FR2895787A1 - Air conditioning system, especially for motor vehicle, incorporates regulating module controlling compressor valve command signal on starting - 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

Air conditioning system traversed by a supercritical fluid

  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 provided 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 the users.

  For example, the air conditioning regulator can control the temperature of the supply air at the outlet 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 real 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, 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 overcome this drawback, patent application FR 2 856 782 has proposed an air-conditioning system, equipped with an expansion device, of the electronic expansion valve type, in which the initial opening degree of the expander, at the start of the air conditioning, is calculated from an estimate of the initial temperature of the fluid at the inlet of the expander, 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 estimate of the fluid pressure at the inlet of the expander that 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 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. .

  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 regulation 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 below the lower threshold of the control signal, the control module is able to replace the current value of the control signal with 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 control module is further able to calculate the difference between the compressor discharge pressure and the compressor cutoff pressure, and to compare said condition to a predefined constant.

  - In the presence of a pressure difference less 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 time interval of length 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 quantity 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 displacement, and the upper limit is defined by the maximum value of the control signal when the compressor is in maximum displacement. The quantity 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 regulation 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 quantity linked to the control signal is the evaporating temperature set point and in that the upper threshold of the control signal is control signal is the maximum value of the control signal when the compressor is in maximum capacity. The quantity 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 control module is able to control the control signal according to a chosen period. - The chosen period is substantially equal to second.

  The invention furthermore proposes 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 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. Other features and advantages of the invention will be apparent from the following detailed description and the accompanying drawings, in which:

  FIG. 1 is a diagram of an air conditioning installation operating according to a supercritical cycle;

  FIG. 2 is a diagram of a motor vehicle air-conditioning installation equipped with a start-up 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 for integration 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 mechanical type.

  FIG. 2 is a diagram showing an air conditioning installation 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. 1.

  The installation is furthermore provided with a regulation module or an air conditioning computer 40 comprising an electronic card 43, a cabin regulator 41 and an air conditioning loop regulator 42. The cabin regulator 41 sets the instruction of the evaporation temperature Te., 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 the mechanical type, the Applicant proposes to regulate the control signal of the compressor.

  The Applicant proposes an air conditioning regulation module during the start-up phase of the air conditioning, suitable for limiting the pressure peaks and the stops of the compressor. 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 control module 40 calculates a current value of the PWM control signal (k) at a given instant and compares this value with an upper threshold of the PWMS? P control signal and a lower threshold of the control signal PWMi ,, f. 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 to prevent a sudden change that could generate a peak pressure.

  In a first embodiment illustrated in FIG. 3, the magnitude related to the control signal is the upper threshold of the PWMSΔp control signal that the PWM control signal must not exceed.

  In a second embodiment illustrated in FIG. 4, the quantity related to the control signal is the TecoäS evaporation temperature setpoint 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 moment considered, ie PWMsäp (k) or Tecoäs (k) as the case may be, so that this magnitude has a speed. progressive variation between a lower bound and an upper bound, in a time interval of length defined by a time constant KPwM or KTe respectively. Since these magnitudes affect the evolution of the PWM control signal over time, their control makes it possible to act on it and to prevent it from exhibiting sudden variations that could generate pressure peaks.

  The time constants KPwM for the first embodiment or KTe for the second embodiment are determined as a function of the operating parameters of the air conditioning circuit.5 The quantity related to the control signal PWMsup (k) or Tecons (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 PWMsäp (k-1) or Tecons (k-1).

  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 regulation module thus makes it possible to control the evolution of the compressor control signal in order to limit the occurrence of pressure peaks and thus maintain the high compressor pressure substantially below the HPeY cut-off pressure of the compressor during the start-up phase. .

  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 relationship related to the difference between the high pressure of the HP compressor (k) and the HP cutoff pressure.

  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 long as the start phase is not completed, as indicated in the test of step 3. If the startup phase is completed, a conventional control law of the compressor, which supplies 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 set point of the evaporation temperature, 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 time k considered and a cutoff pressure HPanêt is less than a constant C, for example equal to 5 bars. The HP shutoff pressure corresponds to the value of the high pressure which causes the compressor to stop. The high pressure of the HP compressor (k) at time k may 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 time k is calculated according to the law conventional control as a function of the evaporation temperature Te, for example as a function of the difference between the measurement of the evaporation temperature Ternes and the setpoint of the evaporation temperature TecoäS.

  On the other hand, if the pressure difference {HP (k) - HP off} is less than the constant C prefixed, the control module calculates the variation of the control signal) PWM (k) at time k, at step 302, as a function of the difference HP (k) - HP off, according to the equation A1 of Appendix A. Then, in step 306, the regulation module calculates the control signal PWM (k) of the valve 140 of the compressor 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 at step 302. At the start of the air-conditioning (first iteration of the start-up control), the value of the PWM control signal (k-1) can be taken equal to the last value of the control signal before the start-up 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 cutoff pressure is detected while maintaining a high pressure level to cool the most. quickly 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 PWMsup (k) that the control signal PWM (k) should not exceed in steps 303 and 308.

  The current value of the upper threshold of the PWMsup control signal (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 PWMrnax 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 bound is represented by the minimum value of the control signal PWMmin. 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%. The current value of the upper threshold of the PWMsup control signal (k) can be calculated for example by means of a first-order filter, from: the upper limit PWMmax, the lower limit PWMm; and the time constant K.PwM.

  The KPwM time constant may vary during the start-up period. it represents the time taken by the control signal to go from the lower limit PWMmin to the upper limit PWMmax. Determining the upper threshold PWMsup (k) also uses the value of the upper threshold PWMsup (k-1) determined at the previous time k-1. This value is normally stored in memory.

