On to the Half-million
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Experience with Leyland 0600 Engine Shows that Early Optimistic Estimates of Life have been
Greatly Exceeded
EVERAL years ago, I presented 5a paper to the Institute of Road
Transport Engineers on the subct of post-war development in :hicles and equipment. One aspect
the paper aroused particular .terest. This was a reference which made to the greatly increased tpectation of life for the larger .pes of oil engine, some of which ere at that time comparatively cent developments.
My own bus fleet was one of the rst in the country to use the Leynd 0600 oil engine and my early }recast was that this unit could be Kpected to run 300,000 miles etween major overhauls. This was :ceived with some scepticism.
300,000 Miles Usual This mileage, however, has now ecome commonplace for the 9.8tre engine. In fairness, however, to lose who found it difficult to accept my figures in 1952, it should be :membered that at that time the majority of maintenance engineers 'could be comparing this high milege with the 100,000 miles usually onsidered to be a satisfactory averge for the smaller units, which were men used in most public service. ehicles. In 1949, when drawing up maintenance schedule for the 0600 nit, my estimate was a maximum of 60,000 miles.
An outstanding example of long mileage between overhauls is :vealed by tests made by Mr. E. V. >yson, general manager of Warringon Transport Department. Mr. >yson has run an engine for 300,000 miles without removing the sump or me cylinder heads, although it should e emphasized that the most careful maintenanCe of fuel-injection eqUipment and essential adjustments have been carried out. A high-quality lubricating oil has been used throughout the test—a matter of first importance.
One important observation on this test is that Mr. Dyson has been able to obtain supplies of oil fuel of exceptional quality.
Although every effort is made to secure fuel having a low sulphur content and other desirable qualities, we at Stockton experience conditions which make it necessary to re-ring at suitable intervals and change pistons and liners in order to avoid unduly high oil consumption and other unfavourable features.
The terrain over which my buses operate is relatively flat, but stops are exceptionally close together and this, together with heavy traffic for long periods, introduces arduous conditions when judged by the number pf gear changes per mile. Most of the vehicles concerned are 56-seaters, having an unladen weight of 7 tons 12 cwt.
When endeavouring to explain concisely how we plan our programme in regard to intermediate docks, I find myself in some difficulty. We have progressively adjusted the mileage between piston overhauls, as experience has shown that by doing this we can expect a higher total mileage from the engine.
At first we removed pistons at 40,000 miles and we found this was necessary, but the employment of a high-suality lubricating. oil and improvements in piston rings made it possible to extend this to 100,000 miles. Total engine life is still undetermined, for several units have reached 400,000 miles, whilst one is still going strong at 430,000 miles...
There seems,, to be no reason to carry out a major overhaul at a fixed mileage, so we now confidently expect our engines to reach 409,000 miles, after which each will be kept under careful observation.
My reason for being rather indefinite about a mileage period for piston docking is that whilst we expect that pistons will need to be re-ringed at 100,000 and 300,000 miles, with new liners and pistons at 200,000 miles, the question of engine condition at these mileages is taken into account.
Warning of Attention At the mileages quoted the engineering-records section informs the engineering department that the appropriate attention is due. Thereafter, oiland fuel-consumption figures are taken into account by the department before the work is put in hand.
No useful object can be served by stripping when an engine is running .really well, and we do, in effect, often run to mileages well in excess of the target figure; this confers benefits of real value. Occasionally, a set of pistons will fail prematurely, which makes an early dock essential. In our experience, the defect is invariably wear in the top-ring groove, and in the ring itself. Failure at this point doeS, in fact, appear to be the limiting factor. If it could be reduced we could expect to carry out the attentions already referred to at approximately 200,000 miles and then carry on to whatever mileage will eventually call for reconditioning.
We have found bore wear to average approximately 0.001 in per 18,000 miles, and this is with. Leyland liners. It may well be that this figure could be improved upon by using a harder but more costly liner and we intend toexperiment along these lines.
Difficult Conditions have every reason to believe that the figure quoted is not particularly good, as many operators have obtained greater mileages for the same degree of bore wear. It -is recognized, however, that the nature of our operating conditions, entailing a great deal of starting from cold, may well have militated against better figures At these half-way docks, which, for most operators, including ourselves, occur after four or five years of service, we find it necessary to carry out certain work in addition to component changes. Most gaskets, as well as oil seals, may call for renewal and by taking them into an all-embracing survey of minor items we are able to avoid trivial, yet irritating, failures.
During the .renewal of waterjacket gaskets we found that the blocks contained a large amount of sediment, and steps taken to improve this condition led to the use of a product known as Trawl Scale, which is introduced into the cooling water. After, two such treatments, the deposit in the water jacket was, much reduced and so softened tha'f eventually it largely dispersed.
They Last for Ever My observations have been mainly confined to pistons and liners, the reason being that most other parts do not appear to suffer wear. For instance, we have yet to replace a crankshaft, few bearing shells have been renewed, camshafts and their bearings are found in excellent condition, whilst timing gears present no problems in maintenance or replacement.
Cylinder heads justify a special reference. The machining accuracy is of a high standard, the fine tolerances and robust design resulting in a component worthy of such an engine. Perfect alignment of valve and seating is promoted by the small
c24 clearance between the valve stem and guide, a flash of chromium on the valve stem keeping wear well under control.
Stellited valve seats and inserts ensure an efficient and durable seal; and although it is our practice lightly to grind valves and seats when the heads are stripped at piston docks., this is kept to a minimum.
In the 18m. miles which have been covered by our Leyland 0600 engines, we have replaced fewer than 300 exhaust valves and 60 inlet valves. The C.A.V. pump has proved entirely satisfactory and is easily maintained. Our standards of maintenance are of the highest order and we have a wellequipped and adequately staffed department for servicing fuelinjection equipment.
Leyland injectors perform yeoman service, and the combination of this equipment has enabled our vehicles to run economically with an adequate performance and usually devoid of any trace of exhaust smoke.
The grade and quality of the lubricating oil used have played a vital part in the performance of our engines. Eight years ago we were using S.A.E. 30 heavy-duty oil. Although this proved satisfactory, the deterioration in oil fuel which was experienced in 1950 introduced difficulties brought about by scraperring plugging and heavy lacquering of the pistons. Messrs. Goodall, Bates and Todd came to our aid and recommended an oil to the specification MIL/0/21Cr4B. This proved satisfactory.
With regard to viscosity, in 1953 we changedto S.A.E. 20, and a year later, S.A.E. 10. The thinner oil appeared to be satisfactory in every way. The drain period which we have used for a number of years is over 10,000 miles and, whilst this may be considered high, the decision was taken only after repeated tests on additive depletion and other characteristics had shown the oil to be in good condition at mileages well in excess of the figure finally adopted.
Efforts have been made to oPerate our, engines in the temperature range 1.75°-180° F_, but we have not been wholly successful. Our experiments have emphasized the fallibility of thermostats. It is intended to fit a more robust, unit, plated to avoid corrosion, and it is -hoped' that this will give the desired reliability.