Cylinder Bore Wear
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and How
ROAD transport engineers have for many years been interested in the reasons for excessive wear on cylinder bores and the methods and means by which this can be reduced. It is obvious that some wear must occur between moving surfaces in more or less intimate contact, even if they are supposed to be constantly separated by a film of oil. However, recently, attention has been concentrated on this matter as a result of the interesting paper on this subject read by Mr. W. A. Robotham, chief engineer, motorcar division, Rolls-Royce, Ltd., before the Institution of Automobile Engineers. We have, in past issues, given a precis of this paper, and referred to certain points raised during the discussion, but feel that the subject is of such great importance that it warrants more general treatment, especially as regards engines for commercial vehicles.
Oval Bores One of the statements made by Mr. Robotham was that the average liner in a motor engine is about 0.012 in. out of round, and that whilst his company went to extremes to secure really round bores, in service it was impossible to note any difference between these and others slightly oval; in other words, it is sometimes possible to go far towards perfection without obtaining a corresponding benefit. Much must, of course, depend upon the conditions of service. It is well known that vehicles engaged on long-distance work, where there are comparatively few variations of throttle, or of the injection pump in the ease of oil engines, the life of cylinders or liners, as the case may be, is greatly increased. It seems, therefore, that it is not so much a question of the number of times that a bore is traversed by a piston, but how it is traversed.
Long Life Nowadays, nitriding is giving excellent results, and in commercial-vehicle engines, including oilers, 100,000 miles without excessive wear can be regarded as not unusual. Similarly, the inclusion of a large percentage of chromium in the casting gives excellent results, as does chromium-plating. The latter can be either what is known as a flash, perhaps 0.0015 in. thick, or even less, or a coating of 0.005 in. for the full length of the bore. For heavy duty, probably the latter method is better. It must not be too highly polished, but have a matt finish, otherwise the oil cannot adhere, and metal-to-metal contact may result.
Full-length plating, of course, increases the cost, and it is a question of balancing increased maintenance against this factor. With commercial vehicles in particular, it is more important to keep them on the road than to save a small amount by being over frugal.
Lubrication now has a considerable part to play in cylinder life, but it must be remembered that film strength has nothing whatever to do with oil viscosity. Thin oils are more a matter of ease of starting than of possessing any particular merit in respect of lubrication, for most oils become what is known as "thin" once they are hot. There may, however, be much more to be done in the direction of employing additives, not only to reduce corrosion and sludge, but also to enable the oil or the additive to adhere to the cylinder walls.
As is well known, excessive wear often occurs at the upper part of the cylinder, close to the top of the stroke, and a theory has been advanced that this may be due to the change from film to boundary lubrication. The very thin film at that point is also exposed to high flame temperatures. Oxygen can pass through it easily, and oxidation of the metal may occur.
In tests carried out by London Transport engineers it was found that a large phosphorus content in the cylinder material considerably increased the resistance to wear. The phosphorus content was increased to as much as 1.26 per cent., and the mileage obtained was 4,500 per 0.001 in. of wear. On the other hand, little less was obtained with some nitrided liners, and as much as 5,500 miles with the Austenitic type. This was with petrol engines. Later, the use of 7.7-litre oil engines showed liner wear of 0.001 in. per 2,000 miles, but by using the aforementioned high phosphorus content, the mileage was raised to 6,800, independently of the degree of hardness, indicating that the last-mentioned feature was not all important, and to keep costs down the comparatively soft liner has been retained.
Continued research showed that the mileage per "thou.," with the liner plated by a well-known process, could be raised to as high as 12,000, a vanadium-chromium type giving similar results; but, again, economics came into the matter, and the extended cylinder life meant passing the life of the bearings, consequently the benefit achieved at additional cost became doubtful.
One type of oil engine of 8.6-litre capacity, when running in the Central London area, gives a life between overhauls of about 45,000 miles, whereas in the country this rises to 70,000_ It has been shown by experience, however, that the bigger the engine the longer its life. The proof of this is that sorne 9.6-litre engines built just before the war are giving up to 90.000 miles. The question of matching materials which are not in sympathy is also important, particularly in relation to piston rings. Using a particular type of liner, variations in piston-ring material have increased wear by from 40-60 per cent.
In petrol engines there is some evidence that wear on a cylinder is found to be at maximum when measured radially opposite the sparking plug, where it is subjected to the hottest flame. It is worthy of note that with chromium-plating, some cases have been observed where the chromium has worn through at the top of the ring travel; wear then has been rapid; the top ring will dig a little hollow into the underlying material, and although this may be narrow, its depth will increase rapidly.
Porous Plating It appears that porous chromiumplating can be carried out quite well on bores which are themselves reasonably porous, the chromium adhering satisfactorily.
Detonation has been suggested as one of the prime causes of excessive cylinder wear, and this may be linked with the heat of the walls, extra wear possibly resulting from the combination of the two factors.
Some authorities are inclined to believe that too much reliance is placed upon filtering the lubricant while it is in use in the engine. They take the view that there should be more frequent draining of the sump, the off then being carefully filtered in a fixed installation before re-use. Others take the view that it is unwise to cut down the consumption of lubricating oil to the minimum, as this may prove to be false economy. It seems, therefore, that an engine with a small amount of wear should automatically experience a continued reduction in the rate of wear as a result of the additional oil permitted between the moving surfaces.
Saving Ring Wear• One important advantage of obtaining longer cylinder life, as by chromiumplating, is that a considerable saving of wear is also effective in connection with the piston rings. It seems also that crankshaft bearings do not become so badly scored, or the crankshaft so much worn—possibly the material from the worn bores, where this is excessive, acts as an abrasive.
We have not referred to the possibilities of corrosion, chemical or electrolytic, nor to what is usually termed "ring flutter." Little seems to be known about the latter, but it has been suggested that it is the result of centrifugal force acting upon the ring, so that it moves up and down in its groove, quite apart from any tendency to move caused by the slight friction between it and the cylinder wall, also the effects of compression and firing.
We think that enough has been said, however, to indicate that the question is still one to which no complete or definite answer has yet been given