THE HEART OF THE SUBMARINE
SINCE the time of the first propelled underwater boat great strides
have been made in the methods of driving the submarine, until at the
present day the _power plant_ seems to be well-nigh perfect.
As the steam engine was improved upon, its value for driving submarines
became better thought of by inventors, not because it was at all
suitable for the purpose, but in virtue of the fact that it was the
first and for a long time the only practical scheme to produce power
on a large scale. For this reason, all the early and even some of the
later submarines were powered with steam engines.
=What a Good Power Plant Is.=—There are certain things an engine must
be and do to make it useful for driving a submarine boat, and among the
chief ones are:
(1) It must be as small and as light as possible and still have great
strength.
(2) It must develop a lot of power for its size and weight.
(3) It must use a small amount of fuel for the power it develops.
(4) The fuel it uses must not be bulky for the power it gives.
(5) It must not give off odors.
(6) All working parts must be easily getatable, and
(7) It must not give off poisonous gases, or vapors that ignite easily,
for either of these are dangerous.
From this you will see that it is not an easy matter to make an
engine for a submarine that will have all of these good features, but
inventors have come pretty close to it, as you will presently learn.
=The Faults of the Steam Engine.=—Now while the steam engine was the
only motive power that could be used in the early submarine for driving
it when both afloat and under water, it lacked nearly every one of the
good features named above; for,
(1) While a steam engine can be made light and strong, a large heavy
boiler is needed, and this makes the boat a very hot and unhealthful
place for the crew; (2) it is very wasteful of fuel, for most of the
heat energy[19] that is stored up in the coal or oil is lost before it
ever reaches the engine; (3) if coal is used, it is too bulky, and if
oil is used it is too liable to give off vapors which will catch on
fire and explode.
These untoward features did not matter so much when the submarine was
afloat, but when she was cruising below the water they were all present
to make life miserable for the crew. But when the _storage battery_
was put into such shape that it could be used, all this was changed and
the conditions were so improved that undersea travel became bearable
and pretty safe as well. We will tell you all about the storage battery
and how it develops power a little further on.
=When the Gasoline Engine Came.=—The next improvement in submarine
power plants came when the gasoline engine was made practical.
This new kind of a _prime mover_[20] was so much better in every way
than the steam engine that nearly all submarines now built are powered
with them in one form or another.
The usual kind of gasoline engine is known as the _four cycle type_
and has from 12 to 16 _cylinders_, the _pistons_ of each of which are
connected to one _crankshaft_, and together they form a _power unit_,
as the complete engine is sometimes called.
A submarine engine of this kind can develop upwards of 5,000 horsepower
and the large units weigh close to 100 tons. Except that it is larger
and is built especially to meet the needs of the submarine it is
exactly like a motor car or an airplane engine which is shown in Fig.
30.
=How the Gasoline Engine Works.=—A single cylinder gasoline engine is
easier to understand than one with four or more cylinders, so I’ll
describe it first.
[Illustration: FIG. 30. EIGHT CYLINDER GASOLINE OR PETROL ENGINE.]
[Illustration: FIG. 31. HOW A FOUR-CYCLE GASOLINE ENGINE WORKS.]
It consists of (1) a _cylinder_ with _valves_ in it and in which a
piston moves to and fro; this is connected to (2) a crankshaft by means
of (3) a piston rod as shown in Fig. 31. To the cylinder is fixed (4)
a _carburetor_ which mixes the gasoline with the air and forms the
explosive gas, or _fuel mixture_ as it is called, and (5) a _high
tension magneto_ which generates an electric current to make the spark
that fires the fuel mixture, etc.
It is easy to understand how a gas-engine works if you will just
remember that for every _power stroke_ there are three other strokes,
making four strokes altogether, or _four cycles_ as it is called.
