A method of controlling a compressor of an air-conditioning and / or cooling system of a vehicle with an electric powertrain

Abstract

A method of controlling a compressor of an air-conditioning and / or cooling system of a vehicle with an electric powertrain is described. The compressor is operated by a dedicated electric motor. The dedicated electric motor and the electric powertrain are powered by a traction battery pack of the vehicle. According to the method, a limit value of power (ĖElectCompLim) absorbable by the dedicated electric motor is received from a vehicle control unit. An operating value of the compression ratio (βComp) of the compressor is received from the vehicle control unit. A limit value of speed (nCompLim) of the dedicated electric motor is determined based on the received limit value of the absorbable power (ĖElectCompLim) and the received operating value of the compression ratio (βComp). The dedicated electric motor is then operated at a speed lower than or equal to the limit value of speed (nCompLim) thus determined.

Description

[0001] A method of controlling a compressor of an air-conditioning and / or cooling system of a vehicle with an electric powertrain

[0002] TEXT OF THE INVENTION

[0003] Field of the invention

[0004] The present invention relates to vehicles with an electric powertrain, particularly Battery Electric Vehicles (BEVs).

[0005] The invention was developed with reference to the management of the compressor of the air-conditioning (of the passenger cabin) and cooling (of the battery pack) system of the vehicle. In BEV vehicles, this compressor is typically operated by a dedicated electric motor, which is powered by the same batteries that supply energy to the electric powertrain.

[0006] Prior art

[0007] Vehicles with an electric powertrain generally comprise a pair of radiating elements positioned to intercept a flow of cooling air while the vehicle is in motion. These radiating elements comprise a condenser and a radiator, with the former positioned in front of the latter in the direction of travel of the vehicle, such that the flow of air intercepted while the vehicle is in motion first strikes the condenser, and then the radiator (after passing through the condenser). A coolant fluid, of which a phase change (from gaseous to liquid) is supported, passes through the condenser. The coolant fluid circulates in a cooling cycle system comprising at least one evaporation element of the fluid itself, which supports a phase change in the opposite direction to the previous one (from liquid to gaseous). The at least one evaporation element comprises an evaporator of an air-conditioning system for the passenger cabin of the vehicle, and preferably at least one chiller for the cooling of a corresponding group of batteries of the vehicle itself. The cooling cycle system comprises a compressor placed between the outlet of the at least one evaporation element and the inlet of the condenser. The compressor increases the pressure of the coolant fluid (in the gaseous state) and is operated by its own (dedicated) electric motor, which is powered by the same batteries that supply energy to the vehicle’s powertrain. In known solutions, the target rotation speed of the compressor is determined as a function of the overall cooling requirements of the air- conditioning and cooling system of the vehicle. Since the compressor is operated by an electric motor powered by the same batteries that power the powertrain, this may result in excessive power output requirements in certain phases of vehicle use.

[0008] Object of the invention

[0009] The object of the invention is to solve the above-mentioned technical problem. Particularly, the object of the invention is to compute (directly) a maximum value of the rotation speed of the compressor to limit the power absorption from the battery pack of the vehicle.

[0010] Summary of the invention

[0011] The object of the invention is achieved by a method having the features that form the subject of the following claims, which form an integral part of the technical teaching provided herein in relation to the invention.

[0012] Brief description of the figures

[0013] The invention will now be described with reference to the attached figures provided purely by way of non-limiting example, in which:

[0014] - Figure 1 illustrates a modelling diagram for determining the torque delivered by the electric motor that drives the compressor as a function of the speed of the driving motor and the compression ratio;

[0015] - Figure 2 illustrates a modelling diagram for determining the efficiency of the electric motor that drives the compressor as a function of the speed of the driving motor and the compression ratio; and

[0016] - Figure 3 illustrates a modelling diagram for determining a maximum speed limit of the electric motor that drives the compressor as a function of the compression ratio and a limit of absorbable power.

[0017] Detailed description

[0018] As mentioned, the invention is applicable to air-conditioning and / or cooling systems comprising at least one compressor operated by its own electric motor, in which the electric motor that drives the compressor is driven in order to reach a certain target rotation speed of the compressor depending on the cooling demand.

