FUELS AND CARBURETION
In order that a thing may burn, it must be provided with oxygen. Oxygen
is found in air, so it is usual to say that air is necessary in order
that anything may burn.
To prove this, light a candle and place an empty bottle over it, upside
down; in a very short time the oxygen in the bottle will be used up,
and the flame will flicker and get smoky, and finally die out. If a
card is laid on the chimney of a coal-oil lamp so that it covers the
opening, that flame also will flicker, get smoky and go out.
In order to deaden the fire in a stove, the dampers are closed to
prevent air from entering; the fire is kept alight by the very small
quantity of air that leaks in below the fire-box. When the drafts are
opened the fire will burn up brightly because a plentiful volume of air
can then enter.
In a similar way, air must be used in a gas engine in order that the
fuel may burn. It is not possible to mix air with a liquid; the first
step in making a gas that will burn is, therefore, to turn the fuel,
whether it is gasoline, kerosene, distillate, or other oil, into a
vapor; this vapor is then mixed with air.
For good results it is very necessary that the vapor and air be in
proper proportions. In the experiment with the candle and the bottle it
was seen that as the air was used up, the candle flame became yellow
and smoky: this is the effect of insufficient air. If there is not
enough air in the mixture, part of the vapor will not be able to burn,
and will only smoke.
If, on the other hand, there is too much air, the mixture, if it will
burn at all, will burn slowly, and the extra volume of air will reduce
the heat.
In a mixture of the proper proportions of air and fuel vapor, the
burning, or _combustion_, will be very rapid, resulting in the sudden
production of the greatest possible amount of heat. This, of course, is
what is necessary if the engine is to produce its fullest power. With
such a mixture, combustion will be complete before the piston has done
more than start outward on the power stroke, and the greatest possible,
or _maximum_, pressure will then be produced.
When a mixture burns slowly, the piston will have gone through much
of the power stroke before combustion is complete, in which case a
considerable part of the pressure that should have been applied at the
beginning of the stroke will be wasted.
A mixture that is not correct will burn unevenly; it may burn better
during one power stroke than during another, which will make the engine
run unsteadily.
If the mixture has too much air in proportion to the amount of vapor,
it is known as a _thin_ mixture, or a _lean_ or _poor_ mixture. It
burns so slowly that it is quite possible for the mixture that started
burning before the beginning of the power stroke to continue burning
through the exhaust stroke, and for enough flame to remain in the
cylinder to set fire to the fresh charge that enters during the next
inlet stroke. This will produce what is known as a _backfire_; that is,
the mixture entering the cylinder will catch fire, and in burning will
blow back through the open inlet valve. This is a dangerous condition,
for the flame might spread to fuel dripping from the carburetor, or to
the fuel tank.
A mixture that has not enough air is called a _rich_ mixture; the air
that is present will burn part of the vapor, while the rest will go out
of the exhaust unburned, or will work past the piston into the oil in
the crank case. This is wasteful of fuel.
The most serious result of a rich mixture, however, is in the
production of _carbon_, and the _carbonization_ of the engine. The
flame of a rich mixture is smoky; the smoke of this flame, as is the
case with smoke from all other sources, is composed of fine particles
of carbon, or soot. These particles of carbon will deposit on all parts
of the combustion space: on the top of the piston, on the valves, on
the spark plugs, and on the inner wall of the cylinder head. At first
it is gummy, but it rapidly hardens and forms a crust that must be
scraped off with a steel tool.
Carbon in an engine will reduce the power through causing
_preignition_, or, in other words, by setting fire to the fresh charge
before the proper point in the stroke. The heat of the combustion will
cause the carbon deposit to become so heated that it will glow, these
glowing particles being sufficient to ignite the incoming fresh charge.
The remedy for this condition is to remove the carbon, which is usually
done by taking off the cylinder head and scraping away the deposit.
It may be added that carbon is also formed by the use of too much
lubricating oil, as will be explained in the chapter on lubrication.
Thus it is seen that if the engine is to run properly, and is to be
kept in good condition, the proportions of the mixture must be very
carefully maintained.
The mixture is formed in a _carburetor_, or _mixer_. This is, roughly,
in the form of a tube through which air is sucked during the inlet
stroke; projecting into it is a fine tube called a _spray nozzle_
through which the fuel enters. In action it is somewhat similar to the
atomizer that is used for spraying the nose and throat. By forcing the
fuel to flow rapidly through this small tube it comes out in the form
of spray, and the tiny drops are picked up by the current of air and
are carried into the cylinder.
