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.
FIG. 30. EIGHT CYLINDER GASOLINE OR PETROL ENGINE.
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.