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Aviation History
1933
1933 - 1227.PDF
FLIGHT. DECEMBER 14. 1933 POSSIBLE FUTURE DEVELOPMENTS OF AIR-COOLED AERO ENGINES By A. H. R. Fedden,* M.B.E., F.R.Ae.S., M.I.A.E., M.I.M.E., M.S.A.E. IN reviewing the possible future development of air craft engines, I propose to confine myself to air-cooled engines for the immediate future—what we may expect during the next ten to fifteen years—as being the most profitable subject for discussion. I do not propose to attempt to consider power plants which are remotely conceivable for aircraft—such hardy annuals as petrol turbines, rocket propulsion, swash-plate engines, etc. Naturally, being connected with a firm producing air- cooled aero engines only, my activities are directed into the exploration of the air-cooled engine field, and in endeavouring to find as many openings as possible for the employment of direct air-cooled power plant for aircraft. I believe there is a great deal of important and interest ing work that will be accomplished during this period, but I suggest that it will be more along the lines of develop ment rather than any radical change, and I hope that you will not think I am too conservative in my opinions. I propose to confine most of my remarks to the four cycle engine, as I am of the opinion that this type of engine is most likely to predominate during the period considered. It may be fairly stated that the direct air-cooled engine holds an important position in aircraft in all countries of the world, and the object of this paper is to endeavour to suggest the lines on which this position can be main tained for the future. TREND OF AIR-COOLED AERO ENGINE DEVELOPMENT The design and development of an aero-engine power plant is nowadays such a comparatively long process, that the aero-engine designer has to lay his plans some years ahead, and, having fixed upon a certain type, it is usually impossible to make any radical change to the layout with out entirely destroying the design. The difficulties) of the aircraft designer are fully appreciated, but the engine maker sometimes looks with longing eyes upon the facility with which aircraft weights are apt to increase when necessity arises. To increase the performance of aircraft, I believe I am correct in stating that one of the easiest ways is to substi tute a larger engine, provided the weight, shape, etc., are suitable. Broadly speaking, this has been the main line of attack to date, and sizes of engines have been, and still are, creeping up. I am quite convinced that a halt must be called to this procedure, as it does not make for efficiency and is apt to lead to a vicious circle, entailing larger and heavier machines, increased drag, larger fuel tanks, etc. By this I do not necessarily mean that the air craft designer can continue to raise his performance curves by the use of less power, but rather that the engine maker must provide him with the same, or more, power from a considerably more efficient engine. At the present time, about 13 to 14 cruising horse-power per litre (61 cu. in.) is accepted as the limit to take from an aircraft engine of current power/weight ratio, if long life between overhauls may be expected. This seems to me, with all the development at the back of an aero engine, a low figure, and should be capable of being con siderably improved upon. I feel that the aircraft engine of the future must follow the trend of the motor-car engine in volume reduction, and we must aim at giving considerably more power from a given capacity, both for take-off and continuous cruising power. In 1920, IS rated horse-power per litre (61 cu. in.) was considered a reasonable output from an aviation petrol engine ; to-day the figure is about 22-26 rated horse power per litre, and I hope to see 40-50 rated horse-power per litre attained within the period under review. I do not visualise any startling reduction in weight, as changes will have to be made to accommodate additional stiffness and bearing loading capacity, but, with the de- * Abstract of paper read before the Royal Aeronautical Society on December 7, 1933. velopment of such a type of engine, many advantages will accrue, such as reduction in scale of the engine, increased overall efficiency, and reduced fuel consumption. Increase in Compression Ratio Raising the compression ratio is one of the most uselul means of obtaining a higher engine performance. Briefly, its effect is to increase the brake mean effective pressure, thermal efficiency, and maximum explosion pressure, and to decrease the residual exhaust gas temperature and fuel consumption. The improvement obtained is limited by detonation, which problem has already been attacked with a fair de gree of success by the fuel technologists. Effects of Increased Compression Ratio on Design The increased gas pressure can be offset, to some extent, by higher inertia loading, obtained by increasing engine speed. Stresses in articulated pins and gudgeon pins will be increased, and these parts will need attention. In certain quarters it is felt that pistons will have to be modiried to withstand the higher loads, but I think that if pistons are of sufficient section to provide adequate cooling, they will be strong enough to stand the increased loads due to increase in compression ratio. The key to the problem would seem to be, therefore, is it possible to provide adequate cooling to the piston without increasing the weight to dangerous limits? I would suggest that this is possible on the smaller bore cylinders, but, when it comes to the larger size pistons, the problem will undoubt edly be serious in order to prevent abnormal piston tem peratures with consequent ring sticking. The provision of satisfactory exhaust valves and seats is rendered easier by the reduction in exhaust gas tem peratures, due to the higher expansion ratio. Fuels From the aspect of detonation, as affected by compres sion ratio and boosting, fuel becomes one of the most im portant factors influencing advance in these directions, and has been an obstacle to progress in England during the past few years. It must be realised that fuel problems in England and Europe are of a somewhat different nature from those exist ing in America. We have no home product in England, with the exception of small experimental quantities, and all our fuels are imported. Our civil air lines, operating over manv different countries, have to use a wide range of fuels, as standardisation is nothing like so easy a problem as in one large continent, such as America. Our Air Force has also to operate in many different countries, and our imperial position is an important factor which seri ously affects the fuel question. Little imagination is needed to realise that an engine running continuously at any considerable throttle open ing, on a fuel which detonates, will encounter a whole series of troubles, which, if allowed to persist, may rapidly entail the total wreckage of the engine. Until quite recently no country, with the exception of America, has taken this problem seriously and standard ised a fuel of high octane number. America has stan dardised an 87 octane number fuel for military aircraft, for some time past, and, in April this year, issued a 92 octane specification. A number of different types of liquid hydrocarbon fuels, suitable for electric ignition engines, are available, the principal being as follows : — (a) The straight run (or normally distilled) fuel. (b) Cracked fuels, which are produced by a high tem perature process, which can be made to rearrange the hydrocarbon groups, giving higher anti-knock values than those obtainable from a straight-run spirit from the same crude. (c) Blended fuels, consisting of mixtures of straight-run spirits, or spirits to which benzol or other aroma tics have been added, to give a higher knock rating. 1261 D 2
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