[0019] The following equation (1) allows to compute the electric power EElectCompabsorbed by the motor that drives the compressor:

[0020] In particular, the absorbed electric power EElectCompis equal to the product of the motor rotation speed nCompand the torque Tcompdeveloped by the motor to move the compressor parts, divided by the efficiency of the motor ηElect.

[0021] The motor torque Tcompdepends on the compression ratio βComp(i.e. , the ratio between the pressure at the compressor outlet PCompOutand the pressure at the compressor inlet PCompIn) and on the motor rotation speed nComp(since the rotation speed nCompinfluences the internal friction of the motor). Therefore, the following equation (2) is valid:

[0022] In particular, the dependence of the torque Tcompon the compression ratio βCompand on the motor rotation speed nCompis qualitatively represented by the graph of Figure 1 , which illustrates the trend of Tcompas a function of βCompfor different values of nComp, specifically for four different values nComp1,nComp2, nComp3, nComp4, listed in decreasing order (i.e., Generally, therefore, the motor torque Tcomp increases (e.g., linearly) as the compression ratio βCompincreases and increases as the motor speed nCompincreases.

[0023] The efficiency ηElectof the electric motor that drives the compressor depends on the motor rotation speed nCompand on the motor torque Tcomp. Since, as we have just seen, the torque Tcompdepends in turn on the compression ratio βCompand the speed nComp, the efficiency ηElectdepends on the speed nCompand the compression ratio βComp. Therefore, the following equation (3) is valid:

[0024] In particular, the dependence of the efficiency ηElecton the compression ratio βCompand on the motor rotation speed nCompis qualitatively represented by the graph of Figure 2, which illustrates the trend of ηElectas a function of nCompfor different values of βComp, specifically for four different values βComp1, βComp2, βComp3, βComp4, listed in decreasing order (i.e. , βComp1> βComp2> βComp3> βComp4). Generally, therefore, the efficiency ηElectdecreases as the compression ratio βCompincreases.

[0025] It should be noted that in this description the writing “y=f(x)" is used to generically indicate a dependence of a certain variable y on one or more variables x, without thereby implying that the law or map of dependence f is the same, and without implying that the law or map of dependence f can be expressed analytically.

[0026] Since both the torque Tcompand the efficiency ηElectdepend on the speed nCompand the compression ratio βComp, by referring to equation (1 ) it is possible to determine that there is a (direct) correlation between the absorbed electrical power EElectComp, the speed nCompand the compression ratio βComp. Therefore, it is possible to determine a correlation (e.g., on an experimental basis, by developing a set of correlation maps) that allows to compute a limit (maximum) value of speed nCompLimof the compressor that drives the motor as a function of the compression ratio βCompand a limit (maximum) value of electrical power EElectCompLimthat can be absorbed. Therefore, the following equation (4) is valid:

[0027] In particular, the dependence of the limit value of speed nCompLimon the compression ratio βCompand on the limit value of absorbable power EElectCompLimis qualitatively represented by the graph of Figure 3, which illustrates the trend of nCompLimas a function of EElectCompLimfor different values of βComp, specifically for four different values βComp1, βComp2, βComp3, βComp4, listed in decreasing order (i.e., βComp1> βComp2> βComp3> βComp4). Generally, therefore, the (maximum) limit value of speed nCompLimincreases (e.g. linearly) as the limit value of absorbable power EElectCompLimincreases and decreases as the compression ratio βCompincreases.

[0028] Correlation maps of the type exemplified in Figure 3 can be generated using data provided by the supplier of the compressor, or they can be generated by performing tests in which the compressor speed is kept constant, the compression ratio is varied, and the electrical power absorbed by the motor is measured under the specific conditions of speed and compression ratio. In addition, the correlation maps thus generated can be saturated by taking into account the maximum nominal speed limit of the motor that drives the compressor (i.e., referring to Figure 3, the correlation curves can be saturated at a certain maximum value nNomLimthat cannot be exceeded even when EElectCompLimincreases or βCompdecreases).

[0029] In view of the above, according to the present invention, a method for controlling a compressor of an air-conditioning and / or cooling system of an electric vehicle comprises the steps of:

[0030] - receiving, from a vehicle control unit, a limit value of power (EElectCompLim) absorbable by the motor of the compressor;

[0031] - receiving, from a vehicle control unit, an operating value of the compression ratio ( βComp) of the compressor;

[0032] - determining, as a function of the limit value of the absorbable power (EElectCompLim) and the operating value of the compression ratio (βComp), a limit value of speed ( nCompLim) of the motor that drives the compressor; and

[0033] - driving the motor that drives the compressor at a speed lower than or equal to said determined limit value.

