Abstract:
An internal combustion engine and control system are provided having improved efficiency through improved exhaust flow control. The engine includes at least one bank of combustion cylinders and a respective exhaust manifold for conveying exhaust to the atmosphere. Further included is an exhaust gas restriction valve (ERV) associated with the exhaust manifold for selectively increasing backpressure on the associated bank of combustion cylinders and for redirecting a portion of the exhaust into an exhaust gas conditioning system for conditioning the portion of the exhaust and returning it to an air intake of the engine. An engine exhaust gas recirculation (EGR) valve in the exhaust gas recirculation system restricts the flow of conditioned exhaust, and an EGR flow controller operates the EGR valve in one of a substantially open and substantially closed condition and controls the flow of conditioned exhaust to the air intake by modulating the ERV.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to an internal combustion engine and, more particularly, to an internal combustion engine with a flow controlled exhaust gas recirculation system. 
       BACKGROUND 
       [0002]    An exhaust gas recirculation system may be used to reduce the generation of undesirable pollutant gases during the operation of internal combustion engines. Exhaust gas recirculation systems generally recirculate exhaust gas generated during the combustion process into the intake air supply of the internal combustion engine. The exhaust gas introduced into the engine cylinders displaces a volume of the intake air supply that would otherwise be available for oxygen. Reduced oxygen concentrations lower the maximum combustion temperatures within the cylinders and slow the chemical reactions of the combustion process, which decreases the formation of oxides of nitrogen (NO x ). 
         [0003]    Many internal combustion engines having such an exhaust gas recirculation system also have one or more turbochargers. Exhaust gas from the combustion cylinders is typically used to drive the turbocharger of the turbocharger which, in turn, drives the compressor of the turbocharger to compress fluid that is subsequently supplied to the combustion cylinders. A portion of the exhaust gas may also be diverted from the exhaust system used to drive the turbocharger and into the exhaust gas recirculation system. 
         [0004]    U.S. Pat. No. 6,263,272 discloses an exhaust gas recirculation system for an internal combustion engine, including a turbocharger, restrictor valve, and exhaust gas recirculation valve. The restrictor valve is upstream of the turbine of the turbocharger, and restricts the flow of exhaust gas into the turbine. This restriction results in an increase in pressure of the exhaust gas provided to the restrictor valve. The increased pressure exhaust gas is provided to the inlet of an exhaust gas recirculation valve. The &#39;272 patent specification states that the restrictor valve may be modulated until exhaust pressure is greater than the pressure of the intake gas. The &#39;672 restrictor valve may also be operated without recirculation of exhaust gas to increase the load on the engine and decrease the warm-up time. 
         [0005]    The foregoing background discussion is intended solely to aid the reader. It is not intended to limit the innovations described herein nor to limit or expand the prior art discussed. Thus the foregoing discussion should not be taken to indicate that any particular element of a prior system is unsuitable for use with the innovations described herein, nor is it intended to indicate any element, including solving the motivating problem, to be essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims. 
       SUMMARY 
       [0006]    In one aspect of the disclosure, an internal combustion engine is provided having improved efficiency through improved exhaust flow control is provided. The engine includes at least one bank of combustion cylinders and at least one respective exhaust manifold for receiving exhaust from the at least one bank of combustion cylinders and conveying the received exhaust to the atmosphere. Further included is an exhaust gas restriction valve (ERV) associated with the exhaust manifold for selectively increasing backpressure on the associated bank of combustion cylinders and for redirecting a portion of the exhaust into an exhaust gas conditioning system for conditioning the portion of the exhaust and returning it to an air intake of the engine. An engine exhaust gas recirculation (EGR) valve in the exhaust gas recirculation system restricts the flow of conditioned exhaust, and an EGR flow controller operates the EGR valve in one of an open and closed condition and controls the flow of conditioned exhaust to the air intake by modulating the ERV. 
