Patent Publication Number: US-9845750-B2

Title: Method and system for exhaust gas heat recovery

Description:
FIELD 
     The present description relates generally to methods and systems for exhaust gas heat recovery at an exhaust gas heat exchanger. 
     BACKGROUND/SUMMARY 
     Engines may be configured with an exhaust heat recovery system for recovering heat of exhaust gas generated at an internal combustion engine. The heat is transferred from the hot exhaust gas to a coolant through an exhaust gas heat exchanger system. The heat from the coolant, circulated through an exhaust gas heat exchanger, may be utilized for functions such as heating the cylinder head, and warming the passenger cabin, thereby improving engine efficiency. In hybrid electric vehicles, the recovery of exhaust heat improves fuel economy by enabling engine temperatures to be maintained longer, thereby allowing for a faster engine shut-off and extended use of the vehicle in an electric mode. 
     Exhaust heat may also be retrieved at an exhaust gas recirculation (EGR) cooler. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold. EGR may be used to reduce exhaust NOx emissions. Further, EGR may be used to assist in the reduction of throttling losses at low loads, and to improve knock tolerance. 
     Various approaches are provided for exhaust heat recovery and EGR cooling. In one example, as shown in US 20120260897, Hayman et al. discloses an engine system wherein exhaust gas from a first group of cylinders is directed to an exhaust tailpipe. Exhaust gas from a second group of cylinders is either directed to an intake manifold via an EGR passage. In addition, based on the EGR requirement of the engine, a portion of the exhaust from the second group of cylinders may be diverted to the exhaust tailpipe via a bypass passage. An EGR cooler coupled to the EGR passage is used to cool the exhaust gas prior to mixing of the EGR with combustion air and entry of the mixture into the intake manifold. 
     However, the inventors herein have recognized potential disadvantages with the above approaches. As one example, it may be difficult to coordinate EGR cooling with exhaust heat recovery. In particular, the heat recovered at the EGR cooler cannot be used for heating a cabin space. As a result, a distinct heat exchanger is required for cabin heating. Likewise, even if heat is extracted from exhaust gas at a heat exchanger, the cooled exhaust gas is not recirculated, resulting in the need for a distinct EGR cooler. The additional components add cost and complexity. 
     The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method comprises recirculating exhaust gas to an engine intake by flowing a first amount of exhaust gas in a first direction through a first section of a heat exchanger, and flowing a second amount of exhaust gas in a second, opposite direction through a second section of the heat exchanger. 
     In one example, an engine system may be configured with a heat exchanger positioned downstream of a catalytic convertor in an exhaust bypass disposed parallel to a main exhaust passage. The heat exchanger may be asymmetrically designed with two separate cooling sections having an EGR passage coupled to the intermediate section of the two cooler sections. A single butterfly valve may be used to enable exhaust gas to be diverted into the bypass passage, and through the heat exchanger in one of two directions, a position of the valve adjusted based on engine operating parameters. Depending on engine requirement, EGR may be cooled to different levels. In one example, during a first operating mode, when required EGR temperature is above a threshold, the valve position may be adjusted such that the exhaust gas may flow through the first section of the two section EGR cooler before being delivered to the engine manifold. In another example, during a second operating mode, when required EGR temperature below the threshold, the valve may be adjusted to flow the exhaust gas in opposite directions through both sections of the EGR cooler before entering the engine intake manifold. During the flow in both first and second operating mode, exhaust heat is transferred to a coolant circulating through the heat exchanger. The warmed coolant may be circulated back to the engine (such as when engine heating is required) and/or utilized for heating a passenger cabin of the vehicle (such as when cabin heating is requested). Alternatively, the extracted heat is transferred to the radiator for dissipation into the atmosphere. 
     In this way, the heating requirements of an engine system may be met using a single heat exchanger. By providing the functions of an EGR cooler and an exhaust gas heat exchanger via a single heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. In addition, the specific configuration of a single heat exchanger in a bypass exhaust passage allows for a shorter EGR passage length, which reduces EGR transport delays. The technical effect of using a butterfly valve to regulate the passage of exhaust gas through the bypass passage is that exhaust gas can be flowed in both directions across the heat exchanger. As such, this improves the heat transfer efficiency and reduces the requirement for long EGR passages. By using a single butterfly valve to control the EGR flow, number of components in the system is reduced which facilitates quicker warm-up of the coolant due to less thermal mass. By using a split EGR cooler it is possible to regulate the degree of cooling of the EGR depending on engine requirements. The separate cooler and heat recovery systems on being combined to one, reduces manufacturing cost. Since the EGR cooler is located downstream from the catalyst convertor, catalyst functionality is not compromised. Overall, by improving the amount of waste heat that can be recovered from exhaust using fewer components, engine fuel economy and performance is improved. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example embodiment of an engine system including an exhaust bypass assembly with a heat exchanger. 
         FIG. 2  shows an example embodiment of an engine system including an exhaust bypass assembly with a split heat exchanger. 
         FIG. 3A  shows an example embodiment of the exhaust bypass assembly of  FIG. 1  operating in a first mode. 
         FIG. 3B  shows an example embodiment of the exhaust bypass assembly of  FIG. 1  operating in a second mode. 
         FIG. 4A  shows an example embodiment of the exhaust bypass assembly of  FIG. 2  operating in a first mode. 
         FIG. 4B  shows an example embodiment of the exhaust bypass assembly of  FIG. 2  operating in a second mode. 
         FIG. 5  shows a flow chart illustrating an example method that may be implemented for adjusting exhaust flow through the exhaust bypass assembly of  FIG. 1 . 
         FIG. 6  shows a table illustrating the different modes of operation of the exhaust bypass assembly of  FIG. 1 . 
         FIG. 7  shows an example operation of the exhaust bypass assembly of  FIG. 1 . 
         FIG. 8  shows a flow chart illustrating an example method that may be implemented for adjusting exhaust flow through the exhaust bypass assembly of  FIG. 2 . 
         FIG. 9  shows a table illustrating the different modes of operation of the exhaust bypass assembly system of  FIG. 2 . 
         FIG. 10  shows an example operation of the exhaust bypass assembly of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to systems and methods for improving exhaust heat recovery using a single heat exchanger/EGR cooler coupled in an exhaust bypass. Example engine systems comprising an exhaust bypass assembly are shown in  FIGS. 1 and 2 . In  FIG. 1 , a pair of valves are used to enable bidirectional flow of exhaust through the heat exchanger. In  FIG. 2 , a single valve is used to enable bidirectional flow of exhaust through one or more sections of a split heat exchanger. The different modes of operation of the systems of  FIGS. 1-2  are elaborated with reference to  FIGS. 3A-3B, 4A-4B, 6, and 7 . An engine controller may be configured to perform a control routine, such as the example routines of  FIGS. 5 and 8 , to vary the position of the system valve(s) to thereby adjust exhaust flow through the heat exchanger in the systems of  FIGS. 1 and 2 , respectively. Example operations of the systems of  FIGS. 1-2  are shown with reference to  FIGS. 7 and 10  respectively. 
       FIG. 1  schematically shows aspects of an example engine system  100  including an engine  10 . In the depicted embodiment, engine  10  is a boosted engine coupled to a turbocharger  13  including a compressor  114  driven by a turbine  116 . Specifically, fresh air is introduced along intake passage  42  into engine  10  via air cleaner  112  and flows to compressor  114 . The compressor may be any suitable intake-air compressor, such as a motor-driven or driveshaft driven supercharger compressor. In engine system  10 , the compressor is a turbocharger compressor mechanically coupled to turbine  116  via a shaft  19 , the turbine  116  driven by expanding engine exhaust. 
     As shown in  FIG. 1 , compressor  114  is coupled, through charge-air cooler (CAC)  18  to throttle valve  20 . Throttle valve  20  is coupled to engine intake manifold  22 . From the compressor, the compressed air charge flows through the charge-air cooler  18  and the throttle valve to the intake manifold. In the embodiment shown in  FIG. 1A , the pressure of the air charge within the intake manifold is sensed by manifold air pressure (MAP) sensor  124 . 
     One or more sensors may be coupled to an inlet of compressor  114 . For example, a temperature sensor  55  may be coupled to the inlet for estimating a compressor inlet temperature, and a pressure sensor  56  may be coupled to the inlet for estimating a compressor inlet pressure. As another example, a humidity sensor  57  may be coupled to the inlet for estimating a humidity of aircharge entering the compressor. Still other sensors may include, for example, air-fuel ratio sensors, etc. In other examples, one or more of the compressor inlet conditions (such as humidity, temperature, pressure, etc.) may be inferred based on engine operating conditions. In addition, when exhaust gas recirculation (EGR) is enabled, the sensors may estimate a temperature, pressure, humidity, and air-fuel ratio of the aircharge mixture including fresh air, recirculated compressed air, and exhaust residuals received at the compressor inlet. 
     A wastegate actuator  92  may be actuated open to dump at least some exhaust pressure from upstream of the turbine to a location downstream of the turbine via wastegate  90 . By reducing exhaust pressure upstream of the turbine, turbine speed can be reduced, which in turn helps to reduce compressor surge. 
     Intake manifold  22  is coupled to a series of combustion chambers  30  through a series of intake valves (not shown). The combustion chambers are further coupled to exhaust manifold  36  via a series of exhaust valves (not shown). In the depicted embodiment, a single exhaust manifold  36  is shown. However, in other embodiments, the exhaust manifold may include a plurality of exhaust manifold sections. Configurations having a plurality of exhaust manifold sections may enable effluent from different combustion chambers to be directed to different locations in the engine system. 
     In one embodiment, each of the exhaust and intake valves may be electronically actuated or controlled. In another embodiment, each of the exhaust and intake valves may be cam actuated or controlled. Whether electronically actuated or cam actuated, the timing of exhaust and intake valve opening and closure may be adjusted as needed for desired combustion and emissions-control performance. 
     Combustion chambers  30  may be supplied one or more fuels, such as gasoline, alcohol fuel blends, diesel, biodiesel, compressed natural gas, etc., via injector  66 . Fuel may be supplied to the combustion chambers via direct injection, port injection, throttle valve-body injection, or any combination thereof. In the combustion chambers, combustion may be initiated via spark ignition and/or compression ignition. 
     As shown in  FIG. 1 , exhaust from the one or more exhaust manifold sections is directed to turbine  116  to drive the turbine. The combined flow from the turbine and the wastegate then flows through emission control device  170 . In general, one or more emission control devices  170  may include one or more exhaust after-treatment catalysts configured to catalytically treat the exhaust flow, and thereby reduce an amount of one or more substances in the exhaust flow. For example, one exhaust after-treatment catalyst may be configured to trap NO, from the exhaust flow when the exhaust flow is lean, and to reduce the trapped NO, when the exhaust flow is rich. In other examples, an exhaust after-treatment catalyst may be configured to disproportionate NO, or to selectively reduce NO, with the aid of a reducing agent. In still other examples, an exhaust after-treatment catalyst may be configured to oxidize residual hydrocarbons and/or carbon monoxide in the exhaust flow. Different exhaust after-treatment catalysts having any such functionality may be arranged in wash coats or elsewhere in the exhaust after-treatment stages, either separately or together. In some embodiments, the exhaust after-treatment stages may include a regeneratable soot filter configured to trap and oxidize soot particles in the exhaust flow. 
