[Aircraft Induction, Fuel, and Oil Systems]
[From the U.S. Government Publishing Office, www.gpo.gov]
W l.%5: l-407/a.
NON-CIRCULATING
Document Reserve
’’rar Department
TECHNICAL MAKUAL
AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS
November, 4, 1941
library
OF
NORTH TEXAS STATE TEACHERS COLLEGE
DENTON, TEXAS
Photomount Pamphlet Binder
Gaylord Bros., Inc.
Makers
TM 1-407
WAR DEPARTMENT
TECHNICAL MANUAL
AIRCRAFT INDUCTION, FUEL,
AND OIL SYSTEMS
November 4, 1941
*TM 1-407
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TECHNICAL MANUAL No. 1-407
WAR DEPARTMENT, Washington, November 4, 1941.
AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS
Prepared under direction of the Chief of the Air Corps
Paragraphs
Section I. Fuels______________________________________________ 1-7
II. Lubricants______________________________________ 8-14
III. Oiling system____________________________________15-20
IV. Fuel systems_____________________________________21-26
V. Carburetion systems______________________________27-33
VI. Fuel injection systems___________________________34—38
VII. Supercharger systems_____________________________39-45
Section I
FUELS
Paragraph
General______________________________________________________________1
Gasoline___________________________________________________________ 2
Alcohol and benzol__________________________________________________ 3
Volatility of liquid fuels__________________________________________ 4
Octane rating------------------------------------------------------- 5
Purity-------------------------------------■----------------------- 6
Summary------------------------------------------------------------- 7
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.
Z>. Oxygen (O2), 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
♦This manual supersedes TM 1-407, September 23, 1940.
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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.
/. 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 internalcombustion 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.
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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 of 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.
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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
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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.
-«—Thermometer
______________________ zzz_-[ce-water bath
~ '"Electric heater ¥
Figure 1.—Distillation apparatus.
Z>. 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 compromse 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°
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F. to 160° F., is satisfactory. The 90 percent point should not exceed 300° F., and approximately 250° F. is ideal.
GAGE
BOMB
Figure 2.—Vapor pressure test apparatus.
SAMPLE CONTAINER
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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
FLAME PROPAGATION q
CYLINDER PRESSURE I \
t. c. Vt c.
—i —-----------------------------------—
IGN. IGN.
NORMAL COMBUSTION DETONATION
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,
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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
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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 actane 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
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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 “leaded5' 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-methyl-pentane) 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.
/. It is necessary to differentiate between detonation and pre-igni-tion. 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.
COPPER DISH
—"I STEAM
I----\ ENTRANCE
~ WATER .^X^STEAM BATH
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.
A Even though all precautions are observed in storing and handling aircraft gasoline, it is not uncommon to discover a small
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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 saystem, 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 anti vapor-locking tendencies to prevent the fuel from vaporizing in the fuel lines. Accurate blending of the proper fractions in gasoline meets this requirement.
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(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.
Section II
LUBRICANTS
Paragraph
General________________________________________________________________ 8
Description and classification of mineral lubricants------------------- 9
Engine lubricating oils----------------------------------------------- 10
Determining properties of lubricating oils---------.------------------ 11
Requirements of aircraft-engine oils__________________________________ 12
Oil reclamation:______________________________________________________ 13
Miscellaneous lubricants---------------------------------------------- 14
8. General.—a. A lubricant may be defined as a substance having greasy properties and used almost exclusively for reducing friction between bearing surfaces; however, it may also be used as a rust preventive on metallic parts subject to corrosion.
b. Common lubricants may be classified as animal, vegetable, and mineral, according to the source from which they are derived.
(1) Animal lubricants, such as tallow, sperm oil, lard oil, etc., are excellent lubricants provided they are not subjected to high temperatures. They are not suitable for lubricating internal-combustion engines, since above certain temperatures they form fatty acids. Porpoise-jaw oil, used for lubricating expensive watches, is an excellent high-quality animal oil.
(2) Certain vegetable lubricants, such as castor oil, olive oil, and cottonseed oil, have satisfactory lubricating qualities but are chemically unstable under conditions prevailing in internal-combustion engines. Straight mineral oils of high quality have almost completely replaced the blends of castor oil formerly used in high-output engines.
(3) Mineral lubricants have many desirable properties for use in the lubrication of internal-combustion engines and are, therefore, described in detail in subsequent paragraphs.
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9. Description and classification of mineral lubricants.— a. Lubricants may be readily classified according to their physical properties as solids, semisolids, and fluids. Solid lubricants, such as mica, soapstone, and graphite in finely powdered form, serve to fill the low spots in a bearing surface to form a perfectly smooth surface and at the same time to provide a slippery film to reduce friction. A finely divided solid lubricant also acts as a mild abrasive, smoothing the surface previously roughened by machining or excessive wear. Solids are fairly satisfactory on slow speed machines but lack the ability to dissipate heat, which is often an essential requirement. Certain solid lubricants have the ability to carry heavy loads and for this reason are often added to fluids to reduce wear between surfaces subjected to high unit pressures.
STATIONARY BEARING
1L m
REVOLVING SHAFT
Figubb 5.—Shaft and bearing with fluid lubricant (cross-section view).
b. The semisolid lubricants include such substances as extremely heavy oils and greases. Modern industrial operations require a great number of such lubricants for special applications. Greases give good service when applied periodically, but because of their consistency they are not suitable for continuous or circulating lubrication systems. A continued discussion of the properties of these lubricants will appear later in this manual under a separate heading. From this point, fluid lubrication will be emphasized, especially the requirements of internal combustion engines.
c. (1) Fluids (oils) are universally used in internal combustion engines for many reasons. They may be readily pumped or sprayed, they provide a good cushioning effect, and are effective in absorbing and distributing heat. In theory, fluid lubrication is based on the actual separation of surfaces so that no metallic contact occurs (fig. 5). In this way, as long as the oil film is unbroken, metallic friction
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is replaced by the internal friction (fluid friction) of the lubricant itself, and obviously under such an ideal condition no wear can occur. Many vital engine parts are given adequate protection by supplying oil under direct pressure, but where this method is impractical a mist or spray will generally be satisfactory. Parts carrying heavy loads at high rubbing velocities are, where possible, lubricated by direct pressure. In the process of circulating through the engine, oil absorbs heat from different parts and will later dissipate most of the heat through suitable coolers or heat exchangers. In this way engine parts are protected from both wear and excessive temperatures.
(2) An ideal fluid lubricant would be capable of providing a strong oil film to prevent metallic friction, and at the same time create a minimum amount of oil drag or viscous friction. Unfortunately, however, the body or viscosity of oils is affected by temperature changes to such an extent that ideal conditions are difficult to attain. Variations in climatic temperatures alone will often create an astounding change in oil viscosity. It is not at all uncommon for some grades of oil to become completely solid in cold weather with consequent high oil drag and impaired circulation. Conversely, at high operating temperatures oil may thin out to such extent that the oil film is broken, which permits rapid wear of the moving parts. The major problem in lubrication is to obtain a satisfactory compromise between the above conditions.
10. Engine lubricating oils.—a. Crude petroleum, which furnishes the fuel for internal-combustion engines, also supplies the most satisfactory oils for their lubrication. The numerous individual compounds contained in crude oil are arranged in groups, such as the paraffin series (saturated hydrocarbons), naphthalene series, olefin series, aromatic series, and others of lesser importance. A single crude sample may contain all of the above groups of the hydrocarbons in varying proportions. As a group, the paraffins are most satisfactory for engine lubricants since they are very stable, have good lubricating qualities, and are affected least by temperature variations. Almost any crude contains a certain percentage of paraffin compounds, but the crudes taken from eastern United States oil fields contain a higher proportion than midcontinent or western crudes.
£>. Since the paraffin hydrocarbons are most desirable in a finished oil, the refining process should endeavor to eliminate all of the undesirable constituents without damage to the paraffin compounds. Two principal methods of refining are in common use: acid extrac-
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 10-11 tion and solvent extraction, the latter being of more recent development.
(1) The acid extraction process involves the treatment of the lubricating stock with sulphuric acid which reacts chemically with many of the undesirable compounds, thus removing them from the mixture. The acid concentration and the duration of the treatment must be carefully controlled, since insufficient exposure will fail to remove the undesired compounds, and an overtreatment will destroy a part of the valuable constituents. In most cases, the type of crude has a direct bearing on the quality of the finished product when acid extraction is employed. Thus, it would be very difficult, if not impossible, to produce a superior lubricating oil by this process, except from crudes containing a high percentage of paraffin hydrocarbons.
(2) Solvent extraction, on the other hand, does not require a chemical reaction but, as the name implies, separates the various compounds by dissolving them with certain selected solvents. Advantage is taken of the fact that the desirable compounds are soluble in particular fluids such as propane, whereas the undesirable ingredients are soluble in other liquids. By the use of one or more solvents the lubricating stock may be accurately divided into the desired fractions. By this process a good lubricating oil may be produced from practically any crude.
(3) Oils extracted by either of the above methods require many treatments other than the basic extraction process. In order to remove heavy wax which will cause cold weather difficulties, the oil is chilled as low as —30° F., and the solid wax is removed by special filters or centrifugal separators. Filtration through fuller’s earth or other decolorizing earth is an essential and valuable part of most refining methods. After being properly purified, the various “cuts” taken by low pressure distillation are accurately blended to give the various grades of oil required by modern industrial operations.
11. Determining properties of lubricating oils. — a. Since mineral oils are produced in many grades, it is highly important to examine particular specifications when selecting oils for use in internal combustion engines. Of special significance are the factors of viscosity, flash point, and pour point, each of which will be discussed in detail.
b. Viscosity is generally considered as the resistance the oil offers to flow. Thus,, if an oil flows slowly, it is described as a viscous oil or an oil of high viscosity. An oil which flows readily is said to possess a low viscosity. As previously stated, the body or viscosity of an oil determines the amount of fluid friction. In general, it is
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desirable to select an oil of the lowest viscosity which will provide an unbroken film, so that friction may be held to a minimum. However, when consideration is given to the fact that the oil must often lubricate through a temperature range of 0° F. to 300° F., the problem becomes a very complex one and worthy of extensive study.
(1) In order to measure oil viscosity, the Saybolt viscosimeter (fig. 6) is employed. By a suitable arrangement, a certain amount of oil is heated to a standard testing temperature, generally 130° F. or
Figure 6.—Saybolt viscosimeter
A. Oil tube thermometer.
B. Bath thermometer.
C. Electric heater.
D. Turntable cover.
E. Overflow cup.
F. Turntable handles.
G. Steam inlet or outlet.
H. Steam U-tube.
J. Standard oil tube.
(sectional view).
