A Catechism of the Steam Engine Part 10

181. _Q._--Then by working an engine expansively, the power of the steam is increased, but the power of the engine is diminished?

_A._--Yes. The efficacy of a given quant.i.ty of steam is more than doubled by expanding the steam four times, while the efficacy of each stroke is made nearly one-half less. And, therefore, to carry out the expansive principle in practice, the cylinder requires to be larger than usual, or the piston faster than usual, in the proportion in which the expansion is carried out. Every one who is acquainted with simple arithmetic, can compute the terminal pressure of steam in a cylinder, when he knows the initial pressure and the point at which the steam is cut off; and he can also find, by the same process, any pressure intermediate between the first and the last. By setting down these pressures in a table, and taking their mean, he can determine the effect, with tolerable accuracy, of any particular measure of expansion. It is necessary to remark, that it is the total pressure of the steam that he must take; not the pressure above the atmosphere, but the pressure above a perfect vacuum.

182. _Q._--Can you give any rule for ascertaining at one operation the amount of benefit derivable from expansion?

_A._--Divide the length of stroke through which the steam expands, by the length of stroke performed with full pressure, which last call 1; the hyperbolic logarithm of the quotient is the increase of efficiency due to expansion. According to this rule it will be found, that if a given quant.i.ty of steam, the power of which working at full pressure is represented by 1, be admitted into a cylinder of such a size that its ingress is concluded when one-half the stroke has been performed, its efficacy will be raised by expansion to 1.69; if the admission of the steam be stopped at one-third of the stroke, the efficacy will be 2.10; at one- fourth, 2.39; at one-fifth, 2.61; at one-sixth, 2.79; at one-seventh, 2.95; at one-eighth, 3.08. The expansion, however, cannot be carried beneficially so far as one-eighth, unless the pressure of the steam in the boiler be very considerable, on account of the inconvenient size of cylinder or speed of piston which would require to be adopted, the friction of the engine, and the resistance of vapor in the condenser, which all become relatively greater with a smaller urging force.

183. _Q._--Is this amount of benefit actually realized in practice?

_A._--Only in some cases. It appears to be indispensable to the realization of any large amount of benefit by expansion, that the cylinder should be enclosed in a steam jacket, or should in some other way be effectually protected from refrigeration. In some engines not so protected, it has been found experimentally that less benefit was obtained from the fuel by working expansively than by working without expansion--the whole benefit due to expansion being more than counteracted by the increased refrigeration due to the larger surface of the cylinder required to develop the power. In locomotive engines, with outside cylinders, this condition of the advantageous use of expansion has been made very conspicuous, as has also been the case in screw steamers with four cylinders, and in which the refrigerating surface of the cylinders was consequently large.

184. _Q._--The steam is admitted to and from the cylinder by means of a slide or sluice valve?

[Ill.u.s.tration: Fig. 33.]

_A._--Yes; and of the slide valve there are many varieties; but the kinds most in use are the D valve,--so called from its resemblance to a half cylinder or D in its cross section--and the three ported valve, shown in fig. 33, which consists of a bra.s.s or iron box set over the two ports or openings into the cylinder, and a central port which conducts away the steam to the atmosphere or condenser; but the length of the box is so adjusted that it can only cover one of the cylinder ports and the central or eduction port at the same time. The effect, therefore, of moving the valve up and down, as is done by the eccentric, is to establish a connection alternately between each cylinder port and the central pa.s.sage whereby the steam escapes; and while the steam is escaping from beneath the piston, the position of the valve is such, that a free communication exists between the s.p.a.ce above the piston and the steam in the boiler. The piston is thus urged alternately up and down--the valve so changing its position before the piston arrives at the end of the stroke, that the pressure is by that time thrown on the reverse side of the piston, so as to urge it into motion in the opposite direction.

185. _Q._--Is the motion of the valve, then, the reverse of that of the piston?

_A._--No. The valve does not move down when the piston moves down, nor does it move down when the piston moves up; but it moves from its mid position, to the extremity of its throw, and back again to its mid position, while the piston makes an upward or downward movement, so that the motion is as it were at right angles to the motion of the piston; or it is the same motion that the piston of another engine, the crank of which is set at right angles with that of the first engine, would acquire.

186. _Q._--Then in a steam vessel the valve of one engine may be worked from the piston of the other?

_A._--Yes, it may; or it may be worked from its own connecting rod; and in the case of locomotive engines, this has sometimes been done.

187. _Q._--What is meant by the lead of the valve?