     All or part of the treated exhaust from emission control  170  may be released into the atmosphere via main exhaust passage  102  after passing through a muffler  172 . An exhaust bypass assembly  160  with a bypass passage  173  may be coupled to the main exhaust passage  102 , downstream of the emission control device  170 . The bypass passage  173  may extend from downstream of the emission control device  170  to upstream of muffler  172 . The bypass passage  173  may be arranged parallel to the main exhaust passage  102 . A heat exchanger  174  may be coupled to bypass passage  173  to cool the exhaust gas passing through the bypass passage  173 . Exhaust gas recirculation (EGR) delivery passage  180  may be coupled to the exhaust bypass passage  173  at a location upstream of heat exchanger  174 . From downstream of the emission control device  170 , exhaust may flow towards muffler  172  via one or more of the main exhaust passage  102  and the bypass passage  173 . 
     A pair of diverter valves  176  and  178  may be used to regulate the amount and direction of flow of exhaust gas through main exhaust passage  102  and bypass passage  173 . In one example, diverter valves  176  and  178  may each be configured as butterfly valves (herein also referred to as butterfly valves  176  and  178 ), although other valve configurations may also be used. Further, depending on operating conditions such as engine temperature, a portion of the exhaust residuals may be diverted through bypass passage  173  and thereon to tailpipe  35  or to the inlet of compressor  114  via exhaust gas recirculation (EGR) valve  52  and EGR passage  180 . Also based on engine operating conditions, a direction of exhaust gas flow through the bypass passage  173  and across the heat exchanger  174  may be varied. In this way, the configuration allows for bidirectional flow of exhaust gas through the heat exchanger. Opening of the pair of butterfly valves and the EGR valve  52  may be regulated to control the flow of exhaust though the bypass passage  173  and the heat exchanger  174 . EGR valve  52  may be opened to admit a controlled amount of exhaust gas to the compressor inlet for desirable combustion and emissions control performance. EGR valve  52  may be configured as a continuously variable valve. In an alternate example, however, EGR valve  52  may be configured as an on/off valve. 
     By adjusting the position of the butterfly valves  176  and  178 , and an opening of EGR valve  52 , flow of exhaust gas through the heat exchanger can be varied. In one example, butterfly valve  176  may be closed while butterfly valve  178  is opened so that exhaust gas may flow from downstream of the emissions control device  170  into the bypass passage  173 , across the heat exchanger  174  in a first direction and thereon to tailpipe  35 . While flowing exhaust through the bypass passage  173  in the first direction, if requested, hot EGR may be drawn from upstream of the heat exchanger and delivered to the inlet of compressor  114  via the EGR delivery passage  180 . Opening of EGR valve  52  may be regulated to control the amount of exhaust entering the EGR delivery passage compared to the amount of exhaust flowing across the heat exchanger  174  towards the tailpipe  35 . In this way, hot LP-EGR may be optionally provided. 
     In another example, butterfly valve  176  may be opened while butterfly valve  178  is closed so that exhaust gas may flow through the main exhaust passage  35 , via emission control device  170  and butterfly valve  176  and enter bypass passage  173  from upstream of the muffler  172 . In this case, the exhaust may flow across the heat exchanger  174  in a second direction, opposite to the first direction, and thereon the cooled exhaust may enter the EGR delivery passage  180 . In this way, cooled LP-EGR may be provided. 
     As the exhaust gas passes through the heat exchanger  174  in either direction, heat from the hot exhaust gas may be transferred to a coolant circulating through the heat exchanger  174 . In one example, the heat exchanger  174  is a water-gas exchanger. Upon transfer of heat from the exhaust gas to the coolant, the warmed coolant may be circulated back to the engine (such as when engine heating is required) and/or through a heater core for heating a passenger cabin of the vehicle (such as when cabin heating is requested). Alternatively, when there are no heating demands, the warmed coolant may be flowed through a radiator for heat dissipation to the atmosphere. 
     In this way, engine system  10  may be adapted to provide external, low-pressure (LP) EGR by tapping exhaust gas from downstream of turbine  116 , the EGR cooled upon passage through a heat exchanger used for EGR cooling and exhaust heat recovery. Detailed description of the operation and structure of exhaust bypass assembly  160  will be discussed with relation to  FIGS. 3A-3B, 5, 6, and 7 . In further embodiments, the engine system may include a high pressure EGR flow path wherein exhaust gas is drawn from upstream of turbine  116  and recirculated to the engine intake manifold, downstream of compressor  114 . 
     One or more sensors may be coupled to EGR passage  180  for providing details regarding the composition and condition of the EGR. For example, a temperature sensor may be provided for determining a temperature of the EGR, a pressure sensor may be provided for determining a pressure of the EGR, a humidity sensor may be provided for determining a humidity or water content of the EGR, and an air-fuel ratio sensor may be provided for estimating an air-fuel ratio of the EGR. Alternatively, EGR conditions may be inferred by the one or more temperature, pressure, humidity and oxygen sensors  55 - 57  coupled to the compressor inlet. In one example, air-fuel ratio sensor  57  is an oxygen sensor. 
     Engine system  100  may further include control system  14 . Control system  14  is shown receiving information from a plurality of sensors  16  (various examples of which are described herein) and sending control signals to a plurality of actuators  18  (various examples of which are described herein). As one example, sensors  16  may include exhaust gas sensor  126  located upstream of the emission control device, MAP sensor  124 , exhaust temperature sensor  128 , exhaust pressure sensor  129 , compressor inlet temperature sensor  55 , compressor inlet pressure sensor  56 , compressor inlet humidity sensor  57 , and EGR sensor. Other sensors such as additional pressure, temperature, air/fuel ratio, and composition sensors may be coupled to various locations in engine system  100 . The actuators  81  may include, for example, throttle  20 , EGR valve  52 , butterfly valves  176  and  178 , wastegate  92 , and fuel injector  66 . The control system  14  may include a controller  12 . The controller  12  may receive input data from the various sensors, process the input data, and trigger various actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. For example, based on engine operating conditions and EGR requirements the controller  12  may regulate the opening of the butterfly valves  176  and  178  to direct exhaust gas in a first or a second direction through the heat exchanger. As another example, based on catalyst temperature inferred from exhaust temperature sensor  128 , opening of EGR valve  52  may be adjusted to draw a desired amount of EGR from the exhaust bypass passage into the engine intake manifold. An example control routine is described with regard to  FIG. 5 . 
     An alternate embodiment of  FIG. 1  is shown in  FIG. 2  and elaborated below with reference to example engine system  200 . All components of the engine system  200  may be identical to that of engine system  100  other than the exhaust bypass assembly  260 . As elaborated below, exhaust bypass assembly  260  may be configured with a split heat exchanger and a single diverter valve. The different modes of operation of the exhaust bypass assembly of  FIG. 2  are elaborated with reference to  FIGS. 4A-4B . 
       FIG. 3A  further elaborates the exhaust bypass assembly introduced in  FIG. 1  and shows an example embodiment  300  of operating the exhaust bypass assembly of  FIG. 1  in a first operating mode. In one example, assembly  300  is an embodiment of assembly  160  of  FIG. 1  and therefore may share common features and/or configurations as those already described for bypass assembly  160 . Exhaust bypass assembly  160  is fluidly coupled to exhaust passage  302  downstream of an emission control device  170 . As discussed in relation to  FIG. 1 , one or more emission control devices  170  may include one or more exhaust after-treatment catalysts configured to catalytically treat the exhaust flow, and thereby reduce an amount of one or more substances in the exhaust flow. 
     Exhaust gas flowing from the engine passes through the emission control device  170  and reaches the exhaust bypass assembly  160  located further downstream along the exhaust passage  302 . An inlet pipe  310  of the exhaust bypass assembly  160  is disposed on the exhaust passage  302  at junction  306  (downstream from emission control device  170 ). Butterfly valve  176  is coupled to the exhaust passage  302  downstream from the junction  306 . The inlet pipe  310  leads to a bypass passage  312  which may be parallel to the exhaust passage  302 . Butterfly valve  178  is coupled to the bypass passage  312 . In one example, the two butterfly valves may be coupled to a common shaft, and the valves may be actuated together. Depending on the desired direction of exhaust flow through the exhaust bypass assembly, one of butterfly valves  176  and  178  may be in open position. In an alternate embodiment, the two butterfly valves may be actuated independently and may be opened simultaneously. 
     Downstream from the second butterfly valve  178 , a heat exchanger may be coupled to the pipe  310 . A coolant may be circulated through the heat exchanger  174  for effective cooling of the exhaust gas passing through it, before the gas either enters the EGR passage  180  or escapes to the atmosphere through tailpipe  335 . Downstream from the second butterfly valve  178  and upstream from the heat exchanger  174 , an inlet to the EGR passage  180  may be located on the bypass passage  312 . Depending on engine requirements, it is possible to flow exhaust gas in both directions through the passage  312  and accordingly cooled or hot EGR may be delivered through the EGR passage  180  to the engine intake manifold. An EGR valve  52  may control the delivery of EGR from the exhaust passage  302  to the passage  328  leading to engine intake manifold. Upstream of the heat exchanger  174 , the bypass passage  312  ends in an outlet pipe  316  leading back to the exhaust passage  302 . The inlet  310  and outlet pipes  316  may be at right angles to the bypass passage  312  and the main exhaust passage  302 . The outlet pipe  316  joins the exhaust passage  302  at a junction  318  located downstream from the junction  306  and first butterfly valve  176 . Further downstream of junction  318 , a muffler  172  may be coupled to the exhaust passage  302 . After passing through the muffler  172  the exhaust gas may be released to the atmosphere through a tailpipe  335 . By locating all components of the exhaust bypass assembly downstream of the emissions control device (including catalyst)  170 , limitations due to catalyst warm-up are eliminated. As such, the first operating mode represents a first setting of the butterfly valves  176  and  178  that enables exhaust flow control. In the first operating mode, the butterfly valve  176  (coupled to the main exhaust passage  302 ) may be in closed position and the butterfly valve  178  (coupled to the bypass passage  312 ) may be in open position. It will be appreciated that various functional modes of the engine system may be possible while operating the assembly in the first operating mode, such as by varying the opening of an EGR valve. When in the first operating mode, due to closing of the first butterfly valve  176 , exhaust gas may not be able to flow through the exhaust passage  302  towards the tail pipe  335 . The exhaust gas may flow into the exhaust bypass assembly though the inlet pipe  310 , as shown by the bold arrows. The second butterfly valve  178  is in open state and the exhaust gas may pass through it and enter the bypass passage  312 . Depending on EGR requirement the EGR valve may be in one of open and closed position. When the EGR valve is in closed position, exhaust gas may not enter the EGR passage  180  and may directly enter the heat exchanger  174  located further downstream on the bypass passage  312 . In the first operating mode, the exhaust gas flows through the heat exchanger  174  in a first direction (from a first end of the heat exchanger proximal to the inlet pipe  310  to the second end of the heat exchanger  174  proximal to the outlet pipe  316 ). After passing through the heat exchanger  174 , the cooled exhaust gas flows through the outlet pipe  316  and exits the exhaust bypass assembly. The exhaust gas re-enters the exhaust passage  302  at the junction  318  and flows downstream towards the muffler  172 . After passing through the muffler  172 , the exhaust gas exits to the atmosphere through the tailpipe  335 . 
