Examples of Steam Engines. Stationery Engines.
191. In classifying engines with regard to their general arrangement of parts and mode of working, account has to be taken of a considerable number of independent characteristics. We have, first, a general division into condensing and non-condensing engines with a subdivision of the condensing class into those which act by surface condensation and those which use injection. Next there is the division into compound and non-compound, with a further classification of the former as double, triple, or quadruple-expansion engines. Again, engines may be classed as single or double-acting, according as the steam acts on one or alternately on both sides of the piston. Again, a few enginessuch as steam-hammers and certain kinds of steam-pumpsare non-rotative, that is to say, the reciprocating motion of the piston does work simply on a reciprocating piece; but generally an engine does work on a continuously revolving shaft, and is termed rotative. In most cases the crank-pin of the revolving shaft is connected directly with the piston-rod by a connecting-rod, and the engine is then said to be direct-acting; in other cases, of which the ordinary beam-engine is the most important example, a lever is interposed between the piston and the connecting-rod. The same distinction applies to non-rotative pumping engines, in some of which the piston acts directly on the pump-rod, while in others it acts through a beam. The position of the cylinder is another element of classification, giving horizontal, vertical, and inclined cylinder engines. Many vertical engines are further distinguished as belonging to the inverted cylinder class; that is to say, the cylinder is above the connecting-rod and crank. In oscillating cylinder engines the connecting-rod is dispensed with; the piston-rod works on the crank-pin, and the cylinder oscillates on trunnions to a allow the piston-rod to follow the crank-pin round its circular path. In trunk engines the piston-rod is dispensed with; the connecting-rod extends as far as the piston, to which it is joined, and a trunk or tubular extension of the piston, through the cylinder cover, gives room for the rod to oscillate. In rotary engines there is no piston in the ordinary sense; the steam does work on a revolving piece, and the necessity is thus avoided of afterwards converting reciprocating into rotary motion.
192. In the single-acting atmospheric engine of Newcomen the beam was a necessary feature; the use of water-packing for the piston required that the piston should move down in the working stroke, and a beam was needed to let the counterpoise pull the piston up. Watts improvements made the beam no longer necessary; and in one of the forms he designed it was dischargednamely, in the form of pumping-engine known as the Bull engine, in which a vertical inverted cylinder stands over and acts directly on the pump-rod. But the beam type was generally retained by Watt, and for many years it remained a favourite with builders of engines of the larger class. The beam formed a convenient driver for pump-rods and valve-rods; and the parallel motion invented by Watt as a means of guiding the piston-rod, which could easily be applied to beam-engine, was, in the early days of engine-building, an easier thing to construct than the plane surfaces which are the natural guides of the piston-rod in a direct-acting engine. In modern practice the direct-acting type has to a very great extent displace the beam type. For mill-driving and the general purposes of a rotative engine the beam type I snow rarely chosen. In pumping engines it is more common, but even there the tendency is to use direct-acting forms.
193. The only distinctive feature of beam-engines requiring special notice here is the "parallel motion," an ordinary form of which is shown diagrammatically I fig. 119. There MN is the path in which the piston-rod head, or crosshead, as it is often called, is to be guided. ABC is the middle line of half the beam, C being the fixed centre about which the beam oscillates. A link BD connects a point in the beam with a radius link ED, which oscillates about a fixed centre at E. A point P in BD, taken so that BP: DP:: EN:: CM, moves in a path which coincides very closely with the straight line MPN. Any other point Fin the line CP or CP produced is made to copy this motion by means of the links AF and FG, parallel to BD and AC. In the ordinary application of the parallel motion a point such as F is the point of attachment of the piston-rod, and P is used to drive a pump-rod. Other points in the line CP produced are occasionally made use of, by adding other links parallel to AC and BD.1
Watts linkage gives no more than an approximation to straight-line motion, but in a well-designed example the amount of deviation need not exceed one four-thousandth of the length of stroke. It was for long believed that the production of an exact straight-line motion by pure linkage was impossible, until the problem was solved by the invention of the Peaucellier cell.2 The Peaucellier linkage has not been applied to the steam-engine, except in isolated cases.
