Method for operating an air conditioning system for a motor vehicle, air conditioning system for a motor vehicle and motor vehicle with air conditioning system

Abstract

The invention relates to a method for operating an air conditioning device for a motor vehicle, wherein in the method an optimal operating mode of the air conditioning device is determined in a predetermined manner as a function of refrigerant pressure values and refrigerant temperature values of a refrigerant contained in a refrigerant circuit (2) of the air conditioning device, independently of characteristic values of components of the refrigerant circuit (2), which are detected by means of at least three sensors included in the air conditioning device, and the air conditioning device comprises a control device (11) which, by means of the detected refrigerant pressure values and refrigerant temperature values, determines a compressor enthalpy gradient (19) via a compression (18) of the refrigerant in at least one compressor (3) as a respective component of the refrigerant circuit (2) and a gas cooler outlet enthalpy gradient (28, 29,30) of the refrigerant downstream of a gas cooler (4) as a component of the refrigerant circuit (2) and a gradient factor describes a quotient of the compressor enthalpy gradient (19) and the gas cooler outlet enthalpy gradient (28, 29, 30) and one of the following three operating modes is set as the optimal operating mode: an efficiency operating mode, a power operating mode and a transitional operating mode.,

Description

[0001] The invention relates to a method for operating an air conditioning device for a motor vehicle, wherein in the method an optimal operating mode of the air conditioning device is determined in a predetermined manner as a function of refrigerant pressure values and refrigerant temperature values of a refrigerant contained in a refrigerant circuit of the air conditioning device, independently of characteristic values of components of the refrigerant circuit, and the air conditioning device comprises a control device which, by means of the detected refrigerant pressure values and refrigerant temperature values, calculates a compressor enthalpy gradient via a compression of the refrigerant in at least one compressor as a respective component of the refrigerant circuit and a gas cooler outlet enthalpy gradient of the refrigerant downstream of a gas cooler as a component of the refrigerant circuit.The invention also relates to an air conditioning device for a motor vehicle and a motor vehicle with an air conditioning device.

[0002] DE 199 35 731 A1 describes a method for operating a vehicle refrigeration system or air conditioning system for a motor vehicle. Depending on component efficiencies, a characteristic curve is determined that calculates an optimal high pressure as a function of the refrigerant temperature at the gas cooler outlet. Optimum performance and efficiency are not determined; pressure and temperature information upstream and downstream of a compressor, as well as an enthalpy analysis of the refrigerant, are not provided here.

[0003] DE 10 2015 104 464 A1 describes a control system for a vehicle's refrigerant circuit. The optimal high pressure of the refrigerant system is determined based on the refrigerant temperature downstream of a gas cooler. Optimum performance and efficiency are not determined, nor are pressure and temperature information upstream and downstream of a compressor or an enthalpy analysis of the refrigerant provided.

[0004] DE 10 2015 103 032 A1 describes a thermal management system for a vehicle. The system can be operated in a high-efficiency mode and a maximum performance mode. Optimum performance and efficiency are not determined; pressure and temperature information upstream and downstream of a compressor or downstream of a gas cooler, as well as an enthalpy analysis of the refrigerant, are not provided.

[0005] DE 10 2022 117 709 A1 discloses a method for operating a temperature control device for controlling the temperature of a motor vehicle. The temperature control device has a refrigerant circuit comprising a compressor train in which a refrigerant compressor is arranged, by means of which the refrigerant is compressed. The refrigerant circuit comprises a condenser train branching off from the compressor train, in which a first expansion valve and a condenser are arranged. The refrigerant circuit comprises a bypass train connected in parallel to the condenser train, in which a second expansion valve is arranged.

[0006] Furthermore, it is known from the prior art that a high pressure in a refrigerant circuit of an air conditioning system is calculated using empirical formulas, for example, as a function of the temperature of the refrigerant downstream of a gas cooler. The use of empirical formulas to operate an air conditioning system can result in the disadvantage that an air conditioning system cannot be operated optimally in its specific embodiment.

[0007] The invention is based on the object of providing a method for operating an air conditioning device for a motor vehicle with particularly low application or adaptation effort to the respective embodiment of the air conditioning device, as well as an air conditioning device for a motor vehicle and a motor vehicle with an air conditioning device.

[0008] The object is achieved by the subject matter of the independent patent claims. Advantageous developments of the invention are described by the dependent patent claims, the following description, and the figures.

[0009] A first aspect of the invention relates to a method for operating an air conditioning device for a motor vehicle, wherein in the method an optimal operating mode of the air conditioning device is determined in a predetermined manner as a function of refrigerant pressure values and refrigerant temperature values of a refrigerant contained in a refrigerant circuit of the air conditioning device, independently of characteristic values of components of the refrigerant circuit, and the air conditioning device comprises a control device,which, using the recorded refrigerant pressure values and refrigerant temperature values, calculates a compressor enthalpy gradient over a compression of the refrigerant in at least one compressor as a respective component of the refrigerant circuit and a gas cooler outlet enthalpy gradient of the refrigerant downstream of a gas cooler as a component of the refrigerant circuit, and a gradient factor describes a quotient of the compressor enthalpy gradient and the gas cooler outlet enthalpy gradient, and one of the following three operating modes is set as the optimal operating mode, in which in particular a predetermined target value of the gradient factor that specifies the optimal operating mode is set by the control device: an efficiency operating mode, a power operating mode, and a transitional operating mode.

[0010] In other words, the air conditioning system can only be operated in one of the three operating modes as the optimal operating mode, a selectable operating mode, or an adjustable operating mode, and therefore not in multiple operating modes simultaneously. In other words, the method is an operating strategy for the refrigerant circuit or a refrigeration circuit that determines an optimal operating point or operating mode, for example, application-dependent, performance-optimized in the performance operating mode or efficiency-optimized in the efficiency operating mode, independently of the system and components, from the measured values (refrigerant temperature values and refrigerant pressure values) of the at least three sensors and thus, contrary to the conventional methodology or the method described above and known from the prior art, finds the optimal operating state based on the material properties, for example of the refrigerant, and the measurement results or measured values.It is, so to speak, a generally applicable regulation for the optimal operation of refrigeration circuits, especially supercritical refrigeration circuits.

[0011] The air conditioning system comprises a refrigerant circuit with refrigerant, with at least one compressor as a respective component, which in the prior art can also be referred to as a compressor, and with at least one gas cooler as a component, which can, for example, comprise several gas coolers and in the prior art can also be referred to as a condenser, and with at least one expansion valve as a respective component and with at least one evaporator as a respective component. In other words, the refrigerant circuit can have one or more compressors, evaporators, gas coolers and expansion valves and / or more than three sensors, which, however, neither limits nor expands the invention. Therefore, the invention is described below only with reference to one compressor, one evaporator, one gas cooler and one expansion valve, whereby several are not excluded.The air conditioning system can, for example, cool a battery, in particular a traction battery, and / or an interior of the motor vehicle. The gas cooler is designed for heat exchange between the refrigerant and the environment. The evaporator is designed, for example, for heat exchange between the refrigerant and the interior of the motor vehicle, in particular air contained therein, and / or for heat exchange between the refrigerant and the battery.

