THE CONVERSION OF HEAT ENERGY INTO MECHANICAL MOTION.
Reciprocating engines--Double-cylinder engines--The function of the
fly-wheel--The cylinder--The slide-valve--The eccentric--"Lap" of
the valve: expansion of steam--How the cut-off is managed--Limit of
expansive working--Compound engines--Arrangement of expansion
engines--Compound locomotives--Reversing
gears--"Linking-up"--Piston-valves--Speed governors--Marine-speed
governors--The condenser.
Having treated at some length the apparatus used for converting water
into high-pressure steam, we may pass at once to a consideration of the
mechanisms which convert the energy of steam into mechanical motion, or
_work_.
Steam-engines are of two kinds:--(1) _reciprocating_, employing
cylinders and cranks; (2) _rotary_, called turbines.
RECIPROCATING ENGINES.
[Illustration: FIG. 17.--Sketch showing parts of a horizontal
steam-engine.]
Fig. 17 is a skeleton diagram of the simplest form of reciprocating
engine. C is a _cylinder_ to which steam is admitted through the
_steam-ways_[3] W W, first on one side of the piston P, then on the
other. The pressure on the piston pushes it along the cylinder, and the
force is transmitted through the piston rod P R to the _connecting rod_
C R, which causes the _crank_ K to revolve. At the point where the two
rods meet there is a "crosshead," H, running to and fro in a guide to
prevent the piston rod being broken or bent by the oblique thrusts and
pulls which it imparts through C R to the crank K. The latter is keyed
to a _shaft_ S carrying the fly-wheel, or, in the case of a locomotive,
the driving-wheels. The crank shaft revolves in bearings. The internal
diameter of a cylinder is called its _bore_. The travel of the piston is
called its _stroke_. The distance from the centre of the shaft to the
centre of the crank pin is called the crank's _throw_, which is half of
the piston's _stroke_. An engine of this type is called double-acting,
as the piston is pushed alternately backwards and forwards by the steam.
When piston rod, connecting rod, and crank lie in a straight line--that
is, when the piston is fully out, or fully in--the crank is said to be
at a "dead point;" for, were the crank turned to such a position, the
admission of steam would not produce motion, since the thrust or pull
would be entirely absorbed by the bearings.
[Illustration: FIG. 18.--Sectional plan of a horizontal engine.]
DOUBLE-CYLINDER ENGINES.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
Locomotive, marine, and all other engines which must be started in any
position have at least _two_ cylinders, and as many cranks set at an
angle to one another. Fig. 19 demonstrates that when one crank, C_1,
of a double-cylinder engine is at a "dead point," the other, C_2, has
reached a position at which the piston exerts the maximum of turning
power. In Fig. 20 each crank is at 45° with the horizontal, and both
pistons are able to do work. The power of one piston is constantly
increasing while that of the other is decreasing. If _single_-action
cylinders are used, at least _three_ of these are needed to produce a
perpetual turning movement, independently of a fly-wheel.
THE FUNCTION OF THE FLY-WHEEL.
A fly-wheel acts as a _reservoir of energy_, to carry the crank of a
single-cylinder engine past the "dead points." It is useful in all
reciprocating engines to produce steady running, as a heavy wheel acts
as a drag on the effects of a sudden increase or decrease of steam
pressure. In a pump, mangold-slicer, cake-crusher, or chaff-cutter, the
fly-wheel helps the operator to pass _his_ dead points--that is, those
parts of the circle described by the handle in which he can do little
work.
THE CYLINDER.
[Illustration: FIG. 21.--Diagrammatic section of a cylinder and its
slide-valve.]
