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gines, much in use in paddle steamers, the beam being placed below the level of the cylinder cover, and worked by a rod or rods descending to it from the cross head of the piston rod; B, direct-acting engines, in which the piston rod acts directly upon the crank; as 1, directacting horizontal engines, stationary; 2, directacting vertical engines, stationary or marine, in some forms known as steeple" engines; 3, oscillating engines. Rotatory, disk, and certain other peculiar forms of engine are also direct-acting. Among the advantages of direct-acting over beam engines are the saving of space, of liability of damage to the cylinder from breaking down and fall of the beam, and of some useless load and friction; the parts transmitting the power in direct-acting being generally less than in beam engines, in the ratio of 2: 5. The Corliss steam engine company, of Providence, construct a non-condensing horizontal engine, which is worked by Corliss's peculiar valve gear. These valves are segmental in form, and in their movement rotative-reciprocating. The steam passages, four in number, are reduced to the shortest practicable length, and each is controlled by a separate valve. The steam is employed expansively, the point of cutting off being controlled and regulated (without appreciable resistance) by the governor, so as to proportion the total pressure of the stroke continually to the desired rate of movement of the machinery; while a peculiarity of the action of the valves is the complete and almost instantaneous manner in which they open and close the passages, thus admitting the steam at or near the full boiler pressure, and preventing the effect known as "wiredrawing." Some of these engines have recently been furnished to orders from Scotland, the centre of steam engine manufacture. Noncondensing stationary engines are direct-acting, and have two principal plans of construction, the horizontal and the vertical. Of these the former are most common. The oblong form of base is now mainly superseded by a base in L form, of which the chief strength is in a vertical web; and the cylinder, crank, and flywheel are so attached to this that the strains arising in working the engine are best met, and the relative positions of the parts the most accurately preserved. Pumping engines are of various construction; among them the most noted and efficient are the so called Cornish engines. These engines economize the power they produce by dispensing with cranks, the fly-wheel, and many other parts to which ordinary forms of engine must impart movement; while for facility of admitting steam of very high pressure, for the great ratio of expansive working they allow, and the small amount of friction involved, they have for pumping taken precedence of all others. For Worthington's "duplex engine," for pumping, see PUMP. IV. Behavior of Steam in the Engine, and how known. The substitution of the elastic force of a vapor for the practically lim

ited and unvarying pressure of the atmosphere, introduced a source of power susceptible of indefinite increase, and restricted only by considerations of safety and of practical advantage. The question, within what limits of steam pressure the maximum of advantage is to be attained, is however one involving so many variable quantities, dependent on the construction of the engine and the conditions under which it is worked, that no general determination of this problem has been obtained; and the desired information must be arrived at, if at all, by repeated experiments for each sort of engine and each set of conditions under which it may be worked. The tendency of such experiments has been to show a gain from the use of comparatively high pressures; so that, in condensing engines, the working pressure (in excess over one atmosphere) per square inch on the piston has been carried up from about 8 or 12 to an average of 25 or 30 lbs. Steam, as commonly employed, is drawn directly from the boiler, and generally in the boiler is saturated, or at maximum density. So existing, the slightest fall of the temperature, owing to abstraction of heat by the surfaces of the cylinder, by radiation, or otherwise, unavoidably determines the condensation of a portion of the steam. It is this instantaneous sensitiveness to cold and facility of condensation that most frequently prevent our attaining the working power of which theoretically the boiler pressure and the engine should be capable, and that oftenest defeat the special expedients resorted to for increasing their efficiency. When steam first enters the cylinder, the space it exists in is at once enlarged by that through which the piston moves; and if the steam space in the boiler and the heat for generating fresh supplies did not much exceed the capacity of the cylinder, the consequence would be a rapid reduction of density and pressure of the acting body of steam. With adequate boiler and furnace, however, the steam removed into the cylinder is continually replaced; and if the pressure be at first somewhat above that of the air, and the steam pipe kept open, the initial pressure of the entering steam may be regarded as, so far as this cause is concerned, maintained from beginning to end of each stroke of the piston. The disturbances in the actual pressure spring mainly from other sources. Even though it were uniform, the pressure on the piston is not equal to that in the boiler; a result due to length and winding of passages, to friction, with usually some condensation. Upon the piston the steam works in a twofold manner: first, by the tension it possesses as delivered freely and continuously into the cylinder; secondly, after the supply is cut off, by the expansion of the volume previously delivered, until in so expanding its pressure may fall to and be balanced by that of the atmosphere, or by the back pressure of the exhaust steam on the other side of the piston. Actuated in the

