The Theory of the Gas Engine

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Appendix

DATA USED IN THE PAPER ON * * THE THEORY

OF THE GAS ENGINE.

17.6 foot-lbe.

24.8

Specific heat of air at

constant volume.

Specific heat of air at

constant pressure Mechanical eqivalent )

of heat foot lbs. [-=1389.6

Centigrade )

Specific heat of air at^

constant volume

in foot lbs. for 1

cubic foot at 17"

C. and 760 mm.

barometer

Specific heat of air at

constant pressure

in foot-lbs. for 1

cubic foot from mm ^

Weight of 1 cubic ft. )

and 760 mm )

Burning completely in oxygen, the following substances are taken as evolving the noted amounts of heat in Centigrade units, per unit weight of substance burned.

Hydrogen 34,170

Carbon 8,000

Carbonic oxide 2,400

Marsh gas 13,080

defiant gas 11,900

DISCUSSION.

Mr. D. Clerk mentioned that Dr. Siemens had worked out the irethod of compression used in engine type 2 in 1860 in so complete a manner that no advance had since been made on it by any one. Dr. Siemens was again working at this type of engine, which, from the fact of it using hot cylinder and regenerator, Mr. Clerk was certain was the best type for the very large gas engines to be developed in the future. With respect to the cold cylinder engine, of which alone he had treated in the paper, he wished again to insist on this : that the theory which sought to explain the so-called sustained pressure on the indicated diagram by the hypothesis of slow inflammation (erroneously termed slow combustion) was a false one. That when maximum pressure was attained in the gas engine cylinder it was certain that the whole mass was completely infiiained, and that no system of stratiflcation producing slow inflammation could do good, but was quite opposed to the conditions of economy.

Dr. Siemens said that one part of the paper dealt with matters regarding the mechanical arrangement of gas engines, and the other with a theoretical question, that of the law of combustion. He would refer to the theoretical part first, because the author appeared to attach great importance to it, and as Dr. Siemens had from time to time given a great amount of consideration to the action of negative combustion or dissociation, it might be of some interest to the members to see how far his views fell in with those set forth by the author. It was well known that by combustion no unlimited degree of temperature could be attained. Thus, in a furnace worked at very high temperature the fuel was not completely burned when it came in contact with the oxygen of the heated or non -heated air. The moment a certain comparatively high temperature was reached the carbon refused to take up oxygen, or the hydrogen refused to take oxygen, and what had been caUed by Bunsen, and, shortly after him, by St. Claire Deville, dissociation, arose. The point of dissociation was not a fixed one ; partial dissociation came into play at a comparatively low temperature, and went on increasing at a higher temperature in very much the same ratio as vapor density increased with temperature. Thus, if aqueous vapor were passed through a tube at a sufficient temperature the whole of the vapor would be dissociated, and the oxygen and the hydrogen would be separated. It was true, if these gases were left to themselves they would, the moment the temperature lowered again associate or bum ; but if precautions were taken to cool them rapidly after they had attained that high temperature they would be found as a mixture of oxygen and hydrogen simply.

The author had stated that the law which governed these actions was not well known and required research, but Dr. Siemens would like to know whether he was aware of the researches of St. Claire Deville on the subject. It might be that the determinationB of St. Claire Deville were not quite correct, but in the meantime they might be regarded as being so. He found that at atmospheric pressure the point of half dissociation of aqueous vapor arose at a temperature of 2,800° Centigrade, and that of complete dissociation at a much higher temperature. Taking that law as determined by the French philosopher, it did seem reasonable to suppose that when a mixture of hydrogen and oxygen, with or vdthout a mixture of nitrogen exploded, the point was reached beyond which the temperature did not increase, and, according to the author, that point was 1,500° Centigrade. If such a temperature was reached in a working cylinder complete combustion would not take place immediately, but only partial combustion would occur, which would go on as the temperature diminished by absorption into the cylinder or by expansion, and that combustion would be completed only in the course of the stroke. In that way the action which had been described with reference to the diagrams was reasonable enough. With regard to the mechanical arrangement of gas engines, the author distinguished between three types. In the first, the mixture of gas and air drawn in at atmospheric pressure was exploded. In the second, with which the author had connected his name as that of the first proposer, the combustion was produced gradually ; the gases were ignited as they flowed into the heating cylinder.

