WHAT IS STEAM?
IF ice be heated above 32° Fahrenheit, its molecules lose their cohesion, and move freely round one another—the ice is turned into water. Heat water above 212° Fahrenheit, and the molecules exhibit a violent mutual repulsion, and, like dormant bees revived by spring sunshine, separate and dart to and fro. If confined in an air-tight vessel, the molecules have their flights curtailed, and beat more and more violently against their prison walls, so that every square inch of the[Pg 14] vessel is subjected to a rising pressure. We may compare the action of the steam molecules to that of bullets fired from a machine-gun at a plate mounted on a spring. The faster the bullets came, the greater would be the continuous compression of the spring.
THE MECHANICAL ENERGY OF STEAM.
If steam is let into one end of a cylinder behind an air-tight but freely-moving piston, it will bombard the walls of the cylinder and the piston; and if the united push of the molecules on the one side of the latter is greater than the resistance on the other side opposing its motion, the piston must move. Having thus partly got their liberty, the molecules become less active, and do not rush about so vigorously. The pressure on the piston decreases as it moves. But if the piston were driven back to its original position against the force of the steam, the molecular activity—that is, pressure—would be restored. We are here assuming that no heat has passed through the cylinder or piston and been radiated into the air; for any loss of heat means loss of energy, since heat is energy.
THE BOILER.
The combustion of fuel in a furnace causes the[Pg 15] walls of the furnace to become hot, which means that the molecules of the substance forming the walls are thrown into violent agitation. If the walls are what are called “good conductors” of heat, they will transmit the agitation through them to any surrounding substance. In the case of the ordinary house stove this is the air, which itself is agitated, or grows warm. A steam-boiler has the furnace walls surrounded by water, and its function is to transmit molecular movement (heat, or energy) through the furnace plates to the water until the point is reached when steam generates. At atmospheric pressure—that is, if not confined in any way—steam would fill 1,610 times the space which its molecules occupied in their watery formation. If we seal up the boiler so that no escape is possible for the steam molecules, their motion becomes more and more rapid, and pressure is developed by their beating on the walls of the boiler. There is theoretically no limit to which the pressure may be raised, provided that sufficient fuel-combustion energy is transmitted to the vaporizing water.
To raise steam in large quantities we must employ a fuel which develops great heat in proportion to its weight, is readily procured, and cheap. Coal[Pg 16] fulfils all these conditions. Of the 800 million tons mined annually throughout the world, 400 million tons are burnt in the furnaces of steam-boilers.
A good boiler must be—(1) Strong enough to withstand much higher pressures than that at which it is worked; (2) so designed as to burn its fuel to the greatest advantage.
Even in the best-designed boilers a large part of the combustion heat passes through the chimney, while a further proportion is radiated from the boiler. Professor John Perry[1] considers that this waste amounts, under the best conditions at present obtainable, to eleven-twelfths of the whole. We have to burn a shillingsworth of coal to capture the energy stored in a pennyworth. Yet the steam-engine of to-day is three or four times as efficient as the engine of fifty years ago. This is due to radical improvements in the design of boilers and of the machinery which converts the heat energy of steam into mechanical motion.
CIRCULATION OF WATER IN A BOILER.
If you place a pot filled with water on an open fire, and watch it when it boils, you will notice[Pg 17] that the water heaves up at the sides and plunges down at the centre. This is due to the water being heated most at the sides, and therefore being lightest there. The rising steam-bubbles also carry it up. On reaching the surface, the bubbles burst, the steam escapes, and the water loses some of its heat, and rushes down again to take the place of steam-laden water rising.
Fig. 1. Fig. 1.
Fig. 2. Fig. 2.
If the fire is very fierce, steam-bubbles may rise from all points at the bottom, and impede downward currents (Fig. 1). The pot then “boils over.”
Fig. 2 shows a method of preventing this trouble. We lower into our pot a vessel of somewhat smaller diameter, with a hole in the bottom, arranged in such a[Pg 18] manner as to leave a space between it and the pot all round. The upward currents are then separated entirely from the downward, and the fire can be forced to a very much greater extent than before without the water boiling over. This very simple arrangement is the basis of many devices for producing free circulation of the water in steam-boilers.
We can easily follow out the process of development. In Fig. 3 we see a simple U-tube depending from a vessel of water. Heat is applied to the left leg, and a steady circulation at once commences. In order to increase the heating surface we can extend the heated leg into a long incline (Fig. 4), beneath which three lamps instead of only one are placed. The direction of the circulation is the same, but its rate is increased.
Fig. 3. Fig. 3.
A further improvement results from increasing the number of tubes (Fig. 5), keeping them all on the slant, so that the heated water and steam may rise freely.
THE ENCLOSED FURNACE.
Fig. 4. Fig. 4.
Fig. 5. Fig. 5.
Still, a lot of the heat gets away. In a steam-boiler the burning fuel is enclosed either by fire-brick or a “water-jacket,” forming part of the boiler. A water-jacket signifies a double coating of metal plates with a space between, which is filled with water (see Fig. 6). The fire is now enclosed much as it is in a kitchen range. But our boiler must not be so wasteful of the heat as is that useful household fixture. On their way to the funnel the flames and hot gases should act on a very large metal or other surface in contact with the water of the boiler, in order to give up a due proportion of their heat.
Fig. 6. Fig. 6.—Diagrammatic sketch of a locomotive type of boiler. Water indicated by dotted lines. The arrows show the direction taken by the air and hot gases from the air-door to the funnel.
[Pg 21]
THE MULTITUBULAR BOILER.
Fig. 7. Fig. 7.—The Babcock and Wilcox water-tube boiler. One side of the brick seating has been removed to show the arrangement of the water-tubes and furnace.
To save room, boilers which have to make steam very quickly and at high pressures are largely composed of pipes. Such boilers we call multitubular. They are of two kinds—(1) Water-tube boilers; in which the water circulates through tubes exposed to the furnace heat. The Babcock and Wilcox boiler (Fig. 7) is typical of this variety.Pg 22 Fire-tube boilers; in which the hot gases pass through tubes surrounded by water. The ordinary locomotive boiler (Fig. 6) illustrates this form.
The Babcock and Wilcox boiler is widely used in mines, power stations, and, in a modified form, on shipboard. It consists of two main parts—(1) A drum, h, in the upper part of which the steam collects; (2) a group of pipes arranged on the principle illustrated by Fig. 5. The boiler is seated on a rectangular frame of fire-bricks. At one end is the furnace door; at the other the exit to the chimney. From the furnace f the flames and hot gases rise round the upper end of the sloping tubes tt into the space a, where they play upon the under surface of h before plunging downward again among the tubes into the space b. Here the temperature is lower. The arrows indicate further journeys upwards into the space c on the right of a fire-brick division, and past the down tubes ss into d, whence the hot gases find an escape into the chimney through the opening e. It will be noticed that the greatest heat is brought to bear on tt near their junction with uu, the “uptake” tubes; and that every succeeding passage of the pipes brings the gradually cooling gases nearer to the “downtake” tubes ss.
[Pg 23]The pipes tt are easily brushed and scraped after the removal of plugs from the “headers” into which the tube ends are expanded.
Other well-known water-tube boilers are the Yarrow, Belleville, Stirling, and Thorneycroft, all used for driving marine engines.
FIRE-TUBE BOILERS.
Fig. 6 shows a locomotive boiler in section. To the right is the fire-box, surrounded on all sides by a water-jacket in direct communication with the barrel of the boiler. The inner shell of the fire-box is often made of copper, which withstands the fierce heat better than steel; the outer, like the rest of the boiler, is of steel plates from ½ to ¾ inch thick. The shells of the jacket are braced together by a large number of rivets, rr; and the top, or crown, is strengthened by heavy longitudinal girders riveted to it, or is braced to the top of the boiler by long bolts. A large number of fire-tubes (only three are shown in the diagram for the sake of simplicity) extend from the fire-box to the smoke-box. The most powerful “mammoth” American locomotives have 350 or more tubes, which, with the fire-box, give 4,000 square feet of surface[Pg 24] for the furnace heat to act upon. These tubes are expanded at their ends by a special tool into the tube-plates of the fire-box and boiler front. George Stephenson and his predecessors experienced great difficulty in rendering the tube-end joints quite water-tight, but the invention of the “expander” has removed this trouble.
The fire-brick arch shown (Fig. 6) in the fire-box is used to deflect the flames towards the back of the fire-box, so that the hot gases may be retarded somewhat, and their combustion rendered more perfect. It also helps to distribute the heat more evenly over the whole of the inside of the box, and prevents cold air from flying directly from the firing door to the tubes. In some American and Continental locomotives the fire-brick arch is replaced by a “water bridge,” which serves the same purpose, while giving additional heating surface.
The water circulation in a locomotive boiler is—upwards at the fire-box end, where the heat is most intense; forward along the surface; downwards at the smoke-box end; backwards along the bottom of the barrel.
OTHER TYPES OF BOILERS.
For small stationary land engines the vertical[Pg 25] boiler is much used. In Fig. 8 we have three forms of this type—a and b with cross water-tubes; c with vertical fire-tubes. The furnace in every case is surrounded by water, and fed through a door at one side.
Fig. 8. Fig. 8.—Diagrammatic representation of three types of vertical boilers.
