In the simple form of valve that appears in Fig. 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 steam is cut off before the piston has travelled quite along the cylinder, and readmitted before the back stroke is accomplished. Even with this lead the working is very uneconomical, as the steam goes 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 steam after the cut-off is so much power saved. The _average_ pressure on the piston is not so high as in the first case; still, from a given volume of 100 lbs.
pressure steam we get much more _work_.
HOW THE CUT-OFF IS MANAGED.
[Ill.u.s.tration: FIG. 27.--A slide-valve with "lap."]
[Ill.u.s.tration: FIG. 28.]
Look at Fig. 27. Here we have a slide-valve, with faces much wider than the steam ports. The parts marked black, P P, are those corresponding 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 this must be so is obvious from a consideration of Fig. 27, 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 a.s.sumed the position of the line S, the eccentric has pa.s.sed 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 make. The steam then begins to work expansively, and continues to do so until the valve a.s.sumes the position shown in Fig. 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 E^2.
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 economize our power more and more.
But in practice a great 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 from the other end a considerable proportion of the steam"s energy will be lost by cooling. In such a case, the difference in temperature between admitted steam and exhausted steam is too great for economy. Yet we want to utilize as much energy as possible. How are we to do it?
COMPOUND ENGINES.
In the year 1853, John Elder, founder of the shipping firm of Elder and Co., Glasgow, introduced the _compound_ engine for use on ships. The steam, when exhausted from the high-pressure cylinder, pa.s.sed into another cylinder of equal stroke but larger diameter, where the expansion continued. In modern engines 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 number of stages of expansion consistent with economical working.
[Ill.u.s.tration: FIG. 29.--Sketch of the arrangement of a triple-expansion marine engine. No valve gear or supports, etc., shown.]
In Fig. 29 we have a triple-expansion marine engine. Steam enters the high-pressure cylinder[4] at, say, 200 lbs. per square inch. It exhausts at 75 lbs. into the large pipe 2, and pa.s.ses 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 from cylinder C. In fact, the condenser exerts a _sucking_ power on the exhaust side of C"s piston.
ARRANGEMENT OF EXPANSION ENGINES.
In the ill.u.s.tration 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 maximum turning power. Where only two stages are used the cylinders are often arranged _tandem_, both pistons having a common piston rod and crank. In order to get a constant turning movement they must be mounted separately, and work cranks set at right angles to one another.
COMPOUND LOCOMOTIVES.
In 1876 Mr. A. Mallet introduced _compounding_ in locomotives; and the practice has been largely adopted. The various types of "compounds" may be cla.s.sified as follows:--(1) One low-pressure and one high-pressure cylinder; (2) one high-pressure and two low-pressure; (3) one low-pressure and two high-pressure; (4) two high-pressure and two low-pressure. The last cla.s.s is very widely used in France, America, and Russia, and seems to give the best results. Where only two cylinders are used (and sometimes in the case of three and four), a valve arrangement permits the admission of high-pressure steam to both high and low-pressure cylinders for starting a train, or moving it up heavy grades.
REVERSING GEARS.
[Ill.u.s.tration: FIGS. 30, 31, 32.--Showing how a reversing gear alters the position of the slide-valve.]
The engines of a locomotive or steamship must be reversible--that is, when steam is admitted to the cylinders, the engineer must be able to so direct it through the steam-ways that the cranks may turn in the desired direction. The commonest form of reversing device (invented by George Stephenson) is known as Stephenson"s Link Gear. In Fig. 30 we have a diagrammatic presentment of this gear. E^1 and E^2 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 from the valve rod, working through a guide, G. In Fig. 31 the link is half raised, or in "mid-gear," as drivers say. Eccentric E^1 has pushed the lower end of the link fully back; E^2 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 admitted to the cylinder.