194. Another "parallel motion" which has been used in steam-engines is shown in fig. 120. AB is a link pivoted on a mixed centre at A, and connected to the middle of another link PQ, which s twice the length of AB. Q is guided to move in a straight line in the direction of AQ. P then moves in an exact straight line through A. This is not a pure linkage, since Q slides in a fixed guide, but the distance through which Q has to be guided is small compared with the stroke of P. if Q is guided to move in the arc of a circle of large radius, by using a radius rod from a fixed centre above or below it, the guiding surface at Q are avoided, but the path P is then only very nearly straight. An example of the linkage in this form, with further modification that A is shifted out, and B is brought nearer to P, occurs in the pumping engine of fig. 130 below.
In by far the greater number of modern steam-engines the crosshead is guided by a block sliding on planed surfaces. In many beam-engines, even, this plan of guiding the piston has taken the place of the parallel motion.
!95. No type of steam-engine is so common as the horizontal direct-acting. A small engine of this type, made by Messrs Tanye, and rated as a 10-horse-power engine, is illustrated in figs. 121 to 124. It furnishes a good example of a very numerous class, acting and serves to illustrate the principal parts of a complete engine. Fig 121 is a side elevation, fig. 122 a plan, fig. 123 a transverse section through the bedplate in front of the cylinder, on the line AB; and fig. 124 is a horizontal section through the cylinder, valve-chest, valve, stuffing-boxes, piston, and crosshead. The bedplate is a single below casting, with tow surfaces planed on it to serve as guides (see fig. 123). At one end the bedplate forms a pillow-block for the shaft, which has another main bearing independently supported beyond the fly-wheel. At the other end the bedplate is shaped so as to form the cylinder cover; the cylinder is bolted to this and overhangs the bed.
The cylinder (of 10 inches diameter and 20 inches stroke) consists of an internal "liner" of cast-iron and fitted within an external cylindrical casting, of which the ports and sides of the valve-chest form part. The space between the liner and the external cylinder serves as a steam-jacket. The use of a separate liner within the main cylinder is now general in large engines. In the front cylinder cover there is a stuffing-box through which the piston-rod passé. The stuffing-box is kept steam-tight by a soft packing which is pressed in by a gland. In some instances the packing consists of metallic rings. The cylinder cover and gland are lined with a brass ring in the hole through which the piston-rod passes. The valve-rod is brought out of the valve-chest in the same way. The piston is a hollow casting into a which the piston-rod is screwed and riveted over. It is packed by two split rings of cast-iron, which are sprung into recesses turned in the circumference of the piston. This mode of packing is used in locomotives and small engines. For large pistons the usual plan is to employ wider split rings, called floating rings, pressed against the sides of the cylinder, not by their own elasticity, but by separate springs behind them in the body of the piston; they are held in place by movable flange called a junk-ring on one face of the piston. One example of the packing of a large piston is shown in fig. 134. The crosshead consists of a steel centre-piece with a round boss, in which the piston-rod is secured by a cotter, and a forked front, where the end of the connecting-rod works on a pin. A pair of pins at top and bottom carry the steel shoes or sliding-blocks, whose distance from the centre is adjustable by nuts to take up wear. There is no crank; the connecting-rod works on a pin fixed in a disk on the end of the shaft in front of the main bearing. The valve-rod, which is worked by an eccentric just behind the bearing, is extended through the end of the valve-chest, and forms the plunger of a feed-pump which is bolted to the end of the chest. Frequently the feed-pump is fixed at any convenient part of the bedplate, and is driven by a separate eccentric, and in some cases its plunger is connected directly to the crosshead. In the main bearing the shaft turns in gun-metal or phosphor-bronze blocks called brasses. In heavy engines these are generally lined with Babbits anti-friction metal or other soft alloy, and in many modern engines the brasses are entirely dispensed with, a lining of Babbits metal being let into the cast-iron surface of the bearing. When the brasses are into two pieces, the plane of division between them is chosen to be that in which the wear is likely to be least. A more satisfactory adjustment is possible when the brasses are in three or more pieces.
