The Engine shown on Plate SS (fig. 2) has the cylinders disposed in pairs, tandem fashion, the two next the crank-shaft being the high-pressure and the intermediate cylinders respectively, while the two rear cylinders are low-pressure and are of unequal diameter. It is in this inequality of diameter that one of the peculiarities of the engine consists. By this arrangement there are obtained two engines, either of which could be worked as an independent tandem-compound engine in case of accident, or which could be worked coupled as tandem compounds (there being then two high- and two low-pressure cylinders), in case of circumstances necessitating a reduction of boiler-pressure below that suit able for the triple-expansion system of working. The engine is thus available for working in three different ways, the necessary changes being made by the arrangement of pipes and valves connecting the cylinders. The arrangement is also one which could be advantageously adopted for converting a tandem-compound engine into the triple-expansion system.
In the engine illustrated the diameters of the cylinders are as follows: high-pressure, 11.o2 inches; intermediate, 15.75 inches; and low-pressures, 2o.oS inches and 35.43 inches. The stroke is 35.43 inches. The engine, running at seventy revolutions per minute with steam at an initial pres sure of one hundred and fifty pounds per square inch absolute, cut-off at from 40 to 5o per cent. in the first cylinder, will indicate three hundred horse-power.
Iii proportioning the cylinders of the engine it has been deemed of less importance to attain equality of power in the two halves of the engine than to secure as small a variation as possible from what is known as "the mean turning moment," it being of special importance for mill purposes to obtain the steadiest possible driving..
Only the high-pressure and intermediate cylinders are steam-jacketed. The front and rear cylinders of each pair are connected by three bolts passing through cast-iron distance-pieces. This arrangement makes a firm connection, and at the same time affords facilities for the examination of the stuffing-boxes, etc. The steam- and exhaust-valves are of the Corliss type. The exhaust-valves are nearest to the ends of each cylinder, and are so placed that they drain the latter. The steam-valves are driven from one sick and the exhaust-valves from the other side of each engine, this simplifying the disposition of the parts and rendering them more access ible. The steam-valves on the high-pressure cylinder are fitted with a trip cut-off gear controlled by the governor, which member actuates a rod car rying cams with serrated faces, the point of the stroke at which the detent is released depending upon those parts of these cams that act upon the releasing rods. Each rod operating the detent gear has a hardened steel chisel-shaped point, which collies into contact with the cam as the valve is opened by the action of the eccentric, the further movement due to the eccentric, after the detaching rod has been stopped by the cam, causing the rod to actuate the detent, and, by releasing the valve from the pull of the eccentric, to leave it free to be closed by the action of a spring. This
'gear has the advantage of throwing exceedingly little work on the gov ernor, the contact of the points of the releasing-rods with the cams being merely momentary. The bed-plate is made with a long foot under the crank-shaft bearing, this foot being extended toward the guides to prevent twisting of the bed under the action of the connecting-rod on the cross head. There are two vertical air-pumps, which are driven from the cross heads thraugh bell-crank levers.
Twin 4 (pt. 88) shows a twin triple compound engine arranged to drive the screw of a vessel. Of the six cyl inders, each has its separate supply of steam, but all work upon the same shaft. The large cylindrical vessel seen in front is a surface condenser— that is, one in which the steam is condensed by contact with metal sur faces cooled on the outer side by a current of water, and not, as in Watt's condenser, by the direct action of a jet of water.
Engines may have from four to eight cylinders. The most common arrangements are—(i) where the high-pressure is above the low-pressure cylinder, the first intermediate beside the high-pressure cylinder, and the second under this and beside the low-pressure; (2) where the high-pressure is above one low-pressure cylinder, with two "first intermediates" beside it, and a "second intermediate" below one of the first intermediates and between the two low.
Oman/pie good exam ple of a quadruple engine is shown on Plate Sg. While this style is intended for stationary purposes, it embodies many features of marine engines, as, for instance, double web-cranks instead of the usual overhung single cranks, and connecting-rods with Tends for the crank-pin bushes. The cylinders are unjacketed and are covered with non-conducting mate rial and lagged with polished teak; they are 12 inches, 16 inches, 22 inches, and 28 inches in diameter respectively, all having a piston stroke of 36 inches. Each pair is bolted to a bedplate of box section, with planed seats for the cylinders and pedestals. These latter are four in number, strongly bolted to the bedplates and adjustable by wedges. They are fitted with heavy gun-metal bushes, each cast in four pieces, rendered easily adjustable by two wedge-bolts. The crank-shaft is to inches in diameter at the inside journals and 7y, inches in diameter at the out side journals; it is constructed on the " built " principle, and is fitted with two sets of double web-cranks placed at right angles to each other. The heavy fly-wheel, 16 feet iu diameter, is built in eight segments bolted together; the rim has fifteen grooves suitable for ropes VA inches in ch.-. cuinference. The valve-gear is of the usual slide-valve pattern for all the cylinders except the high-pressure one, which is fitted with Proell's auto matic expansion-gear and governor. The exhaust from both ends of the cylinder is controlled by a single piston-valve worked by an eccentric and rod off the crank-shaft. The feed-pump is worked off the low-pressure piston-rod cross-head.