The Lancashire boiler is of large size. It has a cylindrical shell, measuring up to 30 feet in length and 7 feet in diameter, traversed from end to end by two large flues, in the rear part of which are situated the furnaces. The boiler is fixed on a seating of fire-bricks, so built up as to form three flues, a and bb, shown in cross section in Fig. 9. The furnace gases, after leaving the two furnace flues, are deflected downwards into the channel a, by which they pass underneath the boiler to a point[Pg 26] almost under the furnace, where they divide right and left and travel through cross passages into the side channels bb, to be led along the boiler’s flanks to the chimney exit c. By this arrangement the effective heating surface is greatly increased; and the passages being large, natural draught generally suffices to maintain proper combustion. The Lancashire boiler is much used in factories and (in a modified form) on ships, since it is a steady steamer and is easily kept in order.
Fig. 9. Fig. 9.—Cross and longitudinal sections of a Lancashire boiler.
In marine boilers of cylindrical shape cross water-tubes and fire-tubes are often employed to increase the heating surface. Return tubes are also led through the water to the funnels, situated at the same end as the furnace.
AIDS TO COMBUSTION.
We may now turn our attention more particularly to the chemical process called combustion, upon[Pg 27] which a boiler depends for its heat. Ordinary steam coal contains about 85 per cent. of carbon, 7 per cent. of oxygen, and 4 per cent. of hydrogen, besides traces of nitrogen and sulphur and a small incombustible residue. When the coal burns, the nitrogen is released and passes away without combining with any of the other elements. The sulphur unites with hydrogen and forms sulphuretted hydrogen (also named sulphurous acid), which is injurious to steel plates, and is largely responsible for the decay of tubes and funnels. More of the hydrogen unites with the oxygen as steam.
The most important element in coal is the carbon (known chemically by the symbol C). Its combination with oxygen, called combustion, is the act which heats the boiler. Only when the carbon present has combined with the greatest possible amount of oxygen that it will take into partnership is the combustion complete and the full heat-value (fixed by scientific experiment at 14,500 thermal units per pound of carbon) developed.
Now, carbon may unite with oxygen, atom for atom, and form carbon monoxide (CO); or in the proportion of one atom of carbon to two of [Pg 28]oxygen, and form carbon dioxide (CO2). The former gas is combustible—that is, will admit another atom of carbon to the molecule—but the latter is saturated with oxygen, and will not burn, or, to put it otherwise, is the product of perfect combustion. A properly designed furnace, supplied with a due amount of air, will cause nearly all the carbon in the coal burnt to combine with the full amount of oxygen. On the other hand, if the oxygen supply is inefficient, CO as well as CO2 will form, and there will be a heat loss, equal in extreme cases to two-thirds of the whole. It is therefore necessary that a furnace which has to eat up fuel at a great pace should be artificially fed with air in the proportion of from 12 to 20 pounds of air for every pound of fuel. There are two methods of creating a violent draught through the furnace. The first is—
The forced draught; very simply exemplified by the ordinary bellows used in every house. On a ship (Fig. 10) the principle is developed as follows:—The boilers are situated in a compartment or compartments having no communication with the outer air, except for the passages down which air is forced by powerful fans at a pressure considerably greater than that of the atmosphere. There is only one “way out”—namely, through the furnace[Pg 29] and tubes (or gas-ways) of the boiler, and the funnel. So through these it rushes, raising the fuel to white heat. As may easily be imagined, the temperature of a stokehold, especially in the tropics, is far from pleasant. In the Red Sea the thermometer sometimes rises to 170° Fahrenheit or more, and the poor stokers have a very bad time of it.
Fig. 10. Fig. 10.—Sketch showing how the “forced draught” is produced in a stokehold and how it affects the furnaces.
[Pg 30]
SCENE IN THE STOKEHOLD OF A BATTLE-SHIP. SCENE IN THE STOKEHOLD OF A BATTLE-SHIP.
[Pg 31]
The second system is that of the induced draught. Here air is sucked through the furnace by creating a vacuum in the funnel and in a chamber opening into it. Turning to Fig. 6, we see a pipe through which the exhaust steam from the locomotive’s cylinders is shot upwards into the funnel, in which, and in the smoke-box beneath it, a strong vacuum is formed while the engine is running. Now, “nature abhors a vacuum,” so air will get into the smoke-box if there be a way open. There is—through the air-doors at the bottom of the furnace, the furnace itself, and the fire-tubes; and on the way oxygen combines with the carbon of the fuel, to form carbon dioxide. The power of the draught is so great that, as one often notices when a train passes during the night, red-hot cinders, plucked from the fire-box, and dragged through the tubes, are hurled far into the air. It might be mentioned in parenthesis that the so-called “smoke” which pours from the funnel of a moving engine is mainly condensing steam. A steamship, on the other hand, belches smoke only from its funnels, as fresh water is far too precious to waste as steam. We shall refer to this later on (p. 72).
BOILER FITTINGS.
The most important fittings on a boiler are:—(1) the safety-valve; (2) the water-gauge; (3) the steam-gauge; (4) the mechanisms for feeding it with water.
THE SAFETY-VALVE.
Professor Thurston, an eminent authority on the steam-engine, has estimated that a plain cylindrical[Pg 32] boiler carrying 100 lbs. pressure to the square inch contains sufficient stored energy to project it into the air a vertical distance of 3½ miles. In the case of a Lancashire boiler at equal pressure the distance would be 2½ miles; of a locomotive boiler, at 125 lbs., 1½ miles; of a steam tubular boiler, at 75 lbs., 1 mile. According to the same writer, a cubic foot of heated water under a pressure of from 60 to 70 lbs. per square inch has about the same energy as one pound of gunpowder.
Steam is a good servant, but a terrible master. It must be kept under strict control. However strong a boiler may be, it will burst if the steam pressure in it be raised to a certain point; and some device must therefore be fitted on it which will give the steam free egress before that point is reached. A device of this kind is called a safety-valve. It usually blows off at less than half the greatest pressure that the boiler has been proved by experiment to be capable of withstanding.
In principle the safety-valve denotes an orifice closed by an accurately-fitting plug, which is pressed against its seat on the boiler top by a weighted lever, or by a spring. As soon as the steam pressure on the face of the plug exceeds the counteracting force[Pg 33] of the weight or spring, the plug rises, and steam escapes until equilibrium of the opposing forces is restored.
On stationary engines a lever safety-valve is commonly employed (Fig. 11). The blowing-off point can be varied by shifting the weight along the arm so as to give it a greater or less leverage. On locomotive and marine boilers, where shocks and movements have to be reckoned with, weights are replaced by springs, set to a certain tension, and locked up so that they cannot be tampered with.
Fig. 11. Fig. 11.—A Lever Safety-Valve. v, valve; s, seating; p, pin; l, lever; f, fulcrum; w, weight. The figures indicate the positions at which the weight should be placed for the valve to act when the pressure rises to that number of pounds per square inch.
Boilers are tested by filling the boilers quite full and (1) by heating the water, which expands slightly, but with great pressure; (2) by forcing in additional water with a powerful pump. In either case a rupture[Pg 34] would not be attended by an explosion, as water is very inelastic.
The days when an engineer could “sit on the valves”—that is, screw them down—to obtain greater pressure, are now past, and with them a considerable proportion of the dangers of high-pressure steam. The Factory Act of 1895, in force throughout the British Isles, provides that every boiler for generating steam in a factory or workshop where the Act applies must have a proper safety-valve, steam-gauge, and water-gauge; and that boilers and fittings must be examined by a competent person at least once in every fourteen months. Neglect of these provisions renders the owner of a boiler liable to heavy penalties if an explosion occurs.
One of the most disastrous explosions on record took place at the Redcar Iron Works, Yorkshire, in June 1895. In this case, twelve out of fifteen boilers ranged side by side burst, through one proving too weak for its work. The flying fragments of this boiler, striking the sides of other boilers, exploded them, and so the damage was transmitted down the line. Twenty men were killed and injured; while masses of metal, weighing several tons each, were hurled 250 yards, and caused widespread damage.
[Pg 35]The following is taken from a journal, dated December 22, 1895: “Providence (Rhode Island).—A recent prophecy that a boiler would explode between December 16 and 24 in a store has seriously affected the Christmas trade. Shoppers are incredibly nervous. One store advertises, ‘No boilers are being used; lifts running electrically.’ All stores have had their boilers inspected.”
THE WATER-GAUGE.
No fitting of a boiler is more important than the water-gauge, which shows the level at which the water stands. The engineer must continually consult his gauge, for if the water gets too low, pipes and other surfaces exposed to the furnace flames may burn through, with disastrous results; while, on the other hand, too much water will cause bad steaming. A section of an ordinary gauge is seen in Fig. 12. It consists of two parts, each furnished with a gland, g, to make a steam-tight joint round the glass tube, which is inserted through the hole covered by the plug p1. The cocks t1 t2 are normally open, allowing the ingress of steam and water respectively to the tube. Cock t3 is kept closed unless for any reason it is necessary to blow steam or water [Pg 36]through the gauge. The holes c c can be cleaned out if the plugs p2 p3 are removed.
Fig. 12. Fig. 12.—Section of a water-gauge.
Most gauges on high-pressure boilers have a thick glass screen in front, so that in the event of the tube breaking, the steam and water may not blow directly on to the attendants. A further precaution is to include two ball-valves near the ends of the gauge-glass. Under ordinary conditions the balls lie in depressions clear of the ways; but when a rush of steam or water occurs they are sucked into their seatings and block all egress.
On many boilers two water-gauges are fitted, since any gauge may work badly at times. The glasses are tested to a pressure of 3,000 lbs. or more to the square inch before use.
THE STEAM-GAUGE.
