Imágenes de página
[ocr errors]
[ocr errors]


following rule will give the effective power of the engine, calculated on Watt's data: “Multiply the area of the piston in square inches by the effective pressure (found as above) and by the motion of the piston in feet per minute, and divide this by 33,000; the quotient is the actual number of horse-power.” For each horse-power of an engine it is calculated that 33 cubic feet of steam is expended per minute, or an evaporation of 1 cubic foot of water per hour. The combustion of 1 lb. of coal is calculated to raise 6 or 8 lbs. of water into steam ; Watt reckoned 73 lbs. of water to be evaporated by the combustion of 1 lb. of coal. In the modern Cornish engines, the same quantity of fuel evaporates 10lbs. of water.

Land engines are generally calculated to consume 10lbs. of fuel per hour for every nominal horse-power, or 5 or 6 lbs. for each actual horse-power. In the Cornish engines the duty of an engine is "expressed by the number of millions of pounds raised one foot high by a bushel, or 94 lbs. of Welsh coal;" a bushel of Newcastle coal will only weigh 84lbs; and, in comparing the duty of a Cornish engine with the performance of an engine in some locality where a different quality of coal is used, it is necessary to pay regard to such variations.” In the engine at Long Benton Colliery, erected by Smeaton, the duty performed was equal to 9:45 millions of pounds, raised 1 foot high by the consumption of 1 bushel of Newcastle coal. In the pre

sent time, what with improved engines and boilers, and

the extensive adoption of the principle of expansive ů

working, the duty of Cornish engines is estimated at d

60,000,000 lbs.; and in some instances the duty has increased to the large amount of 100,000,000 lbs.

raised 1 foot by the consumption of 94 lbs. of fuel. h For much valuable practical information on the power

of engines, the heating surface for each horse-power, &c. &c. we refer the reader to Bourne's Catechism of the Steam Engine.

We now, in concluding the present division, give a description and diagrams of the important instrument, the “indicator," so essentially necessary in computing the effective power of steam-engines. A small cylinder, as a, fig. 115, is placed in connection with the interior of the cylinder, either above or below the piston,-generally it is screwed into the aper

ture made in the cylinder-cover; a stop-cock is placed 16 k in the pipe b, by which the connection between the

interior of steam-engine cylinder and that of the in

dicator can be closed or opened as required. Into fig. 115.

the cylinder a a piston works; within the interior

of the rod c of this, which is made hollow on purpose, a spiral spring is placed; the lower end of this is fixed to the piston, and the upper to a small cross-head, e, supported by side-rods, connected with the cylinder, a a. To the top of the piston-rod a pencil, fg, is attached; the point of the pencil works in contact with a slip of paper wrapped round the small cylinder, h, and kept in contact with it by the vertical strip of brass, ii, on which is marked a scale. The axis of this cylinder, h, is continued downwards, and provided with a pulley, k. This pulley is connected with the parallel motion, or other reciprocating


[ocr errors]
[ocr errors]
[ocr errors]


[ocr errors]
[ocr errors]


part of the engine, by which motion is given to it, causing it to revolve only in one direction: the cylinder, h, makes its return motion to its original position by a spring coiled up in the bottom near ss; the direction of the cord, after leaving the pulley, is changed by a guide-pulley not shown in our diagram. The effect of the two motions, namely, the up and down motion of the piston, and the revolution of the cylinder, h, is to cause the pencil, g, to describe a curve, varying in its outline. Before the connection is made between the interior of cylinder a a and the steam-engine cylinder, what is called the atmospheric line is drawn on the paper; this is effected by pulling the cord, or allowing the engine to act on the pulley. The cock at the pipe b is then opened, when the piston is at the top of its stroke: the steam acting on the cylinder (steam-engine) piston acts also on that of the indicator; this will therefore rise, and with it the pencil, which is made to press slightly on the surface of the paper by a small spring; at the same time the roller, or cylinder, revolves. A line is thus traced on the paper, “ which rises higher up on the cylinder as the pressure of the steam increases, and comes lower upon it as the steam-pressure subsides."

