engine consisted of a vertical cylinder, open at the top and placed above the boiler. The piston was connected by a chain with one end of an overhead rocking beam, the opposite end of the beam having a long pumprod hung from it, also a heavy weight or counterpoise to bring the piston to the top of its stroke when steam was admitted into the cylinder from the boiler. Steam was then shut off, and a jet of cold water allowed to enter the cylinder, condensing the steam and producing a partial vacuum. The pressure of the atmosphere acting on the upper face of the piston forced it down and lifted the pump-rod at the other end of the beam. The condensation of the steam inside of the cylinder was Savery's invention, and Savery claimed and received an interest in the Newcomen engine. About 1711 Newcomen's engine began to be introduced for pumping water out of mines.

In 1763 James Watt was engaged in repairing a model of Newcomen's engine belonging to Glasgow University, and from this date the true development of the steam-engine may be said to have begun. Watt saw that it was absolutely essential that the cylinder should be kept as hot as possible to prevent undue loss of steam, and that all condensation must take place in a vessel separate from the cylinder. To deal with the various inventions and improvements made in the steamengine by James Watt is beyond the scope of this article, which is meant rather to describe the construction and general design of a few of the more important and best types of the modern steamengine.

The plain slide-valve steamengine (Fig. 1) is one of the simplest forms. It consists of a cast-iron cylinder c, here shown in section, fitted with a piston P, which is made steamtight by means of expansible rings R. The piston is connected to the crank D through a piston rod PR and a connecting rod CR. The reciprocating motion of the piston is thus converted into a rotary motion of the crank shaft. The piston is made to move to and fro in the cylinder by the action of the steam, which is admitted alternately to the opposite ends of the cylinder through ports s s. The face of the piston, opposite to that on which the driving pressure is acting, is for the greater part of the stroke in communication with the exhaust port EP. The steam enters and leaves the cylinder through the steam ports, and is discharged into the atmosphere (non-condensing engine), or into a conden

ser (condensing engine) through the exhaust port.

Slide Valve. The admission of the steam and its discharge after it has effected its purpose is determined by a slide valve sv, which is made to travel to and fro across the port openings by the action of an eccentric E keyed to the crank shaft cs. The pressure of the steam acting on the back of the valve keeps it in steam tight contact with the working face of the cylinder. The cavity F in the centre of the valve permits either steam port to communicate with the exhaust port. The amount by which the slide valve overlaps the outer and inner edges respectively of each steam port when in mid position (Fig. 2) is called the outside or steam lap' o, and the 'inside' or 'exhaust lap' I. The object of the outside lap is to cut off the steam before the piston has reached the end of a stroke, so as to take advantage of the expansive energy of the steam, and thus to work much more economically than if the steam were admitted throughout the entire stroke. The inside lap acts in the same way with regard to the steam leaving the cylinder a certain portion of the exhaust steam is retained and compressed by the piston, forming an elastic cushion, which assists in bringing the piston, etc., to rest without shock at the end of each stroke. The steam port s begins to open for the admission of steam just before the piston reaches the end of each stroke, so that at the commencement of a stroke the port is open by a small amount, which is called the 'lead.' The dotted lines show the valve in this position. Admitting steam on the exhaust side just before the end of the stroke completes the cushioning action, and enables the full pressure to be brought to bear on the piston at the commencement of each working stroke. Towards the end of the expansion or working stroke the slide valve is in such a position that the steam can escape through the cavity F into the exhaust port EP. This point is called the release.' The driving pressure is thus relieved, and at the commencement of the return stroke, owing to the further motion of the valve, there is ample opening for the exhaust steam to pass out of the cylinder without producing undue back pressure. A delayed release causes excessive back pressure, and reduces the effective driving pressure on the piston.

The respective positions of the slide valve (in section) and piston with reference to the steam and exhaust ports at the beginning of a stroke, cut-off, release,

and compression, are shown in Fig. 3. The arrows indicate the direction in which the slide valve and piston are moving for each position. The corresponding positions are also shown on a hypothetical indicator diagram (Fig. 4). The cut-off, release, etc., do not occur at the same points on the forward and turn strokes respectively, because the obliquity of the connecting rod to the line of stroke causes the piston to be more advanced in the one stroke and less advanced in the other than it should be to correspond exactly with the crank's position.

