JP2000199424A - Exhaust gas cleanup device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To permit efficient elimination of SOx absorbed by NOx purification means, by controlling so as to vary the air-fuel ratio of the mixture between a lean side and a rich side in detecting degradation of the NOx purification means, and keeping the air-fuel ratio on the rich side when the temperature of the NOx purification means becomes higher than a regeneration temperature. SOLUTION: A NOx purification device 16 is interposed in an exhaust pipe 12 of an engine 1. The device occludes NOx in an exhaust gas lean state where the air-fuel ratio of the mixture is set on a leaner side than the stoichiometric air-fuel ratio and the concentration of oxygen in exhaust gas is relatively high (high NOx ratio), and emits occluded NOx in an exhaust gas rich state where the air-fuel ratio of mixture is set at a rich side and the concentration of oxygen in exhaust gas is relatively low. When degradation of a NOx absorbing agent is detected, the air-fuel ratio of the mixture is varied between the lean side and the rich side at a predetermined period. When the temperature of the NOx purification device 16 becomes higher than a regeneration temperature, the air-fuel ratio is kept on the rich side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an apparatus for purifying nitrogen oxides (NOx) in an exhaust system.
The present invention relates to an exhaust gas purifying apparatus provided with a NOx purifying apparatus incorporating the above absorbent.

[0002]

2. Description of the Related Art When a lean operation in which the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set to a side leaner than the stoichiometric air-fuel ratio is executed, the amount of NOx emission tends to increase, so that NOx is absorbed during the lean operation. Nx that purifies NOx by reducing the absorbed NOx in a timely manner
It is conventionally known to provide an Ox purification device in an engine exhaust system (for example, Japanese Patent No. 2586739).

The NOx absorbent of such a NOx purifying device has an air-fuel ratio set leaner than the stoichiometric air-fuel ratio and has a relatively high oxygen concentration in the exhaust gas (a large amount of NOx) (hereinafter referred to as an "exhaust gas lean state"). ")
On the other hand, while absorbing Ox, the air-fuel ratio is set close to the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, and the oxygen concentration in the exhaust gas is relatively low (hereinafter, “exhaust gas rich state”).
Has the property of releasing absorbed NOx. In the exhaust gas rich state, the NOx purification device converts NOx released from the NOx absorbent into HC,
It is configured to be reduced by CO and discharged as nitrogen gas, and that HC and CO are oxidized and discharged as water vapor and carbon dioxide.

[0004]

SUMMARY OF THE INVENTION NO in a NOx purifying apparatus
Since the x absorbent absorbs not only NOx but also SOx (sulfur oxide), sulfur (S) contained in the fuel gradually accumulates as SOx. Therefore, the problem of sulfur poisoning in which the NOx absorbing ability of the NOx absorbent is significantly reduced (for example, the 80% absorbing ability is reduced to about 20%) is inevitable. SOx absorbed by the NOx absorbent is NOx
Since the absorbent is released from the NOx absorbent at a high temperature (600 ° C. or higher), it can be reduced by enriching the air-fuel ratio in that state. However, when the NOx absorbent is 6
The high temperature of 00 ° C. or higher is limited to the case where the high-load operation is continued for a relatively long time. Therefore, the operating state in which the air-fuel ratio enrichment for actually removing SOx can be performed is limited, and the SOx There was a problem that it was not possible to remove satisfactorily.

[0005] The present invention has been made to solve this problem.
It is an object of the present invention to provide an exhaust gas purifying apparatus capable of raising the temperature to a temperature at which NOx is released and efficiently removing SOx absorbed by the NOx absorbent.

[0006]

In order to achieve the above object, the invention according to claim 1 is provided in an exhaust system of an internal combustion engine, and when the exhaust gas is in a lean state where the oxygen concentration in the exhaust gas is relatively high. In an exhaust gas purifying apparatus for an internal combustion engine provided with a nitrogen oxide purifying means for absorbing nitrogen oxides contained therein, a deterioration detecting means for detecting deterioration of the nitrogen oxide purifying means, and the nitrogen oxide purifying means being provided by the deterioration detecting means. Air-fuel ratio changing means for changing the air-fuel ratio of the air-fuel mixture supplied to the engine between a lean side and a rich side with a stoichiometric air-fuel ratio as a boundary when the deterioration of the means is detected. When the temperature of the nitrogen oxide purifying means becomes higher than the deterioration regeneration temperature by the operation of the air-fuel ratio varying means, the air-fuel ratio is maintained on a richer side than the stoichiometric air-fuel ratio for the deterioration regeneration time. Characterized in that it comprises a reduction reproducing means.

