Abstract:
A diagnostic device includes: a diesel oxidation catalyst (DOC) for oxidizing hydrocarbon (HC) and nitrogen monoxide in an exhaust gas; a selective catalytic reduction (SCR) catalyst for reducing and purifying NOx contained in the exhaust gas; a NOx purification rate calculation unit which calculates, based on NOx values at upstream and downstream sides of the SCR catalyst, a low temperature NOx purification rate and a high temperature NOx purification rate; a HC heat generation rate calculation unit which calculates an HC purification rate based on at least the difference between the exhaust gas heat quantities at upstream and downstream sides of the DOC; and a deterioration determination unit which determines the deterioration in the NO 2  producing capability of the DOC based on the calculated low temperature NOx purification rate, the calculated high temperature NOx purification rate, and the calculated HC purification rate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application, which claims the benefit under 35 U.S.C. §371 of PCT International Patent Application No. PCT/JP2014/074688, filed Sep. 18, 2014, which claims the foreign priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2013-193012, filed Sep. 18, 2013, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a diagnostic device, and in particular to diagnosis of a deterioration of an oxidation catalyst arranged in an exhaust system of an internal combustion engine. 
     BACKGROUND ART 
     As an exhaust gas purifying catalyst to be arranged in an exhaust system (exhaust gas passage) of a diesel engine or the like, there is known an oxidation catalyst (Diesel Oxidation Catalyst: DOC) for oxidizing hydrocarbons (HC) and carbon monoxide (CO) contained in an exhaust gas and also oxidizing nitrogen monoxide (NO) to produce nitrogen dioxide (NO 2 ). There is also known a selective catalytic reduction (SCR) catalyst for selectively reducing and purifying nitrogen compounds (NOx) contained in the exhaust gas. The SCR catalyst uses ammonia (NH 3 ) as a reducing agent that is obtained by hydrolyzing urea solution (urea water). 
     In the SCR catalyst, the purification of NOx in a low temperature range is facilitated in particular when a ratio of NO contained in the exhaust gas to NO 2  produced in an upstream DOC becomes approximately 1 to 1. In other words, when the capability of the DOC to oxide NO (NO 2  producing capability of the DOC) drops due to aging deterioration or the like, the NOx purification rate of the SCR catalyst may be affected. In view of such possibility, there is a demand for diagnosing the deterioration state (level) of the DOC on board. 
     For example, Patent Literature Document 1 discloses a technique that estimates an amount of NO 2  by multiplying a ratio of NO 2  to NO contained in the exhaust gas by a detection value of the NOx sensor disposed downstream of the SCR catalyst, and determines whether the DOC is in a deteriorated state. 
     LISTING OF REFERENCES 
     Patent Literature Document 1: Japanese Patent Application Laid-Open Publication No. 2012-36860 
     It is difficult to directly detect a value of NO 2  contained in the exhaust gas with a sensor. Thus, the value of NO 2  may be estimated by multiplying a ratio of NO 2  by the detection value of the NOx sensor. This is the above-described conventional technique. However, the ratio of NO 2  to NO contained in the exhaust gas changes with the running condition. Thus, if the value of NO 2  is estimated from the detection value of the NOx sensor and the deterioration of the DOC is diagnosed, the diagnosis may not be performed at high accuracy. 
     SUMMARY OF THE INVENTION 
     An object of a diagnostic device disclosed herein is to carry out a deterioration diagnosis of the DOC at high accuracy. 
     A diagnostic device disclosed herein includes: an oxidation catalyst arranged in an exhaust system of an internal combustion engine and configured to oxidize at least hydrocarbons and nitrogen monoxide contained in an exhaust gas; a selective catalytic reduction catalyst arranged in the exhaust system at a position downstream of the oxidation catalyst and configured to reduce and purify NOx contained in the exhaust gas with ammonia being a reducing agent; a first purification rate calculation unit configured to calculate a low temperature NOx purification rate when a catalyst temperature of the selective catalytic reduction catalyst is in a predetermined low temperature range based on a NOx value at an upstream side and a NOx value at a downstream side of the selective catalytic reduction catalyst, and also configured to calculate a high temperature NOx purification rate when the catalyst temperature of the selective catalytic reduction catalyst is in a predetermined high temperature range based on the NOx value at the upstream side and the NOx value at the downstream side of the selective catalytic reduction catalyst; a second purification rate calculation unit configured to calculate a hydrocarbon purification rate of the oxidation catalyst based on at least a difference in exhaust gas heat quantity between an upstream side and a downstream side of the oxidation catalyst; and a determination unit configured to determine whether the oxidation catalyst is in a deteriorated state based on the calculated low temperature NOx purification rate, the calculated high temperature NOx purification rate, and the calculated hydrocarbon purification rate. 
