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Aviation History
1937
1937 - 0074.PDF
-28 FLIGHT. JANUARY 14, 1937. THE NEXT STEP Problem of the Modern High-duty Aero Engines and Their Fuels : Isooctane or Isopropyl Ether? : Air- v. Liquid-cooling TO tell the engine specialists of the latest fuel develop ments and the fuel specialists of the engine problems involved was the basic plan -of the extremely im portant paper which Mr. F. R. Banks, of the Ethyl Export Corporation, read before a joint meeting of the Royal Aeronautical Society, the Institution of Automobile Engineers, and the Institution of Petroleum Technologist? on Friday of last week. The first part of the paper dealt with fuels and the second with engine development. Referring to the big strides made in recent years, Mr. Banks said that Great Britain led the world so far as supercharging was concerned; the Americans taught us the importance of fuel quality and anti-knock value. These two developments had been responsible for great improvements to date, but he could not see, at the moment, anything which would allow us to make such big jumps in future, unless it was a fuel which did not detonate or give rise to pre-ignition. Even then he doubted whether our technique was advanced enough to let us take full advantage of such a fuel. At present the greater proportion of high-duty aero engines in Great Britain, America, and Europe were developed to employ fuels of 87 octane number containing tetraethyl lead. The maximum amount of tetraethyl lead allowed in such fuels for commercial use was 3.6 ml. per Imperial gallon. For mili tary requirements this concentration could be doubled; in special circumstances the concentration might be increased to 4.5 ml. per Imperial gallon for commercial fuels of 87 octane or higher, the average amount being in the region of 3.0 ml. Isooctane Recent developments in the U.S.A. had enabled isooctane to be produced on a scale which allowed it to be considered seriously as a constituent of aviation fuels. It was produced from the raw material isobutylene, which occurred in small amounts in refinery gases. This latter was converted into diisobutvlene, which, on hvdrogenation, yielded isooctane. The boiling point of this was constant at about 99.3 deg. C, which rendeied it unsuitable for use as a complete fuel. It was mostly employed as a blending agent with aviation gasoline, the fuel generally being completed to 100 octane number by the addition of tetra- ethyl lead. A typical 100 octane fuel of this ,^^ type consisted of gaso line having an octane number of about 73 with which was blended technical iso- octane in proportions varying between 40 and 50 per cent, by volume, depending on the octane number of the particular gasoline used and the limit set on lead concentration. The amount of tetraethyl lead was generally about 4.0 ml. per Imperial gallon. The lead response of isooctane was very good, although this did not show up very well by present knock-testing methods. The most recent development in the synthesis of fuels on a production basis was diisopropyl ether. The raw materia! from which this was produced was propylene, present in re finery gases. When blended with ordinary aviation gasolines isopropyl ether would produce fuel of 100 octane with a lead concentration not exceeding 3.6 ml. per Imperial gallon. Its production potentialities appeared far greater than those o; isooctane, but it had one important disadvantage; its calorific value was lower than that of isooctane, with consequent loss in fuel economy. It was, however,' more readily available than isooctane, particularly in European countries. It was not intended to displace isooctane, but to be complementary to it as a blending agent for fuels of high anti-knock value. A 50-50 blend of the two in standard knock reference fuel C-<i had a superior anti-knock value to isooctane alone. -Mr. Banks was not enthusiastic about the alcohols as con stituents oi aviation fuel, pointing out that they appeared to be very critical to operating conditions, and their employment was uneconomical. Anti-knock Tests A chapter of the paper was devoted to the correlation of laboratory knock-test results with those obtained in full-scale engines. The three knock-testing methods in use were ex plained, viz., the C.F.R. Motor Method, the C.F.R. Modified Motor Method (specified by the British Air Ministry), and the U.S. Army Air Corps test. The need for a satisfactory method of determining accurately the onset and degree of detonation was stressed, and the lecturer expressed the view that perhap- the cathode-ray indicator might provide the solution. Turning to the subject of engine development, Mr. Banks paid a tribute to the engine manufacturers for their enthusiasm. The pace of progress had been, however, so rapid that troubles which had been overcome in the early stages had recurred later due to the large increase in engine performance, until, it seemed, they would never be free from some major trouble foi long. That, however, was the price that had to be paid for progress. With regard to points of engine design, Mr. Banks stresse I the need for adequate cooling of the combustion chambf; which latter, he considered, need not be "fancy." The cylinder head should be designed with a view to providing ample valving, efficient valve cooling, and satisfactory posi tioning of sparking plugs, while in the case of liquid-cooled cylinder heads design ol coolant passages should be such that the drawings could be followed in practice in the foundrv Consideration of these points would not onlv give longer life
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