EXPANSION OF STEAM AND ACTION OF THE VALVES.
177. _Q._--What is meant by working engines expansively?
_A._--Adjusting the valves, so that the steam is shut off from the cylinder
before the end of the stroke, whereby the residue of the stroke is left to
be completed by the expanding steam.
178. _Q._--And what is the benefit of that practice?
_A._--It accomplishes an important saving of steam, or, what is the same
thing, of fuel; but it diminishes the power of the engine, while increasing
the power of the steam. A larger engine will be required to do the same
work, but the work will be done with a smaller consumption of fuel. If, for
example, the steam be shut off when only half the stroke is completed,
there will only be half the quantity of steam used. But there will be more
than half the power exerted; for although the pressure of the steam
decreases after the supply entering from the boiler is shut off, yet it
imparts, during its expansion, _some_ power, and that power, it is clear,
is obtained without any expenditure of steam or fuel whatever.
179. _Q._--What will be the pressure of the steam, under such
circumstances, at the end of the stroke?
_A._--If the steam be shut off at half stroke, the pressure of the steam,
reckoning the total pressure both below and above the atmosphere, will just
be one-half of what it was at the beginning of the stroke. It is a well
known law of pneumatics, that the pressure of elastic fluids varies
inversely as the spaces into which they are expanded or compressed. For
example, if a cubic foot of air of the atmospheric density be compressed
into the compass of half a cubic foot, its elasticity will be increased
from 15 lbs. on the square inch to 30 lbs. on the square inch; whereas, if
its volume be enlarged to two cubic feet, its elasticity will be reduced to
7-1/2 lbs. on the square inch, being just half its original pressure. The
same law holds in all other proportions, and with all other gases and
vapors, provided their temperature remains unchanged; and if the steam
valve of an engine be closed, when the piston has descended through
one- fourth of the stroke, the steam within the cylinder will, at the end
of the stroke, just exert one-fourth of its initial pressure.
180. _Q._--Then by computing the varying pressure at a number of stages,
the average or mean pressure throughout the stroke may be approximately
determined?
[Illustration: Fig. 32. Diagram showing law of expansion of steam in a
cylinder.]
_A._--Precisely so. Thus in the accompanying figure, (fig. 32), let E be a
cylinder, J the piston, _a_ the steam pipe, _c_ the upper port, _f_ the
lower port, _d_ the steam pipe, prolonged to _e_ the equilibrium valve, _g_
the eduction valve, M the steam jacket, N the cylinder cover, O stuffing
box, _n_ piston rod, P cylinder bottom; let the cylinder be supposed to be
divided in the direction of its length into any number of equal parts, say
twenty, and let the diameter of the cylinder represent the pressure of the
steam, which, for the sake of simplicity, we may take at 10 lbs., so that
we may divide the cylinder, in the direction of its diameter, into ten
equal parts. If now the piston be supposed to descend through five of the
divisions, and the steam valve then be shut, the pressure at each
subsequent position of the piston will be represented by a series, computed
according to the laws of pneumatics, and which, if the initial pressure be
represented by 1, will give a pressure of .5 at the middle of the stroke,
and .25 at the end of it.
If this series be set off on the horizontal lines, it will mark out a
hyperbolic curve--the area of the part exterior to which represents the
total efficacy of the stroke, and the interior area, therefore, represents
the diminution in the power of a stroke, when the steam is cut off at
one- fourth of the descent. If the squares above the point, where the steam
is cut off, be counted, they will be found to amount to 50; and if those
beneath that point be counted or estimated, they will be found to amount to
about 69. These squares are representative of the power exerted; so that
while an amount of power represented by 50 has been obtained by the
expenditure of a quarter of a cylinder full of steam, we get an amount of
power represented by 69, without any expenditure of steam at all, merely by
permitting the steam first used to expand into four times its original
volume.
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 quantity 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
quantity 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?
[Illustration: 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 brass 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 passage
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 space 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 passage
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 passage will be closed?
_A._--To find how much before the end of the stroke the eduction passage
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 passage 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 passage
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 passage 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 passages, and which was incidental to
the application of lap to the valves. The average consumption of coke 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 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.
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?
[Illustration: 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 class 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 class, 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
brass, 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 passing 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
circumferential 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 circumstances, 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 brass
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 mass
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?
_A._--In locomotive engines the use of cams is inadmissible, and other
expedients are employed, of which those contrived by Stephenson and by
Cabrey operate on the principle of accomplishing the requisite variations
of expansion by altering the throw of the slide valve.
