Energy of Carbon and Hydrogen.—When a pound of carbon is burned, it develops 14,500 heat units, and a pound of hydrogen over 52,000 heat units. Assuming that 85 per cent. of a pound of petroleum is carbon, and 15 per cent. is hydrogen, the heat units of the carbon would be 12,225, and the heat units of the 15 per cent. of hydrogen would be 12,800. The combined value is, therefore, 25,025, which is almost double that of coal gas.
This fact makes the gasoline engine so much more efficient, and for the same horse power the cylinders can be made smaller, and the whole structure much lighter in every way.
Gasoline motors are of two types, one in which an explosion takes place at every revolution of the crank, called the two-cycle, and the other the four-cycle, in which the explosion occurs at every other turn of the crank.
The terms two-cycle is derived from the movement of the piston, as that moves downwardly during the period when the crank is making a half turn, and returns in its upward stroke when the crank completes the turn, or that two half turns of the crankshaft complete the cycle. Four-cycle engines have two such complete movements at each impulse, or require four half turns of the crankshaft to complete the cycle.
The Two-Cycle Type.—In order to clearly distinguish between this and the four-cycle, it would be well to examine the diagram, [Fig. 23]. For a clearer understanding the drawing is explained in detail.
The cylinder A, within which the piston works, has a removable cap B, and at its lower end a removable crank case C. The case is designed to entirely close the lower end of the cylinder so that it is air tight, for reasons which will be explained.
The outer jacket, or casing D, at the upper end of the cylinder, is designed to provide a space E, for the circulation of water, to cool the cylinder during its working period. The crankshaft F passes through the crank case, the latter having suitable bearings G for taking care of the wear.
The piston H is connected up with the rod I, the latter being hinged at a point within the piston, as shown. The crank case has an inlet port, provided with a valve which opens inwardly, so that when the piston moves upwardly the valve will open and air will be drawn into the crank case and space below the piston.