Abstract:
An exhaust heat recovery system (EHRS) for a vehicle is operable to direct exhaust heat to a vehicle transmission under certain operating conditions. In some embodiments, the EHRS may also direct exhaust heat to a heater for vehicle passenger compartment. Preferably, the EHRS is controllable to manage available exhaust heat according to vehicle operating conditions, by prioritizing the heat flow among the engine, the transmission, and the vehicle heater. The EHRS may also operate in a bypass mode during which exhaust heat is not directed to the engine, the transmission or the vehicle heater. A method of managing exhaust heat recovery on a vehicle having an EHRS is also provided.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to an exhaust heat recovery system for a vehicle, and a method of managing exhaust heat. 
       BACKGROUND OF THE INVENTION 
       [0002]    Rapid warm-up of engine fluid, engine oil, and transmission fluid is important to fuel economy during a cold start (i.e., when the vehicle has not been running and the engine and transmission are relatively cold). Engine warm-up is especially challenging for diesel and hybrid applications, as less fuel is burned. Adequate passenger compartment heater performance is also necessary for passenger comfort. Powering the heater using a motor/generator on a hybrid vehicle can negatively impact fuel economy. 
       SUMMARY OF THE INVENTION 
       [0003]    An exhaust heat recovery system (EHRS) for a vehicle is provided that is operable to direct exhaust heat to a vehicle transmission under certain operating conditions. In some embodiments, the EHRS may also direct exhaust heat to a heater for vehicle passenger compartment. Preferably, the EHRS is controllable to manage available exhaust heat according to vehicle operating conditions, by prioritizing the heat flow among the engine, the transmission, and the vehicle heater. The EHRS may also operate in a bypass mode during which exhaust heat is not directed to the engine, the transmission or the vehicle heater. 
         [0004]    Specifically, the EHRS includes an exhaust heat recovery device positioned in the vehicle exhaust system. The device includes a heat exchanger, referred to herein as an exhaust heat recovery device heat exchanger. Under certain vehicle operating conditions, the exhaust heat recovery device is operable to direct exhaust gas through the exhaust heat recovery device heat exchanger, such as by signaling an actuator to open a valve. The EHRS is also operable to bypass the exhaust heat recovery device heat exchanger under other vehicle operating conditions, and simply expel the exhaust heat from the vehicle. The EHRS includes a transmission heat exchanger positioned in thermal communication with the transmission. Conduits containing fluid are arranged to operatively connect the exhaust heat recovery device heat exchanger with the transmission heat exchanger to thereby utilize heat from the exhaust gas to heat the transmission via the fluid. Fluid flow to the transmission heat exchanger may be controlled by a valve positioned downstream of the exhaust heat recovery device heat exchanger and upstream of the transmission heat exchanger. Many different types of valves may be used, such as an H-valve, a rotary valve, or a series of valves. 
         [0005]    In one embodiment, the valve is a rotary valve and is in a first position under the first set of vehicle operating conditions and is further positionable by the controller in a second position under a second set of vehicle operating conditions. Additional conduits may be provided to operatively connect the engine with the rotary valve to direct fluid from the engine to the rotary valve without passing through the exhaust heat recovery device. The rotary valve then further directs the fluid in the additional conduits to the transmission heat exchanger when in the second position. Thus, the engine cooling system can be used to cool the transmission by cooling fluid in the additional conduits under the second set of operating conditions, such as when transmission temperature is above a predetermined maximum transmission temperature, engine loading is above a predetermined threshold engine load, and/or engine speed is above a predetermined threshold engine speed, and cooling the transmission is therefore a priority. 
         [0006]    The conduits may also direct exhaust heat from the exhaust heat recovery device heat exchanger to the vehicle heater. In one embodiment, the conduits operatively connect the exhaust heat recovery device heat exchanger with the heater upstream of the transmission heat exchanger. Another valve may be positioned upstream of the heater to control flow of exhaust heat to the heater according to vehicle operating conditions. A controller is provided to determine the vehicle operating conditions and control the valve or valves accordingly. 