Let us suppose that Fig. 30 denotes one cylinder, crank, rods, etc., of a locomotive. The crank has come to rest at its half-stroke; the reversing lever is at the mid-gear notch. If the engineer desires to turn his cranks in an anti-clockwise direction, he _raises_ the link, which brings the rod of E^1 into line with the valve rod and presses the block _backwards_ till the right-hand port is uncovered (Fig. 31).
If steam be now admitted, the piston will be pushed towards the left, and the engine will continue to run in an anti-clockwise direction. If, on the other hand, he wants to run the engine the other way, he would _drop_ the link, bringing the rod of E^2 into line with the valve rod, and drawing V _forward_ to uncover the rear port (Fig. 32). In either case the eccentric working the end of the link remote from B has no effect, since it merely causes that end to describe arcs of circles of which B is the centre.
"LINKING UP."
If the link is only partly lowered or raised from the central position it still causes the engine to run accordingly, but the movement of the valve is decreased. When running at high speed the engineer "links up"
his reversing gear, causing his valves to cut off early in the stroke, and the steam to work more expansively than it could with the lever at _full_, or _end_, gear; so that this device not only renders an engine reversible, but also gives the engineer an absolute command over the expansion ratio of the steam admitted to the cylinder, and furnishes a method of cutting off the steam altogether. In Figs. 30, 31, 32, the valve has no lap and the eccentrics are set square. In actual practice the valve faces would have "lap" and the eccentric "lead" to correspond; but for the sake of simplicity neither is shown.
OTHER GEARS.
In the Gooch gear for reversing locomotives the link does not shift, but the valve rod and its block is raised or lowered. The Allan gear is so arranged that when the link is raised the block is lowered, and _vice versa_. These are really only modifications of Stephenson"s principle--namely, the employment of _two_ eccentrics set at equal angles to and on opposite sides of the crank. There are three other forms of link-reversing gear, and nearly a dozen types of _radial_ reversing devices; but as we have already described the three most commonly used on locomotives and ships, there is no need to give particulars of these.
Before the introduction of Stephenson"s gear a single eccentric was used for each cylinder, and to reverse the engine this eccentric had to be loose on the axle. "A lever and gear worked by a treadle on the footplate controlled the position of the eccentrics. When starting the engine, the driver put the eccentrics out of gear by the treadle; then, by means of a lever he raised the small-ends[5] of the eccentric rods, and, noting the position of the cranks, or, if more convenient, the balance weight in the wheels, he, by means of another handle, moved the valves to open the necessary ports to steam and worked them by hand until the engine was moving; then, with the treadle, he threw the eccentrics over to engage the studs, at the same time dropping the small-ends of the rods to engage pins upon the valve spindles, so that they continued to keep up the movement of the valve."[6] One would imagine that in modern shunting yards such a device would somewhat delay operations!
PISTON VALVES.
In marine engines, and on many locomotives and some stationary engines, the D-valve (shown in Figs. 30-32) is replaced by a piston valve, or circular valve, working up and down in a tubular seating. It may best be described as a rod carrying two pistons which correspond to the faces of a D-valve. Instead of rectangular ports there are openings in the tube in which the piston valve moves, communicating with the steam-ways into the cylinder and with the exhaust pipe. In the case of the D-valve the pressure above it is much greater than that below, and considerable friction arises if the rubbing faces are not kept well lubricated. The piston valve gets over this difficulty, since such steam as may leak past it presses on its circ.u.mference at all points equally.
SPEED GOVERNORS.
[Ill.u.s.tration: FIG. 33.--A speed governor.]
