ATKINSON'S DIFFERENTIAL GAS ENGINE, 8 H.P.
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:
| Per cent. | |
| 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.
The very large percentage of heat absorbed by the water-jacket should point out to the ingenuity of inventors the first problem to be attacked, viz., how to save this heat without wasting the lubricant or making it inoperative; and in the solution of this problem, I look for the most important improvement to be expected in the engine. The most obvious contrivance would be some sort of intercepting shield, which would save the walls of the cylinder and the rings of the piston from the heat of the ignited gases. I have just learned that something of the kind is under trial. Another solution may possibly be found in the employment of a fluid piston; but here we are placed in a dilemma between the liquids that are decomposed and the metals that are oxidized at high temperatures. Next, the loss by radiation—15 per cent.—seems large; but this is to be attributed to the fact that the inside surface of the cylinder is at each inward stroke exposed to the atmosphere—an influence which contributes to the cooling necessary for lubrication. The remaining 15 per cent., which is carried away by the exhaust, is small compared with the proportion passing away with the exhaust steam of a high-pressure or the water of a condensing engine. As the water in the jacket can be safely raised to 212° Fahr., the whole of the jacket heat can be utilized where hot water is required for other purposes; and this, with the exhaust gases, has been used for drying and heating purposes.
With such advantages, it may be asked: Why does not the gas-engine everywhere supersede the steam-engine? My answer is a simple one: The gas we manufacture is a dear fuel compared with coal. Ordinary coal gas measures 30 cubic feet to the pound; and 1,000 cubic feet, therefore, weigh 33 lb. Taking the price at 2s. 9d. per 1,000 cubic feet, it costs 1d. per lb. The 30 cubic feet at 630θ give 19,000θ all available heat. Although good coal may yield 14,000 units by its combustion, only about 11,000 of these reach the boiler; so that the ratio of the useful heat is 11/19. The thermal efficiency of the best non-condensing engine to that of the gas-engine is in the ratio 4/22. Multiplying together these two ratios, we get (11 / 19)×(4 / 22½) = 44 / 4.28. That is, speaking roughly, 1 lb. of gas gives about ten times as much power as 1 lb. of coal does in a good non-condensing engine. But at 18s. 8d. a ton we get 10 lb. of coal for 1d.; so that with these figures the cheapness of the coal would just compensate for the efficiency of the gas. As to the waste heat passing away from the engine being utilized, here the gas-engine has no advantage; and, so far as this is concerned, the gas is about eight times dearer than coal. The prices of gas and coal vary so much in different places that it is hard to determine in what cases gas or coal will be the dearer fuel, considering this point alone.
But there are other kinds of non-illuminating gases—such as Wilson's, Strong's, and Dowson's—which are now coming into use; and at Messrs. Crossley's works you will have an opportunity of seeing a large engineering factory employing several hundred mechanics, and without a chimney, in which every shaft and tool is driven by gas-engines supplied by Dowson's gas, and in which the consumption of coal is only 1.2 lb. per indicated horse power. The greatest economy ever claimed for the steam-engine was a consumption of 1.6 lb.; and this with steam of very high pressure, expanded in three cylinders successively. Thus in a quarter of a century the gas-engine has beaten in the race the steam-engine; although from Watt's first idea of improvement, nearly a century and a quarter have elapsed.
As regards the steam-engine, it is the opinion of competent authorities that the limits of temperature between which it works are so restricted, and so much of the heat is expended in producing a change of state from liquid to vapor, that little further improvement can be made. With respect to gas-engines, the limits of temperature are much further apart. A change of state is not required, and so very great improvement may still be looked for. It is not impossible even that some of the younger members of our body may live to see that period foretold by one of the greatest of our civil engineers—that happy time when boiler explosions will only be matters of history; that period, not a millennium removed by a thousand years, but an era deferred perhaps by only half a dozen decades, when the use of the gas-engine will be universal, and "a steam-engine can be found only in a cabinet of antiquities."