Heat Engines - Definition. Early History of the Steam Engine.
Heat Engines - Definition.
1. A HEAT-ENGINE is a machine in which heat is employed to do mechanical work. In all practical heat-engines, work is done through the expansion by heat of a fluid which overcomes resistance as it expands---in steam-engines by the expansion of water and water-vapour, in air-engines by the expansion of hot air, in gas-engines by the expansions of a burnt mixture of air and gas. One of the most simple and historically one of the oldest types of heat-engines are guns, in which heat, generated by the combustion of an explosive, does work in giving energy of motion to a projectile. But guns differ so widely form all other types, both in their purpose and in their development that it is convenient to leave them out of account in treating of engines which may serve as prime movers to other mechanism.
Early History of the Steam Engine.
2. The earliest notices of the heat-engines are found in the Pneumatica of Hero of Alexnadria (c. 130 B.C.) Two contrivances described there deserve mention. One is the æolipile, a steam reaction turbine consisting of a spherical vessel pivoted on a central axis and supplied with steam through one of the pivots. The steam escapes by bent pipes facing tangentially in opposite directions, at opposite ends of a diameter perpendicular to the axis. The globe revolves by reaction from the escaping steam, just as a Barkers mill is driven by escaping water. Another apparatus described by Hero (fig. 1)1 is interesting as the prototype of a class of engines which long afterwards became practically important. A hollow altar containing air is heated by a fire kindled on it; the air in expanding drives some of the water contained in a spherical vessel beneath the altar into a bucket, which descends and opens the temple doors above by pulling round a pair of vertical posts to which the doors fixed. When the fire is extinguishing the air cools, the water leaves the bucket, and the doors close. In another device a jet of water driven out by expanding air is turned to account as a fountain.
3. From the time of Hero to the 17th century there is no progress to record, though here and there we find evidence that appliances like those described by Hero were used for trivial purposes, such as organ-blowing and the turning spits. The next distinct step was the publication 1601 of a treatise on pneumatics by Giovanni Batista della Porta, in which he shows an apparatus similar to Heros fountain, but with steam instead of air as the displacing fluid. Steam generated in a separate vessel passes into a closed chamber containing water, from which a pipe (open under the water) leads out. He also points out that the condensation of steam in the closed chamber may be used to produce a vacuum and suck up water from a lower level. In fact, his suggestions anticipate very fully the engine which a century later became in the hands of Savery the earliest commercially successful steam-engine. In 1615 Solomon de Caus gives a plan of forcing up water by a steam fountain which differs from Portas only in having one vessel serve both as boiler and as displacement-chamber, the hot water being itself raised.
4. Another line of invention was taken by Giovanni Branca who designed an engine shaped like 1629. water-wheel, to be driven by the impact of a jet of steam on its vanes, and, in its turn, to drive other mechanism for various useful purposes. But Brancas suggestion was unproductive, and we find the course of invention revert to the line followed by Della Porta and De Caus.
5. The next contributors is one whose place is not easily assigned. To Edward Somerset, second marquis of Worcester, appears to be due the credit of making the useful steam-engine. Its object was to raise water, and it worked probably like Della Portas model, but with a pair water was forced by steam from an independent boiler, or perhaps by applying heat to the chamber itself, while the other vessel was allowed to refill. Lord Worcesters description of the engine in his Century of Inventions (1663) is obscure, and no drawings are extant. It is therefore difficult to say whether the suction of a vacuum was used to raise water as well as the direct pressure of steam. An engine of about two horse-power was in se at Vauxhall in 1656, and the walls of Raglan Castle contain traces of another, but neither Worcesters efforts nor those of his widow were successful in securing the commercial success of his engine.