[0034] In particular, the operating (current) value of the compression ratio (βComp) of the compressor can be computed as the ratio between the discharge pressure and suction pressure of the compressor. Such pressure values can be measured via suitable sensors installed on board the vehicle and / or can be determined (computed, estimated) by a control unit of the vehicle based on respective air-conditioning and / or cooling system operation models and as a function of other measured parameters. In some embodiments, for example, the discharge pressure is measured and the suction pressure is estimated (modelled).

[0035] The estimation or modeling of the compressor suction pressure can be based on the approach described below. In various embodiments, the air-conditioning and cooling system comprises, as mentioned, an evaporator (for the air-conditioning of the vehicle cabin) and a chiller (for the cooling of the battery pack) that are arranged in parallel in the cooling cycle, i.e. , they receive the coolant fluid at the input from the same source (and therefore at the same pressure and temperature) and supply the coolant fluid at the output to the same destination. The evaporator exchanges heat with the cabin air, which flows through the evaporator with a certain mass flow rate The chiller exchanges heat with the battery coolant liquid, which flows through the chiller with a certain mass flow rate Temperature sensors are provided to measure both the inlet (TAircabinEvapln) and outlet (TAircabinEvapln) temperatures of the cabin air, and the inlet (Tclnchilln) and outlet (TcinchillOut) temperatures of the coolant liquid from the respective heat exchangers. There exists a direct dependency between the air flow rate and the speed of the air-conditioning unit blower, and similarly a direct dependency between the liquid flow rate and the speed of the coolant liquid pump. Thus, starting from the speed of the air-conditioning unit blower and the speed of the coolant liquid pump (which are known), the two flow rates and rhcinchni can be determined respectively (for example, there may be a linear dependency relationship between the respective quantities, such as a direct proportionality relationship). Furthermore, the evaporator cooling power (indicated by and the chiller cooling power (indicated by canbe computed as a function of the respective flow rates and the respective specific heats and the respective temperature variations measured by the sensors mentioned above Using the following equations (5) and (6):

[0036] Once the cooling powers , have been determined, it is possible to determine the respective temperatures of the coolant fluid in the evaporator and in the chiller using the following equations (7) and (8), where RncabinEvap is the thermal resistance of the evaporator and R-mchii is the thermal resistance of the chiller:

[0037] In particular, the thermal resistances can be determined as a function of the respective flow rates and as there is a direct correlation between the two quantities. In particular, decreases as increases (e.g., it is inversely proportional to ) , and decreases as increases (e.g., it is inversely proportional to

[0038] Finally, the suction pressure of the compressor can be determined from the temperature as in the vapor phase of the coolant fluid there is a direct correlation between the temperature of the coolant fluid and the pressure of the coolant fluid In particular, the pressure increases as the temperature increases.

[0039] It will be noted that when the evaporator and the chiller are active at the same time, the temperatures and pressures of the coolant fluid at the outlet of the evaporator and the chiller are approximately the same. Therefore, in case of simultaneous activation of the two heat exchangers, it is possible to use the measurements made at the chiller as a reference to compute since the temperature measurements made on the coolant liquid are more accurate than the temperature measurements made on the cabin air. If, however, only one of the evaporator and the chiller is active, then the computation method described above is applied using the measurements made on the respective heat exchanger. In other words: if the evaporator and the chiller are active at the same time, the equation used; if only the chiller is active, the equation is used; if only the evaporator is active, the equation

[0040] Returning now to the steps of the method described above, the limit value ( nCompLim) of the speed of the motor that drives the compressor can be determined as a function of one or more correlation maps stored in a vehicle control unit. The limit value of the motor speed increases as the limit value of the absorbable power increases and decreases as the compression ratio increases.

[0041] In particular, the limit value of the power that can be absorbed by the motor of the compressor can be determined by a vehicle control unit based on additional instantaneous power absorptions from the traction battery of the vehicle, concurrent with the power absorption by the compressor. For example, if the vehicle powertrain requires a certain instantaneous power and the compressor of the air-conditioning and / or cooling system requires a certain instantaneous power and the sum of the two needed powers exceeds the limit value that can be supplied overall by the traction battery, then the power supplied to the compressor can be limited to the maximum value so that the sum of the two supplied powers falls within the limits of the traction battery.