The cylinders of an engine take the place of the muscular system of the
human body. In Fig. 21 we have a cylinder and its slide-valve shown in
section. First of all, look at P, the piston. Round it are white
grooves, R R, in which rings are fitted to prevent the passage of steam
past the piston. The rings are cut through at one point in their
circumference, and slightly opened, so that when in position they press
all round against the walls of the cylinder. After a little use they
"settle down to their work"--that is, wear to a true fit in the
cylinder. Each end of the cylinder is closed by a cover, one of which
has a boss cast on it, pierced by a hole for the piston rod to work
through. To prevent the escape of steam the boss is hollowed out true to
accommodate a _gland_, G^1, which is threaded on the rod and screwed
up against the boss; the internal space between them being filled with
packing. Steam from the boiler enters the steam-chest, and would have
access to both sides of the piston simultaneously through the
steam-ways, W W, were it not for the
SLIDE-VALVE,
a hollow box open at the bottom, and long enough for its edges to cover
both steam-ways at once. Between W W is E, the passage for the exhaust
steam to escape by. The edges of the slide-valve are perfectly flat, as
is the face over which the valve moves, so that no steam may pass under
the edges. In our illustration the piston has just begun to move towards
the right. Steam enters by the left steam-way, which the valve is just
commencing to uncover. As the piston moves, the valve moves in the same
direction until the port is fully uncovered, when it begins to move back
again; and just before the piston has finished its stroke the steam-way
on the right begins to open. The steam-way on the left is now in
communication with the exhaust port E, so that the steam that has done
its duty is released and pressed from the cylinder by the piston.
_Reciprocation_ is this backward and forward motion of the piston: hence
the term "reciprocating" engines. The linear motion of the piston rod is
converted into rotatory motion by the connecting rod and crank.
[Illustration: FIG. 22.--Perspective section of cylinder.]
The use of a crank appears to be so obvious a method of producing this
conversion that it is interesting to learn that, when James Watt
produced his "rotative engine" in 1780 he was unable to use the crank
because it had already been patented by one Matthew Wasborough. Watt was
not easily daunted, however, and within a twelvemonth had himself
patented five other devices for obtaining rotatory motion from a piston
rod. Before passing on, it may be mentioned that Watt was the father of
the modern--that is, the high-pressure--steam-engine; and that, owing to
the imperfection of the existing machinery, the difficulties he had to
overcome were enormous. On one occasion he congratulated himself because
one of his steam-cylinders was only three-eighths of an inch out of
truth in the bore. Nowadays a good firm would reject a cylinder 1/500 of
an inch out of truth; and in small petrol-engines 1/5000 of an inch is
sometimes the greatest "limit of error" allowed.
[Illustration: FIG. 23.--The eccentric and its rod.]
THE ECCENTRIC
is used to move the slide-valve to and fro over the steam ports (Fig.
23). It consists of three main parts--the _sheave_, or circular plate S,
mounted on the crank shaft; and the two _straps_ which encircle it, and
in which it revolves. To one strap is bolted the "big end" of the
eccentric rod, which engages at its other end with the valve rod. The
straps are semicircular and held together by strong bolts, B B, passing
through lugs, or thickenings at the ends of the semicircles. The sheave
has a deep groove all round the edges, in which the straps ride. The
"eccentricity" or "throw" of an eccentric is the distance between C^2,
the centre of the shaft, and C^1, the centre of the sheave. The throw
must equal half of the distance which the slide-valve has to travel over
the steam ports. A tapering steel wedge or key, K, sunk half in the
eccentric and half in a slot in the shaft, holds the eccentric steady
and prevents it slipping. Some eccentric sheaves are made in two parts,
bolted together, so that they may be removed easily without dismounting
the shaft.
The eccentric is in principle nothing more than a crank pin so
exaggerated as to be larger than the shaft of the crank. Its convenience
lies in the fact that it may be mounted at any point on a shaft, whereas
a crank can be situated at an end only, if it is not actually a V-shaped
bend in the shaft itself--in which case its position is of course
permanent.
SETTING OF THE SLIDE-VALVE AND ECCENTRIC.
The subject of valve-setting is so extensive that a full exposition
might weary the reader, even if space permitted its inclusion. But
inasmuch as the effectiveness of a reciprocating engine depends largely
on the nature and arrangement of the valves, we will glance at some of
the more elementary principles.
[Illustration: FIG. 24.]
[Illustration: FIG. 25.]
