Popular Science Monthly/Volume 18/February 1881/Domestic Motors III
|←Horses and their Feet||Popular Science Monthly Volume 18 February 1881 (1881)
Domestic Motors III
By Charles Marshall Lungren
|The Value of Accomplishments→|
THE gas-engine differs from both the steam and hot-air engine in the character of the expansion of the elastic fluid employed and in the mode of applying the heat. In the one the fire is used to convert water placed in a vessel exterior to the engine into steam, which, let into the cylinder, moves the piston by its expansive force; and in the other it is used to expand a volume of air contained in the cylinder or adjacent chamber.
In both, the heat is applied outside of the working cylinder, but the peculiarity of the gas-engine is that the heat is developed within the cylinder itself. A mixture of gas and air is by the operation of the engine drawn into the cylinder, and then exploded, the heat generated expanding the products of combustion, which, exerting a pressure against the piston, give it motion. Simple as is this mode of converting heat into work, the practical realization of it has been found to be exceedingly difficult, and it is only within a very few years that thoroughly serviceable machines have been constructed. The most economical result is obtained from expanding gases when the pressure they exert is a continuous and gradually diminishing one, such as that of steam in the steam-engine. With an explosive mixture, like that in the gas-engine, the expansion takes place with great rapidity, producing a sudden and unsustained pressure, from which it is difficult to get either an economical result or a steady operation of the mechanism. This rapidity of expansion can be decreased, and the pressure obtained approximated to that of the steam-engine, by altering the proportions of air and gas so as to produce a quick combustion instead of an explosion, and by introducing the gaseous mixture into the cylinder gradually, instead of all at once; and it is in this direction that the improvements have taken place which make the latest forms of gas-engine so superior to their predecessors.
Among the first engines to obtain a moderate degree of success were those of Hugon (1858) and Lenoir (1860). Neither of these was, however, very economical in the use of gas, and, previous to the engine of Otto and Langen in 1867, none were produced that were at all satisfactory in this respect. This was, however, objectionable, owing to the intolerable din it made when in operation.
While this engine was at best but a very qualified success, it has led the way to a machine which is very far from being so. In the Otto silent gas-engine, introduced a few years since, and now made in this country and Europe, and in both rapidly going into use, we have a machine that is a very satisfactory solution of the problems involved in the construction of this class of motors. In the matter of fuel it is nearly, if not quite, as economical as a steam-engine of corresponding power, and is, therefore, in actual use much more so, both
because no engineer is required to run it and the expense of keeping up steam, whether running or not, is avoided. The combustible charge introduced into the cylinder is composed of gas and air in such proportions that rapid combustion instead of an explosion takes place, and, to obtain sufficient pressure from its expansion, it is compressed to one third its original volume before being ignited. In this country the engine is made for the trade in three sizes, from two to seven horse-power, but those of larger power are built when desired. The English makers furnish it from one up to forty horse, and as wide a range is given it on the Continent. The engine, as shown in the engraving (Fig. 12), is of the form constructed by the American makers. The cylinder is placed horizontal and overhangs the substantial bed-block of the machine. It is open to the atmosphere at the end toward the fly-wheel, and is closed at the other by a head-piece in which are the appliances for introducing and igniting the gaseous mixture, the construction of which is shown in the sectional cut Fig. 13. The head-plate A closes the cylinder entirely except at l, where there is a passage for the admission of the combustible charge. Between this plate and an outer one, C, is a slide-valve, B. The outer plate is pressed against the valve by the spiral springs shown in Fig. 12. The
pipe supplying the gas opens in its inner face at c, and at m there is a small jet constantly lit while the engine is in operation. The slide-valve B has two channels, i and n, the former placing the air and gas in communication with the cylinder, and the latter serving to ignite the mixture. The piston being at the beginning of its stroke, the valve B is in such a position that the air-inlet a and the gas-inlet c are in communication with the passage I through the port i. The piston then moves outward, drawing in a charge of air and gas which it compresses on its return, the valve B having moved so as to close the opening I. By this movement the channel n becomes filled with gas from the small supply-pipe o, which is ignited at the jet m. Just as the piston has completed the compression of the gaseous mixture, n arrives opposite I and ignites it, the valve continuing its motion so as to close the opening. The piston is driven outward by the expansion of the gases, and on its return they are expelled through the valve q in the side of the cylinder operated by the mechanism of the engine. The slide valve is