         [0007]    In a further aspect, an engine exhaust gas recirculation system is provided having a first valve for selectively directing engine exhaust to an exhaust gas conditioning system and a second valve for restricting an output of the exhaust gas conditioning system to an air intake of the engine, wherein the second valves is a substantially two position valve. A controller controls the first and second valves to achieve a determined level of exhaust gas recirculation to the air intake of the engine. 
         [0008]    In yet another aspect, a method is provided for controlling recirculation of engine exhaust in a recirculation system having a first valve for selectively directing engine exhaust to an exhaust gas conditioning system and a second valve for restricting an output of the exhaust gas conditioning system to an air intake of the engine, wherein the second valves is a substantially two position valve. The method includes controlling the first and second valves to achieve a determined level of exhaust gas recirculation to the air intake of the engine by setting the second valve at one of an on position and an off position and varying the position of the first valve to achieve the determined level of exhaust gas recirculation. 
         [0009]    Other features and advantages of the described systems and methods will be appreciated from the detailed description in conjunction with the attached drawings of which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic illustration of an internal combustion engine in accordance with the disclosure; 
           [0011]      FIG. 2  is a perspective view of the exhaust manifolds and the exhaust gas balance tube in accordance with the disclosure; 
           [0012]      FIG. 3  is an enlarged bottom view of a portion of the exhaust manifolds and the exhaust gas balance tube of  FIG. 2 ; 
           [0013]      FIG. 4  is a schematic illustration of an internal combustion engine of an alternate embodiment having a single bank of combustion cylinders; 
           [0014]      FIG. 5  is a simplified control schematic according to an embodiment of the disclosed principles; and 
           [0015]      FIG. 6  is an EGR valve control plot showing valve positioning and transition response according to an aspect of an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  depicts an internal combustion engine  10  having a plurality of combustion cylinders  11  configured as a first cylinder bank  12  and a second cylinder bank  13  generally parallel to the first cylinder bank. A first exhaust gas line  20  is fluidly connected to the first cylinder bank  12  and a second exhaust gas line  30  is fluidly connected to the second cylinder bank  13 . Compressed air is supplied to the first and second cylinder banks  12 ,  13  by air intake  50 . An exhaust gas recirculation system  40  provides for the recirculation of exhaust gas into the air intake  50  in order to reduce the emissions of the internal combustion engine  10 . 
         [0017]    A first cylinder head  14  is secured to the internal combustion engine  10  adjacent the first cylinder bank  12  and a second cylinder head  15  is secured to the internal combustion engine adjacent the second cylinder bank  13  of combustion cylinders. The first cylinder bank  12  includes a first cylinder group  16  and a second cylinder group  17 . The second cylinder bank  13  includes a first cylinder group  18  and a second cylinder group  19 . While the first cylinder group  16  of first cylinder bank  12  and the first cylinder group  18  of the second cylinder bank  13  are each depicted with seven combustion cylinders  11  and the second cylinder group  17  of the first cylinder bank  12  and the second cylinder group  19  of the second cylinder bank  13  are each depicted with one combustion cylinder  11 , the combustion cylinders of each cylinder bank may be grouped as desired to define or form cylinder groups having different numbers of combustion cylinders. 
         [0018]    First exhaust gas line  20  includes a first exhaust manifold  21  that is fluidly connected to the first cylinder bank  12 . First exhaust manifold  21  has a first end  22  and an opposite exhaust end  23  with a first section  24  and a second section  25  between the two ends. An exhaust gas control valve  26  is positioned between the first section  24  and the second section  25 . A first extension pipe  27  extends between the exhaust end  23  of first exhaust manifold  21  and first turbocharger  60  and fluidly connects the first exhaust manifold to the first turbocharger. 
         [0019]    Second exhaust gas line  30  includes a second exhaust manifold  31  that is fluidly connected to the second cylinder bank  13 . The second exhaust manifold  31  is generally parallel to the first exhaust manifold and has a first end  32  and an opposite exhaust end  33  with a first section  34  and a second section  35  between the two ends. A second extension pipe  37  extends between the exhaust end  33  of the second exhaust manifold  31  and second turbocharger  61  and fluidly connects the second exhaust manifold to the second turbocharger. 