Here, the "predetermined time" is set to a time suitable for raising the temperature of the nitrogen oxide purifying means, for example, 3 seconds or less. Specifically, when the fuel efficiency during the air-fuel ratio fluctuation control is emphasized, the lean time for controlling the air-fuel ratio to the lean side from the stoichiometric air-fuel ratio is set to 2 seconds or less, and the air-fuel ratio is set to the rich side from the stoichiometric air-fuel ratio. Control the rich time to 1 of the lean time
It is desirable to set (lean time + rich time) when the time is equal to or less than 1/2, and when emphasis is placed on early temperature rise of the nitrogen oxide purifying means by air-fuel ratio fluctuation control, especially when the temperature is When the source catalyst is disposed and the nitrogen oxide purifying means is disposed downstream of the three-way catalyst and at a position slightly away from the engine, it is desirable to set the lean time and the rich time to substantially the same value. .

[0008] The "degradation regeneration temperature" is considered to be the lowest temperature at which the nitrogen oxide absorbent of the nitrogen oxide purifying means starts to release SOx, or that the temperature drops when the air-fuel ratio enrichment is started by the degradation regeneration means. Then, the temperature is set to be about 50 to 100 ° C. higher than the minimum temperature. The “degradation regeneration time” corresponds to the degree of deterioration at which deterioration is detected by the deterioration detecting means, that is, the amount of SOx accumulated in the nitrogen oxide purifying means at the time when the deterioration is detected, for example, the accumulated SOx It is set to a time in which almost all can be returned.

It is desirable to provide a temperature detecting means for actually detecting the temperature of the nitrogen oxide purifying means and to judge that the temperature of the nitrogen oxide purifying means has become higher than the degradation regeneration temperature based on the detected value. Alternatively, the determination may be made based on the fact that the execution time of the air-fuel ratio variation control by the air-fuel ratio variation unit has exceeded a predetermined temperature rise time without using the temperature detection unit. In this case, the “predetermined heating time” is set according to the actual measurement time required to reach the degradation regeneration temperature by performing an experiment on a plurality of target devices.

According to this configuration, when the deterioration detecting means detects the deterioration of the nitrogen oxide purifying means, the air-fuel ratio of the air-fuel mixture supplied to the engine is changed to the stoichiometric air-fuel ratio in a cycle set to a predetermined time or less. When the temperature of the nitrogen oxide purifying means becomes higher than the degradation regeneration temperature during the air-fuel ratio variation control, the air-fuel ratio becomes stoichiometric over the degradation regeneration time. Since the temperature is maintained on the rich side from the fuel ratio, the temperature of the nitrogen oxide purifying means is quickly raised to the deterioration regeneration temperature at which SOx can be removed, and SOx absorbed by the nitrogen oxide purifying means is efficiently removed. be able to. As a result, good exhaust gas characteristics can be maintained over a long period of time.

According to a second aspect of the present invention, there is provided an exhaust system for an internal combustion engine, which is configured to purify nitrogen oxides in the exhaust gas lean state where the oxygen concentration in the exhaust gas is relatively high. In the exhaust gas purifying apparatus for an internal combustion engine, the deterioration degree detecting means for detecting the degree of deterioration of the nitrogen oxide purifying means is set to a predetermined time or less in a specific operation state in which the exhaust gas flow rate of the engine is large. The air-fuel ratio changing means for changing the air-fuel ratio of the air-fuel mixture supplied to the engine between the lean side and the rich side with respect to the stoichiometric air-fuel ratio in a cycle, and the temperature of the nitrogen oxide purifying means becomes higher than the degradation regeneration temperature. And a deterioration regeneration unit that maintains the air-fuel ratio on the rich side of the stoichiometric air-fuel ratio for a deterioration regeneration time corresponding to the detected degree of deterioration.

Here, the "specific operation state" is an operation state in which the rotation speed of the engine is higher than a predetermined rotation speed and the absolute pressure in the intake pipe is higher than a predetermined pressure. A condition that the temperature is higher than a predetermined temperature lower than the regeneration temperature may be added, and a condition that the speed of the vehicle on which the engine is mounted is higher than a predetermined speed may be added. In this case, the predetermined rotation speed, the predetermined pressure,
Since the predetermined temperature and the predetermined speed vary depending on the characteristics of the nitrogen oxide purifying means used and the specifications of the engine, appropriate values are set by experiments.

"Predetermined time" and "deterioration regeneration temperature"
Is the same as the exhaust gas purifying apparatus according to claim 1,
The “degradation regeneration time” is set to be longer as the degree of deterioration detected by the degree of deterioration detector is larger, that is, as the amount of SOx accumulated in the nitrogen oxide purifier is larger.

