ECONOMY IN RUNNING A FARM ENGINE.
It is something to be able to run a farm engine and keep out of
trouble. It is even a great deal if everything runs smoothly day in
and day out, if the engine looks clean, and you can always develop the
amount of power you need. You must be able to do this before you can
give the fine points of engineering much consideration.
When you come to the point where you are always able to keep out of
trouble, you are probably ready to learn how you can make your engine
do more work on less fuel than it does at present. In that direction
the best of us have an infinite amount to learn. It is a fact that
in an ordinary farm engine only about 4 per cent of the coal energy
is actually saved and used for work; the rest is lost, partly in the
boiler, more largely in the engine. So we see what a splendid chance
there is to save.
If we are asked where all the lost energy goes to, we might reply in
a general sort of way, a good deal goes up the smokestack in smoke or
unused fuel; some is radiated from the boiler in the form of heat and
is lost without producing any effect on the steam within the boiler;
some is lost in the cooling of the steam as it passes to the steam
cylinder; some is lost in the cooling of the cylinder itself after
each stroke; some is lost through the pressure on the back of the
steam valve, causing a friction that requires a good deal of energy
in the engine to overcome; some is lost in friction in the bearings,
stuffing-boxes, etc. At each of these points economy may be practiced
if the engineer knows how to do it. We offer a few suggestions.
THEORY OF STEAM POWER.
As economy is a scientific question, we cannot study it intelligently
without knowing something of the theory of heat, steam and the
transmission of power. There will be nothing technical in the following
pages; and as soon as the theory is explained in simple language, any
intelligent person will know for himself just what he ought to do in
any given case.
First, let us define or describe heat according to the scientific
theory. Scientists suppose that all matter is made up of small
particles called molecules, so small that they have never been seen.
Each molecule is made up of still smaller particles called atoms. There
is nothing smaller than an atom, and there are only about sixty-five
different kinds of atoms, which are called elements; or rather, any
substance made up of only one kind of atom is called an element.
Thus iron is an element, and so is zinc, hydrogen, oxygen, etc. But
a substance like water is not an element, but a compound, since its
molecules are made up of an atom of oxygen united with two atoms of
hydrogen. Wood is made up of many different kinds of atoms united in
various ways. Air is not a compound, but a mixture of oxygen, nitrogen
and a few other substances in small quantities.
The reason why air is a mixture and not a compound is an interesting
one, and brings us to our next point. In order to form a compound,
two different kinds of atoms must have an attraction for each other.
There is no attraction between oxygen and nitrogen; but there is great
attraction between oxygen and carbon, and when they get a chance they
rush together like long separated lovers. Anthracite coal is almost
pure carbon. So is charcoal. Soft coal consists of carbon with which
various other things are united, one of them being hydrogen. This is
interesting and important, because it accounts for a curious thing in
firing up boilers with soft coal. We have already said that water is
oxygen united with hydrogen. When soft coal burns, not only does the
carbon unite with oxygen, but the hydrogen unites with oxygen and forms
water, or steam. While the boilers are cold they will condense the
water or steam in the smoke, just as a cold plate in a steamy room will
condense water from the steamy air, so sweating.
Now the scientists suppose that two or three atoms stick together by
reason of their attraction for each other and form molecules. These
molecules in turn stick together and form liquids and solids. The
tighter they stick, the harder the substance. At the same time, these
molecules are more or less loose, and are constantly moving back and
forth. In a solid like iron they move very little; but a current of
electricity through iron makes the molecules move in a peculiar way. In
a liquid like water, the molecules cling together very loosely, and may
easily be pulled apart. In any gas, like air or steam, the molecules
are entirely disconnected, and are constantly trying to get farther
apart.
Heat, says the scientist, is nothing more or less than the movement
of the molecules back and forth. Heat up a piece of iron in a hot
furnace, and the molecules keep getting further and further apart,
and the iron gets softer and softer, till it becomes a liquid. If we
take some liquid like water and heat it, the molecules get farther and
farther apart, till the water boils, as we say, or turns into steam.
As steam the molecules have broken apart entirely, and are beating
back and forth so rapidly that they have a tendency to push each other
farther and farther apart. This pushing tendency is the cause of steam
pressure. It also explains why steam has an expansive power.