         [0020]    Exhaust gas from the first cylinder group  16  of the first cylinder bank  12  is received within the first section  24  of the first exhaust manifold  21  and, depending upon the positions of exhaust gas control valve  26  and exhaust gas recirculation valve  44 , may be routed through the exhaust gas recirculation system  40 . The exhaust gas recirculation system  40  includes an exhaust gas recirculation duct  41  that is fluidly connected to the first end  22  of the first exhaust gas line  20  so that exhaust gas from the first cylinder group  16  of the first cylinder bank  12  may be routed or recirculated through the exhaust gas recirculation system and introduced into the combustion air intake  50 . 
         [0021]    Exhaust gas passing through exhaust gas recirculation duct  41  is cooled by one or more cooling components  42 . The flow rate through exhaust gas recirculation duct  41  is monitored by a flow meter  43  such as a venturi-style flow meter. An exhaust gas recirculation valve  44  is provided along exhaust gas recirculation duct  41  to control exhaust gas flow through the exhaust gas recirculation system  40 . Exhaust gas recirculation valve  44 , together with exhaust gas control valve  26 , controls the amount of exhaust gas that is mixed with air that has been compressed by the first turbocharger  60  and the second turbocharger  61  prior to the air entering the first intake manifold  51  and the second intake manifold  52 . The exhaust gas recirculation duct  41  of the exhaust gas recirculation system  40  splits into two separate legs  45 . Each leg  45  fluidly connects to the air intake  50  between the aftercooler  58  and the first intake manifold  51  and the second intake manifold  52 , respectively. 
         [0022]    Air intake  50  includes a first air intake  53  through which atmospheric air enters the first turbocharger  60 , a second air intake  54  through which atmospheric air enters the second turbocharger  61  and a compressed air line  55  through which compressed air is fed to combustion cylinders  11 . Atmospheric air is compressed by the first and second turbochargers  60 ,  61  and passes through first compressed air lines  56  to aftercooler  58 . Cooled compressed air exits the aftercooler  58  and enters second compressed air lines  57  that are each fluidly connected to a respective one of the first and second intake manifolds  51 ,  52 . Each leg  45  of the exhaust gas recirculation system  40  intersects with and fluidly connects to a respective one of the second compressed air lines  57  between the aftercooler  58  and the first and second intake manifolds  51 ,  52 . In this way, exhaust gas may be mixed with intake air provided to the combustion cylinders  11 . 
         [0023]    A portion of exhaust gas from the first cylinder group  16  of the first cylinder bank  12  is, at times, routed through the exhaust gas recirculation system  40  rather than through the first exhaust gas line  20 . For this reason, a duct or exhaust gas balance tube  65  is fluidly connected between the first exhaust gas line  20  and the second exhaust gas line  30  to balance or equalize, to a controllable extent, the amount of exhaust gas passing through the first and second turbochargers  60 ,  61 . More specifically, second exhaust manifold  31  includes an upstream balance tube connection port  66  ( FIGS. 1-3 ) between the first section  34  of second exhaust manifold  31  and the second section  35  of the second exhaust manifold. First exhaust manifold  21  includes a downstream balance tube connection port  67  positioned between exhaust gas control valve  26  and the second section  25  of the first exhaust manifold  21 . In other words, the upstream balance tube connection port  66  fluidly connects one end of exhaust gas balance tube  65  to the second exhaust manifold  31  and the downstream balance tube connection port  67  fluidly connects the opposite end of the exhaust gas balance tube to the first exhaust manifold  21  to permit exhaust gas to pass from the second exhaust gas line  30  to the first exhaust gas line  20 . The exhaust gas balance tube  65  provides a path for exhaust gas to travel from second exhaust gas line  30  towards first exhaust gas line  20  to balance the flow through the first and second turbochargers  60 ,  61 . 
