CROSS REFERENCE TO RELATED APPLICATION

[0001] This PCT application claims the benefit of U.S. Provisional Patent Application Serial No. 61/736,304 filed on December 12, 2012, entitled "Control of Compressor Outlet Pressure Towards Target Value for Vehicle with Electric Traction Motor".

FIELD OF THE DISCLOSURE

[0002J This disclosure relates to vehicles that include a refrigerant system with a variable speed compressor, and more particularly, to vehicles that have an electric traction motor that include such a refrigerant system.

BACKGROUND

[0003] This section provides background information related to the present disclosure which is not necessarily prior art.

|0004] Vehicles with traction motors offer the promise of powered transportation while producing few or no emissions at the vehicle. Such vehicles may be referred to as electric vehicles, however it will be noted that some electric vehicles include only an electric motor, while some electric vehicles include both a traction motor and an internal combustion engine. For example, some electric vehicles are powered by electric motors only and rely solely on the energy stored in an on-board battery pack. Some electric vehicles are hybrids, having both a traction motor and an internal combustion engine, which may, for example, be used to assist the traction motor in driving the wheels (a parallel hybrid), or which may, for example, be used solely to charge the on-board battery pack, thereby extending the operating range of the vehicle (a series hybrid). In some vehicles, there is a single, centrally-positioned electric motor that powers one or more of the vehicle wheels, and in other vehicles, one or more of the wheels have an electric motor (referred to sometimes as a hub motor) positioned at each driven wheel.

[0005] Thermal management systems in such vehicles may include a refrigerant system, which is used to circulate refrigerant to keep thermal loads such as the vehicle cabin, and optionally the vehicle's battery pack and related components sufficiently cool.

[0006] The compressor that is used in such refrigerant systems is typically an electrically powered compressor and is capable of variable speed. The compressor typically has a maximum pressure that it can operate at before venting refrigerant through a pressure relief valve to protect it and the other components in the refrigerant system from damage. In order to prevent such venting events from occurring vehicles are typically configured to limit the maximum allowable pressure of the compressor at a much lower level. The control schemes as the compressor reaches or nears the maximum allowable level, however, are in some instances somewhat inefficient. In some cases, a control system may simply drop the compressor pressure to zero abruptly, when the compressor pressure exceeds a threshold pressure, and then restart the compressor when the pressure drops below the threshold pressure. Such a control strategy can, however, cause the compressor to cycle on and off repeatedly when the pressure is near the threshold, which can cause undue stress on the compressor and can result in a shortened operating life for the compressor.

[0007] Another challenge to a team designing a refrigerant system is that it is undesirable to be too conservative when limiting the pressure attainable by the compressor, since the compressor is needed to provide a selected performance level in order for the refrigerant system to have sufficient cooling capacity to meet the needs of the user of the vehicle. If the compressor pressure were too limited by the control system, the refrigerant system would require a larger compressor in order to meet specified cooling needs. A larger compressor can add cost and size to the refrigerant system. It is thus desirable to operate the compressor close to the limit at which a venting event would occur, but without risk of bringing about a venting event.

[0008] An improved control strategy for the compressor at or near the maximum allowable limit would be beneficial.

SUMMARY

[0009] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0010] In one aspect, a method of operating a refrigerant system of a vehicle having an electric traction motor, wherein the refrigerant system includes a compressor, the method comprising: a) driving the compressor towards a target compressor speed and circulating refrigerant compressed by the compressor through the refrigerant system to cool a thermal load, wherein the target compressor speed is selected based at least in part on the thermal load; and b) changing the compressor speed to a selected proportion of the target compressor speed when a compressor pressure reaches a selected threshold pressure, wherein the selected proportion varies inversely with compressor pressure.

