Title: Gas and Oil Engines, Simply Explained - An Elementary Instruction Book for Amateurs and Engine Attendants
Author: Runciman, Walter C.
Language: English
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  GAS AND OIL ENGINES

  SIMPLY EXPLAINED

  _An Elementary Instruction Book for Amateurs
  and Engine Attendants_


  BY
  WALTER C. RUNCIMAN


  _FULLY ILLUSTRATED_


  LONDON

  Model Engineer Series. The "Model Engineer"
  Series, no. 26.

  1905



CONTENTS

  CHAP.                                                PAGE

        PREFACE                                           5


     I. INTRODUCTORY                                      7

    II. THE COMPONENT PARTS OF AN ENGINE                 13

   III. HOW A GAS ENGINE WORKS                           22

    IV. IGNITION DEVICES                                 33

     V. MAGNETO IGNITION                                 47

    VI. GOVERNING                                        51

   VII. CAMS AND VALVE SETTINGS                          63

  VIII. OIL ENGINES                                      81



PREFACE


My object in placing this handbook before the reader is to provide him
with a simple and straightforward explanation of how and why a gas
engine, or an oil engine, works. The main features and peculiarities in
the construction of these engines are described, while the methods and
precautions necessary to arrive at desirable results are detailed as
fully as the limited space permits. I have aimed at supplying just that
information which my experience shows is most needed by the user and by
the amateur builder of small power engines. In place of giving a mere
list of common engine troubles and their remedies, I have thought it
better to endeavour to explain thoroughly the fundamental principles and
essentials of good running, so that should any difficulty arise, the
engine attendant will be able to reason out for himself the cause of the
trouble, and will thus know the proper remedy to apply. This will give
him a command over his engine which should render him equal to any
emergency.

  WALTER C. RUNCIMAN.

  LONDON, E.C.



GAS AND OIL ENGINES

SIMPLY EXPLAINED



CHAPTER I

INTRODUCTORY


The history of the gas engine goes back a long way, and the history of
the internal combustion engine proper further still. It will be
interesting to recount the main points in the history of the development
of the class of engine we shall deal with in the following pages, in
order to show what huge strides were made soon after the correct and
most workable theory had been formulated.

In 1678 Abbé Hautefeuille explained how a machine could be constructed
to work with gunpowder as fuel. His arrangement was to explode the
gunpowder in a closed vessel provided with valves, and cool the products
of combustion, and so cause a partial vacuum to be formed. By the aid of
such a machine, water could be raised. This inventor, however, does not
seem to have carried out any experiments.

In 1685 Huyghens designed another powder machine; and Papin, in 1688,
described a similar machine, which was provided with regular valves, as
devised by himself, in the _Proceedings of the Leipsic Academy_, 1688.
From this time until 1791, when John Barber took out a patent for the
production of force by the combustion of hydrocarbon in air, practically
no advancement was made. The latter patent, curiously enough, comprised
a very primitive form of rotary engine. Barber proposed to turn coal,
oil, or other combustible stuff into gas by means of external firing,
and then to mix the gases so produced with air in a vessel called the
exploder. This mixture was then ignited as it issued from the vessel,
and the ensuing flash caused a paddle-wheel to rotate. Mention is also
made that it was an object to inject a little water into the exploder,
in order to strengthen the force of the flash.

Robert Street's patent of 1794 mentions a piston engine, in the cylinder
of which, coal tar, spirit, or turpentine was vaporised, the gases being
ignited by a light burning outside the cylinder. The piston in this
engine was thrown upwards, this in turn forcing a pump piston down which
did work in raising water. This was the first real gas engine, though it
was crude and very imperfectly arranged.

In 1801 Franzose Lebon described a machine to be driven by means of
coal-gas. Two pumps were used to compress air and gas, and the mixture
was fired, as recommended by the inventor, by an electric spark, and
drove a piston in a double-working cylinder.

The atmospheric engine of Samuel Brown, 1823, had a piston working in a
cylinder into which gas was introduced, and the latter, being ignited,
expanded the air in cylinder whilst burning like a flame. The fly-wheel
carried the piston up to the top of its stroke, then water was used to
cool the burnt gases, which also escaped through valves, the latter
closing when the piston had reached the top of its stroke. A partial
vacuum was formed, and the atmospheric pressure did work on the piston
on its down stroke. A number of cylinders were required in this engine,
three being shown in the specification all connected to the same
crank-shaft. According to the _Mechanic's Magazine_, such an engine with
a complete gas generating plant was fitted to a boat which ran as an
experiment upon the Thames.

A two-cylinder engine working on to a beam was built in Paris, but no
useful results were obtained.

Wright's engine of 1833 used a mixture of combustible gas and air, which
operated like steam in a steam engine. This engine had a water-jacket,
centrifugal governor, and flame ignition. In 1838 Barnett applied the
principle of compression to a single-acting engine. He also employed a
gas and air pump, which were placed respectively on either side of the
engine cylinder, communication being established between the receiver
into which the pumps delivered and the working cylinder as the charge
was fired. The double-acting engines which Barnett devised later were
not so successful.

From this time to about 1860 very few practical developments are
recorded. A number of French and English patents were taken out,
referring to hydrogen motors, but are not of much practical value.

Lenoir's patent, dating from 24th January 1860, refers to a form of
engine which received considerable commercial support, and consequently
became very popular. A manufacturer, named Marinoni, built several of
these engines, which were set to work in Paris in a short time. Then,
due to sudden demand, the Lenoir Company was formed to undertake the
manufacture of these engines. It was claimed that a 4-horse-power engine
could be run at a cost of 3·4 shillings per day, or just one half the
cost of a steam engine using 9·9 pounds of coal per horse-power per
hour. Many similar exaggerated accounts of their economy in consumption
were circulated, and the public, on the strength of these figures,
bought.

It was understood that 17·6 cubic ft. of gas were required per
horse-power per hour, but it was found that as much as 105 cubic ft.
were often consumed. The discrepancy between the stated figures and the
actual performance of the engine was a disappointment to the using
public, and, as a result, the Lenoir engine got a bad name.

Hugon, director of the Parisian gas-works, who, together with Reithmann,
a watchmaker of Münich, hotly contested Lenoir's priority to this
invention, brought out a modification of this engine. He cooled the
cylinder by injecting water as well as using a water-jacket, and used
flame instead of electric ignition. The consumption was now brought down
to 87·5 cubic ft.

