Method for operating a refrigerant circuit for a motor vehicle at extremely cold ambient temperatures, refrigerant circuit and motor vehicle

Abstract

The invention relates to a method for operating a refrigerant circuit (10) for a motor vehicle, in which at least one measured value is acquired. The at least one measured value corresponds to a parameter of a refrigerant that can be conveyed by a compressor (16) of the refrigerant circuit (10), wherein the refrigerant exhibits the parameter at a suction side (26) of the compressor (16). When a threshold value of the at least one measured value is reached or fallen below, a limit value of the parameter, at the point of which the commissioning of the compressor (16) is prevented, is temporarily lowered. The compressor (16) is then commissioned. The invention further relates to the refrigerant circuit (10) and the motor vehicle comprising the refrigerant circuit (10).

Description

The invention relates to a method for operating a refrigerant circuit for a motor vehicle, in which at least one measured value is acquired. The at least one measured value corresponds to a parameter of a refrigerant that can be conveyed by a compressor of the refrigerant circuit. The refrigerant exhibits the parameter on a suction side of the compressor. Furthermore, the invention relates to a refrigerant circuit with a control device and a motor vehicle with the refrigerant circuit. When a vehicle equipped with a refrigerant circuit is used at very low ambient temperatures, the static pressure of the refrigerant in the circuit is also comparatively low. The refrigerant exhibits this static pressure before the compressor is started. It may be designed to prevent the compressor from starting if a certain ambient temperature threshold, and thus a specific static pressure of the refrigerant in the circuit, is undershot. However, the refrigerant circuit cannot then be used in heat pump mode. In heat pump mode, the refrigerant compressed by the compressor can be used to heat air, either directly or indirectly (i.e., via another medium), which can then be introduced into the passenger compartment of the vehicle. If, however, the use of the refrigerant circuit in heat pump mode is prevented, this heating of the airflow is not possible. This can lead to a reduction in comfort for passengers in the passenger compartment and is therefore disadvantageous. US 2022 / 0205662A1 describes an air conditioning system with a refrigerant circuit. When the ambient temperature reaches or falls below 5°C, a bypass valve is opened, and then a compressor in the refrigerant circuit is activated. Opening the bypass valve causes the gaseous refrigerant compressed by the compressor to expand. This, in turn, causes the liquid refrigerant in a refrigerant circuit accumulator to evaporate. The object of the present invention is to provide a method of the type mentioned at the outset, by means of which increased comfort for occupants of a passenger compartment of a motor vehicle can be achieved, as well as to provide a corresponding refrigerant circuit and a motor vehicle with the refrigerant circuit. This problem is solved by a method with the features of claim 1, a refrigerant circuit with the features of claim 9, and a motor vehicle with the features of claim 10. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims and in the following description. In the inventive method for operating a refrigerant circuit for a motor vehicle, at least one measured value is acquired. This at least one measured value corresponds to a parameter of a refrigerant that can be conveyed by a compressor of the refrigerant circuit, which the refrigerant exhibits on a suction side of the compressor. In the inventive method, when a threshold value of the at least one measured value is reached or fallen below, a limit value of the parameter, at the point of which the commissioning of the compressor is prevented, is temporarily lowered. The compressor is then commissioned. The threshold value, at which point the compressor is typically prevented or not permitted to start, is therefore temporarily or for a short period lowered. This allows the compressor to be started and thus the refrigerant circuit to be activated, even though this would be detrimental to the continuous and stable operation of the refrigerant circuit, given the recorded measurement value. This is based on the understanding that temporarily or for a short period lowering the threshold value is unproblematic. The procedure is based, in particular, on the understanding that, for example, if an ambient temperature of approximately -28 °C or even less than -28 °C is recorded, compressor start-up problems can occur in the refrigerant circuit. Therefore, it may be stipulated that starting or commissioning the compressor is excluded or prevented due to the originally set or specified limit value, unless the limit value is temporarily lowered. This is based on the consideration that at the aforementioned ambient temperatures, the refrigerant pressure or static pressure present in the refrigerant circuit, which is present on both the suction and pressure sides of the compressor when it is not yet in operation, is undesirably low. Thus, the static pressure may already have reached or fallen below a permanently minimum permissible low pressure. For example, at the aforementioned extremely low ambient temperatures and when using a synthetic refrigerant such as R1234yf, a negative pressure, i.e., a pressure of less than 1 bar, can occur in the refrigerant circuit