[0011] In another aspect, a thermal management system for a vehicle having an electric traction motor, the thermal management system comprising:

a refrigerant system including a compressor, a condenser downstream from the compressor, and a thermal load downstream from the condenser and upstream from the compressor; and

a control system that is configured for:

a) driving the compressor towards a target compressor speed and circulating refrigerant compressed by the compressor through the refrigerant system to cool a thermal load, wherein the target compressor speed is selected based at least in part on the thermal load; and

b) changing the compressor speed to a selected proportion of the target compressor speed when a compressor pressure reaches a selected threshold pressure, wherein the selected proportion varies inversely with compressor pressure.

[0012] Optionally, with respect to either of the aspects noted above:

[0013J Step b) may include reducing the compressor speed to the selected proportion of the target compressor speed when the compressor pressure climbs to the selected threshold pressure. [0014] The selected proportion of the target compressor speed may vary continuously with the compressor pressure over a range of compressor pressures.

[0015] The selected proportion of the target compressor speed may change at a first rate of change with the compressor pressure over a first range of compressor pressures, and at a second rate of change with the compressor pressure over a second range of compressor pressures.

[0016] The first rate of change may be less than the second rate of change.

[0017] The selected proportion of the target compressor speed may vary linearly with the compressor pressure over a range of compressor pressures.

[0018] Optionally, the method includes reducing the speed of the compressor to zero if the pressure at the compressor exceeds a maximum allowable pressure that is greater than the selected threshold pressure.

[0019] Step b) may include increasing the compressor speed to the selected proportion of the target compressor speed when the compressor pressure drops to the selected threshold pressure.

[0020] In yet another aspect, a method of controlling the speed of a compressor in a refrigerant system of a vehicle having an electric traction motor, wherein the refrigerant system includes a compressor, the method comprising:

a) determining a compressor pressure during operation of the compressor; b) providing a basic control algorithm for determining a target compressor speed based on a thermal load to be cooled using the refrigerant system; and c) operating the compressor at a selected proportion of a target compressor speed resulting from the basic control algorithm, based on the compressor pressure determined in step a).

[00211 In yet another aspect a thermal management system for a vehicle having an electric traction motor is provided, the thermal management system comprising:

a refrigerant system including a compressor, a condenser downstream from the compressor, and a thermal load downstream from the condenser and upstream from the compressor; and

a control system that is configured for:

a) determining a compressor pressure during operation of the compressor; b) providing a basic control algorithm for determining a target compressor speed based on a thermal load to be cooled using the refrigerant system; and

c) operating the compressor at a selected proportion of a target compressor speed resulting from the basic control algorithm, based on the compressor pressure determined in step a).

[0022) Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS [0023] The drawings illustrate, by way of example only, embodiments of the present disclosure.

[0024] Figure 1 is a side view of a vehicle;

[0025] Figure 2 is a functional block diagram of a refrigerant system of the vehicle;

[0026J Figure 3 is a flowchart of a method of operating the refrigerant system;

[0027J Figure 4 shows a lookup tables for the method of Figure 3; and

[0028] Figures 5a and 5b illustrate the pressure and compressor speed associated with operation of the refrigerant system shown in Figure 2.

DETAILED DESCRIPTION

[0029] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0030J In this specification and in the claims, the use of the article "a", "an", or "the" in reference to an item is not intended to exclude the possibility of including a plurality of the item in some embodiments. It will be apparent to one skilled in the art in at least some instances in this specification and the attached claims that it would be possible to include a plurality of the item in at least some embodiments.

[0031] Figure 1 depicts an electric vehicle 10. The term 'electric vehicle' as used herein denotes a vehicle that includes an electric traction motor 12 (which may be referred to simply as an 'electric motor' for convenience). The electric vehicle 10 may also include an internal combustion engine, not shown, or alternatively it may lack an internal combustion engine. In embodiments wherein an internal combustion engine is provided, the engine may be operated simultaneously with the electric traction motor 12 (parallel hybrid), or it may be operated only when a battery pack (shown at 28) for the electric traction motor 12 has been substantially depleted (or depleted to a minimum acceptable state of charge). In embodiments wherein the engine is provided, the function of the engine may be to propel the vehicle, to charge the battery pack, to both propel the vehicle and charge the battery pack, or for some other purpose. Furthermore, the electric vehicle 10 may be any suitable type of vehicle, such as, for example, an automobile, a truck, an SUV, a bus, a van, a motorcycle or any other type of vehicle. The vehicle 10 includes a body 91 , a plurality of wheels 93, an electric traction motor 12 configured for driving at least one of the wheels 93, and battery pack 28 configured for providing power to the electric traction motor 12.

