Patent Publication Number: US-2011061368-A1

Title: Particulate matter sensor and exhaust gas purifying apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to International Application No. PCT/JP2009/066063 filed on Sep. 15, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a particulate matter sensor and an exhaust gas purifying apparatus. 
     2. Description of the Related Art 
     Conventionally, an exhaust gas purifying apparatus using a particulate matter capturing filter (diesel particulate filter (DPF)) made of a porous ceramic is known as a device for capturing primarily of C (carbon) particulate matter contained in exhaust gas exhausted from diesel engines (see, for example, Patent EP1916394A). In this conventional exhaust gas purifying apparatus, the particulate matter gradually deposits on the diesel particulate filter along with a continuous use of the diesel engine. Therefore, it is possible to prevent the particulate matter in the exhaust gas from being released from the diesel engine into the atmosphere. Thus, it is possible to purify the exhaust gas. 
     However, when the particulate matter excessively deposits on the diesel particulate filter, the particulate matter may leak into an exhaust line on a downstream side of the diesel particulate filter. Therefore, in order to diagnose the leakage of the particulate matter onto the downstream side of the diesel particulate filter, a guiding branch (diverging passage) may be connected to the exhaust line on the downstream side of the diesel particulate filter, and a detection filter may be attached to the guiding branch in order to measure concentration of the particulate matter and detect the leakage of the particulate matter onto the downstream side of the diesel particulate filter. An effective way of preventing the particulate matter from excessively depositing is supposed to be a removal with oxidation by burning the deposited particulate matters, which is deposited on the diesel particulate filter. In order to measure a deposited amount of the particulate matter on the diesel particulate filter, a guiding branch may be connected to the exhaust line on the upstream side of the diesel particulate filter to divert a part of the exhaust gas exhausted from the internal combustion engine as an sample gas, and a detection filter may be installed in the guiding branch to enable detecting (capturing) the particulate matter in the exhaust gas. 
     In the vicinity of the detection filter, the particulate matter sensor is provided to output signals corresponding to a pressure difference between the upstream side and the downstream side of the detection filter in the guiding branch, the concentration of the oxygen or the like in the exhaust line. The output signal of the particulate matter sensor is sent to a diagnostic device or a measurement device, and is used to diagnose the leakage of the particulate matter onto the downstream side of the diesel particulate filter or measurement of the deposited amount of the particulate matter on the diesel particulate filter. Therefore, when the conventional particulate matter sensor is used, it is possible to diagnose the leakage of the particulate matter onto the downstream side of the diesel particulate filter, or measure the deposited amount on the diesel particulate filter. 
     The entire contents of European Patent No. 1916394 are incorporated in the present application by reference. 
     SUMMARY OF THE INVENTION 
     In an aspect of the embodiments of the present invention, a particulate matter sensor includes a detection filter, a differential pressure detecting unit, an on-off valve, and a valve control unit. The detection filter is installed in an exhaust passage connected to an internal combustion engine and is configured to detect particulate matter contained in an exhaust gas passing through the exhaust passage. The differential pressure detecting unit is configured to detect a pressure difference between an upstream side and a downstream side of the detection filter. The on-off valve is installed on the upstream side of the detection filter in the exhaust passage and is configured to control a flow of the exhaust gas toward the detection filter. The valve control unit is configured to control the on-off valve to be opened and closed. 
     Further, in another aspect of the embodiments of the present invention, an exhaust gas purifying apparatus includes a particulate matter sensor, a particulate matter capturing filter, an upstream side exhaust gas introducing unit, and a particulate matter deposit amount calculating unit. The particulate matter sensor includes a detection filter, a differential pressure detecting unit, an on-off valve, and a valve control unit. The detection filter is installed in an exhaust passage connected to an internal combustion engine and is configured to detect particulate matter contained in an exhaust gas passing through the exhaust passage. The differential pressure detecting unit is configured to detect a pressure difference between an upstream side and a downstream side of the detection filter. The on-off valve is installed on the upstream side of the detection filter in the exhaust passage and is configured to control a flow of the exhaust gas toward the detection filter. The valve control unit is configured to control the on-off valve to be opened and closed. The particulate matter capturing filter is configured to capture particulate matter contained in an exhaust gas flowing through a primary exhaust line included in the exhaust passage. The upstream side exhaust gas introducing unit is configured to introduce a part of the exhaust gas flowing through the primary exhaust line from an upstream side of the particulate matter capturing filter toward the particulate matter sensor. The particulate matter deposit amount calculating unit is configured to calculate an amount of the particulate matter depositing on the particulate matter capturing filter based on an amount of the particulate matter detected by the particulate matter sensor. 
