proportions depend upon the initial pressure of the steam and the total range of expansion. For an engine to be economical as regards steam and fuel consumption, it is now fully recognized that the compound system must be adopted, and practically all large modern engines are designed according to that principle.

In a compound engine it is usually understood that the total

Tramways).

Steam-engine

are

and high-speed or quick-revolution engines: the latter mainly used for the direct driving of dynamos. An example of a high-class modern stationary engine is illustrated in Fig. 12. It is a coupled compound engine, cranks at right angles, and fitted with the trip-gear as shown in Fig. 11. The flywheel is grooved for rope-driving, and the governor is of the Proell type. If this engine be provided with a condenser, it may be fixed below the engine-house floor, or placed at the tail end of the lowpressure cylinder, the air-pump in either case being worked off an extension of the piston rod.

By referring to Fig. 11 it will be seen that there is an annular space J between the working barrel or liner and the outer cylinder casting; this space can be filled with steam at a pressure above the mean pressure of the working steam inside the cylinder. Cylinders built on this principle are said to be steam-jacketed the object being to keep the cylinder as hot as possible, and thus reduce cylinder condensation. The liner is usually made of a much harder iron than the main body of the cylinder. The engine bed is of the girder' pattern, and at one end forms the front cylinder cover. The stop valve is shown on the top of the high-pressure cylinder.

For electric light and power stations the engines are usually of the vertical type, having a high speed of revolution, and coupled direct to the dynamo. A type of this form of engine is shown in Fig. 13, in two vertical sections through the cylinders, etc. This Belliss engine is doubleacting, and one piston valve of a special design distributes the steam to both cylinders; arrows show the path of the steam as it passes from the high- to the lowpressure cylinder. The boiler steam enters the valve chamber through the opening s, and E is the exhaust port of the lowpressure cylinder. A separator A is shown in the view to the right; its object is to strain off any water that may come over with the steam. The engine is fitted with a centrifugal governor G, carried on the end of the crank shaft, and connected to an equilibrium throttle valve V. Forced lubrication is commonly adopted in large engines. Oil is pumped to all the bearings by means of a simple pump worked from the eccentric and is delivered at a pressure of from 10 to 20 lbs. per sq. in. through a system of oil channels, and the oil escaping from the bearings drains into the crank chamber, to be used over again.

Fig. 14 represents a vertical

425

section of a Willans ' central valve' triple-expansion engine, with two sets of cylinders. The three cylinders are placed tandem fashion, and the steam is distributed throughout by a hollow piston rod or trunk, in which works a line of piston valves, driven by an eccentric mounted on the crank pin. The Willans engine is single-acting-i.e. the driving pressure of the steam acts on one side only (the upper side) of the piston. The down stroke is therefore the working or effective stroke; consequently the connecting rods, of which there are two to each line of pistons (one on each side of the eccentric), are always in compression, never in tension. It will. be noticed that the trunk is provided with rings of holes or ports at intervals throughout its entire length; these rings of ports are for the purpose of admitting and discharging the steam to and from each cylinder as determined by the piston valves. Each piston valve gives just the same steam distribution as an ordinary slide valve, except that the actual cut-off is effected by the upper ports in the trunk, which at a prearranged point pass down through a gland into the cylinder, and so leave the steam-chest, thus preventing the further supply of steam for that revolution. When the steam has completed its work in any one cylinder, the corresponding piston valve is then in a position to allow the steam to pass (during the up stroke) from above the piston to the space below it, which acts as a receiver, but is really a steam-chest for the next cylinder. It is therefore evident that this particular engine must make three revolutions before the steam originally admitted to the high-pressure cylinder is finally discharged from the low-pressure cylinder into the atmosphere or into a condenser. Immediately below the low-pressure cylinder there is a guide cylinder, the top of which is closed, and on the up stroke the air in it is compressed to the extent necessary to cushion the piston and other parts. The reciprocating parts are thus 'held down' at all times, and do not depend for cushioning upon the compression of steam in the cylinders, as is the case in engines of the ordinary type. The work stored up in the air during compression is given out again on the succeeding down stroke without sensible loss. There are three types of the Willans enginesimple, compound, and triple-expansion, with one, two, or three sets of cylinders and cranks.

Fig. 20 represents the Westinghouse compound engine. Like the Willans crgine, it is single

Steam-engine

acting. Each of the connecting rods is attached at its upper end to a wrist pin within the hollow or trunk' piston. A single piston valve, located above the two cylinders and driven by an eccentric through a rock shaft, controls the admission of steam to both cylinders. In the position of the valve and cylinder shown, steam entering at the left of the valve chest has been cut off from the high-pressure cylinder, and the expanded steam is about to be discharged through the two ports from the high- to the low-pressure cylinder. This engine is largely used in sizes up to 250 H.P. For larger sizes double-acting engines are preferred.