6. This success was reserved for Thomas Savery, who Savery, in 1698 obtained a patent for a water-raising engine, 1698. shown in fig. 2. Steam is admitted to one of the oval vessels A, displacing water, which it drives up through the check-valve B. When the vessel A is emptied of water, the supply of steam is stopped, and the steam already there is condensed by allowing a jet of cold water from a cistern above to stream over the outer surface of the vessel. This produces a vacuum and causes water to be sucked up through the pipe C and the valve D. Meanwhile, steam has been displacing water from the other vessel, and is ready to be condensed there. The valves B and D open only upwards. The supplementary boiler and furnace E are for feeding water to the main boiler; E is filled while cold and a fire is lighted under it; it then acts like the vessel of De Caus in forcing a supply of feed-water into the main boiler F. The gauge-cocks G, G are an interesting feature of detail. Another form of Saverys engine had only one displacement chamber and worked intermittently. In the use of artificial means to condensed the steam, and in the application of the vacuum so formed to raise water by suction from a level lower than that of the engine, Savers engine was probably an improvement of Worcesters; in any case it found what Worcesters engine had failed to find,----considerable employment in pumping mines and in raising water to supply houses and towns, and even to drive water-wheels. A serious difficulty which prevented its general use in mines was the fact that the height through which it would lift water was limited by the pressure the boiler and vessels could bear. Pressure as high as 8 or 10 atmospheres were employed ----and that, too, without a safety-valve---but Savery found it no easy matter to deal with high-pressure steam; he complains that it melted his common solder, and forced him, as Desaguliers tells us, "to be at the pains and charge to have all us joints soldered with spelter." Apart from this drawback the waste of fuel was enormous, from the condensation of steam which took place on the surface of the water and on the sides of the displacement-chamber at each stroke; the consumption of coal, was, in promotion to the work done, some twenty times greater than in a good modern steam-engine. In a track called The Miners Friend, Savery alludes thus to the alternate heating and cooling of the water-vessel: "On the outside of the vessel you may see how the water goes out as well as if the vessel were transparent, for so far as the steam continues within the vessel so far is the vessel dry without, and so very hot as scarce to endure the least touch of the hand. But as far as the water is, the said vessel will be cold and wet where any water has fallen on it; which cold and moisture vanishes as fast as the steam in its descent takes place of the water." Before Saverys engine was entirely displaced by its successor, Newcomens, it was improved by Desaguliers, who applied to it the safety valve (invented by Papin), and substituted, condensation by a jet of cold water within the vessel for the surface condensation used by Savery.
7. So early as 1678 the use of a piston and cylinder (long before known as applied to pumps) in a heat-engine had been suggested by Jean Heautefeuille, who proposed to use the explosion of gunpowder either to raise a piston of to force up water, or to produce, by the subsequent cooling of the gases, a partial vacuum into which water might be sucked up. Two years later Huygens described an engine in which the explosion of gunpowder in a cylinder expelled part of the gaseous contents, after which the cooling of the remainder caused a piston to descent under atmospheric pressure, and the piston in descending did work by raising a weight.
8. In 1690 Denis Papin, who ten years before had invented the safety-valve as an adjunct to his "digester," suggested that the condensation of steam should be employed to make a vacuum under a piston previously raised by the expansion of the steam. Papins was the earliest cylinder and piston steam-engine, and his plan of using steam was that which afterwards took practical shape in the atmospheric engine of Newcomen. But his scheme was made unworkable by the fact he proposed to use but one vessel as both boiler and cylinder. A small quantity of water was placed at the bottom of a cylinder and heat was applied. When the piston had risen the fire was removed, the steam was allowed to cool and the piston did work in its down-stroke under the pressure of the atmosphere. After hearing of Saverys engine in 1705 Papin turned his attention to improving it, and devised a modified form, shown fig. 3, in which the displacement-chamber A was a cylinder, with a floating diaphragm or a piston on the top of the water to keep the water and steam from direct contact with one another. The water was delivered into a closed air-vessel B, for which it issued in a continuous steam against the vanes of a water-wheel. After the steam had done its work in the displacement-chamber it was allowed to escape by the stop-cock C instead of being condensed. Papins engine was in fact non-condensing single-acting steam-pump, with steam-cylinder and pump-cylinder in one. A curious feature of it was the heater D, a hot mass of metal placed in the diaphragm for the purpose of keeping the steam dry. Among the may inventions of Papin was a boiler with an internal fire-box,---the earliest example of a constructions that is now almost universal.