At the second Parisian International Exhibition, 1867, an atmospheric
engine, invented by Otto & Langen about this time, was shown. In this
engine a free piston was used in a vertical cylinder, the former being
thrown up by the force of the explosion. The only work done on the
up-stroke was that to overcome the weight of the piston and piston rod,
and the latter being made in the form of a rack, engaged with a toothed
wheel on the axle as the piston descended, causing the fly-wheel and
pulley to rotate.

Barsanti and Matteucci were engaged in devising and experimenting with
an engine very similar to this some years before, but Otto & Langen, no
doubt, worked quite independently. Barsanti's engine never became a
commercial article; while Otto & Langen's firm, it is said, held their
own for ten years, and turned out about 4000 engines. In 1862 the French
engineer, Beau de Rochas, laid down the necessary conditions which must
prevail in order to obtain maximum efficiency. His patent says there are
four conditions for perfectly utilising the force of expansion of gas in
an engine.

(1) Largest possible cylinder volume contained by a minimum of surface.

(2) The highest possible speed of working.

(3) Maximum expansion.

(4) Maximum pressure at beginning of expansion.

These are the conditions and principles, briefly stated, that combine to
form the now well-known cycle upon which most gas engines work at the
present time.

It was not until 1876, fifteen years after these principles had been
enumerated, that Otto carried them into practical effect when he brought
out a new type of engine, with compression before ignition, higher
piston speed, more rapid expansion, and a general reduction of
dimensions for a given power. Due to this achievement, the cycle above
referred to has always been termed the "Otto" cycle.



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.

[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.



CHAPTER III

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.



CHAPTER IV

IGNITION DEVICES


The ignition devices commonly employed may be divided into three main
classes--the metal tube, the porcelain tube, and the electric ignition.
These again may be subdivided: The first being either iron or nickel
(hecknum as they are sometimes called); the second are of two
kinds--single-ended and double-ended; and the third takes many forms
which many of my readers are possibly well acquainted with, such as the
magneto, the induction coil and trembler, and the high-tension magneto
ignition, the latter device having been used successfully on various
occasions, though not yet universally adopted.

The first-named have one or two advantages over the nickel tube. They
are very inexpensive, and are easily heated to the required temperature;
moreover, they can be made at home, should occasion demand. On the other
hand, they are not so durable, have a very uncertain life, and
consequently need renewing frequently--their average life being not
more than 60 working hours. Fig. 13 gives an outline drawing of an iron
tube, with its burner and chimney fixed in position. The tube is very
similar to a piece of 1/4-in. gas-barrel, closed up at one end and a
taper thread (1/4-in. gas) cut on the other; in fact, gas-barrel may be
used for making these tubes at home--and measure about 7 or 8 in. over
all It is screwed into a firing block, which in turn is screwed into the
combustion chamber end, so that when right home it is in such position
that the tube stands quite vertical. The section of the tube, fig. 13,
shows the condition it gets into after having been in use some time. The
bore, it will be seen, has become almost completely closed up, so that
there is practically no communication between the hot part of the tube
and the combustion chamber. This closing up of the bore is very gradual,
and it is in the early stages of this process that erratic firing is
likely to occur; sometimes the charge will be successfully fired and
sometimes not. It may be as well to mention here that the length of the
tube, although to a certain extent immaterial, should neither be
excessively long nor abnormally short, the precise length varying with
the size of the engine. A 1/4-in. tube, 8 ins. long, may be used
successfully on engines ranging from 1/2 to 6 horse-power, provided a
suitable burner is fitted enabling the tube to be heated at any required
spot. After the first charge has been fired, and the exhaust takes
place, practically all the burnt gases are cleared out of the cylinder,
but a small amount of these will generally remain in the tube and the
bore of the firing block. On the ensuing compression stroke these inert
gases are compressed to the far end of the tube, thus making way for the
explosive mixture to reach the hot portion, and explode, thus sending a
jet of flame into the main volume of the mixture which is immediately
ignited. Hence there is no advantage in having a tube too long, while,
on the other hand, it _must_ not be too short.

[Illustration: FIG. 13.]

[Illustration: FIG. 14.]

[Illustration: FIG. 15.]

The asbestos lining, shown in fig. 13, may be of various thicknesses,
according to the size of the chimney and the tube; the reason for this
will be apparent to many; but being a most important factor in the
heating of the tube, and consequently the working of the engine, it will
be advisable to deal with this point more fully.

Due mainly to the peculiar behaviour of iron tubes under heat and
internal pressure, it is always advisable to look to them first of all
when the engine shows signs of missing fire; and to always examine the
bore of a fresh one, and ascertain that it is perfectly clear before
putting it in. The adjustment of the ignition tube, although one of the
most important and necessary to be made on the whole engine, is in
itself a perfectly simple matter. It must be understood that the
ignition tube cannot, with the ordinary means at our disposal, be kept
at too high a temperature; but it must not be assumed that either the
_size_ of the flame, or the _time_ the flame has been alight, is
conclusive evidence that the tube is, or ought to be, sufficiently hot
to fire the charge successfully. It is an uncommon thing to hear a man
exclaim--after it has been pointed out that his tube is practically
cold--"Why, it's been alight for hours!"

If such is the case with you, reader, you may very rightly assume that
the burner is not properly adjusted, and so does not give the _right
kind of flame_.

In order to get the hottest possible flame, the quantity of gas and air
must be mixed in the right proportions. A common fault is that there is
too much gas allowed to flow through the nipple, compared with the
amount of air being drawn in at the air aperture, fig. 13. The result
is, we get a flame of great length, but one which is not at all suited
to our requirements; and instead of giving up its heat to the tube and
the asbestos lining of the chimney, a large amount of gas we are
presumably burning _in_ the chimney is not being burnt there at all,
for, on applying a light just above the chimney top, a quantity of this
gas we are wasting will be seen to burn with a flickering blue flame.

To put matters right, it is necessary to do one of two things--either
cut down the supply of gas or increase the air-supply. Providing the air
aperture is normal, _i.e._, the same size as it was originally, it is
better to adjust the _gas_, which may be done by tapping up the nipple
N, as indicated in the enlarged sketch, fig. 14, until just the right
amount of gas can flow.

As a rule, if there is too much air, the flame will burn with a loud
roaring noise, and is liable to fire back. The nipple should then be
opened out with a small reamer--the tang of a small file, ground to a
long taper point, makes an admirable tool for this purpose. Whether the
burner is of the ordinary bunsen type, or the ring or stove type, the
above remarks apply, as in every case the flow of gas is governed by the
size of the orifice through which it flows.

