Читать книгу Gas-Engines and Producer-Gas Plants - Rodolphe Edgard Mathot - Страница 7
ОглавлениеFig. 20.—Design of the piston.
The Piston.—Coming, as it does, continually in contact with the ignited gases, the piston is gradually heated to a high temperature. The rear face of the piston should preferably be plane. Curved surfaces are not to be recommended because they cool off badly. Likewise, faces having either inserted parts or bolt-heads are to be avoided, since they are liable to become red-hot and to ignite the mixture prematurely (Fig. 20).
Fig. 21.—Piston with lubricated pin.
Among the parts of the piston which rapidly wear away because constant lubrication is difficult, is the connection with the piston-rod (Fig. 21). It is important that the bearing at the piston-pin be formed of two parts which can be adjusted to take up the wear. The pin itself should be of case-hardened steel. For large engines, some manufacturers have apparently abandoned the practice of locking the pin, by set-screws, in flanges cast in one piece with the piston. Indeed, the piston is often fractured by reason of the expansion of the pins thus held on two sides. It seems advisable to secure the pin by means of a single screw in one of the flanges, fitting it by pressure against the opposite boss. The use of wedges or of clamping-screws, introduced from without the piston to hold the pin, should be avoided. It may happen that the wedges will be loosened, will move out, and will grind the cylinder, causing injuries that cannot be detected before it is too late. The strength of the piston-pin should be so calculated that the pressure per square inch of projected surface does not exceed 1,500 to 2,850 pounds per square inch. It should be borne in mind that the initial pressure of the explosion is often equal to 400 to 425 pounds per square inch. Some manufacturers mount the pin as far to the back of the piston as possible, so as to bring it nearer the point of application of the motive force of the explosion. Other manufacturers, on the other hand, mount the pin toward the front of the piston. No great objection can be raised against either method. In the former case the position of the rings will limit that of the pin.
The number of these rings ought not to be less than four or five, arranged at the rear of the piston. It is to be observed that makers of good engines use as many as 8 to 10 rings in the pistons of fair-sized motors.
Piston-rings of gray pig-iron can be adjusted with the greatest nicety in such a manner that, by means of tongues fitting in their grooves, they are held from turning in the latter, whereby their openings are prevented from registering and allowing the passage of gas. As a general rule, a large number of rings may be considered a distinguishing feature of a well-built engine. In order to prevent a too rapid wear of the cylinder, several German manufacturers finish off the front of the piston with bronze or anti-friction metal in engines of more than 40 to 50 horse-power. It is to be observed, however, that this expedient is not applicable to motors the cylinders of which are comparatively cold; otherwise the bronze or anti-friction metal will deteriorate.
Arrangement of the Cylinder.—The cylinder shell or liner, in which the piston travels, and the water-jacket should preferably be made in separate pieces and not cast of the same metal, in order to permit a free expansion (Figs. 22 and 23). If for want of care or of proper lubrication, which frequently occurs in gas-engines, the cylinder should be injured by grinding, it can be easily renewed, without the loss of all the connecting parts.
Fig. 22.—Head, jacket and liner of cylinder, cast in one piece.
Fig. 23.—Cylinder with independent liner and head.
For the same reason, the cylinder and its casing should be independent of the frame. In many horizontal engines, the cylinders overhang the frame throughout the entire length, by reason of the joining of their front portions with the frames. Although such a construction is attended with no serious consequences in small engines, nevertheless in large engines it is exceedingly harmful. Indeed, in most modern single-acting engines, the pistons are directly connected with the crank-shaft by the piston-rod, without any intermediate connecting-rod or cross-head. The vertical reaction of the motive effort on the piston is, therefore, taken up entirely by the thrust of the cylinder, which is also vertical (Fig. 24). This thrust, acting against an unsupported part, may cause fractures; at any rate, it entails a rapid deterioration of the cylinder joint.
Fig. 24.—Single-acting engines.
Fig. 25.—Engine with inclined bearings.
