Abstract:
An exhaust gas purification device includes a diesel particulate filter (DPF) for capturing particulate matter (PM) in an exhaust gas, a selective catalytic reduction (SCR) device for reducing NOx in the exhaust gas, detecting units for detecting the DPF electrostatic capacity, an estimating unit for estimating the inside temperature of the DPF based on the electrostatic capacity, and a controlling unit for executing forced DPF regeneration. A lower limit temperature is defined as a temperature to trigger PM combustion, and an upper limit temperature is defined as a temperature to avoid filter erosion. The controlling unit executes the forced regeneration with an amount of fuel supplied for causing the inside temperature to reach the lower limit temperature, when the inside temperature is at or above the SCR activation temperature, and executes the forced regeneration with another amount of fuel supplied for causing the inside temperature to reach the upper limit temperature, when the inside temperature is below the SCR activation temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application, which claims the benefit under 35 U.S.C. §371 of PCT International Patent Application No. PCT/JP2014/053738, filed Feb. 18, 2014, which claims the foreign priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2013-039064, filed Feb. 28, 2013, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an exhaust gas purification device of an internal combustion engine. 
     BACKGROUND ART 
     A diesel particulate filter (hereinafter referred to as “DPF”), for example, is known as a filter for collecting particulate matter (hereinafter referred to as “PM”) in an exhaust gas emitted from a diesel engine. 
     The DPF has a limitation on an amount of collecting PM. Thus, it is necessary to carry out forced regeneration that periodically burns and removes the accumulated PM. The forced regeneration is performed by supplying unburned fuel, primarily hydrocarbon (HC), to an oxidation catalyst in an upstream exhaust passage through in-pipe injection or post injection, and raising the exhaust gas temperature to PM burning temperature with the heat generated upon oxidation. 
     Also, a selective catalytic reduction (hereinafter referred to as “SCR”) catalyst is known as a nitrogen compound (hereinafter referred to as “NOx”) catalyst for purifying NOx in an exhaust gas. The SCR catalyst selectively reduces and purifies NOx in the exhaust gas by using ammonia (NH 3 ) produced from a urea solution (urea water) upon hydrolysis with heat of the exhaust gas. 
     LISTING OF REFERENCES 
     PATENT LITERATURE 1: Japanese Patent Application Laid-Open Publication (Kokai) No. 2013-2283 
     PATENT LITERATURE 2: Japanese Patent Application Laid-Open Publication (Kokai) No. 2009-243316 
     SUMMARY OF THE INVENTION 
     The NOx purification capability or performance of the SCR varies with the catalyst temperature (temperature of the exhaust gas flowing into the SCR). In particular, when the temperature is low (when the SCR temperature is below its activation temperature), the absorption of ammonia (NH 3 ) which serves as the reducing agent decreases. This can be a cause of deteriorating the NOx purification rate. Because of this, if the NOx purification capability of the SCR should be improved, it is necessary to raise the temperature of the exhaust gas flowing into the SCR to the activation temperature at an early stage (or quickly). 
     The present invention has been developed in such a viewpoint, and an object of the present invention is to effectively improve the NOx purification rate by raising the temperature of the exhaust gas flowing into the SCR to the activation temperature at an early stage. 
     To achieve the above-mentioned object, an exhaust gas purification device of an internal combustion engine according to the present invention includes a filter that is provided in an exhaust passage of the internal combustion engine and configured to collect particulate matter in an exhaust gas; a urea water spraying unit that is provided in the exhaust passage downstream of the filter and configured to spray urea water into the exhaust gas; a selective reduction catalyst that is provided in the exhaust passage downstream of the urea water spraying unit and configured to reduce and purify a nitrogen compound in the exhaust gas by using ammonia produced from the urea water; an electrostatic capacity detecting unit configured to detect an electrostatic capacity (capacitance) of the filter; a filter temperature estimating unit configured to estimate inside temperature of the filter based on the detected electrostatic capacity; and a filter regenerating unit configured to carry out forced regeneration by supplying fuel to the filter and raising temperature of the filter to or over burning temperature of the particulate matter. The filter regenerating unit is configured to carry out the forced regeneration with an amount of fuel that is supplied to make the inside temperature equal to a predetermined target lower limit temperature, if the calculated inside temperature is equal to or higher than activation temperature of the selective reduction catalyst. The target lower limit temperature is temperature to start burning of the particulate matter. The filter regenerating unit is configured to carry out the forced regeneration with another amount of fuel that is supplied to make the inside temperature equal to a predetermined target upper limit temperature, if the calculated inside temperature is lower than the activation temperature of the selective reduction catalyst. The target upper limit temperature is temperature to avoid erosion of the filter. 
