TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 5 - Carburation Systems

SECTION V: CARBURATION SYSTEMS

27. General.-a. The common explanation of the four-stroke cycle principle of internal combustion engine operation usually begins by stating that "the intake valve opens, and the, piston moves outward, drawing a combustible charge into the cylinder." It will be observed that, although a description of engine operation might begin at any point in the cycle, the logical procedure is to begin with the suction stroke. Such an order is quite proper, since the induction of the fuel charge directly affects the remaining operations in the cycle. Engine speed, power, and efficiency are regulated principally by the quantity and nature of the charge drawn in through the induction system; in fact, all operations which follow may be considered as resulting from the suction stroke. Thus, the induction of the fuel and air is a fundamental operation which must be clearly understood in order to obtain a complete understanding of an internal combustion engine.

b. The study of carburetion deals with many of the laws of chemistry, hydraulics, heat, and other branches of science. It is necessary, therefore, to point out certain established laws and principles which are applied to the operation of carburetion systems.

28. Carburetion principles.-a. The conventional aircraft engine may be classified as a form of heat engine, in which the burning process occurs inside a closed cylinder. Although an engine is often said to develop power, strictly speaking, an engine is merely a mechanism for converting one form of energy into another. In the gasoline engine, for example, heat which is one form of energy is partially converted into mechanical work. The necessary heat is produced by burning suitable fuels, and the heat liberated is utilized to cause expansion and pressure; thus, the original heat energy performs useful work.

b. Combustion is the result of the rapid combination of certain elements with oxygen (O) ordinarily obtained from the atmosphere. For example, hydrogen (H) may be burned in air or oxygen in a manner represented by the following formula: H₂ + O >H₂O. It will be observed that after the chemical reaction has taken place an entirely new substance is formed having no resemblance to the original elements. In this case the product formed is water. In the same manner carbon (C) will combine with oxygen, but in this reaction two different products may result, depending on the amount of oxygen present. In a plentiful supply of oxygen, carbon will combine as follows: C+O₂-->CO₂, (carbon dioxide). However, when the oxygen supply is limited, as is often the case in an engine cylinder, the formula will be: C+O -->CO (carbon monoxide). Both of these gases are often present in the exhaust of an engine, their relative proportions depending on the mixture ratio.

c. Compounds containing hydrogen and carbon such as gasoline, benzene, acetylene, etc., react with oxygen in a similar manner. For example, a hydrocarbon known as heptane (C₇H₁₆), when burned in a correct amount of oxygen, yields carbon dioxide and water or: C₇H₁₆+11(O₂)-->7CO₂+8H₂O. Under normal conditions both the water and carbon dioxide are absorbed as individual gases into the atmosphere. However, if suitable condensers are installed the water may be recovered. The above reaction assumes that the correct amount of air is present, but this condition of a perfect mixture is generally not obtained and often is not desirable. Lean mixtures permit the formation of a large amount of carbon dioxide, whereas a rich mixture increases the percentage of carbon monoxide in the exhaust gases. From this it can readily be shown that there is a definite relation between the fuel-air ratio, or mixture strength, and the composition of the exhaust gases. This will be mentioned later in connection with the measurement of mixture ratios.

d. The carburetion system of an internal-combustion engine deals with the movement of fluids (liquids and gases) through various passages and orifices according to certain well-defined principles. Liquids have a fairly constant volume and density, but gases will expand and contract under the influence of pressure variations. For example, a certain volume of air at sea level is approximately twice as heavy as an equal volume at 20,000 feet altitude. It must be remembered in connection with gases that it is important to know weight or mass of flow in addition to volume. Since it is generally impractical to weigh gases, it is common practice to measure the pressure which they exert by the use of suitable instruments. If pressure is known, the quantity (mass) of a gas in a given volume can be easily determined.

e. The weight of the earth's atmosphere causes it to exert a pressure on all objects and in all directions. At sea level this pressure is approximately 14.7 pounds per square inch, or since a mercurial barometer is often used to measure this pressure it may be expressed as a pressure capable of supporting a mercury column 29.92 inches in height. Pressures are very commonly given in pounds per square inch or inches of mercury. For practical purposes the conversion ratio between these two expressions may be considered as one to two (14.7 to 29.92). Thus a manifold pressure of 28 inches of mercury corresponds to about 14 pounds per square inch.

