JP2005330848A - Catalyst degradation estimating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst degradation estimating device for estimating whether NOx emission control performance of an NOx trap catalyst is degraded or not by sulfur poisoning at low cost. <P>SOLUTION: A catalyst 25 for OBD of which oxygen storage performance is varied in accordance with SOx occlusion state is arranged on the downstream side of the NOx trap catalyst 24, and whether the NOx trap catalyst is degraded or not is estimated by ECU 30 based on the difference of the output amplitude of O<SB>2</SB>sensors 27 and 28 on the upstream side and the downstream side. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a catalyst deterioration estimation device that estimates the presence or absence of deterioration due to sulfur poisoning of a NOx trap catalyst.

  In general, a NOx trap catalyst is used to purify NOx (nitrogen oxides) discharged during the lean operation of the internal combustion engine. The NOx trap catalyst occludes NOx in exhaust gas discharged during lean operation, and releases and reduces the NOx once occluded during operation at the stoichiometric or rich air-fuel ratio. . However, during the lean operation, sulfur oxide (SOx) is also generated by the reaction between the sulfur (S) component contained in the fuel and oxygen in the exhaust gas, and SOx is stored in the NOx trap catalyst in the same manner as NOx. In addition, it is known that SOx is more stable than NOx, and is not released only by stoichiometric or rich operation, but accumulates in the NOx trap catalyst and reduces NOx purification performance.

Therefore, conventionally, when such S poisoning or S deterioration is detected, the engine operation is performed at stoichiometric or rich air-fuel ratio, and the catalyst temperature is increased to release the SOx accumulated in the NOx trap catalyst. I have to. For example, in the exhaust gas purification apparatus described in Patent Document 1, the deterioration of the NOx catalyst is determined using a NOx sensor.
JP 2000-265824 A

In the exhaust emission control device described in Patent Document 1, a NOx sensor is used for determining the deterioration of the NOx catalyst, and high accuracy is required to use the NOx sensor for determining the deterioration, which may increase the cost.
An object of the present invention is to provide a low-cost catalyst deterioration estimation device that estimates whether or not the NOx purification performance has deteriorated due to S poisoning of the NOx trap catalyst.

  In the catalyst deterioration estimation device for estimating the deterioration of the NOx trap catalyst, the invention according to claim 1 is characterized in that the deterioration determination catalyst whose oxygen storage capacity changes according to the SOx occlusion state is provided in the exhaust passage of the internal combustion engine. The NOx trap catalyst deterioration estimation means estimates whether or not the NOx trap catalyst has deteriorated based on the exhaust air fuel ratio downstream of the deterioration determination catalyst that is arranged upstream or downstream and detected by the downstream air fuel ratio detection means. It is characterized by that.

  According to the catalyst deterioration estimation apparatus of claim 1, during operation of the internal combustion engine, the SOx is gradually occluded in each of the NOx trap catalyst and the deterioration determination catalyst disposed upstream or downstream thereof, and the deterioration determination device is used. In the catalyst, the oxygen storage capacity changes according to the SOx occlusion state. As the oxygen storage capacity of the deterioration determination catalyst changes, the exhaust air-fuel ratio downstream of the deterioration determination catalyst changes. As described above, the exhaust air-fuel ratio on the downstream side of the deterioration determination catalyst reflects the oxygen storage capacity and the SOx occlusion state of the deterioration determination catalyst. Further, the deterioration determination catalyst is arranged in series with the NOx trap catalyst in the exhaust passage, and the exhaust gas containing S component flows into the deterioration determination catalyst and the NOx trap catalyst. The exhaust air-fuel ratio on the downstream side also reflects the SOx occlusion state in the NOx trap catalyst. Therefore, the presence or absence of deterioration of the NOx trap catalyst is accurately estimated based on the exhaust air-fuel ratio on the downstream side of the deterioration determination catalyst.

The catalyst deterioration estimating apparatus according to claim 2 is characterized in that the NOx trap catalyst deterioration estimating means detects the exhaust air / fuel ratio upstream of the deterioration determination catalyst detected by the upstream air / fuel ratio detecting means and the downstream air / fuel ratio detecting means, respectively. The deterioration of the NOx trap catalyst is estimated based on the comparison result with the downstream exhaust air-fuel ratio.
The catalyst deterioration estimation device according to claim 3 comprises air-fuel ratio modulation means for forcibly modulating the exhaust air-fuel ratio upstream of the deterioration determination catalyst, and the NOx trap catalyst deterioration estimation means is upstream of the deterioration determination catalyst. The deterioration of the NOx trap catalyst is estimated on the basis of the result of comparison between the forcedly modulated exhaust air / fuel ratio and the exhaust air / fuel ratio downstream of the deterioration determination catalyst.

