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).
Article discussing the importance of fuel quality, detonation, and anti-knock properties for aero-engines.
| Identifier | ExFiles\Box 27a\4\ Scan046 | |
| Date | 14th December 1932 | |
| Reprinted from THE AEROPLANE, December 14, 1932. THIS FUEL BUSINESS [By B. M.{Mr Moon / Mr Moore} E.{Mr Elliott - Chief Engineer} P.] During the last two or three years there has been considerable activity on the technical side in the study of fuels for aero-engines. We have noticed that the question of fuel quality is being more and more closely appreciated by those who manufacture and operate engines and aircraft. Even now there are many who ought to; but do not know the first thing about fuels. The object of this article is to give some idea of the importance of fuel as it affects the economics of engine and therefore aircraft operation. Its scope is limited by limitations of space and therefore, such matter as “detonation” and “anti-knock value” must be treated in the shortest possible manner. FUELS AND DETONATION When everything is boiled down, a chief limiting factor which prevents a further increase of power being got from an engine of given cylinder-capacity is detonation. Detonation is entirely a characteristic of the fuel. An engine with a badly-designed “combustion end” will aggravate a fuel which has quite a good “anti-knock value” into detonating, but all things being equal it is the fuel which limits engine performance. The fact is well known that petrols, or gasolines, as they are called across the Atlantic, vary in detonation or anti-knock characteristics according to the class of crude (crude oil) from which they are produced. The crudes are taken from many points of the earth's surface. To name a few:—U.S.A., Venezuela, Trinidad, Persia, Russia, Borneo, etc. Aero-engine development has however outstripped even those fuels which, in their normal state, have relatively high anti-knock value. The anti-knock value of a petrol, by the way, is its ability or otherwise to resist detonation. A brief explanation here of the “mechanics” of detonation is perhaps desirable. When the air-and-fuel charge in an engine is fired by the sparking plug, the flame which is started at the plug-points does not “flash” the mixture instantaneously. Quite a measurable time is taken to spread to the whole charge in the combustion-chamber. That portion of the charge which has already been burned is therefore behind the advancing flame front, expands rapidly and compresses the unburned portion of the charge ahead of the flame front, so that owing to excessive pressure and temperature it ignites spontaneously before the flame proper reaches it. The rise of pressure in the cylinder-head caused by detonation is not evenly distributed over the surfaces of the piston-head and combustion-chamber, and is so rapid that it takes place over only about one degree of crank-angle movement. The energy is dissipated so rapidly that no advantage can be taken of it and this portion of the charge is literally wasted. The metallic knock, or “pink,” which is heard when an engine is running under detonating conditions is due to a “wave” effect set up by the “explosion” of the detonating portion of the charge which strikes against the walls of the combustion chamber. Detonation is sometimes confused with pre-ignition. Pre-ignition is, as its name implies, ignition of the charge before it can be fired by the sparking plug. It is caused by some part such as the exhaust valve or sparking-plug point becoming overheated. Continued detonation can however bring on pre-ignition by causing overheating of these parts. Not much imagination is needed to see that if an engine runs continuously on a fuel which detonates it cannot give of its best. Also other troubles are likely to occur, such as overheated and cracked pistons, cracked white metal in big-end bearings, etc., and sometimes cause their failure. These troubles have often been blamed on the lubrication, and often, after extensive and intensive investigation of the design of the affected parts and their immediate surroundings, together with changes of lubricant, all without success, a cure has been achieved by a change to a fuel of superior anti-knock value. Engines are being developed each year which for a given cylinder-capacity, give more and yet more power. This increase in specific power has been mainly brought about by the following combination:—Raising the compression-ratio, supercharging, and higher operational speeds (r.p.m.). The first two items definitely inflict more strenuous conditions on the fuel. We have shown that detonation is a function of the combined effect of temperature and pressure. Supercharging, a misnomer really, is not generally used to “boost” engine performance at “ground level” but to restore “ground level” powers (considering the engine at “ground level” to be working with little or no positive pressure in the inlet