Patent Publication Number: US-8539757-B2

Title: Exhaust purification apparatus for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to an exhaust purification apparatus for an internal combustion engine, and in particular, to an exhaust purification apparatus for an internal combustion engine including a heated gas generation apparatus suitable for heating an exhaust purification catalyst such as an NOx catalyst. 
     BACKGROUND ART 
     As is well known, exhaust temperature is lower in internal combustion engines such as diesel engines in which combustion occurs in an excessive oxygen (lean) state than in gasoline engines in which combustion occurs in a stoichiometric state. The low exhaust temperature makes activation of an exhaust purification catalyst difficult; the exhaust purification catalyst is, for example, an NOx catalyst and is installed in an exhaust passage. In particular, in an internal combustion engine mounted in a vehicle, an exhaust purification catalyst is installed away from a combustion chamber of the internal combustion engine or installed under the floor of the vehicle and thus exposed to a wind flow. Thus, disadvantageously, increasing the temperature of the exhaust purification catalyst is difficult, resulting in the difficulty of activating the exhaust purification catalyst. Furthermore, there has been a demand to activate the exhaust purification catalyst earlier during the cold start of the internal combustion engine. 
     To prevent this, such a heated gas generation apparatus as described below has been provided. The apparatus includes a first catalyst and a second catalyst each provided in an exhaust passage located upstream of the exhaust purification catalyst, the first and second catalysts each providing an oxidation function, and a fuel supply valve allowing fuel to be supplied to the first catalyst. The supplied fuel is sequentially combusted by the first and second catalysts to generate hot heated gas. The generated heated gas can be used to heat the downstream exhaust purification catalyst. 
     On the other hand, as an exhaust purification catalyst, a selective reductive NOx catalyst (what is called urine SCR) is known which continuously reduces NOx in exhaust gas using a urea aqueous solution serving as a reducing agent. In an exhaust purification apparatus using the selective reduction NOx catalyst, the urea aqueous solution is evaporated by exhaust heat or catalytic heat and thus hydrolyzed to generate ammonia. The ammonia and NOx react with each other in the catalyst to reduce and purify the NOx. 
     The selective reduction NOx catalyst is expected to be heated by heated gas generated by the heated gas generation apparatus. However, in this case, when hydrocarbon HC that is a combustion residue of fuel is discharged from the heated gas generation apparatus, the discharged HC may disadvantageously suppress the reaction between ammonia and NOx in the NOx catalyst or poison the NOx catalyst with HC. Thus, the amount of fuel supplied needs to be controlled so as to prevent the heated gas generation apparatus from discharging HC. 
     Patent Document 1 discloses that in order to prevent the selective reduction NOx catalyst from being poisoned with SOF, the amount of reducing agent supplied is determined so as to set the concentration of HC to the upper limit value at which the NOx catalyst is prevented from being poisoned with HC. 
     However, performing only such fuel supply amount control is insufficient. That is, if the catalyst in the heated gas generation apparatus is poisoned with HC or degraded and thus fails to fulfill its inherent performance, even when the fuel supply amount is accurately controlled, part of the fuel remains unburned, with HC discharged. 
     Thus, the present invention has been developed in view of the above-described circumstances. An object of the present invention is to provide an exhaust purification apparatus for an internal combustion engine which can detect poisoning of the catalyst in the heated gas generation apparatus with HC or degradation of the catalyst to prevent the heated gas generation apparatus from discharging HC. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Application Laid-Open No. H11-210447 (1999) 
       
    
     SUMMARY OF INVENTION 
     An aspect of the present invention provides an exhaust purification apparatus for an internal combustion engine comprising: 
     a heated gas generation apparatus configured to generate heated gas utilizing part of exhaust gas flowing through an exhaust passage in the internal combustion engine, the heated gas generation apparatus including a catalyst configured to provide an oxidation function, a fuel supply nozzle configured to supply fuel to the catalyst, and a heater; 
     a selective reduction NOx catalyst provided downstream of the heated gas generation apparatus so as to be heated by the heated gas; 
     an incoming gas temperature sensor configured to detect an actual incoming gas temperature of the NOx catalyst; 
     incoming gas temperature estimation means for estimating the incoming gas temperature of the NOx catalyst; 
     diagnosis means for acquiring the actual incoming gas temperature detected by the incoming gas temperature sensor and the estimated incoming gas temperature estimated by the incoming gas temperature estimation means, when at least the fuel supply nozzle of the heated gas generation apparatus is actuated, and detecting one of poisoning of the catalyst with HC and degradation of the catalyst in the heated gas generation apparatus based on a result of comparison of the values acquired. 
     The exhaust purification apparatus includes the diagnosis means for detecting one of the poisoning of the catalyst with HC and the degradation of the catalyst in the heated gas generation apparatus. Thus, when one of the poisoning with HC and the degradation is detected, a required measure such as inhibition of actuation of the heated gas generation apparatus can be taken. As a result, the heated gas generation apparatus can be prevented from discharging HC. 
     Preferably, the exhaust purification unit includes a selective reduction NOx catalyst. 
     Alternatively, the exhaust purification unit may include an oxidation catalyst or a particulate filter. 
     Preferably, the diagnosis means first detects that one of the poisoning with HC and the degradation is occurring and then carries out a predetermined poisoning recovery process. Then, the diagnosis means determines which of the poisoning with HC and the degradation is occurring. This enables diagnosis accuracy to be improved. 
     Preferably, the diagnosis means detects that one of the poisoning of the catalyst with HC and the degradation of the catalyst is occurring if a difference between the actual incoming gas temperature and the estimated incoming gas temperature both acquired at a first timing when at least the fuel supply nozzle of the heated gas generation apparatus is actuated is greater than a predetermined value. Then, the diagnosis means carries out the poisoning recovery process by stopping the fuel supply nozzle while actuating the heater. Thereafter, the diagnosis means actuates at least the fuel supply nozzle again and detects degradation of the catalyst if the difference between the actual incoming gas temperature and the estimated incoming gas temperature both acquired at a second timing when the fuel supply nozzle is actuated is greater than a predetermined value. 
