As has already been pointed out, compression of gas generates heat, and one of the reasons for cooling the cylinders of an internal combustion engine is to prevent the gas from exploding prematurely on the compression stroke of the piston. In the Diesel engine, instead of avoiding such a critical temperature it is deliberately sought, because it is the heat of compression, instead of an electric spark, which ignites the charge. Pure air is drawn into the cylinder on the suction stroke. This air is under ordinary atmospheric pressure when it enters, but on the return stroke of the piston it is compressed to about 500 pounds per square inch. This raises the temperature up to the neighborhood of 1000 degrees Fahrenheit. Into this highly heated air a spray of oil is injected by air at still higher pressure and immediately the oil flashes into flame and the gases resulting from the combustion drive the piston down. This is a very economical type of motor. Any liquid fuel may be used, from light gasoline to heavy crude oils, or the oils that remain after the more volatile fuels have been distilled from them. The heavy oils have more heat value in them than is to be found in light volatile fuels, such as alcohol, gasoline, and kerosene, and because the oils do not throw off any inflammable vapors unless highly heated, they can be stored more safely. This is of highest importance on submarines and Diesel engines are commonly used for propulsion on the surface or to drive the generators which charge the storage batteries for submerged travel. In order to inject the fuel into the cylinder against the pressure of air therein, a powerful air pump is required. This does not need to be of very large capacity, but it must compress the air to from 700 to 1,000 pounds per square inch. It may seem at first as if the work done by the engine in compressing the air in the cylinder and in operating the injector pump represents so much loss, but a moment’s consideration will show that it is all recovered. The air in the cylinder acts like a spring, rebounding when the piston starts down on its power stroke and adding its energy to that of the burning gases, while the air from the injector pump also enters the cylinder and helps to push the piston down. Diesel engines are made to operate on the two-cycle as well as the four-cycle principle. In one type of engine, known as the semi-Diesel, a lower compression is used in the cylinder. This is not high enough to ignite the oil that is sprayed in, and so a hot tube or bulb is employed at the top of the cylinder against which the fuel jet is directed. This bulb is heated to a dark cherry red by means of a torch until the engine has developed enough heat to keep the bulb at the requisite temperature. THE DOUBLE-ACTING JUNKER ENGINE [Illustration: FIG. 52.--THE JUNKER ENGINE IN WHICH TWO PISTONS RECIPROCATE TOWARD AND AWAY FROM EACH OTHER]