FR3033006A1 - QUICK DISCHARGE PUMP - Google Patents

Abstract

The pump (10) comprises a chamber (28) delimited by a piston (26) movable in a body (24) comprising an inlet opening (30) capable of communicating with the chamber (28), non-return means ( 34) allowing the passage of fluid from the opening (30) to the chamber (28), and prohibiting the passage from the chamber (28) to the opening (30); an expulsion aperture (36) between the chamber (28) and a conduit (38), and anti-return means (40), allowing the passage of fluid from the chamber (28) to the conduit (38), and prohibiting passage from the conduit (38) to the chamber (28). The hollow body (24) has a discharge opening (48), communicating with the conduit (38) and able to communicate with the opening (30), and third non-return means (52), movable between an open position , wherein the conduit (38) communicates with the opening (30), and a closed position preventing the passage of fluid between the conduit (38) and the opening (30), the pump (10) having means (54) ) of moving the third non-return means (52).

Description

The present invention relates to a pump for increasing the pressure in an enclosed space, in particular for acting on a pressure-sensitive element, for example a pneumatic actuator. Already known in the state of the art, a pump comprising a hollow body and a piston, housed in the hollow body movably along a longitudinal direction, the piston delimiting with the hollow body a compression chamber of variable volume depending on a position of the piston in the longitudinal direction. The hollow body usually comprises: an inlet opening intended to communicate with a source of fluid and capable of communicating with the compression chamber; first non-return means arranged between the inlet opening and the compression chamber, allowing the passage of fluid from the inlet opening to the compression chamber, and prohibiting the passage of fluid from the compression chamber to the inlet opening, - an expulsion opening, opening into the compression chamber, and communicating with an expulsion duct, - second non-return means, arranged between the compression chamber and the expulsion duct, allowing the passage of fluid from the compression chamber to the duct; expulsion, and prohibiting the passage of fluid from the expulsion conduit to the compression chamber. As the piston moves in a first direction, the volume of the compression chamber increases, and fluid fills this compression chamber as it passes through the inlet opening. As the piston moves in a second direction opposite to the first, the volume of the compression chamber decreases, and the fluid is expelled through the expulsion aperture. The first and second non-return means allow the fluid to circulate in one direction only.

By applying to the piston oscillation movements, in the first direction and then in the second direction along the longitudinal direction, this piston expels a desired amount of fluid in the expulsion conduit. When this expulsion duct is connected to an enclosed space, the pressure in this confined space then increases. The invention particularly aims to improve such a pump, especially for applications where a fast return to the initial position may be desired.

For this purpose, the invention particularly relates to a pump comprising a hollow body and a piston, housed in the hollow body movably along a longitudinal direction, defining with the hollow body a volume compression chamber. variable according to a position of the piston in the longitudinal direction, the hollow body 5 comprising: - an inlet opening, intended to communicate with a source of fluid, and adapted to communicate with the compression chamber, - first means anti-return, arranged between the inlet opening and the compression chamber, allowing the passage of fluid from the inlet opening 10 to the compression chamber, and prohibiting the passage of fluid from the compression chamber to the pressure chamber; an intake opening; an expulsion opening opening into the compression chamber and communicating with an expulsion conduit; second non-return means arranged between the compression chamber; n and the expulsion duct, allowing the passage of fluid from the compression chamber to the expulsion duct, and prohibiting the passage of fluid from the expulsion duct to the compression chamber, characterized in that: the hollow body comprises a discharge opening, communicating with the expulsion duct and adapted to communicate with the inlet opening, and third non-return means, movable between an open position, in which the expulsion duct communicates with the inlet opening, and a closed position preventing the passage of fluid between the expulsion duct and the inlet opening, the pump comprises means for moving the third non-return means. By moving the third non-return means to the open position, the expulsion duct communicates with the inlet opening, so that the high pressure fluid contained in the expulsion duct is evacuated through the opening. admission, until the pressure in the expulsion duct is reduced to the pressure of the fluid source, for example the atmospheric pressure. It is thus possible to quickly discharge the pump, making its use possible for applications where such rapid discharge is desirable. A pump according to the invention may further comprise one or more of the following characteristics, taken alone or in any technically feasible combination. The displacement means comprise: a ferromagnetic element movable between a first position in which the third non-return means are in the open position and a second position in which the third non-return means are in the closed position, a resilient member requesting the ferromagnetic element to its first position, and an electric coil, surrounding the ferromagnetic element, electrically connected to electrical supply means adapted to apply an adjustable current, the position of the ferromagnetic element being a function of the voltage of the current. The third non-return means comprise a non-return valve biased in the closed position by a resilient member, and the displacement means comprise an actuating element integral with the piston intended to cooperate with the non-return valve to pass it. in the open position, by pushing it against the biasing of the elastic member, when the piston is in a predetermined discharge position, and said ferromagnetic element is formed by a rod integral with the piston. The piston is movable, in the longitudinal direction, between: a first end position, in which the actuating element cooperates with the non-return valve, and in which the volume of the compression chamber is maximum, a second extreme position, in which the volume of the compression chamber is minimal, and at least one intermediate position, in which the actuating element is kept at a distance from the non-return valve. - The third non-return means comprise a non-return valve, said ferromagnetic element being formed by the non-return valve. The piston comprises a rod of ferromagnetic material, in particular of soft iron, extending in the longitudinal direction, the pump comprising an electric coil surrounding the rod, electrically connected to electrical supply means capable of applying a variable electric current. , the position of the piston being a function of the voltage of the electric current. - The inlet opening has a filter. The pump comprises a fluid reservoir of variable volume, forming the source of fluid. - The fluid is gas, especially air. The invention also relates to an assembly of a pump and a controllable pressure-sensitive element controlled by the pump, characterized in that the pump is as defined above, the controllable element being connected to the expulsion conduit pump.

