Abstract:
An engine self-diagnosis system capable of performing diagnosis of the light-off performance of a catalyst at a low cost and high accuracy without requiring addition or improvement of a sensor, etc. The engine self-diagnosis system comprises a unit for directly or indirectly detecting performance A of an exhaust cleaning catalyst when temperature of the catalyst is within a predetermined temperature range, and a unit for, based on the detected catalyst performance A, estimating performance B of the catalyst, which is resulted when the temperature of the catalyst is outside the predetermined temperature range.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an engine self-diagnosis system, and more particularly to an engine self-diagnosis system capable of performing diagnosis of an exhaust cleaning catalyst, which is provided in an exhaust passage, at a low cost and high accuracy.  
         [0003]     2. Description of the Related Art  
         [0004]     Recently, higher accuracy in diagnosis of various devices related to the engine exhaust performance has been demanded with severer control applied to self-diagnosis of automobile engines in North America, Europe, Japan, etc. In particular, the diagnosis accuracy of a catalyst for cleaning specific components (HC, CO and NOx) in engine exhaust is important. Generally, a catalyst exhibits the exhaust cleaning function at temperatures not lower than a predetermined value. A shift into the state where the exhaust cleaning rate is not lower than a predetermined value is called catalyst light-off (or catalyst activation). The catalyst diagnosis has hitherto been made on the cleaning capacity after the catalyst light-off. On the other hand, with a recent increase in performance of a catalyst and catalyst control, the amount of the specific components in engine exhaust has been dominantly occupied by the amount exhausted during a period from the engine startup to the catalyst light-off. For that reason, it is important to diagnose the light-off performance of the catalyst.  
       SUMMARY OF THE INVENTION  
       [0005]     As one example of a system for diagnosing the catalyst performance, Patent Document 1 (JP-A-2003-176714) proposes a system for detecting the exhaust component concentration corresponding to both the engine run status and the catalyst operating status by an exhaust component sensor, e.g., an HC sensor, disposed downstream of the catalyst, and the diagnosing, e.g., the light-off performance of the catalyst in accordance with the detected value.  
         [0006]     Also, Patent Document 2 (JP-A-5-248227) proposes a diagnosis system including an O 2  sensor downstream of a catalyst and a sensor for detecting the catalyst temperature. Then, when the catalyst performance is diagnosed in accordance with the O 2  sensor downstream of the catalyst, a reference value for use in diagnosis of the catalyst performance is changed in accordance with the detected catalyst temperature.  
         [0007]     However, any of those proposed diagnosis systems requires an additional new sensor, such as the exhaust component sensor and the temperature sensor, and increases the system cost.  
         [0008]     Meanwhile, Patent Document 3 (JP-A-2001-317345) proposes a system for detecting the timing at which the oxygen storage capacity of a catalyst is activated, based on the correlation between output signals of O 2  sensors disposed upstream and downstream of the catalyst, and diagnosing the light-off performance of the catalyst in accordance with the detected timing.  
         [0009]     Such a diagnosis system requires the O 2  sensor downstream of the catalyst to be activated before the catalyst light-off. In practice, however, to avoid the sensor from causing a trouble, e.g., cracking due to the presence of water in the catalyst, the sensor downstream of the catalyst is generally heated up after the water in the catalyst has been sufficiently evaporated. Hence, an improvement of the O 2  sensor downstream of the catalyst is required in order to activate that sensor before the catalyst light-off as in the above-mentioned diagnosis system.  
         [0010]     Further, Patent Document 4 (JP-A-9-158713) proposes a system in which, in consideration of that the O 2  sensor (or the A/F sensor) downstream of the catalyst is not sufficiently activated during the catalyst light-off, a diagnosis determination value is changed depending on the temperature of the O 2  sensor downstream of the catalyst.  
         [0011]     However, such a diagnosis system also requires the O 2  sensor downstream of the catalyst to be activated to some extent at the timing of the catalyst light-off, and accompanies a risk of sensor cracking as in the diagnosis system proposed by Patent Document 3. Moreover, there is a fear that diagnosis accuracy lowers because the diagnosis is performed during activation of the sensor downstream of the catalyst.  
         [0012]     All of the above-mentioned diagnosis systems have still another problem that, because light-off characteristics are directly detected, it is difficult to distinctively confirm whether the light-off performance of the catalyst, i.e., the catalyst itself, has deteriorated or the performance of means for raising the temperature of the catalyst has reduced.  
         [0013]     In view of the above-described problems in the related art, an object of the present invention is to provide an engine self-diagnosis system capable of performing diagnosis of the light-off performance of a catalyst at a low cost and high accuracy.  
         [0014]     To achieve the above object, the present invention provides an engine self-diagnosis system comprising a unit for directly or indirectly detecting performance A of an exhaust cleaning catalyst when temperature of the catalyst or temperature of exhaust gas flowing into the catalyst is within a predetermined temperature range; and a unit for, based on the detected catalyst performance A, estimating performance B of the catalyst which is resulted, when the temperature of the catalyst is outside the predetermined temperature range.  
         [0015]     In a first form of the present invention embodying the above features, diagnosis (detection) of the catalyst performance after light-off of the catalyst is carried out, and based on the result of the performance diagnosis, the catalyst performance before or during the light-off is estimated for diagnosis. In general, static (steady-state) performance of a catalyst after the light-off is decided dominantly depending on the specific surface area (dispersibility) of a precious metal used in the catalyst. On the other hand, the light-off performance of the catalyst is also decided dominantly depending on the specific surface area of the precious metal. Accordingly, by detecting the catalyst performance after the light-off, the catalyst performance before or during the light-off can be indirectly estimated (see  FIGS. 1 and 17 ).  
         [0016]     In a second form of the engine self-diagnosis system according to the present invention, the catalyst has at least three-way performance.  
         [0017]     In a third form of the engine self-diagnosis system according to the present invention, the catalyst is an HC adsorbing combustion catalyst that adsorbs HC when the catalyst temperature is within a predetermined temperature range, desorbs the adsorbed HC when the catalyst temperature exceeds the predetermined temperature range, and cleans the adsorbed and desorbed HC.  
         [0018]     In a fourth form of the engine self-diagnosis system according to the present invention, the catalyst is a lean NOx catalyst.  
