The use of the term octane number to indicate the quality of a gasoline has become prevalent among gasoline refiners and distributing agencies. It is also beginning to be used by the automotive industry. The octane selector on one low-priced car has been announced.
The actual rating of gasolines by octane number is a process with which very few naval officers or, for that matter, very few civilians are actively concerned, but knowledge of this process and of its significance, as regards both the gasoline and the engine in which it is to be used, can hardly fail to be of considerable interest to all naval officers as actual or potential automobile owners and supervisors of motor-boat operation. To the aviator an understanding of the meaning of octane number and of the methods and reasons for improving it is of active and vital importance.
In the succeeding paragraphs an effort is made to define this comparatively recent development of the gasoline and automotive industries for the edification of that large group of persons who have neither the time nor inclination to delve into the intricacies involved in the chemistry and physics of octane number but who, nevertheless, require a moderate knowledge of its significance as regards gasoline for internal-combustion engines. The relation between octane number and gasoline engines is briefly discussed, together with the manner of determining the octane number and the methods employed for improving it.
Until recently all gasoline, like all Gaul, was divided into three parts, namely, “Hi- Test” or extra good gasoline, standard (straight) gasoline, and cheap (third- grade) gasoline. The dividing lines between these three grades of gasoline were more definitely drawn in price than in the properties of the gasolines themselves. The advent of the high-compression engine, which will knock unless operated on high- grade gasoline, gave rise to the need of some definite classification for gasolines which would be a criterion of their suitability for use in engines of various compression ratios.
The relation between the quality of the gasoline and the compression ratio of the engine for which that gasoline is suitable will bear some explanation. In the first place, the compression ratio of an engine may be defined as the ratio of the cylinder volume above the piston at the bottom of its stroke to the cylinder volume above the piston at the top of its stroke. When the charge of gasoline vapor and air, drawn into the cylinder on the suction stroke, is compressed by the upward travel of the piston, its temperature and pressure both rise. The smaller the final volume above the piston, the more pressure is necessary to squeeze the charge to that size. The more the charge is compressed, the higher the final temperature and pressure that are attained prior to ignition, which occurs just before reaching the top of the compression stroke. Therefore, in a high-compression engine (one with a large compression ratio) the temperatures and pressures at the time of ignition are higher than those encountered in a low-compression engine. Each gasoline has a certain “critical” temperature and pressure above which it will cease to burn evenly and will detonate spontaneously with explosive violence. If this critical point is reached prior to spark ignition, the charge will pre-ignite (spontaneously) and detonate, producing the phenomenon known as knocking, similar to that produced by the spark’s being too far advanced when climbing a hill, or by pre-ignition caused by incandescent carbon deposits. Even the best straight-run gasolines will detonate in engines of compression ratios higher than 7:1.
Generally speaking, however, the knock in a high-compression engine results from the detonation of only a part of the fuel- vapor charge. After ignition by the spark, the charge begins to burn, the flame progressing rapidly but evenly from the point of ignition (the spark plug points) out into the combustion space. As the burning progresses, the temperature and pressure in the cylinder rise rapidly. The portion of the charge not yet reached by the flame is compressed and heated as the flame advances. If the burning is sufficiently rapid, the temperature and pressure in the cylinder will pass the critical point for the vapor charge and the remaining unburned part of the charge will explode with the accompanying detonation knock. This detonation, occurring near the top position of the piston when the turning arm at the crank is small, results in heavy stresses on the engine.
Since a high-compression engine has comparatively high temperature and pressure in the cylinder at the time of ignition, it follows that a gasoline which can be used therein without knocking must have a high critical point relative to these temperatures and pressures, and must be relatively slow burning. These two qualities apparently go hand in hand and are characteristic of high-grade gasolines, whereas low criticals and rapid burning are indicative of the poorer grades. To distinguish between these grades of gasoline the method of rating by octane number was devised.
The octane number of a gasoline is the percentage of iso-octane (by volume) in the mixture of iso-octane and normal heptane which, in a standard engine under similar conditions, exactly reproduces the knock developed in that engine by the gasoline in question.