K. Stirring paddles.
L. Bath vessel.
M. Electric heater receptacle.
N. Outlet cork stopper.
P. Gas burner.
Q. Strainer.
R. Receiving flask.
S. Base block.
T. Tube-cleaning plunger.
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210° F. The oil tube is provided with an outlet orifice through which the oil is permitted to flow into a 60 cubic centimeter flask. The time (in seconds) required for the delivery of 60 cubic centimeters of oil gives the Saybolt universal viscosity at that temperature. For example, if the time required for a particular sample is 120 seconds at 210° F., the oil is said to have a Saybolt viscosity of 120.
(2) If actual Saybolt numbers were used to designate the viscosites of various oils on the market, there would probably be several hundreds of grades listed and the purchaser would be faced with a rather complex problem. In order to simplify the selection of oils, they are often classified under an SAE rating system which divides all oils into seven groups (SAE 10 to 70, inch) according to viscosities at either 130° F. or 210° F. These SAE ratings are purely arbitrary and bear no direct relationship to the Saybolt or other ratings. They are defined, however, in terms of the Saybolt universal viscosity. By reference to the chart (fig. 7), the relation
SAE VISCOSITY NUMBER VISCOSITY RANGE SAYBOLT UNIVERSAL SECONDS
AT 130' E AT 210' E
MINIMUM MAXIMUM MINIMUM MAXIMUM
10 90 LESS THAN 120
20 1 20 LESS THAN 185
30 1 85 LESS THAN_ 255
40 255 LESS THAN 7 5
52 - 75 LESS THAN 105
60 105 LESS THAN 125
70 125 LESS THAN 150
Figure 7.—SAE conversion chart.
between Saybolt seconds and SAE ratings can readily be determined. It will be noted that grades 10 to 40 are tested at 130° F. and the heavier grades at 210° F., a procedure which is quite normal since the heavier oils are intended for use at higher engine temperatures. To determine the SAE rating of an oil sample, it is first tested at the correct temperature in the Saybolt viscosimeter. The reading thus obtained is compared with the ranges listed on the chart. For example, if an oil tests 82 Saybolt seconds at 420323°—41-------3
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210° F. it is evidently an SAE 50 oil, because the 50 classification covers all oils between 75 and 105 seconds. By the same method, an oil testing 130 seconds at 130° F. is found to be an SAE 20 grade. Occasionally the letter “W” will be included in the SAE number giving a designation such as SAE 20W. This letter indicates that in addition to meeting the viscosity requirements at the testing temperature the oil also meets additional low temperature specifications, showing that it is a satisfactory oil for winter use.
(3) Although the SAE scale as explained above has eliminated some confusion in the selection of lubricating oils, it must not be assumed that this specification covers all of the important viscosity
SAYBOLT VISCOSITY vs TEMPERATURE
OIL TEMPERATURE 300'E 210* E I3O* E 100’E 32'E
SAE 50 SAMPLE A SAYBOLT VISCOSITY 44 80 350 840 20000
SAE 50 SAMPLE B SAYBOLT VISCOSITY 42 80 480 1480 65000
Figure 8.—Viscosity temperature chart of two SAE oils.
requirements. An SAE number indicates grade only; it does not indicate quality or any other essential characteristics. It is well known that there are good oils and inferior oils having the same viscosities at a certain temperature and are therefore subject to classification in the same grade. The SAE letters on an oil container are not an endorsement or recommendation of the oil by the Society of Automotive Engineers.
(4) In order to appreciate the difference between apparently similar oils, viscosity must be known at more than one temperature. Figure 8 shows two oils of the same SAE rating but having widely different characteristics, as their temperatures are changed through the range required by engine operation.
(5) This chart clearly indicates a need for a more complete study of the viscosity temperature characteristics of lubricating oils. Sample A is obviously superior to sample B in its ability to withstand temperature changes with less change in body or viscosity. Sample B congeals readily at low temperatures thus causing excessive drag and cold starting difficulty, and at 300° F. is actually lower in viscosity than sample A.
(6) A convenient method of recording viscosity temperature curves is by the use of a special graph prepared by the American
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Society for Testing Materials (A. S. T. M.), a copy of which is shown in figure 9. By intricate spacing of the temperature and viscosity readings on the graph, resultant curves will appear as straight lines. To examine an oil, it is tested in a Saybolt viscosimeter at two temperatures, such as 130° F. and 210° F., and the viscosities are properly located as points on the chart. A straight line is then drawn through the two points so as to include all temperature readings. It is then a simple matter to determine the viscosity at any tempera-
SECONDS VISCOSITY
AT 100°E 4500 1 4000-
3500-SECONDS
3000- VISCOSITY SECONDS .JL VISCOSITY
2500- p 1000 AT 210° E
F 900 p 160 VISCOSITY
2000- 1-800 two INDEX
r700 l-izo r'°°
1500-s. 1-600 f .r’SOl ___ 1
/:°o° /■«
looo- r 4°Q^kbo'^ 8°
900-^-+.-350 /- \
ao°- L 300 Ho 770
700- I k 6 5 - 60
600- r 250 £ 60
500 ~ l~ 200 £ 55 \'sOj L 40
riao f ■ 30
4°°’ 1-160 f 50 \t2°5
r,40r V
300- If V'o
Fi20F-45
r ioo/
200 - I r r 90L 1-80/ I < 40
*-70
100
Figure 10.—Alinement chart for the estimation of viscosity index.
ture between —30° F. and 450° F., although the temperature values below 0° F. and those above 300° F. are of little significance, because most oils reach their chill points near zero temperatures and are seldom subjected to engine temperatures above 300° F.
(7) Upon examination of the oils plotted on the graph in figure 9 certain significant points are observed. Sample A is a good grade of SAE 50 oil; sample B is an inferior grade of SAE 50 oil; sample C is a superior grade of SAE 50 oil, and sample D is a good grade of SAE 60 oil. Sample B line on the chart indicates that at low temperatures this oil is actually more viscous than the SAE 60 oil
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 11
represented by the sample D line on the chart. Sample C line shows less tendency to congeal at low temperatures and to thin out at high temperatures than other samples tested, thus indicating its superior characteristics. Many other comparisons may be readily made by use of the A. S. T. M. graph. It is generally agreed that an oil of 60 to 100 Saybolt seconds at all temperatures represents an ideal lubricant for internal-combustion engines, but it is highly improbable that such a lubricant will ever be available.
(8) For somewhat greater convenience in indicating the viscosity temperature characteristics of oils, a viscosity index chart (fig. 10) may be used. This is an arbitrary chart on which superior oils of any grade will give a reading at the higher end of the scale, and inferior oils will show much lower readings. Indexes are determined by performing two Saybolt tests at different temperatures and locating the readings thus obtained on the proper lines on the left side of the chart. A straight line is then drawn through these two points and projected so as to cross the index scale at the right. The point of intersection of the viscosity line and index curve gives the index rating of the particular oil. The two oils listed in figure 8 furnish an excellent sample of oils having similar SAE ratings but widely different index values. When compared on the index chart, sample A oil is found to have a viscosity index of 95, and sample B (also an SAE 50 oil) has a rating of only 26. This further emphasizes the fact that an SAE rating alone cannot be interpreted as an indication of quality in lubricating oils. In general, high index ratings are very desirable in all grades of oil, a quality which is found particularly in compounds of the paraffin series. Some oils containing a high percentage of paraffin hydrocarbons have indexes well above the 100 figure.
c. The lubrication of cylinder walls and pistons of internal-combustion engines requires consideration of factors other than oil viscosity. Because of the large amount of heat generated in cylinders during the power stroke, the flash test of an oil assumes considerable importance. The test is performed by heating an oil sample in an open cup (fig. 11), and as the temperature rises a flame is periodically passed slowly over the surface of the oil. The first test which momentarily ignites the oil vapor above the cup is recorded as the flash point. Other factors being equal, an oil of high flash test is preferred because of the greater degree of protection afforded cylinders and pistons and the probable lower oil consumption during continuous engine operation. A fire point test, which indicates the temperature of continuous burning, is often performed along with
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the flash test, but this specification is considered of little importance in aircraft engine oils.
d. In addition to the above properties, a good oil must show a high degree of chemical stability in order to resist the action of high temperature, moisture, and acids, all of which are often present in engine crankcases. Here again, the paraffin hydrocarbons (saturated se-
Figure 11.—Open cup flash test apparatus.
ries) show a marked superiority over the other groups of petroleum products.
e. Cold weather starting and operation also require special consideration. It is not at all uncommon for internal combustion engines to be started cold at temperatures of 0° F. or below. A study of the A. S. T. M. chart (fig. 9) reveals that at these low temperatures oil viscosities reach surprisingly high figures, which, of course, necessitates a very high cranking torque in order to rotate the crankshaft. Furthermore, at certain low temperatures oil will congeal or chill completely so that circulation is impossible even if starting is accomplished by special means.
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(1) In regard to the question of whether a low pour point or a good viscosity curve (high viscosity index) is best for low temperature starting of internal combustion engines,, experience indicate© clearly that both are essential. The viscosity curve must be good (viscosity index about 95 or above is desired), and the pour point should be within 5° F. of the average starting temperature. The pour point of an oil is the lowest temperature at which it will pour or flow when it is chilled without disturbances. The oil to be tested is placed in a glass test jar and cooled in a cooling bath until it ceases to flow, when the test jar is removed from the bath and held in a horizontal position for exactly 5 seconds. A sectional view of a pour-test apparatus is shown in figure 12.
COOLANT
THERMOMETER
OIL
METAL
TUBE
BATH
CONTAINER
COOLING
Figure 12.—Pour test apparatus.
(2) Many ingenious methods have been employed for starting aircraft engines in cold weather, the most satisfactory of which appears to be the dilution of the lubricating oil with gasoline prior to stopping the engine whenever a cold start is anticipated. By adding aircraft gasoline directly to the lubricating oil, the oil viscosity is greatly reduced, and cranking is comparatively easy even at subzero temperatures. Upon starting an engine so prepared, the cold diluted oil circulates quite readily, providing adequate lubrication, and as the engine is brought to nomal operating temperature the gasoline evaporates, returning the oil to its original condition. Increased corrosion of some engine parts has been found, caused by the introduction of the corrosive ethyl fluid into the oil, but the better fluidity of the oil at starting temperature and the lower drain on electric or manpower for starting are believed greatly to outweigh any disadvantages. Uiis system of oil dilution, with proper modifications in the oiling system, provides the first real solution to the problem of maintaining comparatively low oil viscosities in cold weather to
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facilitate starting, and at the same time meeting the high temperature requirements of continuous operation.
