Abstract:
A vehicle is disclosed that has multiple coolant paths selected by control of a valve. A valve system is configured to direct coolant from an engine to a heat exchanger according to a difference between a temperature associated with the engine and a temperature associated with the heat exchanger. The valve system is also configured to direct coolant from the engine to an electric heater and to, in response to a heat demanded from the heat exchanger being greater than a heat capability of the electric heater, request the engine to run. A method is disclosed for controlling a valve to change from an isolation position in which the valve isolates coolant circulating through an electric heater and the valve from coolant circulating through an engine to a non-isolation position in which the valve directs coolant from the engine to the electric heater.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/716,075, filed Oct. 19, 2012, the disclosure of which is incorporated in its entirety by reference herein. 
     
    
     BACKGROUND 
       [0002]    To provide passenger compartment comfort, vehicles have the capability to heat or cool the passenger compartment. Conventional vehicles use waste heat from the engine as the sole source of heating for the passenger compartment. With the advent of Battery Electric Vehicles (BEV), there is no longer any waste heat available so that other means of heating the passenger compartment are required. A typical BEV may use an electric heater to warm the passenger compartment. Similarly, Hybrid Electric Vehicles (HEV) pose different problems because the engine may not always be running and generating waste heat for use by the heating system. Plug-in Hybrid Electric Vehicles (PHEV) compound this issue by running with the engine off for significant periods of time. In order to provide optimal fuel economy benefits, it is desired to heat the passenger compartment without having to rely solely on engine waste heat. 
       SUMMARY 
       [0003]    In an illustrative embodiment, a hybrid vehicle includes an engine, an electric heater, a heater core and a valve arranged to route coolant through at least one of the engine and the electric heater. The illustrative system also includes a controller configured to request the engine to be started and to control a valve to route coolant through the engine and the heater core in response to a heat request. The illustrative system includes the capability to run an electric heater and electric heating loop independent from the engine and radiator loops by controlling the valve and electric heater. The illustrative system may provide robust capability to provide heating despite the fault of some of the system components. The illustrative system may also provide modes of operation to improve the effectiveness of heating the passenger compartment. 
         [0004]    An embodiment of a vehicle is disclosed comprising an engine, a heat exchanger or heater core, an electric heater, and a valve system configured to direct coolant from the engine to the heat exchanger according to a difference between a temperature associated with the engine and a temperature associated with the heat exchanger. The temperature associated with the engine may be the temperature of the coolant exiting the engine. The temperature associated with the heat exchanger may be the temperature of the coolant exiting the heat exchanger. The valve system may be further configured to direct coolant from the engine to the electric heater according to the difference between the temperature associated with the heat exchanger and the temperature associated with the engine. The valve system may direct coolant from the engine to the heat exchanger in response to the difference between the temperature associated with the engine and the temperature associated with the heat exchanger being greater than a predetermined threshold. The electric heater may be further configured to heat fluid according to the difference between the temperature associated with the engine and the temperature associated with the heat exchanger. 
         [0005]    Another embodiment of a vehicle is disclosed comprising an engine, a heat exchanger, an electric heater, and a valve system configured to direct coolant from the engine to the electric heater and to, in response to a heat demanded from the heat exchanger being greater than a heat capability of the electric heater, request the engine to run. The valve system may be further configured to direct coolant from the engine to the electric heater according to a difference between a temperature associated with the engine and a temperature associated with the heat exchanger. It may be required that the difference between the temperature associated with the engine and the temperature associated with the heat exchanger be greater than a predetermined threshold. The electric heater may be further configured to heat fluid according to a difference between a temperature associated with the engine and a temperature associated with the heat exchanger. 