Iso-octane is a pure hydrocarbon and is a clear liquid at atmospheric conditions. It burns comparatively slowly in an engine cylinder and has a high critical point. Therefore, it does not tend to knock under normal conditions encountered in an engine. On the other hand, normal heptane, which is also a pure hydrocarbon of different formula but resembling iso-octane in appearance, burns very rapidly and tends to detonate at comparatively low pressures and temperatures. By mixing these two liquids in various proportions, it is possible to reproduce in an engine cylinder the knock of any gasoline now in general use. For this reason they are used as the standards for comparing the knock characteristics of gasolines.
Before turning to the standard engine and method of determining octane rating, it would be well to clear up a fairly prevalent misconception. The octane number does not refer to the amount of iso-octane present in the gasoline. Although both iso-octane and normal heptane do occur in most gasolines, they are always in very small proportions and the octane number has no connection with the amount of iso-octane actually in the fuel. Other hydrocarbons present in much larger quantities actually determine the antiknock quality of the gasoline. The octane number must, therefore, be considered as merely an arbitrary rating (determined in a specially designed laboratory engine) for comparing the antiknock properties of gasolines.
Every gasoline is a complex mixture of various hydrocarbons, some of which, similar to iso-octane, burn slowly and thus inhibit knocking, while others, like heptane, burn rapidly and cause knocking. The percentages of these various hydrocarbons or fractions vary with each gasoline and, to date, it has been impossible to predict accurately from chemical analysis what the final antiknock qualities will be. However, the resultant knocking tendency of any gasoline is characteristic of that gasoline and is dependent on the source of the crude oil from which it is derived and the methods used in refining it.
Aviation gasoline for the navy is tested for its octane rating at the U. S. Naval Engineering Experiment Station, Annapolis, Maryland, in the following manner. A “Series 30” Ethyl Gasoline Corporation engine is used. This engine consists of a single-cylinder engine with auxiliary equipment, driven at a constant speed of 900 r.p.m. by a synchronous motor and operated at a constant temperature, which is maintained by an ethylene-glycol evaporative cooling system. This engine burns the gasoline under test under fixed conditions of compression ratio, spark setting (16° advance), and a throttle opening sufficient to produce an audible knock. The intensity of the knock is measured by means of a Midgley bouncing pin, resting on a diaphragm set into the cylinder wall. When knocking occurs, the pin bounces up, closing the knockmeter circuit. The more severe the knock, the higher the pin bounces and the longer the circuit remains closed. This circuit consists of a thermal resistance, a source of constant voltage, and a pyrometer which indicates the temperature of the thermal resistance. The longer the circuit is closed on each contact, the higher the thermal resistance temperature, hence the higher is the reading of the pyrometer. This pyrometer, known as the knockmeter, is fitted with an arbitrary scale of so-called knock intensities.
The actual procedure is as follows: A run is made, using the gasoline to be tested. When conditions are stabilized and an audible knock obtained, the knockmeter reading is recorded. From previous experience, the operator can estimate the correct proportions of iso-octane and normal heptane which will produce approximately the same knockmeter reading. Accordingly, a mixture of those proportions is prepared and a second run made, using this mixture. If the resultant knockmeter reading is higher than that obtained with the gasoline, there is insufficient octane in the mixture; if lower, too much. Another mixture is then prepared, using the corrected proportions of iso-octane and normal heptane to bring it closer to the gasoline knockmeter reading, and another run is made. The gasoline is then run a second time and if agreement of knockmeter readings has been reached, the final percentage of iso-octane used in the mixture of iso-octane and normal heptane is assigned as the octane number of that gasoline. With practice, it is possible to estimate very closely, merely from knockmeter readings, what percentage of iso-octane should be used, but a check run on the gasoline is always made to avoid the effect of possible engine changes and to ensure accuracy.
The octane ratings of commercial gasolines, as sold to motorists along the highways, vary from about 60 for the lowest grade to 84 for premium fuels. High-compression-aviation engines require gasolines of an octane number in the neighborhood of 80. (The gasoline engines of the Macon require at least 85 octane number fuel.) It is not economical, with present-day refining methods, to produce a straight-run gasoline of an octane number much above 73. Such being the case, how can we obtain a fuel of sufficiently high octane number to accommodate the high- compression engines of our modern aircraft and automobiles?
