Method for operating a refrigerant circuit of a motor vehicle using two Magnus formulas and a motor vehicle

Abstract

The invention relates to a motor vehicle and a method for operating a refrigerant circuit of the motor vehicle, in which, to reduce the probability of ice formation on an evaporator of the refrigerant circuit, a dew point of air with which the evaporator can be supplied is determined. For determining the dew point, the air temperature and relative humidity are detected by means of a sensor device of the refrigerant circuit. A characteristic curve (32) is used to determine the dew point, which indicates a saturation vapor pressure of the air, wherein in a first section (60) of the characteristic curve (32) the dew point is determined using a first calculation method, and wherein in a second section (62) of the characteristic curve (32) the dew point is determined using a second calculation method, which differs from the first calculation method.

Description

[0001] The invention relates to a method for operating a refrigerant circuit of a motor vehicle, in which a dew point of air is determined to reduce the likelihood of ice formation on an evaporator of the refrigerant circuit. The evaporator of the refrigerant circuit can be exposed to air. To determine the dew point, a temperature of the air and a relative humidity of the air are detected by a sensor device of the refrigerant circuit. Furthermore, the invention relates to a motor vehicle with a refrigerant circuit.

[0002] CN 1 04 807 146 A describes a method for antifreeze detection in which the temperature of an evaporator tube and the dew point temperature of the air are determined. An empirical Magnus formula is used to determine the dew point temperature.

[0003] JP 2004-20 118 A describes a control of an air conditioning system during a cleaning process with ozone, whereby a dew point temperature is calculated.

[0004] DE 10 2011 051 285 A1 describes a method for controlling the evaporator of a heat pump in a vehicle to prevent icing. To determine the dew point of ambient air, an ambient temperature signal and a humidity signal are recorded in the vehicle using the sensors installed there.

[0005] It has been discovered that inaccuracies can occur when determining the dew point of air that can be applied to an evaporator of a motor vehicle's refrigerant circuit operating as a heat pump. This is unfavorable. If the dew point of the air passing over the evaporator during heat pump operation of the refrigerant circuit is not determined correctly, unwanted ice formation can occur on the evaporator. This, in turn, adversely reduces the air flow through the evaporator.

[0006] The object of the present invention is therefore to provide a method of the type mentioned at the outset which enables a particularly precise determination of the dew point, and to create a motor vehicle designed to carry out the method.

[0007] This object is achieved by a method having the features of patent claim 1 and by a motor vehicle having the features of patent claim 10. Advantageous embodiments with expedient further developments of the invention are specified in the dependent patent claims and in the following description.

[0008] In the method according to the invention for operating a refrigerant circuit of a motor vehicle, a dew point of air is determined to reduce the likelihood of ice formation on an evaporator of the refrigerant circuit. The evaporator of the refrigerant circuit can be exposed to the air, the dew point of which is then determined. To determine the dew point, a temperature of the air and a relative humidity of the air are detected by a sensor device associated with the refrigerant circuit. To determine the dew point, a characteristic curve is used which indicates a saturation vapor pressure of the air. In a first section of the characteristic curve, the dew point is determined using a first calculation rule. And in a second section of the characteristic curve, the dew point is determined using a second calculation rule which is different from the first calculation rule.

[0009] The calculation rules can be used, in particular, to create the respective sections of the characteristic curve, i.e., the first and second sections of the characteristic curve. By using or utilizing different calculation rules to determine the dew point in the respective sections of the characteristic curve, a particularly precise determination of the dew point is possible. By using the respective calculation rules, it can be ensured that the characteristic curve as a whole allows for a very precise determination of the dew point using the respective section of the characteristic curve.

[0010] The pair of values ​​of air temperature and relative humidity corresponds to a certain absolute water content of the air, which can be expressed, for example, in grams per kilogram. This absolute water content corresponds to a certain air temperature at 100% relative humidity. This air temperature is the dew point of the air, or the dew point temperature. At the dew point, the relative humidity is 100% and the air is saturated with water vapor.

[0011] To prevent ice formation or icing on the evaporator of the refrigerant circuit, it is important to know the dew point of the air that is or can be exposed to the evaporator as precisely as possible. If the air temperature falls below this dew point, the water contained in the air will condense. And if the evaporator's operating temperature is below 0°C, the condensed water can freeze. This can lead to ice formation on the evaporator. Such undesirable ice formation on the evaporator can be specifically prevented if the dew point of the air can be precisely determined. Therefore, the method using the two calculation rules is particularly well suited to reducing the likelihood of ice formation on the evaporator of the refrigerant circuit.

