Abstract:
The invention relates to a heating/cooling circuit for a motor vehicle comprising an evaporator ( 14 ) for cooling air to be fed into an interior space, a heat exchanger ( 16 ) for heating said air to be fed into the interior space, an external heat exchanger ( 22 ) comprising a compressor for transporting coolant, a first expansion organ ( 28 ), allocated to the evaporator ( 14 ), a second expansion organ ( 30 ), allocated to the external heat exchanger ( 22 ) and coolant conduits (L 1  to L 12 ), via which the aforementioned components are interconnected. The compressor ( 24 ), the external heat exchanger ( 22 ) and the second expansion organ ( 30 ) constitute a de-icing circuit of the inventive circuit.

Description:
BACKGROUND OF THE INVENTION 
   (1) Filed of the Invention 
   The invention relates to a heating/cooling circuit for a motor vehicle according to the preamble of claim  1 , an air-conditioning system containing said heating/cooling circuit and a method for controlling the air-conditioning system. 
   (2) Description of Related Art 
   JP-A-4-278153 discloses a heating/cooling circuit for an air-conditioning system with a deicing mode, deicing being carried out by increasing the degree of opening of an expansion valve and by reducing the quantity of air supplied to an interior heat exchanger. In this case, since the degree of opening of the expansion valve is increased, both the interior heat exchanger and the exterior heat exchanger act as condensers. Furthermore, since the quantity of air supplied to the interior heat exchanger is reduced, the radiated heat quantity at the interior heat exchanger is reduced, and consequently the radiated heat at the exterior heat exchanger is increased correspondingly, with the result that the ice is melted, while the passenger compartment is further heated by the interior heat exchanger, albeit with a reduced maximum power. 
   JP-A-5-77636 discloses a heating/cooling circuit for an air-conditioning system with an interior heat exchanger which serves as a condenser and with an exterior heat exchanger which serves as an evaporator in a heating mode, in order to heat the passenger compartment. In a defrosting mode for the exterior heat exchanger, part of the refrigerant is transferred directly from a compressor to the exterior heat exchanger as a result of the opening of a bypass, in order thereby to bypass the interior heat exchanger and an expansion valve. In this way, the exterior heat exchanger acts as a condenser and the ice is melted by the condensation heat in the exterior heat exchanger. The heat capacity of the interior heat exchanger is thereby reduced. 
   In both of the Japanese publications mentioned above, the heat capacity of the interior heat exchanger is reduced, even though a reduction in the temperature in the passenger interior is minimized. In the case of a continuous operation of the air-conditioning system, the defrosting mode has only a slight influence, but, in an air-conditioning system of a motor vehicle which is operated, for example, for only one hour, a defrosting mode of, for example, 30 minutes has some effect, so that there is a certain reduction in temperature in the passenger compartment. 
   According to a heating/cooling circuit, disclosed in EP 0 788 910 A2, for an air-conditioning system in a motor vehicle, a delay mode is described, in which the deposition of ice on the exterior heat exchanger is delayed, with the result that the heating duration can be prolonged and a defrosting mode postponed. 
   Air-conditioning systems of this type still do not satisfy some requirements, particularly with regard to the duration of the defrosting mode. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to avoid or at least reduce the abovementioned disadvantages and to provide an improved heating/cooling circuit for an air-conditioning system of a motor vehicle and also a method for controlling the air-conditioning system. 
   This object is achieved by means of a subject having the features of claim  1 , an air-conditioning system having the features of claim  16  and a method as claimed in claim  17 . 
   According to the invention, there is a heating/cooling circuit for a motor vehicle, with an evaporator for the cooling of air to be supplied to an interior, with a heating heat exchanger for heating the air to be supplied to the interior, with an exterior heat exchanger, with a compressor for the conveyance of refrigerant, with a first expansion member which is assigned to the evaporator, with a second expansion member which is assigned to the exterior heat exchanger, and with refrigerant lines, via which the abovementioned components are connected to one another, a defrosting connection of the circuit comprising the compressor, the exterior heat exchanger and the second expansion member. By means of a defrosting connection of this type, the refrigerant experiences first a pressure increase in the compressor and subsequently, if appropriate even before expansion, a discharge of heat in the iced-up exterior heat exchanger. In this case, the exterior heat exchanger is heated by the circulating refrigerant in the defrosting mode carried out by means of the defrosting connection. In this way, the iced-up exterior heat exchanger can be defrosted quickly, particularly when the engine cooling requirements cannot be satisfied by means of an air quantity which is reduced on account of icing-up. 
