JP2009036172A - Catalyst-degradation diagnostic system for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst-degradation diagnostic system for an internal combustion engine which performs an accurate and reliable diagnosis irrespective of sulfur concentration in a fuel. <P>SOLUTION: An air-fuel ratio of an inflow gas into the catalyst is switched to a lean/rich state at a specified timing. In the case of a lean air-fuel ratio, an oxygen storage capacity OSAb is measured, and in the case of a rich air-fuel ratio, an oxygen discharge capacity OSAa is measured. On the basis of the measurement, an oxygen storage capacity OSC of the catalyst is measured. A measured OSC is compared with a specified degradation-determination value to make a determination on degradation of the catalyst. When a specified condition is established, the timing for switching from rich to lean side is delayed (Δtd), and the stored oxygen capacity is measured again so as to correct the preceding OSC measurement value on the basis of a difference between the preceding and present oxygen storage capacities. Since before making degradation-diagnosis, the OSC measurement value is corrected to a value equivalent to that in use of low-sulfur fuel, in this diagnostic system, permanent degradation discriminated from temporary degradation due to sulfer influence is diagnosed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates 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, the exhaust gas 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, 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 the stoichiometric. However, such an 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 for forcibly switching the air-fuel ratio of the exhaust gas flowing into the catalyst to rich and lean is performed, and the oxygen storage capacity of the catalyst is measured as the active air-fuel ratio control is executed. A method (so-called Cmax method) for diagnosing deterioration of the ink is employed.

JP 2006-291773 A

  On the other hand, sulfur (S) may be contained in the fuel at a relatively high concentration depending on the region of use. When such fuel is supplied, poisoning (S poisoning) occurs in which sulfur components accumulate in the catalyst and the performance of the catalyst decreases. When S poisoning occurs, the oxygen storage / release reaction of the catalyst is hindered and the oxygen storage capacity of the catalyst decreases. However, if the fuel with a low sulfur concentration is refueled, the poisoning state will eventually be resolved. The performance degradation of the catalyst due to S poisoning is temporary. Therefore, in the deterioration diagnosis of the catalyst, it is necessary not to mistakenly diagnose the temporary deterioration due to the S poisoning as permanent deterioration that should be diagnosed. In particular, it is necessary to prevent a catalyst that is still normal while being in the vicinity of the boundary between normality and deterioration (criteria) from being erroneously diagnosed as deterioration.

  In the Cmax method, the air-fuel ratio is normally switched to rich or lean every time the air-fuel ratio sensor downstream of the catalyst is reversed. The amount of oxygen stored in the catalyst is measured when the air-fuel ratio is lean, and the amount of oxygen released from the catalyst is measured when the air-fuel ratio is rich. The average value of the stored oxygen amount and the released oxygen amount is calculated as the value of the oxygen storage capacity of the catalyst, and the deterioration of the catalyst is determined by comparing the value of this oxygen storage capacity with a predetermined deterioration determination value.

  By the way, when a high-sulfur concentration fuel is used, the value of the released oxygen amount particularly when the air-fuel ratio is rich is lowered, and thus the value of the oxygen storage capacity is lowered. The reason is as follows. Oxygen release from the catalyst is carried out through the noble metal of the catalyst, and this released oxygen also reacts with the rich components (HC, CO) in the exhaust gas through the noble metal of the catalyst to purify the rich components. To do. However, the activity of the noble metal is reduced by sulfur, and the reaction rate between the released oxygen and the rich component is reduced. As a result, although the amount of gas passing through the catalyst exceeds the reaction rate and oxygen still remains in the catalyst, the rich component flows out downstream of the catalyst in an unpurified state, and the air-fuel ratio sensor downstream of the catalyst It reverses and the measured value of the amount of released oxygen decreases.

  On the other hand, it has been found that the value of the stored oxygen amount when the air-fuel ratio is lean is not easily affected by sulfur in the fuel and is difficult to decrease. Therefore, by utilizing this fact, the oxygen release ability when the gas entering the catalyst is made rich is not used for detection of deterioration, but the deterioration detection is performed only by the oxygen storage capacity when the gas entering the catalyst is made lean. (See Patent Document 1).

  However, as in this technique, the method of using only the value of the stored oxygen amount at the time of lean air-fuel ratio without using the value of the released oxygen amount at the time of rich air-fuel ratio is one value from the beginning of the two available values. It is abandoned and diagnosed with only the remaining one value, which is not ideal from the viewpoint of reliability and accuracy. For example, when the stoichiometric equivalent value of the air-fuel ratio sensor is shifted to the rich side or the lean side, accurate diagnosis cannot be performed with only one value. Use of these binary values is preferable in terms of reliability and accuracy. In any case, a device that can perform highly accurate and reliable deterioration diagnosis is desired.

  Therefore, the present invention has been made in view of such circumstances, and an object thereof is to provide a catalyst deterioration diagnosis device for an internal combustion engine capable of highly accurate and reliable diagnosis regardless of the sulfur concentration in the fuel. There is to do.

According to the first aspect of the present invention,
An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
Active air-fuel ratio control means for performing active air-fuel ratio control for alternately switching the air-fuel ratio of the exhaust gas flowing into the catalyst to a lean side and a rich side at a predetermined timing;
Measuring means for measuring the oxygen storage capacity of the catalyst in accordance with execution of the active air-fuel ratio control, measuring the amount of oxygen stored in the catalyst when the air-fuel ratio is on the lean side, Measuring means for measuring the amount of oxygen released from the catalyst when the air-fuel ratio is on the rich side, and calculating the oxygen storage capacity of the catalyst based on the stored oxygen amount and the measured value of the released oxygen amount; ,
Determination means for determining deterioration of the catalyst by comparing the oxygen storage capacity measured by the measurement means with a predetermined deterioration determination value;
With
When the predetermined condition is satisfied, the timing at which at least the air-fuel ratio in the active air-fuel ratio control is switched from the rich side to the lean side is delayed from the predetermined timing, and again, the active air-fuel ratio control and at least the stored oxygen amount And the previous oxygen storage capacity measurement value is corrected based on the difference between the previous stored oxygen amount measurement value and the current stored oxygen amount measurement value, and the corrected previous oxygen storage capacity measurement value is obtained. A catalyst deterioration diagnosis apparatus for an internal combustion engine is provided, wherein the deterioration of the catalyst is determined based on the determination.