There is no need to use anything beyond a touch of oil when putting in a
new tube, in order to make a perfectly tight joint; white or red lead
are quite unnecessary, and are liable to make it a troublesome matter to
remove the tube on future occasions. Neither should undue force be
applied when putting in new tubes; it is liable to wear the thread in
the firing block, which results in a partial stoppage of the ignition
hole, as indicated in fig. 15. This is especially the case if we happen
to get hold of a tube with its screwed part slightly smaller than usual.

The asbestos with which the chimney is lined should be about 1/8 in.
thick, and, when renewing, the same thickness should be used as
originally. A thicker board will reduce the annular space round the
tube, and will have a choking effect on the flame--much the same as
referred to above, when there is too much gas and not enough air. A
simple method of lining the chimney is to cut a block of wood to the
inside dimensions of the chimney, less 1/4 in. in width and thickness,
then soften the asbestos cardboard by immersing in water, and bend it
round the wood, cutting off to the required size, _i.e._, till the two
edges form a neat butt joint. It can be allowed to remain on the mould
until dry--when it will retain its shape--or can be put into the chimney
straight away, if it is wanted for use immediately. In the latter case,
however, it will be some fifteen minutes or so before the tube will
attain its working temperature. Asbestos linings gradually become worn
and ragged, and small flakes are apt to detach themselves and fall down
into the burner, which, of course, prevents the flame playing as it
should around the tube. In such cases it is not always necessary to fit
a new lining; if the chimney is removed, the loose flakes shaken out and
the asbestos well damped and patted down with a wooden or steel
foot-rule or other suitably shaped tool, it will be fit for another long
spell of work.

The nickel or hecknum tubes are treated in the same manner as the iron,
but, as we mentioned before, are more durable, but require more heating
to get them up to a workable temperature. Their greater first cost is
compensated to some extent by makers in some cases guaranteeing them for
six months.

Of the porcelain ignition devices, we will deal with the double-ended
tube first, it being the more commonly used of the two in this country.
This form of tube is usually about 3 in. long, 1/2 in. diameter, and
open at both ends. It may be mounted in a metal casting, in form not
unlike the small gas stoves for heating soldering irons. It is heated
the greater part of its length by a couple of rows of gas jets, and is
frequently surrounded by an asbestos lining. The whole arrangement is
in reality a tiny furnace. When in position for working, one end of the
tube is open to the ignition passage leading and communicating with the
combustion chamber, while the other end is sealed, through butting up
against a metal cap or plate. An asbestos washer is interposed between
the tube at each end and the metal it bears against, thus making a more
or less flexible joint. A thumb screw is arranged at the outside end of
the tube, by means of which pressure can be applied to clamp it up
between the washers to the desired extent. Some care has to be exercised
in adjusting this form of tube for running. When heated to the working
temperature it, of course, expands, so that, if tightened up too much
when cold, it is under a fairly high compression; and when the engine is
started, and the explosion takes place, it not infrequently bursts, if
there is not sufficient "give" in the washers to allow for the
expansion. On the other hand, if not clamped up sufficiently tight to
start with, when the explosion occurs, the washer at one or each end is
blown out. This adjustment has to be made to a nicety, and, although a
somewhat difficult matter, success may be attained after one or two
trials. It is advisable, after a new tube has been put in, to start up
the engine gently, _i.e._, with less than the normal supply of gas, and
increase to the full amount gradually whilst running. This may be done
by simply opening the gas-cock on engine partially in the first place.

The single-ended porcelain tube is not so well known here as on the
continent; why, we cannot say; certainly it is preferable in every way.
We give a few illustrations, showing the method of using this tube.

Figs. 16 and 17 show the general arrangement of tube and chimney and the
manner in which they are fixed to the cylinder. The device consists
primarily of three parts--the body or chimney B, the cover C, and the
tube itself T. The body is a light iron casting, carried by a couple of
studs SS, which are either screwed into the firing block F, or direct
into the metal of the cylinder casting if no firing-block is used; the
latter may very well be dispensed with in the smaller-sized engines.

The tube is made of thin porcelain, slightly bell-mouthed at its open
end, and is mounted in a thick metal washer W, as shown in fig. 18 in
section, the joint being made with a little asbestos paper, moistened.

The block F and the face of the body B (fig. 16) are recessed to take
the washer W easily, but the depth of both recesses taken together must
be about 1/16 in. less than the thickness of the washer W; thus, when
the tube is placed in position between the body B and the block F, and
the former screwed up by means of the two nuts, as shown in the figure
16, the effect is to clamp the _washer_ which carries the tube, but _not
the porcelain tube itself_.

[Illustration: FIG. 16.]

[Illustration: FIG. 17.]

[Illustration: FIG. 18.]

The latter is left perfectly free to expand; and yet, owing to its
particular shape, the pressure in the cylinder during the compression
and explosion stroke only tends to make the joint between the tube and
washer more secure. The action of this ignition device depends upon the
tube heater H, which is merely a small bunsen burner, the flame of which
impinges on the tube at one particular spot, raising it to a very high
temperature--almost white heat. Most of my readers will know the
formation of the bunsen flame. It in composed of two distinct zones. The
inner one, marked A in fig. 18, is a perfectly cold part of the flame,
and appears to be a pale-blue coloured cone.

It is the outer zone which is the hot portion of the flame, hence this
part _only_ must be allowed to play on the tube. The tip of the blue
cone A must be kept about 1/4 in. below the tube, in order to ensure the
hottest part of the flame impinging precisely where the heat is
required.

The total length of the whole flame is, to a certain extent, immaterial;
but, generally speaking, it should be adjusted so that the length of the
inner cone A is about 1 in. or 1-1/4 in. The same methods which we
described in the early part of this chapter can be employed in the
adjustment of this burner, but some care should be exercised to get the
correct flame length.

The result of allowing the cold part of the flame to impinge on the tube
is observable in fig. 18. The black spot indicated on the drawing
actually appears as a black or sooty spot when looking at the tube under
these conditions; but in reality no discoloration whatever takes place,
the spot disappearing immediately the cone A is made shorter, or the
burner H lowered in the chimney B, so that the tip of A is just below,
and does not touch the tube at all.

