JP2010185371A - Catalyst deterioration diagnostic device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve measuring accuracy of oxygen storage capacity by excluding deterioration influence of a sensor behind catalyst. <P>SOLUTION: Active air-fuel ratio control for switching an air-fuel ratio on the upstream side of a catalyst between rich/lean is executed in response to overturn of output Vr of the sensor behind catalyst which detects an air-fuel ratio on the downstream side of the catalyst. The oxygen storage capacity of the catalyst is measured in every overturn period of the output Vr of the sensor behind catalyst. The oxygen storage capacity is measured during periods t11 to t12 in which the output Vr of the sensor behind catalyst is normal in the overturn periods. The overturn periods of the output of the sensor behind catalyst are excluded from the measurement periods to exclude the deterioration influence of the sensor behind catalyst. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an apparatus for diagnosing deterioration of a catalyst, and more particularly to an apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine.

For example, in an internal combustion engine for a vehicle, a catalyst for purifying exhaust gas is installed in the exhaust system. Some of these catalysts have an oxygen storage capacity (O ₂ storage capacity). This is because when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes larger than the stoichiometric air-fuel ratio (stoichiometric), that is, when it becomes lean, Excess oxygen present in the gas is adsorbed and held, and when the air-fuel ratio of the catalyst inflow exhaust gas becomes smaller than the stoichiometric, that is, when it becomes rich, the adsorbed and held oxygen is released. For example, in a gasoline engine, air-fuel ratio control is performed so that the exhaust gas flowing into the catalyst is in the vicinity of stoichiometry. However, when a three-way catalyst having oxygen storage capacity is used, the actual air-fuel ratio slightly deviates from stoichiometry depending on operating conditions. However, the air-fuel ratio shift can be absorbed by the oxygen storage / release action of the three-way catalyst.

  By the way, when the catalyst deteriorates, the purification efficiency of the catalyst decreases. On the other hand, there is a correlation between the degree of deterioration of the catalyst and the degree of reduction of the oxygen storage capacity because they are reactions through noble metals. Therefore, it is possible to detect that the catalyst has deteriorated by detecting that the oxygen storage capacity has decreased. In general, active air-fuel ratio control that switches the air-fuel ratio upstream of the catalyst alternately between rich and lean is performed, and the oxygen storage capacity of the catalyst is measured along with the execution of this active air-fuel ratio control to diagnose catalyst deterioration. (A so-called Cmax method) is employed (see, for example, Patent Document 1).

JP-A-5-133264

  In the Cmax method, a post-catalyst sensor for detecting the air-fuel ratio of the exhaust gas is provided on the downstream side of the catalyst. In response to the output of the post-catalyst sensor being inverted, the air-fuel ratio on the upstream side of the catalyst is rich and Alternately switched to lean. Then, the oxygen storage capacity is measured every inversion period of the post-catalyst sensor output, and the deterioration of the catalyst is determined based on this measured value.

  By the way, as the catalyst deteriorates, the post-catalyst sensor usually deteriorates as well. When the post-catalyst sensor deteriorates, its responsiveness deteriorates. In particular, the output change at the time of output reversal becomes slow, and the deterioration degree of the catalyst may not be detected accurately. That is, the true catalyst deterioration degree is detected by adding the catalyst deterioration degree corresponding to the post-catalyst sensor deterioration, and the true catalyst deterioration degree may not be detected accurately.

  Accordingly, an object of the present invention is to provide a catalyst deterioration diagnosis device that can improve the measurement accuracy of the oxygen storage capacity by eliminating the influence of deterioration of the post-catalyst sensor.

According to one aspect of the invention,
An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A post-catalyst sensor for detecting an air-fuel ratio of exhaust gas downstream of the catalyst;
Active air-fuel ratio control means for executing active air-fuel ratio control for alternately switching the air-fuel ratio upstream of the catalyst between rich and lean in response to the output of the post-catalyst sensor being inverted;
Measuring means for measuring the oxygen storage capacity of the catalyst for each inversion period of the post-catalyst sensor output, and measuring the oxygen storage capacity during a period in which the post-catalyst sensor output is steady within the inversion period; ,
There is provided a catalyst deterioration diagnosis device characterized by comprising:

  According to this, the period during which the post-catalyst sensor output is inverted can be excluded from the measurement period, and the influence of the deterioration of the response of the post-catalyst sensor can be eliminated. Therefore, the influence of deterioration of the post-catalyst sensor can be eliminated, and the measurement accuracy of the oxygen storage capacity can be improved.

  Preferably, the measurement unit determines whether or not the post-catalyst sensor output is steady based on a differential value of the post-catalyst sensor output. In this case, preferably, the measurement unit determines that the post-catalyst sensor output is steady when the differential value is within a predetermined range including zero for a predetermined time.

  When the post-catalyst sensor output becomes steady, the differential value becomes a value near zero. Therefore, it is possible to suitably determine whether or not the post-catalyst sensor output is steady by using the differential value.

  Alternatively, the measurement unit determines whether or not the post-catalyst sensor output is steady based on a second-order differential value obtained by further differentiating the differential value of the post-catalyst sensor output. In this case, preferably, the measurement means determines that the post-catalyst sensor output is steady when the second-order differential value is within a predetermined range including zero for a predetermined time.

  When the post-catalyst sensor output becomes steady, the second-order differential value also becomes a value near zero. Therefore, it can be suitably determined whether the post-catalyst sensor output is steady by using the second-order differential value. In particular, when the second-order differential value is used, the post-catalyst sensor output is almost stable, but is not constant, and it is possible to determine the steady state even when slowly changing gradually.

