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
Mr. Goldswortiiy Gurney said it was well known, how long Sir G. Cayley's attention had been devoted to the subject of using heated air as a motive power ; indeed the air-engine might be said to have originated with him. Sir George Cayley was prevented by indisposition from coming to London, to be present at the discussion, but he had transmitted to the Secretary, a few remarks, which had been understood to have received the approval of the Council, and Mr. Gurney would therefore ask that they might be read to the meeting.
The Secretary, by permission of the President, then read the following:
Remarks on the Use of Heated Air as a Motive Power by Sir George Cayley, Bart., Assoc. Inst. C.E.
The free power of Captain Ericsson's engine, arises from the difference of the areas, of the hot and the cold pistons ; both receiving, in opposition to each other, during their combined stroke, an equal average pressure per square inch, from an equal head ; but the conditions of the case are such, that neither piston receives a uniform pressure during the whole of its stroke.When the cold air is doubled in bulk, by a temperature of say 530°, a head of 15 lbs. per square inch is obtained ; and this head must be kept up for the uniform working of the engine : hence, (this head having been previously generated, for starting the engine, by injecting water on the tire, or by mechanical condensation,) only an equal weight of hot air can be permitted to escape at each stroke, with that of the cold air driven in.
The cold pump, working against a head of 15 lbs. per square inch, will move half its stroke, gradually condensing the air within its cylinder, at an average of about 74 lbs. It then proceeds for the other half of its stroke, against the head of 15 lbs. ; averaging ll-251bs. to the square inch, for the whole stroke.
On the other hand, the full pressure of 15 lbs. per inch is allowed to operate on the hot piston, for the first half of its stroke, when the supply from the head must be cut off ; and the remainder be performed, under the gradual diminution of the pressure, by expansion. So that at the end of the stroke the air passes out at par with the atmospheric pressure. Here, as in the former case, half the stroke is done at 15 lbs., and the remainder at 74 lbs., averaging ll-25 lbs. per square inch, as before. Thus the two pistons keep the same ratio, in the average of their strokes, though in the reverse order, as to the increase and decrease of the pressure upon them. Hence, as before remarked, the free power of the engine is as the difference of the areas of the pistons, multiplied by the average pressure upon them (not by the full pressure of the head) minus the friction on both : and again by the number of strokes per minute, and the length of each stroke.
From whatever source the heat be supplied, that expands the cold air to double its bulk, the expansion must have the same dynamic effect. Hence the case of the regenerator, as applied by Captain Ericsson, is reduced merely to the following consideration. Does caloric, generated by combustion, differ from itself, after being applied to a solid metallic body ? Suppose a mass of red-hot iron taken out of a fire, to be put into a vessel full of cold air, will it, or will it not expand it just in proportion to the heat communicated by conduction ? By what process do steam-engines work, but by iron plates, heated by fire on one side, and transmitting that heat to water, and its steam on the other ?
The tendency of heat, is, with reference to mere temperature, ever to equalise itself with all contiguous bodies, according to their specific demands ; and this can only take place, by its being communicated from one body to the other.
Steam-engine power to some extent depends upon the heated air in the fire-flues, being cooled by the iron sides, or pipes of the boiler, in generating the steam ; thus the heat of the air, from combustion, is transferred, and being so transferred, it creates power. In what then does this case differ, in principle, from the metallic meshes of Captain Ericsson's engine, heated by a similar flow of hot air ?
There can exist no doubt of the effective re-application of heat to an almost unlimited extent, by this beautiful invention, due originally to Mr. Stirling, and now carried out to a greater extent by Captain Ericsson.
The approach to perpetual motion by this repetition of the action, has been considered as a refutation of the principle ; but it must be recollected, that a principle of perpetuity pervades some of the most simple acts under natural laws. When a billiard ball is propelled on a smooth horizontal plain, if it were not for the resistance of the air and friction, the law of its nature would cause it to roll on for ever. The slight escape of heat from the imperfect non-conductors used in the caloric engine, resembles the friction that prevents the action of the rolling ball from being perpetual.
A perfect non-conductor of heat, would render the case as nearly interminable, as the very slight loss of it, by the current of air, in its exit, not having had full time to divide its ultimate extra caloric with the last series of meshes, would permit.
There are, as has already been observed, several difficulties attending the construction of air-engines. The main one is the bulk of the cylinders, and other parts of the engine to make good the small pressure per square inch, that can be obtained without extra condensation, which is objectionable on account of the leakage it occasions.Another objection, is the heat the pistons may be exposed to, without considerable precautions being taken to prevent it.
