Abstract:
A method for controlling a vehicle cabin heating system includes setting a target temperature for a vehicle cabin interior space. The method also includes measuring current temperature within the vehicle cabin interior space at a location spaced apart from a first infrared heater and spaced apart from a target surface within the vehicle cabin interior space, the first infrared heater being aimed to heat the target surface. A surface temperature of the target surface is estimated based the current temperature. The first infrared heater is controlled such that the heater is turned on in response to the estimated surface temperature of the target surface falling below the target temperature by a predetermined amount, and the heater is turned off in response to the estimated surface temperature rising above the target temperature by the predetermined amount.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 12/359,804 filed on Jan. 26, 2009. The entire disclosure of U.S. patent application Ser. No. 12/359,804 is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a vehicle cabin heating system. More specifically, the present invention relates to supplying heating a vehicle cabin interior space using at least one infrared heater. 
         [0004]    2. Background Information 
         [0005]    For many years, automotive vehicles have been provided with climate control systems (e.g., heating, ventilating and air conditioning (HVAC) systems) in order to provide a more comfortable environment for the driver and any passengers. Typically, fresh air is supplied to the HVAC system via a ductwork extending from a cowl box of the vehicle to cabin outlets in the vehicle cabin interior space. The conventional approach to heat the vehicle cabin is to utilize waste heat from the power-train system by powering a blower motor and delivering air through a heat exchanger that uses circulating engine coolant. In particular, in such a conventional engine coolant based heating system, a heater core (heat exchanger) is disposed in the ductwork between the cowl box and the cabin outlets, and a blower is provided in the ductwork to deliver the heated air to the cabin. The heater core is normally heated by the engine coolant from the vehicle&#39;s engine. In cold conditions, when the vehicle is first started, the engine is cold. In other words, there may not be significant temperature potential available to quickly warm the cabin with this conventional engine coolant based heating system, the occupants must wait to be comfortable. Thus, the engine coolant is also cold and is insufficient to warm the air entering the vehicle cabin. This results in a period of time in which the cabin remains below the desired cabin temperature. In order to solve this problem, an infrared heater has been proposed in U.S. Pat. No. 3,619,555 to Bassett, Jr. (assigned to General Motors Corporation), in which heater is designed to be energized only when the vehicle engine coolant is below a predetermined temperature and the ignition switch is “on”. 
         [0006]    In recent years, hybrid vehicles and electric vehicles have become more popular. A hybrid vehicle includes an internal combustion engine and an electric motor or a battery as power sources for providing a driving force to a vehicle drive train. In the case of hybrid vehicles, the internal combustion engine is stopped, when travelling in an electric (EV) mode. When the hybrid vehicle is travelling in an electric (EV) mode, the engine coolant will typically stop being circulated through the heater core (heat exchanger) that is disposed in the HVAC ductwork. Thus, the engine coolant temperature will drop while the vehicle is travelling in an electric (EV) mode. In the above mentioned patent, the infrared heater operates when the engine coolant temperature drops below a predetermined temperature. Apparently, the infrared heater will continue to operate until the engine coolant temperature above the predetermined temperature of the coolant. Thus, the proposed heating system of the above mentioned patent is not well suited for hybrid vehicles. Likewise, the proposed heating system of the above mentioned patent is not well suited for electric vehicles. Electric vehicles often have a cooling system for the batteries and/or the motor that somewhat resembles that of an internal combustion engine configuration. However, the waste heat generation is very low compared to an internal combustion engine and as a consequence has a reduced capability to heat the cabin. 
         [0007]    In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved vehicle cabin heating system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the state of the known technology, one object of the present invention is to vehicle cabin heating system that is well suited for a variety of vehicles, including hybrid vehicles and electric vehicles. 
         [0009]    In accordance with one aspect of the present invention, a method for controlling a vehicle cabin heating system includes setting a target temperature for a vehicle cabin interior space and measuring current temperature within the vehicle cabin interior space at a location spaced apart from a first infrared heater and spaced apart from a target surface within the vehicle cabin interior space, the first infrared heater being aimed to heat the target surface. As well, the method includes estimating a surface temperature of the target surface based on the measuring of the current temperature, and controlling the first infrared heater such that the heater is turned on in response to the estimating of the surface temperature determining that an estimated surface temperature of the target surface has fallen below the target temperature by a predetermined amount, and turning the heater off in response to the estimating of the surface temperature determining that the estimated surface temperature has risen above the target temperature by the predetermined amount. 