  According to this, when the predetermined condition is satisfied, the timing for switching the air-fuel ratio in the active air-fuel ratio control from the rich side to the lean side is forcibly delayed, that is, rich delay is executed. Then, the active air-fuel ratio control and the measurement of the stored oxygen amount are executed again. When there is an influence of sulfur, more oxygen is released from the catalyst by execution of rich delay, and the amount of stored oxygen this time measured during lean control immediately after rich delay is increased accordingly. This difference between the measured value of the stored oxygen amount and the previous measured value of the stored oxygen amount reflects the influence of sulfur in the fuel. Therefore, based on this difference, the previous measured value of the stored oxygen amount is corrected to a value equivalent to when the low sulfur concentration fuel is used. And deterioration determination is performed based on the value after correction | amendment which removed this sulfur influence. As in the normal Cmax method, the oxygen storage capacity is basically measured using binary values of the released oxygen amount and the stored oxygen amount. Moreover, since the deterioration diagnosis is performed after correcting the value of the oxygen storage capacity once measured to a value equivalent to that when the low sulfur concentration fuel is used, the influence of sulfur can be removed, and the permanent deterioration can be diagnosed separately from the temporary deterioration. As a result, highly accurate and reliable diagnosis can be performed.

According to a second aspect of the present invention, in the first aspect,
As the difference between the previous stored oxygen amount measurement value and the current stored oxygen amount measurement value is larger, the correction amount for correcting the previous oxygen storage capacity measurement value is increased.

  Even when the air-fuel ratio switching timing is delayed, that is, the delay time is the same, the difference between the previous stored oxygen amount measured value and the current stored oxygen amount measured value increases as the sulfur concentration in the fuel increases. Further, the higher the sulfur concentration in the fuel, the greater the amount of decrease in the oxygen storage capacity measurement value. Therefore, in order to return the previous oxygen storage capacity measurement value to the level equivalent to when low-sulfur concentration fuel is used, the larger the difference between the previous and current stored oxygen amount measurement values, the larger the correction amount, as in the second embodiment. It is preferable to do this.

According to a third aspect of the present invention, in the first or second aspect,
The case where the predetermined condition is satisfied is a case where the previous oxygen storage capacity measurement value is equal to or less than the deterioration determination value.

  What is important in the deterioration diagnosis is that a normal catalyst near the criteria is not erroneously diagnosed as being deteriorated. Therefore, as in the third embodiment, if the previous oxygen storage capacity measurement value is equal to or lower than the deterioration determination value, that is, only when the catalyst is apparently deteriorated, it is necessary and necessary to perform the measurement again. It is possible to perform the measurement again with a sufficient minimum frequency.

According to a fourth aspect of the present invention, in any one of the first to third aspects,
A pre-catalyst sensor that detects the air-fuel ratio of exhaust gas upstream of the catalyst; and a post-catalyst sensor that detects the air-fuel ratio of exhaust gas downstream of the catalyst;
The active air-fuel ratio control means performs air-fuel ratio control so that the pre-catalyst air-fuel ratio detected by the pre-catalyst sensor matches a predetermined target air-fuel ratio, and the output of the post-catalyst sensor changes from the rich side to the lean side. Or at the timing of reversing, the target air-fuel ratio is switched from the lean side to the rich side or vice versa,
When delaying the air-fuel ratio switching timing, the timing for switching the target air-fuel ratio from the rich side to the lean side is delayed by a predetermined delay time from the timing when the output of the post-catalyst sensor is reversed from the lean side to the rich side. Features.

According to a fifth aspect of the present invention, in the fourth aspect,
The current stored oxygen amount is measured from the time when the target air-fuel ratio is switched to the lean side after the delay time has elapsed until the time when the output of the post-catalyst sensor is reversed to the lean side. Features.

  According to the present invention, an excellent effect is achieved that a highly accurate and reliable diagnosis is possible regardless of the sulfur concentration in the fuel.

  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 figure, the internal combustion engine 1 generates power by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston 4 in the combustion chamber 3. To do. 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.

  In the cylinder head of the internal combustion engine 1, an intake valve Vi for opening and closing the intake port and an exhaust valve Ve for opening and closing the exhaust port are provided 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. An air flow meter 5 for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 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, and the exhaust pipe 6 has catalysts 11, 19 made of a three-way catalyst having an oxygen storage capacity. Are attached in series. An exhaust passage is formed by the exhaust port, the branch pipe, and the exhaust pipe 6. Air-fuel ratio sensors for detecting the air-fuel ratio of exhaust gas, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 are installed on the upstream side and the downstream side of the upstream catalyst 11, respectively. 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 that the output value changes suddenly at the theoretical air-fuel ratio. The post-catalyst sensor 18 is installed between the upstream catalyst 11 and the downstream catalyst 19.

