An immense amount of ingenuity has been expended in devising engines in which the rotary motion of the shaft is obtained directly from the piston without the interven tion of reciprocating parts. These machines are called rotary engines; they have never come into general use, and most of them have been defective in construction, as well as founded on a dynamical misconception.
In locomotive engines it is necessary that the whole machinery should be compressed into the smallest possible bulk, and this necessity is the cause of their principal pecul iarities. The engine itself is much the same as an ordinary horizontal engine, and has two cylinders placed side by side near the front of the locomotive. These cylinders are sometimes placed inside the main framing, which runs the whole length of the engine, and sometimes outside it, each plan having certain advantages. Fig. ri is an outline section of .au "inside cylinder" goods locomotive belonging to the Midland railway company. At the back of the locomotive is the fire-box a the bottom of which is formed by the grate b. Fuel is introduced by the door c. The fire-box is inclosed in a d, and the space between is filled with water. This space communicates freely with the barrel ee of the boiler, a long wrought-iron cylinder. From the back of the fire-box numerous small tubes traverse the boiler (through the water) to the smoke box!, and conduct the products of combustion to the chimney g. The steam pipe 1c, is led away from near the top of the dome h, and fitted with a regulator valve 1. At in are a pair of spring safety-valves. Both cylinders discharge their steam through the i vertical blast-pipe p, and by this means a sufficient draught is caused, notwithstanding the small height of the chimney. The cylinders r are placed in the bottom of the smoke box, and partly inclosed in it.
In all marine engines, except the very smallest., two cylinders are used, working cranks at right angles to each other, so as to equalize the motion as far as possible, it being almost impossible to use a fly-wheel of sufficient weight for that purpose on hoard ship. In vessels of war, where it is essential that all the machinery should he kept below the water-line, horizontal engines are used, often of the "trunk" type. In mer
chant vessels, however, and in all cases where there is no necessity for the machinery being kept low down in the ship, the form known as the "steam-hammer" engine, or some modification or it, is now almost universally adopted. These engines derive their name from their reseifiblance (in their earlier designs) to Mr. Nasmyth's steam-hammer, the form of which seems to have suggested their arrangement. They are direct acting, but the cylinders are inverted, and placed right above the propeller shaft.
The two greatest improvements in the modern steam-engine—the surface condenser and the compound engine—have been brought to perfection chiefly in connection with marine engines, and we may therefore mention them here. In the surface-condenser, the steam is condensed by contact with the surface of a great number of small tubes, through which a current of cold sea water is kept constantly flowing. By this means the condensing water and the condensed are kept separate, the former bei,pg returned to the sea, and the latter only sent into the hot-well. The boiler, therefore, is continu ally fed with distilled water, and the wasteful process of "blowing off," to get rid of the unvaporizable matter which would otherwise be deposited in the boiler, is rendered unnecessary.
In " compound" engines, the two. cylinders are of unequal size—the larger, called the low pressure cylinder, having from three to four times the capacity of the smaller or high-pressure cylinder. The steam from the boiler is admitted into the latter in the usual way, and cut off generally at from s to of the stroke; and after doing its work there, it is conducted to the large cylinder, where its reduced pressure, by acting on an increased area, does as much work as in the other cylinder, and thence to the condenser. This system of engine has several notable advantages—amonm which are that the inter nal stresses are More uniform than in ordinary engines; that leakage past the piston becomes of less importance; and that for any given large measure of expansion, the mechanism of the engine is much more simple and less liable to get out of order than for the same degree of expansion carried out independently in two cylinders.