Page:Motors and motor-driving (1902).djvu/138

106 at the bottom,. The piston is connected by the connecting rod  to the crank  by means of which it turns the flywheel.

Compared with the propulsion of the front wheel of the old high-wheeled bicycle, the connecting-rod represents the rider's leg, the crank (r) the crank of the bicycle, and the flywheel  the large wheel of the bicycle.

The force which drives the piston downward, and so operates the fly-wheel, is generated by the explosion of a mixture of gas and air in the combustion chamber. This mixture reaches the combustion chamber through the induction pipe, and the induction or inlet valve. It is fired by an electric spark occurring in the combustion chamber, or a red-hot platinum tube protruding into same, and the exploded charge is ejected through the exhaust valve, as will be hereafter explained.

Let it be supposed that the fly-wheel has been set rapidly revolving, that the piston has been up at the top at and has just descended to the bottom of its stroke. In doing this it sucks down the valve (called the inlet or induction valve), which is otherwise held closed by a spring, and thus draws through the valve from the induction pipe  a mixture of vapour of petrol and air.

When the piston is at the bottom the cylinder is fully charged with this explosive mixture.

The suction having stopped, the inlet valve is closed by its spring, and the cylinder is then air- or rather gas-tight.

The momentum of the fly-wheel then thrusts the piston up to the top again, and in doing so, as there is no escape, the explosive mixture which had previously filled all the space in the cylinder between its head  and  is compressed into the very small space remaining between  and.