         [0024]    It should be noted that while the upstream balance tube connection port  66  is depicted as being positioned between the first section  34  of the second exhaust manifold  31  and the second section  35  of the second exhaust manifold, the upstream balance tube connection port may alternatively be positioned elsewhere along the second exhaust manifold  31  to provide the desired amount of exhaust gas through exhaust gas balance tube  65 . For example, moving the upstream balance tube connection port  66  upstream or towards first end  32  of second exhaust manifold  31  will result in fewer combustion cylinders  11  being included in first cylinder group  18  of second cylinder bank  13  and thus exhaust gas from fewer combustion cylinders will be available for passage through exhaust gas balance tube  65  to first exhaust gas line  20 . 
         [0025]    Downstream balance tube connection port  67  is depicted as being positioned between the exhaust gas control valve  26  and the second section  25  of the first exhaust manifold  21 . However, the downstream balance tube connection port  67  may be positioned at other locations along the first exhaust manifold  21  as well as other positions along the first exhaust gas line  20 , such as that depicted in phantom at  65 ′ in  FIG. 1  and connected to the first extension pipe  27  between the first exhaust manifold and the first turbocharger  60 . 
         [0026]    Exhaust gas balance tube  65  and upstream balance tube connection port  66  engage or meet second exhaust gas line  30  at an angle “β” relative to centerline  92  of second exhaust manifold  31 . In order to minimize pressure drop though the exhaust gas balance tube  65 , it is believed that setting angle “β” at an angle less than ninety degrees will result in acceptable flow characteristics and setting angle “β” at less than approximately eighty degrees will further reduce the pressure drop and still smaller angles will likely reduce the pressure drop to a greater extent. The exact angle may be set by based upon air flow characteristics and desired routing of the exhaust gas balance tube  65  within the physical space limitations of the internal combustion engine. 
         [0027]    Exhaust gas balance tube  65  and the downstream balance tube connection port  67  engage or meet first exhaust gas line  20  at an angle “α” relative to centerline  91  of first exhaust manifold  21 . With this configuration, exhaust gas flowing from the second exhaust gas line  30  through exhaust gas balance tube  65  into first exhaust gas line  20  does not enter first exhaust gas line  20  in a perpendicular fashion relative to first exhaust gas line  20  and thus pressure drop through the exhaust gas balance tube  65  is reduced. In addition, since the exhaust gas traveling downstream through first exhaust gas line  20  drives the first turbocharger  60 , it is desirable that the exhaust gas passing through the exhaust gas balance tube  65  into the first exhaust gas line  20  minimizes any disruption to the flow or momentum of the exhaust gas from first cylinder group  16  of first cylinder bank  12  as it passes downstream balance tube connection port  67 . By positioning the downstream balance tube connection port  67  at an appropriate angle relative to the centerline of the first exhaust gas line  20 , disruption of the flow through the first exhaust gas line may be reduced or minimized. It is believed that setting the angle “α” to less than ninety degrees will result in acceptable flow characteristics. It is further believed that setting the angle “α” at less than approximately seventy-five degrees will result in a configuration that will minimize disruption of air flow within the first exhaust gas line  20 . The exact angle may be set based upon air flow characteristics and desired routing of the exhaust gas balance tube  65  within the physical space limitations of the internal combustion engine. It should be noted that angles “α” and “β” are not necessarily within a horizontal or a vertical plane relative to internal combustion engine  10  nor do they need to be identical angles. 
         [0028]    Exhaust gas from the first cylinder bank  12  and second cylinder bank  13  passes through the first and second turbochargers  60 ,  61 , respectively, and exits the turbochargers through turbocharger exhaust gas lines  62 . Turbocharger exhaust gas lines  62  are fluidly connected to a filter  63  so that the exhaust gas is filtered prior to being discharged or released to the atmosphere through exhaust gas outlet  64 . 
