Patent Publication Number: US-9896989-B2

Title: Deterioration diagnosis device for oxidation catalyst

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Divisional of U.S. application Ser. No. 14/010,743, filed Aug. 27, 2013, which claims priority from Japanese Application Nos. 2012-189824 and 2013-122621, filed Aug. 30, 2012 and Jun. 11, 2013, respectively. The entire disclosures of the prior applications are considered part of the disclosure of the accompanying continuation application and are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device that diagnoses the deterioration of oxidation catalyst used for an exhaust gas purifier of an internal combustion engine. 
     2. Description of the Related Art 
     Recently, emissions regulation of diesel engines has been tightened. Accordingly, removing Particulate Matter (hereinafter also referred to as “PM”) from exhaust air is desired by trapping PM included in the exhaust air from a diesel engine and burning the trapped PM. In this regard, an engine includes a particulate filter (a Diesel Particulate Filter, hereinafter also referred to as “DPF”) to trap the PM at its exhaust passage. The DPF is made of a material such as ceramic and stainless steel. To trap the PM, the DPF includes, for example, a honeycomb structure. If the PM is excessively trapped, the DPF becomes clogged. Therefore, as one method, the DPF includes an oxidation catalyst (Diesel Oxidation Catalyst, hereinafter also referred to as “DOC”) at an upstream location. The DOC oxidizes NO in the exhaust air to NO 2  using an oxidation catalyst. Use of the NO 2  thus generated allows the DOC to also oxidize and burn the PM trapped by the DPF. Thus, the PM is removed to continuously regenerate the DPF. 
     The performance of the DOC to generate NO 2  deteriorates over time due to usage. Accordingly, the regeneration performance of the DPF is also deteriorated. Therefore, a device that diagnoses the deterioration degree of the DOC has been developed (see JP-A-2012-36860). 
     First, this diagnosis device estimates a NO X  value in the exhaust air immediately after being discharged from the engine based on engine revolution and an engine load. Also, the NO X  value at the downstream side of the DOC is directly detected by a NO X  sensor installed at the downstream of the DOC. Next, a NO 2  ratio is calculated using these two NO X  values. The concentration of NO 2  that has passed through the DPF is obtained from the NO 2  ratio. The deterioration degree of the DOC is determined from this NO 2  concentration. 
     SUMMARY OF THE INVENTION 
     A diagnosis device which determines a deterioration degree of a DOC based on a NO X  ratio calculated using two NO X  values has a low diagnosis accuracy. Especially, when estimating a NO X  value in exhaust air immediately after being discharged from an engine, factors other than an engine revolution and an engine load are not taken into consideration. In view of the above, depending on usage conditions, it may be the case that the estimation accuracy is significantly lowered. 
     It is therefore an object of the present invention to provide a deterioration diagnosis device for an oxidation catalyst that can determine the degree of deterioration of an oxidation catalyst with good accuracy, and which is disposed at an exhaust passage of an internal combustion engine to oxidize NO to NO 2 . 
     The above object of the present invention has been achieved by providing (1) a deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst disposed at an exhaust passage of an internal combustion engine, the deterioration diagnosis device comprising: a multi-gas sensor disposed downstream of the oxidation catalyst, the multi-gas sensor including a multi-gas sensor element unit that integrally includes a NO 2  sensor unit and a NO X  sensor unit, the NO 2  sensor unit directly detecting a NO 2  concentration in exhaust gas after passing through the oxidation catalyst, and the NO X  sensor unit directly detecting a NO X  concentration in the exhaust gas; an NO concentration calculation unit configured to calculate an NO concentration in the exhaust gas after passing through the oxidation catalyst based on the NO 2  concentration and the NO X  concentration; and a deterioration judgment unit configured to determine a deterioration degree of the oxidation catalyst from an evaluation value based on the NO concentration calculated by the NO concentration calculation unit. 
     Using this deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst, the actual NO 2  concentration and the actual NO X  concentration downstream of the oxidation catalyst are directly obtained from the exhaust gas using a multi-gas sensor. Based on the NO 2  concentration and the NO X  concentration, the NO concentration is calculated. The degree of deterioration of the oxidation catalyst is diagnosed from an evaluation value based on the NO concentration. Consequently, the diagnosis can be performed with good accuracy. 
     The evaluation value is a value that need only by based on the NO concentration. The evaluation value may be, for example, a ratio between the NO concentration and the NO 2  concentration, a ratio between the NO concentration and the NO X  concentration, or the NO concentration alone. 
     In a preferred embodiment (2) of the above deterioration diagnosis device (1), the multi-gas sensor is disposed downstream of and immediately after the oxidation catalyst. 
     Thus, disposing the multi-gas sensor immediately after the oxidation catalyst allows for directly measuring the exhaust gas that has passed through the oxidation catalyst at the multi-gas sensor. Consequently, the degree of deterioration of the oxidation catalyst can be diagnosed with good accuracy. 
     “The multi-gas sensor is disposed downstream of and immediately after the oxidation catalyst” means that the multi-gas sensor is disposed at the downstream side of the oxidation catalyst in the exhaust passage without an intermediate member interposed within the exhaust passage between the oxidation catalyst and the multi-gas sensor. 
     In another preferred embodiment (3), the above deterioration diagnosis device (1) further comprises a filter disposed in the exhaust passage downstream of the oxidation catalyst, the filter trapping particulate matter, wherein the multi-gas sensor is disposed downstream and immediately after the filter. 
     In case of disposing the multi-gas sensor immediately after the oxidation catalyst, the PM that has passed through the oxidation catalyst accumulates in the multi-gas sensor element unit. In this case, since the sensor output fluctuates, there is a concern that the sensor output cannot be obtained from the multi-gas sensor with good accuracy. In contrast, disposing the multi-gas sensor downstream and immediately after the filter can inhibit PM from accumulating in the multi-gas sensor element unit. Consequently, the degree of deterioration of the oxidation catalyst can be diagnosed with good accuracy while reducing fluctuation of the sensor output. 
     “The multi-gas sensor is disposed downstream and immediately after the filter” means that the multi-gas sensor is disposed downstream of the filter in the exhaust passage without an intermediate member within the exhaust passage interposed between the filter and the multi-gas sensor. 
     In yet another preferred embodiment (4) of the above deterioration diagnosis device of any of (1) to (3) above, the multi-gas sensor element unit includes a plurality of NO 2  sensor units, and the respective NO 2  sensor units have different sensitivity ratios relative to NO 2 . 
     Using this deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst, correction calculation can be precisely performed on NO 2  even in an environment where various inflammable gases coexist and the oxygen concentration is subject to change. Such calculation together with the NO X  sensor output allows for separately detecting NO 2  and NO. 
     In yet another preferred embodiment (5) of the deterioration device of any of (1) to (4) above, the multi-gas sensor element unit has a plate shape extending in an axial direction, the multi-gas sensor element unit includes a temperature detector for controlling a temperature of the NO X  sensor unit, and respective ones of the plurality of NO 2  sensor units at least partially overlap the temperature detector in the axial direction. 
     Using this deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst, the temperature of the multi-gas sensor element unit can be controlled based on the temperature measured at the temperature detector. In view of the above, the temperature of the multi-gas sensor element unit is kept at the most stable value adjacent the temperature detector. Accordingly, the temperature of the NO 2  sensor unit is kept constant within a predetermined range by at least partially overlapping the NO 2  sensor unit and the temperature detector in the axial direction. Consequently, the measurement accuracy of NO 2  is improved. 
     In yet another preferred embodiment (6) of the deterioration diagnosis device of any of (1) to (5) above, the NO 2  sensor unit includes a solid electrolyte body having oxygen ion conductivity (at an activation temperature), a detection electrode and a reference electrode each disposed at a surface of the solid electrolyte body, and an interlayer disposed between the detection electrode and the solid electrolyte body. The interlayer contains a solid electrolyte component having oxygen ion conductivity of equal to or more than 50% by mass, the interlayer also including a first metal oxide of at least one kind of metal oxide selected from the group consisting of metal oxides of Co, Mn, Cu, Ni and Ce. Further, the detection electrode contains Au in an amount equal to or more than 70% by mass, the detection electrode not including the first metal oxide. 