12. Requirements of aircraft-engine oils.—a. The chief lubrication problems presented in an aircraft engine are the high temperatures encountered during operation, the high bearing pressures on working parts, and oil drag at starting temperatures. All of these conditions require an oil of high initial viscosity and preferably one of flat viscosity temperature characteristics. Only such oils will afford the proper cushioning effect, especially between heavily loaded parts to protect them from friction and wear. A high degree of chemical stability is also an essential quality for aircraft lubricants.
&. In order to fulfill the requirements of aircraft-engine operation at atmospheric temperatures ranging from —20° F. to 110° F., many grades of oil are sometimes prescribed. Much attention has been given to this problem in recent years, with the result that it now seems possible to meet all requirements with only two grades of oil of intermediate viscosity. Oils of especially high viscosities are apparently not necessary when suitable oil temperature regulators are installed in the lubrication system. By means of such regulators, oil temperatures are reduced in warm weather, with a consequent increase in viscosity which insures satisfactory lubrication. On the other hand, the light grades formerly required in cold weather are also unnecessary, since the system of oil dilution (par. 11) has removed most of the cold weather difficulties.
c. In summary, the desirable properties of an aircraft oil are as follows:
(1) High viscosity (90-130 Saybolt seconds at 210° F.).
(2) Flat viscosity temperature curve (high viscosity index).
(3) High flash point.
(4) Chemical stability.
(5) Low pour point.
d. Only oils refined from crude petroleum will meet all of the above requirements. Oils containing compounds other than petroleum hydrocarbons are in most cases unsatisfactory for continuous use.
13. Oil reclamation.—a. In service, oil is constantly exposed to many harmful substances which will in time seriously reduce its ability to protect moving parts. The chief agents of contamination are heavy ends of gasoline, acids, moisture, dirt, carbon, and metallic particles. Because of the accumulation of these injurious substances, it is common practice to drain the entire lubrication system at regular intervals and refill with new oil. This practice is open to the objection that it is not generally possible to determine exactly when the
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 13
oil should be drained. If changed too frequently, needless waste and expense will result, and on the other hand, if an oil is used too long, excessive wear and mechanical trouble are probable. Although there is no exact relation between the length of service of an oil and its condition when drained, regular periodic draining as generally practiced is a fairly safe procedure.
b. Oil filters of many types have been designed to remove the solid impurities from oil during circulation. Really efficient filters of the full flow type which may be readily cleaned are very satisfactory for removing solids, but they cannot remove the liquids such as acids and water. Part flow filters of the treated cotton waste type are efficient when new, but they usually require replacement after a moderate amount of use.
~~ Im । ^VAPOR trap
VENT TANK--
FILTER CHAMBER ‘ ~ 5 i|| 11 |H"~ M
SI
FILTER PAD-----' == =g= PUMP fT IggS?
CONDENSER-j J If IlCJ ||^lf ] l jFI
||M HM
\ .................I Ml I Ml
J = PILUTIQN TANKZ STRAINER , (aJ
= Kfl DILUTED OIL ' ' , ' -'
reclaimed oil
— ~== dilution VAPORS SETTLING DRUMS l|
. -- -.— '//////, DILUTION-PENETRATING OIL
\ RECLAIMED OIL DRUM " 0 U-----------
Figure 13.—Oil reclaimer.
c. A very novel oil supply arrangement has been developed for use with aircraft lubrication systems (dry sump with external supply). A small tank or hopper is incorporated within the main oil tank in the direct path of oil circulation. By this arrangement only a small part (10 percent or less) of the total oil supply passes through the engine. The new oil in the tank merely serves to replenish the supply in the hopper at a rate equal to the oil consumption of the engine. The advantages of the hopper tank are as follows:
(1) More rapid consumption of that portion of the oil in use in any given time, permitting less oxidation and other deteriorating effects.
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(2) More rapid warming up of the oil in use after starting.
(3) Less frequent oil change.
d. It will be noted that oil requires periodic change because of contamination and not because of any chemical change in the lubricant itself. Realizing this fact many concerns manufacture oil reclaimers similar to that shown in figure 13 to remove the impurities so as to return the oil to its original condition. These machines generally operate on the principle of distillation to remove foreign liquids and of filtration to remove the solid matter. Experience indicates that the most economical and practicable oil reclamation is to make no attempt at chemical refining, but simply to remove water, acid, fuel, solids, dirt, carbon, metal particles, etc. This can be done by any simple and reasonable efficient vacuum heating chamber, followed by filtration through tight felts or similarly effective filters, or by centrifugal separation of solids. The resulting oil may be slightly inferior in some respects to the original new oil; probably equal to the new oil after 2 or 3 hours of use. Chemical treatment of oils is practicable on a large scale, and if accomplished by experienced personnel with adequate technical supervision results in quite acceptable lubricants. Various stocks may be reclaimed, but if the charging stock is not controlled there will be a corresponding lack of control in the finished product.
14. Miscellaneous lubricants.—a. In certain cases because of special stresses or peculiar design it is impossible or unsatisfactory to lubricate units by means of engine oil. In these cases special lubricants (generally greases) are required. A great variety of such lubricants are available, each of which has certain special properties. For example, a grease for one purpose may require a resistance to the action of water, whereas in another unit a high melting point may be the most important. Other qualities often desired are low shearing stress, chemical stability, and the ability to support unusually heavy loads. To meet this condition a series of EP (extreme pressure) lubricants have been developed. They contain various active chemical compounds which protect parts from wear when temporary high unit pressures break through the normal film of lubricant. At normal loads the EP characteristic is not essential.
b. Since a large number of lubricants must be available in order correctly to service aircraft units and accessories, it is necessary to emphasize the fact that detailed instructions must be carefully followed. A certain lubricant is prescribed for an item of equipment only after extensive tests have been conducted. The substitution of another lubricant will normally be unsatisfactory and often danger
26
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 14-16
ous. For example, identical parts, such as bearings, used in different airplane accessories may require different lubricants. In one case engine oil may be specified, in another case high melting point grease is required, and in still another case graphite grease is correct, even though the bearings are identical. When lubricating airplane parts, the detailed instructions available in technical publications must be closely followed in regard to the type and grade of lubricant and to the actual lubricating procedure.
Section III
OILING SYSTEM
Paragraph
General______________________________________________________________________ 15
Oiling-system units________________________________________________________ 16
Typical aircraft-oiling system_____________________________________________ 17
Oil-dilution system_________________________________________________________ 18
Operating instructions_______________________________________________________ 19
Maintenance__________________________________________________________________ 20
15. General.—a. Although the complete oiling system of an aircraft power plant includes the lubrication system of the engine, only the external units of the system are shown in oiling-system diagrams of specific types of aircraft. For that reason this section deals only with the external units of the system. The internal engine lubrication system is treated separately under another heading.
Z>. The oiling system performs two functions. It provides for an adequate oil supply, the amount determined by the fuel system capacity, and it incorporates a means of cooling the hot oil discharged from the engine. It is designed and located in the aircraft to furnish oil by gravity to the inlet side of the engine pressure pump during practically all positions of flight except inverted.
16. Oiling-system units.—a. General—The major units in an oiling system include the supply tank, the necessary piping and connections, the oil temperature regulator assembly, the oil temperature gage, and the oil pressure gage. Some modern oil systems also incorporate an oil dilution system.
6. Supply tank.—The tanks are usually constructed of aluminum, aluminum alloy, or stainless steel and are of such design as to permit installation in the aircraft as close to the engine as possible. The ideal tank location is on the engine side of the fire wall with its center approximately 20 inches above the engine oil pump when the aircraft is in its ground position. Oil tank capacity should provide for 1 gallon of oil for each 11 gallons of fuel for air-cooled engine installa
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tions, and 1 gallon of oil for each 14 gallons of fuel for liquid-cooled engines. These figures do not include the oil in the piping, regulator, and engine, which usually totals from 1 to 3 gallons.
(1) The tank outlet is usually located in its lowest section in order to permit complete drainage while the aircraft is in ground position or in normal flight attitude. The outlet is so arranged that with the tank filled to one-half its normal capacity, it will not be uncovered in any normal flight attitude. The tank inlet line from the oil regulator enters the top of the tank and is of the same size as the outlet line. The two vent outlets lead from the top of the tank to the engine crankcase. The filler and cap unit are conveniently located for servicing and checking the oil supply in the tank.
7777222 22 7777'
ELBOW TUBINC
Figure 14.—Flexible oil hose connection.
(2) The supply tank used in conjunction with an oil dilution system incorporates a hopper or metal tube installed inside the tank in a vertical position, with the top of the hopper approximately one-half inch from the top of the tank and the base of the hopper extending down inside of the sump to within one-half inch of the bottom. The minimum oil flow area from the tank into the bottom of the hopper must be greater than the area of the oil tank outlet line. The hopper has a volume of 1 to 2 gallons of oil, depending on the oil flow capacity of the engine.
c. Tube fittings, oil lines, and drain cocks.—There are comparatively few fittings used in the plumbing of aircraft oiling systems. Other than a few street-ells and oil-gage fittings, the system is made up of large piping and flexible hose connections. The supply tank is usually secured to the engine mount in a more or less rigid frame. It necessarily follows that there is vibration, which is violent at times, between the fixed oil system units and the engine. This vibration is naturally concentrated in the oil system tubing and is conducive to fatigue failure in piping. This effect is considerably reduced by the use of flexible hose connections.
(1) Rubber hose is used for the flexible connections and is clamped to the tubing or fittings with standard hose clamps (fig. 14). The
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length of the hose exposed to the oil flow should not be less than 14 inch or greater than the tube diameter. Each hose connection used on the oil line connecting the scavenging pump outlet to the oil cooler is secured by four standard hose clamps because of high pressure exerted by the flow of cold oil when the engine is first started.
(2) Piping may be of copper or aluminum tubing. The wall thickness of copper tubing should not be less than 0.040 inch and of aluminum tubing not less than 0.050 inch. The ends of oil pipes are raised to insure a more positive attachment of the hose connections. Avoidance of excessively tightened hose clamps insures a properly installed flexible hose connection. Oil lines are painted with a band of yellow paint near each end for identification purposes.
(3) A drain cock (fig. 15) is installed to drain the system. One inlet of the cock is from the tank and the other from the engine pump, both draining into a common outlet. The cock is manually controlled by rotating a handle from the vertical position, which
Figuke 15.—Oil system drain cock.
opens the cock, to the horizontal position, which closes it. When closed the cock handle is locked in position to prevent movement. In an oil dilution system, an auxiliary valve is incorporated in the drain cock to provide a means of mixing a certain quantity of fuel with the oil.
d. Oil temperature regulator.—There are a number of sizes of oil temperature regulators in service to control the operating tempera
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It consists of a built-up core
ture of the oil. The operating temperatures must be kept within certain limits to control viscosity for proper lubrication.
