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Aviation History
1934
1934 - 1403.PDF
FEBRUARY 22, 1934 16 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT fuel development has been restricted to relatively severe experimental running and type tests which, one sub- mits, are more critical than flight conditions. One is of the opinion that any troubles experienced in America with leaded fuels of high concentration are not so much due to this feature, but rather to the increased " power per litre " of cylinder capacity at which American engines are now running under normal cruising con- ditions in flight. Knock Testing and Assessing Fuels in Relation to Engine Performance The question of testing fuels for anti-knock value is, admittedly, a subject in itself and hardly comes within the scope of this paper, but it has such direct bearing upon the successful development and satisfactory opera- tion of aviation engines that perhaps little excuse is needed to mention it. The whole essence of knock-testing technique is the ability to correlate the results obtained on the fuel- testing unit, with the performance of the fuels in the engine and to be able to assess the knock ratings of the various fuels in their order of merit. It is exeeedingly difficult to arrange a complete set of conditions for the fuel-testing unit which will imitate, accurately, those met with in the engine. Some time ago, the Institution of Petroleum Technologists appointed a sub-committee to formulate a suitable programme in order that experi- ments could be carried out and the data obtained there- from used to enable a satisfactory technique to be evolved for the correlation of laboratory knock test results with actual engine performance. Tribute should be paid to the I.P.T., which is the first body to formu- late a method of knock testing and correlating aviation fuels, to be accepted nationally. This should, however, only be regarded as a preliminary step. These test results are very clearly and completely described by Mr. Pye, who was chairman of the sub-committee, in a paper read before the World Petroleum Congress last year. The running tests were carried out at the R.A.E., Farnborough, on air- and water-cooled units, and at the engine works of the Bristol Aeroplane Co. Single- cylinder units of representative service engines were used, and in no case was a complete engine employed. Due, according to the report, to the " extreme diffi- culty of accurately detecting the onset of detonation in a complete aero engine and to the large quantity of expensive sub-standard fuel which would be required." One contends that, outside the expense, and particu- larly in the case of the air-cooled engine, the question of audible detonation is not necessarily important, but that the temperature effect on the cylinder head, due to the detonative characteristics of the particular fuel used, is more a measure of that fuel's ability to operate satisfactorily in the engine. With these large engines of comparatively high specific power output, a fuel can cause a dangerous rise in the operating temperature of the cylinder head, which on further increase will result in pre-ignition rather than detonation. This is markedly so when the non- knocking fuels are used. This is largely the reason why all the recent fuel tests formulated by the U.S. Army Air Corps specify that the sample tested shall not show a higher reading of the temperature plug than the refer- ence fuel; rather than taking average bouncing pin readings. On© would say that one or two fuel tests carried out on a complete engine, particularly of the air-cooled type, heavily thermocoupled at suitable points, would have yielded more valuable information than the single- cylinder tests. A further important point is that the tests at Farnborough were run at varying speeds, whereas general experience would indicate that constant speed is necessary when matching fuels. ..:--'.X-...v...v^.;:- ••. . " 174 The report also gives an explanation for the test methods finally adopted as a result of the work done, i.e., C.F.R. Motor Method, modified to use a mixture temperature of 260 deg. F., instead of 300 deg. F., and why the method is less severe than that employed for correlating automobile fuels (Motor Method). The explanation offered is: " that when a read vehicle engine in pulling at low speed on full throttle the con- ditions ara not only severe by reason of the low speed, but that in many types there is also provision for a large amount of mixture heating which may vary and exceed even the heating provided by a supercharger." A road vehicle engine is certainly severe on the fuel under the conditions described, but it must be remem- bered that the B.M.E.P. and moan temperatures, when compared with those of an aviation engine, are not so high, and also, the cylinders are not so large with, generally, a greater ratio of surface to volume than the aviation engine. Consequently, audible knock may be severe for the comparatively short time that the vehicle is running under these conditions, but due to the inherent design of the automobile engine it will take a considerable time before detonation becomes great enough to build up excessive heat and/or bring about actual damage, although, admittedly, the performance of the engine, and consequently the vehicle, is impaired. It is not easy to see how these conditions can be more severe than, say, those of an air-cooled engine having large cylinders by comparison and running at full rated load in an aircraft under climbing conditions, where the rate of air flow over the cylinders is at a minimum and as altitude is increased, with a corresponding decrease in air density, the difficulty of dissipating heat from the cylinders is greater. Actually, the operating speed of an automobile engine, under which detonation usually occurs, is not always so low as might at first be supposed, when compared with the normal speed of an aviation engine, although it may be lower in proportion to the maximum speed in the former case. In fact, the C.F.R. road tests showed that maximum knock occurred at road speeds varying from about 15 to 40 in.p.h. corresponding to crankshaft speeds of approximately 900 to 2,500 r.p.m. respectively. The contentions put forward here are not in any way intended as a criticism of the accuracy of the I.P.T. investigation, since the results of the tests have proved their own accuracy and have shown that it is possible to obtain good correlation with a given set of engine con- ditions. Criticism might be made that the engine conditions specified did not accurately represent those met with in actual service, and in addition only fuels of compara- tively moderate knock ratings were tested, whereas, for future engine development, the Air Ministry has already brought out a fuel specification, D.T.D. 230, in which an Octane value of 87 is called for when tested under the modified Motor Method and, also, the use of lead is allowed. None of the fuels in the I.P.T. tests contained lead. This fuel will be used for new types and these engines will undoubtedly produce higher specific power outputs than the previous models, upon which the I.P.T. investigation was made, and will probably give the fuel a harder time in comparison. It remains to be seen therefore, whether the correla- tion as carried out, applies in this particular case. Some correlation tests on complete engines are already in pro- gress in America, and it will be interesting to study the results and learn the conclusions arrived at, when they are published. The latest fuel specification (No. Y-3557-G) evolved by the U.S. Army Air Corps, calls for a nominal Octane value of 92. A. C.F.R. engine is used as the basis, but is considerably modified to conform to Air Corps require- ments. The temperature rise method of assessing the anti-detonation value is retained. The engine speed of
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