     Using this deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst, the NO 2  concentration can be measured at good accuracy by the NO 2  sensor unit with the above-described configuration. In view of above, the diagnosis accuracy of the deterioration degree of the oxygen catalyst is further improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a deterioration diagnosis device for an oxidation catalyst according to an embodiment of the invention; 
         FIG. 2  is a diagram illustrating a map that is used to determine a deterioration degree of the oxidation catalyst; 
         FIG. 3  is a flowchart illustrating a process performed by the deterioration diagnosis device for an oxidation catalyst; 
         FIG. 4  is a sectional view of a multi-gas sensor along a longitudinal direction; 
         FIG. 5  is a block diagram illustrating a configuration of the multi-gas sensor and a gas sensor control device; 
         FIG. 6  is a sectional view illustrating a configuration of a NO 2  sensor unit; 
         FIG. 7  is a diagram illustrating a processing flow of calculating various gas concentrations by the control device of the multi-gas sensor; 
         FIG. 8  is a graph illustrating a relationship between Ip2 and NO X  concentration after correction of O 2  concentration; 
         FIG. 9  a graph illustrating a relationship between an EMF output from a NO 2  sensor and NO concentration and NO 2  concentration; 
         FIG. 10  is a graph illustrating a relationship between an output of concentration conversion of NO 2  sensor and NO concentration and NO 2  concentration before O 2  concentration correction; 
         FIG. 11  is a graph illustrating a relationship between an output of concentration conversion of a NO 2  sensor and NO concentration and NO 2  concentration after O 2  concentration correction; 
         FIG. 12  is a block diagram illustrating a configuration of the multi-gas sensor and the gas sensor control device according to a second embodiment of the invention; 
         FIG. 13  is a sectional view illustrating a configuration of a first NO 2  sensor unit and a second NO 2  sensor unit; 
         FIG. 14  is a top view illustrating another example where the first NO 2  sensor unit and the second NO 2  sensor unit overlap with an oxygen concentration detection cell in the multi-gas sensor according to the second embodiment; 
         FIG. 15  is a top view illustrating yet another example where the first NO 2  sensor unit and the second NO 2  sensor unit overlap with the oxygen concentration detection cell in the multi-gas sensor according to the second embodiment; 
         FIG. 16A  and  FIG. 16B  are graphs illustrating calculation results of concentrations of corrected NO 2  in the case of using C 3 H 6  as an inflammable gas species; 
         FIG. 17A  and  FIG. 17B  are graphs illustrating the calculation results of the concentrations of the corrected NO 2  in the case of using an inflammable gas other than C 3 H 6 ; 
         FIG. 18  is a graph plotting an output ratio of the first NO 2  sensor unit to the second NO 2  sensor unit for each inflammable gas relative to the output of the first NO 2  sensor unit; and 
         FIG. 19  is a block diagram illustrating the deterioration diagnosis device for an oxidation catalyst according to a modification of the above embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details and that the present invention should not be construed as being limited thereto. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings. 
     An embodiment of the invention will next be described in detail with reference to the drawings. Furthermore, using the deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst according to the embodiment, the degree of the deterioration of the oxidation catalyst, which oxidizes NO to NO 2 , can be diagnosed with good accuracy. 
       FIG. 1  is a block diagram illustrating a deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst  250  according to the embodiment mounted to a vehicle. 
     An exhaust gas purifier  550  is mounted in the middle of an exhaust pipe (an exhaust passage)  502  of an engine (a diesel engine)  500 , which is an internal combustion engine of the vehicle. The exhaust gas purifier  550  purifies an exhaust gas discharged from the engine  500 . The exhaust gas purifier  550  includes an upstream side exhaust gas purifier (also referred to as a “DPF device”)  510  and a downstream side exhaust gas purifier (also referred to as a “SCR device”)  520  disposed in this order from the upstream side of the exhaust pipe  502 . The exhaust gas purifier  550  further includes a urea water addition nozzle  531  disposed between the DPF device  510  and the SCR device  520 . 
     The deterioration diagnosis device  250  includes a multi-gas sensor  200 A and a temperature sensor  210 , which are mounted to the DPF device  510 , and a controller (ECU, Engine (electronic) Control Unit)  220 . The ECU  220  diagnoses a deterioration degree of an oxidation catalyst (DOC)  512 , described below, based on the detected outputs of the multi-gas sensor  200 A and the temperature sensor  210 . 
     The DPF device  510  includes a tubular casing. This tubular casing internally includes the oxidation catalyst (Diesel Oxidation Catalyst, hereinafter referred to as “DOC”)  512  and a particulate filter (Diesel Particulate Filter, hereinafter referred to as “DPF”)  514  in this order from the upstream side. The DPF  514  includes, for example, a porous filter (for example, a ceramic filter) that traps Particulate Matter (PM). The DOC  512  includes a honeycomb-shaped carrier made of, for example, metal or ceramic. This carrier supports a catalytic material that oxidizes NO to NO 2 . The DOC  512  oxidizes NO in the exhaust gas to NO 2 . Use of this NO 2  allows the DOC  512  to oxidize and burn PM trapped in the DPF  514 , thus removing the PM. This allows continuous regeneration of the DPF  514 . The regeneration of the DPF  514  is controlled by the ECU  220 . 
     The SCR device  520  has a tubular casing. This tubular casing internally includes a Selective Catalytic Reduction (hereinafter referred to as “SCR”)  522  and a latter part oxidation catalyst (Clean Up Catalyst, hereinafter abbreviated as CUC)  524  in this order from the upstream side. The SCR  520  is a catalyst that reduces NO X  in the exhaust gas to N 2  using ammonia supplied from the upstream as a reducing agent. The SCR  520 , for example, may be a zeolitic catalyst or a vanadium catalyst. The CUC  524  is an oxidation catalyst that removes ammonia that has not reacted at the SCR  522 . 
     The urea water addition nozzle  531  injects urea water from a urea water tank  535  into the exhaust gas at the upstream side of the SCR  522  by an addition device  533 . The urea water injected upstream of the SCR  522  into the exhaust gas is hydrolyzed, thus generating ammonia. This ammonia acts as a reducing agent in the SCR  522 . The addition of the urea water is controlled by the ECU  220 . 
     The ECU  220  calculates NO concentration in the exhaust gas that has passed through the DOC  512 . The ECU  220  further includes a deterioration judgment unit  221 . The deterioration judgment unit  221  determines the deterioration degree of the DOC  512  using a predetermined model (a deterioration judgment map  223  or a calculating formula). The ECU  220  also performs various controls of the engine, the regeneration control of the above-described DPF  514 , and the addition control of the urea water. 
     The ECU  220  is an electronic control unit (ECU) that includes a microcomputer including a central processing unit (CPU), RAM, ROM, and similar member, and a predetermined analog circuit. In the ECU  220 , the CPU executes a computer program stored in the ROM. This allows performing various processes described below. 
     Specifically, the deterioration judgment unit  221  is implemented as a CPU that executes the computer program stored in the ROM. The deterioration judgment map  223  is stored in a storage medium disposed separately from the microcomputer. 
     The DPF device  510  internally includes the temperature sensor  210  at the upstream side of the DOC  512 . The multi-gas sensor  200 A is installed at the downstream side immediately after the DOC  512 . Thus, the multi-gas sensor  200 A is disposed immediately after the oxidation catalyst. This allows the multi-gas sensor  200 A to directly measure the exhaust gas that has passed through the oxidation catalyst. As a result, the degree of deterioration of the oxidation catalyst can be diagnosed with good accuracy. The temperature sensor  210  detects the temperature of the exhaust gas that has flowed into the DOC  512 . This exhaust gas temperature is taken as the catalyst temperature of the DOC  512 . 
     In the details illustrated in  FIG. 4  to  FIG. 6 , described below, the multi-gas sensor  200 A includes a NO 2  sensor unit  42  and a NO X  sensor unit  30 A. The NO 2  sensor unit  42  and the NO X  sensor unit  30 A directly detect the NO 2  concentration and the NO X  concentration in the exhaust gas that has passed through the DOC  512 , respectively. A control device  300  is connected to the multi-gas sensor  200 A. The control device  300  can calculate NO concentration=(NO X  concentration−NO 2  concentration) using the detected NO 2  concentration and the detected NO X  concentration. This NO concentration is a value calculated using the measured values of the NO 2  concentration and the NO X  concentration in the exhaust air. Thus, the NO concentration determined in accordance with this invention expresses an actual NO concentration that is different from the estimated value disclosed in the above-described JP-A-2012-36860. 
     The deterioration judgment map  223  is illustrated in  FIG. 2 . The deterioration degree of the DOC  512  relates to the catalyst temperature of the DOC  512  and NO 2  ratio (NO 2  concentration/NO concentration). At the same catalyst temperature, the smaller the NO 2  ratio (that is, when the NO 2  generation amount is small), the larger the deterioration degree. Accordingly, the deterioration degree of the DOC  512  can be judged as follows. The NO 2  ratio is calculated based on the NO 2  concentration detected by the multi-gas sensor  200 A and the NO concentration calculated by the control device  300 . Then, the deterioration judgment map  223  is referred to using this NO 2  ratio and the temperature detected by the temperature sensor  210 . 
