Air and Gas Engines
241. Under this head we may include all heat-engines in which the working substance is air, of the gaseous products of the combustion of fuel and air, whether the fuel be itself solid, liquid, or gaseous. When air alone forms the working substance, it receives heat from an external furnace by conduction through the walls of containing vessel, as the working substance in the steam-engine takes in heat through the shell of the boiler. An engine supplied with heat in this way may be called an external combustion engine, to distinguish it from a very important class of engines in which the combustion which supplies heat occurs within a closed chamber containing the working substance. The ordinary coal-gas explosive engine is the common type of internal-combustion engine.
242. Compared with an engine using saturated steam, air and gas engines have the important advantage that the temperature and the pressure of the working substance are independent of one another. Hence it becomes possible to use an upper limit of temperature greatly higher than in the ordinary steam-engine, and if the lower limit is not correspondingly raised an increase of thermodynamic efficiency results. It is true that the same advantage might be obtained in the case of steam, by excessive super-heating; but this would mean substantially the conversion of the engine into the type we are now considering, the working substance being then steam gas.
243. A simple, thermodynamically perfect form of external combustion air-engine would be one following Carnots cycle (§ 40), in which heat is received while the air is at the highest temperature r1, the air meanwhile expanding isothermally. After this the supply of heat is stopped, and the air is allowed to expand adiabatically until its temperature falls to the lower extreme r2. At this it is compressed isothermally, giving out heat, and finally the cycle is completed by adiabatic compression, which restores the initial high temperature r1
244. In place of adiabatic-expansion as a means of changing the temperature from r1 to r2 we may follow Stirlings plan (§ 54) of storing the heat in a regenerator, from which it will afterwards be taken up and so produce the elevation of temperature from r2 to r1 which in the above cycle was performed by adiabatic compression. Stirlings air-engine, in which the action approximated to the perfect cycle described in § 54, is diagrammatically shown in fig. 141. A is a closed vessel containing air, externally heated by furnace beneath it. A pipe from the tip of A leads to the working cylinder B. At the top of A is a refrigerator C, consisting of pipes through which cold water circulates. In A there is a displacer D, which is driven by the engine; when this is raised the air in A is head, whereas when D is lowered the air in A is brought into contact with the refrigerator and cooled. On its way from the bottom to the top of A, or vice versa, the air must pass through an annular lining of wire-gauze E. This is the generator. At the beginning of the cycle D is up. The air is then receiving heat at r1, and is expanding isothermally; this is the first stage § 54. Then the plunger D descends. The air is driven through the generator, where it deposits heat, ad its temperature on emerging at the top is r2. Next, the working piston makes its down-stroke (in the actual engine the working cylinder was double-acting, another heating vessel, precisely like A, being connected with the cylinder B above the piston); this compresses the air isothermally, the heat produced by compression being taken up by C. Finally the plunger is raised, and the working air again passes through the generator, taking up the heat is left there, and rising to r1. The theoretical indicator diagram has been given in fig. 13.
245. The actual forms in which Stirlings engines was used are described in two patents by R. & J. Stirling (1827 and 18401). An important feature in them was that the air was compressed (by means of a pump) to a pressure greatly above that of the atmosphere. Stirlings cycle is theoretically perfect whatever the density of the working air, and compression did not in his case increase what may be called the theoretical thermodynamic efficiency. It did, however, very greatly increase the mechanical efficiency, and also, what is of special importance, it increase the amount of power yielded by an engine of given size. To see this its is sufficient to consider that with compressed air a greater amount of heat was dealt with in each stroke of the engine, and therefore a greater amount of work was produced. Practically it also increased the thermodynamic deficiency by reducing the ratio of the heat wasted by external conduction and radiation to the whole heat.
A double-acting Stirling engine of 50 I. H. P., used in 1843 at the Double foundry, appears to have realized efficiency of 0&Mac250;3, and, notwithstanding very inadequate means of heating the air, consumed only 1&Mac250;7 _ of coal per I. H. P. per hour.2 This engine remained at work for three years, but was finally abandoned on account of the failure of the heating vessels. In some forms of Stirlings engine the regenerator was a separate vessel; in others the plunger D was itself constructed to serve as generator by filling it with wire-gauze and leaving holes at top and bottom for the passage of the air through it.