         [0007]    A method of managing exhaust heat recovery on a vehicle having an EHRS as described above includes determining vehicle operating conditions, such as conditions indicative of transmission temperature, engine temperature, engine speed and engine load. Sensors may be used to determine the vehicle operating conditions directly (such as temperature sensors), or the operating conditions may be determined based on other operating conditions or measured values, by a predictive model. The valve positioned upstream of the transmission heat exchanger is then controlled to permit fluid flow from an exhaust heat recovery device to the transmission heat exchanger if the transmission temperature is not greater than a predetermined threshold transmission temperature and the engine temperature is not less than a predetermined threshold engine temperature. This enables the engine to be heated with a higher priority than the transmission at lower temperatures, up to the predetermined threshold engine temperature, while the transmission is then heated at least to a predetermined minimum transmission temperature. The predetermined threshold engine temperature may be correlated with a temperature above which friction losses in the transmission are greater than friction losses in the engine, and may be dependent upon engine load and speed. 
         [0008]    Optionally, the method may also include opening a different valve to permit fluid flow from the exhaust heat recovery device to a vehicle heater if the ambient temperature is not greater than a predetermined threshold ambient temperature, or if operator input indicates that heating of the passenger compartment is requested. 
         [0009]    Still further, the valve controllable to permit exhaust heat flow to the transmission heat exchanger (referred to as the transmission valve) may be positioned to permit fluid communication between the engine and the transmission heat exchanger if the engine load is above a predetermined threshold engine load, engine speed is above a predetermined threshold engine speed, and/or transmission temperature is above a predetermined maximum transmission temperature. The fluid thus communicated does not pass through the exhaust heat recovery device between the engine and the transmission heat exchanger, as it is fluid cooled by the engine cooling system that is then used to cool the transmission. Under these operating conditions, the exhaust heat is not directed from the exhaust heat recovery device to the conduits, although some minimal amount of heat may be transferred from the exhaust system to the conduits simply due to proximity of the components. By cooling the transmission under such engine loading, engine speed, and/or transmission temperatures, transmission durability and fuel economy may be improved. 
         [0010]    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 
         [0011]      FIG. 1  is a schematic illustration of a first embodiment of a vehicle with a first embodiment of an EHRS that directs exhaust heat to a vehicle heater, then to a transmission heat exchanger, and then to a vehicle engine; 
           [0012]      FIG. 2  is a schematic illustration of a second embodiment of a vehicle with a second embodiment of an EHRS that includes valves controllable to direct heat flow to the heater, the transmission, and the engine according to vehicle operating conditions; 
           [0013]      FIG. 3  is a schematic illustration of a third embodiment of a vehicle with a third embodiment of an EHRS that includes valves controllable to direct heat flow to the heater, the transmission, and the engine according to vehicle operating conditions, and to direct cooling fluid from the engine to the transmission under high engine loading; 
           [0014]      FIG. 4  is a schematic illustration of the EHRS of  FIG. 3  with the valves directing cooling fluid from the engine to the transmission under high engine loading, high engine speeds, and/or high transmission temperatures; and 
           [0015]      FIG. 5  is a flowchart illustrating a method of managing exhaust heat recovery. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,  FIG. 1  shows a vehicle  10  that has an engine  12  (labeled E) for propelling the vehicle  10 , a transmission  14  (labeled T) operatively connected to the engine  12 , and a passenger compartment heater  16  (labeled H) for heating a passenger compartment, indicated in phantom as  18 . 
         [0017]    The engine  10  is an internal combustion engine of the gasoline or diesel type, and generates exhaust gas in an exhaust system that includes an exhaust manifold  20  and an exhaust pipe  22  extending therefrom. The exhaust gas, which is relatively hot, exits the vehicle via the exhaust pipe  22 . An exhaust heat recovery system (EHRS)  24  is provided in order to selectively capture some of the exhaust heat for providing heat to the passenger compartment  16 , the transmission  14 , and the engine  12 . The EHRS  24  includes an exhaust heat recovery device (EHRD)  26  positioned in the exhaust system. Specifically, the EHRD  26  includes an exhaust heat recovery device heat exchanger (EHRDHE)  28 , a valve  30 , and an exhaust bypass actuator  32  controllable to selectively open the valve  30  to permit some of the exhaust gas in the exhaust pipe to flow through the EHRDHE  28  in a warm-up mode. An electronic controller  34  is operatively connected to the actuator  32 , and controls the actuator  32  according to vehicle operating conditions received as input signals from various sensors placed on the vehicle  10  (only an exemplary transmission sensor  33 A and engine sensor  33 B are shown). The information received by the controller  34  is indicative of such operating conditions as ambient temperature, transmission temperature (e.g., transmission oil temperature), engine temperature (e.g., engine oil temperature or engine coolant temperature), engine speed, and engine loading. The sensors may directly measure the operating conditions, or may provide information used in a predictive model that predicts or estimates these operating conditions. 