[0032] The battery pack 28 provides power for use by the motor 12 and other high-voltage loads. In the embodiment shown, current from the battery pack 28 to the motor 12 is controlled by a torque control module (TCM) 29.

[0033] The battery pack 28 may be any suitable type of battery pack, such as one made up of a plurality of lithium polymer cells. While one battery pack 28 is shown, it is alternatively possible to have any suitable number of battery packs, such as two or more.

[0034] The vehicle 10 further includes a battery charge control module (BCCM) 30 that is used to control charging of the battery pack 28 when the vehicle 10 is connected to an external electrical source (e.g., a 1 10-volt source or a 220-volt source).

[0035] The battery pack 28 and the BCCM 30 form part of a high-voltage battery system. The battery pack 28 may be maintained within a selected temperature range, which can extend the operating life of the battery pack 28. To remain within the temperature range 28 the battery pack 28 sometimes requires cooling. The BCCM 30 also requires cooling sometimes. When charging the vehicle 10, the BCCM 30 generates heat and sometimes the BCCM 30 requires cooling to prevent overheating. The battery pack 28 and the BCCM 30 together make up all or part of a battery system cooling load 36 shown in Figure 2, although it will be understood that in other embodiments the battery system cooling load 36 may omit the cooling of one or more of these components (e.g. the BCCM 30) and/or may include other components.

[0036] The vehicle 10 further has a cabin 13 that may require cooling for the comfort of any vehicle occupants therein. The cabin 13 may thus be considered another cooling load, which may be referred to as a cabin-related cooling load.

[0037] Figure 2 shows a refrigerant system 100 for the vehicle 10. The refrigerant system 100 is used for the cooling of the two aforementioned cooling loads in the vehicle 10, namely the vehicle cabin 13 and the battery system cooling load 36. It will be noted that, in Figure 2 fluid connections are shown in solid line, while electrical connections are shown in dashed line. Not all electrical and fluid connections are shown for the sake of clarity.

[0038] The refrigerant system 100 includes a battery system heat exchanger 32, a cabin cooling heat exchanger 50, a compressor 40 and a condenser 38. The condenser 38 is downstream from the compressor 40 and upstream from the two heat exchangers 32 and 50. A refrigerant conduit loop 53 transports refrigerant between the condenser 38, the compressor 40 and the heat exchangers 32 and 50. The condenser 38 is positioned downstream from the compressor 40 and upstream from the heat exchangers 32 and 50.

[0039] The heat exchanger 50 has an inlet line 57 and an outlet line 59 which connect to the refrigerant conduit loop 53 via tee connections 61 and 63. A control valve 45 is provided and can control the flow of refrigerant to the heat exchanger 50 (permitting the refrigerant flow to bypass the heat exchanger 50 entirely if desired).

[0040] The heat exchanger 32 has an inlet line 67 and an outlet line 69 which connect to the refrigerant conduit loop 53 via tee connections 71 and 73. A control valve 55 is provided and can control the flow of refrigerant to the heat exchanger 50 (permitting the refrigerant flow to bypass the heat exchanger 32 entirely if desired).

[0041] Other configurations of the refrigerant system 100 are possible.

[0042] The battery system heat exchanger 32 is configured for use in cooling the battery system cooling load 36. The battery system heat exchanger 32 may be a chiller that uses refrigerant to cool a liquid coolant that flows through a battery system coolant circuit 43. The battery system coolant circuit 43 is used to transport the liquid coolant from the battery system heat exchanger 32 through the components that make up the battery system cooling load 36, namely the battery pack 28 and the BCCM 30.