In the third type, the gases, after being compressed and mixed, were admitted into the working cylinder, and suddenly exploded. With reference to the early engine which Dr. Siemens constructed in 1860, the author had stated that it combined other elements, which were entirely wanting in the gas engines of the present day. The gas engine of the present day, taking either of the three types, was, in his opinion, in the condition of the steam engine at the time of Newcomen. The fuel was burnt in a cylinder which it was attempted to keep cold by passing water over it, and it was easy to conceive that the heat so generated, was only partly utilized for maintaining the state of expansion of the heated gases, the cold sides of the cylinder taking a good half of it away at once, thus causing a great loss. Then there was another palpable loss in these engines. After expansion had taken place, after half the heat had been wasted in heating a cylinder which was intended to be kept cool in order to allow the piston to move, the gases were discharged at a temperature of 1,000°, or in the best types about 700°. That amount of heat, representing in one case one-half and in the other two-thirds of the total heat generated, was thrown away. This was heat which could be saved and made useful. Instead of commencing the combustion at a temperature of 60°, if the heat of the outgoing gases were transferred to the incoming gases, combustion might commence at a temperature of nearly 1,000°, and the result would be a very great economy. In the engine which he constructed in 1860

(Fig. 13) all those points were fully taken into account. The combustion of the gases took place in a cylinder without working a piston, and in a cylinder that, could be maintained hot, and the gases after having completed expansive action, communicated their heat by means of a regenerator to the incoming gases before explosion took place. Although the engine was not worked with ordinary gas used for illumination, but by a cheaper kind made in a gas producer, he then thought that a gas engine constructed on that principle would prove to be the nearest approach to the theoretical limits which could never be exceeded, but which might exceed the limits of the steam engine four or five fold. The engine promised to give very good results, but about the same time he began to give his attention to the production of intense heat in furnaces, and having to make his choice between the two subjects, he selected the furnace and the metallurgic process leading out of it ; and that was why the engine had remained where it was for so long a time. But now the time had come when there was a greater demand for engines of a smaller kind to do their best in houses and in small works, and when marine engineers especially had become fully alive to the importance of more economical arrangements. He therefore looked upon the question before the Institution as one of first importance to engineers, and he hoped that it would be well discussed.

Professor Buceeb said that in his work on Thermodynamics, Mr. Verdet had published a calculation of the theoretical efficiency of an ideal gas engine. He assumed that no heat was lost through the sides of the cylinder, and that the explosion was so sudden that the whole of the gas was inflamed before the piston had appreciably moved ; and under those circumstances he found that if the gases used were carbonic oxide, and a sufficient quantity of air to bum it completely, and if the whole of the carbonic oxide was burnt, the temperature to which the gases would rise, on the assumption that their specific heats remained constant, was 4,388° Centigrade. He found that the pressure would rise from 15 lbs. per square inch to 215 lbs., and that the efficiency of the engine would be 41 per cent. — that was, that 41 per cent of the total amount of heat produced by combustion of the gas would be converted into useful work. It was evident from the conditions of Mr. Yerdet's problem that that was a purely theoretical calculation. The condition, for instance, that no heat was lost was one which could not be realized in practice. About four years ago, however, in the course of a series of lectures given by some of his colleagues and himself on coal, he pointed out that Mr. Verdet's calculation was not even theoretically correct; that Bunsen had proved that it was impossible that a mixture of carbonic oxide and air could reach such a temperature as 4,388° Centigrade, which was something like 2,800° above the highest temperature, which Berthelot had shown was consistent with Bunsen's experiments on the subject.

The question then arose what the effect of dissociation would be upon the gas engine, and Professor Rucker attempted to make a rough calculation to show how important it might be. In the first place, he assumed that the highest temperature which could be reached was that given by Bunsen's experiments, and in the next that the specific heats were constant and the inflammation instantaneous. With those conditions only about one-half of the carbonic oxide would be burned when the highest temperature was reached; then, as the piston began to move forward and the temperature fell, more would be consumed. But then there was the very important question as to how the temperature would fall, and in order to calculate that the law of cooling of a body heated to that extremely elevated point must be known. That, of course, he was ignorant of, and he was therefore obliged to make a rough assumption. Assuming that^ as the piston moved forward, the gas burned so as to keep the temperature constant, he found that at the end of the stroke, when the pressure had faUen to that of the atmosphere, a part of the gas was left still unconsumed. Therofore in the half of the gas left unbumed to begin with, there was sufficient to do all work that was done while the piston was moving forward. The only assumption he could make was that the temperature remained constant ; any other, though that certainly was not true, would have involved some still more arbitrary hypothesis as to the law of cooling. Making, then, that rough assumption, he found that instead of a temperature of 4,000° Centigrade the highest reached would be about 2,0u0°; that the pressure, instead of rising to 215 lbs., would rise only to 103 lbs.; and that the efficiency of the engine would be only 25 instead of 41 per cent. That, though a very rough calculation, showed at once what the enormous importance of the phenomenon of dissociation might be. It served the purpose for which it was put forward, and showed that in any theory of the gas engine physicists must make up their minds as to what part dissociation played in it. Passing from the theoretical problem to that Mr. Verdet and himself discussed, namely, the case in which there was only enough air to bum the carbonic oxide completely, to the practical problem in which there was a much larger quantity of air present, a case arose in which dissociation was less important. The larger the quantity of air present the lower the highest temperature would be, and therefore, probably, the smaller the amount of dissociation. St. Claire Deville had shown that carbonic. .«cid was dissociated at ^.emperatures between 1,000° and 1,200°, and water at temperatures between 1,000° ^nd 1.100° Centigrade. Inasmuch, therefore, as in the author's engines, the highest temperature reached was about 1,500° (or 400° or 600* above the limits put by St. Claire Deyille), it followed that if his measurement of the temperature was correct.

