FUELS AND COMBUSTION
All fuels must be put into a gaseous state before they will burn. This
is true of coal as well as of hydro-carbon oils.
Neither coal nor petroleum will burn in its native state, without the
addition of oxygen. This is absolutely necessary to support combustion.
Burning is caused by the chemical union of oxygen with such substances
as will burn.
This burning process may be slow, and extend over a period of years, or
it may be instantaneous, in which latter case the expansion of the
heated gases is so great as to cause an explosion. When a sufficient
amount of oxygen has been mixed with a fuel to permit it to burn, a high
temperature is necessary to cause the immediate burning of the entire
mass.
If such a temperature is not present the course of combustion is not
arrested, but it will, on its own account, start to oxydize, and
eventually be reduced to the same condition that would take place if
exploded by means of a flame.
Solid Fuels.--The great fuels in nature are carbon and hydrogen, carbon
being the substance most widely known and depended upon. Hard coal, for
instance, is composed almost wholly of carbon; whereas soft coal has a
considerable quantity of hydrogen.
As coal was formed by wood, which, through long process of time became
carbonized, it contains considerable foreign matter which will not burn,
forming ash.
Liquid Fuels.--The volatile oils, however, have very little
non-combustible matter. Ordinary petroleum contains about 80 per cent,
of carbon, and from 12 to 15 per cent. of hydrogen, the residue being
foreign matter, all more or less susceptible of being consumed at high
temperatures.
Combustion.--The term _combustion_, in its general sense, means the act
of burning; but in a larger and more correct application it refers to
that change which takes place in matter when oxygen unites with it.
Oxygen is a wonderful element, and will unite with all known substances,
unlike all other elements in this respect. It may take years for it to
form a complete unity. Thus, wood, in time, will crumble, or rot, as it
is called. This is a slow process of combustion, brought about without
applying heat to it, the change taking place in a gradual way, because
oxygen unites with only a small portion of the wood.
Oxidation.--Iron will rust. This is another instance of combustion,
called oxidation. When oxygen unites with a substance it may produce an
acid, or an alkali, or a neutral compound. When wood is burned it
produces an ash, and this ash contains a large amount of potash, or lye,
which is an alkali, or a salt. So when other substances are burnt the
result may be an acid, like sulphur, or it may be unlike either acid or
the alkali.
The unity of oxygen with the food in the body is another instance of
oxidation, which produces and maintains the heat necessary for
existence.
Carbon or hydrogen, as a fuel, are inert without oxygen, so that in
considering the evolution of a force which is dependent on heat, we
should know something of its nature, thereby enabling us to utilize it
to the best advantage.
The Hydro-carbon Gases.--If petroleum, or gasoline, should be put into
the form of a gas, and as such be confined in a receiver, without adding
any oxygen, it would be impossible to ignite it.
The character of the material is such that it would instantaneously
extinguish any flame. Now, to make a burning mixture, at least three
parts of oxygen must be mixed with one of the hydro-carbon, before it is
combustible.
Oxygen and Atmosphere.--The atmosphere is not oxygen. Only one-fifth of
common air is oxygen, the residue being, principally, nitrogen, which is
not a fuel. To produce the proper aëration, therefore, at least fifteen
parts of air must be mixed with one part of hydro-carbon gas.
The term _hydro-carbon_ is applied to petroleum, and its products,
because the elements carbon and hydrogen make up the largest part of the
oil, whereas this is not the case with most of the other oils.
We are now dealing with a fuel such as is needed in _Internal Combustion
Engines_, and it is well to know some of the problems involved in the
use of the fuel, as this will give a better understanding of the
structure of the devices which handle and evolve the gases, and properly
burn them within the engine.
Vaporizing Fuel.--As the pure liquid will not burn in that state the
first essential is to put it into a gaseous form, or to generate a vapor
from it. The vapor thus made is not a gas, in the true sense of that
term, but it is composed of minute globules of finely-divided particles
of oil.
Nearly all liquids will vaporize if permitted to come into contact with
air. The greater the surface exposed to air the more rapidly will it
turn into a vapor.
By forcibly ejecting the liquid from a pipe or spraying device, and
mingling air with it, evaporation is facilitated, and at the same time
the proper admixture of air is provided to make a combustible substance
the moment sufficient heat is brought into contact with it.
This is what actually takes place in a gasoline engine, and all the
mechanism is built with this end in view.
It has been the universal practice to make an explosive mixture of this
character, and then ignite it by means of an electric spark, but it is
now known that such a fuel can be exploded by pressure, and this needs
some explanation.
Explosion by Compression.--The study of the compressibility of gases is
an interesting one. As we have previously stated, the atoms, comprising
the gases, are constantly moving among themselves with great rapidity,
so that they bombard the sides of the receiver in which they are
confined, and also contact with each other in their restless movements.
When compression takes place the speed of the movements of the atoms is
greatly accelerated, the friction of their movements is increased, and
heat is evolved. As the pressure becomes greater the heat increases
until it is of such intensity that the gas ignites, and an explosion
follows.
How Compression Heats.--The theory of the compressibility of gases may
be stated as follows: Let us assume that the temperature of the air is
70 degrees Fahrenheit, and we have a receiver which holds two cubic feet
of this air.
If the contained air is now compressed to a volume of one cubic foot,
the temperature of two cubic feet is compressed into one cubic foot, and
there is now 140 degrees of heat within the receiver.
If this cubic foot of air is again compressed to half its volume, the
temperature is correspondingly increased. While this it not absolutely
true in practice, owing to the immense loss caused by radiation, still,
it will enable the mind to grasp the significance of compression, when
the subject of heat is concerned.
Elasticity of Gases.--The great elasticity of gases, and the perfected
mechanical devices for compressing the same, afford means whereby ten or
twenty atmospheres can be forced into a receiver, and thereby produce
pressures of several hundred pounds, which would mean sufficiently high
temperatures to ignite oils having the higher flash point.
Advantages of Compression.--The compression system permits of the
introduction of a larger quantity of fuel than is usually drawn into the
cylinder, and thereby a greater and more efficient action is produced on
the piston of the engine on account of quicker combustion and therefore
higher gas pressures.
The compression, however, rarely if ever exceeds six atmospheres or
about 90 pounds per square inch.
_The Necessity of Compression._--There are two reasons why compression
is necessary before igniting it. First, because it is essential to put
sufficient gas in the cylinder to make the engine efficient.
To illustrate: Suppose we have a cylinder capable of drawing in 150
cubic inches of gas, and this is compressed down to 25 cubic inches, the
space then occupied by the gas would represent what is called the
clearance space at the head of the cylinder. To compress it to a greater
degree the clearance space might be made smaller, which could be done in
several ways, but whether the gas thus drawn in should be compressed to
30, or 25, or even 10 cubic inches, it is obvious that there would be
no more fuel in the cylinder in one case than in the other. As however
the mean effective pressure, which determines the efficiency of the
motor, increases with the compression pressure, the latter should be as
high as possible, but not so high that premature explosion takes place
owing to the heat created by compression.
Second: The more perfect the mixture of the vaporized product with the
air, the more vigorous will be the explosion. The downward movement of
the piston draws in the charge of air and sprayed jet of gasoline, and
the only time for mixing it is during the period that it travels from
the carbureter through the pipes and manifold to the cylinder.
Having in mind the statement formerly made that compression causes a
more rapid movement of the molecules of a gas, it is obvious that the
upward movement of the piston, in the act of compressing the gas has a
more positive action in causing an intimate mixture of the hydro-carbon
gases than took place when the gases were traveling through the pipes on
their way to the cylinder.