STEAM ENGINE 347 and the engine takes steam at the end D, while the steam from the opposite side of the piston, A, is allowed to pass out under the valve and off through the exhaust port. The valve is shown in this position in the figure. When the eccentric has turned with the shaft, or when the link is shifted so as to bring the end p into action and thus communicate the mo- tion of the other eccentric to the valve, steam enters at the end A and is exhausted from D. Each eccentric produces this change in such a manner that when the piston reaches the end of its stroke this reversal occurs, and the steam and exhaust ports are opened and closed in the manner required to produce the proper distri- bution of steam. One eccentric is adjusted to give the correct distribution when the engine is moving ahead, the other when worked hack- ward. When it is desired to produce a limited lount of expansion of steam, the exterior edges of the face of the valve are carried further apart, and the valve when in mid- position overlaps the steam ports. The throw of the eccentrics is then correspondingly in- creased, and they are moved upon the shaft until they can be secured in new positions in which they bring the edge of the valve to the edge of the port opening as before, admitting steam at the beginning of the stroke. By this process, which is termed giving lead to the valve, the exhaust port is also both opened and closed earlier than before. To remedy this fault, the edges of the interior of the valve are sometimes changed also, and they are given " lap " in either position, as on the steam side or negative. In the latter case they are moved further apart. Zeuner's valve diagram, fig. 12, is a useful graphic representation of the action of this valve. Let A B represent the path of the piston, A O, B being the positions of the crank at each end of the stroke. Then E O, F will be the positions of the crank when the R F Fiu. 12. Zeuner's .Valve Diagram. eccentric and valve are at their middle posi- tions on the forward and the return stroke re- spectively, provided the valve has neither lap nor lead, and the steam and exhaust ports will be opened and closed precisely at the beginning and end of the stroke. If, however, it is de- sired to open the steam port when the crank is at C and the piston at c, approaching A, and if the steam is to be cut off and* expansion to begin when the crank is at D and the piston at d, on the forward stroke, the valve must be at " half throw" when the crank is at E' mid- way between A and D. E' O, F' will then be the positions of the crank when the valve is at mid throw on the forward and return stroke respectively. While the crank is moving from D to G the valve must continually cover the port which has remained open from A to D. The distances E' L, L are thus proportional to the motion of the valve while the port is opened and closed respectively, and ^, -Q mea- sures the lap. H O A is the " angular ad- vance," or the distance by which the eccen- tric must precede its normal position E to insure the desired distribution of steam. A circle E I O being inscribed, the distance I also measures the lap. Similarly, if the ex- haust port is to be opened at I, it must close at J, and the steam is confined and " cush- ioned " behind the piston as the crank moves from J to A. O M or m measures the " ex- haust lap." The figure NQPSRANis the indicator diagram corresponding to such a distribution of steam, the steam pressure be- ing maintained from N to Q, expansion occur- ring from Q to P, exhaust taking place at P, and S R being the line of back pressure on the return stroke. Cushioning takes place at R, and the steam being admitted immediately afterward, the pressure rises again to its maxi- mum at N. This valve and gear only permits a very limited range of expansion in conse- quence of the seriously objectionable effect of the accompanying alteration of the exhaust. A separate expansion valve, moved by an in- dependent eccentric, is usually placed between the steam pipe, S, fig. 11, and the main valve. The piston of the air pump, P, and the cir- culating pump, where a surface condenser is used, are driven by a rod p R from the main piston. The valves // admit the wa- ter, and c c are the delivery valves. The pump is represented as just making a stroke from left to right. Steam is exhausted from the cylinder A through the exhaust pipe N N" to the condenser, and when condensed falls to the bottom, whence the water of conden- sation is raised by the air pump and forced overboard through a delivery pipe and valve not shown. A portion of the stern of the vessel is represented as torn away to show the screw J. A strong and stiff main frame, X X, unites the cylinders with the condenser and also supports the main shaft journal at G. The whole is firmly secured by bolts to the cross floors of the vessel, Z Z, if of iron, or to engine keelsons if of wood. A spring water valve, V, is placed on the cylinder head to allow water which may enter the cylinder with the steam to be forced out without endanger- ing the cylinder or the heads, as it might were
