The principal parts of a simple engine (Fig. 169) are the frame, cylinder,

Fig. 169.   Steam Engine.

Fig. 169. - Steam Engine.

A - Cylinder B - Piston C - Slide Valve

D - Steam Inlet E - Steam Ports F - Exhaust Port

G - Crank

H - Slide Valve Eccentric piston, rods, eccentric, crank shaft, governor, and wheels. The cylinder is the long, round, iron barrel or tube in which the piston works. The piston is a disk, fitting into the cylinder and dividing it into two compartments. Packing rings are provided to make it steam-tight. The piston moves back and forth, forced by the steam which is alternately admitted on each side of it by means of openings called ports. That is, steam is allowed to enter the cylinder by one port, and forces the piston along, the other port being opened by the slide valve into the exhaust port during this stroke. As soon as the piston has reached the end of the cylinder, the first port closes for the admission of steam, while the second port admits steam which pushes the piston back again to its original position.

The back and forth movement, thus imparted to the piston by the steam, is transmitted to the crank and then to the heavy fly-wheel. The fly-wheel by means of belting or rope transmits motion to the smaller wheels or pulleys which drive the machines in factories.

After moving the piston, the steam either escapes into the air, as it does in the case of a steam locomotive, or passes into one or more other cylinders where it exerts its force until it condenses. An engine that allows steam to escape into one cylinder only, is called a simple engine. If the steam expands twice it is called a compound expansion, and if it expands three times it is called a triple expansion.