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).
The causes of cylinder bore wear, including wall temperature, piston temperature, and spark timing, with related communications.
| Identifier | ExFiles\Box 132\5\ scan0124 | |
| Date | 1st January 1937 guessed | |
| 4 CYLINDER BORE WEAR hall. The latter carburettor actually allows a richer mixture for full-throttle operation than its predecessor, but, at part-throttle or cruising speeds, it is leaner. This follows American practice, and so a generous accelerating pump is incorporated which gives a further richness when accelerating at full throttle or part throttle. As the lean part-throttle mixture governs full consumption on the road, it may reasonably be deduced that it may, and probably does, affect cylinder wear. Part throttle is used considerably more than full throttle. Hence, the list of essential factors must include the provision of the leanest possible mixture ratios for prevailing operating conditions. Fig. 1, curve A, shows an unsatisfactory rate of wear at the front of the engine, becoming fair at the rear. Is this correlated with mixture ratio? It is noteworthy that most English carburettors checked by the author during the past year show a mixture variation of as much as 30 per cent between front and back cylinders. To-day at Vauxhall, distribution from the carburettor is corrected to avoid this error, and no calibration is started until this has been done. The carburettor used in the tests shown in Fig. 1 was not corrected and so incorporates this distribution error. The drop in the curve at No. 5 cylinder is most interesting because it is known that, in this particular engine, No. 5 receives a richer mixture than No. 6 cylinder, due to manifolding. This may not be the reason for the difference in wear between Nos. 5 and 6, but the difference is suggestive. The author does not insist that mixture ratio is responsible for the difference in wear between cylinders, but the facts are significant, though there may be other contributory causes. Effect of Cylinder Wall and Piston Temperatures. Another disquieting factor is the effect of cylinder wall temperature on constant normal operation. Tests were conducted under favourable conditions of warm operation with a minimum number of starts, the only variation being the cooling medium, water or alcohol. The terminal temperature, 145 deg. F.{Mr Friese}, was the same, the thermostat being unchanged. The difference in wear rate was almost 3/1 in favour of alcohol. The reason may be that the water keeps the wall temperature 30 deg. cooler than does alcohol, and that this affects bore wear rather seriously. This engine (Fig. 1), like many others, has its water pump in front, and the coldest water in the system strikes the cylinders from the front. The question is, whether variation in water temperature is responsible for more wear in front than in the rear, or whether both mixture and wall temperature are responsible. What can be effected by wall temperature after an engine has reached a stable temperature, and is relatively warm? Does the wall temperature under these conditions lead to incipient corrosion; does 30 deg. F.{Mr Friese} become important at the border line; or does the higher temperature mean more clearance and oil for an under-oiled engine? It is safe to conclude that high and uniform wall temperature should be added to the list of essential factors. There are still further factors, small variations of which may affect wear in engines operating under stabilized conditions. Thus, it is possible to get zero distortion, whereas there are many engines, particularly those among the side valve type, which are not only distorted mechanically by clamping of the cylinder head, but distort under heat through the influence of the exhaust port on the barrel and lack of water around the barrels. The author has already described an engine of this type among the American products. This engine, as pointed out, is notoriously bad, yet it ranks among the best for resistance to bore wear. It is splash-lubricated and has, therefore, a copious supply of oil on the bores, but this would not avail if hot gases were to blow by the rings in proportion to the distortion. The rings in this engine are pinned to prevent rotation, so that, when once bedded in for their position, they do not have to bed in to a varying shape. In this engine, low-tension rings would not be successful. High-tension, high-point rings are used which are individually cast to a special form. Very few engines have pinned rings. The high-tension ring lends itself to this practice. Where high distortion exists, the rings should be pinned to prevent rotation and lessen blow-by, which can destroy all other protections against wear. Pressure behind the top ring may also have a bad influence on wear. This pressure is affected greatly by the fit of the top land in the bore. The length of this land is important, and is a matter of design, but it may be a matter of manufacturing variations. Temperature variation, which affects the diameter of the land, also affects its clearance. The temperature of the piston top may be affected by several conditions, of which an outstanding one may be