It is much easier to form a mixture of gasoline than of kerosene or
distillate, because gasoline vaporizes more readily at ordinary
temperatures. If saucers of gasoline and kerosene are placed in the
sun, the gasoline will evaporate rapidly and completely, leaving only
a faint oily deposit. The kerosene, on the other hand, will evaporate
slowly, and much of it will not evaporate at all.
To make kerosene and distillate evaporate completely, they must be
heated, just as water must be heated to make it evaporate.
In the case of a carburetor for gasoline, the current of air needs
only to be warmed; the spray of gasoline will evaporate on coming into
contact with the warmed air, and much of it will enter the cylinder as
vapor. In order to evaporate kerosene and distillate much more heat
must be provided, and it is usually considered necessary to heat not
only the current of air, but the liquid fuel as well. Methods of doing
this will be explained in the next chapter.
[Illustration]
[Illustration: FIG. 21.—PRINCIPLE OF CARBURETOR]
When kerosene or distillate is used, there are conditions that make
it necessary to add water vapor to the mixture, which prevents the
overheating of the cylinder and reduces the deposit of carbon. The
difficulty of making a complete vapor of kerosene and distillate
results in a tendency on the part of these fuels to carbonize the
cylinders; the use of water aids in keeping the cylinders clean.
The general principle of a carburetor is shown in Figure 21, one
drawing illustrating conditions when the inlet valve is closed and the
other when it is open. It shows an engine cylinder connected with an
inlet pipe or _mixing chamber_, through which there is a swift flow of
air during an inlet stroke.
Projecting into the intake pipe is the _spray nozzle_, which is
connected with a small chamber containing fuel; inside of this chamber
is a _float_, usually made of cork, although it is sometimes a light
metal box. The fuel is intended to fill the chamber to a certain
height, at which the valve will be closed by the float rising on the
fuel. This level is such that the fuel does not quite reach the tip of
the spray nozzle.
During the compression, power, and exhaust strokes, the fuel stands at
this level, for it cannot run out of the spray nozzle, and the float
holds the valve closed. As soon as the inlet valve opens, air rushes
through the intake pipe and sucks fuel out of the spray nozzle. This,
of course, takes fuel out of the float chamber; the float in sinking
opens the valve, and enough fuel enters to restore the level.
The fuel comes out of the nozzle in the form of fine spray; it is in
such small drops that it evaporates quickly, and the resulting mixture
of fuel vapor and air passes into the cylinder. By using a spray nozzle
of the proper size, any desired proportion of fuel and air may be
obtained.
If an engine runs at a single speed, a carburetor as simple as this one
would be satisfactory, for if the suction is always the same, there
will be little or no change in the proportions of the mixture that is
formed.
To get the best results, the proportions of fuel vapor and air should
be the same for all running speeds of the engine. The proportions of
the mixture, however, depend on the violence of the suction, which
changes as the engine speed changes, becoming greater as the speed
increases. The simple carburetor illustrated in Figure 21 can be
adjusted to give a correct mixture for any particular speed, but will
be out of adjustment for any other speed.
The speed of a 1-cylinder engine does not change very greatly; it is
built to run at practically a constant speed, and a simple carburetor
is satisfactory for it. The speed of engines with a greater number of
cylinders may be greatly changed, and the carburetor must be so made
that it will give the same proportions of vapor and air at low speed as
at high.
In the simple carburetor described, the speeding-up of the engine will
result in a greater rush of air through the intake pipe, which in turn
will suck out a much greater quantity of fuel. If the carburetor is
adjusted to give the proper quantity of fuel for the air that passes
at low speed, at high speed it will give far more fuel than will be
required by the quantity of air that then passes. Thus at high speed
the mixture will be too rich.
If, on the other hand, this carburetor is adjusted to give a proper
mixture at high speed, too little fuel will be sucked out when the
engine runs slowly, and the mixture will be too lean.
A carburetor must thus have an additional device that will keep the
mixture correct, regardless of the speed at which the engine runs.
This is sometimes done by changing the size of the spray nozzle so
that a greater or less quantity of fuel flows out, but more usually
by permitting an extra quantity of air to enter the carburetor as
the engine speeds up. This is done with an _extra air intake_, the
principle of which is illustrated in Figure 22.
As will be seen, this carburetor has two openings for air, one being
the main air inlet and the other the extra air inlet. The latter is an
opening provided with a valve which is held on its seat by a spring.
