THE nature of the service required of locomotive engines, especially those employed on fast-train service, makes it necessary that the steam-distribution gear shall be free from complication; and, for convenience in working the engine, it is essential that means should be provided for reversing the motion promptly, without endangering the working-parts. The valve-gear should also be capable of regulating the admission and exhaust of steam, so that the engine shall be able to maintain a high rate of speed, or to exert a great tractive force. These features are admirably combined in the valve-gear of the ordinary locomotive. Designers of this form of engine have given great consideration to the merit of simplicity. Numerous attempts have been made to displace the common D slide-valve, but every move in that direction has ended in failure.

The slide-valve, in a crude form, was invented by Matthew Murray of Leeds, England, towards the end of last century; and it was subsequently improved by Watt to the D form. It received but little application in England till the locomotive era. Oliver Evans of Philadelphia appears to have perceived the advantages possessed by the slide-valve, for he used it on engines he designed years before locomotives came into service. The D slide-valve was better adapted for high-speed engines than any thing tried during our early engineering days, but it was on locomotives where it first properly demonstrated its real value. The period of necessity brought the slide-valve into prominence; and the galaxy of mechanical genius that heralded the locomotive into successful operation recognized its most valuable features, and it soon obtained exclusive possession of that form of engine. Through good and evil report, and against many attempts to displace it, the slide-valve has retained a monopoly of high-speed reversible engines.

The slide-valve in common use is practically an oblong cast-iron box, which rests and moves on the valve-seat. In the valve-seat, separated by partitions called bridges, are three ports, those at the ends being the openings of the passages for conveying steam to and from the cylinders, while the middle port is in communication with the blast-pipe, which conveys the exhausted steam to the atmosphere. On the under side of the valve is a semicircular cavity, which spans the exhaust-port and the bridges when the valve stands in its central position. When the steam within the cylinder has performed its duty of pushing the piston towards the end of the stroke, the valve cavity moves over the steam-port, and allows the steam to pass into the exhaust-port, thence into the exhaust-pipe. The cavity under the valve thus acts as a door for the escape of the exhaust steam. This is a very convenient and simple method of educting the steam; and the process helps to balance the valve, since the rush of escaping steam striking the under part of the valve tends to counteract the pressure that the steam in the steam-chest continually exerts on the top of the valve.

In its primitive form, the slide-valve was made merely long enough to cover the steam-ports when placed in the central position, as shown in Fig. 6. With a valve of this form, the slightest movement had the effect of opening one end so that steam would be admitted to the cylinder, while the other end opened the exhaust. By such an arrangement, steam was necessarily admitted to the cylinder during the whole length of the stroke; since closing at one end meant opening at the other. There were several serious objections to this system. It was very difficult to give the engine cushion enough to help the cranks over the centers without pounding, and a small degree of lost motion was sufficient to make the steam obstruct the piston during a portion of the stroke. But the most serious drawback to the short valve was, that it permitted no advantage to be taken of the expansive power of steam. For several years after the advent of the locomotive, the boiler pressure used seldom exceeded fifty pounds to the square inch. With this tension of steam, there was little work to be got from expansion with the conditions under which locomotives were worked; but, so soon as higher pressures began to be introduced, the loss of heat entailed by permitting the full-pressure steam to follow the piston to the end of the stroke became too great to continue without an attempted remedy. A very simple change served to remedy this defect, and to render the slide-valve worthy of a prominent place among mechanical appliances for saving power.

The change referred to, which so greatly enhanced the efficiency of the slide-valve, consisted in lengthening the valve-face, so that, when the valve stood in the center of the seat, the edges of the valve extended a certain distance over the induction ports, as in Fig. 7. This extension of the valve is called outside lap, or simply lap. The effect of lap is to close the steam-port before the piston reaches the end of the stroke, and the point at which the steam-port is closed is known as the point of cut-off. When the steam is cut off, and confined within the cylinder, it pushes the piston along by its expansive energy, doing work with heat that would be lost were the cylinder left in communication with the steam-chest till the end of the stroke.

