Patent Publication Number: US-2007095050-A1

Title: Apparatus and method for diagnosing deterioration of catalyst, and catalytic converter apparatus for exhaust emission of engine

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to an apparatus and method for diagnosing deterioration of a catalyst for an internal combustion engine (called an engine for short) and a catalytic converter apparatus for exhaust emission of the engine, and more particularly to an apparatus and method for diagnosing deterioration of a catalyst to judge deterioration of a three-way catalyst for purification of exhaust gas disposed in an exhaust gas passage of the engine and a catalytic converter apparatus for exhaust emission of the engine.  
      Deterioration of a catalyst for an engine having a plurality of catalysts for purification of exhaust gas disposed in an exhaust pipe is diagnosed by oxygen sensors disposed before and after the catalyst which is located closest to the engine exhaust port and according to outputs from the two oxygen sensors. Where it is judged according to the outputs from the oxygen sensors that the subject catalyst is ineffective (lost of exhaust emissions reduction capability), all the plural catalysts are replaced.  
      There is another prior art related to a diagnostic apparatus in that a front catalyst (upper stream catalyst) and a rear catalyst (down stream catalyst) are disposed in an exhaust gas passage, oxygen sensors each are mounted on the upper stream of the front catalyst, the intermediate section between the front and rear catalysts and the down stream of the rear catalyst, the front catalyst is diagnosed by the oxygen sensors mounted on the upper and lower streams of the front catalyst, and the front catalyst and the rear catalyst are collectively diagnosed by the oxygen sensor on the upper stream of the front catalyst and the oxygen sensor on the down stream of the rear catalyst (e.g., U.S. Pat. Nos. 5,740,676 and 6,003,309).  
     SUMMARY OF THE INVENTION  
      The emission control regulation is being tightened every year, and a threshold value level of deterioration judging for judging a catalyst as ineffective is becoming low accordingly, so that it is necessary to make judgment of effective/ineffective when the catalyst is slightly deteriorated.  
      But, the prior art cannot detect accurately a slight degree of deterioration of the catalyst. Therefore, it becomes difficult to diagnose accurately the deterioration of the catalyst in compliance with the emission control regulation which is tightened more and more every year.  
      In a case where the deterioration is diagnosed by a conventional catalyst system having a plurality of catalysts disposed in an exhaust pipe, an effective rear catalyst is also replaced when a front catalyst is judged as ineffective, so that the catalyst replacement cost becomes high. And, even if the oxygen sensor is disposed before and after the individual catalysts, the system cost increases.  
      The present invention has been made in view of the above circumstances and provides an apparatus and method for diagnosing deterioration of a catalyst that can perform an accurate diagnosis of the deterioration of the catalyst and a catalytic converter apparatus for exhaust emission in compliance with the emission control regulation being tightened more and more every year and can reduce the cost.  
      In order to complete the object, the apparatus for diagnosing deterioration of a catalyst for an internal combustion engine according to the present invention is an apparatus for diagnosing deterioration of a catalyst for an internal combustion engine, having a first catalyst and a second catalyst and provided with sensors for detecting a particular exhaust component before and after the first catalyst, wherein a relationship between a deterioration property involved in clarification of an exhaust gas of the internal combustion engine by a combination of the first catalyst and the second catalyst and a deterioration property of the first catalyst are determined previously; a deterioration degree of the first catalyst is calculated according to outputs from the sensors disposed before and after the first catalyst; deterioration of the first catalyst is judged according to the deterioration degree; and deterioration of the second catalyst is judged according to the deterioration degree of the first catalyst.  
      The apparatus for diagnosing deterioration of a catalyst for an internal combustion engine according to the invention preferably adds a volume of a space between the first catalyst and the second catalyst to elements contributing to the deterioration of the second catalyst when a relationship of the first catalyst to the deterioration property due to the combination of the first catalyst and the second catalyst is derived.  
      In the apparatus for diagnosing deterioration of a catalyst for an internal combustion engine according to the invention, the sensors for detecting the particular exhaust component are O 2  sensors which output a binary output for the presence or not of oxygen contained in the exhaust gas or sensors which output a signal corresponding to an oxygen density or a fuel concentration contained in the exhaust gas.  
      In order to achieve the object, the method for diagnosing deterioration of a catalyst for an internal combustion engine according to the invention is a method for diagnosing deterioration of a catalyst for an internal combustion engine, having a first catalyst and a second catalyst and provided with sensors for detecting a particular exhaust component before and after the first catalyst, comprising determining previously a relationship between a deterioration property involved in clarification of an exhaust gas from the internal combustion engine by a combination of the first catalyst and the second catalyst and a deterioration property of the first catalyst; calculating a deterioration degree of the first catalyst according to outputs from the sensors disposed before and after the first catalyst; judging deterioration of the first catalyst according to the deterioration degree; and judging deterioration of the second catalyst according to the deterioration degree of the first catalyst.  
      In order to achieve the object, the catalytic converter apparatus according to the invention is a catalytic converter apparatus for exhaust emission which is disposed in an exhaust gas passage of an internal combustion engine, the apparatus being separated into a first catalyst and a second catalyst, and a volume of the first catalyst disposed on the upper stream side of an exhaust gas flow in the exhaust gas passage is smaller than that of the second catalyst.  
      The catalytic converter apparatus according to the invention preferably has a proportion of a volume of the first catalyst to that of the second catalyst determined to be smaller according to a proportion of an unburnt gas conversion efficiency of a combination of the first and second catalysts when they are new to an unburnt gas conversion efficiency at the time when an emission level is judged as failure.  
      The catalytic converter apparatus according to the invention preferably has the first catalyst and the second catalyst stored in one and the same housing.  