[0043] Alternatively, the battery system heat exchanger 32 may be an evaporator for cooling an air flow used to cool the battery pack 28 and BCCM 30.

[0044] The compressor 40 and condenser 38 supply refrigerant to the battery system heat exchanger 32. Refrigerant that has passed through the battery system heat exchanger 32 is returned to the suction of the compressor 40.

[0045] Temperature control of the battery circuit 43 can be facilitated by one or more battery circuit temperature sensors 47. In this example, the battery circuit temperature sensor 47 is located downstream of the battery system cooling load 36.

[0046] The refrigerant system 100 further includes a cabin cooling heat exchanger 50, which is a heat exchanger that uses refrigerant to cool the cabin 13. Specifically, the cabin cooling heat exchanger 50 receives refrigerant from the condenser 38 and uses the refrigerant to cool air that is sent to the cabin 13 (via an air passage 52 and a blower 54) in order to cool the cabin 13. Refrigerant leaving the cabin cooling heat exchanger 50 returns to the compressor 40.

[0047] The cabin cooling heat exchanger 50 may be an evaporator. In the embodiment shown in Figure 2, the cabin cooling heat exchanger 50 is provided with a flow control valve 45 to control the flow of refrigerant to the cabin cooling heat exchanger 50.

[0048] The vehicle 10 may further includes an ambient temperature sensor 82. The ambient temperature sensor 82 is positioned to measure a temperature indicative of the environmental temperature outside the vehicle 10. The temperature sensor 82 can include a thermocouple, a thermopile, a thermistor, or the like.

[0049] The refrigerant that is transported through the refrigerant system 100 may be any suitable refrigerant such as R-134a.

[0050] A pressure sensor 51 may be provided for sensing the discharge pressure from the compressor 40, which may be referred to as the compressor pressure. The pressure sensor 51 may be any suitable type of pressure sensor.

[0051] The vehicle 10 further includes a control system 80. The control system 80 is represented in Figure 2 as a single unit in Figure 2, however the control system 80 may be a complex distributed control system having multiple individual controllers connected to one another over a controller area network. The control system 80 includes a processor 86 and memory 88 coupled together (however it will be understood that the processor 86 may be representative of a plurality of processors in multiple individual controllers and the memory 88 may be representative of a plurality of memories in multiple individual controllers). The processor 86 is capable of executing instructions stored in or originating at the memory 88. The control system 80 further includes an input-output interface (not shown) for connecting to other components of the vehicle 10 to allow the processor 86 to communicate with such components. The input- output interface can include a control system-area network bus (CAN bus) or similar.

[0052] The control system 80 can be electrically connected (shown in Figure 2 as dashed lines) to any of the components of the refrigerant system 100, such as the control valve 45, the compressor 40 (at "a"), the pressure sensor 51 (also at "a"), the battery circuit temperature sensor 47 (also at "a"), and the ambient temperature sensor 82. The control system 80 can be configured, by programming for example, to control and monitor operations of the refrigerant system 100. The control system 80 can be programmed to control the compressor 40 to operate based on refrigerant demand at the cabin cooling heat exchanger 50 and battery system heat exchanger 32, in embodiments wherein the compressor 40 is a variable speed compressor.

[0053J The control system 80 may control the operation of the compressor 40 according to any suitable control algorithm. For example, below a selected threshold pressure, the control system 80 may use a P-1 control algorithm to control the compressor speed, in order to reach a target evaporator temperature. The control algorithm used to control the speed of the compressor 40 below the selected threshold pressure may be referred to as the basic control algorithm. At and above the aforementioned selected threshold pressure, the control system 80 may use a different control algorithm to control the compressor speed, which may be referred to as a modified control algorithm (and which may be a modified version of the basic control algorithm), in order to ensure that the compressor pressure remains below a selected maximum allowable pressure. The overall method of controlling the compressor may be as shown at 300 in Figure 3.

