The American Railway: Its Construction, Development, Management, and Appliances

by Thomas Curtis Clarke

A locomotive boiler consists of a rectangular fireplace or fire-box, as shown at A, in Figure 16, which is a longitudinal section, and Figure 17 a transverse section through the fire-box. The fire-box is connected with the smoke-box B by a large number of small tubes, a a, through which the smoke and products of combustion pass from the fire-box to the smoke-box, and from the latter they escape up the chimney D. The fire-box and tubes are all surrounded with water, so that as much surface as possible is exposed to the action of the fire. This is essential on account of the large amount of water which must be evaporated in such boilers. To create a strong draught, the steam which is exhausted from the cylinders is discharged up the chimney through pipes, and escapes at e. This produces a partial vacuum in the smoke-box, which causes a current of air to flow through the fire on the grate, into the fire-box, through the tubes, and thence to the smoke-box and up the chimney. Probably many readers have noticed, that of late years the smoke-boxes of locomotives have been extended forward in front of the chimneys. This has been done to give room for deflectors and wire netting inside to arrest sparks and cinders, which are collected in the extended front and are removed by a door or spout, L, below.

Fig. 18.—Rudimentary Injector.

To get the water into the boiler against the pressure of steam a very curious instrument, called an injector, has been devised. Formerly force-pumps were used, but these are now being abandoned. The illustration (Fig. 18) shows what may be called a rudimentary injector. B is a boiler and E a conical tube open at its lower end—and connected to a water-supply tank by a pipe, C. A pipe, A, is connected with the steam-space of the boiler and terminates in a contracted mouth, F, inside of the cone E. If steam is admitted to A, it flows through the pipe and escapes at F. In doing so it produces a partial vacuum in E, and water is consequently drawn up the pipe C from the tank. The current of steam now carries with it the water, and they escape at G. After flowing for a few seconds the water has a high velocity and the steam, mingling with the water, is condensed. The momentum of the water soon becomes sufficient to force the valve H down against the pressure below it, and the jet of water then flows continuously into the boiler. A very curious phenomenon of this somewhat mysterious instrument is that if steam of a low pressure is taken from one boiler it will force water into another against a higher pressure. Figure 19 is a section of an actual injector used on locomotives.

Fig. 19.—Injector used on
Locomotives.

Having explained how the steam is generated, it remains to show how it propels a locomotive. It does this very much as a person on a bicycle propels it—that is, by means of two cranks the wheels are made to revolve, and the latter must then either slip or the vehicle will move. In a locomotive the driving-wheels are turned by means of two cylinders and pistons, which are connected by rods to the cranks attached to the driving-wheels or axles. These cranks are placed at right angles to each other, so that when one of them is at the "dead-point" the piston connected with the other can exert its maximum power to rotate the wheels. This enables the locomotive to start with the pistons in any position; whereas, if one cylinder only was used it would be impossible to turn the wheels if the crank should stop at one of its dead-points.

It will probably interest a good many readers to know how the steam gets into the cylinders and moves the pistons and then gets out again, and how a locomotive is made to run either backward or forward at pleasure.

Figure 20 ([p. 118]) shows a section of a cylinder, A A′, with the piston B and piston rod R. The cylinder has two passages, c c and d d, which connect its ends with a box, U, called a steam-chest, to which steam is admitted from the boiler by a pipe, J. The two passages c and d have another one, g, between them, which is connected with the chimney. These passages are covered by a slide-valve, V, which moves back and forth in the steam-chest, alternately uncovering the openings c and d. When the valve is in the position shown in Figure 20, obviously steam can flow into the front end A of the cylinder through the passage c, as indicated by the darts. The valve has a cavity, H, underneath it. When this cavity is over the passage d and g, it is plain that the steam in the back end A′ of the cylinder can flow through d and g and then escape up the chimney. Under these circumstances the steam in the front end A of the cylinder will force the piston B to the back end. When it reaches the back end of the cylinder the valve is moved into the position shown in Figure 21, and steam can then enter d and will fill the back end A′ while that in the front end escapes through c and g. The piston is then forced to the front end by the pressure of the steam behind it. It will thus be seen that the steam enters and escapes to and from the cylinder through the same openings.