      It is known that the catalyst is deteriorated starting from its front portion which is closer to the exhaust port of the internal combustion engine. Accordingly, the present invention divides the catalyst according to a proportion between an unburnt gas conversion efficiency at the time of a combination of new catalysts and an unburnt gas conversion efficiency when an emission level is judged as failure, so that even if the deterioration is little as a whole, the front catalyst provided with an oxygen sensor is deteriorated quickly, and it is easy to detect a deterioration degree. The deterioration of the rear catalyst is determined according to the deterioration degree and the number of replacement times of the front catalyst, so that the front catalyst is not replaced even if it is deteriorated.  
      Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an overall structure view showing one embodiment of a control system of an internal combustion engine (engine) to which the apparatus for diagnosing deterioration of a catalyst according to the invention is applied.  
       FIG. 2  is a block diagram showing one embodiment of an internal combustion engine control unit to which the apparatus for diagnosing deterioration of a catalyst according to the invention is applied.  
       FIG. 3  is a control block diagram of an engine control unit for performing a method for diagnosing deterioration of a catalyst according to the invention.  
       FIG. 4  is a block diagram showing one embodiment of a front catalyst deterioration degree arithmetic processing section which is included in a catalyst deterioration detection unit for performing the method for diagnosing deterioration of a catalyst according to the invention.  
       FIG. 5  is a graph showing an example of a relationship between a correlation value and a deterioration degree of the outputs of the front and rear O 2  sensors.  
       FIG. 6A ,  FIG. 6B ,  FIG. 6C  are a structure view of a general example of a catalytic converter apparatus not according to the present invention and graphs showing examples of behaviors of the outputs of the front and rear O 2  sensors when it must be judged as catalyst ineffective.  
       FIG. 7A ,  FIG. 7B ,  FIG. 7C  are a structure view of the catalytic converter apparatus according to the invention and graphs showing behaviors of the outputs of the front and rear O 2  sensors when it must be judged as catalyst ineffective, and  FIG. 7D  is a graph showing a comparative example of the output of the rear O 2  sensor.  
       FIG. 8  is a graph showing an example of a relationship between a correlation value and a deterioration degree of the outputs of the front and rear O 2  sensors of the catalytic converter apparatus according to the invention.  
       FIG. 9  is a graph showing an example of a relationship between a front catalyst deterioration degree and a rear catalyst deterioration degree of the catalytic converter apparatus according to the invention.  
       FIG. 10  is a graph showing an example of a relationship between a front catalyst deterioration degree and a front catalyst deterioration degree integrated value (rear catalyst deterioration index) of an apparatus for diagnosing deterioration of a catalyst according to the invention.  
       FIG. 11  is an explanatory view showing a catalyst separation example of the catalytic converter apparatus according to the invention.  
       FIG. 12  is a block diagram showing an example of one structure of an effective/ineffective judgment processing section for the front catalyst and the rear catalyst of the apparatus for diagnosing deterioration of a catalyst according to the invention.  
       FIG. 13  is a block diagram showing an example of another structure of the effective/ineffective judgment processing section for the front catalyst and the rear catalyst of the apparatus for diagnosing deterioration of a catalyst according to the invention.  
       FIG. 14  is a block diagram showing in detail the rear catalyst effective/ineffective judging section of the apparatus for diagnosing deterioration of a catalyst according to the invention.  
       FIG. 15  is a flowchart showing a processing flow of overall control performed by an engine control unit including catalyst deterioration judgment of the embodiment.  
       FIG. 16  is a flowchart showing a processing flow for calculation of a catalyst deterioration degree of the front catalyst by a front catalyst deterioration degree arithmetic processing section of the embodiment.  
       FIG. 17  is a flowchart showing a processing flow for judgment of effective/ineffective of the front catalyst and the rear catalyst performed by an effective/ineffective judgment processing section of the embodiment.  
       FIG. 18  is a flowchart showing a processing flow of the front catalyst deterioration judging value integration and the judging value storing performed by an ineffective judgment processing section.  
       FIG. 19  is a flowchart showing a processing flow for judgment of effective/ineffective of the front catalyst and the rear catalyst performed by the effective/ineffective judgment processing section of another embodiment.  
       FIG. 20  is a flowchart showing a processing flow for judgment of effective/ineffective of the rear catalyst performed by the rear catalyst effective/ineffective judging section of the embodiment.  
       FIG. 21  is an overall structure view showing another embodiment of the internal combustion engine (engine) to which the apparatus for diagnosing deterioration of a catalyst of the invention is applied. It is another example of the engine and its periphery controlled by the fuel control device of the invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      One embodiment of an internal combustion engine (called an engine for short) to which the apparatus for diagnosing deterioration of a catalyst according to the invention is applied will be described with reference to  FIG. 1 .  
      An engine  201  has in an intake system a throttle valve  202  which is a metering device for an amount of intake air, an idle speed control valve (ISC valve)  203  which controls the idling speed of the engine  201  by controlling a passage area of a passage which bypasses the throttle valve  202  and is connected to an intake pipe  204 , an intake pipe pressure sensor  205  which detects a pressure within the intake pipe  204 , and fuel injection valves  206  which inject fuel required by the engine  201 .  
      The engine  201  is provided with ignition plugs  208  for igniting a mixture of air and fuel supplied into cylinders (combustion chambers)  207 , and ignition coils (ignition modules)  209  for supplying an ignition energy to the ignition plugs  208  according to the ignition signal being outputted from an engine control unit  250 .  
      The engine  201  is also provided with a cam angle sensor  210  for detecting a cam angle and a water temperature sensor  211  for detecting a temperature of cooling water.  
      An exhaust pipe  221  has a front catalyst (upper stream catalyst=first catalyst)  223  as a three-way catalyst and a rear catalyst (down stream catalyst=second catalyst)  224  as a three-way catalyst which are separately disposed within a single housing  222 .  
