Abstract:
An exhaust gas recirculation (EGR) system and corresponding method for improved combustion efficiency of an internal combustion engine having a number of cylinders, comprising one or more main cylinders connected to an intake manifold and one or more cylinders operable as a dedicated EGR cylinder(s). The dedicated EGR cylinder(s) have two independent intake flow paths for where one flow path provides only intake air and one flow path provides air and recirculated exhaust gas. The exhaust gas output of the one or more dedicated EGR cylinders are connected to an exhaust gas recirculation loop which delivers the exhaust gas output to the main cylinder intake manifold.

Description:
FIELD 
       [0001]    The present disclosure relates to exhaust gas recirculation for internal combustion engines, and more particularly, to improved exhaust gas recirculation for internal combustion engines powered by hydrocarbon (HC) fuels. 
       BACKGROUND 
       [0002]    For certain conventional exhaust gas recirculation (EGR) systems, exhaust gas expelled from all of the cylinders of an internal combustion engine may be collected in an exhaust manifold. A fraction of the collected exhaust gas (e.g. 5% to 10%) may then be routed from the exhaust manifold through a control valve back to an intake manifold of the engine, where it may be introduced to a stream of fresh (ambient) intake air. The remaining fraction of exhaust gas in the exhaust manifold, rather than being recirculated and recycled, generally flows to a catalytic converter of the exhaust system and, after treatment therein, may be expelled to the atmosphere through the exhaust pipe. 
         [0003]    EGR has a history of use in gasoline spark-ignition engines, and affects combustion in several ways. First, the combustion in the cylinders of the engine may be cooled by the presence of exhaust gas, that is, the recirculated exhaust gas may absorb heat from the combustion. Furthermore, the dilution of the oxygen present in the combustion chamber with the exhaust gas, in combination with the cooler combustion, may reduce the production of mono-nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide (NO 2 ). Additionally, EGR may reduce the need for fuel enrichment at high loads in turbocharged engines and thereby improve fuel economy. 
         [0004]    EGR which uses higher levels of exhaust gas may further increase fuel efficiency and reduce emissions of spark-ignition engines. However, with higher levels of exhaust gas, engines may face challenges related to EGR tolerance, which may reduce the expected fuel efficiency improvement. Challenges related to EGR tolerance may be understood to include increasing an engine&#39;s ability to process higher levels of exhaust gas without adversely affecting performance, particularly fuel economy. Thus, even if EGR tolerance may be satisfactory for engine operation at low levels of EGR, an engine may need additional modifications in structure and operational conditions to accommodate higher levels of EGR without adversely affecting engine performance. 
         [0005]    More recently, an engine configuration has been proposed with one or more cylinders of the engine being dedicated to expelling exhaust gas for EGR, which is then directed to the intake manifold. Such cylinders may be referred to as dedicated EGR, or D-EGR, cylinders. Dedicated EGR cylinder(s) may operate at a broad range of equivalence ratios since their exhaust gas is generally not configured to exit the engine before flowing through a cylinder operating at, for example, a stoichiometric or near stoichiometric air/fuel ratio. This may allow the dedicated EGR cylinder to be operated fuel rich to produce higher levels of hydrogen (H 2 ) gas and carbon monoxide (CO) gas and which, may in turn, increase the octane number and promote increased EGR tolerance and knock tolerance by increasing flame/speed burn rates, as well as increasing the dilution limits of the mixture and associated combustion stability of all the cylinders. Examples of engines with a D-EGR cylinder may be found in U.S. Patent Application Publication No. 2012/0204844 entitled “ Dedicated EGR Control Strategy For Improved EGR Distribution And Engine Performance ” and U.S. Patent Application Publication No. 2012/0204845 entitled “ EGR Distributor Apparatus For Dedicated EGR Configuration ”, both in the name of Jess W. Gingrich, which are assigned to the assignee of the present disclosure and hereby incorporated by reference. Attention is also directed to U.S. Publication No. 2014/0196697 entitled “ Internal Combustion Engine Having Dedicated EGR Cylinder ( s )  With Intake Separate From Intake Of Main Cylinders.”   