For example, the controllable element comprises a mobile membrane of an expansion vessel equipping a Rankine system, said Rankine system comprising: an evaporator, a steam engine arranged downstream of the evaporator, a condenser arranged downstream of the steam engine, - a pumping device arranged downstream of the condenser, and - the expansion tank arranged between the condenser and the pumping device, the expansion tank comprising: - an enclosure, 10 said membrane, mounted movably in the enclosure, separating the enclosure into a first part communicating with the condenser and the pumping device, and a second part communicating with the pump's expulsion duct. According to another example, the controllable element is a valve actuator, in particular for a motor vehicle exhaust system. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended figures in which: FIG. 1 is a diagrammatic sectional view of a set of a pump according to a first exemplary embodiment and a controllable element controlled by said pump, the pump having a piston shown in an intermediate position; FIG. 2 is a view similar to FIG. 1 of the pump of FIG. 1, in which the piston is represented in a first extreme position; - Figure 3 is a view similar to Figure 2 of the pump in which the piston 25 is shown in a second extreme position; - Figure 4 shows schematically an assembly of a pump according to a second exemplary embodiment and an element controlled by the pump; - Figure 5 is a view similar to Figure 4 of the pump according to Figure 4 in a discharge situation; Figure 6 schematically shows a pump according to a third exemplary embodiment of the invention; - Figure 7 shows an assembly of a pump according to the invention and a pump-controllable element equipping a Rankine system. FIG. 1 shows an assembly 8 comprising a pump 10 and a controllable element 12 responsive to the pressure, controlled by the pump 10.

In this example, the controllable element 12 is a pneumatic actuator for controlling an exhaust valve of a motor vehicle exhaust system. This controllable element 12 conventionally comprises a body 14 delimiting a chamber 16 in which is housed a sealed membrane 18 5 separating the chamber 16 in first 16A and second 16B compartments. The membrane 18 is displaceable in the chamber 16, so that the volume of the compartments 16A and 16B depends on the position of this membrane 18. The membrane 18 is connected to a rod 20, so that the displacement of the membrane 18 causes the displacement of the rod 20.

Finally, the actuator 12 comprises an elastic member 22, in particular a spring, applying a resilient biasing force, biasing the diaphragm 18 to a first position, shown in FIG. 1, in which the volume of the first compartment 16A is minimal or zero. , and the volume of the second compartment 16B is maximum. The membrane 18 is movable to a second position, wherein the volume of the first compartment 16A is maximum and the volume of the second compartment 16B is minimal or zero. The rod 20 is connected to said exhaust valve. The exhaust valve is movable between a closed position and an open position. More particularly, the exhaust valve is in the closed position when the diaphragm 18 is in said first position, and in the open position when the diaphragm 18 is in the second position. It will be noted that the membrane 18 may take various intermediate positions between these first and second positions, each of these intermediate positions corresponding to an intermediate position of the valve between the closed position and the open position.