         [0018]    When operating conditions indicate that a warm-up mode is desirable (ambient temperature, engine temperature, or transmission temperature are below predetermined minimums), the controller  34  controls the actuator  32  to open the valve  30  (if not already opened). If operating conditions indicate no warm-up is required, the controller  34  controls the actuator  32  to close the valve  30  (if not already in a closed position) to establish a bypass mode in which exhaust gas in exhaust pipe  22  is not in thermal communication with the EHRDHE  28 . A person of ordinary skill in the art would readily understand the various ways to provide such information indicative of vehicle operating conditions to the controller  34 , and would readily understand various algorithms that may be stored on the controller  34  to process the information. 
         [0019]    Conduits filled with fluid are arranged to carry heat from the EHRDHE  28  to the engine  12 , the transmission  14 , and the heater  16 . The conduits may be flexible or rigid tubing, or bored, drilled, cast or otherwise formed passages in any vehicle component. Specifically, conduit  40  runs past the EHRDHE  28 , in thermal communication therewith, such that fluid in conduit  40  is heated. Conduit  40  carries the heated fluid to the heater  16 . After flowing through the heater  16 , the fluid flows through conduit  42  to a transmission heat exchanger  44  in thermal communication with the transmission  14  to heat the transmission  14 . After flowing through the transmission heat exchanger  44 , the fluid flows through conduit  46  to the engine  12  for heating engine oil within the engine  12  according to any known heat transfer mechanisms. Finally, fluid flows from the engine  12  via conduit  46  past the EHRDHE  28  in order to absorb more heat from the exhaust gas before again flowing out through conduit  40 . Accordingly, the EHRS  24  prioritizes heat flow to the heater  16 , then to the transmission  14 , then to the engine  12  when in warm-up mode, and provides substantially no heat flow when in bypass mode. During bypass mode, a minimal amount of heat flow may exist through the EHRDHE  28  to the conduit  40  due to the proximity of the components, even with the valve  30  closed. 
         [0020]    Referring to  FIG. 2 , an alternative embodiment of a vehicle  100  with an alternative embodiment of an EHRS  124  is illustrated. Components of the vehicle  100  and EHRS  124  that are the same as those shown and described with respect to vehicle  10  and EHRS  24  are referred to with the same reference numbers. The EHRS  124  includes a valve  150 , also referred to herein as a first valve or a heater valve. Conduits  40  and  42  are in fluid communication with the valve  150 , but only selectively in fluid communication with conduits  40 A,  42 A leading to and from the heater  16  when the valve is in a first state (shown in solid), referred to as open When the valve  150  is controlled to be in a second state (referred to as closed) by a controller  134  in accordance with predetermined vehicle operating conditions, alternate valve channels, represented in phantom as  150 A, are positioned in communication with conduits  40 ,  42  so that fluid flowing in conduit  40  flows directly to conduit  42 , bypassing the heater  16 . Such a valve is referred to as an “H” valve. However, it should be appreciated that other types of valves may be used. 
         [0021]    The EHRS  124  also includes a valve  152 , referred to herein as a second valve or a transmission valve, that is similarly controllable by the controller  134  in accordance with predetermined vehicle operating conditions to be in either a first state (shown in solid, referred to as open) or a second state  152 A (shown in phantom, referred to as closed). In the first state, shown in solid, conduits  42 B and  46 B are in communication with conduits  42  and  46 , respectively, to permit fluid flow through the transmission heat exchanger  44 . When the valve is in the second state, in accordance with predetermined vehicle operating conditions, alternate valve channels, represented in phantom as  152 A, are moved from the position shown in  FIG. 2  so that they are in communication with conduits  42 ,  46 . Fluid flowing in conduit  42  therefore flows directly to conduit  46 , bypassing the transmission heat exchanger  44 . 
