Discussion about Heated Air Engines

Source: Minutes of the Proceedings of the Institution of Civil Engineers, Vol 12, N° 886
Date: Feb 15, 1853
Title: On the Use of Heated Air as a Motive Power - Minutes of Discussion
Author: Benjamin Cheverton

A brief history of hot air engines

Mr. G. Crispe said, after the philosophical toys of the early experimenters, the first approach to a practical caloric engine was that invented by Sir George Cayley (Assoc. Inst. C.E.), and described in Nicholson's Journal in 1807. For many years his attention was almost exclusively devoted to the subject, and eventually, in 1837, one of these machines was reported by Mr. Goldsworthy Gurney and Mr. G. Rennie (M. Inst. C.E.), to be doing a duty equal to 5 H.P., with an expenditure of 20 lbs. to 22 lbs. of coke per hour.

The chief defects appeared to be the difficulty of lubricating the piston, and the abrasion of the working parts, from the dirt apparently inseparable from the air, which had passed through the incandescent fuel.

In 1816, the Rev. Dr. R. Stirling took out a patent for an air-engine, differing in all important particulars from that of Sir G. Cayley. The new machine resembled the steam-engine in the construction and arrangement of many of its parts, such as the cylinder, piston, piston-rod, parallel motion, beam, crank, etc.

Motion was obtained, by heating the air in communication with one side of the piston, and cooling that in connection with the other, by which means a difference of pressure was obtained upon the opposite sides. The air was alternately heated and cooled, by having the air-chambers, one of which was connected with the top, and the other with the bottom of the cylinder, fixed respectively over two fires, by which their lower ends were kept at a temperature of about 600° Fah., whilst their upper parts were kept cool ; the air, by the aid of a plunger, being made to occupy the top in one chamber, and the bottom in the other, alternately.

Dr. Stirling also introduced and patented at the time, what had since been called the "regenerator," the object of which was to save a portion of that heat, which would otherwise be lost in cooling the air at each stroke of the engine : it consisted of a series of laminae of metal, between which the heated air passed, in its progress from the bottom to the top of the air-chamber. The metallic surfaces became heated by contact with the air, which heat they retained, until the air, in its return from the top to the bottom, in its cool state, had to make its way through them, in the opposite direction ; in the return stroke it took up from them the heat which they had just received ; and in proportion to the amount of heat kept back, by those means, was the saving of fuel effected.

Another patent was taken in 1827 by Messrs. R. and J. Stirling. An engine of the kind described was erected at the Dundee Foundry in 1843, and continued to do the work of the establishment for about four years. Mr. Stirling in describing the machine, at a meeting of this Institution in 1845, stated that the economy of fuel in using the air-engine in place of steam was as 6 to 26.

Mr. Stirling had recourse to surrounding the cylinder with water, for the purpose of cooling the air, and it was evident, that one of the main features of his invention was the "regenerator," for which he at first used wire gauze, then he tried metallic discs, and at last he adopted metal plates.

Whether the theory of this "regenerator" was practically borne out, or not, there could be little doubt of the invention being due to Messrs. Stirling, and up to the period of the announcement of the caloric engines on board the 'Ericsson,' they had certainly obtained the most satisfactory results.

In 1827, Messrs. Parkinson and Crosley, well known as the inventors and manufacturers of the gas-meters bearing their name, patented a modification of the air-engine. It consisted of a close cylindrical air-chamber, with its plunger piston, immersed midway in cold water and a ring of jets of gas encircling its upper part ; the piston rod traversing by a stuffing box. Connected with the upper part was a working cylinder, open at the top, but with a piston and rod fitted within it.

The air contained within the chamber, when at the top, being heated by contact with its hot sides, was in consequence expanded. A portion making its way through the opening into the working cylinder, pressed the piston upwards. By a proper communication of parts, this was made to give motion to the plunger, so that by the time the piston was at the top of its cylinder, the plunger had reached the top of the air-chamber.

The result was evident, the air made its way down past it to the cold end of the chamber, became cooled and contracted ; the piston descended, and after it the plunger : so that the operation might be repeated ad infinitum. It had only been tried as a model, but the rapidity with which the model worked was very remarkable. The speed was usually kept down to about 150 strokes per minute, although there seemed no reason to doubt that it would make 1500 strokes, if the
experiment were ventured on.
It was instructive, as showing how rapidly air might be heated and cooled.

In 1826 Lieutenant (now Captain) Ericsson, a native of the province of Vermeland, in Sweden, came to England for the purpose of introducing an air-engine, which he had invented ; but, after using every exertion, the difficulties were found to be too great, and the project was soon abandoned.
He tried a long series of experiments upon a machine of considerable dimensions at the works of Messrs. Braithwaite and Co., and placed on record in the archives of this Institution the results of his invention.

In 1850 a modification of the system was patented by Captain Ericsson, under the name of Edward Dunn, and that machine, as now improved, was, it appeared, now erected in the caloric ship "Ericsson."

