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
hydrocarbon 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.
[Illustration: Fig. 41.--How Gravity Feed Fuel Tank May Be Mounted Back
of Engine and Secure Short Fuel Line.]
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.
[Illustration: 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 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.
[Illustration: 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
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.
[Illustration: 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 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.
[Illustration: 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
[Illustration: 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, 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.
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 passage is enlarged on account of the
predetermined taper on the metering pin, and the air passage 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 passage through the valve acts as an aspirating tube. When
the valve is closed altogether the primary air passes 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.
[Illustration: 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
[Illustration: 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 illustration at Fig. 48, it will be
seen that there is a hot-air passage 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 constitute the primary
system. In the secondary system A is a cold-air passage, T a butterfly
valve and J a gasoline jet discharging into the cold-air passage. This
system is brought into operation by opening the butterfly T. A
connection between the butterfly T and the throttle, not shown, throws
the butterfly wide open when the throttle is not quite wide open; at all
other times the butterfly 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 passage 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
passage from the cylindrical chamber to the mixing chamber. Air
constantly passes 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 passage. The float chamber is eccentric. In the
air passage 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 passage containing another air valve. As the valve is
opened this jet is uncovered, and air is led past it. For easy starting
an extra passage leads from the float bowl passage to a point above the
throttle. All the suction falls upon this passage when the throttle is
closed. The passage 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 passage. As the throttle is opened the vacuum
in the intake passage is broken, and the plunger falls, causing gasoline
to gather above it. This is immediately drawn through the pick-up
passage and gives the desired mixture for acceleration.
MASTER MULTIPLE-JET CARBURETOR
[Illustration: 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 pass 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 passage 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.
[Illustration: Fig. 50.--Sectional View of Master Carburetor Showing
Parts.]
The only adjustment is for idling, and once that is fixed it need never
be touched. This is in the form of a screw and regulates the position of
the throttle when at idling position. The dash control has high-speed,
normal and rich-starting positions. In installing the Master carburetor
the float chamber may be turned either toward the radiator or driver's
seat. If the float is turned toward the radiator, however, a forward lug
plate should be ordered; otherwise it will be difficult to install the
control. The throttle lever must go all the way to the stop lug or
maximum power will not be secured. In adjusting the idle screw it is
turned in for rich and out for lean.
COMPOUND NOZZLE ZENITH CARBURETOR
[Illustration: Fig. 51.--Sectional View of Zenith Compound Nozzle
Compensating Carburetor.]
The Zenith carburetor, shown at Fig. 51, has become very popular for
airplane engine use because of its simplicity, as mixture compensation
is secured by a compensating compound nozzle principle that works very
well in practice. To illustrate this principle briefly, let us consider
the elementary type of carburetor or mixing valve, as shown in Fig. 52,
A. It consists of a single jet or spraying nozzle placed in the path of
the incoming air and fed from the usual float chamber. It is a natural
inference to suppose that as the speed of the motor increases, both the
flow of air and of gasoline will increase in the same proportion.
Unhappily, such is not the case. There is a law of liquid bodies which
states that the flow of gasoline from the jet increases under suction
faster than the flow of air, giving a mixture which grows richer and
richer--a mixture containing a much higher percentage of gasoline at
high suction than at low. The tendency is shown by the accompanying
curve (Fig. 52, B), which gives the ratio of gasoline to air at varying
speeds from this type of jet. The mixture is practically constant only
between narrow limits and at very high speed. The most common method of
correcting this defect is by putting various auxiliary air valves which,
adding air, tends to dilute this mixture as it gets too rich. It is
difficult with makeshift devices to gauge this dilution accurately for
every motor speed.
[Illustration: Fig. 52.--Diagrams Explaining Action of Baverey Compound
Nozzle Used in Zenith Carburetor.]
Now, if we have a jet which grows richer as the suction increases, the
opposite type of jet is one which would grow leaner under similar
conditions. Baverey, the inventor of the Zenith, discovered the
principle of the constant flow device which is shown in Fig. 52, C. Here
a certain fixed amount of gasoline determined by the opening I is
permitted to flow by gravity into the well J open to the air. The
suction at jet H has no effect upon the gravity compensator I because
the suction is destroyed by the open well J. The compensator, then,
delivers a steady rate of flow per unit of time, and as the motor
suction increases more air is drawn up, while the amount of gasoline
remains the same and the mixture grows poorer and poorer. Fig. 52, D,
shows this curve.
