The fire of coal or gas is below the cylinder d, which is water-jacketed at the upper end, x x. In tank pumping engines, the pumped water cir culates through the water-jacket. At A is the hollow displacing piston, and Ii is the working piston proper. The displaces is coupled to the bell crank k, and so to the crank c, to which the beam a is linked directly by g. These engines do not use less fuel than steam-engines of similar size, but as they require no water or licensed engineer. they have come into considerable use.
The Stirling etigine and the Ericsson engine between themselves embodied all the characteris tic features of the several types of hot-air en gines defined above. The Ericsson engine is an open-cycle, non-regenerating engine, with tem perature changes at constant pressure; the Stir ling engine was a closed-eyele,regeneratingengine, with temperature changes at constant volume. The hot-air engines produced by other inventors of this kind of prime motor have resembled sometimes the Ericsson engine and sometimes the Stirling engine, but have had the details of oper ation worked out in different ways. Two of these only need be mentioned for the purpose of illustration: The Wilcox engine, of which large numbers were made about 1860 to 1815, was, like the Ericsson ship engine, an open-cycle, regenera tive engine, with temperature changes at con stant pressure. Its distinctive characteristic was a peculiar supply cylinder fitted with a piston operated from the main shaft. This supply cyl inder took in the atmospheric air and passed it through the regenerator to the operating cylin der, where it was heated and expanded to per form its work, after which it was exhausted through the regenerator into the external air. The Merrill engine, one of which of 10 horse power was used for some years previous to 1885 to run a factory at Winehendon, Mass., worked on the same principle as the Stirling engine; that is, it used the same volume of air over and over. It had two working cylinders, each of which was double-acting (see STEAM-ENGINE), and two reverser plungers, which affected the transfer of the air between the heating and cool ing devices.
Mention was made at the beginning of this article of hot-air engines employing previously compressed air. These are commonly known as compression engines. In them a constant quan tity of air is constantly changed in volume, being compressed while cold and expanded while hot. There are usually two cylinders, one cold and kept cold by a water-jacket or other means, and the other hot and kept heated by external means. The piston in the hot cylinder is gen
erally timed from one-sixth to one-quarter revo lution in advance of that in the cold cylinder, whereby the air is first changed into time cold cylinder, sometimes through a regenerator. then compressed therein, then changed to the hot cyl inder back through the regenerator, taking up again the stored heat, and finally expanded in the hot cylinder. The first engine of this kind seems to have been invented by Charles Louis Felix FranchoL a Frenchman, in 1853, and it is de serving of brief mention for the clear manner in NVI] jell it illustrates the working principle. llot and cold cylinders of different areas were placed side by side, as shown by Fig. 3. with their pis tons connected to cranks 135° apart. The bot tom of the cold cylinder, .1, was connected to the top of the hot cylinder, /3, and vice versa through heating and cooling chambers, C and D. ing Stirling regenerators. The arrangement of the pistons was such that the air was compressed in the cool cylinder ; passed through the regenera tor into the hot eylinder, where it was expanded; then transferred to the cold cylinder through the cooling chamber, and the cycle repeated. From four to six cylinders. each double-acting, were proposed to be combined in a series. A model of one of these engines was exhibited at the Paris Exposition of 1855. This some principle was embodied in all engine patented by the famous engineer Sir William Siemens, in ltiaitl, but he neglected to put it into practical tine. The Siemens engine employed four cylinders, each hot at one end and cold at the other, all con nected to one shaft, and so arranged that the hot. end of one communicated through a regenera tor with the cold end of the next. The heat was supplied by hot products of combustion from pro ducer gas in a chamber eonnected with the hot ends of the cylinders. while the opposite ends were fitted with refrigerating devices. One of the latest forms of compression engines is the Bider, shown by Fig. 4. In this sectional view C is the cold cylinder or compression cylinder, surrounded by the water-jacket, E, and D is the hot cylinder. The compressed air from the cold cylinder is changed through the regenerator, II, to the hot cylinder, where it is eximmled by the heat from the grate underneath the cylinder.