Road locomotives and traction engines have been frequently constructed for the transportation of both freight and passengers, and for hauling wagons carrying heavy loads. The latter application only has been permanently successful, although repeated attempts have been made to perfect steam carriages of 'high speed. As early as 1759 Dr. Robinson called the attention of Watt to the possibility of constructing a carriage to be driven by a steam engine. The first actual experiment was made, as is supposed, by a French army officer, Nicolas Joseph Cugnot, in 1709. Encouraged by the partial success of the first locomotive, he constructed a second in 1770, which is still preserved in the conscrxatoire des arts et metiers, Paris. Watt patented a road engine in 1784. About the same time his assistant, Murdoch, completed and made a trial of a model locomotive, driven by a "grasshopper engine," having a steam cylinder £ in. in diameter and 2 in. stroke. It is said to have run 6 to 8 m. an hour. In 1786-7 Oliver Evans obtained from the Pennsylvania legislature the monopoly of his method of applying the steam engine in driving flour mills, and from Maryland a similar privilege in regard to propelling wagons.
In the same or the following year William Symington constructed a working model of a steam carriage, which is now in the patent museum at South Kensington, London. In 1804 Oliver Evans completed a flat-bottomed boat to bo used in dredging at the Philadelphia docks, and, mounting it on wheels, drove it by its own steam engine to the river bank. Launching the craft, he propelled it down the river, using its engine to drive its paddle wheels. Evans's "Oruktor Amphibolos," as he named the machine, was the first road locomotive that we find described after Cugnot's time. In 1821 Julius Griffiths of London made a steam carriage to carry passengers on common roads, which was probably the first ever constructed for that purpose only. During the succeeding 10 or 15 years, Messrs. Burstall and Hill and Bramah of London and Edinburgh, Sir Goldworthy Gurney, the Messrs. Seaward, Sir Charles Dance, W. II. James, Walter Hancock, Ogle and Summers, and others in Great Britain, and Harrison Dyar, Joseph Dixon, Rufus Porter, and Mr. James in the United States, attacked this problem with varying success.
Sir Charles Dance made several hundred trips between London and Cheltenham in 1831. Hancock ran between London and Stratford, and Scott Russell from Glasgow to Paisley. From May to October, 1836, Hancock ran several carriages on the Paddington road. The general introduction of railroads, which took place immediately after the establishment of steam locomotion on the Liverpool and Manchester railway in 1829, put an end to what had promised to become an important and successful method of transportation of passengers and light merchandise. In December, 1833, more than 20 steam carriages were in use or under contract in and near London. It was proposed to substitute steam carriages, capable of travelling 12 or 15 m. an hour, for coaches drawn by horses on all mail routes. Hostile legislation procured by opposing interests, and the rapid progress of steam locomotion on railroads, caused an interruption of experiment, and almost nothing was done during the succeeding quarter of a century. It is only within a few years that any business has been founded upon the construction of road locomotives, although the scheme seems to have been at no time entirely given up.
J. Scott Russell, Boydell, and a few others in England, and Messrs. Roper, Dudgeon, Fawkes, Latta, and J. K. Fisher, in the United States, have all labored in this direction. The last named engineer designed his first steam carriage in 1840, and was at work upon the problem till his death in 1873. A few firms have succeeded within a few years in making a business of constructing road locomotives for hauling heavy loads, and in building steam road rollers; but steam carriages of high speed, adapted to the transportation of passengers, have not yet been successfully introduced. The greatest impediments seem to be the roughness and bad construction of the ordinary highway, the frightening of horses, the engineering difficulties of construction, and the limited power of the machine as it has usually been built. The capabilities of the road locomotive are readily determined by experiment, and the following is an abstract of the results of several series of trials. A trial of a road engine was made by the well known French engineer II. Tresca, in presence of Prof. Fleeming Jenkin, and the report was submitted on Jan. 15, 1868. The results were as follows: 1. The coefficient of traction was about 0.25 on a good road with easy grades. 2. The consumption of coal was 4.4 lbs. per horsepower per hour. 3. The consumption of water was 132.2 gallons an hour with the ten-horse engine. 4. The coefficient of adherence, or of friction between the wheels and the soil, was 0.3. 5. A speed of 7 m. an hour caused no special difficulty in managing either the locomotive or its load.
About this same time M. Servel conducted a series of experiments with a similar machine upon paved and upon macadamized roads, during what he described as the most trying of winter weather, He reports the following distribution of weight per cent.:
Fig. 1. - Cugnot's Steam Carriage, 1770.
Fig. 2. - Hancock's Steam Carriage.
Fig. 3. - Fisher's Steam Carriage.
Weight of Iocomotive .....................
" of wagous ..................
" of paying load ..........
The average total weight of three loaded wagons, which was the usual load, was 22,575 kilogrammes, or about 22 tons. The experiment was made in 1867-8 of applying these engines to the towage of boats on the French canals, with very encouraging results. In 1871 several traction engines were exhibited before the royal agricultural society of England at Wolverhampton, and the judges made a series of careful tests, reported in its "Journal " for that year. The coal used on special trial amounted to 3.2 lbs. per indicated horse power per hour, and the evaporation of water was 7.62 lbs. per pound of coal consumed, the average temperature of feed being 175° F. The load drawn up the maximum grade of 264 ft. to the mile on Tottenham hill, which is 1,900 ft. from top to bottom, was 26 tons, and including weight of engine 38 tons, giving a coefficient of traction of 0.35. On a country road 16 m. long it drew 15 tons at an average rate of 3½ m. an hour, using 2.85 lbs. of coal and 1.94 gallon of water per ton of useful load per mile. In October, 1871, Prof. R. H. Thurston conducted a public trial of road engines and steam road rollers, on a well macadamized road at South Orange, N. J. Two road steamers or traction engines and a steam road roller were tried.
The following are the principal dimensions : weight of engine complete, 5 tons 4 cwt. (11,648 lbs.); diameter of steam cylinder, 7¾ in.; stroke of piston, 10 in.; revolutions of crank to one of driving wheel, 17; diameter of driving wheels, 60 in.; length of boiler over all, 8 ft.; diameter of boiler shell, 30 in.; load on driving wheels, 4 tons 10 cwt. (10,080 lbs.). The boiler was of the ordinary locomotive type, and the engine was mounted upon it, as is usual with portable engines. A representation of the engine is given in the article Plough (fig. 10). The engine valve gear consisted of a three-ported valve and Stephenson link, with reversing lever, as generally used on locomotives. The connection between the gearing and the driving wheels was effected by the device called by builders of cotton machinery a Jack-in-the-box gear, or differential gear. By this combination, the ', effort exerted by the engine is made equal at both wheels at all times, even when the engine is turning a corner. The weight of the steam road roller was 15 tons. The engine and boiler were of the same general dimensions as in the road locomotives already described. The whole machine was carried on four large wheels, with broad tread, covering a total width of 6 ft.
Its weight exerts a compressive force of 5,600 lbs. on each foot of width, or 467 lbs. on each inch. The following is a summary of the conclusions deduced from the trial, and published in the " Journal of the Franklin Institute : " A traction engine may be so constructed as to be easily and rapidly manoeuvred on the common road; and an engine weighing over 5 tons may be turned continuously without difficultv on a circle of 18 ft. radius, or even on a road but little wider than the length of the engine. A locomotive of 5 tons 4 cwt. has been constructed, capable of drawing on a good road 23,000 lbs. up a grade of 533 ft. to the mile, at the rate of 4 m. an hour; and one might be constructed to draw more than 63,000 lbs. up a grade of 225 ft, to the mile, at the rate of 2 m. an hour. It was further shown that the coefficient of traction with heavily laden wagons on a good macadamized road is not far from 4/100; the traction power of this engine is equal to that of 20 horses; the weight, exclusive of the weight of the engine, that could be drawn on a level road, was 163,452 lbs.; and the amount of fuel required is estimated at 500 lbs. a day.