or in sections thereof. Furthermore, when using a natural refrigerant such as R744 (carbon dioxide) in the refrigerant circuit, the low ambient temperatures can cause an inversion of the refrigerant and a lubricant, such as a refrigerant oil, in a refrigerant reservoir due to density reversal. This is because, at the very low ambient temperature and the associated very low pressure of the natural refrigerant, the oil or lubricant can float on top of the liquid refrigerant in the refrigerant reservoir. This is disadvantageous because a mixture containing the oil or lubricant and the refrigerant is typically drawn from the bottom of the refrigerant storage tank to supply the lubricant or oil to the compressor during operation of the refrigerant circuit. Therefore, if the refrigerant circuit containing the natural refrigerant is started up at very low ambient temperatures, for example around -28 °C, insufficient lubrication of the compressor can result. This is especially true if the compressor operates continuously under these conditions. Such a situation is detrimental. To avoid such unfavorable operating conditions, a parameter limit can be defined, whereby the compressor is normally or usually prevented from starting up when this limit is reached, but especially when it is undershot. In this case, however, this originally applicable limit is temporarily lowered. Thus, a modified limit in the form of the lowered limit is temporarily set. Accordingly, it is permitted that the compressor may be started even if the originally valid limit value is not exceeded. This ensures, in particular, that the refrigerant circuit can start up and the compressor can be started or put into operation even under extremely cold conditions, i.e., at extremely low ambient temperatures. Consequently, the operating range (towards lowered or reduced limit values) within which the refrigerant circuit can be used in heat pump operation is expanded. This expanded range, within which the refrigerant circuit can be used in heat pump operation, can be based, in particular, on measured values ​​of the refrigerant pressure and / or temperature and / or measured values ​​of the ambient temperature. This process makes it possible, even in extreme cold or very low ambient temperatures, to heat an airflow by operating the refrigerant circuit in heat pump mode, which can then be introduced into the passenger compartment of a vehicle. Therefore, this process provides increased comfort for the vehicle's occupants. In particular, this process can reduce or even eliminate any loss of comfort in the passenger compartment. This also applies if the refrigerant compressed by the compressor is not used alone, but in addition to, for example, electric heating, to warm the airflow that can be introduced into the passenger compartment of the vehicle. It can be stipulated that when the threshold value is reached or fallen below, the parameter's limit is lowered by a fixed amount. This allows, for example, a temporarily or short-term reduction in pressure and consequently a lower refrigerant temperature at the compressor's suction side without preventing the compressor from starting up. Lowering or reducing the limit by a fixed amount is particularly easy to implement in terms of control engineering. If, for example, a natural refrigerant such as R744 is contained in the refrigerant circuit, an original or previously applicable limit of 16 bar can be reduced by a fixed amount of 2 bar, thus lowering it to 14 bar. In this case, the compressor can therefore be started up even if the refrigerant pressure at the suction side of the compressor is 16 bar. Additionally or alternatively, it can be provided that, when an ambient temperature threshold is reached or fallen below, the parameter's limit value is gradually lowered depending on any further decrease in the ambient temperature. In this way, a sliding control limit is essentially defined, up to which the compressor can be operated. This, too, can be implemented very easily using control engineering and / or automation technology. For example, if the ambient temperature falls below -24 °C, the permissible pressure of the refrigerant on the suction side of the compressor can be reduced by 0.5 bar for every further 1 K drop in ambient temperature. This applies, for instance, to the use of refrigerant R744 in the refrigerant circuit, for which the limit at which compressor start-up is normally prevented may be set at, for example, 15 bar until -24 °C is reached. If the temperature falls below -24 °C, the limit can then be reduced by 0.5 bar for every K drop in ambient temperature. This ensures that the compressor, and thus the refrigerant circuit, can be started and operated even at temperatures below -24 °C. This is advantageous. When using a synthetic refrigerant in the refrigerant circuit, the low ambient temperature can cause the refrigerant pressure at the compressor suction side to temporarily drop below 1 bar. Similarly, when using R744 in the refrigerant circuit, the low ambient temperature and resulting low refrigerant temperature can cause the lubricant or oil to float on top of the liquid refrigerant in the receiver. However, this can be tolerated temporarily without damaging the compressor, especially during start-up. At least one measurement can be taken: a pressure value of the refrigerant's static pressure at the compressor's suction side and / or a temperature value of the ambient temperature. Such measurements are usually available for operating the refrigerant circuit anyway, making