  4. The catalyst deterioration estimation apparatus according to claim 2 and 3, wherein the higher the oxygen storage capacity of the deterioration determination catalyst, the lower the exhaust air-fuel ratio on the downstream side of the deterioration determination catalyst and the upstream exhaust air-fuel ratio due to the oxygen storage action of the catalyst. There is a big difference in the behavior. Accordingly, as the oxygen storage function decreases due to the increase in the SOx storage amount in the deterioration determination catalyst, that is, as the SOx storage amount in the NOx trap catalyst and thus the deterioration degree of the catalyst increases, the downstream side of the deterioration determination catalyst. The behavior of the exhaust air / fuel ratio approaches that of the upstream exhaust air / fuel ratio. Therefore, when it is determined that the downstream exhaust air-fuel ratio has approached the upstream exhaust air-fuel ratio based on the comparison result of the upstream and downstream exhaust air-fuel ratios of the deterioration determination catalyst, the NOx trap catalyst has deteriorated. Is estimated.

  In the catalyst deterioration estimation device according to claim 3, since the exhaust air-fuel ratio upstream of the deterioration determination catalyst is forcibly modulated, the relationship between the upstream exhaust air-fuel ratio and the downstream exhaust air-fuel ratio of the deterioration determination catalyst. However, it greatly changes as the oxygen storage capacity of the deterioration determination catalyst decreases, and the estimated sensitivity of deterioration of the NOx trap catalyst is improved.

  According to the first aspect of the present invention, the deterioration determination catalyst whose oxygen storage capacity changes according to the SOx occlusion state is arranged upstream or downstream of the NOx trap catalyst, and both the deterioration determination catalyst and the NOx trap catalyst are arranged. Presence or absence of NOx trap catalyst deterioration is estimated based on the exhaust air-fuel ratio downstream of the deterioration determination catalyst representing the SOx occlusion state. Therefore, the presence or absence of NOx trap catalyst deterioration is estimated without using a highly accurate NOx sensor. Therefore, an inexpensive catalyst deterioration estimation device can be provided.

According to the invention of claim 2, from the comparison result between the upstream air-fuel ratio and the downstream exhaust air-fuel ratio of the deterioration determination catalyst, the oxygen storage of the catalyst accompanying the increase in the SOx occlusion amount in the deterioration determination catalyst. By determining the decrease in capacity, it is possible to estimate the deterioration of the NOx trap catalyst.
According to the invention of claim 3, by forcibly modulating the exhaust air-fuel ratio upstream of the deterioration determination catalyst, the comparison result between the exhaust air-fuel ratio upstream of the catalyst and the exhaust air-fuel ratio downstream is obtained. The estimated sensitivity of deterioration of the NOx trap catalyst based on it can be improved.

[First Embodiment]
Hereinafter, a NOx trap catalyst deterioration estimating apparatus according to a first embodiment of the present invention will be described with reference to the drawings.
An internal combustion engine (hereinafter referred to as an engine) equipped with the NOx trap catalyst deterioration estimation device of the present embodiment is, for example, in-cylinder injection capable of selectively performing fuel injection in an intake stroke injection mode or a compression stroke injection mode. This is a spark ignition type in-line four-cylinder gasoline engine that is selectively operated at a stoichiometric air-fuel ratio, rich air-fuel ratio, or lean air-fuel ratio. The in-cylinder injection type engine is conventionally known, and its configuration will be briefly described with reference to FIG.

The cylinder head 12 of the direct injection engine 11 is provided with an electromagnetic fuel injection valve 14 together with a spark plug 13 for each cylinder. The fuel injection valve 14 is connected to a fuel tank and a fuel pump via a fuel pipe (not shown). The fuel in the fuel tank can be directly injected from the fuel injection valve 14 into the combustion chamber 15.
An intake port is formed in the cylinder head 12 in a substantially upright direction for each cylinder, and one end of an intake manifold 16 is connected to be able to communicate with each intake port. A drive-by-wire (DBW) type electric throttle valve 17 is connected to the other end of the intake manifold 16, and the throttle valve 17 is provided with a throttle sensor 18 for detecting the throttle opening θth. Further, an exhaust port is formed in the cylinder head 12 in a substantially horizontal direction for each cylinder, and one end of an exhaust manifold 19 is connected to be able to communicate with each exhaust port. The engine 11 is provided with a crank angle sensor 20 that detects the crank angle, and can detect the engine rotational speed Ne.

A muffler (not shown) is connected to an exhaust pipe (exhaust passage) 21 connected to the exhaust manifold 19 via a small three-way catalyst 22 and an exhaust purification catalyst device 23 close to the engine 11. The exhaust purification catalyst device 23 includes a NOx trap catalyst 24 and a deterioration determination catalyst 25 arranged on the downstream side thereof.
The proximity three-way catalyst 22 is activated early by the exhaust gas when the engine 11 is cold-started, contains precious metals such as platinum (Pt) and rhodium (Rh), and the exhaust air-fuel ratio is close to the theoretical air-fuel ratio. Sometimes it purifies harmful substances in the exhaust gas. The proximity three-way catalyst 22 suppresses the addition amount of the material having an oxygen storage function to prevent the reducing agent such as CO from being oxidized during NOx release reduction in the NOx trap catalyst 24. Yes.