When a slide-valve is actuated by an eccentric connected directly with the rocker-arm or valve-stem, the point of cut-off caused by the extent of lap, remains the same till a change is made on the valve, or on the throw of the eccentric, unless an independent cut-off valve be employed. Locomotives having the old hook motion worked under this disadvantage; because the hook could not vary the travel of the valve, which is the method usually resorted to for producing a variable cut-off. The link and other simple expansion gears perform their office of varying the cut-off in this way.

In addition to cutting off admission of steam before the end of the stroke, lap requires the valve to be set in such a way that it has also the effect of leading to the exhaust-port being opened before the end of the stroke. The point where the exhaust is opened is usually known as the point of release. The change which causes release to happen before the piston completes its stroke, leads to the closure of the exhaust-port before the end of the return stroke is reached, which imprisons the steam remaining in the cylinder, causing compression. Where a valve has no inside lap, release and compression happen simultaneously; that is, the port at one end of the cylinder is opened to release the steam, and that at the other end is closed, letting the piston compress any steam remaining in the cylinder into the space left as piston clearance.

In some cases the inside edges of the valve cavity do not reach the edges of the steam-ports when the valve is on the middle of the seat, but lap over on the bridge a certain distance, as shown by the dotted lines in Fig. 7. This is called inside lap, and its effect upon the distribution of steam is to delay the release. By this means it prolongs the period of expansion, and hastens compression on the return stroke. Inside lap is an advantage only with slow-working engines. When high speed is attempted with engines having much inside lap, the steam does not have enough time to escape from the cylinders, and the back pressure and compression become so great as to be very detrimental to the working of the engine. As locomotive engineers have it, the engine is "logy."

In locomotive practice, the extent of lap varies according to the character of service the engine is intended to perform. With American standard gauge engines, the lap varies from 2 inch to 14 inch. For high-speed engines, the extent of lap ranges from d to 14. Freight engines commonly get s to w outside lap, and from z to 4 inside lap. With a given travel, the greater the lap the longer will the period for expansion be.

Lap was applied to the slide-valve in this country before its advantage as an element of economy was understood in Europe. As early as 1829, James of New York used lap on the valves of an engine used to run a steam-carriage; and in 1832 Mr. Charles W. Copeland put a lap-valve on a steamboat engine, and his father understood that its advantage was in providing for expansion of the steam. Within a decade after our first steam-operated railroad was opened, the lap-valve became a recognized feature of the American locomotive; but the cause of the saving of fuel, effected by its use, was not well comprehended. Many enlightened engineers attributed the saving to the early opening of the exhaust, brought about where outside lap was used, which they theorized reduced back pressure on the piston; and in that way they accounted for the enhanced economy resulting from the application of lap. It was not till Colburn applied the indicator to the locomotive, that the true cause of economy was demonstrated to be in the additional work taken from the steam by using it expansively.

An improvement on the plain D slide-valve has been effected in a simple and ingenious manner in the Allen valve, which is receiving considerable favor for high-speed locomotives. This valve is shown in Fig. 8. The valve has a supplementary steam-passage, A, A, cast above the exhaust cavity. The valve and seat are so arranged, that, so soon as the outside edge of the valve begins to uncover the steam-port at B, the supplementary passage begins receiving steam at C; and this gives a double opening for the admission of steam to the port when the travel is short. As the travel of the valve is always short when an engine is running at high speed, the advantage of this double opening is very great; for it has the effect of admitting the steam promptly at the beginning of the stroke, and maintaining a full pressure on the piston till the point of cut-off.