      Specifically, the catalytic converter apparatus for exhaust emission is divided into the front catalyst  223  and the rear catalyst  224 , and the front catalyst (the upper stream catalyst)  223  which is mounted on the upstream side of the rear catalyst  224  viewed from an exhaust gas flow through the exhaust pipe  221  has a volume smaller than that of the rear catalyst (the down stream catalyst)  224 . And, an intermediate space  225  is defined between the front catalyst  223  and the rear catalyst  224  within the housing  222 .  
      A front O 2  sensor  226  which detects an oxygen density in exhaust gas and outputs a binary signal is mounted on the upper stream side (viewed from the exhaust gas flow in the exhaust pipe  221 ) of the front catalyst  223  of the exhaust pipe  221 . The housing  222  is provided with a rear O 2  sensor  227  which detects an oxygen density in the exhaust gas within the intermediate space  225  and outputs a binary signal.  
      The engine  201  is operated and stopped by an ignition key switch  212  which is a main switch.  
      The idling speed of the engine  201  in this embodiment is controlled by the idle speed control valve  203 , but where the throttle valve  202  is controlled by means of a motor or the like, the idling speed can be controlled by the throttle valve  202 , and the idle speed control valve  203  can be eliminated.  
      Fuel control including air/fuel ratio control of the engine  201 , ignition timing control, idle control and catalyst deterioration diagnosis are performed by the engine control unit  250 .  
      The engine control unit  250  is, for example, an electronically-controlled type based on a microcomputer and as shown in  FIG. 2 , comprised of an I/O LSI (Input/Output Large-scale integrated circuit)  251  which converts electrical signals of the individual sensors which are mounted on the engine  201  into signals for digital arithmetic processing and converts the control signals for digital calculation into drive signals for a real actuator, an arithmetic unit (MPU)  252  which judges the state of the engine  201  according to the signals for digital arithmetic processing from the I/O LSI  251 , performs arithmetic processing of an amount of fuel, ignition timing, catalyst deterioration diagnosis and the like required by the engine  201  according to a prescribed procedure and sends the calculated values to the I/O LSI  251 , a nonvolatile memory (EP-ROM)  253  which stores the control procedure and control constant of the arithmetic unit  252 , and a volatile memory (RAM)  254  which stores the results calculated by the arithmetic unit  252 .  
      Even if the ignition switch  212  is off and power is not supplied to the engine control unit  250  from a battery power supply, a back-up power supply may be connected to the volatile memory  254  in order to store the memory contents.  
      In this embodiment, the engine control unit  250  receives signals from each of the water temperature sensor  211 , the cam angle sensor  210 , a crank angle sensor  213 , the front O 2  sensor  226 , the rear O 2  sensor  227 , the intake pipe pressure sensor  205 , a throttle opening degree sensor  214  and the ignition switch  212 , and outputs a fuel injection command signal to the fuel injection valves  206 , an ignition command signal to the ignition coils  209  and an opening degree command signal to the idle speed control valve  203 , and a display command to a failure diagnosis display unit  261  according to a catalyst deterioration diagnosed result.  
      Then, one embodiment of a control block of the engine control unit  250  to perform a method for diagnosing deterioration of a catalyst according to the invention will be described with reference to  FIG. 3 .  
      The engine control unit  250  executes a computer program to embody an engine speed calculation unit  101 , a basic fuel calculation unit  102 , a basic fuel correction factor calculation unit  103 , a basic ignition timing calculation unit  104 , an ISC control unit  105 , an acceleration and deceleration judging unit  106 , an air/fuel ratio feedback control coefficient calculation unit  107 , a catalyst deterioration detection unit (catalyst deterioration diagnostic unit)  108 , a basic fuel correction unit  109 , and an ignition timing correction unit  110  by means of software.  
      The engine speed calculation unit  101  counts the electrical signals of the crank angle sensor  213  which is set to a prescribed crank angle position of the engine  201 , mainly the number of inputs per unit time of changes in pulse signals, and calculates the number of revolutions per unit time of the engine  201  by performing arithmetic processing.  
      The basic fuel calculation unit  102  calculates basic fuel which is required by the engine  201  according to the engine speed calculated by the engine speed calculation unit  101  and an intake pipe pressure (engine load) detected by the intake pipe pressure sensor  205  which is mounted on the intake pipe  204  of the engine  201 .  
      The basic fuel correction factor calculation unit  103  calculates a correction factor of the basic fuel, which is calculated by the basic fuel calculation unit  102 , in the individual operation regions of the engine  201  according to the engine speed calculated by the engine speed calculation unit  101  and the intake pipe pressure (engine load) detected by the intake pipe pressure sensor  205 .  
      The basic ignition timing calculation unit  104  determines optimum ignition timing of the engine  201  by map retrieving or the like based on the engine speed calculated by the engine speed calculation unit  101  and the intake pipe pressure(representing engine load) detected by the intake pipe pressure sensor  205 .  
      The ISC control unit  105  determines a target idling speed value in order to keep the idling speed of the engine  201  at a prescribed value and calculates a target flow rate and an ISC ignition timing compensation amount to the ISC valve  203 . The ISC control unit  105  outputs an ISC valve signal based on the target flow rate to the ISC valve  203 . Thus, the ISC valve  203  is driven so as to provide the target flow rate for idling.  
      The acceleration and deceleration judging unit  106  processes electrical signals outputted from the throttle opening degree sensor  214  to judge from the throttle opening degree change amount whether the engine  201  is in an acceleration or deceleration state and calculates an ignition timing compensation amount for the acceleration or deceleration.  
      The air/fuel ratio feedback control coefficient calculation unit  107  performs air/fuel ratio feedback control according to a sensor signal of the front O 2  sensor  226  on the upper stream side of the front catalyst  223 , an engine speed, an intake pipe pressure and an engine water temperature.  