[0012] The air conditioning system comprises a control device with retrievable enthalpy data of the refrigerant, which describes the enthalpy of the refrigerant as a function of its pressure and temperature. Enthalpy here refers to the specific, i.e., mass-related, enthalpy of the refrigerant, which, as known from thermodynamics, can be denoted by "h." Unless otherwise stated, for the purposes of the invention, pressure and temperature always refer to a static value, i.e., a static pressure, which can be denoted by "p," and a static temperature, which can also be indicated by "T." At least three sensors are arranged in the refrigerant circuit, each for sensing the pressure and temperature of the refrigerant.A refrigerant pressure value and a refrigerant temperature value are sensed, recorded, or measured by the first sensor upstream of the compressor, the second sensor downstream of the compressor, and the third sensor downstream of the gas cooler. The sensors can be designed as known from the prior art for air conditioning systems and arranged in the refrigerant circuit. The tuples sensed by the sensors, i.e. pairs or value pairs of the temperature and pressure values of the refrigerant, are transmitted to the control device for reception, for example, via a data line (electrically conductive cable or WLAN (Wireless Local Area Network) connection). Using the two value pairs from the first two sensors, the control device calculates a compressor enthalpy gradient via compression of the refrigerant in the compressor in conjunction with or using the stored enthalpy data.In other words, the control device uses the first pair of values from the first sensor to determine the compressor inlet enthalpy of the refrigerant as it enters or flows into the compressor, and the second pair of values from the second sensor to determine the compressor outlet enthalpy of the refrigerant as it exits or flows out of the compressor. From the two enthalpy values before and after the compressor (i.e., upstream and downstream of the compressor's position in the circuit), the control device determines or calculates the compressor enthalpy gradient over the pressure gradient or along the pressure gradient of the refrigerant, i.e., across the compression.Let h2 and p2 be the enthalpy and pressure of the refrigerant downstream of the compressor, and h1 and p1 be the enthalpy and pressure of the refrigerant upstream of the compressor. The control unit calculates the compressor enthalpy gradient across the compressor dh / dp as follows: dh / dp = (h2-h1) / (p2-p1). The third pair of values from the third sensor describes the pressure and temperature of the refrigerant downstream of the gas cooler, i.e., as it exits or flows out of it. Based on the values from the third sensor, the control unit calculates a gas cooler outlet enthalpy gradient versus pressure along an isotherm (compare pressure(p)-enthalpy(h) diagram in thermodynamics), where the isotherm, i.e., a curve of constant temperature in the pH diagram, is given by the gas cooler outlet temperature of the refrigerant sensed by the third sensor.In other words, the control device calculates the slope of a tangent in the pH diagram to this isotherm at the point of the pressure sensed by the third sensor and the enthalpy of the refrigerant calculated from this pressure and the temperature downstream of the gas cooler outlet using the enthalpy data. To calculate this slope, the control device can, for example, use the enthalpy data to determine, interpolate, or calculate an enthalpy for a pressure higher than the measured gas cooler outlet pressure, as well as an enthalpy for a pressure below or lower than the measured gas cooler outlet pressure.Using the two points resulting from the calculated enthalpy and the pressure assumed for it, which can be determined simultaneously, or just one of the two points and the point given by the values measured by the third sensor, the slope of the tangent at the point of the refrigerant state after the gas cooler outlet or beyond it can be calculated. For example, the pressure above the measured gas cooler outlet pressure can be assumed to be 1-5% higher than the measured gas cooler outlet pressure, and the pressure below the measured gas cooler outlet pressure can be assumed to be 1-5% below the measured gas cooler outlet pressure. The temperature is kept constant here, since the enthalpy slope is to be determined as a function of or via the pressure at the isotherm, i.e. at a constant temperature measured by the third sensor.The slope of the tangent thus determined should be compared with the compressor enthalpy slope. For the purposes of the invention, a slope factor is a quotient with the compressor enthalpy slope in the numerator and the gas cooler outlet enthalpy slope in the denominator. For the purposes of the invention, all enthalpy slopes can be specified as absolute values. The enthalpy slope for the purposes of the invention refers to the slope of the specific enthalpy over pressure, i.e. dh / dp. In other words, the slope factor describes the ratio of the enthalpy of the refrigerant on the gas cooler outlet side to the enthalpy of the refrigerant on the compressor outlet side when the compressor outlet pressure changes.This is based on the idealized assumption that the heat exchange between the refrigerant and the environment, or the cooling of the refrigerant in the gas cooler, occurs isobarically, meaning that the compressor outlet pressure is equal to the gas cooler outlet pressure of the refrigerant. This is a thermodynamically ideal approach that can be adapted by expert action during implementation, for example, due to measurement inaccuracies of the sensors and / or loss-related deviations from the ideal approach.

[0013] The invention also includes embodiments or further developments which result in additional advantages.

[0014] A further development of the method provides that a target gradient factor or a value of the target gradient factor is assigned to a predetermined cooling capacity in the control device, wherein one of the three operating modes is specified as the optimal operating mode by the target gradient factor or the value of the target gradient factor. In other words, the optimal operating mode is specified in such a way that one of the three operating modes (efficiency operating mode, power operating mode, or transitional operating mode) is specified as the optimal operating mode by the target gradient factor, which in turn depends on the predetermined cooling capacity. The target gradient factor can also be referred to as the target value of the gradient factor.

[0015] An assignment of cooling capacity to a target increase factor can be stored in the control device so that a target increase factor can be assigned to the requested or specified cooling capacity. The cooling capacity can be specified, for example, by a user input that can be received by the control device. For example, the user can set a desired target temperature or target temperature in the interior via a display and operating device arranged in the interior of the motor vehicle. The cooling capacity can be specified, for example, via a difference between the actual temperature in the interior and the target temperature, for example stored in a table in the control device. In addition, the cooling capacity can be adjustable using a cooling capacity or fan power that can be set by a user and received by the control device.The set fan power can be assigned to a cooling power or converted into one by the control device.

[0016] A further development of the method provides that the control device regulates a compressor outlet pressure of the refrigerant in the refrigerant circuit so that the gradient factor or a value of the gradient factor corresponds to the target gradient factor or a value of the target gradient factor.

[0017] If a target gradient factor is assigned to the specified cooling capacity, the compressor outlet pressure of the refrigerant is regulated by the control device such that the gradient factor matches the target gradient factor. In other words, the compressor outlet pressure should be regulated by the control device such that the difference between the absolute gas cooler outlet enthalpy gradient and the quotient of the absolute compressor enthalpy gradient (numerator) and the target gradient factor (denominator) is minimal or equals 0. Mathematically transformed, this difference, with a control deviation of 0, is equivalent to the target gradient factor being equal to the gradient factor. To regulate the compressor outlet pressure, it can be provided that the at least one expansion valve has a cross-section that can be changed by the control device, or an expansion cross-section through which fluid can flow, or a throttle cross-section.According to the invention, the refrigerant is in a supercritical state, at least in some regions, in the refrigerant circuit of the air conditioning system. The supercritical state means that the pressure and temperature of the refrigerant are above the respective so-called critical value. In the supercritical state, a fluid can no longer distinguish between the liquid and gaseous states. By reducing the pressure in the refrigerant circuit or in the refrigerant as it flows through or downstream of the expansion valve, the refrigerant can be expanded into its wet vapor region - viewed in the pH diagram. In the wet vapor region, the refrigerant exists in two phases: vapor and liquid. The evaporator can be arranged downstream of the expansion valve, the compressor can be arranged downstream of the evaporator, and the gas cooler can be arranged downstream of the compressor and upstream of the expansion valve.

