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Aviation History
1934
1934 - 1402.PDF
SUPPLEMENT TOFLIGHT U THE AIRCRAFT ENGINEER FEBRUARY 22, 1934 in the direction and general turbulence of the airflow past the engine caused by the ring may disturb air intake and carburetter conditions and lead to variations in mixture strength and/or to irregular gas distribution. Serious cooling difficulties attendant upon the fitting of the Townend Ring rarely arise, except in the rapidly- diminishing number of cases in which the cooling margin of the engine, even without low-drag cowling, is small. The designer of radial engines has realised the import- ance of modern low-drag cowlings and of providing his engines with cooling capacity which will be adequate when such cowlings are fitted, consequently such diffi- culties are steadily growing rarer. Where irregular temperature distribution occurs, and there is strong evidence that it occurs to some extent in all air-cooled engines, the cooling which has to be pro- vided is that which will keep the hottest cylinder down to permissible limits; and the analysis of the reasons for such irregular temperatures and methods for their cure should be of the utimost interest to the engine maker himself, since they are one method by which effective cooling can be appreciably increased. (To be concluded.) ETHYL BY P. R. BANKS, O.B.E., F.R.AE.S., M.I.A.E., M.INST.P.T., M.S.A.E. (Concluded from, p. 4.) General Notes on Engine Operation with Leaded Fuels THE internal appearance of an engine which has run on a fuel containing lead differs somewhat from that usually associated with the more ordinary fuels. The deposit from the use of the former fuel is harder in nature and perhaps more adherent than that of the latter. Its colouration is also different, being white to greyish white on the cooler parts of the combustion chamber and reddish brown on the hotter parts. This is due to the presence of lead bromide. There is, some- times a yellowish tinge to the deposit, which may be accounted for by some lead sulphate present in the deposit. Where a part, such as an exhaust valve, has been running unduly hot, the deposit is generally ex- ceedingly adherent to the valve head and has a dark " steel " grey appearance. The dye which is present in all leaded fuels is particularly useful for the relatively complicated fuel systems used in aviation engine instal- lations, since it shows up, almost immediately, any leakB which may be present. Some queries have arisen regarding the effect of leaded fuels on ths materials used for aircraft fuel tanks. No trouble has been experienced in the; oase of tanks manufactured from the usual aluminium alloys, but with regard to those particular alloys which contain a large percentage of magnesium, such as Elektron, there seems to be some doubt as to the advisability of employing them for fuel tanks at all. One's personal experience is that corrosion trouble is manifest with high magnesium alloys when water is present in the fuel, whether the latter contains lead or not. If it contains lead, then the corrosion attack appears to be somewhat accelerated. From this one deduces that the presence of water is really the decid- ing factor, but it is almost impossible to avoid a certain amount of water collecting in fuel tanks. It is sug- gested, however, that magnesium alloy tanks could be designed with provision for a sump of some material which does not suffer from this corrosion attack, such as pure aluminium, etc. The sump would be deep enough to prevent any water reaching the joint between dt and the tank, in order to avoid the possibility of electrolytic action. Engine Tests and the Influence of Increasing Concentrations of Lead When considering the duration of engine tests in order to ascertain the effect of leaded fuel upon the engine parts, one is of the opinion that no tests of less than 100 hours' duration are of value. In order to promote rapid engine development on leaded fuels, the knock rating of the finished fuel should be decided upon in the first place, after which a basic petrol chosen, having an initial anti-knock value which demands a fairly large amount of lead in order to attain the required final value. This will ensure that the engine is capable of giving satisfactory operation with any concentration met with in service, even although it may eventually be provided with a fuel, the basis of which only requires a very small amount of lead in order to reach the desired anti-knock value. There are many contentions regarding the influence of increasing lead concentrations on engine condition, and in general the consensus of opinion appears to be that an increase in lead concentration gives the engine parts concerned a harder time by increasing the rate of deposition of the products of combustion. One is not substantially at variance with this view, and has always maintained that tests should be carried out on the lines suggested in the previous paragraphs of this section. It is quite feasible to suppose that an increase in the amount of lead must generally show up in the form of greater rate of deposit build up. However, the follow- ing points are put forward as a matter of interest. Firstly, the American view, backed by six or eight years of intensive experiment and use of leaded fuels, is that increasing concentrations of lead tend to increase the rate of attack and deposit build up, which may lead to troubles previously dormant. Secondly, tests have been carried out by the Air Ministry, at the works of the aviation engine firms in this country, over the last two years. The tests, of 100 hours' duration, were made on representative types of engines in service, and the results did not completely bear out American experience. The interesting point about these tests is that valve failure, due to burning, was experienced in some cases and occurred in about 50 to 70 hours of running. Further tests of 100 hours' duration were then made after completely reconditioning the engines concerned, but a fuel having only 1 c.c. of lead per gallon was tried, where previously a " 4 c.c." concentration was employed. The same class of petrol was used as the basis of the fuels, and a similar knock rating to the " 4 c.c." fuel was obtained by the use of added aroma- tics. However, in a directly comparative test with this and the " 4 c.c." fuel, precisely the same degree of valve failure in practically similar periods was ex- perienced with both fuels. Therefore, from the experi- ence in this country and in Europe, one would say that increasing lead concentrations do not necessarily give rise to trouble or to the same ratio increase of deposits in the engine. With regard to the apparent variation between the results obtained here and in Europe, to those indicated by American experience, a satisfactory explanation might be that the modern American engine has been de- veloped over the same period as that of leaded fuels; consequently a certain amount of technique has been evolved to deal particularly with their use. A great deal more flying, with engines using such fuels, has also been done in America, while little or none has been done in this country, and practically all our leaded 174/
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