Fig. 21 is an illustration of one of two engines made at the AllisChalmers works, Milwaukee, for the Glasgow united tramways power station. These engines are of the vertical three-cylinder, three-crank compound type, with the fly-wheel and generator arranged on the end of the crank shaft. The high-pressure cylin der is 42 in. in diameter, and there are two low-pressure cylinders each 62 in. in diameter, with a stroke of 5 ft. All the cylinders are fitted with Corliss valves. Each engine is designed to develop normally 4,000 I.H.P., and a maximum power of 5,000 I.H.P. when running at 75 revolutions per minute, with a steam pressure of 150 lbs. per sq. in. The fly-wheel is 24 ft. in diameter, and weighs about 120 tons. The approximate total weight of each engine is about 700 tons. Each engine is directly connected to a three-phase generator, designed for an output of 2,500 kilowatts at a pressure of 6,500 volts.

A more recent form of engine, built by the Allis-Chalmers Co. for the large power stations of the electric railways in New York, consists of a pair of twocylinder compound engines connected to each end of a crankshaft supported in two bearings. Mounted on the shaft between the bearings is the armature of a 7,500 kilowatt alternating generator, which takes the place of a fly-wheel. Each engine of the pair has a horizontal high-pressure cylinder and a vertical lowpressure cylinder, of 42 and 88 inches diameter respectively, the stroke being 60 inches. The connecting rods from the two cylinders take hold of a single crank pin, 18 inches diameter and 18 inches long, the bearings of each connecting rod being 9 inches long. The cranks at the two ends of the shaft are set at an angle of 45° with each other, and therefore the shaft receives an impulse from one cylinder or another eight times in each revolu

he had proposed as a pumping engine, to driving a model boat on the Fulda at Cassel.

In 1736 Jonathan Hulls took out an English patent for the use of a steam-engine for ship-propulsion, proposing to employ his steamboat in towing. In 1737 he published a well-written pamphlet, describing this apparatus.

In 1752 Bernouil received a

New York 'Subway' Power House.

gine and put it in a boat fitted with paddle wheels, and tried it in the Conestoga river, but the boat sank and was lost. In 1774 Comte d'Auxiron and Jouffroy in France built a paddle-wheel steam vessel, but it sprang a leak and foundered at the wharf before completion. In 1776 Jouffroy ran a boat 14 ft. long and 6 ft. wide by means of one of

French Academy or the French Government and gave up his attempts. In 1786 John Rumsey drove a boat at the rate of four miles an hour against the current of the Potomac at Shepherdstown, W. Va., in the presence of General Washington. He used the method of jet propulsion,' in which a steam pump forced a stream of water aft. In 1787

Steam-engine

John Fitch, at Philadelphia, made a successful trial of a steamboat which had paddles worked at the sides. In a second boat, 60 ft. long and 8 ft. beam, in 1788, he used paddles at the stern. This boat made a number of excursions on the Delaware river, making three or four miles an hour. Another of his boats, in 1790, made seven miles an hour and was placed as a passenger boat in a line from Philadelphia to Trenton, occasionally running to Wilmington. In 1788 William Symmington, in Scotland, successfully ran a steam vessel.

427

ing two vessels of 70 tons each and making over three miles an hour. Five years later, in August, 1807, Robert Fulton ran his first steamer, the Clermont, from New York to Albany, 150 miles, in 32 hours, returning in 30 hours. The boat was 133 ft. long, 18 ft. wide, and 9 ft. deep, and was equipped with an engine built in England by Watt. next month the boat began a regular passenger service between New York and Albany. Fulton, therefore, was the first to make steam navigation an every-day commercial success. In

The

Steam-engine

propeller in the U. S. S. Princeton in 1840 made important changes in marine engines.

In 1853 high-pressure steam was introduced into the British navy; but it was not till 1860 that the Victoria was fitted with engines working with steam pressure of 22 lbs. With the object of reducing the consumption of coal, the Constance, steam-frigate, was in 1862 provided with six cylinders, being thus the earliest example of a warship with a compound engine. The arrangement, however, was not very successful, and no marked improve

25 ft. long. 7 ft. beam, with paddle wheels, five miles an hour. A larger vessel in 1789 made seven miles an hour. Patrick Miller of Dalswinton, who had employed Symmington and furnished the money for the experiment, to the amount, it is said, of over £30,000, dropped the matter just when it was on the verge of success. Several years later Lord Dundas furnished funds to Symmington as engineer, and later to Henry Bell, to continue experiments on steam. boats. In 1802 Symmington built the Charlotte Dundas, which was successfully tried on the Forth and Clyde Canal, tow

1812 Henry Bell constructed the Comet, the first passenger vessel built in Europe, and steam navigation began to be a practical success in Great Britain.

