Table VI is intended to show the parts of the stroke where, under any given arrangement of slide valve, the eduction ports close and open respectively, so that thereby the engineer may be able to estimate how much, if any, of the efficiency he loses, while he is trying to add to the power of the steam by increasing the expansion. In this table there are eight columns marked A, standing over eight marked B; and, at the heads of these columns are eight fractions as before, representing as many different parts of the stroke at which the steam may be supposed to be cut off. TABLE VI.-PROPORTION OF THE STROKE AT WHICH THE The columns marked A express the distance of the piston—in parts of its stroke-from the end of the stroke when the eduction port before it is shut; and the columns marked B, and which stand immediately under the columns marked A, express the distance of the piston from the end of its stroke when the exhausting port behind it is opened-also measured in parts of the stroke.* Suppose we have an engine in which the slide valve is made to cut the steam off when the piston is at 1-3rd from the end of its stroke, and that the lap on the eduction or exhausting side of the valve is 1-8th of the whole length of its stroke. Suppose the stroke of the piston is 6 feet, or 72 inches. It is required to know when the exhausting port before the piston will be In engines moving at a slow rate, the same necessity for an early eduction does not exist as in quick moving engines, such as locomotives; and, in such engines, there will be a loss from opening the eduction much before the end of the stroke, as the moving pressure urging the piston is thus removed before the termination of the stroke. When the valve is closed before the piston previously to the end of the stroke, the attenuated vapour in the cylinder will be compressed, and sometimes the compression will be carried so far that the pressure resisting the piston at the end of the stroke will exceed the pressure of the steam in the boiler. The indicator diagram will, in such cases, appear with a loop at the upper corner, which shows that the pressure before the end of the stroke exceeds the pressure of the steam, and that the first effect of opening the communication between the cylinder and the boiler is to enable the cylinder to discharge its highly compressed vapour backward into the boiler. The act of compressing the steam is what is called cushioning, and in all ordinary diagrams this action is more or less to be seen. closed, and when the one behind it will be opened. At the top of the left hand column marked A, the given degree of expansion (1-3rd) is given; and in the extreme left column we have at the top the given amount of lap (1-8th). Opposite the 1-8th in the first column marked A, we have, 178; and in the first column marked B, 033, which decimals multiplied respectively by 72, the length of the stroke, give the required positions of the piston, thus 72 X 178 12.8 inches distance of the piston from the end of the stroke when the exhausting port before the piston is shut; and 72 × 033 = 2*38 inches = distance of the piston from the end of its stroke when the exhausting port behind it is opened. of Again, let the stroke of the valve be 16 inches, the lap on the exhausting side -inch, the lap on the steam side 3 inches, and the length of the stroke of the piston 60 inches. It is required to ascertain all the particulars of the working of this valve. The lap on the exhausting side is evidently the length of the valve stroke. Then, looking at 16 in the left hand column of the Table V in page 300, we find in the same horizontal line 3'26, or very nearly 3, under at the head of the column, thus showing that the steam 130 will be cut off at one-sixth from the end of the stroke. Again, under at the head of the sixth column from the left in the Table VI, page 302, and in a line with in the left-hand column, we have 053 under A, and 033 under B. Hence, 033 × 60 = 3.18 inches distance of the piston from the end of its stroke when the exhausting port behind it is opened. If, in this valve, the lap on the exhausting side were increased, say to 2 inches or of the stroke, the effect would be to cause the port before the valve to shut sooner in the proportion of 109 to 053, and the port behind it later in the proportion of oc8 to 003. Whereas, if the lap on the exhausting side were removed entirely, the port before the piston would be shut, and that behind it opened at the same time. The distance of the piston from the end of its stroke at that time would be 43 × 60 = 2.58 inches. If we inspect the Table VI, it will be seen that the effect of increasing the expansion by the slide valve is augmenting the loss of power occasioned by the imperfect action of the eduction passages. On referring to the bottom line of the Table, it will be seen that the eduction passage before the piston is closed, and that behind it is opened, thus destroying the whole moving power of the engine when the piston is 092 from the end of its stroke, the steam being cut off at from the end. Whereas, if the steam is only eut off at from the end of the stroke, the moving power is not withdrawn till or of the stroke remains uncompleted. It will also be observed that increasing the lap on the exhausting side has the effect of retaining the action of the steam longer behind the piston, but it, at the same time, causes the eduction port before it to close sooner. A very cursory examination of the action of the slide valve is sufficient to show that the lap on the steam side should always be greater than that on the eduction side. If they were equal, the steam would be admitted on one side of the piston at the same time that it was allowed to escape from the other; but universal experience has shown that when this is the case, a very considerable part of the power of the engine is destroyed by the resistance opposed