EP1806548A1 - Air-conditioning installation containing a supercritical fluid - Google Patents

Abstract

The air conditioning system has a coolant circuit carrying a supercritical fluid and containing a compressor (14) with a control valve, a gas cooler (11), an expander (12) and an evaporator (13). It differs by incorporating a regulating module (40) that controls the compressor valve command signal during the system's starting phase so that the compressor's output pressure is maintained below the cut-off level. The regulating module calculates a current value for the command signal and compares it with a threshold value when the compressor is at minimal power.

Description

The invention relates to air conditioning installations traversed by a supercritical fluid, especially for a motor vehicle.

A supercritical fluid, for example carbon dioxide (R744) is a high pressure refrigerant. The use of these refrigerants has developed in the air conditioning systems of vehicles to limit the harmful effects on the environment of fluorinated compounds, conventionally used as a refrigerant.

An air-conditioning installation traversed by such a fluid comprises a fluid circuit mainly equipped with a compressor, a gas cooler, an expansion device, and an evaporator.

The installation is also equipped with an air conditioning regulator whose role is to control the operation of various components of the air conditioning circuit to provide a cooling capacity meeting the cold demands of users.

For example, the air conditioning regulator can control the temperature of the air blown out of the evaporator. The air conditioning regulator also controls the high pressure in order to adjust the opening opening of the expansion device so as to reach the cooling capacity required by the user.

However, during the start-up phase of the air conditioning, the air conditioning regulator of existing embodiments generally fixes the opening of the expansion device without taking into account the actual operating conditions.

Thus, when the air conditioning circuit is subjected to a very high thermal load, the regulator imposes a large opening of the expansion device, and therefore a high supercritical refrigerant flow rate, during the start-up phase of the air conditioning. On the other hand, if it imposes a weak opening of the expansion device, during the start-up phase, this can generate pressure peaks leading to a stop of the compressor and therefore to the shutdown of the air conditioning so that the thermal comfort of the passenger is not reached.

To remedy this drawback, the patent application FR 2,856,782 has proposed an air conditioning system, equipped with an expansion device, electronic expansion valve type, in which the initial opening degree of the regulator, at the start of the air conditioning, is calculated, from an estimate of the initial temperature fluid at the entrance of the pressure reducer, the initial pressure of the fluid at the outlet of the expander, and the initial flow rate of the fluid in the expander, and an estimation of the pressure of the fluid at the inlet of the expander which maximizes the coefficient of performance. The initialization of the opening of the electronic expander at the start of the air conditioning, is then adapted to the actual conditions of operation of the air conditioning and avoids an overpressure output of the regulator.

However, the installation proposed by the patent application FR 2,856,782 is not suitable when the expansion device is of the mechanical type. In conventional installations using this type of expansion device, a weak initial opening is generally imposed on the expansion device during the start-up period of the air conditioning. Under conditions of high thermal load, this initial opening is not suitable.

The invention improves the situation by proposing an air conditioning installation, in particular for a motor vehicle, comprising a refrigerant circuit traversed by a supercritical fluid, said circuit comprising a compressor provided with a control valve whose degree of The opening varies according to the intensity of a control signal, a gas cooler, an expansion device and an evaporator. The invention provides a regulation module able to control the control signal of the compressor valve during the start-up phase of the air conditioning so as to maintain the compressor discharge pressure substantially below the compressor cut-off pressure.

