Patent Publication Number: US-8534053-B2

Title: Exhaust purification apparatus for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to an exhaust purification apparatus for an internal combustion engine. 
     BACKGROUND ART 
     NOx purification systems are known which use a selective catalytic reduction (SCR) method of using ammonia to reduce and remove nitrogen oxide (hereinafter referred to as NOx) in exhaust gas from a diesel engine (see, for example, Patent Document 1). Such an NOx purification system generally supplies ammonia by adding a urea aqueous solution to a denitration catalyst (hereinafter referred to as an “NOx catalyst”) via an addition valve to hydrolyze the urea aqueous solution. 
     If a urea aqueous solution addition system is abnormal, the following problems may occur. An excessive amount of urea aqueous solution may be added to increase the amount of ammonia slipping through the NOx catalyst or an excessively small amount of urea aqueous solution may be added to reduce an NOx purification rate. Examples of abnormality of the urea aqueous solution addition system include crystallization of the urea aqueous solution, blockage of the addition valve by dirt or the like, inappropriate opening and closing of the addition valve, a decrease in the supply pressure of a urea aqueous solution supply pump, and time degradation of the addition system. 
     Patent Literature 1 discloses the following technique. When a urea aqueous solution is added to an NOx catalyst to purify NOx, a target NOx purification rate is compared with the actual NOx purification rate. If the actual NOx purification rate is lower than the target NOx purification rate, correction is performed to increase the addition amount of urea aqueous solution. If the increase in the addition amount of urea aqueous solution fails to improve the actual NOx purification rate, the system is determined to be abnormal. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laid-Open No. 2003-293743 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     To determine whether or not the actual amount of urea aqueous solution added to the NOx catalyst has reached the target addition amount, it is necessary to measure the actual addition amount of urea aqueous solution. When a dedicated sensor is provided for measuring the actual addition amount of urea aqueous solution, manufacturing costs increase. Furthermore, for example, NOx sensors may be provided before and after the NOx catalyst, respectively, so that the actual addition amount of urea aqueous solution can be estimated from a difference in NOx concentration obtained from outputs from the NOx sensors. However, the NOx sensor has the property of reacting both to NOx and to ammonia. Thus, if the NOx sensor detects both NOx and ammonia, it is difficult to accurately estimate the addition amount of urea aqueous solution based on the outputs from the NOx sensors. Hence, precisely controllably correcting the addition amount of urea aqueous solution is difficult. 
     An object of the present invention is to provide an exhaust purification apparatus for an internal combustion engine which allows the addition amount of reducing agent to be precisely corrected and which prevents possible ammonia slip to be inhibited with a decrease in NOx purification rate suppressed. 
     Solution to Problem 
     An exhaust purification apparatus for an internal combustion engine according to the present invention includes an NOx catalyst provided in an exhaust system of the internal combustion engine to selectively reduce NOx, reducing agent adding means provided upstream of NOx catalyst to supply the NOx catalyst with a urea aqueous solution or ammonia as a reducing agent, a first NOx sensor provided upstream of the reducing agent adding means and a second NOx sensor provided at an outlet of the NOx catalyst, each of the NOx sensors reacting both to NOx and to ammonia, and addition amount correcting means for, when a bed temperature of the NOx catalyst is in a predetermined high-temperature region in which the amount of ammonia converted into NOx increases relatively and an NOx purification rate decreases relatively, performing correction using outputs from the first and second NOx sensors so that the actual amount of reducing agent added by the reducing agent adding means follows a target addition amount. 
     According to this configuration, the NOx purification rate of the NOx catalyst decreases in the predetermined high-temperature region. Hence, the consumption of the added ammonia in connection with NOx purification is reduced, and the conversion of ammonia into NOx is promoted. The resultant NOx flows out from the NOx catalyst. Thus, the second NOx sensor detects an NOx concentration that is higher than that detected by the first NOx sensor, by an amount equal to that of ammonia added. This enables the actual reducing agent addition amount to be more precisely corrected. 