The power stroke of the piston is the stroke made by the explosion of
the fuel mixture and this forces the piston down. This is the stroke
that turns the crankshaft _one-half_ of a revolution and gives it force
enough to carry it around until the next power stroke takes place. Thus
the flywheel of a single cylinder four-cycle engine makes two complete
turns or revolutions to each power stroke of the piston.
Fig. 31 shows how the inlet and exhaust valves are worked, each one
by a little wheel with a lump on one side, or _cam_, as it is called,
which is fixed on a _cam-shaft_ and is turned by the crankshaft. The
cams, of which there are two for each cylinder, are set directly under
the ends of the _valve rods_, and as the cam-shaft revolves, the little
lumps on the cam strike the valve rods at the right moments and this
lets the fuel mixture into the cylinder and lets the used and burnt-up
gases out of it.
Each valve is provided with a stiff spring, which, as soon as the lump
on the cam has turned past the valve rod, lets the latter drop again
and so closes the valve.
Fig. 31 also shows the complete action of a four-cycle engine. The
_suction stroke_ is shown at _A_; as the piston moves down, the cam
forces the inlet valve up and the piston sucks the fuel mixture into
the cylinder. The exhaust valve is closed while this stroke is taking
place.
The _compression stroke_ is shown at _B_; the _momentum_, that is the
stored up energy of the flywheel, carries the piston up and forces the
fuel mixture into a very small space, that is, it _compresses_ it. By
this time the cams have moved past the valve rods and both the inlet
and exhaust valves are closed.
The _power stroke_ is shown at _C_, and this is the only one of the
four strokes that actually counts. When the fuel mixture is compressed
it is exploded by an electric spark and the force of the explosion
drives the piston down and gives the flywheel great momentum.
It is the momentum produced by this stroke that not only furnishes
enough power to carry the flywheel round until another power stroke
takes place, but also furnishes the excess power to do useful work,
such as to drive a dynamo or a propeller. Of course both valves are
closed when this stroke is being made.
The _exhaust stroke_ is shown at _D_ and is one of the up strokes of
the piston. The cam opens the exhaust valve and the piston forces the
burnt gases of the fuel mixture out of the _exhaust port_, and that
clears the cylinder for the next stroke, which will be the _A_ stroke
over again, and so on through the same four cycles just described.
_The Carburetor and What It Does._—This apparatus is connected to the
_fuel tanks_, which usually contains gasoline, by a supply pipe. The
gasoline is forced out of the tank by compressed air being pumped into
the latter and thence it passes into the carburetor.
The carburetor changes the liquid fuel into a fine spray, or vapor, and
mixes it with air, and this is drawn into the cylinder when the inlet
valve opens and the piston is making its suction stroke.
_The Magneto Electric Machine._—This is an electrical device which is
simply a little dynamo. When it is driven by the engine it generates a
_high tension current_ of electricity which will jump between the ends
of two wires ⅛ inch apart, and this makes a spark.
The magneto is connected to a _spark-plug_ which is screwed into the
head of the cylinder. A _timer_ connected to the magneto and the
spark-plug closes the circuit each time the compression stroke is
completed; the instant the circuit is closed the current generated by
the magneto makes a spark at the business end of the spark-plug and
this fires the fuel mixture.
But as good as the gasoline engine is for motor cars, power boats,
and airplanes, it has been found sadly wanting as a power plant for
submarines; this is due not to any fault of the engine but to the
explosive nature of the gasoline which is used.
Gasoline is a very _volatile_ liquid, that is, at ordinary temperatures
and pressures it tends to change from its liquid state to a vapor which
is really a gaseous state. You may have noticed this if you have been
near a place where gasoline is stored, for the whole air is _saturated_
with the vapor given off by it and this is what you smell.
Further, it is quite impossible to store a fuel like gasoline, and to
use it for firing an engine in such a confined space as there is on a
submarine, without the air becoming charged with the vapor, which is
injurious to those who breathe it, and which, should it be accidentally
ignited, would explode with such force that it would wreck and sink the
submarine.