With an ordinary valve cutting off at six inches, and having five inches eccentric throw, the port opening seldom exceeds a inch. It is a hard matter getting the full pressure of steam through such a small opening in the instant given for admission. If an Allen valve is used with that motion, the opening will be double, making w inch, which makes an important difference. The practical effect of a change of this kind is that an engine will take a train along, cutting off at six inches with the Allen valve, when, with the ordinary valve, the links would have to be dropped to eight or nine inches. The valve can be designed to work on any valve-seat, but the dimensions given in Fig. 8 are those that have been found most satisfactory with our large passenger engines. In designing an Allen valve for an old seat, it is sometimes advisable to widen the steam-ports a quarter of an inch or more, by chamfering off the outside edges that amount. Care must be taken to prevent the valve from traveling so far as to put the supplementary port over the exhaust-port, for that would allow live steam to pass through. The proper dimensions can best be schemed out on paper before making the required change on the seat.

In very carefully conducted experiments made on the Boston and Albany Railroad, to compare the performance of the Allen valve with an engine equipped with a common valve, it was found that the Allen valve effected a fuel saving of seven per cent.

On one of the leading railroads in this country, an engineer was running a locomotive on a fast train where it was a hard matter making the card-time. A few minutes could be saved by passing a water-station; but this was done at serious risk, for the tender would nearly always be empty by the time the next water station was reached. The master mechanic of the road determined to equip this engine with the Allen valve: and, after the change was made, there was no risk in passing the water-station; for there always was a good margin of water in the tank when the next watering-place was reached. The engine seemed to steam better, because the work was done with less steam; and there was a decided saving of fuel. The change made the engine smarter, and there seems to be no limit to the speed it can make. This valve can be applied to any locomotive with trifling expense. When an engine is designed specially for the Allen valve, the steam ports and bridges are usually made a little wider than for the ordinary valve. The only real difficulty in adopting the valve is getting the casting properly made, so that the supplementary port will not be too rough for the passage of steam, and the thin shell will be strong enough to stand the pressure.

For high-speed locomotives, where there is great necessity for getting rid of the exhaust steam quickly, the valves are sometimes cut away at the edges of the cavity, so that, when the valve is placed in the middle of the seat, it does not entirely cover the inside of either of the steam-ports. This is called inside clearance. In many instances inside clearance has been adopted in an effort to rectify mistakes made in designing the valve-motion, principally to overcome defects caused by deficiency of valve-travel. The fastest locomotives throughout the country do not require inside clearance, because their valve-motion is so designed that it is not necessary. Inside clearance induces premature release, and diminishes the period of expansion. Consequently inside clearance wastes steam, and ought to be avoided.

There are certain advantages gained in the working of a locomotive, by having the valves set so that the steam-port will be open a small distance for admission of steam, when the piston is at the beginning of the stroke. This opening is called lead. On the steam side of the valve the opening is called steam-lead: on the exhaust side it is called exhaust-lead. Lead is generally produced by advancing the eccentric on the shaft, its effect being to accelerate every event of the valve's movement; viz., admission, cut-off, release, and compression. In the most perfectly constructed engines, there soon comes to be lost motion in the rod connections and in the boxes. The effect of this lost motion is to delay the movement of the valves; and, unless they are set with a lead opening, the stroke of the piston would in some instances be commenced before steam got into the cylinder. It is also found in practice, that this lost motion would cause a pounding at each change in the direction of the piston's travel, unless there is the necessary cushion to bring the cranks smoothly over the centers. Without cushion, the change of direction of the piston's travel is effected by a series of jerks that are hard on the working-parts. So long as the lead opening at the beginning of the stroke is not advanced enough to produce injurious counter pressure upon the piston, it improves the working of the engine by causing a prompt opening for steam admission at the beginning of the stroke. This is the time that a full steam-pressure is wanted in the cylinder, if economical working be a consideration. A judiciously arranged lead opening is therefore an advantage; since it increases the port opening at the proper time for admitting steam, tending to give nearly boiler pressure in the cylinder at the beginning of the stroke. With the shifting link-motion, the amount of lead opening increases as the links are hooked back towards the center notch; the magnitude of the increase, in most cases, being in direct proportion to the shortness of the eccentric-rods. A common lead opening in full gear with the shifting link is z inch, which often increases to a inch in the center notch. The tendency of wear and lost motion is to neutralize the lead, so that, when a locomotive motion gets worn, increasing the lead will generally improve the working of the engine.