         [0010]    These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Referring now to the attached drawings which form a part of this original disclosure: 
           [0012]      FIG. 1  is a partial perspective view of a vehicle cabin interior space of a vehicle equipped with a vehicle cabin heating system in accordance with one embodiment; 
           [0013]      FIG. 2  is a side elevational view of the vehicle cabin interior space of the vehicle illustrated in  FIG. 1  with the vehicle cabin heating system; 
           [0014]      FIG. 3  is a simplified block diagram of the vehicle cabin heating system in accordance with the illustrated embodiment; and 
           [0015]      FIG. 4  is a flow chart showing an example of steps executed by the controller of the vehicle cabin heating system to heat the vehicle cabin interior space. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
         [0017]    Referring initially to  FIG. 1 , a hybrid vehicle  10  is illustrated with a vehicle cabin interior space  12  equipped with a vehicle cabin heating system in accordance with one embodiment. The hybrid vehicle  10  is selectively operable in both an electric mode in which only an electric motor of the hybrid vehicle  10  is operated, and a hybrid mode in which both an internal combustion engine of the hybrid vehicle  10  is operated by itself or in conjunction with the electric motor. The operations of the electric motor and the internal combustion engine are well known and thus, we not be discussed herein. Of course, it will be apparent to those skilled in the art from this disclosure that the vehicle cabin heating system illustrated herein can also be adapted to electric vehicles and conventional internal combustion engine vehicles as needed and/or desired. 
         [0018]    In the case of the hybrid vehicle  10 , the front portion of the vehicle cabin interior space  12  is at least partially defined by a dashboard  14  and a windshield  16 , which is supported by a pair of A-pillars  18 . The vehicle cabin heating system will typically be primarily installed behind the dashboard  14 . Basically, in illustrated embodiment, the vehicle cabin heating system includes a pair of lower infrared heaters  20  and a pair of upper infrared heaters  22  that form a first heating system for complementing a second heating system that is preferably a conventional coolant based heating system. 
         [0019]    As schematically, shown in  FIG. 2 , the second heating system (engine coolant based heating system) has a blower  24  and a heater core  26 , with ductwork  28  for directing the heated air to the vehicle cabin interior space  12 . Basically, in vehicles with liquid-cooled engines, the engine heat (by-product of the combustion process) contained in the coolant is used to warm the vehicle cabin interior space  12 . The heater core  26  consists of tubes and fins, and has the same basic design as the engine radiator. Engine coolant from the internal combustion engine of the vehicle  10  flows through the tubes of the heater core  26 , while air flows through the fins of the heater core  26 . The blower  24  can be a constant-speed or adjustable-speed electric blower, which forces the air into the vehicle cabin interior space  12 . 
         [0020]    In the second heating system (engine coolant based heating system), the entire air flow is usually directed through the heater core  26  while a valve controls the heating output by regulating the flow of coolant through the heater core  26 . However, alternately, the flow of coolant through the heater core  26  is unrestricted, and the heat is regulated by dividing the air flow before it reaches the heater core  26 . In other words, in this alternative configuration, a portion of the air flows through the heater core  26 , while the rest is directed around the heater core  26 . The two currents are subsequently reunited in the plenum chamber. An air flap can be used to regulate the distribution of the two currents, thereby determining the amount of heat taken from the coolant. Preferably, at least one air temperature sensor  29  is provided in the ductwork  28  at a location downstream of the air flow through the heater core  26 . Since the second heating system (engine coolant based heating system) can be a conventional heating system that is commonly used in most vehicles, the second heating system (engine coolant based heating system) will not be discussed and/or illustrated in further detail herein. 
