JP4186259B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is applied to an air-fuel ratio control system for an internal combustion engine that performs lean combustion in an air-fuel ratio lean region, and a NOx occlusion reduction type catalyst for purifying nitrogen oxide (NOx) in exhaust gas generated during lean combustion. The present invention relates to an exhaust gas purification device for an internal combustion engine having
[0002]
[Prior art]
In recent years, in an air-fuel ratio control device for an internal combustion engine, a technique for performing so-called lean burn control in which fuel is burned on the lean side of the stoichiometric air-fuel ratio is being used in order to improve fuel efficiency. When such lean combustion is performed, the exhaust gas discharged from the internal combustion engine contains a large amount of NOx, and a lean NOx catalyst for purifying this NOx is required. This lean NOx catalyst is known as a NOx occlusion reduction type catalyst. It stores NOx when the air-fuel ratio of the exhaust gas is lean, and also stores the stored NOx when the oxygen concentration of the exhaust gas is lowered, that is, when it is enriched. Reduce and release.
[0003]
In the “exhaust gas purification device for an internal combustion engine” of Japanese Patent No. 2692380, an air-fuel ratio sensor is disposed in the exhaust passage downstream of the lean NOx catalyst (NOx absorbent), and the NOx catalyst of the NOx catalyst accompanying the enrichment of the air-fuel ratio is disclosed. After the NOx releasing action is started, it is determined that the NOx releasing action of the NOx catalyst is completed when the air-fuel ratio detected by the air-fuel ratio sensor switches from lean to rich. In this case, a decrease in the amount of NOx that can be stored in the NOx catalyst means that the NOx catalyst has deteriorated. Therefore, the NOx storage amount decreases, that is, the NOx catalyst deteriorates based on the reaction time required for NOx release. It was able to be detected.
[0004]
[Problems to be solved by the invention]
By the way, the air-fuel ratio sensor (O2 sensor) installed on the downstream side of the NOx catalyst changes its output abruptly with the theoretical air-fuel ratio (λ = 1) as a boundary. Therefore, even if the gas composition slightly changes, the change follows. Sensor output changes.
[0005]
For example, when platinum Pt and barium Ba are supported on the support, if the air-fuel ratio on the upstream side of the NOx catalyst is switched from lean to rich, theoretically,
Ba (NO3) 2 + HC, CO → Ba + N2 + H2 O + CO2
It becomes. Therefore, until the reaction between the NOx occluded in the NOx catalyst and the rich components (HC, CO) is completed, the air-fuel ratio on the downstream side of the catalyst stays at the theoretical air-fuel ratio. It is thought that it will move to the rich side. However, in practice, by enriching the air-fuel ratio, a small amount of rich component flows out downstream of the catalyst before the stored NOx disappears, and the sensor output changes to the rich side due to the rich component.
[0006]
Therefore, there is a case where the time when the stored NOx disappears does not correspond to the behavior of the sensor output, and the conventional publication cannot accurately detect the deterioration of the NOx catalyst. In addition, since the reaction time of the sensor changes depending on how much the exhaust gas supplied to the NOx catalyst is actually rich or how rich the exhaust gas is, the above-mentioned conventional publication accurately reduces the NOx occlusion amount. It was difficult to detect.
[0007]
The present invention has been made paying attention to the above problems, and an object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can accurately detect deterioration of a lean NOx catalyst.
[0008]
[Means for Solving the Problems]
The exhaust gas purifying apparatus according to the present invention includes, as a premise, a lean NOx catalyst provided in the engine exhaust system, performs lean combustion in the air-fuel ratio lean region, and converts NOx in the exhaust gas discharged during lean combustion to lean NOx. The catalyst is occluded, and the air-fuel ratio is temporarily controlled to be rich to release the occluded NOx from the lean NOx catalyst.
[0015]
Contract Claim 1 In the invention described in the above, the NOx purification rate calculated by the catalyst is calculated from the ratio between the NOx inflow amount flowing into the lean NOx catalyst during lean combustion and the rich gas amount required for NOx purification in the lean NOx catalyst during rich combustion. A purification rate calculating means; and a deterioration detecting means for detecting the deterioration of the lean NOx catalyst based on the calculated NOx purification rate.
[0016]
That is, when the lean NOx catalyst deteriorates and the NOx storage capacity decreases, the amount of rich gas required for NOx purification in the catalyst decreases. Therefore, by obtaining the NOx purification rate as a factor of the rich gas amount required for NOx purification, and detecting catalyst deterioration from the NOx purification rate, Even if the amount of rich component flows out to the downstream side of the catalyst and the sensor output value changes to the rich side before the stored NOx disappears as the air-fuel ratio becomes richer Therefore, it is possible to accurately detect the deterioration of the lean NOx catalyst.
[0017]
Claims above 1 In the invention described in claim 2 As described above, it is preferable to detect that the degree of deterioration of the lean NOx catalyst is larger as the amount of rich gas required for NOx purification in the lean NOx catalyst decreases and the NOx purification rate decreases.
[0018]
Claim 3 In the present invention, the NOx inflow amount A flowing into the lean NOx catalyst during lean combustion is calculated based on the detection result of the upstream sensor, and the lean NOx catalyst during rich combustion is calculated based on the detection result of the upstream sensor. The rich gas inflow amount B flowing into the gas is calculated. Further, the surplus gas amount C discharged from the lean NOx catalyst during the rich combustion is calculated based on the detection result of the downstream sensor. And the NOx purification rate calculating means is based on the calculated NOx inflow amount A during lean combustion, rich gas inflow amount B and surplus gas amount C during rich combustion,
(BC) / A
The NOx purification rate is calculated from the result of the calculation.
[0019]
In this case, when the deterioration of the lean NOx catalyst proceeds, the surplus gas amount C discharged from the catalyst with respect to the rich gas inflow amount B during the rich combustion increases, and as a result, the NOx purification rate decreases. And the effect of catalyst deterioration is detected by the fall of a NOx purification rate.
[0020]
And the above claims 1 , 3 In the present invention, when calculating the NOx purification rate as the deterioration determination parameter, “NOx inflow amount (A)” flowing into the lean NOx catalyst and “rich gas amount (rich gas inflow amount B required for NOx purification) in the lean NOx catalyst”. −excess gas amount C) ”, but“ NOx inflow amount (A) ”includes information on lean combustion such as lean degree and lean time, and“ rich gas amount (BC) ”includes rich degree. And information related to rich combustion such as rich time. Therefore, when the “rich gas amount required for NOx purification” changes as a reflection of catalyst deterioration, correction according to various conditions during lean combustion or rich combustion is appropriately added to the rich gas amount. Become. Therefore, catalyst deterioration can always be accurately detected regardless of changes in combustion conditions. In this case, the inconvenience that the detection of deterioration is restricted by the combustion condition is avoided.