reciprocated by a crank on the end of the lay-shaft, shown running lengthwise of the cylinder, which revolves but half as fast as the main shaft. The combustible mixture can therefore be drawn in only once in two complete strokes, but whether a fresh supply is taken or not is determined by the governor, which acts to maintain a constant speed under varying loads. It is of the ordinary ball form, and is placed in the cup-shaped receptacle pendent from the cylinder. It actuates by its movement a lever controlling the gas-valve, so that this is opened and closed in accordance with changes in the speed. The regulation is delicate, and the speed nearly if not quite as uniform as in a steam-engine. The speed can be changed at will by increasing or diminishing the amounts of air and gas which may be drawn in each time. An automatic device is provided, which closes the gas-valve, should the engine by any accident stop in a position in which this would be left open. The oiling is committed almost entirely to the engine itself, the only work required in this connection being the filling of the oil-cups. They are placed upon the top of the cylinder, and by means of the small shaft and pulley driven from the lay-shaft deliver a given number of drops of oil to the slide-valve, cylinder, and piston at each revolution. The exhaust is rendered noiseless by being passed into a chamber, from which it escapes into the atmosphere under slight pressure. The cylinder is water-jacketed to keep it cool, the circulation of the water being maintained by the heat received, the warmer water rising to the supply-tank and the cool taking its place.
As before stated, the engine is very economical of gas. The amount used per hour per indicated horse-power is stated by the makers to be twenty-one and a half cubic feet, which, with gas at two dollars a thousand, is a trifle above four cents. In first cost the engine is somewhat more expensive than a good steam-engine, including boiler, of the same power, the price ranging from five hundred dollars for the two horse to eight hundred and fifty for that of seven horse-power. The former occupies a floor-space of about three feet by seven, and weighs fourteen hundred pounds, and the latter covers somewhat more space, and is of double the weight.
The heat generated by the combustion of the gas has been very fully utilized in this engine, but not to the greatest extent practicable. A certain portion of it is carried off by the water in the jacket, and is therefore wasted. If, instead of being allowed to escape without doing any useful work, it was employed to convert a small quantity of water injected into the cylinder into steam, overheating of the cylinder would be prevented, and at the same time this heat would be utilized. Besides the power gained, the use of steam is of value in giving a more sustained pressure on the piston and in lubricating the cylinder. Numerous attempts have been made to realize its advantages, both in hot-air and gas engines, but in most cases with no considerable gain in economy. The engine of Hugon, mentioned above, employed it, but apparently with little advantage.
Quite recently a gas-engine has been brought out in which the difficulties seem to have been mostly overcome, and which appears to approach the limit of economy as closely as it is possible to do. It is the invention of M. Simon, and was first brought to extended public notice at the Paris Exposition of 1878, where it was exhibited in sizes of from one to four horse. A diluted mixture of air and gas is used, as in the Otto, but the compression is done not in the power but in a separate cylinder. It is admitted to the cylinder by a slide-valve of similar construction to that of the Otto, but the manner of using it differs materially from that employed in the latter machine. In the Otto the entire combustible charge is introduced into the cylinder before ignition takes place, and, though the proportion of air is such that the combustion is not an explosive one, still it is completed within a time that covers but a small fraction of the stroke. In the Simon, on the other hand, the combustible mixture is introduced in small quantities that are successively inflamed, producing a gradual expansion of the gases that, along with the steam also admitted, exert a pressure upon the piston more nearly like that in the steam-engine than is attained in any other machine. The ignition is accomplished by a jet always lit, placed just inside of several thicknesses of wire gauze, to prevent the flame retreating into the combustible mixture without the cylinder. The water that is afterward admitted to the cylinder is first used to jacket it, and thus becomes slightly warm. From the jacket it is passed into a steam generator, where a portion of it is vaporized by the heat of the exhaust-gases. This steam is then admitted into the cylinder along with the combustible charge, and further heated and expanded by the ignition of the latter. The water in the steam generator circulates through the jackets of both the compression and power cylinders, first taking heat from the compression cylinder, then from the power cylinder, and reaching the generator quite warm. The heat is therefore utilized to the utmost, and as a consequence the economical result is superior to any before attained. According to M. Simon, the consumption of gas is a little less than eighteen feet an hour per horsepower. The motor occupies a little larger floor-space than the Otto, though it is of less weight, and is somewhat higher in price.
