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.

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, and valves are in the same relative positions as formerly, and the same series of operations is repeated again and again. Of course, it is not always the case that both air and gas valve are opened on the charging stroke; that depends upon the method employed to govern the speed of the engine. Supposing it were governed on the hit and miss principle (to be explained hereafter), the gas valve would be allowed to remain closed during the charging stroke, and air alone would be drawn into the cylinder, then compressed, but not being explosive would simply expand again on the working stroke, giving back nearly all the energy which was absorbed in compressing it, and finally be exhausted in the same manner as the burnt gases are.

Fig. 8.—Second out stroke, showing position of valves during working stroke.

Fig. 9.—Second inward stroke, showing position of valves during the exhaust stroke.

Fig. 10.—First out-stroke, showing position of valves during the charging stroke.

Fig. 10 shows diagrammatically the position of crank, piston, and valves during the charging stroke.

Fig. 11.—Cross Section of Cylinder.

In figs. 1 and 2 we gave drawings of two gas engines, which are typical examples of modern practice. Huge strides have been made in recent years in gas-engine work, as regards both workmanship and efficiency, so that to-day we have in the gas engine a machine whose mechanical efficiency compares favourably with that of any other power generator, and whose thermal efficiency is very much greater.

Fig. 12.—Longitudinal Section of Cylinder.

Figs. 11 and 12 show respectively a sectional end and side elevation of the cylinder, from which it will not be difficult for the reader, however unacquainted he may be with gas-engine work, to see how the various requirements and peculiarities of the engine should be considered and provided for.

A most important desideratum in any machine or engine is that it shall be as simple in construction as ever possible; complicated mechanism should only be introduced when such addition or complication compensates adequately for what must necessarily be a higher first cost, and incidentally the greater wear and tear and attention involved. Figs. 11 and 12 show what has been done to simplify the construction of the gas engine in recent years. The main feature in this case is the very get-at-able position of the two main valves—the air valve F and the exhaust E. These valves, as may be seen from the drawing, are capable of withdrawal after the cover of the combustion chamber has been removed. The latter is an iron casting, shaped and faced up to make an absolutely tight joint; no asbestos or any packing is used to make this joint—and is held in place by four studs, as shown. Thus, all that is necessary is to remove the four nuts, lift the cover off, then pull out the pins which keep the spiral springs in position, and withdraw the valves. The latter are seated direct on to the metal of the cylinder casting, the gun-metal bushes A and B acting as guides. Further reference to A (the mixer), which serves a twofold purpose, will be made later on.

The gas valve and cock are mounted in a separate casting, which is carried by a couple of studs, the joint between this and cylinder being made with a piece of rubber insertion. The gas enters at the gas-cock, passes through the valve and port G, and round the annular space in the bush or "mixer" A, previously mentioned, and thence through a number of small holes in same, immediately below the seat of the air valve F. At the same time, pure air is drawn in via the air box (as explained hereafter), through port L (fig. 11), and thence up the centre of bush A and over the small holes through which the gas is flowing. The two then thoroughly mix and enter the combustion chamber together as the air valve F is opened. This device produces a perfectly homogeneous mixture, which conduces in no small measure to perfect combustion when the explosion takes place, and upon which, to a very great extent, depends the efficiency of the engine. Besides