TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 1 - Fuel

SECTION I: Fuels

 1. General.-a. Combustion or burning, in a chemical sense, signifies the combination of one or more elements with oxygen, resulting in the formation of oxides of the elements, accompanied in all cases by the liberation of heat. The heat produced may be considered as the result of a chemical reaction which converts the potential (stored up) energy in the fuel into heat, which in turn may be utilized inside a closed cylinder and converted, in part, into kinetic energy (energy of motion). Energy may be transferred and transformed in many ways, but it is never created or destroyed.

b. Oxygen (0₂), which is essential to combustion, is present in the air to the extent of approximately 20 percent. The remaining 80 percent is chiefly nitrogen, a very inert gas which does not enter chemically into the process of combustion. Oxygen, a very active gas, will combine with a great number of elements and compounds, and in each instance the amount of heat liberated will depend on the chemical nature of the substance involved. Hydrogen and carbon alone or in any of their various combinations produce large quantities of heat when burned and constitute our most important fuels for the production of heat and power.

c. Hydrogen (H) is a very light and inflammable gas. When mixed with air and ignited it combines with oxygen to form the oxide H₂O, or water. The combustion of pure hydrogen is a very rapid process which, if confined, may produce a very high pressure.

d. Carbon (C) is a solid existing in three forms: The familiar carbon present in soot or lampblack, graphite, and the diamond. Although differing in physical properties, these are all pure carbon, and under proper conditions one form may be converted into another without any change in chemical structure. At a very high temperature, carbon will pass from the solid into the vapor state. Carbon in its natural state does not exist as a liquid. When ignited in a plentiful supply of air or oxygen, carbon burns with a clear flame to form carbon dioxide (CO₂.), an inactive and harmless gas. However, when the supply of oxygen is insufficient (rich mixture), a certain quantity of carbon monoxide (CO) will also be produced. Carbon monoxide is a poisonous gas which may cause death when present in the air to the extent of only 4 parts in 10,000 (0.04 of 1 percent) by volume. The gas is colorless and odorless, and thus gives practically no warning of its presence.

e. As fuels, hydrogen and carbon are seldom available, in their natural state, but occur in combination with each other, forming compounds known as hydrocarbons (CH group). There are many thousands of these compounds in existence, and all are classed as fuels. These hydrocarbons are present in large quantities in coal and petroleum, occurring as solids, liquids, and gases.

1. Due to the abundance of crude petroleum and because of the various products which may be extracted therefrom, it has become our most important source of hydrocarbon compounds for internal combustion engine fuels and lubricants. Although crude samples from different fields usually vary in composition, all crudes are made up of a great number of hydrocarbon compounds arranged in groups, the paraffin series being the most common in the United States. The chemical nature of petroleum is so complex that complete analysis is seldom attempted, but by distillation the crude may be separated into fractions in order to produce the desired commercial products, including gasoline, kerosene, and lubricating oil.

2. Gasoline.-a. Gasoline is a blend or mixture of hydrocarbon liquids ranging in boiling point from approximately 90° F. to 425° F. There are no exact limits established for this mixture range. Because of this latitude in boiling point and in various other characteristics, it is impossible to list the detailed specifications of gasoline. Any sample must be subjected to a number of tests before its exact properties may be determined. Only after such testing can a gasoline be pronounced satisfactory for use, as a fuel in a particular type of internal-combustion engine.

b. Of the many methods employed for producing gasoline, three are of sufficient importance to warrant a brief description of the apparatus and procedure involved. These are the fractional distillation process, the cracking process, and the absorption process.

(1) The fractional distillation process was the first to be developed and produces what is known as straight-run gasoline. In this process the crude is heated to a moderate temperature in a retort to vaporize progressively the various hydrocarbon liquids. The lighter and more volatile compounds are first vaporized, followed in order by those of higher boiling points. These vapors are then led through condensers which return them to the liquid state. By proper regulation of the vaporization and condensation, the hydrocarbons may be separated into various grades of gasoline, fuel oil, lubricating oil, etc., although further treatment and purification are often necessary. The fractional, distillation process is accomplished at atmospheric pressure, and during the process no effort is made to change the chemical nature of any of the fractions.

(2) The cracking process is employed principally as a means of increasing the yield of gasoline from a given amount of crude. Very often petroleum fractions which are neither suitable for gasoline nor lubricating oil may be cracked, thus obtaining a considerable quantity of gasoline. The cracking process is a form of destructive distillation in which the crude or a portion of it is placed in a sealed retort and subjected to a high temperature and high pressure.