196. When a condenser is used with a small horizontal engine it is usually place behind the cylinder; and the air-pump, which is within the condenser, is a horizontal plunger or piston-pump worked by a "tail-rod"that is, a continuation of the piston-rod past the piston and through the back cover of the cylinder. Figs. 125 and 126 shows in section one of Messrs Tangyes small condensers fitted with a double-acting air-pump to be driven by a tail-rod. The condenser proper is the chamber A, and into it the injection-water steam continuously through perforations in pipe B, which has a cock outside to regulate the supply. The pumps draws condensed water down to the lower part of the vessel at either end alternately through the valves C, and forces it up thence through the valves D to a chamber E, from which the delivery-pipe leads out. The pump is a gun-metal piston working in a cylinder fitted with a gun-metal liner. The valves are flat India-rubber rings held down in the centre by a spring, which allows then to open by rising bodily, as well as by bending.
197. The engine of figs. 121-4 makes 85 revolutions per minute, and its mean piston speed is consequently about 280 feet per minute. In some special forms of small horizontal engine the design is adapted to a much more rapid reciprocation of the moving masses, and the piston speed is raised to a value-seldom exceed in the largest land engines, although still higher values are now common in marine practice. Experience shows that the weight of engine of any one type varies roughly as the piston area. Their power depends on the product of piston area, piston speed, and pressure; and hence, so long as the pressures are similar, the ratio of power to weight is nearly proportional to piston speed. Cases present themselves in which it is desirable to make this ratio as great as possible; and apart from this, an engine making a large number of revolutions per minute is a convenient motor for certain high-speed machines.
A good example of a small horizontal engine, specially designed by the symmetry and balance of its parts, by largeness of the bearing surfaces, and by very perfect lubrication, to stand the strains which are caused by high speed, is the Armington & Sims engine, made in America by the patentees and in England by Messrs Greenwood & Batley. The bedplate is symmetrical about the line of motion of the crosshead; it supplies two very long main bearings for the shaft, at each end of which there is an overhung fly-wheel. The bearings have an adjustable side-block to take up wear. They are formed entirely of white-metal, cast on to the cast-iron pillow-blocks. In the middle are two disks, forming crank-cheeks, which are weighted opposite the crank-pin, so that they balance the pin and that part of the connecting-rod which may be treated as having its mass applied there. The crank-pin and the crosshead-pin are wide enough to give a large bearing area. The crosshead-block is a hollow bronze casting, giving an exceptionally large surface of contact with the guides. The valve is a piston-valve of the Trick type, which works sufficiently tight without packing. The valve-rod and eccentric-rod are connected through a block which slides on a fixed guide. The governor, which has been already illustrated in fig. 100, is contained within one of the fly-wheels. An engine of this type, with a cylinder 12 inches in diameter and a stoke of 12 inches, makes 275 revolutions per minute, has a piston speed of 550 feet per minute, and indicates about 80 horse-power. Other good examples of high speed combined with double action are furnished by the Porter Allen engine1 and by the very light engines which Mr. Thorneycroft and others have introduced for driving fans to supply air to the closed stokeholes of torpedo-boats. In these a speed of 1000 revolutions per minute is made possible by the use of light reciprocating parts and large bearing surfaces.