In order to move the membrane 18, the pressure is varied in the first compartment 16A. Thus, by increasing this pressure, a pressure force is opposed to the resilient biasing force of the elastic member 22, which allows to move the membrane 18 when the pressure force is greater than the elastic force. Conversely, when the pressure decreases, the pressure force becomes less than the elastic force, so that the diaphragm 18 is returned to its first position by the elastic member 22. The pressure in the first compartment 16A is controlled by the pump 10. The pump 10 comprises a hollow body 24 and a piston 26 housed in the hollow body 24 movably along a longitudinal direction X. The hollow body 24 has 35 for example a generally cylindrical shape extending in the longitudinal direction X.

The piston 26 delimits with the hollow body 24 a compression chamber 28 of variable volume as a function of a position of the piston 26 in the longitudinal direction X. The piston 26 is movable in the longitudinal direction X between a first extreme position, 2, in which the volume of the compression chamber 28 is at a maximum, and a second end position, shown in FIG. 3, in which the volume of the compression chamber 28 is minimal. The piston can take any intermediate position between the first and second extreme positions, and in particular a so-called rest position, shown in Figure 1, which will be described later in more detail.

The hollow body 24 has an inlet opening 30 for communicating with a source of fluid and capable of communicating with the compression chamber 28. The source of fluid is, for example, the atmosphere surrounding the pump 10, in which case the fluid is air. In this case, the inlet opening 30 is advantageously equipped with an air filter to prevent any contamination of the hollow body 24 by particles and / or unwanted liquids. Alternatively, the source of fluid is formed by a reservoir connected to the inlet opening 30. Such a reservoir has a variable volume, so that the pressure remains constant in the reservoir when fluid is drawn in by the pump 10 In this case, the fluid may also be air, or alternatively oil or any other conceivable fluid. In the example described, the inlet opening 30 communicates with the compression chamber 28 through orifices 32 formed in the piston 26. First non-return means 34, arranged between the inlet opening 30 and the compression chamber 28, allow the passage of fluid from the inlet opening 30 to the compression chamber 28, and prohibit the passage of fluid from the compression chamber 28 to the inlet opening 30. In the As described above, these first non-return means 34 are formed by a non-return membrane capable of being detached from the piston 26 when fluid passes from the inlet opening 30 to the compression chamber 28, and to apply against the piston 26 so as to close the orifices 32 to prevent the passage of fluid from the compression chamber 28 to the inlet opening 30. The hollow body 24 also comprises an expulsion opening 36, opening into the chamber of compressi 28, and communicating with an expulsion conduit 38.

Second non-return means 40, in particular a conventional non-return valve, are arranged between the expulsion opening 36 and the expulsion duct 38, in order to allow the passage of fluid from the chamber. 28 when the piston 26 moves from the rest position 38, as shown in FIG. towards its second end position, shown in FIG. 3, the fluid contained in the compression chamber 28 is expelled through the expulsion opening 36, this expulsion being authorized by the second non-return means 40. this movement, the non-return diaphragm of the first non-return means 34 is applied against the through holes 32 of the piston 26, so that the fluid does not escape through these through-holes 32. When the piston 26 moves since his second extreme position shown in Figure 3 to its rest position shown in Figure 1, the volume of the compression chamber 28 increases, and its pressure decreases so that fluid is drawn into the compression chamber 28. Due to second non-return means 40, the fluid is not sucked from the expulsion conduit 38. On the other hand, the first non-return means 34 allow the passage of fluid through the through openings 32, so that the fluid is drawn from the fluid source, through the inlet opening 30 and the through openings 32. By repeating the movements of the piston 26 described above, fluid is progressively introduced into the expulsion conduit 38 As shown in FIG. 1, the expulsion duct 38 communicates with the first compartment 16A of the actuator 12, which is an enclosed space, so that the gradual introduction of fluid into the expulsion duct 38 aug The pressure in this expulsion duct 38 and in the first compartment 16A is moved to a desired pressure. This increase in pressure causes the displacement of the membrane 18, as indicated above. The piston 26 comprises a rod 42 of ferromagnetic material, in particular soft iron, extending in the longitudinal direction X in the hollow body 24. The pump 10 then comprises an electric coil 44, surrounding the rod 42, electrically connected to electrical supply means adapted to apply a variable current to the electric coil 44. Thus, the position of the piston 26 is a function of the voltage of this electric current. More particularly, an increase in electric current tends to drive the piston 26 to its second extreme position. The pump 10 further comprises an elastic member 46, applying a resilient force biasing the piston 26 to its first extreme position. Thus, the magnetic force induced by the electric current flowing in the coil 44 is opposed to the elastic force applied by the elastic member 46 to the piston 26. The greater the electric current, the greater the magnetic force, the piston 26 moving towards its second end position when the magnetic force is greater than the elastic force of the elastic member. On the other hand, by decreasing the electric current, the magnetic force decreases, and the piston 26 moves towards its first extreme position when this magnetic force is less than the elastic force applied by the elastic member 46. The piston 26 remains in its position. rest position, shown in Figure 1, when a relatively low holding current, for example 5 volts, is applied to the coil 44, to generate an opposite magnetic force and equal in value to the elastic force. The hollow body 24 of the pump 10 according to the invention comprises a discharge opening 48, communicating with the expulsion duct 38 and able to communicate with the inlet opening 30. For example, this discharge opening 48 communicates with the expulsion duct 38 via a bypass line 50.

