HOW A GAS ENGINE WORKS
The gas engine of the present day, although from a structural point of
view is very different to the early engine, or even that of fifteen
years ago, is, in respect to the principle upon which it works, very
similar. The greater number of smaller power engines in use in this
country work on what is known as the Otto or four-cycle principle; and
it is with this class of engine we propose to deal.
Reference to the various diagrams in the text will help considerably,
and make it an easy matter for any reader hitherto totally unacquainted
with such engines to see why and how they work.
Coal-gas consists primarily of five other gases, mixed together in
certain proportions, these proportions varying slightly in different
parts of the country:--Hydrogen (H), 50; marsh gas (CH4), 38;
carbon-monoxide, 4; olefines (C6H4), 4; nitrogen (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, 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.
[Illustration: FIG. 8.--Second out stroke, showing position of valves
during working stroke.]
[Illustration: FIG. 9.--Second inward stroke, showing position of valves
during the exhaust stroke.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
All that we require of the cooling water is that it shall keep certain
working parts of the engine at a reasonable temperature; for instance,
the cylinder must not be so hot as to deprive the lubricating oil of its
property to lubricate, neither must the exhaust valve become so hot as
to cause it to seize in the bush and stick up; but, beyond such
considerations as these, the higher the temperature is at the
commencement of each explosion the more efficient will the engine be.
The object, then, is to do as little cooling as possible, and to apply
the cooling effect at the right parts; hence the passages and chambers
through which the cooling water circulates should be so arranged that
those which require to be kept at a low temperature are in close
proximity to the cooling water. On some of the engines of days gone by,
the exhaust valve was carried in a large iron casting, this in turn
being bolted to the cylinder casting and communicating with the
combustion chamber by means of a port. Such an arrangement was found to
be not only clumsy but inefficient; the water passages were small and
difficult to get at; they readily furred up; and moreover, the joint
between this casting and the cylinder was necessarily a water _and_
explosion joint, and the fewer we have of these the better.
The method--if it may be called a method--of overcoming or preventing
the exhaust valve becoming too hot is, in the case of figs. 11 and 12,
simply one of judicious arrangement and design. The cooling water enters
by the inlet K (fig. 11), and circulates round the exhaust valve port X
and valve E immediately, before becoming heated, thus keeping the
hottest of the working parts of the engine at a suitable temperature;
and the valve seat, being in direct metallic communication with the cold
water, does not become burnt or pitted. On the other side of the exhaust
valve we have the air valve and its passages, through which cool air is
continually being drawn; this also helps to keep the exhaust valve cool.
From this, then, we may conclude that overheating of the cylinder will
not occur under normal conditions, given an engine of good design; but,
if this trouble does arise, we may safely look first of all for some
defect in the cooling water circulation. Some waters contain a greater
amount of impurities than others, and consequently the water space may
furr up more rapidly in one district than in another. But this deposit,
even under the worst conditions, accumulates very slowly, and the
operation of cleaning out the water-jacket is a very infrequent
necessity. The exhaust valve, however, may become overheated if it is
allowed to get into bad condition, _i.e._, leaky. Its seat should be
well looked after, or the hot gases will blow past when it is
presumably shut; and if this defect, slight though it may be to begin
with, is allowed to develop, both the seat, the valve head, and the
spindle will become burnt away and pitted, perhaps badly, due to the
excessive heat.