Air-engine, a form of heat-engine in which air is the working substance. Captain Ericsson called his latest air-engine a caloric-engine (see CALORIC).
It is a well-known law, applicable to all heat-engines, that (presupposing the merely mechanical part of the machine to be perfect) the heat converted into work bears the same proportion to the total heat given to the working substance as the range of temperature bears to the absolute temperature of the source of heat. Thus, supposing an engine to receive steam (and the law is the same for steam, air, or any other substance whatever) at the temperature of 275° F., and discharge it at that of 120° F., the fraction of heat which it can convert into work will be 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 in the machinery; but these being equally likely to occur in all prime movers, we need not consider them here. The lowest limit of temperature 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, comparatively 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 temperature 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 utilises the heat which would otherwise be rejected into the chimney, and so prevents considerable waste of fuel.
The more heat carried away by the discharged air—the higher its temperature, in other words—the smaller evidently is, ceteris paribus, the range of temperature of the machine, and the less, therefore (as already explained), will be its efficiency. The distinctive principle of the Messrs Stirling's air-engine, as of the later air-engines, consists in utilising a great part of this wasted heat, and thus economising fuel. This is effected by means of a 'regenerator,' or, more properly, 'economiser,' consisting of a chamber filled with metallic sieves of wire-gauze, through which the hot air is made to pass outwards 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 up by the regenerator, and its temperature thus reduced, the range of temperature of the machine is correspondingly increased. The fresh air entering the cylinder for the next stroke is compelled to pass inwards through the regenerator, and abstracts from it the heat left in it. In this way it does not require to receive so much heat in the furnace as would otherwise be the case, and thus economises fuel.
The figure shows one of Stirling's air-engines. E is the plunger which works in the receiver CAB. This receiver is double, the inner lining being pierced with small holes to admit of communication between the annular space and the interior. The regenerator is placed in the space AC. By the motion of the plunger the air is alternately admitted to the upper and under portions of the regenerator, passing through the regenerator. The upper portion is always in communication with G, the cylinder, in which the piston H works. At D is placed the 'refrigerator,' a coil of copper tube through which cold water passes. This refrigerator abstracts the heat wasted through imperfect action of the regenerator.
This method of preventing waste of heat was first discovered by the Rev. Dr Stirling, 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 with a steam-engine of the same power; and in order to obviate this objection, compressed air was used, the idea originating with Mr James Stirling, C.E. Several other difficulties were successfully surmounted by the Messrs Stirling, and eventually 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 three 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.
Captain Ericsson, in his attempt to introduce his caloric-engine 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 give perpetual motion—and under these circumstances it is not to be wondered at that his machines (notwithstanding some not very creditable manoeuvring on the part of their up-holders) entirely failed, and that in two years (1835) 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 utilised as the source of heat. These have been called solar engines. For a very different kind of air-engine, see COMPRESSED AIR ENGINE.