CHAPTER II
THE COMPONENT PARTS OF AN ENGINE
Having recounted very briefly the chief points in the development of the gas engine from its beginning, we may proceed to deal with matters of perhaps more practical interest to those who we are assuming have had little or no actual experience in making or working internal combustion engines.
The modern gas engine comprises comparatively few parts. Apart from the two main castings--the bed and cylinder--a small engine, generally speaking, consists of four fundamental members, viz., the valves and their operating mechanism, the cams and levers; the ignition device for firing the charge; and the governing mechanism for regulating the supply and admission of the explosive charge. There are innumerable designs of each one of these parts, and no two makes are precisely alike in detail, as every maker employs his own method of achieving the same end, namely, the production of an engine which comprises maximum efficiency with a minimum of wear and tear and attention.
Therefore, before dealing with each of these primary parts in an arbitrary manner, and with the cycle of operations in detail, we propose to make the reader familiar with the general arrangement and method of working which usually obtains in the smaller power engines. In the following illustrations these parts are shown. A (fig. 1) is the ignition device which carries the ignition tube to fire the charge. H and I (fig. 2) are the main valves, and GC (fig. 1.) is the gas-cock. The side or cam shaft N (sometimes called the 2 to 1 shaft), the cams which move the levers M, the latter in turn operating the valves, and causing them to open and close at the proper time, are shown in fig. 11. A bracket bolted up to the side of cylinder forms a bearing for one end of the side shaft, and also carries a spindle at its lower end on which the levers oscillate, transmitting the motion imparted to them by the cams to the valves. The main cylinder casting and the bed need no description. In some cases the bed is in two portions, though now a great many makers are discarding the lower portion altogether, having found that it is cheaper, and quite as satisfactory, to use a built-up foundation instead, and, if necessary, to cut a trough for the fly-wheel to run it. This arrangement, however, only obtains where larger engines are concerned. A half-compression handle by which the exhaust cam is moved laterally on the side shaft as required is not needed on very small engines.
Further reference will be made to this in another chapter, and, although this is not a necessity on a _small_ engine, it is always employed on engines over 2 B.H.P. In fig. 1, HW is the cooling water outlet and CW the inlet. A small drain cock is shown at DC, through which the water in the cylinder water-jacket may be drawn off when required. The pipes leading to the inlet and outlet of this supply are connected to the cooling water tank by means of a couple of broad, flat nuts and lead washers, one inside and the other outside the tank, the latter, when clamped up well, making a perfectly water-tight joint. The outlet pipe making an acute angle with the side of tank, the washers used there should be wedge-shape in section. It is also desirable to fit a stop-cock SC, so that the pipes can be disconnected from the engine entirely, or the water-jacket emptied without running the whole of the water out of the tank. The exhaust pipe EP is made up of gas-barrel. It should lead from the engine to the silencer or exhaust box (if one is found to be necessary) as directly as possible, _i.e._, with no more bends than are needed, and what there are should not be acute. The silencer can be inside or outside the engine-room, whichever is most convenient; but both it and the exhaust piping should be kept from all direct contact with wood-work, and at the same time in a readily accessible position.
Beyond the exhaust-pipe and box and the water-tank, the gas bag GB and gas meter (where small powers are concerned, the ordinary house or workshop lighting meter may be used without inconvenience) are the only other accessories which are included in a small installation.
Fig. 2 gives a sectional view, showing the cylinder and liner. The latter is a very desirable feature in any type of gas engine, but especially in the larger sizes; for at any future time, should it be found necessary to re-bore the liner, it can be removed with comparative ease, and is, moreover, more readily dealt with in the lathe than the whole cylinder casting would be.
The liner is virtually a cast-iron tube, with a specially shaped flange at either end. At the back end the joint between it and the cylinder casting has to be very carefully made. This is a water _and_ explosion joint; hence it has not only to prevent water entering the cylinder from the water-jacket, but also to be sufficiently strong to withstand the pressure generated in the cylinder when the charge is fired. For this purpose specially prepared coppered asbestos rings are used, which will stand both water and intense heat. Sometimes a copper ring alone is employed to make the joint. At the front end the liner is just a good fit, and enters the bed easily, and a couple of bolts fitted in corresponding lugs on the liner, pass through the back end of cylinder casting, so that by tightening up these the joint at back end is made secure. A small groove is cut on a flange, and a rubber ring, of about 1/4-in. sectional diameter, is inserted here when the liner is fitted into the cylinder casting. This makes the water-jacket joint at the front end.
Lugs are provided on the bed and cylinder castings, and are bored to receive steel bolts--three are sufficient, provided the metal in and around these lugs is not pinched. In some cases a continuous flange is provided on both bed and cylinder, and a number of bolts inserted all the way round. This, however, is unnecessary, and has a somewhat clumsy appearance. When these bolts are tightened up, the cylinder and liner are clamped firmly to the bed; but the liner being free at the open end, can expand longitudinally without causing stresses in the cylinder casting.
The combustion chamber K is virtually part of the cylinder, and has approximately equal to one-fourth the total volume of the cylinder. The shape varies somewhat in different makes of engines; in some it is rectangular, with all the corners well rounded off; in others it is practically a continuation of the cylinder, _i.e._, it is circular in cross-section, with the back end more or less spherical; while, again, it is made slightly oval in cross-section; but in every case the corners should be _well_ curved and rounded off, so that there is no one part which is liable to become heated disproportionately with the rest of the casting; in fact, in the whole cylinder casting there should be no sudden change, but a uniformity in the thickness of the metal employed. This point should be carefully remembered, although it applies more particularly to those parts of the casting subjected to higher temperatures than the rest.
The main bearings are usually of brass or gun-metal, and are adjusted for running in the same manner as any steam or other engines would be. The "brasses" are in halves, and are held down by the cast-iron caps, as shown in fig. 1.
These bearings require extremely little attention, and do not show the wear and tear of running nearly so soon as the connecting-rod brasses. These, too, are usually of brass or gun-metal; but there are various forms of construction employed in connection with the back end or piston pin bearings. On very small engines the connecting rod is swollen at the back end in the forging, and then machined up and drilled, as shown in fig. 3. In this hole the brasses are inserted after being scraped up to a good fit on the piston pin.
A flat is cut on one of the brasses, and a set screw is fitted, as shown, to prevent any movement of the latter after the final adjustment has been made. A lock nut should be used in conjunction with this set screw. Another method, and one more generally used on larger engines, is shown in fig. 4. In this case the brasses are larger than in the former, where they are virtually a split bush; here they have holes drilled in them to take the bolts, the latter usually and preferably being turned up to the shape shown in fig. 5.