[0054] At step 302, the compressor 40 may be operated at a selected compressor speed and refrigerant compressed by the compressor may be circulated through the refrigerant circuit to cool a thermal load that may include one or both of the evaporator 50 and the chiller 32. The selected compressor speed may be selected based on a selected target temperature for the evaporator 50, which may be selected based at least in part on the thermal load. For example, the vehicle cabin 13 may be relatively hot, and the user may have set a certain target temperature for the cabin 13 and a certain fan setting for the fan 54. The vehicle's HVAC control system, which may be considered to be part of the control system 80 may determine a target temperature for the evaporator 50 based on these inputs (i.e. the current and target cabin temperatures and the fan setting), and possibly based on other inputs also, such as whether the vehicle 10 is in an 'ECO' mode or a 'COMFORT' mode).

[0055] During operation of the vehicle 10, the compressor pressure may rise during step 302. At step 304 the control system 80 determines whether the compressor pressure has exceeded a first threshold pressure. If the compressor pressure reaches the selected first threshold pressure, the control system 80 may be configured (e.g. programmed) to change the compressor speed to a first proportion of the selected compressor speed, using the modified control algorithm at step 306. For example, upon reaching the first threshold pressure, the control system 80 may be programmed to continue to determine a selected target compressor speed based on the aforementioned inputs and may determine a selected first proportion of that selected target compressor speed based on the current compressor pressure.

[0056] The selected first proportion may be represented by PI , and may decrease as the compressor pressure increases. The decrease may be continuous, or in stages. In an embodiment, the first proportion PI may be determined in part from a lookup table stored in the memory 88 and in part based on a calculation. An example of a lookup table is shown at 400 in Figure 4. In the lookup table 400 there are a plurality of threshold pressures including a first threshold pressure PR1 , a second threshold pressure PR2, a third threshold pressure PR3, a fourth threshold pressure PR4 and a fifth threshold pressure PR5, which may be a maximum allowable pressure for the compressor. The maximum allowable pressure may be set to a value that is sufficiently below a pressure at which the compressor 40 would release refrigerant through a pressure relief valve (PRV) so that if the compressor 40 was at the maximum allowable pressure and a sudden spike in compressor pressure occurred for some unforeseen reason, the compressor 40 would likely still remain below the pressure at which the PRV would open.

[0057] Associated with each threshold pressure is a proportion modifier value PM1 , PM2.. .PM5, which indicates how much to modify the target compressor speed that is determined by the P-I control algorithm when the compressor pressure is at the particular pressure noted in the lookup table 400. Thus these proportion modifier values relate directly to proportion values PI , P2....P5 which are not shown in the table. For example a proportion modifier value of 0% equates to a proportion value of 100%. A proportion modifier value of 5% equates to a proportion value of 95%. A proportion modifier value of 15% equates to a proportion value of 85%. It is alternatively possible for the proportion values to be stored in the table 400 instead of proportion modifier values.

[0058] When the instantaneous compressor pressure (represented by PR) is equal to PR1 (i.e. 2500 kPa), the proportion modifier value used by the control system 80 (represented by PM) is equal to PM1 (i.e. 0%), which means that the proportion (represented by P) used by the control system 80 is equal to PI , which is 100%). This means that, when the compressor pressure is 2500 kPa the control system 80 adjusts the current to the compressor based on a target compressor speed that is 100% of the target compressor speed calculated using the P-I control algorithm. For compressor pressures PR that are within a first range (between PR1 and PR2) the proportion modifier value PM and therefore the proportion value P may vary based on pressure. Optionally, the variance in PM and P may be linear based on the compressor pressure. For example, if the compressor pressure PR is 2537.5 kPa (i.e. halfway between PR1 and PR2) the control system 80 may use a proportion modifier value PM of 2.5% (i.e. halfway between 0% and 5%) and the resultant proportion P of the target compressor speed used is 97.5%.