       FIG. 3  is a flowchart for a deterioration determination processing flow performed by the deterioration judgment unit  221 . First, the deterioration judgment unit  221  determines whether or not a deterioration judgment condition is met (Step S 2 ). A “Yes” determination is made in Step S 2  when, for example, the temperature detected by the temperature sensor  210  is within a predetermined range, and the process progresses to Step S 4 . If a “No” determination is made in Step S 2 , the process returns. In Step S 4 , the deterioration judgment unit  221  obtains the NO 2  ratio (NO 2  concentration/NO concentration) from the control device  300 , described below. In Step S 6 , the deterioration judgment unit  221  obtains the value of the catalyst temperature from the temperature sensor  210 . Next, in Step S 8 , the deterioration judgment unit  221  refers to the deterioration judgment map  223  using the NO 2  ratio and the catalyst temperature, and determines the deterioration degree of the DOC  512 . If a “Yes” determination is made in Step S 8  (if the DOC  512  is determined to be deteriorated), the deterioration judgment unit  221  notifies a driver of a vehicle or similar person of the deterioration of the DOC  512  using, for example, a predetermined buzzer or an indicator, and then the process is terminated (Step S 10 ). If a “No” determination is made in Step S 8 , the process returns to the first process in the processing flow. 
     As described above, in this embodiment, the NO 2  ratio is calculated using the actual NO 2  concentration and the actual NO concentration. In view of the above, the degree of the deterioration of the oxidation catalyst can be determined with good accuracy. In Step S 8 , the deterioration degree is determined from the map where the deterioration of the oxidation catalyst is correlated using the NO 2  ratio and the catalyst temperature. In Step S 8 , the deterioration degree may be determined from the map where the deterioration of the oxidation catalyst is correlated using the absolute value of the NO concentration and the catalyst temperature instead of using the NO 2  ratio. 
     The NO 2  ratio is calculated by the control device  300 . The control device  300  calculates the NO concentration by NO concentration=(NO X  concentration−NO 2  concentration) using the NO 2  concentration and the NO X  concentration directly detected at the NO 2  sensor unit  42  and the NO X  sensor unit  30 A. 
       FIG. 4  is a sectional view of the multi-gas sensor  200 A along a longitudinal direction. The multi-gas sensor  200 A is an assembly where a multi-gas sensor element unit  100 A is assembled. The multi-gas sensor element unit  100 A detects the NO 2  concentration and the NO X  concentration. The multi-gas sensor  200 A includes the plate-shaped multi-gas sensor element unit  100 A extending in an axial direction, a tubular metal shell  138 , a tubular ceramic sleeve  106 , an insulating contact member  166 , and a plurality of (in  FIG. 3 , only two terminal are illustrated) connection terminal  110 . The tubular metal shell  138  includes a thread portion  139  to secure the multi-gas sensor  200 A to the exhaust pipe at the outer surface. The tubular ceramic sleeve  106  is disposed so as to surround the peripheral area of the multi-gas sensor element unit  100 A in the radial direction. The insulating contact member  166  is disposed so that the inner wall surface of a contact insertion hole  168  penetrating in the axial direction surrounds the peripheral area of the rear end portion of the multi-gas sensor element unit  100 A. The connection terminal  110  are disposed between the multi-gas sensor element unit  100 A and the insulating contact member  166 . 
     The metal shell  138  has an approximately tubular shape. The metal shell  138  includes a penetration pin hole  154  and a shoulder portion  152 . The penetration pin hole  154  penetrates in the axial direction. The shoulder portion  152  projects in the radially inward direction of the penetration pin hole  154 . The metal shell  138  holds the multi-gas sensor element unit  100 A in the penetration pin hole  154  while the front end side of the multi-gas sensor element unit  100 A is disposed at the outside of the front end side of the penetration pin hole  154 . Also, electrode terminal portions  80 A and  82 A are disposed at the outside of the rear end side of the penetration pin hole  154 . The shoulder portion  152  includes an inward tapered surface inclined with respect to a planar surface vertical to the axial direction. 
     In the penetration pin hole  154  of the metal shell  138 , a ring-shaped ceramic holder  151 , powder filled layers  153  and  156  (hereinafter also referred to as talc rings  153  and  156 ), and the above-described ceramic sleeve  106  are laminated in this order from the front end side to the rear end side. The ring-shaped ceramic holder  151 , the powder filled layers  153  and  156 , and the ceramic sleeve  106  surround the peripheral area of the multi-gas sensor element unit  100 A in the radial direction. A crimp packing  157  is disposed between the ceramic sleeve  106  and a rear end portion  140  of the metal shell  138 . For maintaining air tightness, a metal holder  158  is disposed between the ceramic holder  151  and the shoulder portion  152  of the metal shell  138 . The metal holder  158  holds the talc ring  153  or the ceramic holder  151 . The rear end portion  140  of the metal shell  138  is crimped so that the ceramic sleeve  106  is pressed to the front end side of the metal shell  138  via the crimp packing  157 . 
     Meanwhile, as illustrated in  FIG. 4 , double protectors (an outer protector  142  and an inner protector  143 ), which are made of metal (for example, stainless steel), are installed to the outer periphery of the front end side (the lower side in  FIG. 4 ) of the metal shell  138  by, for example, welding. The outer protector  142  and the inner protector  143  cover the projection portion of the multi-gas sensor element unit  100 A and include a plurality of hole portions. 
     A shell  144  is secured to the outer periphery of the rear end side of the metal shell  138 . A grommet  150  is disposed at the opening of the rear end side (the upper side in  FIG. 4 ) of the shell  144 . The grommet  150  includes a lead wire insertion hole  161 . A plurality of lead wires  146  (only three wires are illustrated in  FIG. 4 ) is inserted through the lead wire insertion hole  161 . The respective plurality of lead wires  146  are electrically connected to the electrode terminal portions  80 A and  82 A of the multi-gas sensor element unit  100 A. For simplification, in  FIG. 4 , only the electrode terminal portions  80 A and  82 A are illustrated as electrode terminal portions of the front face and the reverse face of the multi-gas sensor element unit  100 A. In practice, a plurality of the electrode terminal portions is formed according to the number of electrodes of the NO X  sensor unit  30 A and the NO 2  sensor unit  42 , described below, or the number of similar members. 
     The metal shell  138  includes the insulating contact member  166  at the rear end side (the upper side in  FIG. 4 ) of the multi-gas sensor element unit  100 A projecting from the rear end portion  140 . This insulating contact member  166  is disposed at the peripheral area of the electrode terminal portions  80 A and  82 A, which are formed at the front and the reverse faces at the rear end side of the multi-gas sensor element unit  100 A. This insulating contact member  166  has a tubular shape. The insulating contact member  166  includes the contact insertion hole  168  penetrating in the axial direction. This insulating contact member  166  includes a flange portion  167  projecting from the outer surface in the radially outward direction. The flange portion  167  abuts the shell  144  via a supporting member  169 . Accordingly, the insulating contact member  166  is disposed inside of the shell  144 . The connection terminal  110  at the insulating contact member  166  side electrically connects to the electrode terminal portions  80 A and  82 A of the multi-gas sensor element unit  100 A. The connection terminal  110  is in a continuity state with the outside through the lead wire  146 . 
       FIG. 5  is a block diagram illustrating configurations of the control device (controller)  300  and the multi-gas sensor element unit  100 A, which is connected to the control device  300 , according to the embodiment. In  FIG. 5 , a cross section of the multi-gas sensor element unit  100 A housed in the multi-gas sensor  200 A along the longitudinal direction is illustrated for convenience of explanation. 
     The multi-gas sensor  200 A (the multi-gas sensor element unit  100 A) and the control device  300  are mounted to the vehicle with an internal combustion engine (an engine) (not shown). The control device  300  is electrically connected to the ECU  220 . The end of the lead wire  146  extending from the multi-gas sensor  200 A is connected to the connector. This connector is electrically connected to the connector on the control device  300  side. 
     Next, the configuration of the multi-gas sensor element unit  100 A will be described. The multi-gas sensor element unit  100 A includes the NO X  sensor unit  30 A and the NO 2  sensor unit  42 . The NO X  sensor unit  30 A may include a similar configuration as known NO X  sensors. In the detailed description below, the NO 2  sensor unit  42  is formed at the outer surface of the NO X  sensor unit  30 A. 
     The NO X  sensor unit  30 A is constructed by laminating an insulation layer  23   f , a solid electrolyte body for NO 2  sensor  25 , an insulation layer  23   e , a first solid electrolyte body  2   a , an insulation layer  23   d , a third solid electrolyte body  6   a , an insulation layer  23   c , a second solid electrolyte body  4   a , and insulation layers  23   b  and  23   a  in this order. A first measuring chamber S 1  is defined between the layers of the first solid electrolyte body  2   a  and the third solid electrolyte body  6   a . Exhaust gas is introduced from the outside via a first diffusion resistive element  8   a  disposed at the left end (the inlet) of the first measuring chamber S 1 . A protective layer  9  including a porous material is disposed outside of the first diffusion resistive element  8   a.    
