US2746670A - Free piston motor-compressors - Google Patents

Description

May 22, 1956 R. HUBER 2,746,670

FREE PISTON MOTOR-COMPRESSORS Filed May 11 1951 W7/% A TTORNE6 United States Patent 2,746,670 FREE PISTON MOTOR-COMPRESSORS ApplicationMay 11, 1951, Serial No. 225,691 Claims priority, applicationFrance May 13, 1950 4 Claims. (Cl. 230-1-56) The present invention relates to free piston motor compressors in which the movement of the movable system or systems in one direction (outward stroke) is produced by the combustion of fuel in the power cylinder of the motor compressor, whereas the movement in theopposed direction (inward stroke) is produced by the energy of a return energy accumulator which has stored up for this purpose a portion of theenergy supplied by the combustion of fuelv during the preceding outward stroke.

in the present specification, the term free piston motor compressor designates a machine the compressor portion of which discharges most of the air it compresses to the outside directly, that is to say without preliminary passage through the power cylinder of the machine.

The object of my invention is toprovide a motor compressor of this kind which is better adapted to meet the requirements of practice than those existing up to the present time.

A preferred embodiment of my invention will :be .hereinafter described with reference to the accompanying drawing, given merely by way of example and in which:

Fig. 1 diagrammatically shows a free piston motor. generator made according to my invention.

Fig. 2 diagrammatically :shows differentpositions of a regulating device belonging to this motor-generator.

Fig. 3 is a diagram showing diagrammatically, in accordance with variations of the discharge pressure of the motor compressor, the mean pressures existing in the return energy accumulator.

The motor compressor proper may-be of any suitable construction, such as that shown by Fig. l. Sucha motor compressor includes a power. cylinder 1 provided, in the central part thereof, with at least one, fuel injector; 2 and in which operate, in opposed directions, two power pistons 3 and 4 which control respectively the inlet portv5 and the outlet port 6 of said power cylinder. The power portion of the motor compressor, constituted by said elements 1 to 6, works on the two stroke diesel cycle.

Power piston 3 is connected with a low pressure compressor piston '7 working in a low pressure compressor cylinder 8 provided with a suction valve 9 and a discharge valve 10.

On the other hand, powerpiston4 is rigid with a high pressure compressor piston 11 which works ina high pressure compressor cylinder 12 provided. with an inlet valve 13 and a delivery valve 14. The space 15 into which is delivered the .air discharged from the low pressure compressor cylinder communicates with the inlet of the high pressure cylinder 12 through a conduit 16. The air discharged from the high pressnrecylinder -12 is fed to a reservoir 17, from which it canflowthrougha conduit 18 to the place where it is to beused.

The two movable system-s3, 7 andA, 11 are connected together bya synchronizingmechanism of a known type (not shown), constituted for instance by Connecting rods and cranks, or by pinions andrackaand whichvisintended to give these systems, at any time, respective positions symmetrical with reference to the center of the power cylinder 1.

In such a motor compressor, the outward stroke is produced by the combustion of fuelinjected into the power cylinder in the space between the two power pistons 3 and 4, when they are close to their inner dead centers. During this outward stroke, the low pressure compressor piston 7 compresses and delivers through discharge valves 10 the air present in low pressure cylinder 8, while the high pressure cylinder subjects the air already compressed in the low pressure cylinder to a second stage of compression to discharge it into reservoir 17.

The return or inward strokes of free pistons 3, 7 and i, 11 are ensured on the one'hand by the compressed air cushions which, at the end of the outward stroke of these pistons, remain entrapped in the clearance spaces of compressor cylinders 8 and 12 and, on the other hand, by a return energy pneumatic accumulator constituted by the inside of a cylindrical chamber 3a formed in piston 3 and which cooperates with a stationary piston 19. The energy stored up in this accumulator depends, on the one hand, upon the length of the outward stroke of power piston 3 and, on the other hand, upon the mass of air which constitutes the cushion present in chamber 3a. The energy stored up in the accumulator increases, on the one hand, when the mass of air forming the cushion increases and, on the other hand, when the outward stroke of the movable systems increases, which compensates, at least partly, for the reduction undergone by the return energy of the air cushions of the dead spaces of the compressor cylinders when the outward stroke increases.

if the motor compressor worked always with exactly the same delivery pressure, it would not be necessary to provide a device for regulating the return energy of accumulator 3a, 19. However, even in a motor compressor which normally supplies air always at the same pressure (nominal pressure), some variations in the delivery pressure, due to variations in the load of the compressor are unavoidable. This is why the motor compressor must be fitted with a regulating device which varies the energy brought into play in the accumulator in accordance with the variations which mayoccur in the value of the'delivery pressureof the compressor.