     The first operating mode can be selected when in one or more functional modes, such as a first functional mode where EGR is not required and a second where hot EGR is required for engine operations. 
     In the first functional mode, selected when EGR is not required for engine operation, the assembly may be operated in the first operating mode with the EGR valve  52  held closed. EGR may not be required when the engine temperature is below a threshold, the threshold based on a catalyst light-off temperature. In the first functional mode, the exhaust gas does not enter the EGR passage and flows downstream through the heat exchanger in the first direction. At the heat exchanger  174 , the exhaust gas may be cooled and the heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger  174 . The coolant with the heat recovered from the exhaust gas may be circulated back to the engine (under conditions when engine heating is required) and/or circulated through a heater core of the vehicle and thereafter the recovered heat may be utilized for functions such as warming the passenger cabin, thereby improving engine efficiency. Under circumstances when the heat recovered at the heat exchanger  174  is not required for heating vehicle components, the heat may be transferred to the radiator for dissipation. In the first functional mode, after passing through the heat exchanger  174 , the cooled exhaust gas flows out of the exhaust bypass assembly and exits to the atmosphere through the tailpipe  335 . 
     In the second functional mode, selected when engine temperature is above a threshold, hot EGR may be desired for engine operation and consequently the EGR valve  52  may be opened. Opening of the EGR valve may be adjusted based on the request for hot EGR, the opening increased as the request for hot EGR increases. The exhaust gas after passing through the emissions control device  170  may enter the bypass assembly though the inlet pipe  310 . The entrance to the EGR passage  180  is located on the bypass passage  132  between the second butterfly valve  178  and the heat exchanger  174 . Therefore, depending on opening of the EGR valve  52 , a first amount of the hot exhaust gas may enter the EGR passage  180  before passing through the heat exchanger  174 . After passing through the EGR valve  52 , the exhaust gas may enter the EGR passage  180  and pass through the passage  328  leading to engine intake manifold to be delivered upstream of a compressor inlet. The remaining (second) amount of the exhaust gas that does not enter the EGR passage  180  may pass through the heat exchanger  174  (in first direction) and exit the exhaust bypass assembly. Thereafter, this second amount of the exhaust gas may pass through the muffler  172  and exit to the atmosphere through tailpipe  335 . In this way, based on engine requirements, it is possible to deliver hot EGR from the exhaust passage  302  to the intake manifold through the exhaust bypass assembly. 
       FIG. 3B  shows a schematic view  300  of an example embodiment of an exhaust bypass assembly  160 . The features of the exhaust bypass assembly  160  is described in relation to  FIG. 3A . In  FIG. 3B  a second operating mode of the exhaust bypass assembly  160  is discussed as compared to the operation mode discussed in  FIG. 3A . 
     As such, the second operating mode represents a second setting of the butterfly valves  176  and  178  that enables exhaust flow control. In the second operating mode, the butterfly valve  176  (coupled to the main exhaust passage  302 ) may be in open position and the butterfly valve  178  (coupled to the bypass passage  312 ) may be in closed position. Similar to the first operating mode, various functional modes of the engine system may be possible while operating the assembly in the second operating mode, such as by varying the opening of an EGR valve. 
     When in the second operating mode, due to opening of the first butterfly valve  176  (and closing of second butterfly valve  178 ), exhaust gas may not enter the exhaust bypass assembly through the inlet pipe  310  but rather pass through the first butterfly valve  176  and continue downstream along the exhaust passage  302 . As the exhaust gas reaches the junction  318 , a first amount of the exhaust gas may enter the exhaust bypass assembly through the outlet pipe  316  (used as inlet in this mode), as shown by bold arrows. A second amount of the exhaust gas may continue downstream along the exhaust passage  302  and exit to the atmosphere through the tail pipe  335  after passing through the muffler  172 . The first amount of exhaust gas entering the bypass assembly may be controlled by the opening of the EGR valve  52 . The first amount of exhaust gas entering the exhaust bypass assembly may continue to flow through the bypass passage  312  and enter the heat exchanger  174 . In the second mode, the exhaust gas flows through the heat exchanger in a second (opposite to the first) direction (from a second end of the heat exchanger proximal to the outlet pipe  316  to the first end of the heat exchanger  174  proximal to the inlet pipe  310 ). This first amount of exhaust gas may enter the EGR passage  180 . Since the second butterfly valve  178  is closed, the exhaust gas may not pass through the bypass assembly and return to the exhaust passage  302  via the inlet pipe  310 . The second operating mode can be selected when in one or more functional modes, such as a third functional mode where cold EGR is required for engine operations. 
     In the third functional mode, cooled EGR may be desired by the engine. EGR may be required when the engine temperature is above a threshold where the threshold may be based on a catalyst light-off temperature. To deliver EGR, the EGR valve  52  may be opened, opening of the EGR valve may be increased based on EGR demand. The cooled exhaust gas on exiting the heat exchanger  174  may enter the EGR passage  180 . The exhaust gas may pass thorough the EGR valve  52  and the passage  228  to enter the engine intake manifold. 
     Heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger  174 . As previously described ( FIG. 3A ), depending on demand, the recovered heat may be transferred to the heater core for further utilization. When the engine temperature and vehicle cabin temperature is high, heat may be transferred from the heat exchanger  174  to a coolant circulating through a radiator, and heat is dissipated from the radiator to atmosphere. 
     In this way cooled EGR may be delivered to the intake manifold after passing through the heat exchanger  174 . EGR travel length from the exhaust passage to the intake manifold is short and therefore the latency of EGR delivered is low. In this way, exhaust gas may flow through the heat exchanger in both directions depending on EGR requirement and the heat recovered at the heat exchanger may be utilized in other vehicle operations. 
     Selecting between the first ( FIG. 3A ) and second operating mode ( FIG. 3B ) may be based on engine temperature. For example, when the engine temperature is below a threshold, EGR may not be desired by the engine and consequently the exhaust bypass assembly may be operated in the first mode. In another example, when the engine temperature is above a threshold, cold EGR may be desired by the engine and consequently the exhaust bypass assembly may be operated in the second mode. In this way the functions of an EGR cooler and an exhaust gas heat exchanger may be carried out via a single heat exchanger. 
     In an alternate embodiment, the two butterfly valves  176  and  178  may be actuated independently. During high load conditions, both the butterfly valves may be maintained in open position to allow exhaust gas to flow concurrently through the main exhaust passage  302  and the bypass passage  312  towards the tailpipe  335 . In this way heat may be extracted (at the heat exchanger  174 ) from a part of the exhaust gas but during high load conditions, due to high back pressure, a large amount exhaust gas may directly exit to atmosphere via the tailpipe  335  without entering the exhaust bypass assembly, thereby preventing coolant boil in the heat exchanger  174 . 
     Moving on to  FIG. 2 , example engine system  200  is an alternate embodiment of engine system  100  as shown in  FIG. 1 . Components previously introduced in  FIG. 1  are numbered similarly and not reintroduced. Similar to the embodiment of  FIG. 1 , engine system  200  of  FIG. 2  may further include control system  14  for controlling engine operations. As seen in  FIG. 2 , engine system  200  includes an exhaust bypass assembly  260  with a bypass passage  273  coupled to exhaust passage  35 , downstream of emission control device  170 . In this assembly, a single diverter valve  276  may be provided to regulate the direction of exhaust flow through the bypass passage. In one example, diverter valve  276  may be configured as a butterfly valve (herein also referred to as butterfly valve  276 ), although other valve configurations may also be used. By using a single butterfly valve, thermal mass of the exhaust bypass assembly may be reduced, thereby improving heat recovery from the exhaust gas (by a coolant) at the heat exchanger  274 . Depending on operating conditions, a portion of the exhaust residuals may be diverted through bypass passage  273  and EGR valve  52  to EGR passage  180  and the inlet of compressor  114 . The bypass passage is fitted with a split heat exchanger  274 , with two or more sections for cooling exhaust gas. The first section of the heat exchanger and the second section of the heat exchanger may be symmetrically sized. In an alternate embodiment, the heat exchanger may be asymmetrically sized wherein the first section of the heat exchanger is larger than the second section of the heat exchanger. Alternatively, the second section of the heat exchanger may be larger than the first section of the heat exchanger. 
     Depending on engine operating conditions, warm or cold EGR may be desired. The exhaust gas may flow through one or both sections of the split heat exchanger  274  in order to achieve different levels of cooling. By adjusting the position of the butterfly valve  276 , and an opening of EGR valve  52 , flow of exhaust gas through the heat exchanger can be varied. In one example, when desired EGR temperature is higher than a threshold temperature, exhaust gas may flow through the first section  413  of the split heat exchanger  274  before entering the EGR passage  180 . In another example, when required EGR temperature is lower than the threshold, exhaust gas may flow in opposite directions through both sections of the split heat exchanger  274  before entering the EGR passage  180 . The entrance to the EGR passage  180  may be located in a central region between the two sections of the split heat exchanger  274 . 
     As the exhaust gas passes through one or both sections of the heat exchanger  274 , heat from the hot exhaust gas may be transferred to a coolant circulating through both sections of the heat exchanger  274 . Upon transfer of heat from the exhaust gas to the coolant, the warmed coolant may be utilized in heating a cylinder head (such as when engine heating is requested) and/or heating a passenger cabin of the vehicle (such as when cabin heating is requested). Alternatively, when there are no heating demands, the warmed coolant may be flowed through a radiator for heat dissipation to the atmosphere. 
     In this way, engine system  10  may be adapted to provide external, low-pressure (LP) EGR by tapping exhaust gas from downstream of turbine  116 . EGR valve  52  may be opened to admit a controlled amount of cooled exhaust gas to the compressor inlet for desirable combustion. EGR valve  52  may also be configured as a continuously variable valve. In an alternate example, however, EGR valve  52  may be configured as an on/off valve. Detailed description of the operation and structure of exhaust bypass assembly  260  will be discussed with relation to  FIGS. 4A-4B, 8, 9 and 10 . In further embodiments, the engine system may be a naturally aspirated engine without the presence of a turbo-charger and compressor.  FIG. 4A  further elaborates the exhaust bypass assembly introduced in  FIG. 2  and shows a schematic view of an example embodiment  400  of an exhaust bypass assembly  260 . In one example, assembly  260  is an embodiment of assembly  260  of  FIG. 2  and therefore may share common features and/or configurations as those already described for bypass assembly  260 . Exhaust bypass assembly  260  is fluidly coupled to exhaust passage  302  downstream of an emission control device  170 . As discussed in relation to  FIG. 2 , one or more emission control devices  170  may include one or more exhaust after-treatment catalysts configured to catalytically treat the exhaust flow, and thereby reduce an amount of one or more substances in the exhaust flow. 
     Exhaust gas flowing from the engine may pass through the emission control device  170  and reach the exhaust bypass assembly  400  located further downstream along the exhaust passage  302 . An inlet pipe  410  of the exhaust bypass assembly  400  may be disposed on the exhaust passage  302  at junction  406  (downstream from emission control device  170 ). 
     A butterfly valve (valve)  276  maybe coupled to the exhaust passage  302  downstream from the inlet pipe  410 . The inlet pipe  410  may lead to a bypass passage  412  which may be parallel to the exhaust passage  302 . Depending on the direction of exhaust flow through the exhaust bypass assembly, the butterfly valve  276  may be in open or closed position. 