     A second diffusion resistive element  8   b  is disposed at the end opposite the inlet of the first measuring chamber S 1 . A second measuring chamber (corresponding to the “NO X  measuring chamber” of the invention) S 2  is defined at the right side of the first measuring chamber S 1 . The second measuring chamber S 2  communicates with the first measuring chamber S 1  via the second diffusion resistive element  8   b . The second measuring chamber S 2  penetrates the third solid electrolyte body  6   a . The second measuring chamber S 2  is formed between the layers of the first solid electrolyte body  2   a  and the second solid electrolyte body  4   a.    
     An elongated plate-shaped heater  21  is buried between the insulation layers  23   b  and  23   a . The heater  21  extends along the longitudinal direction of the multi-gas sensor element unit  100 A. The heater  21  stabilizes operation of the NO X  sensor unit  30 A. That is, the heater  21  raises the temperature of the NO X  sensor unit  30 A to its activation temperature and enhances oxygen ion conductivity of the solid electrolyte body. The chief material forming each of the insulation layers  23   a ,  23   b ,  23   c ,  23   d ,  23   e  and  23   f  is alumina. The first diffusion resistive element  8   a  and the second diffusion resistive element  8   b  include a porous material such as alumina. The heater  21  includes, for example, platinum. 
     A first pumping cell  2  includes the first solid electrolyte body  2   a , an inner first pumping electrode  2   b , and an outer first pumping electrode  2   c . The first solid electrolyte body  2   a  includes zirconia having oxygen ion conductivity as the chief material. The inner first pumping electrode  2   b  and the outer first pumping electrode  2   c , which is disposed opposite the inner first pumping electrode  2   b , sandwich the first solid electrolyte body  2   a . The inner first pumping electrode  2   b  faces the first measuring chamber S 1 . The chief material forming both the inner first pumping electrode  2   b  and the outer first pumping electrode  2   c  is platinum. The surface of the inner first pumping electrode  2   b  is covered with a protective layer  11  including a porous body. 
     The insulation layer  23   e , which is equivalent to the top surface of the outer first pumping electrode  2   c , is partially hollowed out. A porous body  13  fills the hollowed portion. This allows communication between the outer first pumping electrode  2   c  and the outside to allow for entrance and exit of gas (oxygen). 
     An oxygen concentration detection cell  6  includes the third solid electrolyte body  6   a , a detection electrode  6   b , and a reference electrode  6   c . The third solid electrolyte body  6   a  is formed from zirconia as a chief material. The detection electrode  6   b  and the reference electrode  6   c  sandwich the third solid electrolyte body  6   a . The detection electrode  6   b  is disposed downstream of the inner first pumping electrode  2   b  and faces the first measuring chamber S 1 . The chief material forming both the detection electrode  6   b  and the reference electrode  6   c  is platinum. 
     The insulation layer  23   c  is cut out so that the reference electrode  6   c  in contact with the third solid electrolyte body  6   a  is internally disposed therein. A porous body fills the cutout portion. Thus, a reference oxygen chamber  15  is formed. A weak current is preliminarily passed through the oxygen concentration detection cell  6  at a constant value using an Icp supply circuit  54 . This transports oxygen from the first measuring chamber S 1  to the inside of the reference oxygen chamber  15 . The reference electrode  6   c  is thereby exposed to a predetermined oxygen concentration, which serves as a reference. 
     A second pumping cell  4  includes the second solid electrolyte body  4   a , an inner second pumping electrode  4   b , and a counterpart second pumping electrode  4   c . The second solid electrolyte body  4   a  is formed from zirconia as a chief material. The inner second pumping electrode  4   b  is disposed on the surface facing the second measuring chamber S 2  of the second solid electrolyte body  4   a . The counterpart second pumping electrode  4   c  forms a counterpart electrode of the inner second pumping electrode  4   b . The chief material forming both the inner second pumping electrode  4   b  and the counterpart second pumping electrode  4   c  is platinum. 
     The counterpart second pumping electrode  4   c  is disposed at the cutout portion of the insulation layer  23   c  on the second solid electrolyte body  4   a . The counterpart second pumping electrode  4   c  is opposed to the reference electrode  6   c  and faces the reference oxygen chamber  15 . 
     The inner first pumping electrode  2   b , the detection electrode  6   b , and the inner second pumping electrode  4   b  are each connected to the reference electric potential. 
     Next, the NO 2  sensor unit  42  will be described. The NO 2  sensor unit  42  is formed on the insulation layer  23   f , which forms the outer surface of the NO X  sensor unit  30 A. That is, the insulation layer  23   f  is partially cut out into a rectangular shape, and the solid electrolyte body for the NO 2  sensor  25  is exposed to the surface. A reference electrode  42   a  and a detection electrode  42   b  of the NO 2  sensor unit  42  are formed on the exposed portion. 
     More specifically, as illustrated in  FIG. 6 , the reference electrode  42   a  and an interlayer  42   c  of the NO 2  sensor unit  42  are formed on the solid electrolyte body for NO 2  sensor  25 . The reference electrode  42   a  and the interlayer  42   c  are separated in a lateral direction. The interlayer  42   c  includes the detection electrode  42   b  on the surface. The NO 2  concentration in the gas to be measured is detected based on a change in electromotive force between the reference electrode  42   a  and the detection electrode  42   b . The solid electrolyte body  25 , for example, includes partially stabilized zirconia (YSZ). In  FIG. 6 , for convenience of explanation, the cross section of the NO 2  sensor unit  42  along the lateral direction is illustrated. 
     The detection electrode  42   b  contains Au in an amount equal to or more than 70% by mass and does not include a first metal oxide, which will be described below. In view of this, inflammable gas is less likely to burn on the surface of the detection electrode  42   b . The interlayer  42   c  contains a solid electrolyte component having oxygen ion conductivity in an amount equal to or more than 50% by mass. The interlayer  42   c  includes a first metal oxide, which is at least one kind of metal oxide selected from a group consisting of metal oxides of Co, Mn, Cu, Ni and Ce. 
     NO 2  passes through the detection electrode  42   b  and reacts with oxygen ion (electrode reaction) at the surface boundary of the detection electrode  42   b  and the interlayer  42   c  below. Accordingly, the detection electrode  42   b  and the interlayer  42   c  function as a unit for detecting NO 2  gas. Here, if the first metal oxide is present at the surface boundary of the detection electrode  42   b  and the interlayer  42   c , sensitivity to a gas other than NO 2  gas (such as HC gas) is reduced; therefore, only the selectivity for the NO 2  gas improves. The reason therefor is not clear; however, it is probably because the first metal oxide interposed at the surface boundary modifies an electrode reaction site. 
     The detection electrode  42   b  does not include the first metal oxide. Consequently, burning of the NO 2  gas inside of the detection electrode  42   b  is suppressed. As a result, the NO 2  gas reaching the surface boundary of the detection electrode  42   b  and the interlayer  42   c  does not decrease, thus improving detection sensitivity. 
     From a perspective that the first metal oxide is not included in the detection electrode  42   b  but included in the neighboring member, it is assumed that the first metal oxide is contained in the solid electrolyte body  25  without using the interlayer  42   c . However, the solid electrolyte body  25  includes, for example, partially stabilized zirconia. In view of this, the solid electrolyte body  25  is baked at a high temperature (around 1500° C.). During this baking, there is a possibility that the first metal oxide is volatilized from the solid electrolyte body  25 . Accordingly, as in this embodiment, the interlayer  42   c  is preferably disposed between the solid electrolyte body  25  and the detection electrode  42   b.    
     The first metal oxide included in the interlayer  42   c  is at least one kind of metal oxide selected from a group consisting of metal oxides of Co, Mn, Cu, Ni and Ce. Especially, when the metal oxide is Co 3 O 4 , a fluctuation in NO 2  sensitivity caused by H 2 O included in the detected gas to the NO 2  gas sensor decreases, which is preferable. The first metal oxide forms as a metal oxide or a complex oxide. The solid electrolyte component included in the interlayer  42   c  may have the same composition as or may have a different composition from the solid electrolyte body  25  constituting the gas sensor in accordance with the invention. 
     The interlayer  42   c  preferably contains the first metal oxide in a proportion of 1 to 50% by mass. If the proportion of contained first metal oxide is less than one % by mass, selectivity for the NO 2  gas may not be sufficiently obtained. On the other hand, if the proportion of contained first metal oxide exceeds 50% by mass, the proportion of the solid electrolyte component in the interlayer  42   c  decreases. Consequently, the oxygen ion conductivity of the interlayer  42   c  may decrease. 