The adjustment of the length of cone A may be accomplished in two
ways--(1) by keeping the supply of gas constant, and varying the amount
of air admitted at aperture K, fig. 18; (2) by keeping the supply of air
constant, and varying the amount of gas admitted through nipple N. The
first method is to be preferred when it is necessary to make any slight
adjustment due to the variation of gas pressure during the day, and may
be accomplished by fitting a small sliding shield G, as shown in the
figs. 16 and 17, and moving it round so that it covers, more or less,
the aperture K. Thus the length of cone A may be adjusted to a nicety in
a very few seconds. This shield keeps all draughts and puffs of wind
from the fly-wheel away from the aperture, and helps the flame to burn
very steadily. In the first place, of course, the flame will be
regulated by opening out or tapping up the nipple N (an enlarged sketch
of which is given in fig. 14), so that cone A is just about 1-1/4 in.
long when air aperture is full open; but once this is done, any future
adjustment can be made by throttling the air-supply, or raising or
lowering the burner bodily, the set screw keeping it in any desired
position (see fig. 17).

From the foregoing remarks it will be seen that the most noteworthy
features of this form of ignition are the ease and certainty with which
the tube can be fixed in a few moments; that when the two nuts on the
studs SS have been tightened up there is no likelihood of the joints
being "blown," for, as we said before, only the metal washer is clamped
up, the porcelain tube itself being as free to expand as it was before.
It is also at once obvious when any adjustment of the flame is
necessary; there need be no uncertainty as to whether the tube is hot
enough or not.



CHAPTER V

MAGNETO IGNITION


The third form of ignition we have to deal with is the electric.

There are a great number of different types made and used, but for
gas-engine use perhaps that known as the magneto ignition is the most
satisfactory. With this form, neither accumulators, dry batteries, or
spark coils are required, and consequently a greater simplicity is
arrived at than would otherwise be the case.

In fig. 19 we show diagrammatically the ordinary form of magneto
machine. Virtually it is a small dynamo which is fixed to the side of
cylinder casting, and is operated in the manner shortly to be described.
As we do not propose to enter into more than a brief explanation of why
and how this apparatus generates current to produce the required spark,
perhaps a simple analogy will make matters most intelligible to any
reader not well acquainted with electrical phenomena. We know that when
a current of electricity is flowing in a wire, and the wire be suddenly
broken, a spark will occur at the point of breakage. This fact may be
observed in an ordinary electric bell when ringing; at the tip of the
contact breaker a number of tiny sparks may be seen to occur, due to the
rapid make and break of the current flowing in the circuit. Precisely
the same action takes place in our magneto-igniter, but, instead of a
multitude of tiny sparks, we produce one at a time, at definite
intervals, viz., at the commencement of each explosion stroke.

[Illustration: FIG. 19.]

In the later form of magneto machines there is a soft iron sleeve
between the magnet poles and the armature. The former is connected to a
system of levers by which a reciprocating motion is imparted to it by
means of a suitably arranged cam on the side shaft. It has been found
that better results are obtained by causing the magnetic field to move
relative to the armature winding than to move the latter through a
stationary field. Reference to the diagrams, figs. 20 and 21, will make
this clear.

In fig. 19 the cam C is shown just on the point of allowing the lever L
to fly back into its normal position, due to the action of the springs
comprising a dashpot S. As the cam rotates, it pushes the lever L to the
left, the sleeve (or virtually the armature A) is also rotated through a
portion of a revolution comparatively slowly; but as soon as L is
released, the sleeve (or armature) flies back again almost
instantaneously and for the moment is generating a current in the same
manner as would any ordinary continuous current dynamo.

[Illustration: FIG. 20.]

[Illustration: FIG. 21.]

At the instant the maximum current is being generated, the circuit is
broken by means of the contact breaker D, fig. 19, which we show in
detail in fig. 22. The latter is mounted on the end of the combustion
chamber, and consists of two parts, D and P.

[Illustration: FIG. 22.]

D is an easy fit in the hole bored to receive it, and has a mushroom
valve head and seating, as shown, so that it moves readily when struck
by the projection E on the rod R (fig. 19); but yet, acting in the
manner of a non-return valve, it allows no gas to escape when the
explosion takes place in the cylinder. D is therefore in direct metallic
communication with the engine frame and earth.

P is a fixed metal pin, carefully insulated from all contact with the
engine frame and earth. To this pin one end of the armature winding is
connected, whilst the other end is connected to the engine frame.

Thus a closed circuit is formed, and when the current is generated it
flows from one terminal of magneto through wire to pin P, on to D,
through D to earth (_i.e._, engine frame), and so back to other terminal
on magneto.

And as the circuit is broken between D and P, we obtain a spark, as
previously explained, which may be timed to take place by adjusting the
position of cam C on side shaft relatively to the position of piston.

It may be said that the position of the magneto-igniter is immaterial;
it will be fixed in different positions on different types of engines,
and so long as the operating mechanism is simple and effective, _i.e._,
as direct as is practicable, it works well, and requires little
attention. The timing of the spark will be dealt with in the chapter on
Cams and Valve Settings.



CHAPTER VI

GOVERNING


The devices for governing the speed of the engine may be divided,
broadly speaking, into two classes--the inertia or hit and miss
governor, and the centrifugal. Of the latter type we will give an
instance first. In figs. 23 and 24 the governor gear is shown
diagrammatically, consisting of a couple of weights WW suspended from a
vertical spindle. These fly apart when caused to revolve by the bevel
wheel gearing BB, and raise the sleeve S to a greater or lesser extent.
A recess in the latter engages a lever arm L, through which the vertical
movement of the sleeve S is converted into a horizontal movement of the
sleeve T. The latter is carried by the valve lever P, and is virtually a
roller which engages with one or other of the steps of the cam C,
according to the speed of the engine. The object of this arrangement is
to keep the ratio of air to gas uniform throughout all variations of
load. The gas and air valve are shown as both being operated by the same
lever P, the accurate timing of the latter being obtained by means of
set screws.

[Illustration: FIG. 23.]

[Illustration: FIG. 24.]

Messrs Dougill & Co.'s engines are fitted with a step down cam and
governor such as this. The centrifugal governor is often arranged so
that instead of the charge being merely reduced in volume, the whole
charge is cut out, and no explosion whatever takes place. (In this
respect the same results are obtained as when a hit and miss governor is
used, and the latter form therefore is to be preferred, especially on
small engines, where the difference between the indicated power and the
brake power is always, even under the best conditions, fairly great.)

In this case the governor lever only operates the gas valve; the air
valve being opened on every charging or suction stroke, whether gas is
admitted or not.

Another application of the centrifugal governor is to suspend a distance
piece on the end of the governor lever, so that at normal speed this
distance piece is interposed between the gas valve spindle and the lever
operating it. In that case the gas valve will be opened. But if the
speed is above the normal, the distance piece will be raised clear of
the valve spindle, and the opening mechanism (driven by a cam on the
side shaft) will simply move forward and recede again without ever
touching the gas valve.

