ENGINES OF WAR
The phrase which I have used for the title of this chapter is often
given a very wide meaning which includes all kinds and varieties of
devices used in warfare. In this case I am giving it its narrower sense,
taking it to indicate the steam-engines and oil-engines which are
employed to drive our battleships, cruisers and destroyers, our
submarines and our aircraft. They are inventions of the highest
importance, which have played a large part in shaping modern warfare.
The type of engine almost invariably used on ships of war other than
submarines is the steam turbine. Great Britain, for the most part, uses
that particular kind associated with the name of the Hon. Sir C. A.
Parsons, while the United States rather favour the Curtiss machine.
Other nations have adopted either one of these or else something very
similar.
All turbines are very simple in their principle, far more so that the
older type of steam-engine, called, because the essential parts of it
move to and fro, the "reciprocating" steam-engine.
In these latter machines there are a number of cylinders with closed
ends and with very smooth interiors, in each of which slides a disc-like
object called a piston. The steam enters a cylinder first at one end
and then at the other, thus pushing the piston to and fro. The movement
of the piston is communicated to the outside by means of a rod which
passes through a hole in the cover at one end of the cylinder, the to
and fro motion being converted into a round and round motion by a
connecting-rod and crank just as the up and down motion of a cyclist's
knees is converted into a round and round motion by the lower leg and
the crank. The lower part of a cyclist's leg is, indeed, a very accurate
illustration of what the connecting-rod of a steam-engine is.
As is evident to the hastiest observer, some arrangement has to be made
whereby the steam shall be led first into one end and then into the other
end of the cylinder: also that provision shall be made for letting the
steam out again when it has done its work. Moreover, such arrangements
must be automatic. Hence, every reciprocating engine has special valves
for this purpose and such valves need rods and cranks (or something
equivalent) to operate them. Further, to get the best results the steam
must not simply be passed through one cylinder but through several in
succession. Engines where the steam goes through only one cylinder are
called "simple," where it goes through two they are "compound," where
three "triple-expansion," where four "quadruple-expansion." Generally
speaking, each cylinder has its own connecting-rod and crank, also its
own set of rods, etc., for working its valves. Hence, a high-class marine
reciprocating engine is of necessity a complicated mass of cylinders,
rods, cranks and other moving parts continually swinging round or to and
fro at considerable speeds, all needing oiling and attention and all
liable at times to give trouble.
And now compare that with the turbine, which has TWO parts, only one of
which moves. That part, moreover, is tightly shut up inside the other
one, being thereby protected from any chance of damage from outside and
likewise rendered unable to inflict any damage upon those in attendance
upon it.
At first sight it seems very strange that the turbine should be the
newer of the two, for it is simply an improved form of the old
time-honoured picturesque windmill which used to top every hill and
grind the corn for every village and hamlet.
The old windmill had four sails against which the wind blew, driving the
whole four round as everyone knows. The new turbine has a great many
sails, only we now call them blades, and the steam blows them round. The
old windmill had to have another smaller set of sails at the back for
the purpose of keeping the main sails always in that position in which
they would catch the full force of the breeze. In the turbine we need
not do that, for we shut the windmill up in a kind of tunnel and cause
the steam to blow in at one end and out at the other.
The difference between the various kinds of turbine lies simply in the
manner in which the steam is guided in its passage through the machine.
After that general description we can take a more detailed view of the
Parsons turbine. The casing or fixed part is a huge iron box suitably
shaped for standing firmly and rigidly upon the floor of the
engine-room. It is made in two halves, the upper of which can be easily
lifted off when necessary. Often, indeed, this upper half is hinged to
the lower, so that it can be opened like the lid of a box.
Inside, the casing is cylindrical, comparatively small at one end but
increasing by steps till it is very much larger at the other end. At
each end is a bearing or support in which the rotor or moving part is
held and in which it can turn freely.
The rotor or part which rotates is a strong steel forging shaped
somewhat to follow the lines of the inside of the casing. It does not
entirely fill the casing but leaves a space all round and all the way
along, which space is intended to accommodate the blades. The ends of
the rotor are smaller than the body since they are intended to fit into
the bearings, and one of the ends is prolonged so as to be available for
coupling to the propeller-shaft of the ship.