   Preferably, the second expansion member follows the compressor. In this case, preferably, the second expansion member precedes the exterior heat exchanger. Since the suction pressure is determined essentially by the coldest point in the defrosting circuit, in this case said suction pressure is determined by the iced-up exterior heat exchanger, so that it is possible to have a higher defrosting capacity than if the exterior heat exchanger were arranged upstream of the expansion member. For this purpose, in the defrosting mode, the refrigerant is compressed in the compressor, conducted to the expansion member, expands there and discharges its heat in the exterior heat exchanger, the heat melting ice which is located in the exterior heat exchanger. 
   Preferably, the second expansion member is controllable, in particular continuously, and can be activated electrically. 
   So that the capacities of the heat exchangers can be matched even more effectively to the demand, preferably the compressor power, too, can be controlled by the adjustment of the stroke volume or of the compressor rotational speed. 
   Preferably, a sensor is provided in the air upstream of the exterior heat exchanger, this sensor preferably determining the exterior temperature of the air. This may preferably be carried out directly upstream of the exterior heat exchanger, but also at any other points at which an exterior temperature changed substantially by influences cannot be measured, such as, for example, on the intake tract of the air-conditioning system. The sensor is preferably a temperature sensor. 
   Preferably, a sensor is provided in the refrigerant downstream of the exterior heat exchanger, in particular in the region located on the low-pressure side, that is to say anywhere between the outlet of the exterior heat exchanger, the outlet of the evaporator and the inlet into the compressor. This sensor may determine, for example, the temperature of the refrigerant. According to an alternative, the sensor may determine the pressure which is closely related to the temperature. 
   The heating/cooling circuit preferably comprises a control device which controls the heating/cooling circuit as a function of the temperatures determined by means of the temperature sensors, that is to say switches to the defrosting mode and/or back again to the normal mode, as required. 
   Preferably, CO 2  is used as refrigerant for the heating or cooling circuit according to the invention, since CO 2  is optimally suitable for a heat-pump mode, such as can be carried out by means of the circuit according to the invention. When CO 2  is used as refrigerant, preferably an internal heat exchanger is provided for the exchange of heat between a high-pressure-side and a low-pressure-side section of the heating/cooling circuit. The result of using CO 2  as refrigerant is that some of the operating points lie in supercritical states, so that the discharge of heat often takes place without condensation. 
   Preferably, a heating bypass line capable of being shut off is provided in the heating/cooling circuit for the refrigerant-side bypass of the heating heat exchanger. This line is opened by means of a valve which is open solely in the defrosting mode. The line supplies the expansion member with the refrigerant coming from the compressor. 
   In order to keep the refrigerant pressure losses between the lines and heat exchangers as low as possible and so that the optimal operating point according to the conditions can be set in the cooling or heating mode, a heating bypass line capable of being shut off is provided, for the refrigerant-side bypass of the heating heat exchanger and of the expansion member assigned to the exterior heat exchanger, and a cooling bypass line capable of being shut off is provided, for bypassing the evaporator and the associated expansion member. Furthermore, the evaporator or heating heat exchanger, as components, can thus be disconnected completely if their functioning is not required or would even be disadvantageous. 
   So that in the cooling mode, when only the evaporator is operating and cools the air to be supplied to the vehicle interior, no refrigerant accumulates in the heating heat exchanger, which is also necessarily cooled by the air cooled by the evaporator, since the air flows through said heating heat exchanger, a throttlable line is provided between a line connected to the heating heat exchanger and a line at the low pressure level of the evaporator. 
   The invention is explained in detail below by means of an exemplary embodiment, with reference to the drawing in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a heating/cooling circuit according to the invention in the cooling operating mode; 
       FIG. 2  shows the heating/cooling circuit according to the invention in the heating operating mode; 
       FIG. 3  shows the heating/cooling circuit according to the invention in the reheat operating mode; 
       FIG. 4  shows the heating/cooling circuit according to the invention in the defrosting operating mode. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A heating/cooling circuit  10  according to the invention for an air-conditioning system for a motor vehicle has an evaporator  14  arranged in an air-conditioning unit  12 , which, as a rule, is arranged in an instrument panel of the motor vehicle, and a heating heat exchanger  16  following the evaporator  14  on the air side. Via a blower (not illustrated), circulating air or fresh air can be conveyed through the air-conditioning unit  12 , as is illustrated by an arrow, in which case the air can be cooled in the evaporator  14  and heated in the heating heat exchanger  16 . 
