PNEUMATIC TOOLS
The best known of these is the pneumatic hammer. It consists of a cylinder, inside which moves a solid piston having a stroke of from half an inch to six inches. Air is supplied through flexible tubing from a compressing pump worked by steam. The piston beats on a loose block of metal carried in the end of the tool, which does the actual striking. The piston suddenly decreases in diameter at about the centre of its length, leaving a shoulder on which air can work to effect the withdrawal stroke. By a very simple arrangement of air-ports the piston is made to act as its own valve. As the plane side of the piston has a greater area than that into which the piston-rod fits, the striking movement is much more violent than the return. Under a pressure of several hundreds of pounds to the square inch a pneumatic hammer delivers upwards of 7,000 blows per minute; the quick succession of comparatively gentle taps having the effect of a much smaller number of heavier blows. For the flat hammer head can be substituted a curved die for riveting, or a chipping chisel, or a caulking iron, to close the seams of boilers.
The riveter is peculiarly useful for ship and bridge-building work where it is impossible to apply an hydraulic tool. A skilled workman will close the rivet heads as fast as his assistant can place them in their holes; certainly in less than half the time needed for swing-hammer closing.
Even more effective proportionately is the pneumatic chipper. The writer has seen one cut a strip off the edge of a half-inch steel plate at the rate of several inches a minute. To the uninitiated beholder it would seem impossible that a tool weighing less than two stone could thus force its way through solid metal. The speed of the piston is so high that, though it scales but a few pounds, its momentum is great enough to advance the chisel a fraction of an inch, and the individual advances, following one another with inconceivable rapidity, soon total up into a big cut.
Automatic chisels are very popular with ornamental masons, as they lend themselves to the sculpturing of elaborate designs in stone and marble.
Their principle, modified to suit work of another character, is seen in percussive rock drills, such as the Ingersoll Sergeant. In this case the piston and tool are solid, and the air is let into the cylinder by means of slide valves operated by tappets which the piston strikes during its movements. Some types of the rock-drill are controllable as to the length of their stroke, so that it can be shortened while the "entry" of the hole is being made and gradually increased as the hole deepens. For perpendicular boring the drill is mounted on a heavily weighted tripod, the inertia of which effectively damps all recoil from the shock of striking; for horizontal work, and sometimes for vertical, the support is a pillar wedged between the walls of the tunnel, or shaft. An ingenious detail is the rifled bar which causes the drill to rotate slightly on its axis between every two strokes, so that it may not jam. The drills are light enough to be easily erected and dismantled, and compact, so that they can be used in restricted and out-of-the way places, while their simplicity entails little special training on the part of the workman. With pneumatic and other power-drills the cost of piercing holes for explosive charges is reduced to less than one-quarter of that of "jumping" with a crowbar and sledgehammers. With the hand method two men are required, usually more; one man to hold, guide, and turn the drill; and the other, or others, to strike the blows with hammers. The machine, striking a blow far more rapidly than can be done by hand, reduces the number of operators to one man, and perhaps his helper. So durable is the metal of these wonderful little mechanisms that the delivery of 360,000 blows daily for months, even though each is given with a force of perhaps half a ton, fails to wear them out; or at the most only necessitates the renewal of some minor and cheap part. The debt that civilisation owes to the substitution of mechanical for hand labour will be fully understood by anyone who is conversant with the history of tunnel-driving and mining.
Another application of pneumatics is seen in the device for cutting off the ends of stay bolts of locomotive boilers. It consists of a cylinder about fifteen inches in diameter, the piston of which operates a pair of large nippers capable of shearing half-inch bars. The whole apparatus weighs but three-quarters of a hundredweight, yet its power is such that it can trim bolts forty times as fast as a man working with hammer and cold-chisel, and more thoroughly.
Then there is the machine for breaking the short bolts which hold together the outer and inner shells of the water-jacket round a locomotive furnace. A threaded bar, along which travels a nut, has a hook on its end to catch the bolt. The nut is screwed up to make the proper adjustment, and a pneumatic cylinder pulls on the hook with a force of many tons, easily shearing through the bolt.