Third non-return means 52 are arranged between this discharge opening 48 and this bypass line 50. These third non-return means are movable between an open position, shown in FIG. 2, in which the expulsion duct 38 communicates with the inlet opening 30, and a closed position preventing the passage of fluid between the expulsion duct 38 and the inlet opening 30. The pump 10 comprises means 54 for moving these third means against In the first example described, the third non-return means 52 comprise a non-return valve 52A urged in the closed position by an elastic member 52B. The displacement means 54 comprise an actuating element 56, in particular formed by a striker, integral with the piston 26, for example carried by the rod 42. This actuating element 56 is intended to cooperate with the non-return valve for move it to the open position, pushing it against the bias of the elastic member 52B, when the piston 26 is in the first extreme position. This first extreme position therefore forms a discharge position. It will be noted that the piston 26 passes into its first extreme position when the electric current flowing in the coil 44 is zero, in which case no magnetic force opposes the elastic force applied by the elastic member 46 to the piston 26. Discharge can thus be carried out in a simple and rapid manner by interrupting the electric current flowing in the coil 44.

FIG. 4 shows a pump 10 according to a second exemplary embodiment of the invention. In this FIG. 4, elements similar to those of the preceding figures are designated by identical references. In this example, the pump 10 is an oil pump. In this case, the actuator 12 is a jack, comprising a piston 58 movable in a body 60, separating said body 60 into a first compartment 62 communicating with the discharge duct 38 of the pump 10, and a second compartment 64 The cylinder 12 also comprises an elastic member 66 urging the piston 58 to a first position in which the first compartment 62 has a minimum or zero volume. The rod 20 is integral with the piston 58 on the one hand, and a valve element 68 on the other hand, so that this valve element 68 is displaceable according to the position of the piston 58, via In accordance with this second embodiment, the inlet opening 30 is connected to a reservoir 70 filled with oil, having a flexible pouch 72 of variable volume depending on the quantity of oil in this pocket. 72. The pump 10 comprises an intake pipe 74 extending between the inlet opening 30 and the compression chamber 28. The first non-return means 34 then comprise a non-return valve allowing the passage of fluid from the intake duct 74 to the compression chamber 28 and prohibiting the passage of fluid from the compression chamber 28 to the intake duct 74. In this embodiment, the piston 26 has no opening passage. However, as before, the volume of the compression chamber 28 depends on the position of the piston 26.

