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Fuel Consumption Reduced but Oil Consumption Increased: Engine Life Extended: Lubrication and Wear Problems Surveyed
DIVERSITY of subject matter was the keynote of the papers presented at a conference on lubrication and wear,held in London by the Institution of Mechanical Engineers this week.
Principal paper of interest to commercial-vehicle operators was that entitled, "Lubrication of Road Vehicle Engines and Worm-driven Axles, with Particular References to Vehicle Fuel Consumption," presented by Mr. A. T. Wilford, BSc., director of research, London Transport. He said that reductions in fuel consumption varying-between 6 per cent. and 10 per cent. had been claimed by operators using lowviscosity lubricants in oil engines in goods and passenger vehicles.
Investigations into the lubrication of worm-driven axles showed evidence of a significant reduction in fuel consumption by using either castor oil or a castor oil-synthetic ester blend in place of the oxidation-inhibited mineral oil normally used in back axles of London buses.
Mr. Wilford's paper and three others which have a bearing on road-vehicle operation are summarized below. They are: "Control of Wear in Piston Engines," by Mr. J. A. Edgar, B.Sc., chief research engineer, Martinez Research Laboratory, Shell Oil Co.; "Field Testing of Big End Bearings," by Mr. D. W. C. Baker, B.Sc., and Mr. E. D. Brailey; and "How Crankcase Lubricating Oils of Internal Combustion Engines Alter During Use," prepared by Mr. J. B. Matthews, B.Sc., Ph.D., research adviser, Shell Research, Ltd. Mr. Baker is metallurgist, Central Electricity Authority, and was formerly with the Glacier Metal Co., Ltd., Mr. Brailey is senior development engineer, Glacier Metal Co., Ltd.
In another paper it was suggested that a desirable property of bearing materials might be low hardness to encourage deep embedding of abrasive particles and hence reduce crankshaft-journal wear caused by the persistent action of embedded matter.
Towards Reduced Wear
A low melting point would usually imply a low softening temperature, which should also reduce wear, because a particle becoming heated by friction against the shaft would embed readily into a material of low softening temperature. ,Furthermore, a soft lowmelting-point phase might form a thin film on the bearing surface and thereby reduce the adhesion of particles to it.
Yet another of the 100-odd papers presented at the conference gave information on polytetrafluoethylene (P.T.F.E.), a plastics having basically good anti-friction and wear properties. Unfortunately, when used in solid form, the plastics has poor bearing properties because of its low mechanical strength, D24 high coefficient of thermal expansion and low thermal conductivity. There are two ways of overcoming 'this difficulty; either a filler can be added to the plastics, or the plastics can be impregnated into a porous metal.
The latter type of bearing material is designed for use at temperatures up to 250° C. with no oil lubrication.
The rapid rise in the use of lithium grease was referred to under the heading of recent advances in grease lubrication of ball bearings. The large proportion—perhaps a third—of petrol greases which contained lithium soaps now in use in industrial plants was stated to be in sharp contrast with the situation 10 years ago, when lithium soaps were largely a curiosity.
In most respects, the performance of lithium greases equalled and frequently surpassed those exhibited by soda soap greases and they had the desirable advantage of not being washed out of bearings by large quantities of water. in volume, however, the calcium soap greases still predominated.
Biggest Advance in 10 Years
mR. WILFORD said that the most striking advance in lubrication practice affecting internal-combustion engines during the past 10 years had been the widespread adoption of lubricating oils of appreciably lower viscosity than those formerly used. This change had taken place as a result of trials conducted from the outset under service conditions, the pioneer work having been done by public service vehicle operators, among whom London Transport had been prominent.
Use of low-viscosity lubricating oils in oil engines in buses and coaches had become established practice, but a somewhat more cautious approach had been made by operators of heavy goods vehicles, although the engine oils at present employed were of lower viscosity than was the case a few years ago. There was, perhaps, rather less scope for very thin oils in petrol engines, though the trend was towards a reduction in viscosity. or the use of multigrade oils.
Success with low-viscosity oils in internal-combustion engines had led to experiments with alternative and thinner grades of oils for the lubrication of worm-driven axles. This was another instance where the preliminary work was being carried out by bus operators under service conditions. A substantial proportion of the total operating costs of a vehicle was attributable to fuel, and with a public service vehicle this amounted to about 15 per cent. Economies arising from reductions in weight were being constantly pursued, but benefits could seldom be realized until existing vehicles became due for replacement.