Either of these engines could doubtless be made in sizes small enough to drive simply a sewing-machine or a scroll-saw, though probably not at prices that would allow of their extended use. Only one gas-engine appears to have been so far made of such small power—that invented by M. de Bisschof, and shown in Fig. 14. It is much simpler than the above engines, but is also a much less perfect machine, though sufficiently economical for the use for which it is designed. The ordinary machine is about one man-power, and is furnished at something over a hundred dollars; a larger one of four times the power costing a hundred and ninety. It is compact, ornamental in design, and runs smoothly. No oiling is required, and, once started, it may therefore be left to itself for a considerable time. One of them, indeed, is reported to have run for forty-seven days without stopping, no attention whatever being given to it in that time. It is said to have gone quite largely into use in France, but it has not yet made its appearance in the American market.
In the engine as shown in the engraving, the cylinder is placed upright, and it and the base-plate are cast in one piece. A number of
ribs upon the former increase the radiating surface to such an extent that a water-jacket is unnecessary. The motor can therefore be readily moved from one place to another, an advantage of considerable value in the uses for which it is intended. The mixture of gas and air is admitted and discharged at the lower end of the cylinder through an opening, alternately placed in communication with the gas-supply and exhaust by a sliding valve. The firing of the gaseous charge is done by means of two gas-jets, shown on the right side of the engine. The lower one remains permanently lit, and serves to relight the upper one, which is extinguished at each ignition in the cylinder. This latter is placed directly opposite a small opening in the cylinder at about one third of the piston-stroke from its base. The piston in its upward movement draws in a charge of the mixed gases during this lower third of its stroke, and they are then ignited by the jet, the remaining two thirds of the stroke being completed by the impulse due to their expansion. The atmospheric pressure and the fly-wheel carry the piston through its return-stroke, when the above motions are repeated. The supply of gas, both to the cylinder and the ignition-jets, is regulated by the pinch-cocks on the base of the machine, to the left. Before using, the machine is heated somewhat by a small burner placed below the cylinder. In the man-power machine the consumption of gas is eleven and a half feet an hour, which is a better result than is obtained with any other heat-engine of such low power. The motor seems to be in every way adapted to use in the household, and is probably as simple and perhaps as economical a heat-engine as can be made for the purpose.
The burning of a combustible mixture gradually, as is done in the Simon engine, was first successfully accomplished in the machine invented by Mr. George B. Brayton, and known in the market as the Ready Motor or Hydrocarbon engine. When first introduced, a dilute mixture of gas and air was employed, but in those now made the vapor of petroleum is substituted for the gas, with the advantage of a more satisfactory operation and a reduced cost of running. The working cylinder is surrounded by a water-jacket, and is placed upright in a substantial frame. It is open to the atmosphere below, the oil and air being supplied at the top. The oil is contained in a tank of from five to ten gallons' capacity, and is delivered to the engine by a small pump. Air is compressed by an air-pump in reservoirs, at the base of the machine, from which it is supplied to the cylinder. Only one of these reservoirs is used at a time, the other being kept charged so as to furnish an air-pressure with which to start the machine. The burner, by means of which the oil is introduced in the proper form into the cylinder and ignited, constitutes the main feature of the machine, and is at once simple and ingenious. It consists of a small chamber in the head of the cylinder, lined with a strip of felt against which the oil and a jet of air are delivered. The felt becomes saturated with oil, which the air-blast, passing through, carries in the form of a spray against the sheets of perforated metal and wire gauze which separate this chamber from the cylinder. Another and larger blast of air, in passing through the gauze, becomes carburetted by the petroleum vapor, and, entering the cylinder, is ignited by a jet placed immediately below the sheets of gauze. The jet remains constantly lit, and is prevented from retreating into the chamber above by the wire gauze. By simple mechanism the supply of air is cut off, when a part of the stroke has been made, and the combustion of the vapor ceases, the expanding products of combustion carrying the piston the remainder of the stroke. As the cut-off can be made at any point of the stroke, and the expanding gases allowed to do the work of the remainder, the piston is subjected to a pressure entirely similar to and as readily controlled as that exerted by steam in the steam-engine. The piston is lubricated by its lower edge dipping into a shallow pan, F, at the bottom of the cylinder, containing oil, and the other parts in the ordinary manner. The motor is easily and quickly stopped and started, and when running requires but little attention. It is made in any size desired under ten horse, but those constructed for the trade are of three and five, the former being sold at four hundred and fifty dollars and the latter at six hundred. They are of about the same weight as the Otto, of corresponding power, and occupy a somewhat less space.