     Preferably, the diagnosis means sets a time for the poisoning recovery process based on the difference between the actual incoming gas temperature and the estimated incoming gas temperature both acquired at the first timing. 
     Preferably, the incoming gas temperature estimation means calculates a heat storage quantity of the catalyst based on an integrated value of a difference between quantities of heat input to and output from the catalyst, and estimates the incoming gas temperature based on the heat storage quantity of the catalyst and flow rates of gas flowing into and out from the catalyst. The present invention very advantageously provides an exhaust purification apparatus for an internal combustion engine which can detect one of the poisoning of the catalyst in the heated gas generation apparatus with HC and the degradation of the catalyst to prevent the heated gas generation apparatus from discharging HC. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a preferred embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view illustrating the operation of the preferred embodiment; 
         FIG. 3  is a time chart showing how the temperature of gas flowing into an NOx catalyst varies during diagnosis; 
         FIG. 4  is a graph relating the quantity of heat input to a catalyst in a heated gas generation apparatus; 
         FIG. 5  is a graph showing the relationship among the incoming gas flow rate into the catalyst, exhaust flow rate, and the catalyst temperature; 
         FIG. 6  is a flowchart showing the procedure of diagnosis and urea aqueous solution supply control; 
         FIG. 7  is a map defining the relationship between a temperature difference and a poisoning recovery process time; and 
         FIG. 8  is a schematic diagram showing various positions at which the heated gas generation apparatus is installed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A preferred embodiment of the present invention will be described below with reference to the attached drawings. 
     An example in which the present invention is applied to a vehicle diesel engine will be described below.  FIG. 1  shows an exhaust purification apparatus for an internal combustion engine according to the present embodiment. Exhaust gas flows from the left to right of the figure (see  FIG. 2 ). The upstream side of the gas flow direction is hereinafter also referred to as the “front”. The downstream side of the gas flow direction is hereinafter also referred to as the “rear”. 
     An exhaust passage  1  is defined by an exhaust pipe  2  and includes an upstream end that is in communication with an exhaust port in the engine main body (not shown in the drawings). An NOx catalyst  3  is provided in the exhaust passage  1  to remove NOx from all of the exhaust gas flowing through the exhaust passage  1 . The NOx catalyst  3  forms an exhaust purification unit. 
     The NOx catalyst  3  includes a selective reduction NOx catalyst (SCR: Selective Catalytic Reduction). The NOx catalyst  3  receives a supplied reducing agent containing a urea aqueous solution and continuously reduces and removes NOx from exhaust gas. For example, the NOx catalyst  3  includes a base material such as zeolite or alumina which carries rare metal such as Pt on a surface thereof or carries transition metal such as Cu or Fe on the surface thereof through ion exchange, or carries a titania/vanadium catalyst (V 2 O 5 /WO 3 /TiO 2 ) on the surface thereof. The NOx catalyst  3  has a catalytic temperature within an active temperature range and can reduce NOx while urea aqueous solution is being supplied or added to the NOx catalyst  3 . When added to the NOx catalyst, a urea aqueous solution is evaporated and hydrolyzed to generate ammonia. The ammonia reacts with NOx in the NOx catalyst, and the NOx is reduced. This reaction can be expressed by a chemical formula as follows.
 
NO+NO 2 +2NH 3 →2N 2 +3H 2 O
 
     A urea aqueous solution supply nozzle  4  is provided upstream of the NOx catalyst  3  to supply urea aqueous solution. The urea aqueous solution supply nozzle  4  is connected to and controlled by an electronic control unit (hereinafter referred to as the ECU)  100 . A urea aqueous solution N is shown by an alternate long and two short dashes line. 
     As described above, the diesel engine according to the present embodiment has a relatively low exhaust temperature. Thus, the NOx catalyst  3  tends to be difficult to activate. Furthermore, in vehicle engines, the NOx catalyst  3  is installed away from a combustion chamber or installed under the floor of the vehicle and thus exposed to a wind flow. Thus, the NOx catalyst  3  tends to be difficult to activate. Moreover, there has been a demand to activate the NOx catalyst  3  earlier during the low-temperature start of the engine. 
     Thus, to activate the NOx catalyst  3  earlier after the engine has been started and maintain the active state, a heated gas generation apparatus  10  is provided upstream of the NOx catalyst  3  to generate heated gas utilizing part of the exhaust gas flowing through the exhaust passage  1 . Since the exhaust purification apparatus includes the heated gas generation apparatus  10 , heated gas generated by the heated gas generation apparatus  10  can be supplied to the NOx catalyst  3 . The heated gas can be used to heat the NOx catalyst  3 , thus facilitating earlier activation of the NOx catalyst and maintenance of the active state. 
     The heated gas generation apparatus includes a catalyst configured to provide an oxidation function and a fuel supply nozzle configured to supply fuel to the catalyst. The heated gas generation apparatus allows fuel supplied by the fuel supply nozzle to react with part of exhaust gas in the catalyst. The fuel is thus oxidized and combusted to generate heated gas. In the present embodiment, as shown in  FIG. 1 , the heated gas generation apparatus  10  includes two catalysts arranged in series and in two stages, that is, a front catalyst  11  and a rear catalyst  12 . The fuel supply nozzle  13  injects and feeds fuel toward a front inlet portion of the front catalyst  11 , which is located in the front stage. The supplied fuel F is shown by an alternate long and two short dashes line. Alternatively, the number of catalysts may be one or at least three. Each of the catalysts  11  and  12  includes an oxidation catalyst that allows hydrocarbon HC contained in the supplied fuel to react with oxygen O 2 . Each of the catalysts  11  and  12  is constructed by dispersively forming a coat material on the surface of a carrier containing cordierite and then placing, in the coat material, a large number of rare metal particles such as Pt which serve as active spots. However, any catalyst may be used for the heated gas generation apparatus  10  provided that the catalyst provides an oxidation function. For example, a three-way catalyst may be used. 