(1) In measuring pressures, confusion is often caused by the fact that in some cases it is necessary to know only the extent of a pressure above or below atmospheric rather than the actual total value. For example, the statement is made that a fuel pump generates a pressure of 3 pounds per square inch in the fuel lines, and the gage connected to the system registers 3 pounds. Actually, the total or absolute pressure in the line at sea level is 14.7 pounds plus 3 pounds or 17.7 pounds per square inch. It must be remembered that an absolute pressure includes the atmospheric pressure, whereas a relative pressure is based on the assumption that atmospheric pressure is zero. Bourdon tube instruments such as fuel-pressure gages, steam gages, etc., and many other instruments indicate relative or differential pressure.

(2) Further confusion is encountered when it is necessary to measure pressures below atmospheric, often erroneously called negative pressures. Actually, these subatmospheric indications are positive in value. A perfect vacuum exerts a zero pressure, and all pressures above this figure are inherently positive. For a clear understanding of pressures and pressure variations in carburetion systems, it is important that recognition be given these facts.

f. Since carburetion involves the movement of fluids at various velocities, consideration of the relation between velocity and pressure is also essential. The principal factor to be observed in this connection is the fact that fluids in motion will undergo pressure changes in a manner inversely related to speed or velocity. That is, as the speed of a moving column is increased, there will occur a decrease in the pressure exerted by the fluid. The application of this principle is utilized in many devices such as atomizers, spray guns of many types, water injectors for steam boilers, and in carburetor equipment. The venturi tube (fig. 43) furnishes an excellent example of the relation between pressure and velocity in a moving column of air. An inspection of the shape of the venturi tube reveals that since the cross sectional area  is reduced at the throat, the velocity at this point must be correspondingly higher. Because of this high velocity, the pressure in the throat of the tube will be lowered. In carburetor operation, the pressure drop (often improperly termed a negative pressure) existing at the venturi throat may be utilized in inducing liquid flow toward this point, so that correct mixing of fuel and air is accomplished. Various methods of utilizing this principle may be employed in carburetors of different designs, but in all cases the laws of fluid pressure and velocity are applied in some manner.

FIGURE 43.-Velocities and pressures in a venturi tube.

g. The subject of carburetion is intimately associated with the properties and behavior of the atmosphere which furnishes the necessary oxygen for combustion. As previously stated, the atmospheric pressure at sea level is 14.7 pounds per square inch, with minor variations according to weather conditions. This pressure is of great significance, because one of the most important factors in regulating engine power output is the weight (or mass) of air which may be taken into the cylinders in a given time period. A decrease in power when operating an engine at altitudes above sea level is obvious, for even though the proper volume of air is induced the mass of air consumed will be lower. At the same time, the mixture ratio will generally become rich as a result of the reduced air density.

h. Air contains not only its two chief ingredients, nitrogen and oxygen, but also a certain quantity of water. The capacity of air for holding water vapor varies with temperature, the capacity being greater as the temperature is raised. Air containing the maximum possible amount of moisture at a given temperature is said to be saturated; partial saturation is expressed as the relative humidity in percent. For example, a relative humidity of 50 percent indicates that the air contains only half as much moisture as it could contain if completely saturated. Temperature changes cause a variation in relative humidity, even though the total quantity of water vapor in the. air remains the same.

(1) To illustrate this point, air at 60° F. and 50 percent relative humidity is heated to 80° F. without the addition of moisture. At the increased temperature the percent of saturation is reduced. The relative humidity in this case will drop below 50 percent. On the other hand, a reduction in air temperature increases relative humidity, possibly to the point of saturation. If the cooling is continued after saturation is obtained, precipitation must occur. The temperature at which moisture condenses from the atmosphere is known as the dew point. Relative humidity and dew point are somewhat related; in general, a high humidity will cause the dew point to be observed quite near the atmospheric temperature. A temperature of 70° F. and a dew point of 65° F. indicate a high humidity, since in this case a drop of only 5° will permit precipitation. Condensation of water vapor accounts for the water often observed on the outside of cold water pipes in damp weather. If conditions are such that precipitation occurs below 32° F., the moisture will be deposited in the form of frost or ice. Such formations are known to occur in some carburetion systems under certain conditions of operation.

(2) During engine operation a decided drop in temperature will be noted in the carburetor barrels as a result of the rapid vaporization of fuel leaving the discharge nozzles. It is commonly known that a liquid absorbs heat as it enters the vapor state, this property being expressed as the latent heat of vaporization of a liquid. When large quantities of a liquid are vaporized, the process often produces a temperature drop sufficient to freeze water; in fact, the principle involved is exactly the same as that employed in the operation of a mechanical refrigerator. Whenever the proper conditions of temperature and humidity are present, ice may form in carburetor passages in dangerous quantities.