It should be noted here that the final pressure of com-' pression in the power cylinder depends upon the amount of energy stored up in the accumulator and that this pressure, on the other hand, is one of the factors which determine the number of oscillations of the movable systems per unit of time, this number being higher as .the final pressure of compression in the power cylinder is higher.

Furthermore, it should benoted that the finalpressure of compression in the power cylinder can vary within rather wide limits.

As above stated when a motor compressor is arranged to work with a given delivery pressure called nominal pressure, .thisdoes not mean that the delivery pressure is constantly equal to said nominal value and slight variations are unavoidable. This is due inv particular to the fact that the obtainment of the desired delivery pressure (pressure in reservoir 17) is obtained by means of a regulating device, of conventional construction and not shown by the drawing, which controls the fuel injection (ml) in accordance with the pressure in reservoir 17. When this pressure exceeds the desired value due to the fact that there is no load, the fuel injection is cut oil and the motor compressor ceases feeding air to said reservoir 17. But it will be understood that since there is no load (i. e. since there .is no outflow from reservoir 17) the slight excess of pressure which has caused the regulating device to cut ofifuelinjection remains, and the pressure in reservoir 17 is then slightly higher than when the motor compressor is working on full load (i. e. reservoir 17 is delivering compressed air to receiver machines).

In other words, the nominal pressure which is to be achieved at the delivery of the motor compressor is not a fixed value, but a range of values, with an upper limit (corresponding to no load) and a lower limit (corresponding to full load), the interval between these limits (hereinafter called regulation difierence") being small, for instance some tenths of an atmosphere.

Starting from these considerations, I arrange, according to my invention, the regulating device of the return energy accumulator so that the final pressure of compression in the power cylinder is relatively high, for instance 60 atmospheres or more, as long as the delivery pressure is lower than or at most equal to the lower limit of the nominal delivery pressure, that is to say the value which corresponds to the maximum load of the motor compressor, and that the final pressure of compression in the power cylinder decreases quickly as soon as the delivery pressure exceeds said lower limit of its nominal value, to come to a minimum value when the delivery pressure reaches the upper limit of its nominal value.

In other words, according to my invention, the return energy regulating device is arranged in such manner as to work in accordance with two different laws one of which determines the operation of this device as long as the delivery pressure has not yet reached its minimum nominal value, whereas the other one determines its operation when the delivery pressure is within the limits of the above mentioned regulation difference.

I thus obtain, in the period for which the motor compressor is working on a load lower than the maximum load thereof, a reduction of the number of oscillations per unit of time, which reduction is the greater as the load of the motor compressor gets nearer to its minimum value, and possibly to its zero value. This reduction of the number of oscillations per unit of time gives for the motor compressor the same etfect as a reduction of the length of stroke of the movable systems. The reduction of the number of oscillations, for loads lower than the maximum load, makes it possible to reduce the differences between the maximum and minimum strokes of the movable units.

When the return energy accumulator is a pneumatic accumulator, the energy brought into play in this accumulator is regulated by varying the mass of air contained in the accumulator, which mass is, for a given length of stroke, proportional to the mean pressure in the accumulator.

It is clear that there are different ways, according to my invention as above set forth, of devising a motor compressor and the device for regulating its return energy.

A particularly advantageous embodiment of my invention is shown by the appended drawing. According to this construction, the device for regulating the mass of air in accumulator 3a, 19 includes a slide valve device 20 connected through conduit 21 with the inside of accumulator cylinder 3a. Slide valve 20 is adapted to connect, according to its position, conduit 21 either with the compressed air. reservoir 17, through a conduit 22 having a throttled portion 23, or with the atmosphere through a tube 24.

Slide valve 20 is made rigid with a piston 25 which cooperates with a cylinder 26 the upper chamber of which communicates through a throttled passage 27 with conduit 21, whereby the mean pressure of the accumulator air cushion is transmitted to said chamber of cylinder 26 and acts upon the upper face of piston 25. The other face of this piston is subjected on the one hand to the action of a compression spring 28 working in opposition to the accumulator mean pressure and on the other hand to the action of an adjustable pulling spring 29 exerting a force of the same direction as that of the accumulator mean pressure acting upon the upper face of piston 25. Pulling spring 29 is adjusted in accordance with the delivery pressure existing in the reservoir 17 of the accumulator.

