MIL-L-2104A; British Standard No. 1905/1952 was also of the same quality level. The meaning . of the expression. " extreme-pressure [E.P.] oil," was most difficult to define, particularly as the properties were exhibited only under certain boundary lubrication conditions. It might reasonably be said, however, that such a lubricant was one with a load-carrying capacity greater than would be expected by its viscosity, by an amount approaching, say, 100 per. cent.
An oil which showed extremepressure properties in one application did not necessarily reveal them to the same degree in another. Although certain anti-oxidants increased the loadcarrying capacity of mineral oils by as much as 150 per cent., when measured on the J.A.E. gear rig or Falex rig, or under those conditions which gave load-carrying capacities almost comparable with those of hypoid oils, they were not appreciably better than straight mineral oils when used in hypoid axles.
Extreme-pressure Oils
A lubricant that reduced wear was not necessarily synonymous with one having a high load-carrying capacity.
The term, "extreme-pressure oil," had probably been applied more to hypoid lubricants than to others, In this application, the high sliding velocity, coupled with high unit loading, called for different properties to satisfy high-torque low-speed conditions from those required for high speed, where the torque was relatively low and the gears failed by scuffing.
It was comparatively easy to produce a chemical additive which would enable 4n oil to satisfy one or other of these conditions, but extremely difficult to 'obtain one to meet both.
Foaming or frothing was a problem in some engines, and the higher the viscosity of the oil, the greater was the tendency for it to hold air bubbles. The most effective anti-foam agents Were silicone polymerS, which were now used exclusively. They were effective even in a concentration measured in parts per million.
In engines which stood idle .for long periods, or Were not operated long enough to become thoroughly warm, excessive rutting of cylinder walls, piston rings and crankshafts might occur. This tendency was aggravated when the fuel used contained bromine compounds. When the oil used was .a straight mineral 'grade, corrosion of copper-lead bearings -was also possible. Special anti-rust oils had been developed, their effect -being. to reduce moisture rusting .and to neutralize
bromine corrosion,. • .
Mr. Towle concluded his. paper with remarks on the use of low-viscosity oils in hypoid and worm-drive rear axles and in gearboxes and automatic transmission systems. S.A.E. 10VV oil had already been successfully used in hypoid axles, he said.