We must not forget the pneumatic borer for cutting holes in wood or metal, or enlarging holes already existing. The head of the borer contains three little cylinders, set at an angle of 120°, to rotate the drill, the valves opening automatically to admit air at very high pressures behind the pistons. Any carpenter can imagine the advantage of a drill which has merely to be forced against its work, the movement of a small lever by the thumb doing the rest!
Next on the list comes the pneumatic painter, which acts on much the same principle as the scent-spray. Mechanical painting first came to the fore in 1893, when the huge Chicago Exposition provided many acres of surfaces which had to be protected from the weather or hidden from sight. The following description of one of the machines used to replace hand-work is given in Cassier's Magazine: "The paint is atomized and sprayed on to the work by a stream of compressed air. From a small air-compressor the air is led, through flexible hose, to a paint-tank, which is provided with an air-tight cover and clamping screws. The paint is contained in a pot which can be readily removed and replaced by another when a different colour is required. This arrangement of interchangeable tins is also important as facilitating easy cleaning. The container is furnished with a semi-rotary stirrer, the spindle passing through a stuffing-box in the cover, and ending in a handle by which the whole thing complete may be carried about. The compressor is necessarily fixed or stationary, but the paint-tank, connected to it by the single air-hose, can be moved close to the work, while the length of hose from the tank to the nozzle gives the freedom of movement necessary. Air-pressure is admitted to the tank by a bottom valve, and forces the paint up an internal pipe and along a hose from the tank to the spraying nozzle, to which air-pressure is also led by a second hose. The nozzle is practically an injector of special form. The flow of paint at the nozzle is controlled by a small plug valve and spring lever, on which the operator keeps his thumb while working, and which, on release, closes automatically. When it is required to change from one colour to another, or to use a different material, such as varnish, the can, previously in use, is removed, and air, or, if necessary, paraffin oil, is blown through the length of hose which supplies the paint until it is completely clean." The writer then mentions as an instance of the machine's efficiency that it has covered a 30 feet by 8 feet boiler in less than an hour, and that at one large bridge yard a 70 feet by 6 feet girder with all its projecting parts was coated with boiled oil in two hours—a job which would have occupied a man with a brush a whole day to execute. Apart from saving time, the machine produces a surface quite free from brush marks, and easily reaches surfaces in intricate mouldings which are difficult to get at with a brush.
The pneumatic sand-jet is used for a variety of purposes: for cleaning off old paint, or the weathered surface of stonework; for polishing up castings and forgings after they have been brazed. At the cycle factory you will find the sand-jet hard at work on the joints of cycle frames, which must be cleared of all roughness before they are fit for the enameller. The writer, a few days before penning these lines, watched a jet removing London grime from the face of a large hotel. Down a side street stood a steam-engine busily compressing air, which was led by long pipes to the jet, situated on some lofty scaffolding. The rapidity with which the flying grains scoured off smoke deposits attracted the notice of a large crowd, which gazed with upturned heads at the whitened stones. A peculiarity about the jet is that it proves much more effective on hard material than on soft, as the latter, by offering an elastic surface, robs the sand of its cutting power.
After merely mentioning the pneumatic rammer for forcing sand into foundry moulds, we pass to the pneumatic sand-papering machine, which may be described briefly as a revolving disc carrying a circle of sand-paper on its face revolved between guards which keep it flat to its work. The disc flies round many hundreds of times per minute, rapidly wearing down the fibrous surface of the wood it touches. When the coarse paper has done its work a finely-grained cloth is substituted to produce the finish needful for painting.
[CHAPTER V]
THE PEDRAIL: A WALKING STEAM-ENGINE
Have you ever watched carefully a steam-roller's action on the road when it is working on newly laid stones? If you have, you noticed that the stones, gravel, etc., in front of the roller moved with a wave-like motion, so that the engine was practically climbing a never-ending hill. No wonder then that the mechanism of such a machine needs to be very strong, and its power multiplied by means of suitable gearing.
Again, suppose that an iron-tyred vehicle, travelling at a rapid pace, meets a large stone, what happens? Either the stone is forced into the ground or the wheel must rise over it. In either case there will be a jar to the vehicle and a loss of propulsive power. Do not all cyclists know the fatigue of riding over a bumpy road—fatigue to both muscles and nerves?