The method of ascertaining the pressure on the piston of the engine, from the diagram thus obtained, is very simple, and will be easily understood by reference to fig. 116. Suppose abcd to be the slip off paper, on which the indicator diagram has been taken, and ef the atmospheric line, the divisions at the ends, as at bc, correspond to the divisions of the scale i on the indicator, fig. 115. Divide the length of the diagram into any number of equal di

fig. 116. visions, and through these draw lines at right angles to the atmospheric line ef; measure the lengths of the spaces thus formed by the intersection of the diagram with the lines, as the length from m to n, and from r to s, and so on (the measurements must be taken from a scale corresponding to that in the indicator-scale), and all the lengths together ; divide this by the number of spaces, and the quotient is the mean effective pressure on the piston in pounds per square inch. We have already described the rule for calculating the effective pressure of the engine. The indicator is not only useful to ascertain the amount of power exerted by the strokes of a steam-engine, but it serves also point out particular defects in the working. Thus the nearer the diagram attains to the form of a parallelogram, the more perfect is the working of the engine. Where certain deviations from the square at the corners are indicated on the diagram, certain defects are made known.

For full information on the practical working of the indicator, and the method of ascertaining the defects as indicated by the diagram, we must refer the reader to other and more practical works—as the Indicator and Dynamometer, by Professor Main and D. Brown ; published by Hebert,

; Cheapside.




The obtaining of a rotatory motion by the direct action of the steam, without involving the use of reciprocating motion as in the piston-rod of a cylinder engine, has long been a favourite problem with many mechanics, on the supposition that the various parts of an ordinary engine, as the piston, beam, connecting-rod, and crank, were not merely inconvenient, but that they acted as counteracting agencies to the full development of the power of the engine. Numerous attempts have been made to substitute an engine in which the main shaft received motion directly from the action of the steam; this acting on certain mechanical arrangements, placed within a case or exterior covering. The great objection which the advocates of the rotatory class of engines have raised, is the assumed loss of power sustained by the use of the crank. It is, however, an easy matter to prove the fallacy of this opinion; suffice it to state, that the opinion that there is a loss of power by the use of the crank, arises from a misconception of the principles of its action. There is no doubt that an efficient rotatory engine would be highly useful for certain applications, as the driving of a screw-propeller, where direct action is required; and a great saving of space and material would also be effected; but so far as the assumed gain of power which would result from their introduction is concerned, it may be taken as a general truth, supported by the best mechanical authority, that no advantage of this kind is possessed by them over the ordinary reciprocating kind. The great desideratum now hoped for, by the introduction of a rotatory engine among those mechanics who devote their time to its attainment, is not a gain of power, but only a simpler and more convenient mode of applying it. “Such a gain,” says the reporter to the jury, section A. class v. Great Exhibition, "might indeed result from a freer access of the steam to the piston, from a diminution of the friction or the jar of the working parts, or from a more complete expansion ; but, thanks to the more general diffusion of information in mechanics, practical men now know that there is no more possibility of increasing the work of an engine by merely altering the direction of any of its working parts, than there is of increasing the quantity of water which a reservoir will supply, by varying the pipes which serve to distribute it."

Although eminent authorities on engineering matters have expressed opinions inimical to the idea that a good rotatory engine will be introduced to the superseding of the reciprocating engine; still it is right to state that others, perhaps of equal standing, hold the contrary. The result, however, of the various discussions entered into on the point, seems to be that, if the engines can be kept tight, and the uniformity of wear in the packing effected, the great difficulty attendant on bringing them into practical operation will have been obviated. That this difficulty is one of no ordinary


kind, may be gathered from the statement of one of our most practical engineers, that he would as soon think of inventing perpetual motion, as of overcoming it."

We now proceed to illustrate the principle of a few of the most celebrated engines of this class yet introduced.

Rotatory engines are of several kinds; the Æloipile of Hero, already described, is an engine of the reaction species. A modern modification of this is exemplified in Avery's engine, introduced by Ruthven of Edinburgh, and to which at one time considerable attention was attracted.