As before, the compression chamber 26 has an expulsion opening 36, opening into the compression chamber 28, and communicating with the expulsion conduit 38. The second non-return means 40 are arranged in this expulsion opening. 36, between the compression chamber 28 and the expulsion duct 38, to allow the passage of oil from the compression chamber 28 to the expulsion duct 38, and prohibit the passage of fluid from the duct expulsion 38 to the compression chamber 28. The operation of this pump is identical to that of the pump according to the first embodiment. In accordance with this second embodiment, the discharge opening 48 is formed in a discharge line 76 extending between the expulsion line 38 and the intake line 74. The third non-return means 52 are housed in this discharge pipe 76. The displacement means 54 then comprise a ferromagnetic element 78 movable between a first position in which the third non-return means 52 are in the open position and a second position in which the third anti-return means 52 are in the open position and a second position in which the third anti-return means back 52 are in closed position. These displacement means 54 also comprise an elastic member 80 biasing the ferromagnetic element 78 towards its first position, as well as a coil 82 surrounding the ferromagnetic element 78, electrically connected to electrical supply means suitable for applying a current 10 adjustable, the position of the ferromagnetic element being a function of the voltage of the current. More particularly, the third non-return means 52 comprise a non-return valve, said ferromagnetic element 78 being formed by this non-return valve. As shown in Fig. 5, when the current is interrupted in the coil 82, no magnetic force is applied to the check valve 78, so that it is only subjected to the elastic force. the resilient member 80, which drives it to its first position. In this position, the expulsion duct 38 communicates with the intake duct 74 so that the high-pressure oil contained in the expulsion duct 38 is discharged to the tank 70. Thus, a discharge of the pump can This is done in a very simple way by interrupting the current flowing in the electric coil 82. In contrast, in normal operation of the pump, a holding current flows continuously in the coil 82 to maintain the valve 78 in its closed position. The magnetic force thus applied to the valve 78, added to the pressure force of the oil contained in the expulsion duct 38, keeps the valve in the closed position against the elastic force applied by the elastic member. 80. FIG. 6 shows a pump 10 according to a third exemplary embodiment of the invention. In this FIG. 6, elements similar to those of the preceding figures are designated by identical references. According to this third embodiment, the pump 10 is an air pump. For example, the inlet opening 30 is connected to a tank 70, having a pocket 72 of variable volume, forming an air source. The air flows from the inlet opening 30 to the compression chamber 28 through at least one intake pipe 74, passing through non-return means 34.

When the volume of the compression chamber 28 decreases, the air is expelled to an expulsion line 38 through an expulsion opening 36 having the second non-return means 40. As in the second In one embodiment, the discharge opening 48 is arranged between the expulsion duct 38 and the intake duct 74, and comprises a valve 78 of ferromagnetic material, surrounded by an electric coil 82. The operation of this pump 10 according to the third embodiment is similar to the operation of the pump 10 according to the second embodiment described above. FIG. 7 shows a Rankine system 100 using a pump 10 according to the invention, for example according to any one of the embodiments described above. More particularly, the pump 10 is advantageously an air pump. The Rankine system 100 conventionally comprises an evaporator 102, a steam engine 104 arranged downstream of the evaporator 102, a condenser 106 arranged downstream of the steam engine 104, and a pumping device 108 arranged downstream of the steam engine 104. condenser 106, as well as an expansion tank 110 arranged between the condenser and the pumping device 108. The Rankine 100 system is intended to recover the heat of exhaust gas emitted by a gasoline or diesel internal combustion engine, by evaporating a working fluid, for example ethanol water, R134, R245 or any other fluid having the required characteristics, in the evaporator 102 generally arranged downstream of a pollution control system. The working fluid is then expanded in the steam engine 104, such as a piston engine, turbine, or any other means for converting pressurized gas at a certain temperature into mechanical work. The steam engine 104 thus provides a mechanical work, which may for example subsequently be converted into electricity. Once relaxed, the fluid is condensed in the condenser 106, which is formed by a heat exchanger with a cold source, for example the cooling water of the motor vehicle or the ambient air. Once returned to the liquid state, the fluid is pumped by the pumping device 108 to be reintroduced into the evaporator 102. The pressure obtained in this evaporator 102 is imposed by the steam engine 104. Such a Rankine system operates in closed loop, without loss of working fluid.