     The diagnostic device disclosed herein is capable of performing a precise deterioration diagnosis of the DOC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic overall configuration diagram illustrating an intake and exhaust system of an engine to which a diagnostic device according to an embodiment of the present invention is applied. 
         FIG. 2  is a schematic diagram useful to explain conservation of an energy generated upon oxidation of HC supplied to a DOC. 
         FIG. 3  is a schematic side view useful to explain a heat loss from the DOC due to an influence of forced convection. 
         FIG. 4  shows a comparison of an NOx purification rate of an SCR catalyst between when the upstream DOC is in a normal state and when the upstream DOC is in a deteriorated state. 
         FIG. 5( a )  shows a comparison of an NO oxidization capability (NO 2  producing capability) of the DOC between when the DOC is in the normal state and when the DOC is in the deteriorated state.  FIG. 5( b )  shows a comparison of an HC oxidization capability (HC purification capability) of the DOC between when the DOC is in the normal state and when the DOC is in the deteriorated state. 
         FIG. 6  is a flowchart illustrating control performed by the diagnostic device according to this embodiment. 
         FIG. 7  is a schematic overall configuration diagram illustrating an intake and exhaust system of an engine to which a diagnostic device according to another embodiment of the present invention is applied. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a diagnostic device according to an embodiment of the present invention will be described with reference to the accompanying drawings. Same parts are designated by same reference numerals, and such parts have same names and functions. Accordingly, redundant detailed descriptions of such parts will be omitted. 
     As shown in  FIG. 1 , a diesel engine (hereinafter simply referred to as “engine”)  10  has an intake manifold  10   a  and an exhaust manifold  10   b . An intake passage  11  for introducing fresh air is connected to the intake manifold  10   a , and an exhaust passage  12  for discharging an exhaust gas to the atmosphere is connected to the exhaust manifold  10   b.    
     On the intake passage  11 , disposed are an air cleaner  30 , an a mass air flow (MAF) sensor  31 , a compressor  32   a  of a turbo charger, and an intercooler  33 . The air cleaner  30 , the MAF sensor  31 , the compressor  32   a  and the intercooler  33  are arranged in this order from the upstream side. On the exhaust passage  12 , disposed are a turbine  32   b  of the turbo charger, an upstream aftertreatment device  14 , and a downstream aftertreatment device  20 . The turbine  32   b , the upstream aftertreatment device  14  and the downstream aftertreatment device  20  are arranged in this order from the upstream side. It should be noted that in  FIG. 1  reference numeral  36  denotes an outside air temperature sensor. 
     The upstream aftertreatment device  14  includes a cylindrical catalyst casing  14   a , a DOC  15 , and a DPF  16 . The DOC  15  is arranged upstream of the DPF  16  in the catalyst casing  14   a . In addition, an in-pipe injector (injector for injecting a fuel into the exhaust pipe)  13  is arranged on an upstream side of the DOC  15 , a DOC inlet temperature sensor  18  is arranged on the upstream side of the DOC  15 , and a DOC outlet temperature sensor  19  is arranged on a downstream side of the DOC  15 . A differential pressure sensor  17 , which is used to detect (measure) a difference in pressure between the upstream and downstream sides of the DPF  16 , is arranged across the DPF  16 . 
     The in-pipe injector (exhaust pipe injector)  13  injects unburnt fuel (HC) into the exhaust passage  12  in accordance with an instruction signal received from an electronic control unit (hereinafter referred to as “ECU”)  40 . It should be noted that if post injections by way of multiple injections of the engine  10  are employed, the in-pipe injector  13  may be omitted. 
     The DOC  15  includes, for example, a ceramic support having a cordierite honeycomb structure or the like, and catalytic components supported on a surface of the ceramic support. Once unburnt HC is supplied to the DOC  15  by the in-pipe injector  13  or the post injection, the DOC  15  oxidizes HC to raise the temperature of the exhaust gas. In addition, the DOC  15  oxidizes NO in the exhaust gas to NO 2  to increase the ratio of NO 2  to NO in the exhaust gas. 
