Aviation Engines Part 8

A very simple and compact fuel supply system is shown at Fig. 41. In this instance the fuel container is placed immediately back of the engine cylinder. The carburetor which is carried as indicated is joined to the tank by a short piece of copper or flexible rubber tubing. This is the simplest possible form of fuel supply system and one used on a number of excellent airplanes.

As the sizes of engines increase and the power plant fuel consumption augments it is necessary to use more fuel, and to obtain a satisfactory flying radius without frequent landings for filling the fuel tank it is necessary to supply large containers.

When a very powerful power plant is fitted, as on battle planes of high capacity, it is necessary to carry large quant.i.ties of gasoline. In order to use a tank of sufficiently large capacity it may be necessary to carry it lower than the carburetor. When installed in this manner it is necessary to force fuel out of the tank by air pressure or to pump it with a vacuum tank because the gasoline tank is lower than the carburetor it supplies and the gasoline cannot flow by gravity as in the simpler systems. While the pressure and gravity feed systems are generally used in airplanes, it may be well to describe the vacuum lift system which has been widely applied to motor cars and which may have some use in connection with airplanes as these machines are developed.

STEWART VACUUM FUEL FEED

One of the marked tendencies has been the adoption of a vacuum fuel feed system to draw the gasoline from tanks placed lower than the carburetor instead of using either exhaust gas or air pressure to achieve this end.

The device generally fitted is the Stewart vacuum feed tank which is clearly shown in section at Fig. 42. In this system the suction of a motor is employed to draw gasoline from the main fuel tank to the auxiliary tank incorporated in the device and from this tank the liquid flows to the carburetor. It is claimed that all the advantages of the pressure system are obtained with very little more complication than is found on the ordinary gravity feed. The mechanism is all contained in the cylindrical tank shown, which may be mounted either on the front of the dash or on the side of the engine as shown.

[Ill.u.s.tration: Fig. 42.--The Stewart Vacuum Fuel Feed Tank.]

The tank is divided into two chambers, the upper one being the filling chamber and the lower one the emptying chamber. The former, which is at the top of the device, contains the float valve, as well as the pipes running to the main fuel container and to the intake manifold. The lower chamber is used to supply the carburetor with gasoline and is under atmospheric pressure at all times, so the flow of fuel from it is by means of gravity only. Since this chamber is located somewhat above the carburetor, there must always be free flow of fuel. Atmospheric pressure is maintained by the pipes A and B, the latter opening into the air. In order that the fuel will be sucked from a main tank to the upper chamber, the suction valve must be opened and the atmospheric valve closed. Under these conditions the float is at the bottom and the suction at the intake manifold produces a vacuum in the tank which draws the gasoline from the main tank to the upper chamber. When the upper chamber is filled at the proper height the float rises to the top, this closing the suction valve and opening the atmospheric valve. As the suction is now cut off, the lower chamber is filled by gravity owing to there being atmospheric pressure in both upper and lower chambers. A flap valve is provided between the two chambers to prevent the gasoline in the lower one from being sucked back into the upper one. The atmospheric and suction valves are controlled by the levers C and D, both of which are pivoted at E, their outer ends being connected by two coil springs. It is seen that the arrangement of these two springs is such that the float must be held at the extremity of its movement, and that it cannot a.s.sume an intermediate position.

This intermittent action is required to insure that the upper part of the tank may be under atmospheric pressure part of the time for the gasoline to flow to the lower chamber. When the level of gasoline drops to a certain point, the float falls, thus opening the suction valve and closing the atmospheric valve. The suction of the motor then causes a flow of fuel from the main container. As soon as the level rises to the proper height the float returns to its upper position. It takes about two seconds for the chamber to become full enough to raise the float, as but .05 gallon is transferred at a time. The pipe running from the bottom of the lower chamber to the carburetor extends up a ways, so that there is but little chance of dirt or water being carried to the float chamber.