The advantages claimed for the traction engine over horse power are: no necessity for a limitation of working hours; a difference in first cost in favor of steam; and in heavy work on a common road the expense by steam is less than 25 per cent.' of the average cost of horse power, a traction engine capable of doing the work of 25 horses being worked at as little expense as six or eight horses. - Railroad Locomotives. Steam carriages for use on railroads, or locomotives, came into public notice subsequently to the introduction of steam carriages on the common road, but they soon displaced the latter, and have now become the most usual means of transportation. In 1802 Trevi.-thick, a Cornish miner, patented a high-pressure locomotive with a fly wheel on the crank shaft. Draught was secured by means of bellows. This little engine was so powerful that the inventor found the adhesion of the driving wheels to the rails insufficient, and proposed the use of gearing which should engage a rack laid down between the rails. Blenkinsop pursued Trevithick's plans, and made a locomotive which ran 10 m. an hour. Blackett in 1812 made a better distribution of weight, and obtained ample adhesion.
John Stevens of Ho-boken, N. J., in 1812 memorialized the legislature of the state of New York, urging the building of railways, and showing their advantages. He published a pamphlet in which he predicted that trains of carriages would be drawn on railways at 20 or 30 m. an hour, and that they might attain 40 or 50 m. an hour; and he further says: "I can see nothing to hinder a steam carriage from moving on these ways (rails) with a velocity of 100 m. an hour." Subsequently Stevens applied his steam boiler, patented in 1805, to a locomotive, which was used however only experimentally. George Stephenson in 1814 introduced the locomotive in Great Britain. The steam blast of Hack-worth, the tubular boiler of Seguin, and the link motion of Stephenson constitute the essential features of the modern locomotive. (See Railroad.) Locomotives have gradually and steadily increased in size and power from the date of their introduction. The Rocket, which first proved conclusively in 1829 the value of steam locomotion, weighed 4¼ tons.
In 1835 Robert Stephenson, who had constructed it with his father, writing to Robert L. Stevens, said that he was making his engines heavier and heavier, and that the engine of which he enclosed a sketch weighed nine tons and could draw "100 tons at the rate of 10 m. an hour, on a level." Locomotives are now built weighing 70 tons, and powerful enough to draw more than 2,000 tons at a speed of 20 m. an hour. The modern locomotive consists of a boiler of the form shown in the article Steam Boilee, mounted upon a strong light frame of forged iron, by which it is connected with the wheels. The largest engine yet constructed in the United States is said to be one in use on the Philadelphia and Reading railroad, having a weight of about 100,000 lbs., which is carried on 12 driving wheels. A locomotive has two steam cylinders, either side by side within the frame, and immediately beneath the forward end of the boiler, or on each side and exterior to the frame. The engines are non-condensing and of the simplest possible construction. The whole machine is carried upon strong but flexible steel springs. The steam pressure is usually more than 100 lbs. The pulling power is generally about one fifth the weight under most favorable conditions, and becomes as low as one tenth on wet rails.
The fuel employed is wood in new countries, coke in bituminous coal districts, and anthracite coal in the eastern part of the United States. The general arrangement and the proportions of locomotives differ somewhat in different localities. In fig. 4, a British express engine, O is the boiler, N the fire box, X the grate, G the smoke box, and P the chimney. S is a spring and R a lever safety valve, T is the whistle, L the throttle or regulator valve, E the steam cylinder, and W the driving wheel. The force pump, B C, is driven from the cross head, D. The frame is the base of the whole system, and all other parts are firmly secured to it. The boiler is made fast at one end, and provision is made for its expansion when heated. Adhesion is secured by throwing a proper proportion of the weight upon the driving wheel W. This is from about 6,000 lbs. on standard freight engines, having several pairs of drivers, to 10,-000 lbs. on passenger engines, per axle. The peculiarities of the American type are the truck or bogie supporting the forward part of the engine, the system of equalizers, or beams which distribute the weight of the machine equally over the several axles, and minor differences of detail.
The cab or house protecting the engine driver and fireman is an American device, which is gradually coming into use abroad also. The American locomotive is distinguished by its flexibility and ease of action upon even roughly laid roads. The cost of passenger locomotives of ordinary size is about $12,000; heavier engines sometimes cost $20,-000. The locomotive is usually furnished with a tender, which carries its fuel and water. The standard passenger engine on the Pennsylvania railroad has four driving wheels, 5½ ft. diameter; steam cylinders, 17 in. diameter and 2 ft. stroke; grate surface 15½, sq. ft., and heating surface 1,058 sq. ft. It weighs 63,100 lbs., of which 39,000 lbs. are on the drivers and 24,100 on the truck. The shell of the boiler is 49¼ in. diameter and 20 ft. 24 in. long. The fire box is of steel, 6 ft. 2 in. long outside, 3½ ft. wide, and 5 ft. 4 in. high. The tubes are of iron, 142 in number, 2¼ in. diameter, and 11 ft. 7 in. long. The steam dome is 30 in. outside diameter, the smoke stack 14½ in. The feed water is supplied by one pump of 2 in. diameter and 2 ft. stroke, and by a No. 8 Giffard injector. The valves are 16½ in. wide by 8½ in. long, and have 5 in. travel. The steam ports are 151/1 5/6 in. wide and 1¼ in. long, and the exhaust port 15 15/16 by 24 in.
The lap of the valve is, outside ¾ in., inside 1/64 in. The eccentrics have a throw of 41/8, in. The freight engine has six driving wheels, 54 5/8 in. in diameter. The steam cylinders are 18 in. in diameter, stroke 22 in., grate surface 14.8 sq. ft., heatinc; surface 1,096 ft. It weighs 68,500 lbs., of which 48,000 are on the drivers and 20,500 on the truck. The boiler is nearly of the same dimensions as that of the passenger engine, but the tubes are 2½; in. in diameter, 12 ft. 9 9/16 in. long, and 119 in number. The stack is 18 in. in diameter. The pump is 2¼ in. in diameter, and has a stroke of 22 in. The valve has £ in. inside lap, 1/16 in. outside. The former takes a train of five cars up an average grade of 90 ft. to the mile. The latter is attached to a train of 11 cars. On a grade of 50 ft. to the mile, the former takes 7 and the latter 17 cars. Tank engines for very heavy work, such as on grades of 320 ft. to the mile, which are found on some of the mountain lines of road, are made with five pairs of driving wheels, and with no truck. The steam cylinders are 20 1/8 in. in diameter, 2 ft. stroke; grate area, 15¾ ft.; heating surface, 1,380 ft.; weight with tank full, and full supply of wood, 112,000 lbs.; average weight, 108,000 lbs.
Such an engine has hauled 110 tons up this grade at the speed of 5 m. an hour, the steam pressure being 145 lbs. The adhesion was about 23 per cent, of the weight. In checking a train in motion, the inertia of the engine itself absorbs a seriously large portion of the work of the brakes. This is sometimes reduced by reversing the engine and allowing the steam pressure to act in aid of the brakes. To avoid injury by abrasion of the surfaces of piston, cylinder, and the valves and valve seats, M. Le Chatelier introduces a jet of steam into the exhaust passages when reversing, and thus prevents the ingress of dust-laden air and the drying of the rubbing surfaces. The valve motion consists of the simplest forms of three-ported valve, moved by two eccentrics attached to a Ste-phenson link. In drawing a train weighing 150 tons at the rate of 60 m. an hour, about 800 effective horse power is required. A speed of 80 m. an hour has been attained several times. The locomotive engine has a maximum life which may be stated at about 30 years. The annual cost of repairs is from 10 to 15 per cent, of its first cost.