According to this configuration, in a specific operation state in which the exhaust gas flow rate of the engine is large, the air-fuel ratio of the air-fuel mixture supplied to the engine is shifted between the lean side and the stoichiometric air-fuel ratio in a cycle set to a predetermined time or less. When the temperature of the nitrogen oxide purifying means is higher than the degradation regeneration temperature, the air-fuel ratio is shifted from the stoichiometric air-fuel ratio to the rich side over the degradation regeneration time corresponding to the detected degree of degradation. Since the temperature is maintained, the temperature of the nitrogen oxide purifying means is quickly raised to the degradation regeneration temperature at which SOx can be removed while achieving a high nitrogen oxide purifying rate in the specific operation state, and the nitrogen oxide purifying efficiency is increased. SOx absorbed by the substance purifying means can be removed. As a result, good exhaust gas characteristics can be maintained over a long period of time.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter referred to as "engine") and a control device thereof, including an exhaust gas purification device according to a first embodiment of the present invention. A throttle valve 3 is arranged in the middle of an intake pipe 2 of a cylinder engine 1. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3, and outputs an electric signal corresponding to the opening of the throttle valve 3 to output an electronic control unit for engine control (hereinafter referred to as “ECU”) 5. To supply.

A fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). The ECU 5 is electrically connected to the ECU 5 and controls a valve opening time of the fuel injection valve 6 based on a signal from the ECU 5.

On the other hand, an intake pipe absolute pressure (PBA) sensor 7 is provided immediately downstream of the throttle valve 3, and the absolute pressure signal converted into an electric signal by the absolute pressure sensor 7 is supplied to the ECU 5. . Further, an intake air temperature (TA) sensor 8 is mounted downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies the electric signal to the ECU 5.

The engine water temperature (TW) sensor 9 mounted on the main body of the engine 1 is composed of a thermistor or the like, detects the engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5. Around the camshaft or the crankshaft (not shown) of the engine 1, the engine speed (N
E) A sensor 10 and a cylinder discrimination (CYL) sensor 11 are mounted. The engine speed sensor 10 outputs a TDC signal pulse at a crank angle position before a predetermined crank angle with respect to the top dead center (TDC) at the start of the intake stroke of each cylinder of the engine 1 (every 180 ° crank angle in a four-cylinder engine). The cylinder discriminating sensor 11 outputs a cylinder discriminating signal pulse at a predetermined crank angle position of a specific cylinder. These signal pulses are supplied to the ECU 5.

The exhaust pipe 12 is provided with a NOx purifying device 16 as a nitrogen oxide purifying means. The NOx purifying device 16 includes a NOx absorbent that absorbs NOx and a catalyst for promoting oxidation and reduction. As the NOx absorbent, when the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is set leaner than the stoichiometric air-fuel ratio, and when the oxygen concentration in the exhaust gas is relatively high (NOx is large), NOx On the other hand, in the exhaust gas rich state where the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is set near the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio and the oxygen concentration in the exhaust gas is relatively low, NOx absorbed
A storage type having a characteristic of releasing NOx or an adsorption type of adsorbing NOx in an exhaust gas lean state and reducing it in an exhaust gas rich state is used. NO
The x purification device 16 is configured to perform N
While the NOx is absorbed by the Ox absorbent, NOx released from the NOx absorbent is reduced by HC and CO to be discharged as nitrogen gas in an exhaust gas rich state, and HC and CO are oxidized to form steam and carbon dioxide. It is configured to be emitted as carbon. Storage type NO
As the x absorbent, for example, barium oxide (Ba0) is used, and as the adsorption type NOx absorbent, for example, sodium (Na) and titanium (Ti) or strontium (Sr) and titanium (Ti) are used. Noble metals such as rhodium (Rh), palladium (Pd) and platinum (Pt) are used in both the occlusion type and the adsorption type.

The NOx purifying device 16 is a NOx purifying device having a built-in NOx absorbent. As will be described later, a short-period air-fuel ratio fluctuation control for varying the air-fuel ratio of the air-fuel mixture supplied to the engine 1 in a relatively short cycle. Is executed in a specific engine operating state, NOx can be efficiently reduced by the action of only the catalyst without the action of absorbing NOx into the NOx absorbent.

If NOx is absorbed up to the limit of the NOx absorption capacity of the NOx absorbent, that is, up to the maximum NOx absorption amount, NOx can no longer be absorbed, so that the air-fuel ratio is enriched to release and reduce NOx in a timely manner. That is, reduction enrichment is performed. The NOx purifying device 16 is provided with a catalyst temperature sensor 17 for detecting the temperature (hereinafter referred to as “catalyst temperature”) TCAT, and the detection signal is E.
It is supplied to CU5. The catalyst temperature TCAT indicates the temperature of the NOx absorbent and the catalyst.