246. Another mode of using the regenerator was introduced in America by Ericsson, in an engine which also failed, partly because the heating surfaces became burnt, and partly because their area was insufficient. In Ericssons engines the temperature of the working substance is changed (by passing through the regenerator) while the pressure remains constant. Cold air is compressed by a pump into a receiver, from which it passes through a regenerator into the working cylinder. In so passing it absorbs heat from the regenerator and expands. The air in the cylinder is then further expanded by taking in heat form a furnace under the cylinder. The cycle is completed by the discharge of the air through the regenerator. The indicator diagram approximates to a form bounded by two isothermals and two line of constant pressure.3
247. Externally-heated air-engines are now employed only for very small powerfrom a fraction of 1 H.P. up to about 3 H.P. Powerful engines of this type are impractically on account of their relatively enormous bulk. Those that are now manufactured resemble the original Stirling engine very closely in the main features of their action, and comprise essentially the same organs.4
248. Internal-combustion engines form a far more important class of motors. The earliest example of this class appears to have been the hot-air engine of Sir George Cayley,5 of which Wenhamss6 and Bucketts7 engines are recent forms. In these engines coal of coke is burnt under pressure in a closed chamber, to which the fuel is fed through a species of air-jock. Air for combustion is supplied by a compression pump, and the engine is governed by means of a distributing valve which supplies a greater of less proportion of the air below the fire as the engine runs slow of fast. The products of combustion, whose volume is increase by their rise in temperature, pass into a working cylinder, raising the piston. When a certain fraction of the stroke is over the supply of hot gas is stopped, and the gases in the cylinder expand, doing more work and becoming reduced in temperature. During the return stroke they are discharged into atmosphere, and the pump takes in a fresh supply of air. Fig. 142 is a diagram section of the Buckett engine. A is the working piston, the form of which is such as to protect tight sliding surface (at the top) from contact with the hot gases; B is the compressing pump, C the valve by which the governor regulates the rate at which fuel is consumed, and D the air-lock through which fuel is supplied.
249. In engines of this class the degree to which the action is thermodynamically efficient depends very largely on the amount of cooling the gases undergo by adiabatic of nearly adiabatic expansion under the working piston. Without a large ratio of expansion the thermodynamic advantage of a high initial temperature is lost; but, as the gases have to be discharged at atmospheric pressure, a large ratio of expansion is possible only when there is much initial compression. Compression is therefore an essential condition, without which a heat-engine of this type cannot be made efficient. It is also, as has already been pointed out, essential in all air-engines to the development of a fair amount of power by an engine of moderate bulk.
250. Internal-combustion engines using solid fuel have hitherto been little used, and that only for small powers. Several small engines employ liquid fuel (namely, petroleum) injected in a state of spray, or even vaporized before entering the combustion-chamber. In some forms, of which the Brayton petroleum engine is a type, combustion occurs as the fuel is injected; an others the action approaches closely that of gas-engines, that is to say, of engines in which fuel (generally coal-gas) is supplied in a perfectly gaseous state, and is burnt in a more or less explosive manner. These last are the only heat-engines that have as yet entered into serious competition with steam-engines.
251. The earliest gas-engine to brought into practical use was that of Lenoir (1860). During the first part of the stroke air and gas, in proportions suitable for combustion, were drawn into the cylinder. At about half-stroke the inlet valve closed, and the mixture was immediately exploded by an electric spark. The heated products of combustion then did work on the piston during the remainder of the forward stroke, and were expelled during the back stroke. The engine was double-acting, and the cylinder was prevented from becoming excessively heated by a casing through which water was kept circulating. The water-jacket has been retained in nearly all later gas-engines.
An indicator diagram form a Lenoir engine is shown in fig. 143.8 After explosion the line falls, partly from expansion, and partly from the cooling action of the cylinder walls; on the other hand, its level is to some extent maintained by the phenomenon of after-burning, which will be discussed later. In this engine, chiefly because there was no compression, the heat removed by the water-jacket bore an exceedingly large proportion to the whole heat, and the efficiency was comparatively low; about 95 cubic feet of gas were used per horse-power per hour. Hugons engine, introduced five years later, was a non-compressive engine very similar to Lenoirs. A novel feature in it was the injection of a jet of cold water to keep the cylinder from becoming too hot. These engines are now obsolete; the type they belonged to, in which the mixture is not compressed before explosion, is now represented by one small engineBischoffsthe mechanical simplicity of which atones for its comparatively wasteful action in certain cases where but little power is required.
252. In 1866 Otto and Langen introduced a curious engine,9 which, as to economy of gas, was distinctly superior to its predecessors. Like them it did not use compression. The explosion occurred early in the stroke, in a vertical cylinder, under a piston which was free to rise without doing work on the engine shaft. The piston rose with great velocity, so that the expansion was much more nearly adiabatic than in earlier engines. Then after the piston had reached the top of its range the gases cooled, and their pressure fell below that of the atmosphere; the piston consequently came down, this time in gear with the shaft, and doing work. The burnt gases were discharge during the last part of the down-stroke. A friction-coupling allowed the piston to be automatically thrown out of gear when rising, and into gear when descending. This "atmospheric" gas engine used about 40 cubic feet of gas per horse-power per hour, and came into somewhat extensive use in spite of its noisy and spasmodic action. After a few years it was displaced by a greatly improve type, in which the direct action of Lenoirs engine was restore, but the gases were compressed before ignition.