198. Fig. 127 shows a large non-compound horizontal Corliss engine for mill-driving, by Messrs Hick, Hargreaves, & Co. The cylinder is 34 inches in diameter, the stroke 8 feet, and the speed 45 revolutions per minute, giving a mean piston speed of 720 feet per minute. The cylinder is steam-jacketed round the barrel in the space between the liner and the outer cylinder, and also at the ends, which are cast hollow for this purpose. In large horizontal engines the weight of the piston tends to cause excessive wear on the lower side of the cylinder. In the example shown a part of the weight is borne b a tail-rod, ending a block, which slides on a fixed guide behind the cylinder. To further diminish wear the piston is sometimes made much wider from front to back than the one shown here; and the device is sometimes resorted to of giving the piston-rod "camber"that is to say, an upward curvature in the middle portion, which the weight of the piston reduces to straightness. Fig. 127 illustrates a common method of attaching the air-pump and condenser in large horizontal engines. The condenser is placed in a well in front of the cylinder, and air-pump, which is a vertical bucket-pump, is worked by a bell-crank lever, connected with the crosshead by a link. The fly-wheel of this engine is grooved for rope-gearing; it is cast in segments, which are bolted to one another and to the spokes, and the spokes are secured by cotters in tapered sockets in the nave. It is large and heavy; to suit the inequality of driving effort which is caused by the use of a single cylinder and a very early cut-off in engines of this class. To facilitate starting and valve-setting, mill engines are often provided with an auxiliary called a "barring" engine. The barring engine turns a toothed pinion, which gears into a toothed rim in the fly-wheel, and is contrived to fall automatically out of gear as soon as the main engine starts.
199. When uniformity of driving effort or the absence of dead-points is specially important, two independent cylinders are often coupled to the same shaft by cranks at right angles to each other, an arrangement which allow the engine to be started readily from any position. The ordinary locomotive is an example of this form. Among fixed engines of the larger kind, winding engines, in which ease of starting, stopping, and reversing is essential, are very generally made by coupling a pair of horizontal cylinders, with cranks at right angles to each other, on opposite sides of the winding-drum, with the link-motion as the means of operating the valves.
200. Non-compound engines of so large a size as that of fig. 127 are comparatively uncommon. Horizontal engines of the larger class are generally compound either (1) by having high and a low pressure cylinder side by side, working on two cranks at exactly or nearly right angles to each other (2) by placing one cylinder behind the other, with the axes of both the same straight line. The latter is called the tandem arrangement. In it one piston-rod is generally common to both cylinders; occasionally, however, the piston-rods are distinct, and are connected to one another by a framing of parallel bars outside of the cylinders. Another construction, rarely followed, is to have parallel cylinders with both piston-rods acting on one crank by being joined to opposite ends of one long crosshead. In some recent compound engines are large cylinder is horizontal, and the other lies above it in an inclined position, with its connecting-rod working on the same crank-pin.
In tandem engines, since the pistons move together, there is no need to provide a receiver between the cylinders. It is practicable to follow the "Woolf" plan of allowing the steam to expand directly from the small into the large cylinder; and in many instanced this is done. In point of fact, however, the connecting-pipe and steam-chest from an intermediate receiver of considerable size, which will cause loss by "drop" (§ 113) unless steam be cut off in the large cylinder before the end of the stroke. Hence it is more usual to work with a moderately early cut-off in the low-pressure cylinder than to use the "Woolf" plan of admitting steam to it throughout the whole stroke. Unless it is desired to make the cut-off occur before half-stoke, a common slide-valve will serve to distribute steam to the large cylinder. For an earlier cut-off than this a separate expansion-valve is required on the low-pressure cylinder, to supplement the slide-valve; and in any case, by providing a separate expansion-valve, the point of cut-off is made subject to easy control, and may be adjusted so as to avoid drop or to divide the work as may be desired between the two cylinders.1 For this season it is not unusual to find an expansion-valve, as well as a common slide-valve, on the low-pressure cylinder even to tandem engines. In many cases, however, the common slide-valve only is used. On the high-pressure cylinder of compound engines, the cut-off is usually effected either by an expansion slide-valve or by some form of Corliss of other trip-gear.
For mill engines the compound tandem and compound coupled types are now the most usual, and the high-pressure cylinder is very generally fitted with Corliss gear. In the compound coupled arrangement the cylinder are on separate bedplates, and the fly-wheel is between the cranks.
