Patent Publication Number: US-2012031077-A1

Title: Pm sensor, pm amount sensing device for exhaust gas, and abnormality detection apparatus for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to a PM sensor, a PM amount sensing device for exhaust gas, and an abnormality detection apparatus for an internal combustion engine. 
     BACKGROUND ART 
     Conventionally, as disclosed in, for example, Japanese Patent Laid-Open No. 08-284644, there is known an internal combustion engine equipped with a particulate filter for filtering particulate matter in the exhaust gas. Hereinafter, the particulate matter is also referred to simply as “particulates”, or “PM”. 
     The conventional internal combustion engine described above is equipped with a pressure sensor for detecting a differential pressure of a filter. When exhaust gas containing a large amount of particulates flows into a filter, the amount of particulates in the filter increases accordingly. The differential pressure of the filter also changes following that as well. Therefore, by sensing the differential pressure of the filter, it is possible to sense the amount of particulates in the exhaust gas. 
     Besides, as the configuration for sensing the amount of particulates, the configurations of Japanese Patent Laid-Open No. 2007-32490 and Japanese Patent Laid-open No. 2008-64621 are well known. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Laid-Open No. 08-284644 
         Patent Literature 2: Japanese Patent Laid-Open No. 2007-32490 
         Patent Literature 3: Japanese Patent Laid-open No. 2008-64621 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As exhaust emission regulations have been tightened in recent years, there is a growing need for sensors for sensing the amount of particulates. At the current technological level, however, there is no advent of on-board type PM sensor or PM amount sensing device, which can withstand practical use environments. Thus, there are urgent needs for developments of PM sensors and PM amount sensing devices for sensing the amount of particulates. Moreover, when an abnormality occurs in a particulate filter of an internal combustion engine, an immediate countermeasure needs to be taken. Thus, technological advancements are also desired in the abnormality sensing technique for particulate filters. 
     The present invention has been made to solve the above described problems, and has an object to provide a PM sensor and a PM amount sensing device for exhaust gas, which are capable of sensing the amount of particulate matter. 
     It is another object of the present invention to provide an abnormality detection apparatus for an internal combustion engine, which is capable of detecting abnormality of a particulate filter. 
     Solution to Problem 
     To achieve the above-mentioned purpose, a first aspect of the present invention is a PM sensor, comprising: 
     an inlet port through which a portion of gas that is drawn from an exhaust path of an internal combustion engine is allowed to flow in; 
     a filter for filtering particulate matter (PM) in the gas that has flowed in through the inlet port; 
     a heater attached to the filter and capable of changing the temperature of the filter; 
     an outlet port through which the gas that has passed through the filter is allowed to flow out to the exhaust path; and 
     an oxygen concentration sensor element disposed in the outlet port side and adapted to change its output according to an oxygen concentration of the gas that has passed through the filter. 
     A second aspect of the present invention is the PM sensor according to the first aspect, further comprising 
     an oxygen concentration sensor element disposed between the inlet port and the filter and adapted to change its output according to an oxygen concentration of gas that has flowed in through the inlet port. 
     A third aspect of the present invention is the PM sensor according to the second aspect, wherein 
     the oxygen concentration sensor element of the outlet port side and the oxygen concentration sensor element of the inlet port side are air-fuel ratio sensor elements. 
     A fourth aspect of the present invention is the PM sensor according to the third aspect, wherein 
     the air-fuel ratio sensor element includes a heater and upon being activated, is heated to a predetermined temperature by the heater, and 
     the filter and the air-fuel ratio sensor element are spaced apart such that the filter comes to a level of temperature at which particulate matter in the filter is not removed when the temperature of the air-fuel ratio sensor element is at the predetermined temperature. 
     To achieve the above-mentioned purpose, a fifth aspect of the present invention is a PM amount sensing device for exhaust gas, comprising: 
     a filter provided in an exhaust path of an internal combustion engine and for filtering particulate matter (PM) in exhaust gas that flows through the exhaust path; 
     an oxygen concentration sensor element disposed in a downstream of the filter in the exhaust path, and adapted to change its output according to an oxygen concentration of the gas that has passed through the filter; 
     a heater attached to the filter; 
     heating control means for controlling the heater such that the filter is heated until particulate matter in the filter is removed; 
     temperature reduction control means for controlling the heater such that a temperature of the filter is not higher than a temperature at which particulate matter in the filter is not removed, after the control of the heating control means; 
     acquisition means for acquiring an output of the oxygen concentration sensor element after a temperature of the filter becomes not higher than the temperature; and 
     calculation means for calculating an amount of particulate matter in the exhaust gas based on the output acquired by the acquisition means. 
     A sixth aspect of the present invention is the PM amount sensing device for exhaust gas according to the fifth aspect, wherein 
     the acquisition means includes means for acquiring an output of the oxygen concentration sensor element when a predetermined time period has elapsed after a temperature of the filter has become not higher than the temperature, and calculating an integrated value of an amount of exhaust gas that has flowed into the filter before an acquisition timing of the output by the acquisition means after a temperature of the filter has become not higher than the temperature, and 
     the calculation means calculates an amount of particulate matter in the exhaust gas per unit time and per unit volume based on the output acquired by the acquisition means, the predetermined time, and the integrated value. 
     A seventh aspect of the present invention is the PM amount sensing device for exhaust gas according to the fifth aspect or the sixth aspect, further comprising 
     an oxygen concentration sensor element disposed in an upstream of the filter in the exhaust path, and capable of changing its output according to an oxygen concentration of exhaust gas that flows into the filter, wherein 
     the calculation means calculates an amount of particulate matter in the exhaust gas based on a difference between an output of the oxygen concentration sensor element of the upstream side of the filter and an output of the oxygen concentration sensor element of the downstream side of the filter. 
     An eighth aspect of the present invention is the PM amount sensing device for exhaust gas according to the seventh aspect, wherein 
     the oxygen concentration sensor element of the downstream side of the filter and the oxygen concentration sensor element of the upstream side of the filter are air-fuel ratio sensor elements. 
     A ninth aspect of the present invention is the PM amount sensing device for exhaust gas according to the eighth aspect, further comprising 
     calibration means for calibrating an output deviation between the air-fuel ratio sensor of the downstream side of the filter and the air-fuel ratio sensor of the upstream side of the filter. 
     To achieve the above-mentioned purpose, a tenth aspect of the present invention is a PM amount sensing device for exhaust gas, comprising: 
     a filter provided in an exhaust path of an internal combustion engine and for filtering particulate matter (PM) in exhaust gas that flows through the exhaust path; 
     a heater attached to the filter; 
     temperature reduction control means for controlling the heater such that a temperature of the filter is not higher than a temperature at which particulate matter in the filter is not removed; 
     heating control means for controlling the heater such that a temperature of the filter becomes not lower than a temperature at which particulate matter in the filter is removed, after a predetermined period has elapsed since a temperature of the filter becomes not higher than the temperature through the control by the temperature reduction control means; 
     electric energy sensing means for sensing electric energy consumption consumed by the heater for removing particulate matter in the filter when the control by the heating control means is being performed; 
     calculation means for calculating an amount of particulate matter of the exhaust gas based on the electric energy consumption sensed by the electric energy sensing means. 
