Concentric cylinders were employed by Rowan, the steam being admit ted into a small cylinder and expansion continued in the larger one which surrounded it; the outer piston being ring-shaped, and having two rods fastened to the same cross-head as that of the inner one.
In the end-to-end double-cylinder type the steam commences its action in one end of a small cylinder, and completes it in the opposite end of a large one, the piston being attached to one rod. The space between the two pistons communicates with the condenser and is at all times a par tial vacuum. In Garrett's double-piston engine the steam commences its action in one end of the cylinder and finishes its expansion in the oppo site end, the former end having its capacity diminished by a plunger of large diameter passing through a stuffing-box and having one end fixed to the piston.
Elder's treble-cylinder compound the pis ton of the small cylinder drives one crank, and those of the two lateral large or low-pressure cylinders work a pair of cranks pointing in a direc tion opposite to the middle one; Rowan's treble-cylinder engine has the rods of the small piston and of the two large lateral pistons attached to one cross-head.
Triple and Quadruple is also accomplished in three or even four cylinders or sets of cylinders working successively. Where there are three grades of expansion the system is said to be triple (or treble) expansion, irrespective of the number of cylinders employed. The cylinders may be placed ill the saute vertical axis or may be arrai4;ed side by side. Where there arc more they may be arranged in pairs, in threes, or in fours, respectively side by side.
cylinders of triple-expansion engines may be either three, four, live, or six in number. One arrangement is for the " intermediate" to be under the high-pressnre and alongside the low pressure cylinder; another, for the low-pressure cylinder to be tinder the high-pressure cylinder and beside the "intermediate;" or all three may be in line. There may he two cylinders for high pressure, one over the "inter mediate" and the other over the low-pressure cylinder, or there may be two low-pressure cylinders beside each other, one with the high-pressure cylinder over it and the other with the " intermediate." There may be two high-pressure cylinders, each over a low-pressure cylinder, which latter have the " intermediate" between them, or there may he three low-pressure cylinders, side by side, having one high-pressure cylinder and two "intermediate" above them. A very convenient, although in
some respects complicated, arrangement is that by which the steam may be worked through all three cylinders of a triple-componnd engine with successive expansions; or two of the cylinders may be worked high pres sure and one low pressure; or two cylinders may he worked at the same degree of expansion at low pressure and the other at high pressure; or all three may be worked high pressure; or any one or any two may be thrown out altogether. Triple-expansion engines have proved more economical than the ordinary compound, the fuel consumption being about twenty five per cent. less. This is very largely due to the higher steam-pressure. Their wear and tear is rather less when three cranks are employed than where there are but two, as in the ordinary compound.
4 and 5 (i51. 86) illustrate a set of triple-expansion marine engines of eighteen'hundred horse-power (indicated), constructed by Craver() & Co., of Genoa, Italy. To economize space the three valves are placed behind the cylinders, and are worked by levers from ordinary link motions. The cylinders are perfectly free among themselves, to the end that they may expand and contract without restraint.
The following are the principal dimensions, which are given because such engines are not very common, and their proportions are not familiar even to professional engineers: Inverted-cylinder Triple-expansion illcrrine Engine.—Figures i and 2 (pl. 87) show a very recent example of a triple-expansion engine constructed for the screw steamer " Ivy." The engines are of the ordi nary inverted-cylinder marine type, with three cranks, and are designed for an initial working pressure of i6o pounds per square inch in the high-pressure cylinder. The high-pressure, intermediate, and low-pres sure cylinders are respectively 26, and 44 inches in diameter, and their common stroke is 36 inches. The high-pressure cylinder has a pis ton-valve, and the others have ordinary slides. The valve-gear consists of double eccentrics and double-bar link-motion. The cranks are of forged iron S inches in diameter in the body, and their pins are inches in diameter and 9 inches long. The piston-rods are 4 inches in diameter; the connecting-rods are 6 feet long and 4 inches in minimum diameter. The condenser is of the " surface " type and has Soo square feet of cooling surface.