         [0029]    Under certain operating conditions, it may be desirable to reduce the shaft speed of the first and second turbochargers  60 ,  61  so that the turbochargers may be maintained within a desired operating range. In order to do so, the amount of exhaust gas passing through the first and second exhaust gas lines  20 ,  30  may be reduced by venting or releasing a desired amount of exhaust gas from the exhaust gas lines. Such exhaust gas may be released in a relatively consistent manner from both the first and second exhaust gas lines  20 , by utilizing a wastegate  70  that is fluidly connected at wastegate interconnection  74  to exhaust gas balance tube  65  to permit exhaust gas to be released from the wastegate. A wastegate valve  71  controls or regulates the flow of exhaust gas through wastegate  70 . By fluidly connecting wastegate  70  to exhaust gas balance tube  65 , exhaust gas within the first and second exhaust gas lines  20 ,  30  may be reduced in a relatively uniform manner so that a reduction in shaft speed of the first and second turbochargers  60 ,  61  will also occur in a relatively uniform manner. 
         [0030]    Under certain other operating conditions, it may be desirable to reduce the pressure within the compressed air line  55 . In such case, a compressor bypass  72  and its associated compressor bypass valve  73  may be used to control or regulate the venting or release of compressed air from the compressed air line  55 . However, because work has been performed (i.e., energy used) to compress the air within the compressed air line  55 , such energy is wasted if the compressed air is vented or released to the atmosphere. In order to increase the efficiency of internal combustion engine  10 , the compressor bypass  72  fluidly connects the compressed air line  55  at aftercooler  58  (but before the compressed air is cooled within the aftercooler) with the exhaust gas balance tube  65  at compressor bypass interconnection  75 . In this way, energy used to compress the atmospheric air within the first and second turbochargers  60 ,  61  is conserved by re-routing the compressed air into the exhaust gas system via the exhaust gas balance tube  65  when the pressure of air in the compressed air line  55  is higher than exhaust gas pressure within the exhaust gas balance tube  65 . In other words, rather than wasting the energy used to compress the air that is being vented or released to the atmosphere, some of the energy may be saved by re-routing the compressed air into the exhaust gas system which is subsequently used to drive the first and second turbochargers  60 ,  61 . In an alternate design, the compressor bypass may extend from any portion of compressed air line  55 , including a portion positioned after the aftercooler  58 . In addition, the compressor bypass may be routed to fluidly connect to the exhaust gas system at a location other than the exhaust gas balance tube  65  including either or both of the first and second exhaust gas lines  20 ,  30 . 
         [0031]    Referring to  FIGS. 2-3 , the first exhaust manifold  21  and the second exhaust manifold  31  are each formed of a plurality of interconnected exhaust manifold elements  80 . More specifically, first exhaust manifold  21  includes seven non-direction specific exhaust manifold elements  81  that are each fluidly connected to one of the combustion cylinders  11  of the first cylinder group  16 . The first exhaust manifold  21  further includes one modular pulse exhaust manifold element  82  positioned adjacent exhaust end  23  of the first exhaust manifold  21  and fluidly connected to the single combustion cylinder  11  of the second cylinder group  17  of the first cylinder bank  12 . Each of the non-direction specific exhaust manifold elements  81  and the modular pulse exhaust manifold element  82  is mechanically and fluidly connected to an adjacent manifold element by connecting members  83 . The connecting members  83  may be formed with a bellows, a slip-fit joint or another structure that is capable of expanding and contracting to compensate for thermal expansion of the exhaust manifold elements  80 . Each exhaust manifold element  80  includes a generally cylindrical hollow duct component  84  and a hollow pipe component  85  for fluidly connecting a combustion cylinder  11  to the duct component  84 . The duct components  84  of the exhaust manifold elements  80  are spaced apart in an array connected by the connecting members  83  to form a generally linear tube-like duct portion  88  of the first exhaust manifold for directing exhaust gas from each combustion cylinder towards the exhaust end  23  of the first exhaust manifold. In other words, each of the connecting members  83  and duct components  84  is positioned along and forms a section of the generally linear tube-like duct portion  88 . 