     An eleventh aspect of the present invention is the PM amount sensing device for exhaust gas according to the tenth aspect, wherein 
     the electric energy sensing means comprises: 
     determination means for determining whether or not particulate matter in the filter is removed after a start of the control by the heating control means; 
     electric energy calculation means for calculating electric energy consumption of the heater during a period from a start of the control by the heating control means until it is determined that the particulate matter in the filter is removed; and 
     calculation means for calculating the electric energy consumption consumed by the heater for removing particulate matter in the filter, based on the electric energy consumption calculated by the electric energy calculation means. 
     A twelfth aspect of the present invention is the PM amount sensing device for exhaust gas according to the eleventh aspect, comprising: 
     an upstream side oxygen concentration sensor disposed in an upstream of the filter in the exhaust path, and adapted to change its output according to an oxygen concentration of gas that flows into the filter; and 
     a downstream side oxygen concentration sensor disposed in a downstream of the filter in the exhaust path, and adapted to change its output according to an oxygen concentration of gas that flows out from the filter, wherein 
     the determination means determines whether or not the particulate matter in the filter is removed based on a difference between an output of the upstream side oxygen concentration sensor and an output of the downstream side oxygen concentration sensor. 
     To achieve the above-mentioned purpose, a thirteenth aspect of the present invention is an abnormality detection apparatus for an internal combustion engine, comprising: 
     an oxygen concentration sensor disposed in a downstream of a particulate filter provided in an exhaust path of the internal combustion engine, and adapted to change its output according to an oxygen concentration of gas that flows out from the particulate filter; 
     heating means for heating the particulate filter so as to regenerate the particulate filter; and 
     detection means for detecting an abnormality of the particulate filter based on an output of the oxygen concentration sensor of the downstream after the regeneration of the particulate filter. 
     The abnormality detection apparatus for an internal combustion engine according to the thirteenth aspect, further comprising 
     an oxygen concentration sensor disposed in an upstream of the particulate filter and adapted to change its output according to an oxygen concentration in exhaust gas, wherein 
     the detection means detects an abnormality of the particulate filter based on a difference between an output of the oxygen concentration sensor of the upstream and an output of the oxygen concentration sensor of the downstream. 
     The abnormality detection apparatus for an internal combustion engine according to the fourteenth aspect, wherein 
     the oxygen concentration sensor disposed in each of the upstream and the downstream of the particulate filter respectively is an air-fuel ratio sensor. 
     Advantageous Effects of Invention 
     According to a first aspect of the present invention, an oxygen concentration sensor element exhibits an output with lower oxygen concentration as the amount of particulates in a filter increases. Based on the output of the oxygen concentration sensor element, it is possible to detect the amount of particulates in the gas that flows into the filter. Further, since the particulates in the filter can be removed by heating with a heater, it is possible to repeatedly perform the sensing of the amount of particulates. 
     According to a second aspect of the present invention, an oxygen concentration sensor element is provided in each of the upstream side of filter and the downstream side of filter. The difference between the outputs of these oxygen concentration sensor elements correspond with high precision to the amount of particulates in the filter. Thus, based on the difference between the outputs of these oxygen concentration sensor elements, it is possible to sense with high precision the amount of particulates in the gas that flows into the filter. 
     According to a third aspect of the present invention, an air-fuel ratio sensor element is used as the oxygen concentration sensor element in the first and second aspects of the present invention. The air-fuel ratio sensor has a proven track record as the sensor for sensing the oxygen concentration of exhaust gas. By using an air-fuel ratio sensor element, it is possible to sense the amount of particulates in the exhaust gas with high reliability. 
     According to a fourth aspect of the present invention, the following effects can be obtained. The air-fuel ratio sensor generally operates while being heated to a predetermined activation temperature. On the other hand, when the temperature of the filter rises to not lower than a specific temperature, particulates will burn off without being accumulated in the filter. According to the fourth aspect of the present invention, it is ensured that the filter can hold particulates even while the temperature of the air-fuel ratio sensor is at the activation temperature. As a result, it is possible to sense the amount of particulates in the exhaust gas even while the air-fuel ratio sensor is at the activation temperature. 
     According to a fifth aspect of the present invention, after the filter is heated to a sufficiently high temperature, a heater is controlled such that the temperature of the filter is lowered to a level at which particulates can be trapped. After the heater control, particulates go on being trapped in the filter, and the output of the oxygen concentration sensor element is acquired. The greater the amount of particulates in the filter, the lower the oxygen concentration in the gas in the downstream of filter becomes, and the output of the oxygen concentration sensor element exhibits a lower oxygen concentration value. Therefore, based on the output of the oxygen concentration sensor element, it is possible to calculate the amount of particulates in the gas that flows into the filter. This allows the sensing of the amount of particulates in the exhaust gas. 
     According to a sixth aspect of the present invention, it is possible to calculate the amount of particulates in the exhaust gas per unit time and per unit volume. 
     According to a seventh aspect of the present invention, an oxygen concentration sensor element is provided in each of the upstream side of filter and the downstream side of filter. The difference between the outputs of these oxygen concentration sensor elements corresponds with high precision to the amount of particulates in the filter. Thus, based on the difference between the outputs of these oxygen concentration sensor elements, it is possible to sense with high precision the amount of particulates in the gas that flows into the filter. 
     According to an eighth aspect of the present invention, an air-fuel ratio sensor element is used as the oxygen concentration sensor element. The air-fuel ratio sensor has a proven track record as the sensor for sensing the oxygen concentration of exhaust gas. Thus, by using an air-fuel ratio sensor element, it is possible to sense the amount of particulates in the exhaust gas with high reliability. 
     According to a ninth aspect of the present invention, the output discrepancy among a plurality of air-fuel ratio sensors can be calibrated. This makes it possible to perform the sensing of the amount of particulates with a higher precision. 
     According to a tenth aspect of the present invention, it is possible to sense the amount of particulates. The greater the amount of particulates in the exhaust gas, the greater the amount of particulates to be trapped in the filter per unit time becomes. The greater the amount of particulates in the filter, the greater the electric energy consumption of heater needed to remove the particulates in the filter becomes. Therefore, it is possible to calculate the amount of particulates in the gas that flows into the filter based on the electric energy consumption of heater. 
     According to an eleventh aspect of the present invention, it is possible to accurately calculate the electric energy consumption that has been consumed at the heater until the particulates in the filter has been removed. 
     According to a twelfth aspect of the present invention, it is possible to determine with high precision whether or not the particulates in the filter are removed. 