29. Fuel and air mixtures.-a. Internal-combustion engines having carburetion systems are fairly sensitive to the proportioning of the fuel and air charge. In general, engines operating on gasoline will require approximately 15 pounds of air in order to burn 1 pound of gasoline completely. However, a theoretically perfect mixture ratio is not essential in all cases. Certain conditions may require the use of mixtures either richer or leaner than this average ratio. Gasoline and air mixtures can be ignited when the ratio is as rich as 7: 1 and as lean as 20: 1, but these values are most extreme and are therefore of little importance. In general, the useful mixture ratios are between 11: 1 and 16: 1, the exact setting being determined by consideration of power output, cylinder cooling, and other factors. It must be remembered in connection with fuel and air mixtures that proportions are expressed on the basis of weight, since a volumetric measurement of air would be subject to inaccuracies resulting from pressure and temperature variations. Fuel air ratios may be given either as a direct ratio, such as 12 to 1, or may be designated as a decimal fraction such as 0.083. The latter expression is best understood by converting it to a common fraction as follows: 0.082  . In either case the ratio is the same but the decimal fraction is probably more convenient, and so is quite often used in the calibration of instruments for indicating fuel air ratios.

FIGURE 44.-Effect of fuel air (F/A) ratio on engine power.

b. The relation between power output and mixture ratio is best shown by a curve similar to that in figure 44. It will be noted that the mixture strength for maximum power is not one particular point, but for practical purposes any ratio between 0.087 and 0.075 gives approximately the same output. In this case, 0.087 setting is known as the rich best power mixture and the 0.075 as the lean best power. Normal carburetor settings (full rich) are generally on the rich side of the rich best power position, and best economy (but not full power) will be obtained from mixture ratios leaner than 0.075. Since any of these mixture ratios may be obtained by the adjustment of a manually operated mixture control, it is well to consider the circumstances under which a particular setting is desirable. Although specific instructions concerning mixture ratios are given for each type of aircraft engine, it may be generally stated that the rich mixtures should be used at high-power output, and the leaner settings are desirable at a lower cruising power. Failure to observe the instructions pertaining to the use of the mixture control may easily result in engine overheating and detonation, either of which will affect the reliability and useful life of the engine. In case of doubt, a comparatively rich mixture is advisable.

c. A common method of adjusting a manually operated mixture control is to observe the tachometer reading closely as the control is moved. By careful observation of the r. p. m. a fair indication of mixture strength can be obtained. However, this method is not applicable to engines having constant speed propellers, since in this case variations in mixture ratio will not appreciably alter the tachometer reading. At best this system is only approximate and must be rechecked with changes of operating conditions. In order to provide a continuous instrument indication of mixture ratio two methods are employed, direct measurement and exhaust gas analysis.

(1) Direct measurement involves the continuous indication of both fuel flow and mass air flow. Separate instruments may be used for these two readings, and the relative flow rates can be compared. Mixture ratio is determined by observing the comparative fuel and air consumptions. For example, a high air flow and a low fuel flow indicate a lean mixture, and, conversely, a low air flow and a high fuel flow are an indication of a rich mixture. Special scales can be provided in order to simplify the determination of the fuel air ratio. When properly corrected for variations in altitude and temperature, such instruments give very satisfactory results in laboratory work.

(2) Exhaust analysis is based on the change in exhaust composition according to the mixture ratio of the charge being burned in the cylinders. As previously explained, the mixture strength will alter the chemical nature of the exhaust gases, particularly the carbon dioxide and the carbon monoxide content. As the exhaust composition varies, the specific heat and thermal conductivity of the gases will be altered to such an extent that a sensitive electrical instrument will respond to such changes. Generally, a filtered sample of the exhaust gas is passed through an analyzing cell which contains resistance elements forming a part of an electrical circuit. In this way changes in exhaust analysis will cause the instrument to indicate such changes. The indicator proper is usually calibrated directly in fuel air ratios. Certain factors other than mixture strength, such as detonation and preignition, will cause inaccurate readings, but such indications are generally not confusing if the basic principles of the instrument are understood.