  Alternatively, the measuring means determines whether the post-catalyst sensor output is steady based on the differential value of the post-catalyst sensor output and a second-order differential value obtained by further differentiating the differential value. judge. In this case, preferably, the measuring means has a first predetermined time in which the differential value is within a first predetermined range including zero, and the second order differential value is in a second predetermined time including zero. 2 is within a predetermined range, it is determined that the post-catalyst sensor output is steady.

  According to the present invention, an excellent effect that the influence of deterioration of the post-catalyst sensor can be eliminated and the measurement accuracy of the oxygen storage capacity can be improved is exhibited.

It is the schematic which shows the structure of embodiment of this invention. It is a schematic sectional drawing which shows the structure of a catalyst. It is a time chart for demonstrating the content of active air fuel ratio control. FIG. 4 is a time chart similar to FIG. 3 for illustrating a method for measuring the oxygen storage capacity. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a time chart for demonstrating the oxygen storage capacity measuring method of this embodiment. It is a figure which shows transition of the oxygen storage capacity measurement value in a normal Cmax method. It is a figure which shows transition of the oxygen storage capacity measured value in this embodiment. It is a time chart for demonstrating the 1st aspect of stationary determination. It is a flowchart which shows the routine of oxygen storage capacity measurement processing. It is a time chart for demonstrating the 2nd aspect of stationary determination. It is a time chart for demonstrating the 3rd aspect of stationary determination.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the configuration of the present embodiment. As shown in the drawing, the internal combustion engine 1 generates power by burning a fuel / air mixture in a combustion chamber 3 in a cylinder block 2 and reciprocating a piston 4 in the combustion chamber 3. The internal combustion engine 1 is a vehicular multi-cylinder engine (only one cylinder is shown), and is a spark ignition type internal combustion engine, more specifically, a gasoline engine.

  The cylinder head of the internal combustion engine 1 is provided with an intake valve Vi that opens and closes an intake port and an exhaust valve Ve that opens and closes an exhaust port for each cylinder. Each intake valve Vi and each exhaust valve Ve are opened and closed by a camshaft (not shown). A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

  The intake port of each cylinder is connected to a surge tank 8 serving as an intake air collecting chamber via a branch pipe for each cylinder. An intake pipe 13 that forms an intake manifold passage is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. In the intake pipe 13, an air flow meter 5 for detecting the intake air amount (the amount of air flowing into the internal combustion engine) and an electronically controlled throttle valve 10 are incorporated in order from the upstream side. An intake passage is formed by the intake port, the surge tank 8 and the intake pipe 13.

  An injector (fuel injection valve) 12 that injects fuel into the intake passage, particularly into the intake port, is provided for each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture. The air-fuel mixture is sucked into the combustion chamber 3 when the intake valve Vi is opened, compressed by the piston 4, and ignited and burned by the spark plug 7. It is done.

On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 6 forming an exhaust collecting passage through a branch pipe for each cylinder. These exhaust ports, branch pipes and exhaust pipe 6 form an exhaust passage. The exhaust pipe 6 is provided with a catalyst composed of a three-way catalyst having oxygen storage capacity, that is, an upstream catalyst 11 and a downstream catalyst 19 in series on the upstream side and the downstream side. An air-fuel ratio sensor, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 for detecting the air-fuel ratio of the exhaust gas are provided on the upstream side and downstream side of the upstream catalyst 11 or at positions immediately before and immediately after. The pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, can continuously detect an air-fuel ratio over a relatively wide range, and outputs a signal having a value proportional to the air-fuel ratio. On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor and has a characteristic (Z characteristic) in which the output value changes suddenly with the theoretical air-fuel ratio as a boundary. The output characteristics of the pre-catalyst sensor 17 and the post-catalyst sensor 18 are shown in FIG.

  The spark plug 7, the throttle valve 10, the injector 12, and the like described above are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 14 that detects the crank angle of the internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. The opening sensor 15 and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the throttle valve 10, the injector 12, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel injection timing, throttle opening. Control the degree etc.

  The ECU 20 normally feeds back the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 so that the air-fuel ratio detected by the pre-catalyst sensor 17, that is, the pre-catalyst air-fuel ratio A / Ff matches the target air-fuel ratio A / Ft. Control. On the other hand, the catalysts 11 and 19 simultaneously purify NOx, HC and CO simultaneously with high efficiency when the air-fuel ratio of the exhaust gas flowing into the catalyst 11 and 19 is the stoichiometric air-fuel ratio (stoichiometric, for example, A / Fs = 14.6). Therefore, the ECU 20 sets the target air-fuel ratio A / Ft equal to the stoichiometric air-fuel ratio during normal operation of the internal combustion engine so that the pre-catalyst air-fuel ratio A / Ff detected by the pre-catalyst sensor 17 matches the stoichiometric air-fuel ratio. Feedback control is performed on the amount of fuel injected from the injector 12. As a result, the air-fuel ratio of the exhaust gas flowing into the catalyst 11 is kept in the vicinity of the theoretical air-fuel ratio, and the maximum purification performance is exhibited in the catalyst 11.

