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/JP2014/076962, filed Oct. 8, 2014, which claims the foreign priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2013-210700, filed Oct. 8, 2013, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an exhaust purification system, and in particular to an exhaust purification system that includes a filter for collecting particulate matter contained in exhaust gas emitted from 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”) contained in exhaust gas emitted from a diesel engine. 
     The DPF can only collect a limited amount of PM. Thus, so-called forced regenerations need to be performed, in which accumulated PM is periodically burned and removed. In a forced regeneration, unburned hydrocarbons (HC) are supplied to a diesel oxidation catalyst (hereinafter referred to as “DOC”) on an upstream side in an exhaust gas flowing direction through in-pipe injection (fuel injection into an exhaust pipe) or post-injection to cause oxidation, and to raise the temperature of the exhaust gas to a PM burning temperature. 
     Known techniques for detecting an amount of accumulated PM collected by a DPF include, for example, a technique of estimating the amount on the basis of a pressure difference across the DPF and a technique of estimating the amount from the electrostatic capacity (capacitance) between electrodes provided in the DPF (e.g., see PATENT LITERATURE DOCUMENTS 1 and 2). 
     LISTING OF REFERENCES 
     PATENT LITERATURE DOCUMENT 1: Japanese Patent Application Laid-Open Publication No. 2011-247145 
     PATENT LITERATURE DOCUMENT 2: Japanese Patent Application Laid-Open Publication No. 2009-97410 
     The technique of estimating the amount of accumulated PM on the basis of the pressure difference across the DPF, however, faces an issue that the amount of accumulated PM cannot be estimated accurately because the sensitivity drops particularly in an operation range in which the flow rate of the exhaust gas decreases. The technique of estimating the amount from the electrostatic capacity between the electrodes enables the amount of accumulated PM to be estimated without being affected by a running condition of a vehicle or the like. However, the dimensions or the arrangement of the electrodes needs to be decided individually in accordance with the shapes, the pitch, or the like of cells disposed in the DPF. This creates an issue that the technique cannot flexibly deal with the specifications or the like of the DPF. 
     SUMMARY OF THE INVENTION 
     The system disclosed herein has an object to detect an amount of accumulated PM in a DPF with high accuracy. 
     A system disclosed herein includes an oxidation catalyst provided in an exhaust passage of an internal combustion engine; a filter provided in the exhaust passage at a position downstream of the oxidation catalyst to collect particulate matter contained in exhaust gas; an electrostatic capacity detecting unit that detects an electrostatic capacity (capacitance) of the oxidation catalyst; an accumulation amount estimating unit that estimates an amount of accumulated particulate matter in the filter on the basis of the electrostatic capacity entered from the electrostatic capacity detecting unit; and a filter regenerating unit that executes a forced regeneration to burn and remove the particulate matter that has accumulated at least in the filter by injecting fuel to the oxidation catalyst when the amount of the accumulated particulate matter entered from the accumulation amount estimating unit exceeds a predetermined amount. 
     The system disclosed herein can detect an amount of PM accumulated in a DPF with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall configuration diagram schematically illustrating an exhaust purification system according to an embodiment of the present invention. 
         FIG. 2  is a functional block diagram of an ECU according to an embodiment of the present invention. 
         FIG. 3(A)  illustrates an example of a DOC accumulation amount map according to an embodiment of the present invention, and  FIG. 3(B)  illustrates an example of a map of correlation between the DOC accumulation amount and a DPF accumulation amount according to an embodiment of the present invention. 
         FIG. 4  illustrates an example of a temperature characteristics map according to an embodiment of the present invention. 
         FIG. 5  illustrates an example of an injection amount correction map according to an embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating contents of control according to an embodiment of the present invention. 
         FIG. 7  illustrates a graph comparing the electrostatic capacity between electrodes to a sensor value of an exhaust gas temperature sensor. 