The engine is economical in the consumption of fuel, and with the present abundant supply of oil is the cheapest heat-engine of small power yet made. Five gallons of crude petroleum are used, it is stated, in the three, and seven and a half in the five-horse engine for ten hours' running. This is at the rate of one sixth of a gallon or one and a quarter pound an hour per horse-power in the smaller machine, and somewhat less in the larger. As the calorific effect of petroleum is almost double that of coal, the engine is nearly as efficient as a steam-engine of large size, and much more so than one of the same power, while, on account of the cheapness of petroleum, the expense for fuel is no greater. The oil used can be obtained in comparatively small quantities at from six to seven cents a gallon, and at the latter price the cost of a horse-power per hour would be a little less than one and a quarter; cent, an expense considerably below the best results obtained in any of the engines using gas. This comparison is with the present cost of operating the latter motor, which is not one by which its possible economy is to be judged. The gas now used is the ordinary illuminating kind, and is high-priced. With a cheap fuel-gas, such as will assuredly come largely into use at no distant day, the cost would probably not be more than half that at present, and possibly less. This would bring it quite near that of the Brayton—near enough, at least, to make the advantage of the greater cleanliness and convenience of gas outweigh the gain in cheapness possessed by oil. An important objection to this engine is the one arising from the danger that accompanies the use of coal-oil. The motor itself is indeed quite safe, as much so as the gas-engine, and, if no more oil were stored than that in the tank from which the supply is drawn while working, the danger would be small. But when a considerable quantity is kept on hand in places where there is much valuable property, as in city buildings, the danger is sufficient to warrant increased insurance rates, and in some cases the prohibition of the machine. In situations where gas can not be procured, and where the use of oil would not be attended with the danger incident to crowded localities, it would probably be found one of the most satisfactory motors that can be had. Made in sufficiently small sizes of compact form, and with the oil receptacle and engine on one base, it might easily become a serviceable motor for domestic use. As the quantity of oil used in such a machine would be small, it need not be in any way more dangerous than an ordinary lamp or oil-stove, and if properly finished would probably require but little more care. The oil used would of course have to be of a high grade, such as is used or should be used in lamps, and would cost considerably more than that suitable to the larger machines, but the expense of running would still be quite small.
Such are some of the best of the machines which the demand for comparatively small motors has, up to the present, called forth, and in the list those desiring such a power can scarcely fail to find something tolerably well suited to their wants. The various forms of heat-engines have been brought very close to the limit of possible simplicity, and show with some clearness what may be expected from further development along the same lines. They have been mainly designed to meet the requirements of industrial users, because the largest and most constant demand is from these; but they are all capable of a reduction to the scale suitable in the household. For this purpose the gas-engine appears, on all accounts, to be the best adapted. Efficient and serviceable heat-engines are, of necessity, somewhat complicated, and require in their main parts an excellence and accuracy of workmanship that make it difficult to construct them cheaply. The gas-engine seems to be susceptible of greater simplicity of construction than any other of these, and can therefore be made at less cost. Present prices are undoubtedly high, but, with a sufficient demand and the competition that would result, they would decrease considerably, and it is not improbable that an efficient and economical machine of about one man-power could, under such conditions, be furnished at a price not exceeding fifty dollars.
