Source: Minutes of proceedings of the Institution of Civil Engineers, Vol. IV, Page 355 - Part 2
Date: June 10th, 1845
Title: Description of Stirling's Improved Air Engine
Author: James Stirling, M. Inst. C. E.
Mr. Stirling exhibited the model of the engine, and explained that the object was to raise the temperature of the air in the air-vessel to such an extent as to obtain a difference of 500 F (approx. 260° C) between the heated and the cooled states. The air when heated might be taken at 650° F (approx. 343° C) and after it had passed through the capillary passages, and the refrigerator, its temperature was 150° F (approx. 65° C).
The machine might be termed a differential engine, because the power was produced by the difference of pressure, arising from the alternatives of temperature. At the first view its principle appeared somewhat paradoxical, because the thin metallic plates forming the capillary passages, by which the caloric was absorbed from the air, in its ascent, were used to give out the caloric to the air in its descent. The working pressure varied during each stroke from 160 lbs. to 240 lbs. per square inch (approx. 11 to 16.5 bars) .
In order to understand the action of the engine, it must be remembered, that when the heated air came in contact with the lower end of the metallic laminae forming the capillary passages, the absorption of caloric commenced and went on gradually as it advanced towards the water-pipe refrigerator at the upper end, where a small portion of the heat, not previously extracted, was thrown off; and it must be evident that in the reverse action, when the air began to descend, it came into contact with the surfaces of the laminae, from which it again absorbed the caloric, which had been left by the air in its ascent.
A few degrees of heat which, had been conveyed away by the water in the refrigerator, required to be supplied at each stroke, and this was given by the heated air-vessel; thus keeping up its original temperature and pressure. When the air was in a heated state, the pressure was communicated to the piston, through the column of air, which constantly occupied the passages, and the air consequently reached the cylinder at a temperature not higher than 150° F (approx. 65° C).
The higher the pressure under which the engine was worked, the greater was the effect produced. A small engine on this principle had been worked to a pressure of 360 lbs. on the square inch (approx. 24.8 bars). It might, eventually, be found more advantageous to work at a higher pressure, with a view to economy of fuel, as the friction did not appear to increase in proportion to the power exerted. It was found that the power derived from the pressure of the atmosphere (15 lbs. per square inch or 1 bar) was barely sufficient to overcome the friction of the engine. As the pressure augmented so the practical effect was increased. It must be remembered also that the pressure was sustained by an air force-pump, and that each air-vessel supplied itself, up to a certain pressure, by a self-acting valve. Any leakage was thus compensated.
If the connecting pipes and passages could be made infinitely small and short, the absolute power indicated by theory would be given out by the engine.
In answer to a question from Mr. Lowe, Mr. Stirling agreed, that Jeffrey's Respirator, used by consumptive patients, was based upon the same principle of the alternate absorption and giving out of caloric to air traversing the capillary passages.
Mr. Walker was at a loss to understand by what action the air was driven through the capillary passages; the rapidity of the alternate heating and cooling was equally extraordinary. From the statement in the paper it was evident, however, that a considerable power was advantageously obtained by a small expenditure of fuel, which was the main point of interest for the Institution.
Mr. Cottam said, it was evident that if it was practicable to arrive at the theoretical condition of the absorption of all the caloric by the thin laminae, during the upward passage of the air and the giving it out again during the downward passage, there would not be any loss of heat. That, however, was not practicable, and as water was a bad absorbent, but a good conductor of heat, the difference carried off by the water, in the pipes of the refrigerator, was necessarily the only loss.
Mr. Stirling explained that the air was set in motion by the alternate action of the plungers within the air-vessels.
With respect to the economy of the system; it must be premised, that some disadvantage resulted from the necessity of adhering to the length and number of strokes, fixed by those parts of the steam engine which were retained. The consumption of fuel, as compared to the steam-engine, which the air-engine had replaced, was as 6 to 26; the same amount of work being now performed by about 6 cwt. (672 lbs. or 305 kg) of coals, which had formerly required about 26 cwt. (2912 lbs. or 1321 kg).
The steam-engine which had been removed was not, however, of the best construction, nor had the boiler any close covering.
The average power required for the works had been ascertained to be equal to about 700,000 lbs. raised 1 foot high per minute or 21 horses power. The quantity of water passed through the refrigerator was 4 cubic feet per minute (approx. 113 l/mn), and the amount of heat communicated to it was 16° F to 18° F.
Mr. Gordon thought it was necessary to reject completely the idea of converting parts of a steam-engine into an air-engine; it should rather be an object to construct an engine, which should communicate, by direct action, all the power derived from the pressure of the expanded air. The adhesion to the use of parts of a steam-engine had, in his opinion, increased the practical difficulties in the construction and working of the machine.
