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Aviation History
1937
1937 - 0077.PDF
JANUARY 14, 1937. FLIGHT. 29 to components, but would also make possible" lower consump tion and higher performance for a given fuel. . The three main valve arrangements were mentioned by the lecturer—the four-valve pent-house as developed by Bristol, which allows good breathing and high piston speeds due to large valve area and small valve mass; the two-valve hemi spherical head favoured in the United States for easy machin ing and cooling with clean entry, but having the definite objection in large-bore engines of big valves weighing as much as one pound; and the four-valve flat-head type usual in liquid- cooled engines, which allowed straightforward valve operation as well as the advantages of the first type. On the subject of exhaust valves Mr. Banks said that in engines such as the Pegasus and Cyclone (in which latter the output was over 100 b.h.p. per exhaust valve) the valve material must be specially developed to resist hot corrosion attack by the free lead oxide not entirely eliminated from the high-duty fuels. Recently, in the Rolls-Royce laboratory, the rate of lead attack had been shown to increase rapidly until 7000 C. was reached, when it remained constant until goo0 C. before again increasing rapidly. Valve Burning The two methods of combating valve burning were effective valve cooling, and the use of steel having a high resistance to corrosion Practically all aero engines now had stellited valve seats, and a new material, " Brightray," was also avail able. In America sodium-cooled valves of the hollow-head tvpe were chiefly used, but in England the stem only was sodium-filled, with a resulting higher valve temperature which must eventually limit engine performance by depreciating the anti-knock value of the fuel. To avoid distortion, stiff heads were necessary. Shrunk and screwed N.M.C. inserts were, the speaker said, giving good service in general in this country. Deposits on sparking plugs, chiefly lead oxide, sulphate, and bromide, had given considerable trouble, even though reasonably good insulators except at high temperatures. They appeared to be hygroscopic or moisture-retaining, which caused difficulty in starting. Of a large number of metals tried for the central electrode, pure nickel seemed to be one of the best. The cooler plug in the cylinders, which received a slightly richer mixture, sometimes showed more deposit. A further rather extra ordinary plug trouble that had been experienced was reported by Mr, Banks, namely, "pin-holing" of the mica, for which an interesting ionisation theory was submitted. A new develop ment in insulating material was aluminium oxide in the form of corundum, which had a high thermal conductivity and was impervious to attack by the products of combustion of leaded fuel. The main difficulty was that of producing uniformity of quality. The future might see the incision of a compact non-detachable plug. A promising plug from the erosion point of view was the new K.L.G. with six platinum, iridium earth wires; this was capable of lasting out a type test of ioo hours' duration without attention. Plugs of 34 mm. size were now used mainly in new types in this country. Cold corrosion, the lecturer said, was not now regarded as a serious problem; it was apparently the result of bromine liberated in combustion, and cobalt-chrome steel was a par ticular offender, while the austenitics were highly resistant. A very effective anti-corrosion material had been developed. Corrosion of fuel tanks, said Mr. Banks, in answer to'queries, had only occurred with high magnesium alloys in the presence of free water, and then regardless of whether the fuel was leaded or not. Three similar preventives depending on pro tective coating formation' had been found for this. Application of 100-octane Fuel On the suoject of suitability of 100-octane fuel for various aircraft, Mr. Banks said that short-range civil aircraft would do well to use. the fuel as the improved pay-load from 30 per cent, increase in power more than compensated for the 200 per cent, increase in price. He realised the airscrew limitations and the difficulty of turning the added power into thrust. >ut thought constant-speed airscrews would eliminate this problem. For a second class of aircraft engaged in long- rang, operation, 100-octane fuel would show great advantage in fuel economy for an engine of low boost pressure and high compression ratio, probably about 8: r average. The develop ment <>1 such engines would, however, present great difficulties, increased compression ratio was harder on fuel and engine ioB*ti ^ koos* pressures. Assuming no detonation or pre- g ition, a reduction in fuel consumption and specific and total ind • WaS assured- The lecturer regretted that no