Patent Publication Number: US-2011061374-A1

Title: Exhaust gas treatment system

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust gas treatment system, and more particularly to an exhaust gas treatment system including an injection valve for injecting urea water into exhaust gas and a selective catalytic reduction catalyst. 
     A conventional exhaust gas treatment system is disclosed in Japanese Unexamined Patent Application Publication No. 2009-68424. The system includes an exhaust passage through which engine exhaust gas flows, a diesel particulate filter (DPF) provided in the exhaust passage, a urea SCR catalyst located downstream of the DPF in the exhaust passage as viewed in the direction of exhaust gas flow, and an ammonia oxidation catalyst located downstream of the urea SCR catalyst. The system further includes an injection nozzle for supplying urea water into the exhaust passage and a burner for accelerating activation of the SCR catalyst. The injection nozzle is provided between the DPF and the urea SCR catalyst, and the burner is provided upstream of the DPF in the exhaust passage. In the system, use of combustion heat produced by the burner shortens the time it takes to activate the SCR catalyst after a cold start of the engine, resulting in shortening of the time it takes to start NOx reduction. 
     In the system, however, since the burner is located apart from the injection nozzle, the combustion space of the burner needs to be provided in the exhaust passage separately from the mixing space for mixing urea water and exhaust gas. In this case, the combustion space needs to be provided with a structure or means for accelerating uniform mixing of fuel and exhaust gas, and the mixing space needs to be provided with a structure such as a mixer for uniform mixing of urea water and exhaust gas. Thus, the combustion space and the mixing space provided separately in the exhaust passage results in increased entire length of the exhaust passage. This leads not only to increased size of the system but also to increased heat loss by radiation in the exhaust passage. 
     The present invention is directed to providing an exhaust gas treatment system that allows shortening of the entire length of an exhaust passage and also allows uniform mixing of urea water and combustion gas resulting from the combustion of fuel. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, an exhaust gas treatment system includes an exhaust passage through which exhaust gas from an internal combustion engine is allowed to flow, a selective catalytic reduction catalyst provided in the exhaust passage, a mixing chamber provided upstream of the selective catalytic reduction catalyst in the exhaust passage as viewed in the direction of exhaust gas flow, a fuel injection valve for injection of fuel, an air injection valve for injection of air, and a urea water injection valve for injection of urea water. The fuel injection valve, the air injection valve and the urea water injection valve each has an injection port directed to the mixing chamber. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an engine equipped with an exhaust gas treatment system according to a first embodiment of the present invention; 
         FIG. 2  is an enlarged fragmentary sectional view of the exhaust gas treatment system of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along the line III-III of  FIG. 2 ; 
         FIG. 4  is a fragmentary sectional view of an exhaust gas treatment system according to a second embodiment of the present invention; and 
         FIG. 5  is a cross-sectional view taken along the line V-V of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe the embodiments of the exhaust gas treatment system according to the present invention with reference to the accompanying drawings. In the embodiments, the exhaust gas treatment system is intended to be installed in a vehicle with a diesel engine as an internal combustion engine. 
     Referring to  FIG. 1 , the engine  11 , which is equipped with an exhaust gas treatment system  10 , has plural cylinders  12  into which fuel is injected from respective fuel injectors  13  that are connected to a common rail  14  where fuel is stored at a high pressure. The engine  11  is operated under the control of an engine control unit (ECU)  15 . Although not shown in the drawings, the ECU  15  includes microprocessors for calculation, ROMs and RAMs for storing data and programs, and interfaces for connection to other devices. 
     An intake passage  17  is connected to an intake manifold  16  of the engine  11 , and an exhaust passage  22  is connected to an exhaust manifold  21  of the engine  11 . The engine  11  is equipped with a turbocharger  18  that uses exhaust gas to increase intake pressure. The turbocharger  18  has a compressor  19  provided in the intake passage  17  and a turbine  20  provided in the exhaust passage  22 . 
     Part of the exhaust passage  22  located downstream of the turbine  20  of the turbocharger  18  as viewed in the direction of exhaust gas flow has a bend where the direction of exhaust gas flow is bent at a right angle. In the following description, the part of the exhaust passage  22  located upstream of the bend will be hereinafter referred to as the upstream exhaust passage  22 A, and the part of the exhaust passage  22  located downstream of the bend will be hereinafter referred to as the downstream exhaust passage  22 B. There is provided in the downstream exhaust passage  22 B a selective catalytic reduction (SCR) catalyst  23 . The SCR catalyst  23  includes a casing  24  accommodating therein a filter  25  (particulate filter) that serves as a DPF for capturing particulate matter (PM) in exhaust gas passing therethrough. The filter  25  supports thereon a catalytic component that accelerates selective catalytic reaction between NOx in exhaust gas and ammonia as a reducing agent, serving to remove NOx. 
