Fig. 34.—"Hit-and-miss" governor.
Governors should be provided with means to permit the manual variation of the speed while the engine is in operation.
For small motors, one of the most widely used admission devices is that of the "hit-and-miss" type. As its name indicates, this admission arrangement allows
a given quantity of gas to enter the cylinder for a number of consecutive intervals, until the engine is about to exceed its normal speed. Thereupon the governor cuts off the gas entirely. The result is that, in this system, the number of admissions is variable, but that each admitted charge is composed of a constant proportion of gas and air.
The governors employed for the "hit-and-miss" type are either "inertia" or "centrifugal" governors.
Inertia governors (Fig. 33) are less sensitive than those of the centrifugal type. They are generally applied only to industrial engines of small power, in which regularity of operation is a secondary consideration.
Centrifugal governors employed for gas-engines with "hit-and-miss" regulation are, as a general rule, noteworthy for their small size, which is accounted for by the fact that, in most systems, merely a movable member is placed between the admission-controlling means and the valve-stem (Fig. 34). It follows that this method of operation relieves the governor of the necessity of overcoming the resistance of the weight of moving parts, more or less effectually lubricated, and subjected to the reaction of the parts which they control.
In engines equipped with variable admission devices for the gas or the explosive mixture, the governor actuates a sleeve on which the admission-cam is fastened (Fig. 35). Or, the governor may displace a conical cam, the reaction of which, on contact with the lever,
destroys the stability of the governor. These conditions justify the employment of powerful governors which, on account of the inertia of their parts, diminish the reactionary forces encountered.