[0021]
For example, if the lean combustion is extended, the NOx inflow amount A increases. At the same time, a large amount of NOx is occluded in the NOx catalyst, and the amount of rich gas (BC) required for NOx purification naturally increases. Therefore, there is no inconvenience that the NOx purification rate is changed in accordance with the change of the combustion condition and the catalyst deterioration is erroneously detected although the degree of catalyst deterioration is unchanged. Further, when rich combustion is extended, the rich gas inflow amount B during rich combustion increases, but at the same time, the surplus gas amount C during rich combustion also increases. Therefore, there is no inconvenience that the NOx purification rate is changed along with the change of the combustion state and the catalyst deterioration is erroneously detected.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. In the air-fuel ratio control system according to the present embodiment, the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set to be leaner than the stoichiometric air-fuel ratio, and lean combustion is performed based on the target air-fuel ratio. Implement control. As a main configuration of the system, a NOx occlusion reduction catalyst (hereinafter referred to as NOx catalyst) is provided in the middle of the exhaust system passage of the internal combustion engine, and a limit current type air-fuel ratio sensor (A / A) is provided upstream of the NOx catalyst. F sensor) and an oxygen sensor (O2 sensor) are disposed downstream. An electronic control unit (hereinafter referred to as ECU) having a microcomputer as a main body takes in the detection results of the A / F sensor and the O2 sensor, and feedback-controls the air-fuel ratio based on the detection results. The detailed configuration will be described below with reference to the drawings.
[0023]
FIG. 1 is a schematic configuration diagram of an air-fuel ratio control system in the present embodiment. As shown in FIG. 1, the internal combustion engine is configured as a four-cylinder four-cycle spark ignition engine (hereinafter referred to as engine 1). The intake air passes through the air cleaner 2, the intake pipe 3, the throttle valve 4, the surge tank 5, and the intake manifold 6 from the upstream, and is mixed with the fuel injected from the fuel injection valve 7 for each cylinder in the intake manifold 6. The Then, it is supplied to each cylinder as an air-fuel mixture having a predetermined air-fuel ratio.
[0024]
A high voltage supplied from an ignition circuit 9 is distributed and supplied to a spark plug 8 provided in each cylinder of the engine 1 via a distributor 10, and the spark plug 8 ignites an air-fuel mixture in each cylinder at a predetermined timing. . Exhaust gas discharged from each cylinder after combustion passes through the exhaust manifold 11 and the exhaust pipe 12, passes through a NOx catalyst 14 provided in the exhaust pipe 12, and is then discharged to the atmosphere. The NOx catalyst 14 mainly stores NOx in the exhaust gas during combustion at a lean air-fuel ratio, and reduces and releases the stored NOx with a rich component (CO, HC, etc.) during combustion at a rich air-fuel ratio.
[0025]
The intake pipe 3 is provided with an intake air temperature sensor 21 and an intake pressure sensor 22, the intake air temperature sensor 21 is the intake air temperature (intake air temperature Tam), and the intake pressure sensor 22 is the negative pressure in the intake pipe downstream of the throttle valve 4. The pressure (intake pressure PM) is detected. The throttle valve 4 is provided with a throttle sensor 23 for detecting the opening of the valve 4 (throttle opening TH), and the throttle sensor 23 outputs an analog signal corresponding to the throttle opening TH. The throttle sensor 23 also includes an idle switch and outputs a detection signal indicating that the throttle valve 4 is substantially fully closed.
[0026]
A water temperature sensor 24 is provided in the cylinder block of the engine 1, and the water temperature sensor 24 detects the temperature of cooling water circulating in the engine 1 (cooling water temperature Thw). The distributor 10 is provided with a rotational speed sensor 25 for detecting the rotational speed of the engine 1 (engine rotational speed Ne). The rotational speed sensor 25 is arranged at equal intervals every two rotations of the engine 1, that is, every 720 ° CA. 24 pulse signals are output.
[0027]
Further, a limit current type A / F sensor 26 is disposed upstream of the NOx catalyst 14 in the exhaust pipe 12, and the sensor 26 detects the oxygen concentration (or not yet) of exhaust gas discharged from the engine 1. A linear and linear air-fuel ratio signal (AF) is output in proportion to the CO concentration in the fuel gas. An O2 sensor 27 is disposed downstream of the NOx catalyst 14, and the sensor 27 outputs an electromotive force signal (VOX2) that varies depending on whether the exhaust gas is rich or lean.
[0028]
The ECU 30 is configured as a logical operation circuit centering on the CPU 31, ROM 32, RAM 33, backup RAM 34, and the like, and has a bus for an input port 35 for inputting detection signals of the sensors and an output port 36 for outputting control signals to the actuators. 37 is connected. The ECU 30 inputs detection signals (intake air temperature Tam, intake air pressure PM, throttle opening TH, cooling water temperature Thw, engine speed Ne, air-fuel ratio signal, etc.) from the various sensors described above through the input port 35. Based on these values, control signals such as the fuel injection amount TAU and ignition timing Ig are calculated, and these control signals are output to the fuel injection valve 7 and the ignition circuit 9 via the output port 36, respectively.
[0029]
Next, the operation of the air-fuel ratio control system configured as described above will be described.
FIG. 2 is a flowchart showing a fuel injection control routine executed by the CPU 31, and this routine is executed for each fuel injection of each cylinder (in this embodiment, every 180 ° CA).
[0030]
When the routine of FIG. 2 starts, the CPU 31 first reads the sensor detection result (engine speed Ne, intake pressure PM, cooling water temperature Tw, etc.) indicating the engine operating state in step 101, and then in the ROM 32 in step 102. The basic injection amount Tp corresponding to the engine speed Ne and the intake pressure PM at that time is calculated using the basic injection map stored in advance. In step 200, the CPU 31 sets the target air-fuel ratio AFTG according to the routine shown in FIG.
[0031]
Thereafter, in step 103, the CPU 31 sets an air-fuel ratio correction coefficient FAF based on the deviation between the actual air-fuel ratio AF (sensor measurement value) and the target air-fuel ratio AFTG at that time. In the present embodiment, air-fuel ratio F / B control based on the modern control theory is performed, and the FAF value is set according to the setting procedure disclosed in, for example, Japanese Patent Application Laid-Open No. 1-110853. However, detailed description thereof is omitted.
[0032]
After setting the FAF value, in step 104, the CPU 31 uses the following formula to calculate the final fuel injection from the basic injection amount Tp, the air-fuel ratio correction coefficient FAF, and other correction coefficients FALL (various correction coefficients such as water temperature and air conditioner load). The quantity TAU is calculated.
[0033]
TAU = Tp / FAF / FALL
After calculating the fuel injection amount TAU, the CPU 31 outputs a control signal corresponding to the TAU value to the fuel injection valve 7 and once ends this routine.
[0034]
The F / B control is executed when the F / B condition is established such that the coolant temperature Tw is equal to or higher than a predetermined temperature, is not in a high rotation / high load state, or the A / F sensor 26 is in an active state. If the F / B condition is not satisfied, air-fuel ratio open control is executed (FAF = 1.0).
[0035]
Next, the procedure for setting the target air-fuel ratio AFTG (the process of step 200) will be described with reference to the flowchart of FIG. In this process, the target air-fuel ratio AFTG is appropriately set so that rich combustion is temporarily performed during the lean combustion. That is, in the present embodiment, the lean time TL and the rich time TR are set so as to have a predetermined time ratio based on the value of the cycle counter counted for each fuel injection, and according to the respective times TL and TR. Thus, lean combustion and rich combustion are alternately performed.