         [0019]     Thus, because the catalysts used in the second, third and fourth forms are all ones using precious metals, the diagnosis principle employed in the first form is also applicable.  
         [0020]     In a fifth form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for directly or indirectly detecting the temperature of the catalyst; a unit for directly or indirectly detecting the catalyst performance A when the catalyst temperature detected by the detecting unit is within a temperature range in which an exhaust cleaning rate is not smaller than a predetermined value; and a unit for, based on the detected catalyst performance A, estimating the catalyst performance B resulted when the catalyst temperature detected by the detecting unit is within a temperature range in which the exhaust cleaning rate is smaller than the predetermined value (see  FIG. 2 ).  
         [0021]     Thus, in the fifth form, the temperature ranges after and before the light-off are defined respectively depending on that the exhaust cleaning rate is not smaller than or is smaller than the predetermined value.  
         [0022]     In a sixth form of the engine self-diagnosis system according to the present invention, the catalyst performance B is a catalyst temperature T 0  at which the exhaust cleaning rate is not smaller than the predetermined value (see  FIG. 3 ).  
         [0023]     Thus, in the sixth form, the catalyst performance B estimated from the directly detected catalyst performance after the light-off is specifically defined as the light-off temperature.  
         [0024]     In a seventh form of the engine self-diagnosis system according to the present invention, the system further comprises catalyst deterioration determining unit for determining that the catalyst has deteriorated, when the catalyst temperature T 0  at which the exhaust cleaning rate is not smaller than the predetermined value exceeds a predetermined temperature (see  FIG. 4 ).  
         [0025]     Thus, in the seventh form, when the estimated light-off temperature exceeds the predetermined temperature, a time from the engine startup to the catalyst light-off is prolonged and particular components (HC, CO and NOx) in exhaust are increased. In such a condition, therefore, it is determined that the catalyst has deteriorated.  
         [0026]     In an eighth form of the engine self-diagnosis system according to the present invention, the catalyst is an HC adsorbing combustion catalyst, and the catalyst deterioration determining unit determines that the HC adsorbing combustion catalyst has deteriorated, when the catalyst temperature T 0  at which the exhaust cleaning rate is not smaller than the predetermined value exceeds a predetermined temperature (see  FIG. 4 ).  
         [0027]     The function of the HC adsorbing combustion catalyst is mainly divided into HC adsorbing performance and adsorbed-HC cleaning capacity. However, the adsorbed-HC cleaning capacity developed by a precious metal as a primary component generally deteriorates in a shorter term. Hence, deterioration diagnosis of the HC adsorbing combustion catalyst is realized by diagnosing the light-off performance in the adsorbed-HC cleaning capacity of the HC adsorbing combustion catalyst.  
         [0028]     In a ninth form of the engine self-diagnosis system according to the present invention, the catalyst is a lean NOx catalyst, and the catalyst deterioration determining unit determines that the lean NOx catalyst has deteriorated, when the catalyst temperature T 0  at which the exhaust cleaning rate is not smaller than the predetermined value exceeds a predetermined temperature (see  FIG. 4 ).  
         [0029]     Thus, this ninth form is based on the fact that the light-off performance in the NOx storage capacity of the lean NOx catalyst also depends on the precious metal in the catalyst.  
         [0030]     In a tenth form of the engine self-diagnosis system according to the present invention, the catalyst performance A is exhaust cleaning capacity (see  FIG. 5 ).  
         [0031]     Thus, in the tenth form, the performance detected after the catalyst temperature exceeds the predetermined temperature (i.e., after the light-off) is defined as the exhaust cleaning capacity of the catalyst.  
         [0032]     In an eleventh form of the engine self-diagnosis system according to the present invention, the catalyst performance A is oxygen storage capacity (see  FIG. 6 ).  
         [0033]     Thus, in the eleventh form, the performance detected after the catalyst temperature exceeds the predetermined temperature (i.e., after the light-off) is defined as the oxygen storage capacity of the catalyst. The oxygen storage capacity (OSC) of a catalyst is decided depending on both the specific surface area (dispersibility) of a precious metal used in the catalyst and the content of an auxiliary catalyst such as ceria (or zirconia). Because the content of the auxiliary catalyst is hardly changed from the initial value, the OSC is substantially decided by sintering (cohesion) of the precious metal. Accordingly, the light-off performance (catalyst characteristic B) of the catalyst is estimated in terms of a sintering degree of the precious metal by diagnosing the OSC.  
         [0034]     In a twelfth form of the engine self-diagnosis system according to the present invention, the catalyst performance A is given as exhaust cleaning capacity, and the catalyst performance B is given as the catalyst temperature T 0  at which the exhaust cleaning rate is not smaller than the predetermined value (see  FIG. 7 ).  
         [0035]     Thus, in the twelfth form, the performance detected after the catalyst temperature exceeds the predetermined temperature (i.e., after the light-off) is defined as the exhaust cleaning capacity of the catalyst, and the catalyst performance B estimated from the exhaust cleaning capacity after the light-off is specifically defined as the light-off temperature.  
         [0036]     In a thirteenth form of the engine self-diagnosis system according to the present invention, the catalyst performance A is given as oxygen storage capacity, and the catalyst performance B is given as the catalyst temperature T 0  at which the exhaust cleaning rate is not smaller than the predetermined value or the oxygen storage capacity is not smaller than a predetermined value (see  FIG. 8 ).  
         [0037]     Thus, in the thirteenth form, the performance detected after the catalyst temperature exceeds the predetermined temperature (i.e., after the light-off) is defined as the oxygen storage capacity of the catalyst, and the catalyst performance B estimated from the oxygen storage capacity after the light-off is specifically defined as the light-off temperature.  
         [0038]     In a fourteenth form of the engine self-diagnosis system according to the present invention, the system further comprises an exhaust component detecting unit disposed downstream of the catalyst (see  FIG. 9 ).  
         [0039]     Thus, in the fourteenth form, exhaust components downstream of the catalyst are directly detected by the exhaust component detecting unit, and the exhaust cleaning rate after the light-off is detected based on the detected exhaust components. Then, the light-off performance is estimated based on the detected cleaning capacity.  