  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 catalysts 11 and 19 simultaneously purify NOx, HC and CO when the air-fuel ratio A / F of the exhaust gas flowing into the catalysts 11 and 19 is a stoichiometric air-fuel ratio (stoichiometric, for example, A / Fs = 14.6). In response to this, the ECU 20 adjusts the air-fuel ratio so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11, that is, the pre-catalyst air-fuel ratio A / Ffr matches the stoichiometric air-fuel ratio during normal operation of the internal combustion engine. Control. Specifically, the ECU 20 sets a target air-fuel ratio A / Ft equal to the theoretical air-fuel ratio, and makes the pre-catalyst air-fuel ratio A / Ffr detected by the pre-catalyst sensor 17 coincide with the target air-fuel ratio A / Ft. The fuel injection amount injected from the injector 12 is feedback-controlled. 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 following description applies to the downstream catalyst 19 as well. 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, even if the pre-catalyst air-fuel ratio A / Ffr slightly varies from the stoichiometric air-fuel ratio during normal air-fuel ratio control, three exhaust gas components such as NOx, HC and CO are simultaneously purified. can do. Therefore, in normal air-fuel ratio control, it is also possible to purify the exhaust gas by making the pre-catalyst air-fuel ratio A / Ffr oscillate minutely around the theoretical air-fuel ratio and repeating the absorption and release of oxygen.

  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 this embodiment, the degree of deterioration of the catalyst 11 is detected by detecting the oxygen storage capacity of the catalyst 11. Here, the oxygen storage capacity of the catalyst 11 is represented by the size of the oxygen storage capacity (OSC; O ₂ Strage Capacity, the unit is g), which is the maximum amount of oxygen that the current catalyst 11 can store.

  Hereinafter, the catalyst deterioration diagnosis in the present embodiment will be described.

  In the present embodiment, the catalyst deterioration diagnosis is performed based on the above-described Cmax method. When a fuel with a high sulfur concentration is used and the oxygen storage capacity is measured as a low value corresponding to the deterioration catalyst due to the influence of the sulfur, the stored oxygen amount is measured again by delaying the air-fuel ratio switching timing, and the previous (1 The previous oxygen storage capacity measurement value is corrected based on the difference between the stored oxygen amount measurement value for the second time and the second time. This correction eliminates the influence of sulfur, and the previous measured value of oxygen storage capacity is corrected to a value equivalent to when a low-sulfur concentration fuel is used as the reference fuel. Then, the deterioration of the catalyst is determined based on the previous oxygen storage capacity measurement value after the correction.

  First, the catalyst deterioration diagnosis based on the Cmax method, which is a basic method, will be described below with reference to FIG.

  In FIG. 3A, the broken line indicates the target air-fuel ratio A / Ft, and the solid line indicates the output of the pre-catalyst sensor 17 (converted value to the pre-catalyst air-fuel ratio A / Ffr). In FIG. 3B, the solid line indicates the output (output voltage V) of the post-catalyst sensor 18. The output of the post-catalyst sensor 18 changes abruptly with the theoretical air-fuel ratio as a boundary. Here, an output value corresponding to the theoretical air-fuel ratio is preset as a rich / lean determination threshold value Vref, and when the sensor output value V is lower than the threshold value Vref, the air-fuel ratio is lean, and the sensor output value V is higher than the threshold value Vref. Sometimes the air-fuel ratio is determined to be rich. In FIG. 3C, the solid line indicates the integrated value of the amount of oxygen released from the catalyst, that is, the amount of released oxygen OSAa. In FIG. 3D, the solid line indicates the integrated value of the amount of oxygen occluded by the catalyst, that is, the amount of occluded oxygen OSAb.

  As shown in the figure, at the time of diagnosis, active air-fuel ratio control is performed in which the air-fuel ratio of the exhaust gas flowing into the catalyst is alternately switched between the lean side and the rich side at a predetermined timing. In this active air-fuel ratio control, as in the previous stoichiometric feedback air-fuel ratio control, the feedback air-fuel ratio control is performed so that the pre-catalyst air-fuel ratio A / Ffr detected by the pre-catalyst sensor 17 matches the target air-fuel ratio A / Ft. Is done. Since there is a transport delay and a sensor response delay, the detected value of the pre-catalyst air-fuel ratio A / Ffr is slightly delayed with respect to the change in the target air-fuel ratio A / Ft.

  For example, before the time t1, the target air-fuel ratio A / Ft is set to a predetermined air-fuel ratio that is leaner than the stoichiometry (lean air-fuel ratio, for example, 15.1), and the lean gas flows into the catalyst 11. At this time, the catalyst 11 continues to absorb oxygen and reduces and purifies the lean component (NOx) in the exhaust. However, when the catalyst absorbs oxygen until it is saturated, that is, full, it can no longer absorb oxygen, and a lean component or lean gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 changes to the lean side, and eventually reaches the threshold value Vref (time t1). At this time, the target air-fuel ratio A / Ft is switched to a predetermined air-fuel ratio (rich air-fuel ratio, for example, 14.1) that is richer than stoichiometry.

  This time, rich gas flows into the catalyst 11. At this time, the catalyst 11 continues to release the oxygen stored until then, and oxidizes and purifies the rich components (HC, CO) in the exhaust gas, but when all the stored oxygen is eventually released from the catalyst 11, At that time, oxygen can no longer be released, and the rich component or rich gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 changes to the rich side, and eventually reaches the threshold value Vref (time t2). At this time, the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio.

  Again, when the lean gas flows into the catalyst 11, the catalyst 11 absorbs oxygen until it is full, and the output of the post-catalyst sensor 18 reaches the threshold value Vref (time t3), the target air-fuel ratio A / Ft is rich. It is switched to the fuel ratio.

  In this way, the target air-fuel ratio A / Ft is switched from the lean side to the rich side or vice versa at the timing when the output of the post-catalyst sensor 18 reverses from the rich side to the lean side or vice versa.

  As the active air-fuel ratio control is executed, the oxygen storage capacity OSC of the catalyst is measured.

