Chapter V
Properties of Liquid Fuels—Distillates of Crude Petroleum—Principles of Carburetion Outlined—Air Needed to Burn Gasoline—What a Carburetor Should Do—Liquid Fuel Storage and Supply—Vacuum Fuel Feed—Early Vaporizer Forms—Development of Float Feed Carburetor—Maybach’s Early Design—Concentric Float and Jet Type—Schebler Carburetor—Claudel Carburetor—Stewart Metering Pin Type—Multiple Nozzle Vaporizers—Two-Stage Carburetor—Master Multiple Jet Type—Compound Nozzle Zenith Carburetor—Utility of Gasoline Strainers—Intake Manifold Design and Construction—Compensating for Various Atmospheric Conditions—How High Altitude Affects Power—The Diesel System—Notes on Carburetor Installation—Notes on Carburetor Adjustment.
There is no appliance that has more material value upon the efficiency of the internal combustion motor than the carburetor or vaporizer which supplies the explosive gas to the cylinders. It is only in recent years that engineers have realized the importance of using carburetors that are efficient and that are so strongly and simply made that there will be little liability of derangement. As the power obtained from the gas-engine depends upon the combustion of fuel in the cylinders, it is evident that if the gas supplied does not have the proper proportions of elements to insure rapid combustion the efficiency of the engine will be low. When a gas engine is used as a stationary installation it is possible to use ordinary illuminating or natural gas for fuel, but when this prime mover is applied to automobiles or airplanes it is evident that considerable difficulty would be experienced in carrying enough compressed coal gas to supply the engine for even a very short trip. Fortunately, the development of the internal-combustion motor was not delayed by the lack of suitable fuel.
Engineers were familiar with the properties of certain liquids which gave off vapors that could be mixed with air to form an explosive gas which burned very well in the engine cylinders. A very small quantity of such liquids would suffice for a very satisfactory period of operation. The problem to be solved before these liquids could be applied in a practical manner was to evolve suitable apparatus for vaporizing them without waste. Among the liquids that can be combined with air and burned, gasoline is the most volatile and is the fuel utilized by internal-combustion engines.
The widely increasing scope of usefulness of the internal-combustion motor has made it imperative that other fuels be applied in some instances because the supply of gasoline may in time become inadequate to supply the demand. In fact, abroad this fuel sells for fifty to two hundred per cent. more than it does in America because most of the gasoline used must be imported from this country or Russia. Because of this foreign engineers have experimented widely with other substances, such as alcohol, benzol, and kerosene, but more to determine if they can be used to advantage in motor cars than in airplane engines.
Distillates of Crude Petroleum
Crude petroleum is found in small quantities in almost all parts of the world, but a large portion of that produced commercially is derived from American wells. The petroleum obtained in this country yields more of the volatile products than those of foreign production, and for that reason the demand for it is greater. The oil fields of this country are found in Pennsylvania, Indiana, and Ohio, and the crude petroleum is usually in association with natural gas. This mineral oil is an agent from which many compounds and products are derived, and the products will vary from heavy sludges, such as asphalt, to the lighter and more volatile components, some of which will evaporate very easily at ordinary temperatures.
The compounds derived from crude petroleum are composed principally of hydrogen and carbon and are termed “Hydrocarbons.” In the crude product one finds many impurities, such as free carbon, sulphur, and various earthy elements. Before the oil can be utilized it must be subjected to a process of purifying which is known as refining, and it is during this process, which is one of destructive distillation, that the various liquids are separated. The oil was formerly broken up into three main groups of products as follows: Highly volatile, naphtha, benzine, gasoline, eight to ten per cent. Light oils, such as kerosene and light lubricating oils seventy to eighty per cent. Heavy oils or residuum five to nine per cent. From the foregoing it will be seen that the available supply of gasoline is determined largely by the demand existing for the light oils forming the larger part of the products derived from crude petroleum. New processes have been recently discovered by which the lighter oils, such as kerosene, are reduced in proportion and that of gasoline increased, though the resulting liquid is neither the high grade, volatile gasoline known in the early days of motoring nor the low grade kerosene.
Principles of Carburetion Outlined
The process of carburetion is combining the volatile vapors which evaporate from the hydrocarbon liquids with certain proportions of air to form an inflammable gas. The quantities of air needed vary with different liquids and some mixtures burn quicker than do other combinations of air and vapor. Combustion is simply burning and it may be rapid, moderate or slow. Mixtures of gasoline and air burn quickly, in fact the combustion is so rapid that it is almost instantaneous and we obtain what is commonly termed an “explosion.” Therefore the explosion of gas in the automobile engine cylinder which produces the power is really a combination of chemical elements which produce heat and an increase in the volume of the gas because of the increase in temperature.