     The catalysts  11  and  12  are shaped like cylinders with given diameters and include metal casings  11 A and  12 A, respectively, in the outer peripheral portion thereof. The catalysts  11  and  12  are arranged in the exhaust passage  1  with a gap of a predetermined length L therebetweeen. The catalysts  11  and  12  are also arranged coaxially with the exhaust passage  1 . The catalysts  11  and  12  are arranged in the exhaust passage  1  so as to be floated by support members (not shown in the drawings). Each of the catalysts  11  and  12  includes a large number of cells extending in the axial direction thereof and through which gas flows. Each of the catalysts  11  and  12  is of what is called a flow through type in which gas passes independently through each cell or of what is called a wall flow type in which the front and rear ends of the cells are alternately closed in a zigzag manner so that the front end of one cell is closed, whereas the rear end of the adjacent cell is closed, allowing gas introduced into the one cell to be discharged from the adjacent cell through a partition wall (containing, for example, porous ceramic) located between the cells. 
     In the present embodiment, the outer diameter D 2  of the rear catalyst  12  is larger than the outer diameter D 1  of the front catalyst  11 . Furthermore, the outer diameters D 1  and D 2  are smaller than the inner diameter (d) of the exhaust passage  1 . Thus, rear catalyst  12  has a larger cross-sectional area than the front catalyst  11 . The cross-sectional areas of both catalysts  11  and  12  are each smaller than that of the exhaust passage  1 . The catalysts  11  and  12  account for a part of the cross-sectional area of the exhaust passage. Thus, part of the exhaust gas flowing through the exhaust passage  1  is introduced into the front catalyst  11 . Part of the exhaust gas having failed to be introduced into the front catalyst  11  is introduced into the rear catalyst  12 . 
     The part of the exhaust gas flowing through the exhaust passage  1  which is introduced into the front catalyst  11  is hereinafter referred to as the first part. The part of the exhaust gas flowing through the exhaust passage  1  which is introduced into the rear catalyst  12  is hereinafter referred to as the second part. Furthermore, heated gas discharged from the front catalyst  11  is hereinafter referred to as first heated gas. Heated gas discharged from the rear catalyst  12  is hereinafter referred to as second heated gas or simply heated gas. On the other hand, a main passage  14  with an annular cross section is formed radially outside the front catalyst  11  and the rear catalyst  12  so that exhaust gas (referred to as the remaining part) having failed to be introduced into the catalysts  11  and  12  flows through the main passage  14 . 
     An introduction passage  15  is provided through which part of the exhaust gas having failed to be introduced into the front catalyst  11  is introduced into the rear catalyst  12 . In the present embodiment, the introduction passage  15  is defined by a tubular member  16  extending forward from the casing  12 A of the rear catalyst  12  so as to cover the outer peripheral portion of the front catalyst  11 . That is, an annular space formed between the tubular member  16  and the front catalyst  11  forms the introduction passage  15 . The tubular member  16  includes a rear end connected to the casing  12 A and is tapered so as to have a diameter decreasing frontward. Thus, the cross-sectional area of the introduction port  15  increases rearward that is, toward the downstream side. This configuration enables a reduction in the passage resistance of the main passage  14  and the introduction port  15 . The front end of the tubular member  16  is positioned slightly behind the front end of the front catalyst  11 . The tubular member  16  partly covers at least the rear half of the front catalyst  11 . 
     Various modifications may be made to the configuration of the multi-stage catalyst. For example, the front catalyst  11  and the rear catalyst  12  need not necessarily be coaxial or parallel. The front catalyst  11  and the rear catalyst  12  may be slightly offset or inclined from each other. The tubular member  16  may have a constant inner diameter and the introduction passage  15  may have a constant cross-sectional area. Furthermore, the tubular member  16  may be reversely tapered so as to have a larger diameter at the front end than at the rear end. The cross-sectional shape of the front catalyst  11  and the rear catalyst  12  is not limited to a circle but may be any other shape such as an ellipse, a semicircle, or a polygon. 
     The fuel supply nozzle  13  is fixedly supported by the exhaust pipe  2  and injects fuel F from the upstream side of the front catalyst  11  toward the front end surface  11 F or inlet surface of the front catalyst  11 . The fuel is light oil, which is fuel for engines, but fuel other than that for engines may be used. The fuel supply nozzle  13  may be configured to supply directly to the inside of the front catalyst  11 . The fuel supply nozzle  13  is controlled by the ECU  100 . 
     A heater  17  is provided in an inlet portion of the front catalyst  11 . The heater  17  is electrically driven and located immediately in front of the front end surface  11 F of the front catalyst  11 . In particular, a heating section  17 A located at the tip of the heater  17  is positioned near the center of the front end surface  11 F. The heating section  17 A of the heater  17  may be located inside the front catalyst  11 , which may then be directly heated. The heater  17  may also be controlled by the ECU  100 . The heater  17  according to the present embodiment is located such that the fuel F supplied by the fuel supply nozzle  13  is injected against the heating section  17 A of the heater  17 . 
     The provision of the heater  17  facilitates the evaporation, oxidation, and combustion of the fuel supplied by the fuel supply nozzle  13  as well as the generation of heated gas and the activation of the front catalyst  11  and the rear catalyst  12 . 
     Furthermore, a dispersion member  20  is provided between the front catalyst  11  and the rear catalyst  12 , which are arranged adjacent to each other, to disperse the first heated gas discharged from the front catalyst  11 . The dispersion member  20  includes a cylindrical member  21 . The cylindrical member  21  has the same outer and inner diameters as those of the casing  11 A of the front catalyst  11 . The cylindrical member  21  is located coaxially with the front catalyst  11 . The front end of the cylindrical member  21  is open and is connected to the rear end of the casing  11 A of the front catalyst  11 . Thus, the cylindrical member  21  projects rearward from the front catalyst  11 . The rear end of the cylindrical member  21  is closed by a closing plate  22  located opposite the rear end surface  11 R of the front catalyst  11 . 
     A plurality of holes  24  are formed in a peripheral side surface portion  23  extending in the axial direction between the front and rear ends of the cylindrical member  21 . The first heated gas discharged from the front catalyst  11  into the cylindrical member  21  is then discharged in a plurality of radial directions through the respective holes  24 . On the other hand, no hole is formed in the closing plate  22  of the cylindrical member  21 . Thus, the closing plate  22  forms a baffle plate with which the first heated gas discharged from the front catalyst  11  collides to change its flow direction. 
