Fig. 59.—Spark Plug
A spark plug is illustrated in [Figure 59]. It consists of a metal shell screwed into the cylinder, enclosing an insulator of porcelain, mica, or some similar material. Through the insulator passes the center electrode, which is a rod of metal, with its lower end separated by a short distance from the shell or from a wire attached to the shell. This separation is the gap across which the sparking current passes, and at which the spark occurs.
Spark plugs receive the pressure of the power stroke, and must be strongly made in order to withstand it. A leaky spark plug will cut down the power of the engine, just as a leaky valve will.
CHAPTER VIII
TRANSMISSION
The parts of a tractor by which the power of the engine is applied to the driving wheels are called the transmission, and include the clutch, the change speed gear, the differential and the drive.
It has been shown that a gas engine delivers power only when it is running at speed; it cannot run until some outside power drives it through the inlet and compression strokes.
The tractor cannot move until the engine is running and delivering power, and it follows, therefore, that it must be possible to disconnect the engine from the driving mechanism in order that it may run independently. This is done by means of a clutch, which is a device that connects two shafts, or disconnects them.
Fig. 60.—Internal Clutch