         [0006]    A method is disclosed for controlling a valve to selectively fluidly connect coolant loops in a vehicle heating system. The method comprises controlling the valve to change from an isolation position in which the valve isolates coolant circulating through an electric heater and the valve from coolant circulating through an engine to a non-isolation position in which the valve directs coolant from the engine to the electric heater in response to a difference between a temperature associated with the engine and a temperature associated with a heat exchanger being greater than a first threshold or in response to a difference between a heat demanded from the heat exchanger and a heat capability of the electric heater being greater than a second threshold. The method may further comprise requesting the engine to run in response to the difference between the heat demanded from the heat exchanger and the heat capability of the electric heater being greater than the second threshold. The method may further comprise controlling the valve to direct coolant from the engine to the heat exchanger in response to the electric heater being inoperative. The method may further comprise controlling the electric heater to heat the circulating coolant according to the difference between the temperature associated with the engine and the temperature associated with the heat exchanger. The temperature associated with the engine may be the temperature of the coolant exiting the engine. The temperature associated with the heat exchanger may be the temperature of the coolant exiting the heat exchanger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic representation of a hybrid vehicle. 
           [0008]      FIGS. 2 and 3  are schematic representations of vehicle components implementing climate control strategies. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0010]    Vehicles may have two or more propulsion devices, such as a first propulsion device and a second propulsion device. For example, the vehicle may have an engine and an electric motor, a fuel cell and an electric motor, or other combinations of propulsion devices as are known in the art. The engine may be a compression or spark ignition internal combustion engine, or an external combustion engine, and the use of various fuels is contemplated. In one example, the vehicle is a hybrid vehicle (HEV), and additionally may have the ability to connect to an external electric grid, such as in a plug-in electric hybrid vehicle (PHEV). The PHEV structure is used in the figures and to describe the various embodiments below; however, it is contemplated that the various embodiments may be used with vehicles having other propulsion devices or combinations of propulsion devices as is known in the art. 
         [0011]    A plug-in Hybrid Electric Vehicle (PHEV) involves an extension of existing Hybrid Electric Vehicle (HEV) technology, in which an electric battery supplements an internal combustion engine and at least one electric machine to further gain increased mileage and reduced vehicle emissions. A PHEV uses a larger capacity battery than a standard hybrid vehicle, and it adds a capability to recharge the battery from an electric power grid, which supplies energy to an electrical outlet at a charging station. This further improves the overall vehicle system operating efficiency in an electric driving mode and in a hydrocarbon/electric blended driving mode. 
         [0012]      FIG. 1  illustrates an HEV  10  powertrain configuration and control system. A power split hybrid electric vehicle  10  may be a parallel hybrid electric vehicle. The HEV configuration as shown is for example purposes only and is not intended to be limiting as the present disclosure applies to HEVs, PHEVs, or other vehicle types of any suitable architecture. In this powertrain configuration, there are two power sources  12 ,  14  that are connected to the driveline, which include a combination of engine and generator subsystems using a planetary gear set to connect to each other, and the electric drive system (motor, generator, and battery subsystems). The battery subsystem is an energy storage system for the generator and the motor. 
         [0013]    The charging generator speed will vary the engine output power split between an electrical path and a mechanical path. In a vehicle  10  with a power split powertrain system, unlike conventional vehicles, the engine  16  requires either the generator torque resulting from engine speed control or the generator brake torque to transmit its output power through both the electrical and mechanical paths (split modes) or through the all-mechanical path (parallel mode) to the drivetrain for forward motion. During operation using the second power source  14 , the electric motor  20  draws power from the battery  26  and provides propulsion independently of the engine  16  for forward and reverse motions. This operating mode is called “electric drive” or electric-only mode or EV mode. 
         [0014]    The operation of this power split powertrain system, unlike conventional powertrain systems, integrates the two power sources  12 ,  14  to work together seamlessly to meet the driver&#39;s demand without exceeding the system&#39;s limits (such as battery limits) while optimizing the total powertrain system efficiency and performance. Coordination control between the two power sources is needed. As shown in  FIG. 1 , there is a hierarchical vehicle system controller (VSC)  28  that performs the coordination control in this power split powertrain system. Under normal powertrain conditions (no subsystems/components faulted), the VSC interprets the driver&#39;s demands (e.g. PRND and acceleration or deceleration demand), and then determines the wheel torque command based on the driver demand and powertrain limits. In addition, the VSC  28  determines when and how much torque each power source needs to provide in order to meet the driver&#39;s torque demand and to achieve the operating point (torque and speed) of the engine. 
         [0015]    The battery  26  is additionally rechargeable in a PHEV vehicle  10  configuration (shown in phantom), using a receptacle  32  which is connected to the power grid or other outside electrical power source and is coupled to battery  26 , possibly through a battery charger/converter  30 . 