In 1804 Col. John Stevens, of Hoboken, N. J., built a small steamboat, driven by twin screws, and later built a paddle-wheel steamer, which had a successful trial in 1807; just too late to anticipate Fulton, who had been granted a monopoly of the waters of the state of New York. The Phoenix in 1808 was taken to Philadelphia by way of the sea, and thereafter ran on the Delaware river.

The introduction of the screw

ment was effected until 1865, when the Pallas was launched for the British navy, with two cylinders instead of six, one being four times larger than the other. The steam entered the smaller one at high pressure, in this case 60 lbs., and thence passed into the larger one, which it filled by expansion. She had surface condensers, and was, on the whole, economical in fuel. The success of this ship caused the general adoption of compound engines throughout the fleet. Between 1875 and 1880 two improvements were brought about in engine-making-viz. hollow compressed steel shafting,

which greatly increased durability, and at the same time reduced weight; and tandem engines-i.e. the placing of the two cylinders in line instead of side by side, thus effecting a great gain in space. The inverted cylinder engine, so called from the manner in which the cylinders are placed above the crank, was the next improvement in naval engines. In 1878 the introduction of the triple-expansion system resulted in an increase of steam pressure from 90 lbs., the limit of the compound type, to at least 150 lbs., with at the same time an

R

Engines for ship propulsion are now practically all of the vertical three or four cylinder triple - expansion type, with three or four cranks. The highpressure, and sometimes the intermediate, cylinder is fitted with a piston valve, while the low-pressure cylinder is usually provided with a double-ported slide valve. The direction of motion of a marine engine must be capable of being reversed, and for this purpose each cylinder is fitted with some form of reversing gear. In the Stephenson gear or link motion (shown diagram

B

FIG. 24. Stephenson's Link Motion. (For explanation, see text.)

economy of fuel. The Rattlesnake, torpedo-boat catcher, of 1886 (550 tons), was the first ship for the British navy to be fitted with engines of the tripleexpansion vertical type. A further development was achieved

in the cruisers Blake and Blenheim (9,000 tons) of 1890-1. In these ships there are four sets of vertical triple-expansion engines, working in pairs. Some modification of this inverted vertical direct-acting engine had much earlier become general in the mercantile marine, and two, three, and four cylinders have long been

common.

matically in Fig. 24), the type usually adopted, there are two eccentrics. E E, symmetrically placed relatively to the crank c, and keyed to the shaft. One of these makes the engine run forward and the other makes it run backward. Each eccentric is connected by a rod R to the opposite ends of a curved slotted bar or link L, which can be moved transversely with respect to a block в fitted in the slot or on the bar. The end of the slide valve rod s is attached to the above block, and by moving the link the slide valve may be brought under the influence of either eccentric.

The movement of the link is ef fected through suitable levers by a handle H or (in large engines) by an auxiliary steam-engine. In the upper figure the block is in the middle of the link (midgear), and the valve is equally acted upon by both eccentrics; in this position the engine will not run in either direction. In the lower figure the valve is under the direct influence of the eccentric E, and is in the full-gear position. With this gear it is also possible to vary the point at which the steam is cut off. By placing the link in any position between mid-gear and full back or forward gear, the travel of the valve is reduced, and the steam cut off at an earlier point of the stroke. All marine engines are provided with surface condensers.

In many of the best designed marine engines vibration greatly reduced by having four cranks arranged at certain definite angles with respect to one another according to the Yarrow-Schlick-Tweedle system. In twin-screw steamers there is an engine for driving each propeller shaft.

An illustration is given in Fig. 25 of one of two sets of fourcylinder triple-expansion engines for a first-class cruiser. The engines are together capable of developing 30,000 I.H.P. when running at 120 revolutions per minute, steam being supplied by forty-three water-tube boilers of the Belleville type, working at a pressure of 300 lbs. per sq. in.

The dimensions of the cylinders on each engine are as follows:

High-pressure cylinder.....
Intermediate cylinder

Diameter.

Low-pressure (two) cylinder..... Stroke..

43 in. ...71 in.

.81 in. 48 in.

All the cylinders are fitted with separate liners, and are steamjacketed. The high-pressure cylinder is fitted with one piston valve, the intermediate cylinder with two piston valves, and the low-pressure cylinders are fitted with double-ported slide valves. There are four main surface condensers, having a collective cooling surface of 32,000 sq. ft. Four 24-inch centrifugal circulating pumps, each driven by independent engines, are provided for the main condensers. Two air-pumps are provided for each set of engines, worked by levers from the main engines in the usual manner. The shafting is of forged steel, and is hollow, the crank and thrust shafts having an external diameter of 21 in.; and the tunnel shafts 19 in. in diameter, each with an 11-inch hole. The propeller shafts are 21 in. in diameter, with a 12-inch