The dust from the coke was, in some constructions of the first air-engines, where the whole heat generated by combustion, was received by the air in an air-tight generator, found inconvenient for the sufficient lubrication of the pistons, although in a great measure shielded from the heat, by the plungers. It may prove difficult to combine the economical production of heat, by consuming the fuel in an air-tight vessel; with the principle of its continued application by the regenerator, as the fine wire meshes may get so clogged up by the loaded vapour, or, external oxidation, as to impede that rapid absorption and restoration of caloric, on which their efficacy depends ; but it must be hoped that even such a primary result, will not eventually prove beyond the remedy of chemical science, applied with mechanical skill ; and should this combination be ever practically carried out, the consumption of fuel in the air-engine will be reduced to an infinitesimal quantity.
The attention of some of the members of this Society, will therefore not be ill-directed, when applied to the utmost practicable improvement in the means of using air as a motive power.
For locomotive purposes, on shore, it seems probable, that steam and air can only come into competition with each other, by their expansion - the weight of water necessary for the condensation of the steam ; and the slowness with which air can be cooled, render both inapplicable, by condensation, for speedy locomotion.
When water has been converted into steam, from its mean temperature, by the application of 1127° of heat, it may then, if the supply be cut off, be doubled in volume, under atmospheric pressure, at a temperature of 692° (212° + 480°), a very inconvenient heat for any piston, and if steam has to be brought up to this temperature, by an external fire, and its flues, the whole of the air heated by combustion must part from the boiler, under the best construction, at a temperature rather exceeding 692° ; a heat considerably greater than that required to work the air-engine, is, therefore, in this case, totally wasted.
But steam will exceed atmospheric pressure (15 lb. to the square inch), when remaining in contact with the boiling water, at a temperature of 250°, when its density is very nearly doubled, and the caloric required is increased in much the same ratio ; it is, therefore, by far the most economical way to form steam, required expansively, by using the air-tight generator, and driving the whole heated air, in streamlets of minute bubbles, rising through the water. The difficulties, as to the dust and the heat of the pistons, stated to have been experienced, only had reference to the engine of 5 H.P, then under notice at [Millbank].
The report of the meeting of June 10, 1845, is incorrect in two points. It is there stated, that the slide-valves were torn by the dust (which was correct), and that the conical valves, though, in some degree, avoiding the evil, were considerably abraded, also the passages, the pistons, and the cylinders were destroyed by the heat and dust. It is true that the pistons were injured, but neither the cylinders, nor passages, nor conical valves were hurt. It is stated also that the evil of the heat of the piston was attempted to be obviated by appending a drum "below the piston ;" now this drum, or plunger, was placed above the piston in every engine made, and therefore, could not have been spoken of as being below it.
A one horse-power engine, previously made, had very little, if any, of these defects, and it was from two accidental circumstances a much better engine, though prior in the order of construction. The hot cylinder, being made of sheet copper, which cools with double the rapidity of iron, and much more than double in this case, from the quadruple thickness of the iron ones, kept the working part of the piston from injury, aided by the plunger ; and the more than double area of the generator, in proportion to the air driven through it in a given time, nearly cured the evil of dust ; it had also conical valves, which were quite uninjured.
From circumstances distinct from the nature of the evils the Millbank engine had shown, and which probably a water jacket would have much remedied, no further experiments were made with it.
The one-horse engine, when examined by Mr. Gurney, worked perfectly well for several days in succession, while being tested by friction both to ascertain its power and its consumption of fuel. It is no bad omen that so small an engine, where there was so much more heated and rubbing surface, in proportion to its power, than in larger engines, without any attempt to economize heat by non-conducting covers, should have produced a horse-power by the expenditure of 7 lbs. of coke per hour. It is right to state these matters, that a true view of the case may not be wanting, towards a further experimental investigation of the subject. The exact particulars of this engine are these :
In the single-horse engine ;
- the cold piston 10.5 inches in diameter ;
- the hot one, 13.5 inches, both a foot stroke ;
- the fire place 10 inches in diameter, by 11 deep, within a generator containing about 20 cubic feet of air ;
- head of pressure, by mercurial gauge, from 8 to 9 lbs., made ninety strokes per minute, that is, ninety revolutions of the fly-wheel.
The weight of 550 lbs., raised one foot high, per second, being taken as the test of one-horse power.The air, or mixed air and steam engines, with the internal combustion of the fuel, may be made to imitate animal muscular power, in as instantaneous and violent an increase of action, by a small jet of water on the fire ; this instantly produces the effect for a limited time. Oil of tar, or other inflammable fluids, might perhaps prolong this action considerably.
Although the experimental air-engines have not had the advantage of Mr. Stirling's, or Captain Ericsson's renovators of heat, yet having made use of all the heat generated by the combustion taking place within the reservoir, containing the head of expanded air, very much compensates for that deficiency, and is undoubtedly the most economical way of communicating heat to the air.