(1) The oil temperature regulator, commonly known as the oil cooler, is placed in the oil system between the oil outlet of the engine and the oil inlet of the supply tank.
section surrounded by a metal jacket. A thermostatically controlled valve is incorporated in the unit to control the flow of oil automatically either through the core or around the shell, depending upon the temperature of the oil. The oil is cooled by the air moving through the core tubes while the oil passes around the tubes. Oil coolers are usually cylindrical in shape and are so designed that the relief valve assembly regulates the amount of oil forced through it to the supply tank. Various diameters of different core areas are used, depending upon the amount of oil circulated through the particular engine installation.
BY-PASS THERMOSTATIC
1^-^' RELIEF VALVE OPEN
IBl /j ~ OUT f/]\ r
--— BAFFLE \ \ \
I \ \
| OIL COOLER \ I
SHELL,_________yy
u 1
HS==7==S*P-DRAIN PLUG
Figure 16.—Oil temperature regulator with thermostatic valve open.
(2) A thermostatically operated valve is incorporated in oil temperature regulators to control the passage of oil automatically through the cooler. When the oil scavenged from the engine is comparatively cool, the thermostatic valve (fig. 16) is open, permitting the oil to flow around the core jacket to the top of the supply tank without flowing through the core. When the oil reaches a normal temperature, the thermostatic valve (fig. 17) closes and directs the oil through the core for cooling purposes. Although it is possible to vary the operating temperature of the oil slightly by incorporating a number of gaskets in the thermostatic regulator valve assembly, it
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must be assumed that these valves are nonadjustable. They are usually designed to operate and shunt the oil through the cooler when
BY-PASS THERMOSTATIC RELIEF VALVE CLOSED
|3B? If Oin/7\\
hj'Co-Q-KJ \u 0,l cooler
\GirG77y / / _ SHELL \7 / /
- ■1 V7
I?rT"'F---DRAIN PLUG
Figure 17.—-Oil temperature regulator with thermostatic valve closed.
the oil reaches a temperature of approximately 60° C. A thermometer bulb is installed in a well incorporated in the inlet line from the supply tank to the engine pump. It registers the inlet oil temperature on the temperature gage in the aircraft cockpit.
(3) Where a shutter assembly is installed in the air exit side of the regulator core, it may be operated to maintain desired oil temperatures. Where there is no provision for a shutter assembly, it is permissible in cold weather to blanket the core area partially with fiberboard or other suitable material in order to obtain proper oil temperatures.
e. Pressure gage.—The oil pressure gage in the aircraft cockpit is connected to the pressure line of the lubrication system in the engine. A restricted orifice is placed in the oil pressure gage line at the engine end. The gage line may be filled with instrument oil in cold weather, giving a comparatively rapid and true indication of oil pressure during engine warm-up.
17. Typical aircraft-oiling system.—a. By referring to the oiling system diagram (fig. 18), a general idea of the circulation of the oil to and from the engine may be obtained. Assuming that the supply tank has been serviced to its proper level, the oil flows by gravity through large tubing to the inlet side of the engine-oil pressure pump, which forces the oil under pressure through the internal
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lubricating system. The engine scavenging pumps return the oil
TAN K-----
£// / //Imax.
// X. // 1 ,V' I-1
// 1 //■ L-1 LEVEL
J \ff\ X> ^0IL
a 7/ //.. .1 V^VEVEL
yy /xl C0CK N0RMAL
/f/g WMf^G '^/7
7/ n
/t/ / J/ DILUTION VALVE
CONTROL "™" ~*
X/ Y oil drain
$ COCK ASSY J
/V^ FUEL PRESSURE °
GAGE
Figure 18.—Typical aircraft-oiling system.
from the engine-oil sumps to the top of the supply tank through the oil-temperature regulator. In most instances the crankcase is vented through suitable piping to the top of the supply tank. Two vent lines are used and are located so that at least one vent is open in all normal flight attitudes.
&. The thermometer well, for the installation of the thermometer bulb, is incorporated at a tangent in the oil piping between the outlet of the tank and the engine oil pressure pump; therefore, the indicator gives a temperature reading of oil entering the engine. The reading obtained at this point is more valuable than one taken from some other point in the oil return line.
c. The drain valve is also located in the tank to engine pressure pump piping at the lowest point to permit complete drainage of the tank and engine. It should be noted that the drain cock does not drain the oil from the oil temperature regulator, which must be drained separately by removing the plug located at the lowest point on the assembly.
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 18-19
18. Oil-dilution system.—a. Due to difficulties experienced in starting aircraft engines in cold weather, an oil-dilution system has been developed to dilute the oil immediately before the engine is stopped when a cold start is anticipated. Inasmuch as the high cranking torque of a cold engine is due to the high viscous drag of the oil, particularly between the pistons and cylinder walls, it is evident that a decided thinning of this oil immediately before the engine is stopped will greatly reduce the cranking torque and facilitate subsequent starting.
b. By referring to figure 18, the units comprising the oil-dilution system are shown, and with the exception of the hopper in the supply tank may be readily incorporated in any aircraft-oiling system. A line is connected from the fuel pressure line to a special Y drain cock in which a spring loaded poppet valve is installed. The valve is operated manually from the cockpit, and before the engine is stopped in cold weather a small amount of fuel is allowed to enter the oil-in line at this point by holding the dilution control open for a short time with the engine operating. This operation permits the diluted oil to replace the heavy oil throughout the entire engine, thereby facilitating starting in cold weather. Inasmuch as some of the diluted oil is returned to the hopper in the supply tank during the last minutes of operation, this diluted oil will be the first oil used at the next start, thereby insuring a more positive flow to the engine pump. A typical hopper installation is shown in figure 19.
(1) Because use of the oil-dilution system increases the combustible vapor in the discharge from the engine crankcase breather, a pipe is installed to conduct these vapors from the breather to the cowl line.
(2) With oil dilution, there is a tendency for the diluted oil from the tank to seep into the engine crankcase and into the cylinders when the engine is at rest. The only satisfactory solution to this problem is the use of a check valve installation in the strainer assembly in the engine. This valve is spring loaded so that the normal gravity pressure of the oil cannot cause seepage into the engine; however, it opens readily under the influence of pump pressure when the engine is in operation.
19. Operating instructions.—a. When servicing an oil tank, no attempt should be made to fill the tank completely, since a definite air space must be provided to accommodate expansion. In most tanks, the filler cap is so located that the correct level is automatically established. In some tanks a level cock is also provided to permit an accurate check of the filling operation.
420323°—41----5
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b. During engine warm-up on the ground it is most important to check carefully the operation of the lubrication system. When an engine is first started, the oil pressure reading will generally be very high due to the high oil viscosity. However, even at high pressures the rate of oil circulation may be quite low, so extreme care is necessary to prevent engine wear. In general, the engine r. p. m. should be kept low until a rise is observed in oil temperature, indicating
OIL RETURN _ / W. II .
TO HOPPER X. (w
FILLER UNIT TANK VENT \ TANK VENT
1 A 4
I! *IR VENTS
ii V. I1 I , 'I
\\ \\ii ।
\s x;*i Maximum oil ______ «--------------------- --------
level i normal
1 r’-X". OIL LEVEL
|l MAIN SUPPLY If ---------
[j TANK HOPPER
ASSEMBLY
!; H
FRESH OIL ||
•| MAINTAINS PROPER >,
> (f LEVEL IN HOPPER ’[
Jf= — = = r~l mb* ........’ =
OIL OUTLET
Figure 19. —Typical hopper tank.
proper circulation. Take-off can be safely accomplished after only a moderate period of ground operation, provided the oil pressure indication is steady and within the proper range, and the engine is otherwise satisfactory. Unusually quick take-off is possible with airplanes having oil dilution systems. Prolonged periods of ground operation are definitely harmful to most high output aircraft engines.
20. Maintenance.—The principal points to be checked in the maintenance of oiling systems may be enumerated as follows:
a. The tank should be inspected for proper mounting and for signs of leaks and dents.
34
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 20-22
6. The oil-temperature regulator supports should be checked and the core inspected for restrictions.
c. The oil-dilution valve linkage will sometimes bind and fail to operate properly unless carefully adjusted.
d. The oil tubing must be properly installed and supported, the hose connections must be in good condition, and the clamps should be tested for tightness.
e. Screens and filters accumulate a large amount of dirt and sludge and are, therefore, cleaned periodically.
/. AU plugs and drains must be carefully checked for leakage and for proper safetying.
Section IV
FUEL SYSTEMS
Paragraph
General_________________________________________________________________ 21
Basic fuel-system circuits______________________________________________ 22
Fuel-system units_______________________________________________________ 23
Typical fuel-system circuits___________________________________•________ 24
Operation_______________________________________________________________ 25
Maintenance_____________________________________________________________ 26
21. General.—a. A well-designed fuel system must provide for the storage of the required amount of fuel in the available space within the airplane structure, and for the delivery of the fuel to the carburetor at the proper rate and pressure. The system must be positive and reliable under all conditions of flight and, if possible, simple in operation. Indicators, such as the fuel-pressure gage and tank-contents gages, are installed to give a continuous indication of the functioning of the system.
b. The continued development of military airplanes has been accompanied by many new problems relative to fuel systems. The practice of installing numerous small tanks rather than a few larger ones has resulted in increased plumbing and complication, but, has also permitted a more efficient use of the available fuel storage space. The use of external superchargers also increases the demands on the fuel system, especially in regard to pump capacity at high altitudes. The trend of late developments in carburetors is also toward increased pressures for the delivery and discharge of the fuel. It is obvious that the fuel system of a modern airplane is a very complicated arrangement requiring careful installation, adjustment, and inspection.
22. Basic fuel-system circuits.—Since each particular fuel system has certain special features, it is necessary to begin the discussion by considering first the simpler designs.
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AIR CORPS
a. The gravity fuel system (fig. 20) though elementary in design is still in use on a number of training-type airplanes. The advantages
Water ^SumB^^atn
Primer —---------_j==r^=»«.
JL f--...............—-p-j
Fuel Code
II / Carburetor
I I Ctrai/ver
Figure 20. —Gravity fuel-feed system.
are simplicity and reliability, but this system cannot be used on tactical airplanes because of structural arrangement and higher pressure requirements. The actual pressure available from a gravity system can be calculated as approximately 1 pound per square inch for each 40 inches head of fuel. Thus, it may be estimated that in order to produce a delivery pressure of 3 pounds per square inch, a vertical head of 120 inches of fuel is necessary.