     The above-described configuration may further has a filter provided upstream of the NOx catalyst to collect a particulate matter contained in exhaust gas. The bed temperature of the NOx catalyst is increased into the predetermined high-temperature region by a recovery process of oxidizing the particulate matter in order to recover a collection capability of the filter. 
     The above-described configuration may further has heating means for increasing the temperature of the NOx catalyst into the predetermined high-temperature region. 
     In the above-described configuration, the addition amount correcting means may use the outputs from the first and second NOx sensors to determine the difference between the NOx concentration measured before the NOx catalyst and the NOx concentration measured after the NOx catalyst, and then use the NOx concentration difference to correct the actual amount of reducing agent added by the reducing agent adding means. In this case, the addition amount correcting means may correct the actual addition amount of reducing agent using, in addition to the outputs from the first and second NOx sensors, at least one of a predetermined relationship between a bed temperature of the NOx catalyst and an ammonia slip amount, a predetermined relationship between the bed temperature of the NOx catalyst and a NOx purification rate, and a predetermined relationship between the bed temperature of the NOx catalyst and a rate at which ammonia is converted into NOx. 
     The above-described configuration may further comprises an oxidation catalyst provided downstream of the NOx catalyst to oxidize ammonia slipping through the NOx catalyst, into NOx. The second NOx sensor may be provided downstream of the oxidation catalyst. 
     Advantageous Effects of the Invention 
     The present invention enables a decrease in NOx purification rate and possible ammonia slip to be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the configuration of an exhaust purification apparatus for an internal combustion engine according to an embodiment of the present invention; 
         FIG. 2  is a flowchart showing an example of processing executed by an ECU; 
         FIG. 3  is a timing chart showing an example of temperature characteristics of an NOx catalyst; 
         FIG. 4  is a diagram showing the relationship between the amounts of NOx and urea aqueous solution flowing into and out of the NOx catalyst; 
         FIG. 5A  is a graph showing the relationship between the bed temperature of the NOx catalyst and the amount of ammonia slip; 
         FIG. 5B  is a graph showing the relationship between the bed temperature of the NOx catalyst and an NOx purification rate; 
         FIG. 5C  is a graph showing the relationship between the bed temperature of the NOx catalyst and the rate at which urea aqueous solution is converted into NOx; and 
         FIG. 6  is a schematic diagram showing the configuration of an exhaust purification apparatus for an internal combustion engine according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the attached drawings. 
       FIG. 1  is a diagram showing the configuration of an exhaust purification apparatus for an internal combustion engine according to an embodiment of the present invention. 
     An internal combustion engine  10  is, for example, a diesel engine. An exhaust passage  15  in the internal combustion engine  10  includes a DPF (Diesel Particulate Filter)  30  serving as a filter, and an NOx catalyst  35 . 
     In the exhaust passage  15 , a urea aqueous solution addition valve  70  and an addition valve downstream mixer  80  are provided between the DPF  30  and the NOx catalyst  35 ; the urea aqueous solution addition valve  70  serves as reducing agent adding means to add a urea aqueous solution to the exhaust passage  15 , and the addition valve downstream mixer  80  is provided downstream of the urea aqueous solution addition valve  70  to mix exhaust gas EG with a urea aqueous solution. 
     Furthermore, in the exhaust passage  15 , an NOx sensor  90 A and an exhaust temperature sensor  95 A are provided upstream of the urea aqueous solution addition valve  70 . An NOx sensor  90 B and an exhaust temperature sensor  95 B are provided downstream of the NOx catalyst  35 . Detection signals from these sensors are input to an electronic control unit (ECU)  100 . 