=The Last Word in Submarine Engines.=—Having tried out both steam and
gasoline engines for submarine work and both having been found wanting,
further experiments were made in engine building.
Now, since it was known that the chief fault of the gasoline engine
lay in the fuel it used and not in the engine itself, inventors worked
hard to make an engine that would burn a fuel so much heavier and less
volatile than gasoline that all danger from vapor would be done away
with.
Many engines were built along this line, but all failed until Diesel
(pronounced Dé-sel), a German inventor, found a way to make an engine
that would burn a heavy oil. The Diesel engine is now used in every
submarine that is built, nearly; and for this reason I want you to
understand exactly how it is made and how it works.
_How the Diesel Engine Works._—The Diesel engine works on the same
general principle as the gasoline engine—that is, by the explosion of
a fuel mixture in the cylinders—but it is different from the gasoline
engine in the way in which the fuel is admitted into the cylinder and
fired.
In the Diesel engine, a rough diagram of which is shown in Fig. 32,
there are two valves in the head of the cylinder, one of which lets in
the heavy fuel mixture and the other one admits compressed air to the
cylinder. The exhaust valve is at the bottom of the cylinder, and the
lower part of the cylinder is built to form an air compressor.
[Illustration: FIG. 32. A TWO-CYCLE DIESEL ENGINE.]
The way it works is like this:
(a) When the compressed air valve opens, the compressed air is forced
into the cylinder and this drives the piston down when the valve closes.
(b) The power that this stroke gives to the flywheel forces the piston
up again, and this compresses the air as shown in _A_, Fig. 32. Now
when you compress air, it heats it, and the amount of heat developed
depends on how much the air is compressed; you can even feel the heat
that is set up by compressing the air in a toy pop-gun; or you may have
noticed that when an automobile tire is pumped up fast it gets hot. Of
course, while the air is being compressed in the cylinder of the engine
the compressed air valve stays closed.
(c) When the fuel inlet valve opens, the heavy fuel is forced into the
cylinder by means of compressed air which presses on the fuel in the
supply tank. The instant the fuel strikes the hot air in the cylinder
it ignites and burns, and as it burns it expands just as the gases of
burning powder in a cartridge expand. This forces the piston down and
makes the power stroke as shown in _B_, Fig. 32.
Here, then, is another great advantage of the Diesel engine over the
ordinary gasoline engine: it does not need an electric spark or any
other kind of flame to fire it.
(d) As soon as the piston has reached the down end of its power stroke,
the exhaust ports are opened and more compressed air flows through the
compressed air valve; this blows what is left of the burnt gases out
of the cylinder through the ports, and as soon as this is done the
piston starts to go up, when it compresses whatever air there is in the
cylinder again.
While this stroke is being made the air compressor piston in the
bottom of the cylinder, which is a part of the regular piston, has
been compressing the air needed to perform the above operations. Since
there is only one waste stroke to every power stroke, the engine is
a _two-cycle_ one, and herein lies its third big advantage over the
gasoline engine.
[Illustration: _Courtesy of Scientific American_
THE ENGINE ROOM OF A MODERN SUBMARINE SHOWING THE DIESEL ENGINES]
The reason that the Diesel engine is better adapted to burn heavier
oils than the gasoline engine is because the latter uses an electric
spark to ignite the fuel mixture and this must be very light and
volatile, since the spark is not hot enough to fire the heavier oils.
Again, in a gasoline engine the fuel mixture _explodes_—that is, it
burns very rapidly, like the powder charge in a cartridge—whereas in a
Diesel engine the fuel mixture _expands_ when it is ignited, very much
as steam expands in the cylinder of a steam engine.
Using heavy oils very greatly reduces the cost of operating an engine,
for oils of this kind are usually the _by-products_ obtained in the
making of petroleum and gasoline. These heavy oils are of little value
for any other purpose than fuel; and, also, since the oil is heavy, it
is more easily handled than gasoline.
