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temperature, and its pressure can thus be raised up to its initial pressure, and heat will be applied to the cylinder-covers and piston, which would otherwise be abstracted from the steam from the boiler, and condensation is prevented to that extent. Cushioning.—Another great advantage from compression is that the compressed steam acts as a cushion to the piston, and prevents a sudden shock at the end and beginning of each stroke, when the motion of the piston is reversed, and the power used in compressing the steam (with the exception of loss from friction) is returned by the re-expansion of the compressed steam on the reversal of the piston. By properly adjusting the quantity of cushion, the momentum of the piston may be balanced, and the engine may be made to run smoothly and noiselessly. Back-Pressure causes loss of power, the extent of which depends upon the quantity of water mixed with the exhaust-steam, and also upon the amount of resistance opposed to the escape of the exhaust-steam from the cylinder, in the shape of contracted ports and passages and bent exhaust pipes. Bends and elbows in the exhaust-pipe cause great back-pressure, but in non-condensing engines the back-pressure is never less than the pressure of the atmosphere, plus the power required to expel the exhaust steam from the cylinder. In condensing-engines, the condenser and airpump are employed to remove the back-pressure or pressure of the atmosphere, but as a perfect vacuum is never obtained and there is always some resistance to the escape of the steam from the cylinder, there is always a back-pressure of at least 2 lbs. per square inch in condensing-engines. Ratio of Expansion.—In order to obtain all the available power, the steam should be brought on to the piston at its highest pressure and cut off quickly, so that the pressure does not fall during the closing of the port, as expansion cannot begin properly until the port is closed, and the full expansive force of the steam should be used as nearly as possible to the end of the stroke, and then exhausted freely, therefore the steam must be cut off at such a part of the stroke that it will expand to the lowest practicable point before exhausting. In practice the best results have been obtained by expanding the steam 6 times in single-cylinder steam-jacketed condensing-engines; 4 times in single-cylinder condensing-engines without steam jackets; 3} times in single-cylinder steam-jacketed non-condensing engines; 3 times in single cylinder non-condensing engines without steam jackets, but with well-clothed cylinders; 8 times in double-expansion condensing steam-jacketed engines; 6 times in double-expansion condensing engines without jackets, but with well-clothed cylinders; Io times in tripleexpansion surface-condensing engines: and 12 times in quadruple expansion surface-condensing engines. In all cases the utmost feasible ratio of expansion is the number of times the total back pressure is contained in the total initial pressure. Lowest Absolute Terminal-Pressure.—In non-condensing engines, the exhaust port being open to the atmosphere, there is a back pressure of 15 lbs. per square inch, plus the power necessary to drive the engine against its own friction, and to expel the exhaust steam from the cylinder, which is on an average 5 lbs. ; so that the lowest terminal absolute pressure to which steam can be economically expanded is 20 lbs. In condensing engines, there is always a pressure in the condenser to be provided against, as well as the resistance to the escape of the steam from the cylinder, and the power necessary to drive the engine against its own friction, so that the lowest terminal absolute pressure to which steam can be economically expanded, is 8 lbs. per square inch. When the steam is expanded to a lower terminal pressure than this, the result will be loss of power. Economical Working.—To secure the utmost economy, it is necessary to work at a good rate of expansion with dry steam, and this can only be obtained by keeping the steam in the cylinder at such a point, that it will be as nearly as possible totally free from condensation; for this purpose the steam jacket was designed. Steam-Jackets.-The object of the steam-jacket is to maintain a uniform temperature in the cylinder, and obtain dry steam throughout the stroke, by preventing condensation in the cylinder. The effect of the jacket is to remove the condensation from the inside of the cylinder, where it retards the effective working of the piston, to the outside of the cylinder into the jacket, whence it can easily be drained off and returned to the boiler as feed-water. To enable the jacket to work properly, means must be provided to keep it clear of both air and water, otherwise they will destroy its action. It is essential to supply the jacket with dry steam, of a temperature not less than that of the steam on entering the cylinder, and the jacket should be drained automatically by a steam-trap. Exhaust-steam should not be used for heating the jacket, as the cylinder would be enveloped in steam of a less temperature than that inside the cylinder, and the heat would flow from the hotter to the colder steam and cause loss of power. A simple form of steam-jacket, cast in one piece with the cylinder, as shown in Fig. 15; the cylinder covers are also jacketed. The jackets of the cylinder and cover should be connected by at least 6 holes, and care must be used in making the joint to prevent these holes from being filled up with red lead, but pieces of tube screwed into the cylinder-cover effectually prevents this taking place. The jacket is filled with steam from the boiler, and condensation in the cylinder during expansion is prevented by the heat passing from the jacket to the expanding steam.

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Another method of jacketing a cylinder is shown in Fig. 16. It is formed by fitting a liner of either hard close-grained cast-iron, or compressed steel, in the cylinder. The liner has a flange at one end, by which it is bolted to the cylinder, the other end of the liner is not fixed to the cylinder, but is free, to allow it to expand, and it is fitted in some cases with a plate, covering a recess filled with packing, to prevent leakage of steam.