     Referring to  FIG. 2 , the upstream exhaust passage  22 A and the downstream exhaust passage  22 B are of a cylindrical shape, and the upstream end of the downstream exhaust passage  22 B is closed by a planar end wall  22 C. The upstream exhaust passage  22 A is connected to the downstream exhaust passage  22 B at a position adjacent to the end wall  22 C so as to be perpendicular to the peripheral wall of the downstream exhaust passage  22 B. The downstream exhaust passage  22 B accommodates therein a cylindrical partition member  26  that is concentric with the downstream exhaust passage  22 B. The outer diameter of the partition member  26  is smaller than the inner diameter of the downstream exhaust passage  22 B. The partition member  26  is mounted at one end to the end wall  22 C of the downstream exhaust passage  22 B and opened at the other end to the interior of the downstream exhaust passage  22 B. That is, the upstream end of the partition member  26  is closed by the end wall  22 C, and the downstream end of the partition member  26  is opened. The partition member  26  forms therein a mixing chamber  27 . The partition member  26  has plural circular openings  28  formed therein for allowing exhaust gas to flow into the mixing chamber  27 . The mixing chamber  27  is provided upstream of the SCR catalyst  23  in the downstream exhaust passage  22 B as viewed in the direction of exhaust gas flow. The downstream exhaust passage  22 B cooperates with the partition member  26  to form a space between the inner peripheral surface of the downstream exhaust passage  22 B and the outer peripheral surface of the partition member  26 . Exhaust gas introduced into the downstream exhaust passage  22 B flows through the space and further through the openings  28  of the partition member  26  into the mixing chamber  27 . A baffle plate  29  with plural holes  30  is provided downstream of the partition member  26  in the downstream exhaust passage  22 B. The baffle plate  29  is provided between the mixing chamber  27  and the SCR catalyst  23 . The baffle plate  29  serves to uniformly mix exhaust gas and combustion gas flowing therethrough. 
     The exhaust gas treatment system  10  includes a fuel injection valve  31 , an air injection valve  32  and a urea water injection valve  33  which are mounted to the end wall  22 C of the downstream exhaust passage  22 B so as to be directed toward the mixing chamber  27 . The fuel injection valve  31 , the air injection valve  32  and the urea water injection valve  33  are arranged in parallel to one another so that the air injection valve  32  is located between the fuel injection valve  31  and the urea water injection valve  33 . The injection directions of the fuel injection valve  31 , the air injection valve  32  and the urea water injection valve  33  are substantially parallel to the direction of exhaust gas flow (horizontal direction in  FIGS. 1 and 2 ) in the downstream exhaust passage  22 B. The injection ports  31 A,  32 A and  33 A of the respective injection valves  31 ,  32  and  33  are aligned in vertical direction, as shown in  FIG. 3 . The injection ports  31 A,  32 A and  33 A of the respective injection valves  31 ,  32  and  33  are directed to the mixing chamber  27 , as shown in  FIGS. 2 and 3 . 
     As shown in  FIG. 1 , the fuel injection valve  31  is connected through a fuel pipe  34  to a fuel tank  36 . The fuel pipe  34  is provided with a fuel pump  35  by which the amount of fuel to be injected is controlled. With the operation of the fuel pump  35 , fuel is supplied from the fuel tank  36  through the fuel pipe  34  to the fuel injection valve  31  and then injected in the form of a mist into the mixing chamber  27 . The fuel pump  35  is connected to the ECU  15 , and the fuel injection valve  31  is operated under the control of the ECU  15 . Fuel for injection from the fuel injection valve  31  is diesel fuel used for the engine  11 . 
     The air injection valve  32  is connected through an air pipe  37  to an air pump  38 . With the operation of the air pump  38 , air is supplied from the air pump  38  to the air injection valve  32  and then injected into the mixing chamber  27 . The air pump  38  is connected to the ECU  15 , and the air injection valve  32  is operated under the control of the ECU  15 . The air injection valve  32  is operated simultaneously with the fuel injection valve  31 . 