[0018] Initially or at the start of the process, the high pressure, i.e. the pressure of the refrigerant downstream of the compressor outlet, can be determined using empirical formulas known from the state of the art, for example, using the Lialo approach (High pressure [bar] = Temperature after gas cooler [°C] * 2 + 20). Alternatively, the empirical Kauf approach can be used to initially set the high pressure downstream of the compressor (High pressure [bar] = Temperature after gas cooler [°C] * 2.6 + 7.54). Starting from this initially set high pressure, the enthalpy gradients across the compressor and at the gas cooler outlet can be determined. Starting from the initially set high pressure, the high pressure can be adjusted using the control device so that the gradient factor corresponds to the target gradient factor.

[0019] Adjusting the high pressure, i.e., the compressor outlet pressure, as a function of the enthalpy gradients results in the advantage that, regardless of the system state and the components used in the air conditioning system, the system can be operated particularly well (e.g., particularly efficiently or with optimized performance) with the least possible application effort or adjustment effort. This is precisely because empirical formulas for adjusting the high pressure are not used consistently, as is known from the state of the art, but at most only for setting an initial high pressure at the beginning of the process.In other words, the air conditioning system can be operated particularly optimally without knowing its concrete system topology and / or the characteristics of individual or all components of the refrigeration circuit, i.e. without knowing the design and / or arrangement of the components in the refrigeration circuit or having to record them for high-pressure control.

[0020] Alternatively, in addition to setting the operating mode or the optimal operating mode using the target gradient factor, the operating mode can be set depending on a user selection. The user selection can be received from a user, for example, via a display and operating device located in the interior and connected to the control device via a data line, and can specify the operating mode in which the air conditioning system is operated.

[0021] In addition to the described components (compressor, gas cooler, expansion valve, and evaporator), the air conditioning system may comprise further components in the refrigeration circuit, such as an accumulator arranged upstream of the compressor, as is known from the prior art for air conditioning systems, and / or an internal heat exchanger (IWT) for stabilizing and improving the operation of the air conditioning system by exchanging heat within the refrigerant circuit between a high-pressure side (for example, from the compressor outlet via the gas cooler to the expansion valve) and a low-pressure side (for example, between the expansion valve via the evaporator to the compressor inlet). The pressure on the low-pressure side is lower than on the high-pressure side. The accumulator may be arranged between the compressor and the evaporator in the refrigerant circuit.

[0022] In other words, the above description of the procedure can be summarized as follows: Method for operating an air conditioning device for a motor vehicle in one of the three following operating modes, in an efficiency operating mode, in a power operating mode and in a transitional operating mode, depending on a predetermined cooling capacity for cooling an interior of the motor vehicle, wherein • the air conditioning system has a refrigerant circuit with refrigerant and • comprises a control device, wherein the control device calculates a compressor enthalpy gradient via a compression of the refrigerant in at least one compressor of the refrigerant circuit and a gas cooler outlet enthalpy gradient of the refrigerant downstream of a gas cooler of the refrigerant circuit by means of detected refrigerant pressure values and refrigerant temperature values.

[0023] The method is characterized by a gradient factor describing a quotient of the gas cooler outlet enthalpy gradient and the compressor enthalpy gradient. A target gradient factor is assigned to the specified cooling capacity in the control device, which specifies the operating mode. The control device regulates the compressor outlet pressure of the refrigerant in the refrigerant circuit so that the gradient factor matches the target gradient factor. This form of control can be implemented without incorporating the characteristics of the aforementioned components, which can ensure universal applicability.

[0024] A further development of the method provides that the compressor outlet pressure is regulated by adjusting a variable opening cross-section of at least one expansion valve included in the refrigerant circuit and controllable by the control device. In other words, the pressure upstream of the expansion valve, i.e. the high pressure, should be adjustable by the control device using the expansion valve. As is known from the prior art for expansion valves, which can also be referred to as throttle valves in the prior art, for air conditioning systems, the expansion valve can have a variable cross-section through which fluid can flow. The compressor can be designed such that a speed at which the compressor can be operated can be adjusted, whereby a volume flow of refrigerant to be delivered by the compressor can be adjusted.This is based on the idealized assumption that the refrigerant in the evaporator exchanges heat isobarically with, or absorbs heat from, air from the interior and / or the traction battery, so that the pressure downstream of the expansion valve is equal to the pressure in the evaporator and equal to the compressor inlet pressure. This results in the advantage that the high pressure in the refrigerant circuit only needs to be adjusted via the expansion valve. The compressor outlet pressure or high pressure can, for example, be between 50 and 170 bar, in particular between 75 and 135 bar.

[0025] A further development of the method provides that the efficiency operating mode corresponds to a target gradient factor or a value of the target gradient factor of 1. A target gradient factor of 1 means that the enthalpy gradient across the compressor or across the compression in the compressor should be equal in magnitude to the enthalpy gradient of the refrigerant at the gas cooler outlet at the isotherm specified by the gas cooler outlet temperature. Due to the course of the isotherms in the supercritical range, for example for the refrigerant R744 (carbon dioxide), the enthalpy gradient of the refrigerant at a high pressure smaller than the efficiency high pressure can be greater than 1 (by "gradient" here we mean dh / dp). The gas cooler outlet enthalpy gradient can be negative, in particular, across the high pressure range relevant to the invention, particularly when using R744 or carbon dioxide as the refrigerant.With a gradient factor of less than 1, i.e. a quotient of the enthalpy gradient across the compressor (numerator) and the greater gradient of the enthalpy along the isotherms (denominator) on the gas cooler outlet side, this means that with an increase in high pressure, the enthalpy of the refrigerant on the gas cooler outlet side decreases more sharply (negative gradient) than the enthalpy of the refrigerant increases after or at the compressor outlet. In other words, this means that with an increase in compressor power, a relatively higher heat output can be absorbed by the refrigerant, particularly after expansion on the low-pressure side in the evaporator, i.e. by the air in the interior, and thus a cooling capacity (specific enthalpy h in conjunction with the refrigerant mass flow) is increased proportionally more than the additional energy required for compression.An increased increase in cooling capacity compared to the increase in compressor capacity is given until both enthalpy gradients are equal in magnitude, i.e. the gradient factor corresponds to a value of 1.

[0026] This results in the advantage that, regardless of the system used, an efficiency optimum or an efficiency high pressure can be determined, and the air conditioning system can be operated at this efficiency high pressure. This approach is based on the assumption that, with a (moderate) compressor outlet pressure increase, the temperature of the refrigerant at the gas cooler outlet does not change or this change is negligible. This means that the enthalpy gradient along the same isotherms remains constant with a pressure increase or decrease toward the efficiency optimum or, within the scope of the invention, can be assumed to be constant for determining the efficiency high pressure.