If the engine is allowed to stand long enough so that the tank becomes empty, it will be replenished after the motor has been cranked over four or five times with the throttle closed. The installation of the Stewart Vacuum-Gravity System is very simple. The suction pipe is tapped into the manifold at a point as near the cylinders as possible, while the fuel pipe is inserted into the gasoline tank and runs to the bottom of that member. There is a screen at the end of the fuel pipe to prevent any trouble due to deposits of sediment in the main container. As the fuel is sucked from the gasoline tank a small vent must be made in the tank filler cap so that the pressure in the main tank will always be that of the atmosphere.

EARLY VAPORIZER FORMS

The early types of carbureting devices were very crude and c.u.mbersome, and the mixture of gasoline vapor and air was accomplished in three ways. The air stream was pa.s.sed over the surface of the liquid itself, through loosely placed absorbent material saturated with liquid, or directly through the fuel. The first type is known as the surface carburetor and is now practically obsolete. The second form is called the "wick" carburetor because the air stream was pa.s.sed over or through saturated wicking. The third form was known as a "bubbling" carburetor.

While these primitive forms gave fairly good results with the early slow-speed engines and the high grade, or very volatile, gasoline which was first used for fuel, they would be entirely unsuitable for present forms of engines because they would not carburate the lower grades of gasoline which are used to-day, and would not supply the modern high-speed engines with gas of the proper consistency fast enough even if they did not have to use very volatile gasoline. The form of carburetor used at the present time operates on a different principle.

These devices are known as "spraying carburetors." The fuel is reduced to a spray by the suction effect of the entering air stream drawing it through a fine opening.

The advantage of this construction is that a more thorough amalgamation of the gasoline and air particles is obtained. With the earlier types previously considered the air would combine with only the more volatile elements, leaving the heavier const.i.tuents in the tank. As the fuel became stale it was difficult to vaporize it, and it had to be drained off and fresh fuel provided before the proper mixture would be produced.

It will be evident that when the fuel is sprayed into the air stream, all the fuel will be used up and the heavier portions of the gasoline will be taken into the cylinder and vaporized just as well as the more volatile vapors.

[Ill.u.s.tration: Fig. 43.--Marine-Type Mixing Valve, by which Gasoline is Sprayed into Air Stream Through Small Opening in Air-Valve Seat.]

The simplest form of spray carburetor is that shown at Fig. 43. In this the gasoline opening through which the fuel is sprayed into the entering air stream is closed by the spring-controlled mushroom valve which regulates the main air opening as well. When the engine draws in a charge of air it unseats the valve and at the same time the air flowing around it is saturated with gasoline particles through the gasoline opening. The mixture thus formed goes to the engine through the mixture pa.s.sage. Two methods of varying the fuel proportions are provided. One of these consists of a needle valve to regulate the amount of gasoline, the other is a knurled screw which controls the amount of air by limiting the lift of the jump valve.

DEVELOPMENT OF FLOAT-FEED CARBURETOR

The modern form of spraying carburetor is provided with two chambers, one a mixing chamber through which the air stream pa.s.ses and mixes with a gasoline spray, the other a float chamber in which a constant level of fuel is maintained by simple mechanism. A jet or standpipe is used in the mixing chamber to spray the fuel through and the object of the float is to maintain the fuel level to such a point that it will not overflow the jet when the motor is not drawing in a charge of gas. With the simple forms of generator valve in which the gasoline opening is controlled by the air valve, a leak anywhere in either valve or valve seat will allow the gasoline to flow continuously whether the engine is drawing in a charge or not. The liquid fuel collects around the air opening, and when the engine inspires a charge it is saturated with gasoline globules and is excessively rich. With a float-feed construction, which maintains a constant level of gasoline at the right height in the standpipe, liquid fuel will only be supplied when drawn out of the jet by the suction effect of the entering air stream.

