Abstract:
A cooling system is provided for a vehicle having an electric power producing device and a power-plant operable to propel the vehicle. The cooling system includes a primary heat exchanger arranged relative to the power-plant. The primary heat exchanger is operable to receive coolant from the power-plant, reduce temperature of said coolant, and return the reduced temperature coolant to the power-plant. The cooling system additionally includes an auxiliary heat exchanger arranged relative to the primary heat exchanger, and operable to receive the reduced temperature coolant from the primary heat exchanger. The auxiliary pump further reduces the temperature of said coolant, and provides the further reduced temperature coolant to the electric power producing device. The electric power producing device may be employed in a hybrid vehicle, where the electric power producing device is a motor-generator operable to propel the vehicle.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a cooling system for a vehicle and a method of controlling such a cooling system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Modern vehicles employ various electric power producing devices configured to satisfy a range of objectives. One example of such a device is an electric motor-generator employed in conjunction with a power-plant, such as an internal combustion engine, as part of a hybrid propulsion system. Another example of such a device is a power electronics module. 
         [0003]    As a by-product of generating power for propelling the vehicle, the power-plant produces heat energy. To ensure efficient and reliable performance of the power-plant, such heat energy is typically removed via a coolant. Likewise, as a consequence of generating electrical power, the aforementioned electric devices also generate heat, which similarly must be removed. 
       SUMMARY OF THE INVENTION 
       [0004]    In view of the foregoing, a cooling system for a vehicle having an electric power producing device and a power-plant operable to propel the vehicle is provided. The cooling system includes a primary heat exchanger arranged relative to the power-plant. The primary heat exchanger is operable to receive coolant from the power-plant, reduce temperature of the coolant, and return the reduced temperature coolant to the power-plant. The cooling system also includes an auxiliary heat exchanger arranged relative to the primary heat exchanger, operable to receive the reduced temperature coolant from the primary heat exchanger. The auxiliary heat exchanger is arranged to further reduce the temperature of the coolant, and provide the further reduced temperature coolant to cool the electric power producing device. The electric power producing device may be employed in a hybrid vehicle, where the electric power producing device is a motor-generator operable to propel the vehicle. 
         [0005]    The auxiliary heat exchanger is further arranged to reduce the temperature of the coolant and provide the further reduced temperature coolant to the electric power producing device. The cooling system may have the reduced temperature coolant returned to the power-plant at a predetermined flow rate, and provide the further reduced temperature coolant to the electric power producing device at a flow rate lower than the predetermined flow rate. 
         [0006]    The cooling system may further include a primary pump operable to return the reduced temperature coolant to the power-plant. The cooling system may additionally include an auxiliary pump controlled by an electronic controller to supply the further reduced temperature coolant from the auxiliary heat exchanger to the electric power producing device. In the alternative, the cooling system may include an orifice configured to control flow of the further reduced temperature coolant from the auxiliary heat exchanger to the electric power producing device. In either case, the flow rate of the further reduced temperature coolant to the electric power producing device may be controlled to approximately 0.5 to 2 liters/minute. 
         [0007]    In an alternate embodiment, a method of controlling a cooling system for a hybrid vehicle having a power-plant and a motor-generator operable to propel the vehicle is provided. The method includes receiving coolant of a first temperature from the power-plant via a primary heat exchanger arranged relative to the power-plant. The method additionally includes reducing the temperature of the coolant via the primary heat exchanger, and returning a first portion of the reduced temperature coolant to the power-plant. The method also includes delivering a second portion of the reduced temperature coolant from the primary heat exchanger to an auxiliary heat exchanger arranged relative to the motor-generator. The method additionally includes further reducing temperature of the second portion of coolant via the auxiliary heat exchanger, and controlling delivery of the further reduced temperature second portion of the coolant to the motor-generator. The method further includes delivering the second portion of coolant from the motor-generator to the power-plant. 
         [0008]    The returning of the reduced temperature coolant to the power-plant may be accomplished at a predetermined flow rate, and the providing of the further reduced temperature coolant to the motor-generator may be accomplished at a flow rate lower than the predetermined flow rate. The controlling the flow rate of the further reduced temperature coolant may be performed by a controller. 
         [0009]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagrammatic view of a first embodiment of a vehicle cooling system; 
           [0011]      FIG. 2  is a schematic diagrammatic view of a second embodiment of a vehicle cooling system; 
           [0012]      FIG. 3  illustrates a plot of operating temperatures versus coolant flow rate for a motor-generator cooled by the cooling system shown in  FIGS. 1 and 2 ; and 
           [0013]      FIG. 4  schematically illustrates, in flow chart format, a method in accordance with the embodiment for controlling the cooling system shown in  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The cooling system according to the preferred embodiment includes an electric power producing device employed in a vehicle and cooled by a heat transfer fluid, i.e. coolant, also utilized to cool a power-plant, such as an internal combustion (IC) engine or a fuel cell. The contemplated coolant is typically is a solution of a suitable organic chemical (most often ethylene glycol, diethylene glycol, or propylene glycol) in water. The fluid cooled electric power producing device may be a power electronics module, or a motor-generator employed as part of a hybrid propulsion system to drive such a vehicle, as understood by those skilled in the art. 
