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Aviation History
1933
1933 - 1228.PDF
FLIGHT, DECEMBER 14, 1933 Fig. 1. Bristol "Pegasus " Engine—Effect of Fuel Octane Values on Engine Performance DTD. Specifica tion 134 224 230 Minimum octane value. Determined according to A.M. spec. 73 // 87 — Determined according to U.S.A. Army spec. Y.3557G 69 Lead content for representa tive fuel mis. of T.E.L. per imp. gall. Nil 73 Nil 85 4 92 6 Relative power output At normal r.p.m. B.h.p. 525 560 665 735 Lbs. B.h.p. 1-95 1 • 84 1 -54 1 -42 At maximum r.p.m. B.h.p. Lbs./B.h.p. 570 1 -8 610 1-68 715 1-43 790 1•32 Fuel consumption i Lbs./b.h.p.; hr. at normal power 0-55 0-53 0-49 0-47 i : Galls./hr. cruising at 400 b.h.p. 27-5 26-5 24-5 23-5 N.B—Weights of these engines suitably adjusted for increased stresses. (d) Hydrogenated fuels, in which the original hydro carbons have been converted by a hydrogen treat ment into compounds of higher knock rating. (e) Alcohol fuels, which are of a very high knock rating, but the use of which is usually limited to racing, or other special events, where cost and high con sumption, due to low calorific value of the fuel, are not factors of prime importance. All these fuels are available in varying degrees in dif ferent parts of the world, although the hydrogenation pro cess is as yet only in an experimental stage. The characteristics of a straight-run fuel depend greatly on its origin, and thus the quality and type of fuel found in any part of the world will obviously depend on a number of factors, such as the type of crude base most readily available, transport facilities, magnitude of sales, etc. The British Air Ministry have, in the past, standardised an aircraft fuel with octane number of 73, while 80 octane number has been available for military aircraft engines in certain European countries during the last year, and 70-73 octane number the average employed on civil air lines. Fuel of 77 octane number has been standardised by the Air Ministry this autumn, and it is proposed to introduce a fuel of 87 octane number in 1934. The necessary increase in octane value can be obtained, with a blended spirit, by the addition of a considerable percentage of aroma tics, but the chief disadvantage of this com bination is that its knock rating is seriously impaired by elevated temperatures. For all high- efficiency engines, and, in par ticular, air-cooled engines, this is a serious failing, and, un doubtedly, the most generally satisfactory combination is a straight run fuel of high naphthene content, and good lead susceptibility, with the addition of a small percentage of tetra-ethyl lead. Such a fuel has other im portant advantages over a petrol- benzol mixture, as trouble can be expected with benzol mixture at high altitude, or in cold climates, owing to the high freezing point of benzol. More over, supplies of benzol would be unobtainable in the event of hostilities, as it would be re quired for the manufacture of explosives. In this connection, I should like to stress the necessity of putting forward, for international agreement, the method of determination of octane numbers, as considerable confusion exists at the moment, and different countries have adopted varying types of test engines and conditions in the standardisation of fuels. With suitable engine modifications, the difference in B.M.E.P. obtained between an 87 and 73 octane fuel is of the order of 20 lb., and the increase of power obtainable from such an engine as the Bristol " Pegasus," using fuels of varying octane numbers, is shown in the tabulation in Fig. 1, together with the specific weight reduction and the comparative fuel consumption based on a cruising power of 400 brake horse-power. I hope that in the above very elementary review of the possible hydrocarbon fuels available for electric ignition internal-combustion engines, I have emphasised sufficiently the great value of the lead taken by America in introducing a fuel of high octane value. Such a fuel is of paramount importance, both for civil and military engines, and the necessity of solving the problem of its supply, on a commercial basis, cannot be too strongly emphasised. I do not consider that I am stating the matter too highly when I say that there is no question -1-070 ^04* m C 10-40 j?0^ •V 09Q X<K~0 £ ?&« g £ eoo r» i "^ ! •£»«!. ! 3>fe '.. -^ i X i ' • i > — K CONS t *^. Sfe sS; ~ ^J ^ >^, •**. -^C.- :i; loo ~~ •*=: !•.= ^ -- -— ,. fea -** ^ -> ~-> *». — -^ —' --. ..„ BHP ! -4-:==H i .~^_ +*-i E*r;s - --— > * WO &64 V> PEGASUS IIM E * iOO i—, — -- „ -^ p J4GINE • •— -- iV> ?-0 t<K — '?« ... pc « 5 a -_ O R Q H 5 4*5 N" 15101 TANT SPEED POWER-CONSUMPTTOt 5 oo PM PW "fftt P* PM »40 t CURVES WEAKE^I MIXTURE FOR MAXIMUM POWt MIXTURE FOR iVfe ytDROP IN RPM MIXTURE FOR S*A DROP IN RPi* fANDWO T»T CONpmONS-AtO KPPKl i r>*«:? X s a. »45* 040i 2 i jD c | VED • B * t 1 o T 990 i 5^ 5*o i& DO y W w» J m A a tr^GWE R PM. eooo ewo A I f\ IH // ft / K *,'• en // / JO eeoo ft // be >o z\ £3 DC oo 7IO k40 a 0 990 MO »D MO no ua %0 vzc ItO 990 ENGINE R.PM. PEGASUS II-M3 PI5KH 50 HRS WEAK NOCTURE TEST 6uyi*P». dooBHP a nooKFneoJornhhfffft O^m^ cowomoMti or rm FULL THROTTLE rOWTW CUfrVE Kf ^OOO FT •'•• • • qjRVCS W COMMENCEMENT Of TEST ——— CURVES WtXlNCl^lON OF TEST STAMOfcROTTST CONOTiOMS A.I D fcPPWCWTO Fig. 2. 1262
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