  The time constant of the filter KPwM can be calculated at each iteration of the control process from the discharge pressure of the compressor HP, the outside temperature Text and the rotational speed N of the compressor. In the example of a first-order filter, the upper threshold value PWMsUp (k) is obtained according to equation A2, where Tech is 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 PWM control signal (k) obtained in step 304 or in step 306 is between the current value of the upper threshold PWMsup (k), obtained at step 303, and a lower threshold represented by the minimum value PWM, nm 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 PWMsup (k) obtained in step 303, the upper threshold value PWMsUp (k) is assigned to the PWM signal (k). However, when the value of the PWM signal (k) determined in step 304 or in step 306 is smaller than the minimum value of the control signal PWMm; n, this minimum value PWMm; n 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 start-up 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 start-up phase, 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 found that the control down the control signal between 470 seconds and 480 seconds to prevent the high pressure (curve a) does not exceed the cutoff pressure HP shutdown, for example between 130 and 140 bar (13 to 14 Mpa), and therefore 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 Figure 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) - HPoffer} is less than the constant C prefixed, the control signal of the compressor PWM (k), at time k, is calculated according to the relation Al similarly to step 306 of FIG. 3. In step 404, if the pressure difference {HP (k) - HPanet} is greater than the constant C, the control signal of the PWM compressor ( k), at instant k, is calculated according to the conventional control law as a function of the difference between the measurement of the evaporation temperature Ternes and a predetermined setpoint of evaporation temperature Tecons (k) at the instant k. This conventional control law can be for example a proportional integral integral 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 KTe. The current value of the Tecons (k) evaporation temperature setpoint is calculated so that this quantity has a gradual rate of change between an upper bound and a lower bound, in a length interval defined by a time constant KTE. The rate of variation is in particular decreasing from the lower limit to the upper limit, as illustrated in the diagram of step 303.

  The upper limit is represented by the maximum temperature setpoint Tecons 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 Teeons min requested in the passenger compartment by a passenger. It is generally between 2 C and 5 c.

  The current value of the TeCons evaporation temperature setpoint (k) can be calculated, for example, using a first-order filter, from: - the upper limit Tec; ons max. - the lower limit TeCOns min. and - the time constant KTe.

  The time constant KTe may vary during the start-up period. It represents the time taken by the control signal to go from the upper bound Teeons max to the lower bound Teeons min • The determination of the current value of the setpoint of evaporation temperature Tee0ns (k) also uses the value of the upper threshold Tee0ns (k-1) determined at the previous time k-1. This value is normally stored in memory.

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

  In the example of a first order filter, the value of the evaporation temperature set point Tee ns (k) is obtained according to equation A3, where Tech 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 pressure peaks.

  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 embodiment, the upper threshold of the PWMmaX control signal represents the value of the control signal when the compressor is maximum displacement, and the lower threshold of the PWMm control signal; n represents the value of the control signal when the compressor is minimum displacement. These thresholds PWMmaX and PWMm; ,, 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 value PWMmax, the PWMma value is assigned to the PWM signal (k). Otherwise, if the value of the PWM signal (k) determined in step 304 or in step 306 is smaller than the minimum value of the PWMm control signal; ,, the PWMmiä 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. In addition, the high pressure of the HP compressor is kept below the HPaunching 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 Al: OPWM (k) = a (HP (k) - HPpoint) 2 + b (HP (k) -HPoff) + c1 A2: PWMsnp (k) = Tech + (1 + K'wAl max) 1 + KM M max Tech Tech A3: KTe * T (k - 1) cons Te0 ,,. S (k) = Tech ù + (1 + K 'e) + K Tech Tech Kpwu max * PWMsup (k -1) 1 * (PWMma ù PWMmin) * (Te cons max ù Tecon.s - min)

Claims (17)

claims   1. An air conditioning installation, 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 in a function of 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) capable of 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.   2. 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 moment, and to compare (310) said value. at an upper threshold of the control signal and at 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.   3. 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 by the upper threshold of the control signal (312).   4. Installation according to one of claims 2 and 3, characterized in that in the presence of a current value of the control signal below the lower threshold of the control signal, the control module is able to replace the current value of the control signal by said lower threshold of the control signal (312).   5. 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 at the control valve of the compressor (140).   6. Installation according to one of claims 2 to 5, characterized in that the control module is further adapted to calculate the difference between the compressor discharge pressure and the compressor cutoff pressure, and to compare said condition to a predefined constant (300). 16   7. Installation according to claim 6, characterized in that in the presence of a pressure difference less 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 (302).   8. 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). 10   9. Installation according to one of claims 2 to 8, characterized in that the regulation module is able to determine the current value of a quantity related to the control signal at the given given instant (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.   10. 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 terminal 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.   11. Installation according to claim 9, characterized in that the magnitude related to the control signal is the evaporating temperature set point, and in that the rate of change of this magnitude is decreasing in the time interval between the upper limit defined by the maximum temperature regulation of the air conditioning and the lower limit defined by the temperature setpoint required in the passenger compartment. 30   12. 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 capacity (403).   13. Installation according to one of claims 9 to 12, characterized in that said magnitude is calculated with a first order filter from the minimum bound, the upper bound, the time constant, the previous value. of said magnitude and a sampling period of chosen duration.   14. Installation according to one of claims 9 to 13, characterized in that the time constant is calculated from the speed of rotation of the compressor, from the outside temperature to the habitable of the vehicle and the discharge pressure of the compressor.   15. Installation according to one of claims 2 to 14, characterized in that the control module is adapted to control the control signal according to a chosen period.   16. Installation according to claim 15, characterized in that the period chosen is substantially equal to 1 second.   17. 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, 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.