         [0022]    The valves  150 ,  152  allow an improved heat flow balance during warm-up between the heater  16 , the engine  12 , and the transmission  16  according to vehicle operating conditions. For example, when ambient temperature is relatively warm (i.e., above a predetermined threshold ambient temperature), and the engine  12  is relatively cold (e.g., has just been started, referred to as a cold start), warming up the engine  12  is a priority. Thus, the controller  134  will open valve  30  and position both valves  150 ,  152  in the second state  150 A,  152 A, respectively (i.e., closed), so that fluid flows from conduit  40 , to conduit,  42 , and then to conduit  46 , providing heat to the engine  12 , but bypassing the heater  16  and transmission heat exchanger  44 . Once the engine  12  is sufficiently warm (i.e., engine temperature is above a predetermined minimum engine temperature), the controller  134  will place valve  152  in a first state (i.e., open) so that fluid heated by the exhaust gas in the EHRD  26  flows from conduit  40 , to conduit  42 , through the transmission heat exchanger  44  via conduits  42 B,  46 B, and through conduit  46  to the engine  12 , before returning via conduit  48  to the EHRDHE  28 . 
         [0023]    When operating conditions are such that engine temperature is above the predetermined threshold engine temperature, the transmission temperature is below a predetermined minimum threshold transmission temperature, and ambient temperature is above a predetermined minimum ambient temperature, warming up the transmission  14  is the highest priority to improve overall efficiency. The predetermined threshold engine temperature is correlated with a temperature-dependent friction level of the engine oil for which it is determined that overall operating efficiency is increased by warming the transmission fluid rather than by increasing the engine oil temperature. The predetermined threshold engine temperature may be referred to as a friction cross-over point, and reflects a temperature at which friction reduction (e.g., decreased motoring torque and spin losses) is better achieved by heating the transmission rather than the engine. 
         [0024]    Once the engine  12  is warmed to the predetermined threshold engine temperature and the transmission  14  reaches at least the predetermined minimum transmission temperature (e.g., 65 degrees Celsius), then the actuator  32  is controlled so that EHRS  124  operates in bypass mode, with the exhaust gas in the exhaust pipe  22  bypassing the EHRDHE  28 , and minimal or no heat added to the conduit  40  by the exhaust gas (any heat added to the conduit  40  by EHRDHE  28  is due to thermal leakage, not to controlled direction of exhaust heat). 
         [0025]    When vehicle operating conditions indicate that ambient temperature is below a predetermined threshold ambient temperature, and the engine  12  is below a predetermined threshold engine temperature (i.e., a cold start), then heating both the passenger compartment  18  and the engine  12  are a greater priority than heating the transmission  14 . Accordingly, the controller  134  controls the valve  30  to direct exhaust gas to the EHRDHE  28 , places the valve  152  in the second state  152 A (i.e., closed), and places the valve  150  in the first state (i.e., open). Accordingly, heated fluid flows from the EHRDHE  28  through conduits  40  and  40 A to heat the heater  16 , then through conduits  42 A and  42  directly to conduit  46 , bypassing the transmission heat exchanger  44 , to heat the engine  12  before returning to the EHRDHE  28  via conduit  48 . After the engine  12  is sufficiently warmed-up (i.e., above the predetermined threshold engine temperature, the controller  134  also opens the valve  152  to allow the transmission  14  to be heated as well. When the transmission temperature then reaches the predetermined minimum transmission temperature, both the transmission  14  and the engine  12  are sufficiently warm, and the controller  134  controls actuator  32  so that valve  30  is closed, and the EHRS  124  operates in bypass mode, with the exhaust gas in the exhaust pipe  22  bypassing the EHRDHE  28 . Little or no heat is added to the conduit  40  (any heat added to the conduit  40  by EHRDHE  28  is due to thermal leakage, not to controlled direction of exhaust heat). Heat may be provided to the heater  16  during bypass mode via another heat source other than the exhaust gas in exhaust pipe  22  to ensure passenger comfort (e.g., via cooling jacket heat flow). 