It is of the utmost importance that a person in charge of a boiler should know what pressure the[Pg 37] steam has reached. Every boiler is therefore fitted with one steam-gauge; many with two, lest one might be unreliable. There are two principal types of steam-gauge:—(1) The Bourdon; (2) the Schäffer-Budenberg. The principle of the Bourdon is illustrated by Fig. 13, in which a is a piece of rubber tubing closed at one end, and at the other drawn over the nozzle of a cycle tyre inflator. If bent in a curve, as shown, the section of the tube is an oval. When air is pumped in, the rubber walls endeavour to assume a circular section, because this shape encloses a larger area than an oval of equal circumference, and therefore makes room for a larger volume of air. In doing so the tube straightens itself, and assumes the position indicated by the dotted lines. Hang an empty “inner tube” of a pneumatic tyre over a nail and inflate it, and you will get a good illustration of the principle.
Fig. 13. Fig. 13.—Showing the principle of the steam-gauge.
[Pg 38]
Fig. 14.
Fig. 14.—Bourdon steam-gauge. Part of dial removed to show mechanism.
In Fig. 14 we have a Bourdon gauge, with part of the dial face broken away to show the internal mechanism. t is a flattened metal tube soldered at one end into a hollow casting, into which screws a tap connected with the boiler. The other end (closed) is attached to a link, l, which works an arm of a quadrant rack, r, engaging with a small pinion, p, actuating the pointer. As the steam pressure rises,[Pg 39] the tube t moves its free end outwards towards the position shown by the dotted lines, and traverses the arm of the rack, so shifting the pointer round the scale. As the pressure falls, the tube gradually returns to its zero position.
The Schäffer-Budenberg gauge depends for its action on the elasticity of a thin corrugated metal plate, on one side of which steam presses. As the plate bulges upwards it pushes up a small rod resting on it, which operates a quadrant and rack similar to that of the Bourdon gauge. The principle is employed in another form for the aneroid barometer (p. 329).
THE WATER SUPPLY TO A BOILER.
The water inside a boiler is kept at a proper level by (1) pumps or (2) injectors. The former are most commonly used on stationary and marine boilers. As their mechanism is much the same as that of ordinary force pumps, which will be described in a later chapter, we may pass at once to the injector, now almost universally used on locomotive, and sometimes on stationary boilers. At first sight the injector is a mechanical paradox, since it employs the steam from a boiler to blow water into the boiler. In Fig. 15 we have an illustration of the principle of[Pg 40] an injector. Steam is led from the boiler through pipe a, which terminates in a nozzle surrounded by a cone, e, connected by the pipe b with the water tank. When steam is turned on it rushes with immense velocity from the nozzle, and creates a partial vacuum in cone e, which soon fills with water. On meeting the water the steam condenses, but not before it has imparted some of its velocity to the water, which thus gains sufficient momentum to force down the valve and find its way to the boiler. The overflow space o o between e and c allows steam and water to escape until the water has gathered the requisite momentum.
Fig. 15.
Fig. 15.—Diagram illustrating the principle of a steam-injector.
[Pg 41]
Fig. 16. Fig. 16.—The Giffard injector.
A form of injector very commonly used is Giffard’s (Fig. 16). Steam is allowed to enter by screwing up the valve v. As it rushes through the nozzle of the cone a it takes up water and projects it into the “mixing cone” b, which can be raised or lowered by the pinion d (worked by the hand-wheel wheel shown) so as to regulate the amount of water admitted to b.[Pg 42] At the centre of b is an aperture, o, communicating with the overflow. The water passes to the boiler through the valve on the left. It will be noticed that the cone a and the part of b above the orifice o contract downward. This is to convert the pressure of the steam into velocity. Below o is a cone, the diameter of which increases downwards. Here the velocity of the water is converted back into pressure in obedience to a well-known hydromechanic law.
An injector does not work well if the feed-water be too hot to condense the steam quickly; and it may be taken as a rule that the warmer the water, the smaller is the amount of it injected by a given weight of steam.[2] Some injectors have flap-valves covering the overflow orifice, to prevent air being sucked in and carried to the boiler.
When an injector receives a sudden shock, such as that produced by the passing of a locomotive over points, it is liable to “fly off”—that is, stop momentarily—and then send the steam and water through the overflow. If this happens, both steam and water must be turned off, and the injector be restarted; unless it be of the self-starting variety, which automatically[Pg 43] controls the admission of water to the “mixing-cone,” and allows the injector to “pick up” of itself.
For economy’s sake part of the steam expelled from the cylinders of a locomotive is sometimes used to work an injector, which passes the water on, at a pressure of 70 lbs. to the square inch, to a second injector operated by high-pressure steam coming direct from the boiler, which increases its velocity sufficiently to overcome the boiler pressure. In this case only a fraction of the weight of high-pressure steam is required to inject a given weight of water, as compared with that used in a single-stage injector.
[1] “The Steam-Engine,” p. 3. [2] By “weight of steam” is meant the steam produced by boiling a certain weight of water. A pound of steam, if condensed, would form a pound of water. [Pg 44]Chapter II.
THE CONVERSION OF HEAT ENERGY INTO MECHANICAL MOTION.
Reciprocating engines—Double-cylinder engines—The function of the fly-wheel—The cylinder—The slide-valve—The eccentric—”Lap” of the valve: expansion of steam—How the cut-off is managed—Limit of expansive working—Compound engines—Arrangement of expansion engines—Compound locomotives—Reversing gears—”Linking-up”—Piston-valves—Speed governors—Marine-speed governors—The condenser.
HAVING treated at some length the apparatus used for converting water into high-pressure steam, we may pass at once to a consideration of the mechanisms which convert the energy of steam into mechanical motion, or work.
Steam-engines are of two kinds:—(1) reciprocating, employing cylinders and cranks; (2) rotary, called turbines.
RECIPROCATING ENGINES.
[Pg 45]Fig. 17. Fig. 17.—Sketch showing parts of a horizontal steam-engine.
Fig. 17 is a skeleton diagram of the simplest form of reciprocating engine. c is a cylinder to which steam is admitted through the steam-ways[3] w w, first on one side of the piston p, then on the other. The pressure on the piston pushes it along the cylinder, and the force is transmitted through the piston rod p r to the connecting rod c r, which causes the crank k to revolve. At the point where the two rods meet there is a “crosshead,” h, running to and fro in a guide to prevent the piston rod being broken or bent by the oblique thrusts and pulls which it imparts through c r to the crank k. The latter is keyed to a shaft s carrying the fly-wheel, or, in the case of a locomotive, the driving-wheels. The crank shaft revolves in bearings. The internal diameter of a cylinder is called its bore. The travel of the piston is called its stroke. The distance from the centre of the shaft to the centre of the crank pin is called the crank’s throw, which is half of the piston’s stroke. An engine of this type is called double-acting, as the piston is pushed alternately backwards and forwards by the steam. When piston rod, connecting rod, and crank lie in a straight line—that is, when the piston is fully out, or fully in—the crank is said to be at a “dead point;” for, were the crank turned to such a position, the admission of steam would not produce motion, since the thrust or pull would be entirely absorbed by the bearings.
Fig. 18. Fig. 18.—Sectional plan of a horizontal engine.
[Pg 47]
DOUBLE-CYLINDER ENGINES.
Fig. 19. Fig. 19.
Fig. 20. Fig. 20.
Locomotive, marine, and all other engines which must be started in any position have at least two cylinders, and as many cranks set at an [Pg 48]angle to one another. Fig. 19 demonstrates that when one crank, c1, of a double-cylinder engine is at a “dead point,” the other, c2, has reached a position at which the piston exerts the maximum of turning power. In Fig. 20 each crank is at 45° with the horizontal, and both pistons are able to do work. The power of one piston is constantly increasing while that of the other is decreasing. If single-action cylinders are used, at least three of these are needed to produce a perpetual turning movement, independently of a fly-wheel.
THE FUNCTION OF THE FLY-WHEEL.
A fly-wheel acts as a reservoir of energy, to carry the crank of a single-cylinder engine past the “dead points.” It is useful in all reciprocating engines to produce steady running, as a heavy wheel acts as a drag on the effects of a sudden increase or decrease of steam pressure. In a pump, mangold-slicer, cake-crusher, or chaff-cutter, the fly-wheel helps the operator to pass his dead points—that is, those parts of the circle described by the handle in which he can do little work.
THE CYLINDER.
Fig. 21. Fig. 21.—Diagrammatic section of a cylinder and its slide-valve.
The cylinders of an engine take the place of the[Pg 49] muscular system of the human body. In Fig. 21 we have a cylinder and its slide-valve shown in section. First of all, look at p, the piston. Round it are white grooves, r r, in which rings are fitted to prevent the passage of steam past the piston. The rings are cut through at one point in their circumference, and slightly opened, so that when in position they press all round against the walls of the cylinder. After a little use they “settle down to their work”—that is, wear to a true fit in the cylinder. Each end of[Pg 50] the cylinder is closed by a cover, one of which has a boss cast on it, pierced by a hole for the piston rod to work through. To prevent the escape of steam the boss is hollowed out true to accommodate a gland, g1, which is threaded on the rod and screwed up against the boss; the internal space between them being filled with packing. Steam from the boiler enters the steam-chest, and would have access to both sides of the piston simultaneously through the steam-ways, w w, were it not for the
SLIDE-VALVE,
a hollow box open at the bottom, and long enough for its edges to cover both steam-ways at once. Between w w is e, the passage for the exhaust steam to escape by. The edges of the slide-valve are perfectly flat, as is the face over which the valve moves, so that no steam may pass under the edges. In our illustration the piston has just begun to move towards the right. Steam enters by the left steam-way, which the valve is just commencing to uncover. As the piston moves, the valve moves in the same direction until the port is fully uncovered, when it begins to move back again; and just before the piston has finished its stroke the steam-way[Pg 51] on the right begins to open. The steam-way on the left is now in communication with the exhaust port e, so that the steam that has done its duty is released and pressed from the cylinder by the piston. Reciprocation is this backward and forward motion of the piston: hence the term “reciprocating” engines. The linear motion of the piston rod is converted into rotatory motion by the connecting rod and crank.