     The DPF  30  is a filter configured to collect particulate matter (PM) contained in the exhaust gas (EG). As is well known, the DPF  30  comprises, for example, a honeycomb member composed of metal or ceramics. A recovery process needs to be carried out on the DPF  30  when a predetermined amount of PM is accumulated. Specifically, unburned fuel is fed into exhaust gas by post injection or the like and combusted in the DPF  30 . Thus, the collected PM is combusted to recover the filter function. The temperature of the DPF  30  for the recovery process is for example, between 600° C. and 700° C. A technique for determining whether or not a predetermined amount of PM has been accumulated is well known and will thus not be described. Furthermore, the DPF  30  may be configured to carry an oxidation catalyst comprising rare metal. 
     The urea aqueous solution addition valve  70  is fed with urea aqueous solution from a tank  75  and operates to add an amount of urea aqueous solution corresponding to a control signal from the ECU  100 . 
     The NOx catalyst  35  uses, as a reducing agent, ammonia generated by hydrolyzing a urea aqueous solution added via the urea addition valve  70  to selectively reduce NOx contained in the exhaust gas EG to nitrogen gas and water. Specifically, the urea aqueous solution added into the exhaust gas EG is hydrolyzed by heat from the exhaust gas EG, into ammonia, which serves as a reducing agent. The ammonia is adsorbed and held on the NOx catalyst  35 . The ammonia adsorbed and held on the NOx catalyst  35  reacts with NOx and is thus reduced to water and harmless nitrogen. When the amount of ammonia adsorbed on the NOx catalyst  35  exceeds a saturated adsorption amount, ammonia slip may occur. When the amount of ammonia adsorbed on the NOx catalyst  35  is excessively small, NOx may fail to be sufficiently purified. In this case, instead of the urea aqueous solution, ammonia may be directly supplied. Moreover, when a recovery process is executed on the DPF  30 , the high-temperature exhaust gas also increases the bed temperature of the NOx catalyst  35 . For example, an increase in the temperature of the DPF  30  up to about 700° C. also increases the temperature of the NOx catalyst  35  up to about 600 and several tens of degrees. When the temperature of the NOx catalyst  35  becomes so high as described above, the purification capability and ammonia adsorption capability of NOx are reduced or lost as described below. 
     The NOx catalyst  35  has a well-known structure. For example, the NOx catalyst may be composed of zeolite containing Si, O, and Al as main components, as well as Fe ions. Alternatively, the NOx catalyst may comprise a base material composed of aluminum alumina oxide and a vanadium catalyst (V2O5) carried on the surface thereof. The NOx catalyst  35  is not particularly limited to these examples. 
     The ECU  100  comprises hardware including a CPU (Central Processing Unit), backup memories such as a ROM (Read Only Memory), a RAM (Random Access Memory), and an EEPROM (Electronically Erasable and Programmable Read Only Memory), an input interface circuit including an A/D converter and a buffer, and an output interface circuit including a driving circuit, as well as required software. The ECU  100  not only controls the internal combustion engine  10  but also, based on signals from the NOx sensors  90 A and  90 B and the exhaust temperature sensors  95 A and  95 B, controls the amount of urea aqueous solution added via the urea aqueous solution addition valve  70  and controllably corrects the amount of urea aqueous solution as described below. 
     Now, an example of a urea aqueous solution addition amount correcting process executed by the ECU  100  will be described with reference to the flowchart shown in  FIG. 2 . The processing routine shown in  FIG. 2  is executed when the recovery process is carried out on the DPF  30 . 
     First, the bed temperature of the NOx catalyst  35  is acquired (step S 1 ). The bed temperature of the NOx catalyst  35  can be estimated from detected temperatures from the exhaust temperature sensors  95 A and  95 B. Alternatively, the bed temperature of the NOx catalyst  35  can directly be measured. Then, the ECU  100  determines whether or not the bed temperature of the NOx catalyst  35  exceeds a predetermined threshold Th (step S 2 ). 
       FIG. 3  shows the relationship between the bed temperature of the NOx catalyst  35  and the NOx purification rate. 
     The amount of NOx flowing into the NOx catalyst  35  is defined as Nin. The following amount of NOx is defined as Nout: the amount of NOx flowing out from the NOx catalyst  35  when a sufficient amount of urea aqueous solution is added which is required to purify all of the NOx amount Nin. Then, the NOx purification rate C of the NOx catalyst  35  is defined by: 
     