     Downstream of the inlet pipe  410 , across the butterfly valve  276  the bypass passage  412  may be fitted with a split heat exchanger  274 , with two or more sections for cooling exhaust gas flowing through it. The cooling sections of the heat exchanger may be symmetric or asymmetric in design. In this example, the heat exchanger  274  has two symmetric cooling sections  413  and  414  respectively. Depending on the temperature of EGR required, the exhaust gas may pass through one or both sections of the heat exchanger  274 . A coolant may be circulated through both sections of heat exchanger  274  for cooling of the exhaust gas before the gas either enters the EGR passage  180  (for delivery to engine intake manifold) or escapes to the atmosphere through tailpipe  335 . Coolant does not flow through the intermediate portion  415  between the two sections of the split heat exchanger  274 , from where the EGR passage  180  initiates. If faster and more effective cooling is required for the EGR, exhaust gas may flow through both sections of the split Heat exchanger  274 . When relatively warmer EGR is required, the exhaust gas may flow through the first section  412  of the split heat exchanger  274 . An EGR valve  52  may control the delivery of EGR from the exhaust passage  302  to the passage  328  leading to engine intake manifold. 
     Upstream of the split heat exchanger  274 , the bypass passage  412  ends in an outlet pipe  416  leading back to the exhaust passage  302 . The inlet  410  and outlet pipes  416  may be at right angles to the bypass passage  412  and the exhaust passage  302 . The outlet pipe  416  joins the exhaust passage  302  at a junction  418  located downstream from the junction  406  and butterfly valve  276 . Further downstream, a muffler  172  may be coupled to the exhaust passage  302 . After passing through the muffler  172  the exhaust gas may be released to the atmosphere through a tailpipe  335 . 
     By adjusting the position of the valves, the exhaust bypass assembly  400  may be operated in a first or a second operating mode. As such, a first operating mode represents a first setting of the butterfly valve  276  coupled to the main exhaust passage  302  that enables exhaust flow control. In the first operating mode, the butterfly valve  276  may be in open position. It will be appreciated that various functional modes of the engine system may be possible while operating the assembly in the first operating mode, such as by varying the opening of an EGR valve. When in the first operating mode, due to opening of the first butterfly valve  276 , a first amount of exhaust gas may flow from junction  406 , downstream of the exhaust catalyst  170  into the exhaust bypass via the inlet pipe  410  without flowing through the valve  276 . The first amount of exhaust may then flow through the bypass passage  412  and into the first section  413  of the split heat exchanger  274 , proximate to the inlet pipe  410 . The first amount of exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe  410  to the intermediate portion  415  of the split heat exchanger  274 ) through the first section  413  of the split heat exchanger  274 . In the first operating mode, a second amount of exhaust gas may flow from downstream of the exhaust catalyst  170  towards the tailpipe  335  through the valve  276 . At junction  406  (upstream of the muffler  172 ), the exhaust gas may flow into the exhaust bypass via the outlet pipe  416  (used for inlet in this case). The second amount of exhaust may then flow through the bypass passage  412  and into the second section  414  of the heat exchanger  274 , proximate to the outlet pipe  416 . The second amount of exhaust gas may flow in a second direction (from a second end of the heat exchanger proximal to the outlet pipe  416  to the intermediate portion  415  of the split heat exchanger  274 ) through the second section  413  of the split heat exchanger  274 . The first amount may be adjusted relative to the second amount via adjustments to the butterfly valve  276  coupled in an exhaust passage downstream of an exhaust catalyst  170 . Adjustments may be carried out by increasing an opening of the valve  276  to increase the second amount relative to the first amount, and decreasing the opening of the valve to increase the first amount relative to the second amount. 
     The first and second amounts of the exhaust gas after being cooled by one of the two sections of the split heat exchanger  274 , combine at an intermediate portion  415 . From the intermediate portion  415 , the EGR may flow through the EGR passage  180 , the EGR valve  52  and passage  428  before entering the engine intake manifold. A third amount of the exhaust gas may not enter the exhaust bypass assembly through the inlet pipe  410  and the outlet pipe  416  and this portion of exhaust gas may flow downstream through the main exhaust passage  302 . This third portion of the exhaust gas may pass through the muffler  172  and exit to the atmosphere uncooled via the tailpipe  335 . 
     The first operating mode can be selected when in one or more functional modes, such as a first functional mode where cold EGR is required for engine operations. 
     In the first functional mode, when cold EGR is required for engine operations, the EGR valve  52  and intake throttle may be held open. The opening of the EGR valve may be increased responsive to an increase in engine load or a decrease in engine temperature, and opening of the EGR valve may be decreased responsive to decrease in engine load or an increase in engine temperature. As exhaust gas flows through both sections of the split heat exchanger  274 , exhaust gas may be cooled and the heat from the exhaust gas may be transferred to a coolant. The coolant may be circulated through the heater core of the vehicle and thereafter the recovered heat from the exhaust gas may be utilized for functions such as heating the cylinder head, and warming the passenger cabin, thereby improving engine efficiency. Under circumstances when the heat recovered at the split heat exchanger  274  is not required for heating vehicle components, the heat may be transferred to the radiator for dissipation to atmosphere. 
     In this way, by using both the sections of the split heat exchanger  274 , it is possible to cool the exhaust gas more effectively and to a low temperature. Also, in this operational and functional mode coolant from both the sections of the split heat exchanger  274  may be used to recover heat from the exhaust gas which may be utilized in other vehicle operations. Also, exhaust gas may be delivered from the exhaust passage  302  to engine intake manifold from two takeoff locations on the exhaust passage, thereby improving EGR flow distribution. 
       FIG. 4B  shows a schematic view of an example embodiment of an exhaust bypass assembly  400 . The features of the exhaust bypass assembly  400  is described in relation to  FIG. 4A . In  FIG. 4B , a second operating mode of the exhaust bypass assembly  160  is discussed as compared to the first operating mode discussed in  FIG. 4A . 
     As such, the second operating mode represents a second setting of the butterfly valve  276  that enables exhaust flow control. In the second operating mode, the butterfly valve  276  may be in closed position. Similar to the first operating mode, various functional modes of the engine system may be possible while operating the assembly in the second operating mode. 
     When in the second operating mode, due to the closing of the butterfly valve  276  the entire exhaust gas may enter the exhaust bypass assembly through the inlet pipe  410  and may continue to the bypass passage  412  and the first cooler section  413  of the split heat exchanger  274 . The exhaust gas may flow in the first direction (from a first end of the heat exchanger proximal to the inlet pipe  410  to the intermediate portion  415  of the split heat exchanger  274 ) through the first section of the split heat exchanger  274 . Therefore after passing through the first cooler section  413 , warm exhaust gas may enter the EGR passage  180 . A part of the exhaust gas may not enter the EGR passage  180  and may pass through both the cooler sections  413  and  414  (in first direction, from first end of the cooler proximal to the inlet pipe  410  to the second end of the cooler proximal to the outlet pipe  416 ) of the split heat exchanger  274  and exit the exhaust bypass assembly through the outlet pipe  416 . Further, this portion of the exhaust gas may pass through the muffler  172  and exit to the atmosphere through the tail pipe  335 . The second operating mode can be selected when in one or more functional modes, such as a second functional mode where warm EGR is required for engine operations. 
     In the second functional mode, warm (cooled to a lesser degree) EGR may be desired for engine operations. Consequently, the EGR valve  52  may be opened, opening of the EGR valve may be increased based on EGR demand. After flowing through the first cooler portion  413  of the split heat exchanger  274 , exhaust gas may enter the EGR passage  180  and pass through the EGR valve  52  the passage  428  before entering the engine intake manifold. At the cooler portion  413 , heat from the exhaust gas may be transferred to a coolant and as previously described ( FIG. 4A ), the recovered heat may be transferred (by circulating coolant) to the heater core for further utilization and/or to the radiator for dissipation. 
     In one example, coolant temperature may be used to determine selection between the two operating modes of the exhaust bypass assembly. If the coolant temperature is above a threshold temperature, first operating mode may not be used as higher cooling load may cause coolant boil. Also, if heat requirement from vehicle cabin and/or other parts of the vehicle is low, second operating mode may be used where less heat is recovered from the EGR (EGR passing through one portion of split heat exchanger  274 ). 
     In another example, in a third functional mode, EGR may not be required by the engine e.g., during cold start. In that case depending on coolant temperature and heat requirement by the vehicle (such as cabin heating requirement) the exhaust bypass assembly may be operated in one of first and second operating mode. If coolant temperature is high and/or heat requirement of the vehicle is low, the first operating mode may be used. In the first operating mode, the butterfly valve  276  is in open position and in this case the EGR valve is closed, a large part of the exhaust gas may not enter the exhaust bypass assembly and directly flow downstream towards the tailpipe  335  (for exiting to atmosphere) through the exhaust passage  302 . A small part of the exhaust gas may flow into the exhaust bypass assembly through the inlet pipe  410  and pass though both sections of the split heat exchanger  274  before returning to the exhaust passage via the outlet pipe  416 . If coolant temperature is low and/or heat requirement of the vehicle is high, the second operating mode may be used. In the second operating mode, the butterfly valve  276  is in closed position and in this case the EGR valve is also closed. The exhaust gas may not flow downstream through the exhaust passage  302  past the closed butterfly valve  276 . The entire exhaust gas may flow into the exhaust bypass assembly through the inlet pipe  410  and pass through both sections of the split heat exchanger  274  before returning to the exhaust passage via the outlet pipe  416 . In this way heat may be recovered by a coolant at both sections of the split heat exchanger which may be utilized for aforementioned purposes. 
     Under high load conditions, the butterfly valve  276  on the exhaust passage  302  may be maintained in open position. Under such conditions, due to high back pressure, a large amount exhaust gas may directly exit to atmosphere via the tailpipe  335  without entering the exhaust bypass assembly, thereby preventing coolant boil in the heat exchanger  274 . 
       FIGS. 1, 2, 3A-3B and 4A-4B  show example configurations of exhaust bypass assembly with relative positioning of the various components. If shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. 
       FIG. 5  illustrates an example method  500  that may be implemented for adjusting exhaust flow through the exhaust bypass assembly of  FIG. 1 . Instructions for carrying out method  500  and the rest of the methods included herein may be executed by a controller based on instructions stored on a memory of the controller and in conjunction with signals received from sensors of the engine system, such as the sensors described above with reference to  FIGS. 1 and 2 . The controller may employ engine actuators of the engine system to adjust engine operation, according to the methods described below. 
     At  502 , the routine includes estimating and/or measuring current engine operating conditions. Conditions assessed may include, for example, engine temperature, engine load, engine speed, manifold vacuum, throttle position, exhaust pressure, exhaust air/fuel ratio, etc. 
     At  504 , the routine includes determining if the vehicle engine is operating under cold start conditions. An engine cold-start condition may be confirmed when the engine is started after a prolonged period of engine inactivity, when the engine temperature is lower than a threshold (such as below an exhaust catalyst light-off temperature), and while ambient temperatures are below a threshold. 
