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When the valve is opened upon the steam side, the cylinder receives steam through both ports at that end of the cylinder, and both ports at the other end of the cylinder are at the same time open to the eduction.
A. To set the engine going, the steam must be raised until the pressure in the steam pipe is at least equal to three pounds on the square inch; and when the cylinder jacket is fully warmed, and steam issues freely from the jacket cock, open all the valves or regulators; the steam will then forcibly blow out the air or water contained in the eduction pipe, and to get rid of the air in the cylinder, shut the steam valve after having blown through the engine for a few minutes.
In locomotive engines the eduction pipe passes into the chimney and the force of the issuing steam has the effect of maintaining a rapid draught through the furnace as before explained.
A. Yes; and of the slide valve there are many varieties; but the kinds most in use are the D valve, so called from its resemblance to a half cylinder or D in its cross section and the three ported valve, shown in fig. 33, which consists of a brass or iron box set over the two ports or openings into the cylinder, and a central port which conducts away the steam to the atmosphere or condenser; but the length of the box is so adjusted that it can only cover one of the cylinder ports and the central or eduction port at the same time.
The bottom of the trunnion packing space is contracted to the diameter of the eduction pipe, so as to prevent the packing from being squeezed into the jacket; but the eduction pipe does not fit quite tight into this contracted part, but, while in close contact on the lower side, has about 1/32nd of an inch of space between the top of the pipe and the cylinder, so as to permit the trunnions to wear to that extent without throwing a strain upon the pipe.
The internal diameter of the steam pipe is 13 inches; diameter of steam trunnion journal 25 inches; diameter of eduction trunnion journal 25 inches; thickness of metal of trunnions 2-1/4 inches; length of trunnion bearings 11 inches; projection of cylinder jacket, 8 inches; depth of packing space in trunnions, 10 inches; width of packing space in trunnions, or space round the pipes, 1-1/2 inch; diameter of crank pin 10-1/4 inches; length of bearing of crank pin 15-1/2, inches.
For the escape of the steam into the eduction passage momentarily impaired the vacuum subsisting there, and owing to the smallness of the passage leading to the space above the balance piston, the vacuum subsisting in that space could not be impaired with equal rapidity.
Shortly after this time, lap began to be applied to the valves of locomotives, and it was found that not only was there a benefit from the operation of expansion, but that there was a still greater benefit from the superior facility of escape given to the steam, inasmuch as the application of lap involved the necessity of turning the eccentric round upon the shaft, which caused the eduction to take place before the end of the stroke.
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