On moderately level roads, the engine requires a pint of oil to each 25 m., and a ton of coal to each 40 or 50 m. run. (SeeRailroad.) - SeeHolley, "American and European Railway Practice " (New York, 1861); Weissenborn, "American Locomotive Engineering" (26 nos. 4to, plates 2 vols, fol., New York, 1861); Vose, " Manual for Railroad Engineers," (Boston, 1872); and Forney, " Catechism of the Locomotive " (New York, 1874). STEAM ENGINE. Hero of Alexandria (about 250 B. C.) described, in his Spiritalia or Pneumatica, several insignificant contrivances illustrating the power of steam. The first modern reference to its actual or possible use is not definitely known. Blasco de Garay is believed by Spanish writers to have applied steam to the propulsion of a ship at Barcelona, A. D. 1543. Giambattista della Porta, in his Spiritalia (1601), described his apparatus for raising water by filling a vertical tube by condensing steam within it and then forcing the water upward by pressure. Salomon de Caus, engineer and architect to Louis XIII., in Les raisons des forces mouvantes, avec diverses machines tant utiles que plaisantes (1615), says that " water will, by the aid of fire, mount higher than its level," and describes a globe filled with water, and an attached vertical pipe through which the water was elevated by the expansion of steam generated by heating the vessel.
Giovanni Branca published at Rome in 1629 an account of a mechanical application of a steam jet to the impulsion of a wheel against the vanes of which the jet impinged, and proposed its application to many useful purposes. The marquis of Worcester, in his " Century of Inventions" (1663), described an apparatus consisting of steam boilers worked alternately and of pipes conveying steam from them to a vessel in which its pressure operated to force water upward as suggested by De Caus. This was set up at Vauxhall, near London, and was the first instance of the application of steam to practical use. The separate boiler was the essential feature of this invention, and this is the basis of the claim that Worcester was one of the inventors of the steam engine. Sir Samuel Mor-land in 1683 constructed these engines commercially, and with an intelligent understanding of their principles and of the more important properties of steam. Denis Papin, of Blois, about 1690 invented an engine having a piston which separated the steam from the water in the cylinder, receiving steam from the boiler in Worcester's combination. He also invented the lever safety valve.
Thomas Savery patented, July 25, 1698, a machine consisting of a duplicate set of boilers, steam reservoirs, and forcing tubes, which were worked alternately, and applied it extensively to the drainage of mines, and occasionally to raising water to turn mill wheels. Savery recharged his reservoirs by the use of surface condensation, and his apparatus was capable of working an indefinite period without stopping. Desaguiliers in 1716 improved upon it by applying the Papin safety valve, and by using jet instead of surface condensation. This engine elevated 5,000,000 lbs. of water one foot with each hundred weight of coal consumed; it gave a " duty " therefore of 5,000,000. Thomas New-comen, John Cawley, and Savery patented in 1705 the first steam engine really deserving the name. It consisted of a cylinder containing a piston driven upward by steam from a separate boiler, and forced downward by atmospheric pressure when the steam below the piston was removed by condensation. The engine was used only for pumping, the pump rod and piston rods being attached to opposite ends of a beam, as in modern engines. Steam was first condensed by the application of cold water to the exterior, as in the original Savery engine, but soon after a jet within the cylinder was used.
The boiler was supplied with gauge cocks to indicate the height of water, and a safety valve. Humphrey Potter, an ingenious boy mechanic, in 1713 made the valve gear automatic by leading cords from the beam. Henry Beighton in 1718 substituted for the latter the plug rod and more substantial apparatus still known to engineers. The improved Newcomen engine came into use during the 18th century throughout Europe. Brindley and John Smeaton devised some improvements in detail and proportion, and the latter built large engines of this type, attaining a duty of 9,500,000. Smeaton says that he had seen engines with cylinders 75 in. in diameter. His largest was 72 in., and its power that of 150 horses " acting together." James Watt, an instrument maker at the university of Glasgow, when repairing a model Newcomen engine (fig. 2) in 1703, began a series of improvements which finally rendered the steam engine universally applicable. To avoid losses of heat in the steam cylinder, which he estimated to amount to three fourths of all supplied, he attached (1705) the separate condenser, thus saving also three fourths of the injection water needed in the Newco-mcn engine. He first tried surface condensation, but soon adopted the jet condenser and air pump.
The piston bad previously been kept from leaking by flooding it from above with water. Watt substituted oil and tallow, He closed the top with a cylinder head, passing the piston rod through a "stuffing box" to prevent leakage about it, and admitting steam above the piston, instead of air, during the down stroke, thus avoiding the cooling effect of the atmosphere. He then protected the cylinder by non-conducting coverings to intercept the heat previously lost by radiation from its exterior, and applied the "steam jacket," introducing a space intermediate between the cylinder and the external covering, in which space steam was retained. He thus converted the atmospheric steam engine of Newcomen into the type known as the engine of Watt. The firm of Boulton and Watt began building these engines at Soho, near Birmingham, in 1773.
Fig. 4. - British Express Engine.
Fig. 1. - Hero's Steam Engine.
Fig. 2. - Newcomen Engine Model.
Watt proposed to secure economy of steam by its expansion in 1769, and in 1776 he adopted a form of cut-off which was patented in 1782. His later pumping engines attained a duty of 20,000,000. The crank and fly wheel were patented by Wasborough in 1781, and Watt adopted the " sun and planet wheels " as the next best expedient for obtaining rotary motion, and applied them in his double-acting engine patented July 4, 1782. This engine is shown in fig. 3, with the parallel motion, governor, and other details patented in 1781. Admitting steam and condensing on both sides of the piston, the power of the engine was doubled. Jonathan Hornblower in 1781 patented a compound or double cylinder engine, in which the steam used at high pressure in one cylinder was exhausted into a second, whence, after acting expansively and with reduced pressure, it was discharged. Woolf in 1804 patented the combination of this engine with the Watt condenser, and a few such engines were built. Oliver Evans devised in 1779 the high-pressure non-condensing steam engine, He introduced it into saw and grain mills, and applied it to the propulsion of vessels and locomotives. It still remains the most commonly used of all forms of the steam engine.
Trevithick and Vivian introduced engines built on Evans's plan into Great Britain in 1802, which carried occasionally 00 to 80 lbs. of steam pressure. Col. John Stevens of Hoboken, N. J., built the direct-acting, high-pressure, and condensing engine, with a sectional steam boiler, in 1804. Joseph Dixon coupled two engines with cranks at right angles in 1823. The detachable, adjustable, or drop cut-off valve gear was patented by Frederick E. Sickels of New York in 1842, and the application of the governor to determine the point of cut-off was made by Zachariah Allen and George II. Corliss of Rhode Island, and patented by the latter in 1849. This completed the growth in general design of the now distinctive American expansive steam engine. Recently the revival of the double cylinder engine, with high steam, considerable expansion, and rapid motion of piston, which have proved economically successful, has been the onlv marked feature of this branch of engineering progress. It is estimated that the total steam power of the world is about 15,000,000 horse power, and that were horses actually employed to do the work which these engines would be capable of doing were they kept constantly in operation, the number required would exceed 60,000,000. - Form of the Steam Engine. In all engines the principal organs of the machine are present, but their forms and proportions, and their arrangement, differ greatly in different classes.
In general, the piston, P, fig. 3, is accurately fitted into a steam cylinder, O, within which it moves from end to end with slight friction, and without permitting the escape of steam past its edges. The piston rod is attached at one end to this piston, and, passing through the cylinder head, is attached at the other extremity to a cross head, which is so guided that it is compelled to move in a vertical line, and thus a side strain upon the rod which would produce friction and leakage, even were it not to cause actual bending and fracture, is prevented. In fig. 3 the cross head is guided by a parallel motion p m, an arrangement of rods of which one set vibrate about centres, thus displacing the centres of vibration of the other set just sufficiently to compensate the tendency of the latter to throw the cross head out of line by their sweep through their own curved path. This compensation permits the head of the piston rod to be securely guided in the vertical line. In later practice, a more common method of obtaining a rectilinear motion of the cross head is to place guides at each end of it, by which its extremities are kept in the desired line of motion. The sliding friction of the cross-head gibs upon these guides is slight, and is kept within proper limits by lubrication.