Possibilities of saving fuel by changes in its characteristics were also narrowly limited. It remained, therefore, to consider whether any immediate benefits could be obtained from a reduction in friction losses.
It had been estimated that under the average conditions of load and speed encountered throughout a day's work, some 45 per cent, of the indicated horsepower output of the engine was dissipated, in overcoming engine and chassis friction. Under bus-operating conditions the mechanical efficiency of the engine itself was 65-70 per cent., so that about two-thirds of the friction losses were attributable to the engine.
Saving of 9.4 Per Cent.
An indication that a significant saving in fuel consumption might be obtained by the use of an engine lubricating oil of lower viscosity had been derived from an analysis of fleet fuel-consumption records inLondon Transport in 1946. It had -been demonstrated that under otherwise equal conditions, the fleet fuel consumption over a period when the mean atmospheric temperature was 21° C. was 9.4 per cent. lower than at a time when the mean atmospheric temperature was 2'° C.
It had been suggested that this difference was partially attributable to a reduction in the effective viscosity of the lubricating oil at the higher atmospheric temperature when, naturally, the engine operating temperature was also higher.
Before 1947, an S.A.E. 30-type oil had been in use for compression-ignition engines, whilst for spark-ignition engines a more viscous oil, within the S.A.E. 40 range, had generally been employed. If, as was sometimes the case, a winter grade was employed, this had been normally of S.A.E. 30 viscosity.
Service tests had commenced with an oil of S.A.E. 20W viscosity, but at a later stage S.A.E. 10W oil had been tested, and this grade subsequently became the standard lubricant for all London Transport buses and coaches.
When the S.A.E. low oil had been in use in a substantial proportion of the fleet, the mean economy in fuel consumption throughout a year resulting from a change from an S.A.E. 30 oil to one of S.A.E, lOW viscosity amounted to some 6 per cent.
• The effect of the S.A.E. IOW oil on engine life between overhauls had been determined by analysing the records of a total of nearly 1,400 engines constituting two approximately equal groups. Data accumulated during a period of over two years had shown that even on the most pessimistic assumption, the saving in fuel costs derived from the use of the thinner oil far outweighed any possible increase in engine maintenance costs.
Engine-life data had also shown that the overall saving might be greater than that estimated from savings in fuel costs alone. Some confirmation that this might indeed be true had been obtained from the extension in engine life which had been observed since the SAE. lOW heavy-duty oil became standardized.
Service tests on a comparatively large scale using an S.A.E. 5W oil had disclosed no adverse results, neither had any significant effect on fuel consumption, compared with engines using the S.A.E. lOW oil, been noted.
The magnitude of the benefit to be obtained from the use of low-viscosity oils was largely determined by operating conditions, and it bad been reported that savings of 7-10 per cent, in fuel consumption on public-service vehicles in city operation had been obtained when changing from S.A.E. 30 to S.A.E. 5W oil. For trucks under normal and heavy loads the saving was 4-7 per cent.
A saving of 10 per cent., based upon figures for a fleet of 400 vehicles operating in the Thames Valley area, had been reported from another source, whilst an operator in the North of England found an improvement in fuel consumption of at least 6 per cent, when using S.A.E. 10W H.D.-type oil in place of S.A.E. 10 plain oil. However, the saving might vary between less than 5 per cent, and more than 10 per cent., depending on operating conditions and engine temperature.
Another operator had obtained a reduction in fuel consumption of 6 per cent, when changing from S.A.E. 30 to S.A.E. 10, both oils being of the plain variety. Based on many operators' experience, another authority had claimed an average saving in fuel consumption of 8 per cent, as a result of changing from S.A.E. 30 to S.A.E. 5 oiI.
Increased Oil Consumption So far as could be ascertained, the use of S.A.E. 10W oil in place of S.A.E. 30 oil led in itself to an increase in lubricating-oil consumption of about 15 per cent. The cost of this was, of course, very small compared with the saving in fuel consumption. Still another benefit to be obtained from the use Of low-viscosity engine oil was the reduction in premature failures of batteries.
There was ample evidence for concluding that a valuable reduction in bus fuel consumption of around 2,1 per cent. was to be obtained by using either castor oil or a castor oil-synthetic ester blend in place of the oxidation-inhibited mineral oil in worm-driven axles. In view of its superior load-carrying capacity in comparison with a mineral oil, the performance given by castor oil had not been unexpected.
