A plate illustrating a four-cylinder triple-ex pansion engine of a modern second-class cruiser for the United States Navy is shown with various parts indicated. The engine framing is sup ported on a bed plate made in sections and bolted by holding-down bolts to structural parts of the vessel. The engine framing is made of steel col umns braced by various cross rods. A more gen eral practice is to have east or wrought steel in verted I frames on one side which support the guides, and in the merchant service the con denser is generally cast in one piece with lower portions of the I frames. The cylinders are supported on top of framing and bolted to it by various fastenings. In large engines each cylin der is a separate casting; generally the valve chest is cast with the cylinder, making the whole a rather intricate casting. Cast iron is generally used for cylinders. Cylinders are fitted with liners, a, which form the bearing surface for the piston. The liners are bolted at the lower end to the bottom of the cylinder and the joint at the top is packed. Liners are of cast steel or cast iron; steel is stronger. but cast iron gives a bet ter wearing surface. The space between the cyl inder and liner is commonly used as a steam jacket. Lately the economy of steam jacketing, especially for fast-moving engines, has been ques tioned. All cylinders are fitted with covers, c, of cast iron or steel, and these are secured to the cylinders by bolts and nuts and the joints packed by gaskets. For large cylinders, smaller openings called bull's-eyes, d, are fitted for pur poses of examination. The pistons are cone shaped, made of forged or cast steel or cast iron, and fitted with cast iron spring packing rings, e. The piston rod is secured to the piston by its taper and the piston rod nut, f, on top. The opening in the bottom of the cylinder for the piston rod is fitted with a stuffing box. g. sup plied with some form of metallic The lower end of the piston is secured to the cross head 11, made of forged steel, which has a slip per, i, sliding on the crosshead guide. j. The crosshead also has journals. k, for the upper end of the connecting rod. The lower end of the con necting rod is attached to the crank pin. I, by means of the crank pin brasses. in. The crank shaft is supported by the main bearings, n, which are supported by n bed-plate. All large hearings are lined with anti-friction metal.
The valves o for the H. P. and I. P. cylin ders are piston valves, either single or double ported, hollow or solid. For the L. P., the double-ported flat slide valve, p, fitted with a relief ring is used. Piston valves are employed with high pressure because in them the pressure is on all sides and there is no force holding the valve against its seat. They are fitted with spring rings to make them tight. Valves are made of cast iron, cast steel, or forged steel. The valve seats are generally liners of cast iron. The valve stem is secured to the valve by its taper or shoulder and the nut q and the upper end r of the rod is fitted as a guide. The lower end of the valve rod is connected by means of a bearing to the link block s, which works in the link 1. To the ends of the link are attached the eccentric rod u, and the lower ends of these rods are bolted to the eccentric straps v, which move around eccentrics w. The eccentrics are secured to the shaft and fitted so that the posi tion can be slightly changed. There are two ec centrics, one to give go-ahead motion and the other backing. The link is moved by an arm attached to the reversing shaft a-, which is operated by the reversing engine 2/, and this en gine is controlled by the reversing lever. The link arm is attached to the adjustable cut off block, by means of which the cut-off can be varied from .5 to .75 of stroke. This is the
Stephenson gear, which is most generally used; others are Marshall's, Joy's, Morton's, etc.
In the cut shown, the air pump A is operated by a cross beam B attached to the L. P. cross head. In many merchant vessels the circulat ing, bilge, and feed pumps are operated from such a beam. For large installments these pumps are as a rule independent and of the Blake, Worthington, Snow, or other patent type. The office of the air pump is to pump the con densed water and vapor from the condenser to the feed tank and produce a vacuum. Surface condensers are now always fitted, and the steam and condensed fresh water are kept separate from the circulating sea water. This keeps salt out of the system. The condenser consists of an approximately cylindrical vessel, having a water chamber and a tube sheet at each end. Brass tubes connect the two tube sheets and cold sea water is pumped through the tubes by means of the circulating pump, thus cooling and condens ing the exhaust steam surrounding the tubes. Condensers are made of bronze or cast steel and the tubes of brass. Circulating pumps are cen trifugal and operated by a vertical simple or compound engine.
The feed tank is generally fitted with a filter chamber for purifying the water. Feed heaters, using auxiliary exhaust steam to heat the water before reaching the boilers, are fitted for pm. poses of economy and make the service less hard on boilers. The feed pumps are vertical, single, or duplex plunger pumps. Cylinders are lagged (i.e. covered with non-heat-conducting material) to prevent loss of heat. All cylinders are fitted with relief valves set at appropriate pressures, drains for conducting off any water that may ac cumulate. and indicator pipes. cocks, and reduc ing motion for taking indicator cards. Pres sure gauges are supplied to indicate the pressure in the steam pipe, the various receivers, and the vacuum in the condenser. Revolution counters are attached, which automatically record the number of revolutions of the engines. A water ser vice is supplied, consisting of a system of piping by means of which sea water can be circulated through such parts as the thrust bearing and crosshead guides, or sprayed on various other bearings where heating is likely.
The oil services on a modern engine are very elaborate, as all working bearings must be sup plied with a lubricant. The best practice is the manifold system, where each bearing has small pipe leading np to one of the several mani folds where it is fed by means of a wick.' The manifold can be filled from a reservoir placed above the level of the engine. Besides oil, graph ite and various preparations of tallow and grease are used for lubrication.
The steam pressures now used are 150-300 pounds. It is not likely that much higher pres sures than 250 will be soon used, on account of the great strength of parts necessary to with stand the pressure, the difficulty of keeping tight joints, and the high temperature of steam, which heats the working surfaces and prevents proper lubrication.
Steam is expanded in triple-expansion engines 6 to 9 times; in quadruple, S to 12 times. The ratio of the area of the H. P. to that of L. P. cylinder varies from 1 to 5 to 1 to 10. there be ing a greater ratio with increased pressures.
The revolutions vary from SO or 100 per minute in very long stroke engines to 400 to 500 in high-speed torpedo boats. The piston speed is limited to about 1000 feet per minute. The length of stroke for large merchant vessels is four to six feet; for naval engines not over four feet; with smaller engines the stroke is less. A relatively long stroke results in greater economy.
Of late the steam turbine is beginning to he in troduced in place of the reciprocating engine for fast vessels. See STEAM TURBINE.