         FIG. 8  is an overall configuration diagram schematically illustrating an exhaust purification system of an internal combustion engine according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An exhaust purification system according to embodiments of the present invention will be described with reference to the appended drawings. Identical components are given identical reference numerals and symbols, and their names and functions are identical as well. Therefore, detailed descriptions of such components are not repeated. 
     As illustrated in  FIG. 1 , a diesel engine (hereinafter simply referred to “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 the exhaust gas to the atmosphere is connected to the exhaust manifold  10   b.    
     On the intake passage  11 , disposed are an air cleaner  13 , a mass airflow sensor  14 , a compressor  15   a  of a turbo charger  15 , an intercooler  16 , and so on in this order from the upstream side with respect to an intake air flowing direction. On the exhaust passage  12 , disposed are a turbine  15   b  of the turbo charger  15 , an exhaust gas aftertreatment device  20 , and so on in this order from the upstream side with respect to an exhaust gas flowing direction. 
     The exhaust gas aftertreatment device  20  includes a catalyst casing  20   a , a DOC  21 , and a DPF  22 . The DOC  21  is disposed upstream of the DPF  22  in the catalyst casing  20   a . An in-pipe injection device (device for injecting fuel into the exhaust passage)  23  is provided upstream of the DOC  21 . 
     The in-pipe injection device  23 , which constitutes a part of a filter regenerating unit according to the present invention, injects unburned fuel (primarily HC) into the exhaust passage  12  in response to an instruction signal (pulse current) entered from an electronic control unit (hereinafter referred to as “ECU”)  50 . It should be noted that the in-pipe injection device  23  may be omitted if post-injection by means of multiple-injection of the engine  10  is employed. 
     The DOC  21  includes a ceramic carrier having, for example, a cordierite honeycomb structure and a catalyst component supported on a surface of the ceramic carrier. The DOC  21  has a number of cells, which are defined by porous partition walls, arranged along the exhaust gas flowing direction. The DOC  21  collects the PM contained in the exhaust gas into fine pores of the partition walls and the surfaces of the partition walls. As unburned fuel (HC) is supplied to the DOC  21  by the in-pipe injection device  23  or through post-injection, the DOC  21  oxidizes the HC to raise the exhaust gas temperature. 
     The DOC  21  of this embodiment has a plurality of electrodes  27  that are disposed so as to face each other with at least one or more partition walls interposed therebetween to form a capacitor. The outer peripheral faces of the electrodes  27  are covered with corrosion-resistive insulating layers (not shown). The electrodes  27  are electrically connected to the ECU  50  via an electrostatic capacity detecting circuit (not shown). The electrodes  27  and the electrostatic capacity detecting circuit (not shown) serve as a preferred example of an electrostatic capacity detecting unit according to the present invention. 
     The DPF  22  includes, for example, a number of cells, which are defined by porous partition walls, arranged along the exhaust gas flowing direction. The upstream sides and the downstream sides of these cells are plugged in an alternating manner. The DPF  22  collects the PM contained in the exhaust gas into the small cavities in the partition walls or onto their surfaces. When an estimated amount of accumulated PM reaches a predetermined amount, a so-called forced regeneration is carried out to the DPF  22 , i.e., the accumulated PM in the DPF  22  is burned and removed. In the forced regeneration, unburned fuel (HC) is supplied to the DOC  21  by the in-pipe injection device  23  or through post-injection, and the temperature of the exhaust gas flowing into the DPF  22  is raised to a PM-burning temperature (e.g., approximately 500 degrees C. to 600 degrees C.). 
     The ECU  50  controls the engine  10 , the in-pipe injection device  23 , and other components. The ECU  50  includes a CPU, a ROM, a RAM, input ports, output ports, and other components which are known in the art. 
     As illustrated in  FIG. 2 , the ECU  50  also includes, as some of its function elements, a PM accumulation amount estimating unit  51 , a DOC internal temperature estimating unit  52 , a forced regeneration controlling unit  53 , and an injection amount correcting unit  54 . The description continues with a premise that these functional elements are included in the ECU  50 , which is an integrated piece of hardware. Alternatively, some of these functional elements may be provided in separate pieces of hardware. 