     An incoming gas temperature sensor  30  is provided in an inlet portion of or immediately in front of the NOx catalyst  3  to detect the temperature (hereinafter referred to as the incoming gas temperature) of exhaust gas (hereinafter referred to as the incoming gas) flowing into the NOx catalyst  3 . Furthermore, although not shown in the drawings, an outgoing gas temperature sensor is provided in an outlet portion of or immediately behind the NOx catalyst  3  to detect the temperature (hereinafter referred to as the outgoing gas temperature) of exhaust gas (hereinafter referred to as the outgoing gas) flowing out from the NOx catalyst  3 . The incoming gas temperature sensor  30  and the outgoing gas temperature sensor are connected to the ECU  100 . 
     The ECU  100  estimates the temperature Tc of the NOx catalyst  3  based on the incoming and outgoing gas temperatures detected by the sensors. 
     Now, the operation and effects of the present embodiment will be described. 
       FIG. 2  shows the flow of gas by arrows. When the heated gas generation apparatus  10  is actuated, if the engine is in operation to allow exhaust gas to flow through the exhaust passage  1 , the fuel F is supplied by the fuel supply nozzle  13 . On the other hand, the first part G 1  of the exhaust gas is introduced into the front catalyst  11 . The second part G 2  of the exhaust gas flowing into the introduction passage  15  is introduced into the rear catalyst  12 . During cold start or the like when the front catalyst  11  and the rear catalyst  12  are insufficiently activated, the heater  17  is simultaneously activated. 
     In the front catalyst  11 , the oxygen and fuel F contained in the first part G 1  of the exhaust gas react with each other to oxidize or combust the fuel F. In particular, since only part of the exhaust gas is introduced into the front catalyst  11 , the gas flows slowly in the front catalyst  11 , resulting in a sufficient reaction time. Thus, the first part G 1  of the exhaust gas becomes hot, first heated gas, which is then discharged from the front catalyst  11 . If the heater  17  has been actuated, since at least part of the fuel F can be oxidized and combusted, the fuel oxidation reaction in the front catalyst  11  and the activation of the front catalyst  11  are facilitated. Furthermore, first heated gas is generated which is hotter than that obtained when the heater is not actuated. 
     The first heated gas discharged from the front catalyst  11  may contain a non-oxidized part of the fuel, particularly HC. If the first heated gas is fed to the downstream NOx catalyst  3  without any change, problems such as poisoning of the NOx catalyst  3  with HC may occur. Thus, the first heated gas is fed to the rear catalyst  12 , in which the first heated gas reacts with the second part G 2  of the exhaust gas. Thus, the HC in the first heated gas is oxidized or combusted again and thus removed. As is the case with the front catalyst  11 , only the second part G 2  of the exhaust gas and the first heated gas are introduced into the rear catalyst  12 . Hence, the gas flows slowly in the rear catalyst  12 , resulting in a sufficient reaction time. 
     Thus, the second heated gas discharged from the rear catalyst  12 , that is, the heated gas discharged from the heated gas generation apparatus  10 , does not contain a sufficient amount of HC to affect the catalytic ability of the NOx catalyst  3  but is hot enough to heat the NOx catalyst  3 . The oxidation reaction in the rear catalyst  12  enables the temperature of the first heated gas to be maintained or preferably enables the first heated gas to be reheated. Consequently, the rear catalyst  12  provides second heated gas significantly hotter than the remaining part G 3  of the exhaust gas having passed through the main passage  14 . 
     The second heated gas thus discharged from the rear catalyst  12  is fed to the NOx catalyst  3  to heat the NOx catalyst  3 . 
     As described above, the present embodiment can efficiently generate heated gas available for heating the NOx catalyst  3 , which is an exhaust purification catalyst. The present embodiment can thus reliably achieve early activation of the NOx catalyst  3  after the start of the engine and maintenance of the active state after the activation of the NOx catalyst. 
     Furthermore, if the front catalyst  11  and the rear catalyst  12  are insufficiently activated, the heater  17  is actuated to heat the first part of the exhaust gas and the front catalyst  11 . Thus, the front catalyst  11  and the rear catalyst  12  can be activated earlier, and the NOx catalyst  3  can be activated earlier. Furthermore, even if the activated front catalyst  11  and rear catalyst  12  become inactive owing to a decrease in exhaust temperature or the like, the front catalyst  11  and the rear catalyst  12  can be quickly activated by actuating the heater  17 . 
     The provision of the above-described dispersion member  20  is effective as follows. That is, given that the dispersion member  20  is not provided, the first heated gas (which is relatively rich and hot) discharged from the front catalyst  11  tends to migrate linearly rearward in the axial direction without any change to flow into the rear catalyst  12 . Since the rear catalyst  12  has a larger diameter than the front catalyst  11 , the first heated gas tends to flow only into the central portion of the rear catalyst  12 , in other words, the portion of the rear catalyst  12  positioned immediately behind the front catalyst  11 . On the other hand, the second part G 2  (which is lean and cool) of the exhaust gas having passed through the introduction passage  15  tends to be introduced only into the outer peripheral portion of the rear catalyst  12 , in other words, the portion of the rear catalyst  12  positioned immediately behind the introduction passage  15 . Thus, the first heated gas and the second part G 2  of the exhaust gas are introduced into different areas of the rear catalyst  12 . As a result, oxygen is insufficient in the central portion of the rear catalyst  12 , whereas the temperature and fuel are insufficient in the outer peripheral portion of the rear catalyst  12 . This prevents efficient generation of heated gas. 
     However, when the dispersion member  20  is provided as in the case of the present embodiment, the first heated gas discharged from the front catalyst  11  can be dispersed and mixed with the second part G 2  of the exhaust gas by the dispersion member  20  so as to be evenly fed through the whole rear catalyst  12 . Specifically, the first heated gas flowing out from the front catalyst  11  in the axial direction collides with the closing plate  22 , serving as a baffle plate, to change its flow direction (see reference character (a) in  FIG. 2 ). The first heated gas then flows out radially outward through the plurality of holes  24  in the peripheral side surface portion  23  (see reference numeral (b) in  FIG. 2 ). Then, the first heated gas mixes with the second part G 2  of the exhaust gas flowing through the introduction passage  15 . The resultant mixed gas flows to the gap between the front catalyst  11  and the rear catalyst  12 , located behind the closing plate  22 , and thus flows evenly through the rear catalyst  12 . 
