SPECIAL TYPES OF ENGINES
In describing various special types of motors, attention is first
directed to that class which depend on the development of heat in
various gases, and this also necessitates some explanation of ice-making
machinery, and the principles underlying refrigeration.
It is not an anomaly to say that to make ice requires heat. Ice and
boiling water represent merely the opposites of a certain scale in the
condition of matter, just as we speak of light and darkness, up and
down, and like expressions.
We are apt to think zero weather is very cold. Freezing weather is a
temperature of 32 degrees. At the poles 70 degrees below have been
recorded. In interstellar space,--that is, the region between the
planets, it is assumed that the temperature is about 513 degrees
Fahrenheit, below zero, called absolute zero.
The highest heat which we are able to produce artificially, is about
10,000 degrees by means of the electric arc. We thus have a range of
over 10,500 degrees of heat, but it is well known that heat extends
over a much higher range.
Assuming, however, that the figures given represent the limit, it will
be seen that the difference between ice and boiling water, namely, 180
degrees, is a very small range compared with the temperatures referred
to.
In order to effect this change power is necessary, and power requires a
motor of some kind. Hence it is, that to make a lower temperature, a
higher degree of heat is necessary, and in the transit between a high
and a low temperature, there is considerable loss in this respect, as in
every other phase of power mechanism, as has been pointed out in a
previous chapter.
In order that we may clearly understand the phenomena of heat and cold,
let us take a receiver which holds a cubic foot of gas or liquid, and
exhaust all the air from it so the vacuum will be equivalent to the
atmospheric pressure, namely, 14.7 pounds per square inch.
Alongside is a small vessel containing one cubic inch of water, which is
heated so that it is converted into steam, and is permitted to exhaust
into the receiver. When all the water is converted into steam and fills
the receiver we shall have the same pressure inside the receiver as on
the outside.
It will be assumed, of course, that there has been no loss by
condensation, and that the cubic inch of water has been expanded 1700
times by its conversion into steam.
In a short time the steam will condense into water, and we now have,
again, a partial vacuum in the receiver, due, of course, to the change
in bulk from steam to water. Each time the liquid is heated it produces
a pressure, and the pressure indicates the presence of heat; and
whenever it cools a loss of pressure is indicated, and that represents
cold, or the opposite of heat.
Now, putting these two things together, we get the starting point
necessary in the development of power. Let us carry the experiment a
step further. Liquids are not compressible. Gases are. The first step
then is to take a gas and compress it, which gives it an increase of
heat temperature, dependent on the pressure.
If the same receiver is used, and say, two atmospheres are compressed
within it, so that it has two temperatures, and the exterior air cools
it down to the same temperature of the surrounding atmosphere, we are
ready to use the air within to continue the experiment.
Let us convey this compressed gas through pipes, and thus permit it to
expand; in doing so the area within the pipes, which is very much
greater than that of the receiver, grows colder, due to the rarefied
gases within. Now bearing in mind the previous statement, that loss of
pressure indicates a lowering of temperature, we can see that first
expanding the gas, or air, by heat, and then allowing it to cool, or to
produce the heat by compressing it, and afterwards permitting it to
exhaust into a space which rarefies it, will make a lower temperature.
It is this principle which is used in all refrigerating machines,
whereby the cool pipes extract the heat from the surrounding atmosphere,
or when making ice, from the water itself, and this temperature may be
lowered to any extent desired, dependent on the degree of rarefaction
produced.
Let us now see how this applies to the generation of power in which we
are more particularly interested.
All liquids do not evaporate at the same temperature as water. Some
require a great deal more than 212 degrees; others, like, for instance,
dioxide-of-carbon, will evaporate at 110 degrees, or about one half the
heat necessary to turn water into steam.
On the other hand, all gases act alike so far as their heat absorption
is concerned, so that by using a material with a low evaporative unit,
less fuel will be required to get the same expansion, which means the
same power.
To illustrate this, let us assume that we have equal quantities of
water, and of dioxide-of-carbon, and that is to be converted into a gas.
It will take just double the amount of fuel to convert the water into a
gaseous state. As both are now in the same condition, the law of heat
absorption is the same from this time on.
The dioxide-of-carbon engine is one, therefore, which uses the vapor of
this material, which, after passing through the engine, is condensed and
pumped back to the boiler to be used over and over.
In like manner, also, ether, which has a low point of vaporization, is
used in some engines, the principle being the same as the foregoing
type.