Heat, then, is the movement of the molecules back and forth. There are
three fixed ranges in which they move; the small range makes a solid;
the next range makes a liquid; the third range makes a gas, such as
steam. These three states of matter as affected by heat are very sharp
and definite. The point at which a solid turns to a liquid is called
the melting point. The melting point of ice is 32° Fahr. The point at
which it turns to a gas is called the boiling point. With water that is
212° Fahr. The general tendency of heat is to push apart, or expand;
and when the heat is taken away the substances contract.
Let us consider our steam boiler. We saw that some different kinds of
atoms have a strong tendency to rush together; for example, oxygen
and carbon. The air is full of oxygen, and coal and wood are full
of carbon. When they are raised to a certain temperature, and the
molecules get loose enough so that they can tear themselves away from
whatever they are attached to, they rush together with terrible force,
which sets all surrounding molecules to vibrating faster than ever.
This means that heat is given out.
Another important thing is that when a solid changes to a liquid, or a
liquid to a gas, it must take up a certain amount of heat to keep the
molecules always just so far apart. That heat is said to become latent,
for it will not show in a thermometer, it will not cause anything to
expand, nor will it do any work. It merely serves to hold the molecules
just so far apart.
HOW ENERGY IS LOST.
We may now see some of the ways in which energy is lost. First, the air
which goes into the firebox consists of nitrogen as well as oxygen.
That nitrogen is only in the way, and takes heat from the fire, which
it carries out at the smokestack.
Again, if the air cannot get through the bed of coals easily enough,
or there is not enough of it so that every atom of carbon, etc., will
find the right number of atoms of oxygen, some of the atoms of carbon
will be torn off and united with oxygen, and the other atoms of carbon,
left without any oxygen to unite with, will go floating out at the
smokestack as black smoke. Also, the carbon and the oxygen cannot unite
except at a certain temperature, and when fresh fuel is thrown on the
fire it is cold, and a good many atoms of carbon after being loosened
up, get cooled off again before they have a chance to find an atom of
oxygen, and so they, too, go floating off and are lost.
If the smoke could be heated up, and there were enough oxygen mixed
with it, the loose carbon would still burn and produce heat, and there
would be an economy of fuel. This has given rise to smoke consumers,
and arranging two boilers, so that when one is being fired the heat
from the other will catch the loose carbon before it gets away and burn
it up.
So we have these points:
1. Enough oxygen or air must get into a furnace so that every atom of
carbon will have its atom of oxygen. This means that you must have a
good draft and that the air must have a chance to get through the coal
or other fuel.
2. The fuel must be kept hot enough all the time so that the carbon and
oxygen can unite. Throwing on too much cold fuel at one time will lower
the heat beyond the economical point and cause loss in thick smoke.
3. If the smoke can pass over a hot bed of coals, or through a hot
chamber, the carbon in it may still be burned. This suggests putting
fuel at the front of the firebox, a little at a time, so that its smoke
will have to pass over a hot bed of coals and the waste carbon will be
burned. When the fresh fuel gets heated up, it may be pushed farther
back.
From a practical point of view these points mean, No dead plates in a
furnace to keep the air from going through coal or wood; a thin fire so
the air can get through easily; place the fresh fuel where its smoke
will have a chance to be burned; and do not cool off the furnace by
putting on much fresh fuel at a time.
(Later we will give more hints on firing.)
HOW HEAT IS DISTRIBUTED.
We have described heat as the movement of molecules back and forth at a
high rate of speed. If these heated molecules beat against a solid like
iron, its molecules are set in motion, one knocks the next, and so on,
just as you push one man in a crowd, he pushes the next, and so on till
the push comes out on the other side. So heat passes through iron and
appears on the other side. This is called “conduction.”
All space is supposed to be filled with a substance in which heat,
light, etc., may be transmitted, called the ether. When the molecules
of a sheet of iron are heated, or set vibrating, they transmit the
vibration through the air, or ether. This is called “radiation.” Heat
is “conducted” through solid and liquid substances, and “radiated”
through gases.