         [0016]    The vehicle  10  may be operated in electric mode (EV mode) in which the battery  26  provides all of the power to the electric motor  20  to operate the vehicle  10 . In addition to the benefit of saving fuel, operation in EV mode may enhance the ride comfort through lower noise and better driveability, e.g., smoother electric operation, lower noise, vibration, and harshness (NVH), and faster response. Operation in EV mode also benefits the environment with zero emissions from the vehicle during this mode. 
         [0017]    A Plug-in Hybrid Electric Vehicle (PHEV) shares characteristics of both an ICE and a BEV. A PHEV may have some driving range in which propulsion is provided only by an electric motor  20  powered from a battery pack  26 . Once the battery pack  26  charge has been depleted to a certain level, the engine  16  may be started. The engine  16  may provide power to propel the vehicle and to recharge the battery pack  26 . In electric only mode, the engine  16  will not be running. Since the engine  16  is not running, there will be no engine heat generated that can be used for heating the passenger compartment. A PHEV may start the engine  16  in response to a need for passenger heating. This, however, interferes with the electric only operation and may impact fuel economy and emissions. 
         [0018]    One possible system for providing passenger compartment heating for a PHEV is shown in  FIG. 2 . The system provides two sources of coolant heating. The system may utilize heat from the engine  40  to heat the coolant as in a conventional ICE vehicle. The system may also provide heat via an electric heater  42  as in a BEV system. Having multiple sources of heat allows flexibility during normal operating conditions and some redundancy during fault modes. The system allows the coolant from the different heat sources to flow through the heater core. The addition of a Heater Core Isolation Valve (HCIV)  44  allows the passenger heater system to select the source of heated coolant. A vehicle system controller (VSC) module ( 28   FIG. 1 ) may control the operation of the system. The VSC ( 28   FIG. 1 ) may determine the heating mode based on the passenger-heating request and the status of the various components in the heating system. A desired heater core coolant temperature is generated by or provided to the VSC ( 28   FIG. 1 ). To ensure robust operation, the VSC ( 28   FIG. 1 ) may attempt to work with missing or failed control elements by choosing an appropriate operating mode. The goal of the heating system is to maintain the heater core temperature at the desired heater core coolant temperature in the most fuel efficient manner possible. 
         [0019]    The electric heater  42  may be a positive temperature coefficient (PTC) type heater. PTC heating elements are made of small ceramic stones that have self-limiting temperature properties. These properties have fast heating response times and the ability to automatically vary its wattage to maintain a pre-defined temperature. As such, PTC heaters may be a good choice for providing controlled electrical heat to a vehicle cabin. 
         [0020]    The system may also have an auxiliary water pump  46  to force coolant to flow through the heating system. A coolant sensor  48  may be included to measure the coolant temperature. The coolant flows through a heater core  50  that allows heat to be transferred from the coolant to air entering the passenger compartment. The heat may be transferred from the coolant in the heater core using a blower  52  to pass air over the heater core  50  and into the passenger compartment. 
         [0021]    The system may also have a water pump  54  to force fluid to flow through the engine  40 . The water pump  54  may be electrically or mechanically driven. In certain modes, the water pump  54  may force fluid through the heating components as well. The system may also have a radiator  56  to dissipate heat in the coolant. The system may also have a thermostat  58  to control the flow of coolant between the radiator  56  and the engine  40 . The system may also have a degas bottle  60  that may act as a coolant reservoir, remove air from the coolant, and provide pressure relief. The cooling system may further include an exhaust gas recirculation (EGR)  62  system that recirculates a portion of the engine&#39;s exhaust gas back to the engine cylinders. 
         [0022]    A system having multiple coolant paths may allow coolant heating to be handled differently depending on operating conditions. Referring to  FIG. 3 , the system has multiple distinct coolant paths. The electric-only heating loop (EOHL)  166  consists of an electric heater  142 , an auxiliary water pump (AWP)  146 , an engine coolant temperature (ECT) sensor  148  and a heater core  150 . In the EOHL  166 , the electric heater  142  heats the coolant. The auxiliary water pump  146  forces the coolant to flow through the heater core  150  and the electric heater  142 . The temperature sensor  148  measures the coolant temperature in the EOHL  166  so that control and monitoring functions can be performed. The EOHL  166  may run independently of the engine  140  and the engine-radiator loop  174 . 