     Further, in another aspect of the embodiments of the present invention, an exhaust gas purifying apparatus includes a particulate matter sensor, a particulate matter capturing filter, a downstream side exhaust gas introducing unit, and a filter failure diagnosing unit. The particulate matter sensor includes a detection filter, a differential pressure detecting unit, an on-off valve, and a valve control unit. The detection filter is installed in an exhaust passage connected to an internal combustion engine and is configured to detect particulate matter contained in an exhaust gas passing through the exhaust passage. The differential pressure detecting unit is configured to detect a pressure difference between an upstream side and a downstream side of the detection filter. The on-off valve is installed on the upstream side of the detection filter in the exhaust passage and is configured to control a flow of the exhaust gas toward the detection filter. The valve control unit is configured to control the on-off valve to be opened and closed. The particulate matter capturing filter is configured to capture particulate matter contained in an exhaust gas flowing through a primary exhaust line included in the exhaust passage. The downstream side exhaust gas introducing unit is configured to introduce a part of the exhaust gas flowing through the primary exhaust line from a downstream side of the particulate matter capturing filter toward the particulate matter sensor. The filter failure diagnosing unit is configured to diagnose the particulate matter capturing filter based on an amount of the particulate matter detected by the particulate matter sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  illustrates an entire structure of an exhaust gas purifying apparatus of an embodiment of the present invention; 
         FIG. 2  illustrates an important portion of the exhaust gas purifying apparatus of  FIG. 1 ; 
         FIG. 3  is a flowchart illustrating a control routine of driving an on-off valve of the exhaust gas purifying apparatus of  FIG. 2 ; and 
         FIG. 4  illustrates the entire structure of an exhaust gas purifying apparatus of another embodiment according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to diagnose the leakage of the particulate matter on the downstream side of the diesel particulate filter or measure the amount of the particulate matter captured by the conventional diesel particulate filter (EP0916394A), it is necessary to guide the exhaust gas to the above detection filter. Under a structure in which the detection filter is constantly exposed to the exhaust gas passing through the exhaust line, an increment rate of the particulate matter depositing on the detection filter is relatively high. As a result, the detection filter needs to be operated for a relatively long time. Therefore, a regeneration mechanism such as an electrothermal heater for regenerating the detection filter by burning to remove the particulate matter deposited on the detection filter is ordinarily provided in the vicinity of the detection filter. 
     However, when such a regeneration mechanism is provided, the structure of the entire exhaust gas purifying apparatus becomes bulky, and portions having high temperatures increase. Thus, there occur inconveniences such that heat resistances of each part need to be enhanced or the distances between the each part and the detection filter needs to be large. 
     The embodiment of the present invention may provide a particulate matter sensor and exhaust gas purifying apparatus which be omitted a regeneration mechanism for regenerating a detection filter even though the detection filter operates for a long period of time. 
     A particulate matter sensor of the embodiment of the present invention detects particulate matter in an exhaust gas exhausted from an internal combustion engine, and includes an exhaust passage through which the exhaust gas flows, a detection filter installed in the exhaust passage and configured to detect (capture) the particulate matter contained in the exhaust gas passing through the exhaust passage, a differential pressure detecting unit configured to detect a pressure difference between an upstream side and a downstream side of the detection filter, an on-off valve installed on the upstream side of the detection filter in the exhaust passage and configured to permit and prohibit a flow of the exhaust gas toward the detection filter, and a valve control unit configured to control the on-off valve to be opened and closed. 
     In this embodiment of the invention, the on-off valve permitting or prohibiting a flow of the exhaust gas to the detection filter is installed on the upstream side of the detection filter capable of detecting (capturing) the particulate matter inside the exhaust passage through which the exhaust gas exhausted from the internal combustion engine flows. In a case where the on-off valve is opened, the flow of the exhaust gas onto the side of the detection filter is permitted, and the exhaust gas is guided to the detection filter. In a case where the on-off valve is closed, the flow of the exhaust gas onto the detection filter side from the exhaust passage is prohibited, and the exhaust gas is not guided to the detection filter. Therefore, it is possible to drastically reduce (limit) the amount of time during which the detection filter is exposed to the exhaust gas. Thus, a deposit rate of the particulate matter on the detection filter may be drastically lowered. As a result, a regeneration mechanism, which regenerates the detection filter in order to allow for long term operation, may be omitted. 