former manner through the entire stroke, the piston should advance under a uniform impulsion, its speed being constantly accelerated, and its momentum at last suddenly expended on some fixed parts of the machine, involving injury to the latter, and waste of power; when the latter method is, at the proper point in the stroke, made to supersede the former, the pressure gradually falling may be caused to approximate so nearly to, or to fall so slightly below, the back pressure, that the impulsion of the piston and its velocity shall gradually decline, and terminate naturally at or near the end of the stroke. So much cutting off and expansive working of steam is practically desirable in all engines; but this is not what is technically intended by "cutting off" and "expansion." In the mode of working specially so named, the steam being of comparatively high tension, the supply to the cylinder is cut off at an earlier period in the stroke; and expansion is availed of, not merely for avoiding waste, but as a positive means of deriving from a given volume of steam an augmented total of pressure, and so of performance. In strict language, the whole process is expansive acting; since so long as its pressure is in excess of that of the bodies that confine it, the steam must continue to push these bodies before it, in the tendency to arrive at equilibrium; and, with open ports, its expansive energy acts from the water in the boiler as its fixed point or fulerum, as, after cutting off, it acts from the fixed end of the cylinder. . For practical purposes, however, the distinction to be drawn at the point between full and expansive working is a real and important one. In actual working, again, the pressure of full stroke is seldom or never maintained quite uniform; owing to time consumed in shutting the ports, or to great speed of the piston, or to both, the density of the steam is reduced, and the pressure begins to decline before cutting off is complete; if this result is marked, wiredrawing" of the steam is said to occur, the effect being as if the entering charge were gradually drawn or spun into steam of reduced density and pressure. In order that the steam may gain admission within the cylinder by the very moment at which the stroke should commence, the valve motion-the eccentric on the shaft, for example-must be so set in advance of the crank that the steam port shall be already uncovered or opened by a small fraction of the movement of the valve by the time the piston is prepared to return. This anticipative opening of the steam port is called the lead; and it may be greater or less, even to commencing the steam supply in front of the piston while nearing the end of its stroke, for the purpose of "cushioning" it, i. e., arresting its movement against the steam itself, as against a spring; the point of effecting this being simply determined by the adjustment of the valve gear, with the form and dimensions of the valves. Properly, one complete advance and

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return of the piston constitutes a stroke; the respective half strokes, however, being termed forward stroke and return stroke. Let us trace the distribution of the steam during a stroke starting from either end of the cylinder. Suppose the piston, fig. 4, to have risen already to its highest position in the cylinder; now, in obedience to the position and throw of the eccentric, or to a connection with the governor, three events have just previously occurred: the returning valve has closed the exhaust from before or above the piston, locking up before it a residue of steam; it has at the same moment, or a little before, opened the exhaust beneath the piston, relieving it of the pressure that impelled it forward; and very shortly after these two events it has opened the steam communication above the piston by the amount of lead allowed, and commenced the steam supply. Under this last, having come to rest, the piston commences its forward stroke; the exhaust from before (beneath) it, having been previously opened, is maintained during nearly this whole movement; but first, and at the proper fraction of the movement, the steam supply behind (above) the piston is cut off. Shortly before completion of this forward stroke the exhaust in advance of it is closed, and that from behind it opened; and at a very small distance from the end the lead or admission of steam from beneath occurs, and in a moment more the arrested piston is ready for the return stroke. Thus, on the two sides of the piston there are at the same time proceeding two complete cycles of events, but in different parts of their course. In each cycle there are these four events, in their order: 1, admission of steam; 2, suppression or cutting off; 3, release, or exhaust; 4, arrest, or lock-up, prior to readmission of steam.-The pressure, and generally the behavior of the steam during a stroke in either end of the cylinder, is known by use of the indicator. This, in a usual form, fig. 6, is a

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long, small cylinder, having within it a piston, which is fixed to the lower end of a spiral spring that attaches above and within to the r top of the cylinder, and which, moving with little w friction and carrying outside the cylinder case an index, is made by this to show and also to register the pressures of steam exerted upon it; thus it can indicate for any steam space the work the steam in it is capable of, or is performing. O is the cylinder; 8 a tube with screw thread for fitting into an orifice in the cylinder cover, or in any passage, or the boiler, as may be desired; c the cock