       SUMMARY 
       [0006]    An exhaust gas recirculation (EGR) system for improved combustion efficiency of an internal combustion engine having a number of cylinders, comprising one or more main cylinders connected to an intake manifold and one or more cylinders operable as a dedicated EGR cylinder(s). The dedicated EGR cylinder(s) have two independent intake flow paths for where one flow path provides only intake air and one flow path provides air and recirculated exhaust gas. The exhaust gas output of the one or more dedicated EGR cylinders are connected to an exhaust gas recirculation loop which delivers the exhaust gas output to the main cylinder intake manifold. 
         [0007]    The present disclosure also relates to a corresponding method of using exhaust gas recirculation (EGR) to improve the combustion efficiency of an internal combustion engine comprising operating one or more main cylinders as non-dedicated EGR cylinders, which main cylinders are connected to an intake manifold. The operation of the one or more cylinders as dedicated EGR cylinder(s) are such that its exhaust, during all or some engine cycles, is recirculated via an EGR loop as EGR gas. The one or more EGR cylinder(s) have two independent intake flow paths where one flow path provides only intake air and one flow path provides air and recirculated exhaust gas. The exhaust gas output of the one or more dedicated EGR cylinders are connected to an exhaust gas recirculation loop which delivers the exhaust gas output to the main cylinder intake manifold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The above-mentioned and other features of this disclosure, and the manner of attaining them, will become more apparent and better understood by reference to the following description of embodiments described herein taken in conjunction with the accompanying drawings, wherein: 
           [0009]      FIG. 1  illustrates, for comparison purposes, a dedicated EGR configuration of the prior art. 
           [0010]      FIG. 1A  illustrates the basis for calculation of the dedicated EGR configuration of  FIG. 1 . 
           [0011]      FIG. 2  illustrates for comparison purposes another dedicated EGR configuration of the prior art. 
           [0012]      FIG. 3  illustrates an internal combustion engine  10  in accordance with the present disclosure. 
           [0013]      FIG. 3A  illustrates the basis for the calculation of the EGR rates for the configuration shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    It may be appreciated that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention(s) herein may be capable of other embodiments and of being practiced or being carried out in various ways. Also, it may be appreciated that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting as such may be understood by one of skill in the art. 
         [0015]    The following description is directed to various configurations of emissions systems, particularly exhaust gas recirculation (EGR) systems, apparatuses and methods to be used with an internal combustion engine, particularly of a motor vehicle such as an automobile. With an EGR system, one or more cylinders of the internal combustion engine may be used to generate exhaust gas, which may then be recirculated and mixed with an intake stream of fresh (ambient) air to provide a mixed charge (mixture) of exhaust gas and air to the cylinders of the engine. 
         [0016]    For the purposes of this disclosure, an engine configured such that substantially an entire output of exhaust gas from a cylinder is to be recirculated for EGR may be referred to herein as an engine having a dedicated EGR cylinder. 
         [0017]      FIG. 1  illustrates a dedicated EGR configuration described, for comparison purposes, in U.S. Patent Application Publication No. 2012/0204844.  FIG. 1A  identifies the basis for calculation of the EGR rate for such configuration. The D-EGR system is defined as follows: (1) cylinders one through three are the main cylinders; (2) cylinder four is the dedicated cylinder; and (3) all four cylinders receives air and recirculated exhaust gasses. To simplify the calculations, the following assumptions are made: (1) all cylinders are operating at the same air-fuel ratio; and (2) the intake and exhaust flows for each cylinder are equal. The following relationships then apply:
       The total flow to all of the cylinders (I 1 +I 2 +I 3 +I 4 ) equals the air flow plus the exhaust flow from cylinder  4  (A+E 4 )       
 
