Part 3 (1/2)

In the si 24, the valve faces are just wide enough to cover the steam ports If the eccentric is not _square_ with the crank, the admission of steam lasts until the very end of the stroke; if set a little in advance--that is, given _lead_--the stea the cylinder, and readmitted before the back stroke is acco is very uneconooes to the exhaust at practically the same pressure as that at which it entered the cylinder Its property of _expansion_ has been neglected But supposing that steam at 100 lbs pressure were admitted till half-stroke, and then suddenly cut off, the expansive nature of the steam would then continue to push the piston out until the pressure had decreased to 50 lbs per square inch, at which pressure it would go to the exhaust Now, observe that all the work done by the steae_ pressure on the piston is not so high as in the first case; still, froet much more _work_

HOW THE CUT-OFF IS MANAGED

[Illustration: FIG 27--A slide-valve with ”lap”]

[Illustration: FIG 28]

Look at Fig 27 Here we have a slide-valve, with faces much wider than the stea to the faces of the valves shown in previous diagrams (p 54) The shaded parts, L L, are called the _lap_ By increasing the length of the lap we increase the range of expansive working Fig 28 shows the piston full to the left; the valve is just on the point of opening to admit steam behind the piston The eccentric has a throw equal to the breadth of a port + the lap of the valve That this27, where the valve is at its central position

Hence the very simple formula:--Travel of valve = 2 (lap + breadth of port) The path of the eccentric's centre round the centre of the shaft is indicated by the usual dotted line (Fig 28) You will notice that the ”angle of advance,” denoted by the arrow A, is now very considerable By the time that the crank C has assumed the position of the line S, the eccentric has passed its dead point, and the valve begins to travel backwards, eventually returning to the position shown in Fig 28, and cutting off the steam supply while the piston has still a considerable part of its stroke to ins to work expansively, and continues to do so until the valve assu 27

If the valve has to have ”lead” to admit steam _before_ the end of the stroke to the other side of the piston, the _angle of advance_ must be increased, and the eccentric centre line would lie on the line E2

Therefore--total angle of advance = angle for _lap_ and angle for _lead_

LIMIT OF EXPANSIVE WORKING

Theoretically, by increasing the _lap_ and cutting off the steam earlier and earlier in the stroke, we should econoreat difficulty is met with--namely, that _as the steam expands its temperature falls_ If the cut-off occurs early, say at one-third stroke, the great expansion will reduce the temperature of the metal walls of the cylinder to such an extent, that when the next spirt of steam enters froy will be lost by cooling In such a case, the difference in tereat for econoy as possible How are we to do it?

COMPOUND ENGINES

In the year 1853, John Elder, founder of the shi+pping firine for use on shi+ps The steah-pressure cylinder, passed into another cylinder of equal stroke but larger diaines the expansion is extended to three and even four stages, according to the boiler pressure; for it is a rule that the higher the initial pressure is, the larger is the nu

[Illustration: FIG 29--Sketch of the arrangeear or supports, etc, shown]

In Fig 29 we have a triple-expansion h-pressure cylinder[4] at, say, 200 lbs per square inch It exhausts at 75 lbs into the large pipe 2, and passes to the intermediate cylinder, whence it is exhausted at 25 lbs or so through pipe 3 to the low-pressure cylinder Finally, it is ejected at about 8 lbs per square inch to the condenser, and is suddenly converted into water; an act which produces a vacuum, and diminishes the back-pressure of the exhaust fro_ power on the exhaust side of C's piston

ARRANGEMENT OF EXPANSION ENGINES

In the illustration the cranks are set at angles of 120, or a third of a circle, so that one or other is always at or near the position of es are used the cylinders are often arranged _tande a co movement they les to one another

COMPOUND LOCOMOTIVES

In 1876 Mr A Mallet introduced _coely adopted The various types of ”compounds” may be classified as follows:--(1) One low-pressure and one high-pressure cylinder; (2) one high-pressure and t-pressure; (3) one low-pressure and two high-pressure; (4) two high-pressure and t-pressure The last class is very widely used in France, Aive the best results Where only two cylinders are used (and soeh and low-pressure cylinders for starting a train, or rades

REVERSING GEARS

[Illustration: FIGS 30, 31, 32--Showing how a reversing gear alters the position of the slide-valve]

The engines of a locomotive or steamshi+p must be reversible--that is, when steaineer h the steam-ways that the cranks may turn in the desired direction The coe Stephenson) is known as Stephenson's Link Gear In Fig 30 we have a diagraear E1 and E2 are two eccentrics set square with the crank at opposite ends of a diameter

Their rods are connected to the ends of a link, L, which can be raised and lowered by means of levers (not shown) B is a block which can partly revolve on a pin projecting fro 31 the link is half raised, or in ”ear,” as drivers say Eccentric E1 has pushed the lower end of the link fully back; E2 has pulled it fully forward; and since any movement of the one eccentric is counterbalanced by the opposite movement of the other, rotation of the eccentrics would not cause the valve to move at all, and no steam could be ad 30 denotes one cylinder, crank, rods, etc, of a locomotive The crank has co lever is at the ineer desires to turn his cranks in an anti-clockwise direction, he _raises_ the link, which brings the rod of E1 into line with the valve rod and presses the block _backwards_ till the right-hand port is uncovered (Fig 31)