Abstract:
A method according to an exemplary aspect of the present disclosure includes, among other things, controlling a climate control system of an electrified vehicle by shutting off refrigerant flow to a rear portion of a heat pump subsystem in response to a defrost request.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to methods and systems for maximizing defrost functionality of an electrified vehicle equipped with a climate control system that employs dual evaporators and dual heater cores. During certain conditions, the climate control system can be controlled by shutting off refrigerant flow to a portion of a heat pump subsystem in response to a defrost request. 
       BACKGROUND 
       [0002]    Electrified vehicles, such as hybrid electric vehicles (HEV&#39;s), plug-in hybrid electric vehicles (PHEV&#39;s), battery electric vehicles (BEV&#39;s), or fuel cell vehicles differ from conventional motor vehicles because they are powered by electric machines (i.e., electric motors and/or generators) instead of or in addition to an internal combustion engine. High voltage current for powering these types of electric machines is typically supplied by one or more high voltage battery assemblies. 
         [0003]    Some electrified vehicles are equipped with a climate control system that employs a heat pump subsystem for warming, cooling and/or dehumidifying a passenger cabin. For example, the heat pump subsystem can be operated in a heating mode in which the passenger cabin is heated, a cooling mode in which the passenger cabin is cooled, and a dehumidification mode in which a vehicle windshield defrost function is supported. It is desirable to improve operation of the climate control system during certain conditions. 
       SUMMARY 
       [0004]    A method according to an exemplary aspect of the present disclosure includes, among other things, controlling a climate control system of an electrified vehicle by shutting off refrigerant flow to a rear portion of a heat pump subsystem in response to a defrost request. 
         [0005]    In a further non-limiting embodiment of the foregoing method, the controlling step is performed if the electrified vehicle is operating in EV mode. 
         [0006]    In a further non-limiting embodiment of either of the foregoing methods, the controlling step is performed if a fuel level of the electrified vehicle is below a predefined threshold. 
         [0007]    In a further non-limiting embodiment of any of the foregoing methods, the method includes closing an expansion valve to shut off the refrigerant flow to the rear portion. 
         [0008]    In a further non-limiting embodiment of any of the foregoing methods, the rear portion includes a rear evaporator of the heat pump subsystem. 
         [0009]    In a further non-limiting embodiment of any of the foregoing methods, the method includes shutting off coolant flow to a portion of a coolant subsystem in response to the defrost request. 
         [0010]    In a further non-limiting embodiment of any of the foregoing methods, the method includes closing a valve to shut off the coolant flow to the portion. The portion includes a rear heater core of the coolant subsystem. 
         [0011]    In a further non-limiting embodiment of any of the foregoing methods, the method includes turning off a blower of a rear housing of a ventilation subsystem in response to the defrost request. 
         [0012]    In a further non-limiting embodiment of any of the foregoing methods, the controlling step includes commanding operation of the heat pump subsystem in dehumidification mode in response to the defrost request if the electrified vehicle is operating in EV mode and a fuel level of the electrified vehicle is below a predefined threshold. 
         [0013]    In a further non-limiting embodiment of any of the foregoing methods, the heat pump subsystem is a dual evaporator/dual heater core vapor compression heat pump system. 
         [0014]    A method according to another exemplary aspect of the present disclosure includes, among other things, operating a heat pump subsystem of a climate control system of an electrified vehicle in a dehumidification mode and shutting off refrigerant flow to an evaporator of the heat pump subsystem if a defrost request has been made. 
         [0015]    In a further non-limiting embodiment of the foregoing methods, the operating step and the shutting off step are performed if the electrified vehicle is operating in EV mode and a fuel level of the electrified vehicle is below a predefined threshold. 
         [0016]    In a further non-limiting embodiment of either of the foregoing methods, the heat pump subsystem is a dual evaporator/dual heater core vapor compression heat pump system. 
         [0017]    In a further non-limiting embodiment of any of the foregoing methods, the method includes actuating an expansion valve to shut off the refrigerant flow. 
         [0018]    In a further non-limiting embodiment of any of the foregoing methods, the method includes at least one of shutting off coolant flow to a heater core of a coolant subsystem of the climate control system and turning off a blower of a ventilation subsystem of the climate control system. 