201. The general arrangement of vertical engines differs little from that of horizontal engines. The cylinder is usually supported above the shaft by a cast-iron frame resembling an inverted A, whose sides are kept parallel for a part of their length to serve as guides for the crosshead. Sometimes one side of the frame only is used, and the engine is stiffened by a wrought-iron column between the cylinder and the base on the other side. Wall-engines are a vertical form with a flat frame or bedplate, which is made to be bolded against a wall; in these the shaft is generally at the top. Vertical engines are compounded like horizontal engines, either by coupling parallel cylinders to cranks at right angles (as in the ordinary marine form, which will be illustrated later § 218), or, tandem fashion, by placing the high-pressure cylinder above the other. In vertical condensing engines the condenser is situated at the base, and the air-pump, which has a vertical stroke, is generally worked by a lever connected by a short link to the crosshead. In some cases the pump is horizontal, and is worked by a crank on the main shaft.
202. Engines making 400 to 1600 revolutions per minute have been extensively applied, in recent years, to the driving of dynamos and other high-speed machines. These are for the most part single-acting; steam is admitted to the back of the piston only, and the connecting-rod is in compression throughout the whole revolution. Besides simplifying the valves, this has the important advantage that alternation of strain at the joints may be entirely avoided, with the knocking and wears of the brasses which it is apt to cause. To secure, however, that the connecting-rod shall always push, there must be much cushioning during the back of exhaust stroke. From a point near the middle of the back stroke to the end the piston is being retarded; and, as this must not be done by the rod (which would thereby be required to pull), cushioning must begin there, and the work spent upon the cushion must at every stage be at least as great as the loss of energy on the part of the piston and rod. In some single-acting this cushioning is done by compressing a portion of the exhaust steam; in others the rod is kept in compression by help of a supplementary piston, on which steam from the boiler presses; in Mr. Williams engine the cushioning is done by compressing air.
203. A very successful example of the multiple-cylinder single-acting high-speed is the three-cylinder engine introduced by Mr. Brotherhood in 1873, the most recent form of which is shown in figs. 128 and 129. Fig 128 is a longitudinal and fig. 129 a transverse section. Three cylinders set at 120° apart, project from a closed casing, the central portion of which forms the exhaust. The pistons are of the trunk typethat is to say, there is a joint in the piston itself which allows the piston-rod to oscillate, and so makes a separate connecting-rod unnecessary. The three rods work on a single crank-pin, which is counterbalanced by masses fixed to the crank cheeks on the other side of the shaft. Steam is admitted to the back of the pistons only. It passes first through a throttle-valve, which is controlled by a centrifugal spring-governor (fig. 128), and is then distributed to the cylinders by three piston-valves A, worked by an eccentric, the sheave of which is made hollow so as to overhang one of the main bearings (fig. 128). Release takes place by the piston itself uncovering exhaust ports in the circumference of the cylinder, and the rocking motion of the piston-rod is taken advantage of to open a supplementary exhaust port (B, fig. 129), which remains open during a sufficient portion of the back stroke. The flexible coupling C shown in fig. 128, in which the twisting moment of the shaft is transmitted through disks of leather, prevents straining of the shaft and bearings through any want of alignment between the shaft of the engine and that of the mechanism it drives. Besides its use as a steam-engine, Mr. Brotherhoods pattern has been extensively applied in driving torpedoes by means of compressed air. As a steam-engine it is compounded by placing a high-pressure cylinder outside of and tandem with each low-pressure cylinder.