• Le module de régulation est apte à calculer une valeur courante du signal de commande, à un instant donné, et à comparer ladite valeur à un seuil supérieur du signal de commande et à un seuil inférieur du signal de commande, le seuil inférieur correspondant à la valeur prise par le signal de commande lorsque le compresseur est en cylindrée minimale.
• En présence d'une valeur courante du signal de commande supérieure au seuil supérieur du signal de commande, le module de régulation est propre à remplacer la valeur courante du signal de commande par le seuil supérieur du signal de commande.
• En présence d'une valeur courante du signal de commande inférieure au seuil inférieur du signal de commande, le module de régulation est propre à remplacer la valeur courante du signal de commande par ledit seuil inférieur du signal de commande.
• L'installation est apte à appliquer la valeur courante du signal de commande obtenue à la vanne de contrôle du compresseur.
• Le module de régulation est en outre apte à calculer la différence entre la pression de décharge du compresseur et la pression de coupure du compresseur, et à comparer ladite condition à une constante prédéfinie.
• En présence d'une différence de pression inférieure à ladite constante, la valeur courante du signal de commande est calculée en fonction de l'écart entre la mesure de la température d'évaporation et une consigne de température d'évaporation.
• En présence d'une différence de pression supérieure à ladite constante, la valeur courante du signal de commande est calculée à partir de la différence de pression et d'une valeur du signal de commande déterminé à un instant précédent.
• Le module de régulation est apte à déterminer la valeur courante d'une grandeur liée au signal de commande à l'instant donné considéré de sorte que cette grandeur ait une vitesse de variation progressive entre une borne inférieure et une borne supérieure, dans un intervalle de temps de longueur définie par une constante de temps.
• La grandeur liée au signal de commande est le seuil supérieur du signal de commande, et la vitesse de variation de cette grandeur est croissante dans l'intervalle de temps entre la borne inférieure définie par la valeur minimale du signal de commande lorsque le compresseur est en cylindrée minimale, et la borne supérieure définie par la valeur maximale du signal de commande lorsque le compresseur est en cylindrée maximale.
• La grandeur liée au signal de commande est la consigne de température d'évaporation, et la vitesse de variation de cette grandeur est décroissante dans l'intervalle de temps entre la borne supérieure définie par la consigne de température maximale de la climatisation et la borne inférieure définie par la consigne de température demandée dans l'habitacle.
• Le seuil supérieur du signal de commande correspond à la valeur maximale du signal de commande lorsque le compresseur est en cylindrée maximale.
• La grandeur liée au signal de commande est la consigne de température d'évaporation et en ce que le seuil supérieur du signal de commande correspond à la valeur maximale du signal de commande lorsque le compresseur est en cylindrée maximale.
• La grandeur est calculée avec un filtre de premier ordre à partir de la borne minimale, de la borne supérieure, de la constante de temps, de la valeur précédente de ladite grandeur et d'une période d'échantillonnage de durée choisie.
• La constante de temps est calculée à partir de la vitesse de rotation du compresseur, de la température extérieure à l'habitable du véhicule et de la pression de décharge du compresseur.
• Le module de régulation est apte à contrôler du signal de commande selon une période choisie.
• La période choisie est sensiblement égale à seconde.

Optional features of the air conditioning system of the invention, complementary or substitution, are set out below:

• The regulation module is able to calculate a current value of the control signal, at a given instant, and to compare said value with an upper threshold of the control signal and with a lower threshold of the control signal, the lower threshold corresponding to the value taken by the control signal when the compressor is in minimum displacement.
• In the presence of a current value of the control signal greater than the upper threshold of the control signal, the control module is able to replace the current value of the control signal with the upper threshold of the control signal.
• In the presence of a current value of the control signal lower than the lower threshold of the control signal, the regulation module is able to replace the current value of the control signal by said lower threshold of the control signal.
• The installation is able to apply the current value of the control signal obtained to the control valve of the compressor.
• The regulation module is furthermore able to calculate the difference between the compressor discharge pressure and the compressor cut-off pressure, and to compare said condition with a predefined constant.
• In the presence of a pressure difference smaller than said constant, the current value of the control signal is calculated as a function of the difference between the measurement of the evaporation temperature and an evaporation temperature setpoint.
• In the presence of a pressure difference greater than said constant, the current value of the control signal is calculated from the pressure difference and a value of the control signal determined at a previous time.
• The regulation module is able to determine the current value of a quantity linked to the control signal at the given instant considered so that this quantity has a progressive variation speed between a lower bound and an upper bound, in a range of length time defined by a time constant.
• The magnitude related to the control signal is the upper threshold of the control signal, and the rate of change of this magnitude is increasing in the time interval between the lower limit defined by the minimum value of the control signal when the compressor is in operation. minimum displacement, and the upper limit defined by the maximum value of the control signal when the compressor is in maximum capacity.
• The magnitude related to the control signal is the evaporation temperature setpoint, and the rate of variation of this quantity is decreasing in the time interval between the upper limit defined by the maximum temperature setpoint of the air conditioning and the lower limit. defined by the temperature setpoint required in the passenger compartment.
• The upper threshold of the control signal corresponds to the maximum value of the control signal when the compressor is in maximum capacity.
• The magnitude related to the control signal is the evaporating temperature set point and in that the upper threshold of the control signal corresponds to the maximum value of the control signal when the compressor is in maximum capacity.
• The magnitude is calculated with a first-order filter from the minimum bound, the upper bound, the time constant, the previous value of the said quantity, and a sampling period of the chosen duration.
• The time constant is calculated from the rotational speed of the compressor, the outside temperature of the vehicle and the discharge pressure of the compressor.
• The regulation module is able to control the control signal according to a chosen period.
• The chosen period is substantially equal to second.