[0012] The respective calculation rule can be used in the respective section of the characteristic curve in such a way that the respective calculation rule is used to determine the course of the characteristic curve in the respective section. Accordingly, the first calculation rule can be used to determine the course of the first section. And the second calculation rule, which is different from the first calculation rule, can be used to determine the course of the characteristic curve in the second section. In particular, by selecting calculation rules in the respective section that are particularly well suited to correctly describe the saturation vapor pressure of the air, the dew point can be determined very precisely.Because if the respective sections of the characteristic curve indicate the saturation vapor pressure of the air particularly realistically, the dew point of the air can also be specified very precisely, which is determined based on the pair of values ​​of the temperature of the air and the relative humidity of the air using the characteristic curve.

[0013] In particular, using the respective calculation rule for each section of the characteristic curve leads to greater precision in determining the dew point than would be the case if one of the two calculation rules were used for both sections of the characteristic curve. Therefore, using the first calculation rule in the first section of the characteristic curve and the second calculation rule in the second section of the characteristic curve is very advantageous with regard to the precise determination of the dew point.

[0014] Preferably, a respective Magnus formula is used as the first calculation rule and as the second calculation rule. This is based on the realization that Magnus formulas offer a very simple way to specify the saturation vapor pressure for water vapor. Thus, the use of a first Magnus formula as the first calculation rule and the use of a second Magnus formula as the second calculation rule allows for a very precise creation or derivation of the characteristic curve, which can be used to determine the dew point based on the pair of values ​​of the air temperature and the relative humidity of the air.

[0015] Preferably, the first calculation rule is used when the air temperature detected by the sensor device is less than 0°C. In this case, the second calculation rule is used when the air temperature detected by the sensor device is more than 0°C. This is based on the knowledge that using one and the same calculation rule for a temperature range from less than 0°C to more than 0°C, each beyond the zero-degree limit, can lead to inaccuracies when determining the dew point using the characteristic curve. This can be largely avoided by using the first calculation rule when the air temperature detected by the sensor device is less than 0°C and by using the second calculation rule when the air temperature detected by the sensor device is more than 0°C.

[0016] Preferably, when the air temperature detected by the sensor device is below 0°C, a Magnus formula valid for the saturation vapor pressure above a flat ice surface is used as the first calculation rule. And when the air temperature detected by the sensor device is above 0°C, a Magnus formula valid for the saturation vapor pressure above a flat water surface is used as the second calculation rule. This is based on the knowledge that respective Magnus formulas are available for specifying both the saturation vapor pressure above flat water surfaces and the saturation vapor pressure above flat ice surfaces. Therefore, these Magnus formulas can be used very simply and with little effort in the method.

[0017] This also ensures that the respective sections of the characteristic curve accurately reflect the actual conditions with regard to the dew point, which corresponds to a specific absolute water content of the air, whose temperature and relative humidity are measured by the sensor device. The implementation of these Magnus formulas, which apply to the saturation vapor pressure above the flat ice surface or the flat water surface, in a corresponding control device for the refrigerant circuit is particularly easy to implement.

[0018] Preferably, the first calculation rule is the Magnus formula Ei(t)=6.112 hPa⋅exp(22.46⋅t272.62°C+t) with E i (t) = saturation vapor pressure above the flat ice surface and t = air temperature in °C and as the second calculation rule the Magnus formula Ew (t)=6.112 hPa⋅exp(17.62⋅t243.12°C+t) with E w (t) = saturation vapor pressure above the flat water surface and t = air temperature in °C used.

[0019] These Magnus formulas provide particularly precise results to specify the saturation vapor pressure of the air through the characteristic curve used to determine the dew point.

[0020] Preferably, the characteristic curve is used by a control device of the refrigerant circuit, wherein data values ​​of the air temperature and the relative humidity of the air, detected by the sensor device, are transmitted to the control device. The control device can be integrated into the sensor device, in particular if the sensor device is designed as a dew point sensor. However, the characteristic curve can also be used by a control device of the refrigerant circuit to determine the dew point, which control device is designed separately from the sensor device and which is provided, for example, by a control unit of the refrigerant circuit. The dew point can advantageously be taken into account by the control device in order to trigger the implementation of at least one measure which serves to reduce the likelihood of ice formation on the evaporator.