[0036]
The processing of FIG. 3 will be described step by step. First, the CPU 31 determines in step 201 whether or not the current cycle counter is “0”. On the condition that the cycle counter = 0, the CPU 31 leans based on the engine speed Ne and the intake pressure PM in step 202. Time TL and rich time TR are set. If step 201 is NO (when the cycle counter ≠ 0), the CPU 31 skips the process of step 202.
[0037]
Here, the lean time TL and the rich time TR correspond to the number of fuel injections at the lean air-fuel ratio and the number of fuel injections at the rich air-fuel ratio, respectively. A higher value is set as the atmospheric pressure PM is higher. In the present embodiment, the rich time TR is obtained by map search based on the relationship of FIG. On the other hand, the lean time TL is calculated from the rich time TR and a predetermined coefficient α.
TL = TR ・ α
As required. The coefficient α may be a fixed value of about “50”, but may be variably set according to the engine operating state such as Ne or PM.
[0038]
Thereafter, the CPU 31 increments the period counter by “1” in step 203, and determines whether or not the value of the period counter has reached a value corresponding to the lean time TL in subsequent step 204. When the cycle counter <TL, the CPU 31 proceeds to step 205 and sets the target air-fuel ratio AFTG as a lean control value based on the engine speed Ne and the intake pressure PM at that time. After setting the AFTG value, the CPU 31 ends this routine and returns to the original routine of FIG.
[0039]
The AFTG value is obtained, for example, by searching the target air-fuel ratio map shown in FIG. 5, and a value corresponding to, for example, A / F = 20 to 23 is set as the AFTG value (however, the lean combustion is not performed at the time of steady operation, for example) If the condition is not satisfied, the AFTG value is set near the stoichiometric value). In such a case, the air-fuel ratio is lean-controlled by the AFTG value set in step 205.
[0040]
When the cycle counter ≧ TL, the CPU 31 proceeds to step 206 and sets the target air-fuel ratio AFTG as a rich control value. The AFTG value may be a fixed value in the rich region, or may be variably set by searching the map based on the engine speed Ne and the intake pressure PM. When the map search is performed, the AFTG value is set so that the richness becomes stronger as the engine speed Ne is higher or the intake pressure PM is higher.
[0041]
Thereafter, in step 207, the CPU 31 determines whether or not the value of the cycle counter has reached a value corresponding to the total time “TL + TR” of TL and TR time. If the cycle counter <TL + TR, the CPU 31 ends this routine. The process returns to the original routine of FIG. In such a case, the air-fuel ratio is richly controlled by the AFTG value set in step 206.
[0042]
On the other hand, if cycle counter ≧ TL + TR and step 207 is affirmatively determined, the CPU 31 clears the cycle counter to “0” in step 208, thereafter ends this routine and returns to the original routine of FIG. As the cycle counter is cleared, step 201 is affirmed at the next processing, and the lean time TL and the rich time TR are newly set. Then, the above-described lean control and rich control are performed again based on the lean time TL and the rich time TR.
[0043]
FIG. 6 is a time chart for explaining the air-fuel ratio control operation by the routines of FIG. 2 and FIG.
In FIG. 6, during the period from time t1 to t2 (period counter = 0 to TL), the air-fuel ratio is lean-controlled and NOx in the exhaust gas is occluded in the NOx catalyst 14. In the period from time t2 to t3 (period counter = TL to TL + TR period), the air-fuel ratio is richly controlled, and the stored NOx of the NOx catalyst 14 is reduced by the unburned gas components (HC, CO) in the exhaust gas. Released. Thus, the lean control and rich control of the air-fuel ratio are repeatedly performed according to the lean time TL and the rich time TR.
[0044]
On the other hand, FIG. 7 is a flowchart showing a routine for detecting deterioration of the NOx catalyst 14, and this routine is executed by the CPU 31 for each fuel injection of each cylinder. In the present embodiment, during rich control, the output VOX2 of the O2 sensor 27 on the downstream side of the NOx catalyst (for convenience, this is referred to as “rear O2 output”) is monitored, and the NOx storage capability of the NOx catalyst 14 is based on the peak value. Is estimated. The degree of catalyst deterioration is detected based on the estimated NOx storage capacity.
[0045]
That is, as seen in FIGS. 9A and 9B, the peak value of the rear O2 output VOX2 is different if the degree of catalyst deterioration is different. In FIG. 9B, the peak value is larger than that in (a). Therefore, it can be determined that the catalyst deterioration is progressing. 10 (a) and 10 (b) show the case where the rich time is extended. In this case, the rear O2 output VOX2 reaches the maximum value (about 1V) on the rich side regardless of the presence or absence of catalyst deterioration. Therefore, it becomes difficult to detect deterioration based on the peak value of the rear O2 output.
[0046]
Hereinafter, the process of FIG. 7 will be described in detail. First, in step 301, the CPU 31 determines whether or not an execution condition for deterioration detection is satisfied. The execution condition for the deterioration detection includes that the rich time is shorter than a predetermined value. For example, in the state of FIG. 9, the condition is established because the peak value of the rear O2 output VOX2 can be determined, and in the state of FIG. 10, the condition is not established because the peak value of the rear O2 output VOX2 cannot be determined. In addition,
The richness is within a predetermined range,
-The lean time or rich time at the time of lean combustion is within a predetermined range,
-The catalyst is in a steady operating state where the catalyst temperature is around 350 ° C.
Etc. may be included in the implementation conditions. Then, if the execution condition is satisfied, the CPU 31 proceeds to step 302, and if the execution condition is not satisfied, the routine is once ended.
[0047]
Thereafter, the CPU 31 determines whether or not the counter CCATDT is “0” in step 302, and proceeds to step 303 on condition that CCATDT = 0. In step 303, the CPU 31 determines whether it is the timing for starting the rich control. If step 303 is YES, the CPU 31 proceeds to step 304 and sets a predetermined value “KCCATDT” in the counter CCATDT. The predetermined value KCCATDT may be about three times as long as the rich time TR.
[0048]
For example, at time t2 in FIG. 6, step 303 is YES, and at this time t2, a predetermined value KCCATDT is set. If step 303 is NO, the CPU 31 ends this routine as it is.
[0049]
When the predetermined value KCCATDT is set at the beginning of rich control as described above, step 302 becomes NO from the next time, and the CPU 31 decrements the counter CCATDT by “1” in step 305, and then proceeds to step 306.
[0050]
In step 306, the CPU 31 determines whether the counter CCATDT is “0”. If CCATDT ≠ 0, the CPU 31 proceeds to step 307 to determine whether or not the rear O2 output VOX2 is larger than the maximum value Vmax up to the previous time. If VOX2> Vmax, the CPU 31 proceeds to step 308 to update the maximum value Vmax by the rear O2 output VOX2 at that time, and if VOX2 ≦ Vmax, the routine is terminated as it is. That is, the peak value of the rear O2 output VOX2 is obtained by repeatedly performing Steps 307 and 308.
[0051]
On the other hand, if CCATDT = 0 and the determination in step 306 is affirmative, the CPU 31 proceeds to step 309 and stores the NOx in the NOx catalyst 14 based on the calculated rear O2 output maximum value Vmax (rear O2 output peak value). Estimate the amount. At this time, it is estimated that the larger the maximum value Vmax of the rear O2 output, the smaller the NOx occlusion amount.
