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, comparing British and American engine design practices and theories.
| Identifier | ExFiles\Box 132\5\ scan0122 | |
| Date | 1st January 1937 guessed | |
| 2 CYLINDER BORE WEAR Austenitic rings are not recommended by reliable British ring manufacturers for use with cast iron, because experience has shown that best results are obtained with bore and rings of like materials. The austenitic combination is very costly and, although it is generally reported to give some advantage under extreme corrosion conditions, there is a very fine margin of difference between other, considerably cheaper, materials and austenitic liners and rings. The main problem is not one of liners, but of improvement in the cast iron integral bore, because this is still the most common practice. For ten years the author designed engines for one of the two American cars which enjoy a bore life of 12,000 miles per 0·001 inch wear. When a practice has been successful for ten years and has been applied to eight million engines, the assumption is justified that such a practice is worth following. This practice can be summarized as follows :— (1) Thermostatically controlled cooling for the water to not less than 145 deg. F.{Mr Friese}, 165 deg. F.{Mr Friese} being preferred. (2) Crankcase ventilation to dissipate gas blow-by and eliminate contamination of the lubricant. (3) Adequate lubrication for the cylinder bores, implying a copious supply. (4) Complete control of lubricant at the piston rings, not at the source of supply. (5) Control of gas blow-by at the piston rings. These factors are reasonably common to transatlantic practice and it is the author's firm belief, supported by experience here, that these factors must be applied in any engine if reasonable wear is to ensue, regardless of the material used. Considerable work has been done on corrosion due to continuous cold operation. However, it is not an after an engine is warmed up. Engine wear after an engine is warmed up is a more definite problem, as there are many engines with a wear of 0·001 inch per 2,500 miles that are started in a warm garage and only given one start every 50-100 miles. It is to this, the usual type of wear, that the author addresses himself. It will be of interest to examine why two American manufacturers led the American industry for the two consecutive years recorded. Briefly, two cars were twice as good as the average, and one was half as good as the average. There is a difference of four to one between the best and the worst. Therefore, regardless of the test conditions, this gives a comprehensible difference. Examining all the characteristics of the two good engines, a remarkable list of differences is found :— Car A Aluminium pistons. Side valve. No water between cylinders. Small bore, long stroke. Low axle ratio. Four piston rings (two high tension, 3/32-inch, plain faced compression rings, two 1/8-inch slotted, high-tension oil rings, one above, one below gudgeon pin). All rings pinned against turning. Bad distortion bores. Heavy splash lubrication. Crankcase ventilator. Relatively lean part-throttle mixture ratio. Car B Iron pistons. Overhead valve. Water all round bores. Big bore, short stroke. High axle ratio. Three rings (two compression, one oil). Rings loose. Very good distortion. Heavy splash lubrication. Two engines more unlike in design could hardly exist. What factors bring them together at the head of their class? The wear under discussion is due, not to excessive cold operation, at least not artificial cold, but to hard going under the vicissitudes of American weather over a distance of 25,000 miles. It is well to remember that these cars are at the head of the class having a very good general average of bore wear. The similarity of the heavy splash lubricating systems is unquestionably significant. Both these engines had highly developed oil and blow-by control, though the methods were different. Oil and blow-by control are the important elements which give an advantage over average cars. The car which, for two years in succession, showed most bore wear, had the shortest piston—one on which it was difficult to get good rings—and a crank-pin construction notorious for its lack of oil throw-off, due to a peculiar type of bearing. Here, in the worst case, is found very low degree of bore oiling. This at least shows that oil on the cylinder wall is necessary to ensure good wearing quality. Excess of oil may not serve any useful purpose, but a great deal of oil may be put on to the walls, provided that the rings, aided by the piston, will keep the oil out of the combustion chamber. The effect of oil as a deterrent to wear from any cause except dirt and abrasion, is well established, but there is some difference of opinion as to whether oil can be used in optimum quantities, because of the belief that piston rings to British standards will not give the desired control. There is substance in this belief, and for this reason, American practice rather than British practice in piston rings has been recommended. The author has observed, and has been a party to, considerable development in American piston rings. Since their use has been highly satisfactory, he sees no reason why American practice should not be adopted here. However, before comparing American and British ring practice, it would be well to discuss the general problem of bore wear. COMMUNICATIONS ON CYLINDER BORE WEAR 23 cerning corrosion as a main factor in bore