BRIEF TREATISE ON CARBURETION
A carburetor is a metering device whose function is to mechanically
blend liquid fuel with a certain amount of air to produce as nearly a
homogeneous mixture as possible, and in such proportions as will result
in as perfect an explosive mixture as can be obtained.
If a gas is used as a fuel it is of course not so difficult to obtain
a homogeneous mixture due to the intimacy with which a gas will
mechanically mix with air. This intimacy is a result of the minuteness
of the molecules of both the gas and the air. With a liquid fuel, such
as gasoline, however, it is quite different, especially with low test
gasoline. If it were possible to completely transfer the liquid fuel
into its vapor the latter would act as a gas and would, therefore,
mix with the air to form a homogeneous mixture. It should be, and is,
therefore, the aim of the carburetor designer to produce an instrument
which will atomize the fuel and break it up into small particles so
that every minute particle of the fuel will be surrounded by a correct
proportion of air when it is discharged into the intake manifold of the
motor. To facilitate the vaporization of these minute particles of fuel
it is advisable to preheat the air taken into the carburetor, thereby
furnishing the necessary heat units required to vaporize the fuel by
virtue of its physical property known as its latent heat of evaporation.
There is a range of proportions of air to vapor, for a given fuel,
between which combustion will obtain. This range extends from that
proportion known as the upper limit of combustion to that known as the
lower limit of combustion. The upper limit is reached when the ratio of
air to vapor is a maximum at which combustion will take place; that
is to say, any addition of air in excess of this maximum will render
the mixture non-combustible. The lower limit is reached when the ratio
of air to vapor is a minimum at which combustion will take place, any
decrease of air below this minimum producing a non-combustible mixture.
It should be remembered that the limits of combustion of any fuel with
air are dependent upon the temperature and pressure.
[Illustration:
Carburetor Flange
Throttle Valve
Throttle Stem or Shaft
Large Venturi
Idle Discharge Jet
Idle Adjustment Needle
High Speed Adjustment Needle
Small Venturi
Float Needle
Air Bleeder
Mixture Control Valve or Choker
Float
Accelerating Well
Idling Tube
Strainer
Float Needle Seat
High Speead Needle Seat
Strainer Body
Gasoline Connection
Drain Plug
Fig. 26. Stromberg Model M Carburetor--Sectional View]
Under given temperature and pressure the rate at which the combustible
mixture will burn depends upon the ratio of air to vapor. This rate
of burning is known as the rate of propagation, and it is apparent
that it is desirable to obtain a mixture whose rate of propagation is
a maximum, because the force of the explosion will depend upon the
rapidity with which the entire mixture is completely ignited.
The limits of combustion of gasoline (.70 sp. gr.) can be taken
approximately as follows: lower limit, 7 parts air (by weight) to 1
part gasoline, upper limit, 20 parts air to 1 part gasoline.
=The Stromberg Plain Tube Model M Carburetor.=--A plain tube carburetor
is one in which both the air and the gasoline openings are fixed in
size, and in which the gasoline is metered automatically, without the
aid of moving parts by the suction of air velocity past the jets.
Fig. 26 shows a longitudinal section of a type M plain tube carburetor,
and shows the location of the gasoline when the motor is at rest.
The various parts are indicated by names and arrows. An elementary
requirement of a carburetor is that as a metering device it shall
properly proportion the gasoline and air throughout the entire
operating range.
[Illustration: Fig. 27. Stromberg Carburetor Model M--Air Bleeder
Action]
In the carburetor under discussion this mixture proportioning is
properly maintained by the use of what is termed the air bleed jet.
Fig. 27 shows the principle of the action of the air bleeder. The
gasoline leaves the float chamber, passes the point of the high speed
adjusting needle, and rises through a vertical channel “B.” Air is
taken in through the air bleeder “C,” and discharged into the gasoline
channel before the latter reaches the jet holes in the small venturi
tube “E.” The result is that the air thus taken in breaks up the flow
of gasoline and produces a finely divided emulsion. Upon reaching the
jet holes of the small venturi tube this emulsion is discharged into
the high velocity air stream in the form of a finely divided mist. If
the reader will recall how thoroughly a soap bubble divides itself when
it bursts, he will readily appreciate how completely the air bleed jet
will atomize the fuel.
Before explaining the operation of the accelerating well it is
advisable to know the reason for its existence. Suppose we had a large
tube such as the intake manifold of a motor through which air and
particles of gasoline were flowing due to a certain suction at one end.