     The interlayer  42   c  is preferably porous because this improves the detection sensitivity of NO 2  gas and selectivity for NO 2  gas. 
     Whether the interlayer  42   c  includes the first metal oxide or not can be confirmed by analyzing the cross section of the NO 2  sensor unit using an Electron Probe Microanalyser (EPMA) (usually, an average value from analyses at three locations). 
     The detection electrode  42   b  contains Au in an amount equal to or more than 70% by mass. This ensures that the detection electrode  42   b  is capable of serving as a current collector. If the Au content of the detection electrode  42   b  is less than 70% by mass, the capability of the detection electrode  42   b  to serve as a current collector is lowered, thus making the detection of the NO 2  gas difficult. 
     The detection electrode  42   b  is preferably a porous electrode that includes a second metal oxide, which is at least one kind of metal oxide selected from a group consisting of metal oxides of Zr, Y, Al and Si. This allows the detection electrode  42   b  to achieve sufficient gas permeability, and allows NO 2  to pass through the detection electrode  42   b  and easily reach the surface boundary of the detection electrode  42   b  and the interlayer  42   c  below. As a result, selectivity only for NO 2  gas can be ensured. The detection electrode  42   b  preferably contains the second metal oxide in a proportion of 5 to 30% by mass. 
     On the other hand, the reference electrode  42   a  is an electrode having a surface at which inflammable gas burns. The reference electrode  42   a  is, for example, constituted by a material formed of Pt alone or a material mainly constituted of Pt. 
     The reference electrode  42   a  is preferably directly disposed on the solid electrolyte body  25 . That is, the interlayer  42   c  preferable is not present below the reference electrode  42   a . As one method for interposing the interlayer  42   c  between the detection electrode  42   b  and the solid electrolyte body  25 , the detection electrode  42   b  is formed on the interlayer  42   c  after the interlayer  42   c  is formed on the entire surface of the solid electrolyte body  25 . In this case, the interlayer  42   c  is also present below the reference electrode  42   a . However, if the reference electrode  42   a  includes Pt, since the baking temperature of Pt is high (approximately equal to or more than 1400° C.), the first metal oxide included in the interlayer  42   c  may be volatilized near the reference electrode  42   a.    
     When, for example, the first metal oxide includes Ce, which is difficult to volatilize, the interlayer  42   c  may be present below the reference electrode  42   a.    
     Next, referring again to  FIG. 5 , an exemplary configuration of the control device  300  will be described. The control device  300  includes a (analog) control circuit  59  and a microcomputer  60  on a circuit board. The microcomputer  60  controls the entire control device  300 . The microcomputer  60  includes a central processing unit (CPU)  61 , a RAM  62 , a ROM  63 , a signal input/output unit  64 , an A/D converter  65 , and a clock (not shown). The CPU  61  executes a program preliminary stored in, for example, the ROM  63 . 
     The control circuit  59  includes a reference voltage comparison circuit  51 , an Ip1 drive circuit  52 , a Vs detection circuit  53 , the Icp supply circuit  54 , an Ip2 detection circuit  55 , a Vp2 application circuit  56 , a Vh heater drive circuit (a heater circuit)  57 , and a NO 2  sensor unit electromotive force detection circuit  58 , which will be described below. 
     The control circuit  59  controls the NO X  sensor unit  30 A and detects a first pumping current Ip1 and a second pumping current Ip2, which flow in the NO X  sensor unit  30 A. The control circuit  59  outputs a detection result to the microcomputer  60 . 
     The NO 2  sensor unit electromotive force detection circuit  58  detects NO 2  concentration output (an electromotive force) between a pair of electrodes  42   a  and  42   b  and outputs the NO 2  concentration output to the microcomputer  60 . 
     In detail, the outer first pumping electrode  2   c  of the NO X  sensor unit  30 A is connected to the Ip1 drive circuit  52 . The reference electrode  6   c  is connected to the Vs detection circuit  53  and the Icp supply circuit  54  in parallel. The counterpart second pumping electrode  4   c  is connected to the Ip2 detection circuit  55  and the Vp2 application circuit  56  in parallel. The heater circuit  57  is connected to the heater  21 . 
     The pair of electrodes  42   a  and  42   b  of the NO 2  sensor unit  42  are each connected to the NO 2  sensor unit electromotive force detection circuit  58 . 
     The circuits  51  to  57  have the respective following functions. 
     The Ip1 drive circuit  52  supplies the first pumping current Ip1 between the inner first pumping electrode  2   b  and the outer first pumping electrode  2   c  and detects the first pumping current Ip1 at that time. 
     The Vs detection circuit  53  detects a voltage Vs between the detection electrode  6   b  and the reference electrode  6   c  and outputs the detection result to the reference voltage comparison circuit  51 . The reference voltage comparison circuit  51  compares the reference voltage (for example, 425 mV) and the output from the Vs detection circuit  53  (a voltage Vs) and outputs the comparison result to the Ip1 drive circuit  52 . The Ip1 drive circuit  52  controls the direction and the amount of the Ip1 current so that the voltage Vs is equal to the reference voltage. This adjusts the oxygen concentration in the first measuring chamber S 1  to a predetermined value so as not to decompose NO X . 
     The Icp supply circuit  54  supplies a weak current Icp which flows between the detection electrode  6   b  and the reference electrode  6   c . This transports oxygen from the first measuring chamber S 1  to the inside of the reference oxygen chamber  15 . As a result, the reference electrode  6   c  is exposed to a predetermined oxygen concentration, which becomes a reference. 
     The Vp2 application circuit  56  applies a constant voltage Vp2 (for example, 450 mV) to a degree where the NO X  gas in the gas to be measured is decomposed into oxygen and N 2  gas between the inner second pumping electrode  4   b  and the counterpart second pumping electrode  4   c . This decomposes the NO X  into nitrogen and oxygen. 
     The Ip2 detection circuit  55  detects the second pumping current Ip2 flowing in the second pumping cell  4  when the oxygen generated by the decomposition of the NO X  is pumped to the counterpart second pumping electrode  4   c  side from the second measuring chamber S 2  via the second solid electrolyte body  4   a.    
     The Ip1 drive circuit  52  outputs the value of the detected first pumping current Ip1 to the A/D converter  65 . The Ip2 detection circuit  55  outputs the value of the detected second pumping current Ip2 to the A/D converter  65 . 
     The A/D converter  65  converts these values into digital values and outputs the digital values to the CPU  61  via the signal input/output unit  64 . 
     Next, one exemplary control using the control circuit  59  will be described. First, upon start of the engine, electric power is supplied from an external power source to the deterioration diagnosis device for determining a deterioration degree of an oxidation catalyst  250 . Then, the heater circuit  57  operates the heater  21 . The heater circuit  57  heats the first pumping cell  2 , the oxygen concentration detection cell  6 , and the second pumping cell  4  to an activation temperature. The Icp supply circuit  54  flows a weak current Icp between the detection electrode  6   b  and the reference electrode  6   c . This transports oxygen from the first measuring chamber S 1  to the inside of the reference oxygen chamber  15 . As a result, the reference electrode  6   c  is exposed to a predetermined oxygen concentration, which becomes a reference. 
     When the NO X  sensor unit  30 A is heated to an appropriate temperature by the heater  21 , accordingly, the temperature of the NO 2  sensor unit  42  on the NO X  sensor unit  30 A also rises to a desired temperature. 
     When each cell is heated to the activation temperature, the first pumping cell  2  pumps oxygen in the gas to be measured (exhaust gas) which has been introduced into the first measuring chamber S 1  from the inner first pumping electrode  2   b  to the outer first pumping electrode  2   c.    
     At this time, the oxygen concentration inside of the first measuring chamber S 1  assumes a value corresponding to a voltage Vs developed between electrodes of the oxygen concentration detection cell  6  (a voltage across the terminal s). In view of the above, the Ip1 drive circuit  52  controls the first pumping current Ip1 flowing in the first pumping cell  2  so that the voltage Vs across the electrodes becomes the reference voltage. This adjusts the oxygen concentration in the first measuring chamber S 1  to a concentration so as not to decompose the NO X . 
     The gas to be measured in which the oxygen concentration has been adjusted further flows toward the second measuring chamber S 2 . As a voltage between electrodes of the second pumping cell  4  (a voltage between terminal s), the Vp2 application circuit  56  applies a constant voltage Vp2 (a voltage higher than a control voltage value of the oxygen concentration detection cell  6 , for example, 450 mV) to the degree that the NO X  gas in the gas to be measured is decomposed into oxygen and N 2  gas. This decomposes the NO X  into nitrogen and oxygen. The second pumping current Ip2 flows through the second pumping cell  4  so that the oxygen generated by the decomposition of the NO X  is pumped from the second measuring chamber S 2 . In this respect, the second pumping current Ip2 and the NO X  concentration are in linear relationship. Thus, the NO X  concentration in the gas to be measured can be detected based on the second pumping current Ip2 detected by the Ip2 detection circuit  55 . 