[0021] Preferably, the control device controls at least one further component of the refrigerant circuit depending on the determined dew point for the purpose of reducing the likelihood of ice formation in the evaporator. This makes it very easy to ensure that no ice formation occurs on the evaporator, or at most, that ice formation occurs to a negligible extent.

[0022] Preferably, the control device reduces the refrigerant pressure prevailing on an inlet side of a compressor of the refrigerant circuit to reduce the likelihood of ice formation on the evaporator. Such a reduction in the suction pressure in the refrigerant circuit ensures a less pronounced temperature drop at the evaporator during operation of the refrigerant circuit and thus leads to a reduction in the likelihood of ice formation on the evaporator.

[0023] Additionally or alternatively, the control device can change the mass flow of refrigerant delivered by the compressor of the refrigerant circuit to reduce the likelihood of ice formation on the evaporator. This measure has also proven effective in at least largely preventing ice formation on the evaporator.

[0024] Additionally or alternatively, the control device can, for the purpose of reducing the likelihood of ice formation on the evaporator, cause a change in the expansion of the refrigerant by means of an expansion device of the refrigerant circuit located upstream of the evaporator. This also contributes to the goal of reducing the likelihood of ice formation on the evaporator.

[0025] Preferably, the sensor device of the refrigerant circuit detects the temperature and relative humidity of the ambient air that is applied to the evaporator of the refrigerant circuit when the refrigerant circuit is operating in heat pump mode. This is based on the knowledge that the refrigerant circuit is preferably operated as a heat pump when a passenger compartment of the motor vehicle is to be heated by operating the refrigerant circuit as a heat pump. This is usually the case when the ambient air temperature, i.e. the air surrounding the motor vehicle, is low. In order to achieve absorption of the heat contained in the ambient air by the refrigerant evaporating in the evaporator, the evaporator must be set to an operating temperature that is even lower than the ambient air temperature.At low ambient temperatures, particular care must be taken to ensure that no ice forms on the evaporator of the refrigerant circuit.

[0026] Accordingly, it is advantageous to use the refrigerant circuit's sensor device to detect the temperature and relative humidity of the ambient air when the refrigerant circuit's evaporator is exposed to ambient air during heat pump operation. This allows timely countermeasures to be taken before ice forms on the evaporator.

[0027] Preferably, during heat pump operation, a refrigerant cooler in the refrigerant circuit is pressurized with air, which is introduced into a passenger compartment of the motor vehicle. The refrigerant, previously compressed by the compressor in the refrigerant circuit, is cooled at the refrigerant cooler. This heat released into the air can be used to increase the temperature in the passenger compartment of the motor vehicle. This allows the temperature in the passenger compartment of the motor vehicle to be adjusted very quickly as desired.

[0028] Additionally or alternatively, the refrigerant cooler can be designed as a heat exchanger, through which the refrigerant flows on the one hand and a liquid coolant on the other. In this case, the liquid coolant, which is heated in the heat pump operation of the refrigerant circuit, can transfer heat to the air, which is then introduced into the passenger compartment of the motor vehicle. This is also advantageous for quickly setting a pleasantly warm temperature in the passenger compartment of the motor vehicle.

[0029] The refrigerant circuit can therefore be operated as either an air-source heat pump or a water-source heat pump. The liquid coolant, which absorbs the heat released by the refrigerant at the refrigerant cooler, can be referred to as cooling water, or simply as water. This applies even if the liquid coolant contains non-aqueous additives.

[0030] The heat pump operation of the refrigerant circuit enables a pleasant indoor climate to be created in the passenger compartment of the vehicle for the occupants.

[0031] The motor vehicle according to the invention has a refrigerant circuit, wherein a control device of the refrigerant circuit is designed to determine a dew point of air with which the evaporator can be acted upon in order to reduce the probability of ice formation on an evaporator of the refrigerant circuit. To determine the dew point, a temperature of the air and a relative humidity of the air can be detected by means of a sensor device of the refrigerant circuit. The control device is designed to use a characteristic curve that indicates a saturation vapor pressure of the air to determine the dew point. The control device is designed to determine the dew point in a first section of the characteristic curve using a first calculation rule.The control device is further designed to determine the dew point in a second section of the characteristic curve using a second calculation rule that is different from the first calculation rule.