The NOx trap catalyst 24 of the exhaust purification catalyst device 23 includes, for example, noble metals such as platinum (Pt) and rhodium (Rh), alkali metals such as barium (Ba), potassium (K), and sodium (Na), and alkaline earths. A metal NOx trapping agent that stores NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and releases NOx when the exhaust air-fuel ratio is rich to reduce and purify nitrogen (N ₂ ) or the like It is what you have. Note that the NOx trap catalyst 24 preferably has a three-way function for purifying harmful substances in the exhaust gas when the exhaust air-fuel ratio is close to the stoichiometric air-fuel ratio.

The deterioration determination catalyst 25 is an OBD (vehicle diagnosis) catalyst that absorbs SOx in exhaust gas and changes its oxygen storage capacity in accordance with the SOx occlusion state. In this embodiment, the deterioration determination catalyst 25 is an oxide of cerium (Ce). It is composed of a three-way catalyst to which an oxygen storage agent containing some ceria is added as an S trap agent. This oxygen storage agent has an oxygen storage capacity that maintains oxygen (O ₂ ) adsorbed during lean operation in an adsorbed state during rich operation. However, when S is adsorbed, oxygen cannot be adsorbed, and the amount of S traps is reduced. Oxygen storage capacity decreases with the increase. In addition, it is necessary to carry | support more oxygen storage agents for OBD than the quantity required for the effect | action of a three way catalyst. Instead of the oxygen storage agent containing ceria, a material having an oxygen storage function such as lanthanum (La) or nickel (Ni) may be added in an amount sufficient to function as an S trapping agent.

  Furthermore, in the case of using an oxygen storage agent containing ceria which is an oxide of cerium (Ce), a composite oxide in which zirconia or the like is compounded with ceria is often used as a recent oxygen storage agent. However, in this case, since it is mainly ceria in the composite oxide that functions as the S trap agent, it is necessary that the ratio of ceria in the composite oxide be a predetermined value. The weight ratio of ceria in a suitable composite oxide is 50% or more of the whole, more preferably 80% or more.

  Here, the oxygen storage capacity of the OBD catalyst 25 changes according to the S trap amount in the catalyst 25, and the S trap amount of the OBD catalyst 25 has a correlation with the S trap amount of the NOx trap catalyst 24. ing. The OBD catalyst 25 is different from a normal OBD three-way catalyst used for thermal degradation determination in that it is mainly used for estimating S degradation of the NOx trap catalyst 24. That is, the OBD catalyst 25 may be used exclusively for estimating the S deterioration of the NOx trap catalyst 24. In the case of such an OBD catalyst 25, only the S trap agent is added without adding the noble metal. .

In the exhaust pipe 21, an upstream O2 sensor 27 is provided between the NOx trap catalyst 24 and the OBD catalyst 25, and a downstream O ₂ sensor 28 is provided downstream of the OBD catalyst 25. Yes. The upstream O ₂ sensor 27 functions as upstream air-fuel ratio determination means for detecting the exhaust air-fuel ratio upstream of the OBD catalyst 25, while the downstream O ₂ sensor 28 is exhaust air downstream of the OBD catalyst 25. It functions as a downstream air-fuel ratio determining means for detecting the fuel ratio. Reference numeral 26 represents a high-temperature sensor that is provided between the three-way catalyst 22 and the NOx trap catalyst 24 and detects an exhaust gas temperature corresponding to the catalyst temperature. Note that the high temperature sensor 26 may be provided close to the NOx trap catalyst 24 to detect the catalyst temperature.

Reference numeral 30 represents an ECU (Electronic Control Unit) having an input / output device, a storage device, a central processing unit, a timer counter, and the like. The ECU 30 performs overall engine control, and includes an upstream O ₂ sensor 27 and It functions as NOx trap catalyst deterioration estimation means for estimating deterioration of the NOx trap catalyst 24 based on the output of the downstream O ₂ sensor 28.

Regarding engine control, the ECU 30 calculates a fuel injection amount and ignition timing from detection information input from various sensors connected to the input side thereof, and an ignition coil and fuel for the spark plug 13 connected to the output side of the ECU 30. The injection valve 14 and the like are driven to inject an appropriate amount of fuel at an appropriate timing and ignite at an appropriate timing.
Further, the ECU 30 obtains a target in-cylinder pressure (target average effective pressure Pe) corresponding to the engine load based on the throttle opening degree information θth from the throttle sensor 18 and the engine rotational speed information Ne from the crank angle sensor 20. The fuel injection mode is set according to the target average effective pressure Pe and the engine rotational speed information Ne. For example, the compression stroke injection mode is selected when both the target average effective pressure Pe and the engine rotation speed Ne are small, while the intake stroke injection mode is selected when the target average effective pressure Pe or the engine rotation speed Ne increases. Further, a target air-fuel ratio is set from the target average effective pressure Pe and the engine speed Ne, and the fuel injection amount is determined based on the target air-fuel ratio.

