TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Supplement No. 1 - July 15 1942

SECTION I -Self -Sealing Fuel Tanks and Lines   1-5

SECTION II - The Injection Carburetor  6-10

SECTION I: SELF-SEALING FUEL TANKS AND LINES

1.  General.--a. The development of self-sealing containers for fuel is relatively recent and is peculiar to combat aircraft. These tanks and lines are designed to prevent or minimize loss of fuel when punctured by bullets or shell fragments. Leakage of fuel may result in a disastrous fire, or loss of fuel may prevent the aircraft from returning to its base.

b. The safety feature of self-sealing tanks is not gained without sacrifice; for tanks of this kind are heavier, more expensive and require a great deal of maintenance to keep them serviceable.

2. Construction. -a. Although many variations in construction for self-sealling-fuel cells have appeared in a short but rapid period of development, the fundamental concept as to component parts is common to all types. The fuel container consists essentially of three elements as indicated in Figure la.

(1) The innermost element is the liner which surrounds and contains the fuel itself. This is generally a thin sheet which may be rubbercoated fabric or cloth, Neoprene, HyCar, Buna synthetic rubber sheet or some flexible plastic material. This layer is highly resistant to gasoline so as to prevent the fuel from reaching the sealant, or second element, which becomes active only on contact with the fuel. Some manufacturers coat the gasoline side of the inner liner with a fuel resistant lacquer to preserve the life of the liner.

(2) The second element or sealant is usually the thickest of the three elements and may consist of one layer or a composite of several layers of rubber latex, sponge or a partially vulcanized rubber compound. Upon contact with fuel (such as occurs upon penetration by bullets), this layer becomes partially soluble, gums up and swells to several times its original size thereby preventing further escape of the gasoline from the cell proper. The sealing action is shown in Figure 1b

FIGURE 1.-Self-sealing fuel tank.

(3) The third or outer element is the covering for the fuel cell and is generally leather, fabric, or a composite plastic of relatively heavy construction. This material provides the required strength and rigidity for the complete self-sealing bag.

b. The same principles of construction used for self-sealing fuel tanks are also employed in making self-sealing fuel hose. As shown in Figure 2, the arrangement and function of the various layers in a self -sealing hose is much the same as that described above. In a large number of combat airplanes self-sealing fuel hose has replaced the rigid metal tubing formerly employed.

3. Repair. --a. The effectiveness of any repair procedure applied to a self-sealing fuel cell is largely dependent upon the analysis of damage to the cell and the selection of the appropriate repair procedure to mend that damage. Injuries will vary from minor punctures which may be very easily repaired to extensive shattering of the cell which may make repair exceedingly difficult or impossible. The determination of this division between repairable and non-repairable damage will depend on the availability of spare fuel cells, repair materials, time, etc. Thorough inspections should be made before arriving at a decision regarding repairability.

b. The actual theory of repair is quite similar for practically all types of self-sealing fuel cells. The variations in the construction of the different manufacturers' cells have made it necessary to recognize minor differences in repair technique. The primary object of repair is to:

(1) Effect a positive gasoline-proof seal of the inner-liner,

(2) Prevent further deterioration of the sealing element, and

(3) Restore the strength and rigidity required in the cell to withstand the loads imposed upon it in combat service.

c. The general construction of all types of tanks will invariably consist of the three basic elements previously described. For this reason, it is obvious that repairs to these tanks may be made with substitute materials provided they possess the characteristics of the substances they replace to a satisfactory degree. The materials recommended for repair work by the tank manufacturers are those which will restore the original construction as nearly as possible. This is most desirable and in most instances results in a very satisfactory repair, but it should be remembered that substitute materials, intelligently selected and applied may also be used to produce acceptable repairs.

d. As described above, when fuel hose is punctured by gun fire, the intermediate layer produces a sealing action similar to the second element of the fuel tank. However, since damage of this nature will normally render it impracticable to attempt repairs of the hose, complete replacement of the entire section is generally necessary.

