CALORIC ENGINE. This•was the name given by capt. Ericsson to his latest air engine. There seems no reason for the change of name, unless it were meant to distin guish it from the previously well-known, though hitherto unsuccessful air-enginesof the Messrs. Stirling. We shall in this article treat air and caloric engine as synonymous terms.
It is a well-known law, to all thermo-dvnamie.engines. that (Presupposing the merely mechanical part of the machine to be perfect) the heat converted into work hears the same proportion to the total heat given to the fluid that the range of tem perature bears to the highest absolute temperature of the fluid. '1'1111s supposing an engine to receive steam * at the temperature of 275 F., and discharge it at that of 120°F., the 273-120 fraction of heat which it can convert into work will be 275+461 or about 21 per cent of the total heat of the fluid. This proportion would be, of course, greatly reduced in practice, owing to imperfections ia the machinery, but these being equally likely to occur in all prime movers, we need not consider them here. The lowest limit of tem perature available being practically constant, fixed either by the temperature of the atmosphere, or that obtainable in a condenser, it follows that greater economy can only be looked for in the direction of increase of initial temperature. In ordinary steam engines, in which the pressure and temperature increase simultaneously, the latter is limited by the former, which in its turn is kept, by considerations of safety, compara tively low. When, however, superheated steam (steam to which additional heat has been imparted without the corresponding addition of pressure) or heated air is used, the tem perature is limited only by the power of the metals composing the machine to resist the destructive action of heat, or the chemical action of the fluid at that temperature. Heated air possesses the advantage over superheated steam as a motive power, that with it an explosion, in the usual sense of the word, is rendered almost impossible, and that if one were to occur, it would be comparatively harmless. It also, of course, enables the boiler to be dispensed with.
Air-engines, in their principal working parts, are Very similar to ordinary steam engines. The heated air is introduced into a cylinder, in which works a tightly-fitting piston, which is thus compelled to move up and down, and transfers its motion to a revolving shaft by means of a piston and connecting rod in the usual manner. The motion of the piston results in all cases from the expansion of the heated air; the air is heated by means of a furnace, is introduced below the piston, raises it, and then is allowed to escape into the atmosphere. Air-engines are almost invariably single acting; they are sometimes worked simply by heated air, and sometimes with the air which, having passed through the furnace, is mixed with all the gaseous products of combustion. The latter method has the immense advantage that it utilizes the heat which would otherwise be rejected into the chimney The total efficiency of the machine is thus increased, although the efficiency of the engine proper, between the given pair of temperatures, remains the same.
The more heat carried away by the discharged air—the higher its temperature, in other words—the smaller evidently is, csrteris paribus, the range of temperature of the machine, and the less. therefore (as already explained), will be its efficiency. The dis tinctive principle of the Messrs. Stirling's air engine, as of the later C. E., consists in utilizing a great part of this wasted beat, and thus economizing fuel. This is effected by means of a "regenerator," or, more properly, "economizer," consisting of a chamber filled with metallic sieves of wire-gauze, through which the hot air is made to pass out wards from the cylinder, after having performed its work on the working piston of the engine. As much of the heat of the escaping air is taken np by the regenerator. and its temperature thus reduced, the range of temperature of the machine is correspond ingly increased. The fresh air entering the cylinder for the next stroke was compelled to pass inwards through the regenerator, and abstracted from it the heat left in it. In this way it did not require to receive so much heat in the furnace as would otherwise have been the case, and thus economized fuel.
This method of preventing waste of heat was first discovered by the Rev. Dr. Stir ling, who obtained a patent for it in 1816. In working with air at the ordinary pressure of the atmosphere, however, the engine was found to require to be of large dimensions as compared to a steam-engine of the same power; and in order to obviate this objec tion, compressed air was used, the idea originating with Mr. James Stirling, c.E. Sev eral other difficulties were successfully surmounted by the Messrs. Stirling, and event ually- two improved engines were constructed, one of which was tested to fully 40 horse power. This latter engine did all the work of the Dundee foundry company regularly for upwards of 3 years, during which period they employed no other motor. At the end of this period it was laid aside, principally owing to the repeated failure of one of the heating vessels.
Capt. Ericsson, in his attempt to introduce his C. E. in the ship which bore his name, experienced precisely the same difficulties and disappointments, and tried nearly the same remedies as the Messrs. Stirling. There seems little doubt, however, that he actually believed his " regenerator" was to make the same heat do work over and over again—to be a kind of perpetual motion—and tinder these circumstances it is not to be wondered at that his machines (notwithstanding some not very creditable maneuvering on the part of their upholders) entirely failed, and that 2 years (1855) they were replaced by steam-engines.
Air engines have recently been constructed, in which the solar rays, concentrated by means of an arrangement of mirrors, are utilized as the source of heat. These have been called solar engines.