As I mentioned before, the air which runs it is regulated by a reducing valve, but there is another important part of the power plant, and this is the heater for heating the compressed air before it is admitted to the engine.
FIG. 41. THE PENDULUM CONTROL.
Compressed air is as good to run an engine with as steam, for both of them have tremendous powers of expansion. As soon as the air is released from the supply tank—which keeps it compressed in a space that is many times smaller than it would take up when it is free—and has passed into the cylinders of the engine, it begins to expand, or to spread out in every direction, exactly as steam does.
Now when air is pumped into the tank it gets very hot and this heat is stored up in the air as energy; this makes the air when it is released expand with much force and gives it the power to do useful work.
But as the air is let out of the tank through the reducing valve it expands and loses its energy—or latent heat, as it is called—and this makes it lose its power to keep on expanding to its greatest extent and so it gets weaker and weaker.
To overcome this bad feature of compressed air, a heater is fixed on the engine, and just as the compressed air reaches the cylinders of the engine it is suddenly heated and this gives it all the expansive force it needs. The heater consists of a small oil-burner which is so fixed that the instant the torpedo is shot from the tube an electric spark ignites the oil, and there is, in consequence, neither the loss of time nor power.
The engine is often of the cylinder and piston type and is built quite like an automobile engine, except that the inlet valve—which lets the air into the cylinders—is disk-shaped so that it can operate all the cylinders one after another. The exhaust ports open outside of the torpedo, and it sets up a tell-tale white streak of bubbles on the surface of the water.
Some torpedoes as, for instance, the Bliss-Leavitt, use a rotary engine. But whichever kind is used, the power plant develops from 30 to 50 horsepower, and thus each torpedo weighs about as much and is as costly as a high-priced motor car.
The Propeller-Shaft and Propellers.—The Propeller-Shaft.—The engine drives the propeller-shaft, or, as it is called in England, the cardan-shaft. To the end of this shaft outside of the torpedo is fixed one of the propellers.