FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1939
1939 - 0622.PDF
224 FLIGHT. MARCH 2, 1935 BRISTOLIAN in DERBY Mr. A. H. R. Fedden Lectures on Sleeve-valves to a Rolls-Royce Audience T_Ii£RE were at least two points upon which most people agreed when Mr. A. H. R. Fedden repeated, before the Derby Branch of the Institution of Auto mobile Engineers, his R.Ae.S. lecture * on Bristol sleeve-valve engines. The first was that there was room for both sleeve- and poppet-valve engines. The second was that whether one used as a basis of comparison for two en gines, h.p. per sq. in. of piston area or output per litre, the only true basis was the performance, side by side, of two units of similar power, and thus depended upon drag and weight. It was to be expected that Mr. Fedden, carrying the Bristol gospel into the home camp of Rolls-Royce, would meet a barrage of difficult questions and he is to be con gratulated upon the simple and honest way in which he answered most of them, and the neat way in which he side-stepped a few. The lecture was followed, as in London, by the showing of the cinema film of sleeve-valve operation, and the dis cussion was then opened by Mr. Elliot. His main points were that the sleeve-valve, while very suitable for air- cooled radial engines, would not be so efficient in the case of in-line liquid-cooled units. The chief of several reasons was the increase in length which was necessitated. For a poppet-valve engine of Rolls-Royce type, the inter-cylinder distance could be as low as 0.65m., while for a sleeve-- valve engine the minimum was 1.75m., making a difference of over 6in. in the length of the engine. Again, the relative weights of cylinder assembly for poppet and sleeve designs were 2.68 lb. and 7.2 lb. respectively. There followed a series of questions posed by members of the Rolls-Royce technical staff, among whom were Messrs. Ellor, Pearson, Cantrell, Lovesy and Dory. Some thirty questions were answered by Mr. Fedden as follows : — There was room for both types of valve gear, and he recalled the Barr and Stroud motor cycle engine. The difficulty in that case was the materials, but these were now good. Pilots had told him that machines with sleeve-valve engines were quieter than with poppet-valve engines. Both the Scylla type and a twin-engined military machine had been found to be quieter with sleeve-valve engines. Referring to length, he realised that in the case of in-line engines, weight and crankshaft harmonics entered into the question. Identical liquid- and air-cooled sleeve-valve units were to be run side by side. Breathing was better with bores of sin. ; Rolls-Royce would be able to tell him. of experience with Gin. bores. Poppet- valve ports on engines of the 500 c.c. motor cycle type were perhaps better from the point of view of breathing than were sleeve ports. Results of Failures The theory that wreckage in the case of a sleeve-valve engine would be greater had been shown in practice to be wrong. Sleeve drive failures were localised; damage from piston failures was about the same as with a poppet-valve engine. It was admitted that the Bristol Company were worried at first, and it was realised that sleeves must be dead reliable. It had taken much time to achieve this. With regard to the question of what advantage a user obtained from the sleeve-valve engine, Mr. Fedden said that in the case of Bristol radial engines the output was better than with the push-rod type of comparable size; there were no oil leaks or valve maintenance; fewer spares were required; the engines were cleaner; they cost less, and they could be pro duced more quickly. Major troubles centred on (a) pistons, (b) the ball-sleeve drive, and (c) how to make sleeves. The advantage of sleeves over push-rods, and the troubles experienced with valve bounce, etc., in large radial engines probably did not affect in-line engines with overhead cam shafts to the same extent. With regard to limitations of output, the figure of 6.2 b.h.p./sq. in. of piston area was not the maximum which could be obtained. Mr. Fedden had great regard for the high- output-short-life policy seen in Rolls-Royce racing engines. In general, Bristol policy was reliability at any cost. (The figure * Reoorted in some detail k: Flight of February 16. of 9 b.h.p./sq. in. of piston area was mentioned in connection with Rolls-Royce engines.) It was hard to make a neat double-row four-valve radial engine, and he did not know how to deal with the breathing Installation drag and weight was a true basis of comparison as already mentioned. Bristol thought they had "a bit un their sleeve" in this connection. In any case, the engine was neat, clean, and good from the production point of view. Answering a question in connection with cylinder arranee- ment, Mr. Fedden had in mind two-, three-, and four-row radials, and thought sleeves would be an advantage for all of these. The "X" type engine could be considered as a six- deep radial with four cylinders per bank. DTD 49 D low-carbon steel was found at the present time to be suitable for sleeves. Fins had been" left off the bottom of the cylinder barrels simply to give access to the holding-down nuts. The accumulation of carbon in the ports was not serious in an engine which had done 440 hours in the air (this was the maximum period so far). Thanks to the excellent work of the metallurgists, the expansion properties of the materials were not at all bad. In answer to a question on the various designs of sleeve drive used during the development period, Mr. Fedden said that the integral ball which he illustrated was difficult to assemble and expensive to produce. With regard to the oil film in large-bore cylinders, no troubles had been experienced with the film in 5|in. x /fin. cylinders (the largest so far). Due to the moving oil films on each side of the sleeve, it was almost '' transparent"' to heat. The power drop on a sleeve-valve engine after about 100 hours' test was in the region of 1 per cent. Answering the Throttle In answer to a question on acceleration in service, two Empire boats had so far put in about 40 hours' flying with sleeve-valve engines, and it could be said that, at any rate, they were as good in this connection as the poppet-valve units. Answering a question concerning the smoothness of power acceleration and the low-speed effect of sleeve valves on ignition units: The sparking plug position in the sleeve-valve heads was not quite so good as on the poppet-valve engines. The valve gear was, however, quite quiet when idling, and good reports had been received from squadrons of Vildebeeste IVs, A special sparking plug had been developed to suit the engine. Fuel of a little over 100-octane value was used for high-power tests on a single-cylinder unit. The oil feed and cooling were normal. Concerning a question as to the various designs of induction manifold during development, the control of swirl had been found to be very important, the difficulty being too much swirl. Out-of-balance forces on present engines had not presented any difficulties. Tests in Canada had shown that sleeve valves did not present any extra cold-starting problems, and the torque on cold days was much the same for sleeve or poppet-valve engines. The oil cooler on the Northrop monoplane used as a flying test bed for the Hercules was not disproportionate considering the output of the engine. The heat losses of an engine were roughly 2.5 per cent, oil, 45 per cent, exhaust, and 15 percent. cooling air. The exhaust gases were definitely cooler on a sleeve-valve engine in view of the increased compression ratio used with a fuel of given octane value. Exhaust manifolds in service naa a life of some 150 to 200 hours. Troubles might be experi enced later when higher power outputs were obtained. The mechanical efficiency of Perseus and Mercury engmes at 1,800 r.p.m. was, near enough, the same. Sleeves had not behaved any better towards detonation tlian poppet-valve engines, but tests on very weak mixture had De quite satisfactory. . ,• Other points mentioned were the fact that the oil cin ulau^s on sleeve-valve engines was increased by about 20 per cen • ^ compared with poppet-valve engines, but some 70. .h.p. were taken from 25 litres capacity. Oil circulation in Amer engines was a great deal up. ^g The Bristol Taurus engine had as small a diameter as v.^ sible considering the cooling necessary for an engine o) an output. Considerable difficulty was being experience-a ^ the Continental small-diameter double-row popp< ^ engines. Some form of cooling similar to that on. ks0f Havilland Gipsy XII might be applied for the rear ca multi-row small-diameter radials.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
