The disadvantage of the hot-air motor as compared with the steam engine is that water takes up only -F7J-6-6 of the space occupied by the stec..m raised from it at atmospheric pressure, so that while a comparatively small boiler and pump may serve for an engine of great power, in the hot-air engine it is necessary to pump into the motor air of nearly as great a vol ume as is needed in the cylinder; that is, in the steam-engine the pinup is very small as compared with the boiler, while in the hot-air engine it must be nearly or quite as large as the working cylinder. This may be readily seen in Figure 4 (pl. io6).
Only those hot-air engines which permit the use of air considerably compressed act economically. If the air be compressed by the pump, its tem perature will be considerably increased; by reason of this it can absorb very little heat for the purpose of mechanical exhaustion. As it is not well for the cylinder that the air have a temperature of over 3oo° Cent. (on account of the difficulty of getting a lubricant to stand such high temperatures), the limit of practical ability of exhaustion is soon reached, and the neces sary size for a given horse-power is very great.
Ericsson' s 1S6o, Ericsson constructed a hot-air engine, such as shown in Figure 6, consisting of a single horizontal cylinder projecting (to the right) into a furnace, whose fire-doors and the channels surrounding the cylinder-like jackets are seen in the Figure. The cylinder is both a working cylinder and a pump cylinder, having two pistons, which by a very ingenious crank-and-lever mechanism are so moved toward and from each other that the drawing in of air, its com pression, heating, and expansion, take place in the order required for proper work. While a ball-governor is provided, a heavy fly-wheel serves to steady the motion and by its momentum to force back the piston. The trouble with this type was that it was soon destroyed by the heat and required too many repairs, the metal fire-box burning out, and also tend ing to destroy the cylinder and the piston.
The Ericsson Improved Motor (151. 107, fig. 5), as at present built for pumping, is a single-cylinder engine in which are two pistons, one called the " main " or air-piston (b), which receives and transmits power, and the other the " transfer-piston " (c), whose office is to transfer the air contained in the machine alternately and at the proper time from. one end of the cylinder (a') to the other.
The cylinder is provided at its upper end with a water-jacket (x), through which all the water passes on its way from the well to the tank.
This jacket keeps the upper end of the cylinder cool, while the lower end is exposed to the fire and becomes as hot as it is practicable to make it. By the peculiar arrangement of connections between air- and trans fer-pistons, the proper relative motions between these pistons are obtained. The operation is as follows: After the lower end of the cylinder has been sufficiently heated, the engine must be started by hand by giving it one or two revolutions. The air contained in the machine is first compressed in the cold part of the cylinder, and is then transferred to the lower end, where it is instantly heated and expanded, thus furnishing the power. This, like all other hot-air engines, is only single-acting. The momentum of the fly wheel continues the rotation until it receives an additional impulse by the repetition of the above-mentioned conditions, which occur once in every revolution. The same air is used continuously, and is cooled, com pressed, heated, and expanded in regular order.
The Rider Compression Hot-air Engine, as arranged for pumping, is shown hi Figure 4 (tr. 107). The compression-piston A extends down ward to the base of the engine, closely fitting the compression-cylinder II, which also extends downward nearly to the bottom of the cooler /1". The lower part of the compression-cylinder B is sufficiently smaller than the inside shell of the cooler IC to form a thin annular passage for the air, which becomes cooled on its way to the bottom, and through which pas sage it flows on its way back to the heater. The power-piston C likewise extends downward into the heater E, which in shape resembles the bottom of a champagne-bottle—that is, rising in the centre and presenting to the action of the fire a narrow annulus all ai'ound the bottom. Within this heater is the "telescope," a thin iron cylinder about of an inch less in diameter than the interior of the heater. This cylinder is fitted to the interior of the power-cylinder and extends nearly to the bottom of the heater. Its off= is to cause the air which flows from the compression cylinder to be presented in a thin sheet all around the interior surface of the heater, and particularly at the lower and hotter portion. The same air is used continuously, as there is neither influx nor escape, the air being merely shifted from one cylinder to another.