Fig. 22. Fig. 22.—Perspective section of cylinder.
The use of a crank appears to be so obvious a method of producing this conversion that it is interesting to learn that, when James Watt produced his “rotative engine” in 1780 he was unable to use the crank because it had already been patented by one Matthew Wasborough. Watt was not easily daunted, however, and within a twelvemonth had himself patented five other devices for obtaining rotatory motion from a piston rod. Before passing on, it may be mentioned that Watt was the father of the modern—that is, the high-pressure—steam-engine; and that, owing to the imperfection of the existing[Pg 52] machinery, the difficulties he had to overcome were enormous. On one occasion he congratulated himself because one of his steam-cylinders was only three-eighths of an inch out of truth in the bore. Nowadays a good firm would reject a cylinder 1⁄500 of an inch out of truth; and in small petrol-engines 1⁄5000 of an inch is sometimes the greatest “limit of error” allowed.
Fig. 23. Fig. 23.—The eccentric and its rod.
THE ECCENTRIC
is used to move the slide-valve to and fro over the steam ports (Fig. 23). It consists of three main parts—the sheave, or circular plate s, mounted on the crank shaft; and the two straps which encircle it, and in which it revolves. To one strap is bolted the “big end” of the eccentric rod, which engages at its other end with the valve rod. The straps are semicircular and held together by strong bolts, b b, passing through lugs, or thickenings at the ends of the semicircles. The sheave has a deep groove all round the edges,[Pg 53] in which the straps ride. The “eccentricity” or “throw” of an eccentric is the distance between c2, the centre of the shaft, and c1, the centre of the sheave. The throw must equal half of the distance which the slide-valve has to travel over the steam ports. A tapering steel wedge or key, k, sunk half in the eccentric and half in a slot in the shaft, holds the eccentric steady and prevents it slipping. Some eccentric sheaves are made in two parts, bolted together, so that they may be removed easily without dismounting the shaft.
The eccentric is in principle nothing more than a crank pin so exaggerated as to be larger than the shaft of the crank. Its convenience lies in the fact that it may be mounted at any point on a shaft, whereas a crank can be situated at an end only, if it is not actually a V-shaped bend in the shaft itself—in which case its position is of course permanent.
SETTING OF THE SLIDE-VALVE AND ECCENTRIC.
The subject of valve-setting is so extensive that a full exposition might weary the reader, even if space permitted its inclusion. But inasmuch as the effectiveness of a reciprocating engine depends largely on the nature and arrangement of the valves, we[Pg 54] will glance at some of the more elementary principles.
Fig. 24. Fig. 24.
Fig. 25. Fig. 25.
In Fig. 24 we see in section the slide-valve, the ports of the cylinder, and part of the piston. To the right are two lines at right angles—the thicker, c, representing the position of the crank; the thinner, e, that of the eccentric. (The position of an eccentric is denoted diagrammatically by a line drawn from the centre of the crank shaft through the centre of the sheave.) The edges of the valve are in this case only broad enough to just cover the ports—that is, they have no lap. The piston is about to commence its stroke towards the left; and the eccentric,[Pg 55] which is set at an angle of 90° in advance of the crank, is about to begin opening the left-hand port. By the time that c has got to the position originally occupied by e, e will be horizontal (Fig. 25)—that is, the eccentric will have finished its stroke towards the left; and while c passes through the next right angle the valve will be closing the left port, which will cease to admit steam when the piston has come to the end of its travel. The operation is repeated on the right-hand side while the piston returns.
Fig. 26. Fig. 26.
It must be noticed here—(1) that steam is admitted at full pressure all through the stroke; (2) that admission begins and ends simultaneously with the stroke. Now, in actual practice it is necessary to admit steam before the piston has ended its travel, so as to cushion the violence of the sudden change of direction of the piston, its rod, and other moving parts. To effect this, the eccentric is set more[Pg 56] than 90° in advance—that is, more than what the engineers call square. Fig. 26 shows such an arrangement. The angle between e and e1 is called the angle of advance. Referring to the valve, you will see that it has opened an appreciable amount, though the piston has not yet started on its rightwards journey.
“LAP” OF THE VALVE—EXPANSION OF STEAM.
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[Pg 57] 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.
Fig. 27. Fig. 27.—A slide-valve with “lap.”
Fig. 28. 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.[Pg 58] 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 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 make. The steam then begins to work expansively, and continues to do so until the valve assumes 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 e2. Therefore—total angle of advance = angle for lap and angle for lead.
[Pg 59]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, passed 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.
[Pg 60]Fig. 29. Fig. 29.—Sketch of the arrangement of a triple-expansion marine engine. No valve gear or supports, etc., shown.
[Pg 61]
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 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 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 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 maximum turning power. Where only two stages are used the[Pg 62] 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 classified 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 class 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.
Figs. 30, 31, 32. 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[Pg 63] 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. 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[Pg 64] 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 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 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 e1 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 e2 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[Pg 65] 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[Pg 66] that when the link is raised the block is lowered, and vice versâ. 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[Pg 67] 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 circumference at all points equally.
SPEED GOVERNORS.
Fig. 33. Fig. 33.—A speed governor.
Practically all engines except locomotives and those[Pg 68] 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 balls 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 balls on links, k k.[Pg 69] Two more links, l l, connect the balls with a weight, w, which has a deep groove cut round it at the bottom. When the shaft revolves, the balls 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 balls 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[Pg 70] the crank shaft without the intervention of bevel gearing.
Fig. 34. Fig. 34.
The Hartwell governor employs a link motion. You must here picture the balls 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.
[Pg 71]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[Pg 72] again in the boilers. On the sea, where fresh water is not obtainable in large quantities, 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.
Fig. 35. Fig. 35.—The marine condenser.
Fig. 35 is a sectional illustration of a marine condenser. Steam enters the condenser through the large pipe e, and passes 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[Pg 73] transversely by a diaphragm, d. Passing 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 passes 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.
Chapter III.
THE STEAM TURBINE.
How a turbine works—The De Laval turbine—The Parsons turbine—Description of the Parsons turbine—The expansive action of steam in a Parsons turbine—Balancing the thrust—Advantages of the marine turbine.
MORE than two thousand years ago Hero of Alexandria produced the first apparatus to which the name of steam-engine could rightly be given. Its principle was practically the same as that of the revolving jet used to sprinkle lawns during dry weather, steam being used in the place of water. From the top of a closed cauldron rose two vertical pipes, which at their upper ends had short, right-angle bends. Between them was hung a hollow globe, pivoted on two short tubes projecting from its sides into the upright tubes. Two little L-shaped pipes projected from opposite sides of the globe, at the ends of a diameter, in a plane perpendicular to the axis. On fire being applied to the[Pg 75] cauldron, steam was generated. It passed up through the upright, through the pivots, and into the globe, from which it escaped by the two L-shaped nozzles, causing rapid revolution of the ball. In short, the first steam-engine was a turbine. Curiously enough, we have reverted to this primitive type (scientifically developed, of course) in the most modern engineering practice.
HOW A TURBINE WORKS.
In reciprocating—that is, cylinder—engines steam is admitted into a chamber and the door shut behind it, as it were. As it struggles to expand, it forces out one of the confining walls—that is, the piston—and presently the door opens again, and allows it to escape when it has done its work. In Hero’s toy the impact of the issuing molecules against other molecules that have already emerged from the pipes was used. One may compare the reaction to that exerted by a thrown stone on the thrower. If the thrower is standing on skates, the reaction of the stone will cause him to glide backwards, just as if he had pushed off from some fixed object. In the case of the reaction—namely, the Hero-type—turbine the nozzle from which the steam or water issues[Pg 76] moves, along with bodies to which it may be attached. In action turbines steam is led through fixed nozzles or steam-ways, and the momentum of the steam is brought to bear on the surfaces of movable bodies connected with the shaft.
THE DE LAVAL TURBINE.
In its earliest form this turbine was a modification of Hero’s. The wheel was merely a pipe bent in S form, attached at its centre to a hollow vertical shaft supplied with steam through a stuffing-box at one extremity. The steam blew out tangentially from the ends of the S, causing the shaft to revolve rapidly and work the machinery (usually a cream separator) mounted on it. This motor proved very suitable for dairy work, but was too wasteful of steam to be useful where high power was needed.
Fig. 36.
Fig. 36.—The wheel and nozzles of a De Laval turbine.
In the De Laval turbine as now constructed the steam is blown from stationary nozzles against vanes mounted on a revolving wheel. Fig. 36 shows the nozzles and a turbine wheel. The wheel is made as a solid disc, to the circumference of which the vanes are dovetailed separately in a single row. Each vane is of curved section, the concave side directed towards the nozzles, which, as will be[Pg 77] gathered from the “transparent” specimen on the right of our illustration, gradually expand towards the mouth. This is to allow the expansion of the steam, and a consequent gain of velocity. As it issues, each molecule strikes against the concave face of a vane, and, while changing its direction, is robbed[Pg 78] of its kinetic energy, which passes to the wheel. To turn once more to a stone-throwing comparison, it is as if a boy were pelting the wheel with an enormous number of tiny stones. Now, escaping high-pressure steam moves very fast indeed. To give figures, if it enters the small end of a De Laval nozzle at 200 lbs. per square inch, it will leave the big end at a velocity of 48 miles per minute—that is, at a speed which would take it right round the world in 8½ hours! The wheel itself would not move at more than about one-third of this speed as a maximum.[7] But even so, it may make as many as 30,000 revolutions per minute. A mechanical difficulty is now encountered—namely, that arising from vibration. No matter how carefully the turbine wheel may be balanced, it is practically impossible to make its centre of gravity coincide exactly with the central point of the shaft; in other words, the wheel will be a bit—perhaps only a tiny fraction of an ounce—heavier on one side than the other. This want of truth causes vibration, which, at the high speed mentioned, would cause the shaft to knock the bearings[Pg 79] in which it revolves to pieces, if—and this is the point—those bearings were close to the wheel M. de Laval mounted the wheel on a shaft long enough between the bearings to “whip,” or bend a little, and the difficulty was surmounted.
