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).
Study on cylinder bore wear, piston ring practice, blow-by, and the use of thermostats in engines.
| Identifier | ExFiles\Box 132\5\ scan0132 | |
| Date | 3rd February 1939 guessed | |
| 14 CYLINDER BORE WEAR by.{R.W. Bailey - Chief Engineer} The apparatus used at Vauxhall (Fig. 16, Plate 2) for the purpose, follows closely the accepted practice of the four largest American ring makers. A large gas meter with a special large dial for small unit measurement is attached to the crankcase at its inlet and to a vacuum pump at its outlet. A water manometer is attached to the crankcase. Since all engines are subject to crankcase leakage, tests are first made motoring the engine with spark plugs removed, and the vacuum pump is set to give a depression of ¾ inch water gauge in the crankcase. Readings of the number of cubic feet registered at the meter for all speeds every three minutes, give a record of leakage into the engine with this depression, which is equal to the operating depression when the crankcase ventilator is working properly. When these tests are complete, the engine is run under full power at all speeds, and the vacuum pump is adjusted to maintain the same depression in the crankcase as before. The difference in the number of cubic feet registered at each speed when the engine is motored and under power, gives the measure of the blow-by.{R.W. Bailey - Chief Engineer} All the curves reproduced were made by this method, which lends itself to general as well as to specialized use. The Use of Thermostats. The use of thermostats to give the quickest possible warm-up is well known, but as users appear to believe that thermostats give more trouble than they are worth, it must be emphasized that thermostats are necessary. If ordinary care is used in manufacture and positioning, they should last many thousands of miles. Figs. 17 and 19, Plate 2, illustrate the application of thermostats of various models. There are two types, (1) the bypass type which allows a certain amount of water to be short-circuited around the system, but not through the radiator, and (2) the bleed type, which shuts off all circulation except a predetermined amount through a small bleed. This bleed need only be large enough to ventilate the system for filling. Both systems do very well and are very widely used. The position of these thermostats at Vauxhall is located with reference to the varying temperatures of water in different heights through the head. The desirable control should be at the mean temperature and thus must be located at a point where mixing of the different strata of water has already occurred. The omission of thermostats is an invitation to trouble. Time required for warming-up affects the time the strangler is in use and this, coupled with the cold walls, is a serious handicap to overcome. Small engines do not escape this evil; in fact, cold operation, due to many causes, is one of the principal reasons why small engines are credited by many with inherently bad cylinder bore life. Fig. 18, Plate 2, Piston Ring Practice. It must be remembered that proportion alone does not make a ring, although the lack of proportion will give bad results. The selection of metals must be given consideration, according to the experience of the author in truck and motor car practice, grey iron rings give adequate results. It must be recognized, however, that the type of engine or usage may require metallurgical characteristics beyond the scope of grey iron. Tables 1 and 2 give details of the rings which were originally used and of the rings which replaced them. In American practice, by far the largest number of rings are individually cast to have a special radial pressure, their composition and properties being as follows:— Total carbon, 3·25-3·75. Combined carbon, 0·30-0·80. Silicon, 2·75-3·25. Manganese, 0·50-0·80. Phosphorus, 0·85 max. Sulphur, 0·10 max. Rockwell hardness No. “B” 98-106. Scleroscope hardness No. 47-55. Tensile strength, 20,000 lb. per sq. in. There is a tendency in England to deprecate the need for this type of ring. English rings made to give a correct radial pressure and tension, are doing very well in service. Certainly rings that give 6,000 miles per gallon after 20,000 miles of service, with mediocre bores, are good rings, and the author believes that they work well because the tension is high and they are properly designed to have the correct initial radial pressure. Ring wear has been suggested as a possible handicap for high-tension rings. If too dry they will, undoubtedly, wear, but when normally used, they prove themselves by their service. Reference has been made (p. 1) to tests conducted by the Atlantic Refining Company in America, on six American cars. The ring wear in these engines averaged 0·002 inch per side for each, for 100,000 miles. Now, assuming that this was not a good bore wear test due to continuous driving, at least it must be admitted that the rings, which were of the high-tension high-point type and shows a design where everything possible has been done to offset any prolonged cold operation. Water is circulated through the cylinder head under pressure, the water in the cylinder block does not circulate under pressure, but merely moves thermally when heated. This combination, coupled with a thermostat in the circulating part of the system, enables the metal to be warmed in the shortest time without danger from overheating. ring will conform at once to the curve of the cylinder wall with sufficient wall pressure at all points. “An ideal oil-control ring obviously should have uniform radial wall pressure. This can be accomplished initially, but cannot be maintained, owing to wear. Therefore, it is necessary to design this ring so that it will pass through a period of efficient life above and below the uniform pressure line. It will be noted that the point pressure is somewhat lower at the time of installation, when compared with the compression ring. It should be noted how the efficient-life line passes through a zone above and below the uniform pressure line until it reaches zero point pressure, representing the end of the efficient life of the oil-control ring. In developing the oil-control ring, it has been possible to double the radial-wear factor, which has increased the efficient life to approximately 30,000 miles. It is then obvious that an oil-control ring so designed is at a point of maximum efficiency when half worn out.” PRESSURES MEASURED BY RADIAL WALL PRESSURE SCALE COMPRESSION -RING OIL CONTROL -RING Fig. 12. Radial Wall Pressure Chart Teetor and Bramberry thus point out that the efficient life of oil control rings is in proportion to the point pressure and that the useful life ends when this pressure is zero. Compression rings must have high point pressures for different reasons. With so-called ideal pressure rings (Fig. 12), the points of maximum pressure under operating conditions, are always low. That rings actually flutter the author is certain, but that the exact cause may be obscure. The irregularity of the bore has been suggested as the exciter of the movement. This the author does not believe because he has seen too much evidence of ring flutter in a bore that was well known to be regular. He still believes that the pressure behind the ring, rising and falling with the piston movement at the top of the pressure stroke, acts like the pick on a banjo string and starts the ring vibrating unless it is adequately damped. 11 CYLINDER BORE WEAR Fig. 13 gives some interesting curves of blow-by.{R.W. Bailey - Chief Engineer} Curves A, B, and C refer to three different 3¼ × ⅛-inch rings, by three manufacturers. It will be seen how alike the characteristics are. The average break occurs at 3,600 r.p.m. Curve D refers to a 3¼ × ³/₃₂-inch ring with the same radial thickness and a tension proportional to its width. The radial pressure pattern is substantially the same. This ring is very satisfactory from the blow-by standpoint, as the data show. These curves may suggest that the natural frequency of the rings is important. This is an attractive theory which the author formerly accepted. To-day, however, he views it with much suspicion, since the natural frequency of piston rings is somewhere between 10,000 and 12,000 cycles per minute. He believes the fact that rings will vibrate is important, but that the actual period is not important. BLOW-BY—CU. FT. PER MIN. SPEED—R.P.M. Fig. 13. Blow-by Curves A, B, C 3¼ × ⅛-inch rings. D 3¼ × ³/₃₂-inch ring. Fig. 14 is an interesting piece of evidence which should warn investigators to make certain that their blow-by data are taken after rings are properly run in. This diagram shows three different blow-by breaking points for the same rings, after varying degrees of running-in. Had the author not known this ring, he might have assumed that the first curve | ||