Before explaining the principle upon which the gas-engine and every other hot-air engine depends, I shall remind you of a few data with which most of you are already familiar. The volume of every gas increases with the temperature; and this increase was the basis of the air thermometer - the first ever used. It is to be regretted that it was not the foundation of all others; for it is based on a physical principle universally applicable. Although the volume increases with the temperature, it does not increase in proportion to the degrees of any ordinary scale, but much more slowly. Now, if to each of the terms of an arithmetical series we add the same number, the new series so formed increases or decreases more slowly than the original; and it was discovered that, by adding 461 to the degrees of Fahrenheit's scale, the new scale so formed represented exactly the increment of volume caused by increase of temperature. This scale, proposed by Sir W. Thomson in 1848, is called the "scale of absolute temperature." Its zero, called the "absolute zero," is 461° below the zero of Fahrenheit, or 493° below the freezing point of water; and the degree of heat measured by it is termed the "absolute temperature." It is often convenient to refer to 39° Fahr. (which happens to be the point at which water attains its maximum density), as this is the same as 500° absolute; for, counting from this datum level, a volume of air expands exactly 1 per cent. for 5°, and would be doubled at 1,000° absolute, or 539° Fahr.
Whenever any body is compressed, its specific heat is diminished; and the surplus portion is, as it were, pushed out of the body - appearing as sensible heat. And whenever any body is expanded, its specific heat is increased; and the additional quantity of heat requisite is, as it were, sucked in from surrounding bodies - so producing cold. This action may be compared to that of a wet sponge from which, when compressed, a portion of the water is forced out, and when the sponge is allowed to expand, the water is drawn back. This effect is manifested by the increase of temperature in air-compressing machines, and the cold produced by allowing or forcing air to expand in air-cooling machines. At 39° Fahr., 1 lb. of air measures 12½ cubic feet. Let us suppose that 1 lb. of air at 39° Fahr. = 500° absolute, is contained in a non-conducting cylinder of 1 foot area and 12½ feet deep under a counterpoised piston. The pressure of the atmosphere on the piston = 144 square inches × 14.7 lb., or 2,116 lb.
If the air be now heated up to 539° Fahr. = 1,000° absolute, and at the same time the piston is not allowed to move, the pressure is doubled; and when the piston is released, it would rise 12½ feet, provided that the temperature remained constant, and the indicator would describe a hyperbolic curve (called an "isothermal") because the temperature would have remained equal throughout. But, in fact, the temperature is lowered, because expansion has taken place, and the indicator curve which would then be described is called an "adiabatic curve," which is more inclined to the horizontal line when the volumes are represented by horizontal and the pressures by vertical co-ordinates. In this case it is supposed that there is no conduction or transmission (diabasis) of heat through the sides of the containing vessel. If, however, an additional quantity of heat be communicated to the air, so as to maintain the temperature at 1,000° absolute, the piston will rise until it is 12½ feet above its original position, and the indicator will describe an isothermal curve. Now mark the difference.
When the piston was fixed, only a heating effect resulted; but when the piston moved up 12½ feet, not only a heating but a mechanical, in fact, a thermodynamic, effect was produced, for the weight of the atmosphere (2,116 lb.) was lifted 12½ feet = 26,450 foot-pounds.
The specific heat of air at constant pressure has been proved by the experiments of Regnault to be 0.2378, or something less than one-fourth of that of water - a result arrived at by Rankine from totally different data. In the case we have taken, there have been expended 500 × 0.2378, or (say) 118.9 θ to produce 26,450 f.p. Each unit has therefore produced 26,450 / 118.9 = 222.5 f.p., instead of 772 f.p., which would have been rendered if every unit had been converted into power. We therefore conclude that 222.5 / 772 = 29 per cent. of the total heat has been converted. The residue, or 71 per cent., remains unchanged as heat, and may be partly saved by a regenerator, or applied to other purposes for which a moderate heat is required.
The quantity of heat necessary to raise the heat of air at a constant volume is only 71 per cent. of that required to raise to the same temperature the same weight of air under constant pressure. This is exactly the result which Laplace arrived at from observations on the velocity of sound, and may be stated thus -
|K= 1 lb. of air at constant pressure||0.2378 × 772 =||183.5 =||100|
|K= 1 lb. of air at constant volume||0.1688 × 772 =||130.3 =||71|
|Difference, being heat converted into power||0.0690 × 772 =||53.2 =||29|
Or, in a hot-air engine without regeneration, the maximum effect of 1 lb. of air heated 1° Fahr. would be 53.2 f.p. The quantity of heat K necessary to heat air under constant volume is to K, or that necessary to heat it under constant pressure, as 71:100, or as 1:1.408, or very nearly as 1:√2 - a result which was arrived at by Masson from theoretical considerations. The 71 per cent. escaping as heat may be utilized in place of other fuel; and with the first hot-air engine I ever saw, it was employed for drying blocks of wood. In the same way, the unconverted heat of the exhaust steam from a high-pressure engine, or the heated gases and water passing away from a gas-engine, may be employed.