The normal speed of the turbine wheel is too high for direct driving of some machinery, so it is reduced by means of gearing. To dynamos, pumps, and air-fans it is often coupled direct.
THE PARSONS TURBINE.
At the grand naval review held in 1897 in honour of Queen Victoria’s diamond jubilee, one of the most noteworthy sights was the little Turbinia of 44½ tons burthen, which darted about among the floating forts at a speed much surpassing that of the fastest “destroyer.” Inside the nimble little craft were engines developing 2,000 horse power, without any of the clank and vibration which usually reigns in the engine-room of a high-speed vessel. The Turbinia was the first turbine-driven boat, and as such, even apart from her extraordinary pace, she attracted great attention. Since 1897 the Parsons turbine has been installed on many ships, including several men-of-war, and it seems probable that the time is[Pg 80] not far distant when reciprocating engines will be abandoned on all high-speed craft.
DESCRIPTION OF THE PARSONS TURBINE.
Fig. 37. Fig. 37.—Section of a Parsons turbine.
The essential parts of a Parsons turbine are:—(1) The shaft, on which is mounted (2) the drum; (3) the cylindrical casing inside which the drum revolves; (4) the vanes on the drum and casing; (5) the balance pistons. Fig. 37 shows a diagrammatic turbine in section. The drum, it will be noticed, increases its diameter in three stages, d1, d2, d3, towards the right. From end to end it is studded with little vanes, m m, set in parallel rings small distances apart. Each vane has a curved section (see Fig. 38), the hollow side facing towards the left. The vanes stick out from the drum like[Pg 81] short spokes, and their outer ends almost touch the casing. To the latter are attached equally-spaced rings of fixed vanes, f f, pointing inwards towards the drum, and occupying the intervals between the rings of moving vanes. Their concave sides also face towards the left, but, as seen in Fig. 38, their line of curve lies the reverse way to that of m m. Steam enters the casing at a, and at once rushes through the vanes towards the outlet at b. It meets the first row of fixed vanes, and has its path so deflected that it strikes the ring of moving (or drum) vanes at the most effective angle, and pushes them round. It then has its direction changed by the ring of f f, so that it may treat the next row of m m in a similar fashion.
Fig. 38. Fig. 38.—Blades or vanes of a Parsons turbine.
[Pg 82]
One of the low-pressure turbines of the Carmania, in
casing. Its size will be inferred from comparison with the man standing
near the end of the casing. One of the low-pressure turbines of the Carmania, in casing. Its size will be inferred from comparison with the man standing near the end of the casing.
[Pg 83]
THE EXPANSIVE ACTION OF STEAM IN A TURBINE.
On reaching the end of d1 it enters the second, or intermediate, set of vanes. The drum here is of a greater diameter, and the blades are longer and set somewhat farther apart, to give a freer passage to the now partly expanded steam, which has lost pressure but gained velocity. The process of movement is repeated through this stage; and again in d3, the low-pressure drum. The steam then escapes to the condenser through b, having by this time expanded very many times; and it is found advisable, for reasons explained in connection with compound steam-engines, to have a separate turbine in an independent casing for the extreme stages of expansion.
The vanes are made of brass. In the turbines of the Carmania, the huge Cunard liner, 1,115,000 vanes are used. The largest diameter of the drums is 11 feet, and each low-pressure turbine weighs 350 tons.
BALANCING OF THRUST.
The push exerted by the steam on the blades not only turns the drum, but presses it in the direction in which the steam flows. This end thrust is counterbalanced by means of the “dummy” pistons, p1, p2, p3. Each dummy consists of a number of discs revolving between rings projecting from the casing, the distance[Pg 84] between discs and rings being so small that but little steam can pass. In the high-pressure compartment the steam pushes p1 to the left with the same pressure as it pushes the blades of d1 to the right. After completing the first stage it fills the passage c, which communicates with the second piston, p2, and the pressure on that piston negatives the thrust on d2. Similarly, the passage e causes the steam to press equally on p3 and the vanes of d3. So that the bearings in which the shaft revolves have but little thrust to take. This form of compensation is necessary in marine as well as in stationary turbines. In the former the dummy pistons are so proportioned that the forward thrust given by them and the screw combined is almost equal to the thrust aft of the moving vanes.
[Pg 85]One of the turbine drums of the Carmania. Note the
rows of vanes. The drum is here being tested for perfect balance on two
absolutely level supports.
One of the turbine drums of the Carmania. Note the rows of vanes. The drum is here being tested for perfect balance on two absolutely level supports.
[Pg 86]
ADVANTAGES OF THE MARINE TURBINE.
(1.) Absence of vibration. Reciprocating engines, however well balanced, cause a shaking of the whole ship which is very unpleasant to passengers. The turbine, on the other hand, being almost perfectly balanced, runs so smoothly at the highest speeds that, if the hand be laid on the covering, it is sometimes almost impossible to tell whether the machinery is in motion. As a consequence of this smooth running there is little noise in the engine-room—a pleasant contrast to the deafening roar of reciprocating engines. (2.) Turbines occupy less room. (3.) They are more easily tended. (4.) They require fewer repairs, since the rubbing surfaces are very small as compared to those of reciprocating engines. (5.) They are more economical at high speeds. It must be remembered that a turbine is essentially meant for high speeds. If run slowly, the steam will escape through the many passages without doing much work.
Owing to its construction, a turbine cannot be reversed like a cylinder engine. It therefore becomes necessary to fit special astern turbines to one or more of the screw shafts, for use when the ship has to be stopped or moved astern. Under ordinary conditions these turbines revolve idly in their cases.
The highest speed ever attained on the sea was the forty-two miles per hour of the unfortunate Viper, a turbine destroyer which developed 11,500 horse power, though displacing only 370 tons. This velocity would compare favourably with that of a good many expresses on certain railways that we could name. In the future thirty miles an hour will certainly be attained by turbine-driven liners.
[7] Even at this speed the wheel has a circumferential velocity of two-thirds that of a bullet shot from a Lee-Metford rifle. A vane weighing only 250 grains (about ½ oz.) exerts under these conditions a centrifugal pull of 15 cwt. on the wheel! [Pg 87]Chapter IV.
THE INTERNAL-COMBUSTION ENGINE.
The meaning of the term—Action of the internal-combustion engine—The motor car—The starting-handle—The engine—The carburetter—Ignition of the charge—Advancing the spark—Governing the engine—The clutch—The gear-box—The compensating gear—The silencer—The brakes—Speed of cars.
THE MEANING OF THE TERM “INTERNAL-COMBUSTION ENGINE.”
IN the case of a steam-boiler the energy of combustion is transmitted to water inside an air-tight vessel. The fuel does not actually touch the “working fluid.” In the gas or oil engine the fuel is brought into contact and mixed with the working fluid, which is air. It combines suddenly with it in the cylinder, and heat energy is developed so rapidly that the act is called an explosion. Coal gas, mineral oils, alcohol, petrol, etc., all contain hydrogen and carbon. If air, which contributes oxygen, be added to any of these in due proportion, the mixture becomes highly explosive. On a light being applied, oxygen and carbon unite, also hydrogen and oxygen, and violent heat is generated, causing a violent molecular bombardment of the sides of the vessel containing the mixture. Now, if the mixture be compressed it becomes hotter and hotter, until a point is reached at which it ignites spontaneously. Early gas-engines did not compress the charge before ignition. Alphonse Beau de Rochas, a Frenchman, first thought of making the piston of the engine squeeze the mixture before ignition; and from the year 1862, when he proposed this innovation, the success of the internal-combustion engine may be said to date.
[Pg 88] [Pg 89]Fig. 39. Fig. 39.—Showing the four strokes that the piston of a gas-engine makes during one “cycle.”
[Pg 90]
ACTION OF THE ENGINE.
The gas-engine, the oil-engine, and the motor-car engine are similar in general principles. The cylinder has, instead of a slide-valve, two, or sometimes three, “mushroom” valves, which may be described as small and thick round plates, with bevelled edges, mounted on the ends of short rods, called stems. These valves open into the cylinder, upwards, downwards, or horizontally, as the case may be; being pushed in by cams projecting from a shaft rotated by the engine. For the present we will confine our attention to the[Pg 91] series of operations which causes the engine to work. This series is called the Beau de Rochas, or Otto, cycle, and includes four movements of the piston. Reference to Fig. 39 will show exactly what happens in a gas-engine—(1) The piston moves from left to right, and just as the movement commences valves g (gas) and a (air) open to admit the explosive mixture. By the time that p has reached the end of its travel these valves have closed again. (2) The piston returns to the left, compressing the mixture, which has no way of escape open to it. At the end of the stroke the charge is ignited by an incandescent tube i (in motor car and some stationary engines by an electric spark), and (3) the piston flies out again on the “explosion” stroke. Before it reaches the limit position, valve e (exhaust) opens, and (4) the piston flies back under the momentum of the fly-wheel, driving out the burnt gases through the still open e. The “cycle” is now complete. There has been suction, compression (including ignition), combustion, and exhaustion. It is evident that a heavy fly-wheel must be attached to the crank shaft, because the energy of one stroke (the explosion) has to serve for the whole cycle; in other words, for two complete revolutions of the crank. A single-cylinder steam-engine[Pg 92] develops an impulse every half-turn—that is, four times as often. In order to get a more constant turning effect, motor cars have two, three, four, six, and even eight cylinders. Four-cylinder engines are at present the most popular type for powerful cars.