         [0019]    A climate control system according to another exemplary aspect of the present disclosure includes, among other things, a heat pump subsystem configured to circulate a refrigerant. The heat pump subsystem includes a front evaporator, a rear evaporator and an expansion valve adapted to shut off flow of the refrigerant to the rear evaporator in response to a defrost request. 
         [0020]    In a further non-limiting embodiment of the foregoing system, a controller is configured to control operation of the expansion valve. 
         [0021]    In a further non-limiting embodiment of either of the foregoing systems, a coolant subsystem is configured to circulate a coolant for cooling an engine. The coolant subsystem includes a front heater core, a rear heater core, and a valve adapted to shut off flow of the coolant to the rear heater core in response to the defrost request. 
         [0022]    In a further non-limiting embodiment of any of the foregoing systems, an intermediate heat exchanger is adapted to effectuate heat transfer between the refrigerant and the coolant. 
         [0023]    In a further non-limiting embodiment of any of the foregoing systems, a ventilation subsystem includes a front housing that houses the front evaporator and a rear housing that houses the rear evaporator. 
         [0024]    The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
         [0025]    The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  schematically illustrates the powertrain of an electrified vehicle. 
           [0027]      FIG. 2  schematically illustrates a climate control system of an electrified vehicle. 
           [0028]      FIG. 3  schematically illustrates a control strategy for controlling a climate control system of an electrified vehicle. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    This disclosure relates to improving the defrost functionality of an electrified vehicle equipped with a climate control system employing dual evaporators and dual heater cores. The climate control system may be controlled by shutting off refrigerant flow to a portion of a heat pump system and/or a portion of a coolant subsystem in response to a defrost request. In some embodiments, defrost operation is maximized in situations where the electrified vehicle is operating in EV mode, fuel level is below a predefined threshold, and a defrost request has been made. These and other features are discussed in greater detail in the paragraphs that follow. 
         [0030]      FIG. 1  schematically illustrates a powertrain  10  of an electrified vehicle  12 . Although depicted as a PHEV in this embodiment, it should be understood that the concepts described herein are not limited to PHEV&#39;s and could extend to other electrified vehicles, including, but not limited to, HEV&#39;s and BEV&#39;s. 
         [0031]    In one embodiment, the powertrain  10  is a power-split powertrain system that employs a first drive system and a second drive system. The first drive system includes a combination of an engine  14  and a generator  18  (i.e., a first electric machine). The second drive system includes at least a motor  22  (i.e., a second electric machine), the generator  18 , and a battery assembly  24 . In this example, the second drive system is considered an electric drive system of the powertrain  10 . The first and second drive systems generate torque to drive one or more sets of vehicle drive wheels  28  of the electrified vehicle  12 . 
         [0032]    The engine  14 , which may be an internal combustion engine, and the generator  18  may be connected through a power transfer unit  30 , such as a planetary gear set. Of course, other types of power transfer units, including other gear sets and transmissions, may be used to connect the engine  14  to the generator  18 . In one non-limiting embodiment, the power transfer unit  30  is a planetary gear set that includes a ring gear  32 , a sun gear  34 , and a carrier assembly  36 . 
         [0033]    The generator  18  can be driven by the engine  14  through the power transfer unit  30  to convert kinetic energy to electrical energy. The generator  18  can alternatively function as a motor to convert electrical energy into kinetic energy, thereby outputting torque to a shaft  38  connected to the power transfer unit  30 . Because the generator  18  is operatively connected to the engine  14 , the speed of the engine  14  can be controlled by the generator  18 . 
         [0034]    The ring gear  32  of the power transfer unit  30  may be connected to a shaft  40 , which is connected to vehicle drive wheels  28  through a second power transfer unit  44 . The second power transfer unit  44  may include a gear set having a plurality of gears  46 . Other power transfer units may also be suitable. The gears  46  transfer torque from the engine  14  to a differential  48  to ultimately provide traction to the vehicle drive wheels  28 . The differential  48  may include a plurality of gears that enable the transfer of torque to the vehicle drive wheels  28 . In one embodiment, the second power transfer unit  44  is mechanically coupled to an axle  50  through the differential  48  to distribute torque to the vehicle drive wheels  28 . 