     The DPF  16  includes, for example, a large number of cells defined by porous partitions and arranged along the flow direction of the exhaust gas, with the upstream and downstream sides of the cells being plugged alternately. In the DPF  16 , PM in the exhaust gas is collected in pores of the partitions and on surfaces of the partitions. When an amount of accumulated PM reaches a predetermined value, so-called forced regeneration is carried out, i.e., the accumulated PM is burnt for removal. The forced regeneration is carried out by supplying the unburnt fuel (HC) into the DOC  15  through the in-pipe injector  13  or the post injection, and raising the temperature of the exhaust gas flowing into the DPF  16  up to a PM combustion temperature (for example, about 600 degrees C.). The amount of accumulated PM can be obtained (known) from a sensor value of the differential pressure sensor  17 . 
     The DOC inlet temperature sensor  18  detects the temperature of the upstream exhaust gas flowing into the DOC  15  (hereinafter referred to as “DOC inlet exhaust gas temperature”). The DOC outlet temperature sensor  19  detects the temperature of the downstream exhaust gas flowing out of the DOC  15  (hereinafter referred to as “DOC outlet exhaust gas temperature”). The detection values of the temperature sensors  18  and  19  are supplied to the ECU  40 , which is electrically connected to the sensors  18  and  19 . 
     The downstream aftertreatment device  20  includes a cylindrical catalyst casing  20   a , a urea solution injector  21 , and an SCR catalyst  22  disposed in the catalyst casing  20   a . The urea solution injector  21  is arranged upstream of the SCR catalyst  22 . An SCR catalyst inlet temperature sensor  23  and an SCR catalyst inlet NOx sensor  24  are disposed upstream of the SCR catalyst  22 . An SCR catalyst outlet NOx sensor  25  is disposed downstream of the SCR catalyst  22 . 
     The urea solution injector  21  injects a urea solution (urea water) from a urea solution tank (not shown) into the exhaust passage  12  between the upstream aftertreatment device  14  and the downstream aftertreatment device  20  in accordance with an instruction signal received from the ECU  40 . The injected urea solution is hydrolyzed to NH 3  with the exhaust gas heat, and NH 3  is supplied to the SCR catalyst  22  on the downstream side as a reducing agent. 
     The SCR catalyst  22  includes, for example, a ceramic support having a honeycomb structure, and zeolite supported on a surface of the ceramic support. The SCR catalyst  22  absorbs NH 3 , which supplied as the reducing agent, and the absorbed NH 3  reduces NOx contained in the exhaust gas passing therethrough for purification. 
     The SCR catalyst inlet temperature sensor  23  detects the temperature of the upstream exhaust gas that flows in the SCR catalyst  22  (hereinafter referred to as “SCR catalyst inlet exhaust gas temperature). The SCR catalyst inlet NOx sensor  24  detects the value of NOx contained in the exhaust gas that flows into the SCR catalyst  22 . The SCR catalyst outlet NOx sensor  25  detects the value of NOx contained in the exhaust gas that flows out of the SCR catalyst  22 . The detection values of these sensors  23  to  25  are supplied to the ECU  40 , which is electrically connected to the sensors  23  to  25 . 
     The ECU  40  performs various types of control, such as control over the engine  10 , the in-pipe injector  13  and the urea solution injector  21 , and includes a CPU, a ROM, a RAM, input ports, output ports, and other elements which are known in the art. In addition, the ECU  40  includes an NOx purification rate calculation unit  41 , an HC heat generation rate calculation unit  42 , an NOx purification rate determination unit  43 , an HC purification rate determination unit  44 , and a deterioration determination unit  45  as functional components thereof. It is assumed in the following description that all of these functional components are included in the ECU  40 , which is a single piece of hardware, but one or more of these functional components may be included in a separate piece of hardware. 
     The NOx purification rate calculation unit  41  is an example of a first purification rate calculation unit of the present invention, and calculates a low temperature NOx purification rate NC LOW %  of the SCR catalyst  22  and a high temperature NOx purification rate NC HIGH %  of the SCR catalyst  22  on the basis of the expression (1). The low temperature NOx purification rate NC LOW %  is calculated when the exhaust gas temperature at the SCR catalyst inlet, which is obtained by the SCR catalyst inlet temperature sensor  23 , is in a range of 180 to 280 degrees C. The high temperature NOx purification rate NC HIGH %  is calculated when the exhaust gas temperature at the SCR catalyst inlet, which is obtained by the SCR catalyst inlet temperature sensor  23 , exceeds, for example, 280 degrees C. 
     