THE MOTOR CAR.
Fig. 40. Fig. 40.—Plan of the chassis of a motor car.
We will now proceed to an examination of the motor car, which, in addition to mechanical apparatus for the transmission of motion to the driving-wheels, includes all the fundamental adjuncts of the internal-combustion engine.[8] Fig. 40 is a bird’s-eye view of the chassis (or “works” and wheels) of a car, from which the body has been removed. Starting at the[Pg 93] left, we have the handle for setting the engine in motion; the engine (a two-cylinder in this case); the fly-wheel, inside which is the clutch; the gear-box, containing the cogs for altering the speed of revolution of the driving-wheels relatively to that of the engine; the propeller shaft; the silencer, for deadening the noise of the exhaust; and the bevel-gear, for turning the driving-wheels. In the particular type of car here considered you will notice that a “direct,” or shaft, drive is used. The shaft has at each end a flexible, or “universal,” joint, which allows the shaft to turn freely, even though it may not be in a line with the shaft projecting from the gear-box. It must be remembered that the engine and gear-box are mounted on the frame, between which and the axles are springs, so that when the car bumps up and down, the shaft describes part of a circle, of which the gear-box end is the centre.
An alternative method of driving is by means of chains, which run round sprocket (cog) wheels on the ends of a shaft crossing the frame just behind the gear-box, and round larger sprockets attached to the hubs of the driving-wheels. In such a case the axles of the driving-wheel are fixed to the springs, and the wheels revolve round them. Where a Cardan (shaft)[Pg 94] drive is used the axles are attached rigidly to the wheels at one end, and extend, through tubes fixed to the springs, to bevel-wheels in a central compensating-gear box (of which more presently).
Several parts—the carburetter, tanks, governor, and pump—are not shown in the general plan. These will be referred to in the more detailed account that follows.
THE STARTING-HANDLE.
Fig. 41. Fig. 41.—The starting-handle.
Fig. 41 gives the starting-handle in part section. The handle h is attached to a tube which terminates in a clutch, c. A powerful spring keeps c normally apart from a second clutch, c1, keyed to the engine shaft. When the driver wishes to start the engine he presses the handle towards the right, brings the clutches together, and turns the handle in a clockwise[Pg 95] direction. As soon as the engine begins to fire, the faces of the clutches slip over one another.
THE ENGINE.
Fig. 42. Fig. 42.—End and cross sections of a two-cylinder motor.
We next examine the two-cylinder engine (Fig. 42). Each cylinder is surrounded by a water-jacket, through which water is circulated by a pump[9] (Fig. 43). The heat generated by combustion is so great that the walls of the cylinder would soon become red-hot unless some of the heat were quickly[Pg 96] carried away. The pistons are of “trunk” form—that is, long enough to act as guides and absorb the oblique thrust of the piston rods. Three or more piston rings lying in slots (not shown) prevent the escape of gas past the piston. It is interesting to notice that the efficiency of an internal-combustion engine depends so largely on the good fit of these moving parts, that cylinders, pistons, and rings must be exceedingly true. A good firm will turn out standard parts which are well within 1⁄5000 of an inch of perfect truth. It is also a wonderful testimony to the quality of the materials used that, if properly looked after, an engine which has made many millions of revolutions, at the rate of 1,000 to 2,000 per minute, often shows no appreciable signs of wear. In one particular test an engine was run continuously for several months, and at the end of the trial was in absolutely perfect condition.
The cranks revolve in an oil-tight case (generally made of aluminium), and dip in oil, which they splash up into the cylinder to keep the piston well lubricated. The plate, p p, through a slot in which the piston rod works, prevents an excess of oil being flung up. Channels are provided for leading oil[Pg 97] into the bearings. The cranks are 180° apart. While one piston is being driven out by an explosion, the other is compressing its charge prior to ignition, so that the one action deadens the other. Therefore two explosions occur in one revolution of the cranks, and none during the next revolution. If both cranks were in line, the pistons would move together, giving one explosion each revolution.
Fig. 43. Fig. 43.—Showing how the water which cools the cylinders is circulated.
The valve seats, and the inlet and exhaust pipes, are seen in section. The inlet valve here works automatically, being pulled in by suction; but on many engines—on all powerful engines—the inlet, like the exhaust valve, is lifted by a cam, lest it should stick or work irregularly. Three dotted circles show a, a cog on the crank shaft; b, a “lay” cog, which transmits motion to c, on a short shaft rotating the cam that lifts the exhaust valve. c,[Pg 98] having twice as many teeth as a, revolves at half its rate. This ensures that the valve shall be lifted only once in two revolutions of the crank shaft to which it is geared. The cogs are timed, or arranged, so that the cam begins to lift the valve when the piston has made about seven-eighths of its explosion stroke, and closes the valve at the end of the exhaust stroke.
THE CARBURETTER.
A motor car generally uses petrol as its fuel. Petrol is one of the more volatile products of petroleum, and has a specific gravity of about 680—that is, volume for volume, its weight is to that of water in the proportion of 680 to 1,000. It is extremely dangerous, as it gives off an inflammable gas at ordinary temperatures. Benzine, which we use to clean clothes, is practically the same as petrol, and should be treated with equal care. The function of a carburetter is to reduce petrol to a very fine spray and mix it with a due quantity of air. The device consists of two main parts (Fig. 44)—the float chamber and the jet chamber. In the former is a contrivance for regulating the petrol supply. A float—a cork, or air-tight metal box—is arranged to move freely up and down the stem of a needle-valve,[Pg 99] which closes the inlet from the tank. At the bottom of the chamber are two pivoted levers, w w, which, when the float rests on them, tip up and lift the valve. Petrol flows in and raises the float. This allows the valve to sink and cut off the supply. If the valve is a good fit and the float is of the correct weight, the petrol will never rise higher than the tip of the jet g.
Fig. 44. Fig. 44.—Section of a carburetter.
The suction of the engine makes petrol spirt through the jet (which has a very small hole in its end) and atomize itself against a spraying-cone, a.[Pg 100] It then passes to the engine inlet pipe through a number of openings, after mixing with air entering from below. An extra air inlet, controllable by the driver, is generally added, unless the carburetter be of a type which automatically maintains constant proportions of air and vapour. The jet chamber is often surrounded by a jacket, through which part of the hot exhaust gases circulate. In cold weather especially this is a valuable aid to vaporization.
Fig. 45. Fig. 45.—Sketch of the electrical ignition arrangements on a motor car.
IGNITION OF THE CHARGE.
All petrol-cars now use electrical ignition. There are two main systems—(1) by an accumulator and induction coil; (2) magneto ignition, by means of a small dynamo driven by the engine. A general arrangement of the first is shown in Fig. 45. A disc, d, of some insulating material—fibre or vulcanite—is mounted on the cam, or half-speed, shaft. Into the circumference is let a piece of brass, called the contact-piece, through which a screw passes to the cam shaft. A movable plate, m p, which can be rotated concentrically with d through part of a circle, carries a “wipe” block at the end of a spring, which presses it against d. The spring itself is attached to an insulated plate. When the revolution[Pg 101] of d brings the wipe and contact together, current flows from the accumulator through switch s to the wipe; through the contact-piece to c; from c to m p and the induction coil; and back to the accumulator. This is the primary, or low-tension, circuit. A high-tension current is induced by the coil in the secondary circuit, indicated by dotted lines.[10] In this circuit is the sparking-plug (see Fig. 46), having a central insulated rod in connection with one terminal of the secondary coil. Between it[Pg 102] and a bent wire projecting from the iron casing of the plug (in contact with the other terminal of the secondary coil through the metal of the engine, to which one wire of the circuit is attached) is a small gap, across which the secondary current leaps when the primary current is broken by the wipe and contact parting company. The spark is intensely hot, and suffices to ignite the compressed charge in the cylinder.
Fig. 46. Fig. 46.—Section of a sparking-plug.
ADVANCING THE SPARK.
We will assume that the position of w (in Fig. 45) is such that the contact touches w at the moment when the piston has just completed the compression[Pg 103] stroke. Now, the actual combustion of the charge occupies an appreciable time, and with the engine running at high speed the piston would have travelled some way down the cylinder before the full force of the explosion was developed. But by raising lever l, the position of w may be so altered that contact is made slightly before the compression stroke is complete, so that the charge is fairly alight by the time the piston has altered its direction. This is called advancing the spark.
GOVERNING THE ENGINE.
There are several methods of controlling the speed of internal-combustion engines. The operating mechanism in most cases is a centrifugal ball-governor. When the speed has reached the fixed limit it either (1) raises the exhaust valve, so that no fresh charges are drawn in; (2) prevents the opening of the inlet valve; or (3) throttles the gas supply. The last is now most commonly used on motor cars, in conjunction with some device for putting it out of action when the driver wishes to exceed the highest speed that it normally permits.
Fig. 47. Fig. 47.—One form of governor used on motor cars.