Practically all engines except locomotives and those known as "donkey-engines"--used on cranes--are fitted with some device for keeping the rotatory speed of the crank constant within very narrow limits. Perhaps you have seen a pair of b.a.l.l.s moving round on a seating over the boiler of a threshing-engine. They form part of the "governor,"
or speed-controller, shown in principle in Fig. 33. A belt driven by a pulley on the crank shaft turns a small pulley, P, at the foot of the governor. This transmits motion through two bevel-wheels, G, to a vertical shaft, from the top of which hang two heavy b.a.l.l.s on links, K K. Two more links, L L, connect the b.a.l.l.s with a weight, W, which has a deep groove cut round it at the bottom. When the shaft revolves, the b.a.l.l.s fly outwards by centrifugal force, and as their velocity increases the quadrilateral figure contained by the four links expands laterally and shortens vertically. The angles between K K and L L become less and less obtuse, and the weight W is drawn upwards, bringing with it the fork C of the rod A, which has ends engaging with the groove. As C rises, the other end of the rod is depressed, and the rod B depresses rod O, which is attached to the spindle operating a sort of shutter in the steam-pipe. Consequently the supply of steam is throttled more and more as the speed increases, until it has been so reduced that the engine slows, and the b.a.l.l.s fall, opening the valve again. Fig. 34 shows the valve fully closed. This form of governor was invented by James Watt. A spring is often used instead of a weight, and the governor is arranged horizontally so that it may be driven direct from the crank shaft without the intervention of bevel gearing.
[Ill.u.s.tration: FIG. 34.]
The Hartwell governor employs a link motion. You must here picture the b.a.l.l.s raising and lowering the _free end_ of the valve rod, which carries a block moving in a link connected with the eccentric rod. The link is pivoted at the upper end, and the eccentric rod is attached to the lower. When the engine is at rest the end of the valve rod and its block are dropped till in a line with the eccentric rod; but when the machinery begins to work the block is gradually drawn up by the governor, diminishing the movement of the valve, and so shortening the period of steam admission to the cylinder.
Governors are of special importance where the _load_ of an engine is constantly varying, as in the case of a sawmill. A good governor will limit variation of speed within two per cent.--that is, if the engine is set to run at 100 revolutions a minute, it will not allow it to exceed 101 or fall below 99. In _very_ high-speed engines the governing will prevent variation of less than one per cent., even when the load is at one instant full on, and the next taken completely off.
MARINE GOVERNORS.
These must be more quick-acting than those used on engines provided with fly-wheels, which prevent very sudden variations of speed. The screw is light in proportion to the engine power, and when it is suddenly raised from the water by the pitching of the vessel, the engine would race till the screw took the water again, unless some regulating mechanism were provided. Many types of marine governors have been tried. The most successful seems to be one in which water is being constantly forced by a pump driven off the engine shaft into a cylinder controlling a throttle-valve in the main steam-pipe. The water escapes through a leak, which is adjustable. As long as the speed of the engine is normal, the water escapes from the cylinder as fast as it is pumped in, and no movement of the piston results; but when the screw begins to race, the pump overcomes the leak, and the piston is driven out, causing a throttling of the steam supply.
CONDENSERS.
The _condenser_ serves two purposes:--(1) It makes it possible to use the same water over and over again in the boilers. On the sea, where fresh water is not obtainable in large quant.i.ties, this is a matter of the greatest importance. (2) It adds to the power of a compound engine by exerting a back pull on the piston of the low-pressure cylinder while the steam is being exhausted.
[Ill.u.s.tration: FIG. 35.--The marine condenser.]
Fig. 35 is a sectional ill.u.s.tration of a marine condenser. Steam enters the condenser through the large pipe E, and pa.s.ses among a number of very thin copper tubes, through which sea-water is kept circulating by a pump. The path of the water is shown by the featherless arrows. It comes from the pump through pipe A into the lower part of a large cap covering one end of the condenser and divided transversely by a diaphragm, D.
Pa.s.sing through the pipes, it reaches the cap attached to the other end, and flows back through the upper tubes to the outlet C. This arrangement ensures that, as the steam condenses, it shall meet colder and colder tubes, and finally be turned to water, which pa.s.ses to the well through the outlet F. In some condensers the positions of steam and water are reversed, steam going through the tubes outside which cold water circulates.