       
         
           
             
               
                 
                   C 
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                         Nin 
                         - 
                         Nout 
                       
                       Nin 
                     
                     × 
                     
                       100 
                       ⁢ 
                       
                           
                       
                       [ 
                       % 
                       ] 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
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                     1 
                   
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     As shown in  FIG. 3 , in an NOx purification temperature region (for example, a temperature region of lower than about 400° C.) R 1  in which the NOx purification rate C is larger than 0%, the NOx catalyst  35  purifies and converts much of inflow NOx into nitrogen gas N 2  and water H 2 O. A relatively lower temperature region of the NOx purification temperature region R 1 , most of the ammonia generated by hydrolysis of a urea aqueous solution is consumed for purification of NOx. The remaining ammonia is adsorbed on the NOx catalyst  35 . Then, an increase in the bed temperature of the NOx catalyst  35  correspondingly degrades the purification capability of the NOx catalyst, while enhancing the oxidation capability thereof. This increases the rate at which the ammonia is converted into NOx. 
     At a temperature Ta at which the NOx purification rate C is 0% as shown in  FIG. 3 , half of the ammonia is consumed for NOx purification, with the remaining half oxidized and converted into NOx. In a high-temperature region R 2  from a temperature Ta to a temperature Tb, an increase in the bed temperature of the NOx catalyst  35  from the temperature Ta correspondingly reduces the amount of ammonia used for NOx purification, while increasing the rate at which the ammonia is converted into NOx. At the temperature Tb, almost 100% of the ammonia is converted into NOx. In a temperature region R 3  exceeding the temperature Ta, 100% of the ammonia is theoretically converted into NOx. Thus, the NOx purification rate is −100%. 
     In the present embodiment, the threshold Th in step S 2  is set to a value in the high temperature region R 2  or R 3 , in which the amount of ammonia converted into NOx increases relatively, while the NOx purification rate C decreases relatively. Additionally, the upper limit of the threshold Th is determined by an increase in the temperature of the NOx catalyst  35  resulting from the recovery process carried out on the DPF  30 . 
     In step S 2 , if the bed temperature of the NOx catalyst  35  has not exceeded the threshold Th, the process is terminated, but during the recovery process for the DPF  30 , an addition amount correcting process routine is repeated. In step S 2 , if the bed temperature of the NOx catalyst  35  has exceeded the threshold Th, a predetermined amount of urea aqueous solution is added via the urea aqueous solution addition valve  70  (step S 3 ). In this case, before the recovery process for the DPF  30 , the addition of a urea aqueous solution is temporarily stopped, and a urea aqueous solution is re-added during step S 3 . Alternatively, for example, a urea aqueous solution may be continuously added regardless of the recovery process for the DPF  30 . Furthermore, if a urea aqueous solution is continuously added, the addition amount may be fixed or increased when the threshold Th is exceeded. 
     Then, outputs from the NOx sensors  90 A and  90 B, provided upstream and downstream of the NOx catalyst  35 , respectively, are acquired (step S 4 ). Then, the outputs from the NOx sensors  90 A and  90 B are used to determine the difference between NOx concentration measured before the NOx catalyst  35  and the NOx concentration measured after the NOx catalyst  35  (S 5 ). 
     For example, if the NO purification rate X of the NOx catalyst  35  is −100%, that is, if the bed temperature is in the high-temperature region R 3 , then for example, as shown in  FIG. 4 , all of the urea aqueous solution added to the NOx catalyst  35  (added ammonia amount) is converted into NOx, which then flows out from the NOx catalyst  35 . Moreover, all of the NOx having flowed into the NOx catalyst  35  flows out from the NOx catalyst  35  without being purified. Hence, when the NO purification rate C of the NOx catalyst  35  is −100%, the amount of NOx flowing out from the NOx catalyst  35  is the sum of the amount of NOx resulting from the conversion of the added ammonia and the amount of NOx having flowed into the NOx catalyst  35 . 
     The upstream NOx sensor  90 A detects the NOx concentration corresponding to the amount of NOx flowing into the NOx catalyst  35 . The downstream NOx sensor  90 B detects the NOx concentration corresponding to the total NOx amount. Thus, the difference between NOx concentration measured before the NOx catalyst  35  and the NOx concentration measured after the NOx catalyst  35  can be determined based on the difference between the NOx concentration obtained from the upstream NOx sensor  90 A and the NOx concentration obtained from the upstream NOx sensor  90 B. 