     Thus, the provision of the dispersion member  20  allows facilitation of mixture of the first heated gas discharged from the front catalyst  11  and the newly introduced second part of the exhaust gas. This enables a variation in oxygen concentration, fuel concentration, and temperature among the areas of the rear catalyst  12  to be suppressed, allowing heated gas to be efficiently generated. 
     In particular, the distance between the front catalyst  11  and the rear catalyst  12  may be reduced in order to allow the catalysts  11  and  12  to be more easily mounted in the apparatus and to reduce heat loss. However, this has a strong tendency to cause the first heated gas to flow only into the portion of the rear catalyst  12  positioned immediately behind the front catalyst  11  given that the dispersion member  20  is not provided. However, the presence of the provision member  20  enables such a partial inflow to be prevented even if the inter-catalyst distance L is short. This allows the mixed gas to flow evenly through the whole rear catalyst  12 . 
     Meanwhile, to prevent the heated gas generation apparatus  10  or the rear catalyst  12  from discharging HC, the present embodiment includes diagnosis means for detecting one of poisoning with HC and degradation in at least one of the front catalyst  11  and the rear catalyst  12 . This will be described below. 
       FIG. 3  shows how the incoming gas temperature of the NOx catalyst  3  varies during diagnosis. A solid line indicates an actual incoming gas temperature T 1  detected by the incoming gas temperature sensor  30 . A dashed line indicates an incoming gas temperature T 1   e  estimated by the ECU  100 . The estimated incoming gas temperature T 1   e  is the estimated value of the incoming gas temperature obtained given that neither the front catalyst  11  nor the rear catalyst  12  is poisoned with HC or degraded (that is, both the front and rear catalysts  11  and  12  are normal). A method for estimating the incoming gas temperature will be described below. In the illustrated example, at least one of the front catalyst  11  and the rear catalyst  12  is degraded. 
     First, it is assumed that at time t 0 , the heated gas generation apparatus  10  is actuated to simultaneously actuate the fuel supply nozzle  13  and the heater  17 . The actuation of the heater  17  is optional. Then, if the front catalyst  11  and the rear catalyst  12  were normal, both catalysts would be gradually warmed and the heated gas generation apparatus  10  would gradually discharge hot heated gas. Thus, the actual incoming gas temperature T 1  would rise as shown by the estimated value T 1   e  in the figure. 
     However, in the illustrated example, one of the front catalyst  11  and the rear catalyst  12  is degraded. Thus, the actual incoming gas temperature fails to rise like the estimated incoming gas temperature; the degree of the rise in the actual incoming gas temperature is lower than that in the estimated incoming gas temperature. 
     In the present embodiment, the actual incoming gas temperature T 1  and the estimated incoming gas temperature T 1   e  at a first timing t 1  that is a predetermined time Δt 1  after time t 0  are acquired. The difference T 1   e −T 1  between these values is determined. If the difference is greater than a relatively small predetermined value α as in the illustrated example, the apparatus detects that at least one of the front catalyst  11  and the rear catalyst  12  is poisoned with HC or degraded. On the other hand, although not shown in the drawings, if the difference is not greater than the predetermined value α, the front catalyst  11  and the rear catalyst  12  are expected to be functioning normally. Hence, the catalysts  11  and  12  are not poisoned with HC or degraded and are thus determined to be normal. 
     Upon detecting that one of poisoning with HC or degradation is occurring, the apparatus carries out a predetermined poisoning recovery process for recovering the catalysts  11  and  12  from poisoned with HC. In general, the catalyst poisoned with HC can be recovered by allowing relatively hot exhaust gas to flow through the catalyst for a certain amount of time. Thus, in this case, the fuel supply nozzle  13  is stopped, whereas the heater  17  is actuated so as to heat the first exhaust gas G 1 . Consequently, the heated first exhaust gas G 1  is supplied to the front catalyst  11  and the rear catalyst  12 . 
     The poisoning recovery process is carried out for a predetermined time Δt 2 . Then, at time t 2 , the fuel supply nozzle  13  is actuated again to resume the supply of fuel, and the heater  17  is actuated. At this time, if the cause of the abnormality is only the poisoning with HC, since the poisoning with HC has already been eliminated by the previous poisoning recovery process, the front catalyst  11  and the rear catalyst  12  operate normally. Thus, the heated gas generation apparatus  10  discharges sufficiently hot heated gas to raise the actual incoming gas temperature T 1  like the estimated value T 1   e  in  FIG. 3 . 
     However, in the illustrated example, at least one of the front catalyst  11  and the rear catalyst  12  is degraded. Thus, the actual incoming gas temperature T 1  fails to rise in the same manner as that in which the estimated incoming gas temperature T 1   e  increases. 
     In the present embodiment, the actual incoming gas temperature T 1  and the estimated incoming gas temperature T 1   e  at a second timing t 3  that is a predetermined time Δt 1  after time t 2  are acquired. The difference T 1   e −T 1  between these values is determined. If the difference is greater than the predetermined value α as in the illustrated example, the apparatus detects that at least one of the front catalyst  11  and the rear catalyst  12  is degraded. On the other hand, although not shown in the drawings, if the difference is not greater than the predetermined value α, at least one of the front catalyst  11  and rear catalyst  12  previously poisoned with HC has already been recovered. Hence, the apparatus determines both the catalysts  11  and  12  to be normal. 
     As described above, one of the poisoning with HC and degradation of the catalysts  11 ,  12  is distinctively detected. Thus, diagnosis accuracy can be improved. 
     Now, a method in which the ECU  100  estimates the incoming gas temperature will be described. 