As regards motors and cycles the vibration trouble has been largely reduced by the employment of pneumatic tyres, which lap over small objects, and when they strike large ones minimise the shock by their buffer-like nature. Yet there is still a great loss of power, and if pneumatic-tyred vehicles suffer, what must happen to the solid, snorting, inelastic traction-engine? On hard roads it rattles and bumps along, pulverising stones, crushing the surface. When soft ground is encountered, in sink the wheels, because their bearing surface must be increased until it is sufficient to carry the engine's weight. But by the time that they are six inches below the surface there will be a continuous vertical belt of earth six inches deep to be crushed down incessantly by their advance.
How much more favourably situated is the railway locomotive or truck. Their wheels touch metal at a point but a fraction of an inch in length; consequently there is nothing to hamper their progression. So great is the difference between the rail and the road that experiment has shown that, whereas a pull of from 8 to 10 lbs. will move a ton on rails, an equal weight requires a tractive force of 50 to 100 lbs. on the ordinary turnpike.
In order to obviate this great wastage of power, various attempts have been made to provide a road locomotive with means for laying its own rail track as it proceeds. About forty years ago Mr. Boydell constructed a wheel which took its own rail with it, the rails being arranged about the wheel like a hexagon round a circle, so that as the wheel moved it always rested on one of the hexagon's sides, itself flat on the ground. This device had two serious drawbacks. In the first place, the plates made a rattling noise which has been compared to the reports of a Maxim gun; secondly, though the contrivance acted fairly well on level ground, it failed when uneven surfaces were encountered. Thus, if a brick lay across the path, one end of a plate rested on the brick, the other on the ground behind, and the unsupported centre had to carry a sudden, severe strain. Furthermore, the plates, being connected at the angles of the hexagon, could not tilt sideways, with the result that breakages were frequent.
Of late years another inventor, Mr. J. B. Diplock, has come forward with an invention which bids fair to revolutionise heavy road traffic. At present, though it has reached a practical stage and undergone many tests satisfactorily, it has not been made absolutely perfect, for the simple reason that no great invention jumps to finality all at once. Are not engineers still improving the locomotive?
The Pedrail, as it has been named, signifies a rail moving on feet. Mr. Diplock, observing that a horse has for its weight a tractive force much in excess of the traction-engine, took a hint from nature, and conceived the idea of copying the horse's foot action. The reader must not imagine that here is a return to the abortive and rather ludicrous attempts at a walking locomotive made many years ago, when some engineers considered it proper that a railway engine should be propelled by legs. Mr. Diplock's device not merely propels, but also steps, i.e. selects the spot on the ground which shall be the momentary point at which propulsive force shall be exerted. To make this clearer, consider the action of a wheel. First, we will suppose that the spokes, any number you please, are connected at their outer ends by flat plates. As each angle is passed the wheel falls flop on to the next plate. The greater the number of the spokes, the less will be each successive jar (or step); and consequently the perfect wheel is theoretically one in which the sides have been so much multiplied as to be infinitely short.
A horse has practically two wheels, its front legs one, its back legs the other. The shoulder and hip joints form the axles, and the legs the spokes. As the animal pulls, the leg on the ground advances at the shoulder past the vertical position, and the horse would fall forwards were it not for the other leg which has been advanced simultaneously. Each step corresponds to our many-sided wheel falling on to a flat side—and the "hammer, hammer, hammer on the hard high road" is the horsey counterpart of the metallic rattle.
On rough ground a horse has a great advantage over a wheeled tractor, because it can put its feet down on the top of objects of different elevations, and still pull. A wheel cannot do this, and, as we have seen, a loss of power results. Our inventor, therefore, created in his pedrail a compromise between the railway smoothness and ease of running and the selective and accommodating powers of a quadruped.
We must now plunge into the mechanical details of the pedrail, which is, strictly speaking, a term confined to the wheel alone. Our illustration will aid the reader to follow the working of the various parts.
In a railway we have (a) sleepers, on the ground, (b) rails attached to the sleepers, (c) wheels rolling over the rails. In the pedrail the order, reckoning upwards, is altered. On the ground is the ped, or movable sleeper, carrying wheels, over which a rail attached to the moving vehicle glides continuously. The principle is used by anyone who puts wooden rollers down to help him move heavy furniture about.