A sketch of a neat governor, with regulating attachment, is given in Fig. 47. The governor shaft[Pg 104] is driven from the engine. As the balls, b b, increase their velocity, they fly away from the shaft and move the arms, a a, and a sliding tube, c, towards the right. This rocks the lever r, and allows the valves in the inlet pipe to close and reduce the supply of air and gas. A wedge, w, which can be raised or lowered by lever l, intervenes between the end of r and the valve stem. If this lever be lifted to its highest position, the governing commences at a lower speed, as the valve then has but a short distance to travel before closing completely. For high speeds[Pg 105] the driver depresses l, forces the wedge down, and so minimizes the effect of the governor.
THE CLUTCH.
The engine shaft has on its rear end the fly-wheel, which has a broad and heavy rim, turned to a conical shape inside. Close to this, revolving loosely on the shaft, is the clutch plate, a heavy disc with a broad edge so shaped as to fit the inside of a fly-wheel. It is generally faced with leather. A very strong spring presses the plate into the fly-wheel, and the resulting friction is sufficient to prevent any slip. Projections on the rear of the clutch engage with the gear-box shaft. The driver throws out the clutch by depressing a lever with his foot. Some clutches dispense with the leather lining. These are termed metal to metal clutches.
THE GEAR-BOX.
We now come to a very interesting detail of the motor car, the gear-box. The steam-engine has its speed increased by admitting more steam to the cylinders. But an explosion engine must be run at a high speed to develop its full power, and when heavier work has to be done on a hill it becomes necessary to[Pg 106] alter the speed ratio of engine to driving-wheels. Our illustration (Fig. 48) gives a section of a gear-box, which will serve as a typical example. It provides three forward speeds and one reverse. To understand how it works, we must study the illustration carefully. Pinion 1 is mounted on a hollow shaft turned by the clutch. Into the hollow shaft projects the end of another shaft carrying pinions 6 and 4. Pinion 6 slides up and down this shaft, which is square at this point, but round inside the loose pinion 4. Pinions 2 and 3 are keyed to a square secondary shaft, and are respectively always in gear with 1 and 4; but 5 can be slid backwards[Pg 107] and forwards so as to engage or disengage with 6. In the illustration no gear is “in.” If the engine is working, 1 revolves 2, 2 turns 3, and 3 revolves 4 idly on its shaft.
Fig. 48. Fig. 48.—The gear-box of a motor car.
To get the lowest, or “first,” speed the driver moves his lever and slides 5 into gear with 6. The transmission then is: 1 turns 2, 2 turns 5, 5 turns 6, 6 turns the propeller shaft through the universal joint. For the second speed, 5 and 6 are disengaged, and 6 is moved up the page, as it were, till projections on it interlock with slots in 4; thus driving 1, 2, 3, 4, shaft. For the third, or “solid,” speed, 6 is pulled down into connection with 1, and couples the engine shaft direct to the propeller shaft.
The “reverse” is accomplished by raising a long pinion, 7, which lies in the gear-box under 5 and 6. The drive then is 1, 2, 5, 7, 6. There being an odd number of pinions now engaged, the propeller shaft turns in the reverse direction to that of the engine shaft.
Fig. 49. Fig. 49.
THE COMPENSATING GEAR.
Every axle of a railway train carries a wheel at each end, rigidly attached to it. When rounding a corner the outside wheel has further to travel than the other, and consequently one or both wheels must[Pg 108] slip. The curves are made so gentle, however, that the amount of slip is very small. But with a traction-engine, motor car, or tricycle the case is different, for all have to describe circles of very small diameter in proportion to the length of the vehicle. Therefore in every case a compensating gear is fitted, to allow the wheels to turn at different speeds, while permitting them both to drive. Fig. 49 is an exaggerated sketch of the gear. The axles of the moving wheels turn inside tubes attached to the springs and a central casing (not shown), and terminate in large bevel-wheels, c and d. Between these are small bevels mounted on a shaft supported by the driving drum. If the latter be rotated, the bevels would turn c and d at equal speeds, assuming that[Pg 109] both axles revolve without friction in their bearings. We will suppose that the drum is turned 50 times a minute. Now, if one wheel be held, the other will revolve 100 times a minute; or, if one be slowed, the other will increase its speed by a corresponding amount. The average speed remains 50. It should be mentioned that drum a has incorporated with it on the outside a bevel-wheel (not shown) rotated by a smaller bevel on the end of the propeller shaft.
THE SILENCER.
The petrol-engine, as now used, emits the products of combustion at a high pressure. If unchecked, they expand violently, and cause a partial vacuum in the exhaust pipe, into which the air rushes back with such violence as to cause a loud noise. Devices called silencers are therefore fitted, to render the escape more gradual, and split it up among a number of small apertures. The simplest form of silencer is a cylindrical box, with a number of finely perforated tubes passing from end to end of it. The exhaust gases pouring into the box maintain a constant pressure somewhat higher than that of the atmosphere, but as the gases are escaping from it in a fairly steady stream the noise becomes a gentle hiss rather than a[Pg 110] “pop.” There are numerous types of silencers, but all employ this principle in one form or another.
THE BRAKES.
Every car carries at least two brakes of band pattern—one, usually worked by a side hand-lever, acting on the axle or hubs of the driving-wheel; the other, operated by the foot, acting on the transmission gear (see Fig. 48). The latter brake is generally arranged to withdraw the clutch simultaneously. Tests have proved that even heavy cars can be pulled up in astonishingly short distances, considering their rate of travel. Trials made in the United States with a touring car and a four-in-hand coach gave 25⅓ and 70 feet respectively for the distance in which the speed could be reduced from sixteen miles per hour to zero.
SPEED OF CARS.
As regards speed, motor cars can rival the fastest express trains, even on long journeys. In fact, feats performed during the Gordon-Bennett and other races have equalled railway performances over equal distances. When we come to record speeds, we find a car, specially built for the purpose, covering a mile in less than half a minute. A speed of over 120 miles[Pg 111] an hour has actually been reached. Engines of 150 h.p. can now be packed into a vehicle scaling less than 1½ tons. Even on touring cars are often found engines developing 40 to 60 h.p., which force the car up steep hills at a pace nothing less than astonishing. In the future the motor car will revolutionize our modes of life to an extent comparable to the changes effected by the advent of the steam-engine. Even since 1896, when the “man-with-the-flag” law was abolished in the British Isles, the motor has reduced distances, opened up country districts, and generally quickened the pulses of the community in a manner which makes it hazardous to prophesy how the next generation will live.
Note.—The author is much indebted to Mr. Wilfrid J. Lineham, M. Inst. C.E., for several of the illustrations which appear in the above chapter.
[8] Steam-driven cars are not considered in this chapter, as their principle is much the same as that of the ordinary locomotive. [9] On some cars natural circulation is used, the hot water flowing from the top of the cylinder to the tank, from which it returns, after being cooled, to the bottom of the cylinder. [10] For explanation of the induction coil, see p. 122 [Pg 112]Chapter V.
ELECTRICAL APPARATUS.
What is electricity?—Forms of electricity—Magnetism—The permanent magnet—Lines of force—Electro-magnets—The electric bell—The induction coil—The condenser—Transformation of current—Uses of the induction coil.
WHAT IS ELECTRICITY?
OF the ultimate nature of electricity, as of that of heat and light, we are at present ignorant. But it has been clearly established that all three phenomena are but manifestations of the energy pervading the universe. By means of suitable apparatus one form can be converted into another form. The heat of fuel burnt in a boiler furnace develops mechanical energy in the engine which the boiler feeds with steam. The engine revolves a dynamo, and the electric current thereby generated can be passed through wires to produce mechanical motion, heat, or light. We must remain content, therefore, with assuming that electricity is energy or motion transmitted[Pg 113] through the ether from molecule to molecule, or from atom to atom, of matter. Scientific investigation has taught us how to produce it at will, how to harness it to our uses, and how to measure it; but not what it is. That question may, perhaps, remain unanswered till the end of human history. A great difficulty attending the explanation of electrical action is this—that, except in one or two cases, no comparison can be established between it and the operation of gases and fluids. When dealing with the steam-engine, any ordinary intelligence soon grasps the principles which govern the use of steam in cylinders or turbines. The diagrams show, it is hoped, quite plainly “how it works.” But electricity is elusive, invisible; and the greatest authorities cannot say what goes on at the poles of a magnet or on the surface of an electrified body. Even the existence of “negative” and “positive” electricity is problematical. However, we see the effects, and we know that if one thing is done another thing happens; so that we are at least able to use terms which, while convenient, are not at present controverted by scientific progress.
FORMS OF ELECTRICITY.
Rub a vulcanite rod and hold one end near some[Pg 114] tiny pieces of paper. They fly to it, stick to it for a time, and then fall off. The rod was electrified—that is, its surface was affected in such a way as to be in a state of molecular strain which the contact of the paper fragments alleviated. By rubbing large surfaces and collecting the electricity in suitable receivers the strain can be made to relieve itself in the form of a violent discharge accompanied by a bright flash. This form of electricity is known as static.
Next, place a copper plate and a zinc plate into a jar full of diluted sulphuric acid. If a wire be attached to them a current of electricity is said to flow along the wire. We must not, however, imagine that anything actually moves along inside the wire, as water, steam, or air, passes through a pipe. Professor Trowbridge says,[11] “No other agency for transmitting power can be stopped by such slight obstacles as electricity. A thin sheet of paper placed across a tube conveying compressed air would be instantly ruptured. It would take a wall of steel at least an inch thick to stand the pressure of steam which is driving a 10,000 horse-power engine. A thin layer of dirt beneath the wheels of an electric car can prevent the current which propels the car from passing to the[Pg 115] rail, and then back to the power-house.” There would, indeed, be a puncture of the paper if the current had a sufficient voltage, or pressure; yet the fact remains that current electricity can be very easily confined to its conductor by means of some insulating or nonconducting envelope.