[0027] If the compressor outlet pressure is increased above the efficiency-optimal compressor outlet pressure or efficiency high pressure, the enthalpy gradient at the gas cooler outlet side can become smaller in magnitude, which can make the gradient factor greater than 1.

[0028] A further development provides that the transitional operating mode corresponds to a target gradient factor or a value of the target gradient factor between 1 and 2. If a greater cooling capacity is required than can be provided by the efficiency operating mode, the high pressure is increased beyond the efficiency high pressure, which can reduce the gas cooler outlet enthalpy compared to the gas cooler outlet enthalpy at the efficiency high pressure. With a gradient factor or target gradient factor greater than 1, the cooling capacity increases to a lesser extent than the additionally required compressor capacity (to increase the high pressure). Nevertheless, the cooling capacity increases. The transitional operating mode has the advantage that the required cooling capacity can be increased particularly efficiently, even if it is increased beyond the efficiency optimum.

[0029] A further development provides that the power operating mode corresponds to a target gradient factor or a value of the target gradient factor equal to or greater than 2. The higher the high pressure or compressor outlet pressure, the greater the cooling capacity, but the more inefficient the operation of the air conditioning system becomes. In other words, proportionally more compressor capacity must be supplied to increase the cooling capacity. The power operating mode offers the advantage that a particularly high cooling capacity can be provided without knowing or recording the exact system structure of the air conditioning system. In power operating mode, the maximum high pressure can be limited upwards by a limit value stored in the control device.

[0030] A further development provides that the compressor outlet pressure is limited by a maximum pressure permissible for a component arranged in the refrigerant circuit. In other words, to increase the cooling capacity, the high pressure can only be increased until a maximum pressure stored, for example, in the control device is reached. This maximum pressure can be predetermined, for example, by components installed on the high-pressure side, such as the gas cooler, or a maximum permissible operating pressure for the gas cooler. This results in the advantage of providing a particularly durable or robust air conditioning system that can be operated continuously even at maximum cooling capacity without being damaged. At the same time, maximum performance can be exploited to the point of failure.According to the invention, it can be provided that the maximum permissible pressure can be lower than the maximum permissible operating pressure of the component most critical for pressure failure, i.e., for example, 2-10% lower or stored in the control device.

[0031] A further development provides that the compressor outlet pressure is limited by maintaining a gas cooler outlet temperature within a specified temperature range above the ambient temperature. In other words, the refrigerant at the gas cooler outlet must not exceed a maximum permissible temperature increase relative to the ambient temperature. This has the advantage of ensuring that the air conditioning system maintains its cooling capacity for cooling the air in the interior. As the high pressure or compressor outlet pressure increases, the temperature of the refrigerant downstream of or after the compressor also increases. Along the gas cooler arranged downstream of the compressor, the refrigerant cools by releasing heat to the environment.However, according to the second law of thermodynamics, the refrigerant cannot become colder than the ambient temperature because heat transfer from the refrigerant to the environment requires a temperature gradient from the refrigerant to the environment. This means that after leaving the gas cooler, the refrigerant is always warmer than the ambient temperature. In order to absorb heat from the interior air in the evaporator, the refrigerant must, according to the law described, be colder than the interior air. This means that if the refrigerant is too hot due to compression, meaning that not enough heat can be dissipated via the gas cooler, the temperature of the refrigerant in the evaporator can be too high and the air conditioning system cannot cool sufficiently or to the required extent, meaning that, for example, a setpoint or target temperature cannot be reached.For example, it can be provided that the refrigerant at the gas cooler outlet is a maximum of 3-5°C warmer than the ambient temperature. To detect the ambient temperature, the air conditioning system can have a fourth sensor for detecting the ambient temperature, which can transmit the ambient temperature value it senses to the control system. The value for the upper temperature limit that the refrigerant at the gas cooler outlet must not exceed compared to the ambient temperature can be stored in the control system. If this upper temperature limit is exceeded, it can be provided that the high pressure is reduced. This has the advantage that the cooling capacity of the air conditioning system can be guaranteed.

[0032] A further development provides that the compressor outlet pressure is limited by ensuring that a predetermined enthalpy difference between the enthalpy of the refrigerant downstream of the at least one expansion valve and upstream of the compressor inlet is not undercut. This enthalpy difference, in conjunction with the refrigerant mass flow, indicates the cooling capacity provided by the air conditioning system. This enthalpy difference can be predetermined, for example, by an accumulator of the refrigerant circuit, which can be arranged upstream of the compressor inlet, setting a vapor content of the refrigerant, for example, of 90%. This results in the advantage that a minimum cooling capacity of the air conditioning system can be provided.

[0033] A further development proposes using R744 (carbon dioxide) as the refrigerant in the refrigerant circuit. Using carbon dioxide as the refrigerant offers the advantage that the refrigerant is neither corrosive nor harmful to health and can assume a supercritical state within typical load limits (temperature and pressure) for air conditioning system components.

[0034] A second aspect of the invention relates to an air conditioning device for carrying out one of the described methods, in particular according to the first aspect of the invention. Advantages and advantageous developments of the first aspect of the invention are to be regarded as advantages and advantageous developments of the second aspect of the invention, and vice versa.

[0035] A third aspect of the invention relates to a motor vehicle with an air conditioning system, in particular according to the first and / or second aspect of the invention. Advantages and advantageous developments of the first and / or second aspect of the invention are to be regarded as advantages and advantageous developments of the third aspect, and vice versa. The motor vehicle is designed, for example, as a motor vehicle, in particular as a passenger car or a commercial vehicle, a passenger bus, or a motorcycle. The motor vehicle is preferably an electric vehicle or a hybrid vehicle.

[0036] For applications or application situations that may arise during the method and are not explicitly described here, it may be provided that, according to the method, an error message and / or a request for user feedback is issued and / or a default setting and / or a predetermined initial state is set. For example, emergency operation of the air conditioning system can be provided using the described and known empirical formulas.

[0037] The control device of the air conditioning device can have a data processing device or a processor device that is configured to carry out an embodiment of the method according to the invention. The processor device can for this purpose 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 or further development 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, for example,based on at least one circuit board and / or on at least one SoC (System on Chip).

[0038] A computer-readable storage medium comprises program code which, when executed by a computer or computer network, causes the computer or computer network 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, for example, as a so-called app store server and / or cloud server on the Internet. The computer or computer network can provide a processor circuit with, for example, at least one microprocessor. The program code can 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) must be provided.

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

[0040] Exemplary embodiments of the invention are described below. Shown are: Fig. 1 schematic representation of a refrigerant circuit of an air conditioning system with its components and their arrangement; Fig. 2 schematic representation of the refrigerant circuit as a cycle in the pressure-enthalpy diagram during operation of the air conditioning system and positions of the pressure and temperature sensors; Fig. 3 schematic representation of limits for a maximum and minimum high pressure to be set when operating the air conditioning system in the pressure-enthalpy diagram; Fig. 4 schematic representation of an operation of the air conditioning device in an efficiency operating mode in a pressure-enthalpy diagram; Fig. 5 schematic representation of an operation of the air conditioning device in a power operating mode in a pressure-enthalpy diagram; and Fig. 6 Assignment of a required cooling line to a target gradient factor in a control device of the air conditioning system.