Here, the upstream catalyst 11 to be subjected to deterioration diagnosis will be described in more detail. The downstream catalyst 19 is configured in the same manner as the upstream catalyst 11. As shown in FIG. 2, in the catalyst 11, a coating material 31 is coated on the surface of a carrier base material (not shown), and the coating material 31 is held in a state in which a large number of particulate catalyst components 32 are dispersedly arranged. The catalyst 11 is exposed inside. The catalyst component 32 is mainly composed of a noble metal such as Pt or Pd, and serves as an active point for reacting exhaust gas components such as NOx, HC and CO. On the other hand, the coating material 31 plays the role of a promoter that promotes the reaction at the interface between the exhaust gas and the catalyst component 32 and includes an oxygen storage component capable of absorbing and releasing oxygen according to the air-fuel ratio of the atmospheric gas. The oxygen storage component is made of, for example, cerium dioxide CeO ₂ or zirconia. For example, if the atmosphere gas of the catalyst component 32 and the coating material 31 is richer than the stoichiometric air-fuel ratio, the oxygen stored in the oxygen storage component present around the catalyst component 32 is released, and as a result, the released oxygen As a result, unburned components such as HC and CO are oxidized and purified. On the contrary, when the atmosphere gas of the catalyst component 32 and the coating material 31 is leaner than the stoichiometric air-fuel ratio, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmosphere gas, and as a result, NOx is reduced and purified. The

  By such an oxygen absorption / release action, three exhaust gas components such as NOx, HC and CO are simultaneously purified even if the pre-catalyst air-fuel ratio A / Ff slightly varies from the theoretical air-fuel ratio during normal air-fuel ratio control. can do. Therefore, in normal air-fuel ratio control, it is also possible to purify exhaust gas by causing the pre-catalyst air-fuel ratio A / Ff to oscillate minutely around the theoretical air-fuel ratio and repeat the oxygen absorption and release.

  By the way, in the catalyst 11 in the new state, as described above, a large number of fine particulate catalyst components 32 are uniformly distributed, and the contact probability between the exhaust gas and the catalyst component 32 is kept high. However, when the catalyst 11 deteriorates, some of the catalyst components 32 are lost, and some of the catalyst components 32 are baked and solidified by exhaust heat (see broken lines in the figure). In this case, the contact probability between the exhaust gas and the catalyst component 32 is lowered, and the purification rate is lowered. In addition to this, the amount of the coating material 31 existing around the catalyst component 32, that is, the amount of the oxygen storage component decreases, and the oxygen storage capacity itself decreases.

Thus, the degree of deterioration of the catalyst 11 and the degree of decrease in the oxygen storage capacity of the catalyst 11 are in a correlation. Therefore, in the present embodiment, the deterioration degree of the upstream catalyst 11 is detected by detecting the oxygen storage capacity of the upstream catalyst 11 that has a particularly large influence on the emission. Here, the oxygen storage capacity of the catalyst 11 is represented by the size of the oxygen storage capacity (OSC; O ₂ Storage Capacity, the unit is g), which is the maximum amount of oxygen that the current catalyst 11 can store.

  The catalyst deterioration diagnosis of the present embodiment is basically based on the Cmax method described above. When the deterioration diagnosis of the catalyst 11 is performed, the active air-fuel ratio control is executed by the ECU 20. In the active air-fuel ratio control, the air-fuel ratio upstream of the catalyst 11, that is, the air-fuel ratio of the air-fuel mixture in the combustion chamber 3 and thus the air-fuel ratio of the exhaust gas supplied to the catalyst 11 borders a predetermined center air-fuel ratio A / Fc. Active (forced) and alternating between rich and lean. The air-fuel ratio when switched to the rich side is referred to as rich air-fuel ratio A / Fr, and the air-fuel ratio when switched to the lean side is referred to as lean air-fuel ratio A / Fl.

  The deterioration diagnosis of the catalyst 11 is executed during steady operation of the internal combustion engine 1 and when the catalyst 11 is in the active temperature range. Measurement of the temperature of the catalyst 11 (catalyst bed temperature) may be detected directly using a temperature sensor, but in the present embodiment, it is estimated from the operating state of the internal combustion engine. For example, the ECU 20 estimates the temperature Tc of the catalyst 11 using a preset map based on the intake air amount Ga detected by the air flow meter 5. It should be noted that parameters other than the intake air amount Ga, for example, the engine rotational speed Ne (rpm) may be included in the parameters used for the catalyst temperature estimation.

  Hereinafter, a method for measuring the oxygen storage capacity of the upstream catalyst 11 by the normal Cmax method will be described with reference to FIGS. 3 and 4. In FIGS. 3A and 3B, the output behavior of the pre-catalyst sensor 17 and the post-catalyst sensor 18 when the active air-fuel ratio control is executed is indicated by a solid line. In FIG. 3A, the target air-fuel ratio A / Ft generated inside the ECU 20 is indicated by a broken line. The output value of the pre-catalyst sensor 17 shown in FIG. 3A is a value converted to the pre-catalyst air-fuel ratio A / Ff. The output value of the post-catalyst sensor 18 shown in FIG. 3B is the output value itself, that is, the value of the output voltage Vr.

  As shown in FIG. 3A, the target air-fuel ratio A / Ft is centered on the theoretical air-fuel ratio A / Fs as the center air-fuel ratio, and then has a predetermined amplitude (rich amplitude Ar, Ar> 0) on the rich side. ) Separated by an air-fuel ratio (rich air-fuel ratio A / Fr) and an air-fuel ratio (lean air-fuel ratio A / Fl) separated from the air-fuel ratio by a predetermined amplitude (lean amplitude Al, Al> 0) on the lean side. And alternately. Following the switching of the target air-fuel ratio A / Ft, the pre-catalyst air-fuel ratio A / Ff as an actual value is also switched with a slight time delay with respect to the target air-fuel ratio A / Ft. From this, it will be understood that the target air-fuel ratio A / Ft and the pre-catalyst air-fuel ratio A / Ff are equivalent except that there is a time delay.