     The electrostatic capacity detecting unit may include a pair of electrodes disposed in a corresponding pair of cells that oppose each other with at least one cell in the filter interposed therebetween. 
     The exhaust gas purification device may further include a bypass passage that branches off from the exhaust passage at a position upstream of the filter so as to bypass the filter, and a second filter that is provided in the bypass passage and configured to collect particulate matter in the exhaust gas flowing through the bypass passage. The paired electrodes may be disposed in a corresponding pair of cells that oppose each other with at least one cell in the second filter interposed therebetween. 
     When the forced regeneration is carried out in the second filter, the paired electrodes may function as a heater. 
     The exhaust gas purification device of an internal combustion engine according to the present invention can raise the temperature of the exhaust gas flowing into the SCR to the activation temperature at an early stage, and effectively improve the NOx purification rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall configuration diagram schematically illustrating an exhaust gas purification device of an internal combustion engine according to an embodiment of the present invention. 
         FIG. 2  illustrates a diagram useful to describe changes in the electrostatic capacity, the DPF inlet temperature, and the DPF outlet temperature of the exhaust gas purification device of the internal combustion engine according to the embodiment of the present invention. 
         FIG. 3  is a flowchart showing processing to be executed by the exhaust gas purification device of the internal combustion engine according to the embodiment of the present invention. 
         FIG. 4  is an overall configuration diagram schematically illustrating an exhaust gas purification device of an internal combustion engine according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to  FIGS. 1 to 3 , an exhaust gas purification device of an internal combustion engine according to embodiments of the present invention will be described. Identical parts are given identical reference numerals and symbols, and their names and functions are identical as well. Therefore, detailed description of such parts will not be repeated. 
     As illustrated in  FIG. 1 , a diesel engine (hereinafter, simply referred to as “engine”)  10  has an intake manifold  10   a  and an exhaust manifold  10   b . An intake passage  11  for introducing fresh air is connected to the intake manifold  10   a , and an exhaust passage  12  for discharging an exhaust gas to the atmosphere is connected to the exhaust manifold  10   b . A pre-stage (upstream) post-treatment device  14  and a post-stage (downstream) post-treatment device  20  are provided in the exhaust passage  12 . The post-treatment device  14  is arranged upstream of the post-treatment device  20 . It should be noted that the engine  10  is not limited to a diesel engine. The engine  10  can be other internal combustion engines including a gasoline engine. 
     The pre-stage post-treatment device  14  is constituted by a diesel oxidation catalyst (hereinafter referred to as “DOC”)  15  and a DPF  16  disposed in a casing  14   a . The DOC  15  is arranged upstream of the DPF  16 . An in-pipe injection device  13  is provided upstream of the DOC  15 . A DPF inlet temperature sensor  18  is provided upstream of the DPF  16 . A DPF outlet temperature sensor  19  is provided downstream of the DPF  16 . 
     The in-pipe injection device  13  injects unburned fuel (primarily HC) into the exhaust passage  12 , in response to an instruction signal from an electronic control unit (hereinafter referred to as “ECU”)  40 . The in-pipe injection device  13  may be omitted if post-injection through multiple-injection of the engine  10  is carried out. 
     The DOC  15  includes, for example, a ceramic carrier having a cordierite honeycomb structure, with a catalyst component supported on a surface of the ceramic carrier. Upon unburned fuel (primarily HC) being supplied by the in-pipe injection device  13  or through post-injection, the DOC  15  oxidizes the unburned fuel, thereby causing the exhaust gas temperature to rise. The DOC  15  also oxidizes NO in the exhaust gas to produce NO 2 , thereby causing the ratio of NO 2  to NO in the exhaust gas to increase. 