     According to a thirteenth aspect of the present invention, an oxygen concentration sensor is provided in the downstream of a particulate filter. If the particulate filter is in a condition to be able to normally trap particulates, the particulates will go on accumulating in the filter so that the effect of the accumulation of particulates should manifest itself in the output of the oxygen concentration sensor. Therefore, based on the output of the oxygen concentration sensor, it is possible to detect abnormality of the particulate filter. 
     According to a fourteenth aspect of the present invention, an oxygen concentration sensor element is provided in each of the upstream and downstream of a particulate filter. The difference between the outputs of these oxygen concentration sensor elements corresponds with high precision to the amount of particulates in the particulate filter. Based on the difference between the outputs of these oxygen concentration sensor elements, it is possible to detect abnormality of the particulate filter with high reliability. 
     According to a fifteenth aspect of the present invention, an air-fuel ratio sensor is used as the oxygen concentration sensor in the fourteenth aspect of the present invention. The air-fuel ratio sensor has a proven track record as the sensor for sensing the oxygen concentration of exhaust gas. By using the air-fuel ratio sensor, it is possible to detect the abnormality of particulate filter with high reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram to show the configuration of a PM sensor and PM amount sensing device for exhaust gas according to Embodiment 1 of the present invention. 
         FIG. 2  is a diagram to show the view of the configuration of  FIG. 1  seen from the direction of arrow A. 
         FIG. 3  is a time chart to illustrate the operation of sensing the amount of PM relating to Embodiment 1. 
         FIG. 4  is a flowchart of a routine performed by ECU  50  in Embodiment 1. 
         FIG. 5  is a diagram to show an example of the map of the correlation line between the value of ΔI L  and the amount of particulates (the amount of PM). 
         FIG. 6  is a flowchart of a routine performed by ECU  50  in Embodiment 2 according to the present invention. 
         FIG. 7  is a diagram to show the configuration of an abnormality detection apparatus of an internal combustion engine relating to Embodiment 3 of the present invention. 
         FIG. 8  is a flowchart of a routine performed by ECU  50  in Embodiment 3 according to the present invention. 
     
    
    
     REFERENCE SIGNS LIST 
     
         
           2  an internal combustion engine 
           10  an exhaust pipe 
           20  a partition 
           22 ,  24  an air-fuel ratio sensor (A/F sensor) 
           30  a filter 
           34  a heater control part 
           50  ECU (Electronic Control Unit) 
           130  DPF 
       
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     Configuration of Embodiment 1 
       FIG. 1  is a diagram to show the configuration of a PM sensor and PM amount sensing device for exhaust gas according to Embodiment 1 of the present invention.  FIG. 2  is a diagram to show the view of the configuration of  FIG. 1  seen from the direction of arrow A. The PM sensor and the PM amount sensing device for exhaust gas according to Embodiment 1 are suitable for internal combustion engines of vehicles. 
     The PM sensor and the PM amount sensing device according to Embodiment 1 are mounted on an exhaust pipe  10  of an internal combustion engine  2 . There is no limitation on the number and type of cylinders of the internal combustion engine  2 . It is noted that the internal combustion engine  2  of  FIG. 1  is schematically shown for the sake of convenience. In the exhaust pipe  10 , installed are an air-fuel ratio sensor  22 , a filter  30 , and an air-fuel ratio sensor  24  in sequence in the direction of the flow of exhaust gas. In the following description below, for the sake of simplicity, the air-fuel ratio sensor is also referred to as “A/F sensor.” In Embodiment 1, a partition  20  shown in  FIG. 1  is provided. The partition  20  opens into the left side on the page and the right side on the page of  FIG. 1 . Exhaust gas flows from the left side on the page of  FIG. 1  to the right side on the page of  FIG. 1  through the inside of the partition  20 . 
     The filter  30  is a compact filter for trapping fine particles. The filter  30  is a small-sized version of the so-called diesel particulate filter (DPF). Hereinafter, particulate matter (PM) is also referred to simply as “particulates” or “PM”. 
     A part of the exhaust gas flowing in the exhaust pipe  10  of the internal combustion engine  2  flows into the filter  30 . The filter  30  can filter particulates in the exhaust gas that flows thereinto. According to this, the particulates go on accumulating within the filter  30 . As a result, the filter  30  can capture and collect (that is, trap) particulates. 
     The filter  30  can be formed by imitating the material and specific configuration of a DPF, and making its outer shape smaller than that of the DPF. The detailed structure of the filter  30  needs not necessarily be the same as or analogous to the DPF. As shown in  FIG. 2 , the outer-shape dimension of the filter  30  is smaller compared to the inner diameter of the exhaust pipe  10 . Therefore, a portion of the exhaust gas flows into the filter  30 , and the remaining gas goes on flowing to the downstream of the exhaust pipe  10  without flowing into the filter  30 . 
     A/F sensors  22  and  24  are A/F sensors of limiting current type. The A/F sensor of limiting current type exhibits a different limiting current value according to the oxygen concentration of the atmosphere, in other words, the oxygen concentration of the gas to be detected. The limiting current value proportionally varies according to the oxygen concentration. Therefore, the A/F sensor  22  changes its output according to the oxygen concentration of the exhaust gas in the upstream of the filter  30 . Moreover, the A/F sensor  24  changes its output according to the oxygen concentration of the exhaust gas in the downstream of the filter  30  as well. 
     The A/F sensors  22  and  24  each includes an outer electrode that is exposed to the gas to be detected, that is, the exhaust gas, an inner electrode which is exposed to the atmosphere, and an oxygen-ion conducting electrolyte interposed between the outer electrode and the inner electrode. The oxygen-ion conducting electrolyte favorably utilizes, for example, ZrO 2  which has a high reliability. Since there is not particular limitation on the specific configurations of the A/F sensors  22  and  24 , further description thereof will be omitted. 
     The A/F sensors  22  and  24  are heated to a predetermined activation temperature by a built-in heater, and thereafter perform the sensing of air-fuel ratio at the activation temperature. As shown in  FIG. 1 , the filter  30  and the A/F sensor  22  or  24  are spaced apart by a predetermined distance. The distance between the filter  30  and the A/F sensor  22  or  24  is large enough such that particulates can be present without being burnt within the filter  30  even when the A/F sensors  22  and  24  are at the activation temperature. 
     The filter  30  includes a heater  32  which is a compact heater. The heater  32  connects to a heater control part  34 . The heater  32  can keep the inside of the filter  30  at a high temperature so that particulates within the filter  30  can be removed. This makes it possible to reduce the amount of particulates in the filter  30  to be zero, thereby performing the regeneration of the filter  30  (regeneration of trapping capability). 