As the work performed by a steam-engine is in direct proportion to the pressure exerted by the steam on the side of the piston which is pulling or pushing on the crank-pin, it is important that the steam should press only on one side of the piston at once. Hence, good engines have the valves operated so that, by the time a stroke is completed, the steam, which was pushing the piston, shall escape, and not obstruct the piston during the return stroke, and so neutralize the steam pressing upon the other side. When an engine is working properly, the steam is admitted alternately to each side of the piston; and its work is done against a pressure on the other side not much higher than that of the atmosphere.

When, from any cause, the steam is not permitted to escape promptly and freely from the cylinder at the end of the piston stroke, a pressure higher than that of the atmosphere remains in the cylinder, obstructing the piston during the return stroke, and causing what is known as back pressure. There is seldom trouble for want of sufficient opening to admit steam to the cylinders, for the pressure is so great that the steam rushes in through a very limited space; but, when the steam has expanded two or three times, its pressure is comparatively weak, and needs a wide opening to get out in the short time allowed. This is one reason why the exhaust-port is made larger than the admission-ports. Nearly all engines with short ports suffer more or less from back pressure, but the most fruitful cause of loss of power through this source is the use of extremely contracted exhaust nozzles. Were it not for the necessity of making a strong artificial draught in the smokestack, so that an intense heat shall be created in the fire-box, quite a saving of power, now lost by back pressure, would be effected by having the exhaust opening as large as the exhaust-pipe. This not being practicable with locomotives, engineers should endeavor to have their nozzles as large as possible consistent with steam-making.

Engines with very limited eccentric throw will often cause back pressure when hooked up, through the valve not opening the port wide enough for free exhaust.

Locomotives suffering from excessive back pressure are nearly always logy. The engine can not be urged into more than moderate speed under any circumstances; and all work is done at the expense of lavish waste of fuel, for a serious percentage of the steam-pressure on the right side of the piston is lost by pressure on the wrong side. It is like the useless labor a man has to do turning a grindstone with one crank, while a boy is holding back on the other side. The weight of obstruction done by the boy must be subtracted from the power exerted by the man to find the net useful energy exerted in turning the grindstone. In the same way, every pound of back pressure on a piston takes away a pound of useful work done by the steam on the other side.

Engines that have much inside lap to the valves are likely to suffer from back pressure when high speed is attempted. The inside lap delays the release of the steam; and, where the piston's velocity is high, the steam does not escape from the cylinder in time to prevent back pressure.

Most of engineers are familiar with the tendency of some engines to "run into a hill." That is, so soon as a hill is struck, they suddenly slow down till a certain speed is reached, when they will keep going. This is generally produced by back pressure, its obstructing effect being reduced when the engine is moving slow.

The necessity which requires lap to be put on a slide-valve to produce an early cut-off, in its turn causes compression, by the valve passing over the steam-port, and closing it entirely for a limited period towards the end of the return stroke. As the cylinder contains some steam which did not pass out while the exhaust-port was open, this is now squeezed into a diminishing space by the advancing piston. In cases where too much steam was left in the cylinders through contracted nozzles or other causes, or where, through mistaken designing of the valve-motion, the port is closed during a protracted period, the steam in the cylinder gets compressed above boiler tension, and loss of useful effect is the result. Under proper limits, the closing of the port before the end of the stroke, and the consequent compression of the steam remaining in the cylinder, have a useful effect on the working of the engine by providing an elastic cushion, which absorbs the momentum of the piston and its connections, leading the crank smoothly over the center. Where it can be so arranged, the amount of compression desirable for any engine is the degree that, along with the lead, will raise the pressure of the cylinder up to that of the boiler at the beginning of the stroke. When this can be regulated, the compression performs desirable service by cushioning the working-parts, thereby preventing pounding, and by filling up the clearance space and steam passages, by that means saving live steam. Compression probably does some economical service by reheating the cylinder, which has a tendency to get cooled down during the period of release, and by re-evaporating the water, which forms by condensation of steam in the cool cylinder.