Mr. Stirling could not entirely accord with Mr. Gordon's views. It was true, that as he had before stated, the power had been restricted, by the obligation to use certain parts of the old engine; for instance, if it had been practicable to have increased the number of strokes, from 28 to 36 or 38 strokes per minute, the power would have been augmented nearly in proportion, or the same power could have been produced, from a machine of smaller dimensions. With respect to "direct action", if Mr. Stirling comprehended Mr. Gordon's meaning, there could not be any doubt, that a steam-engine would be a more economical machine than an air-engine, in which the air was merely introduced in a heated state, and was suffered to escape after producing motion. Such, however, was not the case in the machine under consideration; in it, the economy depended upon the reiterated use of the same air, alternately giving out and absorbing the same caloric. The principal practical difficulties of construction and use might be considered, in a great degree, as overcome although, in further practice, many improvements would doubtless be introduced.
Mr Homersham inquired whether any indicator diagrams had ever been taken, from which the actual power exerted by the engine could be estimated.
Mr. Stirling replied, that some indicator diagrams had been taken, but he was doubtful whether they could be relied upon, as there was necessarily considerable friction in the instrument, owing to the necessity of rendering it air-tight by means of a leather cup. Besides, the indicator was affected, to a certain extent, by the motion of the piston, without reference to the expansion and contraction of the air, and the amount of that effect could not be correctly ascertained.
Mr. Field, V.P. could not understand by what means the commencement of the change of pressure was induced, so as first to generate motion in the engine, or to stop it, or reverse it, when in action.
Mr. Stirling explained, that motion was first induced, by changing the position of the plunger by hand, in the same manner as the valves of a steam-engine were moved, and by the action of the eccentric rod the motion was continued; to stop, or to reverse the motion, the usual hand gear was used.
Mr. R. Stephenson said, he understood the process to consist of heating the air in a vessel, whence it ascended to the cylinder between numerous thin laminae, by which the caloric was absorbed, to be again given out to the descending air. Now it appeared to him, that though the ascending process was natural and easy, the reverse action would require a certain expenditure of power, in the depression of the plunger.
Mr. Stirling admitted, that a certain amount of power was necessary to move the plungers either way, and to overcome the friction of the air in the passages; but that no loss arose from any difference of pressure on the upper and lower ends of the plungers, that pressure being always equal, from the well-known principle of fluids pressing equally in all directions.
Mr. J. Smith (Deanslon) corroborated the statements as to the efficiency of the engine; it worked very regularly and smoothly, and apparently with a considerable economy of fuel, which was supplied by a feeding apparatus, with regulating doors to the ash-pit so that no waste of fuel occurred by the unnecessary introduction of cold air, either between the bars or through the fire-door, as was too frequently the case when the fuel was supplied by hand.
Mr. Leslie said, he had given much attention to the engine from its first introduction, and he was satisfied of its advantageous applicability in many cases; the engine under consideration had done a great deal of work very regularly, and with much economy of fuel; indeed the small dimensions of the fire-grate, which was only 24 inches (approx. 56 cm) by 22 inches (approx. 61 cm), sufficed to show, that except the fuel was carelessly used, only a very small quantity could be consumed.
He directed attention to the differential lever for shortening or lengthening the stroke of the plungers, as more or less power was required; by a simple arrangement, this had been made self-acting, and was a very sensitive governor. He thought this might be advantageously adopted in steam-engines. It was extraordinary how very cool the cylinder was, it never became as hot as the cylinder of a steam-engine.
He had noticed this particularly, because it had been imagined that the heated air would soon injure the cylinder and the piston.
Sir George Cayley was much gratified with the apparent success which had attended Mr. Stirling's efforts; he was particularly struck with the clever arrangement of the laminae, and the refrigerating tubes, as by these means many of the difficulties he had encountered in his attempts to construct an air-engine were obviated. It was true, that his attention had been directed towards a different object, that of the production of a light locomotive engine for travelling on turnpike roads, for which purpose it had been his main object to reduce, as much as possible, the weight of the machine, and the number of the moving parts.
Acting upon the principle of air doubling its volume at a temperature of 480° F (approx. 249 ° C), he had sought to produce motion by drawing air through the ignited fuel in the fire-box, and employing its expansive power at about 520° F (approx. 271 ° C), directly upon a piston. In this process many difficulties were experienced, of which the principal were, the destruction of the slide valve, the passages, the piston, and the cylinder, by the heat and the dust. The faces of the slide valves were torn to pieces in a short time, and it became necessary to adopt a conical valve, which, although it in some degree avoided the great evil, was still considerably abraded.
It was necessary to have an extremely well-fitting piston; but, in a very short time, both it and the cylinder were much injured by the same action of the heat and dust. He had attempted to obviate this by appending a drum below the piston, and allowing a thin film of air around the periphery; but even that was not effectual. These were only a few of the difficulties he had to encounter in his experiments. The principal point of general interest was, however, the proportion of power obtained by Mr. Stirling from a given quantity of fuel. Sir G. Cayley had not been so successful as his engine consumed 6 lbs. (approx. 2.7 kg) of coke = 9 lbs. (approx. 4 kg) of coil per horse power.