  As shown in FIG. 3C, in the release cycle from time t1 to time t2, the released oxygen amount OSAa is sequentially integrated every short predetermined cycle. More specifically, from the time t11 when the output of the pre-catalyst sensor 17 reaches the stoichiometric equivalent while moving to the rich side, to the time t2 when the output of the post-catalyst sensor 18 is reversed to the rich side (has reached the threshold value Vref). The released oxygen amount dOSA (dOSAa) per cycle is calculated by the following equation (1), and the released oxygen amount dOSA per cycle is integrated for each cycle. The final integrated value obtained in this way is the released oxygen amount OSAa as an index value representing the oxygen storage capacity of the catalyst.

  Q is the fuel injection amount. When the air-fuel ratio difference ΔA / F is multiplied by the fuel injection amount Q, the excess or insufficient air amount can be calculated. K is the proportion of oxygen contained in the air (about 0.23).

  Similarly, in the storage cycle from time t2 to t3, as shown in FIG. 3D, the output of the post-catalyst sensor 18 is reversed to the lean side from the time t21 when the output of the pre-catalyst sensor 17 reaches the stoichiometric equivalent ( The released oxygen amount dOSA (dOSAb) per cycle is calculated by the above equation (1) until the time point t3 (when the threshold value Vref is reached), and the released oxygen amount dOSA per cycle is integrated every cycle. . The final integrated value obtained in this way becomes the stored oxygen amount OSAb as another index value representing the oxygen storage capacity of the catalyst.

In principle, the amount of oxygen that can be stored in the catalyst is equal to the amount of oxygen that can be released, and thus the amount of released oxygen OSAa and the amount of stored oxygen OSAb should be equal. It is possible to diagnose deterioration by treating only one of the values as the value of the oxygen storage capacity OSC of the catalyst. However, here, from the viewpoint of improving reliability and accuracy, the average value of both values is treated as the value of the oxygen storage capacity OSC of the catalyst. That is, the adjacent release cycle and storage cycle are set as one set, and the oxygen storage capacity of the catalyst is calculated by the following equation (2) using the values of the released oxygen amount OSAa and the stored oxygen amount OSAb respectively measured in these cycles. The capacity OSC is calculated.
OSC = (OSAa + OSAb) / 2 (2)

  This measured value of oxygen storage capacity is a value obtained in one set of absorption / release cycles. Basically, deterioration diagnosis is possible using only this, but in this embodiment, in order to further improve the reliability and accuracy, a plurality of oxygen storage capacities in a plurality of sets of absorption / release cycles are measured, and these measured values are calculated. The average value is used as the final oxygen storage capacity measurement value.

  Next, the deterioration of the catalyst is determined using the measured oxygen storage capacity. That is, the value of the oxygen storage capacity OSC is compared with a predetermined deterioration determination value OSCs. If the value of the oxygen storage capacity OSC is larger than the deterioration determination value OSCs, the catalyst is normal, and if the value of the oxygen storage capacity OSC is less than or equal to the deterioration determination value OSCs. The catalyst is judged 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. This completes the basic catalyst deterioration diagnosis.

  Next, the effect of sulfur in the fuel on the measured oxygen storage capacity will be described.

  FIG. 4 shows the state of oxygen absorption / release when a set of absorption / release cycles is extracted for the sake of simplicity. The left figure (a) shows the case where the low sulfur concentration fuel is used as the reference fuel, and the middle figure (b) shows the case where the high sulfur concentration fuel is used. The (c) diagram on the right side shows a case in which a high sulfur concentration fuel is used and a rich delay (details will be described later) peculiar to the present embodiment is performed.

  In the upper part (A), the broken line indicates the target air-fuel ratio A / Ft, and the solid line indicates the output of the pre-catalyst sensor 17 (converted value to the pre-catalyst air-fuel ratio A / Ffr). In the middle (B) diagram, the solid line indicates the output (output voltage V) of the post-catalyst sensor 18. In the lower part (C), the solid line indicates the oxygen storage rate (%). The oxygen storage rate refers to the ratio (m / M) of the amount of oxygen m currently stored by the catalyst to the amount of oxygen M that can be stored by the current catalyst (this is the oxygen storage capacity OSC).

  As can be seen by comparing FIGS. (A) and (b), the output of the post-catalyst sensor 18 is reversed at an earlier timing when the high sulfur concentration fuel is used than when the low sulfur concentration fuel is used. The released oxygen amount OSAa and the stored oxygen amount OSAb are reduced. The reason is that, as described above, the oxygen absorption / release reaction of the catalyst is hindered by the sulfur component in the exhaust gas.

  Here, according to the tests of the present inventors, it has been found that sulfur in fuel or exhaust greatly hinders oxygen release rather than oxygen storage. That is, in the storage cycle, oxygen can be stored up to an oxygen storage rate of almost 100% even when high sulfur concentration fuel is used, but in the release cycle, the oxygen release amount decreases when high sulfur concentration fuel is used. The reason is as follows. Oxygen release from the catalyst is carried out through the noble metal of the catalyst, and this released oxygen also reacts with the rich components (HC, CO) in the exhaust gas through the noble metal of the catalyst to purify the rich components. To do. However, when the activity of the noble metal is reduced by sulfur, the reaction rate between the released oxygen and the rich component is reduced. As a result, although the amount of gas passing through the catalyst exceeds the reaction rate and oxygen still remains in the catalyst, the rich component flows out downstream of the catalyst in an unpurified state, and the post-catalyst sensor is inverted, The measured value of the amount of released oxygen decreases.

  Therefore, in the present embodiment, as shown in FIG. 5C, rich delay is performed to forcibly delay the timing of switching the target air-fuel ratio A / Ft from the rich side to the lean side in order to intentionally release oxygen longer. . That is, even if the post-catalyst sensor output is reversed from the lean side to the rich side (t2), the target air-fuel ratio A / Ft is not switched from the rich side to the lean side at that time, and the predetermined delay time Δtd from the inversion timing t2 The target air-fuel ratio A / Ft is switched from the rich side to the lean side at a timing td that is delayed by a certain amount.