  During operation of the engine 11 having the exhaust purification device having the above-described configuration, the NOx trap catalyst 24 stores NOx in the exhaust if the exhaust air-fuel ratio is lean, while it stores during the lean operation if the exhaust air-fuel ratio is rich. The released NOx is released and reduced, whereby the NOx purification ability of the NOx trap catalyst 24 is regenerated. When such regeneration of the NOx trap catalyst 24 is actively performed, conventionally known NOx purge control is performed. For example, the air-fuel ratio is controlled so that the exhaust air-fuel ratio becomes rich or stoichiometric.

  Further, the fuel contains a sulfur component. Therefore, the sulfur component is also present in the exhaust gas, and this sulfur component reacts with oxygen to become sulfur oxide (SOx). In a lean atmosphere, the NOx trap catalyst 24 occludes NOx as nitrate and occludes SOx as sulfate. As described above, when SOx is occluded in the NOx trap catalyst 24 that should originally occlude NOx, the NOx purification performance of the NOx trap catalyst 24 is lowered accordingly.

  In a stoichiometric or rich atmosphere with a low oxygen concentration, SOx occluded as a sulfate in the NOx trap catalyst 24 has a higher stability as a salt than a nitrate, so even if the atmosphere has a reduced oxygen concentration. Only the portion is decomposed, and the amount of sulfate remaining in the NOx trap catalyst 24 increases with time. Due to such SOx occlusion (S poisoning), the NOx purification ability of the NOx trap catalyst 24 decreases (S deterioration). When S deterioration occurs, it is necessary to perform S purge control for releasing and reducing SOx by increasing the temperature of the NOx trap catalyst 24 and enriching the exhaust air-fuel ratio.

Hereinafter, estimation of S deterioration of the NOx trap catalyst 24 in the present embodiment will be described.
As described above, the OBD catalyst 25 is disposed in the exhaust pipe 21 downstream of the NOx trap catalyst 24 and contains an S trap agent, and the exhaust pipe 21 contains a large amount of exhaust gas containing S component. Therefore, the exhaust gas containing the S component flows into both the NOx trap catalyst 24 and the OBD catalyst 25. Therefore, in the lean atmosphere, the S component in the exhaust gas is occluded by the S trap agent of the OBD catalyst 25.

  Then, the amount of S traps in the OBD catalyst 25 increases as time elapses from the time when the previous S purge control is completed, like the amount of S traps in the NOx trap catalyst 24. That is, the S trap amount of the OBD catalyst 25 increases as the S trap amount of the NOx trap catalyst 24 increases, and the S trap amount of the OBD catalyst 25 and the S trap amount of the NOx trap catalyst 24 increase. There is a correlation between quantities. Further, as the S trap amount in the OBD catalyst 25 increases, the oxygen storage capacity of the OBD catalyst 25 decreases. That is, there is also a correlation between the oxygen storage capacity of the OBD catalyst 25 and the S trap amount in the NOx trap catalyst 24.

In the present embodiment, the O ₂ sensors 27 and 28 are arranged on the upstream side and the downstream side of the OBD catalyst 25, and the output voltages of the upstream O ₂ sensor 27 and the downstream O ₂ sensor 28 are the OBD catalyst 25. Represents the oxygen concentration in the exhaust gas on the upstream side and the downstream side. Here, when the air-fuel ratio is modulated with a predetermined amplitude and frequency, the difference in the output behavior of the O ₂ sensors 27 and 28 represents the current oxygen storage capacity of the OBD catalyst 25. As illustrated in FIG. 2, each of the O ₂ sensors 27 and 28 is configured such that its output voltage is large in a rich atmosphere and small in a lean atmosphere, and an air near the stoichiometric air-fuel ratio (stoichiometric air-fuel ratio). The output voltage is configured to change greatly as the fuel ratio changes. That is, the oxygen concentration detection sensitivity of each O ₂ sensor becomes high near the stoichiometric air-fuel ratio.

As is apparent from the above description, the difference in the output behavior of the O ₂ sensors 27 and 28 when the air-fuel ratio is modulated indicates that the oxygen storage capacity of the OBD catalyst 25 and thus the amount of S traps in the NOx trap catalyst 24, that is, S degradation. Represents the degree of. Therefore, in this embodiment, the S deterioration of the NOx trap catalyst 24 is estimated based on the difference in output behavior of the O ₂ sensors 27 and 28. As representative of the output behavior, the output difference between the lean side peak and the rich side peak when the air-fuel ratio is modulated, that is, the output amplitude is used. Specifically, the ECU (NOx trap catalyst deterioration estimation means) 30 executes an S deterioration estimation routine shown in FIG. 3 at a predetermined period during engine operation.