their use for temporarily lowering the limit value particularly easy. Furthermore, the ambient temperature, especially after extended periods of inactivity of the vehicle equipped with the refrigerant circuit and thus a homogeneous temperature distribution between the vehicle and the refrigerant circuit, corresponds to the refrigerant's static pressure at the compressor's suction side. Therefore, in many cases, the refrigerant's static pressure can be easily determined from the ambient temperature.If the refrigerant circuit is in the resting state in which the refrigerant has the resting pressure, further sensors installed in the refrigerant circuit can be used to determine, in particular, the resting pressure, for example, at least one sensor located downstream of the compressor or refrigerant compressor on the high-pressure side of the refrigerant circuit. Preferably, a pressure value is temporarily reduced as the limit value. This reduced pressure value is then permitted for the refrigerant on the suction side of the compressor. In other words, it is considered acceptable for commissioning the refrigerant circuit if the refrigerant on the suction side of the compressor has a pressure value corresponding to the reduced limit value. This is particularly advantageous because the refrigerant pressure on the suction side, or upstream of the compressor, can already be used for operating the refrigerant circuit. In particular, the reduced pressure value permitted on the compressor's suction side can be a permissible start-up pressure, i.e., a pressure value at which the compressor is allowed to start. Consequently, the operating pressure value can also be reduced, down to which the low pressure may temporarily fluctuate during compressor start-up. Therefore, reduced or lowered pressure values ​​can be permitted as limit values ​​for compressor start-up and, as a result, for the (temporary) operating pressure of the activated refrigerant circuit. Preferably, to increase the pressure of the refrigerant on the suction side of the compressor, a partial flow of the refrigerant compressed by the compressor is fed to the suction side of the compressor, bypassing any evaporator in the refrigerant circuit, whereby the evaporator is supplied with expanded refrigerant. Accordingly, the partial flow of compressed refrigerant can pass virtually directly from a pressure side of the compressor to the suction side of the compressor via a bypass line, preferably one that includes an expansion element, i.e., without this partial flow passing through the at least one evaporator in the refrigerant circuit that is supplied with the expanded refrigerant.Depending on the implemented topology of the refrigerant circuit, the partial flow can be fed to an inlet side of a refrigerant storage tank located upstream of the compressor, with the refrigerant then passing from the refrigerant storage tank to the suction side of the compressor. This makes it very easy to increase the low pressure in the refrigerant circuit, i.e., the pressure of the refrigerant at the suction side of the compressor. This is advantageous to ensure that any temporary reduction in the limit value only results in a temporary drop below the originally applicable limit. The bypass line, also known as a hot gas bypass in the refrigerant circuit, is used to increase the low pressure, i.e., the refrigerant pressure present on the compressor's suction side. To supply the partial flow of compressed refrigerant to the compressor's suction side, a shut-off device in the bypass line, such as a shut-off valve and / or an expansion valve, can be opened as needed. This shut-off device reduces the refrigerant pressure from a high-pressure to a low-pressure level and regulates the mass flow rate of refrigerant to the low-pressure side. By introducing heat into the refrigerant on the low-pressure side of the refrigerant circuit, it is possible, in particular, to ensure that a minimum low pressure is permanently maintained, which corresponds at least to the original, unreduced limit value. This is equivalent to regaining and permanently restoring the limit value originally defined for the refrigerant circuit, which in this case applies to the low-pressure side of the system or the refrigerant circuit. This is advantageous for the stable and robust operation of the refrigerant circuit. After increasing the pressure of the refrigerant on the suction side of the compressor, the parameter limit value that was valid before the reduction can be used again, at which point the compressor can be regulated down or at which point the commissioning or operation of the compressor is prevented. Additionally or alternatively, to increase the refrigerant pressure on the compressor's suction side, heat can be introduced into a coolant flowing through a chiller in the refrigerant circuit, which is pressurized with expanded refrigerant. In this case, the chiller operates as an evaporator. Furthermore, additionally or alternatively, to increase the refrigerant pressure on the compressor's suction side, heat can be introduced into air, which is then used to heat a refrigerant cooler in the refrigerant circuit, also operating as an evaporator. The refrigerant cooler then operates as an air-source heat pump evaporator. For example, the refrigerant and / or the air can be heated using an electric heating element so that the heat can be transferred to the refrigerant. Such a system can be easily implemented. In particular, when the chiller is used to dissipate heat from an electric drive