[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 denote elements with the same function.

[0043] Fig. 1 shows a refrigerant circuit 2 of an air conditioning system. Components of the refrigerant circuit 2 may include a compressor 3, which may also be referred to as a compressor, a gas cooler 4, which may also be referred to as a condenser, an expansion valve 5, and an evaporator 6. In addition, further components, such as an accumulator and / or an internal heat exchanger (IHE), may be arranged in the refrigerant circuit 2. The refrigerant circuit 2 and the components it comprises may be conventional components of air conditioning systems known from the prior art. The at least one compressor 3 can, in particular, compress and heat gaseous or vaporous refrigerant, and the gas cooler 4 can cool the refrigerant while dissipating heat to the environment.The evaporator 6 can be designed to evaporate the refrigerant, for example after passing through at least one expansion valve 5 for expansion, i.e. pressure reduction, while absorbing heat. For example, heat is absorbed from the air in a vehicle interior or passenger compartment and / or from a traction battery, and these can be cooled as a result. In the refrigerant circuit 2, a first sensor 8 can be arranged, for example, upstream of the compressor 3 or the compressor inlet, which can also be referred to as the compressor inlet, a second sensor 9 can be arranged between the compressor outlet, which can also be referred to as the compressor outlet, and the gas cooler 4, and a third sensor 10 can be arranged downstream of a gas cooler outlet. The sensors 8, 9, 10 can each be designed to sense or detect the pressure and temperature of the refrigerant.The sensed temperature and pressure values, which can also be referred to as measurement data, can be transmitted from the sensors 8, 9, 10 to a control device 11 included in the air conditioning system. For this purpose, the sensors 8, 9, 10 can each be connected to the control device 11 via a data-conducting connection or data link, for example, a conductive cable or via a WLAN connection. The expansion valve 5 can have a variable cross-section. The cross-section of the expansion valve 5 can be adjustable, for example, by the control device 11. For this purpose, the expansion valve 5 can be connected to the control device 11 via a data line for the exchange of data and / or signals.The air conditioning system can, for example, comprise an interior temperature sensor 12 for sensing a temperature or air temperature in the interior of a motor vehicle and / or a temperature of a traction battery, and an ambient temperature sensor 13 for sensing an ambient temperature or ambient air temperature. These two sensors 12, 13 can each be connected to the control device 11 via a data line for transmitting their respective temperature measurements. The control device 11 can be connected via a data connection to a display and operating device 14, which is arranged, for example, in the interior and via which the control device 11 can receive a target or desired temperature specification, for example from a user.Enthalpy data of the refrigerant used in the refrigerant circuit 2 can be stored in the control device 11, for example on a digital data storage device. With the aid of these data, the control device 11 can assign a specific enthalpy of the refrigerant to a pair of values, i.e., a refrigerant pressure value and a refrigerant temperature value, for example, received from one of the sensors 8, 9, 10. The enthalpy data can be stored, for example, in the form of a look-up table. In the case of temperature and / or pressure values or a pair of values that are not explicitly listed in the look-up table, the control device 11 can be configured to interpolate between the values present in the look-up table.The control device 11 can be configured to determine a required cooling capacity or condenser capacity, for example with the aid of a target temperature specification, for example received via the display and operating device 14, a fan and / or cooling capacity received by the display and operating device 14, a temperature, for example in the interior, for example sensed by the interior temperature sensor 12, and / or an ambient temperature, for example sensed by the ambient temperature sensor 13. In the control device 11, a target gradient factor can be assigned to a required or requested cooling capacity, for example in the form of a look-up table. Based on the determined target gradient factor, a refrigerant pressure or a high pressure or a compressor outlet pressure, i.e. a pressure downstream of the compressor 3, can be set by means of the control device 11 by adjusting the cross-section of the expansion valve 5.The arrows in . Fig. 1 can indicate a flow direction of the refrigerant through the refrigerant circuit 2. For example, R744 or carbon dioxide can be used as the refrigerant, which can assume a supercritical state at least in some areas in the refrigerant circuit 2.

[0044] Fig. 2 shows a schematic representation of the refrigerant circuit 2 as a cycle in the pressure-enthalpy diagram or pH diagram when operating the air conditioning system with carbon dioxide as refrigerant and the respective positions of the pressure and temperature sensors 8, 9, 10. Along the abscissa or x-axis in the diagram in Fig. 2, the specific enthalpy of carbon dioxide is plotted in kilojoules per kilogram, along the ordinate or y-axis the static pressure of the refrigerant in bar. Additionally, the diagram shows isotherms 15, i.e. lines of constant temperature of the refrigerant, as well as a wet vapor region 16 in which lines with constant vapor content 17 are shown. The apex of the wet vapor region 16 is the so-called critical point 23. Above and to the right of the critical point 23 in the diagram in Fig. 2 the refrigerant is in a supercritical state. How Fig. 2 As shown in the diagram, the compression 18 in the compressor 3 can cause the pressure and temperature of the refrigerant to rise from the compressor inlet to the compressor outlet, whereby the specific enthalpy can also rise. Downstream of the compressor outlet, the refrigerant can cool isobarically, i.e. at a constant pressure, in the gas cooler 4 up to the gas cooler outlet. Cooling 20 of the refrigerant can therefore take place from the compressor outlet to the gas cooler outlet. Downstream of the gas cooler outlet, the expansion valve 5 can be arranged, via which the refrigerant can expand or relax isenthalpically, i.e. with a constant specific enthalpy, into the wet vapor region 16 in the cycle shown here in an idealized manner. Expansion 21 can therefore take place through the expansion valve 5.Downstream of the expansion valve 5, the refrigerant can absorb heat isobarically and isothermally in the evaporator 6, which heat can be converted into evaporation of the refrigerant 22. As shown in . Fig. As shown in Figure 2, the refrigerant outside the wet vapor region can continue to absorb heat isobarically into the superheated vapor region. Compressor 3 can be located downstream of evaporator 6, and the cycle begins again. Upstream of the compressor inlet and downstream of evaporator 6, the first sensor for sensing the pressure and temperature of the refrigerant can be located in front of the compressor inlet. Downstream of the compressor outlet and upstream of gas cooler 4, the second sensor 9 can be located. As can be seen from Fig. As can be seen from Figure 2, the compressor enthalpy gradient 19 can be determined based on the known enthalpies of the refrigerant upstream of the compressor 3, for example, sensed by the first sensor 8, and downstream of the compressor outlet, for example, sensed by the second sensor 9. The third sensor 10 can be arranged downstream of the gas cooler 4 and upstream of the expansion valve 5. The arrows in Fig. 2 can indicate the direction of the cycle. As in Fig. 2, the cycle can have a high-pressure side 24, for example from the compressor outlet to the expansion valve 5, i.e. immediately upstream of the expansion valve 5, and a low-pressure side 25, for example between the expansion valve 5 and the compressor inlet. The difference in the specific enthalpy in the diagram in Fig. 2 along the low-pressure side 25 can (in conjunction with a given refrigerant mass flow) represent or indicate the available cooling capacity, i.e., the amount of heat that can be extracted, for example, from indoor air by the refrigerant in the evaporator 6. The difference between the specific enthalpy immediately before or at the compressor inlet and at the compressor outlet can (in conjunction with a given refrigerant mass flow) indicate the power required to compress the refrigerant.