         [0000]        I 1+ I 2+ I 3+ I 4= A+E 4   (1)
       The intake flow for each cylinder is the same       
 
         [0000]      I1=I2=I3=I4   (2)
       The intake flow into cylinder  4  equals the exhaust flow of cylinder four       
 
         [0000]      I4=E4   (3)
 
         [0021]    The EGR rate is then defined as the exhaust flow into the intake divided by the exhaust flow into the intake plus the air flow: 
         [0000]    
       
         
           
             
               
                 
                   
                     % 
                      
                     
                         
                     
                      
                     EGR 
                   
                   = 
                   
                     
                       
                         E 
                          
                         
                             
                         
                          
                         4 
                       
                       
                         ( 
                         
                           
                             E 
                              
                             
                                 
                             
                              
                             4 
                           
                           + 
                           A 
                         
                         ) 
                       
                     
                     * 
                     100 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0022]    Combining equations 1, 2 and 3: 
         [0000]      3* E 4= A    (5)
 
         [0023]    Combining equations 4 and 5: 
         [0000]    
       
         
           
             
               
                 
                   
                     % 
                      
                     
                         
                     
                      
                     EGR 
                   
                   = 
                   
                     
                       
                         E 
                          
                         
                             
                         
                          
                         4 
                       
                       
                         4 
                         * 
                         E 
                          
                         
                             
                         
                          
                         4 
                       
                     
                     * 
                     100 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0024]    Equation 6 produces an EGR rate of 25%. As can now be seen, for this 4 cylinder D-EGR embodiment ( FIG. 1 ) the exhaust gas recirculation (EGR) rate of the engine is approximately 25%. This is the case as the exhaust gas of ¼ of all of the cylinders, as shown, is then recirculated to all four cylinders. 
         [0025]      FIG. 2  illustrated a dedicated EGR configuration, described for comparison purposes, in U.S. Patent Application Publication No. 2014/0196697. For this configuration, the exhaust from the D-EGR cylinder is directed only to the non D-EGR cylinders. This was accomplished by dividing the intake manifold to separate the intake for the D-EGR cylinder from the non D-EGR cylinders. Accordingly, all of the exhaust from the D-EGR cylinder may be divided equally among the main cylinders which then provides 33% EGR to each of the three main cylinders. 
         [0026]    In addition, Valve A is added to the passageway that connects the intake manifold from a point after the fresh air has passed through the Mixer and Intercooler to the intake port of the D-EGR cylinder. Valve B is added in the fresh air intake to the D-EGR cylinder. When Valve A closed and Valve B open, approximately 33% of the D-EGR cylinder exhaust is distributed to each of the main cylinders. When Valve A is opened and Valve B is closed, the system reverts to having approximately 25% of the EGR routed to each of the four cylinders. When proportional control is applied to both Valve A and Valve B, it is possible to vary the EGR distribution from the D-EGR cylinders to the main cylinders between 25% and 33%. 
         [0027]      FIG. 3  illustrates an internal combustion engine  10  in accordance with the present disclosure. Internal combustion engine  10  may power a motor vehicle that utilizes a hydrocarbon fuel. Internal combustion engine  10  is shown to have four cylinders  12 ,  14 ,  16  and  18  although such is not intended to limit the present disclosure. One of the cylinders, cylinder  18 , may be understood to be a dedicated EGR cylinder (D-EGR). The engine  10  may be spark ignited as well as compression ignited. Accordingly, it may be appreciated that the present disclosure, while illustrated with respect to a four cylinder engine having three non-dedicated EGR cylinders as the main cylinders and one dedicated EGR cylinder, is applicable to other engine configurations that may have a different number of main cylinders and a plurality of dedicated EGR cylinders. 
         [0028]    As can be seen then in representative  FIG. 3 , the exhaust gas  20  expelled from cylinder  18  may be sampled by an exhaust gas oxygen (EGO) sensor, which may more particularly comprise a universal exhaust gas oxygen (UEGO) sensor  22 . It may then pass through a water gas shift water reactor  24  which provides for the water gas shift (WGS) reaction that converts carbon monoxide and water to carbon dioxide and hydrogen. More particularly, with the WGS reaction, carbon monoxide (CO) gas in the exhaust gas  20  may react with water (H 2 O) vapor to produce carbon dioxide (CO 2 ) gas and hydrogen (H 2 ) gas according to the following reaction: 
         [0000]      CO+H 2 O→CO 2 +H 2  
 