Mr. Stirling said, that his air-engine of 21 horses power consumed 50 lbs. of coal per hour (approx. 23 kg) , which was about 2½ lbs. (approx. 0.9 kg) per horse power per hour.
Mr. Gordon said, that in Mr. Stirling's engine, the intense heat of the fire did not come into actual contact with the piston; the motion was derived from the expansion of the air, caused by the heating of the air-vessel. Sir George Cayley, on the contrary, used the direct products of combustion, consisting of the expanded air and the gases evolved; and hence arose the difficulty of keeping in order the working parts of the engine. The difference between the two systems was, however most apparent in the relative consumption of fuel of the two engines, which, as being the commercial value, was the point of great interest for the public.
It must not be lost sight of, that in adopting the use of air as a prime mover, such distressing accidents as had frequently occurred recently from the explosion of boilers, would be in future entirely avoided.
Mr. Stirling felt all reasonable confidence in the ultimate success of the air-engine as all the changes hitherto made had improved its action, and increased the economy of the fuel consumed. It must be remarked, that Sir George Cayley, in following an entirely different object, had overlooked the great leading principle of repeatedly using the same heat Mr. Stirling was of opinion, that except on that principle, the air could not be economically used as a moving power; his hopes of ultimate success were, therefore, fixed upon the further improvement of the economizing and refrigerating processes, as had been previously explained.
He would be glad to learn from Sir G. Cayley how to overcome the difficulty of using a piston in a hot cylinder, as he would then be able materially to increase the effect of his engine by connecting the cylinder with the hot, instead of the cold end of the air-vessels. In the attempts which he had hitherto made for this purpose, he had always found the inconvenience so great as to induce him to return to the arrangement exhibited in the model. He had endeavoured to protect the piston by joining to it a false piston filled with non-conducting substances; but hitherto he had not obtained that success he wished. Upon his present system he found, that after working for 2½ years, the piston was not perceptibly worn, and the cylinder had attained a very high polish. The laminae were not perceptibly corroded, there was not any dust or dirt in the air-vessels, and the quantity of oil used in the cylinder and piston was literally not more than a thimbleful in three days.
Mr. Field, V.P. said that Mr Stirling appeared to have been successful in preventing the piston and cylinder from being injured by the heated air. He would inquire whether there was any difficulty in rendering tight the stuffing-boxes of the piston and air-vessel rods; and also at what part of the machine the air was introduced to compensate for the leakage which did occur.
Mr. Stirling replied, that the temperature of the rods was so low, that by means of the leather collars employed, there was no difficulty in keeping the stuffing- boxes almost perfectly air-tight. The temperature of the rods, which was generally about 150° F (approx. 65° C), did not appear to vary at any time above 8° F or 10° F. The leakage to be allowed for was, therefore, very slight; and although, for the sake of convenience, the air was admitted into the nosle of the cylinder, it was, in fact, of no importance as far as the effect of the engine was concerned, at what point it was introduced.
Mr. Julius Jeffreys said, the construction of the respirator, which had been alluded to, was so well known, that he would only refer to the principles of its operation, which had a close analogy with that of the laminae of Mr. Stirling's engine. The conducting power of metals had been, in both cases, used for alternately absorbing and giving out the caloric.
He had found, that when the plates were so united as to form one continuous series the result was less positive than when there was a separation between each, and he thought, that if the vertical laminae of the air- engine had been divided into lengths, by some non-conducting medium, they would have been found more effective. When one end of a continuous bar or tube was inserted in a fire, by degrees the mass became heated in its entire length, much more rapidly, than when it was divided into lengths by a non-conducting substance. He thought it would be found, that if the temperature of the first portion was raised, say to 450° F, the next would be at a somewhat lower temperature, and so on seriatim, until the whole of the caloric was extracted, and that in the reversed action, the heat would be given out in the same degree. He therefore considered the separation of the plates as of great importance.
Mr. Stirling was glad to hear the principle so clearly explained. In any early patent he had specified the arrangement followed in the respirator, intending to use a series of perforated plates, or wire gauze. The principle of having each plate separate was theoretically correct, but in practice he had not found any great disadvantage from the conducting power of the continuous laminae; while the positive advantage of their continuity was in other respects very apparent. It must be admitted that when the engine was at rest, the conducting power of the laminae had a tendency to equalize their temperature throughout; but in a few minutes after it had been set in motion, the caloric seemed, by an apparent power of self-adjustment, to have been removed, and properly dispersed along the laminae, so that the temperature of the upper part of the air-vessels was lowered to that point at which it remained permanently during the actual working of the engine. He had tried the effect of dividing the laminae into five portions, but had not observed any beneficial result in the working of the engine; and therefore, after numerous experiments, he had reverted to the continuous laminae, which had been brought before and explained to the meeting.