CHAPTER IV.

ROTATORY ENGINES AND OTHER VARIETIES.

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,

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, ban axle passing through stuffingboxes at the ends of the casing, ca 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 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.

fig. 119.

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 a a, placed within the casing d d. The steam, after working the rollers, passes into the condenser by the pipe f; the air-pump was worked by a

crank fitted on the end of axis a. Packing is introduced into cavities at the end of each projection, as at b, to keep them steam-tight during the revolutions. There is much leakage in an engine of this kind, and the friction is great.

From the great variety of rotatory engines of this class which have been introduced from the time of Watt till now, it is quite an impossibility for us to notice even a small proportion of their number; we must refer

the reader to larger treatises, where several of the most ingenious are illustrated. We propose only giving illustrations of one or two of the most recently introduced, and which, from their admirable arrangement of

fig. 123.

parts, and their general efficiency, are likely to be introduced on a comparatively extensive scale.

A form of rotatory engine, which has been spoken highly of by competent authorities, is that invented by Mr. Isaiah Davies of Birmingham. Through the courtesy of William Johnson, Esq., editor of the Practical Mechanic's Journal, we are enabled to present our readers with a description and illustration of this (as well as of the one following this) ingenious rotatory steam-engine. In fig. 122 we give a transverse section of the engine, which is of the duplex construction. Fig. 123 is an external endelevation corresponding, showing the arrangement of the cam motion for working the valves. Fig. 124 is a plan view, showing an engine or cylinder in elevation; the other in section. a is the main shaft of the engine, which

both directly receives and transmits the power. Two pistons b, are carried loose on this shaft on three feathers; by this arrangement the pistons are carried round by the revolution of the shaft, but are allowed a certain amount of play laterally; that is, they can move backward and forward a slight extent on the shafts. It is in this point that the main objections to rotatory engines have been overcome; this side to side movement of the pistons preventing all wear and injurious binding of the piston: and by means of the end set-up plates the surest adjustment of parts can be obtained, thus obviating the excessive wear found in other engines of this class on the surface of the piston and the ends of the cylinder. The pistons, b, are cylindrical for the greater portion of their circumference, but have each projections cast as in fig. 123, these receiving the actuating pressure of the steam. The axes of the pistons coincide with the centres of the cylinders; and the diameter of the former being less than that of the latter, an annular space is left all round as at c. The pistons are placed on the shaft in such a manner that the projections are exactly opposite each other. The cylinders, cc, are bolted down on the same axial line upon a cast-iron