The exhaust from the low-pressure cylinder or cylinders of a compound engine in which there are only two successive expansions (or from the last cylinder or cylinders, where there are three or four successive expansions) may be either discharged into the atmosphere (in which case the engine is said to be " compound non-condensing") or conveyed to a condenser, in which case the entire system is said to be "compound condensing." Fig ure r (pi. SS) shows a tandem-compound condensing engine, both pistons acting on the same rod, and there being no receiver and but one crank.
The Corliss Engine has the original cock used to effect steam distribu tion developed into an oscillating plug, and the hand-power or simple cords employed to open and close it have been replaced by a beautiful automatic system, by which the governor permits a weight or an air-spring sud denly to close the valve when the time for cut-off (as determined by the governor itself) has arrived in each stroke. The use of four valves reduces to a minimum the waste clearance-space between the valves and the coun terbore. The valves are given partial rotation by rods from a wrist-plate oscillated by an eccentric and giving sudden opening and prompt clos ing, while practically holding the valve still between opening and clos ing times. When it is time to cut off, the admission-valve is sharply detached from the driving mechanism. The detaching mechanism is directly connected with the governor, which has not to do the actual work of valve-moving. When either admission-valve is detached from the driving mechanism, it is closed by a spring, a weight, or a vacuum-pot. Thc oscillations of the governor are controlled by a dash-pot.
7:andein-compound Corliss Evian—Figure 4 (pl. 914 is a good type of the Corliss engine arranged as a "tandem-compound "—that is, with the high- and low-pressure cylinders in the same axial line. In the exam ple given, instead of the admission- and the exhaust-valves of each cylin der being worked from a common "wrist-plate" or disc, as is usual in the Corliss construction, there is in each cylinder one crank-disc to work the two admission-valves and one to work the two exhaust-valves. The two admission-valve discs are connected by a " parallel-rod," as are the two exhaust-valve discs. Each of the limn admission-valve cranks is in communication with the regulating device, and also with the vertical rods extending from the dash-pot pistons and with its admission-yalve wrist plate, so that when the governor and its attached regulator-rods disengage the valve-cranks from the control of the wrist-plate rods, the valve is sud denly closed by the action of the dash-pot rod. The wrist-plates of the high-pressure cylinder (the one nearest the crank) are actuated by eccen tric-rods from eccentrics on the main shaft, and in turn, by the parallel iods shown, give motion to the wrist-plates on the low-pressure cylinder.
A lengthwise central vertical section of a horizontal Corliss engine is shown in Figure i. Both admission-valves are closed, as is also the left hand exhaust-valve, the right-hand exhaust-valve being open. Figure 2 shows the "tangent crab-claw" which hangs to the shaded piece shown in the cut, and which is attached to the valve-crank until the action of the governor depresses the entire claw- or thumb-and-finger-like member and allows the vacuum dash-pot piston, which has been drawn up by the crab-claw as the latter opened the admission-valve, to fall suddenly, thus closing the valve and cutting off steam quickly and sharply. The vacuum dash-pot is shown, with a part of its rod, in Figure 3.
ne Centennial Exhibition CorhIcs Eir.cines were such a marked depart ure from the usual type of construction of their builders, and by reason of their psition were so prominent and familiar, that we have selected them for onr ErentelOicce.
These engines have a double-acting vertical beam, constructed upon the Corliss pattern. The frame is A-shaped, the beam-centre being at the vertex, with the cylinders and main shaft at the base angles; the various parts of the frame are in the hollow or box form, and the corners are flat tened, producing a section almost octagonal. The cylinders are 4o inches in diameter, io feet stroke, and are rated at fifteen hundred horse-power collectively, with a capacity up to twenty-five hundred, the lesser power calling for about twenty-seven and a half pounds mean effective pressure per square inch. The single shaft to which they are connected carried at the Centennial Exhibition a gear-wheel 30 feet in diameter, 24 inches face, having two hundred and sixteen teeth, cut with a pitch of 3.183 inches. It has been stated that this is the largest cut iron gear ever made; it weighs fifty-six tons, and, at thirty-six turns per minute, its periphery travels at the rate of about thirty-eight miles per hour. The crank-shaft carrying this wheel is 19 inches in diameter and 12 feet long. The cranks are of iron gun-metal, and weigh three tons each. The beams are 9 feet wide in the centre, 27 feet long, and each weighs about eleven tons. The connecting-rods, 25 feet long, are manufactured out of "scrap" iron, requiring in their construction ten thousand worn horse-shoes. The piston-rods are steel, inches in diameter, with a speed of 72o feet per minute. The gearing by which motion was imparted to the shaft ing at the Exhibition was in covered ways under the floor. The great gear-wheel drove a pinion ro feet in diameter, and parallel to its axis was a line of shafting diminishing from 9 to 8, 7, and 6 inches, the pinion gear weighing seventeen thousand pounds.