       
         
           
             
               
                 
                   
                     NC 
                     % 
                   
                   = 
                   
                     
                       Σ 
                       ⁡ 
                       
                         ( 
                         
                           
                             NOx 
                             in 
                           
                           - 
                           
                             NOx 
                             out 
                           
                         
                         ) 
                       
                     
                     
                       Σ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         NOx 
                         in 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In the expression (1), NOx in  represents a value of NOx contained in the exhaust gas that flows into the SCR catalyst  22 , and is obtained from the SCR catalyst inlet NOx sensor  24 . NOx OUT  represents a value of NOx contained in the exhaust gas that flows out of the SCR catalyst  22 , and is obtained from the SCR catalyst outlet NOx sensor  25 . 
     The HC heat generation rate calculation unit  42  is an example of a second purification rate calculation unit according to the present invention, and calculates a heat generation rate (purification rate) of HC oxidized in the DOC  15  at the time of a forced regeneration in the DPF  16 . A procedure of estimating the heat generation rate will be described in detail below. 
     As shown in  FIG. 2 , an actual amount C act  of heat generated by HC supplied from the in-pipe injector  13  into the DOC  15  at the time of the forced regeneration can be obtained by adding the amount Q lost  of heat loss, i.e., a quantity of heat dissipated from the DOC  15  to the outside air, to an exhaust gas energy difference ΔQ, which is a difference between an energy Q in  of the exhaust gas on the upstream side of the DOC  15  and an energy Q out  of the exhaust gas on the downstream side of the DOC  15 . 
     The energy Q in  of the upstream exhaust gas is calculated on the basis of the expression (2), and the energy Q out  of the downstream exhaust gas is calculated on the basis of the expression (3).
 