     Here, the front catalyst  11  and the rear catalyst  12  are considered to be one catalyst and are denoted by reference numeral  40 . Terms are defined as follows. The quantity of heat flowing into the catalyst  40  in the heated gas generation apparatus  10  is defined as an “input heat quantity A”. The quantity of heat flowing out from the catalyst  40  in the heated gas generation apparatus  10  is defined as an “output heat quantity B”. The quantity of heat stored in the catalyst  40  in the heated gas generation apparatus  10  is defined as a “heat storage quantity C”. The flow rate of exhaust gas flowing into the catalyst  40  in the heated gas generation apparatus  10  is defined as an “incoming gas flow rate D”. The amount by which the temperature of exhaust gas (containing heated gas) flowing into the NOx catalyst  3  rises is defined as an “incoming gas temperature rise amount E”. The flow rate of the whole exhaust gas flowing through the exhaust passage  1  is defined as an “exhaust flow rate Ge”. 
     The input heat quantity A of heat input to the catalyst  40  is equal to the sum of heat Qh from the heater  17  and oxidation heat Qf from the supplied fuel. As shown in  FIG. 4 , during a period between t 0  and t 1  when both the fuel supply nozzle  13  and the heater  17  are in operation, the heat A=Qh+Qf flows into the catalyst  40  in the heated gas generation apparatus  10 . Furthermore, during a period after t 1  when only the fuel supply nozzle  13  is in operation, whereas the heater  17  is stopped, the heat A=Qf flows into the catalyst  40  in the heated gas generation apparatus  10 . 
     On the other hand, the output heat quantity B of heat output from the catalyst  40  is determined by the heat storage quantity C of heat stored in the catalyst  40  and the incoming gas flow rate D of gas flowing into the catalyst  40 . That is, the output heat quantity B increases consistently with the heat storage quantity C and thus the incoming gas flow rate D. In the present embodiment, B=C×D. 
     The heat storage quantity C of the catalyst  40  is equal to the output heat quantity B subtracted from the input heat quantity A. 
     The incoming gas flow rate D of gas flowing into the catalyst  40  increases consistently with the exhaust flow rate Ge. However, the rate of increase in the incoming gas flow rate D tends to decrease with increasing temperature of the catalyst  40  because the viscosity of the gas inside the catalyst increases consistently with the catalyst temperature. This is indicated in  FIG. 5 . Thus, the incoming gas flow rate D is a function of the exhaust gas flow rate Ge and the temperature of the catalyst  40 . 
     Based on the above-described knowledge, the ECU  100  calculates or updates the heat storage quantity C of the catalyst  40  in accordance with the following equation, where (n) denotes the current value and n−1 denotes the last value.
 
[Expression 1]
 
 Cn=Σ{A   n-1 −( C   n-1   ×D   n-1 )}  (1)
 
     Here, a predetermined value corresponding to the operation conditions of the fuel supply nozzle  13  and the heater  17  is used for the input heat quantity A of heat input to the catalyst  40 . Furthermore, the incoming gas flow rate D of gas flowing into the catalyst  40  is calculated from such a map or function as shown in  FIG. 5  based on the exhaust flow rate Ge and the temperature of the catalyst  40 . The exhaust flow rate Ge may be directly detected, but in the present embodiment, the intake air flow rate Ga of the engine detected by an air flow meter (not shown in the drawings) is used as a substitute for the exhaust flow rate Ge. Furthermore, since the heat storage quantity C of the catalyst  40  is a function of the temperature of the catalyst  40  and the heat capacity, the temperature Dn of the catalyst  40  is calculated based on the heat storage quantity Cn of the catalyst  40  obtained at the same timing. 
     Σ means an integrated value. The ECU  100  sequentially integrates and updates the heat storage quantity C n  at every sampling period from engine start to determine the current heat storage quantity C. 
     Once the heat storage quantity C of the catalyst  40  is calculated, the ECU  100  calculates the incoming gas temperature increase amount E of the gas flowing into the NOx catalyst  3 , at every sampling period in accordance with: 
     
       
         
           
             
               
                 
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     That is, the incoming gas temperature increase amount E n  for every sampling period is a function in which the output heat quantity B n  is divided by the exhaust flow rate Ge n . The incoming gas temperature increase amount E n  increases consistently with output heat quantity B n  and with decreasing exhaust flow rate Ge n . 
     The ECU  100  integrates the incoming gas temperature increase amount E n  at every sampling period from the points t 0  and t 2  of estimation start as shown in  FIG. 3 . On the other hand, ECU  100  separately estimates the incoming gas temperature obtained given that the heated gas generation apparatus  10  is not provided (this incoming gas temperature is hereinafter referred to as the reference incoming gas temperature), based on the engine operation condition. The ECU  100  adds the integrated value of the incoming gas temperature increase amount E n  to the separately estimated reference incoming gas temperature to determine an estimated incoming gas temperature T 1   e.    
     Now, the procedure of diagnosis and urea aqueous solution supply control carried by the ECU  100  will be described with reference to  FIG. 6 . 
     In step S 101 , the ECU  100  determines whether or not the estimated catalyst temperature Tc of the NOx catalyst  3  is lower than a predetermined value Tcs (for example, 200° C.) corresponding to the lower limit of the activation temperature. 
     If Tc≧Tcs, the NOx catalyst  3  is considered to be active. In step S 111 , the urea aqueous solution supply nozzle  4  is actuated to allow the urea aqueous solution supply nozzle  4  to supply a urea aqueous solution. 
     On the other hand, if Tc&lt;Tcs, the procedure proceeds to step S 102  to actuate the fuel supply nozzle  13  and the heater  17 . Thus, both fuel supply by the fuel supply nozzle  13  and heating by the heater  17  are carried out. 
     Then, in step S 103 , the ECU  100  determines whether or not a predetermined time Δt 1  has elapsed since the time of actuation start of the fuel supply nozzle  13  and the heater  17 . If the predetermined time Δt 1  has not elapsed, step S 102  is continued. If the predetermined time Δt 1  has elapsed, the procedure proceeds to step S 104 . This timing of shifting to step S 104  corresponds to the first timing t 1  ( FIG. 3 ). 