d. Improper mixtures will cause certain variations in engine performance and in many cases will seriously damage vital engine parts. Excessively rich mixtures are accompanied by a loss of power. Black smoke (free carbon) will appear in the exhaust when a rich mixture is burned, and carbon monoxide, a colorless but poisonous gas, will also be present. Very lean mixtures cause a loss of power and under certain conditions will result in serious overheating of the engine cylinders. Lean mixtures must be especially avoided when an engine is operating near its maximum output, and it is well to observe closely the cylinder head temperature whenever lean mixtures are used. If leaning is excessive, an engine may backfire through the induction system or stop completely. Backfiring is not to be confused with kickback, which is merely a tendency to reverse the direction of rotation when starting the engine and is caused by a highly advanced ignition timing or preignition. A backfire is caused by slow flame propagation resulting from a lean mixture, so that the charge is still burning when the cycle is completed (end of exhaust stroke). As the intake valve opens to admit the fresh charge to the cylinder, it is immediately ignited by the residual flame of the previous cycle. The flame travels back through the induction system burning all the combustible charge, and often will ignite any accumulation of gasoline near the carburetor.

e. When starting a cold engine, excessive quantities of liquid fuel are required in order that sufficient vapor may be present to form a combustible mixture. The nonvolatile fractions of gasoline (heavy ends) do not assist in starting; in fact, such fractions are often harmful in that they tend to remove oil from the cylinder walls, thus lowering the compression in the cylinder. Since most aircraft gasolines possess only a moderate vapor pressure, some difficulty is generally experienced in starting engines at subzero temperatures. The use of highly volatile fuels for starting at low temperatures will reduce the difficulty providing the operator is familiar with the characteristics of such fuels. In all cases it is well to remember that gasoline and similar liquid fuels will not burn in their liquid state. They must be converted into a vapor or gas and mixed with the proper amount of oxygen bef ore combustion can occur.

30. Fuel vaporization.-a. (1) Although internal-combustion engines may be operated on either gaseous or liquid fuels, it is generally most convenient to utilize the latter type. Liquid fuels are available in large quantities and they represent a comparatively concentrated source of heat, since one volume of the liquid will form several hundred volumes of vapor. The rapid transition of a liquid into the vapor state is known as vaporization.

(2) In carburetion systems, vaporization is accomplished as the fuel, discharged from the carburetor discharge nozzles, travels through the induction system. Because of the high air velocity through the carburetor venturi and the volatile nature of aircraft fuel, the vaporization process is quickly accomplished. As previously mentioned, an absorption of heat is necessary whenever this change occurs, and this heat must ordinarily be extracted from the air flowing through the carburetor. In many cases, it is not at all unusual for the temperature in the mixture chamber to drop 40° to 60° F. (22° to 30° C.) below the temperature of the incoming air when a high rate of fuel flow is present. If the air contains a large amount of moisture, the cooling process may cause precipitation in the form of ice. Such formations generally begin in the vicinity of the throttle (in carburetors using a butterfly throttle) and will often build up to such an extent that engine operation is noticeably affected. The ice will obstruct the carburetor passages, resulting in a decreased flow of mixture and consequently a drop in power output. If not detected, this condition will continue in some cases to such an extent that the reduced power will cause engine failure.

b. (1) Although carburetors of some designs and fuel injectors are free from icing difficulties, the most common remedy is to preheat the air supply entering the carburetor. In this way, sufficient heat is added to replace the heat lost due to vaporization of fuel, and the mixing chamber temperatures cannot drop to the freezing point of water. The general arrangement of the various parts of a carburetion system is shown in figure 45. The air preheater is essentially a tube or jacket through which the exhaust of one or more cylinders is passed, and the air flowing over these heated surf aces is raised to the required temperature before entering the carburetor. A control for adjusting the air preheater valve is installed in the cockpit, so that heat may be applied only when actually required to prevent ice formation.

(2) Consistently high air temperatures are to be avoided because of the increased danger of detonation, especially when operating at high-power output. A mixture thermometer located at the point of minimum temperature (fig. 45), is of great assistance in maintaining the correct preheater adjustment. As long as the mixture temperature is slightly above the freezing point (32° F. or 0° C.) no danger of icing will be present. Changes in manifold pressure or altitude will often require a readjustment of the preheater control in order to maintain the proper carburetor temperature.

c. In addition to the preheater described above, many engines also incorporate a somewhat similar unit located on the outlet side of the carburetor, commonly known as a hot spot. (See, fig. 45.) This device may also be heated by exhaust gases but is generally not controlled from the cockpit. The purpose of the hot spot is to promote the vaporization and distribution of the fuel charge and not to prevent carburetor icing.