         [0040]     In a fifteenth form of the engine self-diagnosis system according to the present invention, the system further comprises an O 2  sensor or an A/F sensor downstream of the catalyst (see  FIG. 10 ).  
         [0041]     Thus, in the fifteenth form, the A/F ratio downstream of the catalyst is directly detected by the O 2  sensor or the A/F sensor, and the exhaust cleaning capacity after the light-off is detected based on the detected A/F ratio. Then, the light-off performance is estimated based on the detected cleaning capacity.  
         [0042]     In a sixteenth form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for detecting oxygen storage capacity of the catalyst based on an output signal from the O 2  sensor or the A/F sensor (see  FIG. 11 ).  
         [0043]     Thus, in the sixteenth form, the A/F ratio downstream of the catalyst is directly detected by the O 2  sensor or the A/F sensor, and the oxygen storage capacity of the catalyst after the light-off is detected based on the detected A/F ratio. Then, the light-off performance is estimated based on the detected oxygen storage capacity.  
         [0044]     In a seventeenth form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for oscillating an O 2  concentration or an air/fuel ratio upstream of the catalyst at a predetermined frequency; a unit for computing a component at the predetermined frequency of the output signal from the O 2  sensor or the A/F sensor; and a unit for detecting oxygen storage capacity of the catalyst based on the computed component at the predetermined frequency (see  FIG. 12 ).  
         [0045]     When the O 2  concentration or the air/fuel ratio upstream of the catalyst is oscillated at the predetermined frequency, the oscillation of the O 2  concentration or the air/fuel ratio downstream of the catalyst exhibits behaviors differing from those upstream of the catalyst due to the oxygen storage capacity of the catalyst if the catalyst (oxygen storage capacity) is in the light-off state. Based on that finding, in the seventeenth form, the oxygen storage capacity is detected by executing frequency analysis of the oscillation of the O 2  concentration or the air/fuel ratio downstream of the catalyst, and the light-off performance is estimated based on the detected oxygen storage capacity.  
         [0046]     In an eighteenth form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for changing an O 2  concentration or an air/fuel ratio upstream of the catalyst by a predetermined value; a response delay time computing unit for computing a response delay time from a time at which the O 2  concentration or the air/fuel ratio upstream of the catalyst is changed by a predetermined value to a time at which an output signal from the O 2  sensor downstream of the catalyst is changed by a predetermined value; and a unit for detecting oxygen storage capacity of the catalyst based on the computed response delay time ( FIG. 13 ).  
         [0047]     When the O 2  concentration or the air/fuel ratio upstream of the catalyst is changed by the predetermined value, the response delay time until the O 2  concentration or the air/fuel ratio downstream of the catalyst is changed depends on the oxygen storage capacity of the catalyst if the catalyst (oxygen storage capacity) is in the light-off state. Based on that finding, in the eighteenth form, the oxygen storage capacity is detected by determining the response delay time until the O 2  concentration or the air/fuel ratio downstream of the catalyst is changed, and the light-off performance is estimated based on the detected oxygen storage capacity.  
         [0048]     In a nineteenth form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for raising temperature of the catalyst; a catalyst temperature estimating unit; an O 2  sensor, an A/F sensor, or an exhaust sensor disposed downstream of the catalyst; a unit for directly detecting, based on an output signal from the O 2  sensor, the A/F sensor, or the exhaust sensor, whether the exhaust cleaning rate of the catalyst is not smaller than a predetermined value and the catalyst is in the light-off state; and an abnormality determining unit for determining the catalyst temperature raising unit to be abnormal, when the unit for directly detecting the catalyst light-off does not detect that the catalyst is in the light-off state, in spite of the catalyst temperature estimated by the catalyst temperature estimating unit reaching an estimated light-off temperature representing the catalyst performance B (see  FIG. 14 ).  
         [0049]     Thus, the nineteenth form is intended to distinctly detect whether the light-off performance of the catalyst has deteriorated or the unit for activating the catalyst in a shorter time (i.e., the catalyst temperature raising unit) has deteriorated. Specifically, when the estimated catalyst temperature (not actual temperature) reaches the estimated light-off temperature, the catalyst should have been (normally) brought into the light-off state. Taking into account that point, whether the catalyst is in the light-off state or not is detected by using, e.g., the O 2  sensor, the A/F sensor, or the exhaust sensor. If the detection result shows that the catalyst is not in the light-off state, this is determined as indicating that the catalyst temperature does not reach the light-off temperature. Then, the catalyst temperature raising unit is determined to be abnormal.  
         [0050]     In a twentieth form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for indicating the catalyst performance A and/or B or information related to the catalyst performance A and/or B.  
         [0051]     In a twenty-first form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for modifying an engine control parameter based on the catalyst performance A and/or B (see  FIG. 15 ).  
         [0052]     Thus, the engine control parameter is modified based on the catalyst performance determined as described above, to thereby further reduce particular components (HC, CO and NOx) in engine exhaust.  
         [0053]     In a twenty-second form of the engine self-diagnosis system according to the present invention, the system further comprises a unit for modifying a control parameter for the catalyst temperature raising unit based on the catalyst performance B represented by a catalyst temperature T 0  at which an exhaust cleaning rate is not smaller than the predetermined value (see  FIG. 16 ).  
         [0054]     Thus, in the twenty-second form, for example, a control parameter for engine startup is modified based on the catalyst light-off performance estimated as described above.  
         [0055]     In a twenty-third form of the engine self-diagnosis system according to the present invention, the control parameter for the catalyst temperature raising unit is a retard amount of ignition timing and/or a period during which the ignition timing is retarded (see  FIG. 16 ).  
         [0056]     Thus, in the twenty-third form, for example, the retard amount of ignition timing and/or the period during which the ignition timing is retarded is modified based on the catalyst light-off performance estimated as described above, to thereby activate the catalyst in a shorter time.  
         [0057]     In addition, the present invention also provides an automobile equipped with the engine self-diagnosis system constituted as described above.  