Numerous experiments have been made to determine the economical value of steam-jackets. The results of several tests are as follows:—

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The economy effected by steam-jacketing a cylinder depends partly upon the rate of expansion. The higher the rate of expansion in a single cylinder the greater is the advantage derived from the use of a steam-jacket, because the higher the rate of expansion the greater the variation of temperature of steam in the cylinder. In one case the saving effected by the use of a steam-jacket was 27 per cent. when the steam was expanded 6 times and 15 per cent, when the steam was expanded 24 times in the cylinder. It may be assumed that the economy to be expected from the use of an efficient steam-jacket is about 25 per cent.

MOVEMENT OF THE PISTON AND SLIDE-VALVE OF A STEAM - ENGINE.

Movement of the Piston relative to that of the Crank.-The piston acts upon the crank through the medium of the connecting-rod, The piston traverses twice the diameter of the path-circle of the crank-pin while the crank-pin describes the circumference, during one revolution of the crank. The varying angularity of the connecting-rod influences the movement of the piston in such a manner that, the piston moves more slowly during one half of its stroke than during the other.

With an indefinitely long connecting-rod, of which the angularity is inconsiderable, the relation of the motion of the crank and the piston is shown in Fig. 17, in which A C is the stroke of the piston, and A B C the half-revolution of the crank-pin, simultaneously described. Let the path of the crank-pin be divided into equal parts at the points 1, 2, 3, 4, and draw vertical lines from the points of division to the line A. C. Then, as the angular speed of the crank-pin is uniform and the divisions of the circular path A B C are equal, the line A C is divided by the perpendiculars into segments representing spaces described by the piston in equal times, and therefore also the varying average velocity of the piston in traversing these spaces. Showing that, the speed of the piston, during one stroke, begins and ends at nothing at the extreme or dead-points, A C; and it accelerates towards B, ** *-*.*.* the position at half-stroke, where it reaches a maximum, and that beyond this point it is retarded until it gains the end of its stroke. The piston moves at the same rate as the crank-pin only for a brief period about the middle of the stroke. The piston is stopped twice and started twice in each revolution of the crank. The stoppage of the piston at the end of each stroke permits an element of time for the steam to get in and out of the cylinder. Movement of the Slide-Valve.—The small circle in Fig. 17 shows the path of the centre of the eccentric, in which the travel of the valve is represented by the diametrical line. The slide-valve travels in a similar manner to the piston. The slide-valve opens the ports for the admission of steam to the cylinder towards the middle of its travel, when its velocity is greatest and its action quickest. Slide-Valves.—The admission, expansion and exhaust of steam in the cylinder is regulated by the slide-valve, a simple form of which is shown in

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steam behind it; and the valve should Fig. 18.-Slide Valve. cut the steam off quickly to prevent wiredrawing; and it should effect the release of the steam from one side of the piston, and its compression on the other without causing unavoidable back-pressure. Lead of a valve is the distance that the port is open at the commencement of the stroke of the piston, for the purpose of obtaining the full pressure of the steam on the piston when it leaves the end of the cylinder at the commencement of its stroke. This is effected by fixing the eccentric a little in advance of the position at right angles to the crank, which causes the port to be slightly open before the piston arrives at the end of its stroke, so that the moment the crank has passed its dead-centre the piston begins its stroke with the full pressure of the steam behind it. The amount of lead depends upon the speed of the piston, the size of the ports, the quantity of steam in the cylinder at the time the valve is opened. The valves of vertical engines are generally given more lead at the bottom than at the top, to balance the momentum of the moving parts as they reach the bottom-centre. Insufficient lead causes the piston to travel a portion of its stroke before it receives the full pressure of the steam : and excessive lead causes an irregular working of the piston, which receives a sudden shock, and the entering steam is compressed, which causes back pressure and loss of power, besides straining the engine. Lap of a Valve.—In order to work expansively, the admission of the steam is cut off and the steam is confined in the cylinder, when the piston has only travelled a portion of its stroke, and this is effected with the common slide-valve by making it sufficiently long, when in middle position, to overlap the extreme edges of the steam-ports. The overlap is called outside-lap. Inside-lap, or lap on the exhaust-side, when it exists to any extent, is given to the valve to delay the release of the steam, but in engines that work at a good speed no inside lap is given, more than is just sufficient to cover the ports on the exhausting side to prevent leakage of steam when the valve is at its half-stroke. Lap of Valve necessary to cut the Steam off at a given part of the Stroke.-Rule: From the length of stroke in inches, deduct the distance in inches moved by the piston when the steam is cut off, divide the remainder by the stroke of the piston in inches, and extract the square r00t of the quotient, then, multiply the result by half the stroke of the valve in inches, and deduct half the lead from the product, the remainder will be the required lap in inches. Point of Cut-off of Steam from a given Lap.—Aoule : To the lap of the valve on the Steam-side in inches add one half the lead, then divide by half the travel of the valve in inches, and multiply the square of the quotient by the length of stroke of the piston in inches; deduct the product from the length of stroke of the piston in inches, and the remainder will be the distance in inches that the piston moves when the steam is cut off. A Trick-Slide-Valve, shown in Fig. 19 has a passage formed on the

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