  In the illustrated example, the rich amplitude Ar and the lean amplitude Al are equal. For example, rich air-fuel ratio A / Fr = 14.1, lean air-fuel ratio A / Fl = 15.1, rich amplitude Ar = lean amplitude Al = 0.5. Compared with the normal air-fuel ratio control, the active air-fuel ratio control has a larger amplitude of the air-fuel ratio, that is, the values of the rich amplitude Ar and the lean amplitude Al are larger.

  The target air-fuel ratio A / Ft is switched in response to the output of the post-catalyst sensor 18 being inverted. As shown in FIG. 5, the output voltage Vr of the post-catalyst sensor 18 changes suddenly with the theoretical air-fuel ratio A / Fs as a boundary. In order to determine the inversion timing of the output voltage Vr, that is, the timing at which the output voltage Vr is inverted from lean to rich or from rich to lean, two inversion thresholds VR and VL relating to the output voltage Vr are determined in advance. Yes. Here, VR is referred to as a rich determination value, and VL is referred to as a lean determination value. VR> VL, for example, VR = 0.59 (V) and VL = 0.21 (V). When the output voltage Vr changes to the lean side, that is, decreases and reaches the lean determination value VL, the output voltage Vr is considered to have been reversed to the lean side, and the post-catalyst air-fuel ratio A / Fr detected by the post-catalyst sensor 18 Is at least leaner than the stoichiometric air-fuel ratio. On the other hand, when the output voltage Vr changes to the rich side, that is, increases and reaches the rich determination value VR, it is considered that the output voltage Vr is reversed to the rich side, and the post-catalyst air-fuel ratio A / Fr is at least greater than the stoichiometric air-fuel ratio. Judged to be rich. The stoichiometric air-fuel ratio is included in a narrow region Y between the air-fuel ratios corresponding to the rich determination value VR and the lean determination value VL (this is referred to as a transition region). The output voltage Vr can only detect whether the post-catalyst air-fuel ratio A / Fr is richer or leaner than the stoichiometric air-fuel ratio, and it is difficult to detect the absolute value of the post-catalyst air-fuel ratio A / Fr.

  As shown in FIGS. 3A and 3B, when the output voltage Vr of the post-catalyst sensor 18 changes from the rich side value to the lean side and becomes equal to the lean determination value VL (time t1), the target The air-fuel ratio A / Ft is switched from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr. Thereafter, when the output voltage Vr of the post-catalyst sensor 18 changes from the lean side value to the rich side and becomes equal to the rich determination value VR (time t2), the target air-fuel ratio A / Ft is changed from the rich air-fuel ratio A / Fr. The lean air-fuel ratio A / Fl is switched. In this way, the air-fuel ratio is actively controlled to be rich or lean at the same time that the output of the post-catalyst sensor 18 is reversed to lean or rich.

  While performing this active air-fuel ratio control, in the normal Cmax method, the oxygen storage capacity OSC of the catalyst 11 is measured as follows, and deterioration of the catalyst 11 is determined.

  Referring to FIG. 3, the target air-fuel ratio A / Ft is set to the lean air-fuel ratio A / Fl before time t1, and the lean gas flows into the catalyst 11. At this time, the catalyst 11 continues to absorb oxygen, but when it fully absorbs oxygen, it can no longer absorb oxygen, and the lean gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Fr changes to the lean side, and when the output voltage of the post-catalyst sensor 18 reaches the lean determination value VL (t1), the target air-fuel ratio A / Ft becomes the rich air-fuel ratio A / Fr. Can be switched to.

  This time, rich gas flows into the catalyst 11. At this time, the oxygen stored in the catalyst 11 continues to be released from the catalyst 11. Therefore, the exhaust gas of the theoretical air-fuel ratio A / Fs flows out to the downstream side of the catalyst 11 and the post-catalyst air-fuel ratio A / Fr does not become rich, so the output of the post-catalyst sensor 18 is not reversed. When oxygen is continuously released from the catalyst 11, all of the stored oxygen is eventually released from the catalyst 11, and at that time, no more oxygen can be released, and the rich gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Fr changes to the rich side, and when the output voltage of the post-catalyst sensor 18 reaches the rich determination value VR (t2), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / Fl. Can be switched to.

  The larger the oxygen storage capacity of the catalyst 11, the longer the time during which oxygen can be absorbed or released. That is, when the catalyst has not deteriorated, the inversion period of the post-catalyst sensor output Vr (for example, the time from t1 to t2) becomes longer, and the inversion period becomes shorter as the deterioration of the catalyst proceeds.

  Therefore, using this, the oxygen storage capacity OSC is measured as follows. As shown in FIG. 4, immediately after the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t1, the pre-catalyst air-fuel ratio A / Ff as an actual value is slightly delayed with the rich air-fuel ratio A / Ff. Switch to Fr. From the time t11 when the pre-catalyst air-fuel ratio A / Ff reaches the stoichiometric air-fuel ratio A / Fs to the time t2 when the post-catalyst sensor output Vr next reverses, the oxygen at every predetermined minute time is obtained by the following equation (1). The storage capacity dOSC is calculated, and the oxygen storage capacity dOSC for each minute time is integrated from time t11 to time t2. Thus, in one reversal period t1 to t2 during the rich control, the oxygen storage capacity OSC (in this case, the amount of released oxygen indicated by OSC1 in FIG. 4) is measured as the final integrated value.

  Here, Q is a fuel injection amount. When the air-fuel ratio difference ΔA / F is multiplied by the fuel injection amount Q, an air amount that is insufficient or excessive with respect to the stoichiometry can be calculated. K is a constant representing the proportion of oxygen contained in air (about 0.23).