MAYBACH'S EARLY DESIGN

The first form of spraying carburetor ever applied successfully was evolved by Maybach for use on one of the earliest Daimler engines. The general principles of operation of this pioneer float-feed carburetor are shown at Fig. 44, A. The mixing chamber and valve chamber were one and the standpipe or jet protruded into the mixing chamber. It was connected to the float compartment by a pipe. The fuel from the tank entered the top of the float compartment and the opening was closed by a needle valve carried on top of a hollow metal float. When the level of gasoline in the float chamber was lowered the float would fall and the needle valve uncover the opening. This would permit the gasoline from the tank to flow into the float chamber, and as the chamber filled the float would rise until the proper level had been reached, under which conditions the float would shut off the gasoline opening. On every suction stroke of the engine the inlet valve, which was an automatic type, would leave its seat and a stream of air would be drawn through the air opening and around the standpipe or jet. This would cause the gasoline to spray out of the tube and mix with the entering air stream.

[Ill.u.s.tration: Fig. 44.--Tracing Evolution of Modern Spray Carburetor.

A--Early Form Evolved by Maybach. B.--Phoenix-Daimler Modification of Maybach's Principle. C--Modern Concentric Float Automatic Compensating Carburetor.]

The form shown at B was a modification of Maybach's simple device and was first used on the Phoenix-Daimler engines. Several improvements are noted in this device. First, the carburetor was made one unit by casting the float and mixing chambers together instead of making them separate and joining them by a pipe, as shown at A. The float construction was improved and the gasoline shut-off valve was operated through leverage instead of being directly fastened to the float. The spray nozzle was surrounded by a choke tube which concentrated the air stream around it and made for more rapid air flow at low engine speeds. A conical piece was placed over the jet to break up the entering spray into a mist and insure more intimate admixture of air and gasoline. The air opening was provided with an air cone which had a shutter controlling the opening so that the amount of air entering could be regulated and thus vary the mixture proportions within certain limits.

CONCENTRIC FLOAT AND JET TYPE

The form shown at B has been further improved, and the type shown at C is representative of modern single jet practice. In this the float chamber and mixing chamber are concentric. A balanced float mechanism which insures steadiness of feed is used, the gasoline jet or standpipe is provided with a needle valve to vary the amount of gasoline supplied the mixture and two air openings are provided. The main air port is at the bottom of the vaporizer, while an auxiliary air inlet is provided at the side of the mixing chamber. There are two methods of controlling the mixture proportions in this form of carburetor. One may regulate the gasoline needle or adjust the auxiliary air valve.

SCHEBLER CARBURETOR

A Schebler carburetor, which has been used on some airplane engines, is shown in Fig. 45. It will be noticed that a metering pin or needle valve opens the jet when the air valve opens. The long arm of a leverage is connected to the air valve, while the short arm is connected to the needle, the reduction in leverage being such that the needle valve is made to travel much less than the air valve. For setting the amount of fuel pa.s.sed or the size of the jet orifice when running with the air valve closed, there is a screw which raises or lowers the fulcrum of the lever and there is also a dash control having the same effect by pus.h.i.+ng down the fulcrum against a small spring. A long extension is given to the venturi tube which is very narrow around the jet orifices, which are horizontal and shown at A in the drawing. Fuel enters the float chamber through the union M, and the spring P holds the metering pin upward against the restraining action of the lever. The air valve may be set by an easily adjustable knurled screw shown in the drawing, and fluttering of the valve is prevented by the piston dash pot carried in a chamber above the valve into which the valve stem projects. The primary air enters beneath the jet pa.s.sage and there is a small throttle in the intake to increase the speed of air flow for starting purposes. The carburetor is adapted for the use of a hot-air connection to the stove around the exhaust pipe and it is recommended that such a fitting be supplied. The lever which controls the supply of air through the primary air intake is so arranged that if desired it can be connected with a linkage on the dash or control column by means of a flexible wire.

[Ill.u.s.tration: Fig. 45.--New Model of Schebler Carburetor With Metering Valve and Extended Venturi. Note Mechanical Connection Between Air Valve and Fuel Regulating Needle.]