The cross head is connected with the working beam, B, by links (usually a pair), and the beam, vibrating about the main centre, transfers the motion by means of the connecting rod, c r, to the crank attached to the main shaft or crank shaft, which carries the fly wheel or balance wheel, W. In this figure the crank is not shown, the sun and planet wheels taking its place. Steam is conveyed to the engine by the steam pipe, at some convenient point in which a stop valve is usually placed. Sometimes this valve is omitted, a throttle valve alone being used, adapted to adjust the supply of steam. The latter is either a disk valve, adjusted by a screw (in which form it is known as a screw stop valve), or it is some variety of slide valve, opening and closing by sliding transversely across the opening through which steam passes. Where the supply of i steam is determined automatically, a governor, G, is attached, which when the speed of the engine tends to exceed the desired maximum closes the throttle valve, and when the speed falls too low opens it.
In the figure, the governor consists of a pair of suspended balls caused to revolve by a belt, or by gearing connecting the spindle with the shaft, which when speed rises are given a high velocity of revolution about the spindle carrying them, and, separating under the action of centrifugal force, move the lever Z, and thus close the throttle valve. There are many varieties of governors. The "fly-ball governor," just described, is most common, but, though simple and quite well adapted to general purposes, it is not perfectly isochronous; i. c, it does not compel the engine to keep the precise speed at which it is set to work. As the governor and valve are rigidly connected, there is but one speed to which the position of the valve and of the governor can be perfectly adapted under any one set of conditions of steam pressure and of load. The valve gear is the system of valves and of actuating mechanism which distribute the steam as the engine passes through its cycles of motion. The steam valves admit steam alternately to each end of the steam cylinder, as the piston moves backward and forward, and the exhaust valves alternately open and close the passages or ports through which the steam escapes, after impelling the piston, into the condenser c in the condensing engine, or into the open air from a non-condensing engine.
These valves are moved automatically by some part of the engine itself. In the kind of engine shown in fig. 3, and in pumping engines which have no crank and revolving shaft, the motion is obtained from a rod depending from the beam, projections on which rod strike the tappets t as they rise and fall. This rod is called the plug rod. In nearly all other engines, the valve gear is actuated by an eccentric, or disk attached to and revolving with the crank shaft. While the piston is moving upward, the steam valve below and the exhaust valve above are open, the steam entering below to drive the piston up, while the steam which had produced the downward stroke escapes through the open exhaust valve at the top into the condenser. During the descent of the piston these conditions are reversed. The condenser may be either a jet condenser, as shown in the figure, or a surface condenser. Its office is to condense the steam ejected from the cylinder, and thus to create a vacuum, so removing the resisting pressure of the atmosphere from before the piston.
With the jet condenser, the steam issuing from the exhaust pipe of the engine is received in a closed vessel, where it is brought into contact with jets of cold water, and thus instantly condensed, and the vacuum so produced pervades the condenser, the exhaust pipe, and the exhausted end of the cylinder. The water of condensation, the remaining uncondensed vapor, and any air which may enter the condenser with the steam, are removed by the air pump p, and thrown into the hot well above the condenser whence they are taken by the hot water pump and discharged. Water is also taken from the hot well by the feed pump and fed to the steam boiler. - Classification of Engines. Steam engines are designated as condensing or non-condensing, according as they are furnished with a condenser or as that detail is omitted. They are high pressure or low pressure, the former term being applied to engines supplied with steam of 50 lbs. pressure to the square inch and upward, and the latter to engines working under 40 lbs. pressure. The latter are almost invariably condensing engines, and high-pressure engines are very generally non-condensing. Reciprocating engines have pistons moving backward and forward in the steam cylinder, as in Watt's engine. When they turn a shaft, they are sometimes called rotative.
Rotary engines have a piston attached to a shaft and revolving with it within a cylinder of which the axis is parallel with the axis of rotation of the piston or vane. Engines are direct-acting where the piston rod acts directly upon the connecting rod, and through it upon the crank, without the intervention of a beam or lever. In back-acting or return connecting rod engines, the shaft lies between the cylinder and the cross head, the connecting rod returning from the cross head to the crank. Beam engines have the working beam already described. Side lever engines have two beams, one on each side of the steam cylinder, and below instead of above the cross head. Oscillating engines have their piston rods attached directly to the crank pin, and as the crank revolves the cylinder oscillates upon trunnions, one on each side of it, through which the steam enters and leaves the steam chest. The valves are within the steam chest, oscillating with the cylinder. In these engines the mechanism actuating the valves is seldom perfectly satisfactory in its operation.
In compound or double-cylinder engines, the steam enters first a high-pressure cylinder, and there usually expands from its initial pressure of from 60 to 100 lbs. down to a much lower density; it is then exhausted into a second steam cylinder, in which it expands still further while completing its work. - Engines are also classified, according to the use for which they are intended, as stationary, pumping, portable, locomotive, or marine engines. The locomotive engine is the simplest form. In it the condenser and the governor are dispensed with, and the valve and its gearing are the simplest possible. The portable engine is usually very similar to the locomotive, and, like the latter, is attached to its steam boiler. It is sometimes provided with a heater to warm the feed water sent into the boiler, and is frequently provided with a governor. It is usually mounted on wheels. Both the locomotive and the portable engine employ high steam pressure without condensation. In both of these forms of engine are secured to the fullest extent lightness and simplicity, and, where properly constructed, cheapness, durability, compactness, and fair efficiency. Draft is usually secured in both by the blast of the exhaust steam.
Engines of this class have attained the remarkable economical result of a horse power developed with the expenditure of less than three pounds of coal per hour. - The oldest form of pumping engine still retained in use is the Cornish. In it the crank shaft and balance wheel are dispensed with, the end of the pump rod being attached directly to the end of the beam opposite the steam cylinder. Steam is first admitted above the piston, driving it rapidly downward and raising the pump rod. At an early point in the stroke the admission of steam is checked by the sudden closing of the induction valve, and the stroke is completed under the action of expanding steam assisted by the inertia of the heavy parts already in motion. The necessary weight and inertia is afforded in many cases, where the engine is applied to | the pumping of deep mines, by the immensely long and heavy pump rods. Where this weight is too great it is counterbalanced, and where too small, weights are added. When the stroke is completed, the "'equilibrium valve" is opened, and the steam passes from above to the space below the piston, and an equilibrium of pressure being thus produced, the pump rods descend, forcing the water from the pumps and raising the steam piston.
The absence of the crank or other device which might determine absolutely the length of stroke compels a very careful adjustment of steam admission to the amount of load. Should the stroke be allowed to exceed the proper length, and should danger thus arise of the piston striking the cylinder heads, the movement is checked by buffer beams. The valve motion is actuated by a plug rod, as in Watt's engine. The regulation is effected by a "cataract," a kind of hydraulic governor, consisting of a plunger pump with a reservoir attached. The plunger is raised by the engine, and then automatically detached. It falls with greater or less rapidity, its velocity being determined by the size of the eduction orifice, which is adjustable by hand. When the plunger reaches the bottom of the pump barrel, it disengages a catch, a weight is allowed to act upon the steam valve, opening it, and the engine is caused to make a stroke. When the outlet of the cataract is nearly closed, the engine stands still a considerable time while the plunger is descending, and the strokes succeed each other at long intervals. When the opening is greater, the cataract acts more rapidly, and the engine works faster.
This has been regarded until recently as the most economical of pumping engines, and it is still generally used in freeing mines of water, and in situations where existing heavy pump rods may be utilized in continuing the motion of the piston during that portion of its stroke which is performed after expansion has begun. The direct-acting steam pump is sometimes used as a pumping engine. (See Pump.) The compound pumping engine has been recently adopted with great success.
Fig. 8. - Watt's Engine, 1784.
Fig. 4. - Leavitt's Pumping Engine.