  Similarly, the oxygen storage capacity (in this case, the stored oxygen amount indicated by OSC2 in FIG. 4) is measured even during the lean control in which the target air-fuel ratio A / Ft is lean. The target air-fuel ratio A / Ft is alternately switched between rich and lean, and the oxygen storage capacity is measured every time rich control and lean control are alternately performed. If a plurality of oxygen storage capacity measurement values are obtained in this way, the average value OSCav is calculated.

  As shown in FIG. 4, the measurement of the oxygen storage capacity during lean control is performed after the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio A / Fl at time t2, and then the pre-catalyst air-fuel ratio A / Ff. From time t21 when the air / fuel ratio reaches the stoichiometric air / fuel ratio A / Fs to time t3 when the target air / fuel ratio A / Ft reverses to the rich side next time, the oxygen storage capacity dOSC for each minute time is calculated by the previous equation (1), The oxygen storage capacity dOSC for each minute time is integrated. The final integrated value is the value of the oxygen storage capacity measured in the inversion period during the lean control. Ideally, the measured value of the oxygen storage capacity is approximately the same when releasing oxygen and storing oxygen.

  Next, the deterioration of the catalyst is determined based on the average value OSCav of the oxygen storage capacity measurement values. That is, the average value OSCav is compared with the predetermined deterioration determination value OSCs. If the average value OSCav is larger than the deterioration determination value OSCs, the catalyst is determined to be normal, and if the average value OSCav is equal to or less than the deterioration determination value OSCs, the catalyst is determined to be deteriorated. When it is determined that the catalyst is deteriorated, it is preferable to activate a warning device such as a check lamp in order to notify the user of the fact.

  As described above, when the catalyst 11 deteriorates from a new state, the post-catalyst sensor 18 also normally deteriorates in the same manner. When the post-catalyst sensor 18 deteriorates, its responsiveness deteriorates. In particular, the output change at the time of output reversal becomes slow, and the degree of deterioration of the catalyst may not be detected accurately.

  In other words, in the case of the post-catalyst sensor 18 whose responsiveness has deteriorated, the output change speed at the time of inversion immediately before the end of the rich control or lean control becomes slow, and accordingly, the cumulative time of the oxygen storage capacity dOSC for every minute time becomes long. . Therefore, a larger oxygen storage capacity value is measured than in the case of a sensor whose responsiveness has not deteriorated. That is, the true catalyst deterioration degree is detected by adding the catalyst deterioration degree corresponding to the post-catalyst sensor deterioration. This leads to a decrease in measurement accuracy, which may cause a misdiagnosis to erroneously determine that the deteriorated catalyst is normal.

  In addition, it is thought that the output fall by deterioration of the post-catalyst sensor 18 is also one cause for the slowing of the output change speed at the time of output reversal. Further, when the catalyst 11 deteriorates, there is a tendency that a so-called blow-through occurs in which the reaction rate of the catalyst cannot catch up with the supply gas amount immediately before the end of the rich control (oxygen release), which is also considered as one cause.

  Therefore, in this embodiment, in order to cope with such a problem, the above-described normal Cmax method is improved particularly in the measurement part, and the oxygen sensor is output during a period when the post-catalyst sensor output is steady within the inversion period of the post-catalyst sensor output. The storage capacity is to be measured. This will be described below.

  As shown in FIG. 6, the target air-fuel ratio A / Ft is switched at every timing t1, t2, t3, t4 at which the post-catalyst sensor output Vr is reversed. Here, the one reversal cycle refers to a period from when the post-catalyst sensor output Vr is reversed once to the next reversal, for example, a period from t1 to t2. Within one reversal period, the measurement of the oxygen storage capacity OSC, that is, the integration of the oxygen storage capacity dOSC at every minute time, shows that the post-catalyst sensor output Vr is steady (or stable) as indicated by the white arrow. ) It is executed during the periods t11 to t12, t21 to t22, t31 to t32. In addition, (C) shows the integrated value of the stored oxygen amount for reference.

  According to this method, the post-catalyst sensor output Vr reverse period (for example, t12 to t2) and the output fluctuation period immediately after the reverse (for example, t2 to t21) can be excluded from the oxygen storage capacity measurement period. The effects of 18 responsiveness deterioration and output reduction can be eliminated. Thereby, the influence by deterioration of the post-catalyst sensor 18 can be eliminated, and the measurement accuracy of the oxygen storage capacity can be improved. In addition, misdiagnosis can be prevented.

  According to this method, the measurement period is shorter than in the normal Cmax method, and a small value of the oxygen storage capacity OSC is measured. However, there is no particular problem because there is a relative difference in the measured value of the oxygen storage capacity between when the catalyst is normal and when it is deteriorated. Rather, as a result of eliminating the deterioration effect of the post-catalyst sensor 18, the relative difference in the oxygen storage capacity measurement value between the normal time and the deterioration time becomes large, and the diagnostic accuracy may be improved.

  7 and 8 show the transition of the measured value of the oxygen storage capacity in the normal Cmax method and this embodiment. (A) shows the case of a new catalyst, (B) shows the case of a normal deterioration catalyst that is deteriorated but still considered normal, and (C) shows a normal deterioration catalyst. This shows a case of an abnormally deteriorated catalyst that is further deteriorated and must be determined as deteriorated. Region A in the figure shows the amount corresponding to the net oxygen storage capacity of the catalyst among the measured values, and region B shows the deterioration influence of the post-catalyst sensor 18 among the measured values.