In the S deterioration estimation routine of FIG. 3, the ECU 30 determines whether or not the engine 11 is operated at the stoichiometric air-fuel ratio (step S1), and during the stoichiometric feedback operation (air-fuel ratio modulation control operation using the O ₂ sensor). If not, the S deterioration estimation process in the current cycle is terminated. On the other hand, if it is determined that the stoichiometric feedback operation is being performed, the ECU 30 determines that the exhaust air-fuel ratio is in the vicinity of the stoichiometry and is modulated and that the S deterioration estimation can be performed satisfactorily using the O ₂ sensors 27 and 28. The respective outputs of the O ₂ sensors 27 and 28 are read (step S2), the difference between the output amplitudes of the O ₂ sensors 27 and 28 is calculated (step S3), and the difference between the output amplitudes is compared with the catalyst deterioration determination value (step S3). Step S4).

When the amount of S traps in the NOx trap catalyst 24 is small, the amount of S traps in the OBD catalyst 25 is also small, and the OBD catalyst 25 has an oxygen storage capability. Therefore, oxygen in the exhaust gas flowing into the OBD catalyst 25 is reduced. The behavior of oxygen concentration in the exhaust gas on the downstream side of the OBD catalyst 25 becomes smaller than the behavior of oxygen concentration in the exhaust gas on the upstream side. Thus, if the behavior of the oxygen concentration in the exhaust gas is small downstream of the OBD catalyst 25, the difference between the output amplitude of the upstream O ₂ sensor 27 and the output amplitude of the downstream O ₂ sensor 28 is large, and the output amplitude The difference is larger than the catalyst deterioration judgment value. Therefore, when the difference between the output amplitudes of the O ₂ sensors 27 and 28 is larger than the determination value, the ECU 30 reduces the S trap amount in the NOx trap catalyst 24, that is, the degree of S deterioration, and thus the catalyst 24 deteriorates to S. It is determined that it has not reached, and the S deterioration determination process in the current cycle is terminated.

On the other hand, when the amount of S trap in the NOx trap catalyst 24 is large, the amount of S trap in the OBD catalyst 25 is also large, and the oxygen storage capacity of the OBD catalyst 25 is reduced. Therefore, oxygen in the exhaust gas flowing into the OBD catalyst 25 flows into the downstream side without being occluded in the catalyst 25. For this reason, the behavior of the oxygen concentration in the exhaust gas on the downstream side of the OBD catalyst 25 is comparable to the behavior of the oxygen concentration in the exhaust gas on the upstream side. As described above, when the behavior of the oxygen concentration in the exhaust gas is approximately the same between the upstream side and the downstream side of the OBD catalyst 25, the difference in output amplitude between the O ₂ sensors 27 and 28 is small, and the difference in output amplitude is determined. Smaller than the value. Accordingly, when the difference in output amplitude is smaller than the determination value, the ECU 30 determines that the amount of S trap in the NOx trap catalyst 24, that is, the degree of S degradation is large, and estimates that the catalyst 24 has undergone S degradation (step S5). ). As described above, the presence or absence of S deterioration of the NOx trap catalyst 24 can be accurately estimated.

  Then, when it is estimated that the NOx trap catalyst 24 has undergone S degradation, S purge control is started in order to remove the SOx stored in the catalyst 24 (step S6). In the S purge control, the NOx trap catalyst 24 is heated to a predetermined temperature by changing the exhaust air-fuel ratio from lean to rich and performing ignition timing retard or additional fuel injection. As a result, the NOx trap catalyst 24 is regenerated and its NOx purification performance is maintained.

  Instead of the S purge control, for example, a warning lamp (engine check lamp) 31 provided on the instrument panel of the vehicle is turned on to notify the driver that the NOx trap catalyst 24 has undergone S deterioration. Also good. Alternatively, the warning lamp 31 may be turned on to give a warning to the driver when S deterioration is estimated despite the S purge control. In any case, the driver can take measures such as replacement of the deteriorated catalyst at a maintenance factory or the like by an alarm by turning on the warning lamp 31.

As described above, the catalyst deterioration estimation device of the present embodiment was created by paying attention to the fact that there is a high correlation between the oxygen storage capacity of the OBD catalyst 25 and the degree of S deterioration of the NOx trap catalyst 24. Therefore, the oxygen storage capacity of the OBD catalyst 25 is determined from the O ₂ sensor outputs respectively arranged on the upstream side and the downstream side of the catalyst 25.
Based on the principle of estimating S deterioration, the presence or absence of S deterioration of the NOx trap catalyst 24 is estimated based on the comparison result of the output amplitudes of the upstream O ₂ sensor 27 and the downstream O ₂ sensor 28 as in the above embodiment. It is not essential to perform the estimation, and for example, the S degradation can be estimated based on the comparison result of the frequencies of the outputs of the O ₂ sensors 27 and 28.