3033006 12 For the correct operation of the Rankine system, it is necessary to minimize external leakage and to prevent air entry. Some fluids, such as ethanol water, are completely liquid at the pressures and temperatures encountered when the vehicle is stationary (atmospheric pressures and temperatures). The air is absolutely forbidden in a closed system because it is compressible. Thus, if an air bubble was sucked by the pumping device 108, the flow would be unpredictable. As a result, when the vehicle is stationary and cold, all the fluid is in liquid form if the internal pressure of the system is at atmospheric pressure. On the other hand, if the volume dedicated to the working fluid is constant, the internal cold pressure would decrease until a pressure of the order of 0.1 bar corresponding to the saturation vapor pressure at around 20 to 40 ° C. vs. In other words, there will remain some of the working fluid in vapor form at very low pressure and ambient temperature. A vehicle such as a car is at a standstill for most of its life. Under these conditions, the pressure inside is so small relative to the atmospheric pressure that the risk of introducing a little air during these long exposures is important. To eliminate any risk of leakage, the system must be at atmospheric pressure during vehicle stops. If the system is at atmospheric pressure at shutdown without air inside, then the entire fluid circuit is in liquid form. When the engine is started, the exhaust gases heat the working fluid to turn it into steam. This vapor has a lower density than the corresponding liquid. The additional volume necessary for the formation of the steam must be provided by an additional element, which is the expansion vessel 110.

The expansion vessel 110 comprises an enclosure 112 and a membrane 114 mounted movably in the enclosure 112, separating the enclosure 112 in a first portion 116 communicating with the condenser 106 and the pumping device 108, and a second part 118, which communicates with the expulsion duct 38 of the pump 10. Thus, the pump 10 makes it possible to manage the pressure in this second part 118 of the enclosure 112. This second part 118 is for example equipped with a pressure sensor. , making it possible to know the pressure in this second part 118. Advantageously, the membrane 114 is metallic, in order to have a satisfactory seal, preventing the migration of gas particles through this membrane during the life of the vehicle.

It should be noted that the system is not generally completely watertight. It is therefore preferable to store in the expansion vessel 110 a buffer volume of working fluid which will decrease as a portion of the system fluid is lost. Such a buffer volume is sufficient to cover the leakage of the system which is about 200 cm3 for the life of the vehicle, which is generally 15 years and 5000 hours of use. In such a Rankine system, once the working fluid is expanded (at the exit of the steam engine 104), it is condensed in the condenser 106. The condenser is fed with a low temperature fluid (for example engine cooling water, or ambient air). The condensation pressure depends on the temperature of this low temperature fluid. For certain working fluids, (water, ethanol water mixture, acetone, toluene, for example), the expansion vessel 110 makes it possible to take into account the increase in volume of said working fluid due to its transition from the liquid state to the vapor state in a portion of the evaporator 102, in the steam engine 104 and in a portion of the condenser 106. The purpose of the association of the expansion vessel 110 with the condenser 106 is to ensure the lowest possible pressure at the output of the steam engine 104 while ensuring that the working fluid is completely liquid at the outlet of the condenser 106 and especially at the inlet of the high pressure pump 108, to ensure that the latter always pumps a liquid. It is therefore necessary to manage the pressure of the working fluid between the condenser 106 and the pump 108. For this, the temperature of the cold source (engine water, generally known at the injection computer, or the engine) is known. ambient air, also measured at the engine intake), as well as the flow of this cold source (the flow of water depends on the engine speed, and the air flow depends on the speed of the vehicle and / or the speed of the fan). This temperature and this flow rate make it possible to calculate the heat flow. It is therefore possible to know the temperature of the working fluid either by calculation or by measuring it directly. The geometric definition of the expansion vessel 110 makes it possible to know at any operating point of the cycle the resulting pressure of the working fluid. However in some cases it is necessary to correct this pressure to be sure of the state of working fluid. To adapt the pressure to the temperature of the cold source is applied in the portion 116 a pressure that is managed by the pump 10. Advantageously, the pressure in the first portion 116 is measured by said pressure sensor, which allows to apply the pressure necessary to ensure that the fluid is liquid at the outlet of the condenser 106, regardless of the temperature of the cold source.

It should be noted that the invention is not limited to the embodiment described, but could have various variants.