We are now in a position to judge what is the practical efficiency of the gas-engine. Some years since, in a letter which I addressed to Engineering, and which also appeared in the Journal of Gas Lighting,2 I showed (I believe for the first time) that, in the Otto-Crossley engine, 18 per cent. of the total heat was converted into power, as against the 8 per cent. given by a very good steam-engine. About the end of 1883 a very elaborate essay, by M. Witz, appeared in the Annales de Chimie et de Physique, reporting experiments on a similar engine, which gave an efficiency somewhat lower. Early in 1884 there appeared in Van Nostrand's Engineering Magazine a most valuable paper, by Messrs. Brooks and Steward, with a preface by Professor Thurston,3 in which the efficiency was estimated at 17 to 18 per cent. of the total heat of combustion. Both these papers show what I had no opportunity of ascertaining, that is, what becomes of the 82 per cent. of heat which is not utilized - information of the greatest importance, as it indicates in what direction improvement may be sought for, and how loss may be avoided.
But, short as is the time that has elapsed since the appearance of these papers, you will find that progress has been made, and that a still higher efficiency is now claimed.
When I first wrote on this subject, I relied upon some data which led me to suppose that the heating power of ordinary coal gas was higher than it really is. At our last meeting, Mr. Hartley proved, by experiments with his calorimeter, that gas of 16 or 17 candles gave only about 630 units of heat per cubic foot. Now, if all this heat could be converted into power, it would yield 630 × 772, or 486,360 f.p.; and it would require only 1,980,000 / 486,360 = 4.07 cubic feet to produce 1 indicated horse power. Some recent tests have shown that, with gas of similar heating power, 18 cubic feet have given 1 indicated horse power, and therefore 4.07 / 18 = 22.6 of the whole heat has been converted - a truly wonderful proportion when compared with steam-engines of a similar power, showing only an efficiency of 2 to 4 per cent.
The first gas-engine which came into practical use was Lenoir's, invented about 1866, in which the mixture of gas and air drawn in for part of the stroke at atmospheric pressure was inflamed by the spark from an induction coil. This required a couple of cells of a strong Bunsen battery, was apt to miss fire, and used about 90 cubic feet of gas per horse power. This was succeeded by Hugon's engine, in which the ignition was caused by a small gas flame, and the consumption was reduced to 80 cubic feet. In 1864 Otto's atmospheric engine was invented, in which a heavily-loaded piston was forced upward by an explosion of gas and air drawn in at atmospheric pressure. In its upward stroke the piston was free to move; but in its downward stroke it was connected with a ratchet, and the partial vacuum formed after the explosion beneath the piston, together with its own weight in falling, operated through a rack, and caused rotation of the flywheel. This engine (which, in an improved form, uses only about 20 cubic feet of gas) is still largely employed, some 1,600 having been constructed. The great objection to it was the noise it produced, and the wear and tear of the ratchet and rack arrangements. In 1876 the Otto-Crossley silent engine was introduced.
As you are aware, it is a single-acting engine, in which the gas and air are drawn in by the first outward, and compressed by the first inward stroke. The compressed mixture is then ignited; and, being expanded by heat, drives the piston outward by the second outward stroke. Near the end of this stroke the exhaust-valve is opened, the products of combustion partly escape, and are partly driven out by the second inward stroke. I say partly, for a considerable clearance space, equal to 38 per cent. of the whole cylinder volume, remains unexhausted at the inner end of the cylinder. When working to full power, only one stroke out of every four is effective; but this engine works with only 18 to 22 cubic feet of gas per horse power. Up to the present time I am informed that about 18,000 of these engines have been manufactured. Several other compression engines have been introduced, of which the best known is Mr. Dugald Clerk's, using about 20 feet of Glasgow cannel gas. It gives one effective stroke for every revolution; the mixture being compressed in a separate air-pump. But this arrangement leads to additional friction; and the power measured by the brake is a smaller percentage of the indicated horse power than in the Otto-Crossley engine.
A number of gas engines - such as Bisschop's (much used for very small powers), Robson's (at present undergoing transformation in the able hands of Messrs. Tangye), Korting's, and others - are in use; but, so far as I can learn, all require a larger quantity of gas than those previously referred to.
I have all along spoken of efficiency as a percentage of the total quantity of heat evolved by the fuel; and this is, in the eyes of a manufacturer, the essential question. Other things being equal, that engine is the most economical which requires the smallest quantity of coal or of gas. But men of science often employ the term efficiency in another sense, which I will explain. If I wind a clock, I have spent a certain amount of energy lifting the weight. This is called "energy of position;" and it is returned by the fall of the weight to its original level. In the same way if I heat air or water, I communicate to it energy of heat, which remains potential as long as the temperature does not fall, but which can be spent again by a decrease of temperature. In every heat-engine, therefore, there must be a fall from a higher to a lower temperature; otherwise no work would be done. If the water in the condenser of a steam-engine were as hot as that in the boiler, there would be equal pressure on both sides of the piston, and consequently the engine would remain at rest. Now, the greater the fall, the greater the power developed; for a smaller proportion of the heat remains as heat.