THE CLAUDEL (FRENCH) CARBURETOR

[Ill.u.s.tration: Fig. 46.--The Claudel Carburetor.]

This carburetor is of extremely simple construction, because it has no supplementary or auxiliary air valve and no moving parts except the throttle controlling the gas flow. The construction is already shown in Fig. 46. The spray jet is eccentric with a surrounding sleeve or tube in which there are two series of small orifices, one at the top and the other near the bottom. The former are about level with the spray jet opening. The sleeve surrounding the nozzle is closed at the top. The air, pa.s.sing the upper holes in the sleeve, produces a vacuum in the sleeve, thereby drawing air in through the bottom holes. It is this moving interior column of air that controls the flow of gasoline from the nozzle. Owing to the friction of the small pa.s.sages, the speed of air flow through the sleeve does not increase as fast as the speed of air flow outside the sleeve, hence there is a tendency for the mixture to remain constant. The throttle of this carburetor is of the barrel type, and the top of the spray nozzle and its surrounding sleeve are located inside the throttle.

STEWART METERING PIN CARBURETOR

The carburetor shown at Fig. 47 is a metering type in which the vacuum at the jet is controlled by the weight of the metering valve surrounding the upright metering pin. The only moving part is the metering valve, which rises and falls with the changes in vacuum. The air chamber surrounds the metering valve, and there is a mixing chamber above. As the valve is drawn up the gasoline pa.s.sage is enlarged on account of the predetermined taper on the metering pin, and the air pa.s.sage also is increased proportionately, giving the correct mixture. A dashpot at the bottom of the valve checks flutter. In idling the valve rests on its seat, practically closing the air and giving the necessary idling mixture. A pa.s.sage through the valve acts as an aspirating tube. When the valve is closed altogether the primary air pa.s.ses through ducts in the valve itself, giving the proper amount for idling. The one adjustment consists in raising or lowering the tapered metering pin, increasing or decreasing the supply of gasoline. Dash control is supplied. This pulls down the metering pin, increasing the gasoline flow. The duplex type for eight- and twelve-cylinder motors is the same in principle as model 25, but it is a double carburetor synchronized as to throttle movements, adjustments, etc. The duplex for aeronautical motors is made of cast aluminum alloy.

[Ill.u.s.tration: Fig. 47.--The Stewart Metering Pin Carburetor.]

MULTIPLE NOZZLE VAPORIZERS

To secure properly proportioned mixtures some carburetor designers have evolved forms in which two or more nozzles are used in a common mixing chamber. The usual construction is to use two, one having a small opening and placed in a small air tube and used only for low speeds, the other being placed in a larger air tube and having a slightly augmented bore so that it is employed on intermediate speeds. At high speeds both jets would be used in series. Some multiple jet carburetors could be considered as a series of these instruments, each one being designed for certain conditions of engine action. They would vary from small size just sufficient to run the engine at low speed to others having sufficient capacity to furnish gas for the highest possible engine speed when used in conjunction with the smaller members which have been brought into service progressively as the engine speed has been augmented. The multiple nozzle carburetor differs from that in which a single spray tube is used only in the construction of the mixing chamber, as a common float bowl can be used to supply all spray pipes.

It is common practice to bring the jets into action progressively by some form of mechanical connection with the throttle or by automatic valves.

The object of any multiple nozzle carburetor is to secure greater flexibility and endeavor to supply mixtures of proper proportions at all speeds of the engine. It should be stated, however, that while devices of this nature lend themselves readily to practical application it is more difficult to adjust them than the simpler forms having but one nozzle. When a number of jets are used the liability of clogging up the carburetor is increased, and if one or more of the nozzles is choked by a particle of dirt or water the resulting mixture trouble is difficult to detect. One of the nozzles may supply enough gasoline to permit the engine to run well at certain speeds and yet not be adequate to supply the proper amount of gas under other conditions. In adjusting a multiple jet carburetor in which the jets are provided with gasoline regulating needles, it is customary to consider each nozzle as a distinct carburetor and to regulate it to secure the best motor action at that throttle position which corresponds to the conditions under which the jet is brought into service. For instance, that supplied the primary mixing chamber should be regulated with the throttle partly closed, while the auxiliary jet should be adjusted with the throttle fully opened.