In Fig. 24 we see in section the slide-valve, the ports of the cylinder,
and part of the piston. To the right are two lines at right angles--the
thicker, C, representing the position of the crank; the thinner, E, that
of the eccentric. (The position of an eccentric is denoted
diagrammatically by a line drawn from the centre of the crank shaft
through the centre of the sheave.) The edges of the valve are in this
case only broad enough to just cover the ports--that is, they have no
_lap_. The piston is about to commence its stroke towards the left; and
the eccentric, which is set at an angle of 90° in _advance_ of the
crank, is about to begin opening the left-hand port. By the time that C
has got to the position originally occupied by E, E will be horizontal
(Fig. 25)--that is, the eccentric will have finished its stroke towards
the left; and while C passes through the next right angle the valve will
be closing the left port, which will cease to admit steam when the
piston has come to the end of its travel. The operation is repeated on
the right-hand side while the piston returns.
[Illustration: FIG. 26.]
It must be noticed here--(1) that steam is admitted at full pressure
_all through_ the stroke; (2) that admission begins and ends
simultaneously with the stroke. Now, in actual practice it is necessary
to admit steam before the piston has ended its travel, so as to
_cushion_ the violence of the sudden change of direction of the piston,
its rod, and other moving parts. To effect this, the eccentric is set
more than 90° in advance--that is, more than what the engineers call
_square_. Fig. 26 shows such an arrangement. The angle between E and
E^1 is called the _angle of advance_. Referring to the valve, you will
see that it has opened an appreciable amount, though the piston has not
yet started on its rightwards journey.
"LAP" OF THE VALVE--EXPANSION OF STEAM.
In the simple form of valve that appears in Fig. 24, the valve faces are
just wide enough to cover the steam ports. If the eccentric is not
_square_ with the crank, the admission of steam lasts until the very end
of the stroke; if set a little in advance--that is, given _lead_--the
steam is cut off before the piston has travelled quite along the
cylinder, and readmitted before the back stroke is accomplished. Even
with this lead the working is very uneconomical, as the steam goes to
the exhaust at practically the same pressure as that at which it entered
the cylinder. Its property of _expansion_ has been neglected. But
supposing that steam at 100 lbs. pressure were admitted till
half-stroke, and then suddenly cut off, the expansive nature of the
steam would then continue to push the piston out until the pressure had
decreased to 50 lbs. per square inch, at which pressure it would go to
the exhaust. Now, observe that all the work done by the steam after the
cut-off is so much power saved. The _average_ pressure on the piston is
not so high as in the first case; still, from a given volume of 100 lbs.
pressure steam we get much more _work_.
HOW THE CUT-OFF IS MANAGED.
[Illustration: FIG. 27.--A slide-valve with "lap."]
[Illustration: FIG. 28.]
Look at Fig. 27. Here we have a slide-valve, with faces much wider than
the steam ports. The parts marked black, P P, are those corresponding to
the faces of the valves shown in previous diagrams (p. 54). The shaded
parts, L L, are called the _lap_. By increasing the length of the lap we
increase the range of expansive working. Fig. 28 shows the piston full
to the left; the valve is just on the point of opening to admit steam
behind the piston. The eccentric has a throw equal to the breadth of a
port + the lap of the valve. That this must be so is obvious from a
consideration of Fig. 27, where the valve is at its central position.
Hence the very simple formula:--Travel of valve = 2 × (lap + breadth of
port). The path of the eccentric's centre round the centre of the shaft
is indicated by the usual dotted line (Fig. 28). You will notice that
the "angle of advance," denoted by the arrow A, is now very
considerable. By the time that the crank C has assumed the position of
the line S, the eccentric has passed its dead point, and the valve
begins to travel backwards, eventually returning to the position shown
in Fig. 28, and cutting off the steam supply while the piston has still
a considerable part of its stroke to make. The steam then begins to work
expansively, and continues to do so until the valve assumes the position
shown in Fig. 27.
If the valve has to have "lead" to admit steam _before_ the end of the
stroke to the other side of the piston, the _angle of advance_ must be
increased, and the eccentric centre line would lie on the line E^2.
Therefore--total angle of advance = angle for _lap_ and angle for
_lead_.