One of the most efficient forms is that designed by E. I). Leavitt, jr., for the Lynn (Mass.) water works, and shown in fig. 4. The two cylinders, A and B, are placed one on each side the centre of the beam 0 D, and are so inclined that they may be coupled to opposite ends of it, while their lower ends are placed close together. At their upper ends a valve is placed at each end of the connecting steam pipe. At their lower ends a single valve serves as exhaust valve to the high-pressure and as steam valve to the low-pressure cylinder. The pistons move in opposite directions, and steam is exhausted from the high-pressure cylinder A directly into the nearer end of the low-pressure cylinder B. The pump, E, of the "Thames-Dit-ton" or "bucket and plunger" variety, takes a full supply of water on the down stroke, and discharges half when rising and half when descending again. The duty of this engine is reported by a board of engineers as 103,023,215 foot pounds for every 100 lbs. of coal burned. The duty of a moderately good engine is usually considered to be from 60 to 70 millions. This engine has steam cylinders of 17½ and 36 in. diameter respectively, with a stroke of 7 ft. The pump had a capacity of about 195 gallons, and delivered 96 per cent.
Steam was carried at a pressure of 75 lbs. above the atmosphere, and was expanded about ten times. Plain horizontal tubular boilers were used, evaporating 8.58 lbs. of water from 98° F. per pound of coal. - The stationary steam engine has a great variety of forms. Since compactness and lightness are not as essential as in portable, locomotive, and marine engines, the parts are arranged with a view simply to securing efficiency, and the design is determined by circumstances. It was formerly usual to adopt the condensing engine in mills and wherever a stationary engine was required. In Europe generally, and to some extent in the United States, where a supply of condensing water is obtainable, condensing engines and moderate steam pressures are still employed. But this engine is gradually becoming superseded by the high-pressure condensing engine, with considerable expansion, and with an expansion gear in which the point of cut-oft' is determined by the governor. The best known engine of this class is the Corliss engine, which is very extensively used in the United States, and which has been copied very generally by European builders.
Fig. 5 represents the Corliss engine as built in the United States by Harris. The horizontal steam cylinder is bolted firmly to the end of the frame, which is so formed as to transmit the strain to the main journal with the greatest directness. The frame carries the guides for the cross head, which are both in the same vertical plane. The valves are four in number, a steam and an exhaust valve being placed at each end of the steam cylinder. Short steam passages are thus secured, and this diminution of clearance is a source of some economy. Both sets of valves are driven by an eccentric operating a disk or wrist plate, which vibrates on a pin projecting from the cylinder. Short links reaching from this wrist plate to the several valves move them with a peculiarly varying motion, opening and closing them rapidly, and moving them quite slowly when the port is either nearly open or almost closed. This effect is ingeniously secured by so placing the pins on the wrist plate that their line of motion becomes nearly transverse to the direction of the valve links when the limit of movement is approached.
The links connecting the wrist plate with the arms moving the steam valves have catches at their extremities, which are disengaged by coming in contact, as the arm swings around with the valve stem, with a cam adjusted by the governor. This adjustment permits the steam to follow the piston further when the engine is caused to " slow down," and thus tends to restore the proper speed. It disengages the steam valve earlier, and expands the steam to a greater extent, when the engine tends to run above the proper speed. When the catch is thrown out, the valve is closed by a weight or a strong spring. To prevent jar when the motion of the valve is checked, a "dash pot" is used, invented by F. E. Sickels. It is a vessel having a nicely fitted piston, which is received by a " cushion" of water or air when the piston suddenly enters the cylinder at the end of the valve movement. In the original water dash pot of Sickels, the cylinder is vertical, and the plunger or piston descends upon a small body of water confined in the base of the dash pot. In the Greene steam engine, fig.
G, the valves are four in number, as in the Corliss. The cut-off gear consists of a bar, A, moved by the steam eccentric in a direction parallel with the centre line of the cylinder and nearly coincident as to time with the piston. On this bar are tappets, C 0, supported by springs and adjustable in height by the governor, G. These tappets engage the arms B B, on the ends of rock shafts E E, which move the steam valves and remain in contact with them a longer or shorter time, and opening the valve during a greater or less part of the piston stroke, as the governor permits the tappets to rise with diminishing engine speed, or forces them down as speed increases. The exhaust valves are moved by an independent eccentric rod, moved by an eccentric set, as is usual with the Corliss and with other engines generally, at right angles with the crank. This engine, in consequence of the independence of the steam eccentric, and of the contemporary movement of steam valve motion and steam piston, is capable of cutting off at any point from beginning to nearly the end of the stroke. The usual arrangement, by which steam and exhaust valves are moved by the same eccentric, only permits expansion with the range from the beginning to half stroke.
The Wright engine has an adjustable expansion valve gear also, and the point of cut-off is determined by the governor. In this machine the steam valves are opened by a cam of such form that when the cam shaft is moved longitudinally, the valve is held open a longer or a shorter time. The position of the cam shaft is adjusted by the governor. Its motion is obtained by gearing it to the main shaft. The Babcock and Wilcox engine has a cut-off valve on the back of the main valve, which is moved by a.small steam cylinder. The point of cut-off is determined by the governor also, by varying the time of admission of steam into the auxiliary cylinder. This engine has the same latitude of expansion as the Greene engine. - The characteristics of the American stationary engine are high steam pressure without condensation, an expansion valve gear with drop cut-off adjustable by the governor, high piston speed, and lightness combined with strength of construction. In other countries this engine is now rapidly coming into general use, but abroad the valve most generally adopted is the form usual in other styles of engine, expansion being obtained by a cut-off valve on the back of the main valve, and regulation secured by attaching the governor to a throttle valve. - The Marine Steam Engine. Marine engines have a great variety of forms, but general practice has now indicated a few which are preferred.
They are almost invariably fitted with condensers. Until recently they were usually driven by steam of moderate pressure, but within a few years the pressure of steam, which in the time of Watt was usually from 5 to 10 lbs. above the atmosphere, has risen to GO lbs. In the earlier days of steam navigation, the paddle wheel was exclusively used. Be-cently the screw has become the sole instrument of propulsion, where deep water permits its use. In shallow water the paddle wheel is still employed. Marino engines are therefore divided into paddle engines and screw engines. - The most common forms of paddle engines in the United States are the overhead beam engine, driven by steam of from 20 to 50 lbs. pressure, and fitted with a jet condenser, and the high-pressure and non-condensing direct-acting engine, used principally on the western rivers. The latter is driven by steam of from 100 to 150 lbs. pressure, and exhausts its steam into the atmosphere. It is the simplest possible form of direct-acting engine. The valves are of the disk or poppet variety, rising and falling vertically. They are four in number, two steam and two exhaust valves being placed at each end of the steam cylinder. The beam engine is a peculiarly American type, seldom if ever seen abroad.
Fig. 7 is an outline sketch of this engine as built for a steamer plying on the Hudson river. This class of engine is usually adopted in vessels of great length, light draught, and high speed. But one steam cylinder is commonly used. The cross head is coupled to one end of the beam by means of a pair of links, and the motion of the opposite end of the beam is transmitted to the crank by a connecting rod of moderate length. The beam has a cast-iron centre surrounded by a wrought-iron strap of lozenge shape, in which are forged the bosses for the end centres, or for the pins to which the connecting rod and the links are attached. The main centre of the beam is supported by a " gallows frame " of timbers so arranged as to receive all stresses longitudinally. The crank and shaft are of wrought iron. The valve gear is usually of the form known as the Stevens valve gear, an invention of Robert L. and Francis B. Stevens. The steam and exhaust valves are worked by independent eccentrics, the latter being set in the usual manner, opening and closing the exhaust passages just before the crank passes its centre. The steam eccentric is so placed that the steam valve is opened as usual, but closed when but about one half the stroke has been made.
This result is accomplished by giving the eccentric a greater throw than is required by the motion of the valve, and permitting it to move through a portion of its path without moving the valve. Thus in fig. 8, if A B be the direction of motion of the eccentric rod, the valve would ordinarily open the steam port when the eccentric assumes the position O C, closing when the eccentric has passed around to O D. With the Stevens valve gear, the valve is opened when the eccentric reaches O E, and closes when it arrives at O F. The steam valve of the opposite end of the cylinder is open while the eccentric is moving from O M to O K. Between K and E, and between F and M, both valves are seated. II B is proportional to the lift of the valve, and O II to the motion of the valve gear when out of contact with the valve lifters. While the crank is moving through an arc E F, steam is entering the cylinder; from F to M the steam is expanding. At M the stroke is completed and the other steam valve opens. The ratio:
---- is the ratio of expansion.