[0052]
Then, at step 310, the CPU 31 determines the degree of deterioration of the NOx catalyst 14 based on the estimated NOx occlusion amount using the relationship shown in FIG. In FIG. 8, as the estimated NOx occlusion amount increases (the rear O2 output peak value decreases), the degree of catalyst deterioration decreases, and conversely, the NOx occlusion amount decreases (the rear O2 output peak value increases). A relationship that increases the degree of deterioration is given. In this case, it is determined that there is “deterioration” if it is within the hatched area in FIG.
[0053]
If it is determined in step 310 that there is deterioration, the CPU 31 turns on an abnormality warning light (MIL: Malfunction Indicator Light) in step 311 to warn the driver that an abnormality has occurred and restores the NOx storage capacity. A reproduction process is performed. Finally, the CPU 31 clears the maximum value Vmax of the rear O2 output to “0” in step 312 and thereafter ends this routine.
[0054]
Note that, in the regeneration process of step 311, for example, a process for recovering sulfur poisoning that is a main cause of catalyst deterioration is executed. Since the regeneration process is not the gist of the present plan, a detailed description thereof will be omitted. However, if the outline is simply described, the temperature (catalyst temperature) of the NOx catalyst 14 is increased by increasing the ratio of rich combustion during lean combustion. At the same time, stoichiometric control or weak rich control is performed at an air-fuel ratio λ = 1. When rich components (HC, CO) are supplied to the catalyst 14 while the NOx catalyst 14 is at a high temperature, the sulfate BaSO4 produced by sulfur poisoning is reduced and sulfur is released. Thereby, the NOx catalyst 14 is regenerated.
[0055]
In addition, if the deterioration state of the NOx catalyst 14 is continuously detected regardless of the catalyst regeneration process, the catalyst 14 is regarded as being in a state in which it cannot be regenerated, and finally it is determined that an abnormality has occurred. The When it is finally determined that an abnormality has occurred, the subsequent lean control is prohibited and, for example, stoichiometric control at λ = 1 is performed. The abnormality warning lamp may be turned on after it is finally determined that an abnormality has occurred.
[0056]
In the present embodiment, steps 307 to 309 in FIG. 7 correspond to estimation means described in claims, and step 310 corresponds to deterioration detection means.
According to the embodiment described in detail above, the following effects can be obtained.
[0057]
In the present embodiment, the NOx storage capacity of the NOx catalyst 14 is estimated based on the peak value of the rear O2 output VOX2 during rich combustion, and the deterioration of the catalyst 14 is detected based on the estimated NOx storage capacity. did. According to this configuration, the NOx occlusion capability can be accurately determined while reflecting how rich the exhaust gas supplied to the NOx catalyst 14 is actually or how rich the exhaust gas is. In this case, even if a small amount of rich component flows out to the downstream side of the catalyst and the sensor output value changes to the rich side before the stored NOx disappears due to the enrichment of the air-fuel ratio, it depends on the state of catalyst deterioration at that time. Appropriate sensor output information can be obtained. As a result, it is possible to accurately detect the deterioration of the NOx catalyst 14.
[0058]
In addition, the deterioration detection execution conditions are set, and the NOx storage capacity is estimated only when, for example, the rich time is shorter than a predetermined value. In this case, the reliability can be improved by detecting the deterioration only when the rich gas amount is equal to or less than the predetermined value.
[0059]
Next, second to fifth embodiments of the present invention will be described. However, in the configuration of each of the following embodiments, components that are equivalent to those of the first embodiment described above are given the same reference numerals in the drawings and the description thereof is simplified. In the following description, differences from the first embodiment will be mainly described.
[0060]
(Second Embodiment)
In the second embodiment, the “NOx purification rate (= NOx purification amount / NOx inflow amount)” is obtained from the ratio between the NOx purification amount in the NOx catalyst 14 and the NOx inflow amount into the catalyst 14, The deterioration of the NOx catalyst 14 is detected according to the NOx purification rate.
[0061]
Here, the “NOx purification amount” can be obtained as the actual rich gas amount required for NOx purification. In such a case, the air-fuel ratio before and after the NOx catalyst is monitored during rich combustion to determine the difference between the rich gas inflow amount and the surplus gas amount, and the NOx purification amount is determined from the difference between the rich gas inflow amount and the surplus gas amount. Actually, the output AF of the A / F sensor 26 on the upstream side of the catalyst during rich combustion is integrated to calculate the rich gas integrated value AFAD as the rich gas inflow, and the O2 sensor on the downstream side of the catalyst in the rich combustion as well. 27 output VOX2 is integrated to calculate a rear O2 output integrated value VOX2AD as an excess gas amount. The difference between the rich gas integrated value AFAD and the rear O2 output integrated value VOX2AD is set as the NOx purification amount (NOx purification amount = AFAD−VOX2AD).
[0062]
Further, the “NOx inflow amount” can be obtained as the NOx amount supplied to the NOx catalyst 14. Actually, the NOx integrated amount CNOXAD as the NOx inflow amount is calculated based on the engine operating state (Ne, PM, A / F) at the time of lean combustion.
[0063]
And
(AFAD-VOX2AD) / CNOXAD
The calculation result of “NOx purification rate” is used, and the deterioration of the NOx catalyst 14 is detected using this NOx purification rate as a deterioration determination parameter.
[0064]
The control operation of the CPU 31 in the present embodiment will be described using the flowcharts of FIGS. 11 and 13 to 16. FIG. 11 shows the procedure for estimating the NOx integrated amount of the NOx catalyst 14, and FIGS. 13 and 14 show the detection procedure for catalyst deterioration. The processes of FIGS. 13 and 14 are performed in place of the process of FIG.
[0065]
In FIG. 11, the CPU 31 first determines in step 401 whether or not the current output AF (the air-fuel ratio on the upstream side of the catalyst) of the A / F sensor 26 is a lean value, and that step is positively determined. Then, the process proceeds to step 402. In step 402, the CPU 31 estimates the NOx amount CNOX (mole) contained in the exhaust gas based on the engine operating state. When estimating the CNOX value, for example, the basic amount of NOx corresponding to the engine speed Ne and the intake pressure PM at that time is obtained using the map of FIG. 12A, and the relationship shown in FIG. An A / F correction value corresponding to the air / fuel ratio is obtained. Then, the NOx basic amount and the A / F correction value are multiplied to obtain a CNOX value (CNOX = NOx basic amount / A / F correction value).
[0066]
Incidentally, in FIG. 12A, the NOx basic amount is set to a larger value as the engine speed Ne is higher or the intake pressure PM is larger. In FIG. 12B, the A / F correction value = 1.0 is set at the theoretical air-fuel ratio (λ = 1), and an A / F correction value of “1.0” or more is set on the lean side. Is done. However, when the air-fuel ratio is leaner than a certain level (for example, A / F> 16), the combustion temperature is lowered, so that no further correction on the increase side is necessary, and the A / F correction value converges to a predetermined value.
[0067]
Thereafter, the CPU 31 calculates a NOx integrated amount CNOXAD in step 403. At this time, the CNOX value calculated in step 402 is added to the previous value of the CNOXAD value, and the sum is used as the current value of the CNOXAD value (CNOXAD = CNOXAD + CNOX).
