Reference to the various diagrams in the text will help considerably, and make it an easy matter for any reader hitherto totally unacquainted with such engines to see why and how they work.
Coal-gas consists primarily of five other gases, mixed together in certain proportions, these proportions varying slightly in different parts of the country:—Hydrogen (H), 50; marsh gas (CH4), 38; carbon-monoxide, 4; olefines (C6H4), 4; nitrogen (N), 4.
Gas alone is not explosive; and before any practical use can be made of it, a considerable quantity of air has to be added, diluting it down to approximately ten parts air to one of pure gas. This mixture is now highly explosive.
The reader will do well to bear these facts constantly in mind, especially when he is repairing, adjusting, or experimenting with a gas engine. We wish to emphasise this at the outset, because a consideration of these facts will keep cropping up throughout all our dealings with the gas engine, and if once a fairly clear conception is obtained of how gas will behave under certain and various conditions, half, or even more than half, our "troubles" will disappear; the cry that the gas engine has "gone wrong" will be heard less often, and users would soon learn that the gas engine is in reality as worthy of their confidence as any other form of power generator in common use.
But to revert to the explanation of the cycle of operations. The cycle is completed in four strokes of the piston, i.e., two revolutions of the crank shaft.
At the commencement of the first out-stroke (the charging or suction stroke) gas and air are admitted to the cylinder through the respective valves (fig. 6), and continue to be drawn in by what may be termed the sucking action of the piston, until the completion of this stroke (the precise position of the closing and opening of the valves will be referred to later on). The next stroke (fig. 7) is the compression stroke. All the valves are closed whilst the piston moves inwards, compressing the gases, until at the end of this stroke, and at the instant of maximum compression, the highly explosive charge is fired by means of the hot tube or an electric spark, as the case may be. The ensuing stroke—the second out-stroke of the cycle—is the result of the explosion, the expanding gases driving the piston rapidly before them; this, then, is the expansion, or working stroke (fig. 8.)
Fig. 6.—Commencement of first out-stroke suction or charging stroke. Gas and air valve about to open.
Fig. 7.—Compression stroke, during which all valves remain closed.