     Furthermore, for example, if the bed temperature of the NOx catalyst  25  is in the high-temperature region R 2 , not all of the added ammonia is converted into NOx. In the high-temperature region R 2 , addition of an excessive amount of urea aqueous solution may cause slippage of an amount of ammonia having failed to be used for NOx purification or converted into NOx. As shown in  FIG. 5A , the slip amount decreases with increasing bed temperature. Additionally, as shown in  FIG. 5B , the NOx purification rate of the NOx catalyst  35  decreases with increasing bed temperature. Moreover, as shown in  FIG. 5C , the rate at which ammonia is converted into NOx increases consistently with the bed temperature. Thus, if for example, the threshold Th is set to a value in the high-temperature region R 2 , the difference between NOx concentration measured before the NOx catalyst  35  and the NOx concentration measured after the NOx catalyst  35  can be corrected to a more accurate value using at least one of the relationships shown in  FIGS. 5A to 5C . In addition, the present embodiment can be appropriately modified such that even for the high-temperature region R 3 , a more accurate NOx concentration difference can be obtained instead of the simple difference between the outputs from the NOx sensors  90 A and  90 B. 
     Then, the amount of ammonia added is calculated from the NOx concentration difference determined in step S 5  (step S 6 ). The actual addition amount of urea aqueous solution is calculated from the calculated ammonia amount (step S 7 ). Then, the calculated actual addition amount of urea aqueous solution is compared with the target addition amount of urea aqueous solution. The process thus determines whether or not the amounts are equal, that is, whether or not there is a deviation of at least a predetermined amount (step S 8 ). If the deviation between the actual addition amount of urea aqueous solution and the target addition amount of urea aqueous solution is equal to or larger than a predetermined amount, the addition amount of urea aqueous solution specified for the urea aqueous solution addition valve  70  is corrected such that the actual addition amount of urea aqueous solution is equal to the target addition amount (step S 9 ). Thus, the actual addition amount of urea aqueous solution is corrected to an appropriate value. Hence, an appropriate amount of urea aqueous solution is added to the NOx catalyst. This enables a decrease in NOx purification rate and possible ammonia slip to be inhibited. 
       FIG. 6  is a diagram showing the configuration of an exhaust purification apparatus for an internal combustion engine according to another embodiment of the present invention. In  FIG. 6 , the same components as those in  FIG. 6  are denoted by the same reference numerals. 
     The exhaust purification apparatus shown in  FIG. 6  comprises a burner  60  provided upstream of a DPF  30  and serving as heating means, and an oxidation catalyst  40  provided downstream of the NOx catalyst. An NOx sensor  90 B and an exhaust temperature sensor  95 B are provided at the outlet of the oxidation catalyst  40 . 
     An air supply path  61  through which air is supplied and a fuel supply path  62  through which fuel is supplied are connected to the burner  60 ; the air supply path  61  and the fuel supply path  62  are arranged in this order from the internal combustion engine  10  side. Fuel supplied through the fuel supply path  62  is combusted, and the resultant combustion gas is fed to an exhaust passage  15 . Furthermore, the amount of air from the air supply path  61  and the amount of fuel from the fuel supply path  62  are controlled to control the air-fuel ratio of the combustion gas. The burner  60  is used to increase the bed temperatures of a DPF  30  and an NOx catalyst  35 . That is, regardless of the recovery process for the DPF  30 , the burner  60  can be used to increase the temperature of the NOx catalyst  35  into the above-described high-temperature regions R 2  and R 3 . 
     The oxidation catalyst  40  is provided to oxidize ammonia having slipped through the NOx catalyst  35 , into NOx. That is, provision of the oxidation catalyst  40  promotes the conversion of a urea aqueous solution (ammonia) added to the NOx catalyst  35  into NOx. Thus, the above-described NOx concentration difference can be accurately determined. As a result, the addition amount of urea aqueous solution can be controllably corrected more precisely. 
     In the above-described embodiment, by way of example, the recovery process for the DPF  30  or the burner  60  is used to set the temperature of the NOx catalyst  35  to a value in a predetermined high-temperature region. However, the present invention is not limited to these aspects. For example, during rapid acceleration or what is called towing in which a high load is applied to the internal combustion engine  10 , the temperature of the exhaust gas increases to allow the temperature of the NOx catalyst  35  to reach the predetermined high-temperature region. Even in this case, the addition amount correction control as described above is possible. 
     Instead of the burner, a heater configured to heat the exhaust gas or NOx catalyst can be used as heating means. 
     In the above-described embodiments, the DPF  30  is located upstream of the NOx catalyst. However, the present invention is also applicable to the case in which the DPF  30  is provided downstream of the NOx catalyst  35 . The present invention is also applicable to the case in which the DPF  30  is not used. 
     REFERENCE SIGNS LIST 
     
         
           10  . . . Internal combustion engine 
           15  . . . exhaust passage 
           30  . . . DPF (filter) 
           35  . . . NOx catalyst 
           60  . . . Burner 
           70  . . . Urea aqueous solution addition valve 
           100  . . . ECU 
           90 A,  90 B . . . NOx sensor 
           95 A,  95 B . . . . Exhaust temperature sensor