[0032] Accordingly, the motor vehicle is designed to carry out the method according to the invention. Advantageously, the motor vehicle enables particularly precise determination of the dew point.

[0033] The advantages and preferred embodiments described for the method according to the invention apply analogously to the motor vehicle according to the invention and vice versa.

[0034] The invention therefore also includes further developments of the motor vehicle according to the invention that have features already described in connection with the further developments of the method according to the invention. For this reason, the corresponding further developments of the motor vehicle according to the invention are not described again here.

[0035] 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.

[0036] For use cases or application situations that may arise during the method and which are not explicitly described here, it may be provided that, in accordance with the method, an error message and / or a request to enter user feedback is issued and / or a default setting and / or a predetermined initial state is set.

[0037] The invention also includes the control device for the motor vehicle. The control device can have a data processing apparatus or a processor device that is configured to carry out an embodiment of the method according to the invention. For this purpose, the processor device can have 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 (Graphical Processing Unit), or an NPU (Neural Processing Unit) can be used as the microprocessor. Furthermore, the processor device can have program code that is configured to carry out the embodiment of the method according to the invention when executed by the processor device. The program code can be stored in a data memory of the processor device.The processor device can be based, for example, on at least one circuit board and / or on at least one SoC (System on Chip).

[0038] As a further solution, the invention also encompasses a computer-readable storage medium comprising program code which, when executed by a computer or computer network, causes the computer to carry out an embodiment of the method according to the invention. The storage medium can be provided at least partially as a non-volatile data memory (e.g. as a flash memory and / or as an SSD - solid state drive) and / or at least partially as a volatile data memory (e.g. as a RAM - random access memory). The storage medium can be arranged in the computer or computer network. However, the storage medium can also be operated on the Internet, for example, as a so-called app store server and / or cloud server. The computer or computer network can provide a processor circuit with, for example, at least one microprocessor.The program code may be provided as binary code and / or as assembly code and / or as source code of a programming language (e.g. C) and / or as a program script (e.g. Python).

[0039] The invention also encompasses combinations of the features of the described embodiments. The invention therefore also encompasses implementations that each comprise a combination of the features of several of the described embodiments, unless the embodiments are described as mutually exclusive.

[0040] Exemplary embodiments of the invention are described below. Shown are: Fig. 1 schematically shows a motor vehicle with a refrigerant circuit which can be operated as a heat pump in order to heat a passenger compartment of the motor vehicle; Fig. 2 several curves indicating a saturation vapor pressure for water vapor as a function of temperature; and Fig. 3 a diagram by means of which, using a characteristic curve which indicates a saturation vapor pressure of humid air, the dew point of the air corresponding to a measured pair of values ​​of a temperature and a relative humidity of air can be determined.

[0041] The exemplary embodiments explained below are preferred embodiments of the invention. In the exemplary embodiments, the described components of the embodiments each represent individual features of the invention that can be considered independently of one another, each of which also develops the invention independently of one another. Therefore, the disclosure is intended to encompass combinations of the features of the embodiments other than those shown. Furthermore, the described embodiments can also be supplemented by further features of the invention already described.

[0042] In the figures, the same reference symbols designate elements with the same function.

[0043] In Fig. 1 shows a highly schematic illustration of a motor vehicle 10 having a refrigerant circuit 12. The refrigerant circuit 12 can be used in heat pump mode to introduce heat into a passenger compartment 14 of the motor vehicle 10. In heat pump mode of the refrigerant circuit 12, the refrigerant compressed by a compressor 16 of the refrigerant circuit 12 is fed to a refrigerant cooler 18. At the refrigerant cooler 18, which, depending on the refrigerant used in the refrigerant circuit 12, can be designed as a gas cooler or a condenser, heat is transferred to another medium. For example, air, which is introduced into the passenger compartment 14 of the motor vehicle 10, can be heated as an additional medium.

[0044] The refrigerant cooled at the refrigerant cooler 18 and under high pressure is expanded by an expansion device 20 of the refrigerant circuit 12 and fed to an evaporator 22 of the refrigerant circuit 12. The refrigerant evaporating in the evaporator 22 absorbs heat from the air 24.

