multiplying that pressure by the number of square inches in the area of
the piston of the engine, the total pressure on the piston can be found.
The pressure does not rise instantly when the steam is first admitted,
nor does it fall quite abruptly when the steam is cut off and
communication opened with the condenser. When the steam is worked
expansively, the pressure falls gradually from the time the steam is
shut off. Now, the amount of work done by any force is reckoned by the
pressure it exerts multiplied into the space through which that pressure
is exerted. Therefore the work done by the steam is known by multiplying
the pressure in pounds on the whole surface of the piston into the
length in feet of the piston’s motion through which that pressure is
exerted. The trace of the pencil on the paper—_i.e._, the _indicator
diagram_—shows the pressures, and also the length of the piston’s path
through which each pressure is exerted, and therefore it is not
difficult to calculate the actual work which is done by the steam at
every stroke of the engine. If this be multiplied by the number of
strokes per minute, and the product divided by 33,000, we obtain what is
termed the _indicated horse-power_ of the engine. The work done per
minute is divided by 33,000, because that number is taken to represent
the work that a horse can do in a minute: that is, the average work done
in one minute by a horse would be equal to the raising of the weight of
1,000 lbs. thirty-three feet high, or the raising of thirty-three pounds
1,000 feet high. The number, 33,000, as expressing the work that could
be done by a horse in one minute, was fixed on by Watt, but more recent
experiments have shown that he over-estimated the power of horses, and
that we should have to reduce this number by about one-third if we
desire to express the actual average working power of a horse. But the
power of engines having come to be expressed by stating the horse-power
on Watt’s standard, engineers have kept to the original number, which
is, however, to be considered as a merely artificial unit or term of
comparison between one engine and another; for the power of a horse to
perform work will vary with the mode in which its strength is exerted.
The source of the power which does the work in the steam engine is the
combustion of the coal in the furnace under the boiler. The amount of
work a steam engine will do depends not only on the quantity of steam
which is generated in a given time, but also upon the pressure, and
therefore the temperature at which the steam is formed.
[Illustration:
FIG. 6.—_Section of Giffard’s Injector._
]
The water constantly evaporating in the boiler of a steam engine is
usually renewed by forcing water into the boiler against the pressure of
the steam by means of a small pump worked by the engine. In the
engraving of Watt’s engine this pump is shown at M. But recently the
feed-pump has been to a great extent superseded by a singular apparatus
invented by M. Giffard, and known as Giffard’s Injector. In this a jet
of steam from the boiler itself supplies the means of propelling a
stream of water directly into the boiler. Fig. 6 is a section of this
interesting apparatus through its centre, and it clearly shows the
manner in which the current of steam is made to operate on the jet of
water. The steam from the boiler passes through the pipe A and into the
tube B through the holes. The nozzle of this tube is of a conical shape,
and its centre is occupied by a rod pointed to fit into the conical
nozzle, and provided with a screw at the other end, so that the opening
can be regulated by turning the handle, C. At D the jet of steam comes
in contact with the water which feeds the boiler, the arrangement being
such that the steam is driven into the centre of the stream of water
which enters by the pipe E, and is propelled by the steam jet through
another cone, F, issuing with such force from the orifice of the latter
that it is carried forward through the small opening at G into the
chamber H. Here the water presses on the valve K, which it raises
against the pressure of the steam and enters the boiler. The water
issuing from the cone, F, actually traverses an open space which is
exposed to the air, and where the fluid may be seen rushing into the
boiler as a clear jet, except a few beads of steam which may be carried
forward in the centre, the rest of the steam having been condensed by
the cold water. The steam, of course, rushes from the cone, B D, with
enormous velocity, which is partly communicated to the water. The pipe,
L, is for the water which overflows in starting the apparatus, until the
pressure in H becomes great enough to open the valve. The supplies of
water and of steam have to be adjusted according to the conditions of
pressure in the boiler, and according to the temperature of the
feed-water. It is found that when the feed-water is at a temperature
above 120° Fahrenheit, the injector will not work: the condensation of
the steam is therefore necessary to the result. For, as the steam is
continually condensed by the cold water, it rushes from D with the same
velocity as into a vacuum, and the water is urged on by a momentum due
to this velocity. We must observe, moreover, that the net result of the
operation is a lessening of the pressure in the boiler; for the entrance
of the feed-water produces a fall of temperature in the boiler, and the
bulk of steam expended is fourteen times the bulk of the water injected:
thus, although the apparatus before actual trial would not appear likely
to produce the required result, the effect is no more paradoxical than
in the case of the feed-pump. The injector has been greatly improved by
Mr. Gresham, who has contrived to make some of the adjustments
self-acting, and his form of the apparatus is now largely used in this
country. The injector is applicable to stationary, locomotive, or marine
engines.