         [0023]    The engine-radiator loop (ERL)  174  provides cooling for the engine  140 . The engine-radiator loop  174  may consist of conventional engine cooling components. A water pump  154  may force coolant to flow through the ERL  174 . A thermostat  158  may regulate the flow of coolant into the engine  140  based on the coolant temperature. The coolant may flow through a radiator  156  to dissipate the heat from the coolant. The system may include a degas bottle  160  to remove air from the coolant system. The thermostat  158  will not allow coolant flow from the radiator  156  to the engine  140  when the coolant temperature is below a certain threshold. As the engine  140  runs, the coolant in the engine  140  will increase in temperature. At a certain temperature, the thermostat  158  will open and allow coolant to flow from the radiator  156  to the engine  140 . Coolant may flow through the engine-bypass loop  172  instead of through the radiator  156  when the thermostat  158  is closed. For coolant to flow, one of the water pumps,  146  or  154 , must be activated to force the coolant to flow through the system. When the thermostat  158  is open, the coolant flows through the radiator  156  where the coolant temperature decreases as heat is dissipated in the radiator  156 . The cooled fluid then flows back into the engine  140  and the process is repeated. Engine coolant temperature  164  may be measured for control and monitoring purposes. 
         [0024]    The ERL  174  and the EOHL  166  may be run independently from one another. Separate coolant temperatures may be achieved in each loop depending on the heating/cooling requirements of each loop at a particular time. The addition of the Heater Core Isolation Valve (HCIV)  144  allows the coolant flow to be modified. The HCIV  144  may be an electrically switched valve that alters the flow of coolant through the system. The HCIV  144  may be a three-way valve that allows one port to be alternately connected to each of the other two ports based on an activation signal. The HCIV  144  may allow the coolant loops to be combined as one larger coolant loop. The HCIV  144  may be switched in such a way to allow coolant to flow from the engine  140  outlet to the electric heater  142  input forming a combined heating loop (CHL)  168 . 
         [0025]    The CHL  168  allows both the engine  140  and the electric heater  142  to heat the coolant. Either the water pump  154  or the auxiliary water pump  146  may force coolant to flow through the engine  140 . When the engine  140  is running, heat from the engine  140  is transferred to the coolant running through the engine  140 . The engine coolant may then run through the HCIV  144  and through the electric heater  142 , the auxiliary water pump  146  and the heater core  150 . Coolant may return to the engine  140  via the thermostat  158  housing. 
         [0026]    The system has the capability to operate in several different modes based on the availability of multiple coolant loops. For example in one mode, the vehicle is run solely on electric power. In this mode, an electric heater  142  may heat the coolant so that the engine  140  does not have to run. The HCIV  144  may be placed in a mode such that coolant is circulated in the electric only heating loop  166 . 
         [0027]    In this mode, the auxiliary water pump  146  forces coolant to flow through the heater core  150 . The electric heater  142  may heat the coolant flowing through the loop as needed. Temperature is measured with a temperature sensor  148 . In addition, no coolant from the engine  140  flows through the heater core  150 . Coolant temperature may be controlled by varying the power output of the electric heater  142 . Variables such as ambient temperature, passenger desired temperature, and blower fan speed may be used for adjusting the electric heater output power. One of the advantages of this arrangement is that coolant does not have to flow through the engine  140 . If the engine  140  is not running, the engine block may be cooler than the desired coolant temperature. Running coolant through the engine block in this condition may cause the coolant to dissipate heat through the engine block—that is, heating up the engine block while decreasing the temperature of the coolant. The net effect may be that the electric heater  142  cannot provide enough heat to overcome the dissipation in the engine block. By isolating the engine  140  from the heating loop, the electric heater  142  can provide enough heat to maintain the desired coolant temperature because it is only heating the coolant flowing in the electric only heating loop  166 . 