There are any number of movements which have been, and there are many
more which could be, devised to give the same result; and it depends
principally upon the form of engine in question which device we adopt.

The simplest and most direct action is, however, always the best;
complicated mechanism is to be deprecated, especially on small engines.
For this reason is the inertia governor more generally fitted to such
engines.

[Illustration: FIG. 25.]

A simple form of this governor is shown in fig. 25. The gas valve V is
shown on its seating. It is screwed into a pecker block B, and pinned as
shown. The latter should be of cast steel, tempered to a straw colour;
or if mild steel or iron is used, it must be well case-hardened, in
order to resist wear. The pecker P (also tempered hard) is mounted on
the cast-iron weight W, which in turn is pivoted on the valve lever L.
It will be seen that the weight W (which is only held in the position
shown by the spring S) will tend to lag behind when a sudden upward
motion is imparted to the lever L. Thus it depends upon the degree of
suddenness with which L moves whether the pecker P remains in the same
relative position to the lever as the latter travels upwards and engages
with the pecker block B, or whether it misses it and simply slides over
the face of the block. The adjustment of the spring S is effected by
screwing up or slacking out the milled nuts T; and on the degree to
which this spring is compressed depends the sensitiveness of the
governor, and consequently the speed of the engine. To obtain accurate
and steady governing with this type of mechanism it is essential that
the weight be perfectly free on its spindle, and that nothing but the
spring S holds, or tends to hold, it in the position shown. On this
account it is advisable to provide a "lip" on the pecker block, as
shown, to keep the area of contact as small as possible. This
effectually prevents any sticking, should a superfluity of oil happen to
get on either block or pecker. For similar reasons there should be some
clearance between A and the pecker, _i.e._, the latter should only bear
at one point and not bed flat against A.

Another form of inertia governor is shown in fig. 26 of the hit and miss
type, which is employed by Messrs Capel & Co. on many of their engines.

It consists of three main parts--the brass arm L carried on a stud D, on
which it is free to move; the weight W, which carries the pecker P
pivoted at the upper end of L; and the pecker block B, which engages the
pecker when the engine requires a charge of gas.

The governing action is dependent upon the shape of the operating cam
from X to Y. (In the case already dealt with, the lever L serves to
operate both air and gas valves, and so one cam only is necessary; but
in this instance the gas valve is operated by a separate cam, and a
greater nicety of adjustment is obtainable.)

[Illustration: FIG. 26.]

If the speed of the engine is sufficiently high, the arm L is thrust
forward at such a rate that the weight W tends to lag behind, with the
result that P is raised above the notch in B, as shown by the dotted
lines in drawing. On the other hand, when the speed is too low, the arm
L will not be thrust forward with so great a degree of suddenness, the
weight W will have time to move with L, and the relative position of W
and P to L will remain the same. Hence, in the first case, when a
_further_ forward movement is given to L by the cam, the pecker P is
clear of B, and omits to open the gas valve V; in the second case, P
engages with B, and the gas valve is held open during the time the
portion of cam Y to Z is passing over the roller R on arm L.

The great drawback to some forms of governors is not that they fail to
govern well when new, but that no provision is made to ensure them
working steadily when a bit worn. The shape of the cam has everything to
do with the regular working of this form of governor.

Supposing our cam was of the shape shown in fig. 27, _i.e._, the
governing and opening portion all in one curve, it would cause the
pecker to move both _forward_ and in an _upward_ direction at the _same
time_, so that at the moment of engaging B, P might still be moving in
an upward direction, which would cause uncertainty of action, especially
if the tips of the engaging members were at all blunt through wear; and,
in all probability, P would fly off B after partially opening the gas
valve.

This behaviour is very undesirable, as the small quantity of gas so
admitted to the cylinder is quite useless, and a sheer waste is
incurred. With the governing arrangement shown in fig. 26, this trouble
does not exist. The cam is so designed that the first rise from X to A
determines whether or not the valve is to be opened; the curve from A to
Y is struck from the centre of the side shaft; thus, during that
portion of the revolution the arm L is stationary, and the pecker at the
same instant takes up a definite position either in the notch in B or on
top of it, and is ready to open the valve if the speed of the engine is
such as to require an explosion, or simply to slide over the top of B,
allowing the valve to remain closed. It is most interesting to observe
the action of this governor; when an engine fitted with one is running
very slowly, the three distinct movements of the pecker P may be clearly
discerned as the respective portions of the cam pass over the small
roller R.

[Illustration: FIG. 27.]



CHAPTER VII

CAMS AND VALVE SETTINGS


With the gas, as with any other kind of engine, the valve settings are
of primary importance. On very small engines it is often the case that
only the exhaust valve is operated mechanically.

Again, there are several well-known makes which operate the gas and
exhaust mechanically while the air valve is opened by suction alone.
Though opinions differ as to which is the best course to take, there can
be little doubt that, with all three valves mechanically operated, a
greater nicety of adjustment is obtainable than would be otherwise
possible. And provided the working parts are neatly made and finished,
they will take but little power to drive them; and such loss would be
compensated by the additional power and efficiency obtained from the
engine, due to satisfactory and correct adjustment.

In fig. 28 we give a diagram showing the exact positions of the crank
when the gas, air, and exhaust valves open and close respectively, under
normal conditions of working. The solid circle represents the first
revolution of the crank shaft, starting from the commencement of the
suction stroke, and the dotted circle the second revolution, during
which the explosion and exhaust strokes take place; the dotted
horizontal line shows the position of crank at the back and front dead
centres.

As a clear conception of why certain things happen under certain
conditions is most desirable, we will first describe the operation of
marking off the cams which operate the respective valve levers, and then
discuss the effect of various "settings" of the valves on the running of
the engine.

[Illustration: FIG. 28.]

Assuming that we are still dealing with the Otto cycle engine, the cam
or side shaft will revolve at precisely half the speed of the crank
shaft. This 2 to 1 motion is obtained by means of toothed wheels, or a
screw gear. In the former case, where plain or bevel cog-wheels are
employed, the one fixed on the crank shaft must be exactly half the
diameter of the one on the side shaft, _i.e._, it must have one half the
number of teeth. On the other hand, if a screw gear is used, the
relative diameters of the two wheels may vary, but the pitch of the
teeth on the one must be twice that of the other. These wheels sometimes
have the teeth or thread formed in the casting, and sometimes they are
cut after a plain casting has been made. The latter kind are, needless
to say, better than the former, which often require filing up in order
to make every tooth alike, and ensure sweet running.