     In step S 104 , the values of the estimated incoming gas temperature T 1   e  and the actual incoming gas temperature T 1  are acquired, and the difference T 1   e −T 1  is calculated. The ECU  100  then determines whether or not the difference is greater than the predetermined value α. If T 1   e −T 1 &gt;α, the ECU  100  determines that at least one of the front catalyst  11  and the rear catalyst  12  is poisoned with HC or degraded. The procedure then shifts to the poisoning recovery process in step S 105 . On the other hand, if T 1   e −T 1 ≦α, the ECU  100  determines that both the front catalyst  11  and the rear catalyst  12  are normal, and terminates the procedure. 
     In step S 105 , the fuel supply nozzle  13  is stopped, and the heater  17  is actuated. Thus, the fuel supply by the fuel supply nozzle  13  is stopped, and only the heating by the heater  17  is carried out. 
     Here, the poisoning recovery process in step S 105  is carried out for a predetermined time Δt 2 . However, if at least one of the front catalyst  11  and the rear catalyst  12  is poisoned with HC, the poisoning condition is considered to be severer when the temperature difference T 1   e −T 1  at the first timing t 1  is greater. Thus, in the present embodiment, the ECU  100  sets the poisoning recovery process time Δt 2  based on the temperature difference T 1   e −T 1 . Specifically, the ECU  100  sets a longer poisoning recovery time Δt 2  for a greater temperature difference T 1   e −T 1  based on such a map or function as shown in  FIG. 7 . This enables a more appropriate poisoning recovery process to be carried out. 
     Then, in step S 106 , the ECU determines whether or not the predetermined time Δt 2  has elapsed since the actuation start of the heater  17 . If the predetermined time Δt 2  has not elapsed, step S 105  is continued. If the predetermined time Δt 2  has elapsed, the procedure proceeds to step S 107 . This timing of shifting to step S 107  corresponds to t 2  shown in  FIG. 3 . 
     In step S 107 , the fuel supply nozzle  13  and the heater  17  are actuated. Thus, both fuel supply by the fuel supply nozzle  13  and heating by the heater  17  are carried out. 
     Then, in step S 108 , the ECU determines whether or not the predetermined time Δt 1  has elapsed since the actuation start of the fuel supply nozzle  13  and the heater  17 . If the predetermined time Δt 1  has not elapsed, step S 107  is continued. If the predetermined time Δt 1  has elapsed, the procedure proceeds to step S 109 . This timing of shifting to step S 109  corresponds to the second timing t 3  ( FIG. 3 ). 
     In step S 109 , the values of the estimated incoming gas temperature T 1   e  and the actual incoming gas temperature T 1  are acquired, and the difference therebetween T 1   e −T 1  is calculated. The ECU  100  then determines whether or not the difference is greater than the predetermined value α. 
     If T 1   e −T 1 ≦α, the ECU  100  determines that at least one of the front catalyst  11  and rear catalyst  12  previously poisoned with HC has already been recovered. The ECU  100  also determines that both the front catalyst  11  and the rear catalyst  12  are normal, and terminates the procedure. 
     On the other hand, if T 1   e −T 1 &gt;α, this is a case where both catalysts  11 ,  12  fails to be recovered to the normal condition in spite of carrying out the poisoning recovery process. Thus, the procedure proceeds to step S 110 , where the ECU  100  determines that at least one of the front catalyst  11  and the rear catalyst  12  is degraded, and the procedure is terminated. In this case, a warning device (not shown in the drawings) is preferably actuated in order to notify the user of the degradation. Furthermore, the catalyst  11  or  12  may operate abnormally to discharge HC. Thus, preferably, the fuel supply nozzle  13  is inhibited from operating or the amount of fuel supplied by the fuel supply nozzle  13  is reduced. 
     As described above, diagnosis is carried out before the NOx catalyst  3  is activated. The present embodiment can thus detect that the catalyst  11  or  12  is poisoned with HC or degraded before a urea aqueous solution starts to be supplied. This enables the supply of a urea aqueous solution to be prevented from being carried out with HC discharged. 
     The embodiment of the present invention has been described. However, other embodiments of the present invention can be adopted. For example, the present invention is applicable to internal combustion engines other than the diesel engine, that is, the compression-ignition internal combustion engine. The present invention is applicable to, for example, a spark-ignition internal combustion engine, particularly a direct-injection lean burn gasoline engine. 
     (1) The number of catalysts in the heated gas generation apparatus is not limited to two. More catalysts, for example, three or four catalysts, may be provided. For example, if two catalysts are provided and undesirable HC is discharged from the second (final) catalyst, the number of catalysts may be increased to, for example, three so that the third catalyst removes HC components. In any case, the number of catalysts may be determined as required so as to prevent the final catalyst from discharging HC. 
     (2) The fuel supply nozzle in the heated gas generation apparatus is essential for the front catalyst, that is, the first catalyst. However, the fuel supply nozzle may also be provided for the intermediate or final catalyst. 
     (3) The heated gas generation apparatus  10  may be installed at any of the following various positions.  FIG. 8  shows the various positions where the heated gas generation apparatus  10  may be installed. In  FIG. 8 , reference numerals  41 ,  42 , and  3  denote an oxidation catalyst, a particulate filter, and a selective reduction NOx catalyst, respectively. Each of the oxidation catalyst, the particulate filter, and the selective reduction NOx catalyst forms an exhaust purification unit. In the illustrated example, the oxidation catalyst  41 , the particulate filter  42 , and the selective reduction NOx catalyst  3  are provided in series in this order from the upstream side (left) of the exhaust passage  1 . 
     The exhaust purification unit refers to any unit configured to purify exhaust gas. The exhaust purification units include exhaust purification catalysts such as an oxidation catalyst, an NOx catalyst, and a three-way catalyst, and a particulate filter configured to catch particulates in the exhaust gas. The particulate filter may be of a self-recovery type that carries a catalyst or may be recovered by heat from an external heater. The particulate filter is hereinafter simply referred to as a “filter”. The NOx catalyst includes a storage reduction type in addition to the selective reduction type. 