Of course, the peds cannot be put on the ground and left behind; they must accompany their rollers and rails. We will endeavour to explain in simple words how this is effected.
To the axles of the locomotive is attached firmly a flat, vertical plate, parallel to the sides of the fire-box. Pivoted to it, top and bottom, at their centres, are two horizontal rocking arms; and these have their extremities connected by two bow-shaped bars, or cams, their convex edges pointing outwards, away from the axle. Powerful springs also join the rocking arms, and tend to keep them in a horizontal position. Thus we have a powerful frame, which can oscillate up and down at either end. The bottom arm is the rail on which the whole weight of the axle rests.
The rotating and moving parts consist of a large, flat, circular case, the sides of which are a few inches apart. Its circumference is pierced by fourteen openings, provided with guides, to accommodate as many short sliding spokes, which are in no way attached to the main axle. Each spoke is shaped somewhat like a tuning-fork. In the V is a roller-wheel, and at the tip is a "ped," or foot. As the case revolves, the tuning-fork spokes pass, as it were, with a leg on each side of the framework referred to above; the wheel of each spoke being the only part which comes into contact with the frame. Strong springs hold the spokes and rollers normally at an equal distance from the wheel's centre.
It must now be stated that the object of the framework is to thrust the rollers outwards as they approach the ground, and slide them below the rail. The side-pieces of the frame are, as will be noticed (see [Fig. 3]), eccentric, i.e. points on their surfaces are at different distances from the axle centre. This is to meet the fact that the distance from the axle to the ground is greater in an oblique direction than it is vertically, and therefore for three spokes to be carrying the weight at once, two of them must be more extended than the third. So then a spoke is moved outward by the frame till its roller gets under the rail, and as it passes off it it gradually slides inwards again.
It will be obvious to the reader that, if the "peds" were attached inflexibly to the ends of their spokes they would strike the ground at an angle, and, of course, be badly strained. Now, Mr. Diplock meant his "peds" to be as like feet as possible, and come down flat. He therefore furnished them with ankles, that is, ball-and-socket joints, so that they could move loosely on their spokes in all directions; and as such a contrivance must be protected from dust and dirt, the inventor produced what has been called a "crustacean joint," on account of the resemblance it bears to the overlapping armour-plates of a lobster's tail. The plates, which suggest very thin quoits, are made of copper, and can be renewed at small cost when badly worn. An elastic spring collar at the top takes up all wear automatically, and renders the plates noiseless. This detail cost its inventor much work. The first joint made represented an expenditure of £6; but now, thanks to automatic machinery, any number can be turned out at 3s. 6d. each.
A word about the feet. A wheel has fourteen of these. They are eleven inches in diameter at the tread, and soled with rubber in eight segments, with strips of wood between the segments to prevent suction in clay soil. The segments are held together by a malleable cast-iron ring around the periphery of the feet and a tightening core in the centre. These wearing parts, being separate from the rest of the foot, are easily and cheaply renewed, and repairs can be quickly effected, if necessary, when on the road. The surface in contact with the ground being composed of the three substances—metal, wood, and rubber, which all take a bearing, provides a combination of materials adapted to the best adhesion and wear on any class of road, or even on no road at all.
Fig. 3
Motive power is transmitted by the machinery to the wheel axle, from that to the casing, from the casing to the sliding spokes. As there are alternately two and three feet simultaneously in contact with the ground, the power of adhesion is very great—much greater than that of an ordinary traction-engine. This is what Professor Hele-Shaw says in a report on a pedrail tractor: "The weight of the engine is spread over no less than twelve feet, each one of which presses upon the ground with an area immensely greater—probably as much as ten times greater—than that of all the wheels (of an ordinary traction-engine) taken together on a hard road. Upon a soft road all comparison between wheels and the action of these feet ceases. The contact of each of the feet of the Pedrail is absolutely free from all slipping action, and attains the absolute ideal of working, being merely placed in position without sliding to take up the load, and then lifted up again without any sliding to be carried to a new position on the road."