motor of the vehicle containing the refrigerant circuit, inefficient operation of the electric motor can generate heat that is then transferred to the refrigerant. This also provides a simple and reliable way to increase the refrigerant pressure on the suction side of the compressor. It has proven further advantageous to operate at least one heating device in the refrigerant circuit, designed to heat expanded refrigerant, to increase the refrigerant pressure on the suction side of the compressor. This allows the expanded refrigerant to be heated directly by means of this heating device, thereby increasing the refrigerant pressure (i.e., the low pressure) on the suction side of the compressor. In this way, a very direct heat input into the refrigerant can be advantageously achieved. If the refrigerant circuit includes a refrigerant receiver, at least one heating device can be arranged, in particular, upstream of the refrigerant receiver. This is based on the understanding that the refrigerant receiver can contain a comparatively large quantity of refrigerant. Accordingly, by operating the heating device arranged upstream of the refrigerant receiver, the quantity of refrigerant contained in the refrigerant receiver can be brought down to the desired low pressure very quickly. Alternatively, the refrigerant receiver can be heated directly by means of a heating device designed as a surface heater and / or by means of a heating device integrated into the refrigerant receiver. This is also advantageous. The refrigerant circuit according to the invention includes a control device. This control device is configured to temporarily lower the limit value of the parameter in the method according to the invention. Consequently, the refrigerant circuit according to the invention can be operated according to the method according to the invention. This is advantageous in order to quickly provide increased comfort for the occupants of a passenger compartment of a motor vehicle when the refrigerant circuit is used in the vehicle. The motor vehicle according to the invention has the refrigerant circuit according to the invention. Accordingly, the method can be advantageously used in the motor vehicle in which the limit value of the parameter is temporarily lowered in order to be able to start the compressor of the refrigerant circuit even at extremely low ambient temperatures. The advantages and preferred embodiments described for the method according to the invention apply analogously to the refrigerant circuit according to the invention as well as to the motor vehicle according to the invention and vice versa. The invention therefore also includes further developments of the refrigerant circuit and the motor vehicle according to the invention, which have features as already described in connection with the further developments of the method according to the invention. For this reason, the corresponding further developments of the refrigerant circuit and the motor vehicle according to the invention are not described again here. The motor vehicle according to the invention is preferably designed as a motor vehicle, in particular as a passenger car or truck, or as a passenger bus. The invention also includes the control unit for the motor vehicle. The control unit can comprise a data processing device or a processor unit (processor circuit) configured to carry out the method according to the invention. For this purpose, the processor unit can comprise at least one microprocessor and / or at least one microcontroller and / or at least one FPGA (Field Programmable Gate Array) and / or at least one DSP (Digital Signal Processor). In particular, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or an NPU (Neural Processing Unit) can be used as the microprocessor. Furthermore, the processor unit can comprise program code configured to carry out the method according to the invention when executed by the processor unit. The program code can be stored in a data memory of the processor unit.The processor setup can be based on at least one circuit board and / or at least one SoC (System on Chip). The invention also includes combinations of the features of the described embodiments. The invention therefore also includes realizations that each exhibit a combination of the features of several of the described embodiments, provided that the embodiments have not been described as mutually exclusive. Exemplary embodiments of the invention are described below. Figure 1 schematically shows a refrigerant circuit of a motor vehicle, wherein a compressor of the refrigerant circuit can be operated even at extremely cold ambient temperatures; and Figure 2 shows a highly schematic representation of the motor vehicle with the refrigerant circuit. The exemplary embodiments described below are preferred embodiments of the invention. In these exemplary embodiments, the described components each represent individual features of the invention, which can be considered independently of one another and each further develops the invention independently. Therefore, the disclosure is intended to include combinations of features of the embodiments other than those shown. Furthermore, the described embodiments can also be supplemented by further features of the invention already described. In the figures, identical reference symbols denote functionally equivalent elements. Figure 1 shows an exemplary and schematic representation of a refrigerant circuit 10, as it can be used in a motor vehicle 12 (see Figure 2). The following section explains