     The NO 2  concentration in the gas to be measured can be detected by detecting the NO 2  concentration output (the electromotive force) between the pair of electrodes  42   a  and  42   b  by the NO 2  sensor unit electromotive force detection circuit  58 . The NO 2  concentration is calculated by storing an NO 2  concentration conversion value based on an electromotive force between the electrodes  42   a  and  42   b  (it is also possible to use a changing rate (sensitivity) between a base electromotive force value when the NO 2  concentration is 0 and an electromotive force value when NO 2  is present) in the microcomputer  60 . This calculation process will be described below. 
     Next, a processing flow of calculating various gas concentrations by the microcomputer  60  of the control device  300  will be described with reference to  FIG. 7 . 
     First, the microcomputer  60  obtains the value of the first pumping current Ip1, the value of the second pumping current Ip2, and the NO 2  concentration output (the electromotive force) (Step S 20 ). Next, the microcomputer  60  calculates a NO X  concentration value after O 2  concentration correction based on Ip1 and Ip2 (Step S 22 ). Even if the NO X  concentration is the same, the higher the O 2  concentration in the gas to be measured, the smaller the Ip2 value. In view of this, the microcomputer  60  corrects the calculated NO X  concentration value to have a true value depending on the O 2  concentration. The current Ip1 is proportional to the O 2  concentration in the gas to be measured. In this manner, the O 2  concentration can be obtained from the Ip1.  FIG. 8  illustrates a relationship between the Ip2 and the NO X  concentration after the O 2  concentration correction. 
     Next, the microcomputer  60  calculates the NO 2  concentration value after the O 2  concentration correction based on Ip1 and the NO 2  concentration output (Step S 24 ). That is, even if the NO 2  concentration is the same, the higher the O 2  concentration in the gas to be measured, the smaller the NO 2  concentration output value. In view of this, the microcomputer  60  corrects the calculated NO 2  concentration value to have a true value depending on the O 2  concentration (Step S 26 ).  FIG. 9  illustrates a relationship between the EMF output of the NO 2  sensor before O 2  concentration correction and the NO concentration and the NO 2  concentration. It can be seen that the NO 2  sensor unit  42  does not have sensitivity to NO.  FIG. 10  illustrates a relationship between the concentration conversion output of the NO 2  sensor before the O 2  concentration correction and the NO concentration and the NO 2  concentration. It can be seen that even if the NO 2  concentration is the same, the higher the O 2  concentration in the gas to be measured, the smaller the NO 2  concentration output value.  FIG. 11  illustrates a relationship between the concentration conversion output of the NO 2  sensor after the O 2  concentration correction and the NO concentration and the NO 2  concentration. 
     Next, the microcomputer  60  calculates the NO concentration where NO concentration=(NO X  concentration−NO 2  concentration) (Step S 28 ), and calculates the NO 2  ratio where NO 2  ratio=(NO 2  concentration/NO concentration) (Step S 30 ). 
     The microcomputer  60  corresponds to “the NO concentration calculation unit” of the invention. The NO 2  ratio corresponds to “the evaluation value” of the invention. 
     Next, a multi-gas sensor according to a second embodiment will be described with reference to  FIG. 12  and  FIG. 13 .  FIG. 12  is a sectional view of a multi-gas sensor  200 B (a multi-gas sensor element unit  100 B) according to the second embodiment along the longitudinal direction (the axial direction).  FIG. 13  is a sectional view illustrating the configuration of NO 2  sensor units  42   x  and  42   y  along the width direction of the multi-gas sensor element unit  100 B. Members of the multi-gas sensor  200 B other than the multi-gas sensor element unit  100 B are the same as the members of the multi-gas sensor  200 A according to the first embodiment. Thus, the illustration of these members is omitted. 
     The multi-gas sensor element unit  100 B of the multi-gas sensor  200 B according to the second embodiment is the same as the multi-gas sensor element unit  100 A, except that the configuration of the NO 2  sensor unit is different and the solid electrolyte body for the NO 2  sensor  25  is not included Like reference numerals designate identical elements throughout the embodiments, and therefore the following description will not further elaborate on such elements. The multi-gas sensor  200 B includes two NO 2  sensor units. Thus, a control device  300 B includes two circuits: a first electromotive force detection circuit  58   a  and a second electromotive force detection circuit  58   b  as an electromotive force detection circuit for NO 2  sensor unit. Except for this, the control device  300 B has the same configuration as the control device  300 . Like reference numerals designate identical elements throughout the embodiments, and therefore the following description will not further elaborate on such elements. 
     As illustrated in  FIG. 13 , the multi-gas sensor element unit  100 B includes the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  separated from one another in the width direction. 
     The first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  are formed on the insulation layer  23   f , which is the outer surface of the NO X  sensor unit  30 A. More specifically, the first NO 2  sensor unit  42   x  includes a reference electrode  42   ax  on the insulation layer  23   f . The top surface and the side surfaces of the first reference electrode  42   ax  are covered with a first solid electrolyte body  42   dx . A first detection electrode  42   bx  is formed on the surface of the first solid electrolyte body  42   dx . NO 2  concentration in a measured gas is detected based on changes in the electromotive force between the first reference electrode  42   ax  and the first detection electrode  42   bx.    
     Similarly, the second NO 2  sensor unit  42   y  includes a second reference electrode  42   ay  on the insulation layer  23   f . The top surface and the side surfaces of the second reference electrode  42   ay  are covered with a second solid electrolyte body  42   dy . A second detection electrode  42   by  is formed on the surface of the second solid electrolyte body  42   dy.    
     The first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  are integrally covered with a protective layer  23   g  including a porous material. 
     The protective layer  23   g  prevents poisoning substances from accumulating on the first detection electrode  42   bx  and the second detection electrode  42   by . The protective layer  23   g  adjusts the diffusion speed of the gas to be measured which has flowed from the outside to the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y . The material forming the protective layer  23   g , for example, may be at least one kind of material selected from the group consisting of alumina (aluminum oxide), spinel (MgAl 2 O 4 ), silica alumina and mullite. The diffusion speed of the gas to be measured is adjusted by adjusting, for example, the thickness, the particle diameter, the particle size distribution, the porosity, and/or the mixing ratio of the protective layer  23   g.    
     The protective layer  23   g  may be disposed in a manner similar to the above-described embodiment. The first NO 2  sensor unit  42   x , the second NO 2  sensor unit  42   y , or similar member may be exposed without disposing the protective layer  23   g . The presence or absence of the protective layer  23   g  is not particularly limited. 
     The first detection electrode  42   bx  and the second detection electrode  42   by  can be formed of a material mainly constituted of Au (for example, a material containing Au in an amount equal to or more than 70% by mass). The first detection electrode  42   bx  and the second detection electrode  42   by  can be formed of Pt alone or a material mainly constituted of Pt (for example, a material containing Pt in an amount equal to or more than 70% by mass). The first detection electrode  42   bx  and the second detection electrode  42   by  are located where NO 2  gas is less likely to burn on their respective surfaces. NO 2  passes through the detection electrode  42   bx  ( 42   by ) and reaches the surface boundary of the detection electrode  42   bx  ( 42   by ) and the underlying reference electrode  42   ax  ( 42   ay ). The NO 2  reacts with oxygen ion at this surface boundary (electrode reaction). This allows detecting the concentration of NO 2 . 
     The first solid electrolyte body  42   dx  and the second solid electrolyte body  42   dy , for example, include partially stabilized zirconia (YSZ). The first reference electrode  42   ax  and the second reference electrode  42   ay  may have a composition similar to the composition of the reference electrode  42   a . The first detection electrode  42   bx  and the second detection electrode  42   by  may have a composition similar to the composition of the detection electrode  42   b . The first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  do not include an interlayer. 
     Here, in this embodiment, the temperature of the oxygen concentration detection cell (corresponding to the “temperature detector” of the invention)  6  is measured. Based on this measured temperature, the heater  21  is heated. In view of the above, the temperature of the multi-gas sensor element unit  100 B is kept at the most stable value adjacent to the oxygen concentration detection cell  6 . Because the multi-gas sensor element unit  100 B is distant from the oxygen concentration detection cell  6  in the axial direction, the change in temperature of multi-gas sensor element unit  100 B becomes larger with fluctuation in the external temperature. 