Steam boilers are now always provided with one of _Bourdon’s_ gauges,
for indicating the pressure of the steam. The construction of the
instrument will easily be understood by an examination of Fig. 7. The
gauge is screwed into some part of the boiler, where it can always be
seen by the person in charge. The stop-cock A communicates with the
curved metallic tube C, which is the essential part of the contrivance.
This tube is of the flattened form shown at D, having its greatest
breadth perpendicular to the plane in which the tube is curved, and it
is closed at the end E, where it is attached to the rod F, so that any
movement of E causes the axle carrying the index-finger, F, to turn, and
the index then moves along the graduated arc. The connection is
sometimes made by wheelwork, instead of by the simple plan shown in the
figure. The front plate is represented as partly broken away, in order
to show the internal arrangement, which, of course, is not visible in
the real instrument, where only the index-finger and graduated scale are
seen, protected by a glass plate.
[Illustration:
FIG. 7.—_Bourdon’s Pressure Gauge._
]
When a curved tube of the shape here described is subjected to a greater
pressure on the inside than on the outside, it tends to become
straighter, and the end E moves outward; but when the pressure is
removed, the tube resumes its former shape. The graduations on the scale
are made by marking the position of the index when known pressures are
applied. The amounts of pressure, when the gauges are being graduated,
are known by the compression produced in air contained in another
apparatus. Gauges constructed on Bourdon’s principle are applied to
other purposes, and can be made strong enough to measure very great
pressures, such as several thousand pounds on the square inch; they may
also be made so delicate as to measure variations of pressure below that
of the atmosphere. The simplicity and small size of these gauges, and
the readiness with which they can be attached, render them most
convenient instruments wherever the pressure of a gas or liquid is
required to be known.
[Illustration:
FIG. 8.—_Steam Generator._
]
A point to which great attention has been directed of late years is the
construction of a boiler which shall secure the greatest possible
economy in fuel. Of the total heat which the fuel placed in the furnace
is capable of supplying by its combustion, part may be wasted by an
incomplete burning of the fuel, producing cinders or smoke or unburnt
gases, another part is always lost by radiation and conduction, and a
third portion is carried off by the hot gases that escape from the
boiler-flues. Many contrivances have been adopted to diminish as much as
possible this waste of heat, and so obtain the greatest possible
proportion of available steam power from a given weight of fuel. Boilers
wholly or partially formed of tubes have recently been much in favour.
An arrangement for quickly generating and superheating steam is shown in
Fig. 8, in connection with a high-pressure engine.