The invention also proposes a method of regulating an air conditioning circuit traversed by a supercritical fluid, particularly for a motor vehicle, comprising a compressor provided with a control valve whose degree of opening varies according to the intensity of a control signal. The invention provides control of the control signal of the compressor valve during the start-up phase of the air conditioning so as to maintain the compressor discharge pressure substantially below the compressor cut-off pressure.

• la figure 1 est un schéma d'une installation de climatisation fonctionnant selon un cycle supercritique ;
• la figure 2 est un schéma d'une installation de climatisation de véhicule automobile à moteur munie d'un dispositif de contrôle de démarrage, selon l'invention ;
• la figure 3 est un organigramme illustrant les différentes étapes mises en oeuvre pour réguler le signal de commande du compresseur pendant la phase de démarrage, selon un premier mode de réalisation de l'invention ;
• la figure 4 est un organigramme illustrant les différentes étapes mises en oeuvre pour réguler le signal de commande du compresseur pendant la phase de démarrage, selon un deuxième mode de réalisation de l'invention ; et
• la figure 5 est un diagramme représentant l'évolution de la pression de décharge du compresseur, de la pression d'aspiration du compresseur et du signal de commande du compresseur en fonction du temps, selon l'invention.

Other features and advantages of the invention will appear on examining the detailed description below, and the attached drawings in which:

• Figure 1 is a diagram of an air conditioning system operating in a supercritical cycle;
• Figure 2 is a diagram of a motor vehicle air conditioning system provided with a starting control device, according to the invention;
• FIG. 3 is a flowchart illustrating the various steps implemented to regulate the compressor control signal during the start-up phase, according to a first embodiment of the invention;
• FIG. 4 is a flowchart illustrating the various steps implemented to regulate the compressor control signal during the start-up phase, according to a second embodiment of the invention; and
• FIG. 5 is a diagram showing the evolution of the compressor discharge pressure, the compressor suction pressure and the compressor control signal as a function of time, according to the invention.

Referring first to Figure 1 which shows a diagram of an air conditioning circuit 10 to be integrated with a motor vehicle.

The air conditioning circuit is flown by a high pressure refrigerant, including carbon dioxide R744.

The circuit comprises an externally controlled compressor 14 provided with a variable aperture control valve 140 as a function of a control signal. The compressor is adapted to receive the fluid in the gaseous state and to compress it. The circuit is furthermore equipped with a gas cooler 11 which cools the gas compressed by the compressor, at a substantially constant pressure, with an internal heat exchanger 9, an expansion device 12, in particular a mechanical expansion device. which lowers the pressure of the fluid from the internal exchanger 9, and an evaporator 13 which moves the fluid from the liquid state to the gaseous state, at a substantially constant pressure, to produce a flow of air conditioning sent to the passenger compartment of the vehicle.

The gas cooler 11 receives a stream of air 16 which under certain operating conditions is set in motion by a motor-fan unit 15, to evacuate the heat taken from the refrigerant.

The evaporator 13 receives an air flow 18 from a blower 20 and produces a flow of conditioned air sent to the cabin.

The internal heat exchanger 9 allows a heat exchange between the portion of the fluid flowing from the gas cooler 11 to the expander 12 and the portion of the fluid flowing from the evaporator 13 to the compressor 14.

The expansion device is in particular of the mechanical type.

Figure 2 is a diagram showing an air conditioning system according to the invention, intended to equip a motor vehicle.

The installation is provided with the air conditioning circuit 10 described with reference to FIG.

The installation is furthermore equipped with a regulation module or an air-conditioning computer 40 comprising an electronic card 43, a cockpit regulator 41 and an air conditioning loop regulator 42. The cockpit regulator 41 sets the instruction of the evaporation temperature Te _cons of the regulator 42.

In the installations of the prior art, when the cockpit regulator 41 supplies an evaporation temperature setpoint to the regulator of the air conditioning loop 42, the control signal of the compressor, during the start-up phase, is calculated according to a regulation law that uses the difference between the measurement of the evaporator temperature of the compressor and the setpoint of the evaporator temperature of the compressor. The measurement of the evaporation temperature can be provided by a temperature probe 130 placed behind the evaporator 13, in its overheating zone, or in the flow of air passing through the evaporator. The evaporating temperature setpoint represents the target temperature requested in the passenger compartment by a passenger of the vehicle.