     Accordingly, the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  preferably at least partially overlap with a position  6   s  of the oxygen concentration detection cell  6  as viewed from the axial direction. In this case, the temperatures of the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  are kept constant within a predetermined range. As a result, the NO 2  measurement accuracy improves. The position  6   s  of the oxygen concentration detection cell  6  is an overlapping portion of the detection electrode  6   b  and the reference electrode  6   c  included in the oxygen concentration detection cell  6  (in this embodiment, both electrodes have approximately the same dimension and are disposed at approximately the same position). Similarly, the NO 2  sensor unit is disposed at the overlapping portion of the detection electrode  6   b  and the reference electrode  6   c.    
     The overlapping state of the first NO 2  sensor unit  42   x  or the second NO 2  sensor unit  42   y  with the oxygen concentration detection cell  6  is not limited to the above-described state. For example, as illustrated in  FIG. 14 , the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  may be separated from one another in the width direction of the multi-gas sensor element unit  100 B (the direction vertical to an axis O direction). The front end side of the first NO 2  sensor unit  42   x  may protrude with respect to the front end of the position  6   s . The rear end side of the second NO 2  sensor unit  42   y  may protrude with respect to the rear end of the position  6   s . As illustrated in  FIG. 15 , the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  may be separated from one another in the ax axis O direction of the multi-gas sensor element unit  100 B. The front end side of the first NO 2  sensor unit  42   x  may protrude with respect to the front end of the position  6   s . The rear end side of the second NO 2  sensor unit  42   y  may protrude with respect to the rear end of the position  6   s.    
     In the second embodiment, the reason for disposing two NO 2  sensor units: the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  is as follows. That is, the NO 2  sensor unit detects not only NO 2  but also another gas constituent (especially, inflammable gas such as propylene). In view of this, if a disturbance (interfering) gas other than NO 2  is included in the detected gas, the detection accuracy of NO 2  is reduced. Therefore, two NO 2  sensor units with a different sensitivity ratio to NO 2  from one another are preferably disposed. In this case, the two NO 2  sensor units detect two unknown concentrations of NO 2  gas and disturbance gas at different sensitivities. In view of the above, the concentration of the NO 2  gas and the disturbance gas can be calculated. Here, “the sensitivity ratio of the NO 2  sensor unit to NO 2 ” is a ratio of the detection sensitivity of NO 2  to that of all gases detected by the NO 2  sensor unit (including NO 2  and the following disturbance gas). 
     That is, the sensor output of the NO 2  sensor unit is expressed as F (x, y, D) assuming that x is NO 2  concentration, y is a disturbance gas concentration output, and D is the O 2  concentration output. The use of the two NO 2  sensor units with different sensitivity ratios allows obtaining the following two formulas: F1 (mx, ny, D) and F2 (sx, ty, D), where m, n, s, and t are coefficients. F1, F2, and D are obtained from the sensor output. Accordingly, the only two unknown variables (x, y) may be solved from the two formulas. Specifically, the formulas can be calculated by removing y from the two formulas and obtaining x as in formulas (1) and (2), which will be described below. For application of this disclosure, obtaining the NO 2  concentration output where influence of the disturbance gas has been removed is sufficient. Obtaining the disturbance gas concentration is not required. 
     A method for changing the sensitivity ratio of the NO 2  sensor unit for NO 2  adds co-electrolyte (the constituent of the solid electrolyte body) or a precious metal such as Pt or Pd to the detection electrode. That is, simply changing the amount of co-electrolyte or precious metal added to the first detection electrode  42   bx  and the second detection electrode  42   by  changes the sensitivity ratio of NO 2  between the first detection electrode  42   bx  and the second detection electrode  42   by . When the amount of co-electrolyte or precious metal to be added is increased, the sensitivity ratio tends to be large. 
     Here, the sensitivity of the NO 2  sensor unit differs depending on the type of disturbance gas (inflammable gas). In view of above, usually, if the constituent of the disturbance gas is not known, the calculation cannot be performed. However, the following point has been demonstrated by the inventor. That is, for example, using the NO 2  sensor unit having a sensitivity ratio 1 and the NO 2  sensor unit having a sensitivity ratio 2, the relational expression between the concentration of a known disturbance gas (propylene) and the concentration of NO 2  is preliminarily generated. Even if the type of the disturbance gas differs, the trend of the relational expression is the same. 
     Accordingly, even if the constituent of the disturbance gas is unknown, the calculation can be performed. That is, for example, whether the inflammable gas is propylene or another inflammable gas (for example, C 3 H 8  or C 4 H 8 ), the ratio of detection value of each gas in the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  is almost constant. 
     Next, details of detection of NO 2  and the disturbance gas by the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  and the calculation of the concentrations of the NO 2  gas and the disturbance gas will be described. 
     An electromotive force according to the NO 2  concentration included in the gas to be measured is generated between the first reference electrode  42   ax  and the first detection electrode  42   bx  of the first NO 2  sensor unit  42   x . The first electromotive force detection circuit  58   a  detects an electromotive force between the first reference electrode  42   ax  and the first detection electrode  42   bx  as a first NO 2  electromotive force. Similarly, an electromotive force according to the NO 2  concentration is also generated between the second reference electrode  42   ay  and the second detection electrode  42   by  of the second NO 2  sensor unit  42   y . The second electromotive force detection circuit  58   b  detects an electromotive force between the second reference electrode  42   ay  and the second detection electrode  42   by  as a second NO 2  electromotive force. 
     The ROM  63  of the microcomputer  60  stores various data (relational expressions) described below. The CPU  61  reads the various data from the ROM  63  and performs various arithmetic operations using the value of the first pumping current Ip1, the value of the second pumping current Ip2, the first NO 2  electromotive force, and the second NO 2  electromotive force. 
     More particularly, the ROM  63  stores “output relational expression between first NO 2  electromotive force and first NO 2  concentration”, “output relational expression between second NO 2  electromotive force and second NO 2  concentration”, “output relational expression between first pumping current Ip1 and O 2  concentration”, “output relational expression between second pumping current Ip2 and NO X  concentration”, “output relational expression between first NO 2  concentration output &amp; second NO 2  concentration output &amp; O 2  concentration output and corrected NO 2  concentration” (correction formula (1): see below), and “output relational expression between NO X  concentration output &amp; corrected NO 2  concentration output and NO 2 /NO ratio” (correction formula (2): see below). 
     The various data may be set as predetermined relational expressions as described above. It is only necessary that the various data (for example, relational expressions) is data that allows calculating various gas concentrations using the output value of the sensor. The various data may be, for example, a table. Alternatively, the various data may be data regarding a gas concentration value (for example, relational expressions or tables) preliminarily obtained using a known gas model. 
     “Output relational expression between first NO 2  electromotive force and first NO 2  concentration” and “output relational expression between second NO 2  electromotive force and second NO 2  concentration” are formulas expressing the relationships between the NO 2  electromotive forces output from the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  and the NO 2  concentration output regarding the NO 2  concentration of the gas to be measured. 
     “Output relational expression between first pumping current Ip1 and O 2  concentration” is a formula expressing the relationship between the first pumping current Ip1 and the O 2  concentration of the gas to be measured. 
     “Output relational expression between second pumping current Ip2 and NO X  concentration” is a formula expressing the relationship between the second pumping current Ip2 and the NO X  concentration of the gas to be measured. 
     “Output relational expression between first NO 2  concentration output &amp; second NO 2  concentration output &amp; O 2  concentration output and corrected NO 2  concentration” is a formula expressing the relationship between the NO 2  concentration outputs (first and second) affected by the oxygen concentration and various inflammable gas concentrations and the corrected NO 2  concentration output where the influence of the oxygen concentration and the various inflammable gas concentrations is removed. 
     “Output relational expression between NO X  concentration output &amp; corrected NO 2  concentration output and NO 2 /NO ratio” is a formula for calculating the NO 2 /NO ratio from the NO X  concentration output and the NO 2  concentration output where the influence of the various inflammable gas concentrations is removed. 
     Next, arithmetic operations obtaining the NO X  concentration and the NO 2 /NO ratio using the first pumping current Ip1, the second pumping current Ip2, the first NO 2  electromotive force EMF, and the second NO 2  electromotive force EMF will be described. The arithmetic operations are performed in the CPU  61  of the microcomputer  60 . 
     When the first pumping current Ip1, the second pumping current Ip2, the first NO 2  electromotive force, and the second NO 2  electromotive force are input, the CPU  61  performs the arithmetic operations obtaining the O 2  concentration output, the NO X  concentration output, the first NO 2  concentration output, and the second NO 2  concentration output. Specifically, the CPU  61  calls “output relational expression between first NO 2  electromotive force and first NO 2  concentration”, “output relational expression between second NO 2  electromotive force and second NO 2  concentration”, “output relational expression between first pumping current Ip1 and O 2  concentration”, and “output relational expression between second pumping current Ip2 and NO X  concentration” from the ROM  63 . The CPU  61  performs a process calculating each concentration output using these relational expressions. 