For these very good reasons the Diesel engine (Fig. 33), is used by
nearly every government at the present time for submarine power plants.
Moreover, like the steam engine, it will develop its greatest power,
nearly, on starting; it does not need any _reducing gears_ to lower its
speed—this is done with a _throttle_ as it is in a steam engine; and
it can be reversed without a reversing gear, which is better than the
steam engine.
The Diesel engines of present-day make range from 900 to 5,000
horsepower, and eight or more cylinders are used for each engine. The
weight of the engine is about 30 pounds per horsepower; or for a 5,000
horsepower engine the weight is in the neighborhood of 70 tons. As
great as this weight may seem, it is much lighter for the horsepower
produced than the old-fashioned gasoline engine.
[Illustration: FIG. 33. TYPE OF DIESEL ENGINE USED IN SUBMARINE.]
=Why An Electric Power Plant Is Needed.=—As you know, a submarine has
two chief conditions, and these are (1) when it is _afloat_, and (2)
when it is _submerged_.
When she is afloat the air in the craft and which the crew must breathe
is being constantly sucked in from the outside, and there is always a
large enough supply to keep the compartments clear and to furnish the
air compressors which fill the tanks. But when the boat is submerged
there is no way of getting a fresh supply unless air is taken from the
tanks or the submarine goes to the surface every little while, like a
whale, and this would hardly do.
=The Dynamo-Motor and Storage Battery System.=—When the storage battery
came into use inventors of submarines were quick to see that the thing
to do was to use two separate and distinct power plants and these are
(1) the steam or gas engine, which is used when the boat is running
afloat; and (2) the electric storage battery system, which is used when
the craft is running submerged.
Three devices must be used for the undersea electric power plant,
namely, (1) the _dynamo_, which generates the electric current; (2) the
_storage battery_, which is charged by the current generated by the
dynamo, and (3) the _motor_, which develops power when a current from
the storage battery is made to flow through it. The diagram shown in
Fig. 34 will make the electric connections clear.
_About the Dynamo._—The _dynamo_[21] (see Fig. 34) is connected to the
crankshaft of the engine and is driven by it. It changes the mechanical
motion of the engine into an electric current. This electrical energy
must then be stored up so that it can be used later when the craft is
submerged and it is wanted.
[Illustration: FIG. 34. A SUBMARINE DYNAMO MOTOR.]
_And Now the Storage Battery._—To store up the electrical energy a
_storage battery_ (see Fig. 35) is used. When the current is made
to flow into this kind of a battery it charges the battery, and if
then the battery is connected to a motor it will deliver an electric
current, and this runs the motor.
The storage battery has been almost as big a bugbear to the submarine
builders as the oil engine. It must be as small and as light as
possible; it must not absorb the oxygen of the air which the crew
breathes, and it must not give off any poisonous gases.
Now, there are two kinds of storage batteries, and these are (1) the
_lead-plate_ storage battery, which is the oldest form, and (2) the
_nickel-steel_ storage battery, which was invented quite recently by
our own Edison. Both kinds are used in submarines.
[Illustration: FIG. 35. A SIMPLE STORAGE BATTERY.]
_Last of All, the Motor._—The motor is the last device by means of
which the electric energy is made to drive the craft when it is
undersea.
Now, a dynamo and a motor are made exactly alike, in fact, a dynamo
is a motor and a motor is a dynamo. That is to say, if you turn the
_armature_ of a dynamo, it will generate a current of electricity, and
if you make a current flow through the coils of a dynamo, the armature
will spin round and it is then a motor.
This being true, however strange it may seem, only one electric machine
is needed to do the work of the dynamo and the motor; for when it is
connected to the shaft of the engine it will generate a current for
charging the storage battery, and this is done while the craft is
afloat; or it will develop power to drive the propellers when it is
connected to the storage battery, and this is done when the submarine
is cruising undersea.