9. While Papin was thus going back from his first notion of a piston-engine to Saverys cruder type, a new inventor had appeared who made the piston-engine a practical success by separating the boiler from the cylinder and by using (as Savery had done) artificial means to condense the steam. This was Newcomen, who in 1705, with his assistant Cawley, gave the steam-engine the form shown in fig. 4. Steam admitted from the boiler to the cylinder allowed the piston to be raised by a heavy counterpoise on the other side of the beam. Then the steam-valve was shut and a jet of cold water entered the cylinder and condensed the steam. The piston was consequently forced down by the pressure of the atmosphere and did wok on the jump. The next entry of steam expelled the condensed water from the cylinder through an escape valve. The piston was kept tight by a layer of water on its upper surface. Condensation was at first effected by cooling the outside of the cylinder, but the accidental leakage of the packing water past the piston showed the advantage of condensing by a jet of injection water, and this plan took the place of surface condensation. The engine used steam those pressure was little if at all greater than that of the atmosphere; sometimes indeed it was worked with the manhole lid off the boiler.
10. About 1711 newcomens engine began to be introduced for pumping mines; and in 1713 a boy named Humphrey Potter, whose duty it was to open and shut the valves of an engine he attended, made the engine self-acting by causing the beam itself to open and close the valves by suitable cords and catches. Potters rude device was simplified in 1718 by Henry Beighton, who suspended from the beam a rod called the plug-tree, which worked the valves by means of tappets. By 1725 the engines, was in common use in collieries, and it held its place without material change for about three-quarters of a century in all. Near the close of its career the atmospheric engines was much improved in its mechanical details by Smeaton, who built many large engines of this type about the year 1770, just after the great step which was to make Newcomens engine obsolete had been taken by James Watt.
Compared with Saverys engine, Newcomens had (as a pumping-engine) the great advantage that the intensity of pressure in the pumps was not in any way limited by the pressure of the steam. If shared with Saverys, in a steam was wasted by the alternate heating and cooling of vessel into which it was led. Though obviously capable of more extended uses, it was in fact almost exclusively employed to raise water,---in some instances for the purpose of turning water-wheels to drive other machinery. Even contemporary writers complain of its "vast consumption of fuel," which appears to have been scarcely smaller than that of the engine of Savery.
11. In 1763 James Watt, an instrument maker in Glasgow, while engaged by the university in repairing a model of Newcomens engine, was struck with the waste of steam to which the alternate chilling and heating of the cylinder gave rise. He saw that the remedy, in his own words, would lie in keeping the cylinder as hot as the steam that entered it. With this view he added to the engine a new organ---an empty vessel separate from the cylinder, into which the steam should be allowed to escape from the cylinder, to be condensed there by the application of cold water either outside or as a jet. To preserve the vacuum in his condenser he added a pump called the air-pump, whose function was to pump from it the condensed steam and water of condensation, as well as the air which would otherwise accumulate by leakage of by being brought in with the steam or with the injection water. Then as the cylinder was no longer used as a condenser he was able to keep it hot by clothing it with non-conducting bodies, and in particular by the use of a steam-jacket, or a layer of hot steam between the cylinder and an external casing. Further and still with the same object, he covered in the top of the cylinder, taking the piston-rod out through a steam-tight stuffing-box, and allowed steam instead of air to press upon the pistons upper surface. The idea of using a separate condenser had no sooner occurred to Watt than he put it to the test by constructing the apparatus shown in fig. 5. There A is the cylinder, B a surface condenser, and C the air-pump. The cylinder was filled with steam above the piston, and a vacuum was formed in the surface condenser B. On opening the stop-cock D the steam rushed over from the cylinder and was condensed, while the piston rose and lifted a weight. After several trials Watt patented his improvements in 1769; they are described in his specification in the following words, which, apart from the their immense historical interest, deserve careful study as a statement of principles which to this day guide the scientific development of the steam-engine:---
"My method of lessening the consumption of steam, and consequently
Fuel, in fire-engines, consists of the following principles:---"First, That vessel in which the powers of steam are to be employed to work the engine, which is called the cylinder in common fire-engines, and which I call the steam-vessel, must, during the whole time the engine is at work, be kept as hot as the steam that enters it; first by enclosing it in a case of wood, or any other materials that transmit heat slowly: secondly, by surrounding it with steam or other heated bodies; and, thirdly, by suffering neither water nor any other substance colder than the steam to enter or touch it during that time.