reliable which was ava'la[)le foi measuring the combustion pressures would be increased to about 1,000 lb. sq. in. for an 8 : 1 compression ratio. Mechanical difficulties .at present made -even an 8 : 1 compression ratio appear high, but the higher the ratio the greater was the fuel economy possible. One of the main difficulties with petrol engines was to reproduce in the air the good figures obtained pn the test beds, in the air. Inter- cooled superchargers giving low temperature boost and capable of being disconnected after take-off to avoid mechanical losses might soon be developed, for as the load which could be carried in flight was determined by that which could be taken off, the take-off h p. became the important factor governing operational economy. Where fuel consumption was not of primary importance, much depended on the supercharger. High boost temperatures seriously affected the anti-knock value of a fuel. Intercoolers appeared necessary for the modern engine, but there were complications. An intercooler would probably have to be placed on the pressure side of a supercharger, and between it and the carburetter, because a homogeneous mixture could not be maintained if the fuel had to pass through the intercooler, and there was risk of damage if an explosion occurred due to the firing of the charge in the induction svstem. This arrangement would eliminate carburetter icing, but would reduce supercharger efficiency because the fuel would not pass through the blower, and the beneficial effect of its latent heat would be lost. The pressure carburetter system was difficult to arrange for a radial engine. An immediate alternative pend ing the development of the intercooler was possibly the two- speed blower, since it reduced the power loss and boost tem perature at take-off. The ultimate goal was probably the exhaust-driven turbo-supercharger with intercooler and car buretter. The principal reasons for the absence of a production-type direct-injection engine were probably cost and complication. Three methods of injection could be used: (a) into the cylinder head, (b) into the induction pipe, (c) into the eve of the blower. It was often undesirable in the case of an engine developed for maximum output to increase the compression ratio, because, apart from the question of high gas-loading, the power might be reduced due to the'fact that the compression space above the piston was made smaller in volume, reducing the weight of the charge which could enter. Reduced Fuel Consumption The lecturer thought that an even greater reduction in specific fuel consumption than the 0.04 lb. per b.h.p. hr. men tioned by a leading engine designer in this country could be obtained by adopting 100 octane fuel instead of " 87." He also assumed it reasonable to suppose, in case of an engine designed for 100 octane fuel and having a com pression ratio approaching S : 1 and moderate or low boost, that a fuel consumption figure of 0.381b. b.h.p./hr. was im mediately possible. Whilst there Were certainly advantages in the particular lines of development adopted by American manufacturers, it was felt that the low-speed engine with large cylinders working at relatively high b.m.e.p. was more critical to fuel quality (anti-knock value) than the equivalent British types. Little had yet been done with automatic mixture control; car manufacturers were ahead of aero-engine people in its development and use. Mr. Banks thought that a cylinder of if-2 litres might be the best compromise in size. In discussing various cylinder arrangements the lecturer proposed a flat type with two banks of eight horizontally op posed cylinders, the crankcase consisting of two four-throw sections with the main airscrew reduction drive pinion and, possibly, the camshaft and accessory drive pinions bolted in between them. It was intended for mounting inside a wing or at the leading edge and with a capacity of 36-40 litres and running at 3,000-3,500 r.p.m. might deliver 1,600-1,800 h.p. Alcohol fuels were extremely critical to temperature and rather too unstable to be considered for normal purposes in modern high duty engines. The lecturer was of the opinion that liquid cooling might be used for large commercial engines rated at 1,500-2,000 h.p. and over. He thought, however, that there was a place for air- and liquid-cooled units, but was of the opinion that for powers in excess of about 1.500 h.p. it would be more satisfactory to employ liquid cooling. Subsequently Mr. Banks commented on miscellaneous sub jects, including "icing-up," sparking-plug testing, sleeve and poppet valves He suggested that with 100 octane fuel and a constant-soeed airscrew, a high-speed, heavily loaded machine would be capable of a maximum rate of climb of more than 4,000 ft./min.
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