What would be the result if we suddenly increased the suction? It would
be this: Due to the fact that air is so much lighter than gasoline,
the air would respond almost instantly to the increased suction and
its flow would be accelerated very suddenly, whereas the particles of
gasoline, owing to that characteristic known as inertia, would not
respond so rapidly, and due to its heavier weight its flow would not
accelerate as much as the air. This would mean that the air would rush
ahead of the gasoline particles, and the proportion of air to gasoline
would be greater until the inertia forces had been overcome and the
gasoline particles responded completely to the increased suction. This
very thing will take place in a carburetor unless provision is made
for it. That is to say a sudden opening of the throttle will tend
toward producing a very lean mixture at the motor due to the lagging
of the gasoline explained above. A lean mixture at this time, when
acceleration is desired, would obviously be detrimental to the result
wanted. It is at this particular time that additional gasoline is most
desirable in order to compensate for the lagging gasoline and maintain
the proper mixture at the motor. In the Stromberg carburetor this is
accomplished by means of the accelerating well shown in Fig. 28. The
operation is as follows: The action is based upon the principle of
the ordinary U tube. If a U tube contains a liquid, and if pressure
is applied to one arm of the tube, or what is the same, if suction is
applied to the other arm, it is self-evident that the level of the
liquid will rise in the arm on which the suction is applied and will
drop in the other arm. So it is in the construction of the accelerating
well. Referring to the illustration, Fig. 28, the space “F” forms the
one arm of the U tube, and the space “B” the other arm. These spaces
communicate with each other through the holes “G” thus forming a
modified form of U tube.
[Illustration: Fig. 28. Stromberg Carburetor Model M--Accelerating Well]
When the motor is idling or retarding in speed, the accelerating well
or space “F” fills with gasoline. Now when the throttle is opened,
thereby increasing the suction in the venturi tube, the following
takes place: atmospheric pressure at the bleeder “C” exerts itself
on the gasoline in the space “F” forcing the liquid down to join the
regular flow from “H” and passing up the space “B” and out into the
high velocity air stream through the small venturi tube. While the
well acts the flow of gasoline is more than double the normal rate of
flow, thereby compensating for the lagging of the gasoline referred to
previously.
Upon close observation it will be noticed that there is a series of
small holes down the wall of the well. Referring to the analogy of
the U tube, these holes directly connect the two arms of the U tube.
It is obvious that the smaller and fewer these holes, the faster will
the well empty, due to the U tube suction, and the larger and more
these holes, the slower will the well empty. It is therefore apparent
that the rate of discharge of the well can be regulated as required by
different motors, different grades of gasoline, different altitudes,
etc., by inserting wells of different drillings. The action of the well
is also dependent upon the size of the hole in the bleeder “C” because
it is the relative area of this hole in the bleeder as compared to the
area of the holes in the well which determine the rate at which the
well will empty.
The foregoing characteristics of the model M carburetor have dealt more
with the open throttle or high speed operation. We shall now consider
the operation when the motor is idling. Earlier types of carburetors,
when high test and very volatile gasoline was employed, were designed
with a mixing chamber in which the gasoline, after being discharged
from the nozzle, would mix with the air and evaporate very freely.
Present day gasoline, however, is considerably heavier and very much
less volatile, and we therefore cannot depend upon its volatility to
accomplish its vaporization.
[Illustration: Fig. 29. Stromberg Carburetor Model M--Idling Operation]
Fig. 29 shows the arrangement and idling operation of the model M
Stromberg carburetor. Concentric and inside of the passage “B” is
located the idling tube “J.” When the motor is idling, that is, when
the throttle is practically closed, the action which takes place is
as follows: the gasoline leaves the float chamber, passes through
the passage “H” into the idling tube through the hole “I,” thence up
through the idling tube “J” to the idling jet “L.” Air is drawn through
the hole “K” and mixes with the gasoline to form a finely divided
emulsion which passes on to the jet “L.” It will be noted that this jet
directs the gasoline-air emulsion into the manifold just above the lip
of the throttle valve. Inasmuch as this throttle valve is practically
closed the vacuum created at the entrance of the jet “L” is very high
and exceeds 8 pounds per square inch. It is obvious, therefore, with
this condition existing, that the gasoline will be drawn into the
manifold in a highly atomized condition. It is well to call attention
here to the fact that the low speed adjusting screw “F” operates a
needle valve which controls the amount of air which passes through the
hole “K,” and it is the position of this needle valve which determines
the idling mixture.