         [0035]    The motor  22  can also be employed to drive the vehicle drive wheels  28  by outputting torque to a shaft  52  that is also connected to the second power transfer unit  44 . In one embodiment, the motor  22  and the generator  18  cooperate as part of a regenerative braking system in which both the motor  22  and the generator  18  can be employed as motors to output torque. For example, the motor  22  and the generator  18  can each output electrical power to the battery assembly  24 . 
         [0036]    The battery assembly  24  is an example type of electrified vehicle battery assembly. The battery assembly  24  may include a high voltage battery pack that is capable of outputting electrical power to operate the motor  22  and the generator  18 . Other types of energy storage devices and/or output devices can also be used to electrically power the electrified vehicle  12 . 
         [0037]    In a non-limiting PHEV embodiment of the electrified vehicle  12 , the battery assembly  24  may be recharged or partially recharged using a charging adapter  54  that is connected to a charging station powered by an external power source, such as an electrical grid, a solar panel, or the like. 
         [0038]    In one non-limiting embodiment, the electrified vehicle  12  has two basic operating modes. The electrified vehicle  12  may operate in an Electric Vehicle (EV) mode where the motor  22  is used (generally without assistance from the engine  14 ) for vehicle propulsion, thereby depleting the battery assembly  24  state of charge up to its maximum allowable discharging rate under certain driving patterns/cycles. The EV mode is an example of a charge depleting mode of operation for the electrified vehicle  12 . During EV mode, the state of charge of the battery assembly  24  may increase in some circumstances, for example due to a period of regenerative braking The engine  14  is generally not permitted to operate under a default EV mode but could be operated as necessary based on a vehicle system state or as permitted by the operator. 
         [0039]    The electrified vehicle  12  may additionally be operated in a Hybrid (HEV) mode in which the engine  14  and the motor  22  are both used for vehicle propulsion. The HEV mode is an example of a charge sustaining mode of operation for the electrified vehicle  12 . During the HEV mode, the electrified vehicle  12  may reduce the motor  22  propulsion usage in order to maintain the state of charge of the battery assembly  24  at a constant or approximately constant level by increasing the engine  14  propulsion usage. The electrified vehicle  12  may be operated in other operating modes in addition to the EV and HEV modes. 
         [0040]      FIG. 2  illustrates a climate control system  56  of an electrified vehicle, such as the electrified vehicle  12  of  FIG. 1 . However, this disclosure extends to other electrified vehicles and is not limited to the specific configuration shown in  FIG. 1 . In  FIG. 2 , devices and fluidic passages or conduits are shown in solid lines, and electrical connections are shown as dashed lines. 
         [0041]    In one embodiment, the electrified vehicle  12  includes a passenger compartment  58 , an engine compartment  60 , and the climate control system  56 . The passenger compartment  58  may be located inside the electrified vehicle  12  and can receive one or more occupants. A portion of the climate control system  56  may be disposed within the passenger compartment  58 . 
         [0042]    Engine compartment  60  is positioned proximate to the passenger compartment  58 . One or more power sources, such as an internal combustion engine  14 , as well as a portion of the climate control system  56  may be housed within the engine compartment  60 . The engine compartment  60  may be isolated from the passenger compartment  58  via a bulkhead  62 . The climate control system  56  can circulate air and/or control or modify the temperature of air that is circulated in the passenger compartment  58 . The internal combustion engine  14  can also be thermally managed by the climate control system  56  to reduce fuel consumption and emissions. 
         [0043]    The climate control system  56  may include a coolant subsystem  64 , a heat pump subsystem  66 , and a ventilation subsystem  68 . Each of these systems is described in detail below. 
         [0044]    The coolant subsystem  64 , or coolant loop, may circulate a coolant, such as glycol, to cool the engine  14 . For example, waste heat generated by the engine  14  when the engine is operational may be transferred to the coolant and then circulated to a radiator  70  to cool the engine  14 . In one embodiment, the coolant subsystem  64  includes a coolant pump  72 , an intermediate heat exchanger  74 , a front heater core  76 , a rear heater core  78 , and a bypass loop  80  that may be fluidly interconnected by conduits or passages such as tubes, hoses, pipes and/or the like. The radiator  70  transfers thermal energy from the coolant to the ambient air surrounding the electrified vehicle  12 . 
