Abstract:
The invention relates to a method for controlling an expansion valve, in particular for vehicle air-conditioning systems operated with CO 2  as the refrigerant, with a valve housing which has a feed opening and a removal opening, with a valve-closing member which closes a valve seat of a passage opening arranged between the feed and removal openings, and with a resetting device which acts in the closing direction of the valve-closing member, an adjusting element, which is assigned to the valve-closing member, on the low-pressure side being activated via a predetermined threshold value as the low pressure rises or as the temperature on the low-pressure side rises and actuates at least one actuating element by means of which a closing force of the resetting device acting on the valve-closing member is reduced such that a passage opening is enlarged or a passage of a bypass valve arranged on the low-pressure side, which passage is arranged parallel to the passage opening, is opened.

Description:
[0001]     Applicant herein incorporates by reference the following foreign priority document: German Appln. No. 10 2006 021 327.0, filed May 5, 2006.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a method for controlling an expansion valve and to an expansion valve, in particular for vehicle air-conditioning systems operated with CO 2  as the refrigerant, in accordance with the precharacterizing clause of Claim  1  and  2 .  
         [0003]     DE 10 2004 010 997 B3 discloses an expansion valve and a method for controlling it, in which the opening and closing movement of a valve-closing member is set as a function of the pressure differential present in a feed opening on the high-pressure side and in a removal opening of the expansion valve on the low-pressure side. A resetting device keeps the valve-closing member in a closed position when the refrigerant circuit is not in operation.  
         [0004]     An expansion valve of this type which operates as a function of the pressure differential between the high-pressure side and the low-pressure side has to satisfy a multiplicity of operating conditions in an optimum manner. A rapid response characteristic and good cooling power have been obtained with an expansion valve of this type at low to average ambient temperatures, for example up to circa 40° C.  
         [0005]     In the case of refrigerant systems, it is desirable to obtain rapid cooling after long inoperative periods of the vehicle, the refrigerant pressure having risen during the inoperative period of the vehicle to a high value as a function of the ambient temperature. It generally takes too long for the maximum cooling power to be obtained under high ambient temperatures.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention is therefore based on the object of developing a method for controlling the expansion valve and an expansion valve of the type mentioned at the beginning to the effect that, after long inoperative periods and at high ambient temperatures, a rapid response characteristic is provided thus resulting in rapid cooling of the space to be cooled.  
         [0007]     This object is achieved according to the invention by a method according to the feature of Claim  1  and by an expansion valve according to the feature of Claim  2 . Further advantageous refinements and developments are provided in the further claims.  
         [0008]     The effect obtained by the method according to the invention for controlling the expansion valve is that an opening characteristic of the valve-closing member of the expansion valve controlled by differential pressure is displaced to a lower opening pressure, as a result of which the response characteristic of the refrigerant circuit is increased. This is based on the effect that, by displacing the valve characteristic towards lower values, a premature opening of the valve-closing member takes place, thus resulting in an increase of the mass flow of refrigerant obtained between the very rapidly rising high pressure and the less steeply dropping low pressure. When the refrigerant circuit is switched on, the compressor is switched on and, on the high-pressure side, reaches, within a few seconds, the maximum permissible pressure which is approximately maintained over a starting period. By increasing the mass flow of refrigerant under such conditions, rapid dropping of the low pressure is achieved because of the cooling of the cooling-circuit components on the low-pressure side which has taken place.  
