All About Engines

120 All About Engines The disadvantage of the ordinary D valve—apart from the power required to drive it—was hinted at in the first chapter. Steam cannot be cut off earlier than half-stroke without “ wire-drawing ” or throttling the steam as it enters. This defect was not important until high-pressure steam began to be used, but then it was vital. It was overcome by the use of two valves, the lower one an ordinary D valve, but having ports right through it, which are opened and closed by two blocks sliding backwards and forwards on the top of the main Fig. 71.—Meyer expansion valve valve. The whole arrange- ment is shown in Fig. 71. The two blocks are fixed on one rod,’ and form what is called Meyer’s expansion valve. The rod is driven by an additional eccentric, and the cut-off can be varied by altering the distance between the blocks. With this ar- rangement the cut-off may be as early as one-tenth of the stroke. In order to avoid the friction which is produced when the whole pressure of the steam in the steam chest acts on the back of the valve, engineers have had to make fresh types, one of the most satis- factory of which is the piston valve (Fig. 72, Plate 8). The valve chamber, instead of being rectangular is cylindrical, or at any rate has a cylindrical liner. The ports which lead into this barrel are opened