204. In other engines of this type a pair of cylinders, or a high and a low pressure cylinder, are set vertically side by side, to work on cranks opposite each other. The cranks and connecting-rods are completely enclosed, and are lubricated by dipping into a mixture of oil and water with which the lower part of the casing is filled. In the Westinghouse engine, where there was two vertical cylinders to which steam is admitted by a piston-valve, the crank-shaft is situated half a cranks length out of the line of stroke, to reduce the effects of the connecting-rods obliquity during the working stroke.1 In Mr. Williams latest form of engine the high and low pressure cylinder are tandem, and the space between the piston forms an intermediate receiver. The piston-rod is hollow, and has a piston-valve in it which controls the admission of steam to the high-pressure cylinder and its transfer to the low pressure cylinder. The piston-valve within the rods takes its differential motion from an eccentric on the crank-pin. The crosshead is itself a piston working in a cylindrical guide, in which it compresses air as its rises during the back stroke in order to cushion the reciprocating parts.2
205. In engines for pumping or for blowing air it is not essential to drive a revolving shaft, and in many forms the reciprocating motion of the steam-piston is applied directly or through a beam to produce the reciprocating motion of the pump-piston or plunger. On the other hand, pumping engines are frequently made rotative for the sake of adding a fly-wheel. When the level of the suction water is sufficiently high, horizontal engines, with the pump behind the cylinder and in line with it, are generally preferred; in other cases a beam-engine or vertical direct-acting engine is more common. Horizontal engines are, however, employed to pump water from any depth by using triangular rocking frames, which serve as bell-crank levers between the horizontal piston and vertical pump-rods.3
Fig. 130 shows a compound inverted vertical pumping engine of the non-rotative class, by Messrs Hathorn, Davey, & Co. Steam is distributed through lift valves, and the engine is governed by the differential gear illustrated in fig. 107, in conjunction in fig. 107, in conjunction with a cataract, which makes the pistons pause at the end of each stroke. The pistons are in line with two pump-rods, and are coupled by an inverted beam which gives guidance to the crossheads by means of an approximate straight-line motion, which is a modification of that of fig. 120. Surface condensers are frequently used with pumping engines, the water which the engine pumps serving as circulating water.
206. In a very numerous class of direct-acting steam-pumps, the steam-piston and the pump-piston or plunger are on the same piston-rod. In some of these a rotative element is introduced, partly to secure uniformity of motion, and partly for convenience of working the valves; a connecting-rod is taken from some point in the piston-rod to a crank-shaft which carries a fly-wheel; or a slotted crosshead fixed to the rod gives rotary motion to a crank-pin gearing into the slot, the line of the slot being perpendicular to that of the stroke. Many other steam-pumps are strictly non-rotative. In some the valve is worked by tappets from the piston-rod. In the Blake steam-pump a tapped worked by the piston as it reaches each end of its stroke throws over an auxiliary steam-valve, which admits steam to one or other side of an auxiliary piston carrying the main slide-valve. In Cameron & Floyds form one of a pair of tappet-valve at the ends of the cylinder is opened by the piston as it reaches the end of the stroke, and puts one or other side of an auxiliary piston, which carries the slide-valve, into communication with the exhaust, so that is thrown over. In the Worthington enginea design which has had much success in America, and is now being introduced in England by Messrs Simpsontwo steam cylinders are placed side by side, each working its own pump-piston. The piston-rod of each is connected by a short link to a swinging bar, which the slide-valve of the other steam-cylinder. In this way one piston begins its stroke when the motion of the other is about to cease, and a smooth and continuous action is secured.
207. The Worthington engine has been extensively applied, on a large scale, to raise water for the supply of towns and to force oil through "pipe-lines" in the United States. In the larger sizes it is made compound, each high-pressure cylinder having a low-pressure cylinder tandem with it on the same rod. Owing to the lightness of the reciprocating masses, and their comparatively slow acceleration, their inertia does not compensate, to any great extent, for the inequality of pressure on the pump-piston that would be caused by an early cut-off in the steam cylinder (see § 186). To meet this difficultly, and make high expansion practicable, an ingenious addition has recently been made to the engine.4 A crosshead A (fig. 131) fixed to each of the piston-rods is connected to the piston-rods of a pair of oscillating cylinders B, B, which contain water and communicate with a reservoir full of air compressed to a pressure of about 300 _ per square inch. When the stroke (which takes place in the direction of the arrow) begins the pistons are at first force in, and work is at first done by the main piston-rod, through the compensating cylinders B, B, on the compressed air in the reservoir. This continues until the crosshead has advanced so that the cylinders stand at right angles to the line of stroke. Then for the remainder of the stroke the compensating cylinders assist in driving the main piston, and the compressed air gives out the energy which it stored in the earlier portion. The volume of the air reservoir is so much greater than the volume of the cylinder B, B that the air pressure nearly constant throughout the stroke. Any leakage from the cylinder or reservoir is made good by a small pump which the engine drives. One advantage which this method of equalizing the effort of a steam-engine piston has (as compared with making use of the inertia of the reciprocating masses) is that the effort, when adjusted to be uniform at one speed, remains uniform although the speed be changed, provided the inertia of the reciprocating parts be small. In the Worthington "high-duty" engine, where this plan is in use, the high and low pressure cylinders are each provided with a separate expansion-valve of the rocking-cylinder type, as well as a slide-valve; the cut-off is early, and the efficiency is as high as in other pumping engines of the beat class.