     The DPF  16  includes, for example, a number of cells defined by porous partition walls disposed along a flowing direction of the exhaust gas. The cells are plugged alternatingly at the upstream side and the downstream side. The DPF  16  collects PM in the exhaust gas into the small cavities and on the surfaces of the partition walls. Upon the amount of accumulated PM reaching a predetermined amount, a so-called forced regeneration for burning and removing the PM is performed. The forced regeneration is performed by supplying unburned fuel (primarily HC) to the DOC  15  by the in-pipe injection device  13  or through post-injection and by raising the temperature of the DPF  16  to the PM-burning temperature (e.g., approximately 600 degrees C.). 
     The DPF  16  of this embodiment is provided with a pair of electrodes  17   a  and  17   b  disposed inside a corresponding pair of cells that oppose each other with at least one cell interposed therebetween. The paired electrodes  17   a  and  17   b  form a capacitor. The paired electrodes  17   a  and  17   b  are electrically connected to the ECU  40 . 
     The DPF inlet temperature sensor  18  detects the temperature of the exhaust gas flowing into the DPF  16  (hereinafter referred to as “inlet temperature T IN ”). The DPF outlet temperature sensor  19  detects the temperature of the exhaust gas flowing out of the DPF  16  (hereinafter referred to as “outlet temperature T OUT ”). The inlet temperature T IN  and the outlet temperature T OUT  are introduced to the ECU  40  that is electrically connected to the DPF inlet temperature sensor  18  and the DPF outlet temperature sensor  19 . 
     The post-stage post-treatment device  20  includes a urea water spraying device  21  and an SCR  22  disposed in a casing  20   a . The urea water spraying device  21  is arranged upstream of the SCR  22 . 
     The urea water spraying device  21  sprays or injects urea water (urea solution) from a urea water tank (not illustrated) into the exhaust passage  12  between the pre-stage post-treatment device  14  and the post-stage post-treatment device  20 , in response to an instruction signal from the ECU  40 . The sprayed urea water undergoes hydrolysis with the heat of the exhaust gas, and ammonia (NH 3 ) is produced. Ammonia (NH 3 ) is then supplied to the SCR  22  on the downstream side as a reducing agent. 
     The SCR  22  includes, for example, a ceramic carrier having a honeycomb structure, with a copper zeolite or an iron zeolite supported on a surface of the ceramic carrier. The SCR  22  adsorbs ammonia (NH 3 ) supplied as the reducing agent and reduces and purifies NOx in the exhaust gas passing therethrough with the adsorbed ammonia (NH 3 ). 
     The ECU  40  controls the engine  10 , the in-pipe injection device  13 , the urea water spraying device  21 , and other components, and includes known CPU, ROM, RAM, input port, output port, and so on. The ECU  40  further includes, as part of its functional elements, an electrostatic capacity calculating unit  41 , a PM accumulation amount estimating unit  42 , a DPF temperature estimating unit  43 , and a regeneration controlling unit  44 . The description continues with a premise that these functional elements are included in the ECU  40 , which is an integrated piece of hardware, but some of these functional elements can be provided in a separate piece of hardware. In this embodiment, the electrostatic capacity calculating unit  41  and the electrodes  17   a  and  17   b  constitute an electrostatic capacity detecting unit according to the present invention. 
     The electrostatic capacity calculating unit  41  calculates an electrostatic capacity (capacitance) C between the electrodes  17   a  and  17   b  on the basis of signals entered from the paired electrodes  17   a  and  17   b . The electrostatic capacity C is calculated by Expression 1, where c represents a dielectric constant of a medium between the electrodes  17   a  and  17   b , S represents the area of the electrodes  17   a  and  17   b , and d represents the distance between the electrodes  17   a  and  17   b.    