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A single slide valve is not suit able for a cut-off earlier than half-stroke, because with it the period of expansion is equal to the period of compression, and it is desirable that under certain circumstances the latter should be independent of the former. Consequently many engines have two sets of valves, one set working at the back of the other, as in Fig. 5, which shows a form known as the Meyer variable expansion gear.' The steam admission, release, and compression are determined by a main valve MV, and the cut-off is effected by the two valves or blocks vv, which slide across the back of the main valve and are operated by a separate eccentric. The cut-off valve spindle s is usually provided with a right and left hand screw working in suitable nuts within each valve, and by rotating this spindle the valves vv may be made to separate, or to come closer together, thus varying the point of cut-off to any desired degree. The slide valve is sometimes made in the form of a piston; an example of a piston valve is shown in Fig. 6. action is identical with that of a simple slide valve. The total pressures on the opposite ends of a piston valve are practically equal; consequently very little force is required to operate it. Piston rings are fitted to the valve to make it steam-tight. In large engines, especially of the marine type, double-ported valves have been adopted. (See Fig. 7.) With this form of valve there are two openings to each steam port instead of one, and the inner openings get their steam from two passages A A cast in the body of the valve. It is therefore evident that, with a given movement of the valve, the area of port opening will be twice that obtained with an ordinary slide valve; consequently the travel of the valve can be considerably reduced.

The

The eccentric, which gives the reciprocating motion to the slide valve, is set with its radius OE (Fig. 8) in a definite position

Steam-engine

with respect to the crank co: thus the angle is termed the angular advance of the eccentric,' and the eccentric radius is 90+ degrees in advance of the crank. EF is perpendicular to CF, and of is equal to lap plus lead. The travel of the valve is equal to twice the throw or radius of the eccentric; also the eccentric radius is equal to the lap of the valve plus the maximum opening of the port to steam.

Corliss Valve Gear.-In steamengines fitted with a slide valve the two ports serve a double purpose: they conduct the exhaust steam out of the cylinder as well as admit the fresh boiler steam. This produces an alternate cooling and heating of the ports, and causes a loss of steam due to condensation; the ports are also somewhat long, and contain a quantity of steam which is to a large extent non-effective. To remedy these defects, and to give

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stantaneously. A general view of the Corliss gear on a large pumping-engine is illustrated in Fig. 10. The steam-valves ss are opened by links connected with a wrist plate wr, which is operated from an eccentric on the crank shaft. The sudden closing of the steam-valves is effected through a 'trip' or trigger mechanism connected with the governor, which causes a catch to be released when the moment for cutting off the supply of steam to the cylinder has arrived, and the valve instantly flies back to its normal position covering the steam-port. The exhaust valves EE are opened and closed through links connected to a wrist plate WIP, which is worked from a second eccentric. The steamvalves are connected by suitable linkwork to dash-pots DP, whose function is to return the valves quickly and noiselessly when the governor releases the trip-catch.

Steam-engine

obtained from an eccentric H keyed to a side shaft м driven by gearing from the crank shaft. The point at which the trigger piece c releases the valve-lifting lever is determined by the governor. The valve is lifted against the action of a spring contained in a cylinder L; this spring assists in bringing about a quick closing of the valve. Each exhaust valve E is also of the double-beat type, and is operated through a cam N on the side shaft. A cross compound engine fitted with this gear is shown in Fig. 18.

Fly-wheel. With a single-cylinder engine there cannot be any turning effort on the crank shaft when the piston is at either extreme end of its stroke (the crank then being on a dead centre), and in that position the engine cannot be started. A flywheel is always keyed to the crank shaft: when the engine is

a better steam distribution, the Corliss valve gear has been introduced, and is much used for engines of moderate speed. In this system each end of the cylinder is provided with two separate valves and ports for the admission and discharge of the steam. The valves are placed as close as possible to the working barrel of the cylinder; consequently the ports are very short. A transverse section through part of the end of a cylinder fitted with Corliss valve gear is shown in Fig. 9. The valve A is a portion of a cylinder, and is made to oscillate through a small angle, about c as a centre, on a cylindrical face in which there is a steam-port s. The dotted lines show the valve full open. In the Corliss gear the steam-valves open to the full extent, and with equal rapidity, whether the cut-off is to be early or late; they remain open as long as required for the admission of steam, and then close almost in

Each dash-pot is fitted with a piston, which is moved upwards by the valve gear during the steam admission; a vacuum is formed under the piston, which is forced down by atmospheric pressure the instant that the tripcatch is released by the governor. Sometimes the piston of the dash - pot is moved upwards against the pressure of a spring in preference to depending upon

a vacuum.

