Action of a complete understanding of the operation of the modern engine it will be necessary to explain some of the more ordi nary terms used to express details of the steam action. The entrance of the steam into the cylinder is usually given the name of admission, al though engineers distinguish between the almost instantaneous admission which takes place (or should take place) before the piston has made any part of its stroke and that fuller admission which continues after the piston has commenced its stroke, and while it is in communication with the steam in the chest. When communication is closed between the cylinder and the steam-chest, "cut-off" is said to have taken place; and from that point the next period is called "expansion." The point at which the exhaust valve is opened and the expanding steam is allowed to enter the exhaust passage (whether to go into another cylinder, into a condenser, or into the air) is called release, the operation which follows being exhaust. Just as admission may take place a trifle before the piston gets to the beginning of its stroke, so the exhaust may be released a trifle before stroke-end, the terms pre-admission and being used. The closing of the ex haust-valve, so as to confine what is left of the exhaust steam in the cyl inder between the advancing piston and the cylinder-head, is called the result being "cushion" or "compression;" and this exhaust-closure is generally- made to take place earlier in high-speed engines than in those which run slower, the compressed steam acting as a cushion to absorb the momentum of the reciprocating parts and to pre vent jarring and racking of the engine. Cushioning also has the effect of heating the compressed steam and the walls of the cylinder and pas sages in communication with such compressed steam, this being of advan tage in preventing the chilling of the new "live" steam which is admitted when "admission " takes place.
Cylinder and most essential points of a reciprocating engine are the cylinder and the piston, with the distributing-valves. In a rotary engine the cylinder is replaced either by a cylinder having a rotating piston or by a case containing two gear-wheels meshing together. A reciprocating engine is usually built with one or more distributing valves, and these are most frequently put in a steam-chest, but many high grade engines have no steam-chest, and it is possible to construct recipro cating engines which use the piston-head as the distributing-valve. The piston may be said to consist of the " head " (that circular portion which fits in the cylinder) and the "rod;" and the head generally consists of a "spider" attached to the rod and a " follower " plate attached to the spicier. There are often packing-rings which serve to make a steam tight joint between the piston-head and the cylinder-bore, and these rings may be steam-packed—that is, driven out by the action of the steam-pres sure (communication being permitted by orifices contrived for that pur pose)—or they may be spring-packed, in which case they are held out by the pressure of springs (generally German silver or similar metal, as steel loses its temper under the high temperature of most steam-cylinders) between them and the rim of the spider. The piston-rod may be screwed
into the head, or passed through and riveted, or passed through and kept by a nut from being pulled out, or passed through and keyed hi; or com binations of these methods may be employed.
Steam-chests.—In a horizontal engine the steam-chest (where there is one) may be above, on the side, or below the cylinder, and in a vertical or an inclined engine may occupy the same relative positions. A chest placed below the cylinder of a horizontal engine has the advantage of permitting the steam which may be condensed in the cylinder, or the water which may be carried over into be drained out through the exhaust-pas sages without accumulating in sufficient quantity to cause damage to the cylinder-heads or the piston. Where the steam-chest is upon the side, this advantage is possessed only in part, and when above, not at all.
to the valves themselves, which effect the steam distribu tion, a detailed study of which is necessary to a thorough understanding of steam-engine design, construction, and operation, they may be (1) flat slides; (2) cylindrical slides, each of these haring a movement lengthwise of the cylinder; (3) cylindrical oscillating (or " rock ") valves, having a motion about an axis at right angles to that of the engine-cylinder, but controlling the ports in the same way as the slide; (4) plugs which oscil late about an axis at right angles to that of the engine-cylinder, but each controlling but one port; or (5) poppets (sometimes written "puppets," and also called "beat-valves "), which are circular discs that open and close circular ports by rising and falling.
operation of the ordinary D-valve in steam distri bution and its effect upon the piston position are exhibited in Figures II and 12 (pl. 8o), where No. I. shows the piston at about the centre of the down stroke, with the slide-valve admitting live steam from the chest above the piston, and opening communication between the lower end-port g of the cylinder and the exhaust-passage o. The valve is moving upward or in the direction opposite to that of the piston. A little later, when the piston has gone lower, the upper edge of the valve will begin to cut off steam from entering the port a', and during the up-stroke the lower end port will begin to close by the advancing inner edge of the valve. No. II. shows the piston at about the middle of its up-stroke, the valve moving downward. The live steam is here entering the lower passage g through the end-port f, while the upper passage e is in communication with the exhaust by the upper end-port e, the arch a' of the valve, and the exhaust port and passage d.