FIGURE 2.-Self-sealing fuel hose.

4. Installation and Removal.--a. During the insertion or removal of liners or bags from the wing and fuselage cavities, care should be taken to prevent the spillage of fuel on the outside of cell lining. Even though the outer layer is gasoline proof, it can be seriously injured by prolonged exposure to fuel. Care must also be taken to prevent physical damage to bulkheads, baffles or stiffeners that may be built into the lining. A preliminary examination of the liners will reveal the location of these structural units.

b. The installation procedure is as follows:

(1) Make certain that the lining is the correct one for the cavity or cell and that it faces in the correct direction.

(2) Make certain that the cavity or cell is free of foreign matter such as loose rivets, nuts, bolts, washers, clamps, drillings, etc.

(3) Insert the lining or bag with as little distortion as possible.

(4) Dust the outer surfaces of the cell with soapstone or tale to lubricate surfaces and facilitate insertion.

c. The removal procedure is as follows:

(1) Make certain that the tank cavity is open as far as possible.

(2) Disconnect all connections.

(3) Remove cell with as little distortion as possible

d. When installing self-sealing fuel hose certain precautions must be observed in order to prevent damage. To secure the hose to a metal plumbing connection, only one hose clamp is required, and over-tightening of this clamp must be avoided in order to eliminate distortion of the hose end.

5. Storage and Handling of Repair Materials.--a. Until such time as standardized repair procedure is adopted, it is deemed advisable to stock repair materials in minimum bulk quantities rather than in standard kits. The life of many of the repair materials (cements especially) is vitally affected by temperature, humidity, storage conditions, etc. In order that maximum usefulness may be obtained, extreme care should be taken to store and handle the materials as directed by the various manufacturers.

b. Any cells which are removed from aircraft and found to be damaged beyond repair should be very carefully examined for undamaged portions which may be used for repairs on other tanks of similar construction. Such salvaged parts should be marked to indicate the type of self-sealing fuel tank from which they were removed, so that they may be effectively used in later repairs.

SECTION II:  THE INJECTION CARBURETOR

6. General.--a. The number of military aircraft engines equipped with the injection or pressure carburetor is now so large that additional information is considered necessary. Because of its greater complexity, this carburetor requires more accurate adjustment and maintenance than the float type, but, when properly maintained, gives very satisfactory performance.

b. The injection carburetor is different from previous types in that it does not have a vented float chamber, but, instead, has a closed fuel system in which the fuel is contained under pressure. The fuel is injected beyond the throttle valve, at the supercharger entrance instead of inside the venturi. In order to operate properly, the injection carburetor must be supplied with vapor-free fuel under a pressure of 12 to 16 pounds per square inch. This necessitates use of a pump of higher capacity than is required by the float-type carburetor. In some installations, the fuel is passed through a vapor eliminator just before reaching the carburetor.

7. Flow of Air. --a. Most of the injection carburetors in use at present are of the downdraft type, as illustrated in Figure 3 of this supplement. Air enters the carburetor barrels from above and passes first through the venturi, then past the familiar butterfly-type throttle valve. The throttle valve regulates air flow just as in the float type carburetor. The air next passes through the carburetor adapter and into the supercharger impeller. In the adapter, fuel is sprayed into the air from a pressure discharge nozzle.

b. A small boost venturi is located with its outlet at the low pressure area of the large venturi. The venturi suction tends to lower the pressure in chamber B, and is used to regulate the fuel metering force. The increased suction obtained by use of two venturii in series results in more positive regulation and more rapid response to changes in air flow.

c. A small bleed is shown below the air diaphragm. This bleed serves the same purpose as the back suction line in the float type carburetors; that is, it tends to lower the pressure in chamber A by allowing air to flow from A into B.

d. Impact tubes located around the leading edge of the larger venturi collect air at average entrance pressure. The flow of this air into chamber A is controlled by a calibrated needle valve, operated by an automatic mixture control unit located in the inlet air scoop on top of the carburetor. By controlling the flow of air into A, compensation for changes in atmospheric temperature and density is made automatically except when the mixture control knob is in the "full rich" position. A valve is then opened which by-passes the automatic mixture control unit and vents air directly from the impact tubes to chamber A, thus raising the air pressure therein to the atmospheric valve.