         [0021]    The first and second heating systems are controlled by an HVAC controller  30  that includes a user interface device  32 . The controller  30  preferably includes a microcomputer with an HVAC control program that controls first and second heating systems as discussed below, to heat the vehicle cabin interior space  12 . Preferably, the controller  30  sets a target surface temperature or temperatures, depending on the heating mode, and controls the lower and upper infrared heaters  20  and  22  towards the target surface temperature(s) based on an effective or estimated surface temperature directly in front of the upper infrared heater(s) in the vehicle cabin interior space  12 , as discussed below. On the other hand, the controller  30  sets a target air temperature or temperatures, depending on the heating mode, and controls the heater core  26  or the air flowing through the heater core based on the target air temperature(s) detected by the air temperature sensor  29 . 
         [0022]    The HVAC controller  30  can also include other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. The microcomputer of the HVAC controller  30  is programmed to control at least the operations of the blower  24 , the doors or flaps in the ductwork  28  and the infrared heaters  20  and  22 . Basically, the HVAC controller  30  controls the blower  24 , the doors or flaps in the ductwork  28  so as to automatically maintain an interior air temperature of the vehicle cabin interior space  12  to a user target air temperature. In other words, sensors monitor the temperatures of the vehicle&#39;s interior and of the air emerging from the ductwork  28 . The HVAC controller  30  processes this information and compares it with the target air temperature. Meanwhile, a solenoid valve installed in the cooling circuit of the heater core  26  opens and closes at a given frequency in response to the signals which it receives from the HVAC controller  30  to regulate the heating of the air entering the vehicle cabin interior space  12 . The adjustments in open/close ratio in the cycle periods regulate the flow rate from the closed position up to the maximum. A servo-actuated adjustment flap can be employed to provide infinitely-variable temperature regulation to allow for separate adjustment between the left and right sides of the vehicle cabin interior space  12 . 
         [0023]    The user interface device  32  is configured to allow a user to set a target cabin interior temperature for the vehicle cabin interior space  12  as well as other settings such as heating mode, blower speed, etc. for both the first and second heating systems. These setting are carried out by the HVAC controller  30 , which is operatively coupled to the components of the first heating system (i.e., the infrared heaters  20  and  22 ) and to the components of the second heating system (i.e., internal combustion engine or electric motor coolant based heating system) to selectively operate the first and second heating systems to heat the vehicle cabin interior space  12 . 
         [0024]    In the case of the hybrid vehicle  10 , the first and second heating systems are preferably controlled by the controller  30  to save electricity during an electric mode in which only the motor of the hybrid vehicle  10  is operated. Thus, as in the illustrated embodiment, the controller  30  stops the coolant based heating system upon determining an ignition “on” position exists and the electric mode exist in the hybrid vehicle  10 , and then only selectively uses the infrared heaters  20  and/or  22  to heat the cabin interior temperature towards the target temperature as seen in the flow chart of  FIG. 4 . When the controller  30  determines an ignition “on” position exists and a hybrid mode exist in the hybrid vehicle  10 , the controller  30  selectively uses both the infrared heaters  20  and/or  22  and the second heating system (i.e., internal combustion engine or electric motor coolant based heating system) to heat the vehicle cabin interior space  12  with the infrared heaters  20  and/or  22  being operated as seen in accordance with the process shown in the flow chart of  FIG. 4  and the conventional second heating system being operated in accordance with conventional methods. 
         [0025]    Preferably, the user interface device  32  allows the user to set to various heating modes. In other words, the user interface device  32  includes a mode input switch in which the user can select a floor heat mode (e.g., operation of only the lower infrared heaters  20 ), a defog or defrost mode (e.g., operation of only the upper infrared heaters  22 ), or a full heat mode (e.g., both the lower and upper infrared heaters  20  and  22 ). During each of these modes, the blower  24  can be either automatically or manually operated so that the blower  24  can be turned “on”, set to a preferred blower speed and turned “off” as needed and/or desired. For example, in the case of the defog or defrost mode, the blower  24  can be automatically turned “on” and set to a preset speed when the defog or defrost mode is selected. In this automatic defog or defrost mode, the user interface device  32  can be set to a plurality of defog or defrost settings with the target surface temperature of the upper infrared heaters  22  and/blower speed changing with each setting. Alternatively, in the case of the defog or defrost mode, the blower  24  can be manually operated and set to one of a plurality of preset speeds when the defog or defrost mode is selected. 
