TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 6 - Fuel Injection Systems

SECTION VI: FUEL INJECTION SYSTEMS

34. General.-a. In some respects the development of aircraft engines has been influenced by the design of internal combustion engines intended for other purposes. This statement applies especially to the carburetor equipment used by automobile engines. With few exceptions, the basic principles involved in the operation of an aircraft carburetor are quite similar to those principles which have governed the design of automobile carburetors for many years. As the industry progressed, numerous modifications and improvements were applied, but the standard items such as floats, jets, needle valves, venturi, etc., appear to be essential parts of practically all carburetors. Thus, after many years of development, the aircraft carburetor as ordinarily constructed remains basically unchanged, and consequently is still open to criticism when installed on modern aircraft engines. The principal faults of the aircraft carburetor appear to be its sensitivity to the various tactical maneuvers required of military airplanes and the tendency to permit ice formation in the carburetor barrels under certain conditions. Since these objections seem inherent to most carburetors and therefore difficult to overcome, the design and development of mechanical fuel injection were undertaken. This step marks one of the most radical improvements in aircraft engines in recent years.

b. Due to the fact that a fuel injector is entirely different in operating principle from the carburetor which it replaces, much experimental work has been necessary to adapt the injector to the aircraft engine. After extensive bench and operating tests on single-cylinder engines, injectors were built for installation on nine-cylinder radial aircraft engines. A number of pursuit airplanes were equipped with injectors, installed with little change in the engine proper, and a satisfactory service test was conducted with these airplanes. Sufficient information was gained from the service test to warrant the redesign of the engine accessory section of certain models for the exclusive use of injector equipment. Such installations are very compact and are arranged in such a manner that the control linkage is simple and direct. Although most injector installations have been made on nine-cylinder radial engines, similar conversions may be accomplished on any type engine, providing the proper study is made of fuel and air consumption, valve timing, intake velocities, and other variable factors.

35. Principle of operation.-a. The term "fuel injection" often gives an impression that such equipment converts an engine to the Diesel principle of operation. This is not the case since, in the Diesel engine, fuel is injected directly into the engine cylinder near the end of the compression stroke, whereas the injector under discussion merely replaces the carburetor by discharging fuel into the induction system at the proper time. Once the fuel charge enters the engine cylinders, compression, ignition, and combustion proceed in the same manner as when carburetor equipment is used.

b. The purpose of the fuel injector system is to deliver a measured quantity of fuel into each intake pipe (one for each cylinder) during a certain part of the suction stroke of that particular cylinder, and to continue to deliver similar fuel charges to all cylinders in an order corresponding to the firing order of the engine. To accomplish this function, the injector proper is mechanically driven by the engine and is provided with a pressure outlet for each engine cylinder. A discharge nozzle is located in each intake pipe so that the fuel may be sprayed into the air stream at the proper place. A complete study of the fuel injector system requires first an understanding of the air flow through the induction system of an engine under various operating conditions.

(1) Although the intake stroke is often considered as occupying 180° travel of the crankshaft of a four-stroke engine, variations are desirable as noted in figure 59(1). It will be observed that the intake stroke has been decidedly lengthened by opening the intake, valve early and closing it late, a practice which is quite common on modern high output engines. During the early part of the stroke there is a gradual acceleration in air flow into the cylinder, reaching maximum velocity at approximately 70° or 80° after top center. Conversely, during the latter part of the stroke, the air flow decelerates until the velocity is almost zero when the intake valve closes. Figure 59(2) reveals the desired relation between fuel flow and air flow in a fuel injection system. It will be noted that the air flow starts before the fuel is injected and continues for a period after injection has been completed. This relationship between fuel flow and air flow has been found most satisfactory in insuring complete vaporization and distribution of the fuel charge. Obviously, this condition necessitates timing the injection to the engine in much, the same manner as valve and ignition operations are timed to the crankshaft.

FIGURE 59-Carburetion and fuel injection timing.

(2) Since aircraft engines operate over a speed range of from 400 r. p. m. to 3,000 r. p. m., it is necessary to provide a method of controlling air and fuel consumption to meet engine requirements within this range. In the fuel injector system, air consumption is controlled in the conventional manner by a manually operated throttle valve in the air intake, and the fuel consumption is controlled by varying the amount of fuel discharged per stroke by the pumps in the injector proper. By proper interconnection of the air throttle and injector stroke levers, speed regulation is accomplished and correct fuel air ratios are maintained. The centrifugal supercharger, which is a necessary part of all modern aircraft engines, remains unchanged in fuel injection systems, except that it handles only air instead of a mixture of air and fuel as in the carburetor system.

c. A typical fuel injector consists of various parts such as filters, pumps, cams, etc., arranged in such a manner that the pump output can be controlled by means of in external lever. Fuel is delivered to the injector by a standard fuel pump at approximately 3 pounds per square inch pressure, and is drawn into the injector pumps and discharged by positive cam action to the proper outlet leading to a discharge nozzle. A continuous series of these fuel impulses is generated by the injector in proper order, so that each discharge nozzle receives the correct quantity of fuel at the proper point in its cycle of operation as shown in figure 59 (2). The fuel pressure in the discharge lines leading to the injector nozzles is quite high during impulse strokes (80 to 275 pounds per square inch), depending upon

FIGURE 60.-Fuel injector discharge nozzle.

the resistance offered by the discharge nozzles. Between injection impulses the high-pressure lines are completely filled with fuel, but the nozzles remain closed until the next impulse is required. The injector is mounted on the engine accessory section and receives its lubrication from the engine oiling system. Special drive couplings are often provided so that correct timing is insured in the event an injector must be removed or replaced.

d. The discharge, nozzle assemblies are located in the lower openings of the intake pipes (fig. 60). The laminated disk nozzles are loaded to prevent fuel from flowing out of phase with the air flow through the intake pipes. The seating pressure of the nozzles is such that pressure is required to open them, and when the injection period is over they instantly close. Fuel leaves the discharge nozzles in a fine spray, impinging on the surrounding deflecting ring which deflects the fuel downstream toward the incoming air. It is then directed toward the cylinder intake ports with the direction of air flow as shown in figure 60. The high air velocity contributes to a rapid and complete vaporization of the fuel charge. Again, the importance of injector timing is emphasized, since such vaporization will occur only when the fuel is injected during the period when air velocities are sufficiently high to sustain the discharged fuel.

36. Control system.-a. As previously mentioned, an injector system requires two controls, one for controlling air consumption, and a second for adjusting the fuel injector discharge. The usual arrangement for these controls is shown in figure 61. Fore and aft

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 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.T he mounting bolts also function as fuel feed lines so that correct 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.