Steam engines are constructed in a great variety of forms, adapted to
the purposes for which they are intended. Distinctions are made
according as the engine is fitted with a condenser or not. When steam of
a low pressure is employed, the engine always has a condenser, and as in
this way a larger quantity of work is obtainable for a given weight of
fuel, all marine engines—and all stationary engines, where there is an
abundant supply of water and the size is not objectionable—are provided
with condensers. High-pressure steam may be used with condensing
engines, but is generally employed in non-condensing engines only, as in
locomotives and agricultural engines, the steam being allowed to escape
into the air when it has driven the piston to the end of the stroke. In
such engines the beam is commonly dispensed with, the head of the
piston-rod moving between guides and driving the crank directly by means
of a connecting-rod. The axis of the cylinder may be either vertical,
horizontal, or inclined. A plan often adopted in marine engines, by
which space is saved, consists in jointing the piston-rod directly to
the crank, and suspending the cylinder on trunnions near the middle of
its length. The trunnions are hollow, and are connected by steam-tight
joints, one with the steam-pipe from the boiler, and the other with the
eduction-pipe. Such engines have fewer parts than any others; they are
lighter for the same strength, and are easily repaired. The trunnion
joints are easily packed, so that no leakage takes place, and yet there
is so little friction that a man can with one hand move a very large
cylinder, whereas in another form of marine engine, known as the
side-lever engine, constructed with oscillating beams, the friction is
often very great.
_THE LOCOMOTIVE._
The first locomotive came into practical use in 1804. Twenty years
before, Watt had patented—but had not constructed—a locomotive engine,
the application of steam to drive carriages having first been suggested
by Robinson in 1759. The first locomotives were very imperfect, and
could draw loads only by means of toothed driving-wheels, which engaged
teeth in rack-work rails. The teeth were very liable to break off, and
the rails to be torn up by the pull of the engine. In 1813, the
important discovery was made that such aids are unnecessary, for it was
found that the “bite” of a smooth wheel upon a smooth rail was
sufficient for all ordinary purposes of traction. But for this
discovery, the locomotive might never have emerged from the humble duty
of slowly dragging coal-laden waggons along the tramways of obscure
collieries. The progress of the locomotive in the path of improvement
was, however, slow, until about 1825, when George Stephenson applied the
blast-pipe, and a few years later adopted the tubular boiler. These are
the capital improvements which, at the famous trial of locomotives, on
the 6th of October, 1829, enabled Stephenson’s “Rocket” to win the prize
offered by the directors of the Liverpool and Manchester Railway. The
“Rocket” weighed 4½ tons, and at the trial drew a load of tenders and
carriages weighing 12¾ tons. Its average speed was 14 miles an hour, and
its greatest, 29 miles an hour. This engine, the parent of the powerful
locomotives of the present day, may now be seen in the Patent Museum at
South Kensington. Since 1829, numberless variations and improvements
have been made in the details of the locomotive. In weight, dimensions,
tractive power and speed, the later locomotives vastly surpass the
earlier types.
[Illustration:
FIG. 9.—_Section of Locomotive_ (A.D. 1837).
]
Fig. 9 represents the section of a locomotive constructed _c._ 1837. The
boiler is cylindrical; and at one end is placed the fire-box, partly
enclosed in the cylindrical boiler, and surrounded on all sides by the
water, except where the furnace door is placed, and at the bottom, where
the fuel is heaped up on bars which permit the cinders to drop out. At
the other end of the boiler, a space beneath the chimney called the
smoke-box is connected with the fire-box by a great number of brass
pipes, open at both ends, firmly fixed in the end plates of the boiler.
These tubes are from 1¼ in. to 2 in. in diameter, and are very
numerous—usually about one hundred and eighty, but sometimes nearly
double that number. They therefore present a large heating surface to
the water, which stands at a level high enough to cover them all and the
top of the fire-box. The boiler of the locomotive is not exposed to the
air, which would, if allowed to come in contact with it, carry off a
large amount of heat. The outer surface is therefore protected from this
cooling effect by covering it with a substance which does not permit the
heat to readily pass through it. Nothing is found to answer better than
felt; and the boiler is accordingly covered with a thick layer of this
substance, over which is placed a layer of strips of wood ¾ in. thick,
and the whole is surrounded with thin sheet iron. It is this sheet iron
alone that is visible on the outside. The level of the water in the
boiler is indicated by a gauge, which is merely a very strong glass
tube; and the water carried in the tender is forced in as required, by a
pump (not shown in the Fig.). The steam leaves the boiler from the upper
part of the _steam-dome_, A, where it enters the pipe, B; the object
being to prevent water from passing over with the steam into the pipe.
