(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.