If we call the higher temperature T and the lower T' on the absolute scale, T - T' is the difference; and the ratio of this to the higher temperature is called the "efficiency." This is the foundation of the formula we meet so often: E = (T - T')/T. A perfect heat-engine would, therefore, be one in which the temperature of the absolute zero would be attained, for (T - O)/T = 1. This low temperature, however, has never been reached, and in all practical cases we are confined within much narrower limits. Taking the case of the condensing engine, the limits were 312° to 102°, or 773° and 563° absolute, respectively. The equation then becomes (773 - 563)/773 = 210 / 773 or (say) 27 per cent. With non-condensing engines, the temperatures may be taken as 312° and 212°, or 773° and 673° absolute respectively. The equation then becomes (773 - 673)/773 = 100 / 773, or nearly 13 per cent. The practical efficiencies are not nearly this, but they are in about the same ratio - 27/13. If, then, we multiply the theoretical efficiencies by 0.37, we get the practical efficiencies, say 10 per cent. and 5 per cent.; and it is in the former sense that M. Witz calculated the efficiency of the steam-engine at 35 per cent. - a statement which, I own, puzzled me a little when I first met it.
These efficiencies do not take any account of loss of heat before the boiler. In the case of the gas-engine, the question is much more complicated on account of the large clearance space and the early opening of the exhaust. The highest temperature has been calculated by the American observers at 3,443° absolute, and the observed temperature of the exhaust gases was 1,229°. The fraction then becomes (3443 - 1229)/3443 = 64 per cent. If we multiply this by 0.37, as we did in the case of the steam-engine, we get 23.7 per cent., or approximately the same as that arrived at by direct experience. Indeed, if the consumption is, as sometimes stated, less than 18 feet, the two percentages would be exactly the same. I do not put this forward as scientifically true; but the coincidence is at least striking.
I have spoken of the illuminating power of the gas as of importance; for the richer gases have also more calorific power, and an engine would, of course, require a smaller quantity of them. The heat-giving power does not, however, vary as the illuminating power, but at a much slower rate; and, adopting the same contrivance as that on which the absolute scale of temperature is formed, I would suggest a formula of the following type: H = C (I + K), in which H represents the number of heat-units given out by the combustion of 1 cubic foot of gas, I is the illuminating power in candles, and C and K two constants to be determined by experiment. If we take the value for motive power of the different qualities of gas as given in Mr. Charles Hunt's interesting paper in our Transactions for 1882, C might without any great error be taken as 22 and K as 7.5. With Pintsch's oil gas, however, as compared with coal gas, this formula does not hold; and C should be taken much lower, and K much higher than the figures given above. That is to say, the heating power increases in a slower progression.
The data available, however, are few; but I trust that Mr. Hartley will on this, as he has done on so many other scientific subjects, come to our aid.
I will now refer to the valuable experiments of Messrs. Brooks and Steward, which were most carefully made. Everything was measured - the gas by a 60 light, and the air by a 300 light meter; the indicated horse power, by a steam-engine indicator; the useful work, by a Prony brake; the temperature of the water, by a standard thermometer; and that of the escaping gases, by a pyrometer. The gas itself was analyzed; and its heating power calculated, from its composition, as 617.5θ. Its specific gravity was .464; and the volume of air was about seven times that of the gas used (or one-eighth of the mixture), and was only 11½ per cent. by weight more than was needed for perfect combustion. The results arrived at were as follows:
|Converted into indicated horse power, including friction, etc.||17.0|
|Escaped with the exhaust gas.||15.5|
|Escaped in radiation.||15.5|
|Communicated to water in the jacket.||52.0|
It will thus be seen that more than half of the heat is communicated to the water in the jacket. Now, this is the opposite of the steam-engine, where the jacket is used to transmit heat to the cylinder, and not from it. This cooling is rendered necessary, because without it the oil would be carbonized, and lubrication of the cylinder rendered impossible. Indeed, a similar difficulty has occurred with all hot-air engines, and is, I think, the reason they have not been more generally adopted. I felt this so strongly that, for some time after the introduction of the gas-engine, I was very cautious in recommending those who consulted me to adopt it. I was afraid that the wear and tear would be excessive. I have, however, for some time past been thoroughly satisfied that this fear was needless; as I am satisfied that a well-made gas-engine is as durable as a steam-engine, and the parts subject to wear can be replaced at moderate cost. We have no boiler, no feed pump, no stuffing-boxes to attend to - no water-gauges, pressure-gauges, safety-valve, or throttle-valve to be looked after; the governor is of a very simple construction; and the slide-valves may be removed and replaced in a few minutes.
An occasional cleaning out of the cylinder at considerable intervals is all the supervision that the engine requires.