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.