These conditions serve to break up the chemical arrangement of the heavy hydrocarbon molecules  and partially convert the heavier products into cracked gasoline. The fuel thus produced is often superior to many grades of straight-run gasoline in antiknock value but requires thorough refining to make it suitable for storage. The reason for this is that the cracked hydrocarbons, which are chemically the olefins and the diolefins, produce gum on aging. Some types of cracked gasoline may be stabilized or inhibited from gum formation by the addition of a small quantity of a suitable anticatalyst.

A high percentage of the total gasoline production at present is the result of the cracking process.

(3) Extracting gasoline from certain compounds present in natural gas produces a fuel of comparatively high volatility, known as casing head or natural gasoline. The most common method of extracting gasoline from natural gas is the absorption process. This is accomplished by forcing natural gas through a heavy oil which absorbs the liquid content of the gas. The oil is then distilled to reclaim the light fraction, which is gasoline. If properly blended with a straight-run or cracked gasoline, it is quite satisfactory as an engine fuel.

3. Alcohol and benzol.-a. Although the petroleum fractions known as gasoline have been employed almost exclusively for internal combustion engine fuels, other liquid fuels have also been investigated and used to some extent. Ethyl (grain) alcohol and benzol appear to be preferred at present.

b. Ethyl alcohol is a compound of hydrogen, carbon, and oxygen which may be prepared from any organic compound such as grain, starch, or sugar. As an engine fuel its chief virtue is that it will withstand a high compression pressure, which in turn promotes efficient engine operation. The particular disadvantages of alcohol as compared with gasoline are its low heat value, low vapor pressure, and a pronounced affinity for water.

c. Benzol is a hydrocarbon compound obtained from coal. It may be compressed to a high degree, but it has a low specific heat value, slow burning rate, high freezing point, and greater cost, and the available supply would be urgently required by other industries in case of military emergency. It has been successfully blended with gasoline as an antiknock compound, but, as such, it is inferior to such antiknock compounds as tetraethyl lead, iso-octane, and iso-pentane.

4. Volatility of liquid fuels.-a. Since liquid fuels are generally used for internal combustion engines, they must always be converted into a vapor state before combustion occurs. This property of a liquid, which enables it to change readily into a vapor, is known as "volatility," a characteristic which may be determined by a distillation test and vapor pressure test.

(1) In the distillation test, the gasoline is heated and vaporized at a constant rate. The boiling temperatures are recorded as the various percentages of fuel are recovered. These percentages determine the volatility range between the initial and end boiling points of the fuel under test. The distillation apparatus is shown in figure 1.

(2) The vapor pressure test is accomplished by sealing a sample of the fuel in a bomb equipped with a pressure gage. The apparatus is then immersed in a constant temperature bath, and the indicated pressure is noted. The higher the corrected vapor pressure obtained from the fuel under test, the more susceptible it is to vapor locking. The apparatus used for this test is illustrated in figure 2.

FIGURE 1.-Distillation apparatus.

b. The volatility of a fuel is quite important in determining whether or not an engine may be started when cold. In this connection, it is well to know that gasoline is not combustible in its liquid form, principally because the molecules of the liquid will not readily mix with the oxygen of the air. Gasoline vapor, however, unites quite readily with oxygen, resulting in very rapid combustion. From this it is evident that an engine fuel should be sufficiently volatile to form combustible vapor at low atmospheric temperatures.

(1) On the other hand, excessively volatile gasolines are very troublesome, because they promote a condition known as "vapor lock." This condition is due to vapor formation in the fuel lines which restricts the liquid flow, resulting in a lean mixture and the possibility of engine failure.

(2) A compromise between the two extremes in volatility of gasolines is generally attained, permitting satisfactory starting characteristics, and at the same time probable freedom from vapor lock under all conditions. For present aircraft engine fuels a maximum vapor pressure of about 7 pounds, with 10 percent distilled at 140°F. to 160° F. is satisfactory. The 90 percent point should not exceed 300° F., and approximately 250° F. is ideal.

FIGURE 2.-Vapor pressure test apparatus.

5. Octane rating.-a. "Octane rating" is a term universally used to designate the antiknock value of the fuel mixture in an engine cylinder. Modern aircraft engines of high power output have been possible principally as a result of the blending of fuels of high octane rating. The use of such fuels has permitted increases in compression ratio and manifold pressure with resultant improvement in engine power and efficiency. However, it must be remembered that even the high octane fuels will detonate under severe operating conditions, or if certain engine control are improperly operated.