128. Mr. Halls "pulsometer" is a peculiar pumping engine without cylinder or piston, which may be regarded as the modern representative of the engine of Savery (§ 6). The sectional view, fig. 132, shows its principal parts. There are two chambers A, A, narrowing towards the top, where the steam-pipe B enters. A ball-valve C allows steam to pass into one of the chambers and closes the other. Steam entering (say) the right-hand chamber forces water out of it past the clack-valve V into a delivery passage D, which is connected with an air-vessel. When the water-level in A sinks so far that steam begins to blow through the delivery-passage, the water and steam are disturbed and so brought into intimate contact, the steam in A is condensed, and a partial vacuum is formed. This causes the ball-valve C to rock over and close the top of A, while water rises from the suction-pipe E to fill that chamber. At the same time steam begins to enter the other chamber A, discharging water from it, and the same series of actions is repeated in either chamber alternately. While the water is being driven out there is comparatively little condensation of steam, partly because the shape of the vessel does not promote the formation of eddies, and partly because there is a cushion of air between the steam and the water. Near the top of each chamber is a small air-valve opening inwards, which allows a little air to enter each times a vacuum, is formed. When any steam is condensed, the air mixed with it remains on the cold surface and forms a non-conducting layer. The pulsometer is, of course, far from efficient as a thermodynamic engine, but its suitable for situations where other steam-pumps cannot be used, and the extreme simplicity of its working parts, make it valuable in certain cases.
209. We have seen that the tendency of modern steam practice is towards higher pressures, and that this means a gain both in efficiency and in power for a given weight of engine. High pressure, or indeed any pressure materially above that of the atmosphere, is out of the question when engine and boiler are to work without the regular presence of an attendant. Mr. Davey has recently introduced a domestic motor which deserves notice from the fact that it employs steam at atmospheric pressure. One form of this successful little engine is shown in fig. 133. The boilerwhich serves as the frame of the engine is of cast-iron, and is fitted with a cast-iron internal fire-box, with a vertical flue which is traversed by a water-bridge. The cylinder, which is enclosed within the upper part of the boiler, and the piston are of gun-metal, and work without lubrication. Steam is admitted by an ordinary slide-valve, also of gun-metal, worked by an eccentric in the usual way. The condenser stands behind the boiler; it consists of a number of upright tubes in a box, through which a current of cold water circulates from a supply-pipe at the bottom to an overflow-pipe at the top. In larger sizes of the motor the cylinder stands on a distinct frame, and the boiler has a hopper fire-box, which will take a charge of coke sufficient to drive the engine for several hours without attention. About 6 or 7 _ of coke are burned per horse-power per hour.
