CHAPTER VII.
THE EFFECT OF ALTERATIONS TO VALVE AND ECCENTRIC.
_Advance of the Eccentric._—It should here be established that to any given example of valve, ports, and eccentric (leaving the piston out of consideration for a moment) belongs a certain cycle of operations, the occurrence of these operations and their _duration, expressed in fractions of a revolution of the rotating eccentric_, being constant for a given case,[3] but the fraction of stroke traversed by the _piston_ between or during any of these operations of the valve will depend upon _where_ in the stroke of the piston the cycle of operations is started, taking any convenient operation as being the commencement of the cycle, for the speed of the piston rises from _nil_ at the commencement of its stroke to a maximum near the middle, falling again to _nil_ at the end.
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Footnote 3:
To make the idea clear one might plot these operations round a circle representing one revolution of the eccentric, when it would be seen that the operations of the _valve_ could be considered quite independently of those of the piston—then it would remain to show what the piston might do, or have done upon it, by comparing its motion under varying conditions with the _unvarying_ operations of the valve, in the manner explained later.
Now we may go on to consider the effect of increasing the advance of an eccentric beyond that which it would ordinarily have (fig. 32). If we suppose our eccentric and valve to give us a cut-off at C and an exhaust at E, expansion lasting 0.17 of the stroke (as shown in full lines, fig. 32), and that by advancing the eccentric we get a cut-off at C^1 0.14 earlier, we must not expect to find that exhaust also will be 0.14 earlier (dotted lines, fig. 32), for although the exhaust edge of the valve always gives exhaust at a certain constant period of time later than steam is cut off by the steam edge, yet in that interval the speed of the piston slackens less in this second case than in the first example, and it therefore travels through a greater distance than in the same time-interval taken later in the stroke in the first case. Hence, with increased advance we get _earlier_ cut-off, with expansion of _lengthened duration_ because _exhaust is not earlier in the same degree_, neither are all the other operations performed by the valve, although they each commence earlier and last longer in the stroke, lead included, but admission, of course, excepted. _The idea may, perhaps, be more easily grasped by keeping the eccentric and valve in view at an unaltering standard of reference, and when it is desired to consider the effect of advancing the eccentric, to imagine instead that it is the setting of the main crank which is altered_—which in effect is the same thing, and is, perhaps, easier to reason out.
[Illustration: FIG. 32.]
_Lengthening or Shortening the Eccentric Rod—Shifting the Valve on its Spindle._—Starting with the valve in its central position, and lengthening or shortening the eccentric rod or shifting the valve on its spindle, or combining both operations in the same sense, the distance between the centre of the crank-shaft and the centre of the valve is altered, and the travel of the valve is shifted bodily to the right or left, _i.e._, the valve when in the centre of its travel (the extent of travel remaining of course unaltered) will be either further from or nearer to the crank-shaft than before, while the position of the ports remains unaltered; therefore the arrow upon the valve at its centre (see fig. 16) lies to the right or left of the centre line of the port-face, and, that arrow being no longer appropriate as a pointer at mid-travel to the zero of the scale on the ports, we must adopt as our index-finger the graduation mark made adjacent to the arrow and lying over the said zero, in order that when the disc may indicate that the valve should move, say, from 0 to 6 the new mark or index-finger, and not the arrow, may be given that movement.
If the valve had previously been properly set for lead, and the alteration shifted it ¹⁄₈ in. to the left, for example, we should find that the lead at the left-hand end would be diminished by that amount, and that subsequently, at cut-off, release, and compression, for both ends of the cylinder, the valve would always be found ¹⁄₈ in. to the left of its proper position. The result would be, as shown in fig. 33, that: At left-hand of cylinder (see lines of dashes):—
Lead decreases } earlier, these operations Cut-off takes place } taking place Exhaust commences } as the valve moves } _towards_ the left.
Compression commences later, this operation taking place as the valve moves _from_ the left.
At right-hand end of cylinder (see lines of dots):—
Lead increases } later, because when Exhaust commences } effecting them the Cut-off takes place } valve is moving } _from_ the left.
[Illustration: FIG. 33.]
Compression commences earlier, for it occurs whilst the valve is moving _to_ the left. The alterations at opposite ends being in opposite sense. (The normal distribution is shown by _full lines_ for comparison.) And although the commencement of the operations is advanced and retarded by the same amount of error, operating in opposite sense on opposite sides of the piston, yet there is a difference in the work done upon the opposite sides because of the commencement of the two cycles of operation at different times, whence we infer differences in the speeds of the piston, in the quantities of steam admitted, and in the expansion-curves.
In some cases such a difference is desirable, and with the working diagram suggested in Chapter I. another series of results may be obtained by using the ordinary valve for distribution on one side of the piston, and for the other side using a differently-proportioned valve with a different index-pointer, as though opposite ends of one valve were differently proportioned, as in certain large vertical engines.
_Adding Extra Lap to a Valve._—In some cases cut-off may be wanted earlier in the stroke, permanently, with expansion of longer duration. Under such circumstances more lap may be added to the valve. If this be done, and the valve with its extra lap be re-set by giving the eccentric greater advance in order to get the same lead as before (travel of valve remaining unaltered), the steam-ports will not be opened to so great an extent as formerly, and will close earlier, partly because of the added lap,[4] and partly because of the increased advance. The increased length of the valve-face will, after cut-off, confine the steam longer in the cylinder, and it should be obvious (the effect of advancing the eccentric having already been shown with reference to fig. 32) that in addition to the earlier commencement and lengthened duration of expansion, the exhaust and compression will begin somewhat earlier because of the increased advance, and the compression will be of longer duration because of the increased length of the valve-face.
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Footnote 4:
To assure one’s self of this, take an extreme case and imagine an excessive amount of lap to be added. In such a case, with travel unaltered, the ports might not be opened to steam at all, the valve simply sliding backwards and forwards without uncovering them; now, taking this lap off piecemeal the port would first be opened to an exceedingly small degree and instantly shut again, the amount and duration of opening increasing with each successive removal of lap.
_General._—It is here seen that in altering either the outside or the inside laps with intent to affect only the steam admission with the former, or only the exhaust with the latter, we must _perforce_ affect other things. For instance, we alter the outside lap, with the intention, say, of simply making cut-off earlier or later as the case may be—but having altered the lengths of the valve-faces, they will alter the duration of expansion and of compression, and the alteration necessitated in the advance of the eccentric in order that the lead may remain unaltered has its own effect in addition. This interdependence, which is often inconvenient, is characteristic of the slide valve worked by an eccentric, and has led to the use of valve gears and valves which permit, more or less, of the independent regulation of some or all of the various operations in the distribution of steam. Some of these gears are used with a slide valve (as in the case of the Joy and other well-known valve gears), while others operate special valves.
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