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FIG. 6.

opening or closing the tube. The index, p, fixed to the piston, rises and descends with it, one of its ends moving along a scale, S, showing the pressure in pounds per square inch on the piston; the other end having a joint by which a pencil is brought down in contact with a sheet of paper called the card, rolled about a vertical drum, D, and held upon it by clasps, as f. The spring is of such length that when the atmosphere entering above rests upon the piston, and steam of one atmosphere balances it from beneath, the piston shall rest in its natural or unstrained position; this point is the 0 of the scale. If the steam pressure exceed one atmosphere, it forces up the piston and index, compressing the spring; if it fall short, the piston and index are by the air carried proportionally down, elongating the spring. The drum can rotate about half way around a vertical axis, and when released is returned with a like uniform movement by a spring within. By the wheel, W, and cord, r, the connection needed to work the drum is made with some part of the machinery. To use the indicator: Connect it with the interior of the cylinder or other steam space; leave the cock open during a few strokes of the engine piston, to bring the indicator cylinder to a like temperature; and the pressures upon the indicator piston and, say, the engine piston may now be considered equal. Now, at beginning of a stroke, bring down the pencil to touch the card; the latter semi-rotating and returning, and the pencil rising and falling with the varying pressures through the stroke, there is traced on the card a curved figure, approaching more or less to an oblong, which is the "indicator diagram." Closing the cock at beginning of the next stroke, let the drum turn once or more while the pencil rests stationary; thus will be traced through or beneath the diagram, as the case may be, at the O level, the "atmospheric line." The lengths of ordinates drawn from this line to points of the curve above it will show the excess above one atmosphere of the steam pressure, and to

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points. Of course, with high pressure engines the diagram will never run below the atmospheric line. In fig. 7 are shown two diagrams, the heavy-lined and dotted-lined figures, taken from a locomotive engine, under different conditions. A B, the atmospheric line, may also represent the length of stroke; the periods of the several events in the distribution of steam are here noted for comparison. The diagram is a picture of the operations in one end of the cylinder; and the indicator has been aptly said, like the stethoscope, to reveal what is transpiring beyond the reach of the eye. When accuracy is desired, diagrams are taken for both ends of a cylinder. A B, fig. 7, may stand for the length of stroke, and the space above this line represent the interior of the cylinder. In the heavy-lined diagram, taken with the slow average speed of piston, 40 feet per minute, the piston is seen here to start under the uniform pressure of 61 lbs. above the atmosphere; to preserve this nearly until cutting off of steam; the pressure during expansion then rapidly declining to about 23 lbs.; on release, still more rapidly; and before the end of stroke to come down completely to one atmosphere. During the return stroke, the back pressure remains thus low, until, upon lock-up, the pressure curve mounts rapidly; and at a, when the "lead" takes place, it sweeps still more rapidly up, regaining the full head by beginning of the next forward stroke. Here, with admission through about the forward stroke, and expansion through slightly more, inspection will show that about the whole work of the steam has been that secured by expansion. The dotted-lined diagram shows the behavior of the steam in the same cylinder, with speed of piston equal to 310 feet per minute, other conditions remaining the same. Here, the steam entering at initial pressure of 62 lbs., the quick recedence of the piston before it allows the pressure curve to fall slightly; from wiredrawing near cutting off, it falls still more rapidly; after release, however, keeping higher than before, since the speed of the piston

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does not allow time for exhaustion; and at no time in the return stroke quite falling to 0. If, when a slide valve is in middle position, the advancing edge on whichever side already overlaps the port on that side, it must have closed that port previous to its reaching such position, i. e., to cut off earlier than in full working, and to work the steam expansively. The effect, the opposite of "lead," is called the "lap;" and the amount of the lap determines the ratio in which expansion shall occur. If the cylinder be colder than the admitted steam, a very sensible condensation occurs, both during admission and the early part of the expansion; and though during the latter part