"Secondly, In engines that are to be worked wholly of partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam-vessels or cylinder, although occasionally communicating wit them; these vessels I call condensers; and, whilst the engines are working, these condensers ought at least to be kept as cold as the air in the neighborhood of the engines, by application of water or other cold bodies.
"Thirdly, Whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam-vessels or condensers by means of pumps, wrought by the engines themselves, or otherwise.
"Fourthly, I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire-engines. In cases where cold water cannot be had in plenty the engines may be wrought by this force of steam only, by steam only discharging the steam into the air after it has done its office. . .
"Sixthly, I intend in some cases to apply a degree f cold not capable of reducing the steam to water, but of contracting it considerably, so that the engines shall be worked by the alternate expansion and contraction of the steam.
"Lastly, Instead of using water to render the pistons and other parts of the engine air and steam tight, I employ oils, wax, resinous bodies, fat animals, quicksilver and other metals in their fluid state."
The fifth claim was for a rotary engine, and need not be quoted here.
The "common fire-engine" alluded to was the steam-engine, or, as it was more generally called, the "atmospheric" engine of Newcomen. Enormously important as Watts first patent was, it resulted for a time in the production of nothing more than a greatly improved engine of the Newcomen type, much less wasteful of fuel, able to make faster strokes, but still only suitable for pumping, still single-acting, with steam admitted during the whole stroke, the piston, as before, pulling the steam by a chain working on a circular arc. The condenser was generally worked by injection, but Watt has left a model of a surface condenser made up of small tubes in every essential respect like the condenser now used in marine engines.
12. Fig. 6 is an example of the Watt pumping-engine of this period. It should be noticed that, although the top of the cylinder is closed and steam has access to the upper side of the piston, this is done only to keep the cylinder and piston warm. The engine is still single-acting; the steam in the upper side merely plays the part which was played in Newcomens engine by the atmosphere; and it is the lower end of the cylinder alone that is ever put in communication with the condenser, There are three valves,--- the "steam" valve a, the "equilibrium" valve b, and the "exhaust" valve c. At the beginning of the down-stroke c is opened to produce a vacuum below the piston and a is opened to admit steam above it. At the end down-stroke a and c are shut and b is opened. This puts the two sides in equilibrium, and allows the piston to be pulled up by the pump-rod P, which is heavy enough to serve as a counterpoise. C is the condenser, and A is air-pump, which discharges into the hot well H, whence the supply of the feed-pump F is drawn.
13. In a second patent (1781) Watt describe the "sun-and-planet" wheels and other methods of making the engine give continuous revolving motion of a shaft provided with a fly-wheel. He had invented the crank and connecting-rod for this purpose, but it had meanwhile been patented by one Pickard, and Watt, rather than make terms with Pickard, whom he regarded as a plagiarist of his own ideas made use of his sun-and-planet motion until the patent on the crank expired. The reciprocating motion of earlier forms had served only for pumping; by this invention Watt opened up for the steam-engine a thousand other channels of usefulness. The engine was still single-acting; the connecting rod was attached to the far end of the beam, and that carried a counterpoise which served to raise the piston when steam was admitted below it.