LIMIT OF EXPANSIVE WORKING.
Theoretically, by increasing the _lap_ and cutting off the steam earlier
and earlier in the stroke, we should economize our power more and more.
But in practice a great difficulty is met with--namely, that _as the
steam expands its temperature falls_. If the cut-off occurs early, say
at one-third stroke, the great expansion will reduce the temperature of
the metal walls of the cylinder to such an extent, that when the next
spirt of steam enters from the other end a considerable proportion of
the steam's energy will be lost by cooling. In such a case, the
difference in temperature between admitted steam and exhausted steam is
too great for economy. Yet we want to utilize as much energy as
possible. How are we to do it?
COMPOUND ENGINES.
In the year 1853, John Elder, founder of the shipping firm of Elder and
Co., Glasgow, introduced the _compound_ engine for use on ships. The
steam, when exhausted from the high-pressure cylinder, passed into
another cylinder of equal stroke but larger diameter, where the
expansion continued. In modern engines the expansion is extended to
three and even four stages, according to the boiler pressure; for it is
a rule that the higher the initial pressure is, the larger is the number
of stages of expansion consistent with economical working.
[Illustration: FIG. 29.--Sketch of the arrangement of a
triple-expansion marine engine. No valve gear or supports, etc., shown.]
In Fig. 29 we have a triple-expansion marine engine. Steam enters the
high-pressure cylinder[4] at, say, 200 lbs. per square inch. It exhausts
at 75 lbs. into the large pipe 2, and passes to the intermediate
cylinder, whence it is exhausted at 25 lbs. or so through pipe 3 to the
low-pressure cylinder. Finally, it is ejected at about 8 lbs. per square
inch to the condenser, and is suddenly converted into water; an act
which produces a vacuum, and diminishes the back-pressure of the exhaust
from cylinder C. In fact, the condenser exerts a _sucking_ power on the
exhaust side of C's piston.
ARRANGEMENT OF EXPANSION ENGINES.
In the illustration the cranks are set at angles of 120°, or a third of
a circle, so that one or other is always at or near the position of
maximum turning power. Where only two stages are used the cylinders are
often arranged _tandem_, both pistons having a common piston rod and
crank. In order to get a constant turning movement they must be mounted
separately, and work cranks set at right angles to one another.
COMPOUND LOCOMOTIVES.
In 1876 Mr. A. Mallet introduced _compounding_ in locomotives; and the
practice has been largely adopted. The various types of "compounds" may
be classified as follows:--(1) One low-pressure and one high-pressure
cylinder; (2) one high-pressure and two low-pressure; (3) one
low-pressure and two high-pressure; (4) two high-pressure and two
low-pressure. The last class is very widely used in France, America, and
Russia, and seems to give the best results. Where only two cylinders are
used (and sometimes in the case of three and four), a valve arrangement
permits the admission of high-pressure steam to both high and
low-pressure cylinders for starting a train, or moving it up heavy
grades.
REVERSING GEARS.
[Illustration: FIGS. 30, 31, 32.--Showing how a reversing gear alters
the position of the slide-valve.]
The engines of a locomotive or steamship must be reversible--that is,
when steam is admitted to the cylinders, the engineer must be able to
so direct it through the steam-ways that the cranks may turn in the
desired direction. The commonest form of reversing device (invented by
George Stephenson) is known as Stephenson's Link Gear. In Fig. 30 we
have a diagrammatic presentment of this gear. E^1 and E^2 are two
eccentrics set square with the crank at opposite ends of a diameter.
Their rods are connected to the ends of a link, L, which can be raised
and lowered by means of levers (not shown). B is a block which can
partly revolve on a pin projecting from the valve rod, working through
a guide, G. In Fig. 31 the link is half raised, or in "mid-gear," as
drivers say. Eccentric E^1 has pushed the lower end of the link fully
back; E^2 has pulled it fully forward; and since any movement of the
one eccentric is counterbalanced by the opposite movement of the other,
rotation of the eccentrics would not cause the valve to move at all, and
no steam could be admitted to the cylinder.