         [0026]    Referring to  FIGS. 3 and 4 , an alternative embodiment of a vehicle  200  with an alternative embodiment of an EHRS  224  is illustrated. Components of the vehicle  200  and EHRS  224  that are the same as those shown and described with respect to vehicles  10  and  100 , and EHRS  24  and EHRS  124  are referred to with the same reference numbers. In addition to valves  30  and  150 , the EHRS  224  includes a valve  252 , referred to herein as a second valve or a transmission valve, that is a rotary valve and is similarly controllable by the controller  234  in accordance with predetermined vehicle operating conditions to be in either a first position, shown in  FIG. 4 , or another position, shown in  FIG. 3 , or still another position shown with passages  47 A,  47 B in phantom in  FIG. 3 , labeled  47 AA and  47 BB, referred to as a second position. The valve  252  has two valve channels  47 A and  47 B that are arranged such that in the state or position of  FIG. 3 , channel  47 A establishes fluid communication between conduit  42  and conduit  46  through valve  252 , while channel  47 B is out of communication (i.e., not aligned with) with conduit  42 C leading to transmission heat exchanger  44 , as indicated by the “X” across the conduit  42 C. Furthermore, with valve  252  in the state shown in solid in  FIG. 3 , fluid cannot flow through valve  252  to transmission heat exchanger  44  from an additional conduit  50  leading from conduit  48 . 
         [0027]    When vehicle operating conditions indicate that ambient temperature is below a predetermined threshold ambient temperature and that engine temperature is below a predetermined threshold engine temperature, warming the heater  16  and the engine  12  are given priority by the controller  134  controlling the valve  30  to direct exhaust heat to the EHRDHE  28 , position valve  150  in a first (open) position, and positions valve  252  in the state or position shown in solid in  FIG. 3 . Accordingly, heated fluid flows from the EHRDHE  28  through conduits  40  and  40 A to heater  16 , then through conduits  42 A and  42 , through valve passage  47 A to conduit  46  to heat the engine  12  before returning via conduit  48  to the EHRDHE  28 . 
         [0028]    When vehicle operating conditions indicate that ambient temperature is above a predetermined threshold ambient temperature, and the engine  12  is below a predetermined threshold engine temperature, then heating the engine  12  is a priority. Thus, the controller  134  controls valve  30  to direct exhaust heat to the EHRDHE  28 , positions valve  150  in a second (closed) position  150 A, and positions valve  252  in the position of  FIG. 3 . Accordingly, heated fluid flows from the EHRDHE  28  through conduits  40  and  42 , bypassing the heater  16 , through valve passage  47 A to conduit  46  to heat the engine  12  before returning via conduit  48  to the EHRDHE  28 . 
         [0029]    When the engine  12  is sufficiently warm (i.e., engine temperature is above the predetermined threshold engine temperature), and the ambient temperature is below the predetermined threshold ambient temperature, the friction cross-over point has been reached and vehicle efficiency is maximized by giving priority to heating the transmission  14  (and heater  16  for passenger comfort) rather than the engine  12 . Thus, the controller  234  controls valve  30  to direct exhaust heat to the EHRDHE  28 , positions valve  150  in a first (open) position, and positions valve  252  in the position of  FIG. 4 . Accordingly, heated fluid flows from the EHRDHE  28  through conduits  40 ,  40 A,  42 A and  42 , heating the heater  16 , through valve passage  47 A to transmission heat exchanger  44 . From the transmission heat exchanger  44 , the fluid flows to the engine  12  through the conduit  46 , with the fluid in conduit  46  now relatively cold, and returning to the EHRDHE via conduit  48 . 
         [0030]    When the engine  12  is sufficiently warm (i.e., engine temperature is above the predetermined threshold engine temperature), and the ambient temperature is above the predetermined threshold ambient temperature, the friction cross-over point has been reached and vehicle efficiency is maximized by giving priority to heating the transmission  14  rather than the engine  12  and the heater  16 . Thus, the controller  134  controls valve  30  to direct exhaust heat to the EHRDHE  28 , positions valve  150  in the second (closed) position  150 A, and positions valve  252  in the state or position of  FIG. 4 . Accordingly, heated fluid flows from the EHRDHE  28  through conduits  40 , and  42 , bypassing the heater  16 , through valve passage  47 A and conduit  42 C to transmission heat exchanger  44 , before flowing to the engine  12  through conduit  46 , and returning to the EHRDHE via conduit  48 . 