14. In 1782 Watt patented two further improvements of the first importance, both of which he had invented some years before. One was the use of double action, that is to say, the application of steam and vacuum to each side of the piston alternately. The other (invented as early as 1769) was the use of steam expansively, in other words the plan (now used in all engines that aim at economy of fuel) of stopping the admission of steam when the piston had made only a part of its stroke, and allowing the rest of the stroke, and allowing the rest of the stoke to be performed by the expansion of the steam already in the cylinder. To let the piston push as well as pull the end of the beam Watt devised his so-called parallel motion, an arrangement of links connecting the piston-rod head with the beam in such a way as to guide the rod to move in a very nearly straight line. He further added the throttle-valve, for regulating the rate of admission of steam, and the centrifugal governor, a double conical pendulum, which controlled the speed by acting on the throttle-valve. The stage of development reached at this time is illustrated by the engine fig. 7 (from Stuarts History of The Steam-Engine), which shows the parallel motion pp, the governor g, the throttle-valve t, and a pair of steam and exhaust valves at each end of the cylinder. Among other inventions of Watt were the "indicator," by which diagrams showing the relation of the steam-pressure in the cylinder to the movement of the piston are automatically drawn; a steam tilt-hammer and also a steam locomotive for ordinary roads,---but this invention was not prosecuted.
In partnership with Matthew Boulton, Watt carried on in Birmingham the manufacture and sale of his engines with the utmost success, and held the field against all rivals in spite of severe assaults on the validity of his patents, Notwithstanding his accurate knowledge of the advantage to be gained by using steam expansively he continued to employ only low pressure---seldom more than 7 _ per inch over that of the atmosphere. His boilers were fed, as Newcomens had been, through an open pipe which rose high enough to let the column of water in it balance the pressure of the steam. He introduced the term "horse-power" as a mode of rating engines, defining one horse-power as the rate at which work is done when 33,000 _ are raised one foot in one minute. This estimate was based on trials of the work done by horses; it is excessive as a statement of what an average horse can do, but Watt purposely made it so in order that his customers might have no reason to complain on this score.
15. In the fourth claim in Watts first patent, the second sentence describes a non-condensing engine, which would have required steam of a higher pressure. This, however, was a line of invention which Watt did not follow up, perhaps because so early as 1725 a non-condensing engine had been described by Leupold in his shown in fig. 8, which makes its action sufficiently clear. Watts aversion to high-pressure steam was strong, and its influence on steam-engine practice long survived the expiry of his patents. So much indeed was this the case that the terms "high-pressure" and "non-condensing" were for many years synonymous, in contradistinction to the "low-pressure" or condensing engines of Watt. This nomenclature no longer holds; in modern practice many condensing engines use as high pressures as non-condensing engines, and by doing so are able to take advantage of Watts great invention of expansive working to a degree which was impossible in his own practice.
16. The introduction of the non-condensing and, at that time, relatively high-pressure engine, was effected in England by Trevithick and in America by Oliver Evans about 1800. Both Evans and Trevithck applied their engines to propel carriages on roads, and both used for boiler a cylindrical vessel with a cylindrical vessel with a cylindrical flue inside the construction now known as the Cornish boiler. In partnership with Bull, Trevithick had previously made direct-acting pumping-engines, with an inverted cylinder set over and in line with the pump-rod, thus dispensing with the beam that had been a feature in all earlier forms. But in these "Bull" engines, as they were called, a condenser was used, or, rather, the steam was condensed by a jet of cold water in the exhaust-pipe, and Boulton and Watt successfully opposed them as infringing Watts patents. To Trevithick belongs the distinguished honour of being the first to use a steam-carriage on a railway; in 1804 he built a locomotive in the modern sense, to run on what had formerly been a hose-tramway in Wales, and it is noteworthy that the exhaust steam was discharged into the funnel to force the furnace draught, a device which, 25 years later, in the hands of George Stephenson, went far to make the locomotive what it is to-day. In this conexion it may be added that as early as 1769 a steam-carriage for roads had been built by Cugnot in France, who used a pair of single-acting high-pressure cylinders to turn a driving axle step by step by means of pawls and ratchet-wheels. To the initiative of Evans may be ascribed the early general use of high-pressure steam in the United States, a feature which for many years distinguished American from English practice.