         [0009]     The obtaining of a relatively large pressure differential directly after an inoperative air-conditioning system is started up is made possible, according to the invention, by an expansion valve, in particular for carrying out the method according to the features of claim  1 , with an adjusting element which is on the low-pressure side and, via a predetermined threshold value, reduces, as a function of a rising temperature on the low-pressure side or a rising pressure, a closing force of the resetting device acting on the valve-closing member such that a refrigerant flow flows through the passage opening. For this purpose, according to a first embodiment, the adjusting, element is used to activate at least one actuating element which acts on the resetting device of the valve-closing member. This improves the response characteristic of the refrigerant circuit. Cooling is obtained even after a few seconds. As an alternative to reducing the closing force acting on the valve-closing member by the at least one actuating element, an embodiment is provided, in which a passage is provided parallel to the passage opening, which passage forms a bypass valve with and is closed by an adjusting element which can be activated on the low-pressure side. When a predetermined threshold value of the low pressure or of the temperature on the low-pressure side is exceeded, the bypass valve opens such that, at high pressures or temperatures on the low-pressure side, in particular when starting under high ambient temperatures, a relatively large refrigerant flow expands and cooling occurs at an even earlier time.  
         [0010]     According to an advantageous refinement of the expansion valve, it is provided that the adjusting element; which can be activated by low pressure, is designed as an expansion bellows, in particular with an inert gas filling, and comprises tappets which act on a resetting device acting on the high-pressure side in order to reduce the closing force of the valve-closing member closing the passage opening. As the low pressure or evaporation pressure of the refrigerant in the expansion valve rises, for example because of a high ambient pressure or heating of the parts of the air-conditioning system, the free length of the expansion bellows is reduced in accordance with the pressure. Tappets arranged on the expansion bellows protrude from the low-pressure side to the high-pressure side and act on a setting valve for regulating the closing force. When the stroke length of the expansion bellows is reduced, the closing force, in particular the prestressing force of a resetting device designed as a spring element, is reduced such that the valve-closing member opens the passage opening at an earlier time and a mass flow of refrigerant flows through the expansion valve.  
         [0011]     According to an alternative refinement of the invention, it is provided that the activation of the valve-closing element which takes place on the low-pressure side is provided using a valve-closing member, in which the resetting device is arranged on the low-pressure side. As a result, a premature opening of the valve-closing member and therefore a displacement of the opening characteristic of the expansion valve to a premature opening time is likewise obtained as a function of the low pressure or as the low pressure rises.  
         [0012]     According to a further advantageous refinement of the expansion valve according to the invention, it is provided that the adjusting element can be activated as the temperature on the low-pressure side rises, the adjusting element comprising a temperature sensor which is arranged on the low-pressure side and likewise obtaining a reduction in the closing force of the valve-closing member in the expansion valve. For example, a wax expansion element which, in the event of rises in temperature, produces an adjustment travel which is provided for opening the valve-closing member is provided as the temperature sensor. An alternative embodiment is provided by a spring element comprising a shape memory alloy which, as the temperature increases, brings about a shortening of the free length of said spring element, and therefore again a premature opening of the valve-closing member is brought about.  
         [0013]     According to an alternative refinement of the invention, the resetting device which keeps the valve-closing member in a closed position is provided on the low-pressure side. The adjusting element acting on the valve-closing member and comprising a temperature sensor can be designed in the same manner as the high-pressure resetting element.  
         [0014]     According to a further advantageous refinement of the invention, it is provided that the temperature sensor of the adjusting element is also provided outside the housing of the expansion valve. As a result, the ambient temperature or a temperature of adjacent parts can be taken as the basis for the control variable.  
         [0015]     According to a further advantageous refinement of the invention, it is provided that the bypass valve which is arranged on the low-pressure side and can be activated as a function of the low pressure has a closing member which is arranged on a membrane element or expansion bellows and closes a parallel passage to the passage opening between the feed and removal openings. The membrane element or the expansion bellows is preferably filled with inert gas thus making possible an activation as a function of low pressure in order to open the bypass. Alternatively, the bellows can be filled with a refrigerant, in particular CO 2 . In this case, the filling is designed in such a manner that, at temperatures, for example below 15° C. or 20° C., the pressure of the filling material corresponds approximately identically to the evaporation pressure.  