205. _Q._--What is Stephenson's arrangement?
[Illustration: Fig. 35.]
_A._--Stephenson connects the ends of the forward and backward eccentric
rods by a link with a curved slot in which a pin upon the end of the valve
rod works. By moving this link so as to bring the forward eccentric rod in
the same line with the valve rod, the valve receives the motion due to that
eccentric; whereas if the backward eccentric rod is brought in a line with
the valve rod, the valve gets the motion proper for reversing, and if the
link be so placed that the valve rod is midway between the two eccentric
rods, the valve will remain nearly stationary. This arrangement, which is
now employed extensively, is what is termed "the link motion." It is
represented in the annexed figure, fig. 35, where _e_ is the valve rod,
which is attached by a pin to an open curved link susceptible of being
moved up and down by the bell-crank lever _f''_ _f''_, supported on the
centre _g_, and acting on the links _f_, while the valve rod _e_ remains in
the same horizontal plane; _d d'_ are the eccentric rods, and the link is
represented in its lowest position. The dotted lines _h' h''_ show the
position of the eccentric rods when the link is in its highest position,
and _l l'_ when in mid position.
206. _Q._--What is Cabrey's arrangement?
_A._--Mr. Cabrey makes his eccentric rod terminate in a pin which works
into a straight slotted lever, furnished with jaws similar to the jaws on
the eccentric rods of locomotives. By raising the pin of the eccentric rod
in this slot, the travel of the valve will be varied, and expansive action
will be the result.
207. _Q._--What other forms of apparatus are there for working steam
expansively?
_A._--They are too numerous for description here, but a few of them may be
enumerated. Fenton seeks to accomplish the desired object by introducing a
spiral feather on the crank axle, by moving the eccentric laterally against
which the eccentric is partially turned round so as to cut off the steam at
a different part of the stroke. Dodds seeks to attain the same end by
corresponding mechanical arrangements. Farcot, Edwards, and Lavagrian cut
off the steam by the application of a supplementary valve at the back of
the ordinary valve, which supplementary valve is moved by tappets fixed to
the valve casing. Bodmer, in 1841, and Meyer, in 1842, employed two slides
or blocks fitted over apertures in the ordinary slide valve, and which
blocks were approximated or set apart by a right and left handed screw
passing through both.[1] Hawthorn, in 1843, employed as an expansion valve
a species of frame lying on the ordinary cylinder face upon the outside of
the valve, and working up against the steam side of the valve at each end
so as to cut off the steam. In the same year Gonzenbach patented an
arrangement which consists of an additional slide valve and valve casing
placed on the back of the ordinary slide valve casing, and through this
supplementary valve the steam must first pass. This supplementary valve is
worked by a double ended lever, slotted at one end for the reception of a
pin on the valve link, the position of which in the slot determines the
throw of the supplementary valve, and the consequent degree of expansion.
208. _Q._--What is the arrangement of expansion valve used in the most
approved modern engines?
_A._--In modern engines, either marine or locomotive, it is found that if
they are fitted with the link motion, as they nearly all are, a very good
expansive action can be obtained by giving a suitable adjustment to it,
without employing an expansion valve at all. Diagrams taken from engines
worked in this manner show a very excellent result, and most of the modern
engines trust for their expansive working to the link motion and the
throttle valve.
[1] In 1838 I patented an arrangement of expansion valve, consisting of two
movable plates set upon the ordinary slide valve, and which might be drawn
together or asunder by means of a right and left handed screw passing
through both plates. The valve spindle was hollow, and a prolongation of
the screw passed up through it, and was armed on the top with a small
wheel, by means of which the plates might be adjusted while the engine was
at work. In 1839 I fitted an expansion valve in a steam vessel, consisting
of two plates, connected by a rod, and moved by tappets up against the
steam edges of the valve. In another steam vessel I fitted the same species
of valve, but the motion was not derived from tappets, but from a moving
part of the engine, though at the moderate speed at which these engines
worked I found tappets to operate well and make little noise. In 1837 I
employed, as an expansion valve, a rectangular throttle valve, accurately
fitting a bored out seat, in which it might be made to revolve, though it
did not revolve in working. This valve was moved by a pin in a pinion,
making two revolutions for every revolution of the engine, and the
configuration of the seat determined the amount of the expansion. In 1855 I
have again used expansion valves of this construction in engines making one
hundred revolutions per minute, and with perfectly satisfactory results.--
J.B.