Now some substances conduct heat readily, and some do so with the
greatest difficulty. Iron is a good conductor; carbon, or soot on the
flues of a boiler, and lime or scale on the inside of a boiler, are
very poor conductors. So the heat will go through the iron and steel to
the water in a boiler quickly and easily, and a large per cent of the
heat of the furnace will get to the water in a boiler. When a boiler is
old and is clogged with soot and coated with lime, the heat cannot get
through easily, and goes off in the smokestack. The air coming out of
the smokestack will be much hotter; and that extra heat is lost.
Iron is a good radiator, too. So if the outer shell of a boiler is
exposed to the air, a great deal of heat will run off into space and be
lost. Here, then, is where you need a non-conductor, as it is called,
such as lime, wood, or the like.
Economy says, cover the outside of a boiler shell with a non-conductor.
This may be brickwork in a set boiler; in a traction boiler it means
a jacket of wood, plaster, hair, or the like. The steam pipe, if it
passes through outer air, should be covered with felt; and the steam
cylinder ought to have its jacket, too.
At the same time all soot and all scale should be scrupulously cleaned
away.
PROPERTIES OF STEAM.
As we have already seen, steam is a gas. It is slightly blue in color,
just as the water in the ocean is blue, or the air in the sky.
We must distinguish between steam and vapor. Vapor is small particles
of water hanging in the air. They seem to stick to the molecules
composing the air, or hang there in minute drops. Water hanging in the
air is, of course, water still. Its molecules do not have the movement
that the molecules of a true gas do, such as steam is. Steam, moreover,
has absorbed latent heat, and has expansive force; but vapor has no
latent heat, and no expansive force. So vapor is dead and lifeless,
while steam is live and full of energy to do work.
When vapor gets mixed with steam it is only in the way; it is a sort of
dead weight that must be carried; and the steam power is diminished by
having vapor mixed with it.
Now all steam as it bubbles up through water in boiling takes up with
it a certain amount of vapor. Such steam is called “wet” steam. When
the vapor is no longer in it, the steam is called “dry” steam. It is
dry steam that does the best work, and that every engineer wants to get.
While water will be taken up to great heights in the air and form
clouds, in steam it will not rise very much, and at a certain height
above the level of the water in a boiler the steam will be much drier
than near the surface. For this reason steam domes have been devised,
so that the steam may be taken out at a point as high as possible above
the water in the boiler, and so be as dry as possible. Also “dry tubes”
have been devised, which let the steam pass through many small holes
that serve to keep back the water to a certain extent.
However, there will be more or less moisture in all steam until it
has been superheated, as it is called. This may be done by passing it
through the hot part of the furnace, where the added heat will turn all
the moisture in the steam into steam, and we shall have perfectly dry
steam.
The moment, however, that steam goes through a cold pipe, or one cooled
by radiation, or goes into a cold cylinder, or a cylinder cooled by
radiation, some of the steam will turn to water, or condense, as it is
called. So we have the same trouble again.
Much moisture passing into the cylinder with the steam is called
“priming.” In that case the dead weight of water has become so great as
to kill a great part of the steam power.
HOW TO USE THE EXPANSIVE POWER OF STEAM.
We have said that the molecules in steam are always trying to get
farther and farther apart. If they are free in the air, they will soon
scatter; but if they are confined in a boiler or cylinder they merely
push out in every direction, forming “pressure.”
When steam is let into the cylinder it has the whole accumulated
pressure in the boiler behind it, and of course that exerts a strong
push on the piston. Shut off the boiler pressure and the steam in the
cylinder will still have its own natural tendency to expand. As the
space in the cylinder grows larger with the movement of the piston from
end to end, the expansive power of the steam becomes less and less,
of course. However, every little helps, and the push this lessened
expansive force exerts on the piston is so much energy saved. If the
full boiler pressure is kept on the piston the whole length of the
stroke, and then the exhaust port is immediately opened, all this
expansive energy of the steam is lost. It escapes through the exhaust
nozzle into the smokestack and is gone. Possibly it cannot get out
quickly enough, and causes back pressure on the cylinder when the
piston begins its return stroke, so reducing the power of the engine.
