Thin Oils Cut Fuel and Maintenance Costs
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Extensive Tests by Ribble Show Saving of 1900 for Every Urn. Miles Run
THE rise in the cost of oil fuel from is. 4d. per gallon in 1946 to 3s. 9d. in 1952 had created an urgent need for economy, said Mr. Tennant. One of the most significant contributions to economy, in recent years, had been the introduction of low-viscosity lubricating oils
His paper was, in effect, a factual account of the test undertaken with such oils by Ribble Motor Services, Ltd., the results of which ultimately led to their adoption in the fleet of 1,250 vehicles.
When the possibilities of low-viscosity lubricants as an aid to fuel economy became apparent some years ago, Ribble decided to run one vehicle on such oils for a specified period before undertaking any large-scale test. The particular engine selected for test was new and as there was every reason to expect that general rates of wear would increase with a reduction in oil viscosity, it was decided to err on the safe side by using a H.D. oil conforming to the U.S. MIL-0-2104 specification.
0.5 m.p.g. Better The initial test carried out with an S.A.E. lOW .H.D. oil proved entirely
satisfactory. During the first major dock at a mileage of 42.000, an improvement in fuel consumption at the rate of 0.5 m.p.g. was evident when compared with similar vehicles operating on the same routes but using a straight mineral S.A.E. 30 oil.
When stripped for examination at the dock mileage, the engine was found to be in excellent condition and was rebuilt without cleaning or the use of any replacement part.
A similar engine run for 42,000 miles under comparable conditions, but using B12
thetigher-viscosity straight mineral oil, showed a 25 per cent. higher rate of cylinder-bore wear and required new scraper and top compression rings on all pistons. Crankshaft bearings and journals on both engines were in good condition and showed no measurable wear. After these preliminary trials it was decided to initiate further tests with
two main objectives. One was to establish on a reasonably large scale what gains in fuel economy could be achieved by reducing the viscosity of the engine lubricant. The other was to determine the relative effect on engine condition of high-additive oils, on the one hand, and straight mineral or lowadditive oils on the other. Both oils were to be of equivalent low viscosity.
On the ground of fuel economy alone, it appeared obvious that, within limits, the lower the viscosity of the oil, the greater would be the gain, but as there were many pre-war engines concerned, it seemed essential to maintain the same margin of safety. Oils of lOW classification were, therefore, selected in preference to the 5W oils which were, at the time., coming into prominence.
To achieve the second objective it was decided to conduct tests on all vehicles at one depot on an S.A.E. 10W oil, conforming to the U.S. 2-10413 Supplementary List 1 specification. The choice here was partially influenced by the sulphur bogey. The oil fuel being used at the time had a main sulphur content of 0.8 per cent., which rose in some deliveries to over I per cent. For comparison with the Supplement 1 oil, Ribble had the choice of lowviscosity straight mineral oils at sensibly the same cost as the standard S.A.E.30 oil, and the inhibited oils at slightly increased cost. While these tests were in progress, a large percentage of the remainder of the fleet was changed over to S.A.E.20 straight mineral oil.
It was obviously desirable to complete the change-over as quickly as possible and, in the case of the inhibited oils, this presented no difficulty. In the Ribble fleet. sump-oil changes are carried out at intervals of 7,000 miles. All vehicles are, however, brought into the garage at intervals of 3,500 miles for greasing, and to expedite the conversion to the new oils changes were, in some instances, also carried out at the greasing stages.
With the Supplement 1 oil, the suppliers were insistent that oil changes should be made only after engine overhaul, or at a time when the interior of the enginecould be thoroughly cleaned. Had this policy been adopted, the change-over at a 40-vehicle depot would have extended over a greater period than 12 months.
Oil-change Procediire It was, therefore, decided to convert at the normal oil-change periods applicable to inhibited lubricants. The only difference in procedure was that with the Supplement 1 oil, the sumps were drained while hot, replenished with the new lubricant and the engines run for 20 min at about 1,000 r.p.m. This oil was then drained off, filters were cleaned or replaced, and the sump again refilled with the new oil.
The test results indicated that the greatest gain in fuel consumption was in vehicles on light operation and, similarly, the increase in lubricating-oil consumption was most marked under the same conditions. It was reasonable to assume, said Mr. Tennant, that in engines normally running at a high load factor, oil temperatures would be appreciably higher than in engines running on a low load.
He quoted these figures:—Long
distance express services (the mean of six engines), oil temperature 110° F., ambient 38° F.; town services (mean of six engines), oil temperature 160° F., ambient 42° F.
Since the spring of 1952, all Ribble depots had progressively gone over to low-viscosity oils. Whilst Mr. Tennant hesitated to give an opinion as to the reason for the variation in fuel gains with the different types of engine, which were Leyland 9.8, 8.6 and 7.4 litre units, the last named gave more economical results than the 9.8 litre engine, except on arduous services, when the converse applied. The 8.6 litre engine normally returned lower fuel averages than both the other types.
Reduced Rate of Wear
Although figures for ultimate comparison of engine life would not he available for another two to three years, excluding the Supplement 1 and H.D. oil tests, the trends were contrary to expectations. It was thought that there would be a higher rate of wear of piston rings and cylinder bores, and a probable reduction in the ultimate life of the crankshaft and its bearings.
However, no appreciable change in the rate of wear of piston rings had been noted and cylinder-bore wear had been 1-educed since the adoption of lowviscosity oils. This was apparent on engines which had been run on the lighter oils from new and from complete overhaul and major dock stages.
The reduction in wear was in the order of 10-15 per cent. on figures previously obtained from vehicles operating on the intermediate types of service, whilst the percentage reduction was less in the case of vehicles used on town services.
'1 hese improved figures probably resulted from the more rapid distribution of the low viscosity oils when starting, and from the additives in the inhibited oils limiting the extent of the corrosion wear which normally occurred under similar conditions, .
It was pointed out that the comparison in bore-wear figures was not strictly true, as the original S.A.E.30 oils were of the straight mineral type, whereas the S.A.E.10W oils contained additives.
Crankshafts and Bearings
Whilst it was too early to give any opinion on the ultimate life of crankshaft journals and of bearing wear. many components had been examined at 42,000 miles and a limited number at 84,000 miles. In all cases, wear figures were extraordinarily low and many of the shafts measured-up to drawing limits.
On S.A.E.30 straight mineral oils, crankshafts from the 9.8-litre engines had been fitted after service mileages tip to 170,400 with maximum wear and °vain)/ not exceeding 0.002 in. Al the worst, some crankshafts from vehicles operating exclusively, on town services had had to be reground on the main journals only to the first undersize. Those engines running on Supplement 1 and H.D. oils, had, without exception, shown reduced rates of wear on piston rings and liners when compared with engines in which straight mineral or inhibited oils had been used.
The speaker said he was not in favour of using low-viscosity engine oils as transmission lubricants, particularly where worm-driven axles were concerned. In the Ribble fleet, S.A.E.30 and, in some instances, S.A.E.20 engine oils had always been used in synchromesh-type gearboxes. The practice originated as one of convenience rather than from choice, and it was not now intended, as a general ntle, to employ anything less than an S.A.E.90 transmission oil for gearboxes and rear axles.
Translated into terms of cash, the saving in fuel costs, less the increase in lubricating-oil cost, amounted to £900 for each Im. miles operated. No allowance had been made for additional mechanical maintenance, as present indications were that no extra costs would he incurred.