Another form of trip-gear' largely used for horizontal engines, especially in Europe, is illustrated in Fig. 11, which shows vertical transverse and longitudinal sections through one end of a cylinder. There are two admission and two exhaust valves. Each admission valve A is of the double-beat drop or equilibrium type, operated by a detachable trigger piece c, which engages with the outer end of the valve-lifting lever B. The motion necessary to lift the valve is

in motion the momentum of the fly-wheel helps the crank over the dead centres, and assists in making the motion of the engine more uniform. With two cylinder engines-cranks at right angles the necessity for a flywheel is not so great; when one is used, it does not need to be quite so heavy as for a singlecylinder engine, in order to give the same uniformity of rotation. It is dispensed with entirely in reversing engines, such as locomotive and marine engines and some forms of heavy rolling-mill engines. In large engines used for driving electric generators the heavy rotating armature often takes the place of a flywheel,

Governor.-Engines that are required to run at a steady speed. must be provided with a governor, whose function is to bring the work done by the steam in the engine cylinder into correspondence with the actual work

FIG. 16.-Watt's Governor. A, As commonly used; E, original form.

introduced by Watt, and consists in varying the pressure of the steam supplied to the engine by opening or closing more or less a valve in the supply pipe. This method of regulation is known as 'throttling,' and the regulating valve as the throttle valve.' It is still extensively used, especially for small engines. The other method consists in varying the volume of steam supplied to the engine by altering the point of cut-off. The second method, being the more efficient of the two, is chiefly used on large stationary engines. In gas and oil engines, regulation is sometimes effected by cutting off the gas or oil supply, thus causing the engine to miss one or more explosions whenever the speed rises too high. Such governors are called hit-and-miss' governors.

Governors are usually of the centrifugal type, of which the pendulum governor of Watt, shown in Fig. 16, may be taken as the simplest. Two heavy balls

FIG. 17.

are fixed at the ends of two links, the other ends of which are pivoted to a vertical spindle, driven from the engine shaft. Owing to centrifugal force, the balls fly outwards, and in doing so raise a sleeve, which slides on the spindle, and is connected with the throttle valve. If the speed of

the engine rises above its normal, the balls fly farther out and the sleeve is raised, closing the throttle valve a little, and reducing the pressure of the steam supplied to the engine. When the speed falls, the balls move inwards, and the throttle valve is opened a little, increasing the pressure of steam supplied to the engine. In the figure the arms are shown jointed to the spindle; but they are sometimes pivoted to a short cross-bar rigidly attached to the spindle, as shown diagrammatically in Fig. 17, or as in Fig. 18, where the arms are crossed, the spindle being slotted out to allow them to pass through.

Fig. 19 shows a modification of Watt's governor, the original form being now seldom used. It is known as the loaded Watt governor,' or the Porter governor,' after its inventor. As will be seen from the figure, there is a large central weight resting on the sleeve and sliding with it on the spindle. Loading a governor in this manner gives it more power to overcome frictional resistances than an unloaded gov

FIG. 18.

ernor with revolving balls of the same weight. The speed of revolution of a loaded governor is much higher than that of an unloaded one. In some similar forms of governor the central weight is replaced by a spring. Such governors are often spoken of as spring controlled.'

In any governor of the Watt, or pendulum type, the height h (Figs. 17 and 18 determines the number of revolutions required to maintain equilibrium. Consequently a governor arranged as shown in Fig. 16 can be made very sensitive, because such an arrangement allows the balls to move a considerable distance for a slight change of height h. Very sensitive governors are desirable only for high-speed engines, and for these the shaft governor is preferable to the pendulum type.

In large stationary engines the governor often controls the steam-supply by operating a trip which allows the admission valve to close. This is the case in the well-known Corliss gear, in which the valve is closed sud

denly by the action of atmospheric pressure on a piston below which is a partial vacuum.

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FIG. 19.-Porter Governor.

The governor merely determines the position of the trip in this gear, and hence is composed of light parts which are easily moved.

Sensibility and Isochronism.If there were no friction, only one position of the governor balls would be possible for any particular speed; and since the steam supply depends upon the position of the balls, the speed of the engine must vary for different loads. When the variation of speed allowed by the governor between no load and full load is small, the governor is said to be 'sensitive.'

Apart from friction, it would be possible to make a governor in which only one speed was possible, the slightest variation from the given speed sending the balls into the extreme up or down position. Such a governor would be called' isochronous.' Friction prevents the attainment of isochronism, and also in practice a certain amount of stability is necessary in order to counteract the tendency to oscillate violently up and down whenever the speed changes from the normal.