         [0016]     An alternative refinement of the bypass which is on the low-pressure side in the expansion valve provides that the bypass valve can be activated as a function of the temperature on the low-pressure side. In this case, the temperature of the refrigerant or the ambient temperature and also the temperature of further parts of the air-conditioning system can be used as the control variable. A control medium which brings about a stroke movement as a function of the refrigerant temperature is preferably provided in a bellows. As an alternative to the bellows or expansion bellows, a wax expansion element or spring element made of a shape memory alloy can be used.  
         [0017]     According to a further advantageous refinement of the pressure-dependent bypass valve, it is provided that the valve-closing member closing the passage is arranged on the high-pressure side and has a closing element, in particular spring element, acting towards the valve seat. As a result, a closed position can be ensured when the vehicle is inoperative.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The invention and further advantageous embodiments and developments of the same will be described and explained in more detail below with reference to the examples illustrated in the drawings. The features to be gathered from the description and the drawings can be used individually by themselves or a number of them can be used in any desired combination according to the invention. In the drawings:  
         [0019]      FIG. 1  shows a schematic illustration of a refrigerant circulating process,  
         [0020]      FIG. 2  shows a diagram which depicts a cooling curve in various operating phases at high ambient temperature with a differential-pressure valve without a “starting aid” in a vehicle,  
         [0021]      FIG. 3  shows a schematic sectional illustration of an expansion valve with a displacement of the opening characteristic taking place as a function of low pressure,  
         [0022]      FIG. 4  shows a schematic sectional illustration of an expansion valve with a displacement of the opening characteristic taking place on the low-pressure side as a function of temperature,  
         [0023]      FIG. 5  shows a schematic sectional illustration of an alternative embodiment of  FIG. 3 ,  
         [0024]      FIG. 6  shows a schematic sectional illustration of an alternative embodiment of  FIG. 4 ,  
         [0025]      FIG. 7  shows a schematic sectional illustration of an alternative embodiment of an expansion valve with a bypass valve controlled on the low-pressure side parallel to the differential-pressure valve, and  
         [0026]      FIG. 8  shows a schematic sectional illustration of an alternative embodiment of an expansion valve with a temperature-controlled bypass valve parallel to the differential-pressure valve.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]      FIG. 1  illustrates a refrigerant circuit  11  which is preferably operated with CO 2 . This refrigerant circuit  11  is used, for example, as a vehicle air-conditioning system. A compressor  12  feeds the compressed refrigerant on the high-pressure side to an outside heat exchanger  14 . The latter is connected to the surroundings and outputs heat outwards. Connected downstream of the latter is an inside heat exchanger  15  which feeds the refrigerant to an expansion valve  16  via a feed line  17 . Upstream of the expansion valve  16  on the high-pressure side there is an input pressure which, for example in summer, can be  120  bar. The refrigerant flows through the expansion valve  16  and passes to the low-pressure side. On the output side, the expansion valve  16  has pressures of between  35  and  45  bar under stationary conditions. The refrigerant, which is cooled by the relaxation of the pressure, passes via a removal line  18  into the inside heat exchanger  21  and will remove heat from the surroundings, as a result of which the cooling, for example of a vehicle interior, is obtained. A collector  22  is connected downstream in the heat exchanger. The refrigerant, which is in vapour form, flows through the inside heat exchanger  15  and passes to the compressor  12 . To operate this refrigerant circuit  11 , a differential-pressure expansion valve  16  is provided which is described in more detail in DE 10 2004 010 997 B3 and to which full reference is made. On the basis of operation described there for the expansion valve  16 ,  FIG. 2 , in which a cooling curve in the vehicle is depicted at a high ambient temperature, is described in more detail below.  