     In Embodiment 1, an ECU (Electronic Control Unit)  50  connects to the A/F sensors  22  and  24 , and the heater control part  34 . The ECU  50  can acquire the outputs of the A/F sensors  22  and  24 , respectively. Hereinafter, for the sake of convenience, the limiting current value of the A/F sensor  22  is also referred to as an output current value I L1 , or an output I L1 ; and the limiting current value of the A/F sensor  24  is referred to as an output current value I L2 , or an output I L2 . Moreover, in Embodiment 1, the ECU  50  prestores the arithmetic processing to calculate the difference between the output I L1  and the output I L2 . Hereinafter, the difference between the output I L1  and the output I L2  is also referred to as ΔI L . 
     Moreover, the ECU  50  can provide the heater control part  34  with a control signal to perform on-off control of the heater  32  and regulation of its heat generation rate. 
     It is noted that in Embodiment 1, although not illustrated, the ECU  50  also connects to a sensor (for example, an intake pressure sensor or an air flow meter) for measuring the amount of intake air of the internal combustion engine  2 , which is located in the upstream of the exhaust pipe  10 . The ECU  50  can measure an intake air amount Ga of the internal combustion engine  2  based on the output of the above described sensor. In Embodiment 1, the ECU  50  stores the routine to calculate an exhaust gas amount Gexh based on the intake air amount Ga. 
     [Operation of Embodiment 1] 
     (PM Detection Principle Relating to Embodiment 1) 
     Having continued diligent research, the present inventors came up with the idea of a method for sensing the amount of particulates based a novel detection principle which has been unknown. That is, when particulates are filtered by a compact filter like the filter  30 , the diffusion distance of the gas (oxygen: O 2 ) that passes through inside the compact filter varies. 
     The greater the amount of particulates in the filter, the longer the diffusion distance of the gas that passes through the compact filter becomes. The amount of O 2  that can pass through the compact filter decreases as the amount of particulates in the filter increases; and as a result of that, the oxygen concentration in the downstream of the compact filter goes on declining. Therefore, it is possible to sense the amount of particulates in the gas that flows into the compact filter based on the oxygen concentration in the downstream of the compact filter. 
     In the above described series of phenomena, the compact filter plays the same role as that of a diffusion-controlled layer in a limiting current type A/F sensor. When the limiting current type A/F sensor is disposed in the downstream of the compact filter, the diffusion distance of oxygen in a layer which is the total of the compact filter and the diffusion-controlled layer of the limiting current type A/F sensor, increases as the amount of particulates in the filter increases. As a result of that, as the amount of particulates within the filter increases, the limiting current value of the limiting current type A/F sensor in the downstream goes on declining. 
     When a limiting current type A/F sensor is disposed respectively in the upstream and the downstream of the compact filter, as the amount of particulates in the filter increases, the difference between the outputs of the limiting current type A/F sensors of the upstream and the downstream increases. Therefore, it is possible to sense the amount of particulates in the gas that flows into the compact filter based on the difference between the outputs of the limiting current type A/F sensors of the upstream and the downstream. 
     (Specific Operation of Embodiment 1) 
     When exhaust gas having a certain air-fuel ratio and a certain amount of particulates flows into the filter  30 , the A/F sensor  22  exhibits a specific output corresponding to the air-fuel ratio. On the other hand, the output of the A/F sensor  24  varies according to the amount of particulates in the filter  30  as described above. As a result of the exhaust gas continually flowing into the filter  30 , the amount of particulates in the filter  30  increases. As the amount of particulates in the filter  30  increases, the oxygen concentration of the atmosphere of the A/F sensor  24  declines, and thereby I L2  declines. As a result of this, since the output I L2  goes on declining while the output I L1  stays constant, ΔI L  increases. 
     Under the condition of the same time period and the same flow rate of exhaust gas, the greater the amount of particulates contained in the exhaust gas, the further ΔI L  increases. Therefore, based on ΔI L , it is possible to calculate the amount of particulates of the exhaust gas that currently flows into the filter  30 . According to this, the amount of particulates generated in the internal combustion engine  2  can be sensed. 
     The method for sensing the amount of PM of Embodiment 1 will be described more specifically using  FIG. 3 .  FIG. 3  is a time chart to illustrate the operation of sensing the amount of PM relating to Embodiment 1. In the operation of sensing the amount of PM of Embodiment 1, three steps of A, B, and C are repeatedly performed. In Embodiment 1, it is supposed that the A/F sensors  22  and  24  are kept constant at the activation temperature. 
     In step A, first, a control signal is sent to the heater control part  34  from the ECU  50  so that the heating of the heater  32  is performed. The heating of the heater  32  will result in that the particulates in the filter  30  is removed (burnt) and the particulates in the filter temporarily becomes zero. Moreover, in Embodiment 1, in order to eliminate the discrepancy of output (output deviation) between the A/F sensor  22  and the A/F sensor  24 , a zero-point correction of output is also performed in step A. This zero-point correction of output allows that ΔI L  indicates with high precision a value corresponding to the amount of particulates in the filter  30 . 
     In step B, the heater  32  is turned off. This will cause the temperature of the filter  30  to be lowered so that particulates start to be accumulated in the filter  30 . In step B, such a state is maintained turning into a standby state until a predetermined time period has elapsed. 
     In step C, upon elapse of the predetermined time period from step B, the ECU  50  acquires the output I L1  and the output I L2  to calculate ΔI L . Based on the above described predetermined time period from step B to step C (that is, the period for trapping particulates) and the total of the exhaust gas amount Gexh that has passed during the time period, the amount of particulates per unit time and unit gas amount is calculated. 
     After step C, step A is performed in succession. Thereafter, by repeatedly performing steps A, B, and C, it is possible to continuously sense the amount of particulates. According to Embodiment 1, it is possible to continuously perform a quantitative sensing of particulates of exhaust gas for every predetermined time period (predetermined cycle) during the operation of the internal combustion engine  2 . 
     As described so far, according to Embodiment 1, it is possible to sense the amount of particulates of the exhaust gas that flows into the filter  30  based on the change amount of the output (the decline amount of the output) of A/F sensor  24 , that is ΔI L . Moreover, according to Embodiment 1, an A/F sensor may be provided in each of the upstream side of the filter  30  and the downstream side of the filter  30 . By measuring the difference ΔI L  between the A/F sensors  22  and  24 , it is possible to sense with high precision the increase in the amount of particulates in the filter  30 . As a result of that, it is possible to sense with high precision the amount of particulates in the gas that flows into the filter. 
     Moreover, according to Embodiment 1, since the particulates of the filter  30  can be heated and thereby removed by the heater  32 , it is possible to repeat the sensing of the amount of particulates. The filter  30  is compact, and the electric energy consumption of the heater  32  will be small even if the heating for removing particles is repeated. Thus, the effect on the fuel economy can be suppressed to be low. 