The steam passes through the _regulator_, C, which can be closed or
opened to any extent required by the handle, D, and rushes along the
pipe, E, which is wholly within the boiler, but divides into two
branches when it reaches the smoke-box, in order to conduct the steam to
the cylinders. Of these there are two, one on each side, each having a
slide-valve, by means of which the steam is admitted before and behind
the pistons alternately, and escapes through the blast-pipe, F, up the
chimney, G, increasing the draught of the fire by drawing the flame
through the longitudinal tubes in proportion to the rush of steam; and
thus the rate of consumption of fuel adjusts itself to the work the
engine is performing, even when the loads and speeds are very different.
Though the plane of section passing through the centre of boiler would
not cut the cylinders, one of them is shown in section. H is the piston;
K the connecting-rod jointed to the crank, L, the latter being formed by
forging the axle with four rectangular angles, thus, __¦¯¯¦__; and the
crank bendings for the two cylinders are placed in planes at right
angles to each other, so that when one is at the “dead point,” the other
is in a position to receive the full power of the piston. There are two
safety valves, one at M, the other at N; the latter being shut up so
that it cannot be tampered with.
Locomotives are fitted with an ingenious apparatus for reversing the
engines, which was first adopted by the younger Stephenson, and is known
as the “link motion.” The same arrangement is employed in other engines
in which the direction of rotation has to be changed; and it serves
another important purpose, namely, to provide a means by which steam may
be employed expansively at pleasure. The link motion is represented in
Fig. 10, where A, B, are two eccentrics oppositely placed on the
driving-shaft, and their rods joined to the ends of the curved bar or
link, C D. A slit extends nearly the whole length of this bar, and in it
works the stud E, forming part of the lever, F, G, movable about the
fixed joint, G, and having its extremity, F, jointed to the rod H, that
moves the slide-valve. The weight of the link and the eccentric rods is
counterpoised with a weight, K, attached to the lever, I K, which turns
on the fixed centre, L. This lever forms one piece with another lever, L
M, with which it may be turned by pulling the handle of O P, connected
with it through the system of jointed rods. When the link is lowered, as
shown in the figure, the slide-valve rod will follow the movement of the
eccentric, B, while the backward and forward movement of the other
eccentric will only be communicated to the end of C, and will scarcely
affect the position of the stud E at all. By drawing the link up to its
highest position, the motion due to eccentric A only will be
communicated to the slide-valve rod, which will therefore be drawn back
at the part of the revolution where before it was pushed forward, and
_vice versâ_; hence the engine will be reversed. When the link is so
placed that the stud is exactly in the centre, the slide-valve will
receive no motion, and remain in its middle position, consequently the
engine is stopped. By keeping the link nearer or farther from its
central position, the throw of the slide-valve will be shorter or
longer, and the steam will be shut off from entering the cylinder when a
smaller or larger portion of the stroke has been performed.
[Illustration:
FIG. 10.—_Stephenson’s Link Motion._
]
Although Fig. 9 represents with sufficient clearness all the essential
parts of a locomotive, it should be observed that as actually
constructed for use on the different lines of railway the machine is
greatly modified in the arrangement and proportions of its parts. A
greater number of adjuncts and subsidiary appliances are also provided
for the more effective and convenient working of the engine, and for
giving control over the movement of the train, and these, in fact,
conduce much to the greater economy and safety with which trains are now
run. As the circumstances and conditions under which railways are worked
vary much in different parts of the world, the locomotive has to be
designed to meet the requirements of each case, and its general
appearance, details and dimensions are accordingly much diversified.
From among the many types of recent locomotives we select for
illustration and a short description the form of express passenger
engine that has lately been designed by Mr. T. W. Worsdell, the engineer
of the North Eastern Railway, and this will give the opportunity of
noticing some of the newest improvements, which are embodied in this
engine. See Plate II.