A proportional type air-fuel ratio sensor 14 (hereinafter referred to as "LAF sensor 14") is mounted at an upstream position of the NOx purification device 16. The LAF sensor 14 detects the oxygen concentration (air-fuel ratio) in the exhaust gas. A substantially proportional electric signal is output and supplied to the ECU 5. The ECU 5 is further equipped with an engine 1 and connected to a vehicle speed sensor 21 as a vehicle speed detecting means for detecting a traveling speed (vehicle speed) VCAR of a vehicle driven by the engine 1, and a detection signal is supplied to the ECU 5. You.

The engine 1 can switch the valve timing of the intake valve and the exhaust valve in two stages: a high-speed valve timing suitable for a high-speed rotation region of the engine and a low-speed valve timing suitable for a low-speed rotation region of the engine. It has a mechanism 30. The switching of the valve timing includes the switching of the valve lift amount. Further, when the low-speed valve timing is selected, one of the two intake valves is stopped to stabilize even when the air-fuel ratio is made leaner than the stoichiometric air-fuel ratio. We try to ensure combustion.

The valve timing switching mechanism 30 switches the valve timing via a hydraulic pressure. An electromagnetic valve and a hydraulic pressure sensor for switching the hydraulic pressure are connected to the ECU 5. The detection signal of the oil pressure sensor is supplied to the ECU 5, and the ECU 5 controls the solenoid valve to control the switching of the valve timing according to the operating state of the engine 1.

The ECU 5 has a function of shaping input signal waveforms from various sensors, correcting a voltage level to a predetermined level, converting an analog signal value to a digital signal value, and the like. 5b, various arithmetic programs executed by the CPU 5b, tables and maps used in the arithmetic programs,
Storage means 5c for storing calculation results and the like;
And an output circuit 5d for supplying a drive signal to the power supply.

The CPU 5b determines various engine operating states based on the various engine parameter signals described above, and according to the determined engine operating states,
Based on the following equation (1), a fuel injection time TOUT of the fuel injection valve 6 that operates to open in synchronization with the TDC signal pulse is calculated. TOUT = TI × KCMD × KLAF × K1 + K2 (1) Here, TI is a basic fuel injection time of the fuel injection valve 6, and a TI map set according to the engine speed NE and the intake pipe absolute pressure PBA is searched. Is determined. TI
The map is set such that the air-fuel ratio of the air-fuel mixture supplied to the engine substantially becomes the stoichiometric air-fuel ratio in the operating state corresponding to the engine speed NE and the intake pipe absolute pressure PBA on the map.

KCMD is a target air-fuel ratio coefficient. The engine speed NE, the intake pipe absolute pressure PBA, and the engine coolant temperature T
It is set according to the engine operating parameters such as W. The target air-fuel ratio coefficient KCMD is proportional to the reciprocal of the air-fuel ratio A / F, that is, the fuel-air ratio F / A, and takes a value of 1.0 at the stoichiometric air-fuel ratio.

The detected equivalent ratio KACT calculated from the detected value of the LAF sensor 14 is equal to the target equivalent ratio KCMD.
Is an air-fuel ratio correction coefficient calculated by PID control so as to coincide with K1 and K2 are other correction coefficients and correction variables calculated according to various engine parameter signals, respectively, and are predetermined values that can optimize various characteristics such as a fuel consumption characteristic and an engine acceleration characteristic according to an engine operating state. Is determined. The CPU 5b supplies a drive signal for opening the fuel injection valve 6 to the fuel injection valve 6 via the output circuit 5d based on the fuel injection time TOUT obtained as described above.

FIG. 2 is a flowchart of a process for calculating the target air-fuel ratio coefficient KCMD applied to the above equation (1). This process is executed by the CPU 5b at regular intervals. In step S11, it is determined whether or not a lean operation is being performed, that is, whether or not a storage value KCMDB of the target air-fuel ratio coefficient KCMD stored in step S19 described later during normal control is smaller than "1.0". As a result, KCMDB ≧ 1.0
If the vehicle is not in the lean operation, step S
The program proceeds to step S17, in which a reduction enrichment time TRR (for example, 1 to 2 seconds) is set in a down count timer tmRR referred to in step S21 to be described later and started. Then
Normal control, that is, setting of the target air-fuel ratio coefficient KCMD according to the engine operating state is performed (step S18). The target air-fuel ratio coefficient KCMD is basically determined by the engine speed N
It is calculated according to E and the intake pipe absolute pressure PBA, and is changed to a value corresponding to those operating conditions in a low-temperature state of the engine coolant temperature TW or a predetermined high-load operating state. Next, the target air-fuel ratio coefficient KCMD calculated in step S18 is stored in the storage value KC.
The data is stored as the MDB (step S19), and the process ends.