By combining these two types of rich and poor mixture carburetors the
Zenith compound nozzle was evolved. In Fig. 52, E, we have both the
direct suction or richer type leading through pipe E and nozzle G and
the "constant flow" device of Baverey shown at J, I, K and nozzle H. One
counteracts the defects of the other, so that from the cranking of the
motor to its highest speed there is a constant ratio of air and
gasoline to supply efficient combustion.
In addition to the compound nozzle the Zenith is equipped with a
starting and idling well, shown in the cut of Model L carburetor at P
and J. This terminates in a priming hole at the edge of the butterfly
valve, where the suction is greatest when this valve is slightly open.
The gasoline is drawn up by the suction at the priming hole and, mixed
with the air rushing by the butterfly, gives an ideal slow speed
mixture. At higher speeds with the butterfly valve opened further the
priming well ceases to operate and the compound nozzle drains the well
and compensates correctly for any motor speed.
[Illustration: Fig. 53.--The Zenith Duplex Carburetor for Airplane
Motors of the V Type.]
With the coming of the double motor containing eight or twelve cylinders
arranged in two V blocks, the question of good carburetion has been a
problem requiring much study. The single carburetor has given only
indifferent results due to the strong cross suction in the inlet
manifold from one set of cylinders to the other. This naturally led to
the adoption of two carburetors in which each set of cylinders was
independently fed by a separate carburetor. Results from this system
were very good when the two carburetors were working exactly in unison,
but as it was extremely difficult to accomplish this co-operation,
especially where the adjustable type was employed, this system never
gained in favor. The next logical step was the Zenith Duplex, shown at
Fig. 53. This consists of two separate and distinct carburetors joined
together so that a common gasoline float chamber and air inlet could be
used by both. It does away with cross suction in the manifold because
each set of cylinders has a separate intake of its own. It does away
with two carburetors and makes for simplicity. The practical application
of the Zenith carburetor to the Curtiss 90 horse-power OX-2 motor used
on the JN-4 standard training machine is shown at Fig. 54, which
outlines a rear view of the engine in question. The carburetor is
carried low to permit of fuel supply from a gravity tank carried back of
the motor.
[Illustration: Fig. 54.--Rear View of Curtiss OX-2 90 Horse-Power
Airplane Motor Showing Carburetor Location and Hot Air Leads.]
UTILITY OF GASOLINE STRAINERS
Many carburetors include a filtering screen at the point where the
liquid enters the float chamber in order to keep dirt or any other
foreign matter which may be present in the fuel from entering the float
chamber. This is not general practice, however, and the majority of
vaporizers do not include a filter in their construction. It is very
desirable that the dirt should be kept out of the carburetor because it
may get under the float control fuel valve and cause flooding by keeping
it raised from its seat. If it finds its way into the spray nozzle it
may block the opening so that no gasoline will issue or may so constrict
the passage that only very small quantities of fuel will be supplied the
mixture. Where the carburetor itself is not provided with a filtering
screen a simple filter is usually installed in the pipe line between the
gasoline tank and the float chamber.
Some simple forms of filters and separators are shown at Fig. 55. That
at A consists of a simple brass casting having a readily detachable
gauze screen and a settling chamber of sufficient capacity to allow the
foreign matter to settle to the bottom, from which it is drained out by
a pet cock. Any water or dirt in the gasoline will settle to the bottom
of the chamber, and as all fuel delivered to the carburetor must pass
through the wire gauze screen it is not likely to contain impurities
when it reaches the float chamber. The heavier particles, such as scale
from the tank or dirt and even water, all of which have greater weight
than the gasoline, will sink to the bottom of the chamber, whereas
light particles, such as lint, will be prevented from flowing into the
carburetor by the filtering screen.
[Illustration: Fig. 55.--Types of Strainers Interposed Between Vaporizer
and Gasoline Tank to Prevent Water or Dirt Passing Into Carbureting
Device.]