Rotary Engines.--Many attempts have been made to produce a rotary type
of steam engine, and also to adapt it for use as an internal combustion
motor.
The problem is a complicated one for the following reasons: First, it is
difficult to provide for cut-off and expansion. A rotating type, to be
efficient, must turn at a high rate of speed, and this makes the task a
more trying one. Second, the apparent impossibility of properly packing
the pistons. The result is a waste of steam, or the gas used to furnish
the power. Third, the difficulty in providing a suitable abutment so as
to confine the steam or gas, and make it operative against the piston.
[Illustration: _Fig. 115. Simple Rotary Engine._]
In Fig. 115 is shown a type of rotary which is a fair sample of the
characteristics of all motors of this form. It comprises an outer
cylindrical shell, or casing, A, having a bore through the ends, which
is above the true center of the shell, to receive a shaft B.
This shaft carries a revolving drum C of such dimensions that it is in
contact with the shell at its upper side only, as shown at D, leaving a
channel E around the other portions of the drum.
The steam inlet is at F, which is one-eighth of the distance around the
cylinder, and the exhaust is at G, the same distance from the point D,
on the other side. The inlet and the outlet pipes are, therefore, at the
contracted parts of the channel.
The drum has a pair of radially-movable blades H H´, which may move
independently of each other, but usually they are connected together,
thus dispensing with the use of any springs to keep their ends in
contact with the shell.
When steam enters the inlet F the pressure against the blade H drives
the drum to the right, and the drum and shell, by contacting at D, form
an abutment. Each charge of steam drives the drum a little over a half
revolution.
A great deal of ingenuity has been exercised to arrange this abutment so
that the blades may pass and provide a steam space for a new supply of
steam. In certain types a revolving abutment is formed, as shown, for
instance, in Fig. 116.
The shell A, in this case, has two oppositely-disposed inlet and outlet
ports, B, C, respectively, and between each set of ports is a revolving
gate, formed of four wings D, mounted on a shaft E, in a housing
outside of the circular path F, between the drum G and shell A.
The drum G is mounted on a shaft H which is centrally within the shell,
and it has two oppositely-projecting rigid blades I. When steam enters
either of the supply ports B, the drum is rotated, and when the blades
reach the revolving gates, the latter are turned by the blades, or, they
may be actuated by mechanism connected up with the driving shaft.
[Illustration: _Fig. 116. Double-feed Rotary Engine._]
Caloric Engine.--This is an engine which is dependent on its action upon
the elastic force of air which is expanded by heat. The cylinder of
such a motor has means for heating it, and thus expanding the air, and a
compressor is usually employed which is operated by the engine itself,
to force compressed air into the cylinder.
It is not an economical engine to work, but it is frequently used in
mines, in which case the compressor is located at the surface, and the
engine operated within the mine, thus serving a double purpose, that of
supplying power, and also furnishing the interior with fresh air.
All engines of this character must run at a slow speed, for the reason
that air does not absorb heat rapidly, and sufficient time must be given
to heat up and expand the air, so as to make it effective.
Adhesion Engine.--A curious exhibition of the action of a gas against a
solid, is shown in what is called an _Adhesion Engine_. Fig. 117 shows
its construction. A plurality of disks A are mounted on a shaft B, these
disks being slightly separated from each other.
The steam discharge pipe C is flattened at its emission end, as shown at
D, so the steam will contact with all the disks. The steam merely
contacts with the sides of the disks, the movement of the steam being
substantially on the plane of the disks themselves, and the action sets
up a rapid rotation, and develops a wonderful amount of power.
[Illustration: _Fig. 117. Adhesion Motor._]
It will be understood that the disks are enclosed by a suitable casing,
so that the steam is carried around and discharged at a point about
three quarters of the distance in the circumference.
This motor is given to illustrate a phase of the subject in the
application of a motor fluid, like steam, or heated gases, that shows
great possibilities. It also points out a third direction in which an
expansive fluid may be used.
Thus the two well-known methods, namely, _pressure_, and _impact_
forces, may be supplemented by the principle of _adhesion_, in which the
expansive force of a gas, passing alongside of and in contact with a
plain surface, may drag along the surface in its train.
Such an exhibition of force has an analogy in nature by what is known as
capillary attraction, which shows _adhesion_. For instance, sap flowing
up the pores of trees, or water moving along the fibers of blotting
paper, illustrates movement of liquids when brought into contact with
solids.