FIGURE 45.-Aircraft engine induction system.

d. In connection with all mixture heaters, it is important to realize that high charge temperatures are undesirable. The application of excessive heat will produce expansion of the charge with resultant loss of density. Since power output depends primarily upon the mass of charge induced into the cylinders, it is obvious that heating the mixture will involve a loss of power, and in the case of the air preheater a decided variation of the mixture may also be observed. Furthermore, high charge temperatures favor detonation and preignition, both of which are to be avoided in the operation of an aircraft engine. In addition to the action of mixture heaters, other factors such as heat conduction and the action of superchargers also produce a noticeable increase in charge temperature.

31. Mixture distribution.-a. After having been properly proportioned and vaporized, the fuel charge must be evenly distributed to the various cylinders in order to complete the carburetion process. This latter step is important not only from the standpoint of power and efficiency, but it also affects the smoothness of operation. The problem of obtaining good distribution is rather difficult, in view of the various engine designs and cylinder arrangements, each of which requires a special study.

(1) In engines having no internal superchargers, consideration of such factors as the number of cylinders, arrangement of cylinders, and firing order is of prime importance, the aim being to insure the delivery of the correct quantity of charge to each cylinder. In such engines the provision of a separate carburetor barrel for each three cylinders will give satisfactory results.

(2) Downdraft carburetors in which the air is taken in at the top and travels downward are almost universally used on gasoline burning engines. (See fig. 45.) Their principal advantages over the older updraft carburetor systems include reduced fire hazards, more direct air passages, and improved charge distribution. Increased power has also been noted in certain installations.

b. The development of the radial aircraft engine presented problems in distribution that were difficult to overcome with conventional carburetion systems. The result was the installation of gear-driven impellers in the induction system which mechanically distributed the charge through suitable passages to the cylinders. In addition to providing correct distribution, these rotary induction systems may also serve as a means for increasing the flow of charge far above the amount that would be induced by cylinder suction alone. In other words, the impeller when driven at higher speeds becomes an efficient internal supercharger. With high-speed superchargers installed, it becomes necessary to provide an instrument to indicate the developed manifold pressure, since this pressure will in many cases greatly exceed the sea-level atmospheric value (29.92 inches Hg).

c. The manifold pressure gage installed on a supercharged engine registers absolute pressure and thus furnishes a convenient means of computing power output at any given r. p. in. In this connection, it will be observed that an unsuperchargd engine is not capable of operating at a manifold pressure higher than approximately 29 inches Hg, and so is limited in power by this maximum pressure. Many supercharged engines, on the other hand, are often operated at manifold pressures of 40 to 50 inches Hg with a proportionate increase in power output. Manifold pressures may be further increased provided fuels of sufficiently high antiknock value (octane rating) are developed. No other factor has been so effective in producing engines of high specific output as the perfection of superchargers to increase the density of the induced charge.

d. When high manifold pressures are utilized, it is often desirable to lengthen the suction stroke by opening the intake valve very early in the cycle; in fact, the intake valve is often opened far before top-center, at which point the exhaust valve is still open from the previous cycle. This practice is permissible, because with high manifold pressures the charge may begin to flow into the cylinder before the piston starts downward. Such a timing, known as the overlapping valve timing, will often result in a considerable increase in power at higher speeds. The principal disadvantage of an overlapping valve timing is the fact that at low speeds a certain reverse flow of exhaust gas will occur, resulting in comparatively rough idling operation.

e. A slight increase in power is often obtained by the use of a carburetor air intake which faces the direction of flight, giving what is known as a ramming air intake. In high-speed airplanes the increase in pressure may amount to 15 to 20 inches of water (1 to 1½ inches Hg), giving a power increase of possibly 3 or 4 percent. Such a gain, although slight, is well worth while, since the installations are quite simple.