_A._--The amount of opening which the valve presents for the admission of the steam, when the piston is just beginning its stroke. It is found expedient that the valve should have opened a little to admit steam on the reverse side of the piston before the stroke terminates; and the amount of this opening, which is given by turning the eccentric more or less round upon the shaft, is what is termed the lead.

188. _Q._--And what is meant by the lap of the valve?

_A._--It is an elongation of the valve face to a certain extent over the port, whereby the port is closed sooner than would otherwise be the case.

This extension is chiefly effected at that part of the valve where the steam is admitted, or upon the _steam side_ of the valve, as the technical phrase is; and the intent of the extension is to close the steam pa.s.sage before the end of the stroke, whereby the engine is made to operate to a certain extent expansively. In some cases, however, there is also a certain amount of lap given to the escape or eduction side, to prevent the eduction from being performed too soon when the lead is great; but in all cases there is far less lap on the eduction than on the steam side, very often there is none, and sometimes less than none, so that the valve is incapable of covering both the ports at once.

189. _Q._--What is the usual proportional length of stroke of the valve?

_A._--The common stroke of the valve in rotative engines is twice the breadth or depth of the port, and the length of the valve face will then be just the breadth of the port when there is lap on neither the steam nor eduction side. Whatever lap is given, therefore, makes the valve face just so much longer. In some engines, however, the stroke of the valve is a good deal more than twice the breadth of the port; and it is to the stroke of the valve that the amount of lap should properly be referred.

190. _Q._--Can you tell what amount of lap will accomplish any given amount of expansion?

_A._--Yes, when the stroke of the valve is known. From the length of the stroke of the piston subtract that part of the stroke which is intended to be accomplished before the steam is cut off; divide the remainder by the length of the stroke of the piston, and extract the square root of the quotient, which multiply by half the stroke of the valve, and from the product take half the lead; the remainder will be the lap required.

191. _Q._--Can you state how we may discover at what point of the stroke the eduction pa.s.sage will be closed?

_A._--To find how much before the end of the stroke the eduction pa.s.sage will be closed:--to the lap on the steam side add the lead, and divide the sum by half the stroke of the valve; find the arc whose sine is equal to the quotient, and add 90 to it.; divide the lap on the eduction side by half the stroke of the valve, and find the arc whose cosine is equal to the quotient; subtract this arc from the one last obtained, and find the cosine of the remainder; subtract this cosine from 2, and multiply the remainder by half the stroke of the piston; the product is the distance of the piston from the end of the stroke when the eduction pa.s.sage is closed.

192. _Q._--Can you explain how we may determine the distance of the piston from the end of the stroke, before the steam urging it onward is allowed to escape?

_A._--To find how far the piston is from the end of its stroke when the steam that is propelling it by expansion is allowed to escape to the atmosphere or condenser--to the lap on the steam side add the lead; divide the sum by half the stroke of the valve, and find the arc whose sine is equal to the quotient; find the arc whose sine is equal to the lap on the eduction side, divided by half the stroke of the valve; add these two arcs together and subtract 90; find the cosine of the residue, subtract it from 1, and multiply the remainder by half the stroke of the piston; the product is the distance of the piston from the end of its stroke when the steam that is propelling it is allowed to escape into the atmosphere or condenser. In using these rules, all the dimensions are to be taken in inches, and the answers will be found in inches also.

193. _Q._--Is it a benefit or a detriment to open the eduction pa.s.sage before the end of the stroke?

_A._--In engines working at a high rate of speed, such as locomotive engines, it is very important to open the exhaust pa.s.sage for the escape of the steam before the end of the stroke, as an injurious amount of back pressure is thus prevented. In the earlier locomotives a great loss of effect was produced from inattention to this condition; and when lap was applied to the valves to enable the steam to be worked expansively, it was found that a still greater benefit was collaterally obtained by the earlier escape of the steam from the eduction pa.s.sages, and which was incidental to the application of lap to the valves. The average consumption of c.o.ke per mile was reduced by Mr. Woods from 40 lbs. per mile to 15 lbs. per mile, chiefly by giving a free outlet to the escaping steam.

194. _Q._--To what extent can expansion be carried beneficially by means of lap upon the valve?

_A._--To 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 two thirds of the stroke have been performed, leaving the residue to be accomplished by the agency of the expanding steam; but if more lap be put on than answers to this amount of expansion, a very distorted action of the valve will be produced, which may impair the efficiency of the engine. If a further amount of expansion than this is wanted, it may be accomplished by wire drawing the steam, or by so contracting the steam pa.s.sage 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.

195. _Q._--Will you explain how this result ensues?

_A._--If the valve be so made as to shut off the steam by the time two thirds of the stroke have 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 that 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 from 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 the aid of wire drawing, be virtually rendered capable of cutting off the steam at one-third of the stroke.