It is necessary that the feet should come down flat on the ground. If they struck it at all edgeways they would "sprain their ankles"; otherwise, probably break off at the ball joint. Mechanism was, therefore, introduced by which the feet would be turned over as they approached the ground, and be held at the proper angle ready for the "step." Without the aid of a special diagram it would be difficult to explain in detail how this is managed; and it must suffice to say that the chief feature is a friction-clutch worked by the roller of the foot's spoke.
To the onlooker the manner in which the pedrail crawls over obstacles is almost weird. The writer was shown a small working model of a pedrail, propelled along a board covered with bits of cork, wood, etc. The axle of the wheel scarcely moved upwards at all, and had he not actually seen the obstacles he would have been inclined to doubt their existence. An ordinary wheel of equal diameter took the obstructions with a series of bumps and bounds that made the contrast very striking.
Fig. 4
An extreme instance of the pedrail's capacity would be afforded by the ascent of a flight of steps (see [Fig. 4]). In such a case the three "peds" carrying the weight of an axle would not be on the same level. That makes no difference, because the frame merely tilts on its top and bottom pivots, the front of the rail rising to a higher level than the back end, and the back spokes being projected by the rail much further than those in front, so that the engine is simply levered over its rollers up an inclined plane. Similarly, in descending, the front spokes are thrust out the furthest, and the reverse action takes place.
With so many moving parts everything must be well lubricated, or the wear would soon become serious. The feet are kept properly greased by being filled with a mixture of blacklead and grease of suitable quality, which requires renewal at long intervals only. The sliding spokes, rollers, and friction-clutches are all lubricated from one central oil-chamber, through a beautiful system of oil-tubes, which provides a circulation of the oil throughout all the moving parts. The central oil-chamber is filled from one orifice, and holds a sufficient supply of oil for a long journey.
We may now turn for a moment from the pedrail itself to the vehicles to which it is attached. Here, again, we are met by novelties, for in his engines Mr. Diplock has so arranged matters, that not only can both front and back pairs of wheels be used as drivers, but both also take part in the steering. As may be imagined, many difficulties had to be surmounted before this innovation was complete. But that it was worth while is evident from the small space in which a double-steering tractor can turn, thanks to both its axles being movable, and from the increased power. Another important feature must also be noticed, viz. that the axles can both tip vertically, so that when the front left wheel is higher than its fellow, the left back wheel may be lower than the right back wheel. In short, flexibility and power are the ideals which Mr. Diplock has striven to reach. How far he has been successful may be gathered from the reports of experts. Professor Hele-Shaw, F.R.S., says: "The Pedrail constitutes, in my belief, the successful solution of a walking machine, which, whilst obviating the chief objections to the ordinary wheel running upon the road, can be made to travel anywhere where an ordinary wheel can go, and in many places where it cannot. At the same time it has the mechanical advantages which have made the railway system such a phenomenal success. It constitutes, in my belief, the solution of one of the most difficult mechanical problems, and deserves to be considered as an invention quite apart from any particular means by which it is actuated, whether it is placed upon a self-propelled carriage or a vehicle drawn by any agency, mechanical or otherwise.... The way in which all four wheels are driven simultaneously so as to give the maximum pulling effect by means of elastic connection is in itself sufficient to mark the engine as a most valuable departure from common practice. Hitherto this driving of four wheels has never been successfully achieved, partly because of the difficulty of turning the steering-wheels, and partly because, until the present invention of Mr. Diplock, the front and hind wheels would act against each other, a defect at first experienced and overcome by the inventor in his first engine."
A PEDRAIL TRACTOR ENGAGED IN WAR OFFICE TRIALS
The inventor, Mr. J. B. Diplock, is standing on the left of the group. Observe the manner in which the feet gradually assume a horizontal position as they approach the ground.
On January 8th, 1902, Mr. Diplock tried an engine fitted with two ordinary wheels behind and two pedrails in front. The authority quoted above was present at the trials, and his opinion will therefore be interesting. "The points which struck me immediately were (1) the marvellous ease with which it started into action, (2) the little noise with which it worked.... Another thing which I noticed was the difference in the behaviour of the feet and wheels. The feet did not in any way seem to affect the surface of the road. Throwing down large stones the size of the fist into their path, the feet simply set themselves to an angle in passing over the stones, and did not crush them; whereas, the wheel coming after invariably crushed the stones, and, moreover, distorted the road surface.