This regulation law is conventionally used in all the operating cycles of the compressor. During the start-up period of the air conditioning, under the conditions of high thermal load, the difference between the measurement of the evaporator temperature of the compressor and the set point of the evaporation temperature is very important. Therefore, the control signal imposed on the compressor generates a large opening of the compressor control valve. When the expansion device is of mechanical type, this results in a very high discharge pressure at the compressor outlet, which may have pressure peaks. These pressure peaks can cause a stop of the air conditioning by setting high pressure cutoff of the compressor.

To improve the operation of the air conditioning system during the start-up phase, especially when the expansion device 12 is of mechanical type, the Applicant proposes to regulate the control signal of the compressor.

The Applicant proposes a climate control module during the start-up phase of the air conditioning, suitable for limiting the pressure peaks and the compressor stops. For this, the regulation module 40 is adapted to control the control signal of the compressor valve during the starting phase of the air conditioning so as to maintain the compressor discharge pressure substantially below the compressor cut-off pressure.

The principle of regulation proposed here is based on management of the start-up phase of the air conditioning by means of a control of the discharge pressure or high pressure of the compressor and a progressive regulation of the control signal of the compressor, depending operating parameters of the air conditioning loop.

The regulation module implements the regulation of the air conditioning during the start-up phase according to a chosen period, for example of 1 second.

To limit the occurrence of pressure peaks, the regulation module 40 calculates a current value of the PWM control signal (k), at a given time, and compares this value with an upper threshold of the PWM control signal _sup and a lower threshold PWM control signal _inf . The control module then adjusts the calculated value of the control signal if it exceeds the aforementioned thresholds.

The regulator module further acts on a magnitude related to the PWM control signal to control the rate of change of the signal itself and prevent a sudden change that could generate a peak pressure.

In a first embodiment illustrated in Figure 3, the magnitude associated with the control signal is the upper limit of the PWM control signal _greater than the PWM control signal must not exceed.

In a second embodiment illustrated in FIG. 4, the quantity linked to the control signal is the setpoint of evaporation temperature Te _cons which is used to regulate the compressor, according to the conventional closed loop control law. This quantity has an influence on the value of the PWM control signal.

The regulation module determines the current value of this quantity linked to the control signal at the instant in question, that is to say PWM _sup (k) or Te _cons (k) as the case may be, so that this quantity has a rate of progressive variation between a lower bound and an upper bound, in a time interval of length defined by a time constant KPWM or K _Te respectively. As these quantities influence the evolution of the PWM control signal over time, their control makes it possible to act on it and to prevent it from having sudden variations that could generate pressure peaks.

The time constants KPWM for the first embodiment or K _Te for the second embodiment are determined according to the operating parameters of the air conditioning circuit.

The magnitude related to the PWM control signal _sup (k) or Te _cons (k) can be calculated with a first-order filter from the lower bound, the upper bound, the time constant, and the value of the quantity calculated at the previous iteration of PWM control _sup (k-1) or Te _cons (k-1).

The notations k and k-1, or k and k + 1, are used to designate two successive iteration instants of the regulation, thus distant from a duration equal to the period of time. Throughout the description, the term "precedent" or the expression "previous value" will be used to designate the value of a magnitude at time k-1, the term "current" or the expression "current value" will be used to denote the value of a magnitude at time k, and the term "next" or the expression "next value" will be used to denote the value of a magnitude at time k + 1.

The controller module is used to control the evolution of the compressor control signal for limiting the occurrence of pressure peaks and thus maintain the high pressure of the compressor substantially below the HP compressor _stop cutoff pressure during the phase of start-up.

In addition, the control module is also adapted to regulate the high pressure in cases where the high pressure of the compressor approaches the compressor cut-off pressure during the start-up phase. For this, the regulation module 40 adjusts the value of the control signal in a chosen relation related to the difference between the high pressure of the compressor HP (k) and the HP cutoff pressure _stop . By thus regulating the PWM control signal, the regulation module 40 makes it possible to prevent the breaking pressure to be exceeded and thus to stop the air conditioning.

The control method of the start phase of Figure 3 will first be described in detail.

