Oil Filtration Merits More Attention
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• Good and Bad Features of Current Methods Discussed in Authoritative Paper. By-pass Type Generally Favoured, but Reconditioning Plant Attractive Where Fleets of Vehicles are Operated STIMILTLUS has certainly been given to the practice of oil L./filtration by the present need for economy in materials, including lubricants. It is timely, therefore, that a study of this subject should feature in the November issue of the Journal of the Institution of Automobile Engineers. It is described as "Notes on Filtration and Distribution. of Lubricating Oil ' and comes from the pen of Mr. T. C. Worth, an authority on filtering.
He approaches the matter with an analysis of the sources of crankcase-oil contamination. Coming from above the piston are burnt or otherwise changed oil, fuel, fuel residues, water, and dust from the induced air. These account for 80 per cent, of the contamination. Formed in the crankcase or entering it direct are oil changed by excessive temperature, and dust, grit and water from the breather, amounting to 15 per cent. The remaining 5 per cent. comprises casting sand, swarf and foreign matter from dirty oil measures, etc.
Investigations of oil impurities in a petrol engine and an oil engine respectively after 12,000 miles showed 1,6 per cent. and 0 per cent, dilution; a trace of and no water; 1 per cent. and 1.1 per cent, of sub-solids; 0.6 and 1.11 per cent. of carbon; and 0.15 and 0,15 per cent, of ash.
Contamination from Above the Piston With regard to the large figure of 80 per cent. for blowpast, Mr. Worth comments that blow-past for an engine in reasonable condition is about 2 per cent of the swept volume, and adds that that is no small amount.
Dilution, he points out, cannot. be remedied by filtration, but is not a serious trouble, because, in the case of petrol, it boils off, and, in the case of an oil 'engine, the fuel
is itself a reasdnable lubricant. He remarks that in America it is common practice in the winter to add deliberately 25 per cent, of kerosene to the lubricant to facilitate starting.
Water is objectionable for its emulsifying properties. Sub-solids are defined as asphaltives, gums, etc., which are
soluble in benzole, but not in petroleum ether. They cause the sticking up of working parts.
As for solids, if abrasive and large, their damaging effecth are obvious. There is a difference of opinion about the maximum permissible size of such particles. One authority he quotes as giving 0.00004 in, as harmless, because it does not bridge the oil film. Such bodies weaken the film, however. A medium-sized petrol engine may gain 1-2 oz. of solids per 1,000 miles, and an oil engine 2-3 oz.
Filtration Methods and Means These are the methods of and means for conditioning oil: Gravitation, straining, adsorption, filter aids, and electrostatic precipitation, and 'the suction strainer, lull-flow filter, by-pass filter and batch-filtration plant.
The first method is found in floating oil inlets, in settling -tanks and in centrifuges. Clogging limits the fineness of the gauze, screen, etc., that may be used for straining. Adsorption, he states, is the chief factor in keeping oil clean, He describes the principle thus. "If a fluid containing solid bodies flow over a surface of a material, some materials show the property of attracting and holding the foreign matter." Of the vegetable fibres, cotton appears to be the best, Adsorption is quite distinct from the straining effect of the fibres, and the author has satisfied himself that it is an electrical phenomenon, the flow of oil over the fibres. causing a change in the polarity of the foreign 'bodies opposite to that in the adsorbing material.
Some experimental work of great interest relating to this electrical phenomenon, which the author has conducted, is
briefly described, His results, however, have not yet proved, conclusive, although they show good promise. Under filter aids.he places the system of adding. a powder to the used oil, which builds up a porous layer forming a filter bed. It is unsuitable for cleaning oil in an engine, . he comments, whilst a fault is that the powder may itself be abrasive. The electrical-discharge method has not yet been advanced to the stage at which it would be a practical proposition to apply it.
-Turning to filtering means, as opposed to methods, he expresses the view that the suction filter may constitute the weakest link in the lubrication system, It is commonly inaccessible and therefore neglected, and pumps are prone to fail if the intake be obstructed. There should be a big area of gauze of not less than 16 mesh, and the filter should he cleanable without dropping the sump or draining ont the oil.
The idear means, he considers, in at least certain major respects, would be the full-flow filter. But, if it worked on the adsorptive principle, it would have a volume equal to that of the oil flowing in one minute. A modern vehicle engine circulates at the rate of 3 gallons a minute, so this condition can, hardly be..fulfilled.
" a Courtesy Title Most full-flow filters, therefore, represent a compromise, and effective filtration combined with elimination of the risk of damage, to the engine through 'clogging of the element can be achieved only by including in the system a by-pass. Mr. Worth remarks: " In fact, very few so-called full-flow filters act as such."
He refers to a type, which he seems to view favourably, in .which a mechanical .device, automatic or otherwise, scrapes accumulations of matter from the element, to prevent clogging.
By means of the by-pass filter, which is designed to deal with only a small part of the oil flow, it is possible to keep clean all the oil in circulation, to use a reasonably small device, to avoid impeding the supply to the bearings and to employ the adsorptive principle. For a 25 h.p. engine the, unit can have a capacity of about one quart.
Only 5-10 per cent, of the flow usually passes through the by-pass: This means, in effect, that every minute a pint, roughly, of dirty oil is taken from the 'sump and replaced by a pint of clean oil. Starting from scratch, this would certainly transform the oil into good condition in a remarkably short time.
Modern American Automobile Practice
The author states that " it is beyond question that a bypass filter can keep oil in an engine in better condition than any other practical method." He adds that the type has not yet attained the popularity it deserves in this country, but that it is standardized on 60 per cent. of American cars, and, he believes, invariably used when hydraulic tappets are used, these being susceptible to dirty oil.
Batch filtration he describes as obviously the most effective way to clean dirty oil, This type of filter is a
plant specially designed for the purpose and in it is treated oil drained-off from the sump. The lubricant can actually be got into better condition by this means than by a by-pass filter.
However, the rejuvenated oil is quickly contaminated again after being put back, and gets steadily worse until removed for further cleaning. The operations involved, too, are relatively laborious.
Referring to a micro-photograph among the illustrations to the paper, Mr. Worth stresses .how out of placg, in a precision machine, are the jagged pointed bodies depicted. The modern automobile engine, he continues, delivers snore than twice the output of its equivalent of 20 years ago. Bearing pressures and .r.p.m. have increased enormously and have brought super finishes and special bearing metals. All are particularly susceptible to grit, and sO filtration 'becomes increasingly important,