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:

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 2 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.

B

3

4

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 AC is divided by the perpendiculars into segments representing spaces described by the piston in equal times, and therec fore 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.

Fig. 17.-Diagram of Piston-
Movement.

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

Fig. 18. The slide-valve should give sufficiently early admission, or preadmission, of the steam to the cylinder to enable the piston to commence its stroke with the full pressure of the steam behind it; and the valve should

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 root 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.-Rule : 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

Fig. 19.-Trick-Slide-Valve.

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

back of the valve through which steam enters the port, as well as from the end of the valve, for the purpose of admitting steam to the cylinder with the smallest possible travel of the valve. The valve and seat are so arranged that the instant the outside edge of the valve begins to open the steam-port, steam flows through the passage in the valve into the steam-port and the piston receives the full pressure of the steam at the instant of admission. This form of valve favours the attainment of high-speed.

Thom's Slide-Valve, shown in Figs. 20 and 21, is an improved form of Trick-valve applied to single-cylinder condensing-engines and to the low

pressure cylinder of compound, or multiple-expansion, engines. Its object is to supply steam for cushioning the piston to balance its momentum, there being little steam left in the cylinder available for compression when the vacuum is good. This is effected by employing the steam-passage at the back of the valve for transferring steam from one side to the other of the piston. When the piston is nearly at the end of its stroke, a communication is formed by the valve with each end of the cylinder and steam passes from one end to the other, as shown by the arrows in Fig. 20, and the ports and passages are filled with steam which would otherwise pass to the condenser. The quantity of steam contained in the clearance-spaces is seldom less

than six per cent. of the quantity of steam used, therefore a saving is effected of that amount of steam.

A Relief-Frame is employed on a slide-valve to diminish friction by relieving the face of the valve of the major portion of the load due to the pressure of the steam on the back of the valve. Thom's Relief-Frame, shown on the back of the valve in Figs. 20 and 21, consists of a slide, fixed

shaft, at an angle of a little more than 90° ahead of the crank. When a slide-valve is worked by link-motion, two eccentrics are employed which form an angle of 180° minus twice the angle of advance. The position of the eccentric is shown in Fig. 23, in which the line A B represents the position of the crank at the beginning of the stroke of the piston, and C, the centre of the eccentric. The resistance to be overcome by an eccentric is the friction of the slide-valve, which is proportional to area of the valve exposed to steam-pressure multiplied by the pressure of the steam. The friction of a slide-valve generally averages about one-tenth of the load upon it.

The Throw of an Eccentric is the distance it is thrown out of the centre, or the amount of eccentricity of the eccentric. It is the distance from the centre of the shaft on which the eccentric is keyed to the centre