[0045] In Fig. 3 limits for the maximum and minimum adjustable high pressure are shown so that a sensible operation of the air conditioning system can be given or guaranteed, for example, a minimum required cooling capacity can be provided and no component of the air conditioning system is damaged. Fig. 3, these limits are shown in the pH diagram for carbon dioxide as a refrigerant. The limits comprise two lines: a high-pressure limit 26, which represents, for example, maximum permissible pressure loads for a component arranged in the refrigerant circuit 2, for example 135 bar, whereby, for example, the gas cooler 4 can limit the maximum permissible high pressure. The second line represents an enthalpy and / or temperature limit of the state of the refrigerant downstream of the gas cooler 4. This line indicates that the state of the refrigerant after exiting the gas cooler 4 must be to the left of this line so that sufficient cooling capacity can be provided on the low-pressure side 25, i.e. downstream of the expansion valve 5 and upstream of the compressor 3.In addition, a lower limit of the high pressure can be specified by an accumulator arranged in the refrigerant circuit 2 between the evaporator 6 and the compressor 3 being able to set a vapor content of the refrigerant of 90 percent, i.e., within the wet vapor region 16. Furthermore, it can be provided that a difference between an ambient temperature and the refrigerant temperature at the outlet from the gas cooler 4 or downstream of the gas cooler 4 of, for example, 1-10°C, in particular 3-5°C, must not be undercut (the refrigerant can never cool down to a lower temperature than the ambient temperature in the gas cooler 4 if the refrigerant is to cool down through heat exchange with the environment). In addition, a volumetric efficiency of the compressor 3 can influence the performance-optimized high pressure. In other words, an increase in the high pressure can influence the cooling capacity, i.e., reduce it despite an increase in the high pressure.

[0046] Fig. Figure 4 shows a schematic representation of the operation of the air conditioning system in an efficiency mode in a pH diagram. As in the previous diagrams, in the diagram in Fig. 4 The specific enthalpy of the refrigerant is plotted along the x-axis and the static pressure of the refrigerant is plotted along the y-axis. Fig. Figure 4 shows, in addition to the wet vapor region 16, the lines with constant vapor content 17 and the isotherms 15, the compression 18 of the refrigerant and the compressor enthalpy gradient 19 present between the compressor inlet and the compressor outlet. The state of the refrigerant upstream or immediately upstream of the compressor inlet can be detected by the first sensor 8. The state of the refrigerant, where state can mean a temperature, a pressure and a specific enthalpy that can be determined therefrom by the control device 11, can be provided by the second sensor 9 arranged downstream or immediately downstream of the compressor outlet in conjunction with the control device 11. The state of the refrigerant downstream or immediately downstream of the gas cooler outlet can be determined by means of the third sensor 10. Prerequisite for the Fig. 4, or the operation of the air conditioning system in the efficiency operating mode, can be that the refrigerant mass flow is constant and the gas cooler outlet temperature of the refrigerant does not change with an increase in the high pressure, i.e., an enthalpy gradient of the refrigerant can be determined along a single gas cooler outlet isotherm 31. Initially or at the beginning of the method, it can be provided that a high pressure, i.e., the pressure on the high pressure side 24, is first determined using an empirical formula (for example, according to Lialo or Kauf). In this example, shown in Fig. 4, this high pressure can be 90 bar and a gas cooler outlet temperature of 40 °C. Using the measured values of the first and second pressure sensors 8, 9, the control device 11 can calculate the compressor enthalpy gradient 19. A further assumption made here can be that the compressor enthalpy gradient 19 does not change when the high pressure is increased and / or decreased, i.e., remains constant. In this example, the high pressure can be 90 bar, and this pressure and the temperature of the refrigerant after the gas cooler outlet can be detected by the third sensor 10 and transmitted to the control device 11.Based on the sensor values, which each include a pressure and a temperature value of the refrigerant, the control device 11 can determine the respective specific enthalpies, for example by means of enthalpy data of the refrigerant used, in this example carbon dioxide or R744, stored in a look-up table. The control device 11 can determine the compressor enthalpy gradient 19 using the determined enthalpy values upstream of the compressor inlet and downstream of the compressor outlet. In order to determine the enthalpy gradient along the gas cooler outlet isotherms 31 at the gas cooler outlet, it can be provided that the control device 11 determines the respective specific enthalpy of the refrigerant while keeping the gas cooler outlet temperature constant around the pressure measured at the gas cooler outlet, i.e. once at a lower pressure than the measured pressure and once at a higher pressure than the measured pressure.The pressure values above and below the measured pressure can be determined, for example, by a deviation of 1 to 5 percent upwards and / or downwards, i.e., above or below the measured pressure. Thus, for example, at a high pressure of 90 bar, a first gas cooler outlet enthalpy gradient 28, shown in . Fig. 4, can be calculated by the control device 11. As in Fig. 4, the first gas cooler outlet enthalpy gradient 28 indicates that, with an increase in high pressure, the specific enthalpy of the refrigerant at the gas cooler outlet can decrease more than the compressor outlet enthalpy of the refrigerant can increase. This can mean that, with an increase in compressor power, the additional cooling capacity provided thereby increases more than the compressor power (assuming the state of the refrigerant at the compressor inlet remains the same). The control device 11 can be configured to calculate a gradient factor, which can describe a quotient of the compressor enthalpy gradient 19 and the gas cooler outlet enthalpy gradient 28, 29, 30 in terms of magnitude. Fig. 4 shows that the first gas cooler outlet enthalpy gradient 28 can be negative, but can be greater in magnitude than the compressor enthalpy gradient 19. In this case, the gradient factor can be less than 1. In this case, it can be provided that the high pressure is increased. For this purpose, the control device 11 can enlarge an opening cross-section of an expansion valve 5 via a control signal, so that the pressure of the refrigerant downstream of the expansion valve 5 is higher than with the previously narrower cross-section. Assuming that the compressor 3 always provides the same pressure increase, the high pressure at the compressor outlet can thus be increased.

[0047] After this pressure increase, the high pressure can be, for example, 105 bar and the control device 11 can Fig. Calculate the third gas cooler outlet enthalpy gradient 30 shown in Figure 4. As in Fig. 4, this can be smaller in magnitude than the compressor enthalpy gradient 19. This can indicate that with an increase in compressor capacity, the cooling capacity increases in a smaller proportion. The gradient factor can be greater than 1 in this example. This can indicate that for the most efficient operation possible, which can be intended for the efficiency operating mode, the high pressure must be reduced. After reducing the high pressure to, for example, 100 bar, a second gas cooler outlet enthalpy gradient 29 can be calculated, which can be equal in magnitude to the compressor enthalpy gradient 19. The gradient factor can be 1 in this case. The gradient factor of 1 can indicate that an efficiency point 32 has been found at the gas cooler outlet, i.e., that the air conditioning system is operating in the efficiency operating mode. As in Fig. 4, the state of the refrigerant at the gas cooler outlet can be within the limits defined by the high pressure limit 26 and the enthalpy and temperature limit 27 after the gas cooler 4.