the operation of the refrigerant circuit 10 as a heat pump, which is advantageous, for example, when it is necessary to rapidly heat the air in a passenger compartment 14 (see Figure 2) of the motor vehicle 12. In heat pump mode, a refrigerant contained in the refrigerant circuit 10 can be compressed by a compressor 16 of the refrigerant circuit 10 and fed to a heat exchanger 18 of the refrigerant circuit 10. An airflow 20, which passes through the heat exchanger 18 and can be introduced into the passenger compartment 14 of the motor vehicle 12, is illustrated by an arrow in Figure 1. In the following explanation of the operation of refrigerant circuit 10, it will be assumed, by way of example, that refrigerant circuit 10 contains the natural refrigerant R744 (carbon dioxide). Therefore, when pressure values ​​for the refrigerant are referenced below, these apply as an example to refrigerant R744. However, the findings and measures described for operation with this natural refrigerant apply analogously if refrigerant circuit 10 is operated with a different refrigerant, such as another natural refrigerant or a synthetic refrigerant. In this case, adjusted pressure values ​​based on the respective material properties of the refrigerant can be used. In particular, if very low ambient temperatures are present in the environment 22 (see Fig. 2) of the motor vehicle 12, this is accompanied by a very low static pressure of the refrigerant in the refrigerant circuit 10. For example, if the temperature in the environment 22 of the motor vehicle 12, i.e., the ambient temperature, corresponds to the temperature of the refrigerant, and if this temperature is approximately -28 °C, the static pressure of the refrigerant in the refrigerant circuit 10 can be approximately 15 bar. The static pressure of the refrigerant is the pressure prevailing in the refrigerant circuit 10 before the compressor 16 is started up. Starting or commissioning the compressor 16 at static pressures in the range of approximately 15 bar, but especially below this pressure, is critical, particularly with regard to the conditions in a preferably provided low-pressure-side refrigerant receiver 24 of the refrigerant circuit 10. In the exemplary variant of the refrigerant circuit 10 shown in Fig. 1, the refrigerant receiver 24 is designed as a low-pressure accumulator. Due to the low static pressure and correspondingly low temperature of the refrigerant, oil or similar lubricant contained in the refrigerant circuit 10 can float on top of the liquid refrigerant present in the refrigerant receiver 24. This occurs because, as a result of a density reversal, the oil is less dense than the very cold, liquid refrigerant. This is particularly true when, after a system start-up, i.e., after the compressor 16 has been commissioned, the operating pressure is further reduced. This can, in turn, lead to an insufficient amount of lubricant or oil in the mixture of refrigerant and lubricant, which is drawn from a bottom area of ​​the refrigerant receiver 24 and conveyed to a suction side 26 of the compressor 16, to lubricate the compressor 16. Therefore, it is disadvantageous to operate the compressor 16 for extended periods if the refrigerant at the suction side 26 of the compressor 16 has a low pressure of only 15 bar or even less than 15 bar. A brief drop below a value of, for example, 15 bar on the suction side 26 of the compressor 16 can occur during system start-up, i.e., when the compressor 16 is commissioned. This is based on the understanding that, for example, due to the inertia of sensors for measuring the refrigerant pressure and the time required to transmit the pressure readings, as well as the reaction times during operation of the refrigerant circuit 10, the pressure on the suction side 26 may briefly fall below the limit of 15 bar, which should ideally not be permanently undercut. In this case, a control device 28 of the refrigerant circuit 10 ensures that the compressor 16 can be started even at very low ambient temperatures of, for example, -28 °C and below, in order to heat the airflow 20 by means of the heat exchanger 18. This is because the pressure limit, for example, 15 bar, which is stored in a memory of the control device 28 and at which, or especially when, the compressor 16 is not started or even operated at all, is temporarily lowered.The limit of 15 bar therefore corresponds to a control limit at which the compressor 16 is usually or normally controlled to prevent the compressor 16 from operating in a situation where the pressure of the refrigerant on the suction side 26 of the compressor 16 permanently falls below the limit of 15 bar. In this case, however, this limit value is temporarily or for a short period of time lowered, after which the compressor 16 can be put into operation. For example, the limit value of 15 bar can be temporarily lowered by a fixed amount, for example 2 bar, if a measured value is recorded that indicates a specific static pressure of the refrigerant at the suction side 26 of the compressor 16 and / or a specific temperature value of the ambient temperature. The static pressure of the refrigerant at the suction side 26 of the compressor 16 can be recorded by means of a sensor 30 of the refrigerant circuit 10, which is preferably designed to detect a pressure and a temperature of the refrigerant. The static pressure prevailing in the refrigerant circuit 10 before the compressor 16 is started can also be measured and subsequently evaluated by means of at least one further sensor (not shown here) in