which there was every reason to believe it was, and if St. Claire Deville's experiments were trustworthy, there was a certain amount of dissociation at the temperatures reached in his gas engine. Passing, however, to the next question, namely, how much dissociation there was, the problem was much more difficult. With regard to that subject a series of papers had recently appeared in the " Comptes Rendus de I'Acad^mie des Science," which were so much to the point that he might be excused for giving a short account of one or two of the leading results at which the experimenters had arrived. The two gentlemen in question were Mr. Mallard (whose experiments on the rate of propagation of inflammation ir gas had been mentioned by the author) and a colleague, Mr. Le Chatelier. They had been making a number of experiments such as those that the author had advocated in his paper. They had made, indeed, what appeared to be one of the first serious attempts to investigate what was going on in gas heated between 1,000° and 1,500® Centigrade. The plan they adopted was as follows : They exploded gases in an iron cylinder, attached to which was a Bourdon manometer ; to that was attached a needle, which registered the pressure on a revolving cylinder. By reading off the curve so obtained, they got information as to the pressure in the cylinder at different times. He could not altogether accept their results without 'further confirmation. Some of the conclusions at which they had arrived were so striking that he thought they must certainly be supplemented by other experiments before they cculd be accepted.

But for the moment he would put aside all difficulties connected with the experiments, and simply state the conclusions. It was found, dealing with gases at very different temperatures, that the curves obtained upon the revolving cylinder showed a point of discontinuity. At the very highest temperatures the curves were somewhat different from what they were at low temperatures, and the aBBumption they made was that at the high temperatnres dissociation had set in, whereas at the lower temperatures there was no dissociation ; therefore the law of cooling would be different in the two cases. If, however, that interpretation of the experiments was accepted, it would be found that the temperatures at which dissociation took place to any considerable extent were higher than those he had mentioned. Thus the authors stated that carbonic acid did not dissociate appreciably below 1,800° Centigrade, and that steam-gas did not dissociate appreciably below 2,000°. Here, then, there were temperatures considerably above those obtained in the gas engine ; if, therefore, the results in question were to be accepted, dissociation could not play a very important part in the matter. But although at first sight the experiments told against dissociation taking place to any large extent, in order to account for the phenomena they observed, Messrs. Mallard and Le Chatelier had had to introduce another hypothesis which practically came to very much the same thing. In all the earlier calculations upon the subject the assumption had been made that the specific heats of the gases were the same at high as at very low temperatures, but within the last few years two or three experimentalists of note had brought forward results tending to show that the specific heat of the gases increased as the temperature rose.

The two most important researches made upon the subject were those by Professor E. Wiedemann and Professor Wiillner, the latter of whom showed that at temperatures between zero and 100^ Centigrade there was an appreciable rise in the specific heat of gases at a constant volume. Messrs. Mallard and Le Chatelier had taken that hint, and they found that in order to explain the facts observed by them on the assumption that there was no dissociation, they must assume an enormous increase in the specific heats of the gases at high temperatures. But there were one or two points which appeared to present diflSQulties iu their way. Wiillner showed that at the temperatures at which he worked, as might be prima facie expected, the increase was much greater in a compound gas Hke water or carbonic acid than in an elementary gas such as oxygen or nitrogen. But Messrs. Mallard and Le Cbatelier completely reversed that, and found that the increase was much greater in the elementary gases than in the compound ones ; and they went so far as to show that oxygen would at a temperature of 1,000° have a specific heat no less than one hundred and sixty -five times greater than that which it had at zero. That result was so astonishing that it could not be accepted without much more proof than had at present been offered. But putting aside for the moment Messrs. Mallard and Le Chatelier's interpretation of the experiments, he wished to consider what they meant from a wider point of view, viz , that those gentlemen had come across a phenomenon which pointed to the fact that a vast, quantity of heat was rendered latent.