      The catalyst deterioration detection unit  108  judges a deterioration degree of the front catalyst  223  according to the sensor signal of the front O 2  sensor  226  which is on the upper stream side of the front catalyst  223 , the sensor signal of the rear O 2  sensor  227  which is on the down stream side of the front catalyst  223 , the engine speed, the intake pipe pressure and the engine water temperature and also judges a deterioration degree of the rear catalyst  224  according to the judged result. The catalyst deterioration detection unit  108  outputs a display command to the failure diagnosis display unit  261  to notify a driver about a catalyst failure according to the ineffective/effective judgment made on the basis of the deterioration degree.  
      The basic fuel correction unit  109  corrects the basic fuel, which is calculated by the basic fuel calculation unit  102 , by using the correction factor of the basic fuel correction factor calculation unit  103 , the air/fuel ratio feedback control coefficient of the air/fuel ratio feedback control coefficient calculation unit  107  and the like and outputs the fuel injection command signal according to the corrected amount of fuel to the fuel injection valves  206  of the individual cylinders. Thus, the fuel injection valves  206  inject a required amount of fuel into the individual cylinders.  
      The ignition timing correction unit  110  corrects the basic ignition timing, which is determined by the basic ignition timing calculation unit  104 , by using the ISC ignition timing compensation amount of the ISC control unit  105 , the ignition timing compensation amount for acceleration or deceleration of the acceleration and deceleration judging unit  106  and the like and outputs the corrected ignition timing command signal to the ignition coils  209  of the individual cylinders. Thus, the ignition plugs  208  of the individual cylinders cause a spark discharge with the required ignition timing to ignite the mixture flowing into the cylinders.  
      In this embodiment, the fuel control is established by detecting an intake pipe pressure, but the fuel control can also be established by detecting an amount of intake air of the engine  201 .  
      One embodiment of the front catalyst deterioration degree arithmetic processing section included in the catalyst deterioration detection unit  108  will be described in detail with reference to  FIG. 4 .  
      The catalyst deterioration detection unit  108  has a filtering processing section  501 , a hold time setting section  502 , a sampling hold processing section  503 , another filtering processing section  504 , a sampling processing section  505 , a correlation value calculation section  506 , and a deterioration degree calculation section  507 .  
      The filtering processing section  501  filters the output voltage of the front O 2  sensor  226 . The filtering is performed by a weighted average or the like.  
      The hold time setting section  502  map retrieves a hold time for combining the output phases of the front O 2  sensor  226  and the rear O 2  sensor  227  according to the engine speed and the intake pipe pressure.  
      The sampling hold processing section  503  samples the output of the front O 2  sensor filtered by the filtering processing section  501  and processes to combine the phases of the output of the front O 2  sensor  226  and the output of the rear O 2  sensor  227  according to the hold time determined by the hold time setting section  502 .  
      The filtering processing section  504  filters the output voltage of the rear O 2  sensor  227 . The filtering processing is performed by the weighted average or the like in the same manner as the filtering of the output voltage of the front O 2  sensor  226 .  
      The sampling processing section  505  samples the output of the rear O 2  sensor filtered by the filtering processing section  504 .  
      The correlation value calculation section  506  calculates a correlation value C for every prescribed interval according to the sampled value of the front O 2  sensor  26  from the sampling hold processing section  503  and the sampled value of the rear O 2  sensor  227  from the sampling processing section  505 .  
      The correlation value C is calculated by following equation (1):  
             C   =       ∑     i   =   1       NUMBER   ⁢           ⁢   OF   ⁢           ⁢   SAMPLES       ⁢         (       Pre   ⁢           ⁢     (   i   )       -   Pre     )     2     ·       (       Post   ⁢           ⁢     (   i   )       -   Post     )     2                 (   1   )             
 
      In the equation, “Pre” denotes a sampled value of the front O 2  sensor, and “Post” denotes a sampled value of the rear O 2  sensor.  
      The deterioration degree calculation section  507  determines a deterioration degree (deterioration judging value) D of the catalyst from the correlation value C for every prescribed interval calculated by the correlation value calculation section  506 .  
       FIG. 5  shows an example of a relationship between the correlation value C and the deterioration degree D outputted from the front and rear O 2  sensors.  
      A line  601  indicates the middle value between the correlation value C and the deterioration degree D. The correlation value C and the deterioration degree D have a certain degree of width between lines  602  and  603  depending on individual variations with the line  601  at the center.  
      The threshold value becomes a line  604  when the deterioration degree D is ineffective under emission control regulation I, and the threshold value of the correlation value C judging that the catalyst deterioration is ineffective becomes a threshold value  606  under the emission control regulation I considering a width  605  of variation  1 . Meanwhile, the threshold value under another emission control regulation II becomes a line  607  when the deterioration degree D is ineffective, and the threshold value of the correlation value D for judging the catalyst deterioration as ineffective can not be set considering a width  608  of variation  2 .  
       FIG. 6A ,  FIG. 6B ,  FIG. 6C  show one example of output behaviors of the front and rear O 2  sensors in a case where a general example of catalyst is judged as catalyst ineffective.  
      As shown in  FIG. 6A , a front O 2  sensor  702  and a rear O 2  sensor  703  are disposed before and after an ordinary catalyst  701  of a single bay.  
      A chart  704  of  FIG. 6B  shows the output of the front O 2  sensor  702 , indicating that it is fluctuated with a prescribed voltage at the center by the air/fuel ratio feedback control of the engine control unit.  
      Meanwhile, the catalyst  701  is deteriorated considerably to an ineffective level in view of the emission control regulation but is not deteriorated so much as a whole. Therefore, the output of the rear O 2  sensor  703  is not fluctuated so heavily as indicated by a chart  705  in  FIG. 6C . Thus, a correlation value for judgment of the catalyst deterioration becomes a small value close to 0.  