253. Dr. Ottos "silent" engine, introduce in 1876, was the first successful motor of the modern type. It is a single-acting engine, generally horizontal in form, and the explosive mixture is compressed in the working cylinder itself. This is done by making cycle of the action extend through two revolutions of the engine. During the first forward stroke gas and air are drawn in by the piston. During the first back-stroke the mixture is compressed into a large clearance space at the end of the cylinder. The mixture is then ignited, and the second forward stroke (which is the only working stroke in the cycle) is performed under the pressure of the heated products of combustion. During the second back-stroke the products are discharged, with the exception of so much as remains in the clearance space, which serves to dilute the explosive mixture in the next cycle. The principal parts of Ottos engine (as made by Messrs Crossley) are shown in the diagram section, fig. 144. The cylinder is kept cool by a water-jacket AA. B is the clearance space into which the mixture is compressed before explosive. Its volume is usually about two-thirds of the stroke, of 40 per cent. of the whole volume to which the gases afterwards expand. C is the exhaust-valve, which is opened during the second back-stroke of each cycle. Gas and air are admitted at D, through a slide-valve E, which reciprocates once in each complete cycle of two revolutions. This slide-valve is shown to a larger scale in fig. 145, in the position it occupies while gas is entering from g and air from a. To ignite the mixture a gasket is kept burning at c. In the slide-valve there is an igniting port d, which is supplied with gas from a groove in the cover. As the slide moves towards the right, the igniting port d carries a flame from c to D. Just before reaching D a little of the compressed mixture from the cylinder enters the igniting port by a small opening which does not appear in the figure, and by the time D is reached the contents of d are so much raised in pressure by their own combustion that a tongue of flame shoots into the cylinder, firing the mixture there. The speed is regulated by a centrifugal governor, which cuts off the supply of gas when the speed exceeds a certain limit. In some small Otto engines of recent construction the inertia of a reciprocating piece is used instead of the inertia of revolving pieces to effect the same end.
254. In Mr. Clerks engine the cycle of operation is essentially the same as in Ottos, but a charging cylinder is introduced, with the effect of allowing an explosion to take place in the working cylinder once in every revolution. As in Ottos, there is a large clearance space behind the piston, and the mixture is compressed into this space by the backward movement of the working piston. The peculiarity of the engine lies in the manner in which the charge is introduced. As the piston advances after an explosion it uncovers exhaust ports in the sides of the cylinder, close to the end of its forward stroke. While it is passing the dead-point there the plunger of the charging cylinder (while has meanwhile taken in a mixture of gas and air) delivers this mixture into the cylinder driving the products of the previous combustion out of the cylinder through the exhaust ports. The charging cylinder is so arranged that the first part of the charge consists almost wholly of air, and this is followed by the explosive mixture of gas and air. The working piston then returns, closing the exhaust ports and compressing the mixture, which is ignited after compression by means of a slide-valve similar to Ottos. In Ottos engine the explosive mixture is diluted, and the sharpness of the explosion thereby reduced, by the residue of burnt products which rill the clearance space at the end of the discharge stroke. In Clerks engine the mixture is diluted by an excess of air. It does not appear that this difference has any material effect on the action.
255. Over 20,000 Otto engines are now in use, of power ranging up to about 40 H.P. Besides the engines which have been named, others are manufactured in which the operations are essentially of the same kind, though in some cases the mechanical details are widely varied. In one of these, Mr. Atkinsons ingenious "differential" engine, the working chamber consists of the space between tow pistons working in one cylinder. During exhaust the pistons come close together; they recede from each other to take in a fresh charge; they approach for compression; and finally they recede again very rapidly and farther than before, after ignition of the mixture, thus giving a comparatively large ratio of expansion. At the same time, by moving bodily along through the cylinder, the pistons uncover admission and exhaust ports and an ignition-tube which is kept permanently incandescent.