To save this the skilled engineer “notches up” his reverse lever, as
they say. The reverse lever controls the valve travel. When the lever
is in the last notch the valve has its full travel. When the lever is
in the center notch the valve has no travel at all, and no steam can
get into the cylinder; on the other side the lever allows the valve to
travel gradually more and more in the opposite direction, so reversing
the engine.
As the change from one direction to the other direction is, of course,
gradual, the valve movement is shortened by degrees, and lets steam
into the cylinder for a correspondingly less time. At its full travel
it perhaps lets steam into the cylinder for three-quarters of its
stroke. For the last quarter the work is done by the expansive power of
the steam.
Set the lever in the half notch, and the travel of the valve is so
altered that steam can get into the cylinder only during half the
stroke of the piston, the work during the rest of the stroke being done
by the expansive force of the steam.
Set the lever in the notch next to the middle notch, or the quarter
notch, and steam will get into the cylinder only during a quarter of
the stroke of the piston, the work being done during three-quarters of
the stroke by the expansive force of the steam.
Obviously the more the steam is expanded the less work it can do. But
when it escapes at the exhaust there will be very little pressure to be
carried away and lost.
Therefore when the load on his engine is light the economical engineer
will “notch up” his engine with the reverse lever, and will use up
correspondingly less steam and save correspondingly more fuel. When the
load is unusually heavy, however, he will have to use the full power of
the pressure in the boiler, and the waste cannot be helped.
THE COMPOUND ENGINE.
The compound engine is an arrangement of steam cylinders to save the
expansive power of steam at all times by letting the steam from one
cylinder where it is at high pressure into another after it exhausts
from the first, in this second cylinder doing more work purely by the
expansive power of the steam.
The illustration shows a sectional view of a compound engine having two
cylinders, one high pressure and one low. The low pressure cylinder is
much larger than the high pressure. There is a single plate between
them called the center head, and the same piston rod is fitted with two
pistons, one for each cylinder. The steam chest does not receive steam
from the boiler, but from the exhaust of the high pressure cylinder.
The steam from the boiler goes into a chamber in the double valve, from
which it passes to the ports of the high pressure cylinder. At the
return stroke the exhaust steam escapes into the steam chest, and from
there it passes into the low pressure cylinder. There may be one valve
riding on the back of another; but the simplest form of compound engine
is built with a single double valve, which opens and closes the ports
for both cylinders at one movement.
[Illustration: WOOLF TANDEM CYLINDER.]
Theoretically the compound engine should effect a genuine economy. In
practice there are many things to operate against this. Of course if
the steam pressure is low to start with, the amount of pressure lost in
the exhaust will be small. But if it is very high, the saving in the
low pressure cylinder will be relatively large. If the work can be done
just as well with a low pressure, it would be a practical waste to keep
the pressure abnormally high in order to make the most of the compound
engine.
An engine must be a certain size before the saving of a compound
cylinder will be appreciable. In these days nearly all very large
engines are compound, while small engines are simple.
Another consideration to be taken into account is that a compound is
more complicated and so harder to manage; and when any unfavorable
condition causes loss it causes proportionately more loss on a compound
than on a simple engine. For these and other reasons compound engines
have been used less for traction purposes than simple engines have.
It is probable that a skilled and thoroughly competent engineer, who
would manage his engine in a scientific manner, would get more out of
a compound than out of a simple; and this would be especially true
in regions where fuel is high. If fuel is cheap and the engineer
unskilled, a compound engine would be a poor economizer.
FRICTION.
We have seen that the molecules of water have a tendency to stick in
the steam as vapor or moisture. All molecules that are brought into
close contact have more or less tendency to stick together, and this is
called friction. The steam as it passes along the steam pipe is checked
to a certain extent by the friction on the sides of the pipe. Friction
causes heat, and it means that the heat caused has been taken from some
source of energy. The friction of the steam diminishes the energy of
the steam.
So, too, the fly wheel moving against the air suffers friction with
the air, besides having to drive particles of air out of its path. All
the moving parts of an engine where one metal moves on another suffer
friction, since where the metals are pressed very tightly together they
have more tendency to stick than when not pressed so tightly. When
iron is pressed too tightly, as under the blows of a hammer in a soft
state, it actually welds together solidly.
