TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 7 - Supercharger Systems

SECTION VII: SUPERCHARGER SYSTEMS

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 (1) , if the density of

 
FIGURE 62.-Relative volumes and pressures.

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 1/2. 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.

b. (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 gas 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).

FIGURE 63.-Effect of temperature on gas volumes.

(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

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(1)) 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. However, the Roots type may be used with good results on certain engines.

The sliding vane type supercharger (fig. 65(2) is also satisfactory but appears to be less desirable in most cases than the centrifugal type (fig. 65(3)).

FIGURE 65-Types of superchargers.

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 outlet 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 for low 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 twospeed 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 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.

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 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.

FIGURE 69.-Manifold and cylinder pressures.

FIGURE 70-Manifold pressure and engine power.

The curves in 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.

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 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.