     
       
         
           
             
               
                 
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     The PM accumulation amount estimating unit  42  estimates the amount of accumulated PM collected by the DPF  16  (i.e., PM DEP ), on the basis of the electrostatic capacity C calculated by the electrostatic capacity calculating unit  41  and an average T AVE  of the inlet temperature T IN  detected by the DPF inlet temperature sensor  18  and the outlet temperature T OUT  detected by the DPF outlet temperature sensor  19 . The amount of accumulated PM (PM DEP ) can be estimated by using an approximation formula, a map, or the like, which may be prepared or obtained in advance through an experiment. 
     The DPF temperature estimating unit  43  estimates the inside temperature of the DPF  16  (hereinafter, referred to as “DPF inside temperature T DPF ”). As shown in  FIG. 2 , the changes in the electrostatic capacity C show a similar response to the detection values of the DPF inlet temperature sensor  18  and the detection values of the DPF outlet temperature sensor  19 . In addition, the changes in the electrostatic capacity C show a faster response than the detection values of the DPF inlet temperature sensor  18  and the detection values of the DPF outlet temperature sensor  19 . The DPF temperature estimating unit  43  of this embodiment estimates the DPF inside temperature T DPF  on the basis of the electrostatic capacity C calculated by the electrostatic capacity calculating unit  41  and the PM accumulation amount PM DEP  estimated by the PM accumulation amount estimating unit  42 . 
     More specifically, although the PM accumulation amount varies with the running condition, variations are limited in a certain short time (e.g., approximately one second). In this embodiment, the PM accumulation amount PM DEP , which is estimated immediately before the current estimation, is taken as a fixed value. Then, sudden or unexpected changes in the DPF inside temperature T DPF , which cannot be followed by the DPF inlet temperature sensor  18  and the DPF outlet temperature sensor  19 , are estimated from instantaneous changes (very fast changes) in the electrostatic capacity C. The estimation of the DPF inside temperature T DPF  may be performed with approximation formula, a map, or the like, which may be prepared or obtained in advance through an experiment. 
     The regeneration controlling unit  44  causes the in-pipe injection device  13  to inject fuel (or to perform the post injection), thereby carrying out the forced regeneration, when the PM accumulation amount PM DEP  estimated by the PM accumulation amount estimating unit  42  reaches the accumulation upper limit PM MAX  that indicates the maximum collectable amount of the DPF  16 . An amount of fuel injected during the forced regeneration is feedback controlled in accordance with the DPF inside temperature T DPF  (i.e., temperature of the exhaust gas flowing into the SCR  22  ) estimated by the DPF temperature estimating unit  43 . 
     More specifically, the ECU  40  stores in advance a target lower limit temperature T 1  (e.g., 600 degrees C.) at which the PM accumulated in the DPF  16  starts burning, and a target upper limit temperature T 2  (e.g., 900 degrees C.) which can avoid erosion of the DPF  16  due to excessively elevated temperature of the DPF  16 . 
     When the DPF inside temperature T DPF  is at or above the activation temperature T ACT  of the SCR  22  (T DPF ≧T ACT ), the regeneration controlling unit  44  feedback controls the amount of fuel to be injected such that the DPF inside temperature T DPF  during the forced regeneration becomes the target lower limit temperature T 1  (lower limit temperature of PM combustion). This effectively suppresses the excessive temperature increase of the SCR  22  during the forced regeneration, and prevents the deterioration of the NOx purification capability. 
     On the other hand, when the DPF inside temperature T DPF  is lower than the activation temperature T ACT  of the SCR  22  (T DPF &lt;T ACT ), the regeneration controlling unit  44  feedback controls the amount of fuel to be injected such that the DPF inside temperature T DPF  during the forced regeneration becomes the target upper limit temperature T 2  (upper limit temperature of erosion). This raises the temperature of the gas fed to the low temperature SCR  22  to the activation temperature T ACT  at an early stage (quickly), and improves the NOx purification capability. 
     Referring now to  FIG. 3 , the control (processing) executed by the exhaust gas purification device of this embodiment will be described. The control starts upon turning on of an ignition key. 