"Coming to the top of the hill, I made the Pedrail walk first over 3-inch planks, then 6-inch, and finally over a 9-inch balk.... One could scarcely believe, on witnessing these experiments, that the whole structure was not permanently distorted and strained, whereas it was evidently within the limits of play allowed by the mechanism. As a proof of this the Diplock engine walked down to the works, and I then witnessed its ascent of a lane, beside the engineering works, which had ruts eight or ten inches deep, and was a steep slope. This lane was composed in places of the softest mud, and whereas the wheels squeezed out the ground in all directions, the feet of the Pedrails set themselves at the angles of the rut where it was hard, or walked through the soft and yielding mud without making the slightest disturbance of the surrounding ground.... I came away from that trial with the firm conviction that I had seen what I believe to be the dawn of a new era in mechanical transport."
Mr. Diplock does not regard the pedrail as an end in itself so much as a means to an end, viz. the development of road-borne traffic. For very long distances which must be covered in a minimum of time the railway will hold its own. But there is a growing feeling that unless the railways can be fed by subsidiary methods of transport more effectively than at present, and unless remote country districts, whither it would not pay to carry even a light railway, are brought into closer touch with the busier parts, our communications cannot be considered satisfactory, and we are not getting the best value out of our roads. For many classes of goods cheapness of transportation is of more importance than speed; witness the fact that coal is so often sent by canal rather than by rail.
Here, then, is the chance for the pedrail tractor and its long train of vehicles fitted with pedrail wheels, which will tend to improve the road surfaces they travel over. Mr. Diplock sets out in his interesting book, A New System of Heavy Goods Transport on Common Roads, a scheme for collecting goods from "branch" routes on to "main" routes, where a number of cars will be coupled up and towed by powerful tractors. With ordinary four-wheeled trucks it is difficult to take a number round a sharp corner, since each truck describes a more sudden circle than its predecessor, the last often endeavouring to climb the pavement. Four-wheeled would therefore be replaced by two-wheeled trucks, provided with special couplings to prevent the cars tilting, while allowing them to turn. Cars so connected would follow the same track round a curve.
The body of the car would be removable, and of a standard size. It could be attached to a simple horse frame for transport into the fields. There the farmer would load his produce, and when the body was full it would be returned to the road, picked up by a crane attached to the tractor, swung on to its carriage and wheels, and taken away to join other cars. By making the bodies of such dimensions as to fit three into an ordinary railway truck, they could be entrained easily. On reaching their destination another tractor would lift them out, fit them to wheels, and trundle them off to the consumer. By this method there would be no "breaking bulk" of goods required from the time it was first loaded till it was exposed in the market for sale.
These things are, of course, in the future. Of more present importance is the fact that the War Office has from the first taken great interest in the new invention, which promises to be of value for military transport over ground either rough or boggy. Trials have been made by the authorities with encouraging results. That daring writer, Mr. H. G. Wells, has in his Land Ironclads pictured the pedrail taking an offensive part in warfare. Huge steel-plated forts, mounted on pedrails, and full of heavy artillery and machine guns, sweep slowly across the country towards where the enemy has entrenched himself. The forts are impervious alike to shell and bullet, but as they cross ditch or hillock in their gigantic stride, their artillery works havoc among their opponents, who are finally forced to an unconditional surrender.
Even if the pedrail is not made to carry weapons of destruction, we can, after our experiences with horseflesh in the Boer War, understand how important it may become for commissariat purposes. The feats which it has already performed mark it as just the locomotive to tackle the rough country in which baggage trains often find themselves.
To conclude with a more peaceful use for it. When fresh country is opened up, years must often pass before a proper high road can be made, yet there is great need of an organised system of transport. Whither ordinary traction-engines, or carts, even horses, could scarcely penetrate, the pedrail tractor, thanks to its big, flat feet, which give it, as someone has remarked, the appearance of "a cross between a traction-engine and an elephant," will be able to push its way at the forefront of advancing civilisation.
At home we shall have good reason to welcome the pedrail if it frees us from those terrible corrugated tracks so dreaded by the cyclist, and to bless it if it actually beats our roads down into a greater smoothness than they now can boast.
