rogen (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.)

[Illustration: FIG. 6.--Commencement of first out-stroke suction or
charging stroke. Gas and air valve about to open.]

[Illustration: FIG. 7.--Compression stroke, during which all valves
remain closed.]

During the last--the second inward--stroke (fig. 9) the exhaust valve is
opened, and the returning piston sweeps all the burnt gases (the product
of combustion) out into the exhaust pipe and so into the atmosphere.
This completes the cycle, and the piston, crank,