     The PM accumulation amount estimating unit  51 , which is an example of an accumulation amount estimating unit according to the present invention, estimates an amount of accumulated PM, which is collected by the DPF  22 , (hereinafter referred to as “DPF accumulation amount PM DEP ”) on the basis of an electrostatic capacity (capacitance) C between the electrodes  27  provided in the DOC  21 . In general, the electrostatic capacity C between the electrodes  27  is expressed by Expression 1, where ∈ represents the dielectric constant of a medium between the electrodes  27 , S represents the area of the electrodes  27 , and d represents the distance between the electrodes  27 . 
     
       
         
           
             
               
                 
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     In Expression 1, the area S of the electrodes  27  and the distance d therebetween are the constants. As more PM accumulates between the electrodes  27 , the dielectric constant ∈ and the distance d change, and so does the electrostatic capacity C. In other words, detecting the electrostatic capacity C between the electrodes  27  enables the amount of accumulated PM collected by the DOC  21  (hereinafter referred to as “DOC accumulation amount PM DOC ”) to be calculated. 
     The ECU  50  stores a DOC accumulation amount map (e.g., see  FIG. 3(A) ) indicating the relation between the electrostatic capacity C between the electrodes  27  and the DOC accumulation amount PM DOC , and also stores a correlation map of DOC-DPF accumulation amount (e.g., see  FIG. 3(B) ) indicating the relation between the DOC accumulation amount PM DOC  and the DPF accumulation amount PM DPF . These maps are prepared in advance through experiments or the like. The PM accumulation amount estimating unit  51  is configured to estimate the DOC accumulation amount PM DOC  by reading out, from the DOC accumulation amount map, a value corresponding to the electrostatic capacity C between the electrodes  27 , and to estimate the DPF accumulation amount PM DPF  by reading out, from the DOC-DPF accumulation amount correlation map, a value corresponding to the DOC accumulation amount PM DOC . It should be noted that the DOC accumulation amount PM DOC  and the DPF accumulation amount PM DPF  may be estimated from other than these maps. For example, the DOC accumulation amount PM DOC  and the DPF accumulation amount PM DPF  may be estimated through an approximation formula or the like, which may be prepared in advance through experiments or the like. 
     The DOC internal temperature estimating unit  52 , which is an example of an internal temperature estimating unit according to the present invention, calculates the internal temperature of the DOC  21  (hereinafter referred to as “DOC internal temperature T DOC ”) on the basis of the electrostatic capacity C between the electrodes  27 . In Expression 1, when the dielectric constant ∈ changes as being affected by the exhaust gas temperature, the electrostatic capacity C also changes correspondingly. In other words, detecting the electrostatic capacity C between the electrodes  27  enables the DOC internal temperature T DOC  to be calculated. 
     The ECU  50  stores a temperature characteristics map (e.g., see  FIG. 4 ) indicating the relation between the electrostatic capacity C and the DOC internal temperature T DOC . The map is prepared in advance by experiments or the like. The DOC internal temperature estimating unit  52  is configured to estimate the DOC internal temperature T DOC  by reading out, from the temperature characteristics map, a value corresponding to the electrostatic capacity C between the electrodes  27 . It should be noted that the DOC internal temperature T DOC  may be estimated from other than the map. For example, the DOC internal temperature T DOC  may be estimated through an approximation formula or the like, which may be prepared in advance through experiments or the like. 
     The forced regeneration controlling unit  53 , which is an example of a filter regenerating unit according to the present invention, controls a forced regeneration on the basis of the DPF accumulation amount PM DPF  entered from the PM accumulation amount estimating unit  51 . Specifically, the forced regeneration controlling unit  53  starts the forced regeneration by causing the in-pipe injection device  23  to execute in-pipe injection in a predetermined amount when the DPF accumulation amount PM DPF  exceeds an upper limit accumulation amount PM MAX  up to which the DPF  22  can collect PM (PM DEP &gt;PM MAX ). The amount of in-pipe injection in the forced regeneration is corrected as necessary by the injection amount correcting unit  54 , which will be described below. 