         [0028]     The vehicle has not been operated, for example, for a long time at high ambient temperature, and high ambient temperatures of more than 40° C. prevail. When the vehicle is inoperative (I), there is usually an equilibrium of the pressure at circa  80  bar on the high-pressure and low-pressure sides of the refrigerant circuit  11 . Directly after the start (II-1) of the refrigerant circuit  11 , the refrigerant pressure on the high-pressure side rises within a few seconds, on account of the power of the compressor, on the high-pressure side to the maximum permissible value of up to, for example, 133 bar. By contrast, on the low-pressure side, the pressure only drops slowly (lower curve). During further operation in the starting phase (II-1), the low pressure drops only with a small gradient. As a result, only a small change in pressure is provided which causes a slow cooling in the interior of the vehicle to take place in the starting phase (II-1). The response characteristic of the air-conditioning system is delayed. During the driving mode (II-2), a stationary state arises which is essentially depicted by the horizontal sections of the upper and lower curves. The third phase (III) depicts the high pressure upstream of the expansion valve  16  (upper curve) and lower pressure downstream of the expansion valve  16  (lower curve) in the idling mode.  
         [0029]     So that an improved response characteristic is provided in the starting phase (II-1) and the duration of the starting phase is shortened, it is proposed according to the invention to increase the opening of the valve under these conditions in order to bring about a relatively large mass flow of refrigerant while the pressure differentials are still comparatively average. For this purpose, it is proposed that this takes place by means of a displacement of the opening characteristic of an expansion component controlled by differential pressure towards a lower opening pressure, with the activation taking place via the rising low pressure or the rising evaporation temperature of the refrigerant or of the ambient temperature. As an alternative to the displacement of the opening characteristic of the valve-closing member in the expansion valve, it is proposed that a bypass valve which is activated on the low-pressure side is provided and, in turn, opens as a function of the temperature or the pressure. The abovementioned exemplary embodiments are based on the evaporation temperature of the refrigerant and the evaporation pressure. These activating means according to the invention are illustrated in more detail in FIGS.  3  to  8 .  
         [0030]      FIG. 3  depicts a schematic sectional illustration of a first expansion valve  16  according to the invention. A feed opening  32  which is connected via a passage opening  34  to a removal opening  36  is provided in a valve housing  31 . A valve-closing member  37  is provided in the passage opening  34 . When there is pressure equalization, said valve-closing member  37  is held on the high-pressure side in a closed position in a valve seat  41  via a resetting device  39  which is preferably designed as a spring element. An adjusting element  42  which comprises an expansion bellows or bellows  44  with an inert gas filling  46  is provided in the removal opening  3 - 6 . At one end, the bellows  44  is supported on a housing wall  47 . On the opposite side, the bellows  44  activates, as a function of the low pressure, at least one actuating element  48 , in particular tappets which extend into the feed opening  32 . These tappets  48  engage on a displacement element  49  which serves to set the prestressing force of the resetting device  39 . In order to limit the stroke movement of the adjusting element  42 , a stop  43  is provided so that the end position of the adjusting element  42  is defined.  
         [0031]     As soon as the low pressure in the removal opening  36  rises, the bellows  44  is compressed on account of the rising pressure and executes a stroke movement. At the same time, a unidirectional stroke movement of the tappets  48  takes place in order to reduce the prestressing force of the resetting device  39  such that the valve-closing member  37  opens even when there is a relatively small pressure differential, and a refrigerant passes earlier in the starting phase from the high-pressure side to the low-pressure side. As a result, a pressure-dependent displacement of the operating characteristic can be obtained for the expansion valve  16 .  
         [0032]      FIG. 4  illustrates an alternative embodiment to  FIG. 3 . The construction of the expansion valve  16  corresponds to that in  FIG. 3 . In a departure from this, a temperature-dependent adjusting element  42  is provided instead of an adjusting element  42  dependent on low pressure or evaporation pressure. This temperature-dependent adjusting element  42  comprises, for example, a spring  52  which is produced from a shape memory alloy. At least one actuating element  48  is provided on the adjusting element  42  and acts on the resetting device  39  and reduces the closing force. As soon as the temperature on the low-pressure side has exceeded a certain desired value, a shortening of the spring  52  made from a shape memory alloy takes place. As a result, the pressure acting on the resetting device  39  is reduced, thus permitting opening of the passage opening  34 . Furthermore, in order to activate the adjusting element  42 , wax expansion elements, bimetals or other two-material elements can be provided as the temperature sensors. Alternatively, the adjusting element  42  can also act directly on the valve-closing member.  
