Page:Encyclopædia Britannica, Ninth Edition, v. 22.djvu/525

 DISTKIBUTIOX OF STEAM.] STEAM-ENGINE 501 with numerous openings to allow the products of combustion to dif- fuse themselves throughout the combustion -chamber. This guards against too intense action on the metallic surfaces, and at the same time serves as a reservoir of heat to rekindle the flame if combustion is intermittent. In getting up steam an auxiliary boiler is used to supply the jet. l VIII. THE DISTRIBUTION OF STEAM. VALVES AND VALVE MOTIONS. 144. In early steam-engines the distribution of steam was effected by means of conical valves, worked by tappets from a rod which hung from the beam. The slide-valve, the invention of which in the form now known as the long Q-slide is credited to Murdoch, an assistant of Watt, came into general use with the introduction of locomotives, and is now employed, in one or other of many forms, in the great majority of engines. The common or locomotive slide-valve is illustrated in fig. 57. The seat, or surface on which the valve slides, is a plane surface formed on or fixed to the side of the cylinder, with three portsor openings, which extend across the great- er part of the cylin- FIG. s-.-Common Slidc-Valve. der's width. The central opening is the exhaust-port through which the steam escapes ; the others, or steam ports, which are narrower, lead to the two ends of the cylin- der respectively. The valve is a box-shaped cover which slides over the seat, and the whole is en- closed in a cham- ber called the valve - chest, to which steam from the boiler is ad- mitted. When the valve moves a sufficient distance to either side of the central posi- tion, steam enters one end of the cylinder from the valve - chest and escapes from the other end of the cylinder through FIG. 58. Eccentric, the cavity of the valve into the exhaust-port. The valve is generally moved by an eccentric on the engine-shaft (fig. 58), which is mechanically equiva- lent to a crank whose radius is equal to the eccentricity, or distance of 0, the centre of the shaft, from P, the centre of the eccentric sheave. The sheave is encircled by a strap forming the end of the eccentric rod, and the rod is connected by a pin-joint to the valve- rod, which comes out of the valve-chest through a steam-tight stuffing-box. The eccentric rod is generally so long that the motion of the valve is sensibly the same as that which it would receive were the rod infinitely long. Thus if a circle (fig. 59) be drawn to represent the path of the eccentric centre during a revolu- tion of the engine, and a perpendicular PM be drawn from any point P on a diameter AB, the distance CM is the displacement of the valve from its middle position at the time when the eccentric centre is at P. AB is the whole travel of the valve. Lap and 145. If the valve when in its middle position did not overlap lead. the steam ports (fig. 60), any movement to the right or the left would admit steam, and the admission would continue until the valve had returned to its middle position, or, in other words, for half a revolution of the engine. Such a valve would not serve for expansive working, and as re- gardstherelativepo- .^^xs^ sition of the crank FIG. 60. Slide- Valve without Lap. FIG. 61. Slide-Valve with Lap. and eccentric it would have to be set so that its middle position coincided with the extreme position i See a paper by Mr T. Urquhart, Afin. Proc. lust. C.E., 1884; also Engineering, June 11-25, 1886. Cut-o iease of the piston ; in other words, the eccentric radius would make a right angle with the crank. Expansive working, however, becomes possible when we give the valve what is called " lap," by making it project over the edges of the steam ports, as in fig. 61, where o is the "outside lap" and i is the "inside lap." Admission of steam (to either side) then begins only when the displacement of the valve from its middle position exceeds the amount of the outside lap, and continues only until the valve has returned to the same dis- tance from its middle position. Further, exhaust begins only when the valve has moved past the middle by a distance equal to i, and continues until the valve has again returned to a distance i from its middle position. Thus on the diagram of the eccentric's travel (fig. 62) we find, by setting off o and i on the two sides of the centre, the posi- tions a, b, c, and d of the eccentric radius at which the four events of ad- mission, cut-off, release, and compres- sion occur for one side of the piston. As to the other side of the piston, it is only necessary to set off o to the right and i to the left of the centre, but for the sake of clearness we may confine our attention to one of the two sides. Of the whole revolution, the part from a to b is the arc of steam admission, from b to c is the arc of expansion, from c to d the arc of exhaust, and from d to a the arc of compres- sion. The relation of these, however, to the piston's motion is still undefined. If the eccentric were set in advance of the crank by an angle equal to ACa, the opening of the valve would be coincident with the beginning of the piston's stroke. It is, however, desirable, in order to allow the steam free entry, that the valve be already some way open when the piston stroke begins, and thus the eccentric may be set to have a position Ca' at the beginning of the stroke. In that case the valve is open at the beginning of the stroke to the extent mm', which is called the "lead." The amount by which the angle between Ca' (the eccentric) and CA (the crank) exceeds a right angle is called the angular advance, this being the angle by which the eccentric is set in advance of the position it would occupy if the primitive arrangement without lap were adopted. The quantities lap, lead, and angular advance (0) are connected by the equation outside lap + lead = half travel x cos B. An effect of lead is to cause preadmission, that is to say, admis- Graphic sion before the end of the back stroke, which, together with the method compression of steam left in the cylinder when the exhaust port of find- closes, produces the mechanical effect of "cushioning," to which ing the reference has already been made. To examine the distribution of distribu- steam throughout the piston's stroke, we may now draw a circle to tion of represent the path of the crank pin (fig. 63, where the dotted lines steam. l Compression Fig. 62. have been added to show the assumed configuration of piston, con- necting-rod, and crank) and transfer to it from the former diagram the angular positions a, b, c, and d at which the four events occur. To facilitate this transfer the diagrams of eccentric path and of crank-pin path may by a suitable choice of scales be drawn of the same actual size. Then by projecting these points on a diameter which represents the piston's path, by circular arcs drawn with a radius equal to the length of the con- <* necting-rod, we find j), the position T of the piston at which admission occvirs during the back stroke, also q and r, the position at cut-off and re- lease, during the stroke which takes fp fsla n place in the direction of the arrow, Patt and s, the point at which compression begins. It is obviously unnecessary to draw the two circles of figs. 62 and Fj g> 04. 63 separately ; the single diagram (fif. 64) contains the solution of the steam distribution with a slide-valve whose laps, travel, and angular advance are known, tl same circle serving, on two scales, to show the motion of the crank and of the eccentric. . 146. A method of representing graphically the relations of valve Oval and piston motion, sometimes convenient in dealing with valve- diagram, gears of a more complex character than the single eccentric, is to set off the valve's and the piston's simultaneous displacement at right angles to each other, as in fig. 65, the valve s motion being exaggerated by using a coarser scale for it than for that of the piston The result is an oval curve, from which the events m the