         [0031]    If vehicle operating conditions indicate that (i) engine load is above a predetermined threshold engine load (e.g., high thermal loading, such as when the vehicle  200  is used for towing); (ii) engine speed is above a predetermined engine speed; and/or (iii) transmission temperature is above a predetermined maximum transmission temperature (e.g., 90 degrees Celsius), cooling of the transmission  14  is a priority to increase overall operating efficiency. Thus, the controller  234  controls valve  30  to operate in a bypass mode, not directing exhaust heat to the EHRDHE  28 . Furthermore, the controller  234  positions valve  150  in the second (closed) position  150 A, and positions valve  252  in the position  47 BB shown in phantom in  FIG. 3 , referred to as a second position, in which valve passage  47 B connects additional conduit  50  to conduit  42 C, and valve passage  47 A does not connect conduit  42  to conduit  46 . Engine coolant cooled by an engine cooling system (not shown) is directed through conduit  48 , additional conduit  50 , through passage  47 B (in position  47 BB) of valve  252 , through conduit  42 C to transmission heat exchanger  44 , and through conduit  46  to engine  12 . 
         [0032]    If engine loading is below the predetermined threshold engine load, engine temperature is above the predetermined threshold engine temperature, and transmission temperature is above the predetermined minimum transmission temperature and below the predetermined maximum transmission temperature (i.e., within an acceptable temperature range in which neither heating or cooling is necessary), then valve  30  is controlled to operate in bypass mode, and valve  252  is positioned in the position of  FIG. 3  in which the valve passages  47 A,  47 B are shown in solid, so that no exhaust heat is used for heating the vehicle  10  and the cooling system of the engine  12  is not used for cooling the transmission  14 . 
         [0033]    Referring to  FIG. 5 , a method  300  of managing exhaust heat recovery is illustrated in a flowchart. The method  300  may be performed by the controller  234  of the EHRS  224  of  FIGS. 3 and 4 , and will be described with respect to EHRS  224 , although the method is not limited to use with the structure of the EHRS  224 . 
         [0034]    The method  300  begins at step  302 , in which the controller  234  determines vehicle operating conditions from various vehicle sensors, such as sensors  33 A and  33 B, either my direct measurement or indirectly, according to a predictive model stored as an algorithm in the controller  234 , as described above. The vehicle operating conditions include the ambient temperature, T AMBIENT , the transmission temperature, T TRANS , such as transmission oil temperature, and the engine temperature, T ENGINE . Optionally, the vehicle operating conditions may include engine speed, SPEED ENGINE , and engine loading, LOAD ENGINE , such as if steps  312  and  314  (described below) are included in the method  300 . 
         [0035]    In step  304 , the controller  234  then determines whether T AMBIENT  is less than or equal to a predetermined ambient threshold temperature, T AMBIENT     —     THRESHOLD . T AMBIENT     —     THRESHOLD  is a temperature selected based on the requirements of the vehicle heater  16 . That is, at or below this temperature, it is determined that the vehicle heater  16  should be supplemented with heating via the exhaust. If T AMBIENT  is less than or equal to T AMBIENT     —     THRESHOLD , then the heater valve  150  is placed in the open position in step  308 . If T AMBIENT  is not less than or equal to T AMBIENT     —     THRESHOLD , then, in step  309 , a determination is made as to whether there is any operator input requesting heating of the passenger compartment  18 . If operator input has been received by the controller  234 , then the method moves to step  308 . If not, then the method  300  moves to step  306 , and the heater valve  150  is closed (i.e., placed in the second state  150 A (i.e., closed) if it had previously been in the open position. 
         [0036]    After either step  306  or step  308 , the method  300  moves to step  310 , in which the controller  234  determines whether T TRANS  is less than or equal to the predetermined minimum transmission temperature, T TRANS     —     MINIMUM . It should be noted that step  310  could be carried out prior to steps  304 ,  306  and  308  if transmission heating is to be given a higher priority than passenger compartment heating. In that case, the method  300  would move from step  302  to step  310 , and only after the remaining steps of the flowchart of  FIG. 5  are carried out would the method  300  move to step  304 . 