     The injection amount correcting unit  54  corrects the fuel injection amount in a forced regeneration on the basis of a temperature difference ΔT between the DOC internal temperature T DOC , which is entered from the DOC internal temperature estimating unit  52 , and a target temperature T TARGT  at which the PM in the DPF  22  is substantially completely burned and removed. Specifically, the ECU  50  stores an injection amount correction map (e.g., see  FIG. 5 ) indicating the relation between the temperature difference ΔT and an injection correction amount ΔINJ needed to compensate for the temperature difference ΔT. The injection amount correction map is prepared in advance by experiments or the like. The in-pipe injection amount INJ Q exh  in a forced regeneration is set by reading out, from the injection amount correction map, an injection correction amount ΔINJ corresponding to the temperature difference ΔT and by adding the read-out injection correction amount ΔINJ or subtracting the read-out injection correction amount ΔINJ to or from a standard injection amount INJ Q std  (INJ Q exh =INJ Q std +/−ΔINJ). The fuel injection after the correction is executed by increasing or reducing the conducting pulse duration of each injection applied to an injector of the in-pipe injection device  23  or by increasing or reducing the frequency of injections. 
     Referring now to  FIG. 6 , a control process of the exhaust purification system of this embodiment will be described. This control starts when an ignition key is turned on. 
     In Step  100  (hereinafter, the term “step” is abbreviated as “S”), the DOC accumulation amount PM DOC  corresponding to the electrostatic capacity C between the electrodes  27  is read out from the DOC accumulation amount map (see  FIG. 3(A) ). In S 110 , the DPF accumulation amount PM DPF  corresponding to the DOC accumulation amount PM DOC  in S 100  is read out from the DOC-DPF accumulation amount correlation map (see  FIG. 3(B) ). 
     In S 120 , it is determined whether the DPF accumulation amount PM DPF  has exceeded the upper limit accumulation amount PM MAX . If the DPF accumulation amount PM DPF  has exceeded the upper limit accumulation amount PM MAX  (Yes), the processing proceeds to S 130  to start a forced regeneration to the DPF  22 . 
     In S 130 , the DOC internal temperature T DOC  corresponding to the electrostatic capacity C between the electrodes  27  is read out from the temperature characteristics map (see  FIG. 4 ). In S 140 , the DOC internal temperature T DOC  is compared to the target temperature T TARGT . If the temperature difference ΔT (absolute value) between the target temperature T TARGT  and the DPF internal temperature T DPF  is greater than 0 (Yes), the processing proceeds to S 150 . On the other hand, if the temperature difference ΔT is 0 (No), the DOC internal temperature T DOC  can be raised to the target temperature T TARGT  even if the in-pipe injection is executed in the standard injection amount INJ Q std . In this case, the processing proceeds to S 170 , and the in-pipe injection is executed in the standard injection amount INJ Q std . 
     In S 150 , the injection amount is corrected by adding the injection correction amount ΔINJ, which is read out from the injection amount correction map in accordance with the temperature difference ΔT, or subtracting the injection correction amount ΔINJ to or from the standard injection amount INJ Q std  (INJ Q exh =INJ Q std +/−ΔINJ). In S 160 , the in-pipe injection is executed on the basis of the corrected in-pipe injection amount INJ Q exh . 
     In S 180 , it is determined whether the DPF accumulation amount PM DPF  has decreased to a lower threshold PM MIN . The lower threshold PM MIN  indicates the end of the regeneration of the DPF  22 . If the DPF accumulation amount PM DPF  has decreased to the lower threshold PM MIN  (Yes), the in-pipe injection is stopped in S 190 , and this control proceeds to “Return.” Thereafter, S 100  to S 190  are iterated until the ignition key is turned off. 
     Effects of the exhaust purification system of this embodiment will now be described. 