Theoretically, according to present chemical data (at the particular degrees of caloric and condensation specified), it would require about one pound and a half of dry charcoal to sustain one horse-power for an hour, by the expansion of air, were there no loss by external cooling : and although the products of this internal combustion may render the application of Captain Ericsson's wire-gauge meshes more difficult ; yet the economy in generating the caloric is so great, as will probably render this ultimate refinement of the engine of more cost and trouble than advantage, in a mercantile point of view.
Engineers should be induced to carry out the internal combustion principle, at present, and subsequently to test the Ericsson principle, in connexion with it. The plunger, which is almost an indispensable part of the air-engine, as by its means the working portion of the piston may be kept at any required moderate temperature, deserves some notice ; as experience has proved, that hitherto it has never been taken advantage of to the proper extent. It was first thought of and applied by Sir G. Cayley to the engine made in the year 1837. It was not, then, perhaps, longer than half the stroke ; in the subsequent engines it has been elongated to be rather more than the length of the stroke ; but although it is a troublesome and weighty appendage, there is reason to believe, that it would be advantageous to make it double the length of the stroke.
Mr. Goldsworthy Gurney resumed :
It was almost needless to enter into the question of Sir G. Cayley's engine, after the remarks that had been read. It was however incumbent on him to corroborate their correctness. In the improved engine there was little inconvenience from the dust, and after the conical valves were adopted there was little trouble from abrasion. The consumption of fuel was about 6 lbs. of coke per horse-power per hour.
The destruction of the heating vessel was the principal difficulty ; the same radical defect must exist in Ericsson's engine, as the plates must be heated to upwards of 500°, or else the air would not attain the necessary temperature. The difference of area between the two cylinders would be found a practical difficulty, and unless some modification of the machine was adopted, something like cutting off the steam at a portion of the stroke, the available effect of the expansion of the air by heat, would be in a great measure lost.
The action of the regenerator, rapidly absorbing and giving out heat, was very remarkable, and afforded an important auxiliary.
Considered as a whole, the arrangement devised by Sir George Cayley was, Mr. Gurney thought, superior to that of Captain Ericsson. In Sir G. Cayley's engine, the air was heated by actual contact with the incandescent fuel. The destruction of the heating vessel, common to all air-engines heated externally, was therefore avoided in this engine, where the combustion was confined to an internal fire-place, much modified, and in which any injured part could be readily renewed.
To use economically the power of air expanded by heat, the supply must be cut off from the power cylinder, as fully explained in the letter just read. The early accounts were written and published in the 'Philosophical Journal' for 1817, several years after the first experiment had been tried at Newcastle, where the engine was so ill-executed and leaky, as to show no free power.
All the principles since applied in the subsequent engines, excepting that of keeping the working piston cool by a plunger, were fully developed in the diagram of the engine then given—even the arrangement adjusted by Capt. Ericsson of placing the cold-air pump, and hot-air power piston, on the same rod, were shown. The plunger, which was almost a sine qua non in every air-engine, was employed in the one-horse power engine made in 1837.
Mr. H. Maxwell Lefroy stated, that his attention having been directed to this subject by Mr. Gordon, he had made an investigation of the quantity, volume, and elastic force, of the gases into which 1 lb. of Jones' anthracite coal was decomposed by combustion, on the assumption that the whole caloric developed by the combustion of the coal should be received and retained by the gases.
The MS. of this portion of the discussion was corrected by Mr. Lefroy, but in consequence of his departure from England it was not possible to submit the proof to him ; the calculations, therefore, remain as in the original MS.
The constituents of 1 lb. of this coal, as given in Sir H. De la Beche's Report on Steam Coal, were:
One part of carbon combining with 2.66 parts of oxygen, the carbon above given would be converted into 0.9144 {1 + 2.66} = 3.346 lbs. carbonic acid.
One part of hydrogen combining with 8 parts of oxygen, the hydrogen above given would be converted into 0.0344 {1 + 8} = 0.3114 lbs. of steam.
Twenty-one parts of oxygen combining with 78 parts of nitrogen in the atmosphere, the nitrogen, combined with the oxygen which would be required for the combustion of the carbon, would be 0.9144 x 2.66 x 78/21 = 9.055 lbs.
Similarly, the nitrogen combined with the oxygen required for 78 the combustion of the hydrogen, would be 0.0346 x 8 x 78/21 = 1.029 lb.
Therefore the total nitrogen, which would be forced into the closed furnace, in combination with the oxygen required to support the combustion of the carbon and hydrogen, in 1 lb. of that coal, was 9.055 + 1.029 = 10.084 lbs.
13,268 units of caloric were developed by the combustion of 1 part of carbon.
Also 62,470 units of caloric were developed by the combustion of 1 part of hydrogen.
Therefore the heat developed by the combustion, that is, the conversion into carbonic acid and steam, of the above given quantities of carbon and hydrogen, would be 0.9144 x 13268 + 0.0346 x 62470 = 14092 units.