pipe), at height. With the exception of the United States, which is definitely “fuel-conscious,” there is no standard fuel in this, or any other, country that will allow a fully-supercharged engine to have its throttle opened up “past the gate” under about 3,000 feet, without detonation. Although an increase in r.p.m. may reduce the time element, because it shortens up the period in which peak-pressures can occur, the increase is often not enough to counterbalance the effect of the other items. In addition to which, the exhaust-valve, whose existence depends upon getting rid of the bulk of its heat through its seating in the cylinder-head, has less time at rest. The reader will see that we come back to the basic limitation of modern engine-performance, i.e., the fuel and its anti-knock value. Our fast military machines, excellent though they are, could with a satisfactory fuel which would allow them to use “full throttle” under all conditions, reach their operating height in still less time than they do now. They could also use, in times of emergency, aerodromes of limited size for full-load take-offs. Finally, the all-round performance of the machines for a given engine size could be improved. What can be offered to the commercial aircraft operators if they are willing to use fuels of improved knock-rating (and they will certainly use them on the advice of the engine manufacturers)? Engines of low frontal-area and weight for a given power. Good fuel-economy owing to the high specific power-output given by the use of high compression and supercharging without detonation. The pay-loads will, therefore, be a greater proportion of the gross weights and the cruising speeds will also be raised. KNOCK PREVENTION In the foregoing it will be noticed that “anti-knock value” and no other quality of the fuel is referred to. Naturally such points as distillation range (or volatility), vapour pressure, freedom from gum and sulphur are all important, but these can be met without much trouble by modern refinery methods. How can the anti-knock value of a petrol be improved? There are several methods, including the treatment of some “cut” of the crude oil itself from which the petrol is normally distilled. This is known as “cracking,” which is basically a high-temperature distillation process which re-arranges the hydro-carbon groups and generally produces petrols of good anti-knock quality. In this instance, however, there are certain disadvantages which although not insuperable are not at the moment worth considering because the anti-knock values of “cracked petrols” although good, are still not high enough for the purposes mentioned and “straight run” or (normally distilled) petrols are available in reasonably large quantities for aviation purposes. Hydrogenation, as its name implies, is a hydrogen treatment and produces petrols which have high anti-knock values. At the moment, however, it is not yet an economical proposition for universal distribution. The normal method of improving the anti-knock value of a petrol has been that of adding benzol. Benzol is a product of coal-tar distillation and is a very high anti-detonant indeed. It has been used for some time, with success, blended with petrols. Certain of its peculiarities are rather a stumbling block to its use for modern aviation purposes. In the first place it has a relatively high freezing point, in fact, the benzene, which is the chief constituent, freezes at about +5°C. This is a definite drawback to its use at great heights or in very cold climates, and only a very limited amount can be added to a petrol if freezing is to be avoided. Benzol is not obtainable in every country and in some European countries where it is available it is often an impure product with a high proportion of sulphur, etc., etc. Furthermore, in times of national emergency, practically all the supplies of benzol would be commandeered for the manufacture of explosives as it is the basis of such materials as T.N.T. In recent years another substance has been “discovered” which is the most potent known for the prevention of detonation when added to a petrol. This is tetra-ethyl lead. In its final form it is more generally known as “Ethyl Fluid.” Although it is not a fuel in itself, it raises the anti-knock value to a high degree when it is added to a petrol in most minute quantities. The concentrations of tetra-ethyl lead in petrol may vary from 0.5 cubic centimetre to 7 cubic centimetres per Imperial gallon (4,545 ccs.) according to the final anti-knock value required and depending upon the basic characteristics of the petrol used. But the fact will be appreciated that even the maximum amount, 7 ccs. per gallon, is exceedingly small. And on an average a little more than half this amount added to a reasonably good petrol will enable an engine to work at “ground level” conditions with supercharge or high compression without trouble. Tetra-ethyl lead behaves very well in high temperatures. | ||