       FIG. 7A ,  FIG. 7B ,  FIG. 7C ,  FIG. 7D  show an example of output behaviors of the front and rear O 2  sensors in a case where the catalyst of this embodiment is to be judged as catalyst ineffective.  
      As shown in  FIG. 7A , the catalytic converter apparatus of this embodiment is splited into the front catalyst  223  and the rear catalyst  224  which are housed within the single housing  222 , and the rear O 2  sensor  227  mounted in the intermediate space  225  which is defined between the front catalyst  223  and the rear catalyst  224 . The front O 2  sensor  226  is mounted on the upper stream side of the front catalyst  223 .  
      The output of the front O 2  sensor  226  is fluctuated with a prescribed voltage at the center by the air/fuel ratio feedback control as indicated by a chart  806  in  FIG. 7B .  
      Meanwhile, where the catalyst as a whole is deteriorated so that the emission level is not conforming to the emission control regulation, the deterioration degree of the front catalyst  223  is larger than that of the rear catalyst  224 , so that the fluctuation width of the rear O 2  sensor  227  mounted in the intermediate space  225  becomes large in comparison with the above case of the mounting position  703  shown in  FIG. 6A  as indicated by the chart  705  in  FIG. 6C . Thus, a correlation value for judgment of the catalyst deterioration indicates a large value to some extent.  
       FIG. 8  shows an example of a relationship between the correlation value C and the deterioration degree D of the outputs of the front and rear O 2  sensors.  
      By configuring the catalyst and the front and rear O 2  sensors as shown in  FIG. 7A , the threshold value of a deterioration degree, which becomes an ineffective level under the emission control regulation, can be shifted from a region  905  to a region  907 , and it is easy to determine the threshold value of the correlation value.  
      In other words, the separation into the front catalyst  223  and the rear catalyst  224  causes that the front catalyst  223  is deteriorated earlier than when it is not separated, and it becomes possible to detect whether the catalyst is ineffective even if deterioration as a whole is little.  
       FIG. 9  shows an example of a relationship between the front catalyst deterioration degree and the rear catalyst deterioration degree in the catalytic converter apparatus of this embodiment (the present invention) shown in  FIG. 7A .  
      A line  1001  indicates a correlation value of the outputs of the front and rear O 2  sensors to show a deterioration degree of the front catalyst  223 . If the line reaches a threshold value  1002  where it is judged as ineffective, the failure diagnosis display unit  261  shows a screen for urging to replace. Then, the front catalyst  223  is replaced, and the deterioration degree of the front catalyst  223  returns to zero as a result of the replacement.  
      Meanwhile, the rear catalyst  224  is deteriorated slowly as indicated by a line  1005 , and according to this embodiment, a deterioration degree of the rear catalyst  224  reaches an ineffective level  1006  at a point  1007  for the meantime the front catalyst  223  was replaced two times.  
      The deterioration property of the rear catalyst  224  varies depending on the distance between the front catalyst  223  and the rear catalyst  224 , the space volume of the intermediate space  225  and the like, and the deterioration degree of the front catalyst  223  may be used when the rear catalyst  224  of a real automobile is judged as deteriorated.  
      In this embodiment, the deterioration degree of the rear catalyst  224  has the integrated value of the front catalyst deterioration degree determined as a rear catalyst deterioration index based on the relationship between the front catalyst deterioration degree and the rear catalyst deterioration degree shown in  FIG. 9 , and ineffective/effective of the rear catalyst is determined according to the rear catalyst deterioration index.  
       FIG. 10  shows an example of a relationship between the front catalyst deterioration degree and the front catalyst deterioration degree integrated value (rear catalyst deterioration index).  
      When a deterioration degree  1101  of the front catalyst  223  reaches a threshold value  1102 , the catalyst is replaced with new one, and the deterioration degree returns to zero. The ineffective level of the rear catalyst deterioration of  FIG. 9  described above is at  1103 , and when the front catalyst deterioration degree integrated value  1101  reaches a threshold value  1105  corresponding to that level, the rear catalyst  224  is judged as ineffective, and the failure diagnosis display unit  261  urges the user or driver to replace the catalyst.  
      After the rear catalyst  224  is replaced, the front catalyst deterioration degree integrated value is cleared, and accumulation is restarted. Upon reaching the threshold value  1105  again, the failure diagnosis display unit  261  urges the user to replace the rear catalyst  224 .  
       FIG. 11  shows an example of separation of a catalyst of the catalytic converter apparatus of this embodiment. Both the front catalyst  223  and the rear catalyst  224  have a columnar shape. The axial length of the front catalyst  223  is L 1 , and the axial length of the rear catalyst  224  is L 2  which is longer than the axial length L 1  of the front catalyst  223 . Because of the difference in the axial length, the volume of the front catalyst  223  is smaller than that of the rear catalyst  224 . The intermediate space  225  having an axial length L 3  is present between the front catalyst  223  and the rear catalyst  224 .  
      A relationship of the front catalyst  223  to the deterioration property due to a combination of the front catalyst  223  and the rear catalyst  224  is derived.  
      It is assumed that an HC (hydrocarbon) transformation efficiency is Efnew when the catalyst is new and an HC transformation efficiency is Efng when the catalyst is ineffective. Then, a split ratio L 1 :L 2  of the front catalyst  223  and the rear catalyst  224  is expressed by following equation (2). The HC transformation efficiency here indicates a transformation efficiency of the combination of the front catalyst  223  and rear catalyst  224 .
 
 L 1: L 2={(Efnew-Efng)/Efnew}:(Efng/Efnew)  (2)
 
      When it is assumed that Efnew=90% and Efng=70%, it becomes that L 1 :L 2 =22%:78%, and a split ratio with allowance included becomes L 1 :L 2 =30%:70%.  