Let us suppose that Fig. 30 denotes one cylinder, crank, rods, etc., of
a locomotive. The crank has come to rest at its half-stroke; the
reversing lever is at the mid-gear notch. If the engineer desires to
turn his cranks in an anti-clockwise direction, he _raises_ the link,
which brings the rod of E^1 into line with the valve rod and presses
the block _backwards_ till the right-hand port is uncovered (Fig. 31).
If steam be now admitted, the piston will be pushed towards the left,
and the engine will continue to run in an anti-clockwise direction. If,
on the other hand, he wants to run the engine the other way, he would
_drop_ the link, bringing the rod of E^2 into line with the valve rod,
and drawing V _forward_ to uncover the rear port (Fig. 32). In either
case the eccentric working the end of the link remote from B has no
effect, since it merely causes that end to describe arcs of circles of
which B is the centre.
"LINKING UP."
If the link is only partly lowered or raised from the central position
it still causes the engine to run accordingly, but the movement of the
valve is decreased. When running at high speed the engineer "links up"
his reversing gear, causing his valves to cut off early in the stroke,
and the steam to work more expansively than it could with the lever at
_full_, or _end_, gear; so that this device not only renders an engine
reversible, but also gives the engineer an absolute command over the
expansion ratio of the steam admitted to the cylinder, and furnishes a
method of cutting off the steam altogether. In Figs. 30, 31, 32, the
valve has no lap and the eccentrics are set square. In actual practice
the valve faces would have "lap" and the eccentric "lead" to correspond;
but for the sake of simplicity neither is shown.
OTHER GEARS.
In the Gooch gear for reversing locomotives the link does not shift, but
the valve rod and its block is raised or lowered. The Allan gear is so
arranged that when the link is raised the block is lowered, and _vice
versâ_. These are really only modifications of Stephenson's
principle--namely, the employment of _two_ eccentrics set at equal
angles to and on opposite sides of the crank. There are three other
forms of link-reversing gear, and nearly a dozen types of _radial_
reversing devices; but as we have already described the three most
commonly used on locomotives and ships, there is no need to give
particulars of these.
Before the introduction of Stephenson's gear a single eccentric was used
for each cylinder, and to reverse the engine this eccentric had to be
loose on the axle. "A lever and gear worked by a treadle on the
footplate controlled the position of the eccentrics. When starting the
engine, the driver put the eccentrics out of gear by the treadle; then,
by means of a lever he raised the small-ends[5] of the eccentric rods,
and, noting the position of the cranks, or, if more convenient, the
balance weight in the wheels, he, by means of another handle, moved the
valves to open the necessary ports to steam and worked them by hand
until the engine was moving; then, with the treadle, he threw the
eccentrics over to engage the studs, at the same time dropping the
small-ends of the rods to engage pins upon the valve spindles, so that
they continued to keep up the movement of the valve."[6] One would
imagine that in modern shunting yards such a device would somewhat delay
operations!
PISTON VALVES.
In marine engines, and on many locomotives and some stationary engines,
the D-valve (shown in Figs. 30-32) is replaced by a piston valve, or
circular valve, working up and down in a tubular seating. It may best be
described as a rod carrying two pistons which correspond to the faces of
a D-valve. Instead of rectangular ports there are openings in the tube
in which the piston valve moves, communicating with the steam-ways into
the cylinder and with the exhaust pipe. In the case of the D-valve the
pressure above it is much greater than that below, and considerable
friction arises if the rubbing faces are not kept well lubricated. The
piston valve gets over this difficulty, since such steam as may leak
past it presses on its circumference at all points equally.
SPEED GOVERNORS.
[Illustration: FIG. 33.--A speed governor.]