the refrigerant circuit 10. For example, a high-pressure sensor can be used for this purpose, which can be located downstream of the compressor 16. From a defined pressure threshold, i.e., when a measured value reaches or falls below a certain threshold, the control pressure or limit value can be reduced by a fixed amount, based on the static pressure and / or the measured ambient temperature. For example, the limit value for the static pressure, at which the start-up of compressor 16 is usually prevented, can be set to 16 bar in the control unit 28. Then, by temporarily lowering this limit value, compressor 16 can be started even if the static pressure of the refrigerant is only 14 bar. It is also possible to introduce a sliding cutoff limit for the low pressure, for example, when the ambient temperature falls below -24 °C. Up to this temperature of -24 °C, the control unit 28 can therefore use a limit of 15 bar, at which point, and especially when this limit is not reached, continuous operation of the compressor 16 should usually or normally be prevented. This limit of, for example, 15 bar can thus be referred to as the standard limit or generally applicable limit. A measuring sensor, or at least a sensor designed to acquire measured values, can exhibit tolerances and, where applicable, aging effects. As a result, it can happen that an evaluation unit, which may be provided in particular by the control unit 28, receives an actual value as a measured value that deviates from a target value. Based on tolerance bands known for the respective sensor and the resulting inaccuracy of the sensor, and / or based on knowledge of the sensor's aging, an acquired measured value can be corrected if necessary, in particular towards a direction that is not critical for the system operation, i.e., for the operation of the refrigerant circuit 10. In the present case, if the ambient temperature drops further, the limit value of this parameter can be gradually lowered, for example by 0.5 bar per K. This ensures that even at ambient temperatures 22 of the vehicle 12 of less than -24 °C, the compressor 16 is started up to supply compressed refrigerant to the heat exchanger 18 for the purpose of conditioning or heating the airflow 20 or cabin supply airflow. When the compressor 16 is then put into operation, measures are preferably taken quickly to establish a stable pressure ratio, i.e., a stable ratio of the refrigerant pressure present at a pressure side 32 of the compressor 16 to the refrigerant pressure present at the suction side 26 of the compressor 16. After the compressor 16 has been put into operation, it is therefore preferably ensured that the low pressure of the refrigerant, i.e., the pressure present at the suction side 26 of the compressor 16, quickly reaches at least its original, unreduced limit value. This can be achieved by introducing heat on the low-pressure side of the refrigerant circuit 10. For this purpose, for example, a partial flow of the refrigerant compressed by the compressor 16 can be supplied to the suction side 26 of the compressor 16 via a bypass line 34 of the refrigerant circuit 10. This bypasses any evaporator of the refrigerant circuit 10, which is supplied with expanded refrigerant during heat pump operation of the refrigerant circuit 10. As an example of such an evaporator usable in the heat pump operation of the refrigerant circuit 10, a chiller 36 is schematically shown in Fig. 1. This chiller is integrated into both the refrigerant circuit 10 and a coolant circuit 38. For example, the heat contained in the coolant can be used to evaporate and heat the expanded refrigerant in the chiller 36. A corresponding water-source heat pump operation will be explained below with reference to Fig. 1. From the pressure side 32 of the compressor 16, the compressed refrigerant enters a heating branch 40 of the refrigerant circuit 10, through which the compressed refrigerant is directed to the heat exchanger 18. For this purpose, a first shut-off valve 42 can be closed and a second shut-off valve 44 open. A bypass line 34 branches off from the heating branch 40, in which a shut-off device 46 is arranged. By opening the shut-off device 46, which can be designed, for example, as a continuously adjustable shut-off valve or as an expansion device, to a certain extent and thus as required, it is ensured that the partial flow of the compressed refrigerant bypasses the chiller 36 and flows to the suction side 26 of the compressor 16. The bypass line 34 can also be used when, in an air-source heat pump operation, the refrigerant circuit 10 is operated at least in refrigerant coolers 48, 50 as evaporators or air-source heat pump evaporators of the refrigerant circuit 10. In the variant of the refrigerant circuit 10 shown by way of example in Fig. 1, the two refrigerant coolers 48, 50 are connected in series to the refrigerant circuit 10 so that the refrigerant can flow through them. For example, the compressed refrigerant coming from the heat exchanger 18 can then be expanded by means of an expansion device 54, which in the air-source heat pump operation is arranged upstream of the two refrigerant coolers 48, 50, when a further shut-off valve 52 of the refrigerant circuit 10 is open. Accordingly, in air-source heat pump operation, the two refrigerant coolers 48, 50 can be supplied with expanded refrigerant and thus used as evaporators. The refrigerant coolers 48, 50, which are exposed to ambient air, are preferably arranged in the area of ​​the front 56 of the vehicle 12 (see Fig. 2) when