The method is implemented at the start of the air conditioning and is repeated at each moment k, as the start phase is not completed, as indicated in the test of step 3.

If the start-up phase is completed, a conventional control law of the compressor, which provides the control signal of the compressor as a function of the evaporation temperature, in particular as a function of the difference between the measurement and the temperature setpoint. of evaporation, is implemented in step 304. This control law can be for example a proportional integral control derivative PID.

If the startup phase is not completed, the following steps are implemented.

In step 300, it is determined whether the difference between the high pressure of the compressor HP (k) at the instant k considered and a _stop HP _stop pressure is less than a constant C, for example equal to 5 bar. The HP _shut- off pressure is the value of the high pressure that causes the compressor to shut down. The high pressure of the HP compressor (k) at time k can be provided by a sensor 142 or estimated. The verification of the condition of step 300 makes it possible to detect that the high pressure is approaching the cut-off pressure, and therefore a risk of stopping the compressor.

In step 304, if this pressure difference {HP (k) - HP _off } is greater than the constant C prefixed, the control signal of the compressor PWM (k), at instant k, is calculated according to the law conventional control according to the evaporation temperature Te, for example as a function of the difference between the measurement of the evaporation temperature Te _mes and the setpoint of the evaporation temperature Te _cons .

On the other hand, if the pressure difference {HP (k) - HP _off } is less than the constant C, the control module calculates the variation of the control signal) PWM (k) at time k, at step 302, depending on the difference HP (k) - HP _off , according to equation A1 of Appendix A.

Then, in step 306, the regulation module calculates the PWM control signal (k) of the compressor valve 140 at time k as a function of the value of the PWM control signal (k-1) at the previous time k-1 and the variation of the control signal) PWM (k) determined in step 302. At the start of the air-conditioning (first iteration of the start-up control), the value of the control signal PWM (k- 1) can be taken equal to the last value of the control signal before the start mode. The value of the PWM control signal (k) thus calculated makes it possible to maintain the discharge pressure of the compressor around the cutoff pressure. Steps 300 to 306 are implemented to avoid stopping the compressor by acting on the PWM control signal as soon as a risk of exceeding the cut-off pressure is detected while maintaining a high pressure level to cool as quickly as possible. possible vehicle interior, that is to say, comply with the instruction requested by the user.

At the end of steps 304 and 306, the detent module determines the current value of the upper threshold PWM _sup (k) that the control signal PWM (k) must not exceed in steps 303 and 308.

The current value of the upper threshold of the PWM control signal _sup (k) is calculated so that this quantity has a progressive rate of change between a lower bound and an upper bound, in a time interval of length defined by a time constant. KPWM. The pace of the rate of variation is in particular increasing from the lower bound to the upper bound, as illustrated in the diagram of step 303.

The upper limit PWM _max is represented by the maximum value of the control signal. This maximum value corresponds to the control signal of the compressor, when it is in maximum capacity. It can be for example equal to 90%.

The lower limit is represented by the minimum value of the control signal PWM _min . This minimum value corresponds to the value of the control signal of the compressor, when it is in minimum displacement. It can be equal for example to 20%.

• la borne supérieure PWM_max,
• la borne inférieure PWM_min, et
• la constante de temps K_PWM.

The current value of the upper threshold of the PWM control signal _sup (k) can be calculated for example using a first-order filter, from:

• the upper terminal PWM _max ,
• the lower limit PWM _min , and
• the time constant K _PWM .

The K _PWM time constant may vary during the start-up period. it represents the time taken by the control signal to go from the lower terminal PWM _min to the upper terminal PWM _max .

Determining the upper threshold PWM _sup (k) also uses the value of the upper threshold PWM _sup (k-1) determined at the previous time k-1. This value is normally stored in memory.

The time constant of the filter K _PWM can be calculated at each iteration of the control process from the discharge pressure of the compressor HP, the outside temperature Text and the rotation speed N of the compressor.

In the example of a first order filter, the upper threshold value PWM _sup (k) is obtained according to the equation A2, where T _ech corresponds to the sampling period of the filter.

The evolution of the threshold of the control signal is thus regulated during the time interval. This regulation makes it possible to avoid abrupt variations of the control signal during the start-up phase, and thus to limit pressure peaks.

In step 310, the control module determines whether the value of the control signal PWM (k) obtained in step 304 or in step 306 is between the current value of the upper threshold PWM _sup (k), obtained in step 303, and a lower threshold represented by the minimum value PWM _min of the control signal.