      Thus, the catalytic converter apparatus is separated into the front catalyst  223  and the rear catalyst  224  at a prescribed split ratio according to the transformation efficiency when they are new, the transformation efficiency when ineffective and the emission measured value. The volume of the front catalyst  223  is smaller than that of the rear catalyst  224 .  
      The deterioration of the rear catalyst  224  becomes slower in comparison with the deterioration of the front catalyst  223  as the intermediate space  225  has a larger volume. Therefore, the relationship between the front catalyst deterioration degree and the rear catalyst deterioration degree shown in  FIG. 9  is determined considering the volume of the intermediate space  225 .  
      In other words, where the relationship of the front catalyst  223  to the deterioration property according to the combination of the front catalyst  223  and the rear catalyst  224  is derived, the volume of the intermediate space  225  is added to the elements contributing to the deterioration of the rear catalyst  224 .  
       FIG. 12  shows an example of one structure of an effective/ineffective judgment processing section (apparatus for diagnosing deterioration of a catalyst) of the front catalyst  223  and the rear catalyst  224 .  
      The effective/ineffective judgment processing section of this embodiment has a front catalyst deterioration judging value calculation section  1401 , a front catalyst effective/ineffective judging section  1402 , a front catalyst ineffective-to-effective invert times counter  1405 , an ineffective-to-effective invert times threshold value setting device  1406 , a comparator  1407 , a rear catalyst effective/ineffective judging section  1409 , a front catalyst deterioration judging value integrating section  1411 , a front catalyst deterioration judgment integrated value storage section  1414 , and switches  1408 ,  1412 ,  1413 .  
      The front catalyst deterioration judging value calculation section  1401  is configured of, for example, the front catalyst deterioration degree arithmetic processing section shown in  FIG. 4 , and the front O 2  sensor output, the rear O 2  sensor output, the intake pipe pressure and the engine speed are inputted, and then the deterioration judging value (deterioration degree) of the front catalyst is calculated by correlated calculation.  
      The front catalyst effective/ineffective judging section  1402  judges effective/ineffective of the front catalyst  223  according to the front catalyst deterioration judging value calculated by the front catalyst deterioration judging value calculation section  1401 .  
      In a case where it is judged as ineffective by the front catalyst effective/ineffective judging section  1402 , it is notified to the driver by the diagnosis display made by the failure diagnosis display unit  261 , the code of the front catalyst ineffective is written into the storage unit  262 , and the count value of the front catalyst ineffective-to-effective invert times of the front catalyst ineffective-to-effective invert times counter  1405  is increased by one increment.  
      The comparator  1407  compares the ineffective-to-effective invert times threshold value which is set in the ineffective-to-effective invert times threshold value setting device  1406  with the count value of the front catalyst ineffective-to- effective invert times counter  1405 . In this comparison, if the count value of invert times is larger than the threshold value, the deterioration judging value of the front catalyst  223  according to the front catalyst deterioration judging value calculation section  1401  is inputted to the rear catalyst effective/ineffective judging section  1409  by the switch  1408 .  
      The front catalyst deterioration judging value integrating section  1411  integrates the deterioration judging values (deterioration degrees) of the front catalyst  223  calculated by the front catalyst deterioration judging value calculation section  1401 . This integration is continued until the switch  1413  is turned off after the rear catalyst  224  is judged as ineffective by the rear catalyst effective/ineffective judging section  1409 , and the integrated value at this point is written as the front catalyst deterioration judgment integration stored value into the front catalyst deterioration judgment integrated value storage section  1414  via the switch  1412 .  
      The rear catalyst effective/ineffective judging section  1409  judges whether the rear catalyst  224  is effective or ineffective according to the deterioration judging value of the front catalyst  223  calculated by the front catalyst deterioration judging value calculation section  1401  or the front catalyst deterioration judgment integrated value integrated by the front catalyst deterioration judging value integrating section  1411 .  
      Where it is judged by the rear catalyst effective/ineffective judging section  1409  that the rear catalyst  224  is ineffective, it is notified to the driver by the diagnosis display made by the failure diagnosis display unit  261 , and a rear catalyst ineffective code is written into the storage unit  263 .  
      The storage units  262 ,  263  are the same storage unit except that the front catalyst ineffective code and the rear catalyst ineffective code have a different memory address.  
       FIG. 13  shows an example of another structure of the effective/ineffective judgment processing section (apparatus for diagnosing deterioration of a catalyst) of the front catalyst  223  and the rear catalyst  224 . In  FIG. 13 , portions corresponding to those of  FIG. 12  are denoted by like reference numerals as those used in  FIG. 12 , and descriptions thereof will be omitted.  
      This embodiment is different from that of  FIG. 12  described above on the point that the embodiment of  FIG. 12  counts the front catalyst ineffective-to-effective invert times, while this embodiment counts the number of replacement times of the front catalyst  223 .  
      In this embodiment, a front catalyst replacement times counter  1505  for counting the number of replacement times of the front catalyst  223 , a replacement times threshold value setting device  1506 , and a comparator  1507  for comparing the replacement times threshold value set in the replacement times threshold value setting device  1506  with the count value of the front catalyst replacement times counter  1505  are disposed, and the output of the comparator  1507  controls the switch  1408  in the same manner as in the embodiment of  FIG. 12  described above. The count value of the front catalyst replacement times counter  1505  is updated by a car repair shop or the like by manually inputting when the front catalyst  223  is replaced.  
      Details of the rear catalyst effective/ineffective judging section  1409  will be described with reference to  FIG. 14 .  
      The rear catalyst effective/ineffective judging section  1409  has a rear catalyst effective/ineffective judgment discrimination portion  1601 , a comparator  1604 , and switches  1602 ,  1603  for switching the input of the comparator  1604 .  