     In the particulate matter sensor of the embodiment, the valve control unit may control the on-off valve to be opened and closed at regular intervals. 
     In the particulate matter sensor of the embodiment, the valve control unit may control the on-off valve to be opened and closed based on an operation state of the internal combustion engine and a state of the exhaust gas. 
     The exhaust gas purifying apparatus of the embodiments of the present invention is installed in a an exhaust pipe (primary exhaust line) through which an exhaust gas flows, and includes a particulate matter capturing filter which captures the particulate matter contained in the exhaust gas further including the above-mentioned particulate matter sensor, an upstream side exhaust gas introducing unit configured to introduce a part of the exhaust gas flowing through the exhaust pipe (primary exhaust line) from the upstream side of the particulate matter capturing filter toward the particulate matter sensor, and a particulate matter deposit amount calculating unit configured to calculate the amount of the particulate matter depositing on the particulate matter capturing filter based on the amount of the particulate matter detected by the particulate matter sensor. 
     Another exhaust gas purifying apparatus of the embodiments of the present invention is installed in an exhaust pipe (primary exhaust line) through which an exhaust gas flows, and includes a particulate matter capturing filter which captures the particulate matter contained in the exhaust gas flowing through the exhaust pipe (primary exhaust line) further including a downstream side exhaust gas introducing unit configured to introduce a part of the exhaust gas flowing through the exhaust pipe (primary exhaust line) from the downstream side of the particulate matter capturing filter toward the particulate matter sensor, and a filter failure (trouble) diagnosing unit configured to diagnose the particulate matter capturing filter based on the amount of the particulate matter detected by the particulate matter sensor. 
     The above-mentioned exhaust gas purifying apparatus is an exhaust gas purifying apparatus applied with the above-mentioned particulate matter sensor. 
     A description of an exhaust gas purifying apparatus is given below, with reference to figures of the embodiment according to the present invention. 
       FIG. 1  illustrates an entire structure of an exhaust gas purifying apparatus  10  of the embodiment according to the present invention.  FIG. 2  illustrates the structure of an important portion of the exhaust gas purifying apparatus  10  of  FIG. 1 . 
     As illustrated in  FIG. 1 , the exhaust gas purifying apparatus  10  of the embodiment includes a Diesel Oxidation Catalyst (DOC)  16  installed in an exhaust line  14 , which is connected to an internal combustion engine  12 , and a particulate matter capturing filter (diesel particulate filter (DPF))  18 . The DOC  16  is a catalyst (catalyst supporting carrier) for removing carbon monoxide, carbon hydride and the like, which are susceptible to oxidation and contained in an exhaust gas exhausted from an internal combustion engine  12 . Meanwhile, the DPF  18  is a filter which may capture particulate matter contained in the exhaust gas exhausted from the internal combustion engine  12 . 
     The exhaust gas purifying apparatus  10  of the embodiment is provided to purify the exhaust gas discharged to the atmosphere from the internal combustion engine  12 . The exhaust gas purifying apparatus  10  of the embodiment may detect a failure (trouble) when the DPF  18  has the failure such as breakage thereby causing leakage of the particulate matter as much as a threshold value or more onto a downstream side of the DPF  18  of the exhaust line  14 . The exhaust gas purifying apparatus  10  provides an alert about the failure, causes a lamp to blink, light up or the like when the failure is detected. The exhaust gas purifying apparatus  10  includes the particulate matter sensor  20  for detecting the failure of the DPF  18 . 
     Referring to  FIG. 2 , an exhaust gas collection line  22  is connected to the primary exhaust line (exhaust line  14 ) on the downstream side of the DPF  18 . The exhaust gas collection line  22  (see  FIG. 2) and 104  (see  FIG. 4 ) and the primary exhaust line (exhaust line  14 ) in this embodiment are typically and inclusively referred to as an exhaust passage in claims. 