The governor shown diagrammatically in Fig. 17 may be made isochronous for small displacements by suitably proportioning the cross-bar from which the arms are hung. In practice it is made rather shorter than this in order to give the necessary stability. This arrangement is known as Farcot's governor, from its in

ventor.

Steam-engine

Hunting. When an engine changes its speed there is always an interval of lag' before the governor produces its effect, due partly to the governor not responding instantly to the change of speed of the engine, and partly to the response of the governor not producing an immediate effect on the engine; for the steam already in the engine will continue to do its work, and if cut-off has occurred, it is not till the next stroke that the action of the governor can begin to take effect. In compound engines in particular, the steam, after passing through the high-pressure cylinder, passes into the lowpressure cylinder, continuing to do work for almost a revolution before the action of the governor can take full effect. The result is that if the governor be too sensitive, a sudden decrease of the load on the engine may increase the speed considerably before the action of the governor makes itself felt; and in consequence the governor moves too far, and reduces the steam supply below what is necessary for the diminished load. This causes the speed to be diminished too much, and the same effect is produced in the opposite direction. oscillation which thus tends to be set up is known as hunting.'

The

Shaft Governors.-The governors of high-speed engines are sometimes fixed directly to the crank-shaft of the engine, often within the fly-wheel, the arms of which serve as centres for the revolving weights. The centrifugal force of the revolving weights is resisted by springs. Governors of this type often regulate by varying the cut-off, by altering the angle of advance and the throw of the eccentric.

The latest development in shaft governors is the inertia-governor.' In this device the revolving weights, when in equilibrium, are so situated that they will move out of position by their own inertia if the speed of the engine changes. This arrangement makes a governor which responds very quickly even for a slight change of speed. For the theory of the governor, see Klein's High-Speed Steam - Engine (New York, 1903).

Hydraulic or Pump Governors. -In this type of governor a small pump driven from the engine pumps water into a small cylinder with a piston held down by a spring, the piston being connected with the regulating mechanism. The water escapes from the vessel by an orifice, the size of which is so regulated that at normal speed the pressure is just sufficient to support the piston against the pressure of the spring. If the speed rises, the

423

pressure in the cylinder is increased and the piston is raised, reducing the steam supply.

Marine Governors. Many forms of governing apparatus have been devised to control marine engines, and so prevent their racing when the screw breaks out of the water as the vessel pitches in a heavy sea. Some of these governors have a pendulum that swings in a foreand-aft plane, cutting off the steam as the stern of the ship rises and admitting full pressure as it falls. Others depend on the pressure of the water on a diaphragm at the stern of the vessel. When the stern rises and the pressure becomes small. the steam is cut off. All these devices have been found to be poor, however, because the governing

Steam-engine

usually from 15 to 18 lbs. per sq. in. (absolute). By condensing the exhaust steam in a suitable condenser fitted with an airpump or its equivalent, a vacuum is formed, and the back pressure reduced to 3 or 4 lbs. per sq. in. (absolute), thus very materially increasing the effective pressure on the piston. A considerable economy is thereby effected in the working of a steamengine by using a condenser. See CONDENSER.

Compound Engines. - Steam contains a large amount of internal or intrinsic energy, obtained during its formation in the boiler from the heat developed by the combustion of the fuel in the boiler furnace. A considerable proportion of this internal energy may be converted into

FIG. 20.-Westinghouse Compound Engine.

action is too tardy, occurring after the screw has broken water, while the steam should be shut off before that happens. As such governors are needed only in stormy weather, they are fitted merely as a precaution, the governing in a storm being usually done by the engineer on watch, who shuts off and turns on the steam at the right instant.

Condensers.-The effective driving pressure on the piston of an engine at any instant is equal to the difference between the forward and the back pressure acting on the opposite faces of the piston. In a non-condensing engine the exhaust steam is discharged against the pressure of the atmosphere and other resistances due to the friction of the steam in the ports, passages, etc.; consequently the back pressure is

mechanical work by allowing the steam to expand in the engine cylinder. The economical working of an engine is greatly augmented by using steam at a high initial pressure and expanding it to the lowest possible practicable terminal pressure; in a condensing engine the terminal pressure is usually about 10 lbs. per sq. in. (absolute), and from 18 to 40 lbs. per sq. in. in a non-condensing engine. Unfortunately, if the full expansion of the steam is carried out in one cylinder, the interior of the latter is subjected to a considerable variation in temperature, and in consequence a large proportion of the incoming steam is condensed during the admission part of the stroke without doing any work. The compound engine is designed to reduce the waste of steam due to