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Aviation History
1934
1934 - 0081.PDF
FLIGHT, JANUARY 25, 1984 j a: LL u E u c o 82 81 BO 79 78 77 75 74 73 72 •*l 70 69 £8 €7 66 0 o» / 1 1 , ,7 1 y T/ o 0 2 / /i r t.Bm u) - .Vol of 9( 0 3 / §pi, §pi> s Benv 0 4 y/ '' il- A. lodir It- B lodir 1 a s !N°77 cNM 0 6 D CC5lEi..otrImpGo|lon o 5J 3TO »4O | S4O S5O 52O 29O 2 To 26O &5O 240 230 \ \ \ THE i"C?ra fxcft'(OCO IN TH 1 UPKt CfMVf AVIATION OAMUni -MI«JC» iHTDl A MID-CDHTfffCNT P *, tf \ -ip K> : 3 SO J \V * ^^"3 * ! 0 4O | 9 a o c ft Fig. 4.—Curve showing the response of T.E.L. and Benzol Fig. 5.—Reduction of cylinder-head temperature from the in fuels of differing characteristics. use of anti-detonants. investigation. This converts the lead oxide to leadbromide which is volatile at the working temperatures prevailing in the cylinder and is evacuated with theexhaust gases in the normal manner. The amount of tetraethyl lead and not fluid is referred to when speakingof concentrations in petrol, since the lead is the active anti-knock agent. The amounts used are so exceedinglysmall that confusion is avoided and greater accuracy is ensured when this procedure is adopted. Subsequent refer-ences in this paper will apply to the concentration of tetra- ethyl lead only, and for the sake of brevity this willgenerally be designated as " lead." The maximum allowable concentration of lead in anyfuel, for commercial purposes, is 3.6 ex. per Imperial gallon. For governmental and/or military purposes theconcentration may be increased to 7.0 c.c. per Imperial gallon. Tetraethyl lead is poisonous and it can be absorbedinto the human system through the skin or by the mouth or by inhalation, but the precautions taken in its manu-facture and blending with petrol in special plants, obviate any risks to the operating personnel. In its diluted statein petrol there is no danger whatever, and fuels containing lead can be handled in the same manner as ordinarypetrols. The Effect of Tetraethyl Lead There are many who think that the lead, in stopping orpreventing detonation, " slows up " the combustion pro- cess considerably, but from the results of many tests thisdoes not appear to be the case. This is not to say that " slowing up " does not occur, but it does not appear tobe of great magnitude, say, in comparison with benzol. With high-benzol mixtures the temperature of the cylinderhead tends to rise, and this feature has been particularly noticeable with air-cooled engines under certain conditions.The exhaust temperature is generally higher under these sonditions and often rises if the amount of benzol in theparticular blend is further increased. The use of fuels containing lead does not in generalraise the cylinder head or exhaust temperatures, rather the reverse in some cases, but for all practical purposes theyremain substantially the same. This naturally assumes that there is no detonation present in either case. There isone peculiarity regarding the effect of lead upon the anti- knock value of a petrol ; it is, that a progressive increasein the lead concentration does not give a corresponding increase in anti-knock or Octane value. Rather, the in-crease in Octane value gets progressively less with increas- ing lead concentrations. In the case of benzol, increasingamounts blended with a petrol usually show progressive improvement in the anti-knock value of the resultant fuel.Fig. 4 shows these features up quite clearly and also shows up the difference in susceptibility of two petrols of unlike characteristics, to additions of anti-knock material. One petrol is highly cracked and the other has a naphthenic base. There is, however, one very important point scored by afuel containing lead over a benzol blend. Suppose two such fuels to have similar knock ratings when tested underthe following conditions: —A Series 30 knock-testing unit running at 600 r.p.m. with a jacket temperature 100 deg.C. Now suppose the fuels to be re-tested on the same engine but with the speed and jacket temperature in-creased to 900 r.p.m. and 190 deg. C. respectively ; it will be found that the knock rating of the fuel containing leadmay drop by, say, 3 or 4 Octane numbers (assuming a direct comparison with a primary reference standard ofOctane/Heptane), but the benzol blend will probably have dropped eight or ten numbers. In fact, benzol hasbeen supposed by many to have an anti-knock value much greater than pure Octane itself, whereas its actual andeffective value, when blended with petrols and tested under conditions similar to those employed for checking aviationfuels, is about the same or even less than that of Octane. That is to say, its blending value, i.e., its value whenamounts up to about 50 per cent, of benzol are blended with petrols, is lower than that determined arithmeticallyby taking its estimated anti-knock value into account and j s J i c £ c UJ u U 4-S 40 e« 1-5 \ X, \ •DO IX) , 1ZO I Cscflng Jaekcf' T«mp« "-— ^^ to iso iso m> Fig. 6.—Variation of Bcnzol-T.E.L. ratio in aviation fuels due to change of engine temperature. ... 81
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