     During cold start conditions, exhaust gas recirculation (EGR) may not be desired by the engine. Thus, if engine cold start conditions are confirmed, the routine moves to  508  in order to operate the exhaust bypass system in the first operating mode. Operating in the first mode, as described in relation to  FIG. 3A , includes moving the first butterfly valve or valve_ 1  (such as first butterfly valve  176  in  FIG. 3A ) located on the exhaust passage to a closed position while moving the second butterfly valve or valve_ 2  (such as second butterfly valve  178  in  FIG. 3A ) located in the bypass passage to an open position. In addition, since EGR is not desired during the engine cold-start, at  510 , the EGR valve may be closed, disabling exhaust gas flow from the exhaust passage to the engine intake manifold. 
     At  512 , by setting the exhaust bypass assembly to the first operating mode, exhaust gas may be flowed from downstream of an exhaust catalyst into the exhaust bypass, and then through a first end of the heat exchanger proximate to the exhaust catalyst, to a second end of the heat exchanger, proximate to the tailpipe. Said another way, exhaust gas may enter the exhaust bypass assembly through an inlet (first) pipe (such as pipe  310  in  FIG. 3A ) and flow through the bypass passage towards a heat exchanger (such as heat exchanger  174  in  FIG. 3A ), and then on to the atmosphere via the exhaust tailpipe. The exhaust gas may flow through the heat exchanger in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to the second end of the heat exchanger proximal to the outlet pipe). Since the EGR valve is closed, exhaust gas may not flow through an EGR passage towards the engine intake manifold. At  514 , waste heat is recovered at the heat exchanger. Specifically, heat from the exhaust gas is transferred to a coolant circulating through the heat exchanger. By transferring the heat from the exhaust gas to the coolant, at a location downstream of the exhaust catalyst, exhaust heat remaining after heating the exhaust catalyst can be advantageously used to warm the coolant circulating through the heat exchanger. As a result, the exhaust gas leaving the heat exchanger is cooled down. 
     At  516 , the coolant warmed via heat transfer at the heat exchanger may be circulated to a heater core so that it can be utilized for heating other components of the vehicle during the engine cold-start, such as a cylinder head, an engine block, and a vehicle cabin space. At  518 , the method includes transferring heat from the heater core to one or more vehicle components based on vehicle heating demands. In case of the cold start, the vehicle cabin temperature may be low and cabin heating may be required. Thus, based on cabin heating demand, as requested by a vehicle operator (e.g., based on a cabin temperature setting), heat may be transferred from the heater core to the cabin. For example, air may be drawn into the cabin via the heater core, thereby enabling the cabin to be warmed. After meeting the cabin heating demand, the warmed coolant may also be circulated to an engine block and cylinder head to raise engine temperatures, thereby improving engine performance during cold conditions. 
     Returning to  506 , if cold start conditions are not confirmed, and the vehicle engine is sufficiently warm, the routine moves to  520 . At  520 , the routine includes determining if hot EGR is desired based on engine operating conditions including engine temperature. In one example, if the engine temperature is above a threshold, it may be determined that hot EGR is desired by the engine. Accordingly, at  522 , the exhaust bypass system may be operated in the second operating mode, as described with reference to  FIG. 3B . Specifically, the first butterfly valve or valve_ 1  (such as first butterfly valve  176  in  FIG. 3A ) located on the exhaust passage may be held in the closed position while the second butterfly valve or valve_ 2  (such as second butterfly valve  178  in  FIG. 3A ) located in the bypass passage may be held in the open position. 
     At  524 , the routine includes determining an amount of hot EGR required. The amount of hot EGR desired by the engine may be dependent on engine operating conditions such as engine load, engine temperature, engine speed, etc. Based on the hot EGR requirement, at  526 , the method includes adjusting an opening of the EGR valve to admit a controlled amount of hot exhaust gas to the intake manifold for desirable combustion and emissions-control performance. In one example, where the EGR valve is a continuously variable valve, a degree of opening of the valve may be increased. In an alternate example, where the EGR valve is configured as an on/off valve, the EGR valve may be transitioned to an open position. Upon opening the EGR valve, at  528 , the method includes flowing exhaust gas from the exhaust manifold into the EGR passage, via the exhaust bypass assembly, upstream of and while bypassing the heat exchanger. In this way low-pressure hot exhaust gas may be delivered from downstream of the turbine in the exhaust passage to the engine intake manifold, without passing through an EGR cooler. It will be appreciated that, based on intake manifold vacuum and exhaust manifold pressure values, a small portion of the exhaust gas may continue downstream through the bypass passage, passing though the heat exchanger, the outlet pipe and exiting to the atmosphere though the tailpipe. 
     Returning to  520 , if hot EGR is not required, it may be inferred that conditions for providing cooled EGR are present. As such, cooled EGR may be provided over a wide range of warmed up engine operating conditions so as to reduce NOx emissions and improve fuel economy. If cooled EGR conditions are confirmed, at  530 , the method includes operating the exhaust bypass system in the second operating mode, as described with reference to  FIG. 3B . In particular, the first butterfly valve or valve_ 1  (such as first butterfly valve  176  in  FIG. 3B ) located in the exhaust passage may be transitioned to an open position while the second butterfly valve or valve_ 2  (such as second butterfly valve  178  in  FIG. 3B ) located in the bypass passage may be shifted to a closed position. 
     At  532 , the routine includes determining a cooled LP-EGR requirement of the engine. As an example, the controller may retrieve the value from a look-up table referenced by engine speed and load. Based on the determined EGR requirement of the engine, at  534 , the method includes increasing the opening of the EGR valve=to admit a controlled amount of cold exhaust gas to the intake manifold for desirable combustion and emissions-control performance. For example, the controller may send a signal to actuate an actuator coupled to the EGR valve, the actuator moving the EGR valve towards the open position. By operating the exhaust bypass assembly in the second mode, at  536 , the method includes flowing exhaust gas from downstream of the exhaust catalyst towards the tailpipe along the main exhaust passage, and then flowing from upstream of the tailpipe exhaust into the second end of the heat exchanger, then through the first end of the heat exchanger to the engine intake via an EGR passage. The exhaust gas may flow downstream through the exhaust passage past the first butterfly valve and enter the exhaust bypass assembly through a second pipe (such as pipe  310  in  FIG. 3B ) situated downstream from the first pipe. In this example, in the second mode, exhaust gas may be flowed across the heat exchanger in a second direction (from a second end of the heat exchanger proximal to the outlet pipe to the first end of the heat exchanger proximal to the inlet pipe). In this way, depending on operating mode, exhaust gas can flow in both a forward and a backward direction through the heat exchanger. As the exhaust gas passes through the heat exchanger, the heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger. By transferring the heat from the exhaust gas to the coolant, waste exhaust heat is recovered and can be used to meet other vehicle heating requirements, such as cabin heating. 
     Opening of the pair of butterfly valves and the EGR valve may be regulated to control the flow of exhaust though the bypass passage and the heat exchanger. A portion of exhaust may not enter the exhaust bypass assembly and may flow downstream through the main exhaust passage and may exit to the atmosphere though the tailpipe. 
     Since cold EGR is required by the engine, the EGR valve may be maintained in open position. Moving on to  538 , after passing through the heat exchanger the cooled exhaust gas may enter the EGR delivery passage leading to the engine intake manifold. In this way low-pressure cold exhaust gas may be delivered from downstream of a turbine in the exhaust passage to the engine intake manifold. 
     At  540 , the heat recovered by the coolant from the exhaust gas at heat exchanger may be utilized for heating other components of the vehicle. For example, the warmed coolant containing the thermal energy from the exhaust gas may be circulated from the heat exchanger to a vehicle heater core, wherefrom the heat is transferred to air flowing through the heater core for cabin heating. If heat is not required by any component of the vehicle, for example if the vehicle cabin temperature is sufficiently high and there is no further demand for cabin heating, the coolant may be circulated through a vehicle radiator wherefrom the heat may be dissipated to the atmosphere. In this way a single heat exchanger may be used for cooling EGR and extracting heat from exhaust gas. 
       FIG. 6  is a table  600  showing example modes of operation of the engine and the heat exchange system (exhaust bypass system) of  FIG. 1 . An engine controller may select one of the operating modes (functional mode) based on engine conditions and heating requirements. Based on the selected operating mode, the position of an exhaust gas recirculation (EGR) valve and dual (coupled) butterfly valves (valve system) may be regulated to control the flow of exhaust gas through a heat exchanger and/or through an EGR delivery passage. The exhaust valve system comprises the dual butterfly valves (as described in  FIG. 1 ) and may be operated in one of first and second modes. In the first valve system mode, the butterfly valve_ 1  (coupled to the main exhaust passage) may be in closed position and the butterfly valve_ 2  (coupled to the bypass passage) may be in open position. In the second valve system mode, the butterfly valve_ 1  may be in open position and the butterfly valve_ 2  may be in closed position. 
     In one example, the controller may operate the EGR valve and exhaust valve system in a first engine operating mode such as during a cold start condition when the engine temperature and vehicle temperature is low. During the low engine temperature (below threshold) conditions, EGR may not be required in the intake manifold. Therefore in this mode the EGR valve may be in closed position. In the first engine operating mode, the exhaust valve system may be operated in the first mode with the first butterfly valve (valve_ 1 ) closed and the second butterfly valve (valve_ 2 ) open. In the first operating mode, exhaust gas from the exhaust passage may enter the exhaust bypass assembly through an inlet (first) pipe and flow through the bypass passage towards a heat exchanger. The exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to the second end of the heat exchanger proximal to the outlet pipe). Since the EGR valve is closed, exhaust gas may flow through the heat exchanger while bypassing an EGR passage. Said another way, flowing exhaust gas in a first direction through the heat exchanger includes exhaust gas entering the Heat exchanger at a first end and exiting at a second, opposite end. The heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger. After passing through the heat exchanger the exhaust gas may return to the exhaust passage via an outlet (second) pipe and exit to the atmosphere through a tail pipe. Heat recovered from the exhaust gas at the heat exchanger may be circulated through the vehicle heater core for utilization in vehicle components. In one example, heat recovered from the exhaust gas may be used to heat the passenger cabin during such cold start conditions. 
     In another example, the controller may operate the EGR valve and exhaust valve system in a second engine operating mode when EGR is not required by the engine, such as at high engine load, conditions. During this time, the engine and the vehicle passenger cabin temperature may be high and transfer of heat from the exhaust gas to the heater core may not be required. In this second engine operating mode, the EGR valve may be in maintained in closed position. The valve system may also be operated in the first mode with valve_ 1  closed and the valve_ 2  open. Exhaust gas from the exhaust passage may enter the exhaust bypass assembly through the first pipe and flow through the bypass passage towards a heat exchanger. The exhaust gas may flow in the first direction through the heat exchanger. Since EGR is not required by the engine and the EGR valve is closed, exhaust gas may not flow through an EGR passage towards the engine intake manifold. The heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger. After passing through the heat exchanger the exhaust gas may return to the exhaust passage via a second pipe and exit to the atmosphere through a tail pipe. Since heat is not required in heating of any vehicle component, the coolant containing the heat may be circulated through a vehicle radiator for dissipation to the atmosphere. 