     The urea water injection valve  33  is connected through a urea water pipe  39  to a urea water tank  41 . The urea water pipe  39  is provided with a urea water pump  40  by which the amount of urea water to be injected is controlled. With the operation of the urea water pump  40 , urea water is supplied from the urea water tank  41  through the urea water pipe  39  to the urea water injection valve  33  and then injected in the form of a mist into the mixing chamber  27 . The urea water pump  40  is connected to the ECU  15 , and the urea water injection valve  33  is operated under the control of the ECU  15 . 
     The exhaust gas treatment system  10  further includes an igniter  42  for igniting fuel injected into the mixing chamber  27 . The igniter  42  is provided in the downstream region of the mixing chamber  27 . The igniter  42  is connected to and operated under the control of the ECU  15 . When fuel is ignited by the igniter  42 , flame is produced in the mixing chamber  27 . When the injection of fuel is continued, the flaming occurs continuously. 
     The exhaust gas treatment system  10  mainly includes the exhaust passage  22 , the SCR catalyst  23  integrated with the filter  25 , the partition member  26 , the baffle plate  29 , the fuel injection valve  31 , the air injection valve  32  and the urea water injection valve  33 , and further includes the ECU  15  and the igniter  42 . 
     The following will describe the operation of the exhaust gas treatment system  10 . 
     Exhaust gas resulting from the combustion in the cylinders  12  of the engine  11  is delivered through the exhaust manifold  21  to the exhaust passage  22  and passes through the turbine  20  of the turbocharger  18 . The direction of exhaust gas flow is bent at a right angle toward the SCR catalyst  23  at the connection between the upstream and downstream exhaust passages  22 A and  22 B, and the exhaust gas flows through the SCR catalyst  23   
     When flame is produced in the mixing chamber  27 , the fuel pump  35 , the air pump  38 , the urea water pump  40  and the igniter  42  are operated under the control of the ECU  15  so that fuel, air and urea water are injected simultaneously into the mixing chamber  27  from the respective injection valves  31 ,  32  and  33  and the injected fuel is ignited by the igniter  42 . In the mixing chamber  27 , the injected fuel, air and urea water are mixed not only with one another but also with the exhaust gas flowing into the mixing chamber  27  through the openings  28  of the partition member  26 , so that the injected fuel, air and urea water are heated by the exhaust gas, which makes it easy for the igniter  42  to ignite the fuel so as to produce flame. Flaming in the mixing chamber  27  is continued when fuel injection is continued. 
     When fuel and urea water are injected simultaneously under the control of the ECU  15 , hydrolysis of urea water proceeds just after the fuel ignition by the heat of the combustion gas flame and water vapor in exhaust gas, so that ammonia is produced. When the combustion of fuel is continued, the injection of air by the air injection valve  32  is also continued. In the present embodiment wherein the urea water injection valve  33  is located apart from the fuel injection valve  31  and adjacent to the air injection valve  32 , the injected air prevents the urea water injection valve  33  from receiving heat directly from the flame produced in a region downstream of the fuel injection valve  31 . Thus, even when the injection of fuel is continued in the mixing chamber  27 , overheating of the urea water injection valve  33  is prevented. 
     In a region downstream of the mixing chamber  27 , the combustion gas resulting from the combustion of fuel in the mixing chamber  27  and containing urea water passes through the baffle plate  29  along with the exhaust gas having flowed through the space outside the mixing chamber  27 . The combustion gas and the exhaust gas are mixed uniformly by the baffle plate  29 , and then the mixture of gases passes through the SCR catalyst  23 . 
     The SCR catalyst  23  receives heat from the mixture of gases to reach a temperature at which the SCR catalyst  23 , that is, the catalytic component supported on the filter  25 , becomes active, so that NOx and ammonia contained in the mixture are removed by selective catalytic reaction in the reaction region of the catalytic component. In addition, PM contained in the mixture is captured and collected by the filter  25  integrated with the SCR catalyst  23 . Thus, exhaust gas purification is achieved by the removal of NOx through the reducing reaction between ammonia and NOx and the capture of PM in the SCR catalyst  23 . 