wear. Engineers should consider deeply the remark “there is a very fine margin of difference between other materials and austenitic ones”. In a recent paper he himself expressed the same sentiment thus: “Exceedingly good results constantly are obtained from irons which are not non-corrosive; this would be a miracle if corrosion was the major, or an important, factor”. The author mentioned the deleterious effects upon bore wear of strong mixtures and of cold walls, but omitted reference to the composition of the gases in the combustion zone (this did not mean exhaust gases), and he himself suggested that the carbon monoxide content of the gases during and immediately after combustion might be the cause of the above effects. It was, he thought, a faux pas for the paper to enter the realm of metallurgy. On the one hand, the author criticized adversely the cylinder block materials used in this country, and one felt that he might have sound grounds for so doing; on the other hand, this impression was swept away by his quotation of the American specification for individually cast piston rings with special radial pressure. This stated that an iron might possess a combined carbon content of not less than 0·30 per cent and not more than 0·80 per cent, which was no whit less futile than if the specification for a motor car pronounced that its best performance should be not less than 30, and not more than 80, m.p.h. He granted that in this country irons of similar calibre existed; but the specification, having already condemned them, aroused his hopes concerning the American specification. These things mattered tremendously; an iron containing 0·80 per cent combined carbon gave a wear result two, three, or four times better than an iron containing 0·30 per cent. It was a question of structure, and, to the best of his own experience, nothing the author did as an engineer would detract from its importance to engineering. Having been associated with the mercantile marine, he himself would as lief trust himself on an Atlantic liner with inferior material in the rudder post as with an iron of 0·30 per cent combined carbon in any vital working part of the engines. Tolerances of this order rendered specifications far worse than useless because they admitted bad metal. The grievous fault of ignoring metallurgy in specifications concerning grey iron was apparently committed also in America. Incidentally, nothing else in this American specification was other than platitude, as the amounts given of each element were those of iron which could be cast individually. The time was due when the irons of which piston rings were made should receive sound consideration. Valuable benefits would then accrue with only inconsiderable increase in cost. Mr. Alex Taub wrote in reply to Mr. Bradbury that until it had been more clearly defined ring flutter must perhaps be considered as a probable cause rather than a definite effect. They were interested in two main effects: (a) the sudden and violent upward change in blow-by at the higher engine speeds, and (b) ring breakage near the open ends, usually in several short pieces. When (a) was eliminated (b) was usually eliminated also. This was certainly his experience. Thus they found themselves attempting to analyse their blow-by “curves”. Because of the nature of the markings in the bore and type of breakage of the ring, “ring flutter” had been tentatively accepted as the cause. For this reason there was a tendency to-day to look upon this “cause” as an effect, and this had resulted in many unsubstantiated theories. He was sure that at certain periods the ring left the bore. He was also reasonably certain that this action was always at the open ends; but he was not certain at all how far around the ring the action extended. He was certain that the degree of seal at the open ends was a major factor affecting the point in the speed range at which the ring began to leave the cylinder walls. This precluded the possibility of the ring harmonics being a controlling factor, but did not cancel the strumming effect of the pressure action at top dead centre. There was a valvular effect from the ring, as explained in the paper, and a high or low pressure fluctuating with the movement of the ring. High tension and high point pressure in rings resulted in an improved seal at the open ends and this, they knew, had been good practice. There should be little difficulty in associating this reasoning with ring breakage at the open end. It was the least flexible area, and when in motion or under deflexion was subject to very high stress. There was another fact that must be considered as a cause of ring breakage near the open ends, namely, that the compressed gases behind the ring might be escaping through the gap, and thus this area of the ring would not be held in contact with the wall to the same extent as the rest of the ring. This could well allow high local blow-by, with resultant rapid deflexion. When properly investigated this might bring some interesting results, and perhaps a new crop of theories. The thoughts contained in Mr. William Howes’s contribution were interesting to designers, amongst whom he himself belonged. But they had not arrived at electronic control yet; they did not even understand thoroughly such tangibles as piston rings. In reply to Mr. Wilson, the state of affairs regarding bore wear and oil consumption had changed in 1939 in the United States mainly because what had formerly been the worst product in this respect was | ||