(1) In this connection it is necessary to consider briefly the nature of combustion in an engine cylinder of high power output. Figure 3 indicated the flame propagation and pressure produced

FIGURE 3.-Flame propagation and cylinder pressure.

in a cylinder during normal combustion and also during detonation. Both of these conditions may be produced in the same engine by operating it with a fuel containing satisfactory antiknock properties, and then again with a fuel of inadequate antiknock properties.

(2) A study of the combustion chamber during normal combustion illustrates how the burning of the charge originates at the sparkplug electrodes (dual ignition) and travels progressively toward the center of the combustion head, meeting at the approximate center of the combustion chamber. The pressure curve reveals that the pressure rise is quite regular, although rapid, reaching a peak value at piston top center or slightly thereafter. A fairly high pressure is then maintained throughout the power stroke, and thus the engine is capable of developing its rated horsepower. The power output is related to the mean effective pressure; however, with detonation, serious danger to engine parts will result from abnormally high peak cylinder pressure.

(3) When detonation occurs, the flame travel is of a somewhat different character. Combustion of the charge is initiated, and for a certain distance the rate of flame propagation is quite normal until possibly four-fifths of the charge is burning. However, at this point a marked change takes place. The combustion accelerates with such rapidity that the remaining charge is burned almost instantaneously, resulting in an unusually rapid pressure rise. The pressure curve ascends to a very high peak and then quickly drops to a lower value, remaining comparatively low throughout the balance of the power stroke. Thus, when detonation occurs, the mean effective pressure and consequent power output are substantially reduced. At the same time the engine is subjected to a series of mechanical shocks. If detonation is permitted to continue, the shocks will become violent and probably terminate in sudden and complete engine failure.

b. Since it is most important that detonation be avoided in the operation of aircraft engines, it is well to consider the principal factors which contribute to this condition. The antiknock value of the fuel, cylinder temperature, induced charge temperature, mixture ratio, and intake manifold pressure are the most important factors and will be discussed briefly. Many other factors of technical interest could also be included, but those given have the greatest significance for the engine operator.

(1) Both the power output and the reliability of an aircraft power plant depend to a great extent on the use of a fuel of high antiknock value or high octane rating. The substitution of an inferior fuel, while permissable in certain emergencies, is attended by serious danger of detonation unless the engine is operated at reduced throttle. The cylinder temperature and the charge temperature are, within certain limits, under the control of the operator, and neither reading should be permitted to exceed the maximum value specified for a particular engine.

(2) With reference to mixture ratio and manifold pressure, it is evident that there is a definite relation between these two factors under conditions of detonation. The operation of a high-output engine at full power usually requires a very rich mixture in order to avoid overheating and detonation. Therefore, excessive leaning of the mixture when operating at high manifold pressure is considered a most dangerous practice. However, when the manifold pressure is reduced to the value recommended for continuous cruising, it is often advisable to lean the mixture slightly in order to lower the fuel consumption. The exact procedure to be followed in the use of the mixture control (altitude control) is variable, depending upon the characteristics of each particular engine.

c. (1) In order to be able to express the antiknock characteristics of a gasoline in accurate numbers, reference is made to the octane rating of the fuel. The octane number system is based on a comparison of any fuel with certain mixtures of iso-octane, and normal heptane. Iso-octane has a very high antiknock value; whereas, heptane detonates readily in an engine cylinder. A mixture of these two liquids will possess an intermediate value, depending on the relative percentage of each of the liquids in the mixture.

(2) To perform the test for octane rating, a single cylinder knocktest engine is utilized. Two float chambers are incorporated in the carburetor in order that the fuel being tested may be fed from one chamber and the iso-octane heptane mixture from the other. The engine, under load conditions which will produce slight detonation, is operated alternately on the fuel to be tested and the known test mixture. An iso-octane heptane test mixture, is found which will exactly match the antiknock value of the fuel under test in the opposite float chamber. Thus, for example, if the fuel under test is equal to a mixture of 70 percent iso-octane and 30 percent normal heptane, the rating of the fuel is 70 octane.

d. Fuels vary extensively in octane rating, and their value is more or less dependent upon this rating. The following table lists the approximate octane rating ranges of the most common fuels:

Military aircraft:

             High output engine grade ----------------------------------- 100

             Basic trainer grade ----------------------------------------- 91

             Primary trainer grade ------- - ----------------------------- 65 and 73

Commercial aircraft:

             Standard grade ------ - ------------------------------------ 80-87

Commercial automotive:

            Best grade ------------------------------------------------- 74-82

            Standard grade -------------------------------------------- 65-74

            Cheap grade ----------------------------------------------- 55--64

e. (1) Efforts are constantly being made to increase the octane rating of all gasolines by careful blending of the hydrocarbons and also by adding small quantities of ethyl fluid, which contains tetraethyl lead, ethylene dibromide and aniline dye. The tetraethyl lead in the, ethyl fluid is a heavy liquid containing lead, which has been found to be highly effective in suppressing detonation. In some fuels the addition of 3 cubic centimeters per gallon results in an increase from ten to eighteen points in octane rating. Some difficulties, such as spark plug fouling and corrosion of certain engine parts, have been encountered as a result of the use of "leaded" fuels, but these objections are rather insignificant when compared with the results obtained from the higher octane number of the ethylized fuel.