Therefore, for example, by performing air-fuel ratio feedback control based on the output of the upstream O ₂ sensor 27, the exhaust air-fuel ratio flowing into the exhaust purification catalyst device 23 becomes a predetermined cycle and a predetermined amplitude between the lean air-fuel ratio and the rich air-fuel ratio. For example, the target combustion air-fuel ratio is modulated around an average air-fuel ratio (modulation center air-fuel ratio) which is a stoichiometric air-fuel ratio. Then, the output frequencies of the upstream O ₂ sensor 27 and the downstream O ₂ sensor 28 at the time of air-fuel ratio modulation are compared.

If the oxygen storage capacity of the OBD catalyst 25 is high, oxygen is occluded in the catalyst 25. Therefore, as shown in FIG. 4, the exhaust air-fuel ratio downstream of the OBD catalyst 25 with respect to the change in the exhaust air-fuel ratio upstream of the OBD catalyst 25 is reduced. Response is slow. The ECU 30 serving as catalyst deterioration estimation means reads the outputs of the upstream O ₂ sensor 27 and the downstream O ₂ sensor 28 during air-fuel ratio modulation, determines the frequency of both outputs, and determines the frequency of the output of the downstream O ₂ sensor 27. A frequency ratio is calculated by dividing by the frequency of the output of the upstream O ₂ sensor 28, and the calculated frequency ratio is compared with the determination value. In the case of FIG. 4, the frequency ratio is smaller than the determination value. If the frequency ratio is smaller than the determination value in this way, the ECU 30 has a high oxygen storage capacity of the OBD catalyst 25, so that the S trap amount in the catalyst 25 and hence the S trap amount in the NOx trap catalyst 24 causes S deterioration. Judge that it is not so large.

  On the other hand, when the oxygen storage capacity of the OBD catalyst 25 becomes low, oxygen is not stored in the catalyst 25 but is discharged to the downstream side, so that the response of the exhaust air / fuel ratio downstream of the catalyst becomes faster as shown in FIG. . In the case of FIG. 5, the frequency ratio is larger than the determination value. If the frequency ratio is larger than the determination value in this manner, the ECU 30 determines that the oxygen storage capacity of the OBD catalyst 25 has decreased, and accordingly, the amount of S traps in the catalyst 25 and thus the NOx trap catalyst 24 is large. It is estimated that S deterioration has occurred in the NOx trap catalyst 24, S purge control is performed, or a warning lamp is turned on.

Furthermore, the presence or absence of S degradation may be estimated by combining the amplitude and frequency of the outputs of the O ₂ sensors 27 and 28.
As described above, in principle, the output amplitude difference between the upstream O ₂ sensor 27 and the downstream O ₂ sensor 28, the frequency ratio of the outputs of the O ₂ sensors 27, 28, or the output amplitude difference and the frequency ratio. Based on the combination, it is possible to estimate the oxygen storage capacity of the OBD catalyst 25 and the presence or absence of S deterioration of the NOx trap catalyst 24, but there is room for improvement from the viewpoint of practicality.

  That is, compared with S trapped by the alkali metal or alkaline earth metal constituting the S trap agent of the NOx trap catalyst 24, S trapped in the ceria used as the S trap agent of the OBD catalyst 25 is generally It is known to be easily released at lower temperatures. However, S is trapped in ceria in various forms. Like S trapped in the NOx trap catalyst 24, S is trapped in a form that is difficult to be released from ceria even at high temperatures. Therefore, there is a correlation between the S trap amount (oxygen storage capability) in the OBD catalyst 25 and the S trap amount (S deterioration) in the NOx trap catalyst 24, and the above-described embodiments and modifications are like that. The S degradation is estimated based on a strong correlation.

However, since the amount of S trapped in a form that is difficult to be released from ceria is not high even at high temperatures, the change in the oxygen storage capacity of the catalyst 25 accompanying the change in the amount of S trap in the OBD catalyst 25 is inevitably small. Accordingly, the present inventor has devised a catalyst deterioration estimation apparatus according to a second embodiment described below with such recognition.
[Second Embodiment]
The catalyst deterioration estimation device according to the modified example of the first embodiment modulates the target combustion air-fuel ratio around the stoichiometric air-fuel ratio by air-fuel ratio feedback control based on the output of the upstream O ₂ sensor 27, thereby reducing the exhaust air-fuel ratio to the lean air-fuel ratio. While changing between the fuel ratio and the rich air-fuel ratio, the exhaust air-fuel ratio downstream of the OBD catalyst 25 is determined to determine the slow response of the exhaust air-fuel ratio downstream of the catalyst. It is estimated that the catalyst 24 has deteriorated S.