         [0032]    All of the non-direction specific exhaust manifold elements  81  and the modular pulse exhaust manifold element  82  have generally identical duct components  84  except as described below. Non-direction specific exhaust manifold element  81  has a non-direction specific pipe component  86  that generally extends from the first cylinder head  14  in a generally straight manner to duct component  84 . In the depicted embodiment, the non-direction specific pipe components  86  are generally perpendicular to axis  91  of first exhaust manifold  21  so that the non-direction specific exhaust manifold elements have a generally “T-shaped” configuration. 
         [0033]    Modular pulse exhaust manifold element  82  has a curved modular pulse pipe component  87  that generally extends from the first cylinder head  14  and fluidly connects the combustion cylinder  11  of the second cylinder group  17  of the first cylinder bank  12  to the duct component  84  of the modular pulse exhaust manifold element  82 . The modular pulse pipe component  87  is configured to direct exhaust gas from a combustion cylinder  11  into the first exhaust manifold in a direction specific or direction biased exhaust flow pattern that includes the generation of a series of pulses of exhaust gas. In addition, the shape of the modular pulse pipe component  87  combined with the duct component  84  directs the exhaust gas towards the exhaust end  23  of the first exhaust manifold  21  and thus towards the first turbocharger  60 . 
         [0034]    The second exhaust manifold  31  is constructed in a manner similar to first exhaust manifold  21  and also has eight exhaust manifold elements  80 . However, all of the exhaust manifold elements are modular pulse exhaust manifold elements  82  in order to direct exhaust gas from the second cylinder bank  13  and through the second exhaust gas line  30  towards the second turbocharger  61 . 
         [0035]    In the embodiment depicted in  FIGS. 1-3 , each of the exhaust manifold elements of the first exhaust manifold  21  associated with the first cylinder group  16  of first cylinder bank  12  is a non-direction specific exhaust manifold element  81  while the exhaust manifold element associated with the second cylinder group  17  of the first cylinder bank  12  is a modular pulse exhaust manifold element  82 . As such, the first exhaust manifold  21  has both non-direction specific exhaust manifold elements  81  and a modular pulse exhaust manifold element  82 . 
         [0036]    By configuring the exhaust manifold elements of the first section  24  of the first exhaust manifold as non-direction specific exhaust manifold elements, exhaust gas may flow more easily towards the exhaust end  23  of first exhaust manifold  21  as well as towards exhaust gas recirculation system  40 . If the exhaust manifold elements of the first section  24  of the first exhaust manifold were modular pulse exhaust manifold elements, the exhaust gas from the first section would be primarily directed towards exhaust end  23  of the first manifold. With such a modular pulse configuration, in order to increase the amount of exhaust gas being recirculated through the exhaust gas recirculation system  40 , the exhaust gas control valve  26  would be closed to a greater extent than if, as disclosed herein, the first exhaust manifold includes both non-direction specific exhaust manifold elements and modular pulse exhaust manifold elements. As a result, the configuration of the first exhaust manifold  21  results in a more efficient structure for the recirculation of exhaust gas. 
         [0037]    The exhaust manifold elements may also include additional features and functionality. For example, non-direction specific exhaust manifold element  81 - 1  adjacent first end  22  of first exhaust manifold  21  has an opening  89  for fluidly connecting first exhaust manifold  21  to exhaust gas recirculation duct  41 . Non-direction specific exhaust manifold element  81 - 7  includes exhaust gas control valve  26  to define the first cylinder group  16  and the second cylinder group  17 . Modular pulse exhaust manifold element  82 - 9  of first exhaust manifold  21  includes the downstream balance tube connection port  67  for fluidly connecting to exhaust gas balance tube  65  and also includes the first extension pipe  27  in the shape of a curved end component for fluidly connecting to first turbocharger  60 . Modular pulse exhaust manifold element  82 - 7  of second exhaust manifold  31  includes the upstream balance tube connection port  66  for fluidly connecting to exhaust gas balance tube  65 . Modular pulse exhaust manifold element  82 - 8  ( FIG. 3 ) of second exhaust manifold  31  includes the second extension pipe  37  in the shape of a curved end component for fluidly connecting to second turbocharger  61 . 