  As shown in FIG. 7, in the case of the normal Cmax method, the degradation effect of the post-catalyst sensor 18 is included in the measured value. This sensor degradation impact is initially small, but gradually increases as the catalyst and sensor degrade. It becomes larger and gradually occupies a larger proportion of the measured values. In other words, as the catalyst and sensor deteriorate, the measurement value error due to the sensor deterioration influence gradually increases. On the other hand, in the catalyst deterioration diagnosis, it is necessary to accurately distinguish between the normal deterioration catalyst (B) and the abnormal deterioration catalyst (C).

  On the other hand, as shown in FIG. 8, in the case of the present embodiment, since the degradation influence of the post-catalyst sensor 18 is not included in the measured value, the net oxygen storage capacity is always measured. In this case, the relative difference C2 between the measured values of the normal deterioration catalyst (B) and the abnormal deterioration catalyst (C) is larger than the relative difference C1 in the case of the normal Cmax method shown in FIG. Rather, there is a possibility that the normal deterioration catalyst and the abnormal deterioration catalyst can be easily distinguished. This is one reason that diagnostic accuracy may be improved.

  In addition, since the oxygen storage capacity measurement value is smaller than that of the normal Cmax method, the deterioration determination value OSCs that is a threshold value for deterioration determination needs to be smaller than that of the normal Cmax method. The value itself is adapted to a suitable value through experiments and the like.

  By the way, in order to measure the oxygen storage capacity during the period when the post-catalyst sensor output Vr is steady, it is necessary to determine whether or not the post-catalyst sensor output Vr is steady. Therefore, in the present embodiment, the steady determination is performed based on any of the following first to third aspects.

  The first mode of steady state determination uses a differential value Vr ′ of the post-catalyst sensor output Vr. As shown in FIG. 9, this focuses on the characteristic that when the post-catalyst sensor output Vr becomes steady, its differential value Vr 'becomes a value near zero.

As shown, ECU 20 is always calculating the differential value Vr post-catalyst sensor output Vr ', the post-catalyst sensor output at a certain time (time) 1 calculation cycle before the post-catalyst sensor output Vr _n in (previous) Vr _n-1 is subtracted and the difference is set as a differential value Vr ′ _n at a certain point in time (current time).

  When the differential value Vr ′ is within a predetermined range including a predetermined time Δt and zero, specifically, when the differential value Vr ′ is within a range of −α ≦ Vr ′ ≦ + α (where α is a small positive predetermined value). Then, it is determined that the post-catalyst sensor output Vr is steady.

  As shown in FIG. 9, the steady periods based on this determination method are the periods t11 to t12, t21 to t22, and t31 to t32 in the figure, and the oxygen storage capacity is measured in each of these steady periods. Specifically, for example, in the steady period from t11 to t12, when the ECU 20 determines that the differential value Vr ′ is within the range of the predetermined time Δt and −α ≦ Vr ′ ≦ + α, −α ≦ Vr ′ ≦ Time t11 that first enters the range of + α is determined as the start of the stationary period, and measurement or integration is started retroactively to time t11 (ie, by a predetermined time Δt). At this time, the ECU 20 uses data such as the pre-catalyst air-fuel ratio A / Ff stored in advance in the buffer. The reason for waiting for the elapse of the predetermined time Δt is that the post-catalyst sensor output Vr fluctuates immediately after the reverse of the post-catalyst sensor output Vr, and the differential value Vr ′ is indicated by P, temporarily or instantaneously −α ≦ This is in order to avoid determining that this time is a steady period in the range of Vr ′ ≦ + α. Thus, the measurement or integration is executed until the end of the steady period, that is, until the time point t12 when the differential value Vr ′ first falls outside the range of −α ≦ Vr ′ ≦ + α.

  FIG. 10 shows a routine of the oxygen storage capacity measurement process according to the first aspect. The illustrated routine is repeatedly executed by the ECU 20 every predetermined calculation cycle (for example, 16 msec).

  First, in step S101, it is determined whether a predetermined precondition suitable for execution of diagnosis or measurement is satisfied. For example, the engine is in a steady operation state such that the fluctuation range of the intake air amount Ga detected by the air flow meter 5 and the engine rotational speed Ne calculated based on the output of the crank angle sensor 14 is within a predetermined range. If the upstream catalyst 11 and the sensors 17 and 18 are in the active state, the precondition is satisfied. Note that the precondition is not limited to the example described here.

  If the precondition is not satisfied, the process proceeds to step S108, and normal air-fuel ratio control is executed. That is, the target air-fuel ratio A / Ft is set to stoichiometric, for example, and the air-fuel ratio is feedback-controlled to stoichiometric.

  On the other hand, if the precondition is satisfied, the process proceeds to step S102, and active air-fuel ratio control is executed.

  Next, in step S103, it is determined whether or not the post-catalyst sensor output Vr is reversed to rich or lean. If reversed, the routine proceeds to step S104 where the target air-fuel ratio A / Ft is switched to lean or rich, and the air-fuel ratio is controlled to lean or rich. Then, the process proceeds to step S105.

  On the other hand, if not reversed, step S104 is skipped and the process proceeds directly to step S105.

  In step S105, it is determined whether or not the post-catalyst sensor output Vr is steady by the above-described method. That is, when it is determined that the differential value Vr ′ is within the range of the predetermined time Δt and −α ≦ Vr ′ ≦ + α, it is determined that the post-catalyst sensor output Vr is steady. It is determined that the sensor output Vr is not steady.

  If it is determined that it is steady, the process proceeds to step S106, and measurement or integration of the oxygen storage capacity OSC is executed. As described above, this measurement or integration is started retroactively when the differential value Vr ′ first enters the range of −α ≦ Vr ′ ≦ + α. On the other hand, if it is determined that it is not steady, the routine proceeds to step S107, where the measured value or integrated value of the oxygen storage capacity OSC is initialized, that is, made zero. This is the end of the routine.

