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.

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.

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.)

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.