Engines that are running fast require more cushioning than those that run slow, or at moderate speeds. The link-motion, by its peculiarity of hastening compression when the links are hooked up, tends to make compression a useful service in fast running.

The reciprocating motion which causes the valves to open and close the steam-ports at the proper periods, is, with most locomotives, imparted from eccentrics fastened upon the driving-axle. An eccentric is a circular plate, or disk, which is secured to the axle in such a position that it will turn round on an axis which is not in the center of the disk. The distance from the center of the disk to the point round which it revolves is called its eccentricity, and is half the throw of the eccentric. Thus, if the throw of an eccentric requires to be 5 inches, the distance between the center of the driving-axle and the center of the eccentric will be 22, inches. The movement of an eccentric is the same as that of a crank of the same stroke, and the eccentric is preferred merely because it is more convenient for the purposes to which it is applied than a crank would be.

On the early forms of locomotives, a single eccentric was used to operate the valve for forward and back motion. The eccentric was made with a half circular slot, on which it could be turned to the position needed for forward or back motion. It was held in the required position by a stop-stud fastened on the axle. Several forms of movable eccentrics were invented, and received considerable application during the first decade of railroad operating; but the best of them provided an extremely defective reversing motion. The first engineer to apply two fixed eccentrics as a reversible gear was William T. James of New York, who made a steam carriage in 1829, and worked the engine with four eccentrics,—two for each side. The eccentrics were connected with a link, but the merits of that form of connection were not then recognized here; for it was not applied to locomotives till it became popular in England, and was re-introduced to this country by Rogers. The advantage of the double fixed eccentrics seemed, however, to be recognized from the time James used them; for the plan was adopted by our first locomotive builders. The first locomotive built by Long, who started in 1833 what was afterwards known as the Norris Locomotive Works, Philadelphia, had four fixed eccentrics.

When a locomotive is running, the wheels turn with something near a uniform speed; but any part which receives a reciprocating motion from a crank or eccentric travels at an irregular velocity. Fig. 9 shows the relative motion of the crank-pin and piston during a half revolution. The points in the path of the crank-pin marked A, 1, 2, B, 3, 4, C, are at equal distances apart. The vertical lines run from them to the points a, b, c, d, e, represent the position of the piston in relation to the position of the crank-pin. That is, while the crank-pin traverses the half-circle, A B C, to make a half revolution, the piston, guided by the

cross-head, travels a distance within the cylinder equal to the straight line A C. The crank-pin travels at nearly uniform speed during the whole of its revolution, but the piston travels with an irregular motion. Thus, while the crank-pin travels from A to 1, the piston travels a distance equal to the space between A and a. By the space between the lines, it will be seen that the piston travels slowly at the beginning of the stroke, gets faster as it moves along, reaches its highest velocity about half stroke, then slows down towards the end till it stops, and is ready for the return stroke.

Certain mechanics and inventors have been terribly harassed over this irregular motion of the piston, and numerous devices have been produced for the purpose of securing a uniform motion to the power transmitted. These inventions have usually taken the shape of rotary engines. Probably the fault these people find with the reciprocating engine is one of its greatest merits, for the piston stopping at the end of each stroke permits an element of time for the steam to get in and out of the cylinder.

The valve travels in a manner similar to the piston; although its stroke is much shorter, and its slow movement is towards the limit of travel. The small circle in the figure shows the orbit of the eccentric's center, and the valve-travel is equal to the rectilinear line across the circle. If the valve opened the steam-ports at the outside of its travel, the slow movement at that point would be an objection, since the operation of opening would be slow: but the valve opens the ports towards the middle of its travel, when its velocity is greatest; and, the nearer to the mid travel the act of opening is done, the more promptly it will be performed. This has a good deal to do with making an engine "smart" in getting away from a station.