The filtering device shown at B is a larger appliance than that shown at
A, and should be more efficient as a separator because the gasoline is
forced to pass through three filtering screens before it reaches the
carburetor. The gasoline enters the device shown at C through a bent
pipe which leads directly to the settling chamber and from thence
through a wire gauze screen to the upper compartment which leads to the
carburetor. The device shown at D is a combination strainer, drain, and
sediment cup. The filtering screen is held in place by a spring and
both are removed by taking out a plug at the bottom of the device. The
shut-off valve at the top of the device is interposed between the
sediment cup and the carburetor. This separating device is incorporated
with the gasoline tank and forms an integral part of the gasoline supply
system. The other types shown are designed to be interposed between the
gasoline tank and the carburetor at any point in the pipe line where
they may be conveniently placed.
INTAKE MANIFOLD DESIGN AND CONSTRUCTION
On four- and six-cylinder engines and in fact on all multiple-cylinder
forms, it is important that the piping leading from the carburetor to
the cylinders be made in such a way that the various cylinders will
receive their full quota of gas and that each cylinder will receive its
charge at about the same point in the cycle of operations. In order to
make the passages direct the bends should be as few as possible, and
when curves are necessary they should be of large radius because an
abrupt corner will not only impede gas flow but will tend to promote
condensation of the fuel. Every precaution should be taken with
four- and six-cylinder engines to insure equitable gas distribution to
the valve chambers if regular action of the power plant is desired. If
the gas pipe has many turns and angles it will be difficult to charge
all cylinders properly. On some six-cylinder aviation engines, two
carburetors are used because of trouble experienced with manifolds
designed for one carburetor. Duplex carburetors are necessary to secure
the best results from eight- and twelve-cylinder V engines.
The problem of intake piping is simplified to some extent on block
motors where the intake passage is cored in the cylinder casting and
where but one short pipe is needed to join this passage to the
carburetor. If the cylinders are cast in pairs a simple pipe of T or Y
form can be used with success. When the engine is of a type using
individual cylinder castings, especially in the six-cylinder power
plants, the proper application and installation of suitable piping is a
difficult problem. The reader is referred to the various engine designs
outlined to ascertain how the inlet piping has been arranged on
representative aviation engines. Intake piping is constructed in two
ways, the most common method being to cast the manifold of brass or
aluminum. The other method, which is more costly, is to use a built-up
construction of copper or brass tubing with cast metal elbows and Y
pieces. One of the disadvantages advanced against the cast manifold is
that blowholes may exist which produce imperfect castings and which will
cause mixture troubles because the entering gas from the carburetor,
which may be of proper proportions, is diluted by the excess air which
leaks in through the porous casting. Another factor of some moment is
that the roughness of the walls has a certain amount of friction which
tends to reduce the velocity of the gases, and when projecting pieces
are present, such as core wire or other points of metal, these tend to
collect the drops of liquid fuel and thus promote condensation. The
advantage of the built-up construction is that the walls of the tubing
are very smooth, and as the castings are small it is not difficult to
clean them out thoroughly before they are incorporated in the manifold.
The tubing and castings are joined together by hard soldering, brazing
or autogenous welding.
COMPENSATING FOR VARYING ATMOSPHERIC CONDITIONS
The low-grade gasoline used at the present time makes it necessary to
use vaporizers that are more susceptible to atmospheric variations than
when higher grade and more volatile liquids are vaporized. Sudden
temperature changes, sometimes being as much as forty degrees rise or
fall in twelve hours, affect the mixture proportions to some extent, and
not only changes in temperature but variations in altitude also have a
bearing on mixture proportions by affecting both gasoline and air. As
the temperature falls the specific gravity of the gasoline increases
and it becomes heavier, this producing difficulty in vaporizing. The
tendency of very cold air is to condense gasoline instead of vaporizing
it and therefore it is necessary to supply heated air to some
carburetors to obtain proper mixtures during cold weather. In order that
the gas mixtures will ignite properly the fuel must be vaporized and
thoroughly mixed with the entering air either by heat or high velocity
of the gases. The application of air stoves to the Curtiss OX-2 motor is
clearly shown at Fig. 54. It will be seen that flexible metal pipes are
used to convey the heated air to the air intakes of the duplex mixing
chamber.
[Illustration: Fig. 56.--Chart Showing Diminution of Air Pressure as
Altitude Increases.]