         [0037]    In step  310 , if it is determined that T TRANS  is less than or equal to T TRANS     —     MINIMUM , then heating of the transmission  14  with the EHRS  124  may be in order, but only if the engine temperature is not less than a predetermined engine temperature, T ENGINE     —     THRESHOLD , below which heating of the engine  12  is given priority over heating of the transmission  14 . Accordingly, in step  316 , if it is determined that T ENGINE  is greater than or equal to T ENGINE     —     THRESHOLD , then in step  318  the transmission valve  252  will be placed in the first position of  FIG. 4  to allow heated fluid to flow to the transmission heat exchanger  44 . The method  300  then returns to step  302  to continue monitoring vehicle operating conditions and managing exhaust heat recovery accordingly. 
         [0038]    If it is determined that T ENGINE  is not greater than or equal to T ENGINE     —     THRESHOLD  (i.e., is less than T ENGINE     —     THRESHOLD ) in step  316 , then the engine  12  is not sufficiently warm to divert any of the exhaust heating to the transmission  14 . Accordingly, the method moves to step  317 , in which the transmission valve  252  is placed in the position shown in solid in  FIG. 3 , if not already in that position. That is, the transmission valve  252  is placed in the position to direct heated fluid to the engine  12  without passing through the transmission heat exchanger  44 . The method  300  returns to step  302 . 
         [0039]    If it is determined that T TRANS  is not less than or equal to T TRANS     —     MINIMUM  in step  310 , then the transmission  14  is at a temperature that does not require additional heating from the EHRS  224  to improve operating efficiency. Thus, the method  300  then determines whether engine loading, engine speed, or transmission temperature indicates that cooling of the transmission is in order for improved transmission durability or system efficiency. Accordingly, the method  300  moves to step  312  to consider loading of the engine  12 . Specifically, the method  300  determines whether the engine load, LOAD ENGINE , is greater than or equal to a predetermined engine load, LOAD ENGINE     —     THRESHOLD . If the LOAD ENGINE  is greater than or equal to LOAD ENGINE     —     THRESHOLD , then efficiency may be improved by using fluid cooled by the engine cooling system to cool the transmission  14 . Accordingly, in step  314 , the transmission valve  252  is placed in the second position (shown in phantom in  FIG. 3 ) to permit fluid cooled by the engine cooling system to pass through the transmission heat exchanger  44 , as described with respect to  FIG. 3  above. The method  300  then returns to step  302 . 
         [0040]    If LOAD ENGINE  is not greater than or equal to LOAD ENGINE     —     THRESHOLD , then the method  300  moves to step  320  to consider whether the speed of the engine  12  is greater than a predetermined engine speed returns, SPEED ENGINE     —     THRESHOLD . If the speed of the engine  12  is greater than SPEED ENGINE     —     THRESHOLD , the method  300  moves to step  314 , and the transmission valve  252  is placed in the second position (shown in phantom in  FIG. 3 ) to permit fluid cooled by the engine cooling system to pass through the transmission heat exchanger  44 , as described with respect to  FIG. 3  above. The method  300  then returns to step  302 . 
         [0041]    If SPEED ENGINE  is not greater than or equal to SPEED ENGINE     —     THRESHOLD , then the method  300  moves to step  322  to consider whether the temperature of the transmission is greater than a predetermined maximum transmission temperature, T TRANS     —     MAXIMUM . If the temperature of the transmission is greater than a T TRANS     —     MAXIMUM , the method  300  moves to step  314 , and the transmission valve  252  is placed in the second position (shown in phantom in  FIG. 3 ) to permit fluid cooled by the engine cooling system to pass through the transmission heat exchanger  44 , as described with respect to  FIG. 3  above. The method  300  then returns to step  302 . 
         [0042]    Accordingly, in the various embodiments described above, exhaust heat that would otherwise be wasted is used for passenger compartment heating, transmission heating or engine heating, and the various conduits and valves provided in the embodiments shown in  FIGS. 1-4  allow the exhaust heat to be managed in order to increase vehicle efficiency. 
         [0043]    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.