  Thereby, a plurality of oxygen storage capacities OSC are measured during the rich control and the lean control. Thereafter, the ECU 20 calculates an average value OSCav of the plurality of oxygen storage capacity measurement values as in the normal Cmax method, and compares the average value OSCav with a predetermined deterioration determination value OSCs. If the average value OSCav is greater than the deterioration determination value OSCs, the catalyst is determined to be normal, and if the average value OSCav is less than or equal to the deterioration determination value OSCs, the catalyst is determined to be deteriorated.

  Note that methods other than the method based on the average value OSCav are also possible. For example, the amount of released oxygen and the amount of stored oxygen are separately measured during rich control and lean control, and these are updated for each measurement to obtain the average value of the final update values. A method of comparing with a predetermined deterioration determination value is possible.

  Next, a second mode of steady determination will be described. This second mode uses a second-order differential value Vr ″ obtained by further differentiating the differential value Vr ′ of the post-catalyst sensor output Vr. As shown in FIG. 11, this is when the post-catalyst sensor output Vr becomes steady. The second-order differential value Vr ″ is focused on the characteristic that it becomes a value near zero.

As shown, ECU 20 is the differential value Vr post-catalyst sensor output Vr 'and, second-order differential value Vr "and is constantly calculating the differential value Vr at a certain point in time (time)' 1 calculation cycle before the _n The (previous) differential value Vr ′ _n−1 is subtracted and the difference is defined as a second-order differential value Vr ″ _n at a certain point in time (current time).

  When the second derivative value Vr ″ is within a predetermined range including a predetermined time Δt and zero, the ECU 20 specifically falls within the range of −γ ≦ Vr ″ ≦ + γ (where γ is a minute positive predetermined value). When there is, it is determined that the post-catalyst sensor output Vr is steady.

  As shown in FIG. 11, the steady periods based on this determination method are the periods t11 to t12, t21 to t22, and t31 to t32 in the figure, and the oxygen storage capacity is measured in each of these steady periods. For example, for the steady period from t11 to t12, when the ECU 20 determines that the second-order differential value Vr ″ is within a predetermined time Δt and −γ ≦ Vr ″ ≦ + γ, the range of −γ ≦ Vr ″ ≦ + γ. The time point t11 that first enters the period is set as the beginning of the stationary period, and measurement or integration is started retroactively to the time point t11 (that is, retroactively by the predetermined time Δt). The reason for waiting for the elapse of the predetermined time Δt is that the second-order differential value Vr ″ is temporarily or instantaneously immediately after inversion of the post-catalyst sensor output Vr, as described above. This is to avoid determining that the stationary period is in the range of −γ ≦ Vr ″ ≦ + γ. In this way, the second-order differential value Vr ″ is the first to be −γ ≦ Vr ″ ≦ Time t1 out of the range of + γ 2, ie, until the end of the stationary period.

  Even with the method based on the second-order differential value Vr ″, it can be suitably determined whether or not the post-catalyst sensor output Vr is steady.

  Further, according to the method based on the second-order differential value Vr ″, it is possible to determine the steady state even when the post-catalyst sensor output Vr is almost stable but is not constant and slowly changes gradually. This point will be described in the third aspect below.

  Also in the second mode, it is possible to perform the oxygen storage capacity measurement process by the routine shown in FIG. In this case, the second-order differential value Vr ″ or the like is used in place of the differential value Vr ′ or the like described in the first mode at the time of steady determination in step S105.

  Next, a third aspect of steady determination will be described. In the third mode, both the differential value Vr ′ and the second-order differential value Vr ″ of the post-catalyst sensor output Vr are used.

  That is, as shown in FIG. 12, the ECU 20 constantly calculates the differential value Vr ′ of the post-catalyst sensor output Vr and the second-order differential value Vr ″. The ECU 20 has the differential value Vr ′ of the first value. When the predetermined time Δt1 is within the first predetermined range including zero and the second-order differential value Vr ″ is within the second predetermined range including the second predetermined time Δt2 and zero, the post-catalyst sensor output Vr Is determined to be steady. Here, the first predetermined range is defined by −β ≦ Vr ′ ≦ + β (where β is a minute positive predetermined value), and the second predetermined range is defined by −γ ≦ Vr ″ ≦ + γ.

  In particular, the predetermined value β that defines the first predetermined range is larger than the predetermined value α of the first aspect, and the range is expanded. The predetermined value β is preferably larger than a predetermined value γ that defines a second predetermined range of the second-order differential value Vr ″, that is, the first predetermined range is preferably larger than the second predetermined range.

  As shown in FIG. 12, the steady periods based on this determination method are the periods t101 to t102 (or t12), t201 to t202, and t301 to t302 in the figure, and the oxygen storage capacity is measured in each of these steady periods. Is done. For example, regarding the steady period from t101 to t102, the differential value Vr ′ is already within the first predetermined range at t11 before t101, but the second-order differential value Vr ″ is not yet within the second predetermined range at this time. Therefore, the steady period (particularly the start period) is set at the time t101 when the second-order differential value Vr ″ enters the second predetermined range, and calculation or integration is started from this time t101. In this case as well, the ECU 20 starts measuring or integrating retroactively to t101 when it is determined that the latter second-order differential value Vr ″ is in the range of the second predetermined time Δt2, −γ ≦ Vr ″ ≦ + γ. To do. Of course, when the differential value Vr ′ enters the predetermined range after the second-order differential value Vr ″, the order is reversed.