[0059] As can be seen in the table 400, if the compressor 40 reaches a pressure PR that is equal to PR2 (i.e. 2575 kPa), the proportion modifier value PM would be equal to PM2 (i.e. 5%), which is indicative of a proportion P that is equal to P2, (i.e. 95%), which means that the control system 80 reduces the target compressor speed to 95% of the target compressor speed determined using the P-I control algorithm. If the compressor 40 reaches a pressure PR that is equal to PR3 (i.e. 2650 kPa), the proportion modifier value is 15%, which is indicative of a proportion P3 of 85%, which means that the control system 80 reduces the target compressor speed to 85% of the target compressor speed determined using the P-I control algorithm. The proportion value P used to modify the target compressor speed may vary linearly or otherwise for compressor pressures that are in the second range of pressures between PR2 and PR3.

[0060] It will be noted that the rate of decrease in the proportion P of the target compressor speed that is used is higher in the second range of pressures than in the first range of pressures. In other words, upon reaching a pressure PR = PR1 , the proportion of the target compressor speed that is used will drop off from 100% to 95% over a range of 75 kPa (i.e. the range between 2500 kPa and 2575 kPa), which corresponds to a rate of decrease of 1 % for every increase of 15 kPa in compressor pressure. By contrast, the proportion of the target compressor speed that is used will drop off from 95% to 85% over a range of 75 kPa (i.e. the range between 2575 kPa and 2650 kPa), which corresponds to a rate of decrease of 1% for every increase of 7.5 kPa in compressor pressure. Thus the rate of decrease over the second range or pressures is two times the rate of decrease over the first range of pressures. As can be seen, the rate of decrease in proportion over the third range (i.e. between PR3 and PR4) corresponds to 1% for every increase of 5 kPa (i.e. (85%-70%) / (2725 kPa - 2650 kPa) = 15% / 75 kPa = 1% I 5 kPa), which is a higher rate of decrease than the rate of decrease in the second range. In the final range, which is the fourth range, the proportion P that is applied to the target compressor speed drops to 0% upon the compressor 40 reaching a pressure PR that is equal to PR5 (i.e. 2800 kPa). A proportion of 0% means that the target compressor speed is reduced to zero. Thus, upon detecting a compressor pressure of 2800 kPa, the control system 80 shuts down the compressor 40, thereby substantially preventing the pressure from increasing significantly beyond 2800 kPa.

[0061] Figures 5a and 5b show several examples of the use of the above noted method to control the compressor 40 and the resultant speed (Figure 5b) and pressure (Figure 5a) of the compressor 40 as a function of time. Three curves are shown representing the response of the compressor 40 to changes in pressure, when starting at three different initial speeds (5000 RPM, 6500 RPM and 8000 RPM). As can be seen, between time TO and time Tl the compressor speed remains substantially constant at some value (e.g. 5000 RPM, 6500 RPM or 8000 RPM) as determined by a basic control algorithm. During this period, however, the pressure rises and reaches a value of PR1. Such a pressure rise can occur for many reasons. For example, the control system 80 may set the compressor speed to a particular value while the vehicle is in motion and is thus receiving good airflow across the condenser 38, and at some point the vehicle 10 may stop, thereby reducing the efficiency of the condenser 38 due to a reduced airflow across the condenser 38. The reduced efficiency of the condenser 38 would result in a pressure rise at the compressor 40. As a result of the pressure rise the control system 80 begins to reduce the compressor speed to a proportion of the speed determined by the basic control algorithm. As can be seen, between time Tl and time T2, the compressor pressure continues to rise even as the speed decreases, and as the pressure rises, the compressor speed drops to a progressively smaller proportion of the target speed determined by the basic control algorithm. At time T2, the pressure reaches PR2. Between time T2 and time T3, the compressor pressure continues to rise at the same rate as between time Tl and time T2 (in this hypothetical scenario), and the rate of decrease in the proportion of the target compressor speed is higher than the rate of decrease that occurs between time Tl and time T2.