      The rear catalyst effective/ineffective judgment discrimination portion  1601  judges whether or not the ineffective judgment of the rear catalyst  224  was done in the past, and if the ineffective judgment was not made at all, the front catalyst deterioration judging value (deterioration degree) is selected for the comparative value and the front catalyst deterioration judgment threshold value is selected for the threshold value by the switches  1602 ,  1603  as inputs to the comparator  1604 , and they are compared by the comparator  1604  according to the structure shown in  FIG. 12  or  FIG. 13 .  
      Where the front catalyst deterioration judging value inputted to the comparator  1604  is larger than the front catalyst deterioration judgment threshold value, the rear catalyst  224  is judged as ineffective.  
      Meanwhile, if it was judged as ineffective even once in the past, the front catalyst deterioration judgment integrated value and the front catalyst deterioration judgment integration stored value as a threshold value are selected as inputs to the comparator  1604  by the switches  1602 ,  1603 , and they are compared by the comparator  1604 .  
      Where the front catalyst deterioration judgment integrated value to be inputted to the comparator  1604  is larger than the front catalyst deterioration judgment integration stored value, the rear catalyst is judged as ineffective.  
      A processing flow of overall control to be performed by the engine control unit  250  including the catalyst deterioration judgment of this embodiment will be described with reference to the flowchart of  FIG. 15 .  
      First, the electrical signals of the crank angle sensor  213 , and mainly the number of inputs per unit time of a change in pulse signal are counted, and the engine speed is calculated by arithmetic processing in step  1701 . In step  1702 , the intake pipe pressure is read by the output from the intake pipe pressure sensor  205 .  
      Subsequently, a basic amount of fuel is calculated based on the engine speed and the intake pipe pressure in step  1703 .  
      Then, a basic fuel correction factor is retrieved based on the engine speed and the intake pipe pressure in step  1704 .  
      The output of the front O 2  sensor  226  which is in front of the catalyst is read in step  1705 .  
      Air/fuel ratio feedback control is performed based on the output signal of the front O 2  sensor  226  to obtain a target air/fuel ratio in step  1706 , thereby determining an air/fuel ratio feedback control coefficient.  
      The output of the rear O 2  sensor  227  which is after the catalyst is read in step  1707 .  
      Then, a deterioration degree (deterioration judging value) of the front catalyst  223  is calculated by correlated calculation according to the front O 2  sensor output and the rear O 2  sensor output in step  1708 .  
      A deterioration degree of the rear catalyst  224  is calculated based on the deterioration degree of the front catalyst  223  described above in step  1709 .  
      The basic amount of fuel is corrected according to the basic fuel correction factor and the air/fuel ratio feedback control coefficient in step  1710 . And, an amount of fuel to inject the corrected amount of fuel is set in step  1711 .  
      Then, a target engine speed for idling is calculated in step  1712 . And, a target flow rate of the ISC valve  203  is calculated from the target engine speed in step  1713 .  
      An ignition timing compensation amount for suppressing the idling speed from varying is calculated in step  1714 .  
      The target flow rate of the ISC valve  203  is outputted to the ISC valve  203  to control the ISC valve  203  in step  1715 .  
      Then, a throttle opening degree is read in step  1716 . And, a time change amount of the read throttle opening degree is determined to judge acceleration or deceleration in step  1717 .  
      An ignition timing compensation amount for acceleration or deceleration is calculated based on the acceleration or deceleration judgment in step  1718 .  
      A basic ignition timing is calculated in step  1719 . And, the calculated ignition timing compensation for idling, acceleration or deceleration is applied to the basic ignition timing in step  1720  to determine the final ignition timing. And, the final ignition timing is set in step  1721 , and ignition is performed at the required ignition timing.  
      Then, a processing flow of the catalyst deterioration degree calculation of the front catalyst by the front catalyst deterioration degree arithmetic processing section shown in  FIG. 4  will be described with reference to the flowchart of  FIG. 16 .  
      First, the output of the front O 2  sensor  226  is read in step  1801 . And, the output of the front O 2  sensor  226  is filtered in step  1802 .  
      An engine speed and an intake pipe pressure are read in step  1803 , and a hold time for combining the phases of the outputs of the O 2  sensors before and after the catalyst according to the engine speed and the intake pipe pressure is map retrieved in step  1804 . The filtering value of the front O 2  sensor  226  is sampled while reflecting the hold time in step  1805 .  
      The output of the rear O 2  sensor  227  is read in step  1806 . And, the output of the rear O 2  sensor  227  is filtered in step  1807 . The filtering value of the rear O 2  sensor  227  is sampled in step  1808 .  
      A correlation value of a prescribed interval is calculated from the sample values of the filtering values of the O 2  sensors before and after the catalyst in step  1809 . The correlation value is calculated by the above-described equation (1).  
      The deterioration judging value (deterioration degree) of the front catalyst  226  is determined from the correlation value in step  1810 .  
      The processing flow of the effective/ineffective judgment of the front catalyst  223  and the rear catalyst  224  by the effective/ineffective judgment processing section shown in  FIG. 12  will be described with reference to the flowchart of  FIG. 17 .  
      The outputs of the O 2  sensors  226 ,  227  before and after the catalyst are read in step  1901 . And, the engine speed and the intake pipe pressure are read in step  1902 .  
      The deterioration judging value of the front catalyst  226  is calculated from the outputs of the O 2  sensors before and after the catalyst, the engine speed and the intake pipe pressure in step  1903 . This deterioration judging value is calculated from the correlation value of the outputs of the front and rear O 2  sensors according to the processing flow shown in  FIG. 16 .  
      Then, the deterioration of the front catalyst  223  is judged for effective/ineffective according to the deterioration judging value in step  1904 .  
      The procedure is branched in step  1905  depending on the judged result effective or ineffective determined in step  1904 . If the judged result is ineffective, the procedure advances to step  1907 , where ineffective code  1  of the front catalyst ineffective is stored, and a failure warning is given by the failure display unit  261  in the next step  1908 .  