 Q   in   =c   exh   ·m   exh   ·T   DOC   _   in   [Formula 2]
 
 Q   out   =c   exh   ·m   exh   ·T   DOC   _   out   [Formula 3]
 
     In the expression (2) and (3), c exh  represents specific heat of the exhaust gas, and m exh  represents the flow rate of the exhaust gas, which is obtained from a detection value of the MAF sensor  31 , the amount of fuel injection of the engine  10 , and so on. It should be noted that the flow rate m exh  of the exhaust gas may be obtained directly from an exhaust gas flow rate sensor (not shown) or the like. T DOC   _   in  represents the exhaust gas temperature at an inlet of the DOC  15 , and is obtained by the DOC inlet temperature sensor  18 . T DOC   _   out  represents the exhaust gas temperature at an outlet of the DOC  15 , and is obtained by the DOC outlet temperature sensor  19 . 
     It can be assumed that the amount Q lost  of heat loss be a sum of an amount Q natural  of heat loss caused by natural convection and an amount Q forced  of heat loss caused by forced convection (Q lost =Q natural +Q forced ). 
     The amount Q natural  of heat loss caused by the natural convection is calculated on the basis of the expression (4).
 
 Q   natural   =h   n   ·A   s ·( T   DOC   _   brick   −T   ambient )  [Formula 4]
 
     In the expression (4), A s  represents an effective area of an outer circumferential surface of the DOC  15  (or an outer circumferential surface of that portion of the catalyst casing  14   a  in which the DOC  15  is arranged). T DOC   _   brick  represents the inside temperature of the DOC  15 , and is calculated as the average of the DOC inlet exhaust gas temperature T DOC   _   in  and the DOC outlet exhaust gas temperature T DOC   _   out . T ambient  represents the temperature of the ambient air, and is obtained by the outside air temperature sensor  36 . h n  represents a heat transfer coefficient of natural convection, and is given by the expression (5). 
     
       
         
           
             
               
                 
                   
                     h 
                     n 
                   
                   = 
                   
                     
                       
                         Nu 
                         n 
                       
                       · 
                       k 
                     
                     
                       L 
                       n 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     In the expression (5), k represents the thermal conductivity of the air. L n  represents a characteristic length of the DOC  15 , and is determined appropriately in accordance with, for example, the volume of the DOC  15 . Nu n  represents a Nusselt number for natural convection. 
     Usually, the DOC  15  has a column shape, and the catalyst casing  14   a , in which the DOC  15  is received, has a substantially cylindrical shape. Therefore, the oxidation heat generated in the DOC  15  is presumably dissipated to the outside air through the entire cylindrical outer circumferential surfaces of the DOC  15  and the catalyst casing  14   a . Assuming that heat dissipation by natural convection causes heat to transfer through the entire cylindrical outer circumferential surface with an axis of the cylindrical outer circumferential surface being horizontally oriented, the Nusselt number Nu n  is given by the expression (6), where Gr represents the Grashof number, and Pr represents the Prandtl number.
 
 Nu   n =0.53×( Gr·Pr ) 0.25   [Formula 6]
 
     The amount Q forced  of heat loss caused by the forced convection is calculated on the basis of the expression (7).
 
 Q   forced   =h   f   ·A   f ·( T   doc   _   brick   −T   ambient )  [Formula 7]
 
     In the expression (7), A f  represents the effective area of the outer circumferential surface of the DOC  15  (or the outer circumferential surface of that portion of the catalyst casing  14   a  in which the DOC  15  is arranged). T DOC   _   brick  represents the inside temperature of the DOC  15 , and is calculated as the average of the DOC inlet exhaust gas temperature T DOC   _   in  and the DOC outlet exhaust gas temperature T DOC   _   out . T ambient  represents the temperature of the outside air, and is obtained by the outside air temperature sensor  36 . h f  represents a heat transfer coefficient of the forced convection, and is given by the expression (8). 
     