256. If the explosion of a gaseous mixture were practically instantaneous, producing at once all the heat due to the chemical reaction, and if the expansion and compression were adiabatic, the theoretical indicator diagram of an engine of the Otto type would have the form shown in fig. 146. OA represent the volume of clearance; AB is the admission, at atmospheric pressure; BC is the compression (which is assumed to be adiabatic); CD is the rise of pressure caused by explosion; DE is adiabatic expansion during the working stroke; and EBA is the exhaust. The height of the point D above C may be calculated when we know the temperature at C (an element of considerable uncertainty in practice), the specific heat (at constant volume) of the burnt mixture, the amount of heat evolved by explosion, and the change of specific density due to the change of chemical constitution which explosion brings about. With the proportion of coal-gas and air ordinarily employed this last consideration may generally be neglected, as the volume of the products would differ by less than 2 per cent. from the volume of the mixture before explosion if both were reduced to the same pressure and temperature.
257. The rise of pressure observed in the indicator diagrams of gas-engine is found to be in all cases much less than the calculated rise of pressure which would be caused by strictly instantaneous explosion. An actual diagram from an Otto engine working in its normal manner is given in fig. 147, where the references letters distinguish the parts of a complete cycle, as in fig. 146. It shows a rapid rise of pressure on explosion, so rapid that the volume has not very materially altered when the maximum of pressure is reached; and the specific heat at constant volume may therefore be used without serious error calculating the amount of heat which this rise accounts for. When this calculation is made,1 it turns out that only about 60 or 70 per cent. of the potential heat of combustion in the mixture is required to produce the rise of temperature corresponding to the point of greatest pressure. The remainder continues to be slowly evolved during the subsequent expansion of the hot gasses. The process of combustiona term evidently more appropriate than explosionis essentially gradual; when ignition takes place it begins rapidly, but it continues to go on at a diminishing rate throughout the stroke. That part which takes place after the maximum pressure is passed is the phenomenon of after-burning to which allusion has been made above.
258. The existence of "alter-burning" is proved not only by the fact that the maximum pressure after ignition is much less than it would be if combustion were then complete, but also by the form which the curve of subsequent expansion takes. During expansion the gases are losing much heat by conduction through the cylinder walls. The water-jacket absorbs rather more than half of the whole heat developed in the engine,2 and the greater part of this of course taken up from the gases during the working stroke. Notwithstanding this loss, the curve of expansion does not fall much below the adiabatic curve; in some cases it even lies higher than the adiabatic curve. This shows that the loss to the sides of the cylinder is being made up continued development of heat within the gas. The process of combustion is especially protracted when the explosive mixture is weak in gas; the point of maximum pressure then comes late in the stroke; and it is probable that the products which are discharged in the exhaust contain some incompletely-burnt fuel. Fig. 148 is the indicator diagram of an Otto engine supplied with a mixture containing an exceptionally large proportion of air: it exhibits well the very gradual character of the explosion in such a case.
259. Much light has been thrown on this subject by the experiments of Mr. Clerk, who has exploded mixtures of gas and air, and also mixtures of hydrogen and air, in a closed vessel furnished with an apparatus for recording the time-rate of variation of pressure. In these experiments the pressure fell after the explosion only on account of cooling action of the containing walls. The temperature before ignition being known, it became possible to calculate form the diagrams of pressure the highest temperature reached during combustion (on the assumption that the specific heat of the gases remained unchanged at high temperature) and to compare this with the temperature which would have been produced had combustion been at once complete. Mixtures of gas and air were exploded, the proportion of gas varying from 1/15 to 1/5, and the highest temperature produced was generally a little more than that which would have been reached by instantaneous combustion of the mixture. With the best proportion of coal-gas to air (1 to 6 or 7) the greatest pressure and hottest state was found one-twentieth of a second after ignition, and the temperature was then 1800° C., instead of 3800°, which would have been the value had all the heat been at once evolved. With the weakest mixtures about half a second was taken to reach a maximum of temperature and its value was 800° C., instead of 1800° C. In this case, however, the degree of completeness of the combustion is not fairly shown by a comparison of these temperatures, since much cooling occurred during the relatively long interval that preceded the instant of greatest pressure.
260. To explain the phenomenon of after-burning or delayed combustion, it has been supposed that the high temperature to which the gases are raised in the first stages of the explosion prevents union from being completed, just as high temperature would dissociate the burnt gases were they already in chemical union,until the fall of temperature by expansion and by the cooling action of the cylinder walls allows the process of union to go on. The maximum attained in the gas engines is thigh enough to cause a perceptible amount of dissociation of the brunt product; it may therefore be admitted that this explanation of delayed combustion is to some extent true. On the other hand, the phenomenon is most noticeable, with mixtures weak in gas, in which the maximum temperature reached is low, and the dissociation effect is correspondingly small. It appears, therefore, that dissociation is not the main cause of the action; apart from it the process of combustion of a gaseous mixture is gradual, beginning fast and going on at a continuously-diminishing rate as the combustible mixture becomes more and more diluted by the portions already burnt. If the mixture is much diluted to begin with, the process is comparatively slow from the first.