The Frame.—Gas-engines driven as they are, by explosions, giving rise to shocks and blows, should be built with frames, heavy, substantial, and broad-based, so as to rest solidly on the ground. This essential condition is often fulfilled at the cost of the engine's appearance; but appearance will be willingly sacrificed to meet one of the requirements of perfect operation. For engines of more than 8 to 10 horse-power, frames should be employed which can be secured to the masonry foundation without a separate pedestal or base. Some manufacturers, for the purpose of lightening the frame, attach but little importance to the foundation and to strength of construction, and employ the design illustrated in place of the crank-shaft bearing (Fig. 25); others, in order to facilitate the adjusting of the connecting-rod bearings, prefer the second form (Fig. 26). It is evident that, in the first case, a part of the effort produced by the explosion reacts on the upper portion of the connecting-rod bearing, on the cap of the crank-shaft bearing, and consequently on the fastening-bolts. In the second case, if the adjustment be not very carefully made, or if the rubbing surfaces are insufficient, the entire thrust due to the explosion will be received by the meeting parts of the two bushings, thus injuring them and causing a more rapid wear. In the construction of large engines, some manufacturers take the precaution of forming the connecting-rod bearings of four parts, adjustable to take up the wear, so that the effort is exerted against the parts disposed at right angles to each other. A form that seems rational is that shown in Fig. 27, in which the reaction of the thrust is taken up by the lower bearing, rigidly supported by the braced frame, in the direction opposite to that of the explosive effort.
Fig. 26.—Engine with straight bearings.
Fig. 27.—Engine with correctly designed bearings.
The sum of the projecting surfaces of the two bearings should be so calculated that a maximum explosive pressure of 405 to 425 pounds per square inch will not subject the bearings to a pressure higher than 425 to 550 pounds per square inch.
Fly-Wheels.—In gas-engines particularly, the fly-wheel should be secured to the crank-shaft with the utmost care. It should be mounted as near as possible to the bearings; otherwise the alinement of the shaft will be destroyed and its strength impaired. If the fly-wheel be fastened by means of a key or wedge having a projecting head, it is advisable to cover the end of the shaft by a movable sleeve. The fly-wheel should run absolutely true and straight even if the explosion be premature. In well-built engines the fly-wheels are lined up and shaped to the rim. The periphery is slightly rounded in order the better to guide the belt when applied to the wheel.
Fig. 28.—Single fly-wheel engine with external bearing.
Furthermore, fly-wheels should be nicely balanced; those are to be preferred which have no counter-weights cast or fastened to the hub, the spokes, or the rim. The system of balancing the engine by means of two fly-wheels, mounted on opposite sides, is used chiefly for the purpose of equalizing the inertia effects. Special engines, employed for driving dynamos, and even industrial engines of high power, are preferably fitted with but a single fly-wheel, with an outer bearing, since they more readily counteract the cyclic irregularities or variations of speed occurring in a single revolution (Fig. 28). If in this case a pulley be provided, it should be mounted between the engine and the outer bearing. The following advantages may be cited in favor of the single fly-wheel, particularly in the case of dynamo-driving engines:
1. The single fly-wheel permits a more ready access to the parts to be examined.
2. It involves the employment of a third bearing, thus avoiding the overhang caused by two ordinary fly-wheels.
3. It avoids the torsional strain to which the two-wheel crank is subjected when starting, stopping, and changing the load, the peripheral resistance varying in one of the fly-wheels, while the other is subjected to a strain in the opposite direction on account of the inertia.
4. Two fly-wheels, keyed as they are to projecting ends of the shaft, will be so affected at the rims by the explosions that the belts will shake.
The third bearing which characterizes the single-fly-wheel system, is but an independent support, resting solidly on the masonry bed of the engine. The bearing with its independent support is sufficiently rigid, and is not subjected to any stress from the crank at the moment of explosion, the reaction of the crank affecting only the frame bearings. With such fly-wheels, reputable firms guarantee a cyclic regularity which compares favorably with that of the best steam-engines. For a duty varying from a third of the load to the maximum load, these engines, when driving direct-current dynamos for directly supplying an electric-light circuit, will insure perfect steadiness of the light; and the effectually aperiodic measuring instruments will not indicate fluctuations greater than 2 to 3 per cent. of the tension or intensity of the current. The coefficient of the variations in the speed of a single revolution will thus be not far from 1⁄60.
