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Facts and Figures about Engines

27th September 1963
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Page 57, 27th September 1963 — Facts and Figures about Engines
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Which of the following most accurately describes the problem?

EVERAL years ago I was privileged to have repeated to me the concern expressed (not for sales purposes) by a director of a famous commercial vehicle mannMug company regarding the difficulty that would he lienced by even the most competent workshops in aiming the high standards embodied by the manno'er into cylinder heads. This really struck home and tight to raise our own standards. Cylinder heads were ne time running in excess of 250,000 miles; this, in opinion, is under-maintenance and I decided that ider heads would be reconditioned at approximately [100 miles with piston rings changed at the same time. -fly afterwards I found that the engineer responsible he maintenance of a very large fleet had passed through same experience and reached the same conclusion, !h was naturally very reassuring. Regular readers of Commercial Motor will, in fact, remember that some :Indy valuable information has been offered by imporoperators which confirms that there is a need for ial consideration in this connection.

)mpression tests carried out on a number of cylinder is indicated that we had two standards of head condione was represented by new equipment, and the other engines reconditioned within the department and based from the manufacturer (though it will be oeciated that the scale of the department does not le a very great number of such observations to be e). The original equipment showed a higher average ing of 18 p.s.i. and this difference appears to be susNi almost up to the time when the first docking attention !eded. In both cases, however, we found that it took t 35-45,000 miles to reach maximum pressure. When peak of running in, tending to regard it as being the very early phase of engine life, it is at first difficult to appreciate that it is so long delayed—at least, on this particular unit. A brief word with the " backroom boys" confirmed that our findings were in no way unusual. It may be as well to point out that the actual pressures achieved are dependent on conditions within the manifold, that is to say on the type of governor employed. Once this point is appreciated, the staff using the special equipment for this particular type of test will soon know what to expect. As I explained last week, we decided to use Dunedin equipment, so it will be to this that I refer. The difference between the pressure found with the hydraulically-governed pump and that of the pneumaticallygoverned unit is likely to vary by an average of 40 p.s.i., the precise difference depending on engine condition.

It is important to understand that the gauge we used is designed for employment on an engine running at working temperature; its construction permits it to be fitted easily and used in this way without difficulty. In our workshops it was at first employed extensively to establish a known set of conditions and, though now used less frequently, it is still depended upon for clarification of any abnormal engine performance.

One undoubted benefit derived from its use is the interest stimulated among the workshop staff and the extension of their knowledge of engine characteristics. For example, cylinder heads overhauled with our new equipment are not expected to achieve maximum pressure for more than twelve months, the gauge will enable us to follow their progress and check whether the anticipated improvement in the standard of machining is realized and maintained for a longer period.

A factor which may deter' experiments with engine components is the length of time taken to prove whether variation has occurred. The manufacturers have their own methods of accelerating wear tests, but results are not always accessible and there is always a -query as to whether the conditions do, in fact, simulate those of operation. The ability to check within pounds the pressure inside an engine can facilitate the formation of policy without excessive delays.

We are told that engine designers aim at evolving a unit which' will have a minimum of "weak spots" and lend itself to partial, or eventually complete reconditioning at periods when either a number of components need attention to restore efficiency or when the engine, as a whole, requires complete overhaul. This has proved to be our experience and the only problem (a most absorbing one) is to strike the balance between the preservation of efficiency, fuel and oil economy, and the reduction to the ultimate of labour demand. Piston rings are the limiting factor in our case, though the employment of a piston fitted with a steel insert in which the top compression ring groove is machined has done much to bring this under control.

I was asked recently whether the type of piston used had played an important part in extending the life of a unit which, when sold with the vehicle, was still running well with a total of over 600,000 miles to its credit. My reply, after very careful consideration, was that during its 15-year life the engine had worn out several sets of pistons and a considerable number of ring sets which, between them, represented most of the improvements made in piston and ring design from 1948 to the 1960s when the last ring dock was carried out Similarly, engine oil, starting with HD30 and getting thinner and thinner and more and more complex in chemical content, has been employed, reflecting much of the progress made in engine lubricant in that period.

Limiting Bore Wear A problem currently receiving our earnest attention is how to limit bore wear, two sets of Cromard liners are now nearing 200,000 miles, one in an engine which has needed no attention and still shows satisfactory consumption figures, and the other fitted in an engine which needed major attention before there was any appreciable wear on the liners. In this latter instance, when the unit was reassembled the liners were treated as though new; consumption figures since refitting indicate that this was a proper assessment.

If ring docks are to be undertaken it would seem that they should be carried out when bore wear is relatively low (and 1 am now referring to standard cast iron liners) so that replacement rings can settle into bores which are still reasonably concentric. The fitment of new rings into bores with wear in excess of 0.010 in. has not in our experience proved a success, though I understand that rings are being developed that may take care of this condition.

No piston and liner changes are made in the chassis. though this was our practice unfit three or four years ago. Prolongation of engine life can bring about unexpected results and our change to partial reconditioning in workshops. instead of in situ, was made when it was found that oil ways were almost completely blocked. Before another five or six years' service was embarked upon, it was vitally necessary to ensure that these were scrupulously cleaned.