If specific heat at constant Tolnme increased, the meaning of it must be that the work done by the heat was done within the molecules of the gas, that the heat was spent in separating or preparing for separation the atoms of those molecules, which were gradually being forced asunder ; whether they were actually forced asunder or not might be a question, but a large amount of work was spent in separating them, or preparing to separate them, by loosening the bonds between them ; and Messrs. Mallard and Le Chatelier's experiments served as much as anything previously hronght forward to illustrate that point. He thought it must be assumed with almost certainty that a large quantity of heat was rendered latent in gases at temperatures between 1,000° and 1,500° Centigrade. All would agree that a certain amount of that heat was spent in dissociation (for Messrs. Mallard and Le Chatelier stated that they harmonized their results with those of St. Claire De- Tiile by supposing that his experiments were more sensitive than their own), and the remainder of the heat would be spent, if not actually in dissociation, in preparing for dissociation. There was one other point in the paper which he thought of interest. The author had pointed out how different the rate of propagation of an explosion woidd be in the case of gaseous mixture which was confined to that in an unenclosed spaca Messrs. Mallard and Le ChateHer had made experiments on that point; they had inflamed gas and air mixture in a tube closed at one end^ and they found that when it was inflamed at the closed end the rate of propagation was much greater than when it was infllamed at the open end. In the one case the gas was merely burning backwards through the tube, in the other the expansion of the gases would spread the inflammation. So enormous was the difference, that in some cases they found that the rate of propagation was one hundred times greater when the gas was Hghted at the closed end of the tube than when it was lighted at the open end. That was a point which strongly confirmed the author's view — that inflammation spread through the gas almost instantaneously. Although, therefore, one could not but feel that on those points there was a great lack of .experimental data, all the facts that were brought together, might, at present, be best explained by the hypothesis that the inflammation spread very rapidly through the gas, and that at high temperatures, say of over 1,000°, a very large amount of heat was rendered latent, either in actual dissociation or in incipient dissociation. Here, then, was an explanation of the curious maintaining of the temperature to which the author had referred. As the gas cooled, the latent heat was given up and the curve was thus kept up to a high temperature by the heat previously absorbed in the molecules of the gas.

Mr. W. R. BousFiELD did not propose to quarrel with the greater part of the facts stated, which were for the most part indisputable, but he thought neither the interpretation which the author had put upon them could be upheld, nor the new and, to most of them, rather startling theory of the action of the gas engine which had been submitted in the paper. He did not say that the phenomena of dissociation played no part in the action of the gas engine ; he did not say that when the explosion took place, there might not be a certain quantity of ammonia and a certain quantity of nitric acid formed, and that the phenomena of dissociation might not take place to a certain extent ; but what he did say was that neither the formation of nitric acid nor the formation of ammonia nor any of the phenomena connected with dissociation could account for the facts mentioned. He would only refer to two of those facts, namely, that notwithstanding the enormous loss of heat through the walls of the cylinder of a gas engine, amounting to 60 per cent, of the total amount of heat put into the cylinder, the curve of the indicator diagram still kept up the theoretical adiabatic line which it should follow, supposing the -whole of the gas were burned at the beginning of the stroke, and the walls of the cyHnder were non-conducting. That was a startling fact which had to be dealt with in one way or another, but the interpretation of the fact seemed to him to be very simple, and even in the paper there were materials for arriving at a conclusion upon it.

The author had stated that a mixture of gas and air took a certain time to ignite, that if ignition was set up at one point it took a certain time before it was communicated to another. There was also the further fact that at the rate of communication of the ignitioD from one point of the dilute mixture to another varied directly with the amount of dilution of the mixture. Supposing for insttiUbe there was a mixture of gas and air in the right proportions for explosion, the ignition would take place at a certain speed ; if more air was put in, the rate would be less ; the greater the quantity, the less the rate at which the ignition traveled. That simple fact he thought sufficient to account for all the phenomena. The diagram which the author had given (Fig. 9) seemed to him, taken in conjunction with the fact to which he had referred, to support the theory which had been put forward by Mr. Otto and by the scientific world in general. In the Otto gas engine the charge varied from a charge which was an explosive mixture at the point of ignition to a charge which was merely an inert fluid near the piston. When ignition took place, there was an explosion close to the point of ignition that was gradually communicated throughout the mass of the cylinder. As the ignition got further away from the primary point of ignition the rate of transmission became slower, and if the engine were not worked too fast the ignition should gradually catch up the piston during its travel, all the combustible gas being thus consumed.

