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.
[Illustration: FIG. 1.--General Arrangement of a Gas Engine and
Accessories.]
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.
[Illustration: FIG. 2.--A Section of a Gas Engine.]
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.
[Illustration: FIG. 3.]
[Illustration: FIG. 5.]
[Illustration: FIG. 4.]
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.