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.]