&. Figure 21 is a schematic diagram of a complete fuel system of a relatively simple type. Certain additional units have been intentionally omitted in order that the basic principles can be more clearly indicated. In tracing the fuel flow, attention is called to the location of the main fuel strainer and the arrangement of the two fuel pumps with a single relief valve. The use of an air vent line from the air intake to the pressure gage and relief valve is also significant. The direction of fuel flow from supply tanks to the carburetor is clearly indicated by arrows near the fuel lines. The exact pressure generated by the fuel pump depends on the adjustment of the relief valve and may be from 3 to 15 pounds per square inch according to the type of carburetor used on the engine. In general, the proper fuel pressure for a float type carburetor or mechanical fuel injector is approximately 3 pounds per square inch; for a diaphragm type variable venturi carburetor, 5 or 6 pounds; and for a pressure injection carburetor, 13 to
36
v»ayiorct I5ros., Inc. «
Makers flj
TM 1-407
AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 22-23
Pressure.
Air Pressure —, ___Jrq?
Fue/-A'ne S' X
/AirIntake / yr(j£[_ \
( X Vafre / TANK
{ p==5 ]
CARB. y TVZZ 11 V J
// W*5 t di
' ^IftraZn&r
Figure 21. —Pressure fuel-feed system.
15 pounds. In all cases, these figures represent the desired differential fuel pressure, and the pressure gage is connected so as to indicate this difference. In many airplanes the carburetor air pressure is appreciably higher than the normal atmospheric pressure,, the increase being delivered by a ramming air intake or by an external supercharger.
23. Fuel-system units.—In order to clarify the principles of operation of complex airplane fuel systems, the various units are discussed separately in the following paragraphs:
a. Tanks.—Fuel tanks are usually constructed of aluminum, aluminum alloy, or stainless steel. The general design is indicated in figure 22, but the shape is variable, depending on the airplane for
SP/LL EASZZV^c^^ ^venT
DRA/N^ E1AZZVT/.ZVK
FINGER
SCR££N
reserve ' ;; !
-----cTg&VZTYOPERATED VALVE
tS/irf 44 - («ii
SUSP COVER
<£TzN£>FR0F1 F1AIN TANK
LMEFRONRESERVE TANKSCREW EMZN COCK
Figure 22.—Sectional view of fuel tank.
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which it is designed. Large tanks require internal baffles for increased rigidity and to prevent objectionable surging of the fuel in flight. In order to counteract corrosion in aluminum and aluminum alloy tanks, a small capsule containing potassium dichromate is installed near the bottom of the tank.
(1) The fillernecks of fuel tanks are so installed that an expansion, space is automatically provided when the tank is serviced. A vent line from the top of the tank leads overboard so as to reduce the danger of fire from fuel or vapors which may be discharged. It is highly important that this vent line be properly installed and free from any obstruction.
(2) Most fuel systems provide a reserve fuel supply, either by the use of a separate reserve tank or by the arrangement shown in figure 22. In either case, the reserve fuel supply is adequate to operate the engine for at least 20 minutes at full rated power.
(3) Some airplanes, especially long-range types, may be equipped with special fuel tanks in order to increase the cruising range for a particular mission. When such tanks are serviced, attention must be given to the total weight of the airplane in order to prevent overloading.
(4) Water may occasionally be found in fuel tanks, either as a result of condensation or careless servicing, therefore, regular drainage of the tank sumps is most important.
b. Lines and fittings.—Fuel lines are generally made from copper or annealed aluminum alloy tubing, the latter being more common on late-model airplanes. The tubing size is governed by the fuel-flow requirements of the engine. Except for the lines between flexible connections, tubing should be properly supported by clamping to structural members. A band of red paint near each end serves to identify a fuel line. Connections between tubing and fuel-system units are made by means of pipe fittings, solderless flared tube fittings, hose connections, or by a combination of these methods.
(1) Standard pipe thread fittings are shown in figure 23. It will be noted that all threads on these fittings are tapered (% inch per foot), and therefore when screwed together make a leak proof connection by the sealing action of the threads. When installing these fittings, no attempt should be made to turn them until the threads are completely engaged. Permanent joint compounds are not required and are generally forbidden. Approved lubricants which do not harden will, however, be found beneficial in making pipe connections. It must be understood that these materials function purely as lubricants and not as sealing compounds for improperly installed fittings.
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
Many fuel-system units, such as fuel tanks, fuel cocks, strainers, pumps, and carburetors, are provided with female pipe threads for the installation of various line fittings.
© COUPLING - TUBE
(2) NIPPLE • CLOSE
(3) BUSHING-PIPE-REDUCING
@ ELBOW-45*
(5) ELBOW-STREET
© ELBOW- PLAIN
© TEE - PLAIN
NOTE: ALL THREADS SHOWN ARE NAT'L ST’D PIPE THREAD.
Figure 23. —Common pipe fittings used in airplane plumbing.
(2) For connecting thin-walled tubing to pipe fittings, the three-piece solderless fittings (fig. 24) are commonly employed. To make this type of connection, the tube is cut to proper length and a nut and
/^-NUT
/“SLEEVE /-TUBE
Figure 24. —Flared-tube union.
sleeve are placed over the tube. The tube is then flared at the open end by an appropriate tool, and the nut is screwed on a nipple, elbow, or similar fitting. In this case it must be noted that the seal is between the tube and fitting, at the flare, not in the threads. A small amount of lubricant on the threads will facilitate tightening, but the use of heavy sealing compounds is unnecessary and possibly dangerous.
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(3) Wherever there is relative motion between two points with a connecting line, a flexible hose connection (fig. 25) is used at each point.
M —-CLAMP
-- —Trai /-hose ^^7^^^T>zz/zzz zz/zzz/zzzZ
y _ ^==g- --C-............
\ / ^-HOSE
K—-TUBING—
Figure 25. —Hose connection.
(4) Cone union fittings which are attached to copper tubing by high melting point solder may be used in fuel systems, but the solderless type is generally preferable.
c. Cocks and dials.—Fuel cocks have a number of uses in airplane fuel systems. They perform such functions as tank selectors, engine selectors, cross feed valves, etc. The size and number of ports of fuel cocks naturally vary according to the type of installation for which they are intended. Fuel cocks must have the full flow capacity of the fuel lines, must be free from leakage, and at the same time operate with little effort. These requirements have produced considerable development on these units, with the result that satisfactory designs are now available.
(1) Fuel cocks having a cone-shaped cock rotor have been used quite extensively in fuel systems and have been fairly satisfactory. The principal disadvantage of this type is the high torque required for operation, resulting in a rather uncertain feel of the operating control. An improved model uses a cam arrangement for unseating the cone between index positions, thus improving the action. External clearance adjustments are provided on this type which must be maintained within proper limits.
(2) A metal cock with synthetic rubber inserts around the ports is also available. This type is simple, operates freely, and provides good sealing properties.
(3) The dial used in conjunction with a fuel cock has the proper number of positions for the particular installations. A diagram of a fuel cock and dial is shown in figure 26.
d. Strainers.—The main strainer installed in a fuel system is a most important item. Its function is to prevent dirt or other foreign matter from entering the fuel pump and carburetor, and by virtue of
40
y!o m BJOS”,nc-
Makers ||jHggjHMHMBliHHilMHlBiiiMHBliMMHiHlHHMlMiiiNMIHi
TM 1-407
AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
INLET
Figure 26.—Fuel cock and indicator dial.
t
~in (A UXOR E S v zll\ /
%^Wu\y \Zoj F\/
its low position to trap any small amount of water which may be present in the system. A drain is provided at the bottom for fre-
OUT
DRAIN
Figure 27.—Fuel strainer.
quent draining, and the entire screen may be easily removed for cleaning. Figure 27 shows a sectional view of a common fuel strainer.
420323°—41----6
41
SCREEN
TM 1-407
23 AIR CORPS
(1) Strainers of fairly coarse mesh are installed in the fuel tanks on the outlet connections as shown in figure 22. Their function is quite obvious and requires no particular comment.
(2) The carburetor body generally incorporates a small strainer of sufficient capacity to stop foreign matter which passes through the man strainers as well as particles which are introduced when fuel lines are removed. The carburetor strainer usually requires less frequent cleaning than the main strainer.
e. Pumps and relief valves.—Fuel pumps are required to furnish fuel prior to starting an engine and to deliver a continuous supply at the proper pressure at all times during engine operation. The various pump types are listed separately as follows:
(1) The hand pump (often called “wobble” pump) is generally located fairly near other fuel-system units and is operated by suitable levers from the pilot’s compartment, The common design is very reliable and quite simple in operation. By reference to figure 28, the path of fuel flow is easily followed. It will be noted that the
OUT
/ff \
\\\ 1 Gravity Operated yates
Figure 28.—Hand fuel pump (wobble pump).
chambers which are not delivering fuel are being filled and will discharge when the handle motion is reversed. These pumps must be installed in a vertical position in order to secure proper action of the check valves. The inlet and outlet connections are in most cases marked on the pump body.
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
(2) The most common engine-driven fuel pumps used in airplane fuel systems are of the eccentric sliding-vane type. Figure 29 shows the principle of operation of this type of pump. When the pump rotor is turned in the direction indicated, the vanes move the fuel from the inlet to the outlet of the pump. Since the pump is symmetrical about a vertical axis it will pump in either direction with equal efficiency. Reversing the direction of rotation has the same effect as changing the pump on the mount 180°. Most pumps of
COUPLING LUBRICATE
J 0R|Vt
lN । 'p ~~
9L,DINC VANES dZn
Figure 29.—Engine-driven fuel pump (vane type).
this type are plainly marked to indicate the discharge port for either direction of rotation. At the point where the drive member enters the pump body it is necessary to provide a seal to prevent fuel leakage. The seal is generally a hardened steel member held by
Figure 30.—Combination fuel pump, relief valve, and by-pass valve.
end pressure against one pump bearing. Very little trouble is experienced with this type seal in service; however, a drain line is provided to drain fuel in case the unit becomes worn or distorted.
(3) The action of a relief valve is best explained by considering its operation in conjunction with a fuel pump (fig. 30). In order to
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furnish a constant fuel supply to the carburetor under all conditions, a fuel pump is designed to deliver much more fuel at any speed than the engine actually requires. Therefore, a spring-loaded relief valve is placed parallel with the pump in order to relieve the surplus fuel to the intake side of the pump. By adjusting the spring tension in the relief valve, the differential pressure generated by the pump is accurately controlled. The bellows or diaphragm incorporated in the relief valve is essential for two reasons: It provides for air venting from a ramming air intake or external supercharger, and by its balancing action helps to maintain constant discharge pressure
PRESSURE ^.ADJUSTMENT
AIR V£nt diaphragm
A FUEL PRESSURE rf
W//// ■' <7/y//777>>. ka
IN 0UT IN0UT
J//j _____ ___ /Sy
SLIDING^- SLEEVE
MINIMUM OUT-PUT MAXIMUM OUT-PUT
Figure 31.—Variable-volume fuel pump.
regardless of variations in pressure on the suction side of the pump. When not connected to air pressure, the interior of the bellows or diaphragm is left open to atmospheric pressure through a small restricted fitting. When properly connected, a single relief valve will accommodate both the hand pump and engine-driven pump. A bypass feature, wThich permits fuel flow through the valve assembly, is also provided so that the unit may be used in varied fuel-system applications.