BALL AND BALL TWO-STAGE CARBURETOR

[Ill.u.s.tration: Fig. 48.--The Ball and Ball Two-Stage Carburetor.]

This is a two-stage vaporizing device, hot air being used in the primary or initial stage of vaporization and cold air in the supplementary stage. Referring to the sectional ill.u.s.tration at Fig. 48, it will be seen that there is a hot-air pa.s.sage with a choke-valve; the primary venturi appears at B; J is its gasoline jet, and V is a spring-loaded idling valve in a fixed air opening. These parts const.i.tute the primary system. In the secondary system A is a cold-air pa.s.sage, T a b.u.t.terfly valve and J a gasoline jet discharging into the cold-air pa.s.sage. This system is brought into operation by opening the b.u.t.terfly T. A connection between the b.u.t.terfly T and the throttle, not shown, throws the b.u.t.terfly wide open when the throttle is not quite wide open; at all other times the b.u.t.terfly is held closed by a spring. The cylindrical chamber at the right of the mixing chamber has an extension E of reduced diameter connecting it with the intake manifold through a pa.s.sage D. A restricted opening connects the float chamber with the cylindrical chamber so that the gasoline level is the same in both. A loosely fitting plunger P in the cylindrical chamber has an upward extension into the small part of the chamber. O is a small air opening and M is a pa.s.sage from the cylindrical chamber to the mixing chamber. Air constantly pa.s.ses through this when the carburetor is in operation. The carburetor is really two in one. The primary carburetor is made up of a central jet in a venturi pa.s.sage. The float chamber is eccentric. In the air pa.s.sage there is a fixed opening, and additional air is taken in by the opening through suction of a spring-opposed air valve. The second stage, which comes into play as soon as the carburetor is called upon for additional mixture above low medium speeds, is made up of an independent air pa.s.sage containing another air valve. As the valve is opened this jet is uncovered, and air is led past it. For easy starting an extra pa.s.sage leads from the float bowl pa.s.sage to a point above the throttle. All the suction falls upon this pa.s.sage when the throttle is closed. The pa.s.sage contains a plunger and acts as a pick-up device.

When the vacuum increases the plunger rises and shuts off the flow of gasoline from the intake pa.s.sage. As the throttle is opened the vacuum in the intake pa.s.sage is broken, and the plunger falls, causing gasoline to gather above it. This is immediately drawn through the pick-up pa.s.sage and gives the desired mixture for acceleration.

MASTER MULTIPLE-JET CARBURETOR

[Ill.u.s.tration: Fig. 49.--The Master Carburetor.]

This carburetor, shown in detail in Figs. 49 and 50, has been very popular in racing cars and aviation engines because of exceptionally good pick-up qualities and its thorough atomization of fuel. Its principle of operation is the breaking up of the fuel by a series of jets, which vary in number from fourteen to twenty-one, according to the size of the carburetor. These are uncovered by opening the throttle, which is curved--a patented feature--to secure the correct progression of jets. The carburetor has an eccentric float chamber, from which the gasoline is led to the jet piece from which the jets stand up in a row.

The tops of these jets are closed until the throttle is opened far enough to pa.s.s them, which it does progressively. The air opening is at the bottom, and the throttle opening is such that a modified venturi is formed. The throttle is carried in a cylindrical barrel with the jets placed below it, and the pa.s.sage from the barrel to the intake is arranged so that there is no interruption in the flow. For easy starting a dash-controlled shutter closes off the air, throwing the suction on the jets, thus giving a rich mixture.

[Ill.u.s.tration: Fig. 50.--Sectional View of Master Carburetor Showing Parts.]