         [0029]    Reacting carbon monoxide (CO) gas in the exhaust gas  20  with water (H 2 O) vapor to produce hydrogen (H 2 ) gas is beneficial by increasing the amount of hydrogen (H 2 ) gas in the exhaust gas  20  from dedicated EGR cylinder  18 . The WGS catalyst performance is highly dependent on exhaust temperature, and the amount of hydrogen (H 2 ) gas exiting the WGS catalyst is dependent on the amount entering and the amount created. The amount of hydrogen (H 2 ) gas entering the WGS catalyst is a function of the fuel, the dedicated EGR cylinder air/fuel ratio and spark timing. The amount of hydrogen (H 2 ) gas created is therefore dependent on exhaust gas temperature and the amount of carbon monoxide (CO) gas in the inlet exhaust. It is possible to manipulate both with the dedicated EGR cylinder air/fuel ratio. Therefore, for a given operating condition, the dedicated EGR cylinder air/fuel ratio may be controlled to increase the amount of hydrogen (H 2 ) gas exiting the WGS catalyst. Examples of WGS catalysts may include iron oxides (Fe 3 O 4 ) or other transition metals and transition metal oxides. 
         [0030]    While it may be possible, based on the configuration of engine  10 , for all of the exhaust gas (i.e. 100%) expelled from cylinder  18  to be optimally recirculated back to the intake system  26 , it should be understood that certain design considerations and operating inefficiencies may only allow a substantial portion of the exhaust gas expelled from cylinder  18  to be recirculated back to the intake system  26 . For example, exhaust gas losses may occur between connection points. Accordingly, it is contemplated that on a volume basis, 90% or more of the exhaust gas expelled from the dedicated EGR cylinder is recirculated to the engine intake system  26 . More preferably, 90-100% of the exhaust gas expelled from cylinder  26  is recirculated, including all values therein, in 0.1% by volume increments. 
         [0031]    During an operation of engine  10 , fresh (ambient) inlet air  28  may enter air inlet  30  of air intake system  26 . The air  28  may then travel within intake passage  32 , during which time it may be compressed by intake compressor  34 . Thereafter, air  28  may enter air/exhaust gas mixer  36  of air intake system  26 , and more particularly as distribution mixer, which is configured to distribute and mix the recirculated and exhaust gas  20  into the stream of air  28  to be introduced to the internal combustion engine  10 . As also shown by  FIG. 3 , each of cylinders  12 ,  14 ,  16  and  18  may receive a hydrocarbon fuel, such as gasoline, from a fuel (common) rail  38  which is in fluid communication with a fuel tank. Hydrocarbon fuel may be provided to each cylinder  12 ,  14 ,  16  and  18  from the fuel rail  38  by a direct fuel injector  40  located in each cylinder  12 ,  14 ,  16  and  18 . It can also be seen that one may position an intercooler  33  in the air only intake pathway  37  leading to D-EGR cylinder  18  that will then allow independent control of air temperature. Another intercooler is identified at  47  for separate cooling of an air/exhaust gas mixture. 
         [0032]    In addition, D-EGR cylinder  18  can be seen to have two independent intake valves  42  and  44 , where valve  42  provides air only and valve  44  flows air and recirculated exhaust gas. Optionally, one may also include a port valve  46  (for control of inlet air flow) as well as for the introduction of fuel to cylinder  18 . However, as will be explained herein, the implementation of just two independent intake valves  42  and  44  for D-EGR cylinder  18  preferably provides the combustion benefits of earlier D-EGR designs while also preferably retaining a relative low control and relatively low hardware complexity. 
         [0033]    Furthermore, illustrated in  FIG. 3  is the presence of the main throttle  48  and D-EGR cylinder throttle  50 . Such throttles will then allow one to separately control the power outputs of the main cylinders  12 ,  14  and  16  and separately control the power output of D-EGR cylinder  18 . For example, the main throttle  48  may be set at a relatively high pressure and the D-EGR throttle  50  may be set at a relatively lower pressure in order to balance the power output of all cylinders  12 ,  14 ,  16  and  18 . 
         [0034]    Attention is next directed to  FIG. 3A  for illustration of the calculation of the EGR rates for the configuration shown in  FIG. 3 . The illustrated and preferred D-EGR system herein is therefore configured as follows: (1) cylinders one through three are the main cylinders; (2) cylinder four is the dedicated cylinder; (3) one intake valve for cylinder four (I 4   b ) receives air and recirculated exhaust gasses (see also valve  44  in  FIG. 3 ); (4) the other intake valve for cylinder four (I 4   a ) receives air only (see also valve  42  in  FIG. 3 ); (5) the exhaust from cylinder four can now be routed back into the intake for the three main cylinders and one of the two intake valves for cylinder four. 
         [0035]    The following assumptions may then be applied for the configurations illustrated in  FIGS. 3 and 3A : (1) all cylinders are operating at the same air-fuel ratio; (2) the intake and exhaust flows for each cylinder are equal; (2) the flow for intake valve  1  and intake valve  2  on cylinder four are equal. Then, the following relationships may be applied:
       The total flow to the main cylinders and intake valve  1  of the dedicated cylinder (I 1 +I 2 +I 3 +I 4   a ) equals the air flow plus the exhaust flow from cylinder  4  (A+E 4 )       
 