At one end of the casing, the smaller one, is the steam inlet and the
steam after emerging from it passes along till it finds its way out at a
very large outlet formed at the bigger end. On its way it has to pass
thousands of small blades so that the progress of each individual
particle of steam is not a straight line but a continual zigzag. There
are rings of blades round the rotor, tightly fixed to its surface. There
are likewise rings of blades affixed to the inner surface of the casing,
the rings upon the casing coming in the spaces between the rings on the
rotor.
Let us imagine that we can see through the casing of a turbine at work
and that looking down upon it from above we can trace the progress of a
particle of steam. It rushes in from the inlet and at once makes
straight for the outlet at the further end. Suddenly, however, it
encounters one of the guide blades (those on the case) and by it is
deflected to one side, we will suppose the left. That causes it to rush
straight at one of the blades upon the rotor against which it strikes
violently, giving that blade a distinct and definite push to the left.
Rebounding, it then comes back towards the right but quickly is caught
by another guide blade and by it hurled back upon a second rotor blade,
giving it a leftward push just as it did to the first. Thus it goes
zigzagging from one set of blades to the other until, tired out, so to
speak, it finally flows away forceless and feeble through the outlet,
having given up all its energy to the blades of the rotor against which
it has struck in its course.
That, then, is the journey of one single particle. Multiply that by an
unknown number of millions and you have a description of what takes
place in the interior of a steam turbine. The blades are so
proportioned, so arranged and so placed that it is very difficult indeed
for a particle of steam to creep past without doing its share of work.
Practically every one is made use of and while, of course, the action of
a single particle of steam would have but a negligible effect, the vast
number engaged cause the rotor to be powerfully blown round.
The reason why the casing and rotor are made larger and larger as one
proceeds from the inlet towards the exhaust or outlet is that the steam
must, if all its energy is to be extracted, expand as it goes and the
enlargement provides room for this expansion.
One of the great advantages of the turbine is that the steam is always
entering at the same end. In the cylinder of a reciprocating engine the
steam enters alternately. It comes in hot but as it does its work and
finally goes out it becomes very much cooler: the next lot of steam
which enters, therefore, is chilled by the cool walls of the cylinder
which have just been cooled by the departure of the previous lot of
steam: so heat is wasted. Wasted heat means fuel lost, and as any given
ship can only carry a limited quantity of fuel, wasted heat means less
range and more frequent returns to the base to coal or to "oil."
Also let me remark again upon the simplicity of the turbine as opposed
to the other sort. The latter consists of a mass of moving and swaying
rods and cranks, to work among which, as the engineers have to do, is a
terrifying and nerve-racking experience. The turbine, on the other hand,
has its only working part enclosed. It is difficult to tell, by looking
at it, whether a turbine is at work or not, so silent and still is it,
so self-contained. The reciprocating engine-room is noisy and full of
turmoil: the turbine room is weirdly still by comparison.
On the whole, too, it makes better use of the steam which it uses, but
it has one decided drawback. It will not reverse, which the other type
of engine does readily.
This means that two turbines have to be coupled together, one with the
blades so set that the steam drives it round correctly to produce
motion ahead and the other set the opposite way so that it drives the
vessel astern. The steam can be sent through either turbine at will and
so motion can be obtained in either direction. Whichever turbine is in
use the other revolves idly.
Unfortunately it is impossible to make a turbine to go slowly and yet be
efficient. Consequently, slow steamers cannot use turbines, but for
warships, which are nearly all fast boats, it has almost displaced the
older type of engine.
The Curtiss turbine is different from the Parsons in that the steam
encounters periodically, in its passage through, a partition perforated
with funnel-shaped holes. Between the partitions it passes blades upon
which it acts just as already described. The chief effect of this is to
permit the machine being made of a rather more convenient shape and
size. Other varieties of turbine are more or less combinations of the
two ideas underlying these two.
When we look at a locomotive in motion we always see steam coming out of
the funnel, but we never see that in the case of a steamer. That is
because all the energy of the steam is taken and used in the latter
case, while in the former much valuable energy goes off up the funnel,
making a puffing noise instead of doing useful work.
On the steamship the steam is led not to the open air but to a vessel
called a condenser the walls of which are kept cool by a continual
circulation of cold water. The steam on entering the condenser at once
collapses into water, leaving a vacuum. A pump called the "air-pump"
removes the water (which was once steam) from the condenser and also any
air which might get in, with the result that the engine is always
discharging its steam into a vacuum. Thus to the pressure of the steam
is added the suction of the vacuum.