8. Flow of Fuel.--a. Fuel for the main metering system enters chamber D through a poppet valve and through the metering jets into C. From C, the fuel flows directly to the discharge nozzle. A diaphragm-operated valve prevents discharge of fuel through the nozzle at pressures below 4 to 5 pounds per square inch. This pressure insures a fine division of the fuel spray, and results in rapid vaporization of the fuel.

b. When the throttle is in idling position, the path of fuel flow is similar except that the flow of fuel is restricted by an idle needle valve which is mechanically synchronized with the throttle lever.

c. The accelerating system is contained in a small, diaphragm-operated automatic unit shown in Figure 3. This unit is mounted near or in conjunction with the discharge nozzle. The pump is actuated by a change in air pressure which accompanies opening of the throttle with the engine operating. The pump cannot be operated with the engine stopped.

d. The economizer system, which is designed to increase the flow of fuel under high power operation, is essentially a spring loaded valve as indicated in Figure 3. At lower engine speeds, this valve remains closed, but, at a certain power setting, a diaphragm opens the valve thus increasing fuel flow. In general, the economizer operates in the power range above normal cruising.

e. Manual adjustments of mixture to correspond with changes in power output of the engine are accomplished by regulation of the flow of fuel with a needle or disc valve. Click-latch positions of the mixture control knob at "automatic rich" and "automatic lean" permit accurate mixture control settings.

f. When the mixture control knob is placed in the "idle cut-off " position, the mixture control valve is closed completely and stops the flow of fuel to the discharge nozzle. An engine equipped with an injection carburetor is started and stopped with the mixture control in this position. Fuel for starting is supplied by the primer pump.

9. Operation.- -a. The position of the butterfly throttle valve controls the flow of air into the engine. The flow of air through the venturi lowers the pressure in chamber B. Since chamber A is vented to the air intake impact tubes through the automatic mixture control unit, there is a greater pressure on the A side of the air diaphragm than on the B side, and the diaphragm will move in the direction of lower pressure, thus opening the fuel inlet poppet valve. Fuel pressure, therefore, builds up in chamber D, forcing fuel past the jets and needle valves to chamber C and the discharge nozzle. When the pressure in C and the nozzle line reaches a value of 4 to 5 Pounds, the nozzle opens and begins discharging fuel. Since the nozzle and chamber C are connected without intervening restrictions, the pressure will always be the same at these places and will never rise appreciably above five pounds.

b. Fuel flows past the poppet valve until the pressure in D exceeds that in C, by an amount equal to the excess of the pressure in A over that in B. The difference between the fuel pressures in the C and D chambers is the pressure with which the fuel is metered through the jets. This fuel metering pressure is held, by means of the regulator system, at a value substantially proportional to the mass air flow. Thus, the venturi suction simply regulates the flow of fuel, while the fuel pump supplies the force needed to meter and discharge the fuel. This is in contrast to the operation of the float type carburetor, where in the venturi suction must perform both duties.

c. Some new models of injection carburetors are equipped with a throttle balancing device. This is an external, spring loaded dash-pot arrangement, fastened to one end of the throttle shaft to balance the effect of engine suction on the butterfly valves. Another unit found on some pressure carburetors is a throttle return mechanism designed to hold the butterfly valves at a fixed power setting in case the cockpit control linkage is shot away.

10. Maintenance.- -a. Maintenance of the injection carburetor is much the same as that of the other types. Idling mixture is adjusted by turning an adjustment screw in the idle needle valve linkage, and idling speed is controlled by the conventional throttle stop screw. Other maintenance includes: inspection for leakage, oiling the throttle shaft bushings, cleaning the strainer and chamber, lubricating the mixture control latch mechanism, and draining the various chambers of the carburetor.