196. _Q._--Is this the usual way of cutting off the steam?

_A._--No; the usual way of cutting off the steam is by means of a separate valve, termed an expansion valve; but such a device appears to be hardly necessary in ordinary engines. In the Cornish engines, where the steam is cut off in some cases at one-twelfth of the stroke, a separate valve for the admission of steam, other than that which permits its escape, is of course indispensable; but in common rotative engines, which may realize expansive efficacy by throttling, a separate expansion valve does not appear to be required.

197. _Q._--That is, where much expansion is required, an expansion valve is a proper appendage, but where not much is required, a separate expansion valve may be dispensed with?

_A._--Precisely so. The wire drawing of the steam causes a loss of part of its power, and the result will not be quite so advantageous by throttling as by cutting off. But for moderate amounts of expansion it will suffice, provided there be lap upon the slide valve.

198. _Q._--Will you explain the structure or configuration of expansion apparatus of the usual construction?

[Ill.u.s.tration: Fig 34.]

_A._--The structure of expansion apparatus is very various; but all the kinds operate either on the principle of giving such a motion to the slide valve as will enable it to cut off the steam, at the desired point, or on the principle of shutting off the steam by a separate valve in the steam pipe or valve casing. The first cla.s.s of apparatus has not been found so manageable, and is not in extensive use, except in that form known as the link motion. Of the second cla.s.s, the most simple probably is the application of a cam giving motion to the throttle valve, or to a valve of the same construction, which either accurately fits the steam pipe, or which comes round to a face, which, however, it is restrained from touching by a suitable construction of the cam. A kind of expansion valve, often employed in marine engines of low speed, is the kind used in the Cornish engines, and known as the equilibrium valve. This valve is represented in fig. 34. It consists substantially of an annulus or bulging cylinder of bra.s.s, with a steam-tight face both at its upper and lower edges, at which points it fits accurately upon a stationary seat. This annulus may be raised or lowered without being resisted by the pressure of the steam, and in rotative engines it is usually worked by a cam on the shaft. The expansion cam is put on the shaft in two pieces, which are fastened to each other by means of four bolts pa.s.sing through lugs, and is fixed to the shaft by keys. A roller at one end of a bell-crank lever, which is connected with the expansion valve, presses against the cam, so that the motion of the lever will work the valve. The roller is kept against the cam by a weight on a lever attached to the same shaft, but a spring is necessary for high speeds. If the cam were concentric with the shaft, the lever which presses upon it would remain stationary, and also the expansion valve; but by the projection of the cam, the end of the lever receives a reciprocating motion, which is communicated to the valve.

199. _Q._--The cam then works the valve?

_A._--Yes. The position of the projection of the cam determines the point in relation to the stroke at which the valve is opened, and its circ.u.mferential length determines the length of the time during which the valve continues open. The time at which the valve should begin to open is the same under all circ.u.mstances, but the duration of its opening varies with the amount of expansion desired. In order to obtain this variable extent of expansion, there are several projections made upon the cam, each of which gives a different degree, or _grade_ as it is usually called, of expansion. These grades all begin at the same point on the cam, but are of different lengths, so that they begin to move the lever at the same time, but differ in the time of returning it to its original position.

200. _Q._--How is the degree of expansion changed?

_A._--The change of expansion is effected by moving the roller on to the desired grade; which is done by slipping the lever carrying the roller endways on the shaft or pin sustaining it.

201. _Q._--Are such cams applicable in all cases?

_A._--In engines moving at a high rate of speed the roller will be thrown back from the cam by its momentum, unless it be kept against it by means of springs. In some cases I have employed a spring formed of a great number of discs of India rubber to keep the roller against the cam, but a few bra.s.s discs require to be interposed to prevent the India rubber discs from being worn in the central hole.

202. _Q._--May not the percussion incident to the action of a cam at a high speed, when the roller is not kept up to the face by springs, be obviated by giving a suitable configuration to the cam itself?

_A._--It may at all events be reduced. The outline of the cam should be a parabola, so that the valve may be set in motion precisely as a falling body would be; but it will, nevertheless, be necessary that the roller on which the cam presses should be forced upward by a spring rather than by a counterweight, as there will thus be less inertia or momentum in the ma.s.s that has to be moved.

203. _Q._--An additional slide valve is sometimes used for cutting off the steam?

_A._--Yes, very frequently; and the slide valve is sometimes on the side or back of the valve casing, and sometimes on the back of the main or distributing valve, and moving with it.

204. _Q._--Are cams used in locomotive engines?