         [0000]        I 1+ I 2+ I 3+ I 4 a=A+E 4   (7)
       The intake flow for each cylinder is the same       
 
         [0000]        I 1= I 2= I 3= I 4 a+I 4 b    (8)
       The total intake flow into cylinder  4  equals the exhaust flow of cylinder four       
 
         [0000]        I 4 a+I 4 b=E 4   (9)
       The flow for intake valve  1  and intake valve  2  on cylinder four are equal       
 
         [0000]      I4a=I4b   (10)
 
         [0040]    We then define the EGR rate for the main cylinders and for intake valve  1  of cylinder four as the exhaust flow into the intake divided by the exhaust flow into the intake plus the air flow: 
         [0000]    
       
         
           
             
               
                 
                   
                     % 
                      
                     
                         
                     
                      
                     EGR 
                   
                   = 
                   
                     
                       
                         E 
                          
                         
                             
                         
                          
                         4 
                       
                       
                         ( 
                         
                           
                             E 
                              
                             
                                 
                             
                              
                             4 
                           
                           + 
                           A 
                         
                         ) 
                       
                     
                     * 
                     100 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0041]    Combining equations 8, 9, and 10: 
         [0000]        I 1= I 2= I 3=2* I 4 a=E 4   (12)
 
         [0042]    Combining equations 7 and 12: 
         [0000]        E 4+ E 4+ E 4+ E 4/2= A+E 4 or 2.5* E 4= A    (13)
 
         [0043]    Combining equations 11 and 13: 
         [0000]    
       
         
           
             
               
                 
                   
                     % 
                      
                     
                         
                     
                      
                     EGR 
                   
                   = 
                   
                     
                       
                         E 
                          
                         
                             
                         
                          
                         4 
                       
                       
                         3.5 
                         * 
                         E 
                          
                         
                             
                         
                          
                         4 
                       
                     
                     * 
                     100 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0044]    Equation 6 identifies an EGR rate of about 28.57% (e.g. 28.57%+/−1.0%) for cylinders one through three and for valve  1  on cylinder  4 . Since the flow rates for valve  1  and valve  2  on cylinder four are preferably equal, and the EGR rate through valve  1  is 28.57% and through valve  2  is 0%, the EGR rate in cylinder  4  is half of the EGR rate through valve  1  or about 14.29% (e.g., 14.29%+/−1.0%). 
         [0045]    It may therefore be appreciated that in such configuration as described herein, utilizing two independent intake flow paths into cylinder by way of intake valves  42  and  44  as illustrated in  FIG. 3 , some EGR is still delivered to the dedicated cylinder  18 , and it is possible to operate such cylinder richer, which then will allow the non D-EGR cylinders  12 ,  14  and  16  to tolerate relatively higher EGR rates. 
         [0046]    To maintain a preferred and relatively high combustion efficiency in combination with relatively higher EGR rates in the main cylinders  12 ,  14  and  16 , increased hydrogen concentrations are desirable. This can be preferably achieved herein by operating the dedicated cylinder  18  at a greater equivalence ratio. Because the D-EGR cylinder(s) herein may operate with less EGR, the amount of over fuelling is much greater than if the D-EGR cylinder(s) herein received an equal portion of the total EGR. With the increased equivalence ratio, relatively more hydrogen and carbon monoxide is produced. This leads to a decreased burn duration and improved combustion in the main cylinders. This then results in relatively higher engine efficiencies. In addition the D-EGR cylinder herein with the identified independent intake flow paths provided by valves  42  and  44  retain some EGR in the dedicated cylinder  18 , which may the mitigate knock. 
         [0047]    While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptions and modifications can be made therein without departing from the scope of the invention as recited in the appended claims.