     Although in the present embodiment fuel and urea water are injected simultaneously into the mixing chamber  27 , urea water may be injected after the injection of fuel. In this case, the injected fuel and air are mixed with exhaust gas and heated by the exhaust gas, which makes it easier for the igniter  42  to ignite the fuel, so that flame is reliably produced by the combustion of fuel. When urea water is injected after the injected fuel is adequately combusted, the urea water is heated by the combustion gas flame and hydrolyzed by water vapor in the exhaust gas. Even when urea water is injected after an elapse of time subsequent to the production of flame, the reducing reaction between NOx and ammonia adsorbed on the catalytic component occurs in the SCR catalyst  23  that has been activated by high temperature gases, which prevents NOx from passing through the SCR catalyst  23  without being removed. 
     In the present embodiment, the filer  25  integrated with the SCR catalyst  23  serves as a DPF. Since excessive accumulation of PM on the filter  25  may cause filter plugging, the filter  25  needs to be regenerated when a predetermined amount of PM is accumulated on the filter  25 . The amount of PM accumulation can be determined, for example, based on the difference of exhaust gas pressure between the upstream and downstream ends of the SCR catalyst  23 . When it is determined by the ECU  15  that the filter  15  needs to be regenerated, fuel and air are injected into the mixing chamber  27 , and flame is produced in the mixing chamber  27  by the igniter  42 . Then high-temperature combustion gas resulting from the combustion in the mixing chamber  27  passes through the filter  25  in the SCR catalyst  23 , so that PM is burned off and the filter  25  is regenerated. 
     During a cold start of the engine, fuel and air are injected into the mixing chamber  27  so as to produce flame, but urea water needs not to be injected immediately. This is because exhaust gas temperature just after a cold start of the engine is low and NOx is not produced in the exhaust gas. Since exhaust gas temperature is increased gradually after a cold start of the engine, urea water is injected, for example, when the exhaust gas reaches a temperature at which NOx starts to be produced. On the other hand, urea water may be injected since the time when the engine is started. In this case, the temperature of the SCR catalyst  23  is then low, and the ammonia produced by hydrolysis of urea water is adsorbed on the catalytic component of the SCR catalyst  23 . 
     The exhaust gas treatment system  10  according to the first embodiment offers the following advantages. 
     (1) Since fuel, air and urea water are injected into the mixing chamber  27 , the injected fuel is combusted in the mixing chamber  27  to produce flame, and the combustion gas is mixed with the injected urea water. The heat of the combustion gas accelerates the hydrolysis of urea water into ammonia in the exhaust passage  22 . The mixing chamber  27  formed in the exhaust passage  22  is used not only for combustion of fuel but also for mixing of combustion gas and urea water, which allows shortening of the entire length of the exhaust passage  22  and also allows uniform mixing of combustion gas and urea water. 
     (2) The air injection valve  32  is located between the fuel injection valve  31  and the urea water injection valve  33  so that the urea water injection valve  33  is spaced apart from the fuel injection valve  31  and located adjacent to the air injection valve  32 . This allows cooling of the urea water injection valve  33  by injection of air, thereby preventing overheating of the urea water injection valve  33 . 
     (3) Exhaust gas is introduced into the mixing chamber  27  and mixed with the injected fuel and air, which allows the fuel and air to be heated by the exhaust gas and to combust easily. 
     (4) The SCR catalyst  23  integrated with the filter  25  (DPF) allows further shortening of the entire length of the exhaust passage  22 . In addition, the heat resulting from the combustion in the mixing chamber  27  can be used for regeneration of the filter  25 . 
     (5) Simultaneous injection of fuel and urea water allows uniform mixing of fuel and urea water. In this case, since the urea water receives the heat of the combustion gas simultaneously with the combustion of the fuel, hydrolysis of urea water is accelerated by the heat, and ammonia is produced easily. On the other hand, when the fuel is combusted and then the urea water is mixed with the combustion gas, the fuel is adequately combusted, so that hydrolysis of urea water is further accelerated by the heat of the combustion gas. 
     (6) Flame is produced in the mixing chamber  27  by combustion of fuel, and the heat of the combustion gas can be used to regenerate the filter  25 . 
       FIGS. 4 and 5  show the second embodiment of the present invention. In the drawings, same reference numerals are used for the common elements or components in the first and second embodiments, and the description of such elements or components for the second embodiment will be omitted. 
     As shown in  FIG. 4 , the exhaust gas treatment system  50  includes a multiple valve unit  51  having a fuel injection valve, an air injection valve and a urea water injection valve integrated as one unit. A fuel passage  52  is formed in the center of the multiple valve unit  51 , extending through the central axis of the multiple valve unit  51 . The fuel passage  52  is opened at one end to the mixing chamber  27  so as to form a fuel injection valve having an injection port  52 A directed to the mixing chamber  27 . The other end of the fuel passage  52  is connected through the fuel pipe  34  to the fuel pump  35  (see  FIG. 1 ). 
