All About Engines

The Modern Reciprocating Engine izt and closed by the movement of two pistons fixed a short distance apart on a rod driven by an eccentric. Steam is admitted to both ends of the liner on the outside of the pistons, or between the pistons, ex- haust then taking place at the ends. This “inside” admission enables simple packing to be used in the stuffing box when steam at high pressure and high temperature is used. Moreover, the greater part of the steam chest is exposed only to relatively low temperatures and pressures, so that it can be made lighter and the joints of the covers give less trouble. Whether the steam is admitted to the inside or outside of the pistons, the valve is “ balanced,” and the only friction is that which is necessary for steam tightness. In order to prevent the packing rings from slip- ping into the ports, these are made in the form of grids instead of slots. No advantage is gained in respect of earlier cut-off, and though piston valves are expensive, yet they are very largely used on high-pressure cylinders because they fit well and take little power to operate them. By this time the reader will imagine that we have done with valves—that every feasible and effective method of controlling the admission and release of steam has had its share of attention. Not a bit of it. The most ingenious, to those of a scientific turn of mind, have yet to be described. As, however, they are usually associated with a governor, it will be necessary to enlarge upon governors before proceed- ing to discuss them.