         [0058]     With the engine self-diagnosis system according to the present invention, the catalyst performance A after the light-off of the catalyst is detected for diagnosis, and the catalyst performance before or during the light-off (i.e., the catalyst performance B) is estimated based on the result of the detection and diagnosis. Therefore, the light-off performance of the catalyst can be diagnosed at a low cost and high accuracy without requiring addition or improvement of a sensor, etc. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0059]      FIG. 1  is a block diagram showing first through fourth forms of an engine self-diagnosis system according to the present invention;  
         [0060]      FIG. 2  is a block diagram showing a fifth form of the engine self-diagnosis system according to the present invention;  
         [0061]      FIG. 3  is a block diagram showing a sixth form of the engine self-diagnosis system according to the present invention;  
         [0062]      FIG. 4  is a block diagram showing seventh to ninth forms of the engine self-diagnosis system according to the present invention;  
         [0063]      FIG. 5  is a block diagram showing a tenth form of the engine self-diagnosis system according to the present invention;  
         [0064]      FIG. 6  is a block diagram showing an eleventh form of the engine self-diagnosis system according to the present invention;  
         [0065]      FIG. 7  is a block diagram showing a twelfth form of the engine self-diagnosis system according to the present invention;  
         [0066]      FIG. 8  is a block diagram showing a thirteenth form of the engine self-diagnosis system according to the present invention;  
         [0067]      FIG. 9  is a block diagram showing a fourteenth form of the engine self-diagnosis system according to the present invention;  
         [0068]      FIG. 10  is a block diagram showing a fifteenth form of the engine self-diagnosis system according to the present invention;  
         [0069]      FIG. 11  is a block diagram showing a sixteenth form of the engine self-diagnosis system according to the present invention;  
         [0070]      FIG. 12  is a block diagram showing a seventeenth form of the engine self-diagnosis system according to the present invention;  
         [0071]      FIG. 13  is a block diagram showing an eighteenth form of the engine self-diagnosis system according to the present invention;  
         [0072]      FIG. 14  is a block diagram showing a nineteenth form of the engine self-diagnosis system according to the present invention;  
         [0073]      FIG. 15  is a block diagram showing a twenty-first form of the engine self-diagnosis system according to the present invention;  
         [0074]      FIG. 16  is a block diagram showing twenty-second and—third forms of the engine self-diagnosis system according to the present invention;  
         [0075]      FIG. 17  is a graph showing the relationship between the catalyst temperature and the OSC index for explaining the diagnosis principle in the present invention;  
         [0076]      FIG. 18  is a schematic view showing an engine self-diagnosis system according to a first embodiment of the present invention, along with an engine to which the self-diagnosis system is applied;  
         [0077]      FIG. 19  is a block diagram showing the internal configuration of a control unit in the first embodiment of the present invention;  
         [0078]      FIG. 20  is a block diagram showing a control system in the first embodiment;  
         [0079]      FIG. 21  is a block diagram for explaining a basic fuel injection amount computing unit in the first embodiment;  
         [0080]      FIG. 22  is a block diagram for explaining a deterioration diagnosis permission determining unit in the first embodiment;  
         [0081]      FIG. 23  is a block diagram for explaining an air/fuel ratio modification term computing unit in the first embodiment;  
         [0082]      FIG. 24  is a block diagram for explaining a target air/fuel ratio computing unit in the first embodiment;  
         [0083]      FIG. 25  is a block diagram for explaining a unit for detecting the oxygen storage capacity after the light-off in the first embodiment;  
         [0084]      FIG. 26  is a block diagram for explaining a frequency component computing unit in the first embodiment;  
         [0085]      FIG. 27  is a block diagram for explaining an oxygen storage capacity computing unit in the first embodiment;  
         [0086]      FIG. 28  is a block diagram for explaining a light-off temperature estimating unit in the first embodiment;  
         [0087]      FIG. 29  is a block diagram showing a control system in a second embodiment;  
         [0088]      FIG. 30  is a block diagram for explaining a target air/fuel ratio computing unit in the second embodiment;  
         [0089]      FIG. 31  is a block diagram for explaining a unit for detecting the oxygen storage capacity after the light-off in the second embodiment;  
         [0090]      FIG. 32  is a block diagram for explaining a response delay time computing unit in the second embodiment;  
         [0091]      FIG. 33  is a block diagram for explaining an oxygen storage capacity computing unit in the second embodiment;  
         [0092]      FIG. 34  is a block diagram showing a control system in a third embodiment;  
         [0093]      FIG. 35  is a block diagram for explaining a light-off temperature estimating unit in the third embodiment;  
         [0094]      FIG. 36  is a block diagram showing a control system in a fourth embodiment;  
         [0095]      FIG. 37  is a block diagram for explaining a deterioration diagnosis permission determining unit in the fourth embodiment;  
         [0096]      FIG. 38  is a block diagram for explaining a light-off temperature estimating unit in the fourth embodiment; and  
         [0097]      FIG. 39  is a block diagram for explaining an ignition timing setting unit in the fourth embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0098]     Embodiments of the present invention will be described below with reference to the drawings.  
       FIRST EMBODIMENT  
       [0099]      FIG. 18  is a schematic view showing an engine self-diagnosis system according to a first embodiment of the present invention, along with one example of a vehicle-loaded engine to which the self-diagnosis system is applied.  
         [0100]     An engine  10  shown in  FIG. 18  is a multi-cylinder engine having four cylinders, for example, and comprises cylinders  12  and pistons  15  slidably inserted in the cylinders  12  numbered # 1 , # 2 , # 3  and # 4 . A combustion chamber  17  is defined above the piston  15 . An ignition plug  35  is disposed to face the combustion chamber  17 .  
         [0101]     Air supplied for combustion of fuel is taken in from an air cleaner  21  disposed at a start end of an intake passage  20  and enters a collector  56  through an airflow sensor  24  and an electronically-controlled throttle valve  25 . Then, the intake air is introduced from the collector  56  to the combustion chamber  17  of each cylinder numbered # 1 , # 2 , # 3  or # 4  through an intake valve  28  that is disposed at a downstream end of the intake passage  20  (i.e., at an intake port). Further, a fuel injection valve  30  is disposed at the downstream end of the intake passage  20 .  
         [0102]     A gas mixture of the air introduced to the combustion chamber  17  and fuel injected from the fuel injection valve  30  is ignited by the ignition plug  35  and is burnt for explosion. Combustion waste gas (exhaust gas) is discharged through an exhaust valve  48  from the combustion chamber  17  to each individual passage portion  40 A forming an upstream part of an exhaust passage  40 . Then, the exhaust gas flows from the individual passage portion  40 A into a three-way catalyst  50 , which is disposed in the exhaust passage  40 , through an exhaust collecting portion  40 B. After cleaning by the three-way catalyst  50 , the exhaust gas is discharged to the exterior.  