         [0015]    Hybrid propulsion systems have been developed in an effort to improve vehicle fuel efficiency and reduce vehicle exhaust emissions. Generally, by shutting off the vehicle&#39;s power-plant when it would otherwise be operating at idle or idle stop, and enabling early fuel cut-off during vehicle deceleration, improved vehicle fuel economy can be achieved. Typically, such hybrid propulsion systems utilize a motor-generator in addition to the power-plant to drive the vehicle. 
         [0016]    In some hybrid propulsion systems a power-plant, such as above, is the primary source of vehicle power. In such systems, a motor-generator is typically employed as a Belt Alternator Starter (BAS). The BAS is typically used for generating electrical energy for use by vehicle accessories, and for quickly restarting and spinning the power-plant up to operating speeds. In other types of hybrid propulsion systems, a motor-generator is employed to assist the power-plant in powering, i.e. driving, the vehicle, and, in certain conditions, even functioning as a sole source of vehicle power. 
         [0017]    Referring now to the drawings in which like elements are identified with identical minerals throughout,  FIG. 1  shows a hybrid vehicle cooling system  10 . The cooling system  10  includes a power-plant  12 , and a motor-generator  14  operatively connected to the power-plant. The power-plant  12  may be an internal combustion (IC) engine, such as a spark ignition or a compression ignition engine, or a fuel cell. Although a motor-generator is described within the cooling system  10 , a combined alternator-starter, a power electronics module, or any other electronic device for producing electrical power, and having a provision for circulation of coolant is similarly envisioned. 
         [0018]    The power-plant  12  may be used to propel the vehicle, while the motor-generator  14 , in this case a motor-generator, may be used to provide rapid restart of the power-plant  12  from shut down mode, i.e. stop or idle stop operation. The motor-generator  14  may also be employed to generate power for propelling the hybrid vehicle while the power-plant  12  is shut down. As understood by those skilled in the art, there may be a number of ways to interconnect the power-plant  12  with the motor-generator  14  to provide these functions. 
         [0019]    The power-plant  12  produces heat energy as a by-product of generating power used to propel the hybrid vehicle. Such heat energy is removed via a coolant, i.e. circulating cooling fluid (not shown), continuously cycling through multiple conduits of the cooling system  10 . The coolant exits the power-plant  12  and is delivered to a primary heat exchanger  18  via a conduit  16 . The heat exchanger  18  is contemplated as a water-to-air radiator configured to ensure sufficient reduction of coolant temperature in order to ensure efficient performance of the power-plant  12 . After the coolant temperature has been reduced inside the heat exchanger  18 , the coolant exits the heat exchanger via conduit  20 . 
         [0020]    The conduit  20  splits into two conduit branches, a conduit  22  configured to deliver the reduced temperature coolant to a thermostat  24 , which is configured to control flow rate of coolant, and a conduit  30 . Thermostat  24  receives from conduit  44  a portion of the coolant returning from a heating and ventilation system (not shown) of the vehicle. Past the thermostat  24 , the reduced temperature coolant delivered by the conduit  22  proceeds via conduit  26  to a primary fluid pump  28 . The reduced temperature coolant is thereby returned to the power-plant  12 , completing the coolant circulation. The baseline volume and pressure of the coolant in the conduits  16 ,  20 ,  22  and  26  are provided via the primary fluid pump  28 , while the thermostat  24  restricts coolant flow to a predetermined flow rate for circulating through the power-plant  12 . 
         [0021]    The conduit  30  diverts some of the reduced temperature coolant after the heat exchanger  18 , and delivers that portion of the coolant to an auxiliary heat exchanger  32  for further temperature reduction. The auxiliary heat exchanger  32  is configured to process coolant at a relatively low flow rate in the range of 0.5-2 liters/minute, thus providing more time to further reduce temperature of the coolant. Operational target of the auxiliary heat exchanger  32  at 30 degrees Celsius ambient temperature is in the range of 40-60 degrees Celsius coolant discharge temperature. Precise target for the operational temperature of the auxiliary heat exchanger  32  would be determined based on the temperature of incoming coolant from the primary heat exchanger  18  and the heat rejection capacity of the auxiliary heat exchanger  32 . After coolant temperature is further reduced inside the auxiliary heat exchanger  32 , the coolant is discharged to conduit  34 . 
         [0022]    The conduit  34  delivers the further reduced temperature coolant to auxiliary fluid pump  36 . The fluid pump  36  pressurizes the further reduced temperature coolant and delivers the coolant to the motor-generator  14 , for removing heat energy produced by the motor-generator during power generation. The fluid pump  36  is controlled by a controller  37  to provide coolant at the aforementioned 0.5-2 liters/minute flow rate. After heat energy of the motor-generator  14  has been removed, the coolant exits the motor-generator via conduit  40 , and is delivered to conduit  22  where it rejoins the reduced temperature coolant delivered by the primary heat exchanger  18  to the thermostat  24 . After the thermostat  24 , the fluid is delivered to the conduit  26 , and, through the pump  28 , back to the power-plant  12 . 