In turbine ships the cooling water for the condensers is circulated by
powerful centrifugal pumps driven by subsidiary engines.
The steam is obtained from boilers of that special variety known as
"water-tube."
The boilers with which most people are familiar are either Lancashire or
Cornish, both sorts being large steel cylinders with two steel flues in
the former and one in the latter running from back to front. The fire is
made in the front part of the flue and the hot gases from it pass to the
back and then along the sides and underneath through flues formed in the
brickwork in which the boiler is set. Locomotive boilers, however, have
no flues, but the hot gases from the fire in the fire-box pass through
tubes which run from end to end through the cylindrical shell, each tube
starting from the fire-box behind and terminating in the smoke-box in
front. Thus we have tubes with fire inside and water outside: hence such
boilers are called "fire-tube" boilers.
On many ships of the merchant type cylindrical boilers are used which
combine the features, to some extent, of the Cornish and the fire-tube,
since there is a flue running from front to back in which the fire is
made and the hot gases return from back to front through a number of
tubes which occupy the space above the fire. Arrived at the front the
gases pass upwards to the chimney.
Water-tube boilers are different from all of these, since in them the
water is inside the tubes while the fires play around the outside. This
enables steam to be got up very quickly, a matter of much importance for
a warship which may be called upon to undertake some operation at a
moment's notice.
The boilers are fed with water from the condensers, so that the same
water is used over and over again. When coal is burnt it is put on the
fires by hand, for although mechanical stoking is a great success on
land, there are special difficulties which prevent its use at sea. It is
becoming more and more the fashion now to burn oil instead of coal in
several types of ships and in those cases the oil is blown in the form
of spray into the furnace. This has many advantages, some of which are
exemplified on a small scale by the difference between using a coal fire
and a gas stove. Like the latter, the oil spray can be quickly lit when
needed and as quickly extinguished. It can be regulated and adjusted
with equal facility. Oil can be taken on board too through a pipe,
silently and quickly and without the terrible dirt and the exhausting
labour involved in coaling a big ship. Oil, too, can be taken on board
at sea, from a tank steamer, almost as easily as it can be taken in
ashore, whereas the difficulty of coaling at sea despite many ingenious
efforts has never been solved quite satisfactorily. Finally, oil can be
stowed anywhere, for the stokers do not need to dig it out with a
shovel. Therefore it can be carried in those spaces between the inner
and outer bottoms which have to be there in order to give strength to
the ship's hull but which would be quite useless for carrying coal. The
advantages of oil fuel, therefore, are many and no doubt it will be used
more and more as time goes on.
For Great Britain, oil fuel has the disadvantage that it has to be
imported whereas the finest steam coal in the world is found in
abundance in South Wales, but the difficulty may eventually be overcome
by distilling from native coal an oil which will serve as well as that
which is now imported.
So much for the turbine, the engine of the big ships: now for the Diesel
oil-engine which drives the submarines. It belongs to that family of
engines called "internal-combustion" since in them the fuel is burnt
actually inside the cylinder and not under a separate contrivance such
as a boiler. There have been oil-engines, so called, for many years, but
they were really gas-engines since the oil was first heated till it
turned into vapour and then that vapour was used as a gas. The Diesel
engine, however, actually burns oil in its liquid state.
To understand how it works let me ask you to conjure up this little
picture before your mind's eye. A hollow iron cylinder is fixed in a
vertical position: its upper end is closed but its lower end is open:
inside it is a piston, free to slide up and down: by means of a
connecting-rod hinged to it and passing downwards through the open lower
end the piston is connected to a crank and flywheel. At the upper end of
the cylinder are certain openings which can be covered and uncovered in
succession by the action of suitable valves.
Now let us assume that that engine is at work, the piston going rapidly
up and down in the cylinder. As it goes down it draws in a quantity of
air through a valve which opens to admit the air at just the right
moment. The moment the piston reverses its movement and starts to go up
again that valve closes and the air is entrapped. The piston continues
to rise, however, with the result that the air becomes compressed in the
upper part of the cylinder.