In step 312, if it is determined that the value of the PWM signal (k) determined in step 304 or step 306 exceeds the upper threshold value PWM _sup (k) obtained in step 303, the value of the upper threshold PWM _sup (k) is assigned to the signal PWM (k). However, when the value of the PWM signal (k) determined in step 304 or step 306 is less than the minimum value of the PWM control signal _min , this minimum value PWM _min is assigned to the PWM signal (k). In other cases, the value of the PWM signal (k) is not changed.

In step 314, the control signal PWM (k) obtained in step 312 is applied to the control valve 140 of the compressor. Steps 300 to 314 are then reiterated at the next time k + 1, if it is determined that the startup phase is not completed (step 3).

The climate control module, according to the first embodiment of the invention, thus makes it possible to control the speed of change of the PWM control signal in time during the start-up period. The invention provides a rate of change of the progressive control signal during this phase by means of a regulation of the upper threshold of the control signal and the level of the control signal. In addition, it makes it possible to detect a risk of stopping the compressor by controlling the level of the high pressure and to avoid the actual stopping of the compressor by acting on the control signal of the compressor. The regulation of the invention thus reduces the risk of pressure peaks and compressor interruptions that result during the startup phase of the air conditioning.

FIG. 5 is a diagram illustrating the evolution over time of the discharge pressure of the compressor (curve a), the suction pressure of the compressor (curve b) and the control signal of the compressor PWM (curve c), during the phase of starting, according to the first embodiment of the invention. The starting phase control method is applied between about 460 seconds and 550 seconds. The start-up phase ends at around 550 seconds. As can be seen in this diagram, a progressive increase is imposed on the PWM control signal (curve c) to avoid compressor cuts. In particular, it is observed that the regulation is down the control signal between 470 seconds and 480 seconds to prevent the high pressure (curve a) exceeds the HP _stop cutoff pressure, for example between 130 and 140 bars (13 to 14 Mpa), and thus a compressor stop. Curve c shows a second effect of regulation during the start-up phase. Indeed, between 510 and 530 seconds, the increase of the PWM signal sent to the compressor valve is limited to about 80% so as to counteract the increase in pressure pressure visible on the curve at 510 seconds. The high pressure (curve a) does not have any pressure peaks likely to exceed the cutoff pressure.

The second embodiment of the invention will now be described with reference to the flowchart of FIG. 4.

Steps 4 and 400 are similar to steps 3 and 300 respectively of Figure 3. These steps will not be described again here.

In step 402, if the pressure difference {HP (k) - HP _off } is less than the constant C prefixed, the control signal of the compressor PWM (k), at time k, is calculated according to the relation A1, similarly to step 306 of FIG.

In step 404, if the pressure difference {HP (k) - HP _off } is greater than the constant C prefixed, the control signal of the compressor PWM (k), at instant k, is calculated according to the law conventional control according to the difference between the measurement of the evaporation temperature Te _mes and a predetermined set of evaporation temperature Te _cons (k) at time k. This conventional regulation law can be for example a derivative integral proportional regulation.

Step 403 makes it possible to determine this instruction. The goal is to impose a gradual decrease of this setpoint over time, for example using a first order filter using a variable time constant K _Te .

The current value of the evaporation temperature setpoint Te _cons (k) is calculated so that this quantity has a progressive rate of variation between an upper bound and a lower bound, in a time interval of length defined by a constant of time K _Te . The pace of the speed of variation is particularly decreasing from the lower bound to the upper bound, as shown in the diagram of step 303.

The upper limit is represented by the maximum temperature setpoint Te _{cons_max} of operation of the air conditioning. It can be for example equal to 15 ° C.

The lower limit is represented by the target temperature target Te _{cons_min} requested in the cockpit by a passenger. It is generally between 2 ° C and 5 ° c.

• la borne supérieure Te_{cons_max,}
• la borne inférieure Te_{cons_min}, et
• la constante de temps K_Te.

The current value of the evaporation temperature set point Te _cons (k) can be calculated for example using a first order filter, from:

• the upper terminal Te _{cons_max,}
• the lower limit Te _{cons_min} , and
• the time constant K _Te .

The time constant K _Te may vary during the start-up period. It represents the time taken by the control signal to go from the upper limit Te _{cons_max} to the lower limit Te _{cons_min} .