   Optionally, a further heating heat exchanger  20  is arranged upstream or downstream of the heating heat exchanger  16  on the air side and is connected via refrigerant lines K to an engine, not illustrated, of the motor vehicle, so that hot coolant can flow through the heating heat exchanger. The functioning of this heating heat exchanger  16  can be controlled, as required, for example, by means of a heating regulating valve  40  or else with the aid of flaps (not illustrated), by means of which the air is led in the desired ratio to the heating heat exchanger  16  or led past the latter. The air thermally controlled in the air-conditioning unit  12  can be supplied to the vehicle interior via suitable outflow devices. 
   In addition to the evaporator  14  and the heating heat exchanger  16 , the heating/cooling circuit  10  has an exterior heat exchanger  22  and a compressor  24 . These components of the heating/cooling circuit are connected to one another via refrigerant lines L 1  to L 12  in the way described below. A refrigerant collector  26  is provided in the line L 7  on the inlet side of the compressor  24 . 
   The evaporator  14  is assigned on the inlet side a first expansion member  28  and the exterior heat exchanger  22  is assigned on the inlet side a further expansion member  30 . The two expansion members  28  and  30  can be activated preferably electrically and can be controlled continuously. Furthermore, valves  32 ,  34 ,  36 ,  38 ,  40  and  44  are provided, the functions of which are described further below. The valves  32 ,  34 ,  36 ,  44  are switching valves which can be activated preferably electrically. The valve  38  is preferably a throttle. The valves  32 ,  34 ,  44  may also form a structural unit (what is known as a 4/3-way valve), in which case construction space and cost benefits may arise. 
   In the cooling mode ( FIG. 1 ), that is to say when the air to be supplied to the vehicle interior is solely to be cooled, the heating/cooling circuit  10  is connected up as follows: 
   Starting from the compressor  24 , the refrigerant is supplied to the exterior heat exchanger  22  by the lines L 1 , L 2  and L 3 . For this purpose, the valve  32  is opened and the valves  34  and  44  are closed. In the exterior heat exchanger  22 , the refrigerant, taking the form of hot gas, is cooled. The refrigerant flows from the exterior heat exchanger  22  to the first expansion member  28  via the lines L 4  and L 5 . When it flows through the expansion member  28 , the refrigerant expands and is supplied via the line L 6  to the evaporator  14  in which the refrigerant evaporates and heat is thus extracted from the air to be cooled. Via a line L 7 , the refrigerant is led back to the compressor  24  via the refrigerant collector  26 . In order to increase capacity, heat exchange between a high-pressure-side section (line L 5 ) and a low-pressure-side section (line L 7 ) takes place in an internal heat exchanger  42 , in the present instance heat being transferred from the high-pressure-side section to the low-pressure-side section. 
   In the cooling mode, the heating heat exchanger  16  is inoperative, and the refrigerant is led via the line L 2  to the latter and is let past the second expansion member  30 , so that the line L 2  is used as a heating bypass line. 
   In the heating mode ( FIG. 2 ) where the air to be supplied to the vehicle interior is solely to be heated, the evaporator  14  is inoperative and the heating heat exchanger  16  is operative. The heating/cooling circuit  10  then operates as a heat pump. The refrigerant is supplied as hot gas from the compressor  24  via the lines L 1  and L 8  to the heating heat exchanger  16 , in which the refrigerant discharges heat and at the same time heats the supply air. The refrigerant is supplied via a line L 9  to the second expansion member  30  and is expanded there. The refrigerant is supplied via a line L 3  to the exterior heat exchanger  22  which can then be operated as an evaporator in which the refrigerant evaporates and extracts heat from the exterior air. The refrigerant is supplied to the compressor  24  again via lines L 4 , L 10  (and, if appropriate, to a small extent L 7 ). The line L 10 , which can be shut off via a valve  36 , serves, in the heating mode, as a cooling bypass line for the refrigerant-side bypass of the evaporator  14 . The internal heat exchanger  42  does not function in the heating mode. 
   In addition, in the heating mode, the further heating heat exchanger  20  can be operated by a heating regulating valve  40  being opened correspondingly, so that hot coolant can flow from the engine to the heating heat exchanger  20 . 
