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.
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.