      If the judged result of the front catalyst  223  is effective, the procedure advances to step  1906 , and the failure warning is cancelled.  
      It is judged in step  1909  whether there is an integration stored value of the deterioration judging value of the front catalyst  223 . If there is not an integration stored value, the procedure advances to step  1910 , where it is judged whether the deterioration judgment of the front catalyst  223  is inverted from ineffective to effective.  
      If it is inverted, the procedure advances to step  1911 , where the number of ineffective-to-effective invert times is counted to judge whether the number of invert times is equal to or more than the threshold value in step  1912 . If the number of invert times is equal to or more than the threshold value, the procedure advances to step  1913 , where the deterioration judging value of the front catalyst  223  is read. And, the deterioration judgment of the rear catalyst  224  is judged for effective/ineffective from the deterioration judging value of the front catalyst  223  in step  1914 .  
      The procedure is branched in step  1915  depending on the judged result effective or ineffective in step  1914 . If the judged result is ineffective, the procedure advances to step  1917 , where ineffective code  2  of the rear catalyst ineffective is stored, and a failure warning is given by the failure display unit  261  in the nest step  1918 .  
      If the judged result of the rear catalyst  224  is effective, the procedure advances to step  1916 , where the failure warning is cancelled.  
      If it is judged in the step  1909  that there is a deterioration judgment integration stored value of the front catalyst  223 , the procedure advances to step  1919 , where the deterioration judgment integration stored value and the front catalyst deterioration judgment integrated value (threshold value) are compared to judge whether the rear catalyst  224  is effective or ineffective.  
      If the front catalyst deterioration judgment integrated value is larger than the deterioration judgment integration stored value, the rear catalyst is ineffective. Then, the code  2  of the rear catalyst ineffective is stored and a failure warning is given in steps  1917 ,  1918 . Meanwhile, if the front catalyst deterioration judgment integrated value is not larger than the deterioration judgment integration stored value, the procedure advances to step  1916 , where the failure warning is cancelled.  
       FIG. 18  shows a processing flow of the front catalyst deterioration judging value integration and judging value storing performed by the ineffective judgment processing section shown in  FIG. 12 .  
      The front catalyst deterioration judging value is read in step  2001 . The front catalyst deterioration judging value is integrated in step  2002 .  
      It is judged in step  2003  whether the rear catalyst  224  is judged as ineffective. If the rear catalyst  224  is judged as ineffective, the procedure advances to step  2004 , where the above-described integrated value is stored, and the integration is stopped in step  2005 .  
      Then, the processing flow of effective/ineffective judgment by the effective/ineffective judgment processing section of the front catalyst  223  and the rear catalyst  224  shown in  FIG. 13  will be described with reference to the flowchart of  FIG. 19 . In  FIG. 19 , steps corresponding to those of  FIG. 17  are denoted by the same step numbers as those used in  FIG. 17 , and descriptions thereof will be omitted.  
      In this processing flow, it is judged in step  2110  whether or not the front catalyst replacement signal to be manually inputted is present, and a replacement times count value is incremented in step  2111 . And, it is judged in step  2112  whether the count value is equal to or more than a threshold value, and it is judged whether the deterioration of the rear catalyst  224  is effective or ineffective.  
      Excepting the above, it is configured such that the processing flow is the same as in the flowchart shown in  FIG. 17 .  
      Then, the processing flow of the rear catalyst effective/ineffective judgment by the rear catalyst effective/ineffective judging section  1409  shown in  FIG. 14  will be described with reference to the flowchart of  FIG. 20 .  
      It is judged in step  2201  whether the deterioration ineffective of the rear catalyst  224  has been judged. If the ineffective judgment has not been made, the procedure advances to step  2202 , where the front catalyst deterioration judging value is selected as a comparative value, and the front catalyst deterioration judgment threshold value is selected as a threshold value in step  2203 .  
      Meanwhile, if it was judged as ineffective, the procedure advances to step  2204 , where the front catalyst deterioration judgment integrated value is selected as a comparative value, and the front catalyst deterioration judgment integration stored value is selected as a threshold value in step  2205 .  
      It is judged in step  2206  whether the comparative value is equal to or more than the threshold value, and if the comparative value is equal to or more than the threshold value, it is judged in step  2207  that the rear catalyst  224  is ineffective, and if the comparative value is not equal to or more than the threshold value, it is judged in step  2208  that the rear catalyst is effective.  
      As described above, the catalytic converter apparatus is separated into the front catalyst  223  and the rear catalyst  224  at a prescribed split ratio according to a transformation efficiency when they are new, a transformation efficiency when ineffective and an emission measured value, and the O 2  sensors  226 ,  227  are mounted before the separated front catalyst  223  and at the split point between them. And, the deterioration of the catalyst is diagnosed by comparing the outputs of the front and rear O 2  sensors, so that the front catalyst is deteriorated earlier than when it is not separated, and it becomes possible to detect ineffective even if the deterioration is slight as a whole. And, the deterioration degree of the rear catalyst  224  is obtained according to the deterioration degree and the number of replacement times of the front catalyst  223 , so that the front catalyst  223  is not replaced even if it is deteriorated.  
      The front catalyst  223  and the rear catalyst  224  are housed in the single housing  222  in the embodiment described above, but the apparatus of the invention is not limited to it. As shown in  FIG. 21 , the front catalyst  223  and the rear catalyst  224  may be housed in separate housings  222 A,  222 B. In  FIG. 21 , portions corresponding to those of  FIG. 1  are denoted by like reference numerals as those used in  FIG. 1 , and descriptions thereof will be omitted.  
      Instead of the O 2  sensors  226 ,  227 , a sensor which outputs a signal corresponding to the oxygen density or fuel concentration contained in the exhaust gas can also be used.  
      It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.