The catalyst deterioration estimation device of the second embodiment is basically based on the above-described estimation principle, but the first is that the frequency of modulation with respect to the exhaust air-fuel ratio flowing into the exhaust purification catalyst device 23 is set to an extremely low frequency. It differs from the modification of 1 embodiment. That is, in the second embodiment, the frequency of modulation with respect to the exhaust air / fuel ratio is set to an extremely low frequency of, for example, 0.5 Hz or less, preferably 0.35 Hz or less. When the exhaust air-fuel ratio is modulated using such an extremely low frequency, forced modulation upstream of the OBD catalyst 25 is performed even when the change in the oxygen storage capacity with respect to the change in the S trap amount in the OBD catalyst 25 is small. Since the slow response of the exhaust air / fuel ratio (downstream O ₂ sensor output waveform) downstream of the catalyst with respect to the change in the exhaust air / fuel ratio (modulation waveform) is greatly changed, the S of the NOx trap catalyst 24 based on the slow response is changed. Degradation estimation sensitivity is improved.

Here, it is extremely difficult to realize ultra-low frequency modulation of 0.5 Hz or less by normal air-fuel ratio feedback control based on the upstream O ₂ sensor output as in the above modification. Therefore, in the second embodiment, the exhaust air / fuel ratio upstream of the OBD catalyst 25 is forcibly modulated by the ECU 30. That is, the ECU 30 has a function as an air-fuel ratio modulation unit in addition to a function as a NOx trap catalyst deterioration estimation unit. In the second embodiment, the upstream O ₂ sensor 27 is not necessary for estimating the S deterioration.

Hereinafter, estimation of S deterioration of the NOx trap catalyst 24 according to the second embodiment will be described.
In order to estimate S deterioration, the ECU (NOx trap catalyst deterioration estimation means) 30 executes an S deterioration estimation routine shown in FIG. 6 at a predetermined cycle during engine operation.
In the S deterioration estimation routine of FIG. 6, the ECU 30 determines whether or not the engine 11 is operated at the stoichiometric air-fuel ratio (step S21), and if not during the stoichiometric operation, ends the S deterioration estimation process of the current cycle. On the other hand, if the stoichiometric operation is being performed, the ECU (air-fuel ratio modulation means) 30 determines that the exhaust air-fuel ratio flowing into the exhaust purification catalyst device 23 has a predetermined period (for example, 0.5 Hz or less, preferably 0.35 Hz or less) and a predetermined amplitude. As shown in FIGS. 7 and 8, the target combustion air-fuel ratio is forced around an average air-fuel ratio (modulation center air-fuel ratio) that is, for example, a stoichiometric air-fuel ratio, so as to change between a lean air-fuel ratio and a rich air-fuel ratio. Modulation is performed (step S22). Then, the output of the downstream O ₂ sensor 28 is read (step S23).

Next, the ECU 30 determines the frequency of the target combustion air-fuel ratio and the frequency of the downstream O ₂ sensor output, and calculates the frequency ratio by dividing the frequency of the output of the downstream O ₂ sensor 27 by the frequency of the target combustion air-fuel ratio. Then, the calculated frequency ratio is compared with the catalyst deterioration determination value (step S25).
As shown in FIG. 9, the catalyst deterioration judgment value (indicated by a broken line in the lower graph of FIG. 9) is an allowable lower limit value of the NOx purification efficiency of the NOx trap catalyst 24 (indicated by a broken line in the upper graph of FIG. 9). It corresponds to. As described above, the S trap amount in the OBD catalyst 25 is considerably smaller than the S trap amount in the NOx trap catalyst 24. Therefore, the horizontal axis ( The scale of (S trap amount) is different. As described above, by performing forced modulation on the target combustion air-fuel ratio at an extremely low frequency of about 0.5 Hz or less, the S trap amount / frequency ratio characteristic line shown in the lower graph of FIG. Thus, the frequency ratio is greatly changed with a slight change in the amount of S traps in the OBD catalyst 25 and the oxygen storage capacity.

  If the oxygen storage capacity of the OBD catalyst 25 is high, oxygen is stored in the catalyst 25. Therefore, as shown in FIG. 7, the response of the exhaust air-fuel ratio downstream of the OBD catalyst 25 to the change in the target combustion air-fuel ratio is Therefore, the frequency ratio becomes smaller than the judgment value. In this case, since the oxygen storage capacity of the OBD catalyst 25 is high, the ECU 30 is not so large that the S trap amount in the catalyst 25 and hence the S trap amount in the NOx trap catalyst 24 causes S deterioration (FIG. 9). In the upper graph, it is determined that the current NOx purification efficiency is higher than the allowable lower limit purification efficiency).