HOW HIGH ALTITUDE AFFECTS POWER
Any internal combustion engine will show less power at high altitudes
than it will deliver at sea level, and this has caused a great deal of
questioning. "There is a good reason for this," says a writer in "Motor
Age," "and it is a physical impossibility for the engine to do
otherwise. The difference is due to the lower atmospheric pressure the
higher up we get. That is, at sea level the atmosphere has a pressure of
14.7 pounds per square inch; at 5,000 feet above sea level the pressure
is approximately 12.13 pounds per square inch, and at 10,000 feet it is
10 pounds per square inch. From this it will be seen that the final
pressure attained after the piston has driven the gas into compressed
condition ready for firing is lower as the atmospheric pressure drops.
This means that there is not so much power in the compressed charge of
gas the higher up you get above sea level.
"For example, suppose the compression ratio to be 4-1/2 to 1; in other
words, suppose the air space above the piston to have 4-1/2 times the
volume when the piston is at the bottom of its stroke that it has when
the piston is at the top of the stroke. That is a common compression
ratio for an average motor, and is chosen because it is considered to be
the best for maximum horse-power and in order that the compression
pressure will not be so high as to cause pre-ignition. Knowing the
compression ratio, we can determine the final pressure immediately
before ignition by substituting in the standard formula:
P^{1} = P(V/V^{1})^{1.3}
in which P is the atmospheric pressure; P^{1} is the final pressure, and
V/V^{1} is the compression ratio, therefore P^{1} = 14.7 (4.5)^{1.3} =
104 pounds per square inch, absolute.
"That is, 104 pounds per square inch is the most efficient final
compression pressure to have for this engine at sea level, since it
comes directly from the compression ratio.
"Now supposing we consider that the altitude is 7,000 feet above sea
level. At this height the atmospheric pressure is 11.25 pounds per
square inch, approximately. In this case we can again substitute in the
formula, using the new atmospheric pressure figure. The equation
becomes:
P^{1} = 11.25 (4.5)^{1.3}--79.4 pounds per square inch, absolute.
"Therefore we now have a final compression pressure of only 79.4 pounds
per square inch, which is considerably below the pressure we have just
found to be the most efficient for the motor. The resulting power drop
is evident.
"It should be borne in mind that these final compression pressures are
absolute pressures--that is, they include the atmospheric pressure. In
the first case, to get the pressure above atmospheric you would subtract
14.7 and in the latter 11.25 would have to be deducted. In other words,
where the sea level compression is 89.3 pounds per square inch above the
atmosphere, the same motor will have only a compression pressure of
68.15 pounds per square inch above the atmosphere at 7,000 feet
elevation.
"From the above it is evident that in order to bring the final
compression pressure up to the efficient figure we have determined, a
different compression ratio would have to be used. That is, the final
volume would have to be less, and as it is impossible to vary this to
meet the conditions of altitude, the loss of power cannot be helped
except by the replacing of the standard pistons with some that are
longer above the wrist-pin so as to reduce the space above the pistons
when on top center. Then if the ratio is thereby raised to some such
figures as 5 to 1, the engine will again have its proper final pressure,
but it will still not have as much power as it would have at sea level,
since the horse-power varies directly with the atmospheric pressure,
final compression being kept constant. That is, at 7,000 feet the
horse-power of an engine that had 40 horse-power at sea level would be
equal to
11.25
------- = 30.6 horse-power.
14.7
"If the original compression ratio of 4.5 were retained, the drop in
horse-power would be even greater than this. These computations and
remarks will make it clear that the designer who contemplates building
an airplane for high altitude use should see to it that it is of
sufficient power to compensate for the drop that is inevitable when it
is up in the air. This is often illustrated in stationary gas-engine
installations. An engine that had a sea-level rating amply sufficient
for the work required, might not be powerful enough when brought up
several thousand feet." When one considers that airplanes attain heights
of over 18,000 feet, it will be evident that an ample margin of engine
power is necessary.