261. Much stress has been laid by some makers of gas-engines on the desirability of having a stratified mixture of gases in the cylinder, with a part rich in gas near the ignition port and a greater proportion of residual product or air near the piston. It has been supposed that stratification of the gases is the cause of their gradual combustion. Mr. Clerks experiments are conclusive against this; the mixture he used, which gave in some case very gradual explosions, were allowed to stand long enough to become sensibly homogeneous. In dealing with weak mixtures it is no doubt of advantage to have a small quantity of richer fluid close to the igniting port to start the ignition of the rest,but beyond this stratification has probably little or not value. And it may be questioned whether, in the ordinary working of a gas-engine, any general stratification can occur, when account is taken of the commotion which the air and gas cause as they rush into the cylinder at a speed exceeding that an express train.
262. A compression gas-engine of the Otto type burns from 20 to 25 cubic feet of coal-gas per hour per indicated horse-power. Good coal-gas has a heating power equivalent to about 500,000 foot-pounds per cubic foot, and hence, with a consumption of 20 cubic feet the efficiency which the engine realizes is nearly 0&Mac250;2. The efficiency of a large steam-engine is about 0&Mac250;14, and in steam-engines that are small enough to be fairly compared with actual gas-engines the efficiency is not more than 0&Mac250;1. The superioty of gas-engines over steam-engines, from the thermodynamic point of view, is well shown by comparing their consumption of fuel. In the steam-engine we find in good engines of large size a consumption of 2 _ or 1_ _ of coal per I.H.P. per hour and by triple expansion this is reduced in large marine engines to about 11/3 _. On the other hand, in small-power engines the consumption is at least 2_ _, and is generally 3 _ or more. When Mr. Dowsons cheap gas,1 which is produced by passing a mixture of superheated steam and air through red-hot anthracite, is used to drive an Otto engine, the consumption of coal has been found to be only 1&Mac250;1 _ per I.H.P. per hour, or less than half the amount used by a steam-engine of similar size. What gives this comparison additional interest is the fact that the gas-producer for a 40 or 50 H.P. engine need not take up more space than the boiler of steam-engine of the same power.
263. In another sense the gas-engine is much less perfect than the steam-engine. The actual efficiency of the latter is about half the ideal efficiency which a perfect engine would show when working through the same range of temperature. In the gas-engine the actual is less than one-fourth of the ideal efficiency. Taking the highest temperature as 1900° C.a value reached in some of Mr. Clerks experimentsand the lowest temperature as 15° C., the efficiency of a perfect engine would be 0&Mac250;87, while that of the actual engine is 0&Mac250;2. This only means that the gas-engine has all the greater margin for future improvement.
264. At present the main cause of waste in gas-engines are the action of the sides of the cylinder and the water-jacket, and the high temperature of the exhaust gases. The water-jacket absorbs about half the whole heat, only to keep the cylinder cool enough to permit of lubrication. The waste gases are discharged at a temperature of about 420°., an so carry away a large amount of heat which might in part be saved by having a greater ratio of expansion, of by the use of a regenerator. Another source of thermodynamic imperfection is the after-burning, which gives heat to the working substance at a temperature lower than the maximum.
In an engine constructed by the late Sir William Siemens it was attempted to do away with or reduce the tow main causes of loss (1) by using a separate combustion-chamber, distinct form the cylinder in which the piston worked, and (2) by passing the exhaust gases through a regenerator, which afterwards gave up heat to the incoming air and gas.2 The late Prof Fleeming Jenkin endeavored to attain the same ends by adapting the Stirling type of engine to internal combustion, a mixture of gas and air being exploded under a displacer like that of fig. 141. Practical difficulties have hitherto prevented regenerative internal-combustion engines from coming into use, but it can scarcely be doubted that their development is only of time. With regard to the probable future of heat-engines, it is important to notice that the internal-combustion engine using gaseous fuel, through already much more efficient than the steam-engine, is crude and full of defects which further invention ought to remove, while the steam-engine has been improved so far that little increase in its efficiency can be expected, and more than a little is impossible. (J. A. E)
The above article was written by James Alfred Ewing, M.A., B.Sc., F.R.S.; M.Instit.C.E.; Professor of Mechanism and Applied Mechanics, Cambridge; Fellow of King's College, Cambridge; Professor of Mechanical Engineering at the Imperial University, Tokyo, Japan, 1878-83; author of Treatise on Earthquake Measurement, Magnetic Induction in Iron and other Metals, The Steam Engine and other Heat Engines, etc.
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