Fig. 29.—Curved spoke fly-wheel.
Straight and Curved Spoke Fly-Wheels.—The spokes of fly-wheels are either straight or curved. In assembling the motor parts it too often occurs that curved spoke fly-wheels are mounted with utter disregard of the direction in which they are to turn. It is important that curved spokes should be subjected to compression and not to traction. Hence the fly-wheels should be so mounted that the concave portions of the spokes travel in the direction of rotation, as shown in the accompanying diagram (Fig. 29). If a single fly-wheel be employed on an engine of the type in which the speed is governed by the "hit-and-miss" system, the fly-wheel should be extra heavy to counteract the irregularities of the motive impulses when the engine is not working at its full load, or in other words, when no explosion takes place at every cycle.
Fig. 30.—Forged crank-shafts.
The Crank-Shaft.—The crank-shaft should be made of the best mild steel. Those shafts are to be preferred the cranks of which are not forged on (Fig. 30), but cut out of the mass of metal; furthermore, the brackets or supports should be planed and shaped so that they are square in cross-section.
Fig. 31.—Correct design of crank-shaft.
Such a design involves fine workmanship and speaks well for the construction of the whole engine. Moreover, it enables the bearings to be brought nearer each other, reduces to a minimum that part of the crank-shaft which may be considered the weakest, and permits a rational and exact counterbalancing of the moving parts, such as the crank and the end of the connecting-rod. The best manufacturers have adopted the method of fastening to the cranks balancing weights secured to the brackets, especially for high-speed engines or for engines of high power. The projecting surface of the crank-pin should, as a rule, be calculated for a pressure of 1,400 pounds per square inch.
Fig. 32.—Crank-shaft with balancing weight.
Cams, Rollers, etc.—The cams, rollers, thrust-bearings, as well as the piston-pin in particular, should be made of good steel, case-hardened to a depth of at least .08 of an inch. Their hardness and the degree of cementation may be tested by means of a file. This is the method followed by the best manufacturers.
Bearings.—All the bearings and all guides should be adjustable to take up the wear. They are usually made of bronze or of the best anti-friction metal.
Steadiness.—The steadiness of engines may be considered from two different standpoints.
Fig. 33.—Inertia governor.
1. Variation of the Number of Revolutions at Different Loads.—This depends chiefly on the sensitiveness of the governor, which should be of the "inertia" or of the "ball" (or centrifugal) type. The first form is rarely employed, except in small engines up to 10 horse-power, and is applicable only to engines in which the "hit and miss" system is employed (Fig. 33). The second form is more widely used, and is applicable to engines having "hit-and-miss" or variable admission devices. In the first form, the governor simply displaces a very light member, whatever may be the size of the engine, for which reason the dimensions are very small. In the second form, on the other hand, the governor acts either on a conical sleeve or on some other regulating member offering resistance. Evidently, in order to overcome the reactions to which it is subjected, it must be as heavy and powerful as a steam-engine governor. Sufficient allowance is made in a good engine for variation in the number of revolutions between no load and full load, not greater than two per cent. if the admission be of the "hit-and-miss" type, and five per cent. if it be of the variable type.
2. Cyclic Regularity.—This term means simply that the speed of the engine is constant in a single revolution. In practice this is never attained. Allowance is made in engines used for driving direct-current dynamos for a variation of about 1⁄60; while in industrial engines a variation of 1⁄25 is permissible. Cyclic variation depends only on the weight of the fly-wheel; whereas variation in the number of revolutions is determined chiefly by the governor.
Governors.—Diagrams are here presented of the principal types of governors—the inertia governor, the ball or centrifugal governor controlling an admission-valve of the "hit-and-miss" type (Fig. 34), and the ball or centrifugal governor controlling a variable gas-admission valve (Fig. 35).