     Moreover, according to Embodiment 1, it is possible to sense the amount of particulates of exhaust gas by utilizing the A/F sensors  22  and  24 . The air-fuel ratio sensor has a proven track record as the sensor for sensing the oxygen concentration of exhaust gas. By using an air-fuel ratio sensor, it is possible to sense the amount of particulates in the exhaust gas with a high reliability. 
     Moreover, an air-fuel ratio sensor generally operates while being heated to a predetermined activation temperature. If the temperature of the filter  30  rises to not lower than a specific temperature (a burning temperature of particulates), particulates will burn off without being accumulated in the filter  30 . In this connection, according to Embodiment 1, the A/F sensors  22  and  24  and the filter  30  are spaced apart. Therefore, it is ensured that the filter  30  can hold particulates even while the temperature of the A/F sensors  22  and  24  are at the activation temperature. As a result, it is possible to sense the amount of particulates in the exhaust gas even while the A/F sensors  22  and  24  are at the activation temperature. Further, according to Embodiment 1, the temperature of the A/F sensors  22  and  24  is kept constant at the activation temperature, and the temperature dependency of the output of the A/F sensors  22  and  24  is small. Therefore, Embodiment 1 does not need the temperature correction of output and a temperature sensor for temperature correction, and therefore is advantageous. 
     [Specific Processing of Embodiment 1] 
     Hereinafter, specific processing performed by the PM amount sensing device for exhaust gas according to Embodiment 1 will be described by using  FIG. 4 .  FIG. 4  is a flowchart of a routine performed by ECU  50  in Embodiment 1. The routine of  FIG. 4  are executed during the startup of the internal combustion engine  2 .  FIG. 5  is a diagram to show an example of the map of the correlation line between the value of ΔI L  and the amount of particulates (the amount of PM). Correlation lines respectively for air-fuel ratios  20  and  25  are shown in  FIG. 5 . In Embodiment 1, the correlation map for air-fuel ratio=20 shown in  FIG. 5  is prestored in the ECU  50 . 
     In the routine shown in  FIG. 4 , first, A/F sensor heating and heater control are performed (step S 100 ). In this step, after the startup of the internal combustion engine  2 , the heater incorporated in each of the A/F sensors  22  and  24  is controlled for heating until the A/F sensors  22  and  24  become activated. At the same time, the heater  32  is also controlled so that the filter  30  is heated to a burning temperature of particulate. 
     Next, after the determination of sensor activation and PM burning, a zero-point correction of output for the A/F sensors is performed (step S 102 ). In this step S 102 , first, it is determined whether or not the A/F sensors  22  and  24  are activated. The determination of sensor activation can be performed by, for example, whether or not the error of the output of the A/F sensor  22  or  24  is within a predetermined range. Moreover, in this step S 102 , the determination of PM burning is also performed. The determination of PM burning is performed to determine whether or not particulates adhered to the filter  30  have burnt off completely. In Embodiment 1, it is determined that particulates have completely burnt off if the heating of the filter  30  by the heater  32  is continued for a predetermined time period. 
     In step S 102 , a zero-point correction of output for the A/F sensors is performed as well. The zero-point correction of output for the A/F sensor is performed to eliminate the discrepancy of output (output deviation) between the A/F sensor  22  and the A/F sensor  24 . This zero-point correction of output, for example, can be performed as follows. First, a power factor k to be multiplied against the output current of the A/F sensor  24  is derived such that the output of the A/F sensor  22  agrees with the output of the A/F sensor  24 . This factor k is multiplied against the output current of the A/F sensor  24 . This allows the difference between outputs to be cancelled every time the processing of step S 102  is performed thus realizing a zero-point correction of output. 
     Next, the heater  32  is turned off (step S 104 ). When the heater  32  is turned off, the temperature of the filter  30  is lowered and, after a while, the filter  30  is sufficiently cooled to a temperature at which particulates can be accumulated within the filter  30 . Thereafter, particulates go on accumulating in the filter  30 . 
     After the heater is turned off, the determination processing of filter temperature is executed for ECU  50  to determine whether or not the temperature of the filter  30  is lowered to a level where particulates can be accumulated. In this filter temperature determination, for example, the determination on whether or not the temperature of the heater  32  is sufficiently lowered may be made based on the comparison between the resistance value of the heater  32  and a predetermined value. It may be determined that the temperature of the filter  30  is sufficiently low when the heater  32  is at a sufficiently low temperature. Alternatively, it may be determined that the temperature of the filter  30  is sufficiently lowered when ΔI L  increases to a predetermined criterion. When a fulfillment of the condition of the determination processing of    
       24  and the storing of the exhaust gas amount may be performed after elapse of the predetermined time period T 0 . When the engine operating region in which sensing of the amount of PM is desired to be performed is determined, or when sensing of the amount of PM is desired to be performed while the amount of generated particulates is considerably large in the view point of sensing accuracy, the operating conditions when the sensing of the amount of PM is performed may be defined in advance. 
     After step S 108 , the processing of ΔI L  calculation is performed (step S 110 ). In this step, first, difference between the output values that are stored in step S 108  is calculated. Next, in Embodiment 1, the difference obtained by that calculation is converted into a reference current value according to the air-fuel ratio and the exhaust gas amount Gexh. In Embodiment 1, the reference current value is supposed to be the output current value of the A/F sensor  22  or  24  when the air-fuel ratio=20, and exhaust gas amount=10 g/s. The reference is unified by this conversion and a final ΔI L  is calculated. 
     Next, the processing to calculate the amount of PM from a correlation line is performed (step S 112 ). In step S 112 , a map in which the correlation line for air-fuel ratio=20 is defined as shown in  FIG. 5  is referred to calculate the amount of PM according to ΔI L  after conversion. Specifically, in this processing, as ΔI L  increases, the calculated amount of PM increases as shown in the map of  FIG. 5 . 
     The following effects are achieved by the above described steps S 110  and S 112 . For example, as shown in  FIG. 5 , the difference ΔI L2  that is obtained when air-fuel ratio=25-coincides with the difference ΔI L2  when air-fuel ratio=20, by being converted into a reference current value. The relationship between the amount of PM and ΔI L  varies according to the air-fuel ratio of exhaust gas. As shown in  FIG. 5 , when ΔI L1  is obtained when air-fuel ratio is 20, the amount of PM corresponding to this ΔI L1  is determined. On the other hand, if ΔI L2  is obtained when air-fuel ratio is 25, it will be the same value, as the amount of PM, as ΔI L2  when air-fuel ratio=20, even if ΔI L2  is larger than ΔI L2 . In Embodiment 1, the difference between the outputs of the A/F sensors  22  and  24  that are obtained at different air-fuel ratios of exhaust gas is converted into a value according to air-fuel ratio=20 through the conversion processing of step S 110 . Besides this conversion being performed, a map is referred to in which the correlation line for air-fuel ratio=20 is defined. This makes it possible to accurately sense the amount of PM based on the outputs of the A/F sensors  22  and  24  even under the situation where the air-fuel ratio varies every moment. 