     The exhaust gas collection line  22  branches off from the primary exhaust line (exhaust line  14 ). Therefore, a part of the exhaust gas passing through the DPF  18  may flow into the exhaust gas collection line  22 . 
     The exhaust gas collection line  22  has a flow passage cross-sectional area smaller than that of the primary exhaust line (exhaust line  14 ). The flow passage cross-sectional area of the exhaust gas collection line  22  is, for example, about from one-hundredth up to one-thousandth of the flow passage cross-sectional area of the primary exhaust line (exhaust line  14 ). 
     The particulate matter sensor  20  is installed inside the exhaust gas collection line  22 . It is determined whether the particulate matter leaks from the DPF  18  based on concentration of the particulate matter in the exhaust gas which flows through the exhaust gas collection line  22 . 
     The particulate matter sensor  20  is made up of detection filter  28  installed inside the exhaust gas collection line  22  and a differential pressure gauge  30 . Meanwhile, the particulate matter sensor  20  may be configured to further include a flow meter and/or a temperature measuring part. A downstream end of the exhaust gas collection line  22  is connected to a portion (e.g. a negative pressure tank, an air intake and the like) having a pressure lower than the pressure of an upstream side of the exhaust gas collection line  22 . Therefore, a part of the exhaust gas inside the primary exhaust line (exhaust line  14 ) passing through the DPF  18  is branched off to a side of the exhaust gas collection line  22 . Thus, the part of the exhaust gas passes through the detection filter  28 . 
     The detection filter  28  is provided to calculate concentration of the particulate matter contained in the exhaust gas exhausted from the internal combustion engine  12 . The detection filter  28  may capture the particulate matter, and may detect the particulate matter. The detection filter  28  is configured by a material similar to the DPF  18  such as a porous ceramic in a cylindrical shape. The detection filter  28  has a soot storage capacity smaller than that of the DPF  18 . 
     The differential pressure gauge  30  outputs an electric signal corresponding to a differential pressure “ΔP” between an inlet and an outlet of the detection filter  28  (a pressure difference “ΔP” between the upstream side and the downstream side). For example, the differential pressure gauge  30  may be a known pressure gauge of a diaphragm type, a gauge type, a bellows type, a thermal type or the like. An operation unit  32  is electrically connected to the differential pressure gauge  30 , mainly configured of a microcomputer. The output from the differential pressure gauge  30  is supplied to the operation unit  32 . The operation unit  32  detects the pressure difference “ΔP” generated between the upstream side and the downstream side of the detection filter  28  inside the exhaust gas collection line  22 . The differential pressure gauge  30  measures the concentration of the particulate matter in the exhaust gas based on a time change of the pressure difference “ΔP”. 
     An on-off valve  34  is installed in the exhaust gas collection line  22 . For example, the on-off valve  34  is an electromagnetic valve or the like. The on-off valve  34  is installed in the exhaust gas collection line  22  between a branching point from the primary exhaust line (exhaust line  14 ) and the detection filter  28 . The on-off valve  34  permits or prohibits a flow of the exhaust gas from the primary exhaust line (exhaust line  14 ) to the detection filter  28 . The on-off valve  34  is electrically connected to the operation unit  32 , which is mainly configured of the microcomputer. The on-off valve  34  opens or closes in accordance with a command from the operation unit  32 . Specifically, the on-off valve  34  permits a flow of the exhaust gas from the primary exhaust line (exhaust line  14 ) to the detection filter  28  when an open command signal is supplied from the operation unit  32 . Meanwhile, the on-off valve  34  is operated to prohibit the flow of the exhaust gas from the primary exhaust line (exhaust line  14 ) to the detection filter  28  in a case where a close command signal is supplied from the operation unit  32 . 
     Referring to  FIG. 3 , operation of the exhaust gas purifying apparatus  10  of the embodiment is described.  FIG. 3  is a flowchart illustrating a control routine of driving an on-off valve of the exhaust gas purifying apparatus of the embodiment, as an example. 