In distinguishing between the operation of the two last-mentioned types, it may be stated that the former bears the same relation to the hit-and-miss gear as it does, for example, to the valve gear of a Corliss steam-engine. In other words, it is an apparatus that indicates without inducing, admission or cut-off. The second type, on the other hand, operates by means of slides and the like, as in the Ridder type of engine, in which it controls the displacement of the cut-off or distribution slide-valve and is subjected to variable forces, depending on the pressure, lubrication, the condition of the stuffing-boxes, and the like.
In gas as well as in steam engines, designs are to be commended which shield the delicate mechanism from strains and stresses that are likely to destroy its sensitiveness, as is the case in the automatic cut-off of the Corliss steam-engine.
Fig. 34.—"Hit-and-miss" governor.
Governors should be provided with means to permit the manual variation of the speed while the engine is in operation.
For small motors, one of the most widely used admission devices is that of the "hit-and-miss" type. As its name indicates, this admission arrangement allows a given quantity of gas to enter the cylinder for a number of consecutive intervals, until the engine is about to exceed its normal speed. Thereupon the governor cuts off the gas entirely. The result is that, in this system, the number of admissions is variable, but that each admitted charge is composed of a constant proportion of gas and air.
The governors employed for the "hit-and-miss" type are either "inertia" or "centrifugal" governors.
Inertia governors (Fig. 33) are less sensitive than those of the centrifugal type. They are generally applied only to industrial engines of small power, in which regularity of operation is a secondary consideration.
Centrifugal governors employed for gas-engines with "hit-and-miss" regulation are, as a general rule, noteworthy for their small size, which is accounted for by the fact that, in most systems, merely a movable member is placed between the admission-controlling means and the valve-stem (Fig. 34). It follows that this method of operation relieves the governor of the necessity of overcoming the resistance of the weight of moving parts, more or less effectually lubricated, and subjected to the reaction of the parts which they control.
In engines equipped with variable admission devices for the gas or the explosive mixture, the governor actuates a sleeve on which the admission-cam is fastened (Fig. 35). Or, the governor may displace a conical cam, the reaction of which, on contact with the lever, destroys the stability of the governor. These conditions justify the employment of powerful governors which, on account of the inertia of their parts, diminish the reactionary forces encountered.
The centrifugal governor should be sufficiently effectual to prevent variations in the number of revolutions within the limits of 2 to 3 per cent. between no load and approximately full load. Under equivalent conditions, the inertia governor can hardly be relied upon for a coefficient of regularity greater than 4 to 5 per cent.
Fig. 35.—Variable admission governor.
The manner of a governor's operation is necessarily dependent on the admission system adopted. And the admission system varies essentially with the size, the purpose of the engine, and the character of the fuel employed.
Fig. 36.—Vertical engine.
Fig. 37.—Section through an engine of the vertical or "steam-hammer" type.
Vertical Engines.—For some years past there seems to have been a tendency in Europe to use horizontal instead of vertical engines, especially since engines of more than 10 or 15 horse-power have been extensively used for industrial purposes. The vertical type is used for 1 to 8 horse-power engines, with the cylinder in the lower part of the frame, and the shaft and its fly-wheel in the upper part (Fig. 36). The only merit to be attributed to this arrangement is a great saving of space. It is evident, however, that beyond a certain size and power, such engines are unstable. In America particularly, many manufacturers of high-power engines (50 to 100 horse-power or more) prefer the vertical or "steam-hammer" arrangement, which consists in placing the cylinder in the upper part, and the shaft in the lower part of the frame as close to the ground as possible (Figs. 37 and 38). The problem of saving space, as well as that of insuring stability, is thus solved, so that it is easily possible to run up the speed of the engine. There is also the advantage that the shaft of a dynamo can be directly coupled up with the crank-shaft of the engine, thus dispensing with a belt, which, at the least, absorbs 4 to 6 per cent. of the total power. It should, nevertheless, be borne in mind that the direct coupling of electric generators to engine-shafts implies the use of extremely large and, therefore, of extremely costly dynamos. Furthermore, by reason of this arrangement, groups of electro-generators can be disposed in a comparatively small amount of space. Some English manufacturers are also beginning to adopt the "steam-hammer" type of engine for high powers, the result being a marked saving in material and lowering of the cost of installation.