to the piston, by the escaping steam not getting away to the condenser quickly enough. Hence, will be seen the necessity of the lap of the eduction side being always less than the lap on the steam side; and the difference should be the greater the higher the velocity of the piston is intended to be, because the quicker the piston moves, the passage for the waste steam requires to be the larger, so as to admit of its getting away to the condenser with as great rapidity as possible. In engines where it is not wished to expand the steam in the cylinder at all, the slide valve is sometimes made with very little lap on the steam side; and under these circumstances, in order to obtain a sufficient difference between the lap on the steam and the eduction sides of the valve, it may be necessary not only to take away all the lap on the eduction side, but to take off still more, so as to cause both eduction passages to be, in some degree, open, when the valve is at the middle of its stroke. But when there is a considerable amount of lap on the steam side, this plan of taking more than all the lap on the eduction side ought never to be resorted to, since it can serve no good purpose, and will materially increase an evil we have already alluded to, viz. :—the opening of the eduction port behind the piston, before the stroke is nearly completed. In the case of engines moving rapidly, as in locomotives for instance, it conduces much to efficiency to begin the eduction before the end of the stroke, as the piston moves slowly at that time; and at that point a very small amount of travel of the piston corresponds to a considerable additional time given for the accomplishment of the eduction. The tables apply equally to the common short slide three ported valves, and to the long D valves. The extent to which expansion can be conveniently carried by means of lap upon the valve is about one-third of the stroke; that is, the valve may be made with so much lap that the steam will be cut off when one-third of the stroke has been performed, leaving the rest to be performed by the expanding steam; but if much more lap be put on than answers to this amount of expansion, a distorted action of the valve will be produced, and the efficiency of the engine will be impaired. By the use of the link motion, however, much of this distorted action can be compensated. If a further amount of expansion than this is wanted, where the link motion is not used, it may be attained by wire-drawing the steam, or by so constructing the steam passage that the pressure within the cylinder must decline when the speed of the piston is accelerated, as it is about the middle of the stroke. Thus, for example, if the valve be so made as to shut off the steam by the time two-thirds of the stroke has been performed, and the steam be at the same time throttled in the steam pipe, the full pressure of the steam within the cylinder cannot be maintained except near the beginning of the stroke, where the piston travels slowly, for as the speed of the piston increases, the pressure necessarily subsides, until the piston approaches the other end of the cylinder, where the pressure would rise again; but the operation of the lap on the valve by this time has had the effect of closing the communication between the cylinder and steam pipe, so as to prevent more steam entering. By throttling the steam, therefore, in the manner here indicated, the amount of expansion due to the lap may be doubled, so that an engine with lap enough upon the valve to cut off the steam at two-thirds of the stroke, may, by means of wire-drawing, be virtually rendered capable of cutting off the steam at one-third the stroke. GEOMETRICAL DEMONSTRATION OF THE SLIDE-VALVE. This subject has been well digested by many authors, both English and Foreign. Of the latter the best authority is Dr. GUSTAV. ZEUNER, of the Swiss Federal Polytechnical School, but his researches and expositions pertain, however, more to the locomotive than the marine type, but in the main his views are correct for either, under relative circumstances. In this country, Mr. WATT, Professor RANKINE, Mr. D. K. CLARKE, C.E., and Mr. M. F. GRAY, claim attention for their productions. The information now given is taken from the Engineer Cadet, by Messrs. BUELL and LONG, of the United States Navy. A three-ported slide-valve in its simplest form is represented in Fig. 6. The valve has the position shown in the sketch, when the piston is at one end of the stroke. When the piston commences to move, the valve also receives motion from the eccentric. If it moves to the right, the port A will be uncovered and steam will be admitted into one end of the cylinder, while the port B will be open to the exhaust port C, and the steam in the other end of the cylinder will be exhausting. When the valve has moved far enough to uncover the port A (at which time the piston will have made half a stroke), it commences to move to the left, and at the termination of the stroke, the port A will again Fig. 6. termination of the stroke. It is evident that the motions of the valve and RR piston are at right angles; for the valve is at half stroke when the piston has completed its stroke, and vice versa. So to set the eccentric of this valve, we have only to make the line joining the centre of eccentric and the centre of the shaft, perpendicular to the line joining the centre of crank-pin and the centre of the shaft. The throw of the eccentric, or the travel of the valve (supposing the eccentric rod to be directly connected to the valve stem), is evidently equal to twice the breadth of the steam port, and the face of the valve is just as wide as the steam port. With the valve set in this manner, the steam is not admitted to the cylinder until the crank is on the centre. But it is found desirable to admit steam a little before the end of the stroke; and this can readily be accom |