Fig. 5. - Corliss Engine.
Fig. 6. - Greene Engine.
Fig. 7. - Beam Engine.
The condenser is placed immediately beneath the steam cylinder. The air pump is placed close beside it, and worked by a rod attached to the beam. Steam vessels on the Hudson river have been driven by such engines at the rate of 23 m. an hour. This form of engine is remarkable for its smoothness of operation, its economy and durability, its compactness, and the latitude which it permits in the change of shape of the long flexible vessels in which it is generally used, without injury by " getting out of line." For paddle engines of large vessels, the favorite type has been the side lever engine, which is now rarely built. For smaller vessels, the oscillating engine with feathering paddle wheels is still largely employed in Europe. This style of engine is shown in fig. 9. It is very compact, light, and moderately economical, and excels in simplicity. The feathering paddle wheel is made with floats or buckets variable in position, and so adjusted by the feathering mechanism that less power is expended in oblique action, raising or pushing downward the water impinged upon, than with the ordinary radial wheel, in which the floats are rigidly attached to the arms. The usual arrangement is such that the feathering wheel has the same action upon the water as a radial wheel of double diameter.
This reduction of the diani-eter of the wheel, while retaining maximum effectiveness, permits a high speed of engine, and thereforo less weight, volume, and cost. The smaller wheel boxes, by offering less resistance to the wind, retard the progress of the vessel less than those of radial wheels. The feathering of the paddle is produced by the use of a rod, E D, fig. 10, which connects an eccentric strap, E F, secured to the vessel, with the short arm A D, by which the paddle is turned upon the pin A. 0 is the centre of the paddle wheel, and C B is one of the arms. Circular hoops, or bands, connect all of the arms, each of which carries a float. They are all thus tied together, forming a very firm and powerful combination to resist external forces. Inclined engines are sometimes used for driving paddle wheels. In these the steam cylinder lies in an inclined position, and its connecting rod directly connects the crank with the cross head. The condenser and air pump usually lie beneath the cross-head guides, and are worked by a bell crank driven by links on each side the connecting rod, attached to the cross head. Such engines are used to some extent in Europe, and they have been adopted in the United States navy for side-wheel gunboats.
They are also used on the ferry boats plying between New York and Brooklyn. Paddle wheels should be immersed usually not more than one third the radius of the wheel for sea-going vessels, and on rivers they are frequently not immersed more than one sixth or one eighth. In the first case the loss by oblique action is about 5 per cent., in the last case about 10 per cent. A loss of 20 or 25 per cent, of the total power applied to the wheel is frequently caused by slip. - Many forms of engines have been used for driving the screw, but they are now almost invariably of one type. The ordinary screw engine is direct-acting. Two engines are placed side by side, with cranks on the shaft at an angle of 90 degrees with each other. In merchant steamers the steam cylinders are usually vertical and directly over the crank pins, to which the cross heads are coupled. The condenser is placed behind the engine frame, or, where a jet condenser is used, the frame itself is sometimes made hollow and serves as a condenser. The air pump is worked by a beam connected by links with the cross head. The general arrangement is like that shown in figs. 13 and 14. For naval purposes such a form is objectionable, since its height is so great that it would be exposed to injury by shot.
In naval engineering the cylinder is placed horizontally, as in fig. 11, which is a sectional view, representing a horizontal, direct-acting naval screw engine, with jet condenser and double-acting air and circulating pumps. A is the steam cylinder, 13 the piston, which is connected to the crank pin by the piston rod D and connecting rod E. F is the cross-head guide. The eccentrics G operate the valve, which is of the "three-ported variety," by a Stephenson link.
Fig. 9. - Oscillating Engine and feathering Paddle Wheel.
Fig. 11. - Horizontal direct-acting Naval Screw Engine.
Reversing is effected by the hand wheel C, which by means of a gear m and a rack k elevates and depresses the link, and thus reverses the valve. As shown in the sketch, this valve is so constructed that, when in precisely the middle of its path, it covers both steam ports as well as the exhaust port. When it is moved to the right, the forward steam port is opened and the engine takes steam at the end D, while the steam from the opposite side of the piston, A, is allowed to pass out under the valve and off through the exhaust- port. The valve is shown in this position in the figure. When the eccentric has turned with the shaft, or when the link is shifted so as to bring the end p iato action and thus communicate the motion of the other eccentric to the valve, steam enters at the end A and is exhausted from D. Each eccentric produces this change in such a manner that when the piston reaches the end of its stroke this reversal occurs, and the steam and exhaust ports are opened and closed in the manner required to produce the proper distribution of steam. One eccentric is adjusted to give the correct distribution when the engine is moving ahead, the other when worked backward.
When it is desired to produce a limited amount of expansion of steam, the exterior edges of the face of the valve are carried further apart, and the valve when in mid-position overlaps the steam ports. The throw of the eccentrics is then correspondingly increased, and they are moved upon the shaft until they can be secured in new positions in which they bring the edge of the valve to the edge of the port opening as before, admitting steam at the beginning of the stroke. By this process, which is termed giving lead to the valve, the exhaust port is also both opened and closed earlier than before. To remedy this fault, the edges of the interior of the valve are sometimes changed also, and they are given "lap" in either position, as on the steam side or negative. In the latter case they are moved further apart. Zeuner's valve diagram, fig. 12, is a useful graphic representation of the action of this valve. Let A B represent the path of the piston, A O, B O being the positions of the crank at each end of the stroke.
Then E O, F O will be the positions of the crank when the eccentric and valve are at their middle positions on the forward and the return stroke respectively, provided the valve has neither lap nor lead, and the steam and exhaust ports will be opened and closed precisely at the beginning and end of the stroke. If, however, it is desired to open the steam port when the crank is at C and the piston at c, approaching A, and if the steam is to be cut off and expansion to begin when the crank is at D and the piston at d, on the forward stroke, the valve must be at " half throw" when the crank is at E' midway between A and D. E' 0, F' O will then be the positions of the crank when the valve, is at mid throw on the forward and return stroke respectively. While the crank is moving from D to G the valve must continually cover the port which has remained open from A to D. The distances E' L, L O are thus proportional to the motion of the valve while the port is opened and closed respectively, and LO/EO measures the lap. HOA is the "angular advance," or the distance by which the eccentric must precede its normal position E to insure the desired distribution of steam. A circle ElO being inscribed, the distance 01 also measures the lap.
Similarly, if the exhaust port is to be opened at I, it must close at J, and the steam is confined and " cushioned " behind the piston as the crank moves from J to A. O M or O m measures the "exhaust lap." The figure N Q P S R A N is the indicator diagram corresponding to such a distribution of steam, the steam pressure being maintained from N to Q, expansion occurring from Q to P, exhaust taking place at P, and S R being the line of back pressure on the return stroke. Cushioning takes place at R, and the steam being admitted immediately afterward, the pressure rises again to its maximum at N. This valve and gear only permits a very limited range of expansion in consequence of the seriously objectionable effect of the accompanying alteration of the exhaust. A separate expansion valve, moved by an independent eccentric, is usually placed between the steam pipe, S, fig. 11, and the main valve. The piston of the air pump, P, and the circulating pump, where a surface condenser is used, are driven by a rod p R from the main piston. The valves f f admit the water, and c c are the delivery valves. The pump is represented as just making a stroke from left to right.