       
         
           
             
               
                 
                   
                     h 
                     f 
                   
                   = 
                   
                     
                       
                         Nu 
                         f 
                       
                       · 
                       k 
                     
                     
                       L 
                       f 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     In the expression (8), L f  represents the characteristic length of the DOC  15 , and is determined appropriately in accordance with, for example, the volume of the DOC  15 . Nu f  represents a Nusselt number of the forced convection. 
     As illustrated in  FIG. 3 , the catalyst casing  14   a  that receives the DOC  15  therein is typically fixed to a lower portion of a chassis frame S of a vehicle body, and a transmission TM and other elements are arranged in front of the catalyst casing  14   a . Accordingly, a wind which flows from in front of the vehicle body into a space below the vehicle body while the vehicle is travelling can be assumed to be a planar turbulent flow which influences only a lower surface of the DOC  15  (or of the catalyst casing  14   a ). Therefore, the Nusselt number Nu f  of the forced convection is given by the expression (9), which is derived by solving a heat transfer equation for planar turbulence.
 
 Nu   f =0.037× Re   0.8   ×Pr   0.33   [Formula 9]
 
     In the expression (9), Re represents the Reynolds number. The Reynolds number Re is given by the expression (10), where v represents the average velocity of the air, ρ represents the air density, L represents the characteristic length of the DOC  15 , and μ represents a dynamic viscosity coefficient. 
     
       
         
           
             
               
                 
                   Re 
                   = 
                   
                     
                       ν 
                       · 
                       ρ 
                       · 
                       L 
                     