But it is doubtful if such a machine would, after all, be the most satisfactory solution of the problem of a domestic motor. The final solution, there is reason to believe, is to be found, not in a heat-engine of any kind, but in a machine that will simply apply, in a convenient form and economical way, a power already furnished. Of such a nature is the water-wheel, which, for simplicity of construction, ease of handling, high efficiency, and small first cost, is unapproached by anything at present, and will probably never be surpassed by any future device. If water under sufficient pressure were everywhere obtainable, there would be no need of looking beyond this very simple and perfect contrivance. Water-power is, however, limited, and is generally least available where small motors are most wanted in populous cities. Doubtless in many locations, where the windmill is employed to supply water to a house, a combination of wind and water power might readily be made which would prove quite satisfactory. A windmill of considerable power could be used to pump water into a properly elevated reservoir, or into a force-tank, from which it could be distributed to small motors attached to the various pieces of apparatus to be driven. But generally, in cities where power could be distributed, people would prefer to have it furnished without thought or care on their part about its production.
For such a, power, one that can at any time be increased to meet the utmost demands, we must, therefore, look to some other agency than water. Compressed air is, without question, an available one, and the motors in which it could be used are comparatively simple, but, as it could only be employed for this one purpose (unless, indeed, sanitary advantages were realized), the present or prospective demand would hardly warrant its adoption. The agent that appears to be the most suitable, and that gives promise of utility in other directions as well, is electricity. Distributed from a central source of supply, all the advantages of a safe and convenient power would be obtained, without any of the disadvantages attendant upon the use of other forms of energy. Of the feasibility of economically distributing the electric current there is a growing confidence among electricians, and the advantages of so transmitting power have been frequently urged of late, not only for moving light machinery, but for doing all the work now done in our factories by the steam-engine.
Distribution accomplished, the machines by which the current is utilized are very simple, and need not be expensive. As remarked by Dr. Paget Higgs, they are so much cast-iron and insulated copper wire, and their construction requires none of the skilled work necessary in the various forms of heat-engine. The construction is practically the same in the essential parts, whether they be used as current generators or as motors. Briefly, such a machine consists of one or more electro-magnets placed so as to revolve before and very close to the poles of another electro-or permanent magnet, the former system of magnets being: termed the armature, and the latter the field.
When permanent magnets are used for the field, the machines are known as magneto-electric, and, when these are replaced by electromagnets, as dynamo-electric. The operation of both kinds depends, as is well known, upon the inductive action between the armature and the field magnets, a current being induced in each of the coils of the former as they approach, and an equal and opposite one being set up as they recede from the poles of the latter. In dynamo-machines the magnetization of the field is due to the currents generated by the machine itself. The soft-iron cores, after they have once been magnetized, always retain some residual magnetism which serves to induce a feeble current in the armature. A portion of this is sent through their coils, increasing their magnetization, which in turn augments the strength of the induced currents, and thus, by this successive action and reaction between the field and the armature, a very powerful magnetization of both is shortly produced. The currents are usually collected-in such a manner that they both have the same direction in the circuit, by a simple device termed a commutator. This has various forms in different machines, but the principle involved is the same in all. The ends of the wire of the revolving coil are connected with the halves of a cylinder separated by an insulating substance. A metallic brush composed of bundles of wire, or thin strips, presses against each of these sections, and, so long as the cylinder remains stationary, the current taken off by the brushes will be an alternating one; but, when the cylinder revolves with the coil, the brushes will change from one half to the other at the moment of the reversal of the current, and its direction in the circuit will, therefore, always be the same. In most machines the armature has many coils, and the commutator cylinder a corresponding number of insulated sections.