32. Carburetor construction and operation.-a. The complication and sensitivity of aircraft carburetors can be directly traced to the unusual demands imposed by aircraft engines under various operating conditions. The carburetor must deliver an accurately metered fuel flow for all conditions of engine speed and load, and provide for manual or automatic mixture correction for altitude and temperature. The carburetor must also be dependable in service and stable in calibration in order to promote maximum safety and efficiency. The design must be such that acceleration, maneuvers, and icing conditions will have no serious effects on normal operation. The resulting assembly is necessarily made up of numerous valves, jets, nozzles, needles, linkages, and similar parts.

b. A curve showing the required mixture ratios at all engine speeds from idling to full rated power is shown in figure 46. A check of the curve reveals that a rich mixture is required at low speeds, and as the power is increased the mixture may be made leaner for better economy. However, at a certain point in the high cruising range the curve rises abruptly indicating a definitely richer mixture. This enrichment is required in order to prevent overheating and detonation. Near full power the mixture should be as rich as possible, consistent with proper combustion and smooth operation. Since full power is used for relatively short periods the high fuel consumption is not a serious matter. In general, lean mixtures must be employed with caution when operating aircraft engines of high specific power output.

c. In order to meet the mixture requirements of engines under all conditions, many intricate parts are required in the carburetor assembly.

FIGURE 46.-Fuel-air ratio requirements of aircraft engines.

The major units are described in some detail in the following order:

(1) In most carburetors, the fuel delivered under pressure to the inlet connection passes into a supply or control chamber. This chamber may contain either a float or diaphragm mechanism as indicated in figure 47.

FIGURE 47.-Carburetor supply chambers.

Either arrangement is satisfactory during normal engine operation, but the diaphragm type is more effective in various positions and during airplane maneuvers. The float type is quite likely to permit excessive fuel flow and possible flooding when the carburetor is inverted. It will be noted that there is no air space in the fuel chamber with the diaphragm control mechanism, and therefore splashing and surging will not occur.

FIGURE 48.-Throttle valves.

(2) Since throttle valves are required in the induction system it is common practice to incorporate them in the carburetor assembly. The throttle provides a means of regulating the air consumption of the engine and thus controls power output. In come carburetors, the throttle action is also used to regulate fuel flow from the idle nozzles in the low-speed range. Figure 48 illustrates the conventional butterfly throttle and also a variable venturi throttle mechanism. In the variable venturi design an appreciable pressure drop is maintained at all engine speeds, thus eliminating the need for independent main and idling metering systems. Also, this type of fuel metering assembly shows little tendency toward carburetor icing when properly designed.

(3) The fuel metered through the carburetor jets may be discharged into the air stream in many ways as indicated in figure 49.

FIGURE 49-Discharge nozzles.

In general, provisions are made for obtaining correct mixing of the f uel and air in order to secure a uniform and well-vaporized charge. Extremely complicated fuel metering nozzles have not been found particularly valuable in improving fuel distribution and are often troublesome in service.

(4) The enrichment of the fuel-air mixture at high-power output (fig. 46) is accomplished in actual carburetor design by the incorporation of auxiliary fuel metering devices. Such devices are variously known as economizers, high-speed jets, enrichening jets, powder compensators, etc.

FIGURE 50.-Typical economizer mechanism.

Regardless of the name applied, all such units serve the same general purpose. The action which operates the enrichening jets may be a throttle linkage, manifold pressure, fuel flow, air-flow, or certain applicable secondary effects. A representative system of this type is illustrated in figure 50. In any case, the fuel flow from the high-speed system is added to the basis flow from the main metering system in order to produce the correct fuel-air ratio for maximum power operation.

(5) The reduced air density at high altitudes has a decided effect on carburetor operation. In addition to the loss of engine power, the fuel mixture becomes rich, since a carburetor is normally calibrated for sea level operation. Since excessively rich mixtures cause a further loss of power and increased fuel consumption, a method of mixture control is necessary. Manual and automatic mixture controls are in common use, both operating on the back-suction principle.

(a) A manually operated back-suction mixture control is shown in figure 51. This type control operates by reducing the differential pressure between the supply chamber and the discharge nozzle.

FIGURE 51-Back suction mixture control.

With the control in the full rich position, normal atmospheric pressure is present in the supply chamber, and the full metering force is applied to the jets. By adjusting the control valve to a leaner setting, a reduced pressure is transmitted from the carburetor venturi to the supply chamber with consequent reduction in fuel flow.

FIGURE 52.-Idle cut-off valve.

A suitable linkage connects the mixture control on the carburetor to the operating lever in the pilot's compartment. Back-suction mixture controls also incorporate an arrangement known as an idle cut-off (fig. 52). When the control is placed in the idle cut-off position, a very low pressure is transmitted to the supply chambers resulting in a complete stoppage of fuel flow. The engine will stop immediately with no tendency toward preignition or afterfiring.