[Illustration: Fig. 30. Stromberg Carburetor--Throttle ¹⁄₅ Open]
As the throttle is slightly opened from the idling position a suction
is created in the throat of the small venturi tube as well as at the
idling jet. When idling the suction is greater at the idling jet,
and when the throttle is open the suction is greater at the small
venturi tube. At some intermediate position of the throttle there
is a time when the suction at the idling jet is equal to that at the
small venturi, and, therefore, at this particular time the gasoline
will follow both channels to the manifold. This condition which is
illustrated in Fig. 30 lasts but a very short while, because as the
throttle is opened wider the suction at the small venturi tube rapidly
becomes greater than that at the idling jet. The result is that the
idling tube and idling jet are thrown entirely out of action, the level
of the gasoline in the idling tube dropping as illustrated in Fig. 31,
where the throttle is shown to be wide open, in which case all of the
gasoline enters the manifold by way of the holes in the small venturi
tube.
[Illustration: Fig. 31. Stromberg Carburetor--Throttle Wide Open]
It will be remembered that at this position of the throttle the
accelerating well has emptied, and therefore there is a direct passage
for air from the bleeder to the gasoline in the main passage giving the
air bleed jet feature explained before. This is being mentioned again
in order to call attention to the fact that care should be taken not
to use too large a bleeder, because the air which enters through the
bleeder partly determines the mixture, and if the bleeder hole is too
large the mixture is very apt to be too lean at high speeds.
Fig. 32 shows an exterior photograph of one of the type M Stromberg
carburetor. Before discussing the installation and adjusting of this
carburetor it will be well to say a few words concerning the use of the
venturi tube and its construction.
The object in using the venturi tube in carburetor design is to produce
a maximum air velocity at the jet and at the same time not cause undue
restriction. This high air velocity creates the suction necessary
to properly atomize the gasoline. The use of the double venturi
tube construction has developed the best possible results. In this
construction the mouth of the smaller venturi tube is located at the
throat of the larger one, and with this arrangement the highest degree
of atomization is attainable, and at the same time the air restriction
is held down to a minimum.
In order that any carburetor may do justice to what is claimed for it,
it is absolutely essential that the motor on which it is installed is
in good condition in other respects because, besides poor carburetion,
there are numerous things about an internal combustion engine which
will cause its poor operation. Therefore, assuming that the following
conditions exist, we can proceed with the installation of the
carburetor and after adjusting it we can expect very good results as to
the operation of the motor.
1. The ignition should be properly timed so that with a retarded spark
the explosion takes place when the piston of the cylinder in which the
explosion occurs is at its upper dead center.
2. The inlet and exhaust valves should be so timed that they open and
close at the proper time during the cycle. In this respect a motor is
usually timed when it comes from the manufacturer.
3. The valves should be ground in so that they form a perfect seal
with the valve seat. Any accumulation of carbon on the upper part of
the exhaust should be removed so as to prevent the valve stem from
sticking in the guide and thereby not permitting the valve to close
upon its seat.
4. Any undue wear of the valve stem guides should be corrected because
the clearance between the stem and the walls of the guide will permit
air to be drawn up into the motor thus ruining the mixture from the
carburetor. Similarly any leaky flange at any joint along the intake
system will produce the same detrimental result.
[Illustration: Fig. 32. Stromberg Model M--Adjustment Points]
5. All piston rings should be tight and leak proof in order to insure
good and even compression in all the cylinders. Without good and even
compression in all the cylinders it is impossible to obtain the maximum
power from the motor, and it is also impossible to obtain good idling
of the motor.
6. It should be seen that the ignition system is delivering a spark to
each spark plug without missing.
7. The spark plugs should be clean, and the accumulation of carbon on
the inside of the plug should not be sufficient to cause fouling or
short-circuiting of the plug. In the case of a short circuited plug it
is impossible to obtain a spark at the end of the high tension cable,
but this does not indicate that the plug is firing. For best results
the gap of the spark plug should never be less than .020″ nor more than
.032″. A good setting is at about .025″.
The foregoing constitute some of the more important troubles to look
for when the motor is not performing satisfactorily.
=Installation and Adjusting.=--We are finally ready to proceed with
instructions for installing and adjusting Model M carburetors.
1. Try the throttle lever and the air horn lever by moving same with
the hand before the carburetor is installed, and be sure that the
butterfly valves are open to the limit when the respective levers come
in contact with their stops. Also be sure that when the throttle valve
is closed, the lower side of the butterfly is adjacent to the hole
through which the idling jet projects.
2. Prepare a paper gasket about .020″ thick to fit the flange of the
carburetor. Shellac same and then attach the carburetor to the flange
of the intake manifold very securely by means of proper cap screws.
The attaching of the gasoline line, hot-air stove, hot air flexible
tubing, and choke control need not be discussed in detail as these
installations are very simple.