Fig. 38.—Side and end elevations of a vertical or "steam-hammer" engine.
Power of the Engine.—The first thing to be considered is that the power of a gas-engine is always given in "effective" horse-power, and that the power of a steam-engine is always given in "indicated" horse-power in contracts of sale. In England and in the United States, the expression "nominal" horse-power is still employed. It may be advisable to define these various terms exactly, since unscrupulous dealers, to the buyer's loss, have done much to confuse them.
"Indicated" horse-power is a designation applied to the theoretical power produced by the action of the motive agent on the piston. The work performed is measured on an indicator card, by means of which the average pressure to be considered in the computation of the theoretical power is ascertained.
The "effective" or brake horse-power is equal to the "indicated" horse-power, less the energy absorbed by passive resistance, friction of the moving parts, etc.
The "effective" work is an experimental term applied to the power actually developed at the shaft. This work is of interest solely to the engine user.
In a well-built motor, in which the passive resistance by reason of the correct adjustment and simplicity of the parts, is reduced to a minimum, the "effective" horse-power is about 80 to 87 per cent. of the "indicated" horse-power, when the engine runs under full load. This reduced output is usually called the "mechanical efficiency" of the engine.
"Nominal" horse-power is an arbitrary term in the sense in which it is used in England and America, where it is quite common. The manufacturers themselves do not seem to agree on its absolute value. A "nominal" horse-power, however, is equal to anything from 3 to 4 "effective" horse-power. The uncertainty which ensues from the use of the term should lead to its abandonment.
In installing a motor, the determination of its horse-power is a matter of grave importance, which should not be considered as if the motor were a steam-engine or an engine of some other type. It must not be forgotten that, especially at full load, explosion-engines are most efficient, and that, under these conditions, it will generally be advisable to subordinate the utility of having a reserve power to the economy which follows from the employment of a motor running at a load close to its maximum capacity. On the other hand, the gas-engine user is unwilling to believe that the stipulated horse-power of the motor which is sold to him is the greatest that it is capable of developing under industrial conditions. Business competition has led some firms to sell their engines to meet these conditions. It is probably not stretching the truth too far to declare that 80 per cent. of the engines sold with no exact contract specifications are incapable of maintaining for more than a half hour the power which is attributed to them, and which the buyer expects. It follows that the power at which the engine is sold should be both industrially realized and maintained, if need be, for an entire day, without the engine's showing the slightest perturbation, or faltering in its silent and regular operation. To attain this end, it is essential that the energy developed by the engine in normal or constant operation should not exceed 90 to 95 per cent. of the maximum power which it is able to yield, and which may be termed its "utmost power". As a general rule, especially for installations in which the power fluctuates from the lowest possible to double this, as much attention must be paid to the consumption at half load as at full load; and preference should be given to the engine which, other things being equal, will operate most economically at its lowest load. In this case the consumption per effective horse-power is appreciably higher. Generally, this consumption is greater by 20 to 30 per cent. than that at full load. This is particularly true of the single-acting engines so widely used for horse-powers less than 100 to 150.
In some double or triple-acting engines, according to certain writers, the diminution in the consumption will hardly be proportional to the diminution of the power, or at any rate, the difference between the consumption per B.H.P. at full load and at reduced load will be less than in other engines. It should be observed, however, that this statement is apparently not borne out by experiments which the author has had occasion to make. To a slight degree, this economy is obtained at the cost of simplicity, and consequently, at the cost of the engine. At all events, the engines have the merit of great cyclic regularity, rendering them serviceable for driving electric-light dynamos; but this regularity can also be attained by the use of the extra heavy fly-wheels which English firms, in particular, have introduced.