The octane rating of gasoline may be improved in three ways: by more selective refining; by blending with high octane number fuels; or by adding small amounts of chemicals which inhibit knocking. Refining methods are constantly being improved, but the octane number of the product is limited either by the high cost of processing and small yield per unit quantity of crude or by the character of the original crude. Some refiners resort to blending with other high octane number fuels, but there are drawbacks to this, too, for without exception the heat units per gallon of fuel are lessened by this blending and the diluents are usually higher in price, thus raising the cost of the blended fuel. Ethanol and methanol, better known as ethyl and methyl alcohols (grain and wood alcohols), are used in conjunction with benzol in many European countries, but to date have not been commercially used in the United States. There is at present some agitation in Congress to require the use of ethyl alcohol as a blending agent for gasoline, but this is with a view to helping the farmer rather than from any intrinsic merit in the blend. It would probably increase the price of fuel to the consumer.
Here again we digress to point out that iso-octane, if used for improving octane ratings, would come under the category of a blending fuel. In this r61e, the large percentages that would be required to effect any substantial improvement and the high cost of $18 per gallon effectively discourage any such use. Iso-octane, per se, has only about one-half the effectiveness of benzol and costs about 100 times as much.
By far the most common method of improving the octane number of a gasoline is by adding a small percentage of a chemical which tends to make the fuel charge burn more evenly throughout the combustion space of the cylinder. Such chemicals are iron carbonyl and tetraethyl lead. There are several other so-called “knock inhibitors” of this type, but tetraethyl lead is by far the most effective and most widely employed. It is used exclusively in the navy for improving the antiknock properties of aviation gasoline. It will be more generally recognized for its use in ethylized gasolines sold as premium fuels , at filling stations.
In refining, the octane rating of a gasoline is regulated by exercising careful control over the low-boiling fractions, chiefly propane, butane, and isopentane, which have very high antiknock values.
In blending, the octane rating of the resulting fuel is a function of the percentage of the blending fuel added. In the case of a motor gasoline of about 63 octane number, adding 40 per cent by volume of benzol increases the octane rating to about 80. In both refining and blending, the method involved is to increase the percentage of the slow-burning constituents with high critical points at the expense of the poorer, knock-encouraging compounds. Chemical knock inhibitors function differently and are required only in very small quantities.
The action of tetraethyl lead in suppressing detonation or knocking in high- compression engines has been variously interpreted. One theory is that the colloidal lead is separated from its suspension in the ethylene dibromide by the heat and pressure and each of the particles scattered throughout the charge becomes oxidized with the evolution of much heat, thus forming a nucleus of incandescent matter, which ignites the portion of the charge in its immediate vicinity. In this way, uniform ignition of the whole charge is secured and there is no pressure wave built up which, hurtling against the sides and top of the cylinder, is said to cause knocking. Another and contradictory theory sets forth that the particles of colloidal lead, under the action of heat and pressure, form lead bromides, which are solids at those temperatures (an endothermic process). These solids plate out on all sharp projections and hot spots that otherwise would tend to cause pre-ignition and detonation. The coating of lead increases the radius of curvature of potential flame propagation at each point. The resultant reduction in the heat intensity, together with the absorption of heat that occurs, prevents pre-ignition and induces normal combustion. In any event, it is known that the presence of inert matter (gas or solid) in an explosive charge increases the temperature required for spontaneous ignition and slows down the rate of propagation of the flame, thus preventing detonation.
In the navy, the process of increasing the octane number of a gasoline by adding tetraethyl lead is commonly called doping I the gasoline. The navy buys its aviation gasoline undoped and the various naval activities dope it according to the octane rating required by the engines in which it is to be used.
