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 possible loss in this direction, however, there is another source of waste which cannot be eliminated, and that is the heat taken away by the cooling water which surrounds the cylinder. As this loss is inevitable, the best thing we can do is to make it as small as possible. Theoretically, it would be no small advantage if we could work at very much higher temperatures than we do at the present time, and it is only certain mechanical difficulties which bar the way and so effectually prevent the already high thermal efficiency of the engine being greatly increased.
It is no easy matter to overcome these difficulties completely, but improvements in this direction are continually being made, so that troubles which attended the gas-engine user years ago no longer exist.