  In this way, oxygen release is continuously performed during the delay time Δtd after the reversal timing t2, so that the amount of released oxygen increases. Further, in the subsequent occlusion cycle, oxygen occlusion is performed from a state in which the oxygen remaining in the catalyst is less, and as a result, the occluded oxygen amount is also increased. It should be noted that the oxygen release rate during the delay time is slower than the oxygen release rate before that, and as shown in the figure, the slope of the oxygen storage ratio is smaller than before that during the delay time and also changes in a quadratic curve. become.

  This increased amount ΔOSA of the absorbed and released oxygen reflects the sulfur concentration in the fuel. That is, even if the delay time is the same, the higher the sulfur concentration, the greater the amount of increase / decrease in oxygen absorption ΔOSA. Therefore, using this fact, it is also possible to estimate the sulfur concentration of the fuel based on the measured value of the increase / decrease amount of absorbed and released oxygen ΔOSA.

  Contrary to the above, it has been found that the amount of occluded oxygen does not change so much even if only the lean delay that delays the timing of switching the target air-fuel ratio A / Ft from the lean side to the rich side is performed. This is because even if only lean delay is performed (that is, if rich delay is not performed), the amount of residual oxygen at the start of occlusion in the catalyst does not change, and almost 100% oxygen at the time when the sensor output after the catalyst is reversed to the lean side. This is because the catalyst is occluded and oxygen is not occluded during the delay time. However, the lean delay is arbitrary, and both rich delay and lean delay may be performed.

  FIG. 5 shows the test results of examining the relationship between the sulfur concentration in the fuel and the measured value of the oxygen storage capacity OSC. As illustrated, the oxygen storage capacity measurement value OSC decreases as the sulfur concentration in the fuel increases. Black circles, white triangles, and black triangles indicate data when the catalyst temperatures are 750 ° C., 700 ° C., and 600 ° C., respectively. In this embodiment, the oxygen storage capacity measured when using a high sulfur concentration fuel is equivalent to when using a low sulfur concentration fuel indicated by the leftmost point in the figure, based on the amount of increase in stored oxygen when rich delay is performed. It corrects to the level of.

  That is, first, as shown in FIG. 4B, the released oxygen amount OSAa and the stored oxygen amount OSAb are measured by a normal method (without performing rich delay), and the oxygen storage capacity OSC is measured based on these. When the predetermined condition is satisfied, rich delay is performed again as shown in FIG. 4C, and at least the stored oxygen amount OSAb is measured (measured twice). Then, the difference between the previous (first) stored oxygen amount measurement value OSAb when the rich delay is not performed and the current (second time) stored oxygen amount measurement value OSAbd when the rich delay is performed is obtained. Based on the above, the previous oxygen storage capacity measurement value OSC is corrected. The previous oxygen storage capacity measurement value OSC ′ after this correction is compared with the deterioration determination value OSCs to determine the deterioration of the catalyst.

  The case where the predetermined condition is satisfied is preferably a case where the previous oxygen storage capacity measurement value is equal to or less than the deterioration determination value. What is important in the deterioration diagnosis is that a normal catalyst near the criteria is not erroneously diagnosed as being deteriorated. Therefore, if the previous measurement of oxygen storage capacity is less than or equal to the deterioration judgment value, that is, if the catalyst is apparently deteriorated, the measurement will be repeated again at the necessary and sufficient minimum frequency. Can be measured.

  Here, the second measurement with rich delay will be described with reference to FIG.

  This second measurement is basically the same as the first measurement. That is, the active air-fuel ratio control is performed as in the first time, and when the output of the post-catalyst sensor 18 is reversed to the lean side while controlling the air-fuel ratio to lean (time t1), the target air-fuel ratio A / Ft is simultaneously switched to rich, The released oxygen amount OSAa is integrated.

  Thereafter, when the output of the post-catalyst sensor 18 is reversed to the rich side (time t2), the integration and measurement of the released oxygen amount OSAa is terminated at this time, but the target air-fuel ratio A / Ft is maintained rich. When a predetermined delay time Δtd elapses from the inversion timing t2, the target air-fuel ratio A / Ft is simultaneously switched to lean and the stored oxygen amount OSAb is integrated. This integration is performed until t3 when the output of the post-catalyst sensor 18 is then reversed to the lean side.

  Since the rich delay is performed immediately before the storage cycle, the amount of oxygen remaining in the catalyst at the start of the storage cycle is smaller than when the rich delay is not performed. Therefore, the final value of the stored oxygen amount OSAb integrated in the storage cycle is increased accordingly. In this way, it is possible to measure the value of the stored oxygen amount OSAb increased by the rich delay. In addition, it is also preferable to repeat such an absorption / release cycle to measure a plurality of stored oxygen amounts OSAb, and to use these average values as the final stored oxygen amount. The value of the released oxygen amount OSAa measured in the second measurement is not used in this embodiment, but can be used for any processing.

  Next, the content of the specific deterioration diagnosis process in this embodiment is demonstrated, referring FIG.7 and FIG.8. FIG. 7 shows a main routine for deterioration diagnosis, and FIG. 8 shows a routine for measuring the amount of absorbed and released oxygen when there is a rich delay. Both routines are repeatedly executed by the ECU 20 at a predetermined cycle. FIG. 8 shows a routine related to a set of intake / release cycles for the sake of simplicity. However, a person skilled in the art can easily assume a routine related to a plurality of sets of absorption / release cycles using the routine related to the set of absorption / release cycles.

First, the main routine of FIG. 7 will be described. First, in step S101, it is determined whether or not the diagnosis permission flag is on. For example, the diagnosis permission flag is turned on when all of the following conditions are satisfied.
1) The pre-catalyst sensor 17 and the post-catalyst sensor 18 are at the activation temperature.
2) The catalyst 11 is at the activation temperature.