the refrigerant circuit 10 is installed in the vehicle 12. In air-source heat pump operation, the refrigerant coming from the two refrigerant coolers 48, 50 can then, when a further shut-off valve 58 of the refrigerant circuit 10 is open, flow back to the suction side 26 of the compressor 16 via the refrigerant receiver 24. When the chiller 36 is used as an evaporator in the refrigerant circuit 10, the refrigerant can be expanded by partially opening a further expansion device 60, which is located upstream of the chiller 36. In this water-heat pump operation of the refrigerant circuit 10, the refrigerant coming from the chiller 36 also returns to the suction side 26 of the compressor 16 via the refrigerant receiver 24. When the chiller 36 is used as an evaporator in the water-source heat pump operation, heat can be introduced into the refrigerant via a coolant heater 62. This refrigerant flows through the chiller 36, which is pressurized with the expanded refrigerant. In this way, the low pressure of the refrigerant can also be increased after the compressor 16 has been started up with a temporarily reduced limit value. The originally applicable limit value can then be restored and, if necessary, exceeded. Similarly, in air-source heat pump operation, an air heater 64 can be used, which can, for example, be designed as an electric heating device to introduce heat into the ambient air with which the at least one refrigerant cooler 48, 50, operated as an evaporator, is supplied. The air heater 64 then serves to raise the low pressure after the compressor 16 has been started up. If both the chiller 36 and the refrigerant coolers 48, 50 are used as evaporators, both the coolant heater 62 and the air heater 64 can be used to increase the pressure of the refrigerant at the suction side 26 of the compressor 16. As shown in Fig. 1, to increase the pressure of the refrigerant at the suction side 26 of the compressor 16, at least one heating device 66, 68 can be used additionally or alternatively, which is designed to directly heat the expanded refrigerant. For example, the first heating device 66 shown in Fig. 1 is arranged between the refrigerant receiver 24 and the suction side 26 of the compressor 16, viewed in the direction of refrigerant flow. The second heating device 68, shown by way of example in Fig. 1, is integrated into the refrigerant circuit 10 upstream of the refrigerant receiver 24 and is designed to directly heat the expanded refrigerant. Furthermore, direct heat input into the refrigerant receiver 24 can be implemented via a surface heating element (not shown here) or via a heating device integrated into the refrigerant receiver 24 (also not shown). The temporary undershooting or falling below the originally valid limit value, achieved by temporarily lowering the limit value and thus temporarily setting a modified, reduced limit value, is therefore permitted in this case. The originally valid limit value is the value at which, or especially at which, the compressor would have been prevented from starting up if the control device 28 had not effected the temporary lowering of the limit value. The low pressure is then increased via at least one heat source, for example in the form of the coolant heater 62 and / or the air heater 64 and / or the at least one heating device 66, 68 and / or by using the bypass line 34, which serves as a hot gas bypass. The standard defined system limit is therefore temporarily undercut, namely during start-up or during a dynamic phase of operation of the refrigerant circuit 10. During a steady-state phase of operation of refrigerant circuit 10, the low pressure is then raised by the heat input on the low-pressure side, thus permanently establishing at least the standard-defined or originally valid limit value. Refrigerant circuit 10 can then continue to operate with the standard-defined limit value. The heat input on the low-pressure side of the refrigerant circuit 10 ensures that at least the pressure limit on the suction side 26 of the compressor 16, which is not normally exceeded, is reached again. This heat input can be achieved through internal measures, particularly by using the hot gas bypass or the bypass line 34, and / or through external measures such as using at least one electric auxiliary heater or increasing the release of waste heat from at least one consumer. Subsequently, a further increase in the low pressure can be advantageous. This is because increasing the refrigerant pressure on the suction side 26 of the compressor 16 also increases the refrigerant density and, consequently, the mass flow rate of refrigerant delivered by the compressor 16 during operation. This results in an increase in the heating potential, which can be used at the heat exchanger 18 to heat the airflow 20. In the exemplary variant of the refrigerant circuit 10 shown in Fig. 1, a refrigerant line 70 can optionally be provided, through which, when the further shut-off valve 52 is closed, the refrigerant coming from the heat exchanger 18 can be expanded by means of a further (also optional) expansion device 72. The expanded refrigerant is then first supplied to the first refrigerant cooler 50 in this refrigerant flow direction and then to the second refrigerant cooler 48. From the second refrigerant cooler 48, the refrigerant can flow via a connecting line 74 of the refrigerant circuit 10 to the chiller 36. In heat pump operation, the two refrigerant coolers 48 and 50 and the chiller 36 are then circulated through in series with the entire refrigerant flow exiting the