77,c Olio of 1890 (ti. io6, fig. 3)—a development of the type of 1876—has a cylinder open at olie end and having in it a piston connected to a fly-wheel by a piston-rod and a connecting-rod. Between the closed end of the cylinder and the piston there is a considerable space, wherein a charge of gas and air, drawn in by an entire out-stroke of the piston, is compressed. The openings for the inlet of this charge and its subsequent ignition arc at the closed end of the cylinder, and these functions are con trolled by a slide-valve found at that end. There is also a lifting-valve, to effect the exhaust of the burned gases after the combustion has occurred. The operation is as follows: On the out-stroke of the piston gas and air are drawn in, which on the return-stroke are compressed into the space referred to. This is done by turning the fly-wheel by hand, or, in case of large machines, by a smaller engine. When compressed, the charge is ignited and there is developed the pressure, which propels the engine during the subsequent stroke. Returning by the momentum of the fly wheel, the piston expels through the exhaust-valve the products of com bustion. When the speed is excessive, a governor prevents a fresh charge of gas from entering the cylinder, the air alone being admitted, thus vary ing the number of effective charges and the gas-consumption in proportion to the load upon the engine. Ignition is effected by various methods, according to the size of the engine, etc. Flame-ignition as well as an electric spark or an incandescent tube is used. The charge ignited is so constituted that the richest part, which will enter into combustion rapidly, is nearest the point of ignition.
Gas-motors proper differ from all other heat-engines in using as a motive fluid atmospheric air and as a mode of heating it atmospheric gas. So far the heating is accomplished by mixing them before ignition, but for all that it is the air, not the gas, which is the working fluid. This is heated and the heating fluid is burned in the cylinder instead of in a separate vessel.
Air being a fluid much poorer than water for conveying heat, the gas and the hot-air engines are in this respect at a disadvantage; but gas motors have the advantage over hot-air engines (p. 303) in the greater temperature that can be given to the air by explosion than by mere heat ing- in contact with metal plates.
The high temperatures attained in the cylinders of gas- and hot-air engines make it necessary, in order to prevent destruction of the metal, to employ larger cylinders in proportion for a given power than in a steam engine, and their friction is much greater. The gas-engine can employ a cooling-jacket without loss of heat for working purposes; the air-engine cannot do so.
The gas-engines of the present day may be divided into those igniting at constant volume without previous compression, those igniting at con stant pressure with previous compression, and those igniting at constant volume with previous compression. The first type is the simplest.
In rS6o the efficiency of the gas-engine was only about four per cent. of the theoretical maximum; in ISS6 it had risen to eighteen, and now it is probably about twenty per cent. In this it has surpassed the steam engine, which gives out only from twelve and a half to fifteen per cent.
of the possible theoretical maximum. At present it takes about zo cubic feet of gas per hour for each indicated horse-power. This could be re duced by complete expansion to the atmospheric pressure to about is cubic feet.
A great disadvantage of the gas-motor at present, as compared with the steam-engine, is the greater cost of each unit of heat supplied in the form of coal-gas than that of each unit supplied in the form of coal. A good gas-producer giving gas stable and free from tar is much needed. To improve any gas-motor, it should be made double-acting, and should be governed by diminishing the power of the impulses instead of their fre quency.
Engines, or so-called " caloric motors," use as a motive fluid ordinary air, usually heated in a vessel separate from the working cylin der. One was constructed in 1827 by John Stirling, and in 1833 the famous Ericsson experimented in this direction, but succeeded only in 1848. In 1853 he produced a marine hot-air engine intended to supply six hundred horse-power; it, however, indicated only three hundred horse-power.
AVer's Caloric engines driven by the gases of com bustion without explosion or compression is Roper's engine, which is well known in America, and is illustrated in Figure 5 (ffi. 1o6). Upon the cylindrical fire-box there is placed the working cylinder, and upon or alongside of this is the compressing cylinder. The air is forced into the furnace through the descending pipe, shown at the back, and after having been mixed with the products of combustion is forced into the working cylinder, where it drives the piston upward by its expansion. After igni tion the fire-box is closed tightly. The disadvantages of this system are that the heat of good combustion is from moo° to 1300° Fahr., that the excessive amount of air required will prevent proper combustion, and that cinders will be passed into the working cylinder. The use of coke does away in great part with the latter objection; gas offers a still better expe dient in this respect.
The Wilcox dolor, shown in Figure 4, consists of two cylinders sus pended for half their height in a furnace, the one in front being the working cylinder and the one behind it the feeding cylinder. The bot o toms of these project conically into the interior, and after being heated by the products of combustion in the fire-box heat the air contained in the cylinders. The lower ends of the massive pistons are hollowed out, so as to correspond,with the form of the bottoms of the cylinders, while the upper ends are level planes. The piston in the working c? hinder acts its ascent, being driven down by the action of the fly-wheel. The piston in the feeding cylinder is moved by a crank placed 75° from the working cylinder crank. The pump draws fresh air into the feeding cylinder and presses the heated air into the working cylinder. Between the two cylin ders there is a "generator" (wrongly so called), which consists of several layers of wire nets or metallic sieves; its function is to withdraw from the air escaping from the working cylinder a portion of the heat in that air, and to transfer it to new air forced through by the feeding cylinder.