                     μ 
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
     The HC heat generation rate calculation unit  42  adds the amount Q lost  of heat loss, which is calculated on the basis of the expressions (4) to (10), to the exhaust gas energy difference ΔQ between the energy Q in  of the exhaust gas on the upstream side, which is calculated on the basis of the expression (2), and the energy Q out  of the exhaust gas on the downstream side, which is calculated on the basis of the expression (3), to calculate the actual amount C act  of heat generated by HC in the DOC  15  at the time of the forced regeneration. The HC heat generation rate calculation unit  41  then divides the actual amount C act  of heat generated by HC by a theoretical amount C theo  of heat generated by an in-pipe injection (or a post injection) to calculate an actual HC heat generation rate C ACT %  in the DOC  15 . 
     The NOx purification rate determination unit  43  is an example of a determination unit of the present invention, and determines the decrease (deterioration) in the NOx purification rate in the SCR catalyst  22  on the basis of the low temperature NOx purification rate NC LOW %  and the high temperature NOx purification rate NC HIGH % , which are calculated by the NOx purification rate calculation unit  41 . More specifically, the ECU  40  stores the NOx purification rate threshold values NC STD % , which are purification rates of NO and NO 2  contained in the exhaust gas flowing into the SCR catalyst  22  when the DOC  15  operations in a normal state (e.g., the solid line in  FIG. 4 ). The threshold values are prepared in advance by experiments or the like. The NOx purification rate determination unit  43  determines that the low temperature NO purification rate has deteriorated when the difference ΔNC LOW %  between the low temperature NOx purification rate NC LOW %  and the NOx purification rate threshold value NC STD %  reaches a predetermined upper threshold value ΔNC MAX , and determines that the high temperature NO purification rate has deteriorated when the difference ΔNC HIGH %  between the high temperature NOx purification rate NC HIGH %  and the NOx purification rate threshold value NC STD %  reaches the predetermined upper threshold value ΔNC MAX . 
     The HC purification rate determination unit  44  is an example of the determination unit of the present invention, and determines the decrease (deterioration) in the HC purification rate in the DOC  15  on the basis of the actual HC heat generation rate C ACT % , which is calculated by the HC heat generation rate calculation unit  42 . Specifically, the ECU  40  stores the HC heat generation rate threshold values C STD %  that indicate the HC heat generation rate when a specified amount of HC is substantially completely oxidized in the DOC  15 . The threshold values C STD %  are prepared in advance by experiments or the like. The HC purification rate determination unit  44  determines that the HC purification rate of the DOC  22  has deteriorated when the difference ΔC %  between the actual HC heat generation rate C ACT %  and the HC heat generation rate threshold value C STD %  reaches a predetermined upper threshold value ΔC MAX . 
     The deterioration determination unit  45  is an example of the determination unit of the present invention, and determines the deterioration of the NO 2  producing capability of the DOC  15  on the basis of the determination result of the NOx purification rate, which is determined by the NOx purification rate determination unit  43 , and the determination result of the HC purification rate, which is determined by the HC purification rate determination unit  44 . The procedure for the deterioration determination will be described in detail below. 
     In general, when the NO oxidation capability (NO 2  producing capability) of the DOC  15 , which is located upstream of the SCR catalyst  22 , drops, the NOx purification capability of the SCR catalyst  22  drops in particular in the low temperature range (e.g., 180 to 280 degrees C.), as shown in  FIG. 4 . On the other hand, there is a tendency that the capability of purifying NOx does not drop in a high temperature range (e.g., 280 degrees C. or higher). In other words, even if the low temperature NOx purification rate NC LOW %  drops, it is assumed that the SCR catalyst  22  is in the normal state as long as the high temperature NOx purification rate NC HIGH %  does not drop. Then it is assumed that the NO 2  producing capability of the DOC  15  has dropped. 
     In general, when the NO 2  producing capability of the DOC  15  drops as shown in  FIG. 5( a ) , there is a tendency that the HC oxidization capability (HC purification rate) drops correspondingly as shown in  FIG. 5( b ) . In other words, when the actual HC heat generation rate C ACT %  drops due to the deterioration of the DOC  15 , it is assumed that the NO 2  producing capability drops correspondingly. 
     The deterioration determination unit  45  determines that the NO 2  producing capability of the DOC  15  has deteriorated when the following three conditions are met, i.e., the low temperature NOx purification rate NC LOW %  has dropped (Condition 1), the high temperature NOx purification rate NC HIGH %  does not drop (Condition 2), and the HC purification rate (actual HC heat generation rate C ACT % ) has dropped (Condition 3). 
     Next, a control flow of the diagnostic device according to this embodiment will be described with reference to  FIG. 6 . 
     In Step (hereinafter, Step is simply referred to as “S”)  100 , it is determined whether or not the SCR catalyst inlet exhaust gas temperature T SCR   _   in , which is obtained by the SCR catalyst inlet temperature sensor  23 , has reached a lower determination temperature (e.g., 180 degrees C.). In S 110 , it is determined whether or not the SCR catalyst inlet exhaust gas temperature T SCR   _   in  has exceeded a higher determination temperature (e.g., 280 degrees C.). 