     In the embodiment, the exhaust gas exhausted from the internal combustion engine  12  flows inside the primary exhaust line (exhaust line  14 ), and passes through the DPF  18 . Thereafter, the exhaust gas is discharged into the atmosphere or flows into the exhaust gas collection line  22 . When the on-off valve  34  is closed to prohibit the exhaust gas from flowing from the primary exhaust line (exhaust line  14 ) to the detection filter  28 , the exhaust gas flown into the exhaust gas collection line  22  is prohibited from flowing into a downstream side, i.e. the detection filter  28 , of the on-off valve  34  in the exhaust gas collection line  22 . On the other hand, when the on-off valve  34  is opened to permit the flow of the exhaust gas from the primary exhaust line (exhaust line  14 ) to the detection filter  28 , the exhaust gas is permitted to flow into the downstream side, i.e. the detection filter  28 , of the on-off valve  34  in the exhaust gas collection line  22 . 
     When the exhaust gas is permitted to flow into the detection filter  28 , the exhaust gas is guided to the detection filter  28 . When the particulate matter is contained in the exhaust gas, the particulate matter is absorbed by the detection filter  28  and deposits on the detection filter  28 . When a state in which the particulate matter deposits on the detection filter  28  changes over a period of time, the pressure difference “ΔP” before and after the detection filter  28  in the exhaust gas collection line  22  changes over the same period of time. 
     The operation unit  32  detects the pressure difference “ΔP” generated between the upstream side and the downstream side of the detection filter  28  in the exhaust gas collection line  22  based on the output signal of the differential pressure gauge  30  during the time which the exhaust gas is permitted by the on-off filter  34  to flow into the detection filter  28 . The operation unit  32  calculates the amount of the particulate matter detected (captured) by the detection filter  28  based on the time change of the pressure difference “ΔP”, and calculates the concentration of the particulate matter in the exhaust gas flowing through the exhaust gas collection line  22 . The operation unit  32  determines existence of the leakage from the DPF  18  based on whether the calculated concentration of the particulate matter is a threshold value or more. When the existence of the leakage of the particulate matter from the DPF  18  is determined as the result of the determination, the exhaust gas purifying apparatus  10  provides an alert about the failure, causes a lamp to blink, light up and the like. 
     Thus, under the situation where the flow of the exhaust gas to the detection filter is permitted by the on-off valve  34 , the differential pressure gauge  30  is used in determining the existence of the failure of the DPF  18  by calculating the concentration of the particulate matter in the exhaust gas on the downstream side of the DPF  18 . Therefore, it is possible to report the failure to a driver of a vehicle having the exhaust gas purifying apparatus installed in it when the failure is determined. 
     The operation unit  32  may supply an open command signal to the on-off valve  34  at regular intervals not constantly. The open command signal may be supplied when the internal combustion engine  12  is driven at a revolution number as large as a predetermined revolution number or more. Specifically, in step S 102  following YES of step S 100 , the open command signal is supplied to the on-off valve  34  for a second predetermined time (e.g. about 1 second through about 30 seconds) shorter than a first predetermined time (e.g. about 5 through about 20 minutes during operation of the internal combustion engine) at every first predetermined time in step S 102 . In step S 104  following NO of step S 100 , the close command signal is supplied to the on-off valve  34 . When the internal combustion engine  12  is driven at about 2000 rpm or more (YES of step S 100 ), the open command signal is supplied to the on-off valve  34  in step S 102 . When the internal combustion engine  12  is driven at less than about 2000 rpm (NO of step S 100 ), the close command signal is supplied to the on-off valve  34  in step S 104 . In this case, the flow of the exhaust gas into the detection filter  28  via the exhaust gas collection line  22  is not always permitted. It is permitted only in a case where a condition of opening the on-off valve  32  is established. 
     A duration time between starting to supply the open command signal to the on-off valve  34  and ending of the supply (e.g. the above second predetermined time) may be as follows. The operation unit  32  calculates at least concentration of the particulate matter in the exhaust gas flowing through the exhaust gas collection line  22 . Then, the duration time is set to be the minimum necessary time for determining the existence of the leakage of the particulate matter from the DPF  18  based on the concentration of the particulate matter. 
     In the exhaust gas purifying apparatus  10 , it is possible to limit a time while the detection filter  28  is exposed to the exhaust gas by closing the on-off valve  34  in comparison with a case where the detection filter  28  is constantly exposed to the exhaust gas. The more the condition of permitting the flow of the exhaust gas into the detection filter  28  is relaxed (the shorter the first predetermined time or the longer the second predetermined time), the less the time is limited. Therefore, the leakage of the particulate matter is determined at the higher frequency. The more the condition of permitting the flow of the exhaust gas into the detection filter  28  is strict (the longer the first predetermined time or the shorter the second predetermined time), the more the time is limited. Therefore, the time is relatively shortened to thereby facilitate restriction of the frequency of determining the leakage of the particulate matter from the DPF  18 . 