         [0026]    Basically, the vehicle cabin heating system uses the first heating system (i.e., the infrared heaters  20  and  22 ) when the heat provided from the conventional second heating system (i.e., the coolant based heating system) is not sufficient to heat the vehicle at start-up or due to an internal combustion engine that generates less heat and therefore lower coolant temperatures, which heat the heater core. Moreover, in the vehicle cabin heating system, the controller  30  is operatively coupled to the lower and upper infrared heaters  20  and  22  to selectively operate the lower and upper infrared heaters  20  and  22  at prescribed conditions, especially when the temperature directly in front of the lower infrared heaters  20  below a prescribed temperature range as described below. The heating operation of the conventional second heating system (i.e., the coolant based heating system) is essentially independent of the heating operation of the first heating system (i.e., the infrared heaters  20  and  22 ). In particular, as explained below, the operation time of the infrared heaters  20  and  22  is controlled by achievement of a target surface temperature rather than cabin interior air temperature. In an extreme case, if the infrared heaters  20  and  22  are very powerful, only short bursts of on-time would be required in most conditions (regardless of the operation of the conventional second heating system). The conventional second heating system would be expected to be operating while a vehicle is in motion (heat being generated; pumps moving cooling fluids for EV and HEV as well as IC engines). When the vehicle is stopped, the engine or electric motor heat generation and fluid pump operation is to be minimized. To help accomplish that objective, the infrared heaters  20  and/or  22  can eventually “turn on” based on the surface temperature decrease due to the absence of the conventional system operation. In that way, the thermal conditions will control the operation of the infrared heaters  20  and  22  when the conventional second heating system is minimized. The surface temperature based operation of the infrared heaters  20  and  22 , while the vehicle is in motion, will reduce the need to consume power with the conventional second heating system. This means energy savings in achieving and maintaining the temperature objectives. Further, the faster delivery of heat to the surfaces is considered a benefit with the infrared heaters  20  and  22  compared to waiting for the conventional second heating system to heat up. 
         [0027]    As seen in  FIGS. 1 and 2 , the lower infrared heaters  20  are integrated into the vehicle cabin and aimed at the occupants&#39; legs to give then a warmth sensation with reduced lamp power consumption. The upper infrared heaters  24  are integrated into the vehicle cabin and aimed at the windshield  16  to reduce moisture accumulation (e.g., defog the windshield  16 ). For these reasons, the lower infrared heaters  20  are disposed in the foot-wells and the upper infrared heaters  24  are disposed in A-pillars  18 . The lower and upper infrared heaters  20  and  22  can be any of a variety of different types and styles with infrared elements that produces only infrared waves. Although some infrared elements are designed to produce a certain type of visible light wave or other types of waves for specific purposes (for example, to indicate when the element is being powered), most infrared elements for use in the infrared heating system produce substantially solely infrared waves. Assorted infrared heaters are commercially available. However, infrared heat lamps in the non-visible range, particularly carbon, are especially suitable for the lower and upper infrared heaters  20  and  22 . Preferably, the lower and upper infrared heaters  20  and  22  emit either IR-B waves, or IR-C waves. The IR-B wavelength range is preferably 2000 to 3500 nm. While the IR-C wavelength range is preferably greater than 3500 nm. The lower and upper infrared heaters  20  and  22  can be operated with about 500 W of input power. 
         [0028]    Long Wave (IR-C waves with wavelengths longer than 2000 nm) would also be effective for heat integration and not being noticeable to the vehicle occupants. For this reason, the lower infrared heaters  20  are preferably IR-C type infrared elements. On the other hand, IR-C waves are not absorbed by the water molecule. Thus, window defogging performance may suffer somewhat if IR-C type infrared elements are selected for the upper infrared heaters  22  over IR-B type infrared elements. For this reason, the upper infrared heaters  22  are preferably IR-B type infrared elements. 