[0045] In the Fig. In the situation shown schematically and by way of example in Figure 1, the air 24 is ambient air, i.e. air 24 which is located in the environment of the motor vehicle 10.

[0046] The evaporator 22 can, unlike in Fig. 1, can be arranged in a front region 26 of the motor vehicle 10. In the front region 26 of the motor vehicle 10, the air 24 can reach the evaporator 22 via air inlets (not shown).

[0047] The refrigerant circuit 12 can also be used in an air conditioning system, in a manner not explained in detail above, in which cooled air is introduced into the passenger compartment 14 of the motor vehicle 10. In the air conditioning system mode of the refrigerant circuit 12, using a further expansion element (not shown here) of the refrigerant circuit 12, the refrigerant cooler 18 can be operated as an evaporator, and the evaporator 22 can be operated as a gas cooler or condenser of the refrigerant circuit 12.

[0048] Heat pump operation of the refrigerant circuit 12 is particularly advantageous when a comparatively low temperature prevails in the environment of the motor vehicle 10. In this case, however, particular care must be taken to ensure that no ice forms on the evaporator 22. This is because, in order to absorb heat from the ambient air, an operating temperature must be set on the evaporator 22 that is even lower than the temperature of the comparatively cold air 24 from the environment of the motor vehicle 10. And ice formation on the evaporator 22 of the refrigerant circuit 12 would result in the air 24 being unable to flow through the evaporator 22, or at least only to a limited extent.

[0049] In order to determine which operating temperatures of the evaporator 22 are uncritical with regard to icing of the evaporator 22, it is useful to determine the dew point 48 (compare Fig. 3) of the air 24, which is supplied to the evaporator 22 during heat pump operation of the refrigerant circuit 12. For this purpose, a sensor device 28 (see Fig. 1) of the refrigerant circuit 12, the temperature of the air 24 and a relative humidity of the air 24 are recorded.

[0050] The temperature of the air 24 and the relative humidity of the air 24 are preferably controlled by a control device 30 (see Fig. 1) of the refrigerant circuit 12 to reach the dew point 48 (compare Fig. 3) of the air 24. The dew point 48 is the temperature of the air 24 at which the air 24 has a relative humidity of 100% and is thus saturated with water vapor. If the air 24 falls below the dew point 48 or the dew point temperature, the water contained in the air 24 condenses. If the evaporator 22 simultaneously has an operating temperature below 0°C, the condensation of the water contained in the air 24 can lead to the undesirable formation of ice on the evaporator 22.

[0051] In order to reduce the probability of ice formation on the evaporator 22 of the refrigerant circuit 12, it is therefore advisable to set the dew point 48 (compare Fig. 3) of the air 24. This allows timely action to be taken to reduce the likelihood of ice formation on the evaporator 22 of the refrigerant circuit 12.

[0052] In the present case, to determine the dew point 48 of the air 24, the control device 30 uses a characteristic curve 32 which is in a Fig. 3. The characteristic curve 32 indicates the saturation vapor pressure of the air 24, i.e., a relative humidity of the air 24 of 100% at a respective temperature of the air 24. By means of the sensor device 28, a pair of values ​​36 is recorded, which indicates the temperature of the air 24 and the relative humidity of the air 24.

[0053] In diagram 34 in Fig. 3 shows a plurality of curves 38, each indicating a specific relative humidity of the air 24. Furthermore, Fig. 3 shows a plurality of lines 40, each indicating a specific temperature of the air 24. For example, the value pair 36 corresponds to a temperature of 0°C and a relative humidity of 55%. This is because one of the curves 38, namely the curve 38 associated with the relative humidity of 55%, intersects one of the lines 40, namely the line associated with the temperature of 0°C.

[0054] This value pair 36 corresponds to an absolute water content of the air 24. The absolute water content of the air 24 is given in the diagram 34 on an abscissa 42, for example in grams per kilogram [g / kg]. On a line starting from the value pair 36 and in Fig. 3, which is parallel to an ordinate 46 of the diagram 34, there are a plurality of further pairs of values ​​of respective temperatures and respective values ​​of relative humidity.