     The heated gas generation apparatus  10  can be installed at a first position P 1  to a fourth position P 4  as shown in  FIG. 8 . If the heated gas generation apparatus  10  is installed at the first position P 1 , an oxidation catalyst  41  is provided after the heated gas generation apparatus  10  in the direction of exhaust gas in the exhaust passage  1  (from left to right as shown by arrows). Similarly, if the heated gas generation apparatus  10  is installed at the second position P 2 , a filter  42  is provided after the heated gas generation apparatus  10 . If the heated gas generation apparatus  10  is installed at the third or fourth position P 3  or P 4 , a selective reduction NOx catalyst  3  is provided after the heated gas generation apparatus  10 . 
     In particular, the heated gas generation apparatus  10  installed at the third position P 3  is positioned upstream of the urea aqueous solution supply nozzle  4 . The heated gas generation apparatus  10  installed at the fourth position P 4  is positioned downstream of the urea aqueous solution supply nozzle  4 . The third position P 3  corresponds to the installation position in the above-described basic embodiment. 
     The heated gas generation apparatus  10  is simplified in the figure but includes the above-described fuel supply nozzle  13  and heater  17 . Furthermore, the incoming gas temperature sensor  30  is also not illustrated but is provided in order to detect the incoming gas temperature of the exhaust purification unit located immediately after the heated gas generation apparatus  10  as described above. 
     Regardless of wherever the heated gas generation apparatus  10  is installed, the heated gas generation apparatus  10  can exert its inherent effect, that is, can heat the downstream exhaust purification unit using generated heated gas. The heated gas generation apparatus  10  installed at the first position P 1  is advantageous for improving the activity of the oxidation catalyst  42  located immediately after the heated gas generation apparatus  10  and thus improving the recovery capability of the succeeding filter  42 . The recovery of the filter  42  is a process of oxidizing and combusting particulates and requires hot exhaust gas. Thus, the supply of heated gas from the heated gas generation apparatus  10  is advantageous. 
     The heated gas generation apparatus  10  installed at the second position P 2  is advantageous for improving the recovery capability of the filter  42  and suppressing thermal degradation of the oxidation catalyst  41 . Fuel is separately added to the oxidation catalyst  41 , which normally reaches a high temperature of 600 to 700° C. Thus, hot gas is supplied to the downstream exhaust purification unit. However, the heated gas generation apparatus  10  located at the second position P 2  similarly generates heated gas and enables a reduction in loads on the oxidation catalyst  41  and in the amount by which the oxidation catalyst  41  needs to raise the temperature. That is, the generation of heated gas can be shared by the oxidation catalyst  41  and the heated gas generation apparatus  10 . Hence, this configuration is advantageous for suppressing thermal degradation of the oxidation catalyst  41 . 
     The advantages of the heated gas generation apparatus  10  installed at the third position  23  are as described above. The heated gas generation apparatus  10  installed at the fourth position P 4  is advantageous for activating the NOx catalyst. The heated gas generation apparatus  10  installed at the third or fourth position is advantageous for promoting hydrolysis of a urea aqueous solution supplied by the urea aqueous solution supply nozzle  4  and improving dispersability of the urea aqueous solution. The heated gas generation apparatus  10  installed at the third or fourth position is advantageous for improving the performance of the NOx catalyst  3 . 
     When diagnosis means is provided for detecting that the catalyst  11  or  12  in the heated gas generation apparatus  10  is poisoned with HC or degraded as in the present invention, the heated gas generation apparatus  10  is prevented from discharging HC. Hence, regardless of the installation position, the diagnosis means is advantageous as follows. 
     If the heated gas generation apparatus  10  is installed at the third position  23 , the diagnosis means is advantageous as described above. If the heated gas generation apparatus  10  is installed at the fourth position, the NOx catalyst  3  can be prevented from being poisoned with HC discharged from the heated gas generation apparatus  10 . 
     If the heated gas generation apparatus  10  is installed at the first position  21 , even when the heated gas generation apparatus  10  discharges HC, the oxidation catalyst  41  located immediately after the heated gas generation apparatus  10  can purify the discharged HC provided that the amount of the HC is very small. However, basically, a sufficient amount of fuel to heat the oxidation catalyst  41  to a desired high temperature is added to the oxidation catalyst  41 . Thus, when the heated gas generation apparatus  10  discharges an unexpected amount of HC, all of the discharged HC may not be purified by the oxidation catalyst  41 . Then, the oxidation catalyst  41  may discharge HC to inhibit the reaction between ammonia and NOx in the downstream NOx catalyst  3  or poison the NOx catalyst  3  with HC. Moreover, the temperature of the oxidation catalyst  41  may be excessively increased to promote thermal degradation. Given that only a combination of the heated gas generation apparatus  10  and the oxidation catalyst  41  is present in the exhaust passage  1 , HC discharged from the oxidation catalyst  41  may be released to the atmosphere. 
     However, the provision of the diagnosis means according to the present invention allows these problems to be solved. 
     Next, if the heated gas generation apparatus  10  is installed at the second position P 2 , HC discharged from the heated gas generation apparatus  10  is likely to simply pass through the filter  42  to the downstream side thereof. If the filter  42  includes a catalyst, discharged HC can be purified. However, the effect of the purification is insignificant and not so promising. Thus, HC discharged from the filter  42  may inhibit the reaction between ammonia and NOx in the downstream NOx catalyst  3  or poison the NOx catalyst  3  with HC. Given that only a combination of the heated gas generation apparatus  10  and the filter  42  is present in the exhaust passage  1 , HC discharged from the filter  42  may be released to the atmosphere. 
     However, the provision of the diagnosis means according to the present invention allows these problems to be solved. 
     If the heated gas generation apparatus  10  is installed at the first position P 1  or the second position P 2 , a plurality of exhaust purification units are provided downstream of the heated gas generation apparatus  10 . This case also falls within the scope of the present invention. Furthermore, the heated gas generation apparatus  10  may be provided at two or more of the first to fourth positions P 1  to P 4 . This case also falls within the scope of the present invention. The embodiments of the present invention are not limited to those described above. The present invention includes any modifications and equivalents embraced in the concepts of the present invention defined by the claims. Thus, the present invention should not be interpreted in a limited manner but is applicable to any other techniques falling within the spirit of the present invention.