     Next, the amount of PM according to the amount of exhaust gas is calculated (step S 114 ). In this step, the amount of particulates per unit time and per unit gas amount are calculated based on the integrated exhaust gas amount Gexh_itg stored in step S 108  and a predetermined time period T o . This makes it possible to perform quantitative evaluation of particulates in the exhaust gas. 
     Next, the heater  32  is heated again and particulates in the filter  30  are removed (step S 116 ). Thereafter, the process returns to step S 102 , and the processing after step S 102  are repeatedly executed. 
     According to the above described processing, it is possible to sense the amount of particulates in the exhaust gas. 
     It is noted that the map in which the relation between ΔI L  and the amount of PM to be stored in the ECU  50  may be a so-called multi-dimensional map in which correlation lines are defined for multiple air-fuel ratios including 20, 25 and others. By utilizing this, the amount of PM may be calculated by directly referring to the correlation lines for each air-fuel ratio without performing the conversion into the reference current value of step S 110 . Moreover, in Embodiment 1, the ECU  50  calculates the exhaust gas amount Gexh based on the intake air amount Ga. Therefore, it is possible to use an integrated value of the intake air amount Ga in place of the integrated exhaust gas amount Gexh_itg. 
     It is noted that in Embodiment 1 described above, the filter  30  corresponds to the “filter” in the first invention, the heater  32  corresponds to the “heater” in the first invention, and the A/F sensor  24  corresponds to the “oxygen concentration sensor element” in the first invention, respectively. Moreover, in Embodiment 1, the A/F sensor  22  corresponds to the “oxygen concentration sensor element” in the second invention. 
     It is noted that in Embodiment 1 described above, the filter  30  corresponds to the “filter” in the fifth invention; the air-fuel ratio sensor  24  to the “oxygen concentration sensor element” in the fifth invention; and the heater  32  to the “heater” in the fifth invention, respectively. Moreover, in Embodiment 1, the “heating control means” in the fifth invention is implemented by the ECU  50  executing the processing of step S 100  or step S 116 ; the “temperature reduction control means” in the fifth invention by the ECU  50  executing the processing of step S 104 ; the “acquisition means” of the fifth invention by the ECU  50  executing the processing of step S 108 ; and the “calculation means” of the fifth invention by the ECU  50  executing the processing of steps S 110  to S 114 , respectively in the routine of  FIG. 4 . 
     Moreover, in Embodiment 1, the predetermined time period T 0  corresponds to the “predetermined time period” in the sixth invention, and the integrated exhaust gas amount Gexh_itg to the “integrated value” in the sixth invention, respectively. 
     Furthermore, in Embodiment 1, the “calibration means” in the ninth invention is implemented by the ECU  50  executing the processing of step S 102  in the routine of  FIG. 4 . 
     [Variant of Embodiment 1] 
     [First Variant] 
     In Embodiment 1, the A/F sensors  22  and  24  utilize an air-fuel ratio sensor of limiting current type. The present invention, however, is not limited to this. As described above, as the amount of particulates in the filter  30  increases, the amount of O 2  that can pass through a compact filter decreases, and consequently the oxygen concentration in the downstream of the filter  30  goes on declining. Embodiment 1 utilizes this phenomenon to sense the amount of particulates in the gas that flows into the filter  30  based on the oxygen concentration in the downstream of the filter  30 . In this connection, any air-fuel ratio sensor of type other than the limiting current type, for example, an air-fuel ratio sensor of two-cell type may be used in place of the A/F sensors  22  and  24 . Moreover, any oxygen concentration sensor other than the air-fuel ratio sensor, which can linearly measure the oxygen concentration of gas, may be used in place of the A/F sensors  22  and  24 . 
     (Second Variant) 
     In Embodiment 1, one A/F sensor is provided for each of the upstream and the downstream of the filter  30 . The present invention, however, is not limited to this. As described above, as the amount of particulates in the filter  30  increases, the amount of O 2  that can pass through a compact filter decreases, and consequently the oxygen concentration in the downstream of the filter  30  goes on declining. Therefore, an A/F sensor may be provided only in the downstream of the filter  30  so that the decline amount of the output (hereafter, ΔI Ld ) of this A/F sensor may be used in place of ΔI L . However, when an A/F sensor or an oxygen concentration sensor is provided only in the downstream of the filter, it is not possible to sense the oxygen concentration of exhaust gas in the upstream of the filter  30 . In this case, for example, the difference between the air-fuel ratio or the oxygen concentration, which is calculated based on the operating condition of the internal combustion engine  2 , and the output of the A/F sensor or the oxygen concentration sensor in the downstream of the filter may be utilized as ΔI L . 
     (Third Variant) 
     In Embodiment 1, the “PM sensor” relating to the first invention is configured by combining the A/F sensors  22  and  24 , the filter  30 , and the heater  32 , respectively as discrete parts. The present invention is, however, not limited to this configuration. A single PM sensor may be fabricated in which the functions of the element parts of the A/F sensors  22  and  24 , the filter  30  and the heater  32  are integrated (unified). 
     Specifically, a filter for filtering PM is provided in a case for the PM sensor which includes an inlet port of exhaust gas and an outlet port of exhaust gas. Further, an air-fuel ratio sensor element part or an oxygen concentration sensor element part is provided respectively in the upstream and the downstream of the filter. A heater for heating the filter is also incorporated. As described so far, there is provided a PM sensor which includes an inlet port and an outlet port of exhaust gas, and incorporates a filter, an oxygen concentration sensor element part, and a heater. When this PM sensor is disposed in the exhaust path, part of the exhaust gas is drawn out via the outlet port to flow into the inside of the case for PM sensor. The exhaust gas that has flowed from the inlet port passes through the filter, and thereafter flows out from the outlet port into the exhaust path again. In this configuration, it is possible to sense the amount of particulates in the exhaust gas by treating the difference in the outputs of the oxygen concentration sensors of the upstream and downstream of the filter, in the same manner as ΔI L  of Embodiment 1. 
     According to the unified PM sensor relating to the present variant, since the effects of the flow rate of exhaust gas and the air-fuel ratio are reduced compared with the configuration of Embodiment 1, it is possible to perform the sensing of the amount of PM with high precision without being subject to these effects. When performing the above described unification, it is preferable that thermal insulation around the filter is sufficiently ensured so that the filter can hold particulates even while the temperature of the air-fuel ratio sensor element is at the activation temperature. It is noted that as described in the second variant above, an air-fuel ratio sensor element part or an oxygen concentration sensor element part may be provided only in the downstream of the filter. 