[0048] Fig. Figure 5 shows a schematic representation of the operation of the air conditioning system in a power operating mode in a pH diagram. If, for example, the cooling capacity provided in the efficiency operating mode is not sufficient to cool the interior to a predetermined target temperature, it may be necessary to increase the cooling capacity. This may mean reducing the specific enthalpy of the refrigerant upstream of the evaporator 6, which, in an idealized view of the cycle, corresponds to the specific enthalpy downstream of the expansion valve 5. Assuming that the state of the refrigerant at the compressor inlet, in Fig. 4 represented by the position of the first sensor 8, constant or remains the same, assuming a constant refrigerant mass flow, the amount of heat per unit time that the refrigerant can absorb from the interior via the evaporator 6 increases (= cooling capacity). To increase the cooling capacity, an increase in the high pressure is effective if the specific enthalpy on the gas cooler outlet side decreases or falls with the increase. Thus, a gas cooler outlet enthalpy gradient is negative. In Fig. 4 or within the range specified by the high pressure limit 26 and the enthalpy and temperature limit after the gas cooler 27, this can be the case with increasing high pressure. In other words, an increase in high pressure can lead to an increase in cooling capacity. As shown in Fig. As shown in Figure 5, the new high pressure can be increased to 120 bar by the control device 11 to increase the cooling capacity compared to operation in efficiency mode. At this pressure on the gas cooler outlet side, the third gas cooler outlet enthalpy gradient 30 calculated by the control device 11 can be smaller in magnitude than the second gas cooler outlet enthalpy gradient 29 in efficiency mode. Additionally or alternatively, it can be provided that, for example, when the high pressure is increased away from the efficiency point 32, the control device 11 calculates whether the newly set or achieved specific enthalpy of the refrigerant at the gas cooler outlet is lower than the specific enthalpy determined previously or at the previously set high pressure. If this is the case, the increase in the high pressure can lead to increased cooling capacity. As shown in Fig. 5, the absolute gradient of the third gas cooler outlet enthalpy gradient 30 can be smaller than the compressor enthalpy gradient 19. In this case, the gradient factor can be greater than 1. In particular, for the power operating mode, it can be provided that the gradient factor should be greater than 2. It can be provided that, even in the power operating mode, the state of the refrigerant at the gas cooler outlet must be within the specified limits 26, 27. For the power mode, it can be provided that the assumption that the gas cooler outlet temperature remains constant during a pressure increase is no longer valid. The high pressure to be set can be determined by the control device 11 as a function of a required cooling capacity or condenser capacity.

[0049] As in Fig. 6, for example, a required cooling capacity or condenser capacity can be assigned a target gradient factor 35, for example stored in tabular form in the form of a look-up table. The control device 11 can thus set the high pressure at which the gradient factor corresponds to the target gradient factor 35. As with the enthalpy data included by the control device 11, the target gradient factor 35 to be assigned to a specifically specified condenser capacity or cooling capacity can be determined by means of interpolation. In the transitional operating mode, the air conditioning device can be operated at a target gradient factor 35 between 1 and 2. In other words, in the transitional operating mode, the target gradient factor 35 can have a value between 1 and 2.

[0050] A particularly preferred embodiment is described below.

[0051] R744 is a refrigerant used in both subcritical and supercritical applications in automotive applications. The operating strategy and control of the optimal high pressure have a significant impact on the following system properties: performance and efficiency. The operating strategy depends on the system used and / or its components. Current status:

[0052] Formulas from the literature are used, whereby the high pressure in the system is defined as a function of the gas cooler outlet temperature and then adjusted. These formulas are used unchanged in the given systems. Two approaches are common: Lialo: High pressure[bar] = Temperature_after_gas cooler[°C] * 2 + 20 Purchase: High pressure[bar] = Temperature_after_gas_cooler[°C] * 2.6 + 7.54

[0053] For future projects, one approach is to apply various formulas to the system. This already distinguishes between optimal performance and optimal efficiency. For efficiency (first suggestion): High pressure [bar] = Temperature after gas cooler [°C] * 2.2 + 26; for power (first suggestion): High pressure [bar] = Temperature after gas cooler [°C] * 2 + 42.6. For example, depending on the operating mode to be set, i.e., efficiency mode or power mode, the high pressure can initially be calculated using the corresponding formula.

[0054] A potential disadvantage here is that the current state of the literature (Kauf / Lialo) does not achieve optimal system conditions because the high pressure is neither optimized for the system nor for current system conditions. The current state described eliminates these disadvantages, but is very application-intensive. For platforms with different vehicles and different components (both in terms of dimensioning and technical implementation), this represents a significant additional effort. For example, different technical implementation concepts (scroll, rotary piston, etc.) and dimensions (5.3ccm and 8.2ccm (ccm: cubic centimeter)) exist for compressors.

[0055] A new idea is proposed, an analytical method based on measured values from sensors such as sensors 8, 9, and 10, and the properties of the refrigerant. This method distinguishes between optimum performance and optimum efficiency, or defines a transition between the two states. This is an implementation for AC operating points (AC: Air Conditioning). Heat pump circuits are initially excluded.

[0056] The advantage of this is that the system operates optimally regardless of the system status and the components used. This requires little or no application effort, such as determining a component load-dependent high-pressure limit.

[0057] The following describes a technical implementation and a basic procedure for regulating the high pressure and determining the relevant parameters.

[0058] In contrast to the state of the art, there is no rigid specification for the high pressure; instead, it is determined based on the measured values in the system and the material properties (specific enthalpy, isotherm, subcooling, isotherm slope) during operation. The following information is required: Relationship between temperature, pressure and specific enthalpy outside the wet steam area (for example as a look-up table in the software of the control device 11); pressure and Temperature information from the following sensors: • Before or upstream of compressor 3 (EKK) • After or downstream of compressor 3 (EKK) • After or downstream of the gas cooler 4 (GK)

[0059] The specific enthalpy can be determined from the pressure and temperature information using the look-up table.

[0060] Calculation of the enthalpy slope of the isotherm [(kJ / kg) / bar]: The enthalpy is determined from the look-up table for the temperature and pressure of the p / T sensor according to GK. The pressure is assumed to be x bar above and below the pressure at the sensor. From these two values, the (pressure-dependent!) gradient of the specific enthalpy at the gas cooler outlet is calculated.

[0061] Determination of the enthalpy gradient at the compressor or via the compression [(kJ / kg) / bar]: The gradient is created from the measured values before and after compressor 3 and the resulting specific enthalpy.

[0062] A framework for setting the high pressure is described below.

