Abstract:
A control process controls an expansion valve (EXV). The expansion valve is situated in an air conditioning circuit in which a coolant circulates and flows. The air conditioning circuit comprises at least a compressor (CMP), a condenser (CND), and an evaporator (EVP). The control process comprises the following steps or stages:
       when overheating is less than a threshold value, opening of the expansion valve is governed by a static control, and   when overheating is higher than the threshold value, opening of the expansion valve is governed by a dynamic control,

Description:
Related Applications 
       [0001]    This application claims priority to and all the advantages of French Patent Application No. FR 10/03507, filed on Sep. 2, 2010. 
       BACKGROUND OF THE INVENTION 
       [0002]    This invention deals with the general domain of control processes of an expansion valve placed in an air conditioning circuit or loop in which circulates a sub-critical cooling fluid (R-134a, R-1234yf for instance). In general, such an air conditioning circuit includes at least a fixed or variable capacity compressor, an expansion valve, a condenser and an evaporator. Optionally, such an air conditioning circuit can include an internal heat exchanger between the high pressure and low pressure parts of the air conditioning circuit. 
         [0003]    The invention deals more specifically with the strategy for controlling the expansion valve of an air conditioning circuit of an air conditioning system to be implemented in an automotive vehicle. 
         [0004]    Control of the expansion valve opening enables to control overheating and under-cooling directly or indirectly so as to optimize the energy efficiency of the air conditioning circuit. Control of the expansion valve opening also permits to protect the air conditioning circuit from temperature and pressure peaks at the compressor outlet. 
         [0005]    There are multiple solutions to control the expansion valve opening. According to these solutions, overheating, in other words, the difference in temperature at the evaporator outlet and the fluid saturation temperature, is controlled by using a derived integral proportional command. It is also possible to control the expansion valve opening on the basis of the temperature at the compressor outlet. 
         [0006]    These solutions pose a problem for the weak thermal charges (for instance for outside temperatures below 25° C.). Indeed, it is very difficult to obtain overheating or under-cooling when the outside air temperature is below 25° C. Rather, one observes a temperature unbalance when exiting the evaporator and/or an increase in the system instability. In such case, checking the overheating does not make sense since this overheating is near the zero value. 
         [0007]    If, on the other hand, a control of under-cooling is looked for at the inlet of the expansion valve, this cooling is also nearly zero during climate conditions showing an outside temperature below 25° C. for instance. 
         [0008]    Consequently, the main purpose of this invention is to overcome the inconveniences or problems of the known systems by proposing a control process for an expansion valve situated on an air conditioning circuit in which a cooling fluid circulates, with the air conditioning circuit comprising at least a compressor, a condenser and an evaporator cooling, with this control process comprising the following steps: 
         [0009]    when overheating is less than a threshold value, opening of the expansion valve is governed by a static control, and 
         [0010]    when the overheating is higher than the threshold value, opening of the expansion valve is governed by a dynamic control. 
         [0011]    Consequently, with the invention, a static control of opening the expansion value is performed as soon as the overheating or the sub-cooling is near zero and thus difficult to control. 
         [0012]    On the other hand, when the overheating or the sub-cooling is controllable, which corresponds to a difference between the temperature or pressure at the compressor outlet and a predetermined temperature or pressure threshold, higher than a predetermined elementary offset, if the parameter to control is higher than its set point value, the expansion valve is controlled dynamically on the basis of this parameter. 
         [0013]    This way, the inconveniences of the systems of the state of the art, in which the absence of overheating or sub-cooling prevents an efficient control of the air conditioning circuit for controlling the opening of the expansion valve. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    According to a beneficial characteristic of the invention, the static control determines the value for opening the expansion valve, on the basis of a set point temperature, of a high pressure of the fluid, of an air temperature at the inlet of the evaporation and of a pulse air flow. 
         [0015]    According to a preferred characteristic, the dynamic control determines the value for opening the expansion valve on the basis of a threshold value below which the fluid temperature and/or pressure oscillates inside the air conditioning circuit. 
