Source: Journal of the Franklin Institute - Vol. 71, N° 421
Date: January 1861
Title: The Caloric Engine
Proceedings of the Polytechnic Association of the American Institute. Oct. 31, 1860, from the Architects and Mechanics’ Journal. Corrected by T.D. Stetson, Esq.
The President proposed three questions to be answered by those explaining the caloric engine: first, as to the burning out of the heaters; for if they are liable to be burned out frequently, it is an objection to its economical use, not only from the expense, but from the frequent interruptions in the working of the engine.
The second, also in the form of an objection, relates to the small amount of force that could be accumulated. The air being heated as it is used, no provision can be made, as in the steam boiler, for a supply of work for a little time. The heating up occurs at the moment, and is liable to disappointment upon the slightest accident or occasion of delay.
The third and strongest objection relates to the small amount of power that is obtained. It is only changing one gas, not into another, but an expansion of the same gas; whereas in making steam, we expand a liquid instantly to 1700 times its bulk. In air engines, double the volume is all that is usually obtained. He would like to hear either some answer to these objections, or evidence that, in spite of these objections, the economy is such as to make the caloric engine valuable.
Mr. G. H. Babcock exhibited drawings of Wilcox’s caloric engine, (an engine recently invented by S. Wilcox, Jr., of Westerly, R.I,) and explained its construction and mode of operation. Although air engineering is younger than steam engineering, much attention has been paid to it, between 200 and 300 patents having been granted for air engines, or improvements upon air engines, in Great Britain alone, and thirty-five in the United States.
These may all be classified in four grand divisions:
In the first class may be placed all those engines in which a reservoir of expanded air is maintained as a reservoir of power, similar in effect to the steam boiler. The air is allowed to escape into an engine of proper character, and worked off like steam from a steam boiler.
The second class includes ail those which use the gaseous products of combustion within the engine. The power may be generated within the engine itself, or in a heater; this class will include the explosive engines.
The third class includes the engine which use a certain quantity of air, which is alternately heated and cooled in opposite portions of the stroke; the difference in temperature in different parts of the stroke generating power in the engine. The Stirling engine belonged to this class.
The fourth class includes all those engines which use 1atmospheric air, drawing their supply from the atmosphere at each stroke, and exhausting again into the atmosphere. This class will include the great experimental caloric ship of Ericsson, the small caloric engine of Ericsson, and also the Wilcox engine; it includes all the caloric engines now in successful use.
The President. —Please to state why these are the only ones in use.
Mr. Babcock.—I do not know that there is any good reason why there are not engines of the third class. The second class, in which the gaseous products of combustion are used, have, in all cases so far, failed because of the excessive heat occasioned by combustion, and of the abrasion occasioned in the cylinder and other working parts by the solid products of combustion. And for the first class I know no positive reason.
The President.—Are there not difficulties connected with accumulating a reserve of power?
Mr. Babcock.—There are practical difficulties in condensing the air. Some power is lost in the condensation, owing to the development of heat in the compression, which is lost, in a great measure. Of all the experiments which have been made, there seem to have been but three which have been practically successful: the Stirling engine, the Ericson engine, and the Wilcox engine. The Stirling engine, after the death of the inventor, having been disused, in consequence of the jealousy of steam engineers, the Ericsson and Wilcox engines are the only ones now in practical operation. Of the Ericsson engine, several hundreds have been constructed, and are now in operation.
The President.—How small a power is used?
Mr. Babcock.—One-man power, which is about a fifth of s horse power, is considered quite a respectable power. To drive an ordinary sewing machine requires about one-sixtieth of a horse power. One of the small air engines would drive quite a number of these machines. The Ericsson engines run up to two horse power in some cases; perhaps more for the double engines. The Wilcox engine has not been fully tested. There is an 18-in. Wilcox engine estimated by the owners as 3½ horse power, in a large bakery in Pawtucket. This is probably above its actual power. It is stated that it does not average over 60 lbs. of coal in 12 hours, and has run with 46 lbs.
The Wilcox engine has two upright cylinders connected at their lower ends next to the fire. One of the cylinders is used as a working cylinder and is single-acting, open at the upper end. The other, termed a changing cylinder, is double-acting, but the piston is always in equilibrium, so that all the resistance it occasions is due to the friction of the air and the friction of the parts. The pistons are connected with cranks upon the main, shaft, which are placed nearly at right angles. This produces a motion nearly corresponding to the theory, which would be that each piston should make its entire stroke while the other is at its dead-point.
Between these two cylinders is the economizer, a chamber so filled with thin metal plates as to allow the free passage of air, and connected with both cylinders at the bottom. It is intended to absorb as much as possible of the heat of the air passed upwards through it, and to return it to the next downward current. It baa been contended that the economizer is of no theoretic value. Practice seems to prove its value in this engine at least, for the engine has been found to run light, other circumstances being equal, twice the number of revolutions with the economizer that it will run without it.
It is also found that the engine will keep running for half an hour or more, by simply passing the same air back and forth; the valve being so set as not to take in any fresh air. Above the economizer is the valve chest, which contains single rolling three way valve, with the three-fold office of induction, education, and equilibrium valve. The heat is applied at the bottom, not by direct radiation, by passing the products of combustion under and around the cylinders. In consequence of cutting off the direct radiation, the heaters are expected to last longer. The heaters are prevented from being overheated by automatic action; the vapor of mercury, which is formed at 600°, operating means which shut off the heat from the cylinders.
The President inquired what was the durability of the beaters.
Mr. Babcock stated that the engines had only been in operation for six months, so that there were no means of knowing. The Stirling engine heaters lasted for two years, and these ought to last as long.
A gentleman inquired how it was lubricated.