[CHAPTER VI]
INTERNAL COMBUSTION ENGINES
OIL ENGINES — ENGINES WORKED WITH PRODUCER GAS — BLAST FURNACE GAS ENGINES
If carbon and oxygen be made to combine chemically, the process is accompanied by the phenomenon called heat. If heat be applied to a liquid or gas in a confined space it causes a violent separation of its molecules, and power is developed.
In the case of a steam-engine the fuel is coal (carbon in a more or less pure form), the fluid, water. By burning the fuel under a boiler, a gas is formed which, if confined, rapidly increases the pressure on the walls of the confining vessel. If allowed to pass into a cylinder, the molecules of steam, struggling to get as far as possible from one another, will do useful work on a piston connected by rods to a revolving crank.
We here see the combustion of fuel external to the cylinder, i.e. under the boiler, and the fuel and fluid kept apart out of actual contact. In the gas or oil-vapour engine the fuel is brought into contact with the fluid which does the work, mixed with it, and burnt inside the cylinder. Therefore these engines are termed internal combustion engines.
Supposing that a little gunpowder were placed in a cylinder, of which the piston had been pushed almost as far in as it would go, and that the powder were fired by electricity. The charcoal would unite with the oxygen contained in the saltpetre and form a large volume of gas. This gas, being heated by the ignition, would instantaneously expand and drive out the piston violently.
A very similar thing happens at each explosion of an internal combustion engine. Into the cylinder is drawn a charge of gas, containing carbon, oxygen, and hydrogen, and also a proportion of air. This charge is squeezed by the inward movement of the piston; its temperature is raised by the compression, and at the proper moment it is ignited. The oxygen and carbon seize on one another and burn (or combine), the heat being increased by the combustion of the hydrogen. The air atoms are expanded by the heat, and work is done on the piston. But the explosion is much gentler than in the case of gunpowder.
During recent years the internal combustion engine has been making rapid progress, ousting steam power from many positions in which it once reigned supreme. We see it propelling vehicles along roads and rails, driving boats through the water, and doing duty in generating stations and smelting works to turn dynamos or drive air-pumps—not to mention the thousand other forms of usefulness which, were they enumerated here, would fill several pages.
A decade ago an internal combustion engine of 100 h.p. was a wonder; to-day single engines are built to develop 3,000 h.p., and in a few years even this enormous capacity will doubtless be increased.
It is interesting to note that the rival systems—gas and steam—were being experimented with at the same time by Robert Street and James Watt respectively. While Watt applied his genius to the useful development of the power latent in boiling water, Street, in 1794, took out letters patent for an engine to be worked by the explosions caused by vaporising spirits of turpentine on a hot metal surface, mixing the vapour with air in a cylinder, exploding the mixture, and using the explosion to move a piston. In his, and subsequent designs, the mixture was pumped in from a separate cylinder under slight pressure. Lenoir, in 1860, conceived the idea of making the piston suck in the charge, so abolishing the need of a separate pump; and many engines built under his patents were long in use, though, if judged by modern standards, they were very wasteful of fuel. Two years later Alphonse Beau de Rochas proposed the further improvement of utilising the cylinder, not only as a suction pump, but also as a compressor; since he saw that a compressed mixture would ignite very much more readily than one not under pressure. Rochas held the secret of success in his grasp, but failed to turn it to practical account. The "Otto cycle," invented by Dr. Otto in 1876, is really only Rochas's suggestion materialised. The large majority of internal combustion engines employ this "cycle" of operations, so we may state its exact meaning:—
(1) A mixture of explosive gas and air is drawn into the cylinder by the piston as it passes outwards (i.e. in the direction of the crank), through the inlet valve.
(2) The valve closes, and the returning piston compresses the mixture.
(3) The mixture is fired as the piston commences its second journey outwards, and gives the "power" stroke.
(4) The piston, returning again, ejects the exploded mixture through the outlet or exhaust valve, which began to open towards the end of the third stroke.
Briefly stated, the "cycle" is—suction, compression, explosion, expulsion; one impulse being given during each cycle, which occupies two complete revolutions of the fly-wheel. Since the first, second, and third operations all absorb energy, the wheel must be heavy enough to store sufficient momentum during the "power" stroke to carry the piston through all its three other duties.
