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Aviation History
1939
1939 - 0443.PDF
FEBRUARY 16, 1939 FLIGHT. 161 Stages in the development of heads for Bristol sleeve-valve engines. On the left is the original design, in which there was difficulty in carrying the air down to the centre of the head. In successive types various arrangements of baffles were tried. On the right is the present radial-finned head in Y alloy. '' During eleven years of mono-sleeve development for aero engines numerous engineers have warned the author against this form of valve mechanism, pointing out that the number of firms throughout the world who have tried to make a success of it and have eventually abandoned the design. He is glad to say that he has no fears on this score, and, although he does' not claim that it is the only solution for the valve mechanism of high-power reciprocating internal-combustion aero engines of the future, he feels convinced that it is going to be a classic one. "The author must admit, however, that, looking back over the years which have elapsed during his association with the development of the mono-sleeve valve, there were several times when failure to realise complete success after long periods of promising running gave him some qualms and called for a good deal of hard thinking. He then began to realise why other firms had given up the conflict, and he became convinced that the successful solution of the mono-sleeve was a long and deep furrow to hoe. " The author has no hesitation in saying that, had it not been for the loyal support of an enthusiastic design and experimental team, considerable patience and encouragement from the Board, and the continued backing of the British Air Ministry, the present Bristol sleeve-valve development might quite well have had to be discontinued some years ago." THE DISCUSSION Dm. D. R. PYE, Director of Scientific Research at the Air Ministry, was asked to open the discussion. He said that Mr. Fedden was the parent of the mono-sleeve valve aero engine but perhaps he (Dr. Pye) could claim to have been the humble but useful wet nurse who had supplied some of the sustenance during the infant's early years. It was a good thing to look back sometimes, and he would recall the background against which the beginning of sleeve- valve research began. Prior to 1928 Mr. Ricardo had done a great deal of work on single cylinders, and had built a sleeve- valve racing car for the Vauxhall company. The units were all of the water-cooled type. He quoted from a paper he wrote for the A.R.C. in 1928, in which he said the sleeve-valve cylinder presented peculiar difficulties and that for the present its claims were bound up with the continuation of water cool ing. A special panel was appointed under the chairmanship of Mr. (now Sir Henry) Tizard, and evidence was heard from a great many people. Excessive oil consumption was a legacy from the double-sleeve engine. It was feared that carbon would build up on the edges of the ports. The panel came to the conclusion that on a given fuel a sleeve-valve engine should be able to run at a higher compression ratio than the poppet valve engine without detonation. That led them to expect better thermal efficiency. It also seemed that when detonation did set in, it came on less suddenly. At the time it seemed important to get, on the naturally-aspirated engines used in those days, the best possible cylinder charging efficiency. Nowadays, with ground boosting, that was not so important. The panel came to the conclusion that on a weight basis there was probably not much to choose between the two types of valve. The thing that seemed important was to get rid " whe °yerneated exhaust valve, and the panel concluded: »> e think that the sleeve-valve is worth development as offering far the greatest scope for advance in the production of high-duty engines of low rating per h.p., and we urge the experimental design of a complete engine." This was based on water-cooled sleeve-valve results, and it was not felt that building a complete air-cooled engine would be justified, but >ts advantages seemed to justify further single-cylinder re- Ta' Tnat was tne Point at which Mr. Fedden and his stag took over the problem. "fk ^e t*len re^erred to Mr. Ricardo's experiments in 1928 with direct petrol injection into the cylinders of a sleeve-valve fngme. .ft had been found that, by choosing the position and nrne of injection, one could get a stratified charge, with a rich mixture at the top of the cylinder and practically pure air near the piston. That made it possible to get high fuel economy under cruising conditions. He felt that this was a line of development worth exploring. MR. J. F. ALCOCK recalled some of the early work of Ricardo, with whom he is associated [Mr. Ricardo is abroad at present]. One main trouble was connected with the sealing of the cylin der head. They thought split rings would catch in the edges of the ports and tried all sorts of solid rings, and no rings at all, but without success. Then they tried, in sheer despera tion, a common or garden variety of piston ring and it worked perfectly. He confirmed the importance of the oval motion of the single sleeve. This helped to spread the lubricant. The oil film had to transfer heat by conduction and so had to be thin. Thus it was important that the cylinder and sleeve materials should have the same coefficient of expansion. DR. W. H. HATFIELD paid a tribute to Mr. Fedden, and jestingly said that a typical procedure when they had one of their many conferences was that Mr. Fedden would begin by talking coefficients of expansion; then he pointed out that he must have greater strength and greater hardness. He must have a high Izod number, and just the right size of crystal and, of course, he could not put up with any non- metallic inclusions. In the end the metallurgists said "Thank goodness he is satisfied with the modulus of elasticity." He thought that, taking it all around, Mr. Fedden would admit that he had been given the materials he asked for. He thought the metallurgical world would meet demands during the next eleven years [the period of development under review] as it had done in the past eleven. MR. J. D. PEARSON challenged some of Mr. Fedden's state ments, particularly about breathing efficiency. He thought that the differences shown by the lecturer could be accounted for by changes in the induction and exhaust systems. On the subject of push-rod-operated valves, he said that he could appreciate the limitations, but with enclosed overhead-cam shaft-operated valves the limit on speed was not imposed by the valve gear. He referred to figures from a Kestrel test unit, which had completed 1,318 hours of running without trouble from the valve gear. He finished up by expressing the hope that he had said enough to convince the audience that there was still a lot of life in the old poppet valve. MR. T. HALL, of Imperial Airways, said he started off as a severe critic of the sleeve-valve engine, but after his experience with it in actual service he had become an advocate of it. MR. TOM THORNYCROFT wanted to know where, approxi mately, the waste heat of a large aero engine went. He thought that in high-speed aero engines the sleeve valve was bound to win. MR. G. MORRIS asked if there was any chance that the sleeve-valve engine had its own " hot-spot." He also quoted a poppet-valve engine which' had been run successfully at a power of 7.9 h.p. per sq. in. of piston area. MR. CANTRELL supported Mr. Fedden's figures, but thought that the air-cooled sleeve valve might possibly have a better application than the radial arrangement. He wanted to know if Mr. Fedden regarded K.E.965 as completely satisfactory for the sleeve. MR. FEDDEN, in replying, said he did not regard Dr. Pye as having been merely " wet nurse " to sleeve-valve develop ment ; he would rather regard him as a fairy godmother who had produced the wherewithal to do the work. If Dr. Hatfield could produce something better in the way of materials they would be able to go still further ahead with development. He admitted that the Rolls-Royce poppet-valve engine had put up a very good show, but still felt that, at any rate for the large air-cooled and radial engine, the sleeve offered the better solution, if everything was taken into account. Mr. Thornycroft had caught him out on the point of heat balance. They had done some work at Bristol and their engines gave an overall efficiency of 34 to 35 per cent., but he would not like to say from memory how the balance was made up.
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