[0055] For all these pairs of values, it is true that they are assigned to the same absolute water content. Where the vertical line 44 in the diagram 34 meets the characteristic curve 32, lies the dew point 48 of the air 24, whose temperature and relative humidity were recorded by the sensor device 28. Fig. In the example shown in Figure 3, the dew point 48 of the air 24, for which the value pair 36 was determined, is approximately -7°C.

[0056] The information relating to Fig. The considerations explained in section 3 make it clear that it is important to use a correct characteristic curve 32 to determine the dew point 48 of the air 24.

[0057] The characteristic curve 32 used by the control device 30 is shown in Fig. 2 in a further diagram 50. In the diagram 50 according to Fig. 2, the saturation vapor pressure, for example in Pascal [Pa], is plotted on a logarithmically divided ordinate 52 and the temperature in degrees Celsius (°C) on an abscissa 54.

[0058] Furthermore, Fig. 2 shows a first curve 56, which indicates the saturation vapor pressure determined using a first Magnus formula. The first Magnus formula is a first calculation rule valid for determining the saturation vapor pressure over a flat ice surface.

[0059] This first Magnus formula can be as follows: Ei(t)=6.112 hPa⋅exp(22.46⋅t272.62°C+t) with E i (t) = saturation vapor pressure above the flat ice surface and t = temperature of the air in °C.

[0060] In the diagram 50 in Fig. Figure 2 shows another curve 58, which was obtained using a second Magnus formula. The second Magnus formula is a second calculation rule that is valid for determining the saturation vapor pressure above a flat water surface. The second Magnus formula can be written as follows: Ew (t)=6.112 hPa⋅exp(17.62⋅t243.12°C+t) with E w (t) = saturation vapor pressure above the flat water surface and t = air temperature in °C

[0061] In the present case, the characteristic curve 32 used by the control device 30 corresponds in a first section 60 to the numerical values ​​for the saturation vapor pressure obtainable using the first Magnus formula, whereby the first Magnus formula is valid for the saturation vapor pressure above a flat ice surface. In a second section 62, the characteristic curve 32, in contrast, corresponds to the course of the second curve 58. Therefore, the saturation vapor pressure indicated by the characteristic curve 32 in the second section 62 of the characteristic curve 32 corresponds to the course of the curve 58 as obtainable using the second Magnus formula. The second Magnus formula is valid for the saturation vapor pressure above the flat water surface.

[0062] Accordingly, in the first section 60 of the characteristic curve 32, the dew point of the air 24 is determined using the first calculation rule or the first Magnus formula. And in the second section 62 of the characteristic curve 32, the dew point of the air 24 is determined using the second calculation rule or the second Magnus formula.

[0063] Out of Fig. 2 shows that the first section 60 of the characteristic curve 32 in the diagram 50 extends from the left, or from low temperatures, to a vertical line 64. The vertical line 64 corresponds to a temperature of 0°C in the diagram 50. And the second section 62 of the characteristic curve follows the vertical line 64 to the right, i.e., toward temperatures above 0°C.

[0064] Accordingly, if the temperature of the air 24 detected by the sensor device 28 is less than 0°C, the first Magnus formula or first calculation rule is used to determine the dew point of the air 24. If the temperature of the air 24 detected by the sensor device 28 is greater than 0°C, the second Magnus formula or second calculation rule is used to determine the dew point of the air 24.

[0065] This is based on the realization that the first Magnus formula, the results of which Fig. 2 by the first curve 56, particularly accurately or realistically reflects the saturation vapor pressure of the air 24 at air temperatures below 0°C. And the second Magnus formula, the results of which are shown in Fig.2, illustrated by the second curve 58, particularly realistically or accurately reflects the saturation vapor pressure of the air 24 at temperatures above 0°C. At a temperature of the air 24 of exactly 0°C, as detected by the sensor device 28, either the first Magnus formula or the second Magnus formula can be used to determine the dew point 48 of the air 24.

[0066] The use of the characteristic curve 32 leads to very precise results in determining the dew point 48 of the air 24 over a very wide temperature range, in particular over the entire temperature range relevant to the operation of the motor vehicle 10, namely from temperatures well below 0°C to temperatures well above 0°C.

[0067] Overall, the examples show how a precise determination of the dew point 48 can be provided for the efficient operation of an air heat pump or water heat pump.