         [0103]     An O 2  sensor  51  is disposed in the exhaust passage  40  downstream of the three-way catalyst  50 , and an A/F (air/fuel ratio) sensor  52  is disposed in the exhaust passage  40  upstream of the three-way catalyst  50  at a position near the exhaust collecting portion  40 B.  
         [0104]     The A/F sensor  52  has a linear output characteristic for the concentration of oxygen contained in the exhaust gas. Because the relationship between the concentration of oxygen in the exhaust gas and the air/fuel ratio is substantially linear, the air/fuel ratio in the exhaust collecting portion  40 B can be determined based on a signal from the A/F sensor  52  for detecting the oxygen concentration. Also, based on a signal from the O 2  sensor  51 , it is possible to detect the oxygen concentration downstream of the three-way catalyst  50 , or whether the exhaust gas is rich or lean with respect to the stoichiometric air/fuel ratio.  
         [0105]     Further, a part of the exhaust gas discharged from the combustion chamber  17  to the exhaust passage  40  is introduced to the intake passage  20  through an EGR passage  41 , as required, for circulation to the combustion chamber  17  of each cylinder through a branched passage portion of the intake passage  20 . An EGR valve  42  for adjusting the EGR rate is disposed in the EGR passage  41 .  
         [0106]     A self-diagnosis system  1  of this embodiment comprises a control unit  100  with a microcomputer incorporated therein for executing various kinds of control of the engine  10 .  
         [0107]     The control unit  100  basically comprises, as shown in  FIG. 19 , a CPU  101 , an input circuit  102 , input/output ports  103 , a RAM  104 , a ROM  105 , etc.  
         [0108]     The control unit  100  is supplied with, as input signals, a signal detected by the airflow sensor  24  and corresponding to the intake air amount, a signal detected by a throttle sensor  34  and corresponding to the opening degree of the throttle valve  25 , a signal detected by a crank angle sensor  37  and indicating the rotation (engine rotation speed)/phase of a crankshaft  18 , a signal detected by the O 2  sensor  51  disposed in the exhaust passage  40  downstream of the three-way catalyst  50  and corresponding to the oxygen concentration in the exhaust gas, a signal detected by the A/F sensor  52  disposed in the exhaust collecting portion  40 B of the exhaust passage  40  upstream of the three-way catalyst  50  and corresponding to the oxygen concentration (air/fuel ratio), a signal detected by a water temperature sensor  19  disposed in the cylinder  12  and corresponding to the temperature of the engine cooling water, a signal detected by an accelerator sensor  36  and corresponding to the amount of depression of an accelerator pedal  39  (which represents a torque demanded by a driver), and a signal detected by a vehicle speed sensor  29  and corresponding to the vehicle speed of an automobile in which the engine  10  is mounted.  
         [0109]     In the control unit  100 , when the signals outputted from the various sensors, such as the A/F sensor  52 , the O 2  sensor  51 , the throttle sensor  34 , the airflow sensor  24 , the crank angle sensor  37 , the water temperature sensor  16  and the accelerator sensor  36 , are inputted, those signals are subjected to signal processing, e.g., removal of noise, in the input circuit  102  and then sent to the input/output ports  103 . Respective values at the input ports are stored in the RAM  104  and are subjected to arithmetic/logical operations in the CPU  101 . A control program describing the contents of the arithmetic/logical operations is written in the ROM  105  beforehand. Values computed in accordance with the control program and representing strokes of various actuators to be operated are stored in the RAM  104  and are sent to the output ports  103 .  
         [0110]     A signal for operating the ignition plug  35  is set as an on/off signal such that it is turned on when a current is supplied to a primary coil in an ignition output circuit  116  and turned off when a current is not supplied to the primary coil. The ignition timing is defined as a time at which the signal is shifted from the on- to off-state. The signal for operating the ignition plug  35 , which has been set at the output port  103 , is amplified by an ignition output circuit  116  to a level of energy sufficient for ignition and is then supplied to the ignition plug  35 . Also, a signal for driving the fuel injection valve  30  (i.e., an air/fuel ratio control signal) is set as an on/off signal such that it is turned on when the fuel injection valve  30  is opened and turned off when it is closed. The driving signal is amplified by a fuel injection valve driving circuit  117  to a level of energy sufficient for opening the fuel injection valve  30  and is then supplied to the fuel injection valve  30 . A driving signal for realizing the target opening degree of the electronically-controlled throttle valve  25  is sent to the electronically-controlled throttle valve  25  through an electronically-controlled throttle valve driving circuit  118 .  
         [0111]     The control unit  100  computes the air/fuel ratio upstream of the three-way catalyst  50  based on the signal from the A/F sensor  52 , and also computes, based on the signal from the O 2  sensor  51 , the oxygen concentration downstream of the three-way catalyst  50 , or whether the exhaust gas is rich or lean with respect to the stoichiometric air/fuel ratio. Further, by using the outputs of both the sensors  51 ,  52 , the control unit  100  executes feedback control for sequentially modifying the fuel injection amount or the intake air amount so that the cleaning efficiency of the three-way catalyst  50  is optimized.  
         [0112]     Performance diagnosis of the three-way catalyst  50  executed by the control unit  100  will be described in more detail below.  
         [0113]      FIG. 20  is a functional block diagram showing a control system in the first embodiment. As shown in the functional block diagram, the control unit  100  comprises a basic fuel injection amount computing unit  110 , an air/fuel ratio modification term computing unit  120 , a deterioration diagnosis permission determining unit  130 , a catalyst characteristic A (after-light-off oxygen storage capacity) detecting unit  140 , and a catalyst characteristic B (light-off temperature) estimating unit  150 .  