Steam is exhausted from the cylinder A through the exhaust pipe N N to the condenser, and when condensed falls to the bottom, whence the water of condensation is raised by the air pump and forced overboard through a delivery pipe and valve not shown. A portion of the stern of the vessel is represented as torn away to show the screw J. A strong and stiff main frame, X X, unites the cylinders with the condenser and also supports the main shaft journal at G. The whole is firmly secured by bolts to the cross floors of the vessel, Z Z, if of iron, or to engine keelsons if of wood. A spring water valve, V, is placed on the cylinder head to allow water which may enter the cylinder with the steam to be forced out without endangering the cylinder or the heads, as it might were it caught there on the return of the piston. The trunk engine, in which the connecting rod is attached directly to the piston and .vibrates* within a trunk or cylinder secured to the piston, moving with it, and extending outside the cylinder, like an immense hollow piston rod, is frequently used in the British navy. It has rarely been adopted in the United States. - In nearly all steam vessels which have been built for the merchant service recently, and in some naval vessels, the compound engine has been adopted.
Figs. 13 and 14 represent the usual form of this engine. Here A A, A' A' are the small and the large, or the high-pressure and the low-pressure cylinders respectively. B B' are the valve chests. C C C is the condenser, which is invariably a surface condenser. The condensing water is sometimes directed around the tubes contained within the casing C C C, while the steam is exhausted around them and among them, and sometimes the steam is condensed within the tubes, while the injection water which is sent into the condenser to produce condensation passes around the exterior of the tubes. In either case, the tubes are usually of small diameter, varying from five eighths to half an inch, and in length from four to seven feet. The extent of heating surface is usually from one half to three fourths that of the heating surface of the boilers. The air and circulating pumps, D D, are placed on the lower part of the condenser casting, and are operated by a crank on the main shaft at E'; or they are sometimes placed as in the style of engine last described, and driven by a beam worked by the cross head.
The piston rods are guided by the cross heads X X working in slipper guides T T, and to these cross heads are attached the connecting rods I I, driving the cranks Y Y. The cranks are now usually set at right angles; in some engines this angle is increased to 120°, or even 180°. Where it is arranged as here shown, an intermediate reservoir, R R, is placed between the two cylinders to prevent the excessive variations of pressure that would otherwise accompany the varying relative motions of the pistons, as the steam passes from the high-pressure to the low-pressure cylinder. Steam from the boilers enters the high-pressure steam chest S, and is admitted by the steam valve alternately above and below the piston as usual. The exhaust steam is conducted through the exhaust passage around into the reservoir R, whence it is taken by the low-pressure cylinder, precisely as the smaller cylinder drew its steam from the boiler. From the large or low-pressure cylinder the steam is exhausted into the condenser.
The valve gear is usually a Stephenson link, L, the position of which is determined, and the reversal of which is accomplished, by a hand wheel U and screw P, which, by the bell crank N M, are attached to the link L L. - The Seven-. Screw steamers are far more efficient than paddle-wheel vessels, not only because the screw is a better instrument of propulsion, but because it permits the use of more efficient machinery, and especially because it utilizes a large amount of energy entirely wasted with the paddle wheel in putting in motion the water, which latter, coming into contact with the hull of the vessel, is set in motion by friction, and the following-current is left behind to expend its vis viva by contact with the surrounding mass of water. The currents so produced, in the case of screw vessels, impinge upon the screw, which works immediately astern of the vessel, and communicate to it a portion of that energy which would otherwise be lost in the creation of such currents. Screws work far below the surface of the water, and lose less by slip than the paddle wheel. Screw engines are quick-working, compact, and light.
Their higher piston speed, their smaller size, and especially their more uniform action upon the propeller and the water, produce greater economy in the use of steam and a more effective application of power than is obtained with the paddle wheel. Incidentally, by permitting the replacing of a considerable weight of machinery and fuel by paying freight, they add greatly to the commercial value of the steam vessel. The forms of screws are exceedingly diverse, but those in common use are not numerous. In naval vessels it is common to apply screws of two blades, that they may be hoisted above water into a "well" when the vessel is under sail, or set with the two blades directly behind the stern post, when their resistance to the forward motion of the vessel will be comparatively small. In other vessels, and in the greater number of full-powered naval vessels, screws of three or four blades are used. The usual form of screw has blades of nearly equal breadth from the hub to the periphery, or slightly widening toward their extremities, as is seen in an exaggerated degree in fig. 15, representing the form adopted for tug-boats, where large surface near the extremity is more generally used than in vessels of high speed running free.
In the Griffith screw, which has been much used, the hub is globular and very large. The blades are secured to the hub by flanges, and are bolted on in such a manner that their position may be changed slightly if desired. The blades are shaped like the section of a pear, the wider part being nearest the hub, and the blades tapering rapidly toward their extremities. A usual form is intermediate between the last and that shown in fig. 15, the hub being sufficiently enlarged to permit the blades to be attached as in the Griffith screw, but more nearly cylindrical, and the blades having nearly uniform width from end to end. The Hirsch screw, tig. 16, is used on the steamship City of Peking. The pitch of a screw is the distance which would be traversed by the screw in one revolution were it to move through the water without slip; i. e., it is double the distance C D, fig. 15. C D' represents the helical path of the extremity of the blade B, and O E F II K is that of the blade A. The proportion of diameter C C to the pitch of the screw is determined by the speed of the vessel. For low speed the pitch may be as small as one and one fourth the diameter. For vessels of high speed the pitch is frequently double the diameter.
The diameter of the screw is made as great as possible, since the slip decreases with the increase of the area of screw disk. Its length is usually about one sixth the diameter. A greater length produces loss by increase of surface causing too great friction, while a shorter screw does not fully utilize the resisting power of the cylinder of water within which it works, and increased slip causes waste of power. Negative slip occurs when the vessel moves at a higher speed than it would attain were the screw to work in a solid nut; it is sometimes observed in badly formed vessels. The slip is decreased by increasing the diameter, and also by increasing the length of the screw. The increased friction above referred to prevents the latter process from being economically carried beyond the maximum given. An empirical value for the probable slip in vessels of good shape, which is closely approximate usually, is given by Prof. Thurston as S=4 M/A' - , in which S is the slip per cent., and M and A are the areas of the midship section and of the screw disk in square feet. The most effective screws have slightly greater pitch at the periphery than at the hub, and an increasing pitch from the forward to the rear part of the screw.
The latter method of increasing pitch is more generally adopted alone. The thrust of the screw is the pressure which it exerts in driving the vessel forward. In well formed vessels, with good screws, about two thirds of the power applied to the screw is utilized in propulsion, the remainder being wasted in slip and other useless work. Its efficiency is in such a case, therefore, CO per cent. Twin screws, one on each side of the stern post, are sometimes used in vessels of light draught and considerable breadth, whereby decreased slip is secured. The following are the dimensions of some of the largest marine screw engines of the well known types. The engines of the British iron-clad Monarch, a vessel of over 8,000 tons displacement, have given an '"indicated power" of 8,528 horses at 65 revolutions a minute, when making a speed of about 15 knots or 17½ statute miles an hour. The steam pressure was 25 lbs. These engines are horizontal, and have steam cylinders 120 in. in diameter, and 4½ ft. stroke of piston; the pistons weigh 8 tons each. The surface condensers contain 10,500 sq. ft. of condensing surface, the tubes being 5/8 in. diameter, and 6 ft. long. The propelling power is a two-bladed Griffith screw, 23½ ft. diameter, 261/3 ft. mean pitch, expanding 5 ft.
The valves are moved by a link motion, of which the reversing gear is worked by a small steam reversing engine, which weighs about 350 tons; the boiler weighs nearly as much more. The cost of engines and boiler was £6(5,500. The City of Peking, a screw steamer built for the Pacific mail company, is a vessel of 5,000 tons. There are two pairs of compound engines, having cylinders of 51 and 88 in. diameter, and 4½ ft. stroke of piston. The crank shafts are 18 in. in diameter. Steam is carried at 60 lbs., and is expanded nine times. The boilers are ten in number, cylindrical in form and with cylindrical flues; they are 13 ft. in diameter, l0½ ft. long, with shells of iron 13/16 in. thick, and have 520 ft. of grate surface, 16,500 sq. ft. of heating surface, and 1,600 sq. ft. of superheating surface. The smoke funnels, or stacks, are 8½ ft. in diameter and 70 ft. high. - Steam Pressure and Engine Power. The steam in the engine exerts a varying pressure from the beginning to the end of the stroke, and these pressures may be determined experimentally by the use of the steam engine indicator.