Automatic Starting.—When the gas-engine was first introduced, starting was effected simply by manually turning the fly-wheel until steady running was assured. This procedure, altogether too crude in its way, is attended with some danger. In a few countries it is prohibited by laws regulating the employment of industrial machinery. If the engine be of rather large size one, moreover, which operates at high pressure—such a method of starting is very troublesome. For these reasons, among others, manufacturers have devised automatic means of setting a gas-engine in motion.
Of such automatic devices, the first that shall be mentioned is a combination of pipes, provided with cocks, by the manipulation of which, a certain amount of gas, drawn from the supply pipe, is introduced into the engine-cylinder. The piston is first placed in a suitable position, and behind it a mixture is formed which is ignited by a naked flame situated near a convenient orifice. When the explosion takes place the ignition-orifice is automatically closed, and the piston is given its motive impulse. The engine thus started continues to run in accordance with the regular recurrence of the cycles. In this system, starting is effected by the explosion of a mixture, without previous compression.
Some designers have devised a system of hand-pumps which compress in the cylinder a mixture of air and gas, ignited at the proper time by allowing it to come into contact with the igniter, through the manipulation of cocks (Fig. 39).
These two methods are not absolutely effective. They require a certain deftness which can be acquired only after some practice. Furthermore, they are objectionable because the starting is effected too violently, and because the instantaneous explosion subjects the stationary piston, crank, and fly-wheel to a shock so sudden that they may be severely strained and may even break. Moreover, the slightest leakage in one of the valves or checks may cause the entire system to fail, and, particularly in the case of the pump, may induce a back explosion exceedingly dangerous to the man in charge of the engine.
These systems are now almost generally supplanted by the compressed-air system, which is simpler, less dangerous, and more certain in its effect.
The elements comprising the system in question include essentially a reservoir of thick sheet iron, capable of resisting a pressure of 180 to 225 pounds and sufficient in capacity to start an engine several times. This reservoir is connected with the engine by piping, which is disposed in one of two ways, depending upon whether the reservoir is charged by the engine itself operatively connected with the compressor, or by an independent compressor, mechanically operated.
Fig. 39.—Tangye starter.
In the first case, the pipe is provided with a stop-cock, mounted adjacent to the cylinder, and with a check-valve. When the engine is started and the gas cut off, the air is drawn in at each cycle and driven back into the reservoir during the period of compression. When the engine, running under these conditions by reason of the inertia of the fly-wheel, begins to slow down, the check-valve is closed and the gas-admission valve opened, so as to produce several explosions and to impart a certain speed to the engine in order to continue the charging of the reservoir with compressed air. This done, the valve on the reservoir itself is tightly closed, as well as the check-valve, so as to avoid any leakage likely to cause a fall in the reservoir's pressure.
In the second case, which applies particularly to engines of more than 50 horse-power, the charging pipe connected with the reservoir is necessarily independent of the pipe by means of which the motor is started. The reservoir having been filled and the decompression cam thrown into gear, starting is accomplished:
1. By placing the piston in starting position, which corresponds with a crank inclination of 10 to 20 degrees in the direction of the piston's movement, from the rear dead center, immediately after the period of compression;
2. By opening the reservoir-valve;
3. By allowing the compressed air to enter the cylinder rapidly, through the quick manipulation of the stop-cock, which is closed again when the impulse is given and reopened at the corresponding period of the following cycle, this operation being repeated several times in order to impart sufficient speed to the motor;
4. By opening the gas-valve and finally closing the two valves of the compressed-air pipe.
The pipes and compressed-air reservoirs should be perfectly tight. The reservoirs should have a capacity in inverse ratio to the pressure under which they are placed, i.e., they increase in size as the pressure decreases. If, for example, the reservoirs should be operated normally at a pressure of 105 to 120 pounds per square inch, their capacity should be at least five or six times the volume of the engine-cylinder. If these reservoirs are charged by the engine itself, the pressure will always be less by 15 to 20 per cent. than that of the compression.