THE DIESEL SYSTEM
A system of fuel supply developed by the late Dr. Diesel, a German
chemist and engineer, is attracting considerable attention at the
present time on account of the ability of the Diesel engine to burn
low-grade fuels, such as crude petroleum. In this system the engines are
built so that very high compressions are used, and only pure air is
taken into the cylinder on the induction stroke. This is compressed to a
pressure of about 500 pounds per square inch, and sufficient heat is
produced by this compression to explode a hydrocarbon mixture. As the
air which is compressed to this high point cannot burn, the fuel is
introduced into the cylinder combustion chamber under still higher
compression than that of the compressed air, and as it is injected in a
fine stream it is immediately vaporized because of the heat. Just as
soon as the compressed air becomes thoroughly saturated with the liquid
fuel, it will explode on account of the degree of heat present in the
combustion chamber. Such motors have been used in marine and stationary
applications, but are not practical for airplanes or motor cars because
of lack of flexibility and great weight in proportion to power
developed. The Diesel engine is the standard power plant used in
submarine boats and motor ships, as its efficiency renders it
particularly well adapted for large units.
NOTES ON CARBURETOR INSTALLATION IN AIRPLANES
A writer in "The Aeroplane," an English publication, discourses on some
features of carburetor installation that may be of interest to the
aviation student, so portions of the dissertation are reproduced
herewith.
"Users of airplanes fitted with ordinary type carburetors will
do well to note carefully the way in which these are fitted, for
several costly machines have been burnt lately through the sheer
carelessness of their users. These particular machines were
fitted with a high powered V-type engine, made by a firm which
is famous as manufacturers of automobiles _de luxe_. In these
engines there are four carburetors, mounted in the V between the
cylinders. When the engine is fitted as a tractor, the float
chambers are in front of the jet chambers. Consequently, when
the tail of the machine is resting on the ground, the jets are
lower than the level of the gasoline in the float chamber.
"Quite naturally, the gasoline runs out of the jet, if it is
left turned on when the machine is standing in its normal
position, and trickles into the V at the top of the crank-case.
Thence it runs down to the tail of the engine, where the
magnetos are fitted, and saturates them. If left long enough,
the gasoline manages to soak well into the fuselage before
evaporating. And what does evaporate makes an inflammable gas in
the forward cockpit. Then some one comes along and starts up the
engine. The spark-gap of the magneto gives one flash, and the
whole front of the machine proceeds to give a Fourth of July
performance forthwith. Naturally, one safeguard is to turn the
petrol off directly the machine lands. Another is never to turn
it on till the engine is actually being started up.
"One would be asking too much of the human boy--who is
officially regarded as the only person fit to fly an
aeroplane--if one depended upon his memory of such a detail to
save his machine, though one might perhaps reasonably expect the
older pilots to remember not to forget. Even so, other means of
prevention are preferable, for fire is quite as likely to occur
from just the same cause if the engine happens to be a trifle
obstinate in starting, and so gives the carburetors several
minutes in which to drip--in which operation they would probably
be assisted by air-mechanics 'tickling' them.
"One way out of the trouble is to fit drip tins under the jet
chamber to catch the gasoline as it falls. This is all very well
just to prevent fire while the machine is being started up, but
it will not save it if it is left standing with the tail on the
ground and the petrol turned on, for the drip tins will then
fill up and run over. And if it catches then, the contents of
the drip tins merely add fuel to the fire.
_Reversing Carburetors_
"Yet another way is to turn the carburetors round, so that the
float chambers are behind the jets, and so come below them when
the tail is on the ground, thus cutting off the gasoline low
down in the jets. There seems to be no particular mechanical
difficulty about this, though I must confess that I did not note
very carefully whether the reversal of the float chambers would
make them foul any other fittings on the engine. It has been
argued, however, that doing this would starve the engine of
gasoline when climbing at a steep angle, as the gasoline would
then be lowered in the jets and need more suction to get into
the cylinders. This is rather a pretty point of amateur motor
mechanics to discuss, for, obviously, when the same engine is
used as a 'pusher' instead of a tractor, the jets are in front
of the floats, and there seems to be no falling off in power.
_Starvation of Mixture_
"Moreover, the higher a machine goes the lower is the
atmospheric pressure, and, consequently, the less is the amount
of air sucked in at each induction stroke. This means, of
course, that with the gasoline supply the mixture at high
altitudes is too rich, so that, in order to get precisely the
right mixture when very high up, it is necessary to reduce the
gasoline supply by screwing down the needle valve between the
tank and the carburetor--at least, that has been the experience
of various high-flying pilots. No doubt something might be done
in the way of forced air feed to compensate for reduced
atmospheric pressure, but it remains to be proved whether the
extra weight of mechanism involved would pay for the extra power
obtained. Variable compression might do something, also, to even
things up, but here, also, weight of mechanism has to be
considered.