With the short valve without lap used on the earliest forms of locomotives, the eccentric was set at right angles to the crank or "square" on the dotted line e, Fig. 10. The least movement of the eccentric from its middle position had the effect of opening the steam-ports. One advantage about an eccentric set in this position, was that it opened and closed the ports when moving the valve at its greatest velocity. Lengthening the valve-face by providing lap entails a change in the location of the eccentric; for, were it left in the right-angle position, the steam-port would remain covered till the eccentric had moved the valve a distance equal to the extent of the lap on one end, and the piston would begin its stroke without steam.

The change made on the eccentric location is to advance it from e to F, being a horizontal distance equal to the extent of lap and lead, and known as the angular advance of the eccentric. The centers F and B represent the full part, or "belly," of the forward and back eccentrics in the position they should occupy, where a rocker is employed, when the piston is at the beginning of the backward stroke. It will be perceived that the eccentrics both incline towards the crank-pin, and the eccentric which is controlling the valve follows the crank-pin. Thus, when the engine is running forward, F follows the crank: when she is backing, B follows.

It is a good plan for an engineer to make himself familiar with the proper position of the eccentrics in relation to the crank, for the knowledge is likely to save time and trouble when any thing goes wrong with the valve-motion. With this knowledge properly digested, a minute's inspection is always sufficient to decide whether or not any thing is wrong with the eccentrics.

In following out the relative motion of the piston and crank, we discover a disturbing factor in what is called the angularity of the connecting rod, which has a curiously distorting effect on the harmony of the motion. When the piston stands exactly in the mid-travel point, the true length of the main rod will be measured from the center of the wrist-pin to the center of the driving-axle. If a tram of this length be extended between these points, this will be found correct, as every machinist accustomed to working on rods knows. Now, if the back end of the tram should be raised or lowered towards the points where the center of the crank-pin must be when the crank stands on the top or bottom quarter, it will be found that the tram point will not reach the crank-pin center, but will fall short a distance in proportion to the length of the main rod. The dotted lines a' and b' in Fig. 11 show how far a rod 72 times the length of the crank falls short. A shorter rod will magnify this obliquity, while a longer rod will reduce it.

As the opening and closing of the steam-ports by the valves are regulated by the eccentrics, which are subject to the same motion as the crank, following it at an unvarying distance, it is evident that their tendency will be to admit and cut off steam at a certain position of the crank's movement. If the motion is planned to cut off at half stroke, it will be apparent, that, in the backward stroke, the piston will be past its mid travel before the crank-pin reaches the quarter, so that end of the cylinder will receive steam during more than half the stroke. On the forward stroke of the piston, however, the crank-pin will reach the quarter before the piston has attained half travel; the consequence being, that in this case steam is cut off too early. The disturbing effect of the angularity of the connecting rod on the steam distribution thus tends to make the cut-off later in the backward stroke than in the forward stroke, resulting in giving the forward end of the cylinder more steam than what is admitted in the back end. The link-motion provides a convenient means of correcting the inequality of valve opening due to the connecting-rod angularity, the details of which will be explained farther on.

An engineer or machinist who wishes to study out this peculiarity of connecting-rod angularity, will find that the use of a tram or long dividers will help him to comprehend it better than any letter-type description. All through the study of the valve-motion, there are numerous difficult problems encountered. The use of a good model will be found an invaluable aid to the study of the valve-motion, and every division of engineers or firemen should make a combined effort to furnish their meeting-room with a model of a locomotive valve-motion. In no way can the spare time of the men connected with locomotive running be better employed than in the wide range for study presented by a well devised model. Great aid can be obtained in the study of the valve-motion from good books devoted to the subject, and they will impart more information than can be obtained by mere contact with the locomotive. The valve and its movements are surrounded with so many complicated influences, that an intelligent man may work for years about a locomotive doing valve setting occasionally, and other gang boss work, yet, unless he studies the valve-motion by the aid of the drawing board, or by models, which admit of changing sizes and dimensions, he may know less about the cause of certain movements than the bright lad who has been a couple of years in the drawing-office. The man who thinks he can study the valve-motion, and understand its philosophy, by merely running the engine, deceives himself. The engineer who never looks at a book or a paper in search of information about his engine, knows very little about any thing not visible to the eye. Yet many men of this stamp, by looking wise, and by exercising a judicious use of silence, pass among their fellows as remarkably profound. But let a fireman, in quest of locomotive knowledge, put a question to such a man, and he is immediately silenced with a "You ought to know better" answer.