The addition of a stated amount of tetraethyl lead to different gasolines does not necessarily result in the same increase in octane number. Each different gasoline has its own degree of response to the addition of tetraethyl lead. The effect on the octane number produced by the addition of tetraethyl lead to any gasoline is known as the lead susceptibility of that gasoline and these lead susceptibilities vary widely with different gasolines. This characteristic of the fuel is determined, for all gasolines offered to the navy, at the Engineering Experiment Station by making runs for octane number on the “Series 30” engine with each gasoline as received, undoped, and then with various concentrations of tetraethyl lead added (generally with 1, 2, and 3 cubic centimeters per gallon). The results of these runs, plotted with octane numbers as co-ordinates and cc. of pure tetraethyl lead as abscissas, constitute the lead susceptibility of that gasoline. Figure 1 represents a typical lead susceptibility curve for an aviation gasoline. Larger amounts than 5 cc. of tetraethyl lead per gallon do not further improve the octane number and, in some cases, even lower it.
Tetraethyl lead is supplied to the various naval stations in the form of ethyl fluid, which is a solution of tetraethyl lead in ethylene dibromide. The function of the ethylene dibromide is to prevent the lead from depositing on the spark plugs, valve seats, and cylinder walls, and to permit it to be carried off in the exhaust gases. Unfortunately, in the fluids supplied to the navy at different times and by different manufacturers, the proportions of pure tetraethyl lead vary. Therefore, these lead susceptibility curves must be used with due caution.
Each naval activity using aviation gasoline has instructions which dictate the octane number of the fuel to be used in its engines. By reference to the susceptibility curve of the contract gasoline supplied it, the activity finds the cc. of pure lead per gallon of gasoline to attain the required octane number. This number of cc. of lead is converted to cc. of fluid per gallon of fuel by multiplying by the fluid-lead ratio inscribed on the container in which the ethyl fluid is delivered. Unless cc. of pure lead are used as abscissas of the susceptibility curve, the interpolation to cc. of ethyl fluid becomes quite involved.
In addition to the octane rating of 73, the navy requires its aviation gasoline to burn without knock or loss of power (be suitable for continued operation) at a compression ratio of at least 6:1 in the variable compression engine designed by the National Advisory Committee for Aeronautics. Actually the compression ratio of this engine is increased for successive runs until the highest useful compression ratio is reached, evidenced by destructive knocking occurring on any increase in compression ratio. The H.U.C.R. (highest useful compression ratio) of a gasoline is that compression ratio at which the brake horsepower of the test engine begins to fall off. At this point the knock is approaching destructive intensity. On this engine it becomes apparent that higher octane fuels burn without knocking under higher compressions than lower octane-number fuels.
The compression ratio is a definite value for any one engine. It can be changed only by modification of the design or by using special cylinder heads or pistons. Different makes of engines may have the same compression ratio, although the shape of the combustion space and the location of the spark plugs may be entirely different. It is obvious that the last two factors will have a decided effect on the flame propagation in the vapor charge; hence, on the proportion of the charge that may detonate. Thus, between engines of different design, having the same compression ratio, there will be quite a variation in the octane ratings of the poorest gasolines that will burn without detonation or knock. However, compression ratio remains the controlling feature of design in the selection of suitable fuels for an engine.
It follows that for a given compression ratio, that engine is most efficiently designed which can develop its designed power while burning the lowest grade (least octane numbered) fuel without knocking. It is interesting to note that in a series of exhaustive tests on automobile engines grouped by compression ratios there were variations of as much as 7 octane numbers in the fuels which burned at a given compression ratio without knock. These variations were due to different shaped combustion spaces, location of spark plugs, effectiveness of cooling, and the different metals used in pistons and cylinder heads.
Automobile manufacturers are rapidly becoming octane conscious and are bending every effort to design their engines along lines that will bring the advantages of high-compression ratio along with the ability to use comparatively low octane- numbered fuels.
The method of rating fuels by octane number, as described herein, has its limitations. For instance, fuels may eventually be in general use which will exceed an octane rating of 100 per cent, in which case no octane rating could be obtained on the above scale, since 100 per cent isooctane is the best mixture that can be obtained with the standards used. Correlation of results is very difficult, because of the difficulty of obtaining exactly similar operating conditions on any two test engines. Eventually, there may be derived a more absolute scale of performances, based on the chemical and physical behavior of the fuel without reference to a standard testing engine. The advantages of such a method of comparison are self-evident; but, in the meanwhile, behavior in a standard engine is our only criterion of comparative performance, and the unit of the scale is octane number