[0042] By doing so, it is possible to preserve the safety of the battery (since the limits of power that can be supplied / absorbed are quickly respected) and / or it is possible to increase the performance of the vehicle (e.g., during an acceleration maneuver of the vehicle, temporarily reducing the power that can be supplied to the compressor allows to keep a greater power that can be supplied to the powertrain). In general, with the method according to the present invention, it is possible to limit the power that can be supplied to the compressor in favor of an electrical load having a higher priority.

[0043] Of course, the details of construction and the embodiments may be varied widely with respect to what is described and illustrated without departing from the scope of the invention as defined by the attached claims.

Claims

CLAIMS1. A method of controlling a compressor of an air-conditioning and / or cooling system of a vehicle with an electric powertrain, wherein the compressor is operated by a dedicated electric motor, and wherein said dedicated electric motor and said electric powertrain are powered by a traction battery pack of the vehicle; the method comprising:- receiving, from a vehicle control unit, a limit value of power (EElectCompLim) absorbable by said dedicated electric motor;- receiving, from said vehicle control unit, an operating value of the compression ratio (βComp) of the compressor;- determining, as a function of said received limit value of absorbable power (EElectCompLim) and said received operating value of the compression ratio (βComp), a limit value of speed (nCompLim) of said dedicated electric motor; and- driving the dedicated electric motor at a speed lower than or equal to said determined limit value of speed ( nCompLim)2. The method of claim 1 , wherein said limit value of speed (nCompLim) of said dedicated electric motor is determined as a function of one or more correlation maps stored in a vehicle control unit.

3. The method of claim 1 or claim 2, wherein the step of determining said limit value of speed ( nCompLim) of said dedicated electric motor comprises:- increasing said limit value of speed ( nCompLim) as said received limit value of absorbable power (EElectCompLim) increases; and / or- decreasing said limit value of speed (nCompLim) as said received operating value of the compression ratio (βComp) increases.

4. The method of any of the previous claims, comprising determining, at said vehicle control unit, said limit value of absorbable power (EElectCompLim) as a function of further instantaneous power absorptions from said traction battery pack of the vehicle, concurrent with said power absorption by said dedicated electric motor.

5. The method of any of the previous claims, comprising determining, at said vehicle control unit, said limit value of absorbable power (EElectCompLim) as a function of a limit value of overall power deliverable bysaid traction battery pack of the vehicle.

6. The method of any of the previous claims, comprising determining, at said vehicle control unit, said limit value of absorbable power (EElectCompLim) as a function of an instantaneous power absorption by said electric powertrain and a limit value of overall power deliverable by said traction battery pack of the vehicle.

7. The method of any of the previous claims, wherein the step of receiving an operating value of the compression ratio (βComp) of the compressor comprises:- measuring a discharge pressure downstream of said compressor;- measuring a speed of a blower of the air-conditioning system and / or a speed of a pump of the cooling system;- determining a flow rate of cabin airthat flows through an evaporator of the air-conditioning system as a function of said speed of said blower, and / or determining a flow rate of coolant liquidthat flows through a chiller of the cooling system as a function of said speed of said pump;- measuring the inlet temperatureand the outlet temperatureof said cabin air at said evaporator, and / or the inlet temperatureand the outlet temperature of saidcoolant liquid at said chiller;- determining a thermal cooling power of said evaporatoas a function of said flow rate of cabin airand a difference between said inlet temperatureand said outlet temperatureof said cabin air; and / or determining a thermal cooling power of said chiller as a function of said flow rate of coolant liquidand a difference between said inlet temperature and saidoutlet temperature of said coolant liquid;- determining a temperature of a coolant fluid that is compressed by said compressor as a function of said inlet temperatureof said cabin air, of said thermal cooling power of said evaporator andof a thermal resistance of said evaporator and / or determininga temperature of said coolant fluid that is compressed by said compressor as a function of said inlet temperatureof said coolant liquid, of said thermal cooling power of said chiller and of a thermalresistance of said chiller- determining a suction pressure upstream of said compressor as a function of the temperature of the coolant fluid; and- determining said operating value of the compression ratioof the compressor as the ratio between said discharge pressure and said suction pressure.

Images