         [0114]     In an ordinary mode, the control unit  100  computes a fuel injection amount Ti for each of the cylinders # 1 -# 4  based on a basic fuel injection amount Tp and an air/fuel ratio modification term Lalpha so that air/fuel ratios of all the cylinders are held at the stoichiometric air/fuel ratio. Then, when deterioration diagnosis is permitted, the control unit  100  oscillates the target air/fuel ratio at a predetermined frequency and estimates the after-light-off oxygen storage capacity (catalyst performance A) of the three-way catalyst  50  in accordance with predetermined frequency components of respective output signals from the A/F sensor  52  and the O 2  sensor  51 . Then, based on the detection result, the control unit  100  estimates the light-off temperature.  
         [0115]     Each of the processing units will be described in more detail below.  
         [0000]     &lt;Basic Fuel Injection Amount Computing Unit  110  ( FIG. 21 )&gt; 
         [0116]     This computing unit  110  computes, based on the engine intake air amount, the fuel injection amount for realizing the target torque and the target air/fuel ratio at the same time under arbitrary operating conditions. Specifically, a basic fuel injection amount Tp is computed as shown in  FIG. 21 . In  FIG. 21 , K is a constant having a value for making adjustment such that the stoichiometric air/fuel ratio is always realized with respect to the intake air amount. Also, Cyl represents the number of engine cylinders.  
         [0000]     &lt;Deterioration Diagnosis Permission Determining Unit  130  ( FIG. 22 )&gt; 
         [0117]     This permission determining unit  130  determines whether the deterioration diagnosis of the three-way catalyst  50  is permitted.  
         [0118]     Specifically, as shown in  FIG. 22 , when Twn≧Twndag, NedagH≧Ne≧NedagL, QadagH≧Qa≧QadagL, ΔNe≦DNedag, ΔQa≦Dqadag, and Tcat≧Tcatdag are all satisfied, a deterioration diagnosis permission flag Fpdag=1 is set to permit the deterioration diagnosis. Otherwise, Fpdag=0 is set to inhibit the deterioration diagnosis.  
         [0119]     In  FIG. 22 , Twn is the engine cooling water temperature, Ne is the engine rotation speed, Qa is the intake air amount, ΔNe is the change rate of the engine rotation speed, ΔQa is the change rate of the intake air amount, and Tcat is the estimated catalyst temperature.  
         [0120]     ΔNe and ΔQa can be each given as the difference between a value computed in the preceding job and a value computed in the current job. Also, because the catalyst temperature depends on the temperature of the exhaust gas flowing into the catalyst and the temperature of the exhaust gas depends on the intake air amount Qa (fuel injection amount), etc., the catalyst temperature can be estimated based on Twn, Qa, an integrated value of Qa, etc. Further details are omitted here for the reason that various methods have already been proposed and are described in many books, papers, etc. Tcatdag is preferably set to a temperature at which the three-way catalyst  50  is in the light-off state at a sufficient level.  
         [0000]     &lt;Air/Fuel Ratio Modification Term Computing Unit  120  ( FIG. 23 )&gt; 
         [0121]     This computing unit  120  executes F/B (feedback) control based on the air/fuel ratio detected by the A/F sensor  52  so that the air/fuel ratio at an inlet of the three-way catalyst  50  is held at the target air/fuel ratio under arbitrary operating conditions. Specifically, as shown in  FIG. 23 , an air/fuel ratio modification term Lalpha is computed with PI control from a deviation Dltabf between a target air/fuel ratio Tabf set by a target air/fuel ratio computing unit  121  and an air/fuel ratio Rabf detected by the A/F sensor. The air/fuel ratio modification term Lalpha is multiplied by the basic fuel injection amount Tp.  
         [0000]     &lt;Target Air/Fuel Ratio Computing Unit  121  (Frequency Response) ( FIG. 24 )&gt; 
         [0122]     This computing unit  121  computes the target air/fuel ratio in a frequency response manner. Specifically, this computation is executed as shown in  FIG. 24 . When Fpdag=1 holds, a target air/fuel ratio Tabf 1 L and a target air/fuel ratio Tab 0  are switched over at a frequency fa [Hz]. Otherwise, an ordinary target air/fuel ratio Tabf 0  is set. In this embodiment, Tabf 0  is a value corresponding to the stoichiometric air/fuel ratio, Tabf 1 R is a value shifted from the stoichiometric air/fuel ratio toward the rich side by a predetermined value, and Tabf 1 L is a value shifted from the stoichiometric air/fuel ratio toward the lean side by a predetermined value. The values of Tabf 1 R(L) and fa are preferably decided based on experiments from the viewpoints of diagnosis accuracy and exhaust performance (emission characteristics).  
         [0000]     &lt;After-Light-Off Oxygen Storage capacity Detecting Unit  140  (Frequency Response) ( FIG. 25 )&gt; 
         [0123]     This detecting unit  140  detects the oxygen storage capacity after the light-off. Specifically, this detection is executed as shown in  FIG. 25 . This detecting unit  140  comprises a frequency component computing unit  141  for computing respective frequency components of an output Rabf of the A/F sensor  52  and an output RVO 2  of the O 2  sensor  51 , and an oxygen storage capacity computing unit  142  for computing the oxygen storage capacity of the three-way catalyst  50  based on the computed frequency components.  
         [0124]     The frequency component computing unit  141  and the oxygen storage capacity computing unit  142  will be described below.  
         [0000]     &lt;Frequency Component Computing Unit  141  ( FIG. 26 )&gt; 
         [0125]     This computing unit  141  computes respective frequency components of the output Rabf of the A/F sensor  52  and the output RVO 2  of the O 2  sensor  51 . Specifically, as shown in  FIG. 26 , powers (Power 1  and Power 2 ) and phases (Phase 1  and Phase 2 ) at the frequency fa [Hz] are computed from both signals Rabf and RVO 2  with processes using DFT (Discrete Fourier Transform).  
         [0000]     &lt;Oxygen Storage capacity Computing Unit  142  ( FIG. 27 )&gt; 
         [0126]     This computing unit  142  computes the oxygen storage capacity of the three-way catalyst  50 . Specifically, as shown in  FIG. 27 , an after-light-off performance deterioration index Ind 13 det 0  is obtained by referring to a map with (Phase 2 -Phase 1 ) and (Power 2 /Power 1 ) being parameters. The map used in obtaining Ind 13 det 0  is preferably decided based on experiments from the relationship between the oxygen storage capacity of the three-way catalyst  50  and the exhaust performance. Also, in the state of (Phase 2 -Phase 1 )≧(predetermined value A) and (Power 2 /Power 1 )≧(predetermined value B), this is determined as indicating that the oxygen storage capacity (catalyst performance) has deteriorated to a limit, whereupon an after-light-off performance deterioration flag Fdet 0 =1 is set. Note that the predetermined value A and the predetermined value B representing the deterioration limit are decided depending on the target exhaust performance (diagnosis performance).  