     In yet another example, the controller may operate the EGR valve and exhaust valve system in a third engine operating mode when hot EGR may be desired for engine operations such as at low engine temperature and low engine load conditions. Therefore in this mode, the EGR valve may be actuated to an open position. The EGR valve opening may be regulated to admit a controlled amount of hot exhaust gas to the intake manifold. The valve system may be operated in the first mode with valve_ 1  closed and the valve_ 2  open. Exhaust gas from the exhaust passage may enter the exhaust bypass assembly through the first pipe and flow through the bypass passage towards the entrance of the EGR passage. Depending on the extent of EGR valve opening, a first amount of hot exhaust gas may flow through the EGR passage and the EGR valve towards the engine intake manifold. In this mode, exhaust gas entering the EGR passage (first amount) does not flow through the heat exchanger. Hence, during supply of hot EGR from the exhaust passage to the intake manifold, heat may not be recovered from the first amount of exhaust gas (at the heat exchanger). A second amount of exhaust gas may not enter though the EGR passage and may flow downstream though the heat exchanger (in first direction) before exiting the exhaust bypass assembly. The heat from the second amount of exhaust gas may be transferred to a coolant circulating through the heat exchanger. Depending on requirements, the heat can be either transferred to one of the heater core and the radiator. 
     In a further example, the controller may operate the EGR valve and exhaust valve system in a fourth engine operating mode when cold EGR may be desired for engine operations such as at high engine temperature and low engine load conditions. Therefore in this mode, the EGR valve may be maintained in open position. The EGR valve opening may be regulated to admit a controlled amount of cold exhaust gas to the intake manifold. The valve system may be operated in the second mode with valve_ 1  open and the valve_ 2  closed. The opening of the butterfly valves and the EGR valve may be regulated to determine the amount of exhaust gas entering the bypass assembly compared to the amount of exhaust gas flowing downstream through the main exhaust passage. A first amount of exhaust gas from the exhaust passage may enter the exhaust bypass assembly through the second pipe and flow through the bypass passage towards the heat exchanger. The exhaust gas may flow in a second direction (from a second end of the heat exchanger proximal to the outlet pipe to the first end of the heat exchanger proximal to the inlet pipe) through the heat exchanger. After flowing through the heat exchanger, the cold exhaust gas may flow through the EGR passage and the EGR valve towards the engine intake manifold. The heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger. Based on demand, heat recovered from the exhaust gas at the heat exchanger may be either circulated through the vehicle heater core for utilization in other vehicle components or transferred to a vehicle radiator for dissipation to atmosphere. A second amount of the exhaust gas may not enter the exhaust bypass assembly but may flow downstream through the exhaust passage towards the tailpipe for exiting to the atmosphere. 
     In this way, dual butterfly valves may be used to regulate exhaust gas flow in opposite directions through the heat exchanger. A single actuator may be used to control the operation of the dual butterfly valves. A single heat exchanger may be effectively used for EGR cooling and heat recovery of heat from the exhaust gas. 
       FIG. 7  shows an example operating sequence  700  illustrating operation of the exhaust bypass assembly of  FIG. 1 . The direction of exhaust gas flow through a heat exchanger and exhaust gas recirculation (EGR) delivery is determined based on engine operations. The horizontal (x-axis) denotes time and the vertical markers t 1 -t 3  identify significant times in the operation of the exhaust bypass assembly system. 
     The first plot (line  702 ) from the top shows variation in engine speed over time. The second plot (line  704 ) shows EGR requirement by the engine for desired combustion and emissions-control performance. EGR requirement may depend on engine operating conditions e.g. engine load, engine speed, engine temperature etc. The third plot (line  706 ) shows variation in vehicle cabin temperature over time. The fourth plot (line  708 ) indicates the opening or closing of a first butterfly valve (valve  1 ) located on the exhaust passage. The fifth plot (line  710 ) indicates opening or closing of a second butterfly valve (valve  2 ) located on the exhaust bypass assembly. The two butterfly valves may be controlled by a single actuator. The sixth and final plot (line  712 ) shows the variation of radiator fan speed over time. Higher the radiator fan speed, greater is the heat dissipated from the radiator to the atmosphere. 
     Prior to time t 1 , the engine starts from rest after a period of inactivity during which the vehicle was not propelled using the engine. The engine may start under cold start conditions with low engine temperature. As seen from the first plot (line  702 ), as the engine starts, the engine speed is low and the engine speed gradually increases to a steady level. During cold start condition, EGR may not be desired by the engine, thereby plot  704  shows little or no EGR flow from the exhaust passage to the engine intake manifold. As the vehicle starts up, the vehicle cabin temperature may be low as the cabin heating may not have been operational prior to engine start-up. The vehicle cabin temperature may gradually increase over time as heat is supplied to the cabin from the vehicle heater core. During this period, as EGR is not desired for engine operations, valve  1  (butterfly valve coupled to the main exhaust passage) may be maintained in closed state and valve  2  (butterfly valve coupled to the exhaust bypass passage) may be maintained in open state. In this mode (first mode of operation as described in relation to  FIG. 3A ), exhaust gas from the exhaust passage, after passing through an emission control device may enter an exhaust bypass assembly. The EGR valve is in closed position preventing any exhaust gas from entering the exhaust delivery passage. A heat exchanger may be coupled to an exhaust bypass passage. As the exhaust gas passes through the heat exchanger, heat from the exhaust gas may be transferred from the exhaust gas to a coolant circulating through the heat exchanger. After passing through the heat exchanger, the cooled exhaust gas may exit the exhaust bypass assembly. The exhaust gas may pass through a muffler coupled to the exhaust passage and exit to the atmosphere through an exhaust tailpipe. 
     The heat recovered from the exhaust gas at the heat exchanger may be utilized for providing heating for the vehicle cabin and/or a plurality of vehicle components e.g., cylinder head. The coolant from the heat exchanger may be circulated from the heat exchanger to the vehicle heater core for utilization in heating different vehicle components. In this example, since prior to t 1 , the vehicle cabin temperature is low and the heat recovered from the exhaust gas may be utilized in heating the cabin to a comfortable temperature as desired by the user. Heat recovered from the exhaust gas (at the heat exchanger) if not required for heating any vehicle component may also be transferred to the vehicle radiator for dissipation to atmosphere. Since during this time period the vehicle cabin may desire heating, heat recovered from the heat exchanger may be entirely transferred to the heater core for utilization in cabin heating. Little or no heat may be dissipated to the atmosphere via the radiator. Thereby, the radiator fan speed may be low prior to time t 1 . 
     After time t 1 , the engine speed is high and EGR may not be desired by the engine. Thereby, between time t 1  and t 2 , the EGR valve may be maintained in closed position and there may be little or no EGR flow from the exhaust passage to the engine intake manifold. During this time period, valve  1  may be continued to be maintained in closed position and valve  2  may be in open position. As seen for time prior to t 1 , during this mode, exhaust gas from the exhaust passage may enter an exhaust bypass assembly and flow through a heat exchanger in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to the second end of the heat exchanger proximal to the outlet pipe). Heat from the exhaust gas may be transferred to a coolant circulating through the heat exchanger. After passing through the heat exchanger, the cooled exhaust gas may return to the exhaust passage and may exit to the atmosphere through an exhaust tailpipe. 
     During this time period, the cabin temperature may not have stabilized to a comfortable level, hence heat may be continued to be required for cabin heating purposes. Heat recovered from the exhaust gas by a coolant at the heat exchanger may be transferred to the heater core for utilization in heating the cabin and/or other vehicle components. During this period, heat recovered at the heat exchanger may be mostly transferred to the heater core (for cabin heating) and not to the radiator (for dissipation to atmosphere) and therefore the radiator fan speed may be low. 
     Over time, the vehicle cabin temperature may stabilize (line  706 ) to a level desired by the operator and heat requirement for cabin heating may decrease. In this example, close to time t 2 , the entire heat recovered from the exhaust gas may not be required by the heater core for heating vehicle components. A significant part of the heat may be transferred to the radiator for dissipation to the atmosphere. 
     At time t 2 , there is a decrease in engine speed. This may be due to lower engine load. At this point, cold EGR may be desired by the engine. In order to deliver cold EGR from the exhaust passage to the engine intake manifold, valve  1  may be opened and valve  2  may be closed. In this mode (second mode of operation as described in relation to  FIG. 3A ), exhaust gas from the exhaust passage, after passing through an emission control device may continue to flow past valve  1  and enter the exhaust bypass assembly downstream of the valve  1 . During this time period, between t 2  and t 3 , the EGR valve is in open position. After passing through the heat exchanger in a second direction (from a second end of the heat exchanger proximal to the outlet pipe to the first end of the heat exchanger proximal to the inlet pipe), the cooled exhaust gas may enter the EGR delivery passage through the EGR valve. As seen from plot  704 , during this time period, the EGR flow into the intake engine manifold increases. In this way cold EGR may be delivered from the exhaust passage to the engine intake manifold. 
     During this time, the vehicle cabin temperature has stabilized (line  706 ) to a level desired by the operator and the entire heat recovered from the exhaust gas (at the heat exchanger) may not be required for cabin heating. The majority of the heat recovered by a coolant from the exhaust gas may be transferred to the radiator for dissipation to atmosphere. Thereby, as seen from line  712 , the radiator speed may be high during this time period. 
     At time t 3 , the engine speed continues to vary with time. At this stage, based on engine temperature hot EGR may be desired for engine operation. In order to deliver hot EGR from the exhaust passage to the engine intake manifold, the valve  1  may be closed, the valve  2  may be opened and the EGR valve may be maintained in open position. During this mode, exhaust gas from the exhaust passage may enter an exhaust bypass assembly. Instead of flowing through the heat exchanger, the hot exhaust gas may enter the EGR passage passing through the EGR valve and may be delivered to the engine intake manifold. Since hot EGR is required, majority of the exhaust gas is not made to pass through the heat exchanger prior to delivery to the intake manifold. Thereby, substantial amount of heat may be retained in the EGR. During this period, since no heat is transferred to the radiator for dissipation, the radiator fan speed is close to zero. 
     In this way, depending on engine operating conditions, cold or hot EGR may be delivered from the exhaust passage to the engine intake manifold. Exhaust gas may flow in opposite directions through the heat exchanger and depending on demand, heat recovered from the exhaust gas at the heat exchanger may be either transferred to the vehicle heater core to the radiator. A single cooler may be used to cool EGR and recover the heat energy from the exhaust gas. 
       FIG. 8  illustrates an example method  800  that may be implemented for flowing exhaust gas recirculation (EGR) through the exhaust bypass assembly of  FIG. 2  in a forward or backward direction so as to provide waste heat recovery. At  802 , the routine includes estimating and/or measuring current engine operating conditions. Engine conditions assessed may include engine temperature, engine load, engine speed, manifold vacuum, throttle position, exhaust pressure, exhaust air/fuel ratio, etc. 
     At  804 , the routine includes determining EGR requirement by the engine. The amount of EGR desired by the engine may be dependent on engine operating conditions e.g., engine load, engine temperature, engine speed etc. Based on EGR requirement by the engine, at  806 , the EGR valve opening may be adjusted to admit a controlled amount of exhaust gas to the intake manifold for desirable combustion and emissions-control performance. EGR valve may be a continuously variable valve. In an alternate example, however, EGR valve may be configured as an on/off valve. 
     At  808 , the routine includes determining the current temperature of the EGR and comparing the current temperature to a desired EGR temperature. Depending on the temperature of the desired EGR, a butterfly valve (such as first butterfly valve  276  in  FIG. 4A ) regulating the entrance to the exhaust bypass assembly may be opened or closed in order to either flow exhaust gas through both sections of the split heat exchanger for effective and faster cooling of the exhaust gas or flow the exhaust gas through a single portion of the split heat exchanger to partially cool the exhaust gas before delivery to the intake manifold. 