17. Amongst the contemporaries of Watt one name deserves special mention. In 1781 Jonathan Hornblower constructed and patented what now be called a compound engine, with two cylinders of different sizes. Steam was first admitted into the smaller cylinder, and then passed over into the larger, doing work against a piston in each. In Hornblowers engine the two cylinders were placed side by side, and both pistons worked on the same end of a beam overhead. This was an instance of the use of steam expansively, and as such was earlier than the patent, though not earlier than the invention, of expansive working by Watt. Hornblower was crushed by the Birmingham firm for infringing their patent in the use of a separate condenser and air-pump. The compound engine was revived in 1804 by Woolf, with whose name it is often associated. Using steam of fairly high pressure, and cutting off the supply before the end of the stroke in the small cylinder, Woolf expanded the steam to several times its original volume. Mechanically the double-cylinder compound engine has this advantage over an engine in single cylinder, that the sum of the stroke in which the same amount of expansion is performed in a single cylinder, that the sum of the forces exerted by the tow pistons in the compound engine varies less throughout the action than the force exerted by the piston of the single-cylinder engine. This advantage may have been clear to Hornblower and Woolf, and to other early users of compound expansion. But another and probably a more important merit of the system lies in a fact of which neither they nor for many years their followers in the use of compound engines were aware--- the fact that by dividing the range of expansion into two parts the cylinders in which these are separately performed are subject to a reduced range of fluctuation in their temperature. This, as will be afterwards pointed out, limits to a great extent a source of waste which is present in all steam-engines, the waste which results from the heating and cooling of the metal by its alternate contact with hot and cooler steam. The system of compound expansion is now used in nearly all large engines that pretend to economy. Its introduction forms the only great improvement which the steam-engine has undergone since the time of Watt; and we are able to recognize it as a very important step in the direction set forth in his "first principle," that the cylinder should be kept as hot as the steam that enter it.
18. Woolf introduced the compound engine somewhat widely about 1874, s a pumping engine in the mines of Cornwall. But here it met a strong competitor in the high-pressure single-cylinder engine of Trevithick, which had the advantage of greater simplicity in construction. Woolfs engine fell into comparative disuse, and the single-cylinder type took a form which, under the name of the Cornish pumping engine, was for many years famous for its great economy of fuel. In this engine the cylinder was set under one end of a beam, from the other end of which hung a heavy rod which operated a pump at the foot of the shaft. Steam was admitted above the piston for a short portion of the stroke, thereby raising the pump-rod, and was allowed to expand for the remainder. Then an equilibrium valve, connecting the space above and below the piston, as in fig. 6, was opened, and the pump-rod descended, doing work in the pump and raising the engine piston. The large mass which had to be started and stopped at each stroke served by its inertia to counter-balance the unequal pressure of the steam, for the ascending rods stored up energy of motion in the early part of the stroke, when then the steam pressure was greatest, and gave out energy in the later part, when expansion had greatly lowered the pressure. The frequency of the stroke was controlled by a device called a cataract, consisting of a small plunger pump, in which the plunger, raised at each stroke by the engine, was allowed to descend more or les slowly by the escape of fluid below it through an adjustable orifice, and in its descent liberated catches which held the steam and exhaust valves from opening. A similar device controlled the equilibrium valve, and could be set to give a pause at the end of the pistons down-stroke, so that the pump cylinder might have time to become completely filled. The Cornish engine is interesting as the earliest from which achieved efficiency comparable with that of good modern engines. For many years monthly reports were published of the "duty" of pounds of work done per bushel of (in some cases) per cwt. of coal. The average duty of engines in the Cornwall district rose from about 18 millions of foot-pounds per cwt. of coal in 1813 to 68 millions in 1844, after which less effort seems to have been made to maintain a high efficiency. In individual cases much higher results were reported, as in the Fowey Consols engine, which in 1835 was stated to have a duty of 125 millions. This (to use a more modern mode of reckoning) is equivalent to the consumption of only a little more than 1_ _ of coal per horse-power-------a result surpassed by very few engines in even the best recent practice. It is difficult to credit figures in even the best recent practice. It is difficult to credit figures which, even in exceptional instances, place the Cornish engine of that period on a level with expansion and higher pressure combine to make a much more perfect thermodynamic machine; and apart from this there is room to question the accuracy of the Cornish engine of that period on a level with the most efficient modern engines---in which compound expansion and higher pressure combine to make a much more perfect thermodynamic machine; and apart form this there is room to question the accuracy of the Cornish reports. They played, however, a useful part in the process of steam-engine development by directing attention to the question of efficiency, and by demonstrating the advantage to be gained by high pressure and expansive working, at a time when the theory of the steam-engine had not yet taken shape.