     According to the embodiment, when the frequency of determining the leakage of the particulate matter from the DPF  18  can be restricted, the condition of opening the on-off valve  32  may be strictly set. Then, it is possible to drastically facilitate reducing a deposit rate that the particulate matter in the exhaust gas deposits on the detection filter  28 . Therefore, regeneration of the detection filter  28  after removing the deposited particulate matter may be omitted in properly operating the detection filter  28  for a long time. Further, a regeneration mechanism for regenerating the detection filter  28  may be omitted. 
     Therefore, it is possible to reduce the size of an entire structure of the exhaust gas purifying apparatus  10  of the Embodiment. Further, the number of parts required to enhance heat resistance may be decreased. Thus, it is possible to easily configure the exhaust gas purifying apparatus  10  which is capable of determining the leakage of the particulate matter from the DPF  18  at a low cost. 
     As described, the preferred embodiment of the present invention has been described. However, the embodiments of the present invention are not limited to the specific embodiment, and various modifications and changes are possible in the scope of claims. 
     For example, in the embodiment, the determination of the leakage of the particulate matter from the DPF  18  and the supply of the opening or closing commands to the on-off valve  34  are carried out by the same operation unit  32 . Instead, it is possible to carry out the determination and the supply of the opening or closing commands by different operation units. 
     In the embodiment, the on-off valve is opened at the regular intervals or when the internal combustion engine  12  is driven at a revolution number as large as the predetermined value or more. However, the embodiments of the present invention are not limited to the embodiment. For example, the on-off valve  34  may be closed when the internal combustion engine  12  is started or runs idle, and opened under other situations. Further, a temperature sensor may be provided to detect the temperature of the exhaust gas, and the on-off valve  34  is closed when the temperature of the exhaust gas is lower than the predetermined value. Under the other situations, the on-off valve  34  may be closed. Moreover, the on-off valve  34  may be opened at regular intervals and when the internal combustion engine  12  is driven at a revolution number as large as a predetermined revolution number or more. 
     Further, the embodiment is related to the system of the exhaust gas purifying apparatus  10  which includes the exhaust gas collection line  22  connected to the downstream side of the DPF  18  in the primary exhaust line (exhaust line  14 ), and the particulate matter sensor  20  inside the exhaust gas collection line  22 . The exhaust gas purifying apparatus  10  determines the leakage of the particulate matter from the DPF  18  based on the amount of the particulate matter detected (captured) by the particulate matter sensor  20 . However, the embodiment of the present invention is not limited thereto. 
     As illustrated in  FIG. 4 , an exhaust gas collection line  102  may be provided on an upstream side of the DPF  18  in the primary exhaust line (exhaust line  14 ). Then, a particulate matter sensor  104  may be installed in the exhaust gas collection line  102 . The concentration of the particulate matter in the exhaust gas flowing through the primary exhaust line (exhaust line  14 ) or the amount of the particulate matter depositing on the DPF  18  is measured based on the amount of the particulate matter detected (captured) by the particulate matter sensor  104 . Such structures may be applied to the system of the exhaust gas purifying apparatus  100 . 
     In this case, the upstream side of the exhaust gas collection line  102  is connected to the upstream side of the DPF  18  in the primary exhaust line (exhaust line  14 ), and the downstream side of the exhaust gas collection line  102  may be connected to the downstream side of the DPF  18 . 
     In this modified example, an on-off valve  106  for permitting and prohibiting flow of the exhaust gas from the primary exhaust line (exhaust line  14 ) to the detection filter (not illustrated but existing inside the particulate matter sensor  104 ) may be installed in the exhaust gas collection line  102 . Further, an operation unit for commanding to open or close the on-off valve  106  at a proper timing described in the above embodiment may be provided. In this case, it becomes possible to restrict a time while the detection filter is exposed to the exhaust gas may be restricted by closing the on-off valve  106 . Therefore, it is possible to obtain effects similar to those of the above embodiment. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.