All About Engines

How a Modern Engine Works *3 attached at their lower ends to the spindle, and at their upper ends to a loose cap. The governor is mounted directly over a valve in the steam pipe, and this valve is operated by a thin steel rod which passes through the main spindle of the governor. We are now in a position to inquire more care- fully what goes on inside the cylinder. When steam is admitted behind the piston it drives the latter forward, filling up the space behind the moving piston as rapidly as it can squeeze through the port. As we shall see later, the port is only open for part of the stroke, but even when the supply is cut off the expansive force tends to propel the piston forward. Moreover, the piston, piston rod, cross-head, and other working parts tend to continue their motion, just as a boy running at full speed cannot pull up immediately. When the crank reaches the farther dead centre this motion must, of course, come to an end, and the stoppage of this motion tends to set up very heavy stresses in the engine. This is pre- vented by causing the slide valve to close the exhaust port and to open the steam port a little before the piston has reached the end of its stroke, so that the steam forms a “ cushion ” which brings the piston gradually to rest. The clearance space in which this cushioning takes place, together with the volume of the steam port, is about one-eighth of the volume of the cylinder. The control, in this way, of the admission and release of steam depends partly on the position of the eccentric with reference to the crank, and partly