The best form now in general use is the Richards indicator, fig. 17. A miniature steam cylinder, A, has within it a closely fitted piston, which by exceedingly nice construction is made to work perfectly steam-tight without friction or leakage. Its rod B is attached to the parallel motion C D E F, which carries a pencil at the middle of F in a perfectly vertical line. To the upper side of this piston and to the cap V of the cylinder is screwed a helical steel spring, of such strength that, resisting the steam pressure beneath the piston, it causes the pencil to rise and fall, as pressures vary, through distances which are proportional to the changes of pressure. A scale, Gr, on the barrel II II, indicates the pressures per square inch which correspond with the position of the pencil at any instant. The barrel II H is connected by means of the string I with some part of the engine having a motion coincident in time with that of the steam piston, but of such extent that at each stroke of the engine the barrel H will be turned about three fourths of a revolution only. A piece of paper or thin card is wrapped upon this barrel, its end being secured by the springs W, and upon this paper the indicator card or diagram is automatically made by the pencil.
The instrument is attached to the steam cylinder by the cock N, which is screwed at O into the cylinder in such a position that steam can at all times enter it, and so that the pressure in the engine and in the indicator shall be the same. The instrument is secured to the cock by the use of the nut with its double screw threads R S, one of which being finer than the other, the cone T may be forced into U very firmly, and perfectly steam-tight. An indicator should be attached to each end of the cylinder, and diagrams taken simultaneously if possible. The instrument being thus attached and steam admitted, after a few moments' working has thoroughly heated the cylinder, the steam is shut off from the indicator, and the pencil is, with its support J K, swung around, until it touches the paper. As the barrel revolves, the pencil makes a horizontal line, which is called the atmospheric line or line of atmospheric pressure. The reading of the barometer will then give the distance of the vacuum line, or the line of absolutely no pressure, below this line of reference.
Steam is again admitted, and the pencil, rising and falling as the steam pressure changes in the cylinder, while the paper is moved laterally with a motion precisely similar to that of the piston, a diagram is made, usually resembling b c d e f a in fig. 18, taken from the work of Mr. Charles T. Porter on the indicator. Steam from the boiler is supplied to the engine at the commencement of the stroke nearly at boiler pressure, and follows the piston at that pressure until at c the steam passage is gradually contracted, and finally closed by the steam valve. The steam thus confined within the cylinder expands as the piston moves forward, diminishing in pressure until it arrives at d, where the exhaust valve gradually opens communication with the condenser; the pressure drops to e at the end of the stroke, and as condensation becomes completed during the return stroke, the minimum pressure is soon readied and retained until at f the closing of the exhaust valve shuts up a small portion remaining in the cylinder, and it is compressed by the returning piston and its pressure thus increased to a, where the end of the return stroke is reached, the steam valve again opens, and a new cycle of operations begins. A B is the atmospheric line, and C D that of absolute vacuum.
In consequence of the slow closing of the steam or cut-oft' valve in this case, the steam is not completely cut off until the point h is reached, where the change in the character of the curve shows that only from h to d does the steam expansion line truly represent the law of change of volume with pressure. From c to h the steam " wire-draws" through the steam port, and the benefit of expansion is not fully secured. Were the steam port closed instantaneously at c, the line c g would be the expansion line, and would closely correspond with that described already (see Steam) for the special conditions under which it may have been formed. It is to secure this sudden closing and this full benefit of expansion that the drop cut-offs of Sickels, Corliss, Greene, and others have been adopted. Referring again to the diagram, should the "lead" be increased, and steam thus admitted earlier in the stroke, the line a b would be formed parallel with but in advance of its present position. With less lead, the point b would be moved also, the line a b becoming inclined to the left. With a greater or less expansion, the point e moves to the right or the left. With a rapidly closing cut-off" valve, the curve c h becomes shorter, and the curve c h g more nearly like c g.
A better vacuum would bring the line e f nearer C D. In a non-condensing engine e f would be above A B. The distance of e f above A B or above C D indicates the back pressure produced by resistances in the exhaust passages, or the degree of imperfection of the vacuum which is due to the presence of both vapor and air in small quantity. With a three-ported valve, such as is used on locomotives, a shorter cutoff would cause an earlier closing of the exhaust on the return stroke, and the point f would fall at the left of its present position. - The mean value of the steam pressure in the cylinder, as determined by measuring the altitude of the diagram at several points, or by obtaining its area with a planimeter and dividing by its length, is termed the mean pressure. The horse power is determined by multiplying the mean pressure by the area of piston and the speed of piston, and dividing by the value of a horse power. That is, H P = (p x A x V) (/33000) , where P is the mean pressure per square inch, A the area of piston in square inches, V the speed of piston or the product of the length of stroke in feet by twice the number of revolutions per minute.
The horse power was assumed by James Watt as equivalent to 33,000 lbs. raised one foot high in a minute, 550 foot pounds a second, or 1,980,-000 foot pounds of work an hour. This is about the maximum which the best London draught horses were then considered capable of performing. An average actual horse power is about 25,000 lbs. a minute, but Watt's figure is retained by engineers. With engines of ordinary proportions, the mean pressure may be determined with considerable accuracy also by the formula p=P (1 + A log.E/E) -B-CP, the assumption being very nearly correct that steam expands in such cases according to Mari-otte's law, the curve of pressure being a hyperbola and the product of pressure and volume constant. The values of the constants A, B, and C, as determined by Francis B. Stevens, are A==2'3, B=5, and C=0'06. P. is the initial pressure and p the mean pressure. With engines working at moderate pressure, with unjacketed cylinders and medium speed of piston, the point of cut-off giving maximum economy is at about 0.4 or 0.5 the stroke. With high steam and rapid motion, and with steam-jacketed cylinders, economy is gained until the steam is expanded four to six times.
In compound engines it is not unusual to expand from eight to twelve times, but experiment has not indicated that such great expansion is attended with economy. The losses which accompany great expansion are due to internal condensation of steam and its reevaporation on the opening of the exhaust valve, when it carries away a large proportion of unutilized heat into the condenser. This loss sometimes exceeds the amount of heat actually utilized. In recent experiments the steam jacket has been found to save 20 per cent, by checking this condensation, which is the principal source of loss of economy in such engines. Superheating the steam sufficiently to cause it to pass through the cylinder "dry" diminishes it also. The minimum expenditure of steam in the best engines is about 16 or 18 lbs. per horse power per hour. The amount used in the single cylinder engine with moderate expansion and comparatively low pressure is seldom less than 30 lbs., and in old styles of engines worked with a pressure of 20 lbs. per square inch and cutting off at three fourths stroke, the consumption of fuel is often C lbs. and of steam 40 to 50 lbs. per horse power per hour.
The expenditure of coal has been reduced by successive improvements, as the increase of steam pressure, greater expansion, surface condensation, high piston speed, the use of the steam jacket, and minor changes in both engine and boiler, until the best steam engines of the present day consume but about 2 lbs. of coal per horse power per hour, in ordinary work, and in some instances as little as 1½ lb. Even the latter, however, is but about one eighth the efficiency which would be given by a perfeet heat engine. - See Tredgold, " Treatise on Steam Engines" (3 vols. 4to, London, 1852); Bourne, "Treatise on the Screw Propeller" (new ed., London, 1873), "Treatise on the Steam Engine" (new ed., 1873), "Handbook of the Steam Engine " (new ed., 1873), and "Examples of Modern Steam, Air, and Gas Engines" (1868 et seq., to be completed in 24 4to parts); Rankine, "Manual of the Steam Engine and other Prime Movers" (7th ed., London, 1874); and Clark, " Steam and Steam Engines " (London, 1875).
Fig. 12. - Zeuner's Valve Diagram.
Figs. 13 and 14. - Compound Marine Engine.
Fig. 15. - Tug-boat Screw.
Fig. 16. - Hirsch Screw.
Fig. 17. - Rieliards Indicator.
Fig. 18. - Diagram of Indicator.