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valve is provided with a sufficient amount of exhaust lap to obtain economic expansion by delaying the release, the exhaust closure becomes excessively early, and an excessive clearance space must be provided, equal to fully 15% of the piston displacement; this entails such a loss that a compromise is generally effected by reducing the exhaust lap, making the release earlier but delaying the closure to a point where not to exceed 10% of clearance space need be provided. This requires a valve having its exhaust edges about equal to the exhaust edges of the ports, generally designated as “line and line,” or, if the valve is cut a little shorter, giving about 3-inch exhaust clearance, the clearance or waste spaces may be reduced to about 8% of the piston displacement. Any further reduction of exhaust lap would result in a greater loss through an early release than would be gained through a reduction of clearance.
In the Allfree system a single valve correctly controls admission, cut-off, and release, the same as in the standard engine, except that a sufficient amount of exhaust lap is used to carry the steam to a point that will give a greater expansion; then, to avoid an early closure, a small piston valve, called the compression valve, is introduced through a section of the ports beneath and to one side of the main valve, and has only the function of controlling the compression and providing
greater freedom for the escape of exhaust steam. While the two valves release at the same instant, the compression valve in closing falls about 14 inches behind the main valve; this allows the exhaust steam that would otherwise be in compression to escape until the piston reaches about 90% of its stroke (at į cut-off), or within 21 or 3 inches of the end of its stroke. The clearance having been reduced to 21% of the piston displacement, a sufficient amount of compression takes place perfectly to cushion the reciprocating parts.
Fig. 42 is a photo view showing the back end of the cylinders and valve chambers; the bore of the compression-valve chamber will be noted inside and a little below the level of the valve stem, in each cylinder. The steam chest or main-valve chamber, is on an angle of 15° and placed close to the cylinder bore, thus securing short and practically straight ports. The compression-valve chamber passes through the longest section of the ports, and is bushed in the usual way; a tube spans the space between the two bushings, forming a separate chamber closed off from the live-steam pressure—see Fig. 44. The steam-chest cover forms the top side of the main-valve chamber and provides a by-pass connection between the exhaust passages, which equalizes the exhaust pressures. The cylinder and steam chest or valve chamber are one casting.
The main steam-valve is of rectilinear form rigidly