         [0029]    As mentioned above, the lower infrared heaters  20  are arranged in the dashboard  14  so that the lower infrared heaters  20  are aimed into the foot-wells to supply heat to the foot-wells of the vehicle cabin interior space  12 . Each of the lower infrared heaters  20  has a temperature sensor  34  that is mounted to a grill or lens of the dashboard  14  in front of the lower infrared heater  20 . The sensors  34  are also used to limit the temperature of the grill or lens surface that is heated and that the passenger(s) may contact. The sensor&#39;s temperature (after mathematical adjustment based on heat transfer properties) is used to control the temperature of a target surface within the cabin (passenger feet, windshield, etc.). Alternatively, only one the temperature sensor is used with the lower infrared heaters  20 . If one of the temperature sensor  34  for each of the lower infrared heaters  20 , then the first heating system can independently adjust the heat output to driver&#39;s side of the vehicle cabin with respect to the passenger&#39;s side of the vehicle cabin. In this way, the user interface device  32  can independently set target surface temperatures for the driver&#39;s side of the vehicle cabin and the passenger&#39;s side of the vehicle cabin. In the illustrated embodiment, a passenger presence sensor  38  is provided in the passenger&#39;s seat to determine whether a passenger is present in a front passenger seat of the vehicle cabin interior space  12 . Using this passenger presence sensor  38 , the controller  30  can automatically perform a stopping operation of the lower infrared heater  20  that is located on the passenger&#39;s side of the vehicle cabin based on a detection result of the passenger presence sensor  38  indicating that the front passenger seat of the vehicle cabin interior space  12  is empty. This passenger presence sensor  38  can be the same one that is used for “enabling” or “disabling” the passenger&#39;s air bag(s) function. 
         [0030]    As mentioned above, the upper infrared heaters  22  are arranged in the A-pillars  18  adjacent the windshield  16  to apply heat across an interior surface of the windshield  16  and to supply heat to an upper area of the vehicle cabin interior space  12 . Each of the upper infrared heaters  22  has a temperature sensor  36  that is mounted to a grill or lens in front of the upper infrared heaters  22 . Alternatively, only one the temperature sensor is used with the upper infrared heaters  22 , or no temperature sensors are used with the upper infrared heaters  22 . If no temperature sensors are used with the upper infrared heaters  22 , then the temperature sensor  34  of the lower infrared heaters  20  can be used to control the operation of the upper infrared heaters  22 . 
         [0031]    Referring now to  FIG. 4 , a flow chart is illustrated that shows an example of operations executed by the controller  30  of the vehicle cabin heating system to heat the vehicle cabin interior space  12  using the lower infrared heaters  20  and/or the upper infrared heaters  22 . The operations of the flow chart are initiated after the vehicle is started (ignition “on”) and the user turns on the vehicle cabin heating system. The process of the flow chart of  FIG. 4  will be executed at prescribed intervals once the user interface is set to a heating application (e.g., a floor heat mode, a defog mode, defrost mode, or a full heat mode activating both the lower and upper infrared heaters  20  and  22 ). 
         [0032]    In step S 1 , the controller  30  determines a target surface temperature Tg, which can be either preset in advance or set by the user. For example, the target surface temperature Tg can be set between 35° C. to 40° C. at the location of the passenger&#39;s foot. If the user selects only to operate the lower infrared heaters  20 , then the then the target surface temperature Tg is set for the lower infrared heaters  20  only. On the other hand, if the user selects a defrost or defog mode, then the target surface temperature Tg is set for the upper infrared heaters  22  only. Of course, the user selects to operate both the lower and upper infrared heaters  20  and  22 , then a first target surface temperature Tg is set for the lower infrared heaters  20  and a second target surface temperature Tg is set for the upper infrared heaters  22  in which the first and second target surface temperatures Tg can be the same or different. Also, the user can set a different target surface temperature Tg for the driver&#39;s lower infrared heater  20  then for the passenger&#39;s lower infrared heater  20 , if desired. 
         [0033]    For the sake of simplicity, the remaining steps in the flow chart of  FIG. 4  will considered a case in which the lower infrared heaters  20  will be operated together using a single temperature sensor. However, the process of the flow chart of  FIG. 4  can be executed by the controller  30  such that all of the lower and upper infrared heaters  20  and  22  operate by a single temperature sensor, or operated individually, or operated based on location (e.g., the lower infrared heaters operated together and the upper infrared heaters operated together). 