  For example, the condition 1) is determined by detecting the element impedance correlated with the element temperature for both sensors by the ECU 20. The condition 2) is determined by estimating or detecting the temperature of the catalyst 11. When estimating the catalyst temperature, the ECU 20 estimates the catalyst temperature based on the engine operating state (for example, the rotational speed and the load), or if there is an exhaust temperature sensor (not shown) provided upstream of the catalyst. Estimate the catalyst temperature based on the value. When detecting the catalyst temperature, a temperature sensor (not shown) is installed in the catalyst, thereby detecting the catalyst temperature directly.

  When such a flag condition is not satisfied, the routine is immediately terminated. On the other hand, when the flag condition is satisfied, the routine proceeds to step S102, where it is determined whether or not the value of the oxygen storage capacity OSC of the catalyst has already been measured by another routine (not shown). If not measured, the routine is terminated.

  On the other hand, when the value of the oxygen storage capacity OSC has been measured, the process proceeds to step S103, and the oxygen storage capacity measurement value OSC is compared with the deterioration determination value OSCs. If the oxygen storage capacity measurement value OSC is larger than the deterioration determination value OSCs, it is immediately determined in step S110 that the catalyst is normal, the process proceeds to step S111, the diagnosis permission flag is turned off (that is, the diagnosis is completed), and the stored oxygen amount with rich delay. The measurement flag (OSAbd measurement flag, which will be described in detail later) is turned off, and the routine ends.

  On the other hand, when the oxygen storage capacity measurement value OSC is equal to or less than the deterioration determination value OSCs, the catalyst may have been permanently deteriorated or may have been temporarily deteriorated due to the influence of sulfur. Therefore, the following steps are executed to determine these.

  In step S104, it is determined whether or not the stored oxygen amount OSAbd with rich delay has been measured. If the measurement has not been completed, the stored oxygen amount measurement flag with rich delay is turned on in step S105, and the routine is terminated. As described above, the stored oxygen amount measurement flag with rich delay is a flag that is turned on to start measurement of the stored oxygen amount with rich delay OSAbd. When this is turned on, the routine of FIG. The stored oxygen amount OSAbd and the released oxygen amount OSAa are measured.

  On the other hand, if the stored oxygen amount with rich delay has been measured in step S104, the difference ΔOSAb between the stored oxygen amount OSAbd with rich delay and the stored oxygen amount OSAb without rich delay is calculated, and this difference ΔOSAb is calculated. It is compared with a predetermined threshold value ΔOSAbs.

  If the stored oxygen amount difference ΔOSAb is greater than or equal to the threshold value ΔOSAbs, it is determined in step S107 that fuel with a high sulfur concentration is being used, and at the same time, a correction coefficient K that is a correction amount for removing the influence of sulfur is calculated. . The correction coefficient K is calculated from a predetermined map (which may be a function) stored in advance as shown in FIG. 9 based on the stored oxygen amount difference ΔOSAb. According to this, the correction coefficient K is 1 when the stored oxygen amount difference ΔOSAbs is smaller than the threshold value ΔOSAbs (that is, no correction), and gradually (proportional) as the stored oxygen amount difference ΔOSAb increases from the threshold value ΔOSAbs. Increase) from 1. Here, since ΔOSAb ≧ ΔOSAbs, a correction coefficient K of 1 or more is always calculated.

  Even with the same or constant delay time Δtd, the higher the sulfur concentration in the fuel, the greater the stored oxygen amount difference ΔOSAb. Further, as shown in FIG. 5, the higher the sulfur concentration in the fuel, the lower the oxygen storage capacity measurement value. Therefore, in order to return the measured value of the oxygen storage capacity to a level equivalent to when the low sulfur concentration fuel is used, it is preferable to increase the correction amount as the stored oxygen amount difference ΔOSAb increases. From this point of view, in the map, a larger correction amount can be obtained as the stored oxygen amount difference ΔOSAb is larger.

  When the correction coefficient K is calculated in this way, the first or previous oxygen storage capacity measurement value OSC already grasped in the previous step S102 is corrected by the correction coefficient K. The corrected oxygen storage capacity measurement value OSC ′ is obtained by the formula: OSC ′ = K × OSC. In this way, the influence of sulfur is removed, and the previous oxygen storage capacity measurement value OSC is corrected to increase to a value corresponding to the use of the low sulfur concentration fuel.

  Next, in step S108, the corrected oxygen storage capacity measurement value OSC 'is compared with the deterioration determination value OSCs. When the corrected oxygen storage capacity measurement value OSC 'is larger than the deterioration determination value OSCs, the routine proceeds to step S110, where the normality determination is made. On the other hand, when the corrected oxygen storage capacity measurement value OSC ′ is equal to or less than the deterioration determination value OSCs, the process proceeds to step S109, where it is finally determined that the catalyst is deteriorated (permanent deterioration). In step S111, the flag processing described above is performed, and the routine is terminated.

  On the other hand, when the stored oxygen amount difference ΔOSAb is smaller than the threshold value ΔOSAbs in step S106, it is considered that the low sulfur concentration fuel is used and there is no influence of sulfur, and the process proceeds to step S109 to determine deterioration.

  Next, an absorption / release oxygen amount measurement routine in the case of rich delay as shown in FIG. 8 will be described. First, in step S201, it is determined whether or not the stored oxygen amount measurement flag with rich delay is ON. When this flag is off, the routine is immediately terminated. On the other hand, when the flag is on, the routine proceeds to step S202, where active air-fuel ratio control is executed, and whether the current target air-fuel ratio (target A / F) in this active air-fuel ratio control is a predetermined rich air-fuel ratio. Is judged.

  When the target air-fuel ratio is a rich air-fuel ratio, the routine proceeds to step S203, where the released oxygen amount OSAa is integrated. In step S204, it is determined whether or not the post-catalyst sensor output V is greater than the threshold value Vref, that is, whether or not the post-catalyst sensor output V is reversed to the rich side.

