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Aviation History
1917
1917 - 0514.PDF
MAY 24. 1917- AIRCRAFT AND MOTOR CAR ENGINE DESIGN. CONTRASTED FROM THE STANDPOINT OF A DESIGNER AND MANUFACTURER OF BOTH TYPES. By LOUIS COATALEN. Concluded from page 488.) The Problem of Weight. WITH regard to the question of weight, the purpose for which the particular aircraft is required is of prime importance. Obviously, in the case of the engine in a machine designed for short flights only, the consumption of fuel and of lubricant is of less importance than the weight of the engine itself, whereas in the case of the heavier sorts of aircraft with which flights of long duration are obtained and for which great power per engine is needed, the consumption assumes much more importance than the actual weight of the engine. In these latter cases efficiency as regards the weight of the power unit has to be arrived at by taking the weight of the engine complete with the amount of fuel and oil that would be consumed in the course of a flight of, say, five or six hours' duration. Thus for short flights the rotary type of engine generally and the air-cooled varieties are apt to show up to advantage, though in them consumption may be compara- tively high, because this is offset by the relative lightness of their starting weight. From several papers that have been read recently with reference to aircraft engines it is evident that, speaking broadly, as regards weight per horse-power, progress in the design of the ordinary water-cooled type is very marked. To the author's own knowledge, in the brief period of two years there has been obtained with this type a reduction in weight from 4.3 lbs. per horse-power to 2.6 lbs. per horse- power. It follows that in designing aircraft engines a variety of points have to be considered with extreme care concerning which the builder of an engine for ordinary car service is not forced to take much trouble. This difference is rendered necessary, firstly, by reason of the amount of material em- ployed, and secondly, on account of the comparatively light weight of the aircraft engine complete. Valve Design : The Intermediate Stage. The design of the engine head, the cylinders, the valves, and the valve gear is one of the cardinal features of successful aircraft engine production. Car engine design allows of the employment of the L-shaped head, or, in some cases, even of the T-shaped type, though the latter is not used to any great extent to-day for automobile vehicle practice. Undoubtedly the L-shaped head has given excellent results in aircraft engine practice in the past, but the author prefers to consider that such examples really represent an intermediate stage in evolution, and that they stand rather for modified or adapted car engine design than for aircraft engine design proper. In point of fact, high efficiency is got with this type of head only by the use of a special design of valve cap that makes provision for the maximum surface of the engine head being served with water by the cooling system. In other words, you employ a form of duplicated valve cap, the removal of the upper and outer member of which reveals a space for water to circulate beneath it when the engine is working. At the bottom of that water is the valve cap proper. Further, to get the best results, it is needful to machine as much of the surface of this transition type of engine as possible. The Number of Valves per Cylinder. For standard car work one exhaust and one inlet valve per cylinder have sufficed for general practice to date, whereas present-day demands on aircraft engine designers are so great that any attempt to attain the requisite degree of efficiency by further exploiting such a scheme of design would lead in- evitably to failure. The necessity for running aircraft engines for long spells at either the maximum or a very high output without impairing the efficiency of the machine by distortion or pitting of the valves, which assuredly would occur with the ordinary design employed for car service, and so forth, has compelled the devotion of much thought and a wealth of experiment to the problem, as a result of which it appears to be accepted as established that the multiple valve system is a necessity. In this connection, to achieve maximum output, the author favours two exhaust valves per cylinder and two inlets. The horse-power obtained by engines designed on this principle and now standardised would appear to justify this conclusion. For example, a six-cylinder engine embodying this feature of valve design and having a cylinder bore of 81J mm. with a piston travel of 156 mm., develops 153 h.p. at 3,000 crankshaft revolutions a minute. That is equivalent to a mean effective pressure of 134 lbs. per square inch. At 2,000 crankshaft revolutions a minute it represents a duty of 21 h.p. per litre capacity. Among the advantages, of the, four valves per cylinder scheme are that a good shape of engine head is obtainable with it as well as the best sparking plug position, because that is in the centre of the head in a vertical position. The inclina- tion of the valves necessary for putting them into place allows of ample water-jacket space being provided round each valve and at the base of the sparking plug. Judged by achievement to date, any other combination of valves per cylinder will not give quite the same degree of efficiency. Take such variants as three valves per cylinder on the principle of two exhaust and one inlet valve. While giving perhaps a better-shaped, because circular, head that can be machined practically all over, this combination prac- tically precludes any other sparking plug position than in the side of the barrel. For obvious reasons, such an arrangement has many disadvantages. Nevertheless, three valves per cylinder scheme gives quite notably good results when exploited in certain ways and for certain special purposes. This may be judged from the fact that the diameter of the inlet valve can be made large enough to-day to give a very high h.p. per litre capacity at 2,000 crankshaft revolutions a minute without involving serious trouble, such as would rise from heat effects, and so forth. The use of more than four valves per cylinder is undesirable. It seems hardly possible to place them efficiently, leaving an even jacket all round each valve, without the employment of very complicated gear. We have an example of this in the Maybach engine, which has three exhaust and two inlet valves per cylinder. In this scheme little water space is provided between the valve seats, while the sparking plug is, besides-, set horizontally in the side of the cylinder barrel. Undoubtedly in any type of engine it is a gain when the surface of cylinder head can be machined. This is not possible, however, with the four valves per cylinder design. Hence in that case a compromise is achieved by making the surface as small as possible and by finishing it as much as possible by hand with files, scrapers, and so on. The Matter of Material. In regard to the material of which the cylinders are fashioned, a tribute is due to the metallurgists and manu- facturers on account of the great improvement in the cast iron available for cylinders of recent years, the very high tensile and good ductility qualities of it being particularly notable. Of course, the advent of aluminium alloy as a material for cylinder castings marks an important stage in the advance of aircraft engine design and production, the rate of progress obtainable by the designer being determined primarily and always by the materials that are available to him. A series of experiments carried out by the author over a period of more than 12 months has proved the superiority of aluminium alloy for this work. It has established the fact that it can give completely satisfactory results when the precisely right alloy is handled with knowledge and is employed on tested designs. Thus a 12-cylinder aircraft engine of 92 mm. bore and 135 mm. piston travel has run at full power for 100 hours without an involuntary stop or untoward incident, otherwise with complete satisfaction. With that sort of confidence Which is founded on reason, as the result of cumulative experience on the one hand and further research on the other, particularly with a view to rendering these special alloys capable of being cast easily in the factory, we may look for both speedy and consistent improvement in those varieties that will be available for this highly specialised Work. Though We are merely on the threshold of realising the possibilities of aluminium alloys for cylinder castings, it cannot be doubted that within a brief period they will be recognised as the standard materials for this Work, cast iron being discarded in favour of them. Progress in Castings and Alloys. The importance of the matter is realised by the aluminium founders of the country. It is encouraging to note the relatively great amount of enterprise being displayed by them in this connection. At the present time the more important aspects of the case embrace the effect of heat on the strength of the given alloy as well as the coefficient of expansion of it.
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