(4) In certain installations the escape of fuel through the pressure relief valve has aggravated vapor formation in the system with resultant loss of pumping efficiency. This effect has been most noticeable in systems using high-pressure carburetors and/or external superchargers when operating at high altitudes. The escape of fuel from the high pressure side of the pump through the relief valve often causes vaporization of fuel in the passages. To remedy this
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■r Gay!ord Bros-> Inc.
Makers
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
condition, a variable-volume fuel pump having no relief valve may be used. In this type of pump a heavy diaphragm, actuated by fuel pressure, regulates the position of the pump sleeve with respect to the rotor. Figure 31 illustrates the principle of the variable-volume pump. Note that the actuating force on the diaphragm is fuel discharge pressure against an adjustable spring. By this arrangement, the pump sleeve assumes the proper position to deliver the required volume of fuel to the carburetor at the proper pressure. Any variation in pressure from the correct setting will produce an immediate readjustment of the sleeve position. The space above the diaphragm is normally vented to the carburetor air intake pressure in order to maintain correct differential pressure. The general installation and hook-up of the variable-volume pump are quite similar to pumps having relief valves.
S-----— FLEXIBLE DRIVE HOUSING
X. __________________________________________^.r. rTH—/I FUEL PUMP
MH C '—FLEXIBLE DRIVE —----------------
I 90* COUPLING
-----ENGINE DRIVEN
Figure 32.—Remote flexible drive-shaft assembly.
(5) The handling of volatile aircraft gasoline requires special care in the location of the fuel pump in the plumbing circuit. In general, the pump should be below the fuel tanks and as near to them as possible in order to avoid vapor locking difficulties. This arrangement often places the pump several feet from the engine and necessitates a remote pump drive.
(tz) The most common pump drive is illustrated in figure 32. The essential units are a flexible drive shaft and the necessary adapters and couplings. Although the drive assembly requires periodic inspection and lubrication, it has contributed to definite improvement in fuel pump performance. This type of drive is suitable only for installations in which the pump is located a relatively short distance from the engine.
(Z») In larger airplanes the desired pump position may be several feet from the engine, and so a hydraulic pump drive is often installed. By reference to figure 33 the operating principles are easily under-
45
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TM 1-407
23 AIR CORPS
----------------—
FUEL TANK
ci. P .. ................: --==^/=--------------1/
CARB. <
r~P
*1/ HYDRAULIC FLUID 'S--j ( ) 1 ..
Z __JUPPLY TANK L O J_fue_l pump ___J
_____________J L________________—________STRAINER Sk
/z '' k--------
I \ GENERATOR MOTOR / I
Figure 33.—Hydraulic fuel-pump drive.
Gaylord Bros., Inc.
Makers
TM 1-407
AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
stood. The hydraulic circuit is entirely independent of other hydraulic units on the airplane. This arrangement permits a most favorable location of the fuel pump and consequently greater reliability in engine operation. An interesting modification of the hydraulic drive is the addition of an oil regulator to vary the flow of hydraulic fluid according to the fuel requirements of the engine. This variable speed hydraulic drive has been used quite effectively on certain airplanes. It is important to distinguish clearly between the variable speed fuel pump and the variable volume pump.
(^/ I ------~ ’_d
LBS. PER SQ. IN.
Figure 35.—Fuel-pressure gage (sectional view).
nection is vented to the carburetor air intake and the fuel connection is connected to the fuel-pressure chamber of the carburetor. Thus, the gage indicates the difference between the fuel pressure entering the carburetor and the air pressure at the carburetor inlet. In some systems having no external supercharger or ramming air intake, the air fitting on the gage is open to normal atmospheric pressure. In order to dampen pressure impulses which cause pointer fluctuation, a restricted fitting is installed at the carburetor end of the fuel-gage line. The correct fuel pressure value depends upon the type carburetor, but the values mentioned in paragraph 22a(2) are approximately correct. In this connection it must be remembered that the fuel-pressure gage indicates the pressure existing at the carburetor only if it is installed at the same level as the carburetor. If the gage is located above the carburetor, the reading will be lower than the carburetor pressure, and if the gage is below the carburetor it will read higher than normal. In certain airplanes, correction must be made for this factor in order to determine the exact fuel pressure at the carburetor inlet.
A. Primer pumps.—The standard primer pump is essentially a manually operated piston pump with inlet and outlet check valves (fig.
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
4-.
36). Fuel for the primer may be supplied from almost any point in the fuel system. The fuel discharged from the pump is generally injected into a number of intake pipes on the engine, in most cases into the upper cylinder pipes on the radial type of engine. The procedure to be followed in priming engines varies according to the type and the atmospheric temperature, a greater priming charge being required in cold weather. When not in use the plunger should be placed in the “off'’ position to prevent fuel from flowing into the induction system when the engine is in operation.
i. Vapor eliminators.-—In many fuel systems an appreciable amount of fuel vapor is often present in the fuel pressure line connected to the carburetor. The vapors are due chiefly to the action of the
Figure 36.—Engine primer.
mechanical pump, especially when the pressure on the pump inlet is quite low. A float type carburetor is not disturbed to any great extent by a small amount of fuel vapor, because the float chamber vents permit the vapor to escape. However, many high-pressure carburetors and mechanical fuel injectors will malfunction due to vapor formation. To remedy this condition vapor eliminators are employed. The general internal mechanism of a vapor eliminator is shown in figure 37. The unit is normally installed in the pressure line immediately preceding the carburetor or injector. In operation, the upper part of the housing contains a small amount of vapor and the lower part contains fuel. When the vapor content increases, the float drops slightly thus opening the needle valve and permitting some vapor to escape. As the vapor escapes, the liquid level rises and the float closes the needle valve to prevent a loss of fuel. The needle valve operation is generally rather intermittent but entirely
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Vapors Outlet to tant
£'v« S Seat
rh
-3px'
--^3
i PH I I I
\ UILLL ;J |
Drain
Fuel Outlet
Figure 37.—Vapor eliminator.
effective in disposing of fuel vapors. The outlet connection returns the vapor to one of the fuel tanks. Some carburetors are built with a small vapor eliminator as an integral part of the carburetor assembly.
pLiquldometevs.—Tank contents gages or liquidometers are practically all of the float type but differ in methods employed in transmitting the indication to the instrument panel. Three methods are in common use: direct mechanical type, hydraulic type, and electrical type (figs. 38, 39, 40). Adjustments are normally provided
I . I
~==4U
Figure 38.—Mechanical liquidometer.
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uaylord Bros., Inc.
Makers
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 23
on all types for both range of operation and pointer position in order that the indicator will be accurate throughout its scale. Each type has particular methods of adjustment so a general system cannot be given. Some designs employ a single indicator with a selector switch for a number of individual tank units.
I JfIO I*—GAGE
<^W'*i'FLEXIBLE LINE
ACTUATING MECHANISM
/ FUEL ' \ I
TANK
1 ZV /
I ' // /
_______ FLOAT__________
| A DRAIN
Figure 39.—Hydraulic liquidometer.
Vo,t99a Compensators \ Fu*' Caf,tent
I y '”%*•* //
—cLwvXv---------------Hr------------------------) Varl!,ble Resistance
Resistance Unit ---------b-------------Ftoot Aetuateei
»o £■•*? ____________________________^2—_______________
Xlr. Switch Manual adjustment / k
Hlllll-
Batterj
Figure 40.—Electric liquidometer.
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AIR CORPS
17^ ] WARNUG SIGNAL LIGHT
I ZTX FUEL PRe55UffE CACB
FUEL PRESSURE SIGNAL^4 J J
ff | I ___________________ ft
(r7 77^ zia
\p / \ FUEL TANK | f- • - R T | FUEL TANK J
N V~ / X'^~— < J y
Nl—/ •3-_ I \ \ \ \ I I SUMP & DRAIN
CAPS / X \ \ \\ _______ ) Ml m-XX \\\vwv.
|| q II I FueLpump incorporating!
<-‘....- ■■ - FUEL OUTLET
fl0*t-d Zii II H yi
/ ) ) The automatic mixture control operates on the same principle as the manual type but requires no particular attention from the pilot. The operating mechanism consists of a sealed bellows which responds to changes in pressure and temperature of the air entering the carburetor. (See fig. 53.) At low altitudes the tapered valve is off its
— LEAN ceo,000 FT.»
Figure 53.—Automatic mixture control.
seat, thus giving the proper fuel flow. With an increase in altitude the reduced pressure causes the bellows to expand, and the valve moves nearer its seat to provide correct mixture compensation. The temperature correction feature is obtained by sealing a certain amount of gas in the bellows.
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AIR CORPS
(■1 |w! 1 I
^SSSSSSsiSSZSSSSSSSSSS^SSSSSS&SSSSSSssss^xsS^^ W $
MAIN JET \ J^n-IROTTLE VALVE
Figure 56.—Typical float-type carburetor.
maneuvers and is quite subject to ice formation. Float type carburetors are built in many individual models in both updraft and downdraft types. Separate main and idling metering systems are generally incorporated.
(2) The variable venturi construction (fig. 57) is also in common use. The specially shaped throttles operate in synchronization and in a definite relation to the main discharge assembly. A strong venturi action is produced by the flow of air between the throttles and the fuel nozzle at all engine speeds. As the throttle opening is
JirrMIXTURE control
VX 17773-------------------rzzg METERING PIN
*^7/ //ZZ/7~77/\■ O O O O d <™R<,TTU,! CONTROLLED!
DIAPHRAGM 'jtq-1--A 7/774.
6 U 77%/P^X 'i | '1 / IDUE **” 61X60
/ if \ ADJUSTMENT
/ / ] 7 LAyuL'W JVJ opEN I Cj) X/
\ / INJECTOR CONTROL A-.TA L_y 7k
[j। t/
Figure 61,—Arrangement of fuel injector controls.
movement of the control knob actuates both levers to the same degree thus controlling engine power output. At idling speed, the air throttle is nearly closed and the injector delivers a small quantity of fuel, but as the throttle is opened both the air flow and fuel flow are increased in approximately the same proportion. A cam is required in the air-throttle linkage to insure correct fuel air ratios at all engine speeds.
b. Individual control of either the fuel or air flow may be obtained in order to meet the requirements of starting, stopping, and altitude operation. This requires that the fuel-control lever be moved without disturbing the setting of the air throttle, so that the mixture may be made rich or lean. The cockpit control is designed to fulfill these conditions. The knob on the control shaft may be rotated so as to increase or decrease fuel flow and thus control mixture proportions. This arrangement replaces the separate mixture control used in conjunction with aircraft carburetors.
c. The injector control knob furnishes a convenient means of starting or stopping aircraft engines in addition to its function as an altitude or mixture control. In starting, the air throttle remains
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 30-38
nearly closed, but the injector output may be increased to provide the necessary priming effect by placing the control knob in its rich position. To stop the engine, the knob is rotated to the full lean position. This causes the engine to stop promptly without afterfiring and effectively clears the cylinders of the products of combustion.
d. Although the mechanical control system gives satisfactory results when properly operated, a fully automatic control is highly desirable. Such a control will adjust the injector according to the air consumption of the engine at any altitude or speed, thus relieving the pilot of a rather difficult task. Furthermore, an automatic control will protect an engine from the serious consequences resulting from improper adjustment of the manually operated control knob.