[0062] Between time T3 and time T4 the rate of decrease in the proportion of the target compressor speed is higher than the rate of decrease between time T2 and the time T3. Similarly, between time T4 and time T5 the rate of decrease is higher than the rate of decrease between time T3 and time T4. At time T5, the compressor pressure reaches 2800 kPa, and the proportion drops to zero. As a result, the compressor speed drops to zero. At that point the compressor pressure remains substantially constant for some period of time. At time T6 the compressor pressure drops below the maximum allowable pressure. Between time T6 and time T7, the compressor pressure may continue to drop, and as a result, using the lookup table 400, the control system 80 increases the proportion of the target compressor speed that the compressor 40 is operated at, until, at time T7, the pressure reaches PR4, and the proportion of the target speed for the compressor 40 is P4 (i.e. 70%). As shown in Figure 5a, the pressure may continue to drop, in which case the control system 80 will increase the proportion of the target compressor speed determined using the basic control algorithm. As can be seen in Figure 5b, the compressor speed may ramp up using the same proportions and the same rates of change that were used to decrease the compressor speed as a result of increasing pressure.

|0063] As noted above, Figure 5b illustrates the reaction of the control system 80 on the compressor speed under three scenarios: where the compressor speed is initially 8000 RPM, where the speed is 6500 RPM, and where the speed is 5000 RPM. As can be seen, if the compressor pressure changed as shown in Figure 5a, the compressor speed would ramp down to zero (i.e. at 5 seconds) at the same time in each of these scenarios.

[0064] An advantage of the control method described above is that, when the pressure initially reaches PR1 the ramp-down in compressor speed is relatively small. By contrast, in a control scheme where the compressor speed is simply reduced by a set amount of RPM or a series of set PRM values (instead of being reduced by a selected proportion of the target speed) the impact on the compressor pressure may change depending on the speed at which the compressor was initially operating. In some cases, the drop in compressor speed can lead to a relatively large and sudden drop in the compressor pressure. Such a drop in compressor pressure can then result in a relatively large increase in compressor speed, depending on the particular control scheme being used. A cycle of large swings in compressor pressure and speed can thus result. For example, if the compressor was running at 4000 RPM and a control system were to drop the RPM by some fixed amount, such as 500 RPM, it would be a relatively significant immediate drop in speed, which could result in a relatively steep drop in pressure. Upon sensing the reduced pressure, the system might increase the speed again. Increasing the compressor speed could drive the pressure right back up beyond the pressure PRl , which would trigger such a system to again reduce the speed. Such 'hunting' can reduce the longevity of the compressor.

[0065] Viewed from another perspective, if the compressor was running at 4000 RPM a drop of 200 RPM may have a desired effect on the pressure, but at 8000 RPM a drop of 200 RPM might not have the desired effect on the pressure. Thus, absolute pressure drops, even ones that vary progressively throughout a range of pressures will have an effect on the compressor pressure that varies depending on the initial R M at which the compressor is running. As a result, it is more difficult to control the compressor pressure in a selected amount of time. By contrast, as can be seen in the graphs in Figure 5b, regardless of the initial speed of the compressor 40, the response in terms of controlling the compressor pressure is similar and is controlled in a progressive way without such 'hunting'.

10066] As shown in the graphs in Figures 5a and 5b, the threshold pressures PR1 -PR5 used to trigger changes in the rate of change in the proportion P of the target compressor speed used to operate the compressor may be the same when the pressure decreases as when the pressure increases. In an alternative embodiment, the threshold pressures that trigger such changes may be different when the pressure is increasing than when the pressure is decreasing. In other words, the threshold pressures shown in the lookup table 400 may be applicable for controlling the proportion of the target compressor speed used to run the compressor 40 as the compressor pressure increases. A different lookup table (not shown) having a different set of threshold pressures and a different associated set of proportions to be used to control the compressor speed when the compressor pressure decreases.