When the engine was worked properly the rate of ignition and the speed of the engine ought to be so timed that the whole of the gaseous contents of the cylinder should have been burned out and have done their work some little time before the exhaust took place, so that their full effect could be seen in the working of the engine. This was the theory of the Otto engine. What was the theory which the author had put forward? He had stated that when gases combined a high temperature was set up ; that a high temperature prevented combination of the gases beyond a certain point ; and therefore, at the moment of ignition, there existed in the cylinder a body of gases heated to a temperature beyond the point of dissociation. A part of those gases being in a state of combination, and having therefore given out a heat which was doing the work of pushing the piston ; a part of the gases, not being in a state of combination, being ready to combine as soon as the temperature was lowered to such a point that they could combine and give out work. Looking at that theory, it seemed as if the point involved was a mere question of words, so far as regarded any question of infringement. In either case, what had to be dealt with was this. The adiabatic line represented the line which was traced out upon the indicator diagram when no heat escaped through the walls of the cylinder, and when the whole heat which the gases lost was converted into work done by the piston ; so that, taking an indicator diagram, and finding the work done as represented by the area included by the curve, the ordinates and the atmosi)heric line, this work ought to be equal to the quantity of heat, represented in foot-lbs., which had been given out by the gas, as shown by the difference of temperatures and specific heat of the gas.

Of course, when heat was escaping through the cylinder, and when the adiabatic line was still kept up to, a considerable amount of energy must be developed somewhere, in order to make up for the energy which went through the walls of the cylinder. The ' only source of energy in the gas engine was the union of combustible gases and oxygen, and it followed that that constant supply of energy must come from the combustion of the gases within the . cylinder. It was therefore a mere question of words, because, whether the energy was developed by the combustion of the gases which took place through the lowering of the temperature below the point of dissociation, or whether that energy was given out through the combustion of the gases which took place from the communication through the mass of an ignition which traveled slowly through it, in either case it was a gradual combustion. It was therefore a mere question of theory, and he did not see in what way it could affect the question of infringement. If Messrs. Crosslev and Mr. Otto had overlooked the theory of dissociation, and had attributed the gradual combustion to something which they ought not to have attributed it to, he did not see how it could affect their position. The real point of difference, however, in a scientific point of view, between the author and himself was this. The author asno

sumed that the ignition was quickly transmitted throagh the cylinder, and took place almost at once near the be-* ginning of the stroke, and that the tdtimate combustion was due to dissociation ; whereas Mr. Bousfield thought with Mr. Otto and many others that the cause of the supply of energy was the gradual communication of ignition through the contents of the cylinder. The author assumed gratuitously that when the point of maximum pressure was reached, that point marked the communication of ignition throughout the whole of the cylinder. That there was absolutely no ground for that as sumption could be very readily showi Neglecting for the moment the loss < heat through the walls of the cylindc the curve representing the increase pressure due to the combustion of i gas, supposing the gases to combine the same rate as they actually did, not to be allowed to expand by the tion of the piston, could be asjerta thus : — Divide the atmospheric line i

Ill

U) into spaces AB, BC, CD, DE, &c., representing equal small spaces of time or equal parts of a revolution. From each of the points A, B, C, &c., raise ordinates AL, BF, CG, &c., to meet the indicator curve in the points F, G, H, &C.J and from the points F, G, H, &c., draw adiabatics to meet AL in L, M, N, Ac. From L, M, N, &c., draw lines parallel to AB to meet their corresponding ordinates in P, Q, E, &c. Then the curve P, Q, E, &c., drawn through these points, would be a curve, the ordinates of which were proportional to the pressure at any time of the contents of the cylinder, supposing these contents to remain confined in the space at the end of the cylinder, and not allowed to expand, and supposing the rate of combustion of these contents to be exactly the same as actually occurred. This curve, therefore, showed the actual progress of the combustion deduced from the working diagram. Even neglecting the loss of heat through the walls of the cylinder, it would be seen that this curve ascended to a point past the point of maximum pressure, viz., till the point K, at the commencement of the part KV (which was supposed to be exactly adiabatic) was reached. From the point S this curve l^ecame in the actual diagram a straight line parallel to AB. If, however, the theoretical diagram, allowing for loss by conduction, were taken, the curve PQRS would ascend throughout the stroke. Hence the maximum point on the diagram was simply the point where the increase of pressure due to combustion was balanced by the decrease of pressure due to the forward motion of the piston, and there was no reason for saying that this maximum point corresponded to complete ignition.