the gas. Since the temperature of the gas rises during compression, much of the work of compression is stored as heat and is given up again in the form of work during expansion.
16. Since heat is produced by the compression of a gas, it is easy to see that, if the compression is carried far enough, almost any desired temperature may be obtained. If the cylinder in Fig. 4 is filled with air, the air may be compressed until the temperature reaches a point equal to that which would be produced by the combustion of the gas. Then, if compressed gas is admitted through the tube in the end of the cylinder it will be ignited by the hot air, the gas burning slowly as it enters and keeping the temperature of the cylinder at the temperature of combustion, or that produced by the compression of the air. By this series of operations, a high initial pressure is obtained without an explosion. The initial pressure is much higher in this case than that obtained by the method described in Art. 15, and because of the high temperature every particle of the fuel is completely consumed.
17. In the internal-combustion engine, or gas engine, as it is generally called, the heat energy of the fuel is converted into mechanical work by raising the temperature, and therefore the pressure, of the gas and air by combustion, and doing work by expanding the gas. The same result might theoretically be produced by simply heating pure air. In fact, this is done in the hot-air engine, which is much used for small pumping outfits. Such a process, however, has the practical objection that it takes some time to heat a body of air, owing to the fact that air is heated by contact with a hot surface only. To heat a gas by combustion requires only enough time for the flame to spread through the mixture.
18. The rise in temperature produced by the combustion of a gas is quite great, often amounting to as much as 2,500° F. If the volume of gas remains constant, the
