From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Detailed analysis of cylinder bore wear, piston ring design, and the prevention of blow-by, including a discussion section.
| Identifier | ExFiles\Box 132\5\ scan0129 | |
| Date | 3rd February 1939 guessed | |
| 8 CYLINDER BORE WEAR engine is such that the ventilator cannot maintain a depression, then harmful dust will follow the air in. In this way, the ejector ventilator may be considered a valuable general criterion of internal conditions. Difficulty may occur when ejector type ventilators are applied to trucks and lorries that maintain a low speed. Under such conditions an air collector, such as is used on the Bedford machine (Fig. 7, Plate 1), must be applied to the bottom of the ventilator outlet to increase the air speed sufficiently to be effective. Grappling with these difficulties with ventilators afforded the inspiration to effect the prevention of blow-by.{R.W. Bailey - Chief Engineer} Piston Ring Design. Considerable progress has been made in the last seventeen years in America in ring design and development. From 1920 to 1927, American rings worked under a uniform radial pressure and were hammered to that pressure. Such rings had an average diametral tension of 7 to 9 lb. The ratio of radial thickness to diameter was 28/1. The rings were finished with sharp corners and there were several methods of casting them, the only limitation specified being that the product must be free from shrinkage strains. From 1927 to 1932, rings were individually cast to ensure long life and less blow-by.{R.W. Bailey - Chief Engineer} This was the period of development of crankcase ventilators. The average tension of these rings was 9 to 11 lb., the ratio of radial thickness to diameter was 24/1 and hammering for shape was eliminated. The individually cast ring, with its more consistent characteristics, easily took the place of the ring shaped with haphazard hammering. From 1932 to 1937 the tension increased to 16 to 18 lb., rings being individually cast with the radial pressure pattern controlled so as to give the maximum effect on a given engine. During this period, blow-by had become a serious factor, and maximum pressure and increased temperatures and speeds were making considerable demands upon the piston ring. Modifications were made in American rings to meet these conditions. Perhaps the most important development in piston rings is the evolution of the pressure shape. This came about through an endeavour to shift the “break” in the blow-by curve. Fig. 8 reproduces a typical blow-by curve. The curve is fairly high through the range from 1 to 1¾ cu. ft. per min., but at 3,400 r.p.m., the break occurs, and at 3,900 r.p.m. the blow-by is 4¼ cu. ft. per min. Experience has indicated that ¾ to 1 cu. ft. throughout the useful range, is an ideal quantity which normal ventilators can handle. The ring shown in this test (a 3/16-inch × ⅛-inch ring) chatters or dithers at speeds above 3,300 r.p.m., a condition which will lead to fracture. Disintegration will follow, even though the rings remain whole; Fig. 9, Plate 1, shows the effects of blow-by on the surface of the ring after a few hours. Rings so injured have lost their ability to seal, a common feature in bores that distort when nothing has been done, either to modify the distortion, or to help the ring to seat quickly. M.{Mr Moon / Mr Moore} O.{Mr Oldham} Teetor, speaking of bore distortion, has said:— “The piston ring cannot conform to distorted contours. Hot blow-by gas passes the rings at these points, which increase the pressure. A heavy oil film collects at the distorted area and the rings cannot make contact with these surfaces with sufficient pressure to reduce the thickness of the oil film. When an area of carbon is formed on a cylinder wall, it cannot be lubricated, and piston rings cannot seal against such a surface. Under such conditions the piston rings, cylinder, and piston scuff, oil consumption cannot be controlled, blow-by is excessive, and wear is inevitable.” The author gives this quotation to show that blowby is indeed a reality, and has been subjected to serious investigation in order to eliminate it. Considerable credit is due to the American piston ring makers, for, in spite of the difficulties of distortion, they have worked out a combination that made a good job out of a bad one. In fact, the engine already referred to as being one of the best American types, is a case in point. It is certainly rather distressing to designers to find an engine that violates all the principles of good design and yet does so much better than engines considered to be superior in design. The specification of this redeemed poor design is given on p. 2 (car A) and includes pinned High-tension rings when pinned have a real opportunity of bedding in. If rings with lower tension or no pinning were used, this job would remain the despair of the ring maker. On the other hand, such measures in an engine that did not require them would lead to trouble from dry bores. The author has always advocated an excess of oil as compared with the supply in the average pressurefed engine, either here or abroad. Sufficient oil is needed to establish a fresh film between all-metal surfaces for every revolution of the engine. With a copious supply of oil and good control, a constant change is going on in the oil film. Control, though maintained by the rings, is aided by piston design. As a weak piston may interfere with the operation of the rings, the piston must be structurally rigid, and its shape must be such that it will meter the oil on the bores in order that a given combination of piston rings may not have more to do than was contemplated in its design. A good piston will lay a new film of oil, and the rings will cut the film