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).
Cylinder bore wear, including analysis of piston ring performance and oil consumption.
| Identifier | ExFiles\Box 132\5\ scan0130 | |
| Date | 3rd February 1939 | |
| 16 CYLINDER BORE WEAR Discussion At the Extra General Meeting in London on 3rd February* Wing Commr. T. R.{Sir Henry Royce} CAVE-BROWNE-CAVE, C.B.E., R.A.F., ret., M.I.Mech.E. (Member of Council), said that one of the most attractive features of the paper was that the author had been able to claim that his data were based on eight to ten million engines. Comparatively few branches of engineering had such an enormous wealth of data at their disposal to enable them to make sure of their facts. One of his pet foibles was the question of what was the best temperature at which an engine should run. The author has gone a long way with him in saying that engines ought to run a great deal hotter, but stopped at 160 deg. F.{Mr Friese} It would be interesting to have from him a clear explanation why he suggested 160 deg. F.{Mr Friese} and not 212 deg. F.{Mr Friese}, because in his own much more limited experience compression-ignition engines and petrol aero-engines ran perfectly satisfactorily under boiling conditions, and there were certain advantages in running evaporatively. The author said that provided a negative pressure equivalent to ½ inch water gauge was maintained in the crankcase, the evil effects of blow-by were enormously reduced. It was difficult to see why that was so. Presumably the reason why it was important to allow the blow-by gas to get out of the crankcase was to minimize the concentration of deleterious gases. If the author had said that in order to reduce the destructiveness of this gas it was necessary to circulate a certain amount of air through the crankcase, he would have agreed with him, but he could not see that a suction of ½ inch of water in the crankcase could make any appreciable difference, bearing in mind that absolute atmospheric pressure was 34 feet of water. The author invited suggestions as to the reasons why piston ring "flutter" took place. He himself took the view that the reason why rings fluttered—i.e. vibrated radially inwards and outwards, leaving the cylinder bore and returning to it—was the action of the ring in sliding over the lubricant on the cylinder wall. At the end of the stroke the ring was pressed fairly closely against the cylinder wall. As soon as it moved it had to shear off the lubricant which lay on the surface. It could not clean the surface completely and a certain amount of oil, depending upon the speed, passed the ring. If that were the case, the ring would have no radial inward force at the ends of the stroke, but at the mid-position it would have maximum radial inward force. That was the fluctuating radial force which in his opinion was probably the cause of ring flutter. The suggestion passed one of the author's tests, in that it would depend upon the stiffness of the ring. His theory was that the "breakdown point" was the point at which the alternating force resonated with the natural period of vibration of the ring, and he would like to know whether, if the engine were accelerated to higher speeds beyond the "breakdown point" in the blow-by curve, this state of resonance could be passed. He asked, in this connexion, if the author would explain what he regarded as the correct tapering of a piston at running temperature. Attention was drawn in the paper to the extreme importance of avoiding small oil leaks, and in that connexion Mr. Scott Paine's recent † suggestion that the whole of the engine installation be painted white might be well worth while. Mr. C. G.{Mr Griffiths - Chief Accountant / Mr Gnapp} WILLIAMS, M.Sc. (Eng.), A.M.I.Mech.E., remarked that for several years he had been actively engaged on the subject of cylinder bore wear through his association with the Institution of Automobile Engineers. At the beginning of the paper the relative rates of wear in English and American cars were discussed, and he agreed that in America much less trouble had been experienced than in this country; but on a recent visit to the United States he had found that the problem was being carefully watched. Rates of wear of the order of 0·001 inch per 5,000-7,000 miles were regarded as typical under the conditions experienced in this country; i.e. very largely short-distance running. His impression was that the bore wear on commercial vehicle engines in the United States was just as big a problem as in this country; in fact, it was possible for us to teach the Americans something, particularly on the metallurgical side. The Americans had found, as had been found in this country, that special materials, such as nitrided cast iron, were needed for heavy-duty service. The author dealt with the causes of the lower rate of bore wear of American as compared with English and he himself agreed that ample lubrication * For Minutes of Proceedings of the meeting, see PROCEEDINGS, 1939, vol. 141, p. 81. † See PROCEEDINGS, 1939, vol. 141, p. 14. [Page Break] CYLINDER BORE WEAR 9 ditions. High-tension rings, properly designed, make this possible. Curve B of the diagram given in Fig. 1, shows the oil consumption over 20,000 miles for the same test run shown by curve A.{Mr Adams} With a wear of 0·0065 inch in one cylinder against 0·0035 inch in the best, the oil mileage, starting at 1,100 miles per gallon at 1,000 miles, increased at 20,000 miles to 6,000 miles per gallon. Such a result could only be obtained by using high-tension rings. In this particular engine the amount of oil on the bores is considerably in excess of normal, yet high-tension rings, combined with (and this is very important) a good piston, give these outstanding results in oil control. Compare this with a typical engine using low-tension rings and a small oil supply. It took twenty hours to consume the first pint of oil and two hours for the second pint, against twelve pints in the next four hours. High-tension rings may be unnecessary, and it may be possible to develop low-tension rings that will do as well. At present, the author knows of no substitute which will give the all-round satisfactory operation of high-tension rings with a pressure pattern designed and cast into each ring. Certainly, he knows of no other type of ring that would give 6,000 miles per gallon of oil. Curve C of Fig. 1 shows another result of a combination of piston and rings where 3,800 miles were run per gallon of oil after 25,000 miles. Again, high-tension rings show to advantage. Without a piston that permits the rings to function unhampered, low oil consumption under continued running may be difficult to maintain. Fig. 10 gives results from two engines operated under the same conditions, using the same piston rings and with the same amount of oil thrown on the bores. The pistons in these engines are different, each being a well-known design based on a great deal of experience. Fig. 10. Oil Consumption in Miles per Gallon Effect of piston design using same rings and oiling. Oil Control. Piston rings control the oil by virtue of the sharpness of their scraping edges. Pistons will rock, deflect, and twist, and if these movements are such as to dull the edges, the rings will not give satisfaction. Oil control implies a delicate balance, and means a great deal more than just keeping the oil out of the combustion chamber. For instance, a combination of rings could be used consisting solely of compression rings with a highly tapered face. For a while these rings would do well, but not over a long and hard mileage. Such rings would control by pushing back the oil on the down stroke that they had helped to drag up during the up stroke, and too large a proportion of the oil would be unchanged and would carbonize and soon start abrasive action and ring sticking. For this reason, the drain type of oil control is preferred. This type of ring "circulates oil", so that a very large proportion of new oil moves up to lubricate the upper rings and the top of the bore. The capacity of the oil ring and piston to pass or circulate oil, is therefore important. Structure or strength is the limiting factor to drainage in both the piston and the ring. Slots in the ring are limited by the breakability of the oil ring, and slots in the piston are limited at the ring groove because the lands may collapse, pinching the oil ring and making it useless. For this reason, carefully placed holes in the piston have replaced the long slot formerly used. Holes may eventually replace slots in the rings. The author has pointed out * some of the factors that interfere with good oil control:— Oil consumption is a sensitive barometer for slight * See footnote † on p. 1. Fig. 11. Pressure Pattern for Poor Rings and Ideal Diagram | ||