         [0033]      FIG. 5  illustrates an alternative embodiment of  FIG. 3 . This embodiment differs in that the resetting device  39  and the valve-closing member  37  are arranged on the low-pressure side. To guide the valve-closing member  37  on the low-pressure side, a holding and guiding element  55  is provided which guides a valve-closing element  37  displaceably within it. Furthermore, the holding and guiding element  55  receives the valve-closing member  37  on the low-pressure side and closes off the bellows  44 . As a result, the holding and guiding element  55  has more than one function. Furthermore, it serves as an actuating element  48 , in particular as a tappet, which joins a stop  43  in order to obtain a limitation of the closing force of the valve-closing member  37 , which closing force acts on the passage opening  34 . When the low pressure increases, the bellows  44  is pressurized and moves to the right, according to the exemplary embodiment. This reduces the prestressing force of the resetting device  39  such that a premature opening of the passage opening  34  takes place. This improves the response characteristic of the refrigerant circuit  11 . Otherwise, the functions are the same as in the embodiment according to  FIG. 3 .  
         [0034]      FIG. 6  illustrates an alternative embodiment to  FIG. 4 . In this embodiment, the resetting device  39  and the valve-closing member  37  are likewise arranged on the low-pressure side. Instead of the actuating element  48  according to the embodiment in  FIG. 4 , in a functionally identical manner an actuating element  48  is provided which is designed as an annular element and increases or reduces the prestressing force of the resetting device  39  on the valve-closing member  37  as a function of the change in travel of the spring  52  made from a shape memory alloy. The annular element moves between two end positions or stops  43  which restrict the displacement of the characteristics.  
         [0035]      FIG. 7  depicts a schematic illustration of a further alternative embodiment of an expansion valve  16 . This embodiment has a basic construction in accordance with the differential-pressure expansion valve described in DE 10 2004 010 997 B3. In addition, parallel to the passage opening  34  there is a parallel passage  61  which is closed by an adjusting element  42  which is arranged on the low-pressure side and, together with the passage  61 , forms a bypass valve  62 . According to the embodiment in  FIG. 7 , this bypass valve  62  is activated on the low-pressure side as a function of evaporation pressure. For this purpose, for example, the bellows  44  is provided which is filled with an inert gas or the inside of which is connected to the atmosphere while the closing force is carried out by means of a compression spring. At an end of the bellows  44  that faces the passage  61  there is a closing member  63  which closes the passage  61  when there are low-pressure values in the refrigerant system  11  that correspond to normal operation and are, for example, below 45 bar.  
         [0036]      FIG. 8  illustrates an alternative embodiment of  FIG. 7 . Instead of a bypass valve  62  which can be activated as a function of the evaporation pressure, a bypass valve  62  which can be activated as a function of temperature is provided. A control medium  66  is provided in the adjusting element  62 , which is designed as a bellows, and undergoes a change in volume as a function of the temperature. The closing member  63  is actuated by an actuating element  48 , for example in the form of a tappet, which is arranged on the bellows, and is preferably provided on the high-pressure side of the passage  61  and is held in a closed position via a closing member  68 , in particular a spring element.  
         [0037]     The filling enclosed in the interior of the bellows has such a pressure and temperature characteristic that, as the temperature of the CO 2  refrigerant rises on the low-pressure side, a higher rise in pressure takes place than on the outside of the bellows. As a result, as the temperature rises on the low-pressure side, a bypass opening can be achieved.  
         [0038]     All of the features described above are each essential in themselves for the invention and can be combined with one another as desired.