[0068]
On the other hand, in the catalyst deterioration detection routine of FIG. 13, the CPU 31 first determines in step 501 whether or not the counter CCATDT is “0”, and proceeds to step 502 on condition that CCATDT = 0. In step 502, the CPU 31 determines whether it is the timing for starting the rich control.
[0069]
If step 502 is NO, the CPU 31 proceeds to step 503 to determine whether or not lean control is currently being performed. When the lean control is being performed, the CPU 31 calculates the O2 output smoothed value VOX2SM from the rear O2 output VOX2 in step 504. That means
VOX2SM = (31/32) VOX2SM + (1/31) VOX2
Is used to calculate the O2 output smoothed value VOX2SM.
[0070]
If step 502 is YES, the CPU 31 proceeds to step 505 to set a predetermined value “KCCATDT” in the counter CCATDT. The predetermined value KCCATDT may be about three times as long as the rich time TR (same as step 304 in FIG. 7). When the predetermined value KCCATDT is set, step 501 becomes NO from the next time, and the CPU 31 decrements the counter CCATDT by “1” in step 506, and then proceeds to step 600.
[0071]
In step 600, the CPU 31 calculates a rear O2 output integrated value VOX2AD in accordance with a routine shown in FIG. In step 700, the CPU 31 calculates the rich gas integrated value AFAD according to the routine shown in FIG.
[0072]
Thereafter, the CPU 31 proceeds to step 507 in FIG. 14 to determine whether or not the counter CCATDT is “0”. If CCATDT ≠ 0, the CPU 31 ends this routine as it is. Further, when CCATDT = 0 in accordance with the countdown in step 506, the CPU 31 proceeds to step 508,
NOXCONV = CNOXAD / (AFAD−VOX2AD)
The deterioration determination value NOXCONV is calculated using the following equation.
[0073]
Thereafter, in step 509, the CPU 31 calculates the NOx purification rate from the NOXCONV value using the relationship shown in FIG. 17, and determines the degree of catalyst deterioration based on the NOx purification rate using the relationship shown in FIG. In FIG. 18, the relationship is such that the higher the NOx purification rate, the smaller the degree of catalyst deterioration, and conversely, the lower the NOx purification rate, the larger the degree of catalyst deterioration. In this case, if there is a hatched area in FIG.
[0074]
If it is determined in step 510 that there is deterioration, the CPU 31 turns on an abnormality warning lamp to warn the driver that an abnormality has occurred in step 511 and performs a regeneration process to restore the NOx storage capacity. (Same as step 311 in FIG. 7). Finally, the CPU 31 clears the values of CNOXAD, VOX2AD, and AFAD to “0” in step 512, and then ends this routine.
[0075]
Next, the calculation procedure of the rear O2 output integrated value VOX2AD (the process of step 600) will be described with reference to the flowchart of FIG. In this process, the CPU 31 first subtracts the O2 output smoothed value VOX2SM (calculated value in FIG. 13, step 504) from the occasional rear O2 output VOX2 at step 601 and sets the difference as the O2 output deviation VOX2DV ( VOX2DV = VOX2-VOX2SM). In the subsequent step 602, the CPU 31 determines whether the absolute value of the O2 output deviation VOX2DV is 0.02 V or more, that is, the rear O2 output VOX2 at that time is “0. It is determined whether or not it has changed to the rich side by “02V” or more.
[0076]
If | VOX2DV | <0.02 V (NO in step 602), the CPU 31 ends this routine as it is and returns to the original routine of FIGS. If | VOX2DV | ≧ 0.02 V (YES in step 602), the CPU 31 calculates “VOX2DV1 value” from the product of the O2 output deviation VOX2DV and the intake air amount QA in step 603 (VOX2DV1 = VOX2DV · QA). The intake air amount QA is calculated based on the engine speed Ne and intake pressure PM at that time.
[0077]
Further, the CPU 31 calculates the rear O2 output integrated value VOX2AD at step 604, thereafter ends this routine, and returns to the original routine of FIGS. In step 604, the calculated VOX2DV1 value is added to the previous value of the VOX2AD value, and the sum is used as the current value of the VOX2AD value (VOX2AD = VOX2AD + VOX2DV1).
[0078]
The procedure for calculating the rich gas integrated value AFAD (the processing of step 700) will be described with reference to the flowchart of FIG. In this process, the CPU 31 first subtracts the output AF (actual air-fuel ratio) of the A / F sensor 26 from the air-fuel ratio reference value AFSD (for example, the theoretical air-fuel ratio 1.0) in step 701 and calculates the difference as a rich deviation AFDV. (AFDV = AFSD−AF). In step 702, the CPU 31 determines whether “AFDV> 0”, that is, whether the actual air-fuel ratio AF at that time is richer than the air-fuel ratio reference value AFSD.
[0079]
When AFDV ≦ 0 (NO in step 702), the CPU 31 ends this routine as it is and returns to the original routine shown in FIGS. If AFDV> 0 (YES in step 702), the CPU 31 calculates the rich gas amount AFDV1 in step 703 based on the product of the rich deviation AFDV and the intake air amount QA (AFDV1 = AFDV · QA). Further, the CPU 31 calculates the rich gas integrated value AFAD in step 704, thereafter ends this routine, and returns to the original routine of FIGS. In step 704, the calculated AFDV1 value is added to the previous value of the AFAD value, and the sum is used as the current value of the AFAD value (AFAD = AFAD + AFDV1).
[0080]
FIG. 19 is a time chart showing a series of operations for detecting catalyst deterioration. Before time t11 in FIG. 19, air-fuel ratio lean control is performed, and the O2 output smoothed value VOX2SM is calculated from the rear O2 output VOX2 at that time (step 504 in FIG. 13).
[0081]
At time t11, air-fuel ratio rich control is started, and a predetermined value KCCATDT is set in the counter CCATDT. Further, the NOx integrated amount CNOXAD is calculated in a period (a period until time t12) until the air-fuel ratio on the upstream side of the catalyst becomes a rich value (the process of FIG. 11).
[0082]
After time t11, until the time t13 when the counter CCATDT becomes “0”, the rich gas integrated value AFAD corresponding to the S1 part of the figure and the rear O2 output integrated value VOX2AD corresponding to the S2 part of the figure are calculated (the above-mentioned) Steps 700 and 600 in FIG. When CCATDT = 0 at time t13, the deterioration determination value NOXCONV is calculated from the CNOXAD value, the AFAD value, and the VOX2AD value, and deterioration detection is performed according to the NOXCONV value (steps 508 and 509 in FIG. 14). After time t13, the O2 output smoothed value VOX2SM is calculated again.
[0083]
In FIG. 19, if the deterioration of the NOx catalyst 14 progresses, the NOx occlusion capacity of the catalyst 14 decreases. Therefore, the rear O2 output integrated value VOX2AD (S2 in the figure) with respect to the rich gas integrated value AFAD (S1 part in the figure). And the NOx purification rate decreases as a result. And the effect of catalyst deterioration is detected by the fall of a NOx purification rate.
[0084]
In the present embodiment, steps 508 and 509 in FIG. 14 correspond to the NOx purification rate calculation means described in the claims, and steps 509 and 510 correspond to the deterioration detection means.
