ADVANCING THE SPARK.
We will assume that the position of W (in Fig. 45) is such that the contact touches W at the moment when the piston has just completed the compression stroke. Now, the actual combustion of the charge occupies an appreciable time, and with the engine running at high speed the piston would have travelled some way down the cylinder before the full force of the explosion was developed. But by raising lever L, the position of W may be so altered that contact is made slightly before the compression stroke is complete, so that the charge is fairly alight by the time the piston has altered its direction. This is called advancing the spark.
GOVERNING THE ENGINE.
There are several methods of controlling the speed of internal-combustion engines. The operating mechanism in most cases is a centrifugal ball-governor. When the speed has reached the fixed limit it either (1) raises the exhaust valve, so that no fresh charges are drawn in; (2) prevents the opening of the inlet valve; or (3) throttles the gas supply. The last is now most commonly used on motor cars, in conjunction with some device for putting it out of action when the driver wishes to exceed the highest speed that it normally permits.