The determination of the current value of the evaporation temperature set point Te _cons (k) also uses the value of the upper threshold Te _cons (k-1) determined at the instant k-1 above. This value is normally stored in memory.

The time constant of the filter K _Te can be calculated at each iteration of the control method from the discharge pressure of the compressor HP, the outside temperature Text and the rotation speed N of the compressor.

In the example of a first-order filter, the value of the evaporation temperature setpoint Te _cons (k) is obtained according to the equation A3, where T _ech corresponds to the sampling period of the filter.

The evolution of the evaporation temperature set point is thus regulated during the time interval. This regulation makes it possible to avoid sudden variations of the PWM control signal during the start-up phase, and thus to limit the peaks of pressure.

In step 410, the control module determines whether the value of the PWM control signal (k) obtained in step 404 or in step 406 is between the upper threshold of the control signal and the lower threshold of the signal control. In this mode of realization, the upper threshold of the PWM _max control signal represents the value of the control signal when the compressor is maximum displacement, and the lower threshold of the PWM control signal _min represents the value of the control signal when the compressor is minimum displacement. These thresholds PWM _max and PWM _min are therefore constant in this embodiment. These thresholds are determined at step 408.

In step 412, if it is determined that the value of the PWM signal (k) determined in step 404 or step 406 exceeds the maximum PWM _max value, the PWM _max value is assigned to the PWM signal (k ). Otherwise, if the value of the PWM signal (k) determined in step 304 or step 306 is smaller than the minimum value of the PWM _min control signal, the PWM _min value is assigned to the PWM signal (k). In other cases, the value of the PWM signal (k) is not changed.

In step 414, the control signal PWM (k) obtained in step 412 is applied to the control valve 140 of the compressor. Steps 400 to 414 are then reiterated at the next time k + 1, if it is determined that the startup phase is not completed (step 4).

By regulating the speed of variation of the evaporation temperature set point, for example by means of the filter, in step 403, the second embodiment makes it possible to impose a progressive increase in the control signal of the PWM compressor, without abrupt variation. The high pressure of the HP compressor is also kept below the HP _shutdown pressure of the compressor during the start-up period, which avoids pressure peaks and repetitive compressor shutdowns.

The regulation module proposed by the invention is therefore particularly suitable for controlling the start-up phase of an air conditioning circuit traversed by a supercritical fluid, even under the conditions of high thermal loads. Indeed, the use of supercritical type fluid such as CO2 requires operation at sometimes very high pressures. Obviously, the higher the pressures, the more difficult it is to maintain a good seal. Therefore, it is particularly desirable, as proposed by the invention, to avoid seeing the high pressure exceed the cutoff threshold of the compressor while achieving the goal of thermal comfort as soon as possible requested by the user of the air conditioning system.

Although the invention applies to any type of expansion device, it is particularly advantageous when the expansion device is of the mechanical type. The invention relates to the air conditioning installation using the start-up phase according to the invention, but it also relates to the method for implementing this start-up phase.

Annex A

• A1: $$\mathrm{\Delta PWM}\left(\mathrm{k}\right)\mathrm{=}\mathrm{at}{\left(\mathrm{HP}\left(\mathrm{k}\right)\mathrm{-}{\mathrm{<figure-callout\; id="2"\; label="HP\; stop"\; filenames="imgf0005.png"\; state="\{\{state\}\}">HP\; </figure-callout>}}_{\mathrm{stop}}\right)}^{\mathrm{2}}\mathrm{+}\mathrm{b}\mathrm{(}\mathrm{HP}\left(\mathrm{k}\right)\mathrm{-}{\mathrm{HP}}_{\mathrm{stop}}\mathrm{)}\mathrm{+}\mathrm{vs}\mathrm{1}$$
  
• A2: $${\mathrm{PWM}}_{\sup }\left(k\right)=\frac{\frac{K_{{\mathit{PWM}}_{\max }}}{T_{\mathit{ech}}}*{\mathit{PWM}}_{\sup }\left(k-1\right)}{\left(1+\frac{K_{{\mathit{PWM}}_{\max }}}{T_{\mathit{ech}}}\right)}+\frac{1}{1+\frac{K_{{\mathit{PWM}}_{\max }}}{T_{\mathit{ech}}}}*\left({\mathit{PWM}}_{\max }-{\mathit{PWM}}_{\min }\right)$$
  