There is a great deal of friction in the steam cylinder, since the
packing rings must press hard against the walls of the cylinder to
prevent the steam from getting through. There is a great deal of
friction between the D valve and its seat, because of the high steam
pressure on the back of the valve. There is friction in the stuffing
boxes both of the valve and the piston. There is friction at all the
bearings.
There are various ways in which friction may be reduced. The most
obvious is to adjust all parts so nicely that they will bind as little
as possible. The stuffing-boxes will be no tighter than is necessary to
prevent leaking of steam; and so with the piston rings. Journal boxes
will be tight enough to prevent pounding, but no tighter. To obtain
just the right adjustment requires great patience and the keen powers
of observation and judgment.
The makers of engines try to reduce friction as much as possible by
using anti-friction metals in the boxes. Iron and steel have to be used
in shafts, gears, etc., because of the strength that they possess; but
there are some metals that stick to each other and to iron and steel
much less than iron or steel stick to each other when pressed close
together. These metals are more or less soft; but they may be used in
boxes and journal bearings. They are called anti-friction metals. The
hardest for practical purposes is brass, and brass is used where there
is much wear. Where there is less wear various alloys of copper, tin,
zinc, etc., may be used in the boxes. One of these is babbit metal,
which is often used in the main journal box.
All these anti-friction metals wear out rapidly, and they must be put
in so that they can be adjusted or renewed easily.
But the great anti-friction agent is oil.
Oil is peculiar in that while the molecules seem to stick tightly
together and to a metal like iron or steel, they roll around upon each
other with the utmost ease. An ideal lubricator is one that sticks so
tight to the journal that it forms a sort of cushion all around it, and
prevents any of its molecules coming into contact with the molecules of
the metal box. All the friction then takes place between the different
molecules of oil, and this friction is a minimum.
The same principle has been applied to mechanics in the ball bearing.
A number of little balls roll around between the journal and its box,
preventing the two metals from coming into contact with each other;
while the balls, being spheres, touch each other only at a single
point, and the total space at which sticking can occur is reduced to a
minimum.
As is well known, there is great difference in oils. Some evaporate,
like gasoline and kerosene, and so disappear quickly. Others do not
stick tightly to the journal, so are easily forced out of place, and
the metals are allowed to come together. What is wanted, then, is a
heavy, sticky oil that will not get hard, but will always form a good
cushion between bearings.
Steam cylinders cannot be oiled directly, but the oil must be carried
to the steam chest and cylinder in the steam. A good cylinder oil must
be able to stand a high temperature. While it is diffused easily in the
steam, it must stick tightly to the walls of the steam cylinder and
to the valve seat, and keep them lubricated. Once it is stuck to the
metal, the heat of the steam should not evaporate it and carry it away.
Again, a cylinder oil should not have any acid in it which would have
a tendency to corrode the metal. Nearly all animal fats do have some
such acid. So tallow and the like should not be placed where they can
corrode iron or steel. Lard and suet alone are suitable for use on an
engine.
When it comes to lubricating traction gears, other problems appear. A
heavy grease will stick to the gears and prevent them from cutting; but
it will stick equally to all sand and grit that may come along, and
that, working between the cogs, may cut them badly. So some engineers
recommend the use on gears of an oil that does not gather so much dirt.
The friction of the valve on its seat due to the pressure of the steam
on its back has given rise to many inventions for counteracting it.
The most obvious of these is what is called “the balanced valve.” In
the compound engine, where the steam pressure is obtained upon both
sides of the valve, it rides much more lightly on its seat--so lightly,
indeed, that when steam pressure is low, as in going down hill or
operating under a light load, plunger pistons must be used to keep the
valve down tight on its seat.
The poppet valves were devised to obviate the undue friction of the D
valve; but the same loss of energy is to a certain extent transferred,
and the practical saving is not always equal to the theoretical. On
large stationary engines rotary valves and other forms, such as are
used on the Corliss engine, have come into common use; but they are too
complicated for a farm engine, which must be as simple as possible,
with least possible liability of getting out of order.