         [0016]    According to another characteristic, the variation of the expansion valve opening is a function of the compressor speed. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    Other characteristics and benefits of this invention will be brought out from the description below, referenced to the attached drawings which show an example of embodiment without any limitations. On the figures: 
           [0018]      FIGS. 1A and 1B  show schematically two examples of thermal circuits in which the invention can be implemented; 
           [0019]      FIG. 2  shows a functional diagram of the opening size of the expansion valve on the basis of the pressure at the compressor outlet; 
           [0020]      FIGS. 3A and 3B  show the variation of a parameter quantifying the overheating on the basis of, respectively, the outside temperature and the high pressure at the compressor outlet; 
           [0021]      FIG. 4  shows a sub-cooling control profile on the basis of the outside temperature; 
           [0022]      FIG. 5  shows an organizational diagram of the control of the expansion valve according to the invention; and 
           [0023]      FIG. 6  shows the evolution of the expansion valve section on the basis of time as obtained with the process according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 1A  represents a first example of the air conditioning circuit for which the invention can be implemented. This air condition circuit includes a compressor CMP; a condenser CND followed by an expansion valve EXP and an evaporator EVP. 
         [0025]      FIG. 1B  shows another example of the air conditioning circuit in which components identical to those of  FIG. 1A  are implemented. An internal heat exchanger (called IHX) permitting heat exchange between the fluid leaving the condenser and entering the expansion valve and the fluid leaving the evaporator and entering the compressor. 
         [0026]    The invention involves the control of expansion valves as implemented in these two types of air conditioning circuits. 
         [0027]      FIG. 2  shows a functional diagram of the command/control of a steam compression thermal circuit comprising an expansion valve with variable opening, and as an option, a variable sized or variable speed compressor (in the case of electric compressors). 
         [0028]    A diagram of the circuit appears on  FIG. 2 , through reference  10 , the expansion valve, through reference EXV, and the compressor, through reference CMP (said compressor can be of the mechanical or electric type). 
         [0029]    Operation of the thermal circuit is continuously evaluated through measurements  20  performed at several points or locations of circuit  10 . A temperature measurement  21  at the outlet of the evaporator is also made. These measurements enable a feedback on the parameters to be controlled during the operation of the thermal circuit. 
         [0030]    In particular, the temperature at the outlet of the evaporator Te can be controlled with measurement  21  observed and calculation  31  of a set point value of this temperature at the outlet of evaporator Te. This feedback enables an action by a control unit of compressor  120  directly on the speed of compressor CMP. 
         [0031]    In addition, another calculation  30  of a set point value SP (Par) of a parameter quantifying overheating or sub-cooling according to the invention, is used to determine a command signal of the expansion valve EXV by using measurements  20 . 
         [0032]    This signal includes several components according to the invention. Measurements  20  and calculation  30  of the set point value of the parameter quantifying the overheating are used within a static control unit  110  which calculates a component CS sent to expansion valve EXV. 
         [0033]    A dynamic control unit  111  also determines a dynamic feedback component CD of the expansion valve control. This feedback is determined solely for predetermined intervals of the parameter and symbolized on  FIG. 3  by reference  112  which symbolizes the presence of a—dead zone—of the parameter for which the feedback is not active. 
         [0034]    According to a beneficial feature of the invention, the control system of the expansion valve also includes a control unit of the pressure peaks  112  using measurements  20  for determining and sending a component CP to expansion valve EXV. 
         [0035]    Also beneficially, the command signal of the expansion valve is used by the control unit of compressor  120  to make a possible feedback for the speed of compressor CMP. 
         [0036]    These last two features are represented by dotted lines on  FIG. 2 . 
         [0037]    Within static control unit  110 , opening of the expansion valve is determined on the basis of the pressures at the inlet and outlet of the compressor. 
         [0038]    According to the invention, the control for the expansion valve opening depends on a static control and a dynamic control for opening. Indeed, when overheating approaches zero, a static control of the expansion valve opening is operated. Given that for outside temperatures below 25° C., overheating is practically zero and as such hard to control, the expansion valve operate as a tube orifice. In other words, below a certain overheating value, the expansion valve is maintained with a fixed diameter opening. When overheating exceeds a value threshold, a dynamic control of the expansion valve opening is then put in operation. 