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of the expansion the remaining steam, becoming more tenuous and dry, re-absorbs part of this water due to condensation, yet there is an absolute and considerable loss, which increases as the steam is earlier cut off. In ordinary engines the waste due to this cause has been found often to exceed 12 per cent.; in exposed locomotive engines, to amount sometimes to nearly 40 per cent. And it is doubtless this source of loss, where the steam is not at all superheated and the heat of the cylinder not kept in, that often defeats attempts at expansive working of steam, and leads to a prejudice against the method, when the fault is in the unfitness of the conditions under which it is tried. No doubt one of the chief actual benefits of superheating steam through a few degrees, before admission to the cylinder, arises from its thus being supplied with a surplus of heat, by parting with a portion of which it keeps up the temperature of the cylinder, while another portion serves to prevent condensation or speedily to re-convert into steam the water due to its momentary occurrence. Back pressure in condensing engines is in part due to air liberated from the boiler water; but, on the principle that the pressure in communicating vessels is never less than that in the coldest part, it is chiefly that of the vapor in the condenser, its temperature being about 104° F., and its pressure 1.06 lbs. per square inch; in practice, the total back pressure is 1 to 3 lbs. or more. Back pressure is made less by enlarging the exhaust port. As to the measurement of the work of the engine: the indicator diagram represents, for each given point in the advance and return of the piston, the effective steam pressure or the back pressure exerted per square inch on the corresponding face of the piston. As the lines of the diagram are curved, its area must be found by a process of reduction or average. Divide the diagram into horizontal sections answering to the pressures, and into a convenient number of vertical sections, as shown in fig. 8; take the mean effective pressure in each of the vertical sections, add these together, divide by the number of such divisions, and the quotient is the effective mean pressure per unit of surface for the whole diagram; multiply this by the area of the piston in like units, and the product is the whole effective work upon one surface of the piston for one stroke. Proceed in the same way with a diagram for the other end of the cylinder; add the two results; take their mean; multiply by the number of single or half strokes of the engine per minute, and divide by 33,000 (see MECHANICS); the resulting quotient is the "indicator horse power" of the engine. In averaging the diagram, fig. 8, add the average pressures for the 10 divisions made in the stroke; their sum, 204.5, divided by 10, gives 20.45 Ibs., the mean unbalanced pressure per square inch on the piston throughout the stroke. If there be two or more engines acting together,

the total indicator power must be found by adding the results given by the cylinders separately. This is the work upon the piston, under the total of resistances of every kind that must be overcome during, and to allow of, its movements; but to find what part of this work is expended upon the useful resistance overcome, or at any other connection between the piston and the useful resistance, so as to learn how much of the retardation of the pis10 9 8 7 6 5 4 3 2 1

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ton is due to the machinery, or possibly to imperfections in it, it becomes necessary to interpose some form of dynamometer at the connection where the observation is to be made, so as to find by its indications how much of the total work upon the piston during a given time reaches and is expended at that connection. "Nominal horse power" is a conventional unit of size of cylinder, not of observed power of the engine. The rules of estimating it have differed with different localities, and they have usually allowed a larger unit of capacity for condensing engines. It is not much in use as a measure of value of engines in the United States; when it is so, the following is a usual form: H. P.:

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stroke in feet X diameter 2 in inches 47

Actual horse power should be reckoned in actual units of pressure, to be known either by use of the indicator, or of a dynamometer showing the power delivered at the crank shaft. For the former, the rule may have the following expressions: Ind. H. P.=

mean press. X diam.2 x .7854 x stroke X 2 x No. of turns 88,000 pressure X diam. 2 x stroke X No. of turns 21,000 pressure X area X stroke X 2 X No. of turns 88,000

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V. Work of Steam in the Engine. The pressure of steam in the boiler, and within the steam chest or the cylinder, is commonly brought under direct observation by means of pressure gauges of various sorts (see PNEUMATICS), while the degree of vacuum in the condenser is indicated by the vacuum gauge. What is at different times called the "vacuum" of the condenser may be either the degree of actual vacuum produced, or the residual pressure. Thus, when the latter sustains 5 inches of mercury, it may be said in one sense that a vacuum equivalent

