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Aviation History
1934
1934 - 0080.PDF
FLIGHT, JANUARY 25, 1934 zoo 190 180 ITP 1*0 1 130 ES 75 8O 85 Ocl-one Number 95 Fig. 2.—Relation between B.M.E.P. and Octane number in American and English engines. («) Raising the compression ratio ; (6) Supercharging ; (c) Increasing the crankshaft speed (r.p.m.).An increase in the compression ratio will improve the power output, lower the specific fuel consumption, andreduce the amount of heat to cooling medium, lowering also the temperature of the gases at exhaust point. A super-charger or blower, whether used for filling cylinders to maintain " ground level " power at altitude or for defi-nitely " boosting " an engine to give a higher specific power, is being used more generally.The limitations to high crankshaft speeds are the load- ings on the reciprocating parts and bearings, but these,although big difficulties to overcome, are not insuperable, and there is a general tendency to increase operationalspeeds of modem engines. These methods of increasing the specific power output impose a heavy duty upon thefuel, and may cause detonation and overheating, with their attendant troubles, such as pre-ignition, damaged pistonsand big ends and overheated exhaust valves. Fig. 3 gives a curve showing the effect of compression ratio on power and fuel consumption at constant speed. In this connection, one would add that where a super- charger is fitted, and if it is desired to increase the power output further by using a greater degree of supercharge in addition to raising the compression ratio, it would be well to make careful bench tests in order to obtain the most suitable, values for each when working in combination. The Choice of An ti-Knock Materials for Aviation Fuels There are at the moment two practical anti-knock materials available to the fuel suppliers of this country in order to make up suitable fuels. These are : — (t) Benzol ; (2) Ethyl fluid (tetraethyl lead).(1) Benzol.—Benzol has been used for some years, with success,as an anti-knock material. It is a fuel in itself of exceedingly highanti-knock value and does not usually detonate in the ordinaryKwise, but can give rise to severe pre-ignition under appro-priate conditions. Benzol is not generally used by itself but isblended with petrols. The amount added to any petrol naturally df -pends upon the anti-knock value of the basic petrol used and thefinal value required. Quite usual quantities of benzol in suchblends may vary between, say. 10 per cent, to 30 per cent, by Fig. volume, or more. From the economic aspect, and havingin view the development of civil and military aviation in the near future, it appears that adequate supplies ofbonzol will be difficult to obtain to meet all needs. In fact, this situation already exists, and in a number ofEuropean countries it is hardly possible, if not impossible, to obtain. In addition to the relatively large quantitywhich must be added to a petrol, to be effective, benzol is expensive even in those countries where its production issatisfactory. In the case of war, all the supplies of henzol would undoubtedly be commandeered for themanufacture of explosives. For peace-time military pur- poses, while cost is important, it is not necessarily theruling factor where engines must have the correct fuel, but even allowing for this, the performance of the modernmilitary aeroplane has been largely responsible for rele- gating benzol to a secondary position as an anti-knockmaterial in aviation fuels, due to the former's capability of operating at considerable altitudes. The principal con-stituent of benzol is benzene. Benzene has a freezing point of about +5 deg. C. Consequently, to avoid freezingtroubles in the fuel system, the amount of benzol which can be added to any petrol must be very much restrictedif aircraft are to operate with safety at altitudes which may vary from 15,000 to 30,000 ft., or, in cold climates.The satisfactory operation of engines in cold climates is also important for civil work. Again, one of the principalconditions of a civil aviation concern is to be able to obtain adequate supplies of uniform material universally,or at least at points along the particular routes operated by it. (2) Ethyl fluid (tetraethyl lead).— Ethyl fluid is not afuel, but may be called a chemical knock suppressor, Its basis is well known and is an organo metallic compoundknown as tetraethyl lead. It is completely soluble in petrol and will not separate out under any normal storageconditions. All Ethyl fluid is coloured and that for aviation use is blue. A definite standard of colour ismaintained for all blends of fuel containing Ethyl fluid. The fluid is supplied in drums to the oil companies con-cerned and, provided that these are kept airtight, it may be stored almost indefinitely. Its distribution to blendingpoints in any country is, therefore, simple. Due to its potency, the amount added to any petrol is almost minute.Naturally, the concentration will vary according to the basic anti-knock value of the particular petrol, and thefinal value required, which is also controlled by the degree of susceptibility of the petrol itself to tetraethyl lead. Some idea of the effectiveness of tetraethyl lead may behad when it is stated that the addition of the first cubic centimetre (1.0 ex.) per gallon (4,545 c.c.'s) to a petrolcan raise the anti-knock value of that petrol by anything from 5 to 15 Octane numbers, again, of course, dependingon the characteristics of the petrol. A further important point is that the addition of Ethyl fluid to any petrol doesnot affect the latter's characteristics. The specific gravity, distillation range, and, what is more important, freezingpoint, remain sensibly constant. ' In fact, the fluid has no effect until it reaches the combustionchamber of the engine, and the petrol may be regarded as a con-venient " carrier " by which it is introduced to and distributed inthe cylinders. Its ease of hand- ling, through standardised blend-ing plants, the convenience of transporting it and its effective-ness as an anti-knock agent, make Ethyl fluid almost ideal for thepurposes of military and civil aviation. Personally, one will goso far as to say that it is the only material in use at the present timewhich can insure the supply of fuels of uniformly high anti-knockvalue (Octane number) nationally and internationally. Tetraethyl lead, the anti-knockconstituent in the fluid, could not be used satisfactorily by itselfbecause the lead oxide, formed during the combustion process inthe engine cylinders, would de- posit at fairly high rate.Ethylene dibromide is included in the fluid as the result of extensive3.—Effect of Compression Ratio onM.E.P. and fuel consumption. 80
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