Where the use of a model can not be obtained, any one beginning the study of the valve-motion can assist himself by making a cross section of the valve and its seat, similar to those published, on a strip of thin wood or thick paper. By slipping the valve on the seat, its position at different parts of the stroke can be comprehended more clearly than by a mere description. With a pair of dividers to represent the motion of the eccentric, and strips of wood to act as eccentric, and valve rod and rocker, and some tacks to fasten them together, a helpful model can be improvised on a table or board. By the time a student gets a rig of this kind going, he will see his way to contrive other methods of self-help.

By the aid of Fig. 10, we will trace the relative movements of the crank and eccentric connections. For the sake of simplicity, the eccentric is represented as connecting directly with the rocker-arm.

The crank-pin being at the point A, or the forward center, the piston must be in the front of the cylinder, or at the beginning of the backward stroke. Owing to the angular advance already referred to, the eccentric center is at F; and, being a certain distance ahead of the middle position, it has pushed the lower arm of the rocker from a to b, drawing back the top arm, which, in its turn, has moved the valve so that it is just beginning to admit steam at the forward port, i. As the crank-pin goes round, the eccentric follows it, opening the steam-port wider till the eccentric reaches the point of its travel nearest A, the limit of the throw. When the eccentric is at this point of its throw, the valve must be at the outside of its travel; and therefore the steam-port is wide open. By this time the crank-pin is getting close up towards the quarter. After passing this point, the forward eccentric begins to draw the bottom rocker-pin towards the axle, and to push the valve ahead, this being the point where the valve changes its direction of motion just as the piston returns when the crank-pin passes the center. When F reaches the point B, the valve is in the same position it occupied at the beginning of the stroke; but, as it is traveling in the opposite direction, a very small movement more closes the port, cutting off steam. When this happens, the crank-pin has reached the point x. When F gets to g, it is on the central point of its throw; so the valve must then be on the middle point of its travel, with the exhaust cavity just covering the outside edges of the bridges, the forward edge being ready to put the steam-port, i, in communication with the exhaust cavity. This releases the steam from the forward end of the cylinder; and at the same moment the inside edge of the valve covers the back port, k, causing the piston-head to compress any steam left in the back part of the cylinder. When the piston reaches the beginning of the forward stroke, the eccentric F has got to the point f, and the valve is beginning to admit steam for the return stroke, the events of which are similar to those described.

In actual practice, the steam distribution is a little different from the manner that has been followed; for the link-motion provides the means of equalizing the cutoff, making it uniform for both strokes. This changes the events of the stroke a little; but the student who engraves in his mind the movements as they are represented in the diagram, will not be far astray.

Many men who have a fair understanding of the action of steam in an engine's cylinders during ordinary working, have no idea of the operations performed in the cylinders when a locomotive is running in reverse motion. All men who have had any thing to do with train service, know, that, when an engine is reversed, the action works to stop the train, even if the locomotive should have no steam on the boiler; but just in what way this result comes round they can not clearly perceive. In hopes of throwing light upon this subject for those who have not studied it out, we will follow the events of a stroke in reversed motion, as we did in the ordinary working.