     If it is determined that the current temperature of the EGR is higher than the desired temperature, the routine moves on to  810 , where the exhaust bypass system may be operated in a first operating mode. Operating in the first mode, as described in relation to  FIG. 4A , includes moving the butterfly valve (such as first butterfly valve  276  in  FIG. 4A ) located on the exhaust passage to an open position. At  812 , by setting the exhaust bypass assembly to the first operating mode, a first amount of the exhaust gas may be entered into the exhaust bypass assembly through an inlet (first) pipe (such as pipe  410  in  FIG. 4A ) and flowed through the bypass passage towards a split heat exchanger (such as heat exchanger  274  in  FIG. 4A ). In this example, the first amount of exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to the intermediate portion of the split heat exchanger) through the first section of the split heat exchanger. As the butterfly valve on the exhaust passage is in open position, a second amount of the exhaust gas may continue downstream along the exhaust passage and may enter the exhaust bypass assembly through a second pipe (such as pipe  416  in  FIG. 4A ). The second amount of exhaust gas may flow in a second direction (from a second end of the heat exchanger proximal to the outlet pipe to the intermediate portion of the split heat exchanger) through the second section of the split heat exchanger. The first and second amount of exhaust gas may enter the EGR delivery passage from the intermediate section of the split heat exchanger. 
     In this way the two amounts of the exhaust gas may flow through the split heat exchanger in opposite directions. In the first operating mode, as both the sections of the split heat exchanger are used simultaneously, the exhaust gas is efficiently cooled to a lower temperature before delivery to the engine intake manifold. The opening of the EGR valve and the butterfly valve may regulate the first and second amounts of exhaust gas entering the bypass assembly. A third amount of exhaust gas may not enter the bypass assembly and my flow downstream through the main exhaust passage towards the tailpipe. 
     At  808 , if it is determined that the current temperature of the EGR is lower than the desired EGR temperature, the routine moves on to  818 , where the exhaust bypass system may be operated in a second operating mode. Operating in the second mode, as described in relation to  FIG. 4B , includes moving the butterfly valve located on the exhaust passage to a closed position. In the second operating mode, since the butterfly valve is in closed position, at  820 , the entire volume of exhaust gas may enter the exhaust bypass assembly through the first pipe and flow through the bypass passage towards a split heat exchanger. The entire volume of exhaust gas may flow in the first direction (from a first end of the heat exchanger proximal to the inlet pipe to the intermediate portion of the split heat exchanger) through the first section of the split heat exchanger before entering the EGR delivery passage. Compared to using both sections of the heat exchanger in the first mode, in the second operating mode, the exhaust gas passes through the first half of the split heat exchanger. Hence in the second operating mode, the exhaust gas is cooled to a lower extent compared to the extent of cooling achieved in the first mode, before delivery to the engine intake manifold. Depending on the EGR valve opening, a part of the exhaust gas entering the bypass passage may not enter the EGR delivery passage but rather flow across both sections of the heat exchanger (from left to right) towards the main exhaust passage and to the tailpipe. 
     At  814 , waste heat from the exhaust gas may be recovered at one or both sections of the split heat exchanger. Heat from the exhaust gas is transferred to a coolant circulating through both sections of the heat exchanger. By transferring the heat from the exhaust gas to the coolant, the exhaust gas leaving the split heat exchanger and entering the EGR passage is cooled down. 
     At  816 , the coolant warmed via heat transfer at the heat exchanger may be circulated to a heater core so that it can be utilized for heating other components of the vehicle during the engine cold-start, such as a cylinder head, an engine block, and a vehicle cabin space. If heat is not required by any component of the vehicle, for example if the vehicle cabin temperature is sufficiently high and there is no further demand for cabin heating, the coolant may be circulated through a vehicle radiator wherefrom the heat may be dissipated to the atmosphere. In this way a single split heat exchanger may be used for cooling EGR and extracting heat from exhaust gas. 
       FIG. 9  is a table  900  showing example modes of operation of the engine and the heat exchange system (exhaust bypass system) of  FIG. 2 . An engine controller may select one of the operating modes (functional mode) based on engine conditions. Based on the selected operating mode, the position of an exhaust gas recirculation (EGR) valve and the butterfly valve (coupled to the main exhaust passage) may be regulated to control the flow of exhaust gas through one or both sections of a split heat exchanger and/or through an EGR delivery passage. 
     In one example, the controller may operate the EGR valve and the butterfly in a first engine operating mode when cold EGR may be desired for engine operations such as at high engine temperature and low engine load conditions. In this operating mode, the EGR valve may be in open position and the butterfly valve coupled to the exhaust passage may also be in open position. The EGR valve opening may be regulated to admit a controlled amount of cold exhaust gas to the intake manifold. A first amount of the exhaust gas may enter the exhaust bypass assembly through an inlet (first) pipe and flow through the bypass passage towards a first section of a split heat exchanger. The first amount of exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to the intermediate portion of the split heat exchanger) through the first section of the split heat exchanger. As the butterfly valve on the exhaust passage is open, a second amount of the exhaust gas may continue to flow downstream along the main exhaust passage and may enter the exhaust bypass assembly through a second pipe. The second amount of exhaust gas may flow in a second direction (from a second end of the heat exchanger proximal to the outlet pipe to the intermediate portion of the split heat exchanger) through the second section of the split heat exchanger. The first and second amount of exhaust gas may enter the EGR delivery passage from the intermediate section of the split heat exchanger. In this way the two parts of the exhaust gas may flow through the split heat exchanger in opposite directions. Both sections of the EGR are utilized to effectively cool the exhaust gas to a lower temperature before delivery to the intake manifold. The opening of the EGR valve and the butterfly valve may regulate the first and second amounts of exhaust gas entering the bypass assembly. A third amount of exhaust gas may not enter the bypass assembly and my flow downstream through the main exhaust passage towards the tailpipe. 
     In another example, the controller may operate the EGR valve and exhaust valve system in a second engine operating mode when, warm (compared to temperature of EGR desired in the first mode) EGR may be desired by for engine operations. In this operating mode, the EGR valve may be maintained in open position and the butterfly valve coupled to the exhaust passage may be actuated to closed position. Consequently, the entire volume of exhaust gas may enter the exhaust bypass assembly through the first pipe and flow through the bypass passage towards the split heat exchanger. The entire volume of exhaust gas may flow in the first direction (from a first end of the heat exchanger proximal to the inlet pipe to the intermediate portion of the split heat exchanger) through the first section of the split heat exchanger before entering the EGR delivery passage. Compared to using both sections of the heat exchanger in the first engine operating mode, in the second operating mode, the exhaust gas passes through the first section of the split Heat exchanger. Hence in the second operating mode, the exhaust gas may be cooled to a lower extent compared to the extent of cooling achieved in the first operating mode, providing warm EGR to the engine intake manifold. Depending on the EGR valve opening, a part of the exhaust gas entering the bypass passage may not enter the EGR delivery passage but rather flow across both sections of the heat exchanger in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to a second end of the heat exchanger proximal to the outlet pipe) and flow towards the main exhaust passage and to the tailpipe. 
     In yet another example, the controller may operate the EGR valve and exhaust valve system in a third engine operating mode when EGR is not required by the engine, such as during high engine load conditions. In the third operating mode, the EGR valve may be actuated to closed position and the butterfly valve coupled to the exhaust passage may be maintained in closed position. Consequently, the entire volume of exhaust gas may enter the exhaust bypass assembly through the inlet (first) pipe and flow through the bypass passage towards the split heat exchanger. In this example, the entire volume of exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to a second end of the heat exchanger proximal to the outlet pipe) through the both first and second sections of the split heat exchanger before exiting the bypass assembly through the outlet (second) pipe. Since the EGR valve is closed exhaust gas may not enter the EGR delivery passage. The cooled exhaust gas may exit to the atmosphere through the exhaust tail pipe. Alternatively, when EGR is not desired by the engine, the butterfly valve opening may be regulated to control the amount of exhaust gas entering the bypass passage compared to the amount of exhaust gas flowing downstream though the main exhaust passage towards the tailpipe. In case of high load and/or during high coolant temperature at the heat exchanger, a large part of the exhaust gas may be prevented from entering the exhaust bypass thereby preventing coolant boil. 
     In this way, a single heat exchanger may be effectively used for cooling EGR and for recovery of heat from the exhaust gas. During all three engine operation modes, based on demand, heat recovered from the exhaust gas at the split heat exchanger may be transferred to the vehicle heater core for utilization in other vehicle components. If heat is not required at any vehicle component, heat may be transferred to a vehicle radiator for dissipation. 
       FIG. 10  shows an example operating sequence  1000  illustrating operation of the heat exchange system of  FIG. 2 . The direction of exhaust gas flow through the heat exchanger, desired exhaust gas recirculation (EGR) temperature and EGR delivery is determined based on engine operations. The horizontal (x-axis) denotes time and the vertical markers t 1  and t 2  identify significant times in the operation of the exhaust bypass assembly. 
     The first plot (line  1002 ) from the top shows variation in engine speed over time. The second plot (line  1004 ) shows EGR requirement by the engine for desired combustion and emissions-control performance. EGR requirement may depend on engine operating conditions e.g., engine load, engine speed, engine temperature etc. The third plot (line  1006 ) indicates the opening or closing of a butterfly valve (valve) located on the exhaust passage. The fourth plot (line  1008 ) shows variation in vehicle cabin temperature over time. The fifth and final plot (line  1010 ) shows the variation of radiator fan speed over time. Higher the radiator fan speed, greater is the heat dissipated from the radiator to the atmosphere. 
     Prior to time t 1 , the engine speed (as seen from line  1002 ) is low. During this time period, cold EGR may be desired by the engine. In order to supply cold EGR from the exhaust passage to the engine intake manifold, the butterfly valve on the exhaust passage is maintained in open position. A first amount of the exhaust gas may enter the exhaust bypass assembly through an inlet (first) pipe and flow through the bypass passage towards a left (first) section of a split heat exchanger. The first amount of exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to the intermediate portion of the split heat exchanger) through the first section of the split heat exchanger before entering the EGR delivery passage. A second amount of the exhaust gas may continue downstream along the main exhaust passage and may enter the exhaust bypass assembly through a second pipe. The second amount of exhaust gas may flow in a second direction (from a second end of the heat exchanger proximal to the outlet pipe to the intermediate portion of the split heat exchanger) through the second section of the split heat exchanger before entering the EGR delivery passage. In this way, exhaust gas may be effectively cooled by passing through two sections of the split heat exchanger before entering the intake manifold via the delivery passage. During this time period a steady flow of EGR (plot  1004 ) is maintained. The opening of the EGR valve and the butterfly valve may regulate the first and second amounts of exhaust gas entering the bypass assembly. A third amount of exhaust gas may not enter the bypass assembly and my flow downstream through the main exhaust passage towards the tailpipe. 