Now it is necessary to remind you at this point that compressing air or
indeed any gas, raises its temperature. This air, therefore, which was
drawn in at the temperature of the outer atmosphere, by the time the
piston has reached the top of its stroke has attained a temperature well
above the ignition point of the oil fuel.
The piston, having arrived at the top of its stroke, the upper part of
the cylinder is filled with hot compressed air: the next moment the
piston commences its descent, but at precisely that same moment a valve
opens and there is projected into the cylinder a spray of oil. Instantly
it bursts into flame, heating the air still more, so that as the piston
descends the air, expanding with the heat, pushes strongly and steadily
upon it. The amount of that push can be varied by varying the duration
of the jet. The longer the jet is injected the more heat is generated
and the more sustained is the push. On the other hand, if the jet is
cut off very quickly the push is only a gentle one.
The power of the engine can thus be adjusted to suit varying
circumstances by a slight variation in the valve which controls the jet.
The piston having thus been driven down to the limit of its stroke, it
commences another upward movement, at which moment another valve opens
and lets out the hot waste gases which have resulted from the burning of
the oil. Thus the cylinder is cleaned out ready for a fresh supply of
pure air to be drawn in on the next ensuing downstroke.
The engine thus works upon a series or cycle of operations which are
repeated automatically over and over again. First comes a downstroke,
drawing in air: then an upstroke, compressing it: then a second
downstroke, during which the fuel burns and the power is generated: and,
finally, a second upstroke during which the waste products of the
burning are ejected. Power, it will be noticed, is only developed in one
out of the four strokes: the other movements having, in single cylinder
engines, to be performed by the momentum of the flywheel.
In most cases, however, the engine has several cylinders in which the
cycles are arranged to follow in succession. Thus, if there are four
cylinders, there is always power being developed by one of them.
The valves are operated automatically by the engine itself just as is
the case with steam-engines. The engine also works a small pump which
provides the very highly compressed air necessary to blow the oil jet
into the cylinder.
Arrangements are often provided whereby the engine when working stores
up a reserve of compressed air which can be used to start it. From the
very nature of its working such an engine cannot develop power until it
has accomplished at least four strokes or two revolutions, so that it
cannot possibly start itself. If, however, compressed air be admitted to
the cylinders to give it a vigorous push or two and so get it going, it
can then take up its own work and go on indefinitely.
In some cases this is not necessary and that of an engine in a submarine
is one of them. In that instance, the electric motor, which drives the
boat when submerged, can be made to give the engine a start.
By altering the rotation in which the valves act the direction can be
reversed. A very simple mechanism can be made to effect this change, so
that reversing is quite easy.
Aircraft are mostly, if not entirely, driven by petrol engines, some of
which are very little different from those of a motor-car or
motor-cycle.
These motor-car engines are so well known that little need be said about
them. It may be well to explain, however, that they, like the Diesel
engines, work on a cycle of four strokes, as follows:--
First stroke (down) draws in a mixture of air and gas.
Second stroke (up) compresses the mixture. Just at the top of this
stroke an electric spark fires the mixture, causing an explosion
which drives the piston downwards, thus making the
Third stroke (down), during which the power is developed.
Fourth stroke (up) expels the waste products of the explosion.
Although all of them work on this same cycle, in which they resemble the
engines of the motor-car, there are several much-used types of
aero-engine in which the mechanical arrangement of the parts is quite
different. Of these the best known is the famous Gnome engine which has
a considerable number of cylinders arranged around a centre like the
spokes of a wheel. The centre is in fact a case which covers the crank,
and the cylinders are placed in relation to it just as the spokes are
placed around the hub of a wheel.
There is only one crank and all the connecting-rods drive on to it.
Owing to their position around it they thus act in succession, giving a
nice regular turning effort.
Further, these engines differ from all others in that the crank is a
fixture while the rest of the engine goes round, exactly the opposite of
what we are accustomed to. The engine, in fact, constitutes its own
flywheel. Rushing thus through the air, the cylinders tend to keep
themselves cool, doing away with the need for cooling water and
radiators. Consequently engines of this type are the very lightest known
in proportion to their horse-power. A fifty horse-power engine can be
easily carried by one man.
It would be possible to go on much longer with this most interesting
subject of engines, but having treated the three types which are most
used in warfare, it is now time to pass on to something else.