to 25 inches of mercury is obtained; in another and less correct sense, that the vacuum corresponds to 5 inches of mercury. In doubleacting condensing engines, the piston is released from effect of the external atmosphere, and its work is performed and estimated independently of it. The indicated power of every steam engine is greater than the available power by the amount of energy expended in overcoming the resistance of the engine. The available power is the useful work the engine can perform in a given time, or rather the power it can impart in such time to the mechanism to be moved by it. The useful effect, or net available power, is a quantity involving three others, the velocity, the load, and the rate of evaporation in the boiler; and this net available power can be expressed and calculated in six different ways: 1, in foot-pounds per unit of time; 2, in horse power; 3, in weight raised per pound of fuel; 4, do. do. per cubic foot of water evaporated; 5, in number of pounds of fuel, or cubic feet of water to a horse power; 6, in number of horse power to a pound of fuel or a cubic foot of water. For the investigations by which formulas for these calculations are obtained, and for the modes of obtaining the requisite numerical solutions by them, the reader is referred to the more comprehensive works presenting the mathematical theory of the steam engine, especially to those of De Pambour, Tredgold, Bourne, and Rankine. The limit beyond which expansion cannot be advantageously carried seems to be that number of volumes found by dividing the initial pressure of the steam by the pressure in the condenser. This result of theory, however, proceeds on the supposition that the steam is maintained in the perfectly gaseous condition. Practical results seem to sustain Prof. Rankine's estimate, to the effect that the gain of efficiency in an ordinary engine, cutting off at one fifth, with superheating by heat from the flues steam of 34 lbs. pressure, is about 15 per cent.; if by heat otherwise wasted, as by carrying the steam pipe through the chimney, about 23 per cent. Though the principle of working steam expansively is very simple, and has long been accepted, the subject is not yet exempt from discussion or differences of opinion among engineers. Mr. King, in his "Practical Notes on Steam" (New York, 1861), estimates that by cutting off at half stroke the saving in fuel may be made nearly 20 per cent.; and for other ratios of expansion, within certain limits, in proportion. Engineers Isherwood, Stimers, and others, as the result of numerous and it would appear carefully conducted experiments with the engines of the steamer Michigan, at Erie, Penn., were led to conclude that the maximum gain by expansion is secured by cutting off at stroke; that to eut off much short of this affords no gain; that the loss by condensation in the cylinder, and by increased friction and back pressure, is generally underrated; and that the use of a variable cut-off

has usually not sufficient advantage over the ordinary throttle-valve, to compensate for its cost and the attention it may require. It may be considered doubtful whether these conclusions will be fully sustained; but they will at least have the good effect to call attention to the extreme to which expansive working has been carried, steam having been cut off in some cases at, or even of the stroke. The ideal "cut-off" arrangement would be that which, first, should close the ports instantaneously at the proper moments, so that the steam should be admitted unreduced by wiredrawing, and enabled to act as it were with explosive force upon the piston; and secondly, which should be completely under control of the governor. The cut-offs now in use are very numerous, among them those of Sickels, Stevens, Allen and Wells, Corliss, Woodruff and Beach, and others. In respect to superheating of steam within limits from 10° to 40° above the temperature of saturation, it may be proper to add that the fears once entertained of its destroying lubrication, burning the surface of the cylinder or passages, &c., have proved quite groundless in practice; while at the same time, the notion that high superheating would greatly increase the effective work of the steam has also been discarded; so that superheating within moderate limits is now resorted to, mainly as a desirable condition for successful expansive working, or otherwise, merely as a means of preventing loss of the steam pressure. In practice, indeed, owing to want of heat conduction in fluids, with radiation of heat into the dome from the water surface, or from other causes, the steam in a high dome, and especially when the water beneath is for some time but slightly agitated, is in effect isolated from the water, and actually very often becomes superheated, unknown to the engineer. In some marine engines, also, steam is in practice surcharged with heat in the dome, by carrying flues through or around it. It is customary to estimate the efficiency of steam in a rough way by considering the effective mechanical force of a cubic foot of water vaporized as 60 horse power. If, then, this quantity of water be converted into steam in an hour, it will give a horse power per hour; and the boiler and engine that could generate and employ the steam of 10 cubic feet of water per hour, would give continually 10 horse power of work upon the piston. The high boiling point of water, but more especially the large degree of latent heat required to vaporize it, renders steam power expensive through necessity of a proportionately great consumption of fuel. Accordingly, various other vapors, as well as gases generated by explosion, have been tried as substitutes for steam. A comparison of the boiling points and latent heat of certain other liquids, with the relative volume and density (air being 1) of their vapors, will show theoretically their eligibility thus to serve as more economical substitutes:

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