         [0045]    The coolant subsystem  64  may additionally include valves  82 ,  84  and  86  for selectively adjusting the flow of coolant through the engine  14 , the radiator  70 , the intermediate heat exchanger  74 , the front heater core  76 , and/or the rear heater core  78 . In one embodiment, the valves  82 ,  84  and  86  are electrically operated valves that are selectively actuated via a controller  88 . Other types of valves could alternatively be utilized within the coolant subsystem  64 . 
         [0046]    In operation, the coolant pump  72  circulates coolant through the coolant subsystem  64 . The coolant pump  72  may be powered by electrical or non-electrical power sources. For example, the coolant pump  72  could be operatively coupled to the engine  14 , or could be driven by an electrically powered motor. The coolant pump  72  receives coolant from the engine  14  and circulates the coolant in a closed loop. For example, when the climate control system  56  is operating in a heating mode, coolant may be routed from the coolant pump  72  to the intermediate heat exchanger  74 , thereby bypassing the radiator  70 , and then to the front heater core  76  and/or the rear heater core  78  before returning to the engine  14 . When the engine  14  is outputting relatively high levels of thermal energy, coolant may flow from the coolant pump  72  to the radiator  70  before returning to the engine  14  via the intermediate heat exchanger  74  and the front heater core  76  and/or the rear heater core  78 . The valve  84  directs coolant from the coolant pump  72  either through the radiator  70  or around the radiator  70  to the valve  82 . Coolant may flow through or around the engine  14  based on the position of the valve  82 . 
         [0047]    The intermediate heat exchanger  74  may facilitate the transfer of thermal energy between the coolant subsystem  64  and the heat pump subsystem  66 . For example, heat may be transferred from the heat pump subsystem  66  to the coolant subsystem  64  or visa-versa. In one embodiment, the intermediate heat exchanger  74  is disposed as part of both the coolant subsystem  64  and the heat pump subsystem  66 . The intermediate heat exchanger  74  can include any suitable configuration. For example, the intermediate heat exchanger  74  may have a plate-fin, tube-fin, or tube-and-shell configuration that facilitates the transfer of thermal energy between the heat pump subsystem  66  and the coolant subsystem  64  without mixing or exchanging the heat transfer fluids of these systems. 
         [0048]    In some conditions, the front heater core  76  and the rear heater core  78  may transfer thermal energy from the engine coolant to air in the passenger compartment  58 . The front heater core  76  and the rear heater core  78  are located at different locations within the passenger compartment  58  in different sections of the ventilation subsystem  68  and could embody any suitable configuration. In one embodiment, the front and rear heater cores  76 ,  78  are configured as plate-fin or tube-fin heat exchangers. However, other heater core configurations are contemplated as within the scope of this disclosure. 
         [0049]    The bypass loop  80  routes coolant in such a way that it is not heated by the engine  14 . The valve  82  may control the flow of coolant through the bypass loop  80 . For example, when in a first position, the valve  82  may prevent coolant from flowing through a bypass line  90  and inhibit the flow of coolant from the engine  14  to the intermediate heat exchanger  74 . In such a position, a second coolant pump  92  may circulate coolant through the bypass loop  80  from the intermediate heat exchanger  74  to the front and rear heater cores  76 ,  78 , then to the bypass line  90 , and back to the second coolant pump  92 . As such, the coolant in the coolant subsystem  64  may be heated independently by the heat pump subsystem  66  via the intermediate heat exchanger  74 . The valve  82  may also inhibit the flow of coolant through the bypass line  90  when positioned in a second position. The second coolant pump  92  may or may not circulate coolant when coolant does not flow through the bypass line  90 . 
         [0050]    The heat pump subsystem  66 , or refrigerant loop, may circulate a refrigerant to transfer thermal energy to or from the passenger compartment  58  and to or from the coolant subsystem  64 . In one embodiment, the heat pump subsystem  66  is configured as a dual evaporator/dual heater core vapor compression heat pump system in which a fluid, such as refrigerant, is circulated through the heat pump subsystem  66  to transfer thermal energy to or from the passenger compartment  58 . 
