From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical bulletin explaining the concept of the Octane Number and its importance for aero-engine fuels.
| Identifier | ExFiles\Box 27a\4\ Scan047 | |
| Date | 10th January 1933 guessed | |
| ture and under high pressure such as those pertaining to high-duty aero-engines, particularly of the air-cooled type A petrol treated with “lead” will under these conditions retain its anti-knock value to a high degree. Ethyl Fluid, in which tetra-ethyl lead is the principal constituent, is completely soluble in petrol and once mixed will not separate out again. It is very uniform in quality. The U.S. Army and Navy Air Services use “leaded” fuels, and so does the Royal Canadian Air Force. A considerable amount of experimental work has been and is being done in Europe with this class of fuel. Its use is recommended by a number of aircraft-manufacturing firms in America and two famous firms in France. The principal feature which occurs to us about the use of tetra-ethyl lead is the fact that it can ensure the supply of aviation fuels of uniformly high standard throughout the world and this is most important for the future development of aviation. It will assist aero-engine manufacturers by guaranteeing that any engines which they send abroad will be able to operate with fuels similar to those recommended by them and used for the engine tests at the works. THE OCTANE NUMBER The anti-knock value of any fuel is assessed or graded in terms of “Octane Number.” The “Octane Number” method of assessment is now generally accepted and herewith is a short explanation of it:— Petrol, or gasoline, are generic terms for the more volatile fuels, distilled from crude oil, for use in engines of the carburetter type. Petrols are known as hydro-carbon fuels,—that is they are composed of hydrogen and carbon in various proportions. In fact, a petrol consists of a number of differently arranged hydro-carbon groups, the predominating one usually characterising the petrol. Some of these groups have pro-knock and some anti-knock tendencies. Now, very briefly, the anti-knock value or Octane Number of a given type of petrol is found by running it in a knock-testing engine, which usually consists of a single-cylinder engine coupled to a synchronous motor, to keep its speed dead constant. The jacket surrounding the cylinder is kept at 100° C. if a “low-temperature” test is desired, or at 150° C. or 190° C. (by means of a liquid with a high boiling-point consisting of Ethylene Glycol and water or Ethylene Glycol alone), if “high-temperature” tests are contemplated. By a combination of speed, ignition-timing, throttle-opening, compression-ratio and jacket-temperature the engine is run under detonating conditions the whole time. Therefore the fuel to be tested will be detonating during the whole test. The degree of detonation is then measured in relation to a standard fuel known as a “reference fuel” of pre-determined anti-knock characteristics, which reference fuel also detonates in the engine. The “values” are ascertained in a comparative manner by an ingenious device known as the “bouncing pin.” A description of this is not possible or really necessary here as the demands of space will not permit. Suffice to say that it measures the differences between two fuels in an arbitrary but comparative manner. The importance of the reference fuel in such a test is obvious, and the search for suitable fuels for this purpose has been going on for some time. Two excellent constituents have been found, namely, Iso-Octane and Normal Heptane. These are pure hydro-carbon compounds and therefore do not consist of the various groups contained in a petrol. They have “constant” qualities which is a very important point when considering a standard reference fuel. Iso-Octane is a very high anti-knock, above that of ordinary petrols. Heptane has definite pro-knock qualities, worse than normal petrols. The boiling points of these two constituents are constant and also very close to one another. The Octane Number of a petrol is, numerically, the percentage by volume of Iso-Octane in a mixture of Octane/Heptane which matches the petrol tested in “detonation” value. 100% Iso-Octane is equivalent to an Octane Number of 100, 100% Heptane equals an Octane Number of 0. Therefore, if it takes 70% Octane in 30% Heptane to give the same reading as the petrol which is being tested in the knock-testing engine, that petrol would be said to have an Octane Number of 70. The custom also is to quote the jacket-temperature of the test engine, at which a particular Octane Number has been ascertained. This is very important, because most petrols show a drop in anti-knock value when operating in high temperature and under great pressure as is the custom in aero-engines, particularly the air-cooled type, as compared with, say, those in an average motor-car engine. Therefore, to rate aviation fuels under equivalent conditions, they are tested with jacket-temperatures on the test engine of about 150° C. or 190° C. For instance, a fuel which has an Octane Number of 80 at 100° C. may drop 4 units and have only an Octane Number of 76 at 150° C. This is particularly the case with benzol mixtures and petrols produced by the “cracking” process. “Straight” petrols containing tetra-ethyl lead only, generally show very little drop at high temperatures, in the region of 1 Octane Number. They have even been known to show a rise of one Octane Number or more, depending, of course, on the basic characteristics of the petrol. Octane and Heptane are exceedingly expensive to produce, so “sub-standard” reference fuels are used for routine testing. These consist of certain “cuts” of “straight” petrols which have known and fairly stable characteristics, and the Octane Numbers required are made up by the addition of benzene (a constituent of benzol) or tetra-ethyl lead. These “sub-standards” are carefully stored to avoid any changes in quality and they are periodically checked and tested against actual mixtures of Octane and Heptane. Some illustrations, of the Octane Numbers of the average petrols sold, may be of interest; A good No. 1 petrol on the British market is about 68 Octane at 100° C. Commercial grades usually vary from about 60-66 Octane at 100° C. The Octane Number of the fuel specified by the Air Ministry (D.T.D. 134) for the R.A.F. is between 75-76 at 100° C. The U.S. Army Air Corps specify an Octane value of 87 at 190° C. for their “Fighting Grade” fuel, which is used in all their high-duty and supercharged engines. This is a very great advance in fuel quality and is the highest Octane Number so far specified by anyone.* To show how the performance of an engine can be improved by the use of fuels which have good anti-knock values, we publish herewith a curve (Fig. 1), which gives the rise in B/M.E.P. possible with fuels of increasing Octane Numbers. [Chart Data] Vertical Axis: Brake Mean Effective Pressure, lb. per sq. in., ranging from 130 to 200. Horizontal Axis: FUEL ANTIKNOCK RATING Top Horizontal Scale: Iso-Octane, per cent, ranging from 60 to 100. Bottom Horizontal Scale: Heptane, per cent, ranging from 40 to 0. Figure Label: FIG. 1 The above diagram is from a paper published in the S.A.E. Journal for April, 1931, entitled “Increasing the Thrust Horse-power from Radial Air-cooled Engines,” by Phillip B. Taylor. In conclusion, we would like to remark that this article only touches the fringe of “this fuel business” and, with the vast amount of research and practical work which has been done on the subject during the past ten or twelve years by workers all over the World, we have been absolutely unable to give more than a “blink” at it. We hope, however, that the “blink” will be enough to enable those interested to open their eyes to the possibilities which are offered for improved engine, and therefore better machine performance by the intelligent choice of suitable fuels. [* This is particularly interesting because for years we have been told that American aero-engines will run on any old gasoline that is put into them. In view of this fact the prospects of American engines sold to some Powers on this side of the Atlantic do not seem any too promising.—ED.{J. L. Edwards}] | ||