"In any case, at present, the higher one goes the more the
power of the engine is reduced, for less air means a less volume
of mixture per cylinder, and as the petrol feed has to be
starved to suit the smaller amount of air available, this means
further loss of power. I do not know whether anyone has evolved
a carburetor which automatically starves the gasoline feed when
high up, but it seems possible that when an airplane is sagging
about 'up against the ceiling'--as a French pilot described the
absolute limit of climb for his particular machine--it might be
a good thing to have the jets in front of the float chamber, for
then a certain amount of automatic starvation would take place.
"When a machine is right up at its limiting height, and the
pilot is doing his best to make it go higher still, it is
probably flying with its tail as low as the pilot dares to let
it go, and the lateral and longitudinal controls are on the
verge of vanishing, so that if the carburetor jets are behind
the float chambers there is bound to be an over-rich mixture in
any case. There is even a possibility of a careless or ignorant
pilot carrying on in this tail-down position till one set of
cylinders cuts out altogether, in which case the carburetor
feeding that set may flood over, just as if the machine were on
the ground, and the whole thing may catch fire. Whereas, with
the jets in front of the floats, though the mixture may starve a
trifle, there is, at any rate, no danger of fire through
climbing with the tail down.
_A Diving Danger_
"On the other hand, in a 'pusher' with this type of engine, if
the jets are in their normal position--which is in front of the
floats--there is danger of fire in a dive. That is to say, if
the pilot throttles right down, or switches off and relies on
air pressure on his propeller to start the engine again, so that
the gasoline is flooding over out of the jets instead of being
sucked into the engine, there may be flooding over the magnetos
if the dive is very steep and prolonged. In any case, a long
dive will mean a certain amount of flooding, and, probably, a
good deal of choking and spitting by the engine before it gets
rid of the over-rich mixture and picks up steady firing again.
Which may indicate to young pilots that it is not good to come
down too low under such circumstances, trusting entirely to
their engines to pick up at once and get going before they hit
the ground.
"On the whole, it seems that it might be better practice to set
the carburetors thwartwise of engines, for then jets and floats
would always be at approximately the same level, no matter what
the longitudinal position of the machine, and it is never long
enough in one position at a big lateral angle to raise any
serious carburetor troubles. Car manufacturers who dive
cheerfully into the troubled waters of aero-engine designs are
a trifle apt to forget that their engines are put into positions
on airplanes which would be positively indecent in a motor car.
An angle of 1 in 10 is the exception on a car, but it is common
on an airplane, and no one ever heard of a car going down a hill
of 10 to 1--which is not quite a vertical dive. Therefore, there
is every excuse for a well-designed and properly brought-up
carburetor misbehaving itself in an aeroplane.
"It seems, then, that it is up to the manufacturers to produce
better carburetors--say, with the jet central with the float.
But it also behooves the user to show ordinary common sense in
handling the material at present available, and not to make a
practice of burning up $25,000 worth or so of airplane just
because he is too lazy to turn off his gasoline, or to have the
tail of his machine lifted up while he is tinkering with his
engines."
NOTES ON CARBURETOR ADJUSTMENT
The modern float feed carburetor is a delicate and nicely balanced
appliance that requires a certain amount of attention and care in order
to obtain the best results. The adjustments can only be made by one
possessing an intelligent knowledge of carburetor construction and must
never be made unless the reason for changing the old adjustment is
understood. Before altering the adjustment of the leading forms of
carburetors, a few hints regarding the quality to be obtained in the
mixture should be given some consideration, as if these are properly
understood this knowledge will prove of great assistance in adjusting
the vaporizer to give a good working proportion of fuel and air. There
is some question regarding the best mixture proportions and it is
estimated that gas will be explosive in which the proportions of fuel
vapor and air will vary from one part of the former to a wide range
included between four and eighteen parts of the latter. A one to four
mixture is much too rich, while the one in eighteen is much too lean to
provide positive ignition.