         [0028]    In another embodiment, one mode of controlling the operation of the system may be referred to as Opportunistic Heating Mode (OHM). In this mode, the electric heater  142  may be used as the main coolant-heating source. Heat from the engine  140  may be used opportunistically when available. In this mode, the engine  140  is not requested to run for climate purposes. When the engine  140  is running, it may be desired to use the engine heat to warm the coolant. Since the coolant is warmed from the engine, it may be more efficient to use this heated coolant rather than using battery energy to heat the coolant with the electric heater  142 . The contribution of the electric heater  142  to the coolant heating may be varied based on the temperature of the coolant  164  coming from the engine  140 . In this mode, the controller may control the HCIV  144  to select the coolant source with the highest temperature. Depending on the current operating conditions, the controller may select between the electric heater  142  and the engine  140  as the source of heated coolant. 
         [0029]    In OHM, the controller may select the loop to operate in based on the coolant temperatures in each loop. One condition for switching between loops may be to determine which loop may provide the highest temperature coolant to the electric heater  142 . The system may determine the temperature of coolant exiting the heater core  150  with a temperature sensor  170  or estimation based on a model. This temperature may then be compared to the estimated or measured temperature  164  of coolant exiting the engine. The controller may then select the maximum of these temperatures to determine which loop the heating system should be operating in. Using the maximum of the temperatures can reduce the energy required by the electric heater  142  to further heat the coolant. 
         [0030]    Running in the combined heating loop  168  during Opportunistic Heating Mode may only be available when the engine  140  is running. In this mode, there may be no need to supplement the coolant heating with the electric coolant heater  142 . If the temperature of coolant from the engine  140  is warm enough, the system may not need to use the electric heater  142 . In OHM, the controller may not request that the engine  140  be turned on if it is currently turned off. In OHM, the system may remain in the combined heating loop for some time after the engine  140  has turned off. The operation in OHM may typically begin in the electric-only heating loop  166  with heat provided by the electric heater  142 . If the engine coolant temperature  164  is greater than the heater core output temperature  170 , the combined heating loop may be activated. After the transition to the combined heating loop, the electric heater output power may be limited. The decision to limit the electric heater output power may depend on if the engine is running or if the vehicle is in a charge sustaining mode. The system may determine a coolant target temperature based on the heating demands of the vehicle occupants. When the engine coolant temperature  164  exceeds the coolant target temperature, the electric heater  142  may not be required for heating. When the engine coolant temperature  164  falls below the heater core output temperature  170 , the system may transition back to the electric-only heating loop  166 . Hysteresis may be provided to prevent excessive cycling between the modes. The electric heater may also supplement the heat generated by the engine. 
         [0031]    In OHM, the system may continue operating when certain faults are present. Various quantities may be measured by the controller or communicated by other controllers. When the coolant target temperature is missing or unknown, the controller may generate its own coolant target temperature. If blower airflow is missing or not available, the system may default to a maximum airflow setting. If cabin temperature is missing or not available, a default cabin temperature calibration may be used. If ambient air temperature is missing or not available, the system may infer an ambient air temperature. 
         [0032]    Another possible mode of operation may be Forced Hybrid Heating Mode (FHHM). FHHM may use both the engine  140  and the electric heater  142  to heat the coolant. The coolant heats up as it flows through the engine  140  and the electric heater  142  may add additional heat to the coolant. This mode may be entered when heat dissipation in the heater core  150  is greater than the heat output of the electric heater  142 . That is, the electric heater  142  is not able to keep up with heat demand from the heater core  150 . In this situation, the electric heater  142  may not be able to heat the coolant to a sufficient temperature to meet the heating requirement of the passenger compartment. 
         [0033]    Estimating or measuring the coolant temperature  170  at the heater core  150  output may be used to help determine this situation. Also, information regarding the electric heater  142  capability may also be used. The actual or estimated coolant temperature at the engine outlet  164  may also be used to determine how to operate in this mode. Once the system determines that the electric heater  142  is not able to meet the demand from the heater core  150 , the system may request that the engine  140  be started to provide heated coolant. This mode may occur more frequently during extreme cold weather conditions. It may be necessary to override any driver-selected modes, such as electric only, in order to provide the desired level of heating in the passenger compartment. 
