AIR-ENGINE. Engines which have for their working fluid heated air instead of steam are called "air-engines." The name " caloric engine " has also been applied to them, but is not to be commended, for they have no more right to that title than steam-enginesthe useful effect of both machines being due to the transformation of heat into mechanical energy, the air in the one case and the steam in the other being merely convenient media through which to effect that transformation.
The utilisation of the expansion of heated air for driving an engine has for many years been a subject which has exercised the ingenuity of inventors. The history of air-engines has, however, been little more hitherto than a history of failures, and they are as far now from super-seding steam-engines as they were fifty years ago. This is owing mostly to the fact that the inventors have too often worked empirically, without any real knowledge of the conditions under which, and under which only, the real advantages of the fluid could be attained, and have therefore continually violated these conditions. There are also certain constructive difficulties in the way of making a successful air-engine which have never been fully over-come. It should be distinctly understood that, regarded simply as a medium for transforming heat into work, air possesses no advantage over steam or any other fluid. Its advantage is, that it can be used with safety at much higher temperatures than steam (and therefore a larger propor-tion of the heat given to it can be transformed into work), and that by employing the gases of combustion in the cylinder much heat can be utilised which with steam-engines is necessarily wasted.
Of the air-engines which have actually worked we have_ (1.) Those in which the changes of temperature take place at a pair of constant volumes ; (2.) those in which the changes of temperature take place at a pair of constant pressures ; and (3.) those in which heat is received and rejected at a pair of constant pressures. The first two classes, fitted with "'économisera," are in theory "perfect" engines ; that is, they are theoretically capable of transforming into work the largest fraction the limits of tem-perature allow of the heat received from the fuel. The third class are not perfect engines, but possess certain practical advantages which will be afterwards mentioned.
The well-known engine invented by the Rev. Dr Stirling in 1816, and subsequently improved by him, in conjunction with his brother, Mr James Stirling, C.E., of Edinburgh, belongs to the first class. In this engine the same mass of air is used again and again, and is compressed at starting to a pressure of 7 to 10 atmospheres. A cylindrical air-receiver, in which a plunger can be moved up and down, is placed over the flue of the furnace. The annular space between the plunger and the sides of this receiver is occu-pied by an immense number of thin sheets of metal, which form the "économiser." In the upper part of the receiver, which communicates freely with one end of a working cylinder of the usual construction, is a "refrigerator," consisting of a coil of tubing through which cold water continually circulates. The plunger is alternately raised and lowered by suitable mechanism, and in its motion causes the great body of air in the machine to occupy alter-nately the bottom or heating end and the top or cooling end of the receiver. It thus undergoes alternate expansion and contraction, and thereby gives motion to the piston of the working cylinder, and thence to a crank shaft in the usual way. The advantages of this engine were, that the air in the cylinder was always cool, and that the great pressure which could be used rendered the size of the machine for a given power very moderate. It was ultimately abandoned because of the failure of the receiver to stand the destructive action of the heat.
The most familiar example of the second class of air-engines is that invented by Captain Ericsson. It differed from Stirling's in many respects, and does not seem in any one particular to have been an improvement on it. Fresh air was drawn from the atmosphere at every stroke, and a very low pressure used, and what was the receiver in Stirling's engine became the working cylinder of Ericsson's. It was thus excessively bulky in proportion to its power, and all the working parts were exposed to the destructive action of intense heat. It is chiefly interesting on account of the enormous scale on which its construction was actually carried out. The engines of the steamship '' Ericsson " had four working cylinders, each 14 ft. in diameter, with other parts in proportion. The trials of this vessel were conducted in a manner which did not allow any confidence to be placed in the results said to be obtained, and steam-engines replaced those of Ericsson within two years.
To the third class of air-engines belong those of Sir George Cayley and several of the older inventors. The best known modern ex-ample is, however, the engine of Mr Philander Shaw, which is shown in our engraving, and which was exhibited at the Paris Exhibition of 1867. The most important feature of this type of engine is the use of the products of combustion themselves, instead of merely the air heated by them, to drive the piston. The con-struction of the engine is very simple : the working piston is fitted with a trunk on its upper side, which, thus reduced in area, serves as a compressing pump, and the products of combustion act directly upon its under side, which is protected by a large drum filled with non-conducting material from the heat. The furnace stands beside the cylinder, and is entirely closed up, means being provided for feeding it with fuel without allowing any air to enter. The air compressed by the pump is delivered into the furnace, where it com-bines with the fuel to form the gases of combustion, and in this way receiving additional heat, expands, and raises the piston of the working cylinder for a portion of its stroke. The admission-valve of the latter is then closed, and the gases expand, without addition of heat, until the piston has completed its stroke, and are then discharged into the atmosphere. By the addition of an " economiser," the efficiency of this type of engine may be very greatly increased ; but its principal advantage is that, by actually using the products of combustion inside the engine, much heat is saved which in other engines is unavoidably sent up the chimney and lost.
One of the principal features of all air-engines is the "economiser" (sometimes erroneously called the "regenerator "), an invention of Mr Stirling's. The object of this apparatus is to store up the heat rejected by the fluid when it falls in temperature, and subsequently to raise the tem-perature of the fluid by re-storing the same heat, so that the only heat which the furnace has to supply is the latent heat of expansion, together with the amount of sensible heat which may be lost through the imperfection of the economiser.
(For a popular explanation of the theory of air-engines, see an admirable paper by the late Professor Rankine in the Edinburgh Philosophical Journal for January 1855 ; and for a complete account of the same, involving the use of the higher mathematics, see the same author's Steam-Engine, pp. 345, et seq. See also Prof. Clerk Maxwell's Theory of Heat, and a series of papers on the subject in Engineering, 1874.) (A. B. W. K.)