to final smoothness. The surfaces are protected and the oil film is constantly refreshed. It is quite easy to see the necessity for high-tension rings for an engine that must run continuously with a minimum of variation, despite many adverse con- --- PAGE BREAK --- DISCUSSION ON CYLINDER BORE WEAR 17 ditions, and measurable, and it agreed with the effect of weak mixtures on, for example, valve seat wear, although he did not suggest that the mechanism was one of oxidation as in the case of valve seat wear. The Institution of Automobile Engineers tests on aluminium and cast iron pistons, to which the author referred (p. 10) showed that both materials gave the same rate of wear under cold starting and normal operating conditions. The tests were undertaken in the endeavour to dispel the popular fallacy that aluminium pistons were entirely responsible for cylinder bore wear. He agreed with the author, however, that that was not the whole story, and that at the two extremes of operating temperatures—i.e. under cold starting conditions and under very hot running conditions—piston scuffing could occur with aluminium, which could in turn cause scuffing of piston rings. Blow-by was not yet, he thought, a major problem in this country, but he had discussed it with a number of manufacturers and all were interested in it. In most cases he believed that the speed at which blow-by occurred was just above the speed at which the engine exhibited maximum brake-horse-power and manufacturers were anxious to cure the problem before it became acute. Higher ring pressures, as the author pointed out, were being used in the United States. When he was in that country he heard a good deal about ring scuffing troubles, and these, he was given to understand, were probably associated with the high-pressure ring. These troubles occurred especially during the breaking-in of the engine, and he had heard much about special treatments and coatings which were being applied to piston rings in order to overcome the difficulty. It was possible that if this country were to adopt high-pressure rings, the necessity for such surface treatments might also become apparent. The subject was at present being investigated by the Institution of Automobile Engineers, and he had hopes that there were alternative methods, other than excessive ring pressures, of overcoming ring flutter and blow-by.{R.W. Bailey - Chief Engineer} * Mr. C. B. DICKSEE, M.I.Mech.E., expressed surprise that the author should suggest that anyone in this country should hold the view that cylinder wear was not a problem. It would, he thought, be generally admitted that cylinder wear had been a very serious problem, particularly on commercial vehicles. The trouble seemed to start about ten years ago, when foundries began producing a very much purer iron. Apparently they eliminated the one impurity—the phosphorus—which was ensuring good cylinder wear. If cylinder irons in the U.S.A. contained more phosphorus than the average irons in this country it might account for some of the was of prime importance, particularly when starting; this point was brought out in tests with which he had been associated. Under cold-running starting conditions ample lubrication minimized corrosion. English manufacturers had very largely realized this; some had fitted larger oil pumps and were beginning to reap the benefit of an increased supply of oil. He was rather surprised, however, that the author made no reference to the widespread use for many years in the United States of light oils. Americans were fortunate in being forced to use light oils, not to reduce cylinder wear but for cold starting; an incidental advantage of light oils was that they started to circulate very quickly and hence reduce bore wear. The Institution of Automobile Engineers had done much to call attention to corrosion, but he agreed with the author that it was possible to over-emphasize its importance: there was no excuse nowadays for an engine to suffer appreciably from corrosion wear. Interest had shifted, as the author pointed out, to high-speed operation, under which scuffing and blow-by occurred, and he himself would add another factor, namely, the difficulties which occurred during the breaking-in of an engine. It was desirable to differentiate between wear occurring under hot-running conditions and that occurring under cold-running conditions, because the effect of many factors was quite different in the two cases. For example, an ample supply of lubricant, while of major importance during warming-up, was relatively less important once the engine had become warm. Surprisingly little oil was required to lubricate cylinder bores once an engine was hot. The Institution of Automobile Engineers had studied the influence of different fuels, e.g. alcohol, and showed that, as indicated in the paper, somewhat higher wear was obtained with alcohol fuels. That was, however, entirely during the warming-up period; when an engine was running normally hot, they failed to detect any difference between any of the fuels tested; even hydrogen fuel gave the same rate of wear as petrol. That finding applied also to the mixture strength; under hot-running conditions no effect at all was observed, and he himself did not see how it could have an effect. The author mentioned the possibility of dilution of the lubricant on the cylinder walls, but when an engine was running hot the amount of dilution was negligible; it only took place under cold-running conditions. The author said that less cylinder bore wear was experienced with leaner mixtures. That variable had been studied by the Institution of Automobile Engineers under warming-up conditions and their finding was that the leaner mixtures gave more wear. That effect, though not marked, was definite | ||