         [0051]    The heat pump subsystem  66  may be controlled to operate in various modes, including but not limited to, a cooling mode, a heating mode, and a dehumidification mode. In the cooling mode, the heat pump subsystem  66  may circulate refrigerant to transfer thermal energy from inside the passenger compartment  58  to outside the passenger compartment  58 . In a heating mode, the heat pump subsystem  66  may transfer thermal energy from the refrigerant to the coolant of the coolant subsystem  64  via the intermediate heat exchanger  74  without circulating the refrigerant through any heat exchanger located in the passenger compartment  58 . In dehumidification mode, the heat pump subsystem  66  may be operated to remove humidity from the passenger compartment  58  and provide heat to the coolant subsystem  64  via intermediate heat exchanger  74 , such as to defrost a windshield of the electrified vehicle  12 , for example. 
         [0052]    In one embodiment, the heat pump subsystem  66  includes a compressor  94 , the intermediate heat exchanger  74 , a first expansion device  98 , a solenoid valve  99 , an outside heat exchanger  100 , a three-way valve  102 , an accumulator  106 , a second expansion device  108 , a front evaporator  110 , a third expansion device  116 , and a rear evaporator  118 . Components of the heat pump subsystem  66  may be in fluidic communication via one or more conduits, such as tubes, hoses or the like. 
         [0053]    The compressor  94  pressurizes and circulates the refrigerant through the heat pump subsystem  66 . The compressor  94  may be powered by an electrical or non-electrical power source. For example, the compressor  94  may be operatively coupled to the engine  14  or driven by an electrically powered motor. In the heating mode of the climate control system  56 , the compressor  94  directs high pressure refrigerant to the intermediate heat exchanger  74 , which in turn may transfer heat from the high pressure refrigerant to coolant passing through the intermediate heat exchanger  74  to heat the coolant of the coolant subsystem  64 . 
         [0054]    The first expansion device  98  is positioned between and in fluidic communication with both the intermediate heat exchanger  74  and the outside heat exchanger  100 . The first expansion device  98  is adapted to change the pressure of the refrigerant of the heat pump subsystem  66 . For example, the first expansion device  98  may be an electronic expansion valve, a thermal expansion valve (TXV) or a fixed area valve, such as a fixed orifice tube, that may or may not be externally controlled. The first expansion device  98  may reduce the pressure of the refrigerant that passes through the first expansion device  98  from the intermediate heat exchanger  74  to the outside heat exchanger  100 . Therefore, high pressure refrigerant received from the intermediate heat exchanger  74  may exit the first expansion device  98  at a lower pressure and as a liquid and vapor mixture in the heating mode. 
         [0055]    The solenoid valve  99  may be positioned in a bypass line  122  that permits a portion of the refrigerant to bypass the first expansion device  98 . The solenoid valve  99  may be opened during cooling mode and closed during heating mode. When opened, a majority of the refrigerant flow is passed through the solenoid valve  99  since it provides the path of least resistance. When the solenoid valve  99  is closed, all refrigerant flow is passed through the first expansion device  98  to meter the refrigerant flow into the outside heat exchanger  100 . 
         [0056]    The outside heat exchanger  100  may be positioned within the engine compartment  60 . In the cooling mode or air conditioning context, the outside heat exchanger  100  may function as a condenser to transfer heat to the surrounding environment by condensing the refrigerant from a vapor to a liquid. In the heating mode, the outside heat exchanger  100  may function as an evaporator to transfer heat from the surrounding environment to the refrigerant, thereby causing the refrigerant to vaporize. 
         [0057]    The three-way valve  102  may be positioned between the outside heat exchanger  100  and both the accumulator  106  and the evaporator  110 . The three-way valve  102  can control the flow of refrigerant that exits the outside heat exchanger  100 . In the heating mode, the three-way valve  102  is actuated to permit refrigerant to flow from the outside heat exchanger  100  to the accumulator  106  along a bypass line  104 , thereby bypassing flow through the evaporator  110 . The three-way valve  102  may alternatively be positioned to permit flow of the refrigerant to the evaporator  110  along line  112 , such as during the cooling mode. 
         [0058]    The accumulator  106  acts as a reservoir for storing any residual liquid refrigerant so that vapor refrigerant rather than liquid refrigerant is provided to the compressor  94 . The accumulator  106  includes a desiccant that absorbs relatively small amounts of water moisture from the refrigerant. 