[0063] Define a range for the adjustable high pressures (reference is the refrigerant temperature after the gas cooler) to prevent the control system from setting impossible / unfavorable values. The upper limit of the high pressure is based on component specifications, such as a maximum permissible operating pressure. The lower limit of the high pressure is based on the control capability (subcooling) and the enthalpy difference on the suction side or low-pressure side 25 (accumulator at approximately 90% of this set vapor content). Furthermore, the difference between the ambient temperature and the refrigerant outlet temperature must be considered (and applied depending on other conditions if necessary). The volumetric efficiency of the compressor may have an influence. The high pressure can be freely adjusted within this range.

[0064] The efficiency-optimized high pressure is described below.

[0065] For the most efficient operation (assuming a constant refrigerant mass flow), a higher high pressure is advisable if the reduction in specific enthalpy on the gas cooler outlet side is greater than the increase on the gas cooler inlet side. Furthermore, gas cooler 4 (GK) must be sufficiently dimensioned, but this is assumed for an efficiency point of 32 (low load). Based on the assumption that the GK outlet temperature (GK-AT) does not change with a moderate increase in high pressure, the gradient of the specific enthalpy at the gas cooler outlet is the tangent of the GK-AT isotherms. The gradient of the isotherms at compressor 3 results from the inlet and outlet conditions at compressor 3.The specific enthalpy decreases faster at the gas cooler outlet than it increases at the gas cooler inlet (GK-E) if the absolute gradient of the isothermal tangent or the first gas cooler outlet enthalpy gradient 28 at the gas cooler outlet is smaller than the gradient at compressor 3 from inlet to outlet. A higher high pressure should be achieved here. The specific enthalpy decreases more slowly at the gas cooler outlet (GK-A) than it increases at the gas cooler inlet (GK-E) if the absolute gradient of the isothermal tangent or the third gas cooler outlet enthalpy gradient 30 at GK-A is greater than the absolute gradient at compressor 3 from inlet to outlet. A lower high pressure should be set here. The efficiency point 32 marks the optimal high pressure in terms of efficiency. The gradient factor in efficiency mode is 1.

[0066] The performance-optimized high pressure is described below.

[0067] For the most efficient and sensible operation, assuming a constant refrigerant mass flow, a higher high pressure is advisable if the specific enthalpy on the gas cooler outlet side decreases with increasing high pressure. Since these are high-load points, gas cooler 4 may be at its design limit. The assumption that the GK-AT does not change with a moderate increase in high pressure may not be correct but is initially assumed. The specific enthalpy decreases at the GK outlet (GK-A) depending on the gradient of the isotherms at the GK-A. In terms of performance, for example, a high pressure of 120 bar, related to Fig. 5, makes sense. In contrast to optimization for efficiency, the criterion for performance-optimized high pressure is as follows: The absolute slope of the tangent of the isotherms at GK-A is X times higher than the slope at compressor 3 (e.g., slope factor 2). The temperature difference between the ambient temperature and GK-AT on the refrigerant side does not exceed a speed-dependent limit of y-Kelvin (e.g., 5 K). The high pressure can be limited by a compressor-specific pressure ratio, above which the volumetric efficiency drops significantly.

[0068] The following describes the decision on whether to apply an efficiency- or performance-optimized approach.

[0069] The decision regarding the use of the efficiency-optimized or performance-optimized variant or the efficiency or performance operating mode can be made via the slope factor. This is then based on the required system performance. This is represented, for example, by the current or forecasted gas cooler performance or condenser performance 34. The Fig.The relationship between required or specified condenser capacity 34 or cooling capacity and target gradient factor 35 shown in Figure 6 is conceivable and should be applicable. In the software, for example software stored on the control device 11, the difference between the gradient of the tangent of the isotherms (St_Tan) or the gas cooler outlet enthalpy gradient and the enthalpy gradient of the compressor (St_EKK) or the compressor enthalpy gradient 19 divided by the gradient factor (St_FK) is used as the input variable for the high pressure controller or for setting the high pressure. The control deviation is thus calculated as Control deviation = St_Tan - (ST_EKK / St_FK). The goal is to have no control deviation. The control system then has the option of changing the opening cross-section of one or more expansion valves 5 to set the high pressure and thus reduce the control deviation.

[0070] Overall, the examples show how analytical high-pressure control can be provided for an air conditioning system with supercritical refrigerant.

Claims

[1] Method for operating an air conditioning device for a motor vehicle, wherein in the method an optimal operating mode of the air conditioning device is determined in a predetermined manner as a function of refrigerant pressure values and refrigerant temperature values of a refrigerant contained in a refrigerant circuit (2) of the air conditioning device, which are detected by means of at least three sensors (8, 9, 10) comprised by the air conditioning device, and independently of characteristic values of components of the refrigerant circuit (2), and the air conditioning device comprises a control device (11) which, by means of the detected refrigerant pressure values and refrigerant temperature values, determines a compressor enthalpy gradient (19) via a compression (18) of the refrigerant in at least one compressor (3) representing one of the components and a gas cooler outlet enthalpy gradient (28, 29,30) of the refrigerant downstream of a gas cooler (4) representing one of the components, and a gradient factor describes a quotient of the compressor enthalpy gradient (19) and the gas cooler outlet enthalpy gradient (28, 29, 30), and one of the following three operating modes is set as the optimal operating mode, in which in particular a predetermined target value of the gradient factor specifying the optimal operating mode is set by the control device (11): an efficiency operating mode, a power operating mode, and a transitional operating mode. [2] Method according to claim 1, wherein in the control device (11) a target gradient factor (35) is assigned to a predetermined cooling capacity, wherein one of the three operating modes is predetermined as the optimal operating mode by the target gradient factor (35). [3] Method according to claim 2, wherein the control device (11) regulates a compressor outlet pressure of the refrigerant in the refrigerant circuit (2) so that the gradient factor corresponds to the desired gradient factor (35). [4] Method according to claim 3, wherein the compressor outlet pressure is controlled by adjusting a variable opening cross-section of at least one expansion valve (5) comprised in the refrigerant circuit (2) and controllable by the control device (11). [5] Method according to one of claims 2 to 4, wherein the efficiency operating mode corresponds to a target slope factor (35) of 1. [6] Method according to one of claims 2 to 5, wherein the transitional operating mode corresponds to a desired gradient factor (35) between 1 and 2. [7] Method according to one of claims 2 to 6, wherein the power operating mode corresponds to a target slope factor (35) equal to or greater than 2. [8] Method according to one of claims 3 to 7, wherein the compressor outlet pressure is kept limited by a maximum permissible pressure for a component arranged in the refrigerant circuit (2). [9] Method according to one of claims 3 to 8, wherein the compressor outlet pressure is limited by maintaining a gas cooler outlet temperature in a predetermined temperature range above an ambient temperature. [10] Method according to one of claims 3 to 9, wherein the compressor outlet pressure is limited in that a predetermined enthalpy difference between the enthalpy of the refrigerant downstream of the at least one expansion valve (5) and upstream of the compressor inlet is not undershot. [11] Method according to one of the preceding claims, wherein R744 (carbon dioxide) is used as refrigerant in the refrigerant circuit (2). [12] Air conditioning device arranged to carry out a method according to one of the preceding claims. [13] Motor vehicle with an air conditioning device according to claim 12.

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