[0085]
As described above, according to the second embodiment, the NOx inflow amount (NOx integrated amount CNOXAD) flowing into the NOx catalyst 14 during lean combustion and the rich gas amount (rich gas integrated) required for NOx purification by the NOx catalyst 14 during rich combustion. NOx purification rate is calculated from the ratio of the value AFAD-rear O2 output integrated value VOX2AD), and the deterioration of the NOx catalyst 14 is detected based on the NOx purification rate. In such a case, as in the first embodiment, the deterioration of the NOx catalyst 14 can be accurately detected.
[0086]
In this case, “NOx integrated amount CNOXAD” includes information related to lean combustion such as lean degree and lean time, and “rich gas integrated value AFAD−rear O 2 output integrated value VOX2AD” relates to rich combustion such as rich degree and rich time. Contains information. Therefore, when the “rich gas amount required for NOx purification” changes as a reflection of catalyst deterioration, correction according to various conditions during lean combustion or rich combustion is appropriately added to the rich gas amount. Become. Therefore, catalyst deterioration can always be accurately detected regardless of changes in combustion conditions. Further, in this case, the inconvenience that the detection of the deterioration is restricted by the combustion condition, such as the deterioration detection can be performed only when the rich time is shorter than the predetermined value, is avoided.
[0087]
For example, if the lean combustion is extended, the NOx integrated amount CNOXAD increases, but at the same time, a large amount of NOx is occluded in the NOx catalyst 14, and the amount of rich gas (AFAD-VOX2AD) necessary for NOx purification naturally increases. Therefore, there is no inconvenience that the NOx purification rate is changed in accordance with the change of the combustion condition and the catalyst deterioration is erroneously detected although the degree of catalyst deterioration is unchanged. When rich combustion is extended, the rich gas integrated value AFAD at the time of rich combustion increases. At the same time, the rear O2 output integrated value VOX2AD at the time of rich combustion also increases. Therefore, there is no inconvenience that the NOx purification rate is changed along with the change of the combustion state and the catalyst deterioration is erroneously detected.
[0088]
(Third embodiment)
FIG. 20 shows an outline of an air-fuel ratio control system according to the third embodiment. As shown in FIG. 20, in the present embodiment, a three-way catalyst 15 serving as a start catalyst is disposed upstream of the NOx catalyst 14. That is, the three-way catalyst 15 has a smaller capacity than the NOx catalyst 14 and is activated early after the engine 1 is started at a low temperature to purify harmful gases. An A / F sensor 26 is disposed upstream of the three-way catalyst 15, and an O 2 sensor 27 is disposed downstream of the NOx catalyst 14.
[0089]
In this case, the upstream three-way catalyst 15 temporarily stores oxygen in the exhaust gas during lean combustion. Therefore, at the time of rich combustion, the rich components (HC, CO) react with the oxygen stored in the three-way catalyst 15, and after the reaction is completed, the rich components are fed to the NOx catalyst 14. Further, it is known that the oxygen storage capacity of the three-way catalyst 15 changes according to the degree of deterioration of the three-way catalyst 15, and for example, when the catalyst deterioration progresses, the oxygen storage capacity decreases.
[0090]
Therefore, in the present embodiment, the degree of deterioration of the three-way catalyst 15 is detected, and air-fuel ratio rich control is performed according to the degree of catalyst deterioration. In such a case, the CPU 31 determines the rich control amount according to the degree of catalyst deterioration at that time, for example, using the relationship of FIG. In FIG. 21, if the degree of catalyst deterioration is small, the oxygen storage capacity of the three-way catalyst 15 is large, so a relatively large rich control amount is set. That is, the duration time of rich control is set to be relatively long. If the degree of catalyst deterioration is large, the oxygen storage capacity of the three-way catalyst 15 is small, so a relatively small rich control amount is set. That is, the duration time of rich control is set to be relatively short.
[0091]
As described above, when a rich control amount (rich time) is set in accordance with the degree of deterioration of the three-way catalyst 15, it is possible to always supply a certain amount of rich gas to the NOx catalyst. Therefore, the deterioration of the catalyst 14 can be detected based on the magnitude of the rear O2 output VOX2. In this case, the degree of deterioration of the NOx catalyst 14 is detected according to the peak value of the rear O2 output VOX2 at the time of rich combustion using the catalyst deterioration detection procedure of FIG.
[0092]
As a technique for detecting the degree of deterioration of the three-way catalyst 15, for example, a technique disclosed in “exhaust system failure diagnosis device for internal combustion engine” of Japanese Patent Application Laid-Open No. 8-338286 by the applicant of the present application can be applied. The catalyst deterioration detection method is briefly described. That is, the CPU 31 performs sub-feedback control so that the rear O2 output VOX2 (output of the O2 sensor 27 on the downstream side of the catalyst) matches the target value, and obtains an integral value of the deviation of the rear O2 output VOX2. Then, the degree of deterioration of the three-way catalyst 15 is detected based on the integrated value of the VOX2 deviation. At this time, it is detected that the catalyst deterioration degree is larger as the integral value of the VOX2 deviation is smaller.
[0093]
The effect | action of this Embodiment is demonstrated using the time chart of FIG. FIGS. 22A and 22B show the behavior of the air-fuel ratio and the like when the three-way catalyst 15 is new and when the catalyst 15 is deteriorated, respectively. 22A, at time t21, the rich control duration is set based on the degree of deterioration of the three-way catalyst 15 at that time, and the rich control is started in accordance with the duration.
[0094]
Thereafter, at time t22, the air-fuel ratio around the three-way catalyst 15 reaches the theoretical air-fuel ratio (λ = 1). At this time, although the air-fuel ratio in front of the three-way catalyst 15 immediately shifts to the richer side than the stoichiometric air-fuel ratio, the oxygen stored in the three-way catalyst 15 exists during the lean control, so the stored oxygen and the rich in the exhaust gas are present. The components (HC, CO, etc.) react and the air-fuel ratio behind the three-way catalyst 15 is temporarily held at the stoichiometric air-fuel ratio. When the reaction between the stored oxygen and the rich component ends, the air-fuel ratio behind the three-way catalyst 15 shifts to the rich side (time t23). Since the rich component is supplied to the NOx catalyst 14 side after time t23, NOx stored in the catalyst 14 is reduced and released.
[0095]
At time t24, the lean control is resumed, and the air-fuel ratio behind the three-way catalyst 15 has a predetermined period (time) in which the lean component in the exhaust gas fed from the upstream side and the rich component stored in the catalyst 15 react. After being held at the stoichiometric air-fuel ratio for t25 to t26), it returns to the lean control value.
[0096]
On the other hand, when the three-way catalyst 15 is deteriorated, as shown in FIG. 22B, the control air-fuel ratio is switched from lean to rich at time t31, and the rich control is continued based on the degree of deterioration of the three-way catalyst 15. Time is set. In this case, since the deterioration of the three-way catalyst 15 has progressed, a relatively small rich control amount is given (see FIG. 21).