We know already in what positions our crank has to be at the opening and
closing of the three valves, and with the aid of the diagram, fig. 28,
we can determine the size of the cams. In fig. 29, S is the side shaft
to which the cams have to be keyed, R the roller on valve lever, the
latter being represented by the centre lines LL, as all we require to
find is the motion this lever will transmit to the valve, the spindle of
which is shown at V.

Fig. 30 shows diagrammatically the position of crank at the opening and
closing of the air valve. From this we see that the angle through which
the crank travels during the time the air valve is open is equal to the
obtuse angle ABC. Now, as the side shaft S revolves at half the speed of
crank, it is obvious that the former will travel through only half that
angle in the same space of time, _i.e._, through an angle equal to ABD.
We can now transfer this angle on to S, fig. 29, and draw two lines SE,
SF, cutting a circle GHJ, representing the back of the cam, which latter
passes in front of the roller R without causing any movement of the
lever L.

[Illustration: FIG. 29.]

[Illustration: FIG. 30.]

[Illustration: FIG. 31.]

It will be seen that by drawing a line forming a tangent to the circle
GHJ at F and another at E, and producing these, they will meet at point
K. Consequently, as the side shaft rotates in the direction indicated,
the lever L will _begin_ to open the valve V when the cam is in the
position shown in fig. 29, reach a maximum opening at K, and finally
close when the cam has moved so that point E is now where F was. With a
cam of this shape, however, a considerable portion of the stroke would
have passed before the valve was raised any _appreciable_ distance off
its seat; it would only be fully open for an instant, viz., when K was
passing over R, and would begin to close again directly.

Moreover, if the engine were running at even a slow speed, the motion
imparted to lever L would be indefinite; and this, especially if the
governor is fitted to the air valve lever, as in fig. 25, is very
undesirable. Therefore, to obtain a definite opening we must set out the
cam, as shown in fig. 31. In this diagram the roller is shown standing
clear of the back of cam by about 1/16 in. A line MN is then drawn,
forming a tangent to both roller R and circle GHJ at points F and O
respectively. This gives us the opening portion of cam. Then from the
centre S with radius SF describe the arc FE (shown dotted in fig. 31),
and set off the angle required (ABD, fig. 30), as previously explained.
Through point E draw a line forming a tangent to circle GHJ, and produce
it towards P. This line gives us the closing portion of cam. The
distance W is of course variable, according to the amount of lift we
give the valve. By comparing these two diagrams it will be seen that in
both cases the valve will be opened the same length of time, but in
first case the motion will be indefinite and uncertain. In practice the
corners are rounded off somewhat, in order to obtain a steady motion;
and when the air cam is also the governing cam, it is advisable to round
off the opening face, as indicated in fig. 32. Upon the shape of this
face both the sensitiveness and the life of the governor gear depends.
If it is nicely rounded off, giving a gradual rise, very little tension
(or compression, as the case may be) of the controlling spring will be
necessary to give the required speed to engine; whereas, if the rise is
sudden, the spring will have to be screwed up tighter, and, if uneven
and lumpy (_i.e._, not a fair curve), the result will, of course, be
erratic governing.

[Illustration: FIG. 32.]

A certain amount of clearance should always be provided between the
roller and the back of cam (compare figs. 29 and 31), that is, the
roller should not bear against the cam, except during that portion of
the stroke in which it is actually operating the valve, viz., from F to
E (fig. 31). A small stop interposed between the lever and some
convenient part of the engine, such as the side-shaft bracket bearing,
answers this purpose.

[Illustration: FIG. 33.]

[Illustration: FIG. 34.]

The size and shape of the exhaust cam is found in the same manner as
above described; the angle through which it operates is greater than
that of the air cam, and is shown in fig. 33. A fair margin should be
allowed for filing or machining these castings up; the shape and sizes
arrived at by the above described method being finished measurements.
Fig. 34 gives the outline of an exhaust cam worked out from the setting
diagram, fig. 33.

[Illustration: FIG. 35.]

[Illustration: FIG. 37.]

[Illustration: FIG. 41.]

[Illustration: FIG. 42.]

We may now consider the relative positions these two cams will occupy
when keyed up on the side shaft. Assuming that we have both cams
finished to the proper shape and size, and the keyway cut in the side
shaft, we can commence to mark off the position of keyway in the air
cam. With the crank in the position shown in fig. 35, the air cam is
slipped on to the side shaft and brought to the position shown in fig.
32. The keyway being already cut in the side shaft, the position for
that in the cam may be scribed off, as shown by dotted lines (fig. 32),
the cam removed, and the keyway cut. It is as well, however, to check
this mark by turning the crank round to position shown in fig. 37,
_i.e._, the closing of air valve. The side shaft will also turn through
exactly half this angle, so that when the cam is again slipped on the
latter, the scriber marks and keyway in shaft should be exactly in line,
as they were in fig. 32, and the fall of the cam--the closing
portion--should just be touching roller R, but not sufficient to keep
the valve open (see fig. 38). The slightest movement of the crank from
this point in a forward direction should result in a little play being
felt in the lever L, assuming that the cam is also moved just enough to
keep the scriber marks in line with the existing keyway.

[Illustration: FIG. 36.]

[Illustration: FIG. 38.]

[Illustration: FIG. 39.]

By these operations it will be at once evident whether the cam is too
large or too small. Supposing it is too small, we will obtain two sets
of marks indicating the position of keyway, as shown in fig. 39, and it
is obvious that we must give the lever less play by screwing up the set
screws shown in fig. 11. The effect of this is to cause the valve to
open earlier and close later than it would if the play were greater; as
it would were the operating portion of cam larger. A minimum amount of
play must always be allowed, however. When two sets of marks are
obtained, the mean must be taken and the keyway cut as shown by the
thick lines in fig. 39. The exhaust cam in larger engines is usually
made with a swelling on the opening portion, as shown in fig. 40, so
that the valve is _very slightly_ opened some time before the crank has
reached the position shown in fig. 41. Fig. 42 shows position of crank
at the close of exhaust valve, and the two last-mentioned diagrams
correspond with the two positions in which the exhaust cam is shown in
fig. 34. The small lump on the back of exhaust cam, fig. 40, is only
required on engines above 3 B.H.P. to relieve the compression on the
compression stroke when starting up. By moving the roller R on valve
lever longitudinally, so that it engages both parts of cam as they pass
in front of it, the exhaust valve is held open during a small portion of
the compression stroke, usually closing when the crank has reached the
bottom centre.