         [0034]    In step S 2 , the controller  30  receives a signal from the temperature sensor  34  to determine a measured temperature Tm, which is indicative of actual temperatures in front of one of the lower infrared heaters  20 , and then proceeds to step S 3 . 
         [0035]    In step S 3 , the controller  30  determines if the lower infrared heaters  20  are currently “on”. If the controller  30  determines that the lower infrared heaters  20  are currently “on”, then the controller  30  proceeds to step S 4 . However, if in step S 3 , the controller  30  determines that the lower infrared heaters  20  are currently “off”, then the controller  30  proceeds to step S 4 . 
         [0036]    In step S 5 , the controller  30  sets the estimated steady state lamp temperature Tlamp to the measured temperatures Tm, and then proceeds to step S 6 . 
         [0037]    In step S 5 , the controller  30  sets an estimated steady state lamp temperature Tlamp to a preset value such as 100°, and then proceeds to step S 6 . 
         [0038]    In step S 6 , the controller  30  computes an effective or estimated surface temperature Tsurf in an area near or on the passenger&#39;s feet. Thus, the controller  30  controls this effective surface temperature Tsurf temperature, which is a temperature at a relatively great distance from the lower infrared heaters  20  as compared to the sensor location of the temperature sensor  34 . For example, the controller  30  can compute the effective surface temperature Tsurf using the following equation: 
         [0000]        T surf=( Tm−B·T lamp)/(1 −B )       where B is a calibration factor (e.g., B=0.05)         
         [0040]    In step S 7 , the controller  30  determines if the measured temperature Tm detected by the temperature sensor  34  is greater than a prescribed temperature limit such as 65° C. Thus, by this step, since the temperature sensor  34  is provided in the grill or lens of the lower infrared heater  20 , the controller  30  can control the temperature of that specific surface, which the passengers may contact, based on the measured temperature Tm. In other words, the controller  30  uses the sensor&#39;s temperature to avoid a surface from becoming too hot to touch (e.g., 65° C.). If the controller  30  determines the measured temperature Tm detected by the temperature sensor  34  is greater than the prescribed temperature limit e.g., 65° C., in step S 7 , the process proceeds to step S 9 . On the other hand, if the controller  30  determines the measured temperature Tm detected by the temperature sensor  34  is less than or equal to the prescribed temperature limit e.g., 65° C., in step S 7 , the process proceeds to step S 8 . 
         [0041]    In step S 8 , the controller  30  determines if the effective surface temperature Tsurf is greater than the target surface temperature Tg plus a prescribed hysteresis value Hys such as 3° C. If the controller  30  determines, in step S 8 , that the effective surface temperature Tsurf is greater than the target surface temperature Tg plus the prescribed hysteresis value Hys, then the controller  30  proceeds to step S 9 , where either the power to the lower infrared heaters  20  is decreased or the lower infrared heaters  20  are simply turned “off”. However, if in step S 8 , the controller  30  determines that the effective surface temperature Tsurf is less than or equal to the target surface temperature Tg plus the prescribed hysteresis value Hys, then the controller  30  proceeds to step S 10 . 
         [0042]    In step S 10 , the controller  30  determines if the effective surface temperature Tsurf is less than the target surface temperature Tg minus the prescribed hysteresis value Hys. If the controller  30  determines, in step S 8 , that the effective surface temperature Tsurf is less than the target surface temperature Tg minus the prescribed hysteresis value Hys, then the controller  30  proceeds to step S 11 , where either the power to the lower infrared heaters  20  is increased or the lower infrared heaters  20  are simply turned “on”. However, if in step S 10 , the controller  30  determines that the effective surface temperature Tsurf is greater than or equal to the target surface temperature Tg minus the prescribed hysteresis value Hys, then the process ends. 
       GENERAL INTERPRETATION OF TERMS 
       [0043]    In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Also as used herein to describe the above embodiment(s), the following directional terms “forward”, “rearward”, “above”, “downward”, “vertical”, “horizontal”, “below” and “transverse” as well as any other similar directional terms refer to those directions of a vehicle equipped with the vehicle cabin heating system. 
         [0044]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.