This engine consists of two hollow arms ab, fig. 117, attached to a central pipe c, which revolves on its axis, and gives motion to the pulley e, from which the power is distributed as required by a belt; at opposite sides of the extremities of the pipes or arms ab, apertures are made, and the steam issuing from these in contrary directions, as in Hero's, cause the arms to revolve with great rapidity : steam is admitted to the arms through the pipe d. The arms are enclosed in a case ff; and the steam, after working, is let off to the atmosphere by a pipe communicating with the bottom of the case ff. The cold-water pump is worked by an eccentric on the horizontal shaft. Although several engines of this class have been introduced, and, according to the patentee, with marked success,

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

as simple and economical, yet their number is by no means increasing. One great objection to it is the high pressure of the steam employed, and the limit to the power of the engine set by the difficulty of increasing the length of the arms ab, so as to obtain increase of

power; the amazing velocity at which the arms revolve, 3000 and 4000 times per minute, makes it liable to speedy disarrangement of parts.

Rotatory engines are sometimes made on the impulse principle, as exemplified in Branca's steam-wheel, already noticed. A modification of this principle is carried out in the rotatory engine patented in 1841 by Corde and Locke.

In fig. 118, aa is the foundation on which the steam-wheel is supported, bb the steam-wheel, provided with floats or buckets at the extremities of the radial arms; the shaft of this, as c, is carried through the outer-casing in which the wheel revolves through stuffing-boxes; the pulley or toothed wheel for communicating motion to the apparatus to be worked by the power of the engine is placed on this shaft outside the casing of the wheel. A vacuum is made in the interior of the casing by the medium of the condenser f, to which the steam is conducted by the pipe e. The exhausting is carried on by a small double-acting cylinder-engine working the airpump. Attention has been much directed to this invention, principally through a very favourable report as to its working capabilities by Josiah Parker, the consulting engineer to the Royal Agricultural Society of England. A novel fact was elicited in the course of the experiments_namely, that when the steam, after, working an ordinary reciprocating cylinderengine, is admitted to the exhausted case, and made to impinge upon the floats of the wheel before finally passing to the condenser, an additional power is obtained equal to one-third, or 33 per cent; and this without any

increase of fuel or increase of condensing apparatus.

The next class of rotatory engine to which we will direct the reader's attention, is that in which the piston is made to revolve round its axis. In the patent granted to Watt in 1769, he included a claim for a rotatory engine; and, from his own statement, it appears that " a steam-wheel moved by force of steam, acting in a circular channel against a valve on one side, and against a column of mercury or other fluid metal on the other side, was executed at Soho upon a scale of six feet, and tried repeatedly; but was given up, as several objections were urged against it.” This failure did not, however, influence Watt to give up his trials; but in 1782 he took

out a patent for two engines on a similar principle: one of these we here append an illustration of. In fig. 119, a a is a circular casing, b an axle passing through stuffingboxes at the ends of the casing, c à piston revolving in the case, d a valve which, turning on a hinge like a door, passes into the recess e; f the pipe admitting the steam to the casing, g that leading to the condenser. The valve e extends the whole depth of the cylinder. On steam being admitted to the casing, it presses on the piston c, and causes

it to revolve; on reaching that part of the fig. 119.

casing near the eduction-pipe g, the piston

strikes the valve d, and forces it into its seat e; the steam-entrance is thus closed, and the steam in the casing rushes to the condenser through g. On the piston passing e, the valve d falls open, as before, admitting steam to the casing to act on the piston. From the force with which the piston strikes the valve d, the machine rapidly falls into disrepair.

Murdoch, an engineer, employed under Mr. Watt at Soho, introduced a rotatory engine, of which, in figs. 120, 121, we give drawings; the steam is admitted by the pipe e, and acts upon the projecting arms b of the rollers aa, placed within the casing dd. The steam, after working the rollers, passes into the condenser by the pipe f; the air-pump was worked by a

« AnteriorContinuar »