• A3: $$T_{\mathit{cons}}\left(k\right)=\frac{\frac{K_{\mathit{You}}}{T_{\mathit{ech}}}*T_{\mathit{cons}}\left(k-1\right)}{\left(1+\frac{K_{\mathit{You}}}{T_{\mathit{ech}}}\right)}+\frac{1}{1+\frac{K_{\mathit{You}}}{T_{\mathit{ech}}}}*\left({\mathit{You}}_{{\mathit{cons}}_{-}\max }-{\mathit{You}}_{{\mathit{cons}}_{-}\min }\right)$$
  

Claims (17)

Air conditioning system, in particular for a motor vehicle, comprising a refrigerant circuit traversed by a supercritical fluid, said circuit comprising a compressor (14) provided with a control valve (140) whose degree of opening varies according to the intensity of a control signal, a gas cooler (11), an expansion device (12) and an evaporator (13), characterized in that it further comprises a regulation module (40) adapted to controlling the control signal of the compressor valve during the start-up phase of the air conditioning so as to maintain the compressor discharge pressure substantially below a compressor cut-off pressure. Installation according to Claim 1, characterized in that the regulation module is able to calculate a current value of the control signal (306, 304) according to a chosen relation, at a given time, and to compare (310) said value with a upper threshold of the control signal and a lower threshold of the control signal, the lower threshold corresponding to the value taken by the control signal when the compressor is in minimum displacement. Installation according to Claim 2, characterized in that, in the presence of a current value of the control signal greater than the upper threshold of the control signal, the control module is able to replace the current value of the control signal with the upper threshold. the control signal (312). Installation according to one of claims 2 and 3, characterized in that in the presence of a current value of the control signal lower than the lower threshold of the control signal, the control module is able to replace the current value of the signal of control by said lower threshold of the control signal (312). Installation according to one of claims 2 to 4, characterized in that the installation is adapted to apply the current value of the control signal obtained to the control valve of the compressor (140). Installation according to one of Claims 2 to 5, characterized in that the regulation module is furthermore able to calculate the difference between the compressor discharge pressure and the compressor cut-off pressure, and to compare the said condition with a constant predefined (300). Installation according to claim 6, characterized in that in the presence of a pressure difference smaller than said constant, the current value of the control signal is calculated as a function of the difference between the measurement of the evaporation temperature and a evaporating temperature setpoint (302). Installation according to one of claims 6 and 7, characterized in that in the presence of a pressure difference greater than said constant, the current value of the control signal is calculated from the pressure difference and a value of the control signal determined at a previous instant (306). Installation according to one of Claims 2 to 8, characterized in that the control module is adapted to determine the current value of a quantity related to the control signal at the given instant under consideration (303, 403) so that this magnitude has a gradual rate of change between a lower bound and an upper bound in a time interval of length defined by a time constant. An installation according to claim 9, characterized in that the magnitude related to the control signal is the upper threshold of the control signal (303), and in that the rate of change of this magnitude is increasing in the time interval between the lower limit defined by the minimum value of the control signal when the compressor is in minimum capacity, and the upper limit defined by the maximum value of the control signal when the compressor is in maximum capacity. Installation according to Claim 9, characterized in that the quantity linked to the control signal is the evaporating temperature set point, and in that the rate of variation of this quantity is decreasing in the time interval between the defined upper limit. by the maximum temperature regulation of the air conditioning and the lower limit defined by the temperature setpoint required in the passenger compartment. Installation according to Claim 11, characterized in that the upper threshold of the control signal corresponds to the maximum value of the control signal when the compressor is in maximum displacement (403). Installation according to one of claims 9 to 12, characterized in that said quantity is calculated with a first-order filter from the minimum limit, the upper limit, the time constant, of the previous value of said magnitude and a sampling period of chosen duration. Installation according to one of claims 9 to 13, characterized in that the time constant is calculated from the compressor rotational speed, the temperature outside the space of the vehicle and the compressor discharge pressure. Installation according to one of claims 2 to 14, characterized in that the regulation module is able to control the control signal according to a chosen period. Installation according to claim 15, characterized in that the period chosen is substantially equal to 1 second. A method of regulating an air conditioning circuit traversed by a supercritical fluid, in particular for a motor vehicle, comprising a compressor provided with a control valve whose degree of opening varies as a function of the intensity of a signal of control, characterized in that the control signal of the compressor valve is monitored during the start-up phase of the air-conditioning so as to maintain the compressor discharge pressure substantially below the compressor cut-off pressure.

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