By unit of caloric, throughout this investigation, was meant the quantity which would raise the temperature of 1 lb. of water from 39° to 40° Fahrenheit, that is, by one degree measured at the temperature of maximum density.
It was assumed that:
- The specific heat of steam is: 0.8470
- The specific heat of nitrogen is: 0.2754
- The specific heat of acidis: 0.2210
- The volume of 1 lb. of steam: 27.66 cubic feet
- The volume of 1 lb. of atmospheric air:13.02 cubic feet
- The volume of 1 lb. of nitrogen: 13.1 cubic feet
- The volume of 1 lb. of carbonic acid: 8.36 cubic feet
- The volume of 1 lb. of Jones' anthracite coal: 1 cubic foot
The pressure being constant, the relation of volume and temperature of a constant quantity of any gas, was defined by the formula V1 = ((459 x t1)/(459 x t2)) x V2 where V1 t1 V2 t2 respectively were corresponding values of the volume and temperature in degrees of Fahrenheit.
As the carbonic acid, nitrogen, and steam occupying the same space (the closed furnace) would have a common temperature, the 14092 units of caloric must be absorbed by these bodies, in the proportion of the products of the quantities of the bodies and their specific heats, respectively.
Therefore, if X = the caloric received by the nitrogen, (X(3.346 x 0.2210 )) / (10.146 x 0.2754) = 0.2649X, would be the caloric received by the carbonic acid, (X(0.3114 x 0.8470 )) / (10.146 x 0.2754) = 0.0946X the caloric received by the steam.
But the sum of these quantities of caloric was 14092 units.
Therefore X(1 + 0.264 + 0.0946) = 14092, from there X = 14092 / 1.359 = 10375.
Therefore the caloric received by the carbonic acid, = 0.256X = 2746, and the caloric received by the steam, = 0.094X = 979.
That is the caloric received by the nitrogen would raise its temperature 10434 / (10.1426 + 0.2754) = 3735° Fahrenheit.
That of the carbonic acid and steam would be increased by the same quantity.
If the temperature of the external air be 60° and it be pumped on under a pressure of three atmospheres, its temperature deduced from the formula V1 / V2 = (459 + t1) / (459 + t2) would be 1098°.
Then the volume of the nitrogen in the furnace under pressure 45 lbs. per inch, and temperature 1098° + 3735° would be 10.146 x (13.1 / 3) x (459 + 1098 + 3735) / (459 + 1098) = 10.146 x 4.37 x 5292 / 1557 = 150.4 cubic feet.
Under the same conditions the volumes of the carbonic acid and steam would be 3.346 x 8.36 / 3 x 5292 / 1557 = 31.53, and 0.3114 x 27.66 / 3 x 5292 / 1557 = 9.73 cubic feet respectively.
Therefore the aggregate volume, under these conditions, of the nitrogen, carbonic acid, and steam in the furnace, would be 150.4 + 31.53 + 9.73 = 191.66 cubic feet.
Now the oxygen in combination with the carbon and hydrogen being 0.9144 x 0.266 + 0.0344 x 8 = 2.4323 + 0.2752 = 2.7075 lbs., and the atmospheric air holding it = 2.7075 x 100 / 21 = 12.89 lbs. the volume of this air under pressure 45 lbs. per inch and temperature 1098° would be 12.89 + 13.02 / 3 = 55.909 cubic feet.
Therefore the net volume of the gases, derived from the combustion of 1lb. of this coal, theoretically available as a mechanical force, was 191.66 — 55.90 = 135.76 cubic feet, under pressure 45 lbs. per inch, temperature 4833°.
If this body be allowed to expand, till its pressure be reduced to 15 lbs. per inch and temperature to 60°, the volume would become 135.76 x 3 x (459 + 4833) / (459 + 60) = 135.76 x 3 x (5292 / 519) = 4153 cubic feet.
The mechanical effect due to the expansion of 1 / 85.78 (the volume of 1 lb. of this coal) into 4153 cubic feet, was (17.28 / 85.78) x 15 x 4153 x 85.78 x 2.721 / 12 = 8,979,842 lbs. raised 1 foot.
To compare the mechanical value of these gases, with that of the steam derived from the combustion of the same quantity of coal, upon the assumption that the whole caloric developed by the combustion, was transmitted into the water, it should be mentioned, that the quantity of water, upon the above hypothesis, convertible into steam at 212° by 1 lb. of this coal, as given by Sir H. De la Beche, was 13563 lbs.
Consequently, the weight of one cubic foot of water being 62.5 lbs., atmospheric pressure 14.75 lbs. per inch, and the volume of the steam 1700 times that of the water, the duty due was, 13563 / 625 x 14.75 x 1700 x 144 = 783,399 lbs. raised 1 foot.
In each case the use of condensation was supposed to be omitted.
(Continuation : see here)