     As shown in  FIG. 5 , an air passage  53  with annular cross section is located concentrically with and radially outward of the fuel passage  52 . The air passage  53  is opened at one end to the mixing chamber  27  so as to form an air injection valve having an injection port  53 A directed to the mixing chamber  27 . The other end of the air passage  53  is connected through the air pipe  37  to the air pump  38  (see  FIG. 1 ). 
     A urea water passage  54  with annular cross section is concentric with the fuel passage  52  and located radially outward of the air passage  53 . The urea water passage  54  is opened at one end to the mixing chamber  27  so as to form a urea water injection valve having an injection port  54 A directed to the mixing chamber  27 . The other end of the urea water passage  54  is connected to the urea water tank  41  through the urea water pipe  39  provided with the urea water pump  40  (see  FIG. 1 ). 
     The multiple valve unit  51  is provided in the end wall  22 C of the downstream exhaust passage  22 B so that fuel, air and urea water are injected into the mixing chamber  27 . The injection directions of fuel, air and urea water are substantially parallel to the direction of exhaust gas flow in the downstream exhaust passage  22 B. 
     According to the second embodiment wherein the fuel injection valve, the air injection valve and the urea water injection valve are integrated in the multiple valve unit  51 , the number of parts of the system can be reduced, and such injection valves can be provided at a time so as to be directed toward the mixing chamber  27 . Further, the air passage  53  is located between the fuel passage  52  and the urea water passage  54  so that the urea water passage  54  is spaced apart from and located radially outward of and the fuel passage  52 . In this case, the injected air prevents the urea water passage  54  from receiving heat directly from the fuel passage  52 , thereby preventing overheating of the urea water passage  54 . 
     The above embodiments may be modified in various ways as exemplified below. 
     An oxidation catalyst may be provided downstream of the SCR catalyst  23  in the exhaust passage  22  so as to remove the excessive ammonia produced from urea water and the ammonia released from the SCR catalyst  23 . 
     Although in the first and second embodiments the filter  25  in the SCR catalyst  23  serves not only as a particulate filter for capturing PM but also as a selective catalytic reduction catalyst, the particulate filter may be provided separately from the selective catalytic reduction catalyst. 
     Although in the first and second embodiments the mixing chamber  27  is formed by the cylindrical partition member  26  with the circular openings  28 , the partition member may be of any shape or structure so as to form the mixing chamber in which fuel, air, urea water, and exhaust gas can be mixed. In addition, the openings  28  of the partition member  26  for introducing exhaust gas into the mixing chamber  27  may be replaced with slits or mesh structure. 
     Although in the first and second embodiments the partition member  26  is provided separately from the baffle plate  29 , the partition member and the baffle plate may be integrated. 
     The baffle plate  29  may be omitted when exhaust gas can be mixed uniformly with combustion gas in a region downstream of the mixing chamber  27  without using the baffle plate  29 . 
     The baffle plate  29  may be of any shape or structure so as to mix the gases uniformly. 
     In the first and second embodiments, the simultaneous injection of fuel, air and urea water, the simultaneous injection of fuel and air, and the injection of urea water after the simultaneous injection of fuel and air are described. Alternatively, only the urea water may be injected depending on the engine operating conditions. 
     In the first embodiment, the injection valves  31 ,  32 ,  33  are arranged in parallel to one another so that the injection ports  31 A,  32 A,  33 A of the respective injection valves  31 ,  32 ,  33  are aligned in vertical direction. Alternatively, the injection ports  31 A,  32 A,  33 A may be aligned in any direction, or the injection valves  31 ,  32 ,  33  may be arranged in any order. 
     In the second embodiment, the fuel passage  52 , the air passage  53  and the urea water passage  54  are concentric with one another in the multiple valve unit  51 . Alternatively, the multiple valve unit  51  may have such a structure that the fuel passage, the air passage and the urea water passage are at the same distance from the central axis of the multiple valve unit  51  and the passages are angularly spaced at regular intervals. In addition, the multiple valve unit  51  may have such a structure that the fuel injection valve  31 , the air injection valve  32  and the urea water injection valve  33  in the first embodiment are simply integrated.