         [0038]    Although the internal combustion engine  10  and associated components depicted in  FIGS. 1-3  include or relate to a pair of cylinder banks, certain aspects of the present disclosure may be used with internal combustion engines having a single, in-line bank of combustion cylinders.  FIG. 4  depicts an internal combustion engine  210  similar to internal combustion engine  10  of  FIG. 1  but having only a single, in-line cylinder bank  212 . Identical or similar components of the embodiment depicted in  FIG. 1  are identified with identical reference numbers. 
         [0039]    Although the described EGR system configuration and resultant operation serve to significantly increase engine efficiency and lower engine emissions, the system is most beneficially operated when the recirculation is accurately controlled to provide the optimal mixture of fresh and recirculated charge. As discussed above, the described system includes an engine EGR valve  44  located between combustion cylinders  11  as well as an exhaust gas restriction valve (ERV)  26 , both of which affect the extent to which recirculation occurs. For example with the EGR valve  44  closed and the ERV  26  open, flow is at a minimum, whereas with the EGR valve  44  open and the ERV  26  closed, flow is at a maximum. 
         [0040]    However, neither state is ideal for system efficiency. For example, the ERV  26  serves a diversion function for recirculation purposes but also provides backpressure to improve engine operation under certain running conditions. Moreover, during certain other running conditions, backpressure may need to be removed, and yet unrestricted exhaust gas recirculation would negatively affect engine performance, efficiency, and emissions. 
         [0041]    To this end, in an embodiment, for low EGR flow conditions, the EGR valve  44  is used to control EGR flow rate with the ERV  26  fully open. For higher flow levels, the EGR valve  44  is fully opened and the recirculation is controlled by modulating the state of the ERV  26 . In an embodiment, the EGR valve  44  is smoothly transitioned between the on and off states when the flow requirement falls within a predefined range about the division between the low flow and high flow control regimes to provide a smooth transition. 
         [0042]    A control architecture for executing the described valve control schema is shown schematically in  FIG. 5 . In particular, the simplified control schematic  250  includes a number of interacting components including a nonlinear proportional integral control module  260  and an ERV feed forward control module  261 . The feed forward strategy in an embodiment takes the desired EGR flow and provides an initial control flow area. To account for system changes that are condition dependent, the feed forward gain may be determined from a map that is function of engine speed and fuel. 
         [0043]    The nonlinear proportional integral control module  260  receives as input the difference between the actual EGR and a desired EGR, and provides a standard PI output based on this information. For executing the PI function, the proportional integral control module  260  also receives a scheduled gain from an ERV control gain schedule  263  and an integral parameter from an integrator and freeze initiation module  264 . 
         [0044]    In an embodiment, the control gains used on the closed loop control are gain scheduled to account for EGR response changes due to operating conditions. The maps may be, as noted above, a function of engine speed and fuel. In addition, non-linear control action may be used as a function of error to improve response during transient conditions. Regarding the integrator and freeze initiation module  264 , the integrator is frozen when the ERV reaches either the maximum or minimum values and is trying to move beyond the limits. This prevents integrator wind-up and associated problems. The integrator may be initialized before starting the closed loop control to guarantee that the control is starting from a known condition 
         [0045]    In the illustrated example, the ERV control gain schedule  263  has as its input the aforementioned difference. Similarly, the integrator and freeze initiation module  264  takes the desired EGR as its input along with a limit value to be discussed later. 
         [0046]    In parallel, the ERV feed forward control module  261  receives as input the aforementioned difference, and outputs a control signal to be summed with the PI output of the proportional integral control module  260 . The summed or controlled output is then fed to an ERV valve characterization function  265  to generate an ERV control area, e.g., in mm 2 . 