For this purpose, this delivery pressure is made to act through a conduit 30 upon a manometric box 31 placed in a casing 32 so that its top end 31a is subjected on the one hand to the action of said delivery pressure and on the other hand to that of a spring 33 which is given a preliminary compression. This manometric box is constituted by a kind of bellows 31 the lower edge of which is fixed in an airtight fashion to the cylindrical wall of cylinder 32 in which it is coaxially mounted. The flat top end 31a of said bellows carries, fixed thereto, a rod 31b hinged to the end of a lever 40 to an intermediate point of which is pivotally mounted the rod 41 of a slide valve 34 controlling the supply of a fluid under pressure fed through conduit 42 to a cylinder 43 containing a piston 35 the rod of which is pivotally connected to the end of a lever 36 to the other end of which pulling spring 29 is secured. In order to vary the tensioning of spring 29 in accordance with two different laws, this lever 36 bears successively upon two different fulcrum abutments 37 and 38 each analogous to the knife-edge of a balance. The effect of these two fulcrum abutments upon the operation of lever 36 is shown on a larger scale by Fig. 2. For all positions of the lever between position I, which corresponds to a delivery pressure )0 lower than the lower limit 17, of the nominal pressure of the motor compressor, and position II, which corresponds to this nominal pressure lower limit, lever 36 bears against fulcrum 37, whereby the displacements of its right hand end are transmitted with a reduction to the left hand end, i. e. to spring 29. Consequently, the variations of tension of spring 29 are relatively small as compared with the variations of the delivery pressure. When the delivery pressure exceeds value p1, lever 36 ceases to bear against fulcrum 37 and comes to bear against fulcrum 38. From this time on, the displacement of the right hand end of lever 36 is transmitted with an amplification to the left hand of this lever and therefore to spring 29. Thus, the law according to which the tension of spring 29 varies as a function of the delivery pressure is different for the pressures lower than or equal to p1 and for the pressures ranging from )1 and pm. This last mentioned pressure, which is that corresponding to position III of lever 36 is the upper limit of the nominal delivery pressure of the motor compressor, which is reached when said compressor is working on no load.

The effect of this change in the law of variation of the energy brought into play in the accumulator is illustrated by Fig. 3, which shows the variation of the mean pressure pm (in ordinates) of the accumulator air cushion as a function of the delivery pressure p in reservoir 17 (in abscissas). This diagram shows that 2111 decreases slowly when the delivery pressure increases from pc to 121. Preferably, the law of variation of pm as a function of p is such that the reduction thereof balances the increase in the return energy which takes place in the air cushions of the clearance spaces of compressor cylinders 8 and 12 at the ends of the outward strokes. Thus, the sum of the return energies acting upon the moving systems remains substantially constant and has, for instance, a value which gives the final pressure of compression in the power cylinder a value approximating 60 atmospheres.

Now, when the delivery pressure exceeds value p1, the change in the law of variation of the tension of spring 29 causes pm to decrease much more quickly, as shown by line a-d, the value of pm at point d of the curve being that existing in the accumulator air cushion when the delivery pressure is [7121. This reduction of the mean pressure of the pneumatic accumulator air cushion causes a substantial reduction of the final pressure of compression in the power cylinder and consequently a reduction of the number of oscillations of the moving systems per unit of time. The reduction of this final pressure of compression may be such that for a delivery pressure equal to the upper limit of the nominal delivery pressure pm, said final pressure is only 30 atmospheres.

ire-

When the same motor compressor must be capable of giving at several different nominal pressures, the above described regulating device can easily be adapted to these different nominal pressures. It suffices, for this purpose, to provide for fulcrum abutment 38 several positions (by insertion thereof in different holes of the machine frame) each of which holes corresponds to a given nominal pressure. Fig. 1 shows a position 382 of said fulcrum abutment for a nominal delivery pressure lower limit p2, with a nominal delivery pressure upper limit 1722,, and a position 383 for a nominal delivery pressure lower limit 113, with a nominal pressure upper limit P39.-

When fulcrum 38 is placed in position 382, fulcrum 37 remains operative for delivery pressures increasing up to pressure p2 (point b of the curve) and lever 36 comes to cooperate with fulcrum 382 only when the delivery pressure exceeds p2, a quick decrease of the mean pressure pm then taking place along sloped curve b-e. Thus, the final pressure of compression in the power cylinder is substantially reduced, for instance from 60 to 30 atmospheres.