Claims

[1] Method for operating a refrigerant circuit (12) of a motor vehicle (10), in which, in order to reduce the probability of ice formation on an evaporator (22) of the refrigerant circuit (12), a dew point (48) of air (24) with which the evaporator (22) can be acted upon is determined, wherein, for determining the dew point (48), a temperature of the air (24) and a relative humidity of the air (24) are detected by means of a sensor device (28) of the refrigerant circuit (12), characterized by , that a characteristic curve (32) is used to determine the dew point (48), which indicates a saturation vapor pressure of the air (24), wherein in a first section (60) of the characteristic curve (32) the dew point (48) is determined using a first calculation rule, and wherein in a second section (62) of the characteristic curve (32) the dew point (48) is determined using a second calculation rule different from the first calculation rule. [2] Method according to claim 1, characterized by that a respective Magnus formula is used as the first calculation rule and as the second calculation rule. [3] Method according to one of the preceding claims, characterized by that when the temperature of the air (24) detected by the sensor device (28) is less than 0°C, the first calculation rule is used and when the temperature of the air (24) detected by the sensor device (28) is more than 0°C, the second calculation rule is used. [4] Method according to claim 2 and 3, characterized bythat when the temperature of the air (24) detected by the sensor device (28) is less than 0°C, a Magnus formula valid for the saturation vapor pressure above a flat ice surface is used as the first calculation rule, and when the temperature of the air (24) detected by the sensor device (28) is more than 0°C, a Magnus formula valid for the saturation vapor pressure above a flat water surface is used as the second calculation rule. [5] Method according to claim 4, characterized by that the first calculation rule was the Magnus formula Ei(t)=6.112 hPa⋅exp(22.46⋅t272.62°C+t) with E i (t) = saturation vapor pressure above the flat ice surface and t = temperature of the air in °C and as the second calculation rule the Magnus formula Ew (t)=6.112 hPa⋅exp(17.62⋅t243.12°C+t) with E w (t) = saturation vapor pressure above the flat water surface and t = temperature of the air in °C is used. [6] Method according to one of the preceding claims, characterized by that the characteristic curve (32) is used by a control device (30) of the refrigerant circuit (12) to determine the dew point (48), wherein data values ​​of the temperature of the air (24) and the relative humidity of the air (24) detected by the sensor device (28) are transmitted to the control device (30). [7] Method according to claim 6, characterized by that the control device (30) controls at least one further component of the refrigerant circuit (12) as a function of the determined dew point (48) for the purpose of reducing the probability of ice formation on the evaporator (22). [8] Method according to claim 7, characterized by that the control device (30) for the purpose of reducing the probability of ice formation on the evaporator (22) - reducing a pressure of the refrigerant prevailing on an inlet side of a compressor (16) of the refrigerant circuit (12) and / or - changing a mass flow of the refrigerant delivered by the compressor (16) of the refrigerant circuit (12) and / or - a change in the expansion of the refrigerant by means of an expansion element (20) of the refrigerant circuit (12) arranged upstream of the evaporator (22). [9] Method according to one of the preceding claims, characterized by , that the temperature and relative humidity of ambient air are detected by means of the sensor device (28) of the refrigerant circuit, with which the evaporator (22) of the refrigerant circuit (12) is acted upon in a heat pump operation of the refrigerant circuit (12), wherein, in heat pump operation, a refrigerant cooler (18) of the refrigerant circuit (12) is supplied with air which is introduced into a passenger compartment (14) of the motor vehicle (10). [10] Motor vehicle (10) with a refrigerant circuit (12), wherein a control device (30) of the refrigerant circuit (12) is designed to determine a dew point (48) of air (24) with which the evaporator (22) can be acted upon in order to reduce a probability of ice formation on an evaporator (22) of the refrigerant circuit (12), wherein for determining the dew point (48) a temperature of the air (24) and a relative humidity of the air (24) can be detected by means of a sensor device (28) of the refrigerant circuit (12), characterized byin that the control device (30) is designed to use a characteristic curve (32) which indicates a saturation vapor pressure of the air (24) to determine the dew point (48), wherein the control device (30) is designed to determine the dew point (48) in a first section (60) of the characteristic curve (32) using a first calculation rule, and wherein the control device (30) is designed to determine the dew point (48) in a second section (62) of the characteristic curve (32) using a second calculation rule which is different from the first calculation rule.

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