Results of the U.S. Potter Race
iklift. EDGAR commented that with the glorification of the advertised horsepower in the United States automobile, came a succession of engineering problems, one of which centred in the camshaft. and -valve gear. Failures had not been confined to a single metallurgical or mechanical system, but it had soon been found that a sulphurphosphorous additive was highly effective in preventing the failure of carburized steel tappets. This palliative had been put to extensive use in the automobile lubricants of the U.S.A.
'There was reason for concluding that this co-operative oil-engineering venture had outlived its original economic justification, but it had stimulated thought towards the general fortification of the friction-preventive properties of engine oils. The objective was the prevention of the occasional failures of engine parts which by under-design or overstress occasionally passed into the region of contact wear.
Under corrosive condi t ono—I ow operating temperature with fuel of a moderately high sulphur content—prolonged tests on one type of oil engine revealed that in all instances the rate of wear was directly related to the alkalinity of the crankcase oil, measured at the time the wear rate was determined.
For piston engines of the type used in vehicles, it had been found from experience that oil of an original alkalinity of 3-6mg caustic potash per gramme would remain in the wearpreventive condition until good judgment dictated its replacement for other causes, chiefly contamination and loss of dispersant (cleanliness) property.
Mr. Edgar referred to tests carried out on hundreds of taxicab engines of similar make and model and stated that when chromium-plated top piston rings replaced the previously used conventional cast-iron parts, there was a 33.7 per cent. reduction in cylinder-bore wear.
Wear saving was not necessarily passed on to the user as such; instead, only an acceptable level of durability was maintained, whilst a part of the wear advantage had been converted by engineering into improved performance or reduced manufacturing cost.
More compact—lessaall—and lighter engines, for example, now had shorter pistons and connecting rods, both potentially disadvantageous to cylinder life. Short, light pistons also demanded narrow-faced piston rings, which were pro-wear in principle. Oil consumption was reduced by piston rings of high unit radial pressure, this force again being in the direction of greater wear.
Journal Wear More Significant
JOURNAL wear, rather than bearing wear, was predbminantly responsible for the increase in clearance which ultimately resulted in knock and failure, said Mr. Baker and Mr. Walley.
This wear was attributable to the presence of abrasive ferrous fragments which entered the bearing with the lubricant and became embedded in the liner surface. Contamination of a bearing liner by foreign particles could occur rapidly with new or reconditioned engines. •
Distribution of the embedded parti-: cies in the bore of the bearing liners was related to the big-end oil-feed system; in particular, a single inclined hole resulted in segregation of the oilborne particles towards one end of the bearing liner. [From illustrations in the paper this would appear to be towards the end of the journal forming an obtuse angle with the oil feed hole axis.]
Factors contributing to high journal wear were a high stroke-to-bore ratio, low crankshaft rigidity, high bearing width-to-bore-diameter ratio and low lubricant viscosity.
Other causes were high engine operating temperature, engine oversjaeeding and the presence and nature of abrasive foreign matter. Three of these conditions might exist during prolonged operation at a relatively low road speed and high power output—for example, over a hills, route, The choice of vehicle transmission ratios might influence the tendency to the use of excessive engine speeds.
No Fundamental Difference
CRANKCASE lubricating oils in use, said Mr. Mathews, became colloidal dispersions of insoluble particles in an oil solution of surface-active substances. This applied to straight mineral oils and to oils containing additives, and there was no fundamental difference between the changes which occurred in petrol engines and oil engines.
Most insoluble particles were generally organic and were usually derived from the combustion gases, where they were initially formed as smoke particles. The remainder were essentially inorganic and resulted from contamination of the oil by dust, wear and corrosion products. When an engine was run on fuel containing tetraethyl lead a high proportion of the inorganic insoluble particles was lead compounds.
Sludge and lacquer deposits in the lubricating-oil system of an engine were mainly formed by deposition of insoluble particles from the oils. In certain circumstances, however, it appeared that lacquer could form directly on metal surfaces by surface chemical reactions occurring in the liquid films.
Under abnormal operating conditions —for example, high oil temperatures and efficient combustion, or low oil temperatures and inefficient combustion —oil insoluble matter could form directly from chemically unstable oils and fuels by precipitation from oil solution of their reaction products.
These insoluble particles could be distinguished from those normally derived from smoke and inorganic particles, and their contribution to sludge and lacquer deposition could be predominant under abnormal conditions.