     If the determination in S 110  is “No,” the SCR catalyst inlet exhaust gas temperature T SCR   _   in  is between the lower determination temperature and the higher determination temperature (e.g., between 180 degrees C. and 280 degrees C.). Then, the control proceeds to S 120 , and the low temperature NOx purification rate NC LOW %  is calculated. On the other hand, if the determination in S 110  is “Yes,” the SCR catalyst inlet exhaust gas temperature T SCR   _   in  is higher than 280 degrees C. Then, the control proceeds to S 130 , and the high temperature NOx purification rate NC HIGH %  is calculated. 
     In S 140 , it is determined whether or not the low temperature NOx purification rate NC LOW %  of the SCR catalyst  22  has dropped. If the determination is “No,” it is assumed that both of the DOC  15  and the SCR catalyst  22  are in the normal state. Then, the control proceeds to “RETURN.” 
     In S 150 , it is determined whether or not the high temperature NOx purification rate NC HIGH %  of the SCR catalyst  22  has dropped. If the determination is “Yes” (if both of the low temperature purification rate and the high temperature purification rate have dropped), it is assumed that the SCR catalyst  22  is in the deteriorated state and/or other faults have occurred. Then, it is determined in S 160  that the SCR catalyst  22  is in the deteriorated state, and the control proceeds to “RETURN.” On the other hand, if the determination is “No,” the low temperature NOx purification rate NC LOW %  has only dropped. Then, it is assumed that the SCR catalyst  22  is in the normal state, and the control proceeds to S 170  to determine the deterioration level of the DOC  15 . 
     In S 170 , it is determined whether or not the forced regeneration has been carried out to the DPF  16 . If the forced regeneration has been carried out (Yes), the control proceeds to S 180 . 
     In S 180 , the actual HC heat generation rate C ACT %  (HC purification rate) of the DOC  15  is calculated. In S 190 , it is determined whether or not the HC purification rate has dropped. When the determination is “No,” the cause thereof may be other than the deterioration of the DOC  15 . Thus, the control proceeds to “RETURN.” On the other hand, when the determination in S 190  is “Yes,” the three conditions are met, namely, the low temperature NOx purification rate NC LOW %  has dropped (Condition 1) (S 140 ), the high temperature NOx purification rate NC HIGH %  is in the normal range (Condition 2) (S 150 ), and the actual HC heat generation rate C ACT %  has dropped (Condition 3) (S 190 ). In other words, it is assumed that the SCR catalyst  22  is in the normal state, and the NO 2  producing capability of the DOC  15  has dropped along with the fact that the HC purification capability of the DOC  15  has deteriorated. The control proceeds to S 200 , and it is determined that the NO 2  producing capability of the DOC  15  has deteriorated. Then, the control proceeds to “RETURN.” 
     Next, beneficial effects of the diagnostic device according to this embodiment will be described below. 
     Conventionally, it is difficult to directly detect the value (amount) of NO 2  contained in the exhaust gas with the sensor. Thus, the NO 2  producing capability of the DOC is diagnosed on the basis of the estimated value or the like, which is obtained by multiplying the NO 2  ratio of the exhaust gas by the detection value of the NOx sensor or the like. However, the ratio of NO to NO 2  in the exhaust gas changes with the running condition of the engine or the like. Thus, when the value of NO 2  is estimated from the detection value of the NOx sensor, there is a possibility that the deterioration of the DOC may not be diagnosed accurately. 
     On the contrary, the diagnostic device of this embodiment does not estimate the value of NO 2  contained in the exhaust gas that flows through the DOC  15 . Rather, the diagnostic device is configured to diagnose the NO 2  producing capability of the DOC  15  on the basis of the three conditions, namely the low temperature NOx purification rate NC LOW %  (Condition 1), the high temperature NOx purification rate NC HIGH %  (Condition 2), and the actual HC heat generation rate C ACT %  (Condition 3). 
     Therefore, the diagnostic device of this embodiment can precisely diagnose the NO 2  producing capability of the DOC  15  without being influenced by the change in the running condition and the like. 
     The diagnostic device of this embodiment is configured to determine the HC purification rate of the DOC  15  on the basis of an actual amount C act  of HC heat generation, which is obtained by adding up the heat loss Q lost  released to the ambient air to the exhaust gas energy difference ΔQ between the upstream side and the downstream side of the DOC  15 . 
     Consequently, the diagnostic device according to this embodiment can precisely calculate an actual amount C act  of HC heat generation while taking the heat loss Q lost  to the outside into consideration. It is therefore possible to effectively improve the determination accuracy of the HC purification rate deterioration as compared to the determination that is made on the basis of only the exhaust gas energy difference ΔQ. 
     It should be noted that the present invention is not limited to the above-described embodiment, and that changes and modifications can be made as appropriate without departing from the scope and spirit of the present invention. 
     For example, the present invention may be applied to a device as illustrated in  FIG. 7  in which the upstream aftertreatment device  14  includes only the DOC  15 , and neither the DPF  16  nor the in-pipe injector  13  is provided. In this configuration, S 170  (forced regeneration of the DPF) in the flowchart of  FIG. 6  is omitted, and the post injection by the engine  10 , for example, is performed. In addition, the engine  10  is not limited to the diesel engine, and the present invention can be widely applied to other internal combustion engines, such as, for example, gasoline engines.