Year by year, the compression of the mixture has been increased, and improvements have been made in the methods of governing the speed of the engine, so that it may be suitable for work in which the "load" is constantly varying. By doubling, trebling, and quadrupling the cylinders the drive is rendered more and more steady, and the elasticity of a steam-engine more nearly approached.
The internal combustion engine has "arrived" so late because in the earlier part of last century conditions were not favourable to its development. Illuminating gas had not come into general use, and such coal gas as was made was expensive. The great oil-fields of America and Russia had not been discovered. But while the proper fuels for this type of motor were absent, coal, the food of the steam-engine, lay ready to hand, and in forms which, though useless for many purposes, could be advantageously burnt under a boiler.
Now the situation has altered. Gas is abundant; and oil of the right sort costs only a few pence a gallon. Inventors and manufacturers have grasped the opportunity. To-day over 3,000,000 h.p. is developed continuously by the internal combustion engine.
Steam would not have met so formidable a rival had not that rival had some great advantages to offer. What are these? Well, first enter a factory driven by steam power, and carefully note what you see. Then visit a large gas- or oil-engine plant. You will conclude that the latter scores on many points. There are no stokers required. No boilers threaten possible explosions. The heat is less. The dust and dirt are less. The space occupied by the engines is less. There is no noisome smoke to be led away through tall and expensive chimneys. If work is stopped for an hour or a day, there are no fires to be banked or drawn—involving waste in either case.
Above all, the gas engine is more efficient, or, if you like to express the same thing in other words, more economical. If you use only one horse-power for one hour a day, it doesn't much matter whether that horse-power-hour costs 4d. or 5d. But in a factory where a thousand horse-power is required all day long, the extra pence make a big total. If, therefore, the proprietor finds that a shilling's-worth of gas or oil does a quarter as much work again as a shilling's-worth of coal, and that either form of fuel is easily obtained, you may be sure that, so far as economy is concerned, he will make up his mind without difficulty as to the class of engine to be employed. A pound of coal burnt under the best type of steam-engine gives but 10 per cent. of its heating value in useful work. A good oil-engine gives 20-25 per cent., and in special types the figures are said to rise to 35-40 per cent. We may notice another point, viz. that, while a steam-engine must be kept as hot as possible to be efficient, an internal combustion engine must be cooled. In the former case no advantage, beyond increased efficiency, results. But in the latter the water passed round the cylinders to take up the surplus heat has a value for warming the building or for manufacturing processes.
Putting one thing with another, experts agree that the explosion engine is the prime mover of the future. Steam has apparently been developed almost to its limit. Its rival is but half-grown, though already a giant.
Some internal combustion engines use petroleum as their fuel, converting it into gas before it is mixed with air to form the charge; others use coal-gas drawn from the lighting mains; "poor gas" made in special plants for power purposes; or natural gas issuing from the ground. Natural gas occurs in very large quantities in the United States, where it is conveyed through pipes under pressure for hundreds of miles, and distributed among factories and houses for driving machinery, heating, and cooking. In England and Europe the petroleum engine and coal-gas engine have been most utilised; but of late the employment of smelting-furnace gases—formerly blown into the air and wasted—and of "producer" gas has come into great favour with manufacturers. The latest development is the "suction" gas engine, which makes its own gas by drawing steam and air through glowing fuel during the suction stroke.
We will consider the various types under separate headings devoted
(1) To the oil-fuel engine,
(2) The producer-gas engine and the suction-gas engine,
(3) Blast-furnace gas engines,
with reference to the installations used in connection with the last two.
All explosion engines (excepting the very small types employed on motor cycles) have a water-jacket round the cylinders to absorb some of the heat of combustion, which would otherwise render the metal so hot as to make proper lubrication impossible, and also would unduly expand the incoming charge of gas and air before compression. The ideal engine would take in a full charge of cold mixture, which would receive no heat from the walls of the cylinder, and during the explosion would pass no heat through the walls. In other words, the ideal metal for the cylinders would be one absolutely non-receptive of heat. In the absence of this, engineers are obliged to make a compromise, and to keep the cylinder at such a temperature that it can be lubricated fittingly, while not becoming so cold as to absorb too much of the heat of explosion.