[0097]
At time t32 when the air-fuel ratio before and after the three-way catalyst 15 reaches the stoichiometric air-fuel ratio (λ = 1), the air-fuel ratio behind the three-way catalyst 15 is once held at the stoichiometric air-fuel ratio, but the three-way catalyst 15 deteriorates. Therefore, the amount of oxygen stored in the catalyst is small, and the air-fuel ratio shifts to the rich side in a shorter time than in the case of FIG. 22A (time t33). That is, the time t32 to t33 in FIG. 22B, which is the time for the stored oxygen of the three-way catalyst 15 to react with the rich component in the exhaust gas, is shorter than the time t22 to t23 in FIG. . Since the rich component is supplied to the NOx catalyst 14 side after time t33, NOx stored in the catalyst 14 is reduced and released. Thereafter, the control air-fuel ratio is returned to the lean value at time t34.
[0098]
According to FIGS. 22A and 22B, the rich time is controlled in accordance with the degree of deterioration of the three-way catalyst 15. As a result, during rich control, the required amount of rich gas is always supplied regardless of whether the three-way catalyst 15 has deteriorated, and the rich gas amount downstream of the NOx catalyst is regulated to a value that allows detection of catalyst deterioration. .
[0099]
As described above, in the third embodiment, the three-way catalyst 15 is provided on the upstream side of the NOx catalyst 14, and in this case, the deterioration of the NOx catalyst 14 is accurately detected as in the above embodiments. can do.
[0100]
In the present embodiment, the A / F sensor 26 is arranged upstream of the three-way catalyst 15 to shorten the distance between the engine 1 and the sensor 26 and the sensor output changes after the air-fuel ratio changes. Response time is shortened. Therefore, the sensor detection accuracy during transient operation is improved.
[0101]
(Fourth embodiment)
In the fourth embodiment, as in the third embodiment, a three-way catalyst 15 as a start catalyst is provided upstream of the NOx catalyst 14, and the air-fuel ratio control system is as shown in FIG. Composed. 23, the difference from FIG. 20 will be described. In FIG. 23, the A / F sensor 26 is provided downstream of the three-way catalyst 15 (between the catalysts 14 and 15).
[0102]
Further, in this embodiment, as described in the second embodiment, the degree of deterioration of the catalyst 14 is detected based on the NOx purification rate of the NOx catalyst 14. That is, the NOx integrated amount CNOXAD flowing into the NOx catalyst 14 is calculated according to the procedure of FIG. 13 and 14, the actual rich gas amount (rich gas integrated value AFAD−rear O 2 output integrated value VOX2AD) required for NOx purification in the NOx catalyst 14 is calculated, and “AFAD−VOX2AD / CNOXAD”. Is the NOx purification rate, and the degree of deterioration of the NOx catalyst 14 is detected according to the NOx purification rate.
[0103]
When the NOx purification rate is used as a deterioration determination parameter, catalyst deterioration can be detected without restricting the amount of rich gas flowing into the NOx catalyst 14 to a predetermined value. Therefore, the process of detecting the deterioration of the three-way catalyst 15 as described in the third embodiment and adjusting the rich control amount according to the detection result is not necessary.
[0104]
In such a configuration, as described above, even if the oxygen storage capacity fluctuates according to the degree of deterioration of the three-way catalyst 15, regardless of the oxygen storage capacity, the actual rich gas amount required for NOx purification and the lean combustion time The NOx purification rate is accurately obtained from the NOx inflow amount. That is, the deterioration of the NOx catalyst 14 can be accurately detected without being affected by the difference in the degree of deterioration of the three-way catalyst 15.
[0105]
(Fifth embodiment)
In the third and fourth embodiments, the three-way catalyst 15 having an oxygen storage capacity is arranged on the upstream side of the NOx catalyst 14, but in the fifth embodiment, the three-way catalyst is used for oxygen storage. Change to one with no capacity or one with low oxygen storage capacity. That is, in the present embodiment, the three-way catalyst is configured by supporting only a noble metal (platinum Pt) having no oxygen storage capacity on a carrier. Specifically, a support made of ceramic such as stainless steel or cordierite is coated with a catalyst layer constituted by supporting only platinum Pt on the surface of porous alumina Al2 O3.
[0106]
In such a case, oxygen stored in the three-way catalyst 15 reacts with rich components (HC, CO) in the exhaust gas, and the supply amount of the rich component to the downstream side is not reduced by that amount. The behavior of the air-fuel ratio before and after is substantially the same. Therefore, unlike the third embodiment, a process of variably setting the rich control amount in accordance with the degree of deterioration of the three-way catalyst 15 becomes unnecessary. As a method for detecting the deterioration of the NOx catalyst 14, either the method according to FIG. 7 or the method according to FIGS.
[0107]
Embodiments of the present invention can be embodied in the following forms in addition to the above.
In the first embodiment, the deterioration detection condition is satisfied only when the lean time TL and the rich time TR are relatively short, and the deterioration detection using the rear O2 output VOX2 is performed only when the condition is satisfied. However, this configuration is changed. For example, the deterioration detection is performed at a predetermined time period, and the lean time TL and the rich time TR are forcibly shortened when the deterioration detection is performed. That is, when the NOx storage capacity is estimated and catalyst deterioration is detected based on the estimated value, the rich time or the rich degree during rich combustion is limited to a predetermined value or less. According to this configuration, there is a clear difference in the rear O2 output VOX2 between when there is no catalyst deterioration and when there is deterioration, and as a result, highly reliable catalyst deterioration detection can be realized.
[0108]
In the first embodiment, the NOx storage capacity of the NOx catalyst 14 is estimated according to the peak value of the rear O2 output VOX2, and the deterioration of the catalyst 14 is detected based on the NOx storage capacity. (1) and (2).
(1) NOx occlusion capacity is estimated from the time integral value (area) of VOX2 change, and catalyst deterioration is detected based on the NOx occlusion capacity. Specifically, the rear O2 output integrated value VOX2AD is calculated based on the rear O2 output VOX2 during the rich control (for example, in accordance with the processing of the second embodiment, FIG. 15), and this rear O2 output integrated value VOX2AD. The NOx storage capacity is estimated accordingly. In this case, it can be considered that as the VOX2AD value is larger, the NOx occlusion ability of the NOx catalyst 14 is lowered and the deterioration of the catalyst 14 is progressing.
(2) During rich control, the amount of change of the rear O2 output VOX2 per unit time is integrated, thereby obtaining the locus of the output value. Then, the NOx storage capacity is estimated from the locus of VOX2, and catalyst deterioration is detected based on the NOx storage capacity. In this case, it can be considered that the larger the locus of VOX2, the lower the NOx occlusion capacity of the NOx catalyst 14, and the deterioration of the catalyst 14 is progressing.
[0109]
In each of the above embodiments, the O2 sensor 27 is disposed on the downstream side of the NOx catalyst 14, and the deterioration of the NOx catalyst 14 is detected using the output of the sensor 27 (rear O2 output VOX2). Change to the limit current type A / F sensor and use this A / F sensor output to detect catalyst degradation as shown in (a) and (b) below.
(A) Deterioration of the catalyst is detected from the peak value of the output of the A / F sensor arranged on the downstream side of the NOx catalyst or the time integral value (area) of the output. This is basically performed according to the processing of FIG. 7, and the rear O2 output VOX2 seen in steps 307 to 309 of FIG. 7 may be changed to “rear A / F sensor output”.