Mr. Babcock explained how the oil was prevented from being burned. A perforated cover was placed on the open end of the working cylinder, the perforations being slightly inclined, so that at each descent of the piston, the cool air impinged upon the metal in numerous streams, to be driven out again on it ascent, thus conveying away the heat. He stated that there was no packing in the piston in the inclosed cylinder, there being always nearly an equilibrium in that cylinder.
Mr. Churchill inquired what were the cubical contents of the economizer.
Mr. Babcock stated that it was about one-fourth the size of one of the cylinders.
A gentleman inquired what was the economy of this engine.
Mr. Babcock said that if the Pawtucket engine had a two horse power, it burned 2½ lbs. of coal per hour to the horse power; which was about one-third of what any steam engine of the same power would require. Large steam engines sometimes run with as little coal in proportion to their power.
Mr. Babcock explained, by the aid of several large and finely executed diagrams, the motions of the several parts, and the pressure under the working piston at each point in the revolution, which attracted much attention. He explained that the pressure was derived from theory, and would be somewhat less in practice; but would not probably vary more than 25 or 80 per cent from that indicated upon the diagrams. The maximum pressure by the diagrams was 22 pounds above the atmosphere; the pressure being about two-thirds that amount at the commencement, increasing to near one-quarter stroke, and thence declining till the exhaust valve opens.
The President.—Inasmuch as all that you do to the air is to double its volume, while in raising steam you increase the volume 1700 times, wherein does the economy lie?
Mr. Babcock.—In the greater facility of heating air, and its small amount of specific heat, which is only one-fourth of that of water. There is also much heat lost in raising water to the boiling point.
The President.—Is this engine free from danger?
Mr. Babcock.—Perfectly free from all danger.
Mr. C. A. Seely expressed his surprise that the peculiar circumstance of the difference of the specific heat of air and water had not been mentioned before. The same heat will heat a pound of air four times as high as a round of water.
Mr. F. Dibben said the Wilcox engine was very similar to the Ericson engine, but in comparing the two, and estimating the contents of the working cylinder, it would be necessary to take the contents of both cylinders in the Wilcox engine, since the two correspond to the one in Ericsson’s. He failed to appreciate the difference between Mr. Wilcox’s engine and the engines in the caloric ship Ericsson, in 1853.
Mr. T. D. Stetson explained one of the great points in which the Wilcox engine differed radically from those in the Ericsson. In that ship, the supply of air was forced in by pumping it through large force pumps, against the pressure which obtained within. In the Wilcox engine, there is a period while the working piston is descending, where the whole interior of the engine is in free communication with the external atmosphere. During that period, the changing piston descends and inhales a full charge of cold air above it, precisely as the air enters an accordion when it is expanded. The descent of the changing piston occasions no resistance, because there is then no pressure against the under side of that piston. The moment it is thus inhaled, the induction port closes, and the dense cold air is subsequently transferred by the rising of the changing piston into the hot part of the engine, when, by its expansion, the working piston is forced up, and power is developed.
The rising of the changing piston occasions no resistance, because while it rises the same pressure obtains on its under as on its upper side, whatever that may be. The two sides are in free communication through the openings in the economizer. As the changing piston rises and compels the air above it to pass down trough the economizer into the hot part of the engine, the pressure rises in consequence of the heat received by the air; but it is felt equally on the upper and under side of the changing piston, and is only sensible on the working piston, which latter receives the pressure on its under side. The upper end of the working cylinder it always opens to the atmosphere.
The question of most interest, Mr. Stateson believed, was not the difference between this and the previous varieties of air engines, but whether either or any had practically solved the problem presented, and was really a successful and important machine. He believed that both Ericsson’s and Wilcox’s engines were fairly entitled to be thus considered. Between five and six hundred of the Ericson engines, and a small number — about a dozen — of the Wilcox engines are now in daily and successful use.
Mr. Roosevelt inquired if the caloric yacht was not lying up. Was she a success? He had seen a boat driven by a “six horse power” caloric engine, which could be driven as well by two men with oars. He could stop any caloric engine by pressure upon the periphery of the fly wheel with an axe
Mr. Stetson said the engine was to his certain knowledge, doing efficiently and satisfactorily the work for which it was purchased, in a great number of instances, without involving any expense for attendance, or increasing the yate of insurance. He confessed that Ericson’s engine are very much over-rated in their power. He had tested one carefully by the friction-brake. It was an l8-inch engine, employed in driving printing-presses at Dodge & Grattan’s, in this city. It was rated by some at four-horse power. Mr. Stetson found that, when diligently fired, it performed with exactly two-thirds of one horse power.
But the extravagance of some estimates should not lead us to under-rate its actual performance. The caloric engine, both of Ericsson and Wilcox, was a success. It was difficult to compare strictly with steam engine. The performance of an engine depends upon many conditions; so that a steam engine of “two-horse power” may do the work of only one man, or of six or eight horses. The expense and trouble of replacing the heaters is very small. The Ericsson heaters are much more exposed than the Wilcox heaters; but even in the Ericsson engine the most exposed parts endured a year or more with moderately hard firing, and were replaced at an expense of only $15. The great economy of the caloric engine, mainly arose from the ease with which it may be kept in operation without a professional engineer.
Mr. J. K. Fisher remarked that some steam engines were worked at less than atmospheric pressure; so that the safety of those steam engines is as great as that of the caloric engines.
Lieut. Bartlett thought there should be no contest between steam end air engines. The steam engine has proved itself to the world. But there is a great want of an economical very small power, which requires little skill or attention. He had not hesitated to say to Mr. Ericsson that his success and his tame would rest upon the fact that he had supplied a little power, which was a very great necessity in the community.