     Heat may be recovered from the exhaust gas at the two sections of the Heat exchanger using a coolant. Depending on demand, the coolant (with the heat recovered from the exhaust gas) may be circulated either through a vehicle heater core and/or through a radiator. In this example, during this time period, the vehicle cabin temperature may be low and heat recovered from the exhaust gas (at the heat exchanger) may be utilized by the heater core to supply heat to the vehicle cabin. Gradually over time, the cabin temperature may increase and stabilize at a level desired by the operator. Prior to time t 1 , since majority of the heat recovered at the heat exchanger is utilized in heating vehicle cabin, little or no heat is transferred to the radiator for dissipation to the atmosphere. Thereby, during this time period, the radiator fan speed may be low. 
     After time t 1 , the engine speed is seen to increase and EGR is no longer desired for engine operations. To inhibit flow of exhaust gas from the exhaust passage to the intake manifold, the EGR valve is closed. During this time period (between time t 1  and t 2 ), the butterfly valve on the exhaust passage is maintained in closed position. Due to closing of the butterfly valve, exhaust gas may not flow downstream along the exhaust passage and the entire volume of exhaust gas enters the exhaust bypass assembly through the first pipe and flow through the bypass passage towards the split heat exchanger. The entire volume of exhaust gas may flow in a first direction (from a first end of the heat exchanger proximal to the inlet pipe to a second end of the heat exchanger proximal to the outlet pipe) through each of the first and second sections of the split heat exchanger before exiting the bypass assembly through the outlet (second) pipe. Since the EGR valve is closed exhaust gas may not enter the EGR delivery passage. The cooled exhaust gas may exit to the atmosphere through the exhaust tail pipe. Heat from the exhaust gas may be recovered by a coolant circulating through both sections of the split heat exchanger. Between time t 1  and t 2 , the vehicle cabin temperature is stable at a level desired by the operator and further cabin heating may not be required. Thereby, majority of the heat from the exhaust gas may be transferred to the radiator for dissipation to atmosphere, causing a rise in radiator fan speed, as seen from line  1010 . Alternatively, when EGR is not desired by the engine, the butterfly valve opening may be regulated to control the amount of exhaust gas entering the bypass passage compared to the amount of exhaust gas flowing downstream though the main exhaust passage towards the tailpipe. In case of high load and/or during high coolant temperature at the heat exchanger, a large part of the exhaust gas may be prevented from entering the exhaust bypass thereby preventing coolant boil. 
     After time t 2 , the engine speed decreases and EGR may again be de desired for engine operation. During this time period, instead of cold EGR, relatively warm EGR may be desired. The EGR valve may be actuated to open position and the butterfly valve on the exhaust passage may be maintained in closed position. The entire volume of exhaust gas may enter the exhaust bypass assembly through the first pipe and flow through the bypass passage towards the split heat exchanger. The exhaust gas may flow in the first direction (from a first end of the heat exchanger proximal to the inlet pipe to the intermediate portion of the split heat exchanger) through the first section of the split heat exchanger before entering the EGR delivery passage. The exhaust gas is cooled by a single section of the heat exchanger, hence, the exhaust gas may be cooled to a lower extent compared to the extent of cooling achieved using both sections of the heat exchanger. In this way, by regulating the butterfly valve, a steady flow of warm EGR may be maintained from the exhaust passage to the engine intake manifold. Depending on the EGR valve opening, a part of the exhaust gas entering the bypass passage may not enter the EGR delivery passage but rather flow across both sections of the heat exchanger in the first direction and continue towards the main exhaust passage and onto the tailpipe. Also, after time t 2 , the vehicle cabin temperature may continue to be stable at a level desired by the operator and further heating may not be required. Thereby, majority of the heat from the exhaust gas may be transferred to the radiator for dissipation to the atmosphere and high radiator fan speed may be maintained. 
     In one example, a method for engine comprises recirculating exhaust gas to an engine intake by flowing a first amount of exhaust gas in a first direction through a first section of a heat exchanger, and flowing a second amount of exhaust gas in a second, opposite direction through a second section of the heat exchanger. The preceding example method may additionally or optionally further comprise, adjusting the first amount relative to the second amount via adjustments to a valve coupled in an exhaust passage downstream of an exhaust catalyst, the heat exchanger positioned in an exhaust bypass coupled across the valve. In any or all of the preceding examples, additionally or optionally, the adjusting includes, increasing an opening of the valve to increase the second amount relative to the first amount, and decreasing the opening of the valve to increase the first amount relative to the second amount. In any or all of the preceding examples, additionally or optionally, the recirculating to the engine intake further includes combining the first amount and the second amount in an exhaust gas recirculation (EGR) passage coupling the heat exchanger to the engine intake, the EGR passage including an EGR valve, wherein the EGR valve and an intake throttle are open during the recirculating, the method further comprising, when the EGR valve and the intake throttle are closed, flowing exhaust gas in the first direction through each of the first and second section of the heat exchanger to an exhaust tailpipe. In any or all of the preceding examples, additionally or optionally, the adjusting further includes, increasing the opening of the valve responsive to an increase in engine load or a decrease in engine temperature, and decreasing the opening of the valve responsive to decrease in engine load or an increase in engine temperature. In any or all of the preceding examples, additionally or optionally, flowing the first amount of exhaust gas in the first direction through the first section of the heat exchanger includes flowing exhaust from downstream of the exhaust catalyst into the exhaust bypass without flowing through the valve, then into the first section of the heat exchanger, proximate the exhaust catalyst, and from a region intermediate the first and second section of the heat exchanger to the engine intake via an EGR passage, and wherein flowing the second amount of exhaust gas in the second direction through the second section of the heat exchanger includes flowing exhaust from downstream of the exhaust catalyst towards the tailpipe through the valve, then flowing exhaust from upstream of the tailpipe into the second section of the heat exchanger in the exhaust bypass, and then from the region intermediate the first and second section of the heat exchanger to the engine intake via the EGR passage. In any or all of the preceding examples, additionally or optionally, the first section of the heat exchanger and the second section of the heat exchanger are symmetrically sized. In any or all of the preceding examples, additionally or optionally, the first section of the heat exchanger is larger than the second section of the heat exchanger. 
     Another example method comprises operating in a first mode with exhaust gas flowing in a first direction through a first and second section of an heat exchanger coupled in an exhaust bypass, and then to an exhaust tailpipe; and operating in a second mode with exhaust flowing through an exhaust passage, then in a second, opposite direction through the second section of the heat exchanger, and then to the engine intake. The preceding example method may additionally or optionally further comprise, during the second mode, flowing exhaust gas in the first direction through the first section of the heat exchanger and then to the engine intake. In any or all of the preceding examples, additionally or optionally, the first and second section of the heat exchanger are asymmetrically sized. Any or all of the preceding examples further comprises, additionally or optionally, selecting between the first mode and second mode based on one or more of engine temperature and engine load. In any or all of the preceding examples, additionally or optionally, the selecting includes selecting the first mode when the engine temperature is below a threshold temperature or when the engine load is higher than a threshold load, and selecting the second mode when the engine temperature is above the threshold temperature or when the engine load is below the threshold load, the threshold temperature based on a catalyst light-off temperature. In any or all of the preceding examples, additionally or optionally, the flowing exhaust gas in the first direction includes flowing exhaust from downstream of an exhaust catalyst into the exhaust bypass, and then through the first section of the heat exchanger proximate the exhaust catalyst, to the second section of the heat exchanger, proximate the tailpipe, and wherein the flowing exhaust gas in the second direction includes flowing exhaust from downstream of the exhaust catalyst towards the tailpipe, then flowing exhaust from upstream of the tailpipe into the second section of the heat exchanger, and then from a region intermediate the first and second section of the heat exchanger to the engine intake via an exhaust gas recirculation (EGR) passage, the EGR passage including an EGR valve. In any or all of the preceding examples, additionally or optionally, during the first mode, when the engine temperature is below the threshold temperature, the EGR valve is closed. Any or all of the preceding examples further comprises, additionally or optionally, during the first mode, transferring heat from the flowing exhaust gas to the heat exchanger, then from the heat exchanger to coolant flowing through a heater core, and then heating a vehicle cabin by drawing heat from the heater core based on a cabin heating demand. In any or all of the preceding examples, additionally or optionally, during the first mode, a valve coupled downstream of the exhaust catalyst in the exhaust passage is closed and wherein during the second mode, the valve is open. 
     In yet another example a vehicle system comprises a vehicle cabin; an engine intake manifold; an engine exhaust manifold including an exhaust catalyst and a muffler coupled to each other via each of an exhaust passage and an exhaust bypass, the bypass coupled parallel to the exhaust passage; an heat exchanger coupled in the exhaust bypass, the heat exchanger including a first section, a second section and an intermediate region positioned between the first and second section; a coolant system fluidly coupling the heat exchanger to a heater core and a radiator; a valve coupled in the exhaust passage; an EGR passage coupling the intermediate section of the heat exchanger to the intake manifold, the EGR passage including an EGR valve; and a controller with computer readable instructions stored on non-transitory memory for: operating in a first mode with the valve closed to flow a first portion of exhaust gas from the exhaust catalyst to the muffler via the exhaust bypass, the first portion flowing in a first direction through the first and second section of the heat exchanger, and flow a second portion of exhaust gas from the catalyst to the engine intake, the second portion flowing in the first direction through the first section of the heat exchanger before being diverted into the EGR passage; and operating a second mode with the valve open to flow a third portion of exhaust gas from the exhaust catalyst to the muffler via the exhaust passage, and flow a fourth portion of exhaust gas upstream of the muffler to the engine intake via the exhaust bypass, the fourth portion flowing in a second, opposite direction through the second section of the heat exchanger before being diverted into the EGR passage. In the preceding example system, additionally or optionally, the controller includes further instructions for: transitioning between the first and second modes responsive to one or more of engine temperature, engine load, and cabin heating demand, the transitioning including: transitioning from second first mode to the first mode responsive to one of an increase in engine load, a drop in engine temperature, and an increase in cabin heating demand. In any or all of the preceding examples, additionally or optionally, when system is operated in the first mode, heat from the exhaust gas released at the heat exchanger is transferred to engine coolant circulating through the heater core, the heat then transferred to the vehicle cabin, and wherein when the system is operated in the second mode, heat from the exhaust gas released at the heat exchanger is transferred to engine coolant circulating through the radiator, the heat then dissipated to atmosphere. 
     In this way, a single heat exchanger, coupled to an exhaust bypass assembly, may be effectively used as for cooling EGR and for extracting heat from the exhaust gas. By using a single heat exchanger system, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. Additionally, the specific configuration of a single heat exchanger in the exhaust bypass assembly facilitates a shorter EGR passage length thereby reducing EGR transport delay. The technical effect of using a single of butterfly valve to regulate the passage of exhaust gas through the bypass passage is that exhaust gas can flow in both directions across the heat exchanger. As such this improves the heat transfer efficiency while reducing the requirement of having long EGR passages. By using a single butterfly valve to control the EGR flow, number of components in the system is reduced facilitating quicker warm-up of the coolant in the EGR cooler due to less thermal mass. By using a split EGR cooler it is possible to regulate the degree of cooling of the EGR depending on engine temperature. Since the EGR cooler is located downstream from the catalyst convertor, catalyst functionality is not compromised. Overall, by improving the amount of waste heat that can be recovered from exhaust using fewer components, engine fuel economy and performance may be improved. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be carried out by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system, where the described actions are carried out by executing the instructions in a system including the various engine hardware components in combination with the electronic controller. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.