37. Fuel injection advantages.—As experiments with fuel injection have progressed, it has become evident that in addition to eliminating certain troubles common to carburetors many other advantages are also possible. Briefly, the principal points in favor of mechanical fuel injection are as follows:
a. Injection operation is not affected in critical positions or by violent aircraft maneuvers. This is quite evident since the injector contains no sensitive floats or needle valves.
b. Increased power is generally obtained. This may be attributed chiefly to the fact that no restrictions such as heaters, venturi tubes, etc., are required in the air intake passages. Full ramming air intakes are commonly employed. Lower mixture temperatures under certain conditions also contribute to increased power and reduce the danger of detonation.
c. Accurate metering of the fuel charge to each cylinder results in improved distribution; therefore, smoother engine operation is generally obtained.
d. Induction-system icing is not present in injection systems. This may be explained by the fact that fuel vaporization, the principal factor in causing ice, is not concentrated in one area as in the case of carburetor barrels. Instead, a small quantity of fuel is vaporized intermittently in a number of intake pipes. At no time will the mixture temperature be low enough to cause precipitation of moisture in the form of ice. This factor removes a constant danger present in many carburetor systems when atmospheric humidity is high. With this danger removed, air-intake heaters are no longer required, thus eliminating one control in the pilot’s cockpit.
38. Maintenance.—Fuel injectors are constructed in such a manner that it is impossible to mount them incorrectly on an engine. The mounting bolts also function as fuel feed lines so that correct
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hook-up is assured. When installing an injector, a special gasket is used between the mounting flange and the engine. The drive coupling is generally machined off center so that correct timing is insured. A timing pin is also provided which indicates injection to number one cylinder. In service, an injector system should be checked for fuel leakage, proper timing, and for correct synchronization of the fuel and air controls. Any free movement or backlash in the control system must be eliminated in order to insure proper fuel mixtures at various engine speeds. Fuel filters located in the injector body require occasional cleaning. Fuel-injector systems are, in general, considered quite satisfactory in regard to service maintenance and adjustment.
Section VII
SUPERCHARGER SYSTEMS
Paragraph
General________________________________________________________________ 39
Types of superchargers_________________________________________________ 40
Gear-driven supercharger----------------------------------------------- 41
Exhaust-driven supercharger-------------------------------------------- 42
Supercharger control___________________________________________________ 43
Supercharger engine efficiencies_______________________________________ 44
Maintenance____________________________________________________________ 45
39. General.—a. (1) The subject of supercharging is based on a study of mass, volume, and density as applied to the properties of gases. Like liquids and solids, gases have weight, but unlike liquids and solids their weight is not of constant value under all conditions. For example, at sea-level pressure it requires approximately 13 cubic feet of air to weigh 1 pound, but at a higher pressure the same volume will be considerably heavier. For practical purposes, mass may be considered as identical with weight, that is, a measure of true quantity. Mass is not to be confused with volume, since volume merely designates the space occupied and does not consider pressure or density. The relation between these factors is explained in certain laws pertaining to the behavior of gases. At constant temperature, the relation between volume and pressure can be best shown by a study of a definite quantity of air in a closed cylinder fitted with a movable piston (fig. 62).
(2) Assuming constant temperature and no leakage past the piston, it is apparent that volume and pressure are inversely related (Boyle’s law). The mass or quantity of air below the piston is the same in all cases, but as the volume is changed the pressure will be affected. Density, or the mass for a given unit of volume, is also explained by a consideration of these properties. In figure 62 (T), if the density of
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 39
the air is taken as standard, the air in (2) will have a density of 2, and the air in (3) will have a density of only These facts have an important bearing on the performance of an internal-combustion engine, as the power developed depends principally upon the mass of induced charge. A nonsupercharged engine is able to induce only a definite volume according to its piston displacement and volumetric efficiency; therefore, in order to increase this mass of charge it is necessary to increase the pressure and density of the incoming charge by the use of a supercharger. Therefore, the function of a supercharger is to increase the quantity of air (or mixture) entering the engine cylinders.
&. (1) The elastic property of gases is also observed when temperature changes occur. If a given quantity of any gas is heated 1° C. the gas will, if not confined, expand 1/273 of its former volume (Gay-Lussac’s law). If heated 273° C. under the same condition, the eras will expand 273/273 or will simply double its former volume, and the density will be reduced to one-half the original value (fig. 63).
v"- ] 20CU.IN.
10 CU' IN~ 5CU.IN.'
'5 LB|NCMER SQ’ 30 LBS PER SQ- 7 !/2 LBS- pER SQ
INCH INCH Z |NCh
® ® ®
Figure 62.—Relative volumes and pressures.
I a
...' FREE TO MOVE
10 CU. IN. 20 CU. IN.
AT 0° C. AT 273° C.
® ®
Figure 63.-—Effect of temperature on gas volumes.
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(2) If a gas is confined so that free expansion cannot occur, an increase in temperature will result in an increased pressure (fig. 64). The relation between temperature and density must be considered in the operation of internal-combustion engines in order to insure the maximum power output.
c. The weight of the earth’s atmosphere is sufficient to exert considerable pressure on objects at sea level. At altitudes above sea level the pressures will not only be lower but the density of the air will also be reduced. At an altitude of 20,000 feet the pressure and density of the atmosphere are only one-half of the sea level value. Superchargers were originally developed to increase the density of the air taken into the cylinders at high altitudes so that full power
15 LBS. PER. SQ. INCH ©
30 LBS. PER. SQ. INCH ®
Figure 64.—Effect of temperature on gas pressures.
output could be realized, and many superchargers are still employed for this purpose. However, with improved engines and better fuels, it is also very profitable to utilize a supercharger at low altitudes to increase the induction system pressure (and charge density) far above the normal atmospheric value. At one time, superchargers were considered merely an engine accessory but are now a vital part of every high-output engine.
40. Types of superchargers.—Although many types of superchargers have been designed, those illustrated in figure 65 give best results on modern engines.
a. The Roots type supercharger (fig. 65®) is generally mechanically driven from the engine crankshaft at a moderate speed. It is fairly efficient but is usually somewhat heavier than other types of equal output and may offer some problems in lubrication. How-
84
FIXED PISTON
* © i * |
TEMP O’C. TEMP
273° C.
Gaylord Bros., Inc.
— Matters
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 40-41 ever, the Roots type may be used with good results on certain engines. The sliding vane type supercharger (fig. 65®) is also satisfactory but appears to be less desirable in most cases than the centrifugal type (fig. 65®).
ROOTS SUPERCHARGER ©
VANE SUPERCHARGER
®
Figure 65.—Types of superchargers.
CENTRIFUGAL SUPERCHARGER ®
b. The centrifugal type supercharger is remarkably efficient for aircraft engines, as it is simple, has few parts, and can be driven at rotative speeds far higher than would be permitted with other types. It is interesting to note that the centrifugal unit was first installed in radial type engines in order to overcome difficulties in charge distribution and not to increase the charge density. As better aircraft fuels were developed, the natural procedure was to increase the gear ratios to obtain much higher manifold pressures. This higher supercharger output may be utilized either in producing a greater power for take-off or in obtaining a higher altitude rating. Centrifugal superchargers may be driven either through a gear train or by an exhaust gas turbine as described in subsequent paragraphs.
c. In addition to their construction features, superchargers are also classified according to their location in the carburetion system. An impeller located in the induction system between the carburetor and engine cylinders is known as an internal supercharger, and when located on the air inlet side of the carburetor it is classified as an external supercharger.
41. Gear-driven supercharger.—a. As previously mentioned, the gear-driven supercharger employed in modern aircraft engines is located in a suitable housing in the induction system between the carburetor and intake pipes (fig. 45). A manifold pressure gage installed in the induction passageway between the supercharger out-
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let and cylinder intake ports registers the output of the supercharger. Inasmuch as the capacity of the supercharger is determined by the speed at which it is rotated and by the position of the carburetor throttle, its maximum output is attained only at maximum r. p. m. of the engine and at wide open throttle position.
b. In order to improve the general performance of aircraft engines having gear-driven superchargers, two impeller speeds may be used. This is accomplished by incorporating a gear shifting control which is set in high or low position as required. The low ratio is used follow altitude operation, which requires a relatively low supercharger output; however, when a certain altitude is reached, the control is placed in the high position in order to increase impeller speed and maintain relatively high manifold pressures. An appreciable improvement in engine power and airplane performance is obtained by the use of a two-speed supercharger, but it must be remembered that the high ratio may be used only under proper conditions. Below a specified altitude the use of high ratio is quite likely to result in detonation and possible damage to the engine. Detailed instructions must be closely followed in operating engines equipped with two-speed superchargers.
42. Exhaust-driven supercharger.—a. The advantage of an exhaust driven external supercharger over a mechanically driven external supercharger for installation on the air inlet side of the carburetor is due to the comparative low power required for its operation at the higher altitudes. The power required to drive a gear-driven supercharger remains constant, but the power to drive an exhaust supercharger decreases with altitude. By referring to figure 66 it will be noted that the turbine wheel which drives the supercharger impeller is driven by the pressure of the exhaust gases expelled from the engine cylinders. Assuming that the exhaust waste gate is closed and the engine is operating at a given speed, the speed of the turbine wheel is determined by the rate of flow of the exhaust gases through the nozzle box. As the atmospheric pressure decreases with altitude, it becomes evident that the exhaust gases will flow through the turbine wheel more freely with increased altitude, resulting in higher supercharger efficiency. In general practice the waste gate is open at low altitudes and closed at high altitudes. An intercooler of proper capacity is installed on the outlet side of the supercharger to reduce supercharged air temperature.
b. Many airplane installations include both a gear-driven internal supercharger and an external turbo supercharger, which give two
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 42-43
stages of pressure rise in the induction system (fig. 67). This combination develops adequate induction pressures at high altitudes for unusually good airplane performance. An intercooler between the two impellers provides the necessary drop in air temperature to prevent overheating and detonation.