         [0059]    The second expansion device  108  may be positioned between and in fluid communication with the outside heat exchanger  100  and the front evaporator  110 . The third expansion device  116  may be positioned between and in fluid communication with the outside heat exchanger  100  and the rear evaporator  118 . The second and third expansion devices  108 ,  116  may have a similar structure as the first expansion device  98  and are configured to change the pressure of the refrigerant similar to the first expansion device  98 . In one embodiment, the second expansion device  108  is closed to inhibit the flow of refrigerant from the outside heat exchanger  100  to the front evaporator  110  in the heating mode. The third expansion device  116  may also be closed to inhibit the flow of refrigerant through the rear evaporator  118  during certain conditions. 
         [0060]    The front evaporator  110  is fluidly connected to the second expansion device  108 . The front evaporator  110  may be positioned inside the passenger compartment  58 . In the cooling mode, the front evaporator  110  receives heat from air in the passenger compartment  58  to vaporize the refrigerant. Refrigerant exiting the front evaporator  110  is routed to the accumulator  106 . In the heating mode, the three-way valve  102  the refrigerant to the accumulator  106 , bypassing the front evaporator  110 . 
         [0061]    The rear evaporator  118  may be positioned inside the passenger compartment  58 , such as relative to third row seating of the electrified vehicle  12 . In the cooling mode, the rear evaporator  118  receives heat from air in the passenger compartment  58  to vaporize the refrigerant. Refrigerant exiting the rear evaporator  118  is routed to the accumulator  106 . In the heating mode, the refrigerant is not routed to the rear evaporator  118  because the third expansion device  116  is closed. 
         [0062]    The ventilation subsystem  68  may circulate air in the passenger compartment  58 . In one embodiment, the ventilation subsystem  68  includes a front housing  124  and a rear housing  126 . For example, the front housing  124  may be positioned under an instrument panel of the electrified vehicle  12  for circulating air in front portions of the passenger compartment  58 , whereas the rear housing  126  may be positioned relative to third row seating of the electrified vehicle  12  for circulating air in rear portions of the passenger compartment  58 . 
         [0063]    The front housing  124  of the ventilation subsystem  68  may house a blower  128  and a temperature door  130 . An air intake portion  132  may receive air  134  from outside the electrified vehicle  12  and/or air from inside the passenger compartment  58 . For example, the air intake portion  132  may receive ambient air from outside the electrified vehicle  12  via an intake passage, duct or opening that is located in any suitable location, such as proximate a cowl, wheel well, or other vehicle body panel. The air intake portion  132  may also receive air from inside the passenger compartment  58  and recirculate this air through the ventilation subsystem  68 . One or more doors or louvers may also be provided to permit or inhibit air circulation. 
         [0064]    The blower  128 , also called a blower fan, is positioned near the air intake portion  132  and can be configured as a centrifugal fan that circulates air through the front housing  124  of the ventilation subsystem  68 . 
         [0065]    In one embodiment, the temperature door  130  is positioned between the front evaporator  110  and the front heater core  76  and may be positioned downstream of the front evaporator  110  and upstream of the front heater core  76 . The temperature door  130  blocks or permits the flow of air  134  through the front heater core  76  to help control the temperature of air in the passenger compartment  58 . For example, the temperature door  130  may permit airflow through the front heater core  76  in the heating mode such that heat may be transferred from the coolant to air passing through the front heater core  76 . This heated air may then be provided to a plenum for distribution to ducts and vents or outlets located in the passenger compartment  58 . The temperature door  130  may be moved between a plurality of positions to provide air having a desired temperature. In the embodiment of  FIG. 2 , the temperature door  130  is shown in a full heat position in which the flow of air  134  is directed through the front heater core  76 . 
         [0066]    The rear housing  126  may also house a blower  136  and a temperature door  138 . An air intake portion  140  may receive air  142  from inside the passenger compartment  58 . One or more doors or louvers may also be provided to permit or inhibit circulation of the air  142 . The blower  136  circulates the air  142  through the rear housing  126  of the ventilation subsystem  68 . The temperature door  138  blocks or permits the flow of air  142  through the rear heater core  78  to help control the temperature of air in the passenger compartment  58 . 
         [0067]    The climate control system  56  may additionally be operated in a dehumidification mode to remove humidity from the passenger compartment  58 . Operating the climate control system  56 , and in particular the heat pump subsystem  66 , in dehumidification mode supports windshield defrosting functions of the electrified vehicle  12 . Under normal operating conditions, the heat pump subsystem  66  is operated in dehumidification mode in which the outside heat exchanger  100  and the front and rear evaporators  110 ,  118  receive refrigerant. In dehumidification mode, the first expansion device  98  is opened, the solenoid valve  99  is closed, and the second and third expansion devices  108 ,  116  are opened. 