The Ericsson caloric engine

The general construction of the modern caloric-engine, as given in the descriptions transmitted to this country, was a working cylinder, beneath which a fire was made to elevate the temperature of the air within it. Under the piston of the cylinder a vessel was fixed and filled with non-conducting materials, called the heat-interceptor, its office being to prevent the heated air from coming in contact with the main body of the piston, and thus to keep it cool - an expedient which was adopted by Sir George Cayley for the same purpose, in connexion with his air-engine, and patented by him in 1838.

Inverted above the working cylinder was the air-pump, or supply-cylinder, with its piston and valves, the relative areas of the upper and lower cylinders being as 2 to 3. Air being taken in through one valve from the atmosphere, and discharged through another valve into the receiver, placed at the side, and communicating with the top of the upper cylinder and the bottom of the lower, or heating-chamber, by the slide-valve, which could be closed at pleasure.

When the slide-valve was closed, to prevent a communication between the receiver and the working cylinder, it, at the same time, opened a communication between the working cylinder, and the atmosphere under the valve, and through an opening into the regenerator, consisting of a number of layers of wire gauze, through which the air had to pass, both in its entrance to, and exit from, the cylinder.

The modus operandi of the engine was this : the engineer, having seen to the lighting of the fires, permitted the temperature of the lower part of the cylinder and the parts adjacent, to reach a temperature of 540° ; then closing the slide-valve, he proceeded, with a hand-pump, to force atmospheric air into the receiver, until it had attained a pressure of some few pounds to the square inch. These arrangements completed, everything was ready for starting. He then, by the aid of such hand-gear as was usual in steam-engines, threw the slide-valve into such a position as would allow the compressed air to make its way into the working cylinder.

Imagining, then, everything to be perfectly cold, regenerator, fire, supply-cylinder, all removed ; still, upon opening the slide-valve (as before described), the piston in the working cylinder would be elevated. When it had reached the top of its stroke, let the slide-valve be closed : no more air could then enter, and that already within the cylinder would make its escape under the slide-valve into the atmosphere, and thus allow the piston to descend by its own weight.

When it once more reached the bottom of the cylinder, if the slide was again opened, there would be a repetition of the action just described ; and this might evidently be repeated, as long as the necessary supply of air could be maintained in the receiver, precisely as a high-pressure steam-engine continued to work, whilst steam was supplied from the boiler.

But since as much air must be supplied to the receiver, at each stroke of the engine, as was removed by the working cylinder, if the air in that cylinder were of the same temperature as the air supplied to the receiver, a pump must be employed for the purpose as large as the cylinder itself, and more power would, in consequence, be required to work it, than the compressed air could possibly give. By the application of heat, however, every cubic foot of air, forced in by the pump, was doubled in volume, consequently the power required to work it was reduced, because a pump of smaller size answered the same purpose, and thus a surplus of available power was obtained.

The office of the heat, then, was not to increase the pressure of the air, when once it had entered the working cylinder (this, under the circumstances, it could not do), but, in reality, to increase its volume with the elevation of the large piston, thus making the same quantity of air fill double the space, and yet keep up the original pressure of the cold air pumped into the receiver.

The regenerator

The 'regenerator', a very important feature in the contrivance, remained yet to be spoken of. As before stated, it consisted of a series of layers of iron-wire gauze, through the meshes of which the heated air had to pass, in its passage from the cylinder to the atmosphere, and which, as it came in contact with each successive layer, parted with more and more of its heat, until, before it quitted the last, it was said to be not more than about 30° or 40° hotter than the atmosphere itself.

The heat left by the escaping air was supposed to be distributed in the regenerator, in such a manner, that next the cylinder the temperature was almost equal to that of the air within, whilst next the slide-valve it was but little above that of the external air. The heat, thus taken up by the regenerator, was retained by it, until the piston, having reached the bottom of the cylinder, a fresh supply of air was allowed to enter it from the receiver ; this cold air, in its passage through the regenerator, became heated, as the escaping air became cooled, by contact with the metal surfaces, and consequently entered the cylinder with its temperature considerably elevated, requiring, in consequence, so much less heat to be imparted by the fuel employed.

It was, however, contended by the opponents of the system,that the action could not be such as was stated, and that the regenerator could not make any difference to the amount of fuel employed It was to be regretted, that those who took this view had not yet offered any adequate philosophical reason for their opinion, because, when once it was fairly shown, that there was some natural law opposed to it, the question was settled, and much valuable time and money would be saved.

In the absence of this, the question must be examined pro and con, as it was now popularly presented.

The air in the cylinder had a temperature of say 540°, i. e., about 480° above that of the atmosphere, at which temperature, according to Dulong and Petit, its original volume was doubled. Upon escaping into the atmosphere, through the regenerator, from the fact of its having been employed in the cylinder under pressure, it expanded, and in so doing took up a certain amount of heat in the latent form (about 8 degrees for every pound of pressure).
For instance, if the air in the cylinder had a temperature of 540°, and a pressure of 5 lbs. per inch, above that of the atmosphere, upon expanding to the pressure of the atmosphere, it would exhibit a temperature of 500° only, the 40° having become latent.