The interest in the electric light during the past few years has resulted in greatly improving such machines, and in the devising of many new forms of varied excellence. A description of one of these, that has attracted wide attention and proved one of the most efficient, will suffice to indicate the general construction and mode of action of such devices. When a magnet is inserted in a closed coil of insulated wire, a momentary current is induced in the coil, and, when it is withdrawn, one of opposite direction occurs. If, instead of withdrawing the magnet, it is passed through the coil, currents will be induced in each of its spirals as the magnet passes them, which will be in one direction during the passage of the first half of the magnet, and in a reverse one during that of the latter half. If two magnets be placed end to end with their like poles in contact, and be bent into the form of a ring, currents can be continuously produced by revolving the ring within an inclosing coil. Mechanical difficulties prevent such an arrangement; but, if, instead of permanent magnets, a ring of soft ironFig. 15. be wound with insulated wire, and revolved between the poles of a magnet, the same results will be obtained and the difficulties avoided. The iron ring becomes a magnet by induction, and as it revolves its poles shift in the reverse direction, so as to always remain opposite those of the inducing magnet. The effect is the same as if the ring remained stationary and the coil revolved. This is the arrangement of the armature adopted in the Gramme machine, and the difference between it and others lies in this mode of inducing the current, instead of by insertion and withdrawal of a magnet from a coil. The manner in which the armature is actually constructed is shown in Fig, 15. The ring is composed of a bundle of soft-iron wires, about which are wound a number of insulated coils. Radial pieces connected with the end of the wire of one coil and the beginning of the next conduct the currents to the commutator cylinder, when they are taken off by brushes. The armature is mounted upon a shaft and revolves between the poles of an electro-magnet, the arrangement being that shown in Fig. 16. When the machine is used on an electro-motor, the current enters the armature by the brushes and is alternately changed in direction by the commutator, so that there is attraction as the armature approaches and repulsion as it leaves the poles of the field magnet.
Besides machines constructed primarily to generate currents, but which may be used as electro-motors as well, there are various others designed especially for the latter purpose. The commonest form of these is a set of electro-magnets arranged radially around the armature, which is rotated under the influence of the changing polarity, or the magnetization and demagnetization of the field-magnets.
One of the best of the machines constructed for motor purposes, and one that has received high praise from competent electricians, is that of M. Marcel Deprez. It consists of a compound permanent horseshoe magnet with a Siemens armature between its poles. This latter is simply a soft-iron cylinder with two longitudinal grooves in which insulated copper wire is wound, the poles being the faces of the cylinder between the coils. It is placed with its axis parallel to the arms of the magnet instead of across them as is ordinarily done, and to this arrangement is due its greater power, as the whole strength of the magnet is utilized. The current enters through the commutator, which reverses it at each half-revolution just as the poles of the armature are passing those of the field-magnet. The armature, therefore, rotates under these alternate attractions and repulsions. The rate of speed can be regulated with nicety by turning the brushes around the commutator cylinder to and from the neutral points. The speed is rendered uniform under a varying load by a very simple centrifugal governor, consisting of a small spring, one extremity of which is attached to an end of the armature coil, and the other rests against the commutator. When the speed increases beyond the normal rate, the free end of the spring is thrown out from contact with the commutator, and the current interrupted until this rate is regained. This governor has proved very sensitive in use, controlling the speed within variations of 700 of its mean rate.
Excellent as this motor is, it has the defect common to all machines in which the armature is approaching or receding from the poles of the field-magnet during but a small part of each revolution. Currents are induced only during these periods, and hence much of the effective power of the field-magnet is lost. M. Trouvé has recently constructed a machine in which this defect is removed in a very simple manner. Instead of making the grooves in the Siemens armature parallel with its axis, they are cut in a spiral form, so that portions of the armature cores are approaching and receding from the poles of the field-magnet during the entire revolution. The impulse received by the armature is, therefore, a continuous one, and dead points are avoided.
The various electro-motors may of course be worked by currents furnished by ordinary batteries, and for running a sewing-machine a few hours a day, at a comparatively small cost. But, as a means of furnishing currents for power for any considerable time, such batteries are out of the question. As pointed out by Professor Ayrton, even if an electro-motor were a perfect machine—that is, if its efficiency were unity—it would be thirty-three times as expensive as a steam-engine, if operated by currents from such a source. The costliness of present batteries is, however, no necessary index of future possibilities. The most feasible way of obtaining power by electricity to-day seems to be through distribution of the current from a central point of supply; but it is not impossible that a battery may yet be produced which, furnishing electricity as cheaply as a machine, will remove the need of distribution, and at the same time greatly enlarge its field of usefulness.