(2) Modern high octane aircraft engine fuels contain a high percentage of iso-octane in addition to the gasoline and ethyl fluid. The iso-octane is a chemically prepared compound (tri-methylpentane) having a high volatility and a 100 octane rating. Small amounts of iso-pentane (bimethyl-propane) may also be added to the fuel to increase further its octane rating and volatility.

f. It is necessary to differentiate between detonation and pre-ignition. During certain conditions of engine operation a phenomenon occurs which, while often confused with detonation, is properly known as pre-ignition or auto-ignition. Pre-ignition is generally attributed to overheating of such parts as spark plug electrodes, exhaust valves, carbon deposits, etc., to such a high degree that the charge is ignited before the spark occurs at the spark plug electrodes. In such cases an engine may continue to operate after the ignition system is turned off, until the fuel supply in the carburetor is exhausted. Special care must be exercised in stopping many high output engines in order to eliminate this condition.

FIGURE 4.-Corrosion-test apparatus.

6. Purity.-a. It is important that the finished gasoline, after refining, be as free from foreign substances as possible. The elimination of sulphur and corrosive sulphur compounds is particularly desirable especially in aircraft gasolines. The apparatus as shown in figure 4 for testing the gasoline for corrosion consists of a spun copper dish and a steam bath. After evaporating a certain amount of gasoline placed in the copper dish, a gray or black discoloration deposited on the inside surface indicates the presence of corrosive sulphur which condemns the sample.

b. Even though all precautions are observed in storing and handling aircraft gasoline, it is not uncommon to discover a small amount of water and sediment in an aircraft fuel system. The sediment is usually retained in the strainers located at various points in the fuel system, and this is not generally considered a source of great danger. The water, however, presents a rather serious problem since it drops to the bottom of the fuel tank and may then be circulated freely through the fuel system. A small quantity of water will flow with the gasoline through the carburetor jets and not be especially harmful. However, an excessive quantity of water upon reaching the carburetor will effectively displace the gasoline passing through the jets and restrict the flow of fuel which may result in engine failure.

c. Efficient water segregators are installed in servicing equipment; therefore, there is little danger of water actually being pumped into airplane fuel tanks. However, under certain conditions of temperature and humidity,. condensation of moisture occurs on the inner surfaces of the fuel tanks. Since the amount of such condensation is proportionate to the unfilled volume of the tank (air space), it is obvious that the practice of servicing an airplane immediately after flight will do much to eliminate this hazard.

d. Whenever water is believed to be present in a fuel system, a small quantity of gasoline may be drained from the lowest point of the system and tested with water-test paper. This paper is coated with a compound which is soluble in water but is not affected by gasoline. A strip of the paper is immersed vertically in the container so that it touches the bottom. If water is present, the coating will be removed from the lower portion of the strip, thus indicating the amount of water. This simple test will prove conclusively whether or not a real danger exists, and is well worth the time and effort required.

7. Summary.-a. In summarizing the information contained in this section, a fuel suitable, for the operation of high-performance aircraft engines should be as follows:

(1) A liquid readily available in large quantities. Gasoline meets this requirement. The manufacture of iso-octane is increasing rapidly.

(2) Of sufficient volatility to permit vaporization at low atmospheric temperatures to insure starting of the engine. A sufficient amount of light fractions in gasoline meets this requirement.

(3) Of antivapor-locking tendencies to prevent the fuel from vaporizing in the fuel lines. Accurate blending of the proper fractions in gasoline meets this requirement.

(4) Of high octane rating to permit the vaporized fuel to be compressed to a high degree without detonation, resulting in a high power output of the engine. A good grade of gasoline with a mixture of ethyl fluid, iso-octane, or both, has a higher octane rating than most other available fuels.

b. From the above summary it will be noted that gasoline and iso-octane make the most desirable fuels for aircraft engines; however, due to the wide variety of grades of gasoline manufactured, only a few of the highest-quality grades are suitable for engines of high performance.