     (Fourth Variant) 
     It is noted that in Embodiment 1, the following variation of the calculation process is possible as well. First, the ECU  50  stores a map (first map) between the value of I L1  and the value of I L2 , and the oxygen concentration. Moreover, the ECU  50  is also made to store a map (second map) of correlation lines that define the relationship between the oxygen concentration difference ΔO 2  between the upstream and the downstream of the filter  30 , and the amount of PM. This second map can be defined such that the larger the oxygen concentration difference ΔO 2 , the larger the amount of PM becomes. After the ECU  50  acquires I L1  and I L2  in step S 108 , an oxygen concentration value corresponding to those values is calculated according to the above described first map. Next, based on the difference of the oxygen concentration values, the amount of PM is calculated according to the second map. Such calculation process may substitute for the processing of steps S 110  and S 112 . 
     Embodiment 2 
     Configuration of Embodiment 2 
     The PM amount sensing device of Embodiment 2 has a configuration in which a circuit for measuring the electric power consumption of the heater  32  is added to the configuration of Embodiment 1. There is no limitation on the specific configuration of this circuit, and any circuit having a current sensor and a voltage sensor for measuring the current and applied voltage of the heater  32  may be used. Since, excepting this point, the hardware configurations of Embodiment 1 and Embodiment 2 are the same, the hardware configuration of Embodiment 2 will not be illustrated for simplifying the description. The PM amount sensing device of Embodiment 2 may be implemented by causing the ECU  50  to execute the routine shown in  FIG. 6  in the above described configuration. 
     In the following description, the electric power consumption of the heater  32  will also be referred to as “P H ”. Moreover, a quantity obtained by a time integration of the electric power consumption P H  of the heater  32 , that is, the electric energy consumption of the heater  32 , is also referred to as “W H ”. 
     [Operation of Embodiment 2] 
     The greater the amount of particulates in the exhaust gas, the greater the amount of particulates to be trapped in the filter  30  per unit time becomes. The greater the amount of particulates in the filter  30 , the greater the electric energy consumption of the heater  32  needed to remove the particulates in the filter  30  becomes. Accordingly, in Embodiment 2, the amount of particulates in the gas that flows into the filter  30  is calculated based on the electric energy consumption of the heater  32 . 
     [Specific Processing of Embodiment 2] 
     Hereinafter, specific processing performed by the PM amount sensing device for exhaust gas according to Embodiment 2 will be described by using  FIG. 6 .  FIG. 6  is a flowchart of a routine performed by ECU  50  in Embodiment 2. In Embodiment 2, a map of the correlation lines between W H  and the amount of PM are prestored in the ECU  50 . This map can be defined such that the larger the electric energy consumption W H  is, the larger the amount of PM becomes, as with the map of Embodiment 1 in  FIG. 5 . 
     In the routine of  FIG. 6 , first, step S 100  described in Embodiment 1 is executed. 
     Next, the storing of I L1 , I L2 , and Gexh, and the calculation of ΔI L  are performed (step S 208 ). In Embodiment 2, successive storage processing to repeatedly store (sample) the outputs I L1  and I L2  of the A/F sensors  22  and  24 , respectively at a predetermined period (for example, for every 8 milliseconds) is provided in the ECU  50 . Moreover, in Embodiment 2, successive storage processing to store the exhaust gas amount Gexh at the same timing with the storing of the outputs I L1  and I L2  is also provided in the ECU  50 . In step S 208 , the ΔI L  calculation processing of steps S 108  and S 110  is repeatedly performed based on the storage values I L1 , I L2 , and Gexh of the above described successive storage processing. In Embodiment 2, the ECU  50  continually executes these processing after step S 208 , and ΔI L  is successively updated to the latest value. 
     Next, step S 104  described in Embodiment 1 is executed and the heater is turned off. Thereafter, as particulates go on accumulating in the filter  30 , the value of ΔI L  that is successively calculated gradually increases. 
     Next, when ΔI L  reaches a predetermined value, time count is started (step S 213 ). This step allows that the time count is started at a stage where a predetermined level of particulates have accumulated in the filter  30 . This makes it possible to carry out the processing thereafter under the situation where particulates are being surely trapped in the filter  30 . Consequently, it is realized that estimation accuracy of the calculation of PM amount is ensured, and the electric energy consumption of the heater under the condition where particulates are not being trapped is reduced. 
     Next, when the time that is started to count in step S 213  reaches a predetermined time period (hereinafter, referred to as “T 1 ”), the heater is turned ON (step S 214 ). After the heater  32  is turned ON, electric power is supplied to the heater  32  at a predetermined amplitude P 0  and a predetermined duty ratio D H . At this time, the heater  32  is controlled so as to be able to heat the filter  30  at lease to a temperature not lower than the temperature at which particulates start to burn. Moreover, in Embodiment 2, time is counted after the heater  32  is turned ON. 
     After the start of the control of the heater  32  in step S 214 , the filter  30  is heated by the heater  32  and particulates in the filter  30  go on burning to be removed. As a result of this, the value of ΔI L  gradually decreases. 
     Thereafter, the electric energy consumption until ΔI L  becomes zero is calculated (step S 216 ). In Embodiment 2, first, the heater  32  is turned ON, and thereafter determination processing on whether or not ΔI L  becomes zero is performed. The counting of time is stopped at the timing when ΔI L =0 is fulfilled, and a time period T H  from the ON time of the heater  32  to a time when ΔI L  becomes zero is obtained. Next, calculation processing to calculate the electric energy consumption W H  based on the time period T H , the above described P o , and the duty ratio D H  (to be specific, for example, multiplication of T H ×P o ×D H =W H ) is executed. The calculated electric energy consumption W H  is assumed to be the electric energy consumed by the heater  32  to remove particulates in the filter  30 . 
     Next, the amount of PM according to the amount of exhaust gas is calculated (step S 218 ). In this step, first, the map of the correlation lines of W H  and the amount of PM stored in the ECU  50  is referred so that the amount of PM according to W H  is calculated. Thereafter, as with Embodiment 1, the amount of particulates per unit time and per unit gas amount are calculated based on the integrated exhaust gas amount Gexh_itg and the predetermined time period T o . 
     Thereafter, the heater  32  is heated again so that particulates in the filter  30  are removed (step S 220 ). Thereafter, the process returns to step S 208 , and the processing after step S 208  are repeatedly executed. 
     According to the above described processing, it is possible to sense the amount of particulates in the exhaust gas. 
     It is noted that in Embodiment 2 described above, the filter  30  corresponds to the “filter” in the tenth invention; and the heater  32  to the “heater” in the tenth invention, respectively. Moreover, in Embodiment 2, the “temperature reduction control” in the tenth invention is implemented by the ECU  50  executing the processing of step S 212 ; the “heating control means” in the tenth invention by the ECU  50  executing the processing of step S 213  and step S 214 ; the “electric energy sensing means” of the tenth invention by the ECU  50  executing the processing of step S 216 ; and the “calculation means” of the tenth invention by the ECU  50  executing the processing of step S 220 , respectively in the routine of  FIG. 6 . 