19. The final revival of the compound engine did not occur until about the middle of the century, and then several agencies combined to affect it. In 1845 MNaught introduced a plan of improving beam engines of the original Watt type, by adding a high-pressure cylinder whose piston acted on the beam between the centre and the fly-wheel end. Steam of higher pressure than had formerly been used, after doing work in the new cylinder, passed into the old or low-pressure cylinder, where it was further expanded. Many engines whose power was proving insufficient for the extended machinery they had to drive were "MNaughted" in this way, and after conversion were found not only to yield more power but to drive were found not only to yield more power but to show a marked economy of fuel. The compound form was selected by Mr. Pole for the pumping engines of Lambeth and other waterworks about 1850; in 1854 John Elder began to use it in marine engines; in 1857 Mr. Cowper added a steam-jacketed intermediate reservoir for steam between the high and low-pressure cylinders, which made it necessary for the low pressure piston to be just beginning when the other piston was just ending its stroke. As facilities increased for the use of high-pressure steam, compound expansion became more and more general, its advantage becoming more conspicuous with every increase in boiler pressure---until now there are few large land engines and scarcely any marine engines that do not employ it. In marine practice, where economy of fuel is a much more important factor in determining the design than it is on land, the principle of compound expansion has lately been greatly extended by the introduction of triple and even quadruple expansion engines, in which the steam is made to expand successively in three or in four cylinders. Even in the building of locomotive engines, where other considerations are of more moment than the saving of coal, the system of compound expansion is beginning to find a place.
The growth of compound expansion has been referred to at some length, because it forms the most distinctive improvement which the steam-engine has undergone since the time of Watt. For the years, the progress of the steam-engine has consisted in its adaptation to particular uses, in the invention of features of mechanical detail, in the recognition and application of thermo dynamical principles, and in improved methods of engineering construction by which it has profited in common with all other machines. These have in particular made possible the use of eight or ten times the pressure of that employed by Watt.
20. The adaptation of he steam-engine to railways, begun by Trevithick, became a success in the hands of George Stephenson, whose engine the "Rocket," when tried along with others on the Stockton and Darlington road in 1829, not only distanced its competitors but settled once and for all the question whether horse traction of steam traction was to be used on railways. The principal features of the "Rocket" were an improved steam-blast for urging the combustion of coal and a boiler (suggested by Booth, the secretary of the railway) in which a large heating surface was given by the use of many small tubes through which the hot gases passed. Further, the cylinders, instead of being vertical as in earlier locomotives, were set in at a slope, which was afterwards altered to a position more nearly horizontal. To these features there was added later the "link motion," a contrivance which enabled the engine to be easily reversed and the amount of expansion to be readily varied. In the hands of George Stephenson and his son Robert the locomotive took a form which has bee in all essentials maintained by the far heavier locomotives of to-day.