         [0047]    The generated ERV control area is then used by an ERV valve area selector  266  to select a final needed valve area. The final needed valve area is limited by a limiting strategy module  267  to produce a limited area which is also fed back to the integrator and freeze initiation module  264  as discussed above. The limited area which is also input to an ERV linearization module  268  to produce a desired hot valve position which is then fed to the appropriate actuation system  269 . Regarding the limited area, the ERV is limited to a maximum and minimum area in an embodiment. The maximum may be the wide-open area. The minimum may be a function of engine speed and fuel to prevent the valve from going to a position that could cause engine damage. 
         [0048]    Regarding the ERV linearization module  268 , the EGR mass flow is a function of ERV valve position, but the gain of the flow to the valve position is non-linear and condition dependent. Thus, in an embodiment, a decoupling function is used to decouple the flow response to the valve position at different operating conditions. The controls are based on area to keep the outputs as linear as possible to the physics of the system. The ERV valve control is position (0-1), so a linearization map is used within the ERV linearization module  268  to convert the area to position. 
         [0049]    Turning now to the cold EGR valve  44 , the control of the valve is structured to be open or closed. When the engine is operating in a quasi-steady state condition, the valve may be ramped rather than stepped between positions to minimize disturbances in the engine output. However, during a hard transient event, the valve may step to the new position to minimize impact on emissions. 
         [0050]    An exemplary control schema  280  is shown in  FIG. 6 . The illustrated schema  280  plots the EGR valve  44  state as a function of desired position. Thus, the valve has two positions, on and off, and four state transitions, namely (1) ramp off, (2) step off, (3) ramp on and (4) step on. Per the illustrated strategy, a hysteresis region  281  is established wherein the controller maintains the valve in whatever state it is in without change while the desired EGR remains in the region. 
         [0051]    The hysteresis region  281  is bounded by a ramp off region  282  on the lower end and a ramp on region  283  on the upper end. If the desired EGR falls within one of these regions, the controller ramps the position of the EGR valve  44  to the appropriate on or off position to avoid an abrupt change. If however, the desired EGR falls below the ramp off region  282  or above the ramp on region  283 , then the controller steps the position of the EGR valve  44  to the new position to avoid a substantial response lag time during which emissions and efficiency may suffer. 
         [0052]    It will be appreciated that each module may receive other inputs, not shown, depending upon the implementation chosen. Moreover, while the description of the control architecture references modules that execute various steps and functions, these modules need not be implemented strictly in hardware. For example, in an embodiment, one or more modules may be a software module, i.e., a computerized execution of computer-executable code read from a computer-readable medium. The computer-readable medium is a nontransitory medium such as, but not limited to a RAM, ROM, EPROM, disc memory, flash memory, optical memory, and so on. 
       INDUSTRIAL APPLICABILITY 
       [0053]    The industrial applicability of the system described herein will be readily appreciated from the foregoing discussion. The present disclosure is applicable to many internal combustion engines. One exemplary type of such an internal combustion engine is one that utilizes an exhaust gas recirculation system. The internal combustion engine may utilize an EGR valve  44  in cooperation with an ERV  26  to selectively recirculate exhaust gases back to the engine combustion chambers as a variable part of the combustion charge. The exhaust gas recirculation improves engine efficiency and emission characteristics, but can also negatively affect these characteristics if not accurately performed. 
         [0054]    The described system includes a controller for coordinating the control of both recirculation valves (EGR valve  44  and ERV  26 ) based on the current operating state of the engine, in order to maintain appropriate exhaust manifold back pressure on some or all cylinders while still allowing the prescribed degree of recirculation to occur. Thus, during normal operation, the EGR valve  44  is maintained in an open state and the ERV  26  is utilized to provide recirculation flow control. However, during low EGR flow conditions, the ERV  26  is fully opened. 
         [0055]    It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
         [0056]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
         [0057]    Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.