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 and prevention of cylinder bore wear, including piston rings, materials, and fuel effects.
| Identifier | ExFiles\Box 132\5\ scan0125 | |
| Date | 1st January 1937 guessed | |
| 20 DISCUSSION ON CYLINDER BORE WEAR use of tinplated pistons. Rings used with tinplated pistons required five times as long as usual to bed themselves in. Tapered rings did help in oil control, but also tended to increase scuffing and blow-by.{R.W. Bailey - Chief Engineer} The scuffing which his company had experienced with high-tension rings had formerly been particularly bad, but manufacturers of the rings had moved from one foundry to another leaving their old equipment behind them, and ring scuffing ceased. He was told in America two years ago that the cupola must not exceed 25 inches in diameter as ring scuffing was affected by the bulk of metal oxidized or at the melting point at one time. A good deal of work had been done at Vauxhall to combat scuffing, and it had been found that it could be done very readily by allowing slightly more oil to pass the rings, but at no time did they have to reduce the ring tension. He would like to make it clear that they did not use the negative pressure in the crankcase to do anything; it was there simply to make sure there was no positive pressure. It was not possible to measure anything less than 1/8 inch water gauge, or they would make it less. All they wanted to ensure was that any gas which got by the rings got out as quickly as possible, and they felt that the most positive way to do it without blowing dust into the sump was to have a measurable vacuum. Almost everyone who had studied piston ring flutter would agree that the markings occurred in a very narrow band, roughly within the first 1/4 inch of the stroke. The pressure behind the ring was by far the most important factor at this point; the tension of the ring itself was small in comparison. As regards piston taper, it had been found that the piston must be larger at the bottom than at the top. It was important that the edge of the skirt should make contact with the bore completely. Then, if a certain amount of wear did take place, the skirt was still working satisfactorily in that part of the bore which was not worn. Over the past twenty years, he had found that it was not possible to keep the variation of oil consumption between individual mass-produced engines within less than 50 per cent; it might even be as much as 300 per cent. Lately, however, by recording the oil consumption of many engines as produced they had been able to establish some fundamental rules, and by controlling the fits they had now produced 500 engines with an oil consumption variation of only 1 per cent. He thought that was a remarkable result. Eight sizes of piston and eight sizes of bore had been classified; red pistons went into red bores, and so on, so that the accuracy of the fit was never varied more than 0·00025 inch. Wing Commander Cave-Browne-Cave suggested that they should paint their engines white. Evidently he had never had to act as designer for a manufacturing company. If they were definitely to point out to customers all the leaks, they as a firm could not even exist; it was all they could do to cope with those that the customers could see by the oil on their garage floor. He was willing to agree that none of the larger engines in America, whether for aircraft or trucks, was as good as the English engines; he had said that the first week he was over here, and he had not changed his mind. It was particularly true metallurgically. They had hitherto made a better mass-produced engine in America, however, because they recognized that quality could go with mass-production. But it was possible to equal in this country the best that America could do and it would be done. He agreed that ring coating was not as necessary on low-tension as on high-tension rings. Two outstanding coatings were available, both consisting of black oxide. He thought that to-day it could be said that cylinder bore wear—speaking of cast iron bores without liners—was fairly well stabilized, and it was possible to provide small cars with bores good for 50,000 to 60,000 miles. But it was still necessary to replace the rings at least twice. Coatings, however, made it possible to increase the ring life to perhaps 40,000 miles, which with that type of vehicle was very good. So far as the material of the cylinder block itself was concerned, someone unfortunately had scared the foundries of this country into making a harder iron. If the tool cost was three times as high as with American iron, the money was simply wasted. They had to buy their iron from one supply, so that until everyone decided to have a respectable iron, no one could get it. With regard to thermostats, he sympathized with Mr. Dicksee, but thought that in any case it was a difficult job to try to cool an engine with lime water. He did not think the thermostat makers made their instruments as good as they might be, and his company had irritated one thermostat company into making them considerably better. It was not possible to do without a thermostat, however, however evaporative cooling type of engine were adopted or air cooling. There were many good points about the air-cooled engine. It attained a high temperature quickly and hid the effects of miscellaneous fuels. There was certainly some difference between fuels. It was not possible to obtain the evidence in the laboratory, but in service conditions, with heating, cooling, idling, starting and so on, considerable differences between fuels revealed themselves. A standard of measurement of blow-by was obviously needed, but if Mr. Williams was not sure CYLINDER BORE WEAR 5 advance may be as much as 7 deg. owing to manufacturing variation in the distributor, 5 deg. is not very much and must represent a part of the reason why the bore wear problem is a constantly shifting target. The explanation given by the author for the possible effect of spark advance may not be the correct, it may be that, with combustion out of phase, the general blow-by is increased. Whatever the reason, Fig. 3. Bore Wear: Comparison of Two Fuels Dotted lines, aviation fuel. Full lines, alcoholized standard fuel. it is still another factor, while variations in the ignition distributor also require attention. The Effect of the Fuel. Another very important factor is the fuel itself. Unquestionably, much work will be done in the near future to rate fuels experimentally for their effect on bore wear. Fig. 3 shows the comparative wear of a German engine using two kinds of fuel. The bad wear was produced when using the German national alcoholized fuel, which is generally of a low order, but is commonly used in Germany. The other fuel was of aviation quality, and free from alcohol. These engines were not designed to ensure the quick warming of fuel or water, and German rings are not particularly effective for blow-by and oil control. Oil is not present in abundance on the bores, and any oil film that exists may be weakened by blow-by.{R.W. Bailey - Chief Engineer} Any difference in wetness of the fuel in the combustion chamber would produce the same effect as rich mixtures. There is, of course, a great difference between the self-vaporizing qualities of a low-grade alcohol fuel and those of a fuel of aviation quality. It may be the temperature difference due to latent heat that affects the bore wear, or it may be a chemical reaction due to the admixture of alcohol. The solvent action of alcohol is well known, as is the affinity of alcohol mixtures for the nebulous solids in petrol and petrol containers. Lubricating oils and petrol have a common origin, and it is fair to assume that the solvent action between the alcoholic fuel mixture and engine oil is rapid. A weak oil film would have little chance. Further, alcohol mixtures, unless properly balanced by light petrol ends, will require a longer warming-up period, and so a longer use of the strangler—whether automatic or manually operated. This would give an abundance of unfriendly fluid in the combustion chamber and on the bore walls. Again, alcohol mixtures lower the operating temperature and this alone will promote bore wear. However, it is the lubricant which is under attack, and it is to that lubricant we must go for bore protection. Nash and Howes* quote Egloff to show that destruction of the oil film apparently occurs when fuels high in alcohol are used. With reference to corrosion, Nash and Howes suggest that the corrosive action of hot mixtures of carburetted air and alcohol is no worse than that of the petrols, yet it is known that bore wear is worse with certain alcohol blends than with certain petrols. To sum up, the effect of alcohol blends on wear is due to: the solvent action of alcohol which dilutes the oil film; the lower operating temperature, and thus the more liquid condition of the fuel in the combustion chamber; and the lengthy warming-up period of the alcohol blend, requiring the prolonged use of rich mixture ratios. The author expects that fuel technicians will protest against his suspicions. They will claim that the fault lies with engine design. One fact, however, is * “Principles of Motor Fuel Preparation”, 1934 (Chapman and Hall, London). 1a, | ||