The suction created by an inlet stroke is exerted in the carburetor,
but at low speed is not sufficient to suck the extra air valve from its
seat. Air then enters only through the main air inlet, and the spray
nozzle is adjusted to give the proper proportion of fuel.
[Illustration: FIG. 22.—PRINCIPLE OF EXTRA AIR INLET]
As the engine speed increases the mixture becomes richer; but there
is also an increase in suction, which becomes strong enough to pull
the extra air valve from its seat. This provides another opening into
the carburetor, through which enough air enters to keep the mixture
in proper proportion. The higher the speed of the engine the more the
valve will open, and the greater will be the quantity of air admitted.
In order to get the fullest power from an engine, the carburetor is
built to give its most perfect mixture at the usual working speed.
This will be the speed at which the engine will run under ordinary
conditions. As the engine will run at this speed most of the time, the
carburetor should then deliver its best mixture on the least possible
quantity of fuel.
As an engine is run at low speed so little of the time, it is not
necessary that the mixture should then be so perfect or that the fuel
should be used so economically.
The design of a carburetor is a complicated matter, because the
production of mixture is due to the flow of air, which is a very
changeable thing. On a cold, damp day, the air will be heavier and
denser than on a day that is hot and dry, and different quantities of
fuel will be necessary for the formation of the mixture. The carburetor
manufacturer cannot make a commercial carburetor that will take care
of such a difference as this; he strikes an average that gives good
general results, and expects the user to change the adjustments when
weather and temperature make it necessary.
The formation of the mixture is affected by the condition of the
engine. When all of the parts of the engine are tight, the suction in
the carburetor is more violent than when there is a leakage of air past
the piston rings, or through a leaky valve or spark plug.
On a dry, hot day the fuel evaporates much more readily than on a
day that is cold and damp; more of the fuel that flows out of the
spray nozzle will be vaporized and the formation of the mixture will
be easier. On a cold, damp day the fuel will not vaporize in the
carburetor to any extent, and much of it will pass to the cylinder in
drops that even there will not vaporize in time to form a mixture.
In order to assure the vaporization of enough fuel to form a mixture
under such conditions, the fuel and the air must be heated to a greater
degree.
As the engine becomes heated up, more and more of the fuel will
vaporize, and the amount flowing out of the spray nozzle may therefore
be cut down.
With fuels like kerosene and distillate, which do not vaporize as
readily as gasoline, it is not unusual to have them condense on the
walls of the inlet pipe, which produces a condition known as _loading_.
This condensation is similar to the sweating of an ice-water pitcher
on a hot day. If an engine is running at a constant speed, loading
does not make much difference, because the carburetor is so adjusted
that it gives a proper mixture. If the engine is suddenly speeded up,
however, the greater rush of air will pick up the condensed fuel, and
the mixture will instantly become too rich, continuing so until this
extra supply of fuel is used up. The result will be to choke the engine
and make it lose power just at the time when extra power is needed.
Loading can be prevented by heating the inlet pipe to such an extent
that the fuel will not condense on it.
The speed of a tractor engine is practically always controlled by a
_throttle_, which is a valve set in the passage of the carburetor.
It operates exactly the same as a damper in a stovepipe; when it is
closed, it shuts the passage and prevents the flow of mixture to the
engine. As it is opened, it permits a greater quantity of mixture to
flow, and it follows, of course, that as the charges of mixture become
larger, the engine runs with more power. A tractor carburetor usually
has two throttles, one being operated by hand and the other by the
governor.
It is usual for a carburetor to be fitted with a _strangler_, or
_choke_, which makes it easier to form a mixture at slow starting
speed. When an engine is cold, the fuel evaporates slowly; and,
furthermore, when an engine is cranked by hand its speed is so low that
the suction in the carburetor is not sufficient to draw out enough fuel
to form a mixture. The strangler is a valve similar in every way to the
throttle, but placed between the main air inlet and the spray nozzle.
When it is closed and the engine is cranked, very little air can enter
the carburetor; the suction is therefore very great. Far more fuel
than usual is then sucked out of the spray nozzle, and of this greater
amount enough reaches the cylinder to form a combustible mixture. The
engine will start, but as soon as it does so, the strangler must be
opened so that the normal amount of air enters. If this is not done,
the excessive suction will draw so much fuel out of the spray nozzle
that the mixture formed will be too rich to burn.