a wet mixture, the evaporation of which lowers the temperature of the piston head, thus increasing clearance between it and the bore. Spark Timing. Another possibility which calls for attention is spark timing. Spark timing materially affects the relative phasing of the maximum pressure point and the relative position of the ring in its groove. Maximum pressure normally comes just after top dead centre. If the spark is over-advanced, pressures are high and occur earlier. Can spark advance affect bore wear? Investigators claim an increase in wear rate with 5 deg. over-advance, of as much as 1½/1 to 2/1. Since variation in the spark DISCUSSION ON CYLINDER BORE WEAR 21 that it existed to any great extent it seemed doubtful if it would be possible to obtain it, since he was the man who could establish a standard. Mr. Arthur Hoare had objected that he failed to give fundamental quantitative data on bore wear. The fact was that they did not exist except in the minds of investigators who were not happy unless they could express a condition mathematically. The problem was indeterminate because of the variations in effectiveness of one cause as compared with another. He had felt it best to present his observations with his own opinions and results, without suggesting a general programme. He had not discussed liners except to point out that the objective of his firm was their elimination in engines of the sizes in which they were interested. Communications Mr. C. H.{Arthur M. Hanbury - Head Complaints} BRADBURY, A.M.I.Mech.E., wrote that the author mentioned the question of ring flutter in connexion with gas blow-by, but had the courage to admit that it was something of a mystery. Some manufacturers were apt to attribute a number of their troubles to flutter when no evidence was available other than markings which might be due to other causes. He asked the following questions: (1) Had the existence of the ring flutter yet been definitely proved by actual measurement with an indicator? (2) Flutter must be either a forced or a self-excited vibration. Could either of these forms of vibration be accounted for mathematically? (3) When vibrating freely, a piston ring normally had two nodes; if fracture occurred, it should break at one or both of these nodes. Was this borne out in practice? He had had actual experience of a very interesting case of blow-by in a large engine running at 375 r.p.m. Intermittent blowing on two cylinders occurred after 200 and 400 hours respectively. Blowing in each case would last from one to three hours at time intervals of approximately one day. In the first cylinder, the third power ring was found to be broken, though not at the theoretical nodes but near the gap. In the second cylinder, the third power ring was found to be badly worn opposite the gap. Renewing the broken ring in the first cylinder cured the blow-by.{R.W. Bailey - Chief Engineer} When the worn ring in the second cylinder was renewed, blow-by continued for about 100 hours and then disappeared. In both cylinders the blow-by could be stopped by reducing the load. The following interesting points would be noted in connexion with this engine:— (1) Blow-by was periodic in both cases. (2) The engine speed was not altered in any way. (3) In both cases blow-by was brought on as a result of running the engine over a certain period of time. (4) In one case blow-by was cured by running. (5) The load on the engine appeared to be of great importance. These points were very conflicting and at present inexplicable. Much as he would like to blame ring flutter, he did not think it could be the cause in this case. Mr. WILLIAM HOWES, M.I.Mech.E., wrote that in attempting to explain the adherence of lubricants to surfaces by means of the electron theory, it seemed plausible that the adherence of oil films to surfaces was determined by the synchronization of the electrons of the lubricants and the electrons of the surface lubricated. If the speeds of the electrons of the atoms of the lubricated surface were of the same order, or synchronized in some way with the speeds of the electrons in the lubricant, then an inter-meshing and close attachment would occur; but if not, there would be no attachment and no so-called oiliness. Physical research indicated that temperature was a question of speed in the circulation of electrons round their protons, so that if the inter-meshing theory of lubrication had any reality, temperature would have considerable influence on the attachment of a lubricant to a surface. The higher temperatures existing in the very attenuated film of oil on an engine cylinder wall might be of such a character as to throw the oil electrons out of mesh with the cooler cylinder wall, causing them to become easily detached, and thus leave the ring surface to make a direct contact with the cylinder wall surface. Mr. W. A.{Mr Adams} WILSON (Hepworth and Grandage, Ltd.) wrote that he was very interested in the author’s remarks showing the trend of wear in the 1936 model American cars and in his verbal mention of the wear rates obtaining in the case of the 1938 productions. It would also be of interest if the author could give some idea of the state of affairs in the pre- | ||