Supposing an engine to be running ahead, and the necessity arises for stopping suddenly, and the reverse-lever is pulled into the back notch. When the crank-pin is on the forward center, and therefore the piston at the forward end of the cylinder, about to begin its backward stroke, the valve has the forward port open a distance equal to the amount of lead, as in Fig. 10. But, as the back-up eccentric has control of the valve., the latter is being pushed forward; and it closes the forward port just as the piston begins to move back. This shuts off all communication with the forward end of the cylinder; and the receding piston creates a vacuum behind it, just as a pump-plunger does under similar circumstances. At this time the back end of the cylinder is open to the exhaust, and the piston pushes out the air freely to the atmosphere. By the time the piston travels about two inches, the valve gets to its middle position; and, immediately after passing that point, it opens the forward end of the cylinder to the exhaust, and closes the back port. When this event happens, the vacuum in the forward end of the cylinder gets filled with hot gases, that rush in from the smoke-box; and the receding piston keeps drawing air into the cylinder in this way during the remainder of the stroke, and air from that quarter seldom gets in without bringing a sprinkling of cinders. The back steam-port is closed only during about two inches of the stroke, while the lap of the valve is traveling over it. About the time the piston reaches four inches of its travel, the back steam-port is open to the steam-chest, and the piston forces the air through the steam-pipes into the boiler during the remainder of the stroke. The forward stroke is merely a repetition of the backward stroke described.

When it is necessary to reverse a locomotive, it is a better plan to hook the lever clear back than to have it a notch or two past the center, as some men persist in doing, under the mistaken belief that they are in some way saving their engine from harsh usage. When the link is reversed full, the cylinders are merely turned into air-pumps. When the links are put near the center, the travel of the valve is reduced; and the periods when the piston is creating a vacuum in one end of the cylinder, and compressing the air in the other, are prolonged. The result is, that, when the exhaust is opened in the first case, the gases rush in violently from the smoke-box, carrying a heavy load of cinders: in the other case, the piston compresses the air in the cylinder so high that it jerks the valve away from its seat in trying to find outlet. This causes the clattering noise in the steam-chest, so well known in cases where engines are run without steam while the reverse-lever is near the center.

A locomotive with the piston-packing in bad order will not hold well running in reverse-motion. Some kinds of piston-packing do not seem to act properly when the engine is reversed, especially at low speed. Where a valve has much inside lap, there will be a vacuum in one end of the cylinder, and compressed air in the other end. With piston-packing that requires pressure to expand it, the void at one end of the cylinder may neutralize the pressure at the other by drawing the air through the piston. This would be most liable to happen where the lever was kept near the center.

Should the throttle-valve close so tight that the compressed air from the cylinders can not pass into the boiler, there is danger of bursting the steam-chest or some part of the, steam-pipes. The compressed air will lift most of the throttle-valves far enough to prevent any great danger from this source. In some engines a relief-valve is secured in the dry pipe, which provides a passage for this compressed air. When the cylinder-cocks of an engine are opened when the motion is reversed, they form an outlet to the compressed air, and also admit air to the sucking end without letting the piston draw air so freely through the nozzles. Many cylinder-cocks are now made so that they will open automatically to permit the piston to draw air through them. The reversed engine will stop nearly as well with the cylinder-cocks opened as when they are closed, and it is much more easily handled with the cocks opened. Where the cocks are kept closed, the rush of hot air from the smoke-box laps every trace of oil from the valve-seat, and a heavy pressure — frequently above that of the boiler — is present in the steam-chest. When the engine stops under these circumstances, its tendency is to fly back; and an engineer has some difficulty in controlling it with the reverse-lever till a few turns empty the chest and pipes.

Numerous attempts have been made to utilize the reversed engine as a brake for stopping the train, and even by this means to save some of the power lost in stopping. Chatelier, a French engineer, experimented for many years on this mechanical problem. He injected a jet of water into the exhaust-pipe, which supplied low-tension steam to the cylinder, instead of hot gas or air coming through the smoke-box. This was pumped back into the boiler on the return stroke. Thus the act of stopping a train was used to compress a quantity of steam, converting the work of stopping into heat, which was forced into the boiler and retained to aid in getting the train into speed again. Modifications of this idea produce the car-starters that pass so frequently through our Patent Office.

As a means of conserving mechanical energy, the Chatelier brake was not a success; but, in the absence of better power brakes, it met with some applications in Europe. Some of our mountain railroads use it, under the name of the water-brake, as an auxiliary to the automatic brake.

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