(b) The automatic mixture control operates on the same principle as the manual type but requires no particular attention f rom 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 seat, thus giving the proper fuel flow.

FIGURE 53-Automatic mixture control.

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.

(c) In some carburetors a single automatic control position is provided, whereas on others two positions are used, known as "automatic rich" and "automatic lean." The automatic lean setting is approximately the lowest, fuel air ratio which may be employed under most favorable conditions of engine operation, that is, normal cruising power. A low specific fuel consumption is possible with this setting, and if other factors are correct the engine will be entirely free from detonation and overheating. Automatic rich is provided for engine operation at a. greater power output and is also used for take-off at high altitudes. The control arrangement for a combination manual and automatic mixture control is shown in figure 54. The exact sequence of positions will vary with different carburetors, but the general principle is the same.

(6) In order to permit proper engine response to a sudden throttle opening, accelerating pumps are included in practically all carburetors. Two methods of operation are in common use as illustrated in figure 55. It will be noted that one type operates from a, mechanical linkage, whereas the other type responds to pressure changes in the carburetor passages.

FIGURE 54-Manual and automatic mixture control.

When properly designed, either type will furnish adequate charge, during acceleration to promote smooth and positive engine operation. At a fixed throttle setting, an accelerating pump delivers no fuel. An important distinction between the two types shown in figure 55 is that the throttle operated type can pump fuel when the engine is not operating, but the diaphragm type will not function unless the engine is running.

(7) The, fuel air ration delivered by an aircraft carburetor in the cruising or power range, is subject to change by the pilot's mixture control only. No other adjustments are included because of the danger of engine failure due, to improper mixtures. Each carburetor setting is accurately checked and will not require recalibration in service. However, due to variation in engines, it is not practicable to include fixed settings for low idling speed operation. At minimum idling speed, two adjustments are generally required, one for idling speed and the other for idling mixture. The speed control is ordinarily a form of throttle stop adjustment, which should be set to give a relatively low r. p. m. consistent with smooth operation and reliability. The proper mixture adjustment is determined by trial with the engine operating. By coordination of the two adjustments,

FIGURE 55.-Accelerating pumps.

satisfactory idling operation is obtained. In this connection it must be remembered that modern high output aircraft engines must not be expected to idle smoothly at extremely low r. p. m.

d. Aircraft carburetors are available in a great number of types, models, and capacities, but three general principles of construction are employed. In order of development, these types may be classified as the float type, the diaphragm variable venturi type, and the pressure metering type. The principal characteristics of these carburetor designs are briefly covered as follows:

(1) The float type carburetor (fig. 56) has been widely used on many types of engines. The metering accuracy of this design is fairly satisfactory under normal conditions, but it is not reliable during 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.

FIGURE 56.-Typical float-type carburetor.

(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 changed, a variable fuel metering jet is also operated in order to provide proper mixture ratios at all speeds.

FIGURE 57.-Diaphragm variable venturi carburetor.

As there is comparatively little obstruction in the path of the fuel discharge, this type of carburetor has been found to be practically nonicing under most operating conditions. The diaphragm fuel supply chamber remains full at all times and is not adversely affected by airplane maneuvers. Certain carburetors of this general design also possess inherent compensation of the fuel-air ratio at various altitudes.

FIGURE 58-Pressure metering carburetor.

(3) In the pressure metering carburetor (fig. 58) the fuel is supplied at high pressure to the carburetor inlet and remains under pressure until discharged into the air stream. Regulation of fuel flow is accomplished by placing the differential pressure, created by venturi action, on a flexible diaphragm. The diaphragm in turn operates a slide valve regulating the fuel flow to the discharge nozzles. Various other devices such as needle valves, jets, economizers, and mixture controls are also required to complete the carburetor. An automatic mixture control of conventional design is generally installed. This type of carburetor has accurate metering characteristics, is non-icing, and functions properly in various positions. A number of sizes are employed for engines of different power ratings.

33. Maintenance.-With the exception of the idling adjustments, aircraft carburetors are considered nonadjustable units; however, a certain amount of inspection and maintenance work is necessary. The fuel strainer should be removed and cleaned periodically, and the carburetor chambers must be drained and flushed to remove dirt and water. Various passage plugs, gaskets, and other units are inspected for fuel leakage. External mechanical linkages and throttle bearings require occasional lubrication. Other than these points, very little maintenance is required on aircraft carburetors.