Water jacket fouling can be of some consequence, though we believe that we shall have eradicated deposits by an annual treatment of the coolant, every engine is certainly very carefully examined to ensure cleanliness in this respect. Some of those first checked showed a surprising degree of fouling, and I believe the annual "spring clean " is a worth-while investment. The cost of removing art engine from a chassis and refitting it, or replacing it with another, may be considered to militate against the practice; but when it is considered that an engine may well last the life of the vehicle and be removed only once and that, in addition, the opportunity is afforded of gaining access to fuel pump linkages and wear at points which are usually inaccessible, it may be considered money well spent.

Second Life

Second life for an engine has been found almost invariably to be shorter than the first, and I gather that this experience is a common one. We sought for many years to overcome this discrepancy and to find the reason for it. The commonly-accepted theory, and the experts do not refute it (though this may be due to caution or a courteous disposition), is that cylinder eccentricities develop in service --or more likely when cylinder liners are withdrawn and new ones fitted. This may be surprising now that pressures required for liner exchanges are relatively low; but since the theory fits the facts, and nothing else is offered, I have directed my thoughts along these lines:—

Pressure checks on second-life engines have shown a condition which, though strange to us, is apparently not unfamiliar to the trade. After the first pressure rise (which we consider to be running in), the mating of reciprocating parts and valve seats, there is a fall-off and a later recovery which must be the pistons and liners "settling down to co-existence ". Our efforts are being directed towards a renewal of the conditions which prevailed when the liners were first fitted. One unit recently fitted had the cylinder bores honed and liners fitted which were on the plus side, so that the necessary degree of interference fit would not be lost. When the bores were measured prior to honing, high spots of approximately 0.001 in. were found and these would have been removed by honing. To carry this work on a routine basis may well mean honing slightly more, and the employment of liners with a ble, increased external diameter.

Lough the special equipment we purchased has enabled gain a new insight into engine performance, I had reason to appreciate that engines which appear to :rforming to much the same standard can, in fact, be tting at very different standards of efficiency due, no it, to wear in reciprocating parts, valves, seats, and t. Enjoying the co-operation of an industrial chemist laboratory facilities particularly suitable for the pur we embarked on a series of tests for additive :tion, the intention being to settle on a realistic oil L policy for my own fleet.

mples were taken each thousand miles and coded 'e dispatch; the conditions of control were, I believe, effective. At 13,000 miles an urgent message was back to us that one engine which had a high mileage s credit had shown such a rapid increase in the ubles found in the oil that it needed to be drained Kliately. The other engine ran up to 21,000 miles 1.t. that time the test was discontinued because it had

d its purpose. That particular unit had given a good of service but our records had indicated that it should excellent condition. Some years afterwards the test repeated, two similar engines being chosen, and the ts were almost precisely duplicated.

a matter of interest, we have adopted a 10,000-mile period; this, it would seem, should take account of vorst as well as the best engine in the fleet, Little Consistency

ere is so little consistency in engine durability and mance, even when the manufacturer's system of of is rigid, that a reference to our own figures could isleading and where I have quoted them it is with the tion of offering a guide to personal investigation. Lave been mystified by variations of some consequence J in the pressure in the bores of some engines as

■ ared with others in the same delivery group and

e mileage and operating conditions had been so close it could have been anticipated that pressures would been similar throughout the group. Information been made available to me which would seem to in the variables and to justify the long-established ;ntion in the motor industry that there are good and engines. I hope to offer full details in a later article is series and to show what one manufacturer is doing ing accuracy and performance to a new level in the nerciat field.

It would have been a source of satisfaction to me to have closed this article by outlining a method'of employing our special equipment to decide precisely when attention was needed to those parts of an engine whose efficiency it is claimed to indicate. In other words, to have established a relationship between bore wear and pressure and possibly take into account oil consumption—though this can be misleading. Our own observations may be of interest, though i would claim nothing authoritative for them. Bores with approximately 0-006 in. wear averaged 550 lb. pressure; at 0.007 to 0-008 in. this had fallen to 536, and a reading of 530 lb. was found with a small number of bores between 0-009 and 0.010 in.

Oil consumption has always been employed as a useful method of checking engine condition, though quite often attention by a fitter keen enough to reach into inaccessible corners with a spanner can do much to improve oil consumption. I must confess that I was not too worried about this until I learned that one authority had expressed the most serious concern about the discharge of oil fumes from engine breathers into the atmosphere.

Incidentally, a cheek on the value of fitting a number of engines with replacement rings, cylinder heads and no other attention showed a 200 per cent improvement in lubricating oil consumption; but unfortunately nothing in fuel oil. Such statistics can be misleading when a " blitz " has been going on into every possible aspect of efficiency and economy. At all events, the savings in this group of engines on lubricating oil alone should be sufficient to pay for the ring dock.

Operators are given to understand that manufacturers could improve the efficiency of engines, so far as exhaust condition is concerned. It may well be, since the reference was a general one, that such improvements would be in fuel-injection equipment rather than in the engine itself, and I hope to deal with this later in this series. Our need, however, is to secure improvements in the vast number of units already in service—for the problem is with us now as a public duty. When the Ministry of Transport finally decide that standards can be set up and control exerted, as I am sure they can, then there will be a very real need for all concerned, manufacturers and operators alike, to show that the industry does care what part it plays in the preservation of public health and that we shall continue to do so, even though I am sure there will never be a once-and-for-all solution.

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Organisations: Ministry of Transport

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