   In the reheat mode ( FIG. 3 ), which serves for dehumidifying and heating the air to be supplied to the vehicle interior, both the evaporator  14  and the heating heat exchanger  16  are operative, so that the air can first cool in the evaporator  14  and the moisture contained in the air can condense out. Before the air is supplied to the vehicle interior, it can be heated again in the heating heat exchanger  16  and optionally also in the further heating heat exchanger  20 . 
   In the reheat mode, the refrigerant is supplied from the compressor  24  to the heating heat exchanger  16  via the lines L 1  and L 8 , so that, in the heating heat exchanger  16 , the refrigerant can discharge heat into the air. The refrigerant is led to the expansion member  30  via the line L 9 . Depending on the setting of the expansion member  30 , said refrigerant can pass without any appreciable pressure loss or else be throttled to an advantageous pressure. The refrigerant is led to the exterior heat exchanger  22  via the line L 3 . Depending on the setting of the intermediate pressure, in the exterior heat exchanger  22  the refrigerant extracts heat from the exterior air or discharges heat into the exterior air. From the exterior heat exchanger  22 , the refrigerant is supplied to the expansion member  28  via the lines L 4 , L 5  and to the evaporator  14  via the line L 6 . The refrigerant is returned to the compressor  24  again from the evaporator  14  via the lines L 7 . 
   Depending on the moisture content and temperature of the air, a specific cooling capacity at the evaporator  14  is required for the desired dehumidification. Furthermore, depending on whether the interior of a vehicle is still cold at the start of a trip or has already heated up during the trip, a widely differing heating capacity is required for the subsequent heating of the air to a comfortable level. By the variation of the expansion members  28 ,  30 , the pressure in the exterior heat exchanger  22  can be set ideally between the pressure at the evaporator  14  and the high pressure at the heating heat exchanger  16 , in which case it must be remembered that the high pressure of the heating heat exchanger  16  is dependent not only on the conveying volume of the compressor  24 , but, in particular, on the position of the expansion members  28  and  30  and must be set to a value advantageous for the corresponding boundary conditions. The expansion members  28  and  30  therefore cannot be set independently of one another. If the pressure in the exterior heat exchanger is high, that is to say, the expansion member  30  is opened wide and the expansion member  28  is largely closed, the exterior heat exchanger discharges heat and the ratio between the heating capacity of the heat exchanger  16  and the cooling capacity of the evaporator  14  is low. Such a setting is therefore advantageous in the case of a low heating requirement. 
   If, by contrast, the expansion member  30  is increasingly closed and the expansion member  28  increasingly opened, the pressure in the exterior heat exchanger  22  becomes increasingly lower and the discharged heat capacity of the exterior heat exchanger  22  falls. This results in a falling specific refrigerating capacity in the evaporator  14 . The ratio of the heating capacity in the heating heat exchanger  16  and refrigerating capacity in the evaporator  14  consequently rises. 
   When the pressure in the exterior heat exchanger  22  becomes so low that the boiling temperature of the refrigerant assigned to it undershoots the temperature of the exterior air, the exterior heat exchanger then acts as an evaporator and absorbs heat from the surroundings. This leads to a further reduction in the specific refrigerating capacity in the evaporator  14 . The ratio of the heating capacity in the heating heat exchanger  16  and the refrigerating capacity in the evaporator  14  thus rises with the falling pressure in the exterior heat exchanger  22 . 
   In order to match the available cooling and heating capacity to the current requirements, the expansion members  28  and  30  must be set in such a way that, on the one hand, a high pressure advantageous for the overall capacity is established at the heating heat exchanger  16  and, at the same time, a pressure level suitable for the ratio of the cooling capacity and the heating capacity prevails in the exterior heat exchanger  22 . The overall capacity can be set via the conveying volume of the compressor  24  or else, within certain limits, via the selection of the pressure level in the heating heat exchanger  16 . 
   Preferably, the second expansion member  30  can be shut off, so that, in the cooling mode, no refrigerant can pass from the line L 3  into the line L 9  and consequently into the heating heat exchanger  16 . Alternatively, this shut-off may take place by means of a nonreturn valve or nonreturn flap arranged upstream or downstream of the expansion member  30 . 
   Preferably, the first expansion member  28 , too, can be shut off, so that, in the heating mode, no refrigerant can pass under high pressure from the line L 5  into the line L 6  and consequently into the evaporator  14 . 