Practically all engines except locomotives and those known as
"donkey-engines"--used on cranes--are fitted with some device for
keeping the rotatory speed of the crank constant within very narrow
limits. Perhaps you have seen a pair of balls moving round on a seating
over the boiler of a threshing-engine. They form part of the "governor,"
or speed-controller, shown in principle in Fig. 33. A belt driven by a
pulley on the crank shaft turns a small pulley, P, at the foot of the
governor. This transmits motion through two bevel-wheels, G, to a
vertical shaft, from the top of which hang two heavy balls on links, K
K. Two more links, L L, connect the balls with a weight, W, which has a
deep groove cut round it at the bottom. When the shaft revolves, the
balls fly outwards by centrifugal force, and as their velocity increases
the quadrilateral figure contained by the four links expands laterally
and shortens vertically. The angles between K K and L L become less and
less obtuse, and the weight W is drawn upwards, bringing with it the
fork C of the rod A, which has ends engaging with the groove. As C
rises, the other end of the rod is depressed, and the rod B depresses
rod O, which is attached to the spindle operating a sort of shutter in
the steam-pipe. Consequently the supply of steam is throttled more and
more as the speed increases, until it has been so reduced that the
engine slows, and the balls fall, opening the valve again. Fig. 34 shows
the valve fully closed. This form of governor was invented by James
Watt. A spring is often used instead of a weight, and the governor is
arranged horizontally so that it may be driven direct from the crank
shaft without the intervention of bevel gearing.
[Illustration: FIG. 34.]
The Hartwell governor employs a link motion. You must here picture the
balls raising and lowering the _free end_ of the valve rod, which
carries a block moving in a link connected with the eccentric rod. The
link is pivoted at the upper end, and the eccentric rod is attached to
the lower. When the engine is at rest the end of the valve rod and its
block are dropped till in a line with the eccentric rod; but when the
machinery begins to work the block is gradually drawn up by the
governor, diminishing the movement of the valve, and so shortening the
period of steam admission to the cylinder.
Governors are of special importance where the _load_ of an engine is
constantly varying, as in the case of a sawmill. A good governor will
limit variation of speed within two per cent.--that is, if the engine is
set to run at 100 revolutions a minute, it will not allow it to exceed
101 or fall below 99. In _very_ high-speed engines the governing will
prevent variation of less than one per cent., even when the load is at
one instant full on, and the next taken completely off.
MARINE GOVERNORS.
These must be more quick-acting than those used on engines provided with
fly-wheels, which prevent very sudden variations of speed. The screw is
light in proportion to the engine power, and when it is suddenly raised
from the water by the pitching of the vessel, the engine would race till
the screw took the water again, unless some regulating mechanism were
provided. Many types of marine governors have been tried. The most
successful seems to be one in which water is being constantly forced by
a pump driven off the engine shaft into a cylinder controlling a
throttle-valve in the main steam-pipe. The water escapes through a leak,
which is adjustable. As long as the speed of the engine is normal, the
water escapes from the cylinder as fast as it is pumped in, and no
movement of the piston results; but when the screw begins to race, the
pump overcomes the leak, and the piston is driven out, causing a
throttling of the steam supply.
CONDENSERS.
The _condenser_ serves two purposes:--(1) It makes it possible to use
the same water over and over again in the boilers. On the sea, where
fresh water is not obtainable in large quantities, this is a matter of
the greatest importance. (2) It adds to the power of a compound engine
by exerting a back pull on the piston of the low-pressure cylinder while
the steam is being exhausted.
[Illustration: FIG. 35.--The marine condenser.]
Fig. 35 is a sectional illustration of a marine condenser. Steam enters
the condenser through the large pipe E, and passes among a number of
very thin copper tubes, through which sea-water is kept circulating by a
pump. The path of the water is shown by the featherless arrows. It comes
from the pump through pipe A into the lower part of a large cap covering
one end of the condenser and divided transversely by a diaphragm, D.
Passing through the pipes, it reaches the cap attached to the other end,
and flows back through the upper tubes to the outlet C. This arrangement
ensures that, as the steam condenses, it shall meet colder and colder
tubes, and finally be turned to water, which passes to the well through
the outlet F. In some condensers the positions of steam and water are
reversed, steam going through the tubes outside which cold water
circulates.
[3] Also called _ports_.
[4] The bores of the cylinders are in the proportion of 4: 6: 9. The
stroke of all three is the same.
[5] The ends furthest from the eccentric.
[6] "The Locomotive of To-day," p. 87.