  When the post-catalyst sensor output V is larger than the threshold value Vref (with inversion), the delay flag is turned on in step S205, and the integration of the delay timer provided in the ECU 20 is executed. Thus, the delay time Δtd after the rich inversion of the post-catalyst sensor output V is measured. Next, in step S206, it is determined whether or not the integrated value of the delay timer has reached a predetermined target value or more. If the determination is yes, the target air-fuel ratio (target A / F) is switched to a predetermined value on the lean side in step S207, and the integrated value of the delay timer is reset to zero. The routine is then terminated.

  On the other hand, if the post-catalyst sensor output V is less than or equal to the threshold value Vref in step S204 (no inversion), and if the integrated value of the delay timer is less than the target value in step S206, the routine is terminated.

  When the target air-fuel ratio is not a rich air-fuel ratio in step S202, that is, when the target air-fuel ratio is a lean air-fuel ratio, the routine proceeds to step S208, where it is determined whether or not the rich-delayed stored oxygen amount measurement flag is off. If it is off, the routine is terminated. If it is on, it is determined in step S209 whether the delay flag is on. As described above, the delay flag is turned on when the rich delay is performed.

  If the delay flag is ON, the stored oxygen amount OSAbd with rich delay is integrated in step S210. On the other hand, if the delay flag is not on (if off), step S210 is skipped.

  Next, in step S211, it is determined whether or not the post-catalyst sensor output V is smaller than the threshold value Vref, that is, whether or not the post-catalyst sensor output V is reversed to the lean side. If the post-catalyst sensor output V is greater than or equal to the threshold value Vref (no inversion), the routine is terminated.

  On the other hand, if the post-catalyst sensor output V is smaller than the threshold value Vref (with inversion), it is determined in step S212 whether the delay flag is on. If the delay flag is off, the process proceeds to step S214, the target air-fuel ratio is switched to the rich air-fuel ratio, and the routine is terminated. On the other hand, if the delay flag is on, the rich delay presence stored oxygen amount measurement flag is turned off in step S213, and the routine is terminated.

  In the routine of FIG. 8, the flow differs depending on whether the initial target air-fuel ratio at the start of active air-fuel ratio control is rich or lean. When the initial target air-fuel ratio is rich (step S202: YES), steps S203 to S206 are executed, and the released oxygen amount OSAa is integrated until the post-catalyst sensor output V reverses to rich, and the post-catalyst sensor output V is rich. When it is reversed, the accumulation of the released oxygen amount OSAa is completed, and at the same time, the delay time is accumulated by the delay timer. When the delay timer integration time reaches the target value, the target air-fuel ratio is switched to lean in step S207.

  Thereafter, steps S208 to S211 are executed. At this time, since the delay flag is already turned on in step S205, the stored oxygen amount OSAbd is integrated until the post-catalyst sensor output V is reversed to lean. When the post-catalyst sensor output V is reversed to lean, the accumulation of the stored oxygen amount OSAb ends, and at the same time, the stored oxygen amount measurement flag with rich delay is turned off in step S213. As a result, the amount of released oxygen OSAa and the amount of stored oxygen OSAbd with rich delay are each measured.

  On the other hand, when the first target air-fuel ratio is lean (step S202: NO), steps S208 to S211 are executed first. At this time, since the delay flag is still OFF, the accumulated oxygen amount OSAbd is not integrated. When the post-catalyst sensor output V is reversed to lean over time, the target air-fuel ratio is simply switched to rich in step S214. As described above, when the first target air-fuel ratio is lean, no oxygen amount is measured during the lean control. This is because rich delay has not been performed at this time.

  Next, when the target air-fuel ratio is switched to rich (step S202: YES), steps S203 to S206 are executed, and the rich oxygen is executed simultaneously with the measurement of the released oxygen amount OSAa as described above, and then step S207. The target air-fuel ratio is switched to lean. Then, steps S208 to S211 are executed, and the stored oxygen amount OSAbd with rich delay is measured as described above. Thus, each of the released oxygen amount OSAa and the stored oxygen amount OSAbd with rich delay is measured. Finally, in step S213, the stored oxygen amount measurement flag with rich delay is turned off.

  By the way, in the rich delay control, the rich gas is forcibly supplied to the catalyst 11 even though the post-catalyst sensor output is inverted to rich and the rich component flows out downstream of the catalyst 11. Therefore, it is desirable to take care not to reduce this rich component or rich gas flowing out or to release it to the atmosphere. As a countermeasure against this, for example, it is preferable to purify the rich component flowing out from the upstream catalyst 11 by the downstream catalyst 19. For example, rich delay may be performed only when the downstream catalyst 19 is in a state where oxygen can be released. This is because the outflow rich component can be purified by the released oxygen from the downstream catalyst 19. At this time, the post-catalyst sensor 18 determines whether the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 19 is rich or lean. When lean, the lean counter is added, and when lean, the lean counter is subtracted. Rich delay may be executed only when the value is equal to or greater than a predetermined value.

  Alternatively, the rich delay time may be set so that the rich control is continued only up to the oxygen storage capacity level of the new upstream catalyst 11. In this way, the outflow amount of the rich component can be limited to the amount of decrease in the oxygen storage capacity from the new time to the deterioration.

  Alternatively, as shown in FIG. 10, an air injector 24 that injects air into the exhaust passage between the upstream catalyst 11 and the downstream catalyst 19 may be installed, and air may be injected from the air injector 24 during rich delay. By doing so, the outflow rich component in the rich delay can be purified by reacting with the jet air in the downstream catalyst 19, and at the same time, the stored oxygen can be suppressed from being released from the downstream catalyst 19.

