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 deg. 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