COLO AIR / !
SCOOP y iTlrCX^—/ EXHAUjT^ MANIFOLD
\ i LtwAZY 1 INTERCOOLER
\ 'a_ y%L////n Yz-—x—5>jCATB
A,R JMi b
OUTLE^ X |
( __L Pl rm i xb|-| TURBINE
k n (/%/// --| -. -—' BUC KE T 3
dwT..1BEARING spp \
1— ■ 25E-_--,-.____________P
j H Z_T URSINE'
--' ' NOZZLE BOX
Figure 66.—Turbine supercharger.
43. Supercharger control.—a. (1) A supercharger of high capacity must be provided with some means of control in order to permit correct manifold pressure at all altitudes. In the case of the turbine-driven type this can readily be accomplished by providing a controlled escape for a certain portion of the exhaust gases. This method of control is quite satisfactory, since the supercharger output can be readily controlled to meet all operating conditions. Generally the waste gate will not be completely closed except when operating at a very high altitude. The control of the geared centrifugal supercharger is somewhat more difficult, since its output is determined principally by the speed at which it is driven.
(2) A common method of limiting the output of a gear-driven supercharger is to employ a throttle valve in the inlet passage to the
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CYLINDER \ \ / -- \
P \ I \ I iREGULATOR >y \
FMSTON Q \ I /___ |--bo 4 -- k=v9
( / ----T1- I ---I _______________J \ Ljf*
CRANKSHAFT-} - v \\ \L I
__" ] \ INTERNAL SUPERCHARGER JLi / |_ > ■ V-" /1 AIR INTAKC ' k j P_zu
— X---TURBO SUPERCHARGER
Figure 67.—Two-stage supercharger system.
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 43-44
supercharger, the carburetor throttles being ordinarily used for this purpose. At low altitudes the throttles must be only partially opened on highly supercharged engines, but as the altitude is increased the throttles may be opened to obtain the correct manifold pressure reading. This arrangement will generally give the greatest power at one particular altitude.
b. To eliminate the need for manual control, superchargers may be equipped with automatic regulators. Such regulators are sensitive to the developed manifold pressure and will adjust the throttle or waste gage control so as to maintain the correct pressure at all times. The control may be set either to maintain a standard reading at all altitudes or may be used to limit the maximum pressure for take-off. Automatic regulation not only relieves the engine operator of serious responsibility but will also protect the engine from excessive stresses and detonation. A typical example of automatic supercharger regulator installation is shown in figure 68.
44. Supercharger engine efficiencies.—a. (1) In order to appreciate the advantages of superchargers and also to realize the dangers that will result from their improper use, it is well to consider the relation between manifold pressure and power output. The pressures obtained in an engine cylinder during the compression and power stroke with various manifold pressures are illustrated in figure 69.
(2) By referring to figure 69, curve (1) assumes that the compression stroke begins with full sea-level pressure in the cylinder, and represents the maximum possible power output of an engine not having a supercharger. Curves (2) and (3) show the results of increasing manifold pressure by supercharging. The higher manifold pressure causes a corresponding increase in compression pressure and consequently a greater output on the power stroke. These curves assume, of course, that the combustion process is entirely free from detonation in each case. Figure 70 shows the horsepower increase resulting from high manifold pressures.
b. It is therefore evident that the installation of a supercharger represents the most effective method of obtaining greatly increased horsepower without increasing the weight or piston displacement of the engine. In this connection it is observed that many aircraft engines are operated continuously (cruising) at manifold pressures higher than the maximum pressure obtainable at full throttle on engines of older design. Improved materials and fuels of higher octane rating have been largely responsible for such development. The curves in
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44 AIR CORPS
BLAST GATE —s I (f —---------' -EXHAUST MANIFOLD
M ---NOZZLE BOX
---PISTON /£> \ ll / xTURBINE WHEEL
r-~z======* ®nS 1
OIL PRESSURE —[//J / _________
/'^V~Y~Y~Y~\<>i''VYVYYuZ^— adjusting spring
SEALED II _
| ____a
/ BELLOWS .
/g\ OIL DRAIN superchaRGER PRESSURE
Figure 68.—Supercharger regulator installation.
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 44
COMPRESSION POWER
CYLINDER / z-\\
PRESSURE / / \\
so'-tu" tlJ_______________IGN__________________xy
T C
Figure 69.—Manifold and cylinder pressures.
1100.----------------------
1000-------------------/---
900-----------------L_____
800----■--------/
u 700-------------4----------
§ /
O /
u 600---------/--------------
CO /
o: /
O 500----/------------------
400—/^---------------------
3ooL—L—L—L,________L___L__
20 25 30 ' 35 40 45" 50"
MANIFOLD PRESSURE
INCHES OF HG
Figure 70.—Manifold pressure and engine power.
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figure 71 show the relative performance of an engine at various altitudes with an external supercharger and also with a single speed and a two speed internal supercharger. These curves clearly show that the single speed gear-driven supercharger gives best engine performance at one particular altitude. The two speed impeller shows fairly good characteristics over a much wider range of altitudes. It will be noted that the power curve for the turbo supercharger is practically flat up , to approximately 25,000 feet altitude.
H 100 I------------------------------------------------]-----------
Z -----------------------------------------------------
U - -___
O (X x-' ...
m 75----------------------------------------------------------
(X w 50---------------------------------------------------------------
O Cl ui tn tr 25--------------------------------------------------------------
o I
°O 5000 10,000 15,000 20,000 25pOO
ALTITUDE
Z----- TURBO SUPERCHARGER
LEGEND <------2 SPEED GEARED SUPERCHARGER
I-----SINGLE SPEED GEARED SUPERCHARGER
Figure 71.—Altitude power curves.
c. In comparing the internal gear and the external turbine superchargers, factors other than performance must be considered. For example, although less efficient, the internal geared type is far lighter, requires no maintenance, and affords good distribution of the fuel mixture. For obtaining a high sea level manifold pressure and a moderate altitude rating, the geared supercharger is practically ideal, but when exceptionally high altitude performance is required the turbine supercharger is definitely superior. The two types must not be regarded as competitive since the type of service required must be considered in selecting the proper supercharger. A high altitude bombardment airplane will generally give best performance with the turbine supercharger, but an attack airplane requiring a high sea-level power would
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AIRCRAFT INDUCTION, FUEL, AND OIL SYSTEMS 44-45
be relatively inefficient if equipped with the turbine type. A nearly ideal condition is obtained by incorporating an internal supercharger of modern output and also an external unit to increase the high altitude performance.
45. Maintenance.—a. Gear-driven internal superchargers are built as integral parts of engine assemblies and therefore require no special maintenance. Lubrication of the impeller bearings and gears is accomplished by the regular engine-lubrication system.
b. Turbo supercharger installations require very careful inspection and maintenance in service. The principal items to be checked on these supercharger installations are enumerated as follows:
(1) The entire supercharger, intercooler, and piping must be checked for security of mounting and general condition.
(2) The turbine wheel should be rotated by hand to test for scraping, binding of shaft, or defective bearings.
(3) The clearance between the nozzle box and turbine wheel should be adjusted as specified.
(4) Worn bearings may be detected by checking the radial and end clearance of the turbine wheel.
(5) Supercharger bearings must be lubricated with either grease or oil depending upon the design of the lubricating system.
(6) The entire exhaust and induction systems should be maintained free from leaks at all joints, couplings, fittings, etc.
(7) The automatic supercharger regulator is inspected for free control movement and for proper operation.
(8) The waste gate control must operate freely at all times in order to permit correct supercharger regulation.
(9) A final check is made of the entire supercharger installation with the engine operating. Special attention is given to the manifold pressure indications at various speeds.
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INDEX
Air and fuel mixtures________________________________
Aircraft—
Engine oils, requirements_______________________
Oiling systems, typical_________________________
Alcohol and benzol___________________________________
Carburetion:
Principles__________________________________
Systems_________________________________________
Carburetors:
Construction and operation____________________________
Maintenance_________________________________
Fuel—
Injection systems_______________________________
Advantages of_______________________________
Controls for____________________________
Maintenance_________________________________
Operation, principles of__________________________
Systems_________________________________________
Circuits:
Basic___________________________________
Typical_________________________________
Maintenance______________________________ _ ~ _
Operation___________________________________
• Units________________________________________
Fuel and air mixtures______________________________
Fuels__________________________________________________
Aircraft_____________________________________
Liquid, volatility of__________________________________
Purity of________________________________________
Vaporization_________________________________
Gasoline_______________________________________
Lubricants_____________________________________
Minerals, description and classification_______________
Miscellaneous____________________________
Lubricating oils for engines___________________________
Properties, determination of___________________________
Mixture distribution___________________________________
Mixtures, fuel and air_________________________________
Octane rating__________________________________
‘ Oil dilution system__________________________________
Paragraph Page
29 59
12 24
17 31
3 4
28 55
27 55
32 66
33 76
34 76
37 81
36 80
38 81
35 77
21 35
22 35
24 53
26 54
25 53
23 37
29 59
1,7 1,11
7 11
4 4
6 10
30 62
2 3
8 12
9 13
14 26
10 14
11 15
31 65
29 59
5 7
18 33
95
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Paragraph Page
Oiling systems----------------------------------------------------- 15 27
Aircraft, typical---------------------------------------------- 17 31
Maintenance___________________________________________________ 20
Operating instructions----------------------------------------- 19 33
Units___________________________________________________ 1® 27
Oils:
Aircraft engine, requirements--------------------------------- 12 24
Engine lubricating-------------------------------------------- 10 14
Properties, determination of----------------------------- 11 15
Reclamation of------------------------------------------ 13 24
Supercharger system----------------------------------------------- 39 82
Controls______________________________________________________ 43 87
Engine efficiencies_________________________________ 44 89
Exhaust-driven___________________________________________ 42 86
Gear-driven______________________________________________ 41 85
Maintenance______________________________________________ 45 93
Types____________________________________________________ 40 84
Vaporization of fuel----------------------------------------- 30 62
[A. G. 062.11 (9-25-41).]
By order of the Secretary of War: G. C. MARSHALL, Chief of Staff.
Official :
E. S. ADAMS, Major General, The Adjutant General.
Distribution :
Bn and H 1,17(6); IBn 1 (10) ; Bn 9 (2); IC 9 (2).
(For explanation of symbols see FM 21-6.)
96
U. *>. GOVERNMENT PRINTING OFFICE: 1941
For sale by the Superintendent of Documents, Washington, D. C.
Photomount Pamphlet Binder Gaylord Bros., Inc. Makers Stockton, Calif.
PAT. JAN. 21. 1908
UNT LIBRARIES DENTON TX 76203
IIIIIIIIIIIIIIIIIIIIII
1001728961