Claims (12)

REVENDICATIONS1. Pump (10) comprising a hollow body (24) and a piston (26), housed in the hollow body (24) movably along a longitudinal direction (X), delimiting with the hollow body (24) a chamber compression device (28) of variable volume depending on a position of the piston (26) in the longitudinal direction (X), the hollow body (24) comprising: - an inlet opening (30) intended to communicate with a fluid source (70), and adapted to communicate with the compression chamber (28), - first non-return means (34), arranged between the inlet opening (30) and the compression chamber (28) permitting the passage of fluid from the inlet opening (30) to the compression chamber (28) and preventing the passage of fluid from the compression chamber (28) to the inlet opening (30), an expulsion opening (36) opening into the compression chamber (28) and communicating with an expulsion conduit (38); etour (40), arranged between the compression chamber (28) and the expulsion conduit (38), allowing the passage of fluid from the compression chamber (28) to the expulsion conduit (38), and prohibiting the passage of fluid from the expulsion duct (38) to the compression chamber (28), characterized in that: - the hollow body (24) has a discharge opening (48) communicating with the expulsion duct ( 38) and adapted to communicate with the inlet opening (30), and third non-return means (52), movable between an open position, in which the expulsion conduit (38) communicates with the opening an inlet (30), and a closed position preventing the passage of fluid between the expulsion duct (38) and the inlet opening (30), the pump (10) comprises means (54) for displacing the third non-return means (52). 2. Pump (10) according to claim 1, wherein the displacement means (52) comprise: - a ferromagnetic element (42; 78) movable between a first position in which the third non-return means (52) are in position open and a second position in which the third non-return means (52) are in the closed position, - an elastic member (46; 80) biasing the ferromagnetic element (42; 78) to its first position, and 3033006 16 - a an electrical coil (44; 82) surrounding the ferromagnetic element (42; 78) electrically connected to electrical supply means adapted to apply an adjustable current, the position of the ferromagnetic element (42; 78) being a function of the voltage of the current. 5 3. Pump (10) according to claim 2, wherein: - the third non-return means (52) comprise a non-return valve (52A) biased in the closed position by an elastic member (52B), and the displacement means (54) comprise an actuating element (56) integral with the piston (26), intended to cooperate with the non-return valve (52A) to move it into the open position, pushing it against the solicitation of the elastic member (52B), when the piston (26) is in a predetermined discharge position, - said ferromagnetic element is formed by a rod (42) integral with the piston (26). 4. Pump according to claim 3, wherein the piston (26) is movable, in the longitudinal direction (X), between: - a first end position, in which the actuating element (56) cooperates with the valve nonreturn (52A), and wherein the volume of the compression chamber (28) is at a maximum, - a second end position, wherein the volume of the compression chamber (28) is minimal, and at least one intermediate position, in which the actuating element (56) is kept at a distance from the non-return valve (52A). 5. Pump (10) according to claim 2, wherein the third non-return means (52) comprise a non-return valve (78), said ferromagnetic element being formed by the non-return valve (78). 25 6. Pump (10) according to any one of claims 1 to 5, wherein the piston (26) comprises a rod (42) of ferromagnetic material, in particular soft iron, extending in the longitudinal direction (X), the pump (10) comprising an electric coil (44) surrounding the rod (42), electrically connected to electrical supply means adapted to apply a variable electric current, the position of the piston (26) being a function of the voltage of the Electric power. 7. Pump (10) according to any one of the preceding claims, wherein the inlet opening (30) comprises a filter. 8. Pump (10) according to any one of the preceding claims, comprising a reservoir (70) of fluid of variable volume, forming the source of fluid. 35 9. Pump (10) according to any one of the preceding claims, wherein the fluid is gas, especially air. 3033006 17 10. Assembly (8) of a pump (10) and a controllable element (12) responsive to the pressure controlled by the pump, characterized in that the pump (10) is according to any one of claims 1 to 9, the controllable element (12) being connected to the expulsion duct (38) of the pump (10). 5 11. The assembly (8) according to claim 10, wherein the controllable element (12) comprises a membrane (114) movable from an expansion tank (110) equipping a Rankine system (100), said Rankine system (100). ) comprising: - an evaporator (102), - a steam engine (104) arranged downstream of the evaporator (102), - a condenser (106) arranged downstream of the steam engine (104), - a pumping device (108) arranged downstream of the condenser (106), and - the expansion tank (110) arranged between the condenser and the pumping device, the expansion tank (110) comprising: - a chamber (112) ), Said diaphragm (114), mounted movably in the enclosure (112), separating the enclosure (112) into a first part (116) communicating with the condenser (106) and the pumping device (108), and a second portion (118) communicating with the expulsion conduit (38) of the pump (10). 12. The assembly of claim 10, wherein the controllable element is a valve actuator, in particular for a motor vehicle exhaust system.