AIR MOTIVE-ENGINES. It has been already explained that air expands, or has its elastic pressure increased by the application of heat, and that its volume contracts, and its pressure becomes less, by a decrease of temperature; and several attempts have been made by taking advantage of this property of air, to substitute it for steam, as a prime mover of machinery. Could this be effected, it would be a great advantage in many situations ; as, for instance, where water is scarce, or in steam vessels or locomotive engines, where (the machinery forming a part of the load,) it is desirable to reduce the weight as much as possible. Having said thus much of the principle of these machines, we shall proceed to describe one or two of the latest arrangements for the purpoae.
The first we shall notice is Messrs. R & J. Stirling's air-engine, for which those gentlemen obtained a patent in 1827. This machine resembles the steam engine in the construction and application of many of its parts, such as the piston and cylinder, reciprocating beam and parallel motion, crank, and fly wheel, as ehewn in Fig. 1. Motion is communicated to the piston in the cylinder by alternately heating a portion of air connected with one side of the piston, and at the same time cooling that in connexion with the other. This is effected by means of the air vessels a a, one of which communicates with the upper part, and the other with the lower part of the cylinder, through curved nozzles, the pipe a forming the communication between one of the nozzles and the top of the cylinder. Fig. 2 represents a section of one of the air vessels whose sides are cylindrical, and top and bottom spherical. Vile air vessel, which is made of cast-iron, and supported in the brick-work by the projecting ledge 11, is furnished with a plunger c C c. The top and bottom of the plunger is made of strong sheet-iron, perforated with very numerous small holes to admit the air. The interior of the plunger is filled with very thin plates of sheet-iron, so bent as to prevent their flat surfaces from coming in contact, that the air may have a free passage between them. These are also perforated with small holes, which holes are not placed opposite to each other, but so arranged as to cause the air to pass through the plunger in a zig-zag direction. The plunger is formed circular, to fit the top and bottom of the air vessel when drawn up and down. The rim c c of the plunger, which moves in a cylindrical receptacle at the circumference of the air vessel, as represented, is not perforated as the other part. It is kept steady by a spring at 11, consisting of a belt of thin sheet-iron, attached at its upper edge to the rim c a number of slits are made at the lower edge of the belt, to admit of its being bent outwards to rest against the air vessel, and act as a spring. The plunger is also kept steady, in its ascent and descent, by the plunger rod d passing through the stuffing-box at the top of its case, and by the guide-rods g g, which work in the guide cases i i, Figs. 1 and 2. The
guides are fixed to a ring h h which is attached to the plunger and the plunger rod by the arms ff, four in number. The guides are supplied with oil by the oil-cup and stop-cock at the top of their cases. The top e e of the air vessel is flanged down in the manner represented at k, with a thin ring of sheet lead between the flanges, to keep the joining air tight. The lower part of the air vessel is heated by a fire placed under it, and its upper part kept cool by a current of cold air, by water, or by other means. The plunger rods of the air vessels a a Fig. 1, are attached by slings to the end of the beam v, so that the motion which elevates one plunger in one of the vessels, depresses that in the other. When the plunger is raised, the cold air in the upper part of the vessel will be heated in passing through the interstices of the plunger in its ascent, which has itself been heated on reaching the hot or lower part of the air vessel ; and during this time the air in the other vessel will be cooled by passing through the interstices of the plunger in its descent, which has itself been cooled by reaching the cold or upper part of the air vessel. These changes of temperature are further augmented by portions of the air being alternately changed from the hot to the cold and from the cold to the hot parts of the vessels, by the alternate occupation of the hot and cold parts by the plunger. Now, as one of the air vessels ts connected with the top and the other with the bottom of the working cylinder, there will be a motion produced on the piston by the alternate application of the expansive force of heated air : and this motion is communicated to the beam o through the piston rod and parallel motion, and joins the beam to the fly-wheel r s. On the axis of the fly-wheel is fixed an eccentric 1, which communicates motion to the plungers in the air vessels through the system of levers 1, 2, 3, 4, and the beam v, and this motion is adjusted so that the change of the plungers shall be effected whenever the piston reaches the top or bottom of the cylinder ; thus applying to that end of the cylinder where the piston is, the hot air, which, by ha increased elasticity, will drive the piston to the other end. This engine is also furnished with an air-pump, the piston rod of which is shown at r for condensing the air into the reservoir w w. The air is permitted to pass through self-acting valves into the curved nozzles, and thence into the cylinder, or the air vessels a a, but is not permitted to return from these vessels or the cylinder into the reservoir w w; which is also provided with a safety valve for the escape of superfluous air, when more is pumped in than is necessary to supply the air vessels. The air-pump is only occasionally required to be set to work.