Referring again to fig. 26, this gas or governor cam may be set out, and
the keyway marked on the same principle as already described for the air
and exhaust valves. An end view of the three cams keyed up on the side
shaft is given in fig. 40A. In small engines it is convenient to have
the air and exhaust cams made in one casting, when one key only will be
required. On some engines, instead of employing a movable roller or
valve lever, the exhaust cam is fitted on side shaft with a
"feather"--_i.e._, a headless key--and the cam being capable of
longitudinal movement, such movement being controlled by a small lever
or handle, called the half-compression lever.

[Illustration: FIG. 40.]

[Illustration: FIG. 40A.]

Having once thoroughly grasped the important part the cams play in the
working of the engine, it will be an easy matter to adjust the valve
settings, and to keep them adjusted correctly. The effect of a wrong
setting will then be strikingly apparent. On small engines a separate
cam to operate the gas valve is not a necessity; and the practice of
fitting the gas valve spindle (or the pecker, the effect would be the
same) with a device for increasing or diminishing its length, is also
unnecessary and unsound.

The wear on a well-designed gas valve operating mechanism is practically
nil; and even if there was wear, the effect would be to cause the valve
to open a trifle later and close sooner than it would otherwise, _i.e._,
it would remain open a shorter time during each charging stroke. This in
turn (other conditions remaining the same) would give us a weaker
mixture; and although too weak a mixture is preferable to a too rich
one, we should have to adopt some means of increasing the richness of
the mixture; otherwise the maximum power of the engine would soon be
seen to diminish.

To get the mixture normal again we must either enlarge the gas inlet or
cut down the air-supply somewhat, and so keep the proportions the same.
That is to say, the quality of the mixture is dependent upon the
relative dimension of the gas and air inlets. We know by actual trial
that if at the completion of the charging stroke the pressure in the
cylinder is approximately that of the atmosphere, better results are
obtained than when the pressure is considerably below that of the
atmosphere. Thus, the larger we make the inlet ports (but still
retaining correct relative dimensions) the more readily will the mixture
be drawn into the cylinder as the piston moves forward, tending to
create a vacuum. Of the two courses open to us to retain a good mixture
it is preferable to open out the gas-supply, for by cutting down the
air-supply, and sucking the gas in, due to the partial vacuum being
formed, we should be keeping the proportions correct at the expense of
reducing the total volume of the explosive mixture (more strictly
speaking, the density of the charge) admitted to the cylinder.

Under normal conditions it is not necessary to create a high vacuum to
suck the gas into the cylinder, but it is as well to understand what
results we would tend to produce, did we work on these lines. Of course,
with small high-speed engines fitted with suction air valve, the vacuum
is higher than it would be in slow-speed engines with mechanically
operated valves. If we take an extreme case as an example, where, to get
any gas to speak of into the cylinder the air-supply would have to be
cut down or throttled to an abnormal extent, we will realise at once
that such a small quantity of both air and gas would have been drawn in,
and consequently the mixture would be so rarefied that on the
compression stroke the pressure would possibly be extremely low and
totally inadequate to produce efficient working. Moreover, working at
such a high vacuum as this would not only prevent us obtaining a normal
explosion in the cylinder, but would upset the working of the exhaust
valve. The latter being held down on its seat during the suction stroke
by means of a spiral spring would be lifted off its seat by suction (the
partial vacuum in the cylinder), and any burnt gases which happened to
be hanging about in the exhaust port or pipe would be drawn into the
cylinder again, and tend to damp the ensuing explosion. Too early
closing of the exhaust should be avoided almost as rigorously as too
late. The latter will affect the working in a similar way to the exhaust
being lifted on the charging stroke by suction; on the other hand, if it
closes too soon, the entire volume of burnt gases will not have been
swept out of the cylinder, and the effect will again be to damp the
following explosion.

The gas valve opens just after the crank is above the back centre and
closes just before the front centre is reached, that is, opening a
little after the air valve and closing a shade before it, thus every
particle of gas is used in the cylinder, due to a draught of air being
drawn in after the gas valve has been closed.

The settings of the valve being of primary importance, no matter what
size engine we are dealing with, and being also the most confusing
matter for anyone unacquainted with gas engines to grasp, it will not be
out of place to suggest a simple method of checking these settings.

Let us begin by pulling the fly-wheel round backwards until we feel the
piston is on the compression stroke, then from this point--the crank
being about 45° above the front centre--pull the wheel round until the
crank is in the position for the exhaust opening (see fig. 28). In this
position there should be but the _slightest_ play in the exhaust lever,
showing that the valve is _just_ on point of opening; and by keeping
one's hand on the lever whilst the fly-wheel is pulled round _very
slowly_ (it is a good plan to get some one else to do the pulling
round), it is possible to ascertain the precise point at which the valve
opens. Next pull round till the crank is in the position for the air
valve opening, and observe that it is set correctly. Then go on to a
trifle above the back centre, where the exhaust valve should close, and
so on till the opening and closing of each valve has been checked. It
will be noticed that the air, and sometimes the gas, valve opens before
the exhaust closes. This overlap is necessary; and it will be found that
the smaller the engine and the higher the speed the greater this overlap
will be to obtain good results, although a good deal of individual
judgment must be used in settling the exact amount of overlap, as the
requisite amount may, to get the best results, vary in different engines
of precisely the same dimensions and type.

When dealing with engines which have no separate gas valve--the gas
being admitted with the air, which is sometimes the case with very small
engines--the above notes referring to the gas setting independently,
will, of course, not hold good.

It may be mentioned with regard to the lump on the opening side of the
exhaust cam, that this if overdone is found to be detrimental on large
engines, and even on small ones. If it is too large, it will cause both
exhaust valve and seat to become burnt and pitted, due to the surface
being exposed to the exceedingly high temperature of the expanding
gases. If it is too large, it is equivalent to opening the exhaust valve
too early, and the effect is the same, viz., a waste of power and damage
to the valve and its seat.

[Illustration: FIG. 43.]

[Illustration: FIG. 44.--Brake Testing.]

The method of grinding in the valves to their seats with emery powder
and oil is so well known that no further description is needed here. We
give, however, in fig. 43 a sketch showing a very expeditious way of
dealing with very badly worn or burnt seats. The sketch explains itself.
Such a tool is readily made; even the cutter could be turned and filed
up to shape and then hardened at home. By lightly tapping in the taper
cotter pin little by little, sufficient pressure is put on the cutter to
make it an easy matter to completely re-face an old seat or form a new
one. A T-wrench or "tommy" can be used to work the cutter spindle. The
lower part of the latter must be the same diameter as the existing valve
spindle; the bush acts as a guide; and as the bevel of the cutter
should be the same as that of the valve, a very little grinding in with
emery powder is required to finish the job off.