  This measurement or integration is executed until time t102 when at least one of the differential value Vr ′ and the second-order differential value Vr ″ is out of the predetermined range. In the illustrated example, both are out of the predetermined range at t12 or t102 at the same time. At this point, the steady period ends, and the calculation or integration ends.

  Also in the third aspect, the oxygen storage capacity measurement process can be performed by the routine shown in FIG. In this case, the differential value Vr ′, the second-order differential value Vr ″, and the like are used in place of the differential value Vr ′ and the like described in the first mode at the time of steady determination in step S105.

  Even with a method based on both the differential value Vr ′ and the second-order differential value Vr ″, it can be suitably determined whether or not the post-catalyst sensor output Vr is steady.

  As described above, when the second-order differential value Vr ″ is used, it is possible to determine the steady state even when the post-catalyst sensor output Vr is almost stable but not constant and slowly changes.

  That is, when the amount of intake air is large, the exhaust gas flow rate increases, and the reaction rate of the catalyst may not catch up slightly, resulting in a slight blow-through state. In this case, as indicated by Q in FIG. 12 (A), after the post-catalyst sensor output Vr is reversed, the minute change is continued slowly. Then, as indicated by R in FIG. 12B, the differential value Vr ′ does not fall within the predetermined range −α ≦ Vr ′ ≦ + α as described in the first aspect, and in a method based only on the differential value Vr ′. It can happen that measurement is impossible.

  However, when the second-order differential value Vr ″ is used, even if the post-catalyst sensor output Vr gradually changes, if the differential value Vr ′ is steady, the second-order differential value Vr ″ is within a predetermined range −γ ≦ Vr ″ ≦ + γ. In this way, it is possible to stably measure and to improve the diagnosis frequency.

  Further, in such a gradual change case, as indicated by R in FIG. 12B, the differential value Vr ′ is in a steady state in a range outside the predetermined range −α ≦ Vr ′ ≦ + α of the first mode. Sometimes. Therefore, in this third mode, in order to enable measurement in this case as well, a predetermined range −β ≦ Vr ′ ≦ + β wider than that in the first mode is set.

  In this aspect using both the differential value Vr ′ and the second-order differential value Vr ″, there is a possibility that the accuracy of steady determination can be improved. That is, if the responsiveness of the post-catalyst sensor 18 is deteriorated, the sensor Although the sensor output is not steady at the time of output reversal, it is conceivable that the second-order differential value Vr ″ may have more opportunities to enter the second predetermined range. In this case, if there is only the second-order differential value Vr ″, it may be erroneously determined to be steady, but there is a possibility that such erroneous determination can be suppressed by combining the differential value Vr ′.

  Although the embodiment of the present invention has been described in detail above, various other embodiments of the present invention are conceivable. For example, the use and form of the internal combustion engine are arbitrary, and may be other than for vehicles, for example, a direct injection type or the like. In the above embodiment, the steady determination is made based on at least one of the differential value and the second-order differential value of the post-catalyst sensor output, but other methods are possible. For example, the steady determination may be made based on the post-catalyst sensor output itself, or may be determined to be steady when the post-catalyst sensor output is within a predetermined range for a predetermined time.

  The predetermined range of the differential value or the second-order differential value is set symmetrically around zero in the above-described embodiment (for example, a range of ± α around zero), but can also be set asymmetrically.

  The present invention includes all modifications, applications, and equivalents included in the spirit of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 Internal combustion engine 5 Air flow meter 6 Exhaust pipe 11 Upstream catalyst 12 Injector 17 Pre-catalyst sensor 18 Post-catalyst sensor 19 Downstream catalyst 20 Electronic control unit (ECU)
OSC Oxygen storage capacity Vr Post-catalyst sensor output Vr ′ differential value Vr ”second order differential value Δt predetermined time Δt1 first predetermined time Δt2 second predetermined time α predetermined value β predetermined value γ predetermined value

Claims (7)

An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A post-catalyst sensor for detecting an air-fuel ratio of exhaust gas downstream of the catalyst;
Active air-fuel ratio control means for executing active air-fuel ratio control for alternately switching the air-fuel ratio upstream of the catalyst between rich and lean in response to the output of the post-catalyst sensor being inverted;
Measuring means for measuring the oxygen storage capacity of the catalyst for each inversion period of the post-catalyst sensor output, and measuring the oxygen storage capacity during a period in which the post-catalyst sensor output is steady within the inversion period; ,
A catalyst deterioration diagnosis device comprising: The catalyst deterioration diagnosis apparatus according to claim 1, wherein the measurement unit determines whether or not the post-catalyst sensor output is steady based on a differential value of the post-catalyst sensor output. The catalyst deterioration diagnosis according to claim 2, wherein the measurement means determines that the post-catalyst sensor output is steady when the differential value is within a predetermined range including zero for a predetermined time. apparatus. 2. The measuring means determines whether or not the post-catalyst sensor output is steady based on a second-order differential value obtained by further differentiating the differential value of the post-catalyst sensor output. The catalyst deterioration diagnostic apparatus according to 1. The catalyst according to claim 4, wherein the measurement unit determines that the post-catalyst sensor output is steady when the second-order differential value is within a predetermined range including zero for a predetermined time. Deterioration diagnostic device. The measuring means determines whether the post-catalyst sensor output is steady based on a differential value of the post-catalyst sensor output and a second-order differential value obtained by further differentiating the differential value. The catalyst deterioration diagnosis apparatus according to claim 1, wherein The measurement means has the differential value within a first predetermined range including zero for a first predetermined time, and the second differential value within a second predetermined range including zero for a second predetermined time. When it is, it determines with the said post-catalyst sensor output being steady. The catalyst deterioration diagnostic apparatus of Claim 6 characterized by the above-mentioned.

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