It will be seen that the slide valve is hollowed within, as at H, and that the exhaust port I leads from this hollowed portion while the live steam from the boiler enters through pipe J and fills the space K of the chest.
In [Fig. 11] live steam has been entering port F, thus driving the piston to the right. At the same time the exhaust steam at the right side of the piston is discharging through the port G and entering the hollow space within the slide valve. In [Fig. 12] the conditions are reversed, and now live steam enters port G, and the exhaust passes out through port F.
When the engine crank reaches the point 3, which is directly opposite 1, the reverse action takes place with the slide valve, and it is again moved to its original position, shown in [Fig. 12].
Importance of the Valve.—Every improvement which has been made in the engine has been directed to the valve. The importance of this should be fully understood. As the eccentric is constantly turning it is a difficult matter to so arrange the valve as to open or close it at the correct time, absolutely, and many devices have been resorted to to accomplish this.
Expanding the Steam.—As all improvements were in the direction of economizing the use of steam, it was early appreciated that it would be a waste to permit the steam to enter the cylinder during the entire period that the engine traveled from end to end, so that the valve had to be constructed in such a way that while it would cut off the admission of steam at half or three-quarters stroke, the exhaust would remain on until the entire stroke was completed.
Some engines do this with a fair degree of accuracy, but many of them were too complicated for general use. In the form of slide valve shown the pressure of the steam on the upper side, which is constant at all times, produces a great wearing action on its seat. This necessitated the designing of a type of valve which would have a firm bearing and be steam tight without grinding.