         [0034]    In the Forced Hybrid Heating Mode, the engine  140  may be started and stopped as required by the heating demand. The engine  140  may be started and the combined heating loop  168  may be selected. As the engine  140  runs, the coolant temperature will rise. When the coolant temperature has exceeded a coolant target temperature by a certain amount, the engine  140  may no longer be required for coolant heating. The engine  140  may then be turned off. The HCIV  144  may remain in the combined heating loop until the engine coolant temperature  164  has dropped below the heater core output temperature  170 . When the engine coolant temperature  164  has dropped below the heater core output temperature  170 , the HCIV  144  may be switched to the electric-only heating loop  166 . Once the engine  140  is turned off, the electric heater  142  may be used to keep the coolant temperature at the desired heater core coolant temperature. If the electric heater  142  cannot maintain the coolant temperature at the desired heater core coolant temperature, the engine  140  may be turned on again and the HCIV  44  may be switched to the combined heating loop  168 . This may be repeated as necessary in order to maintain the temperature at the desired heater core coolant temperature. 
         [0035]    FHHM may also be entered for certain faults within the heating system. When the HCIV  144 , auxiliary water pump  146 , electric heater  142  or coolant temperature sensor  148  stop working, it may be advantageous to use the combined heating loop. 
         [0036]    If the electric heater  142  stops working, it may not be able to heat fluid in the electric heating loop  166 . In this situation, it may be desirable to switch to the combined heating loop  168  by switching the HCIV  144  to the combined heating loop  168 . This allows heated coolant from the engine  140  to flow through the heater core  150 . This provides some redundancy as heating capability is not completely lost due to an electric heater  142  fault. In addition, the system may require that the engine  140  be running in order to generate heat. 
         [0037]    In the event of an electric heater  142  fault, the Forced Hybrid Heating Mode may cycle the engine  140  on and off at a higher frequency. The engine  140  may be kept running to increase the coolant temperature above the desired heater core coolant temperature. Above the desired heater core coolant temperature, the engine  140  may be turned off. Once the engine  140  is turned off, the coolant temperature may begin decreasing and eventually fall below the desired heater core coolant temperature. If the electric heater  142  cannot provide heat to the coolant, the coolant temperature will decrease at a faster rate. Because of this, the engine  140  may be required to turn on sooner. Information regarding the electric heater  142  fault may be known and used by the system to alter the operation. The system may choose to activate different heating loops via the HCIV  144  if the electric heater  142  has stopped working. 
         [0038]    If the auxiliary water pump  146  fails, it may not be able to move fluid through the electric-only heating loop  166 . In such a situation, it may be desirable to switch to the combined heating loop  168  by switching the HCIV  144 . This way, the water pump  154  may be utilized to pump fluid through both the engine radiator  156  and electric heater  166  loops. The water pump  154  may require information about auxiliary water pump  146  faults so that the speed of the water pump  154  may be increased to compensate for the faulted auxiliary water pump  146 . 
         [0039]    If the coolant temperature sensor  148  in the heater loop stops working, the system may not know the temperature in the electric heating loop  166 . In this situation, the system may run in the electric-only heating loop  166  with the electric heater  142  at a predetermined maximum heating capability. The system may be placed in an open loop control mode since feedback is not available from the temperature sensor  148 . The system may also be switched to the combined heating loop  168  and the engine temperature sensor measurement  164  can be used. 
         [0040]    If the HCIV  144  stops working, the fault detection may indicate the faulted position of the valve. The indicated position may be used and the system operated in that mode to the best of its capabilities. A warning lamp and diagnostic trouble code may be set to indicate the fault. Because of the fault, the heating system may not perform as desired. 
         [0041]    The system may also incorporate a Cold Engine Lock Out (CELO) feature. A CELO feature may inhibit the blower fan operation until the coolant has reached a certain threshold. The system may request that the engine be turned on to assist in heating the coolant. Once the coolant has achieved a certain threshold, the blower fan speed can be increased to allow heated air to flow into the passenger compartment. 
         [0042]    The auxiliary water pump  146  may be run whenever heating is required in the passenger compartment. If no heating is requested, the auxiliary water pump  146  may not need to be activated. Similarly, the water pump  154  in the engine-radiator loop  174  may only need to be activated when the engine  140  is running. The water pump  154  may also be required to be on when running in the combined heating loop  168 . 
         [0043]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.