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.

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 ? 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.

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.

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.

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.

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.

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.