     At Step (hereinafter, abbreviated as “S”)  100 , it is determined whether or not the PM accumulation amount PM DEP  estimated from the electrostatic capacity C has reaches the accumulation upper limit PM MAX . If the PM accumulation amount PM DEP  is equal to or greater than the accumulation upper limit PM MAX  (YES), the control proceeds to S 110 . On the other hand, when the PM accumulation amount PM DEP  is less than the accumulation upper limit PM MAX  (NO), the control proceeds to Return. 
     At S 110 , it is determined whether or not the DPF inside temperature T DPF  estimated from the electrostatic capacity C and the PM accumulation amount PM DEP  has reached the activation temperature T ACT  of the SCR  22 . When the DPF inside temperature T DPF  is equal to or higher than the activation temperature T ACT  (YES), the control proceeds to S 120 . At S 120 , the amount of fuel to be injected during the forced regeneration is feedback controlled such that the DPF inside temperature T DPF  becomes the target lower limit temperature T 1  (lower limit temperature of PM combustion). Then, the control proceeds to Return. 
     On the other hand, when S 110  determines that the DPF inside temperature T DPF  is lower than the activation temperature T ACT  (NO), the control proceeds to S 130 . At S 130 , the amount of fuel to be injected during the forced regeneration is feedback controlled such that the DPF inside temperature T DPF  becomes the target upper limit temperature T 2  (upper limit temperature of erosion). Then, the control proceeds to Return. 
     Operations and advantages of the exhaust gas purification device of an internal combustion engine according to this embodiment will now be described. 
     In general, the NOx purification capability of the SCR  22  drops in particular when the temperature of the SCR  22  is low (when the temperature of the SCR  22  is below the activation temperature). Because of this, if the NOx purification capability of the SCR  22  should be improved, it is necessary to raise the temperature of the exhaust gas flowing into the SCR  22  to the activation temperature at an early stage (quickly). 
     In the exhaust gas purification device of the internal combustion engine according to this embodiment, the DPF inside temperature is precisely estimated from the change in the electrostatic capacity C that has faster response than the exhaust gas temperature sensor, and an amount of fuel to be injected during the forced regeneration is controlled on the basis of the DPF inside temperature. In particular, when the DPF inside temperature has reached the activation temperature of the SCR  22 , an amount of fuel to be injected is feedback controlled such that the DPF inside temperature becomes the lower limit temperature of the PM burning. On the other hand, when the DPF inside temperature has not reached the activation temperature of the SCR  22 , then an amount of fuel to be injected is feedback controlled such that the DPF inside temperature becomes the upper limit temperature of the filter erosion. 
     Therefore, the exhaust gas purification device of the internal combustion engine according to this embodiment can precisely estimate the inside temperature of the DPF  16  from the electrostatic capacity C, which shows faster response than the exhaust gas temperature sensor. Also, the exhaust gas purification device of this embodiment can utilize the PM combustion heat (heat generated upon burning of the PM) during the forced regeneration to raise the temperature of the low temperature SCR  22  to the activation temperature at an early stage (quickly). As a result, it is possible to effectively improve the NOx purification capability of the SCR  22 . 
     It is to be noted that the present invention is not limited to the above-described embodiment and can be implemented with changes and modifications, as appropriate, within the scope that does not depart from the spirit of the present invention. 
     For example, as illustrated in  FIG. 4 , a bypass passage  12   a  may be connected to the exhaust passage  12  so as to bypass the DPF  16 , and a DPF  16  a for measurement (second filter) with a small capacity may be provided in the bypass passage  12   a . In this case, it is preferred that a pair of electrodes  17   a  and  17   b  be disposed inside a corresponding pair of cells that oppose each other with at least one cell in the DPF  16   a  interposed therebetween. It is also preferred that the bypass passage  12   a  be provided with an orifice  12   b  (throttle) that regulates the flow rate of the exhaust gas. When the forced regeneration of the DPF  16   a  is executed, the paired electrodes  17   a  and  17   b  may be used as a heater by applying a voltage across the electrodes  17   a  and  17   b.    
     The NOx catalyst is not limited to the SCR  22 . The NOx catalyst may be a lean NOx trap (LNT) that absorbs and retains NOx at a lean air-fuel ratio and reduces NOx at a rich air-fuel ratio. This configuration can also achieve the same operation and advantages as the above-described embodiment.