Caloric Engine, a prime mover driven directly by heat, without the intervention of steam. The first advantage of such engines is evidently the absence of steam boilers and the dangers incident to their use; and a second is the avoidance of the loss connected with the change of heat into motion by the intervention of steam, chiefly due to the great specific heat of water, and the still greater consumption of heat as latent heat in the act of evaporation. This is one of the causes that prevent the best steam engine from giving one tenth of the theoretical mechanical equivalent of the heat produced by the fuel consumed. Thus far, however, the results obtained from caloric engines have not answered the high expectations of their inventors and others. There are at present two kinds of such engines in use: first, those worked by the force of expansion of atmospheric air when heated, and secondly, those worked by the expansion of the products of combustion. Montgolfier in France, the inventor of the hot air balloon, was also the first, a century ago, to attempt to apply the expansion of heated air as a motive power; but it was not till 1816 that the invention assumed a practical shape through the labors of Dr. Stirling in England, who had then a caloric engine in use to pump water from a quarry, and in 1818 obtained a patent for it.
In 1827 a more efficient form of this kind of engine was employed by the Messrs. Stirling of Scotland; and in the same year Parkinson and Crosby of London constructed such an engine,in which a gas flame was used to generate the heat. Niepce of France, the colaborer of Daguerre in his photographic researches, also entered the field; and in 1833 Lieut. John Ericsson, while residing in England, brought out his first caloric engine. - The principle on which caloric engines are based is that when air is heated, and by confinement prevented from expanding, it will exert a pressure against the walls of the vessel in which it is contained, increasing with the temperature exactly in the same ratio as the air would expand if not confined. As air at 32° F., under a pressure of one atmosphere, expands to double its volume when heated to 522°, it exerts then a pressure of two atmospheres; at 1011°, of three atmospheres, etc. The air being enclosed in a cylinder with a movable heavy piston, this pressure will raise the piston with the force of 15 or 30 lbs. per square inch; and if then the air below is cooled or allowed to escape, the piston will move back downward by its own weight. This is the simplest form of caloric engine, and entirely similar to the oldest (Newcomen's) steam engines.
As the specific heat of the air is very small, it takes little heat to expand it, compared with that required to make steam from water; and this is the chief reason why in later times many practical minds have given their attention to the solution of this problem; several kinds of engines have resulted. The first caloric engine of Ericsson consisted of two cylinders and pistons of different sizes; the piston rods were so connected by a walking beam, that the ascent of one corresponded with the descent of the other (see fig. 1). The air in the small cylinder A was by the descent of its piston driven into the larger cylinder, but during its passage heated and thus expanded in a proper apparatus by a fire; this expansion being more than necessary to fill the large cylinder B, it exerted a pressure on its piston exceeding that on the smaller piston A. At the return stroke the hot air escaped from B, and a new supply of cold air was taken up in A by proper valves.
Fig. 1. - Ericsson's First Caloric Engine.
The improved Ericsson caloric engine, which he introduced in 1850, is represented in fig. 2. The cylinder A contains the fire box B, out of which the products of combustion pass through the channel G around the cylinder in the jacket H, and after heating those parts goes up the chimney J. The cylinder A is open at the other end, and possesses two pistons, the supply piston C, protected by non-conductors, and carrying a cap of light metal e e, fitting over the fire box B; and the working piston D, with two valves h. Two rods, not visible in the figure, are attached to the latter piston, which set the fly wheel h in motion by means of cranks acting on its axle c, and which at the same time, by an ingenious system of levers, move the rod E C and the supply piston C in such a manner that it moves through twice the length of stroke of the working piston D, and is always slightly ahead in its stroke. The working piston carries also a valve ring g, which alternately closes and opens the communication of the space between the pistons D and C with the space A. The valves k in the piston D are opened by an excess of pressure from without, and vice versa; the valve F, on the contrary, is opened by a projection on the axis c and closed by the spring f.
If now the pistons are moving from left to right, the space between D and C enlarges by the more rapid motion of C, and the valves h admit cold air, while the air in the space A and around the cap e e escapes by the valve F. At the return stroke from right to left this valve F closes; the valve ring g opens and permits the air between the pistons to flow into the space A, but in order to reach it, the cold air must pass the space between the cap e e, the outer cylinder, and the hot sides of the fire box B. Notwithstanding the short time that this contact lasts, the air takes a temperature of about 480° F., and this causes an increase of bulk of nearly double the volume, without pressure on the piston C, as the air passes by the annular valve g and fills the space between the pistons C and D. The pressure of this heated air on the piston D increases rapidly with the motion of the pistons to a maximum, which takes place when they have attained their greatest velocity; from this point to the end of the stroke the tension is reduced to about the atmospheric pressure. It is seen that the latter motion, produced by inward pressure on the piston D, is the power which drives the engine.
Many modifications have been made of the principles on which this arrangement is founded, but the high expectations entertained in regard to its economy, where a considerable power is required, have by no means been realized. About 1853 a large ship was built in New York, called the Ericsson, provided with caloric engines of the most colossal size, constructed under the supervision of the inventor; but the experiment was unsuccessful, and the owners were compelled to substitute steam for the caloric engine. - At present such engines are only used where a small power, say two or one horse or less, is required. For such purposes the second kind of caloric engines above referred to, in which the expansion of the products of combustion is utilized, has been the most successful. An engine of this class, that of Roper, is represented in section in fig. 3, and in perspective in fig. 4. The cold air is drawn in by the air pump at the left, through the opening A, its return being prevented by the valve B, and forced into the furnace through the valve D. Two dampers, E and H, serve to pass the air either under the grate through the fire or partially over the fire.
This air, being heated and mingled with the products of combustion, carbonic acid, watery vapor, etc, passes by the channels indicated by the arrows under the working piston, raising it by the difference in pressure on its large surface, with the small surface of the air pump; by the return or down stroke the air is expelled by the opening of the proper valves through the upward chimney seen at the right hand. This return stroke is made by the momentum of a large fly wheel. The piston consists of a long hollow drum, of which the packing is only at the top around the portion marked 1, in order to keep it at a distance from the fire and heat, which otherwise would soon destroy it.
Fig. 2. - Ericsson's Improved Engine.
Fig. 3. - Roper's Caloric Engine (section).
Fig. 4. - Roper's Caloric Engine (in perspective).