Finally, when fulcrum 38 is placed in position 383, fulcrum 37 remains operative for values of pressure pm down to that corresponding to point 0 of the diagram of Fig. 3, for which the minimum nominal delivery pressure lower limit is p3, and when the delivery pressure increases beyond this value p3, up to p3a, the mean pressure Pm decreases at a quicker rate, along line cf of Fig. 3.

It will further be seen that the slopes of lines ad, b-e, c-f are the steeper as the nominal delivery pressure is higher.

In all cases, the quick decrease of the accumulator energy as soon as the delivery pressure exceeds the nominal value lower limit corresponding to the maximum power causes a great reduction of the number of oscillations of the moving systems per unit of time.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efficient embodiments of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

What I claim is:

1. In combination, a free piston motor compressor including at least one power cylinder and one compressor cylinder fixed with respect to each other, a movable system constituted by a power piston and a compressor piston rigid with each other and freely movable in said two cylinders, respectively, an energy accumulator having a portion thereof operatively connected with said movable system for accumulating energy from said movable system during the power stroke of said power piston and giving back said energy thereto during the return stroke thereof, a regulating device operatively connected to said accumulator operative in response to variations in the delivery pressure of said compressor for varying the energy stored up by said accumulator on every cycle of said movable system so as to reduce said energy when said delivery pressure increases, said device being adjustable to have two different laws of variation of said energy in response to variation of said delivery pressure, one for which the ratio of energy variation to delivery pressure variation is smaller than for the other, and means 6 responsive to variations in the dehvery pressure of the compressor for adjusting said regulating device to work according to the first of said two laws as long as said delivery pressure is below the lower limit of the motor compressor nominal delivery pressure and according to the second of said two laws when said delivery pressure is between the lower and upper limits of said nominal delivery pressure.

2. In combination, a free piston motor compressor including at least one power cylinder and one compressor cylinder fixed with respect to each other, a movable system constituted by a power piston and a compressor piston rigid with each other and freely movable in said two cylinders, respectively, a pneumatic energy accumulator having a portion thereof operatively connected with said movable system for accumulating energy from said movable system during the power stroke of said power piston and giving back said energy thereto during the return stroke thereof, a regulating device operatively connected to said accumulator operative in response to variations in the delivery pressure of said compressor for varying the mass of air present in said accumulator on every cycle of said movable system so as to reduce said mass of air when said deliverypressure increases, said device being adjustable to have two different laws of variation of said mass of air in response to variation of said delivery pressure, one for which the ratio of mass of air variation to delivery pressure variation is smaller than for the other, and means responsive to variations in the delivery pressure of the compressor for adjusting said regulating device to work according to the first of said two laws as long as said delivery pressure is below the lower limit of the motor compressor nominal delivery pressure and according to the second of said two laws when said delivery pressure is between the lower and upper limits of said nominal delivery pressure.

3. A combination according to claim 1 including a part movable in response to variations of the compressor delivery pressure, the means for operating said regulating device including a lever operatively connected at one end to said part and at the other end to said regulating device, and two fulcrum abutments for cooperating with said lever, said abutments being positioned to be both in contact with said lever when the delivery pressure is equal to the lower limit of the delivery pressure nominal value.

4. A combination according to claim 1 including a part movable in response to variations of the compressor delivery pressure, the means for operating said regulating device including a lever operatively connected at one end to said part and at the other end to said regulating device, and two fulcrum abutments for cooperating with said lever, said abutments being positioned to be both in contact with said lever when the delivery pressure is equal to the lower limit of the delivery pressure nominal value, one of said abutments being positioned to cause the lever to reduce the movements imparted thereto by said part when transmitting it to said regulating device and the other abutment being positioned to amplify these movements.

References Cited in the file of this patent UNITED STATES PATENTS 1,726,491 Johnson Aug. 27, 1929 2,434,877 Welsh et al Jan. 20, 1948 FOREIGN PATENTS 237,642 Great Britain 1925

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