         [0000]     &lt;Light-Off Temperature Estimating Unit  150  ( FIG. 28 )&gt; 
         [0127]     This computing unit  150  computes (estimates) the light-off temperature of the three-way catalyst  50 . Specifically, as shown in  FIG. 28 , a (estimated) light-off temperature T 0  is obtained, for example, by using a map with the after-light-off performance deterioration index Ind_det 0  being a parameter. The map used in obtaining T 0  is preferably decided based on experiment results shown in  FIG. 17 , by way of example, from the relationship between a deterioration amount of the oxygen storage capacity after the light-off and a change (rise) amount of the light-off temperature. As an alternative, T 0  may be estimated using, e.g., a catalyst model. Also, in the state of T 0 ≧(predetermined value C) or the after-light-off performance deterioration flag Fdet 0 =1, this is determined as indicating that the three-way catalyst  50  has exceeded its performance limit, whereupon a deterioration indicator lamp illuminating flag Fdet=1 is set, for example, to illuminate a deterioration indicator lamp  27  for providing an indication to the exterior. Note that the predetermined value C representing the deterioration limit (in light-off performance) of the three-way catalyst  50  is decided depending on the target exhaust performance (diagnosis performance).  
         [0128]     As understood from the above description, with the self-diagnosis system  10  of this embodiment, the target air/fuel ratio is oscillated at the predetermined frequency, and the after-light-off oxygen storage capacity (catalyst performance A) of the three-way catalyst  50  is detected in accordance with the predetermined frequency components of the output signals from the A/F sensor  52  and the O 2  sensor  51 . The light-off temperature (catalyst performance B) is then estimated based on the detection result. Therefore, the light-off performance of the catalyst can be diagnosed at a low cost and high accuracy without requiring addition or improvement of a sensor, etc.  
       SECOND EMBODIMENT  
       [0129]      FIG. 29  is a functional block diagram showing a control system in a second embodiment. As shown in the functional block diagram, a control unit  100  similar to that in the first embodiment comprises a basic fuel injection amount computing unit  110 , an air/fuel ratio modification term computing unit  120  including a target air/fuel ratio computing unit  221 , a deterioration diagnosis permission determining unit  130 , (the units  110  and  130  being the same as those in the first embodiment), a catalyst characteristic A (after-light-off oxygen storage capacity) detecting unit  240 , and a catalyst characteristic B (light-off temperature) estimating unit  250 .  
         [0130]     In an ordinary mode, the control unit  100  computes a fuel injection amount Ti per cylinder based on a basic fuel injection amount Tp and an air/fuel ratio modification term Lalpha so that air/fuel ratios of all the cylinders are held at the stoichiometric air/fuel ratio. While that process is the same as that in the first embodiment, this second embodiment differs in the following point. When the deterioration diagnosis is permitted, the air/fuel ratio is shifted from the stoichiometric air/fuel ratio by a predetermined value for a predetermined time, and the after-light-off oxygen storage capacity (catalyst performance A) of the three-way catalyst  50  is detected in accordance with a response delay time between respective output signals from the A/F sensor  52  and the O 2  sensor  51 . Then, based on the detection result, the control unit  100  estimates the light-off temperature (catalyst characteristic B).  
         [0131]     The units  221 ,  240  and  250  executing processing in a different manner from that in the first embodiment will be described in more detail below.  
         [0000]     &lt;Target Air/Fuel Ratio Computing Unit  221  (step response) ( FIG. 30 )&gt; 
         [0132]     This computing unit  221  is substituted for the target air/fuel ratio computing unit  121  (see  FIG. 24 ) included in the air/fuel ratio modification term computing unit  120  (see  FIG. 23 ) in the first embodiment. Specifically, the target air/fuel ratio computing unit  221  executes the processing shown in  FIG. 30 . When Fpdag=1 holds, the target air/fuel ratio is set to a diagnosis-mode target air/fuel ratio Tabf 1 . Otherwise, an ordinary target air/fuel ratio Tabf 0  is set. More specifically, a response delay time occurs from a time at which the output of the A/F sensor  52  has reached a level corresponding to Tabf 1  to a time at which the output of the O 2  sensor  51  has reached a level corresponding to Tabf 1 . This response delay time depends on the oxygen storage (release) performance of the three-way catalyst  50 . In this embodiment, Tabf 0  is a value corresponding to the stoichiometric air/fuel ratio, and Tabf 1  is a value shifted from the stoichiometric air/fuel ratio toward the lean side by a predetermined value. The value of Tabf 1  is preferably decided based on experiments from the viewpoints of diagnosis accuracy and exhaust performance.  
         [0000]     &lt;After-Light-Off Oxygen Storage capacity Detecting Unit  240  (Step Response) ( FIG. 31 )&gt; 
         [0133]     This detecting unit  240  detects the oxygen storage capacity after the light-off. Specifically, as shown in  FIG. 31 , this detecting unit  240  comprises a response delay time computing unit  241  for computing the response delay time from an output Rabf of the A/F sensor  52  to an output RVO 2  of the O 2  sensor  51 , and an oxygen storage capacity computing unit  242  for computing the oxygen storage capacity of the three-way catalyst  50  based on the computed response delay time.  
         [0134]     The response delay time computing unit  241  and the oxygen storage capacity computing unit  242  will be described in more detail below.  
         [0000]     &lt;Response Delay Time Computing Unit  241  ( FIG. 32 )&gt; 
         [0135]     This computing unit  241  computes the response delay time from the output Rabf of the A/F sensor  52  to the output RVO 2  of the O 2  sensor  51 . Specifically, as shown in  FIG. 32 , when Fpdag=1 holds and the target air/fuel ratio computing unit  221  sets the diagnosis-mode target air/fuel ratio Tabf 1 , a response delay time T_det is given as a period from a time at which Rabf≧Tabf 1 —K_Tabf 1  is met to a time at which RVO 2 ≦KRVO 2  is met.  
