Page:Motors and motor-driving (1902).djvu/139

Rh This is what is known as compression. The explosive mixture has to be thus compressed before it is fired.

At this point the explosive mixture is fired either by means of an electric spark or by a heated tube. The systems of firing are dealt with in the chapter on Ignition (Chapter VIII.).

It is sufficient at present to note that the highly compressed explosive mixture is fired, and as there is no outlet for the suddenly expanded gases (for the force. of the explosion only tends to close tighter the inlet valve and the outlet or exhaust valve, which are referred to later), the whole force of the explosion goes to thrust down the piston from  to. It is this thrust which gives the fly-wheel its momentum, its swing; it is this thrust, in fact, which makes the car move.

At this point, when the piston is down at the bottom, at, another valve, the exhaust valve is opened (by an arrangement which is explained hereafter), and is kept open during the whole of this up-stroke from  to , the consequence being that the exploded mixture is thrust out through this exhaust valve, which closes immediately the piston gets to the top again.

This is the whole operation:—

Fig. 3, Diagram A. A spot is shown upon the fly-wheel before the beginning of the operation.

Fig. 3, Diagram B, shows that during the suction stroke the fly-wheel has made half a revolution.

Fig. 3, Diagram C, shows that during the compression stroke a further half-revolution is made and the spot has returned to its starting-point.