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).
Article by Alex Taub discussing the causes and factors affecting cylinder bore wear in engines.
| Identifier | ExFiles\Box 132\5\ scan0079 | |
| Date | 1st March 1939 | |
| THE AUTOMOBILE ENGINEER MARCH, 1939 CYLINDER BORE WEAR* By Alex Taub† THOUGH no one simple theory covers the probable causes of cylinder bore wear, this does not mean that corrosion should be disclaimed as a prime factor. Recently, at the General Discussion on Lubrication and Lubricants arranged by the Institution, the general reporter, in summarising the results of the discussion on engine lubrication, observed that there were two schools of thought regarding bore wear, one which believed the corrosion theory, and one which disbelieved it. The author was erroneously placed among the negatives. The author is certain that no one quarrels with the corrosion “theory,” but he does not think it should be considered the only factor. Bore Wear in the United States.—Recently the author stated that, years ago, he had worked on the question of corrosion, or etching as it was called in America. Investigations were carried out by Mougey, of the General Motors Corporation, on the factors affecting etching or abnormal wear led to the installation of thermostats and crankcase ventilators in all General Motors’ products. By 1931 this work was completed and a definite improvement in regard to wear ensued. Continuance of this work led to greater improvement and, by 1934, reboring of the average American car, after running less than 30,000 miles, was usually unnecessary. Reboring might have been imperative in a few cases after running 40,000 miles, but they were very few. The average mileage before reboring is required is about 70,000 miles. It is the author's contention, based upon his experience and observations here and in America, that the bore wear situation in Europe falls very much below this standard. The author has discussed‡ the wear of forty American cars, all 1936 models, which showed, except for three cars, an average of 5,700 to 7,000 miles per 0·001in. of wear. The average rate for the same cars in 1937, with the same exceptions, was slightly lower because, in several cases, No. 1 cylinders showed higher wear than the rest. The three cars held out as exceptions in 1936 included one showing 3,500 miles per 0·001in. wear, and two showing 12,000 miles per 0·001in. wear. It is interesting to note that, for 1937, the car with the poorest rate of wear is still the poorest, and that the two cars at the top of the list are still at the top. This is the result, not of minor changes or of a laboratory experiment, but of conditions which, for two years, have covered an entire industry, representing 90 per cent. of the world's production of motor cars. To clarify the account of these test conditions, it may be added that a major oil company which ran an independent test with six cars from the 1936 group for 100,000 miles, found an average bore wear of 1/40,000 miles per 0·001in. This was unquestionably an easy test, whereas the test referred to for the entire American industry was comparable, when fairly severe public usage and considerable winter driving were taken into account. To deny that the cylinder wear problem exists in England does not solve the problem. The industry has been offered austenitic sleeves and rings as a remedy. Satisfactory results are obtained abroad without these expensive products. Why, therefore, should the industry here be condemned for insisting that bores with a life of 75,000 miles are available in the form of sleeveless cylinders with cast iron rings? 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 0·001in. 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 summarised as follows: [Chart] Fig. 1. Rate of wear and miles per gallon of oil. A.{Mr Adams} Rate of wear for each cylinder. B. and C. Miles per gallon of oil. (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 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 necessary to have continuously cold operation unless an engine is not permitted to warm up. Engine wear after an engine is warmed up is a more definite problem, as there are many engines with a wear of 0·001in. 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:— [Table] Car A Aluminium pistons. Side valve. No water between cylinders. Small bore, long stroke. Low axle ratio. Four piston rings: (two high tension, 1/8in., plain faced compression rings, two 1/8in. slotted, high-tension oil rings, one above, one below piston rings). 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 crankpin construction notorious for its lack of oil throw-off, due to a peculiar type of bearing. Here, in the worst case, is found a 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. Factors Affecting Bore Wear.—The factors affecting bore wear are complex, and, although an engine may include all the known means of protection against wear, wear still occurs, even under conditions considered to be favourable. Such engines may be fully warmed and, with a minimum of starts, may be operating on a fast or mixed schedule. It is this fact which prompts the author to argue against over-emphasis on corrosion. Curve A of Fig. 1 indicates cylinder wear rate for a most interesting case. This engine * Being a paper recently presented to The Institution of Mechanical Engineers. † Vauxhall Motors, Ltd. ‡ Paper read before the University of London. See The Automobile Engineer, 1937, vol. 27, p. 134. | ||