         [0000]     &lt;Oxygen Storage capacity Computing Unit  242  ( FIG. 33 )&gt; 
         [0136]     This computing unit  242  computes the oxygen storage capacity of the three-way catalyst  50 . Specifically, as shown in  FIG. 33 , an after-light-off performance deterioration index Ind_det 0  is obtained by referring to a map with the response delay time T_det and the intake air amount Qa being parameters. The map used in obtaining Ind_det 0  is preferably decided based on experiments from the relationship between the oxygen storage capacity of the three-way catalyst  50  and the exhaust performance. Also, in the state of Ind_det 0 ≧Ind_det_NG, this is determined as indicating that the oxygen storage capacity (catalyst performance) has deteriorated to a limit, whereupon an after-light-off performance deterioration flag Fdet 0 =1 is set. Note that Ind_det_NG representing the deterioration limit is decided depending on the target exhaust performance (diagnosis performance).  
         [0000]     &lt;Light-Off Temperature Estimating Unit  250 &gt; 
         [0137]     This estimating unit  250  is substantially the same as the estimating unit  150  in the first embodiment, and therefore a detailed description thereof is omitted here.  
       THIRD EMBODIMENT  
       [0138]      FIG. 34  is a functional block diagram showing a control system in a third embodiment. As shown in the functional block diagram, a control unit  100  similar to that in the first and second embodiments comprises a basic fuel injection amount computing unit  110 , an air/fuel ratio modification term computing unit  120 , a deterioration diagnosis permission determining unit  130 , (these three units being the same as those in the first embodiment), a catalyst characteristic A (after-light-off exhaust cleaning capacity) detecting unit  340 , and a catalyst characteristic B (light-off temperature) estimating unit  350 . In this third embodiment, a NOx sensor  53  is disposed downstream of the three-way catalyst  50  instead of the O 2  sensor. An output signal from the NOx sensor  53  is also supplied to the control unit  100 .  
         [0139]     In an ordinary mode, the control unit  100  computes a fuel injection amount Ti per cylinder based on a basic fuel injection amount Tp and an air/fuel ratio modification term Lalpha so that air/fuel ratios of all the cylinders are held at the stoichiometric air/fuel ratio. While that process is the same as that in the first embodiment, this third embodiment differs in the following point. When the deterioration diagnosis is permitted, the target air/fuel ratio is oscillated at a predetermined frequency, and the after-light-off exhaust cleaning capacity (catalyst performance A) of the three-way catalyst  50  is detected in accordance with an output signals of the NOx sensor  53  at that time. Then, based on the detection result, the control unit  100  estimates the light-off temperature (catalyst characteristic B).  
         [0140]     The units  340  and  350  executing processing in a different manner from that in the first and second embodiments will be described in more detail below.  
         [0000]     &lt;After-Light-Off Exhaust Cleaning Capacity Detecting Unit  340  ( FIG. 35 )&gt; 
         [0141]     This detecting unit  340  detects the exhaust cleaning capacity after the light-off. Specifically, the detection is executed as shown in  FIG. 35 . An after-light-off performance deterioration index Ind_det 0  is obtained by referring to a map with an output value RNOx of the NOx sensor  53  and an intake air amount Qa being parameters. The map used in obtaining Ind_det 0  is preferably decided based on experiments from the NOx cleaning capacity of the three-way catalyst  50 . Also, in the state of Ind_det 0 ≧Ind_det_NG, this is determined as indicating that the exhaust cleaning capacity (catalyst performance) has deteriorated to a limit, whereupon an after-light-off performance deterioration flag Fdet 0 =1 is set. Note that Ind_det_NG representing the deterioration limit is decided depending on the target exhaust performance (diagnosis performance).  
         [0000]     &lt;Light-Off Temperature Estimating Unit  350  ( FIG. 34 )&gt; 
         [0142]     This estimating unit  350  is substantially the same as the estimating units in the first and second embodiments, and therefore a detailed description thereof is omitted here.  
         [0143]     While this third embodiment employs the NOx sensor, similar processing to that described above can also be executed by using, for example, an HC sensor or a CO sensor.  
       FOURTH EMBODIMENT  
       [0144]      FIG. 36  is a functional block diagram showing a control system in a fourth embodiment. As shown in the functional block diagram, a control unit  100  similar to that in the first through third embodiments comprises a basic fuel injection amount computing unit  110 , an air/fuel ratio modification term computing unit  120 , a deterioration diagnosis permission determining unit  430 , a catalyst characteristic A (after-light-off oxygen storage capacity) detecting unit  440 , a catalyst characteristic B (light-off temperature) estimating unit  450 , and an ignition timing setting unit  160  for, based on the estimated light-off temperature, setting a retard amount of ignition timing at the startup and a period during which the ignition timing is retarded. In this fourth embodiment, the deterioration diagnosis permission determining unit  430  and the catalyst characteristic B (light-off temperature) estimating unit  450  are constituted respectively as shown in  FIGS. 37 and 38 . The ignition timing setting unit  160 , which is not disposed in the above-described embodiments, is constituted as follows.  
         [0000]     &lt;Ignition Timing Setting Unit  160  ( FIG. 39 )&gt; 
         [0145]     This setting unit  160  sets the ignition timing. Specifically, the setting is executed as shown in  FIG. 39 . Basic ignition timing ADVO is decided based on Tp (basic fuel injection amount) and Ne (engine rotation speed). When the estimated catalyst temperature does not reach the light-off temperature, i.e., in the state of Tcat≦T 0 , a value obtained by referring to a map with the light-off temperature T 0  being a parameter is set as a retard amount ADVRTD of the ignition timing. Then, a value obtained by subtracting the retard amount ADVRTD of the ignition timing from the basic ignition timing ADVO is set as ignition timing ADV.  
         [0146]     While the embodiments have been described above in connection with the case using the three-way catalyst, the present invention is not limited to the three-way catalyst so long as a catalyst has the three-way performance, and the present invention is also applicable to the cases using an HC adsorbing combustion catalyst, a lean NOx catalyst, etc. In particular, the present invention can be advantageously applied to the case using the HC adsorbing combustion catalyst because the light-off temperature is a very important factor in deciding the performance of that catalyst.