(B) In the processing of FIGS. 13 and 14, the output integrated value of the A / F sensor on the downstream side of the catalyst is calculated instead of the rear O2 output integrated value VOX2AD. That is, the integrated output value of the A / F sensor is calculated as the surplus gas amount on the downstream side of the catalyst. In this case, the NOx purification amount in the NOx catalyst 14 is calculated from the difference between the integrated output value of the A / F sensor upstream of the catalyst during rich combustion and the integrated output value of the A / F sensor downstream of the catalyst during rich combustion. (Rich gas amount required for NOx purification) is calculated. Then, catalyst deterioration is detected according to the NOx purification amount.
[0110]
The output of the O2 sensor or A / F sensor is converted into a physical quantity for use. For example, using the relationship shown in FIG. 24, the O2 sensor output is converted into a rich excess amount (mole), and the data of the NOx catalyst 14 of the rich excess amount is obtained using any of the peak value, time integral value (area), and locus data. Perform deterioration detection. Alternatively, the A / F sensor output is converted into a rich excess amount (mole) using the relationship shown in FIG. 25, and the NOx catalyst is obtained using any of the rich excess peak value, time integral value (area), and locus data. 14 deterioration detection is performed.
[0111]
In the third embodiment, the method for detecting deterioration of the three-way catalyst 15 is changed. For example, the technique disclosed in the “exhaust gas purifying catalyst deterioration detecting device” of Japanese Patent Laid-Open No. 9-31612 by the applicant of the present application is applied. In this method, the amount of gas components to be purified in the catalyst from when the engine is started until the three-way catalyst is warmed up (data reflecting the amount of unpurified gas components) is calculated, and the unpurified gas components are calculated. The degree of deterioration of the three-way catalyst is detected based on the amount. In this case, catalyst deterioration can be detected with high accuracy while taking into account the increase in emissions before the catalyst activity. In addition, before the three-way catalyst is warmed up, the difference in the purification rate due to the difference in the degree of catalyst deterioration is large, and it becomes possible to easily and accurately detect the catalyst deterioration.
[0112]
In the fifth embodiment, the following configuration can be applied as the three-way catalyst 15 having a small oxygen storage capacity.
-A three-way catalyst is formed by not supporting a promoter having a large oxygen storage capacity on the carrier or reducing the amount supported. In this case, ceria CeO2, barium Ba, lanthanum La and the like are known as promoters having a large oxygen storage capacity.
-A three-way catalyst is constructed by reducing the amount of noble metals (Rh, Pd) supported by oxygen. In particular, it is preferable that the loading is 0.2 g / liter or less for rhodium Rh and 2.5 g / liter or less for palladium Pd.
[0113]
In each of the above-described embodiments, the calculation using the modern control theory is performed in the F / B control of the air-fuel ratio, but the calculation using PID, PI control or the like may be performed instead. Further, the air-fuel ratio may be controlled open during lean combustion.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system in an embodiment.
FIG. 2 is a flowchart showing a fuel injection control routine.
FIG. 3 is a flowchart showing a routine for setting a target air-fuel ratio AFTG.
FIG. 4 is a map for performing rich time according to engine operating conditions.
FIG. 5 is a map for setting a lean target air-fuel ratio according to an engine operating state.
FIG. 6 is a time chart showing the behavior of air-fuel ratio control.
FIG. 7 is a flowchart showing a catalyst deterioration detection routine.
FIG. 8 is a graph showing the relationship between the NOx occlusion amount and the degree of catalyst deterioration.
FIG. 9 is a diagram showing sensor output waveforms before and after catalyst deterioration.
FIG. 10 is a diagram showing sensor output waveforms before and after catalyst deterioration.
FIG. 11 is a flowchart showing a NOx amount estimation routine.
FIG. 12 is a relationship diagram for use in calculating the amount of NOx.
FIG. 13 is a flowchart showing a catalyst deterioration detection routine.
FIG. 14 is a flow chart showing a catalyst deterioration detection routine following FIG. 13;
FIG. 15 is a flowchart showing a routine for calculating a rear O2 output integrated value VOX2AD.
FIG. 16 is a flowchart showing a calculation routine of a rich gas integrated value AFAD.
FIG. 17 is a diagram showing a relationship between a deterioration determination value NOXCONV and a NOx purification rate.
FIG. 18 is a graph showing the relationship between the NOx purification rate and the degree of catalyst deterioration.
FIG. 19 is a time chart for explaining the operation in the second embodiment;
FIG. 20 is a configuration diagram showing an outline of a control system in the third embodiment.
FIG. 21 is a diagram showing a relationship between a three-way catalyst deterioration degree and a rich control amount.
FIG. 22 is a time chart for explaining the operation in the third embodiment;
FIG. 23 is a configuration diagram showing an outline of a control system in the fourth embodiment.
FIG. 24 is a diagram for converting the O2 sensor output into a rich excess amount.
FIG. 25 is a diagram for converting the A / F sensor output into a rich excess amount.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 3 ... Exhaust pipe, 14 ... NOx catalyst (NOx occlusion reduction type catalyst), 26 ... A / F sensor as oxygen concentration sensor (upstream sensor), 27 ... Oxygen concentration sensor (downstream side) O2 sensor as sensor), 30 ... ECU, 31 ... estimating means, deterioration detecting means, CPU as NOx purification rate calculating means.

Claims (3)

Equipped with a lean NOx catalyst provided in the engine exhaust system, causing lean combustion in the air-fuel ratio lean region, storing NOx in the exhaust gas discharged during lean combustion with the lean NOx catalyst, and temporarily reducing the air-fuel ratio In the exhaust gas purifying apparatus for an internal combustion engine, which is controlled to be rich and releases the stored NOx from the lean NOx catalyst,
And NOx inflow amount flowing into the lean NOx catalyst during rie down combustion, NOx purification ratio calculating means for calculating a NOx purification rate by the catalyst from the ratio between the rich gas amount required for NOx purification at the same lean NOx catalyst during rich combustion When,
Exhaust gas purifying apparatus of an internal combustion engine, characterized in that it comprises a deterioration detecting means for detecting deterioration of the lean NO x catalysts on the basis of the calculated N Ox purification rate. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the lean NOx catalyst detects that the degree of deterioration of the lean NOx catalyst is larger as the amount of rich gas required for NOx purification in the lean NOx catalyst decreases and the NOx purification rate decreases . An upstream sensor that is disposed upstream of the lean NOx catalyst and detects an oxygen concentration in the exhaust gas;
A downstream sensor that is disposed downstream of the lean NOx catalyst and detects an oxygen concentration in the exhaust gas;
Means for calculating a NOx inflow amount A flowing into the lean NOx catalyst at the time of lean combustion based on the detection result of the upstream sensor;
Means for calculating a rich gas inflow amount B flowing into the lean NOx catalyst at the time of rich combustion based on the detection result of the upstream side sensor;
Means for calculating an excess gas amount C discharged from the lean NOx catalyst at the time of rich combustion based on the detection result of the downstream sensor,
The NOx purification rate calculating means is based on the calculated NOx inflow amount A during lean combustion, rich gas inflow amount B and surplus gas amount C during rich combustion,
(BC) / A
The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein a NOx purification rate is calculated from the calculation result of the above.

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