[0067] It will be noted that Figure 5b is intended to illustrate the response of the refrigerant system 100 to changes in pressure under different target compressor speeds which are held constant at 5000 RPM, 6500 RPM and 8000 RPM. Figure 5b is not necessarily intended to illustrate the response of the refrigerant system under real-world conditions where the basic control algorithm may change the target compressor speed based on the compressor speed that results from the modified control algorithm.

[0068] The basic control algorithm may be any suitable type of algorithm such as, for example, a PID control algorithm, a PI control algorithm as noted above, a P control algorithm or some other algorithm that incorporates feedback. For example, the basic control algorithm may determine a target temperature for the evaporator 50 based on user-selected settings on the HVAC system. The control system 80 may determine a current temperature for the evaporator 50 via an evaporator temperature sensor 84, and may compare the current and target evaporator temperatures to control the compressor speed.

[0069] It will be noted that, as the compressor pressure reaches a pressure where a proportion of the compressor speed is used so as to reduce the compressor pressure, the flow of refrigerant to the evaporator 50 will drop, which will result in a rise in temperature in the evaporator 50. In an example, when the vehicle 10 is being driven along the highway, the compressor 40 may be controlled to a particular speed via the basic control algorithm to maintain a particular target evaporator temperature. When the vehicle 10 stops, the efficiency of the condenser 38, as noted above, drops, and as a result, the pressure in the compressor 40 rises. In addition, the temperature at the evaporator 50 may rise due to the reduced cooling capacity of the refrigerant leaving the condenser 38. As a result, the basic control algorithm may initiate an increase in the compressor speed, which results in a further increase in compressor pressure. Upon climbing past a threshold pressure, the modified control algorithm is used and applies a speed reduction to the compressor 40, thereby inhibiting the compressor 40 from increasing further in pressure. Sensing this temperature rise, the control system 80 may increase the target compressor speed in an effort to bring the evaporator temperature back down towards the target evaporator temperature. The net result between the speed increase initiated by the basic control algorithm and the speed reduction initiated by the modified control algorithm may result in a net increase in compressor pressure. In the event that the compressor pressure rises, the modified control algorithm progressively increases the amount of reduction that will be applied to the target compressor speed, in an effort to reduce the pressure. At the same time, as the evaporator 50 is no longer reaching the target temperature, the basic control algorithm may continue to increase the target compressor speed. As the basic algorithm applies a progressively higher target speed and as the modified algorithm applies a progressively smaller proportion thereof to the compressor 40, the net change in compressor speed may be small initially. Eventually, (e.g. within 30 seconds) the basic algorithm reaches a maximum permitted target compressor speed (e.g. 8000 RPM) and cannot increase the target speed beyond that. At that point the modified algorithm operates by progressively reducing the proportion of the target compressor speed with which to run the compressor 40 which results in an actual reduction in compressor speed, until the compressor pressure levels off or drops. As the compressor pressure drops the proportion used by the modified control algorithm increases, until the pressure falls below the range in which the modified algorithm is used. Thus, it can be seen that even though the basic control algorithm may attempt to raise the target compressor speed, the modified algorithm will prevent the compressor 40 from increasing the pressure unabated.

[0070] While the pressures in the table 400 are shown as being fixed values, it will be noted that these pressure values can vary depending on certain factors. For example, in some configurations the vehicle may include a radiator that is positioned behind the condenser and that receives an airflow that has first passed over the condenser 38. The radiator is used for cooling coolant that controls the temperature of the vehicle's powertrain (including for example the motor 12). If the vehicle's powertrain becomes too hot, the control system 80 may modify the pressure values in the table 400 downwardly so as to reduce the amount of heat the condenser 38 is likely to transfer to the airflow during periods where the motor 12 may need to be cooled at high efficiency. Such a control method is described in copending US Provisional Patent application 61 /709,357, the contents of which are incorporated herein in their entirety.

[00711 While the foregoing provides certain non-limiting example embodiments, it should be understood that combinations, subsets, and variations of the foregoing are contemplated.