heat exchanger 18. If, on the other hand, the additional shut-off valve 52 is open, the refrigerant coming from the heat exchanger 18 can be divided between the two refrigerant coolers 48 and 50, which then operate as evaporators, and the chiller 36. When the refrigerant circuit 10 is used in cooling mode, the refrigerant coming from the pressure side 32 of the compressor 16 does not flow into the heating circuit 40. Instead, with the first shut-off valve 42 open and the second shut-off valve 44 closed, the compressed refrigerant flows towards the two refrigerant coolers 48 and 50, first entering the first refrigerant cooler 50 (in this flow direction) and then the second refrigerant cooler 48. Depending on the refrigerant used in the refrigerant circuit, the refrigerant coolers 48 and 50 can be operated as condensers or as gas coolers in cooling mode. The refrigerant cooled in the refrigerant coolers 48, 50 can be expanded during cooling operation and fed to the chiller 36 and / or a further evaporator of the refrigerant circuit 10, designed as an interior evaporator 76. The further expansion device 60 upstream of the chiller 36 can be used to expand the refrigerant supplied to it. An expansion device 78 is also connected upstream of the interior evaporator 76, by means of which the refrigerant supplied to the interior evaporator 76 during cooling operation can be expanded. From the chiller 36 and / or the interior evaporator 76, the expanded refrigerant returns via the refrigerant receiver 24 to the suction side 26 of the compressor 16. The interior evaporator 76 can also be used in heat pump operation, for example to dehumidify the airflow 20, which is then heated by means of the heat exchanger 18. Further components of the refrigerant circuit, such as an internal heat exchanger 80 and preferably provided check valves 82, 84, need not be explained in detail here. The same applies to a cooler 86, which is designed in particular as a low-temperature cooler and can be integrated into the coolant circuit 38. The cooler 86, together with the at least one refrigerant cooler 48, 50, can be arranged in the area of ​​the vehicle front 56 and be part of a radiator package or a radiator assembly of the motor vehicle 12. Overall, the examples show how a system start-up of the refrigerant circuit 10 can be provided under extremely cold conditions.

Claims

Method for operating a refrigerant circuit (10) for a motor vehicle (12), in which at least one measured value is recorded, wherein the at least one measured value corresponds to a parameter of a refrigerant that can be conveyed by a compressor (16) of the refrigerant circuit (10), which the refrigerant has on a suction side (26) of the compressor (16), characterized in that when a threshold value of the at least one measured value is reached or fallen below, a limit value of the parameter, at the reaching or falling below of which a commissioning of the compressor (16) is prevented, is temporarily lowered, whereupon the compressor (16) is commissioned. Method according to claim 1, characterized in that, when the threshold value is reached or fallen below, the limit value of the parameter is reduced by a fixed amount. Method according to one of the preceding claims, characterized in that, when the threshold value of an ambient temperature is reached or fallen below, the limit value of the parameter is gradually lowered depending on a further decrease in the ambient temperature. Method according to one of the preceding claims, characterized in that at least one measured value is a pressure value of a static pressure of the refrigerant at the suction side (26) of the compressor (16) and / or a temperature value of an ambient temperature. Method according to one of the preceding claims, characterized in that the limit value is temporarily reduced to a pressure value which is permitted for the refrigerant on the suction side (26) of the compressor (16). Method according to one of the preceding claims, characterized in that, in order to increase the pressure of the refrigerant at the suction side (26) of the compressor (16), a partial flow of the refrigerant compressed by the compressor (16) is supplied to the suction side (26) of the compressor (16) bypassing any evaporator (36, 48, 50, 76) of the refrigerant circuit (10), which is supplied with expanded refrigerant. Method according to one of the preceding claims, characterized in that, in order to increase the pressure of the refrigerant at the suction side (26) of the compressor (16), heat is introduced into a coolant which flows through a chiller (36) of the refrigerant circuit (10) supplied with expanded refrigerant, and / or heat is introduced into air which is supplied to a refrigerant cooler (48, 50) of the refrigerant circuit (10) operated as an evaporator. Method according to one of the preceding claims, characterized in that at least one heating device (66, 68) of the refrigerant circuit (10) is operated in order to increase a pressure of the refrigerant on the suction side (26) of the compressor (16) and is designed for heating expanded refrigerant, in particular arranged upstream of a refrigerant collector (24) of the refrigerant circuit (10). Refrigerant circuit (10) with a control device (28), wherein the control device (28) is configured to effect the temporary reduction of the limit value of the parameter in a method according to one of the preceding claims. Motor vehicle (12) with a refrigerant circuit (10) according to claim 9 .

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