     Conventionally, the technique of estimating an amount of accumulated PM with a differential pressure sensor encounters a problem that the sensitivity drops particularly in a low load operation range in which the flow rate of the exhaust gas decreases or toward the end of a forced regeneration. In contrast, the exhaust purification system of this embodiment estimates the DPF accumulation amount PM DPF  in the DPF  22  on the basis of the electrostatic capacity C between the electrodes  27  provided in the DOC  21 . In other words, an amount of accumulated PM in the DPF  22  is estimated with high accuracy on the basis of the electrostatic capacity C between the electrodes  27  that has good sensitivity even in the low load operation range or toward the end of a forced regeneration. 
     Accordingly, the exhaust purification system of this embodiment enables the amount of accumulated PM to be estimated with high accuracy without being affected by the running condition of a vehicle or the like. Since the electrodes  27  are provided in the DOC  21 , the dimensions or the arrangement of the electrodes  27  need not be set individually in accordance with the shapes, the pitch, or the like of the cells in the DPF  22 . Thus, it is possible to flexibly cope with the specifications of the like of the DPF  22 . 
     Typically, the electrostatic capacity C between the electrodes  27  has characteristics of responding more quickly to a change in the exhaust gas temperature than the sensor value of the exhaust gas temperature sensor does, as illustrated in  FIG. 7 . In other words, the use of the electrostatic capacity C between the electrodes  27  disposed in the DOC  21  makes it possible to detect the internal temperature accurately, as compared with the use of the sensor value of the exhaust temperature sensor(s) provided across the DOC  21 . 
     In the exhaust purification system of this embodiment, the in-pipe injection amount (or the post-injection amount) in a forced regeneration is corrected (adjusted) on the basis of the temperature difference ΔT between the target temperature T TARGT  and the DOC internal temperature T DOC , which is calculated from the electrostatic capacity C between the electrodes  27 . In other words, as compared to the conventional technique of correcting the injection amount with the use of the sensor value of the exhaust gas temperature sensor, the in-pipe injection amount in the forced regeneration can be optimized by this embodiment because the DOC internal temperature T DOC  is accurately detected. 
     Accordingly, the exhaust purification system of this embodiment enables the fuel injection amount in the forced regeneration to be controlled with accuracy and can thus effectively improve the regeneration efficiency of the DPF  22 . In addition, an exhaust gas temperature sensor(s) do(es) not need to be provided across the DOC  21 . Therefore, the cost and the size of the overall apparatus can be effectively reduced. 
     It should be noted that the present invention is not limited to the above-described embodiment, and can be implemented with modifications, as appropriate, within the scope that does not depart from the spirit of the present invention. 
     For example, it suffices that the number of pairs of the electrodes  27  is at least one, and the illustrated embodiment is not limiting in this regard. The engine  10  is not limited to a diesel engine, and an embodiment can be applied widely to other internal combustion engines including a gasoline engine. 
     As illustrated in  FIG. 8 , a bypass passage  25  may be connected to the exhaust passage  12  so as to bypass the DOC  21 , and the DOC  21   a  for measurement may be disposed in the bypass passage  25 . The DOC  21   a  has a small capacity. In this configuration, electrodes  27  are preferably provided in the DOC  21   a , and an orifice  25   a  (restriction) for regulating the flow rate of the exhaust gas is preferably provided in the bypass passage  25 . When a forced regeneration of the DOC  21   a  is executed, a voltage may be applied to the electrodes  27  to cause the electrodes  27  to function as a heater.
An exhaust purification system includes: a diesel oxidation catalyst (DOC) provided on an exhaust passage of an engine; a diesel particulate filter (DPF) provided on the exhaust passage at a position downstream of the DOC to collect particulate matter contained in exhaust gas; electrodes that detect a capacitance of the DOC; a particulate matter accumulation estimating unit that estimates an amount of particulate matter accumulated in the DPF on the basis of the detected capacitance; and a forced regeneration control unit that injects fuel into the DOC and performs forced regeneration that burns and removes at least the particulate matter accumulated in the DPF when the estimated accumulated particulate matter amount surpasses a predetermined amount.