A rich mixture should be avoided because the excessive fuel used will
deposit carbon and will soot the cylinder walls, combustion chamber
interior, piston top and valves and also tend to overheat the motor. A
rich mixture will also seriously interfere with flexible control of the
engine, as it will choke up on low throttle and run well on open
throttle when the full amount of gas is needed. A rich mixture may be
quickly discovered by black smoke issuing from the muffler, the exhaust
gas having a very pungent odor. If the mixture contains a surplus of air
there will be popping sounds in the carburetor, which is commonly termed
"blowing back." To adjust a carburetor is not a difficult matter when
the purpose of the various control members is understood. The first
thing to do in adjusting a carburetor is to start the motor and to
retard the sparking lever so the motor will run slowly leaving the
throttle about half open. In order to ascertain if the mixture is too
rich cut down the gasoline flow gradually by screwing down the needle
valve until the motor commences to run irregularly or misfire. Close the
needle valves as far as possible without having the engine come to a
stop, and after having found the minimum amount of fuel gradually
unscrew the adjusting valve until you arrive at the point where the
engine develops its highest speed. When this adjustment is secured the
lock nut is screwed in place so the needle valve will keep the
adjustment. The next point to look out for is regulation of the
auxiliary air supply on those types of carburetors where an adjustable
air valve is provided. This is done by advancing the spark lever and
opening the throttle. The air valve is first opened or the spring
tension reduced to a point where the engine misfires or pops back in the
carburetor. When the point of maximum air supply the engine will run on
is thus determined, the air valve spring may be tightened by screwing in
on the regulating screw until the point is reached where an appreciable
speeding up of the engine is noticed. If both fuel and air valves are
set right, it will be possible to accelerate the engine speed uniformly
without interfering with regularity of engine operation by moving the
throttle lever or accelerator pedal from its closed to its wide open
position, this being done with the spark lever advanced. All types of
carburetors do not have the same means of adjustment; in fact, some
adjust only with the gasoline regulating needle; others must have a
complete change of spray nozzles; while in others the mixture
proportions may be varied only by adjustment of the quantity of entering
air. Changing the float level is effective in some carburetors, but this
should never be done unless it is certain that the level is not correct.
Full instructions for locating carburetion troubles will be given in
proper sequence.
It is a fact well known to experienced repairmen and motorists that
atmospheric conditions have much to do with carburetor action. It is
often observed that a motor seems to develop more power at night than
during the day, a circumstance which is attributed to the presence of
more moisture in the cooler night air. Likewise, taking a motor from sea
level to an altitude of 10,000 feet involves using rarefied air in the
engine cylinders and atmospheric pressures ranging from 14.7 pounds at
sea level to 10.1 pounds per square inch at the high altitude. All
carburetors will require some adjustment in the course of any material
change from one level to another. Great changes of altitude also have a
marked effect on the cooling system of an airplane. Water boils at 212
degrees F. only at sea level. At an altitude of 10,000 feet it will boil
at a temperature nineteen degrees lower, or 193 degrees F.
In high altitudes the reduced atmospheric pressure, for 5,000 feet or
higher than sea level, results in not enough air reaching the mixture,
so that either the auxiliary air opening has to be increased, or the
gasoline in the mixture cut down. If the user is to be continually at
high altitudes he should immediately purchase either a larger dome or a
smaller strangling tube, mentioning the size carburetor that is at
present in use and the type of motor that it is on, including details as
to the bore and stroke. The smaller strangling tube makes an increased
suction at the spray nozzle; the air will have to be readjusted to meet
it and you can use more auxiliary air, which is necessary. The effect
on the motor without a smaller strangling tube is a perceptible
sluggishness and failure to speed up to its normal crank-shaft
revolutions, as well as failure to give power. It means that about
one-third of the regular speed is cut out. The reduced atmospheric
pressure reduces the power of the explosion, in that there is not the
same quantity of oxygen in the combustion chamber as at sea level; to
increase the amount taken in, you must also increase the gasoline speed,
which is done by an increased suction through the smaller strangling
aperture. Some forms of carburetors are affected more than others by
changes of altitude, which explains why the Zenith is so widely employed
for airplane engine use. The compensating nozzle construction is not
influenced as much by changes of altitude as the simpler nozzle types
are.