In fig. 44 we give a diagram showing the method of testing for Brake
H.P. of engine, as it is frequently interesting to make such a simple
test after any alterations or adjustments have been made.

Two spring balances and a rope or cord (according to the size of the
engine), fitted with a few wood blocks as shown in section, fig. 44, to
keep the rope on the rim of fly-wheel, is all that is required for this
test. The following formula may be used for arriving at the B.H.P.:--

  B.H.P. = (S1 - S2) 3.14 x D x R / 33000

  S1 = Reading in lbs. of spring balance No. 1.
  S2 = Reading in lbs. of spring balance No. 2.
  D =  Diameter of fly-wheel and diameter of brake
       rope in feet.
  R =  Revolutions of fly-wheel per minute.

  As 3.14 x D / 33000 will always remain the same for any given engine and
  gear, we may call that expression C; then the B.H.P. may be written--

  B.H.P. = (S1 - S2) C R



CHAPTER VIII

OIL ENGINES


The small oil engine is practically the same as the gas engine, with the
addition of a vaporiser for converting the oil into gas, or vapour, to
be exploded in the cylinder; consequently the one may be converted into
the other in many cases without much trouble. The difficulty of
producing an efficient oil engine lies principally in devising a
satisfactory and reliable vapouriser--one which will work equally well
under all loads. The heat supplied to the chamber must be sufficient to
vaporise the oil, but not great enough to decompose it. There are
various methods of vaporising the oil, and many types of vaporisers are
employed to attain the same end. There are some in which a charge of oil
is drawn by suction into a hot chamber in which it is converted into
vapour and at the same time mixed with a small quantity of hot air; this
rich mixture is then passed into the combustion chamber of the engine,
in the same manner as coal-gas would be, where it is further diluted
with more air drawn in through the air valve. Other arrangements cause
a jet of oil to be injected into a chamber containing hot air, in the
form of spray, which immediately converts the oil into vapour, and is
then passed into the cylinder, compressed, and fired. Then, again, we
can pump oil through a spraying nipple into the vapouriser (which is
kept at a suitable temperature) whilst the cylinder is being filled with
air on the suction stroke. On the following compression stroke the air
is driven into the vapouriser, which communicates with the cylinder
through a narrow neck, and mixes intimately with the oil vapour.
Gradually, as the pressure rises, due to compression, the charge becomes
more and more explosive, until at the completion of this stroke it has
attained the proper proportions of air and oil vapour, and is fired by
the temperature of the vapouriser and that caused by a high compression;
that is, the charge is fired automatically; and once the engine is
running, no heating lamp is required to keep the vapouriser at the
correct temperature. It is necessary, however, to raise it to the
workable temperature at starting. This is known as the Hornsby-Akroyd
method.

Capel's arrangement is also simple and efficient, and has the additional
advantage of being capable of being fitted to their existing gas
engines, the conversion being made in a very short time. This vapouriser
consists primarily of a tubular casting A, on the outside of which are
formed a series of vertical ribs, shown in plan, fig. 46, running to
within a short distance of the flange at one end, as shown in the
section, fig. 45, thus providing an annular space C between the upper
ends of the ribs and the flange. This casting is enclosed by an outer
casing B, which fits well over the inner tube. It has also a number of
small holes drilled near the lower end communicating with the channels
between the ribs. Thus it will be seen that when the gas valve is opened
and suction takes place, air is drawn in through these holes, passes up
into the annular space C below the top flange, from there travels to the
opposite side of vapouriser, and mixes with the oil which is also being
drawn in through a small nipper at N, fig. 45. Both then pass between a
series of pegs, where they become thoroughly mixed, and finally pass on
to the inlet valve V, fig. 47, and so into the cylinder, where the
complete charge is mixed up and compressed and fired in the usual
manner. Iron ignition tubes may be used, and one heating lamp serves a
double purpose in keeping the tube and vapouriser hot at the same time.
This lamp is fed by means of a pump actuated from the side shaft. The
plunger of the pump is loaded with a spiral spring, which may be
adjusted to give any desired pressure, and is kept constant and steady
by means of an air vessel. This pump is shown in fig. 48. It is actuated
by means of a rod and lever from the side shaft of engine. The plunger P
works in a barrel B, which is carried by a small reservoir R, the latter
being in communication with the main oil tank by means of the pipe H.

[Illustration: FIG. 46.]

[Illustration: FIG. 45.]

[Illustration: FIG. 47.]

[Illustration: FIG. 48.]

[Illustration: FIG. 49.]

[Illustration: FIG. 50.]

[Illustration: FIG. 51.]

The plunger is loaded with a spiral spring, and has a ball valve, as
shown. Intermediate between this small reservoir and the main oil tank
is another set of valves, shown in fig. 49. It will be seen that the
suction of the pump will draw the oil up, the small and lower ball
valve, of course, allowing it to pass freely. On the down stroke the
lower valve will be automatically closed, and the oil will be put under
pressure, this being determined by the load on the plunger valve, which
is adjustable by means of the screw S, fig. 48. When the required
pressure in the pipe P, figs. 45 and 49, has been attained, the plunger
valve lifts on each stroke and the surplus oil flows through the plunger
into the small reservoir R. The latter is at about the same level as
another still smaller reservoir M (shown in figs. 47 and 50), a flow of
oil being established between the two by means of a pipe Q (see figs. 48
and 50). In the reservoir R is fitted an overflow pipe, so that the oil
cannot rise beyond a certain level; hence the head of oil in the smaller
one M is always constant. On the suction stroke a partial vacuum is
formed in the engine cylinder, consequently the pressure in the
vapouriser drops somewhat below that of the atmosphere, and this small
difference in pressure is enough to cause the oil to rise in the small
passage X, fig. 45, beyond its normal level, and overflow into the
vapourising chamber, as previously described. The valve or nipper N is
shown open in the diagram, fig. 45, and all that is required to stop the
engine when running is to drop the small handle L, fig. 45, when the
valve will close, due to the spring S. The air vessel shown in fig. 49
is in communication with the pipe leading to the blow lamp. A pressure
gauge can also be fitted, although it is not in any way a necessity.

The ratchet wheel and pawl shown in fig. 48 are part of the lubricator.
The wheel drives a brass or gun-metal plug, producing an intermittent
rotary motion. The plug has a small hole in its periphery, which becomes
filled with oil when it is at the upper part of its travel, and empties
the oil out into a discharge pipe T, when it is inverted, and is then
led away and applied to the piston at the required spot. Fig. 51 shows
this arrangement in section.





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