210. From the earliest days of the rotative engine attempts have been made to avoid the intermittent reciprocating motion which an ordinary piston-engine first produces and then converts into motion of rotation. Murdoch, the contemporary of Watt, proposed an engine consisting of a pair of spur-wheels gearing with one another in a chamber through which steam passed by being carried round the outer sides of the wheels in the spaces between successive teeth.1
In a more modern wheel-engine (Dudgeons) the steam was admitted by ports in side-plates into the clearance space behind teeth in gear with one another, just they had passed the line of centres. From that point to the end of the arc of contact the clearance space increased in volume; and it was therefore possible, by stopping the admission of steam at an intermediate point, to work expansively. The difficulty of maintaining steam-tight connexion between the teeth and the side-plates on which the faces of the wheels slide is obvious; and the same difficulty has prevented the success of many other forms of rotary engine. These have been devised in immense variety, in many cases, it would seem, with the idea that a distinct mechanical advantage was to be secured by avoiding the reciprocating motion of a piston.2 In point of fact, however, very few forms entirely escape having pieces with reciprocating motion. In all rotary engines, with the exception of steam turbines,where work is done by the kinetic impulse of steam,there are steam chambers which alternately expand and contract in volume, and this action usually takes place through a more or less veiled reciprocation of working parts. So long as engines work at a moderate speed there is little advantage in avoiding reciprocation; the alternate starting and stopping of piston and piston-rod does not affect materially the frictional efficiency, throws no deleterious strain on the joints, and need not disturb the equilibrium of the machine as a whole. The case is different when very high speeds are concerned; it is then desirable as far as possible to limit the amount of reciprocating motion and to reduce the masses that partake in it.
211. A recent interesting and successful example of the rotary type is the spherical engine of Mr. Beauchamp Tower,3 which, like several of its predecesssors,4 is based on the kinematic relations of the moving pieces in a Hookes joint. Imagine a Hookes joint, uniting two shafts set obliquely to one another, to be made up of a central disk to which the two shafts are hinged by semicircular plates, each plate working in a hinge which forms a diameter of the central disk, the two hinges being on opposite sides of the disk and at right angles to one another. Further, let the disk and the hinged pieces be enclosed in a spherical chamber through whose walls t he shafts project. As the shafts revolve each of the four spaces bounded by the disk, a hinged piece, and the chamber wall will suffer a periodic increase and diminution of volume, between limits which depend on the angle at which the shafts are set. In Mr. Towers engine the arrangement is modified by using spherical sectors, each a quarter sphere, in place of semicircular plated, for the pieces in which the shafts terminate. The shafts are at 135°. Each of the four enclosed cavities then alters in volume from zero to a quarter sphere, back to zero, again to a quarter sphere, and again back to zero, in a complete revolution of the shafts. In practice the central disk is a plate of finite thickness, whose edge is kept steam-tight in the enclosing chamber by spring-packing, and the sectors are reduced to an extent corresponding to the thickness of the central disk. One shaft is a dummy and runs free; the other is the driving-shaft. Steam is admitted and exhausted by ports in the spherical sectors, whose backs serve as revolving slide-valves. It is admitted to each cavity during the first part of each periodical increase of the cavitys volume. It is then cut off and allowed to expand as the cavity further enlarges, and is exhausted as the cavity contracts. If the working shaft, to which the driven mechanism serves as a flying-wheel, revolves uniformly, the dummy shaft is alternately accelerated and retarded. Apart from this, the only reciprocating motion is the small amount of oscillation which the comparatively light central disk undergoes.
Another rotary engine of the Hookes-joint family is Mr. Fieldings,1 in which a gimbal-ring and four curved pistons take the place of the disk. Two curved pistons are fixed on each side of the gimbal-ring, and as the shafts revolve these work in a corresponding pair of cavities, which may be called curved cylinders, fixed to each shaft.
212. Attempts have been made form time to time to devise steam-engines of the turbine class, where rotation of a wheel is produced either by reaction from a jet of escaping steam or by impact of a jet upon revolving blades. A revolving piece which is to extract even a respectable fraction of the kinetic energy of a steam jet must move with excessive velocity. In Mr. C. A. Parsonss steam-turbine this difficulty is overcome and a moderate degree of efficiency is secured by using a series of central-flow turbine wheels, in the form of perforated disks, all on one shaft, with fixed disks between which are perforated to serve as guide-blades. Steam passes form end to end the series, giving up a small portion of its energy to each, but retaining little at the end.
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