     Moreover, in Embodiment 2, the “determination means” in the eleventh invention is implemented by the ECU  50  executing the determination processing on whether or not ΔI L  is zero; and the “electric energy calculation means” in the eleventh invention by the ECU  50  executing the calculation processing to calculate electric energy consumption W H  based on the time period T H , the above described P o , and the duty ratio D H , in step S 216  of  FIG. 6 . 
     Moreover, although the hardware configuration is not illustrated in Embodiment 2, the A/F sensor  22  corresponds to the “upstream side oxygen concentration sensor” in the above described twelfth invention; and the A/F sensor  24  not shown corresponds to the “downstream side oxygen concentration sensor” in the twelfth invention. 
     [Variant of Embodiment 2] 
     In the specific processing of Embodiment 2, the outputs of the A/F sensors  22  and  24  when the predetermined time period T 1  elapsed are stored in step S 214 . The present invention, however, is not limited to this configuration. The ECU  50  may store, in place of the time period T 1 , the outputs of the A/F sensors  22  and  24  when the integrated exhaust gas amount Gexh_igt reaches a predetermined amount. 
     The control of the heater  32  is not limited to the duty control as in step S 214 . For example, electric power may be supplied to the heater  32  such that the resistance value (temperature of the heater  32 ) indicates a predetermined value. In this case, the electric energy consumption may be calculated such as by monitoring electric power consumption of the heater  32 . 
     In Embodiment 2, variants include one as shown below. In this variant, when the predetermined time period has elapsed (or the exhaust gas integrated value has reached a predetermined amount) after the heater-off processing in step S 212 , the processing after the heater-on processing in step S 214  is executed. That is, in the present variant, the comparison of ΔI L  with a predetermined value in step S 213  is eliminated. 
     Moreover, variants described in Embodiment 1 may be combined with Embodiment 2. 
     Embodiment 3 
     Configuration of Embodiment 3 
       FIG. 7  is a diagram to show the configuration of an abnormality detection apparatus of an internal combustion engine relating to Embodiment 3 of the present invention. The abnormality detection apparatus of Embodiment 3 can detect abnormality of a diesel particulate filter (DPF)  130  provided in the exhaust pipe  10 . This abnormality detection apparatus can be used for OBD (On-board Diagnosis) while being mounted on a vehicle. 
     In Embodiment 3, it is supposed that the internal combustion engine  2  is a diesel engine, and a heating mechanism (not shown) for regenerating the DPF  130  is provided. The ECU  50  can control the heating mechanism to regenerate the DPF  130 . 
     There are already various known configurations regarding the heating mechanism for the regeneration of DPF. Therefore, although detailed description will not be made, the DPF  130  may be heated by, for example, so-called post injection. To be specific, an exhaust fuel-addition valve may be provided in the exhaust path of the internal combustion engine  2 . The exhaust fuel-addition valve is provided to add fuel to the exhaust gas flowing in the exhaust path. By performing fuel addition with the exhaust fuel-addition valve at an appropriate timing, it is possible to regenerate the DPF  130 . Moreover, so-called post injection may be performed to perform fuel addition. Moreover, a heater may be attached to the DPF  130  to heat the DPF  130  by this heater. 
     As shown in  FIG. 7 , A/F sensors  22  and  24  are provided in the upstream and the downstream of the DPF  130  as with the filter  30  of Embodiment 1. In the DPF  130  as well, as in the filter  30 , as the amount of particulates increases, ΔI L  increases. If the DPF  130  is in a condition to be able to normally trap particulates, the particulates will go on to accumulate in the DPF  130 , and the effect of the accumulation of particulates should manifest itself in ΔI L . Therefore, it is possible to detect abnormality of the DPF  130  based on ΔI L . 
     [Specific Processing of Embodiment 3] 
       FIG. 8  is a flowchart of the routine to be executed by the ECU  50  in Embodiment 3. It is supposed that the routine of  FIG. 8  is executed during the startup of the internal combustion engine  2 . In the following description, description will be omitted or simplified as appropriate on overlapping points in the contents with those of Embodiments 1 and 2. 
     In the routine of  FIG. 8 , first, heating to activate the A/F sensor is performed as with step S 100  of Embodiment 1 (step S 300 ). 
     Then, DPF regeneration control is performed (step S 302 ). In this step, the ECU  50  controls the heating mechanism, which has been already described, so that particulates in the DPF  130  are removed. 
     Next, steps S 102 , S 106 , S 108 , and S 110  are executed as in Embodiment 1. Thereby, the determination processing of A/F sensor activation, the determination processing of PM burning in DPF  130 , the zero-point correction processing of the output of the A/F sensor, the calculation processing of the integrated exhaust gas amount Gexh_itg, and the calculation processing of ΔI L  are successively executed. 
     Next, the amount of PM is calculated (step S 304 ). In this step, based on ΔI L , the amount of PM is calculated according to correlation lines as with the processing of step S 112  of Embodiment 1. In Embodiment 3 as well, a map of correlation lines as shown in  FIG. 5  is created and stored in the ECU  50 . 
     Next, it is determined whether or not the amount of PM is not more than a predetermined value (step S 306 ). As described so far, if the DPF  130  is in a condition to be able to normally trap particulates, particulates should go on accumulating in the DPF  130 . When, contrary to such an expectation, the amount of PM in the DPF  130  indicates a value not more than the predetermined value, it is considered that an abnormality of some kind has occurred in the DPF  130 . Therefore, the determination on whether or not the amount of PM is not more than the predetermined value is performed in Embodiment 3. When this condition is negated, it is judged that the DPF  130  is normally trapping particulates, and the routine of this round ends. 
     When the condition of step S 306  holds, it is determined that there is an abnormality in DPF  130  (step S 308 ). When the abnormality detection apparatus of Embodiment 3 is being used for OBD, alerting the driver by, for example, lighting an alarm lamp is performed. 
     According to the above described processing, it is possible to perform the detection of abnormality in a particulate filter. 
     It is noted that, in Embodiment 3, after the amount of PM is calculated from ΔI L , determination based on the comparison between the amount of PM and the predetermined value is performed. The present invention, however, is not limited to this arrangement. The comparison determination may be made by comparing ΔI L  with a predetermined value without performing the conversion to the amount of PM. 
     It is noted that in Embodiment 3 described above, the DPF  130  corresponds to the “particulate filter” in the thirteenth invention, and the A/F sensor  24  to the “oxygen concentration sensor” in the thirteenth invention, respectively. Moreover, the “heating means” in the thirteenth invention is implemented by the ECU  50  executing the processing of step S 302  in the routine of  FIG. 8 , and the “detection means” in the thirteenth invention by the ECU  50  executing the processing of steps S 110 , S 304 , S 306 , and S 308  in the routine of  FIG. 8 , respectively. 
     Moreover, in Embodiment 3 described above, the A/F sensor  22  corresponds to the “oxygen concentration sensor” in the fourteenth invention.