21. The first practical steamboat was the tug "Charlotte Dundas," built by William Symmington, and tried in Forth and Clyde Canal in 1802. A Watt double-acting condensing engine, placed horizontally, acted directly by a connecting-rod on the crank of a shaft at the stern, which carried a revolving paddle-wheel. The trial was successful, but steam towing was abandoned for fear of injuring the banks of the canal. Ten years later Henry Bell built the "Comet," with side paddle-wheels, which ran as a passenger steamer on the Clyde; but an earlier inventor to follow up Symmingtons success was the American Robert Fulton, who, after unsuccessful experiments on the Seine, fitted a steamer on the Hudson in 1807 with engines made to his designs by Boulton and Watt, and brought steam navigation for the first time to commercial success.
22. The early inventors had little in the way of theory to guide them. Watt had the advantage, which he acknowledges, of knowledge of Blacks doctrine of latent heat; but there was no philosophy of the relation of work to heat until long after the inventions of Watt were complete. The theory of the steam-engine as a heat-engine dates from 1824, when Carnot published his Reflixions sur la Puissance Motrice du Feu, and showed that heat does work only by being let down from a higher to a lower temperature. But Carnot had no idea that any of the heat disappears in the process, and it was not until the doctrine of the conservation of energy was established in 1843 by the experiments of Joule that the theory of heat-engines began a vigorous growth. From 1849 onwards the science of thermodynamics was developed with extraordinary rapidity by Clausius, Rankine, and Thomson, and was applied, especially by Rankine, to practical problems in the use of steam. The publication in 1859 of Rankines Manual of Steam Engine formed an epoch in the history of the subject by giving inventors a new basis, outside of mere empiricism, from which they could push on the development of the steam-engine. Unfortunately, however, for its bearing on practice, the theory of the steam-engine was to a great extent founded on certain simplifying assumptions which experience has now shown to be far from correct. It was assumed that the cylinder and piston might be treated as behaving to the steam like non-conducting bodies, ---that transfer of heat between the steam and the metal was negligibly small. Rankines calculations of steam-consumption, work, and thermodynamic efficiency involve this assumption, except I the case of steam-jacketed cylinders, where he estimates that the steam in its passage through the cylinder takes just enough heat from the jacket to prevent a small amount of condensation which would otherwise occur as the progress of expansion goes on. If the transfer of heat from steam to metal could be overlooked, the steam which enters the cylinder would remain during admission as dry as it was before it entered, and the volume of steam consumed per stroke would correspond with the volume of the cylinder up to the point of cut-off. It is here that the actual behaviour of steam in the cylinder diverges most widely form the behaviour which the theory assumes. When steam enters the cylinder it finds the metal chilled by the previous exhaust, and a portion of it is at once condensed. This has the effect of increasing, often very largely, the volume of boiler steam required per stroke. As expansion goes on the water that was condensed during admission begins to be re-evaporated from the sides of the cylinder, and this action is often prolonged into the exhaust. In a later chapter the effect which this exchange of heat between the metal of the cylinder and the working fluid produces on the engine will be discussed, and an account will be given of experimental means by which we may examine the amount of steam that is initially condensed and trace its subsequent re-evaporation. It is now recognized that an theory which fails to take account of these exchange of heat fails also to yield even comparatively correct results in calculating the relative efficiency of various steam pressure of various ranges of expansion. But the exchanges of heat are so complex that there seems little prospect of submitting them to any comprehensive theoretical treatment, and we must rather look for help in the future development of engines to the scientific analysis of experiments with actual machines. Much careful work of this kind has already been done by Hirn and others, and there is room for much more. Questions relating to the influence (on heat-engine economy) of speed, of pressure, of ratio of expansion, of jacketing, of compound expansion, or of superheating must in the main be settled by an appeal to experiment,---experiment guided and interpreted at every step by reference to the principles of thermodynamics and of the theory of steam.
Read the rest of this article:
Steam Engine - Table of Contents