Now since the air could not, by contact with the metallic surfaces, make them hotter than itself, it would at once appear, that these 40° of heat were lost, but the 500° remaining were tangible heat ; which, for comparison, might be likened to that of hot iron, - heat which could be measured by a thermometer, and which would pass from one body to another by contact.

If then the layers of wire gauze, through which the air had to pass, were of a low temperature, this air would assuredly pass out from it at a lower temperature than it entered. And it could not be doubted, that if cold air was made to pass through the hot gauze, its temperature would be raised, and that so much heat as it received in this way, so much less would the fuel have to give. In reference to either the quantity, or pressure of the air in the cylinder, it would be seen, that it made no difference whatever, whether it was heated by the regenerator, or by the bottom plates of the cylinder itself.

The question might arise, however, whether the air, in passing through the gauze at the rate of 12 feet per second, had time to yield up, or to take up the heat in any quantity ; in reference to this, the rapidity with which the air was capable of being heated and cooled in Parkinson's engine, had been already referred to, and many other examples of a similar character were familiar to engineers.

Objections to hot air engines

The practical objections to the general arrangement of the engine were numerous, and of such a character as, even supposing all others to be removed, would doubtless prove fatal to its ultimate success. For instance, the theoretical effect due to the air in this engine - supposing it to make ten strokes per minute, and its volume to be doubled by heat - was 763 HP. Its actual effect in the working cylinder should be 6OO HP ; and out of this, 374 HP would be required to work the pump, leaving available only 226 HP, or less than one-third of the power due to the air.

Again, from the fact of the pressure obtainable being small, and operating only upon one side of the piston, it became necessary to have an enormous extent of piston surface. In the 'Ericsson,' (Ericsson ship) for instance, four engines, such as that described, were employed with working cylinders 14 feet in diameter ; also four air-pumps, or supply cylinders, each between 11 and 12 feet in diameter, creating a vast amount of friction and liability to leakage ; and in a case, be it remarked, where leakage was of the utmost importance.
It was not possible, as in a steam-engine, to push on the fires to overcome the difficulty, as such a proceeding, carried beyond a certain point, would evidently be fatal to the heating-vessels.

Mr. Stirling was compelled to abandon his engine, chiefly on account of the difficulty experienced in preserving his air vessels. The plan which Captain Ericsson at present employed for heating them, was the same as that which Mr. Stirling commenced with thirty years since, and which he afterwards greatly improved upon, and yet failed. There was then no apparent reason for believing, therefore, that, by the present arrangement, Captain Ericsson could be more fortunate than his predecessor.

At the meeting of the British Association, at Liverpool, in September1854, Mr. W. J. Macqnorn Rankine, Assoc. Inst. C.E., read a paper "On the means of realizing the advantages of the Air Engine ;" in which were explained the fundamental laws of the mechanical action of heat, and their application to determine the efficiency of theoretically perfect engines, working between given limits of temperature ; then the " various causes of waste of heat and power in steam-engines were classified, and the actual efficiency of steam-engines was compared with their maximum theoretical efficiency, and also with the maximum actual efficiency, which might reasonably be supposed to be attainable in the steam-engine, by means of any probable mechanical improvements.

The causes of waste of heat and power in air engines, were then classified in a manner analogous to that applied to steam-engines ; and the actual efficiencies of those previous air-engines, as to which satisfactory experimental data had been obtained, namely, Stirling's engine and Ericsson's engine of 1852, were compared with the efficiencies of theoretically perfect engines, working between the same limits of temperature, the results, so far as they related to the consumption of coal of the specified quality per horse-power per hour being:

Consumption of a theoretically perfect engine :

  • Stirling's engine: 0.73 lbs.
  • Ericsson's engine: 0.82 lbs.

Actual Consumption :

  • Stirling's engine: 2.20 lbs.
  • Ericsson's engine: 2.80 lbs.

Thus showing that an air-engine had actually been made to work successfully, and economically.

A description was then given of the improved air-engine of Messrs. J. B. Napier and W. J. Macquorn Rankine. In this engine, the heating surface was increased to any required extent, by means of tubes employed in a peculiar manner. The waste of heat, by its communication to the air, at improper periods of the stroke, was prevented by a sort of plunger called the heat-screen, which prevented any access of the air to the heating surface, except when it was in the act of expanding, and so performing work.

The engine might be made of the same size with a steam-engine of the same power, or smaller, according to the degree of condensation at which the air was employed. Independently of the amount and value of the saving of fuel, which would result from the introduction of the air-engine, it possessed the important and incontestable advantage, that even should an air-receiver burst (which was very unlikely), the explosion would be harmless, for its force would not be felt beyond the limits of the engine itself, and hot air did not scald. In the Mechanic's Magazine of October 21, 1854, there is given a diagram and short description of this engine.

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