The plan of causing the steam to work expansively has already been
mentioned on pages 8 and 9, as used by cutting off the steam when part
of the stroke of the piston has been made. Another mode by which the
expansive principle has long been made use of in stationary and marine
engines is to allow the steam from the boiler to enter first a smaller
cylinder and from that, at the end of the stroke, to pass into a larger
one in which, as it expands, it exercises a diminished pressure. This
arrangement has been called the compound or double-cylinder engine, and
was known to possess certain advantages where high pressure steam was
made use of. Indeed, in marine engines the principle of “triple
expansion” is now quite commonly adopted—that is, the steam passes
successively into three cylinders of successively greater diameter. Mr.
Webb, the locomotive engineer of the London and North Western Railway,
appears to have been the first to make the “compounding” system a
practical success as applied to the locomotive. In Mr. Webb’s
arrangement there are three cylinders, two smaller ones for the
high-pressure steam from the boiler, and between these a single large
low-pressure cylinder which receives the steam that has done its work
from both the smaller cylinders. In Mr. Worsdell’s engine the original
and simpler locomotive construction of two cylinders has been adhered
to, and thus the general plan of the engine is unchanged except in the
larger size of the low-pressure cylinder. In the present engine the
stroke is 24 in.; the high-pressure cylinder has its internal diameter
20 in. and the low-pressure cylinder a diameter of 28 in. The
boiler-shell is made of steel, the fire-box is of copper, and there are
203 brass tubes, 1¾ in. diameter and 10 ft. 11 in. long, connecting the
fire-box with the smoke-box. The frame, and indeed most parts of the
engine, are also made of steel. The driving-wheels, which here are a
single pair, have a diameter of 7 ft. 7¼ in. The total “wheel-base” is
nearly 21 ft., and it will be observed that the forepart of the engine
is supported on a four-wheeled _bogie_. The _bogie_ is capable of a
certain amount of horizontal motion by turning round a swivel, but this
movement is controlled by springs, so that, notwithstanding the length
of the frame, the engine is enabled to take curves with great facility,
while its motion is perfectly steady even at the highest speeds. The
working pressure of the steam in the boiler is 170 lbs. on the square
inch. The steam which leaves the high-pressure cylinder is conveyed to
the low-pressure cylinder by a pipe that is led round the inside of the
smoke-box, and thus enters the larger cylinder after taking up heat that
would otherwise be wasted, so that its elastic force is fully
maintained. This circumstance, no doubt, contributes to the very marked
economy of fuel that has been effected by the compound engines. How
great the economy is found in the working will be seen by the following
results, which are taken from the actual records. The same train was
taken over the same rails in ordinary quick passenger traffic for
several journeys which, as performed in the same time by the compound
engine and by another otherwise similar non-compound engine, required
for the compound, 25,254 lbs. of coal; for the non-compound, 32,104
lbs.; or, the consumption of coal by the former was 28 lbs. per mile; by
the latter, 36 lbs. per mile. This represents a saving of about 21 per
cent. of the fuel. As the steam enters the high-pressure cylinder first,
it would not be possible to start the engine if it had stopped at one of
the “dead-points” on that side, without a special arrangement for
admitting the steam directly to the other cylinder in such cases. This,
of course, is required only for the first stroke, and Mr. Worsdell and
M. von Borries have contrived for this purpose an ingenious valve,
brought into operation when required by a touch from the engineer, and
then immediately adjusting itself automatically, so as to restore the
steam connections to their normal condition.
[Illustration:
PLATE II.
NORTH EASTERN RAILWAY LOCOMOTIVE.
]
[Illustration:
FIG. 10_a_.—_G.N.R. Express Passenger Locomotive._
]
Another type of the high-speed passenger engines used for express trains
on several of the great English railways is well represented by one of
the Great Northern Company’s locomotives, as depicted in Fig. 10_a_. In
this there are a single pair of driving wheels of very large diameter,
namely, 8 ft. 2 in., so that each complete movement of the pistons will
carry the engine forwards a length of nearly 26 ft. There are outside
cylinders, and therefore the driving axle is straight, and the leading
wheels are in two pairs, mounted on a _bogie_ which is capable of a
certain amount of independent horizontal rotation.