  On the other hand, when the oxygen storage capacity of the OBD catalyst 25 is lowered, oxygen is not stored in the catalyst 25 but is discharged to the downstream side, so that the response of the exhaust air / fuel ratio downstream of the catalyst becomes faster as shown in FIG. The frequency ratio becomes larger than the determination value. In this case, the ECU 30 determines that the oxygen storage capacity of the OBD catalyst 25 has decreased, and accordingly, the amount of S traps in the catalyst 25 and thus the NOx trap catalyst 24 is large, and the NOx trap catalyst 24 has been deteriorated by S. Estimation is performed (step S26), and S purge control is performed (step S27). The warning lamp 31 may be turned on instead of the S purge control.

Here, the S degradation is determined by comparing the frequencies of both the target combustion air-fuel ratio and the lower O ₂ sensor output, but the amplitudes of the two may be compared as in the first embodiment.
This is the end of the description of the embodiment and the modification of the present invention. However, the present invention is not limited to the above-described embodiment and modification, and various modifications can be made.
For example, in the first and second embodiments and the modification of the first embodiment, the case where the present invention is applied to an in-cylinder injection type spark ignition in-line four-cylinder gasoline engine has been described. The present invention can be applied to other engines such as an intake pipe injection type lean burn engine. In the first embodiment and its modification, the OBD catalyst 25 is arranged on the downstream side of the NOx trap catalyst 24, but may be arranged on the upstream side of the NOx trap catalyst 24 as shown in FIG. In FIG. 10, reference numeral 29 indicates a three-way catalyst.

It is the schematic which shows the NOx trap catalyst degradation estimation apparatus by 1st Embodiment of this invention with the internal combustion engine with which the apparatus is equipped. FIG. ₂ is a diagram showing air-fuel ratio / output amplitude characteristics of upstream and downstream O ₂ sensors shown in FIG. 1. It is a flowchart of S degradation estimation routine in 1st Embodiment. FIG. 2 is a diagram showing a response of an exhaust air / fuel ratio downstream of the catalyst to a change in the exhaust air / fuel ratio upstream of the catalyst when the oxygen storage capacity of the OBD catalyst shown in FIG. 1 is high. FIG. 5 is a view similar to FIG. 4 when the oxygen storage capacity of the OBD catalyst is lowered. It is a flowchart of S degradation estimation routine in 2nd Embodiment of this invention. It is a figure which shows the response of the exhaust air-fuel ratio downstream of a catalyst with respect to the change of the target air-fuel ratio when the oxygen storage capability of the OBD catalyst is high in the second embodiment. FIG. 8 is a view similar to FIG. 7 when the oxygen storage capacity of the OBD catalyst is lowered. It is a figure which shows the relationship between S trap amount and NOx purification efficiency characteristic in a NOx trap catalyst, and S trap amount and frequency ratio in an OBD catalyst. FIG. 6 is a partial schematic diagram showing an arrangement relationship between an OBD catalyst and a NOx trap catalyst in a modification of the first embodiment.

Explanation of symbols

11 Engine 21 Exhaust Pipe 23 Exhaust Purification Catalyst Device 24 NOx Trap Catalyst 25 Degradation Determination Catalyst (OBD Catalyst)
27 Upstream O ₂ sensor 28 Downstream O ₂ sensor 30 ECU
31 Warning light

Claims (3)

In a catalyst deterioration estimation device that estimates the deterioration of a NOx trap catalyst that stores NOx in exhaust gas when the exhaust air-fuel ratio is lean, and releases and reduces the stored NOx when the exhaust air-fuel ratio is rich,
A deterioration determination catalyst that is disposed upstream or downstream of the NOx trap catalyst in the exhaust passage of the internal combustion engine and that stores SOx and whose oxygen storage capacity changes according to the SOx storage state;
Downstream air-fuel ratio detecting means for detecting an exhaust air-fuel ratio downstream of the deterioration determining catalyst;
A catalyst deterioration estimation device comprising: NOx trap catalyst deterioration estimation means for estimating deterioration of the NOx trap catalyst based on an exhaust air fuel ratio detected by the downstream air-fuel ratio detection means. An upstream air-fuel ratio detecting means for detecting an exhaust air-fuel ratio upstream of the deterioration determining catalyst,
The NOx trap catalyst deterioration estimating means is configured to detect the NOx trap catalyst based on a comparison result between the exhaust air / fuel ratio detected by the upstream air / fuel ratio detecting means and the exhaust air / fuel ratio detected by the downstream air / fuel ratio detecting means. 2. The catalyst deterioration estimating apparatus according to claim 1, wherein the deterioration is estimated. Air-fuel ratio modulation means for forcibly modulating the exhaust air-fuel ratio upstream of the deterioration determination catalyst,
The NOx trap catalyst deterioration estimating means compares the exhaust air / fuel ratio forcibly modulated by the air / fuel ratio modulating means and the exhaust air / fuel ratio detected by the downstream air / fuel ratio determining means, and based on the comparison result, 2. The catalyst deterioration estimation device according to claim 1, wherein the deterioration of the NOx trap catalyst is estimated.

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