If the gasoline mixture is not properly proportioned the rate of burning will vary, and if the mixture is either too rich or too weak the power of the explosion is reduced and the amount of power applied to the piston is decreased proportionately. In determining the proper proportions of gasoline and air, one must take the chemical composition of gasoline into account. The ordinary liquid used for fuel is said to contain about eight-four per cent. carbon and sixteen per cent. hydrogen. Air is composed of oxygen and nitrogen and the former has a great affinity, or combining power, with the two constituents of hydro-carbon liquids. Therefore, what we call an explosion is merely an indication that oxygen in the air has combined with the carbon and hydrogen of the gasoline.
Air Needed to Burn Gasoline
In figuring the proper volume of air to mix with a given quantity of fuel, one takes into account the fact that one pound of hydrogen requires eight pounds of oxygen to burn it, and one pound of carbon needs two and one-third pounds of oxygen to insure its combustion. Air is composed of one part of oxygen to three and one-half portions of nitrogen by weight. Therefore for each pound of oxygen one needs to burn hydrogen or carbon four and one-half pounds of air must be allowed. To insure combustion of one pound of gasoline which is composed of hydrogen and carbon we must furnish about ten pounds of air to burn the carbon and about six pounds of air to insure combustion of hydrogen, the other component of gasoline. This means that to burn one pound of gasoline one must provide about sixteen pounds of air.
While one does not usually consider air as having much weight, at a temperature of sixty-two degrees Fahrenheit about fourteen cubic feet of air will weigh a pound, and to burn a pound of gasoline one would require about two hundred cubic feet of air. This amount will provide for combustion theoretically, but it is common practice to allow twice this amount because the element nitrogen, which is the main constituent of air, is an inert gas and instead of aiding combustion it acts as a deterrent of burning. In order to be explosive, gasoline vapor must be combined with definite quantities of air. Mixtures that are rich in gasoline ignite quicker than those which have more air, but these are only suitable when starting or when running slowly, as a rich mixture ignites much quicker than a weak mixture. The richer mixture of gasoline and air not only burns quicker but produces the most heat and the most effective pressure in pounds per square inch of piston top area.
The amount of compression of the charge before ignition also has material bearing on the force of the explosion. The higher the degree of compression the greater the force exerted by the rapid combustion of the gas. It may be stated that as a general thing the maximum explosive pressure is somewhat more than four times the compression pressure prior to ignition. A charge compressed to sixty pounds will have a maximum of approximately two hundred and forty pounds; compacted to eighty pounds it will produce a pressure of about three hundred pounds on each square inch of piston area at the beginning of the power stroke. Mixtures varying from one part of gasoline vapor to four of air to others having one part of gasoline vapor to thirteen of air can be ignited, but the best results are obtained when the proportions are one to five or one to seven, as this mixture is said to be the one that will produce the highest temperature, the quickest explosion, and the most pressure.
What a Carburetor Should Do
While it is apparent that the chief function of a carbureting device is to mix hydrocarbon vapors with air to secure mixtures that will burn, there are a number of factors which must be considered before describing the principles of vaporizing devices. Almost any device which permits a current of air to pass over or through a volatile liquid will produce a gas which will explode when compressed and ignited in the motor cylinder. Modern carburetors are not only called upon to supply certain quantities of gas, but these must deliver a mixture to the cylinders that is accurately proportioned and which will be of proper composition at all engine speeds.
Flexible control of the engine is sought by varying the engine speed by regulating the supply of gas to the cylinders. The power plant should run from its lowest to its highest speed without any irregularity in torque, i.e., the acceleration should be gradual rather than spasmodic. As the degree of compression will vary in value with the amount of throttle opening, the conditions necessary to obtain maximum power differ with varying engine speeds. When the throttle is barely opened the engine speed is low and the gas must be richer in fuel than when the throttle is wide open and the engine speed high.
When an engine is turning over slowly the compression has low value and the conditions are not so favorable to rapid combustion as when the compression is high. At high engine speeds the gas velocity through the intake piping is higher than at low speeds, and regular engine action is not so apt to be disturbed by condensation of liquid fuel in the manifold due to excessively rich mixture or a superabundance of liquid in the stream of carbureted air.
Liquid Fuel Storage and Supply
The problem of gasoline storage and method of supplying the carburetor is one that is determined solely by design of the airplane. While the object of designers should be to supply the fuel to the carburetor by as simple means as possible the fuel supply system of some airplanes is quite complex. The first point to consider is the location of the gasoline tank. This depends upon the amount of fuel needed and the space available in the fuselage.
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 quantities 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.
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 assume 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 cumbersome, and the mixture of gasoline vapor and air was accomplished in three ways. The air stream was passed 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 passed 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 constituents 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.
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 passage. 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 passes 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.
The form shown at B was a modification of Maybach’s simple device and was first used on the Phœnix-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 passed 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 pushing 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 passage 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.
The Claudel (French) 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, passing 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 passages, 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.