  Alternatively, a wide air-fuel ratio sensor similar to the pre-catalyst sensor 17 may be used for the post-catalyst sensor 18 so that rich delay can be executed until the detected air-fuel ratios of the pre-catalyst sensor 17 and the post-catalyst sensor 18 become equal. When these detected air-fuel ratios are equivalent, the amount of oxygen stored in the upstream catalyst 11 is considered to be almost zero, and the rich component of the inflow gas cannot be purified. Therefore, at this time, it is preferable to end the rich delay.

Alternatively, as shown in FIG. 10, an air-fuel ratio sensor (wide area air-fuel ratio sensor or O ₂ sensor) 22 is installed in the exhaust passage downstream of the downstream catalyst 19, and the rich delay is only performed when the output of the air-fuel ratio sensor 22 is not rich. May be executable. For example, when the output of the air-fuel ratio sensor 22 is inverted to rich, the rich delay is terminated. The rich output of the air-fuel ratio sensor 22 means that the downstream catalyst 19 has not fully purified the rich component of the inflowing gas. In this case, it is preferable to stop or end the rich delay.

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 type of the internal combustion engine are arbitrary, and may be other than for vehicles, for example, a direct injection type or the like. A wide air-fuel ratio sensor similar to the pre-catalyst sensor may be used for the post-catalyst sensor, and an O ₂ sensor similar to the post-catalyst sensor may be used for the pre-catalyst sensor. A wide range of sensors that detect the exhaust air-fuel ratio, including these wide-range air-fuel ratio sensors and O ₂ sensors, are referred to as air-fuel ratio sensors. The present invention can be applied to any catalyst having an oxygen storage capacity.

  According to the results of FIG. 5, the difference between the oxygen storage capacity when using low-sulfur concentration fuel and the oxygen storage capacity when using high-sulfur concentration fuel differs depending on the catalyst temperature. The difference becomes larger at lower temperatures. Therefore, based on this result, it is preferable to change or correct the correction amount (correction coefficient K) according to the catalyst temperature. Specifically, it is preferable to set the correction amount to a larger value as the catalyst temperature is lower.

  It is also possible to make the rich air-fuel ratio during rich delay different from the value of the normal rich air-fuel ratio.

  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.

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 catalyst deterioration diagnosis by Cmax method. It is a time chart showing the state of oxygen absorption and release, (a) when using low sulfur concentration fuel, (b) when using high sulfur concentration fuel, (c) when using high sulfur concentration fuel and rich delay Show the case. The test result which investigated the relationship between the sulfur concentration in a fuel and an oxygen storage capacity measurement value is shown. It is a time chart for demonstrating the 2nd measurement accompanying a rich delay. It is a flowchart which shows the main routine of a deterioration diagnosis. It is a flowchart which shows the amount measurement routine of absorption / release oxygen in the case of rich delay. A correction coefficient calculation map is shown. It is the partial schematic which shows the structure of a modification.

Explanation of symbols

1 internal combustion engine 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 OSCs Degradation judgment value OSAa Oxygen release amount OSAb Oxygen storage amount OSAbd Oxygen storage amount with rich delay ΔOSAb Oxygen storage amount difference A / Ft Target air-fuel ratio Δtd Delay time K Correction coefficient

Claims (5)

An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
Active air-fuel ratio control means for performing active air-fuel ratio control for alternately switching the air-fuel ratio of the exhaust gas flowing into the catalyst to a lean side and a rich side at a predetermined timing;
Measuring means for measuring the oxygen storage capacity of the catalyst in accordance with execution of the active air-fuel ratio control, measuring the amount of oxygen stored in the catalyst when the air-fuel ratio is on the lean side, Measuring means for measuring the amount of oxygen released from the catalyst when the air-fuel ratio is on the rich side, and calculating the oxygen storage capacity of the catalyst based on the stored oxygen amount and the measured value of the released oxygen amount; ,
Determination means for determining deterioration of the catalyst by comparing the oxygen storage capacity measured by the measurement means with a predetermined deterioration determination value;
With
When the predetermined condition is satisfied, the timing at which at least the air-fuel ratio in the active air-fuel ratio control is switched from the rich side to the lean side is delayed from the predetermined timing, and again, the active air-fuel ratio control and at least the stored oxygen amount And the previous oxygen storage capacity measurement value is corrected based on the difference between the previous stored oxygen amount measurement value and the current stored oxygen amount measurement value, and the corrected previous oxygen storage capacity measurement value is obtained. A catalyst deterioration diagnosis apparatus for an internal combustion engine, wherein deterioration of the catalyst is determined based on the determination. The correction amount for correcting the previous oxygen storage capacity measurement value is increased as the difference between the previous stored oxygen amount measurement value and the current stored oxygen amount measurement value is larger. A catalyst deterioration diagnosis device for an internal combustion engine. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to claim 1 or 2, wherein the predetermined condition is satisfied when the previous oxygen storage capacity measurement value is equal to or less than the deterioration determination value. A pre-catalyst sensor that detects the air-fuel ratio of exhaust gas upstream of the catalyst; and a post-catalyst sensor that detects the air-fuel ratio of exhaust gas downstream of the catalyst;
The active air-fuel ratio control means performs air-fuel ratio control so that the pre-catalyst air-fuel ratio detected by the pre-catalyst sensor matches a predetermined target air-fuel ratio, and the output of the post-catalyst sensor changes from the rich side to the lean side. Or at the timing of reversing, the target air-fuel ratio is switched from the lean side to the rich side or vice versa,
When delaying the air-fuel ratio switching timing, the timing for switching the target air-fuel ratio from the rich side to the lean side is delayed by a predetermined delay time from the timing when the output of the post-catalyst sensor is reversed from the lean side to the rich side. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to any one of claims 1 to 3. The current stored oxygen amount is measured from the time when the target air-fuel ratio is switched to the lean side after the delay time has elapsed until the time when the output of the post-catalyst sensor is reversed to the lean side. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to claim 4,

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