         [0068]    The controller  88  may be part of an overall vehicle control unit, such as a vehicle system controller (VSC), or could alternatively be a stand-alone control unit separate from the VSC. In one embodiment, the controller  88  includes executable instructions for interfacing with and operating the various components of the climate control system  56 . The controller  88  may include inputs  144  and outputs  146  for interfacing with the various components of the climate control system  56 . The controller  88  may also include a central processing unit  148  and non-transitory memory  150  for executing the various control strategies and modes of the climate control system  56 . 
         [0069]      FIG. 3 , with continued reference to  FIGS. 1 and 2 , schematically illustrates a control strategy  200  for controlling operation of the climate control system  56  of the electrified vehicle  12 . For example, the control strategy  200  may be executed during certain conditions to maximize operation of a windshield defrost function of the electrified vehicle  12 . Of course, the electrified vehicle  12  is capable of implementing and executing other control strategies within the scope of this disclosure. In one embodiment, the controller  88  of the climate control system  56  is programmed with one or more algorithms adapted to execute the control strategy  200 , or any other control strategy. In other words, the control strategy  200  may be stored as executable instructions in the non-transitory memory  150  of the controller  88 . 
         [0070]    As shown in  FIG. 3 , the control strategy  200  may begin at block  202  in response to a defrost request. An occupant may perform a defrost request by actuating a defrost button, knob or other input of a climate control instrument panel located within the passenger compartment  58  and which is in electrical communication with the controller  88 . A defrost request indicates to the controller  88  that the heat pump subsystem  66  should be operated in dehumidification mode (either series or parallel) in order to defrost a windshield or other window of the electrified vehicle  12 . 
         [0071]    The control strategy  200  initiates a series of system checks before initiating dehumidification mode. For example, at block  204 , the control strategy  200  may determine whether the electrified vehicle  12  is operating in EV mode. If the electrified vehicle  12  is not operating in EV mode, the control strategy ends at block  206 . However, if it is determined that the electrified vehicle  12  is operating in EV mode, the control strategy  200  may proceed to block  208 . 
         [0072]    At block  208 , the control strategy  200  may determine whether the fuel level of the engine  14  is below a predefined threshold. For example, a fuel tank that holds fuel for powering the engine  14  may include a sensor for determining the amount of fuel held by the tank. In one embodiment, the sensed fuel level may be compared against a threshold fuel level value that is stored on the controller  88 . The control strategy  200  ends at block  206  if it is determined that the fuel level is not below the predefined threshold. 
         [0073]    The control strategy  200  can proceed to block  210  if the fuel level is below a predefined threshold. Refrigerant flow to the rear evaporator  118  of the heat pump subsystem  66  may be shut off at block  210 . In one embodiment, the third expansion device  116  is actuated (i.e., closed) to block refrigerant flow through the rear evaporator  118 . 
         [0074]    Optionally, at block  212 , coolant flow to the rear heater core  78  of the coolant subsystem  64  may be shut off if the electrified vehicle  12  is operating in EV mode and has a fuel level below a predefined threshold. In one embodiment, coolant flow to the rear heater core  78  is blocked by closing the valve  86 . The blower  136  of the rear housing  126  of the ventilation subsystem  68  may also optionally be turned off at block  214  in response to a defrost request, operation in EV mode and a fuel level below a predefined threshold. 
         [0075]    Finally, at block  216 , the control strategy  200  may command operation of the climate control system  56  in dehumidification mode. By operating the climate control system  56  in dehumidification mode after closing the third expansion device  116  and/or the valve  86  in response to a defrost request, the defrost function of the electrified vehicle  12  can be maximized even during conditions where its operation might otherwise be less than optimal. 
         [0076]    Although the different non-limiting embodiments are illustrated as having specific components or steps, the embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from any of the non-limiting embodiments in combination with features or components from any of the other non-limiting embodiments. 
         [0077]    It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should be understood that although a particular component arrangement is disclosed and illustrated in these exemplary embodiments, other arrangements could also benefit from the teachings of this disclosure. 
         [0078]    The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would understand that certain modifications could come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.