         [0039]      FIG. 3  shows a control curve of diameter D of the expansion valve, on the basis of the compressor outlet pressure. One observes that for pressures below a value P 1 , generally in the range of 8 to 12 bar, the size of the valve opening depends on high pressure HP and low pressure LP, while for pressures above P 1 , the size of the valve opening is linear on the basis of high pressure HP of the circuit. In addition, value P 1  is the one from which a dynamic control of the expansion valve takes place. Indeed, when high pressure HP is below P 1 , the overheating value is too weak to be able to put into effect a dynamic control of the opening of the expansion valve. The static control of the expansion valve is then put into effect. As is shown on this figure, the opening diameter of the expansion valve evolves very little for high pressure values below P 1 . 
         [0040]    In addition, when the air conditioning circuit is started up, with the low pressure undefined, the control process for opening the expansion valve uses a low pressure calculation on the basis of the set point value of the temperature at the evaporator outlet, the efficiency of the evaporator and the air temperature at this evaporator inlet. 
         [0041]    In addition, below P 1  and in particular below P 0 , the variation of the expansion valve opening must be very small and depends beneficially on the outside temperature, the air mass flow, the temperatures at the inlet and outlet of the evaporator and the air speed at the condenser. The greater the air mass flow, the larger the expansion valve diameter will be and the lower the temperature, the greater the expansion valve diameter. As such, these parameters can be used to evaluate the initial opening of the expansion valve and to put in place a dynamic feedback according to the invention. 
         [0042]    Indeed, in order to implement the invention, it is necessary to make a calculation of a set point value for an overheating or under-cooling quantifying parameter. It is on this set point value that the derived integral proportional component will be calculated during the dynamic control of the expansion valve since the overheating or the under-cooling is found to be controllable. 
         [0043]      FIGS. 4A and 4B  give two examples for establishing a set point value SP(Te) for the temperature parameter at the inlet to the compressor Te (or, consequently, at the outlet of the evaporator), on the basis respectively of the outside temperature Text and the high pressure HP, in other words, the pressure at the compressor outlet. This set point value SP (Te) is used for the static control. 
         [0044]      FIG. 4A  shows a first curve C 1  which corresponds to determining the set point value ST(Te) in the case of an air conditioning circuit which operates with fresh air, while curve C 2  corresponds to an air condition circuit operating on recycled air. 
         [0045]    One observes that for an outside temperature below 25° C. or, for a compressor outlet pressure below 7 bar, there is no change in the set point value. This means that then one cannot speculate on overheating to optimize operation of the air conditioning circuit. The invention will be particularly useful in these areas of outside temperatures and pressure at the compressor outlet. 
         [0046]      FIG. 5  represents schematically a curve determining a set point value SP(Te) for a sub-cooling quantifying parameter, for instance, here also, the evaporator outlet temperature. 
         [0047]    Here also, one observes that for values below 25° C., it is no longer possible to optimize the performance of the air conditioning circuit by modifying the sub- cooling or overheating set point value. 
         [0048]    Consequently, on the basis of  FIGS. 4 and 5 , one concludes that for outside temperatures below 25° C., the set point value cannot be set at 15 K for instance because the thermal charge is weak or even very weak and it is difficult to obtain high overheating or sub-cooling values without destabilizing the system. 
         [0049]    Consequently, according to the invention, as soon as the difference between the compressor outlet temperature and the temperature threshold predetermined at 25° C. is higher than a predetermined elementary offset or as soon as the difference between the compressor outlet pressure and the pressure threshold predetermined at 7 bar is higher than a predetermined elementary offset and that the parameter is higher than the set point value, the expansion valve opening is controlled by a command signal comprising a component calculated by backfeed on the value of the parameter which quantifies overheating or under-cooling. 
         [0050]    For that, the following measurements are made continuously during the operation of the air conditioning circuit: air flow passing through the evaporator, the target temperature of the pulsed air and of high pressure in the circuit. One observes here that the target temperature and the high pressure values are known with a certain margin of error. 
         [0051]      FIG. 6  represents an organizational diagram for controlling the expansion valve opening of the invention. A stage A in which the parameters for the air conditioning circuit are collected, takes place. These parameters are for instance the air temperature at the evaporator inlet, the set point temperature, the high pressure value and the pulsed air flow. This is followed by a stage B in which the expansion valve opening is selected on the basis of the parameters collected during stage A. Stages A and B correspond to the static control of the expansion valve. As such, the expansion valve opening is set at a value D 0 . Here one observes that a circular section expansion value is considered but that the invention involves any other type of valve from the moment that the section of its opening is variable. 