     “Output relational expression between first NO 2  electromotive force and first NO 2  concentration” and “output relational expression between second NO 2  electromotive force and second NO 2  concentration” are formulas set so that the NO 2  concentration in the gas to be measured and the NO 2  concentration conversion output of the sensor have approximately a linear relationship in all ranges of EMF where the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  can output in an operating environment. Converting with such conversion formulas allows calculation using changes in gradients and offset in the correction formula performed later. 
     When the O 2  concentration output, the NO X  concentration output, the first NO 2  concentration output, and the second NO 2  concentration output are obtained, the CPU  61  performs an operation using the correction formula described below. Thus, the CPU  61  obtains the NO 2  concentration and the NO 2 /NO ratio of the gas to be measured.
 
 x=F ( A,B,D )=( eA−c )*( jB−h−fA+d )/( eA−c−iB+g )+ fA−d   Correction formula (1):
 
NO 2 /NO= x /(C− axax )  Correction formula (2):
 
     Here, x represents the NO 2  concentration. A represents the first NO 2  concentration output. B represents the second NO 2  concentration output. C represents the NO X  concentration output. D represents the O 2  concentration output. F in the formula (1) represents that x is a function of (A, B, D). “a” represents a correction coefficient. “c”, “d”, “e”, “f”, “g”, “h”, “i”, and “j” are coefficients calculated using O 2  concentration output D (coefficients determined by D). The first NO 2  concentration output (A), the second NO 2  concentration output (B), the NO X  concentration output (C), and the O 2  concentration output (D) are substituted in the above-described correction formula, and the formula is executed. Thus, the NO 2  concentration and the NO 2 /NO ratio of the gas to be measured and similar parameters are obtained. 
     The correction formula (1) is a formula determined based on the properties of the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y . The correction formula (2) is a formula determined based on the property of the NO X  sensor unit  30 A. These formulas are exemplary correction formulas. The correction formula and/or the coefficient or similar parameter may be appropriately changed according to a gas detection property. Whether the oxidation catalyst is deteriorated or not can be determined using not only the NO 2 /NO ratio but also appropriate values such as NO 2 /NOx, NO/NOx, NO 2  concentration, and NO concentration. 
     Next, the actual NO 2  concentration output performed by the multi-gas sensor  200 B according to the second embodiment before and after the correction process using the correction formulas (1) and (2) will be described. 
       FIG. 16A  is a graph plotting the concentration conversion output of the first NO 2  sensor unit and the second NO 2  sensor unit when NO 2 =0-150 ppm is introduced under the conditions of O 2 =2, 7, or 15% and C 3 H 6 =0, 20, 50, or 100 ppm. The concentration conversion outputs of the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  vary significantly due to influence of the C 3 H 6  concentration and the O 2  concentration. 
     Meanwhile,  FIG. 16B  is a graph plotting the corrected NO 2  concentration output, which is obtained by substituting the values of the NO 2  concentration output and the O 2  concentration output in the correction formula (1), relative to the NO 2  introduction concentration. The present inventor found that use of the correction formula (1) allows calculation of an accurate NO 2  concentration where the influence of C 3 H 6  and O 2  is removed. 
     The corrected NO 2 /NO ratio can be calculated with the correction formula (2) obtained from the NO X  sensor output characteristics. 
       FIG. 16A  and  FIG. 16B  are calculation results of the concentration of corrected NO 2  in case of using C 3 H 6  as an inflammable gas species. However, in an actual exhaust gas environment, various types of inflammable gases exist besides C 3 H 6 .  FIG. 17A  and  FIG. 17B  illustrate the calculation results of the corrected NO 2  concentration output when another inflammable gas coexists. The formula used for the correction is a formula created based on the relationship between NO 2  and C 3 H 6  completely the same as the formula used in  FIG. 16A  and  FIG. 16B . 
       FIG. 17A  is a graph plotting the concentration conversion output of the first NO 2  sensor unit and the second NO 2  sensor unit when NO 2 =0-150 ppm is introduced under the conditions of O 2 =7% and various inflammable gases=0, 20, 50, or 100 ppm. The inflammable gases are C 3 H 6 , C 3 H 8 , C 4 H 8 , C 4 H 10 , CO and NO, respectively. The concentration conversion outputs of the first NO 2  sensor unit  42   x  and the second NO 2  sensor unit  42   y  vary significantly by the influence of each inflammable gas concentration and the O 2  concentration. 
     Meanwhile,  FIG. 17B  is a graph plotting the corrected NO 2  concentration output, which is obtained by substituting the values of the NO 2  concentration output and the O 2  concentration output in the correction formula (1), relative to the NO 2  introduction concentration. Use of the correction formula (1) was found to allow calculation of an accurate NO 2  concentration where the influence of various types of inflammable gases is removed even if such inflammable gases are present. 
       FIG. 18  is a graph plotting an output ratio of the first NO 2  sensor unit  42   x  to the second NO 2  sensor unit  42   y  for each inflammable gas relative to the output of the first NO 2  sensor unit  42   x . It was found from this graph that even if the type of the inflammable gas changes, the output ratio of the first NO 2  sensor unit  42   x  to the second NO 2  sensor unit  42   y  relative to the inflammable gas exhibits an almost similar trend. That is, even if the type of the inflammable gas changes, the influence rate of the inflammable gas relative to the two NO 2  sensor outputs are similar. In view of the above, if the correction formula is created using any of inflammable gases, the NO 2  concentration can be corrected even if another inflammable gas coexists in the gas to be measured. The multi-gas sensor  200 B according to the second embodiment has the above-described output characteristics to various inflammable gases and NO 2 . Accordingly, this multi-gas sensor  200 B can calculate the NO 2  concentration where the influence of various inflammable gases is removed. 
     As described above, use of the multi-gas sensor  200 B and the correction method according to the second embodiment allows precise correction calculation on NO 2  concentration even in an environment where various inflammable gases coexist and the oxygen concentration changes. Calculating the NO 2  sensor output and the NO X  sensor output allows separately detecting NO 2  and NO. 
     The present invention is not limited to the above embodiments. Various modifications and variations of the embodiments described above will be apparent to those skilled in the art. 
     For example, in the above embodiments, the microcomputer  60  disposed in the control device  300  calculates the NO concentration and the NO 2  ratio. The calculated NO 2  ratio is output to the deterioration judgment unit  221  in the ECU  220 . However, this should not be construed in a limiting sense. The NO concentration calculation unit may be disposed in the ECU  220 , such that the NO 2  concentration and the NO X  concentration after the O 2  concentration correction calculated by the microcomputer  60  may be output to the NO concentration calculation unit in the ECU  220 . In that case, the NO concentration calculation unit may calculate the NO concentration and the NO 2  ratio in the ECU  220 . When the NO concentration calculation unit is disposed in the ECU  220 , the microcomputer  60  may calculate the NO concentration and output the calculation result to the NO concentration calculation unit in the ECU  220 . In that case, the NO concentration calculation unit may calculate the NO 2  ratio in the ECU  220 . 
     In the above embodiments, the deterioration degree of the oxidation catalyst (DOC)  512  is judged by the NO 2  ratio and the catalyst temperature. However, this should not be construed in a limiting sense. The deterioration degree of the oxidation catalyst (DOC) may be judged by a parameter different from the catalyst temperature (for example, vehicle information such as an engine drive condition) and the NO 2  ratio. The deterioration degree may be judged by the NO 2  ratio alone and without using the catalyst temperature or a different parameter. 
     In the embodiments, the deterioration degree of the oxidation catalyst (DOC)  512  is determined using the NO 2  ratio obtained from the NO 2  concentration/NO concentration as an evaluation value. However, this should not be construed in a limiting sense. The deterioration degree of the oxidation catalyst may be determined using the NO 2  ratio obtained from the NO X  concentration/NO concentration as an evaluation value. The deterioration degree of the oxidation catalyst  512  may be judged using the NO concentration value. 
     In the above embodiment, the multi-gas sensor  200 A is installed at the downstream side immediately after the DOC  512  in the DPF device  510 . As illustrated in  FIG. 19 , the multi-gas sensor  200 A may be installed at the downstream side immediately after the DPF  514 . Thus, disposing the multi-gas sensor  200 A at the downstream side immediately after the DPF  514  inhibits PM from accumulating on the multi-gas sensor element unit  100 A. This allows for reducing fluctuation of the sensor output of the multi-gas sensor element unit  100 A and for diagnosing the degree of the deterioration of the DOC  512  at good accuracy. 
     The invention has been described in detail with reference to the above embodiments. However, the invention should not be construed as being limited thereto. It should further be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto. 
     This application is based on Japanese Patent Application No. 2012-189824 filed Aug. 30, 2012 and Japanese Patent Application No. 2013-122621 filed Jun. 11, 2013, the above-noted applications incorporated herein by reference in their entirety.