MAGNETISM.
The most familiar form of electricity is that known as magnetism. When a bar of steel or iron is magnetized, it is supposed that the molecules in it turn and arrange themselves with all their north-seeking poles towards the one end of the bar, and their south-seeking poles towards the other. If the bar is balanced freely on a pivot, it comes to rest pointing north and south; for, the earth being a huge magnet, its north pole attracts all the north-seeking poles of the molecules, and its south poles the south-seeking poles. (The north-seeking pole of a magnet is marked N., though it is in reality the south pole; for unlike poles are mutually attractive, and like poles repellent.)
There are two forms of magnet—permanent and temporary. If steel is magnetized, it remains so; but soft iron loses practically all its magnetism as soon as the cause of magnetization is withdrawn. This is what we should expect; for steel is more[Pg 116] closely compacted than iron, and the molecules therefore would be able to turn about more easily.[12] It is fortunate for us that this is so, since on the rapid magnetization and demagnetization of soft iron depends the action of many of our electrical mechanisms.
THE PERMANENT MAGNET.
Magnets are either (1) straight, in which case they are called bar magnets; or (2) of horseshoe form, as in Figs. 50 and 51. By bending the magnet the two poles are brought close together, and the attraction of both may be exercised simultaneously on a bar of steel or iron.
LINES OF FORCE.
In Fig. 50 are seen a number of dotted lines. These are called lines of magnetic force. If you lay a sheet of paper on a horseshoe magnet and sprinkle it with iron dust, you will at once notice how the particles arrange themselves in curves similar in shape to those shown in the illustration. It is supposed (it cannot be proved) that magnetic force streams away from the N. pole and describes a[Pg 117] circular course through the air back to the S. pole. The same remark applies to the bar magnet.
ELECTRICAL MAGNETS.
Fig. 50. Fig. 50.—Permanent magnet, and the “lines of force” emanating from it.
If an insulated wire is wound round and round a steel or iron bar from end to end, and has its ends connected to the terminals of an electric battery, current rotates round the bar, and the bar is magnetized. By increasing the strength and volume of the current, and multiplying the number of turns of wire, the attractive force of the magnet is increased. Now disconnect the wires from the battery. If of iron, the magnet at once loses its attractive force; but if of steel, it retains it in part. Instead of a simple horseshoe-shaped bar, two shorter bars riveted into a plate are generally used for electromagnets of this type. Coils of wire are wound round[Pg 118] each bar, and connected so as to form one continuous whole; but the wire of one coil is wound in the direction opposite to that of the other. The free end of each goes to a battery terminal.
In Fig. 51 you will notice that some of the “lines of force” are deflected through the iron bar a. They pass more easily through iron than through air; and will choose iron by preference. The attraction exercised by a magnet on iron may be due to the effort of the lines of force to shorten their paths. It is evident that the closer a comes to the poles of the magnet the less will be the distance to be travelled from one pole to the bar, along it, and back to the other pole.
Fig. 51. Fig. 51.—Electro-magnet: a, armature; b, battery.
Having now considered electricity in three of its forms—static, current, and rotatory—we will pass to some of its applications.
THE ELECTRIC BELL.
A fit device to begin with is the Electric Bell, which has so largely replaced wire-pulled bells. These last cause a great deal of trouble sometimes, since if a wire snaps it may be necessary to take up carpets and floor-boards to put things right. Their installation is not simple, for at every corner must be put a crank to alter the direction of the pull, and the cranks mean increased friction. But when electric wires have once been properly installed, there should be no need for touching them for an indefinite period. They can be taken round as many corners as you wish without losing any of their conductivity, and be placed wherever is most convenient for examination. One bell may serve a large number of rooms if an indicator be used to show where the call was made from, by a card appearing in one of a number of small windows. Before answering a call, the attendant presses in a button to return the card to its normal position.
In Fig. 52 we have a diagrammatic view of an electric bell and current. When the bell-push is pressed in, current flows from the battery to [Pg 120]terminal t1, round the electro-magnet m, through the pillar p and flat steel springs s and b, through the platinum-pointed screw, and back to the battery through the push. The circulation of current magnetizes m, which attracts the iron armature a attached to the spring s, and draws the hammer h towards the gong. Just before the stroke occurs, the spring b leaves the tip of the screw, and the circuit is broken, so that the magnet no longer attracts. h is carried by its momentum against the gong, and is withdrawn by the spring, until b once more makes contact, and the magnet is re-excited. The hammer vibrations recur many times a second as long as the push is pressed in.
Fig. 52. Fig. 52.—Sketch of an electric-bell circuit.
[Pg 121]
The electric bell is used for so many purposes that they cannot all be noted. It plays an especially important part in telephonic installations to draw the attention of the subscribers, forms an item in automatic fire and burglar alarms, and is a necessary adjunct of railway signalling cabins.
THE INDUCTION OR RUHMKORFF COIL.
Reference was made in connection with the electrical ignition of internal-combustion engines (p. 101) to the induction coil. This is a device for increasing the voltage, or pressure, of a current. The two-cell accumulator carried in a motor car gives a voltage (otherwise called electro-motive force = E.M.F.) of 4·4 volts. If you attach a wire to one terminal of the accumulator and brush the loose end rapidly across the other terminal, you will notice that a bright spark passes between the wire and the terminal. In reality there are two sparks, one when they touch, and another when they separate, but they occur so closely together that the eye cannot separate the two impressions. A spark of this kind would not be sufficiently hot to ignite a charge in a motor cylinder, and a spark from the induction coil is therefore used.
[Pg 122]Fig. 53. Fig. 53.—Sketch of an induction coil.
We give a sketch of the induction coil in Fig. 53. It consists of a core of soft iron wires round which is wound a layer of coarse insulated wire, denoted by the thick line. One end of the winding of this primary coil is attached to the battery, the other to the base of a hammer, h, vibrating between the end of the core and a screw, s, passing through an upright, t, connected with the other terminal of the battery. The action of the hammer is precisely the same as that of the armature of an electric bell. Outside the primary coil are wound many turns of a much finer wire completely insulated from the[Pg 123] primary coil. The ends of this secondary coil are attached to the objects (in the case of a motor car, the insulated wire of the sparking-plug and a wire projecting from its outer iron casing) between which a spark has to pass. As soon as h touches s the circuit is completed. The core becomes a powerful magnet with external lines of force passing from one pole to the other over and among the turns of the secondary coil. h is almost instantaneously attracted by the core, and the break occurs. The lines of force now (at least so it is supposed) sink into the core, cutting through the turns of the “secondary,” and causing a powerful current to flow through them. The greater the number of turns, the greater the number of times the lines of force are cut, and the stronger is the current. If sufficiently intense, it jumps any gap in the secondary circuit, heating the intermediate air to a state of incandescence.
THE CONDENSER.
The sudden parting of h and s would produce strong sparking across the gap between them if it were not for the condenser, which consists of a number of tinfoil sheets separated by layers of paraffined paper. All [Pg 124]the “odd” sheets are connected with t, all the “even” with t1. Now, the more rapid the extinction of magnetism in the core after “break” of the primary circuit, the more rapidly will the lines of force collapse, and the more intense will be the induced current in the secondary coil. The condenser diminishes the period of extinction very greatly, while lengthening the period of magnetization after the “make” of the primary current, and so decreasing the strength of the reverse current.
TRANSFORMATION OF CURRENT.
The difference in the voltage of the primary and secondary currents depends on the length of the windings. If there are 100 turns of wire in the primary, and 100,000 turns in the secondary, the voltage will be increased 1,000 times; so that a 4-volt current is “stepped up” to 4,000 volts. In the largest induction coils the secondary winding absorbs 200–300 miles of wire, and the spark given may be anything up to four feet in length. Such a spark would pierce a glass plate two inches thick.
It must not be supposed that an induction coil increases the amount of current given off by a battery. It merely increases its pressure at the expense of its volume—stores up its energy, as it[Pg 125] were, until there is enough to do what a low-tension flow could not effect. A fair comparison would be to picture the energy of the low-tension current as the momentum of a number of small pebbles thrown in succession at a door, say 100 a minute. If you went on pelting the door for hours you might make no impression on it, but if you could knead every 100 pebbles into a single stone, and throw these stones one per minute, you would soon break the door in.
Any intermittent current can be transformed as regards its intensity. You may either increase its pressure while decreasing its rate of flow, or amperage; or decrease its pressure and increase its flow. In the case that we have considered, a continuous battery current is rendered intermittent by a mechanical contrivance. But if the current comes from an “alternating” dynamo—that is, is already intermittent—the contact-breaker is not needed. There will be more to say about transformation of current in later paragraphs.
USES OF THE INDUCTION COIL.
The induction coil is used—(1.) For passing currents through glass tubes almost exhausted of air[Pg 126] or containing highly rarefied gases. The luminous effects of these “Geissler” tubes are very beautiful. (2.) For producing the now famous X or Röntgen rays. These rays accompany the light rays given off at the negative terminal (cathode) of a vacuum tube, and are invisible to the eye unless caught on a fluorescent screen, which reduces their rate of vibration sufficiently for the eye to be sensitive to them. The Röntgen rays have the peculiar property of penetrating many substances quite opaque to light, such as metals, stone, wood, etc., and as a consequence have proved of great use to the surgeon in localizing or determining the nature of an internal injury. They also have a deterrent effect upon cancerous growths. (3.) In wireless telegraphy, to cause powerful electric oscillations in the ether. (4.) On motor cars, for igniting the cylinder charges. (5.) For electrical massage of the body.