         [0052]    Initial value D 0  is determined on the basis generally of high pressure HP and the outside temperature, of the progress speed and of the power of the motor-fan unit on the front, and possibly of the low pressure BP as we have seen before. 
         [0053]    A stage C illustrates the progress in time over an increment corresponding to the duration of the period at which the expansion valve control signals are sent by a controller CNT. 
         [0054]    A test stage D receives the measurements made for the air conditioning circuit and determines where there oscillations in the air conditioning circuit. In particular, this stage D analyses whether there are oscillations of low pressure, or high pressure, or of temperature at the evaporator outlet. 
         [0055]    In the event that oscillations are observed (case O), in a stage E, the process stores the DN−1 opening value for the present expansion value in a register; this opening value is entered as DH. A stage F then permits to increase the opening value of the expansion valve by a value d 1 . Consequently, the expansion valve opening has a value DN=DN−1+d 1 . During a stage G, the process verifies whether the parameters of stage A have been modified. If the parameters have changed, a stage G 1  permits upon opening the valve to adopt the value D 0  defined during stage A. If the parameters of stage A remain unchanged during stage G, a stage H verifies whether the value DN is below or equal to the stored value DH of stage E for which the air conditioning circuit undergoes oscillations. If DN is less than or equal to the stored value DH during stage E, the opening value of the valve is then defined as follows: DN=DH+d 1  during a stage I. This stage I permits to modify the opening of the expansion valve to stop the air conditioning circuit oscillations. According to a stage J, controller CNT receives the opening value DN of the valve to be sent to the expansion valve. 
         [0056]    For stage H, if value DN is higher than value DH, the value DN is kept (stage H 1 ) for opening the expansion valve since the oscillations no longer take place with such an opening. This value Dn is then transmitted to the controller for stage J. 
         [0057]    A stage K illustrates the physical modification of the expansion valve opening. In other words, the command signal is sent by the controller to the expansion valve so that its opening can take the value recorded by the controller. 
         [0058]    After stage K, the process goes back to stage C. 
         [0059]    At stage D, if no oscillation has been detected, then the opening value for the expansion valve takes the value DN=DN−1−d 1  during a stage D 1 . Then, this value is transmitted to the controller at stage J. 
         [0060]    Please note that increment d 1  for opening the expansion valve is beneficially calculated in the form of a derived integral proportional component. 
         [0061]      FIG. 7  shows successive values of the command signal for diameter D_EXV of the expansion valve on the basis of time. One sees here that the opening value is modified for each period T. One sees also that the section of the expansion valve has diminished progressively over three levels with the absence of oscillation, then, when the DH value has been reached, it has been necessary to go up with a small increment d 1 . Diameter DS is found to be a stable value for opening the expansion valve under the conditions of operation then observed. 
         [0062]    The invention permits also to be prepared for pressure peaks and drive couple peaks of the compressor, since the speed of said compressor varies greatly (in particular, acceleration) even while it is operating close to its maximum capacity. 
         [0063]    Indeed, the invention allows that the size of the expansion valve opening is controlled on the basis of the compressor speed. Then, the compressor operates simultaneously with the static and dynamic control of the expansion valve opening. 
         [0064]    Thus, the variation of the expansion valve opening is beneficially also a function of the compressor speed. Modification of the opening is determined to compensation the variation of the coolant flow due to the speed variation of the compressor. 
         [0065]    Beneficially, the variation of the expansion valve opening on the basis of the compressor speed is effective from the time when the compressor has reached 75% of its maximum capacity. 
         [0066]    Expansion valve control also allows to protect the air conditioning circuit against pressure or temperature peaks at the compressor outlet. 
         [0067]    Opening of the expansion value is also controlled statically on the basis of pressure and temperature at the evaporator outlet since overheating or sub-cooling is near zero and dynamically when overheating our sub-cooling is controllable, in other words, when the temperature or pressure at the evaporator outlet exceeds threshold values; in this case, control is a function of the compressor speed when the compressor operates near its full capacity so as to compensate for the coolant flow rate variation. 
         [0068]    Finally, please note that several implementations can be achieved according to the principles of the invention.