         [0023]      FIG. 2  shows an alternative hybrid vehicle cooling system  10 A where all like elements are numbered identically as those appearing in  FIG. 1 . The cooling system  10 A is configured identically from the power-plant  12  up through the auxiliary heat exchanger  32 . The further reduced temperature coolant is discharged from the auxiliary heat exchanger  32  to the conduit  34 , which delivers the coolant to an orifice  42 . The orifice  42  is configured to restrict the flow of the further reduced coolant to the motor-generator  14  down to the motor-generator coolant flow requirement of 0.5-2 liters/minute. 
         [0024]    As a consequence of the orifice  42  restricting coolant flow, the coolant remains inside the auxiliary heat exchanger  32  for a longer period of time, thereby permitting a larger coolant temperature drop. The further reduced temperature coolant is delivered to the motor-generator  14  via the conduit  38 . After heat energy of the motor-generator  14  has been removed, the coolant exits the motor-generator via conduit  40 A, and is delivered to the conduit  44  upstream of the thermostat  24  (shown in  FIG. 2 ). Orifice  45  is positioned just upstream of the thermostat  24  in conduit  22 , and configured to establish a coolant pressure drop required to create coolant flow in coolant system  10 A. Alternately, the orifice  45  may be incorporated into the physical structure of the thermostat  24  to accomplish the same result. After the coolant passes through the thermostat  24 , it is delivered to the conduit  26 , and, through the pump  28 , back to the power-plant  12 . 
         [0025]      FIG. 3  illustrates a plot of experimentally determined operating temperatures of the motor-generator  14  versus coolant flow rate. Although not shown, characteristically, the motor-generator  14  follows typical construction of an electric motor. As such, a motor-generator generally employs a steel stator with wire windings, wherein the stator has its outer portion pressed into an aluminum housing which includes a coolant jacket. Typically, during motor-generator operation, heat is generated in the wire windings. Excess amount of heat may, however, render the motor-generator inoperative. Hence, it is generally desirable to remove excess heat while the motor-generator is in operation. 
         [0026]    Excess heat may be removed from a motor-generator by radiation to ambient air, or by forced cooling via conduction to a purposefully channeled and circulated coolant. In the case of the motor-generator  14 , the heat is conducted from the windings to the steel stator. From the stator, the heat is conducted to the aluminum housing, and from there it is taken away by coolant circulated through dedicated cooling passages (not shown), but that are in fluid communication with passages  38  and  40  of  FIG. 1 , or with passages  38  and  40 A of  FIG. 2 . For sensing actual temperature of the stator, the motor-generator  14  may also incorporate a thermal sensor (not shown) in contact with the wire windings. 
         [0027]    As can be seen from  FIG. 3 , difference between temperature of the windings and temperature of the coolant, designated by a trend line  46 , is only reduced from 52 to 47 degrees Celsius, when coolant flow rate is increased from 0.25 to 10 liters/minute. Hence, the magnitude of the stator temperature drop is relatively insensitive to coolant flow rate. When coolant flow rate is increased from 0.25 to 10 liters/minute, difference between temperature of the outer portion of the steel stator in contact with the housing and temperature of the coolant, designated by a trend line  48 , is only reduced from 14 to 9 degrees Celsius. Hence, the magnitude of the temperature drop of the outer portion of the steel stator is similarly insensitive to coolant flow rate. Therefore, a relatively low coolant flow rate, in the range of 0.5-2 liters/minute, can be utilized to generate a larger temperature drop in the auxiliary heat exchanger  32 , in order to provide effective cooling for the motor-generator  14 . 
         [0028]      FIG. 4  depicts a method  50  of controlling the cooling system  10  or  10 A shown in  FIGS. 1 and 2 , respectively. The method  50  is described with reference to  FIGS. 1 and 2 , and the above description of the coolant system  10 . The method commences at block  52 , and then proceeds to block  54 . In block  54  increased temperature coolant is received from the power-plant  12 . The method then advances to block  56 . In block  56 , temperature of the coolant is reduced by the primary heat exchanger  18 . The method then returns a first portion of the reduced temperature coolant to the power-plant  12  in block  58 , and delivers a second portion of the reduced temperature coolant to auxiliary heat exchanger  18  in block  60 . 
         [0029]    According to the method, following block  60 , the temperature of the second portion of the reduced temperature coolant is then reduced further by the auxiliary heat exchanger  18  in block  62 . The method then proceeds to block  64 , where the delivery of the further reduced temperature second portion of coolant to the motor-generator  14  is controlled. Following block  64 , the second portion of coolant is delivered from the motor-generator  14  to the power-plant  12 . At this point the method  50  returns to block  52  and commences again. The method functions continuously according to the preceding description while the vehicle is in operation. 
         [0030]    Although the method was described with respect to the motor-generator  14  employed in a hybrid vehicle propulsion system, the method may also be applied to cooling any electric power producing device having a provision for circulation of coolant. 
         [0031]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.