FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1942
1942 - 2164.PDF
418 FLIGHT OCTOBER 15TH, ig^jj Fig. 2. Turbine rotor, showing profile of blading. cient engine were substantially heavier or occupied more space per b.h.p. than oui present units, it would inevitably be of little or no interest to aircraft designers. That is why the potentially more efficient diesel engine does not seriously rival the four-stroke petrol' engine at present, though con siderations of long-distance flight on civil and freight- carrying aircraft may yet bring it forward. The highly developed aircraft petrol engine'has been reduced in weight from 4.3 lb. per b.h.p. in 1920 to 1.4 lb. per b.h.p. in 1937. To-day, mainly due to supercharger improvements, improved metals, fuels and greater knowledge of combus tion, the larger units scale only a fraction over 1.0 lb. per b.h.p. Growth of Engine Sizes In time of war, development is likely to be tremendously accelerated. The pre-war "big" engine of 1,000 h.p. has already become a "medium-power" engine, and units of more than 2,000 h.p. are now not uncommon. These larger units have up to 24 cylinders, 96 valves and 2 crankshafts, and would seem to mark what is a practical limit to the size of a single reciprocating unit, pending further developments with airscrews. Reasonable estimates of requirements in the not too distant future suggest that engines of 3,000/3,500 h.p. will be demanded by aircraft designers. Multi-bank radial, "X," flat and H-type units in this class are, in fact, being seriously investigated by the engine-building con cerns in several countries. For an output of this order or over it is conceivable that it may be desirable to operate two or more standardised engines in conjunction. Any substantial increase of the diameter of individual cylinders would involve structural and cooling difficulties. Simi larly, an increase in the length of the piston stroke would raise inertia loads and, unless the rotational speed were reduced, would result in unfavourable piston speeds. Either would lead to an undesirable increase in the specific weight per h.p., thus rendering the engine less attractive for aircraft installation. For more powerful engines the tendency is to employ a larger number of cylinders, thus double-row radials are now affecting eighteen cylinders instead of fourteen, and multi-bank engines are on the drawing boards. This multiplicity of cylinders means that the total energy is applied to the driving shaft in a series of small impulses occurring in rapid succession. A six-cylinder engine has three power impulses per revolution or, in other terms, the firing interval is 120 deg. of crankshaft rotation. On a twenty-four cylinder unit of the same capacity, the LSION SYSTEMS firing interval would be 30 deg. of crankshaft rotation, and there would thus be twelve smaller power impulses per Jf revolution. When this feature is considered, it is obvious that the running gear (pistons, connecting rods and crank shaft) and also the general engine structure (cylinders, heads and crankcase) can be of relatively light construction. Conversely, imagine what the result would be if, instead of using twenty-four cylinders as on the Napier "Sabre," we built a six-cylinder engine to give the same output of about 2,400 h.p. Not only would the weight and space be increased, but the aircraft structure would probablv have to be redesigned and strengthened to accommodate it with safety. Rotary Motion the Aim It will be seen that the greater the number of cylinders employed the nearer is the approach to a continuous appli cation of energy. Of course, the number of cylinders cannot be increased indefinitely ; the cost, the complication and the number of component parts assume staggering pro portions. Logically, however, continuous instead of inter mittent power is the aim. When multiplying the number of cylinders ceases to offer practical advantage, an improved operating method must be sought. This aim is not novel, but whilst existing types of engines could fulfil all requirements it remained largely academic £ Fig. 3, Diagrammatic section of 2,000 h.p. unit for Swiss Federal Railway locomotive. A Axial-type rotary air compressor. B Air preheater. C Fuel injection nozzle. D Air admission ring. E Combustion chamber. F Mixing chamber. G Axial Flow combustion gas turbine. H Exhaust outlets to atmosphere. in character. To-day it has become a practical necessity and researches and experiments are being vigorously con ducted. The next step is from the reciprocating engine to the turbine which, due to the purely rotary motion and the - absence of rubbing contact of the working parts, would enable power to be generated continuously at high rota tional speeds. Only by such means, it would appear, can ' huge power outputs be obtained from relatively light and compact power units suitable for the needs of future aircraft. In this connection it is of more than passing interest to recall the evolution of steam power. For more than a century the reciprocating steam engine virtually per formed the work of the world. The demand for higher power output in smaller space for marine propulsion was the practical need which led Sir Charles Parsons to develop the steam turbine. In spite of deep-rooted prejudice and determined oppo sition from the advocates of the then existing reciprocating engines, the value of the turbine was proved and it was soon unrivalled on land and sea wherever huge power outputs were required. Allied Problems Two avenues of development confront aircraft research engineers, and curiously they are allied in principle though entirely different in their objects. I refer to the exhaust turbo-supercharger as applied to reciprocating engines and
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