   When, in the cooling mode, only the evaporator  14  is operative, the air cooled in the evaporator  14  will cool the heating heat exchanger  16  which follows on the air side, with the result that it may happen that, in the course of time, refrigerant accumulates in the heating heat exchanger  16  and the refrigerant lines L 8  and L 9  and is then lacking in the remaining circuit. Moreover, the system being at a standstill, inadmissibly high pressures could occur in the heating heat exchanger  16  at high temperatures as a result of enclosed refrigerant. In order to avoid this accumulation, a line L 11  throttlable via a throttle  38  is provided, which makes a connection between a line, preferably the inflow line L 8 , connected to the heating heat exchanger  16  and a line or component which is at a low system pressure, that is to say is located between the expansion member  28  and the inlet of the compressor  24 . In the exemplary embodiment illustrated, this is a return line L 7  of the evaporator  14 . This throttle  38  may be formed by a valve, as in the exemplary embodiment, a contraction in the line L 11  or capillary, a porous body or the like. In order to avoid an accumulation of refrigerant, the throttle  38  may have very small dimensioning, so that there is not appreciable influence on the further functioning of the circuit  10 . 
   In a mode in which the ambient air serves as a heat source, that is to say when heat is absorbed from the ambient air in the exterior heat exchanger  22  (heating mode, partially also the reheat mode), there is the risk of icing-up of the exterior heat exchanger  22 , since, due to the extraction of heat, atmospheric moisture, that is to say water, is precipitated on the exterior heat exchanger  22 , said water freezing at temperatures below 0° C. and icing up the exterior heat exchanger  22 . The airstream, indicated by an arrow in  FIGS. 1 to 4 , is thereby impeded when it flows through. Since this airstream also flows via the coolant cooler (not illustrated), which discharges the engine heat, a lack of action of air upon the coolant cooler, along with an unfavorable driving state (high load), may put the engine at risk due to overheating, and it is therefore necessary to monitor the state of the exterior heat exchanger  22 . For this purpose, the temperature difference between the temperature T 1  of the air at the inlet and the temperature T 2  of the refrigerant at the outlet of the exterior heat exchanger  22  is determined. Icing-up occurs, that is to say a defrosting of the exterior heat exchanger  22  is necessary, when the temperature difference becomes too great. Conventional reference values for the temperature difference T 1 −T 2  are 5 to 10 K for an ice-free exterior heat exchanger  22  and 10 to 20 K for an iced-up exterior heat exchanger  22 , although there is dependence on the exterior temperature. 
   For the defrosting mode ( FIG. 4 ), the heating/cooling circuit  10  can be operated in such a way that a discharge of heat takes place in the exterior heat exchanger  22 , so that the ice which has formed is removed again. 
   In the defrosting mode, the refrigerant is led from the compressor  24  via the lines L 1  and L 12  to the expansion member  30  where it is expanded to a low pressure. Subsequently, it is supplied via the line L 3  to the exterior heat exchanger  22 , in which it can discharge its heat, by means of which the ice can be melted and therefore removed. Via the lines L 4  and L 10 , the refrigerant is led via the refrigerant collector  26  to the internal heat exchanger  42 , inoperative in this operating mode, and the compressor  24 . 
   In the defrosting mode, the valves  36  and  42  are open, while the valves  32  and  34  are closed. Furthermore, the first expansion member  28  is preferably closed and the second expansion member  30  is controlled. 
   The end of the defrosting operation can be determined as follows: the temperature T 2  of the refrigerant at the outlet of the exterior heat exchanger  22  is determined and is compared with a limit value. The limit value is conventionally around 5 to 10° C. If the limit value is overshot, the defrosting operation is concluded and the defrosting mode is terminated, so that there can be a changeover again to the heating mode or reheat mode. 
   According to the exemplary embodiment, the defrosting behavior is assisted in that, during the defrosting mode, the airstream flowing through the exterior heat exchanger  22  is minimized or completely prevented. In order to free the exterior heat exchanger of the melted ice, that is to say of the melt water, as quickly as possible, a large to preferably the maximum airstream is briefly conducted via the exterior heat exchanger  22  at the end of the defrosting mode, so that the melt water is blown out of the exterior heat exchanger  22  and premature renewed icing-up can be prevented. 
   Preferably, CO 2  is used as refrigerant, since CO 2  has good thermodynamic properties which make it suitable for a heat-pump mode. When CO 2  is used, for increase in capacity an internal heat exchanger  42  is provided, which brings about a heat exchange between a high-pressure-side section (line L 5 ) and a low-pressure-side section (line L 7 ).