CALORIC ENGINE, or HOT-AIR ENGINE. An engine in which the pressure acting on the piston is produced by increasing the tem perature of air through the application to it of heat by transfer through a separating metal wall. This definition distinguishes the hot-air engine from the compressed-air engine (see COMPRESSED AIR ENGINE) on one hand, and the internal com bustion engine (see GAS-ENGINE) on the other hand. There are, however, hot-air engines which employ both previous compression, and internal combustion. The action of hot-air engines, like that of all other heat-engines, consists in admit ting the air at a high temperature and pressure, and by allowing it to perform work on the piston reducing its pressure and temperature, after which it is either exhausted into the atmosphere and a fresh supply is introduced, or else it is again heated for a repetition of the former process. In their principal working parts hot-air engines 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 com pelled to move up and down and transfer its mo tion to a revolving shaft by means of piston and connecting rods and the other usual meehanisms of steam-engines. (See STEAM-ENGINE.) Hot air engines are of several types, tvhieh may be described and explained as follows: Closed-cycle engines are those which operate continuously with the same mass or weight of air, only taking in a fresh charge to replace leakage or to in crease the mass in use. Open-cycle engines; are those in which at each stroke a new charge is drawn in from the atmosphere, and after being heated and expanded is exhausted again into the atmosphere. Regenerative and non-regenerative engines are those which, respectively, use or do not use a regenerator to absorb the heat of the exhaust air and to restore it to the incoming cooler air. Finally, closed-cycle engines may be divided into two subclasses, which differ by hav ing the temperature change take place in the air at constant pressure or at constant volume. Each of these types is identified with the name of some designer or engineer. Hot-air engines have been designed in great numhers, but the limited extent to which they have been used makes most of them but little more than names. The hot-air engine as defined at the beginning of this article seems to have been invented by the Rev. Robert Stirling, an Englishman, in 1816. Ilis first successful engine was built in 1827, and one afterwards was used in a foundry in Dun dee, Scotland. developing 20 brake horse-power on the consumption of 50 pounds of coal per hour. In this engine, shown diagrammatically in Fig. I, the same volume of air was alternately heated and cooled, producing a variation of pressure which actuated a working piston. The heating and cooling were effected by changing the air by means of a plunger, from end to end of a cyl inder. A, one end of which was kept hot by a fire and the other cool by a coil of water-pip', C. On
its way from end to end the air passed through a passage partly tilled with thin plates of metal, E, which alternately absorbed the heat from the air and gave it back on the return. This was the first application of the economizer or regenerator, and its invention is said to be due to James Stir ling, a civil engineer. This engine failed through the giving out of the heaters, which required to be kept red-hot. In 1844 Franchot. a Frenchman. patented an arrangement of the Stirling engine with large and eflicient heating and cooling sur faces. Further attempts at improvement were made by Rankine and Napier, and by Professor .Tenkin in England. and also by Lanberau in France. The Stirling engine belongs to the type of hot-air engines with temperature changes at constant volumes. Another hot-air engine which has much the same classic and historical interest as the Stirling engine is that invented by John Ericsson (q.v.). The engines of Ericsson dif fered from the Stirling engine in that they drew their supply of air from the atmosphere at each stroke, heated it, allowed it to expand while doing work, then exhausted it again into the ex ternal air; they belonged to the type of hot-air engine with temperature changes at constant pressure. The first engine was installed in the ship Ericsson in 1852, but the idea dates from 1833, and it enjoys the distinction of having been built on the largest scale and of having made the most noted failure of any hot-air en gine. To Ericsson is to be credited also the term 'calorie engine,' which he applied as a sort of trade name to his invention. Briefly described, Ericsson's first engine was designed for a 2:200 ton seagoing ship; it was intended for 600 horse power. but actually ran at about :300 horse power. There were four cylinders, each 14 feet in diameter and having 6 feet stroke, and the en gine ran at nine revolutions per minute. As stated above, the engine proved a failure after several attempts had been made to remedy its faults. First the 14-foot cylinders were removed and replaced by others, which also failed; and finally the engine was replaced entirely by a steam-engine. Afterwards Ericsson made an other attempt to drive a ship by an air-engine. The Primcra was built and fitted with horizontal engines drawing their supply from, and exhaust ing into, an artificial atmosphere of high pres sure. As in the former attempt, however, the heating surface proved inadequate and the avail able pressure was too small to give much power, so that again steam was substituted after a short trial. Economizers were employed with both of his large engines, but in the small engines, to the design of which Ericsson turned his attention after the failure of his large motors. the econo mizer was abandoned. Ericsson's first design for a small motor was brought out in 1800, and in 1880 the latest form for pumping purposes was produced. Fig. 2 shows this type of engine.