Page:EB1911 - Volume 25.djvu/855

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833
STEAM ENGINE


the crank, and hence to find the displacement of the valve at any position of the crank we have only to draw CQ in fig. 26 parallel to the crank, when CQ represents the displacement of the valve to the scale on which the diameter of each valve circle represents the half-travel of the valve. CQo is the valve displacement at the beginning of the stroke shown by the arrow. Draw circular arcs ab and cd with C as centre and with radii equal to the outside lap and the inside lap i respectively. Ca is the position of the crank at which pre- admission occurs. The lead is a Qo. The greatest steam opening is ajB. The cut-off occurs when the crank has , . . a , . , , the direction Cb. Cc is the position

of the crank at release, and Cd marks the end of the exhaust

03. In this diagram radii drawn from C mark the angular positions of the crank and their intercepts by the valve circles determine the corresponding displacement of the valve. It remains to find the corresponding displacement of the piston. For this Zeuner employs a supplementary graphic construction, shown in fig. 27. Here ab or a b represents the connecting rod, and be or b'c the crank. With centre c and radius ac a circle ap is drawn, and with centre b and radius ab another circle aq. Then for any position of the crank, as cb , the intercept pq between the circles is easily seen to be equal to aa , and is therefore the distance by which the piston has moved lrom its extreme position at the beginning of the stroke. In practice this diagram is combined with that of fig. 26, by drawing both about the same centre and using different scales for valve and piston travel A radius vector drawn from the centre parallel to the crank in any position then shows the valve's displacement from the valve's middle position by the intercept CQ of fig. 26, and the piston's displacement from the beginning of the piston's motion by the intercept pq of fig. 27.

64. In the figures which have been sketched the events refer to the front end of the cylinder, that is, the end nearest to the crank (see hg. 23) 10 determine the events of steam distribution at the back end, the lap circles shown by dotted lines in fig. 26 must also be drawn, Ca' being the outside lap for the back end, and Cc' the inside lap. These laps are not necessarily equal to those at the other end of the valve. From the diagrams it will be seen that, especially with a short connecting-rod, the cut-off and release occur earlier and the compression later at the front than at the back end it the laps are equal, and a more symmetrical steam distribution can be produced by making the inside lap greater and the outside lap less on the side which leads to the front end of the cylinder On the other hand, an unsymmetrical distribution may be desirable as in a vertical engine, where the weight of the piston assists the steam during the down-stroke and resists it during the up-stroke and this may be secured by a suitable inequality in the laps

65. By varying the ratio of the laps and i to the travel of the valve, we produce effects on the steam distribution which are readily traced by means of the diagram. Reduction of travel (which is equivalent to increase of both and i) gives later pre- admission, earlier cut-off, later release and earlier compression- the ratios of expansion and of compression are both increased. Increase of angular advance accelerates all the events and causes a slight increase in the ratio of expansion.

66. In designing a slide-valve the breadth of the steam ports in the direction of the valve's motion- is determined with reference to the volume of the exhaust steam to be discharged in a given time the area of the ports being generally such that the mean velocity ot the steam during discharge is less than 100 ft. per second The travel is made great enough to keep the cylinder port fully open during the greater part of the exhaust; for this purpose it is 2\ or 3 times the breadth of the steam port. To facilitate the exit of steam the inside lap is always small, and is often wanting or even negative. During admission the steam port is rarely quite un- covered, especially if the outside lap is large and the travel moderate. Large travel has the advantage of giving freer ingress and egress of steam, with more sharply-defined cut-off, compression and release but this advantage is secured at the cost of more work spent in moving the valve and more wear of the faces. To lessen the neces- sary travel without reducing the area of steam ports, double-ported valves are often used. An example is shown below in fig. 39

67 Reversal of Motion with Slide- Valve. —The eccentric must stand in advance of the crank by the angle 90° + 0, as in fig. 28, where CK is the crank, and CE the corresponding position of the eccentric when the engine is running in the direction of the arrow a. To set the engine in gear to run in the opposite direction (b) it is only necessary to shift the eccentric into the position CK when it wil still be 90 +6 in advance of the u„ «■-> -i Cr 5" ln - the ? lder e "g in es this reversal was effected by temporarily disengaging the eccentric-rod from the valve-rod tn the crank turned back through an angle equal to ECE', the eccentric meanwhile remaining