After having properly installed the carburetor on the motor, turn
both the high and low speed adjusting screws, A and B, completely
down clockwise so that the needle valves just touch their respective
seats. Then unscrew (anti-clockwise) the high speed adjusting screw
A about three turns off the seat, and turn the low speed adjusting
screw B anti-clockwise about one and one-half turns off the seat.
These settings are not to be considered as final adjustments of the
carburetor. They are merely to be taken as starting points because the
motor will run freely with these settings.
After the motor has been started, permit it to run long enough to
become thoroughly warm then make the high speed adjustment. Advance
the spark to the position for normal running. Advance the gas throttle
until the motor is running at approximately 750 r. p. m. Then turn down
on the high speed screw A gradually notch by notch until a slowing down
of the motor is observed. Then turn up or open the screw anti-clockwise
until the motor runs at the highest rate of speed for that particular
setting of the throttle.
To make the low speed adjustment B proceed as follows: Retard the spark
fully and close the throttle as far as possible without causing the
motor to come to a stop. If upon idling the motor tends to roll or load
it is an indication that the mixture is too rich and therefore the
low speed screw B should be turned away from the seat anti-clockwise,
thereby permitting more air to enter into the idling mixture. It is
safe to say that the best idling results will be obtained when the
screw B is not much more or less than one and one-half turns off the
seat.
After satisfactory adjustments have been made with the motor vehicle
stationary, it is most important and advisable to take the vehicle out
on the road for further observation and finer adjustments. If upon
rather sudden opening of the throttle the motor backfires, it is an
indication that the high speed mixture is too lean, and in this case
the high speed screw A should be opened one notch at a time until the
tendency to backfire ceases. On the other hand if when running along
with throttle open, the motor rolls or loads, it is an indication that
the mixture is too rich, and this condition is overcome by turning the
high speed screw A down (clockwise) until this loading is eliminated.
STROMBERG MODEL L CARBURETOR
There are three adjustments; the high speed, the extremely low speed or
idle, and the “economizer.”
The high speed is controlled by the knurled nut “A” which locates the
position of the needle “E” past whose point is taken all the gasoline
at all speeds. Turning nut “A” to the right (clockwise) raises the
needle “E” and gives more gasoline, to the left, or anticlockwise, less.
[Illustration: Fig. 33. Stromberg Model “L”--Adjustment Points]
If an entirely new adjustment is necessary, use the following practice.
Put economizer “L” in the 5th notch (or farthest from float chamber)
as an indicator, turn nut “A” to the left, anticlockwise, until needle
“E” reaches its seat, as shown by nut “A” not moving when throttle is
opened and closed. When needle “E” is in its seat it can be felt to
stick slightly when nut “A” is lifted with the fingers. Find adjustment
of “A” where it just begins to move with the throttle opening, then
give 24 notches to the right or clockwise (the notches can be felt).
Then move the economizer pointer “L” back to the 0 notch (toward float
chamber). This will give a rich adjustment. After starting and warming
up the motor, thin out the mixture by turning “A” anticlockwise, and
find the point where the motor responds best to quick opening of the
throttle, and shows the best power.
The gasoline for low speed is taken in above the throttle through a
jet at “K” and is regulated by dilution with air as controlled by the
low speed adjusting screw “B.” Screwing “B” in clockwise gives more
gasoline; outward, less. The best adjustment is usually ¹⁄₂ to 3 turns
outward from a seating position. Note that this is only an idling
adjustment and does not effect the mixture above 8 miles per hour.
When motor is idling properly there should be a steady hiss in the
carburetor; if there is a weak cylinder or manifold leak, or if the
idle adjustment is very much too rich, the hiss will be unsteady.
The economizer device operates to lean out the mixture by lowering the
high speed needle “E” and nut “A” a slight but definitely regulated
amount at throttle positions corresponding to speeds from 5 to 40 miles
per hour. The amount of drop and consequent leaning is regulated by the
pointer “L.”
After making the high speed adjustment for best power, with pointer “L”
in 0 notch, as above described, place throttle lever on steering wheel
to a position giving about 20 miles per hour road speed. Then move
pointer “L” clockwise (away from float chamber), one notch at a time,
till motor begins to slow down. Then come back one notch.
The amount of economizer action needed depends upon the grade of
gasoline and upon the temperature.
In the mid-west the best economizer adjustment will usually be the
third or fourth notch. With Pennsylvania gasoline and in the South, the
2nd notch; while on the Pacific coast no economizer is necessary unless
distillate (which should not be below 59 degrees Baume) is used. Also
fewer notches economizer action will be necessary in summer than in
winter.