When KCMDB <1.0 in step S11 and the engine is in the lean operation, it is set by the processing of FIG. 3 and the exhaust gas flow rate of the engine 1 is large and the catalyst temperature TCAT is such that the SOx can be removed. It is determined whether or not the SOx removal flag FHLSOx indicating “1” indicates that the engine is operating at a high temperature (600 ° C. or higher) (hereinafter referred to as “SOx removable operation state”) (step 1). S12). Then, when FHLSOx = 1 and the engine 1 is in the operating state where SOx can be removed,
An SOx removal process (FIG. 7) for enriching the air-fuel ratio and reducing SOx is executed (step S24).

When FHLSOx = 0 in step S12, the condition is set by the processing of FIG. 3, and the engine is operated in a state where the exhaust gas flow rate of the engine 1 is large and the catalyst temperature TCAT is high (hereinafter referred to as "specific operation state"). It is determined whether or not the specific operation state flag FHL indicating that there is a certain state is "1" (step S13). And FHL
= 1 and the engine 1 is in a specific operation state,
The short-period air-fuel ratio fluctuation control shown in FIG. 5 is executed (step S23). On the other hand, when FHL = 0 and the engine 1 is in an operation state other than the specific operation state, compared to the short-period air-fuel ratio fluctuation control. The processing of step S14 and subsequent steps in which the period of the air-fuel ratio fluctuation becomes longer is executed. The SOx removal operation state corresponds to a state in which the catalyst temperature TCAT is particularly high in the specific operation state, and is an operation state included in the specific operation state.

First, in step S14, an increment value ADDNOx to be used in the next step S15 is determined according to the engine speed NE and the intake pipe absolute pressure PBA. The increment value ADDNOx is a parameter corresponding to the amount of NOx discharged per unit time during the lean operation. As the engine speed NE increases, the intake pipe absolute pressure PB
It is set to increase as A increases.

In step S15, step S
The increment value ADDNOx determined in 14 is applied, and the NOx amount counter CNOx is incremented. This gives N
Ox emissions, that is, NOx absorbed by the NOx absorbent
A count value corresponding to the quantity is obtained. CNOx = CNOx + ADDNOx

In the following step S16, it is determined whether or not the value of the NOx amount counter CNOx has exceeded an allowable value CNOxREF. If the answer is negative (NO), the routine proceeds to step S17, where the normal control, that is, the setting of the target air-fuel ratio coefficient KCMD according to the engine operating state is performed. The allowable value CNOxREF is the maximum NO of the NOx absorbent.
The value corresponding to the NOx amount slightly smaller than the x-absorption amount, or NO with about a half of the maximum NOx absorption amount with a margin.
It is set to a value corresponding to the x amount.

In step S16, CNOx> CNOxR
When EF is reached, the target air-fuel ratio coefficient KCMD is set to 14.
A predetermined enrichment value KCMDR1 corresponding to a value of about 0
Is set, and reduction enrichment is executed (step S2).
0). Then, it is determined whether or not the value of the timer tmRR is "0" (step S21), and this process is immediately terminated while tmRR> 0. When tmRR = 0, the value of the NOx amount counter CNOx is set to "0". (Step S
22). As a result, the answer to step S16 becomes negative (NO) from the next time, so that the control shifts to the normal control.

According to the processing of FIG. 2, the lean operation continuation time in the operation state where FHL = 0 and the lean operation is possible other than the specific operation state, that is, the NOx amount counter CNO
The time when the value of x reaches the allowable value CNOxREF from 0 is
It varies depending on the engine operating condition, but generally 8 to 3
It is about 0 seconds. Therefore, in an operation state other than the specific operation state, the lean operation continuation time is 8 to 3
In about 0 seconds, the reduction enrichment execution time (= TRR) is 1
The air-fuel ratio fluctuation control for about 2 seconds from is executed.

FIG. 3 shows steps S12 and S13 in FIG.
5 is a flowchart of an operation state determination process for setting a SOx removal flag FHLSOx and a specific operation state flag FHL referred to in FIG. This processing is executed by the CPU 5 at regular intervals.
b. First, the vehicle speed VCAR is equal to the predetermined vehicle speed SVCA.
R (for example, 60 km / h) (step S31), and when VCAR> SVCAR,
When the engine speed NE is equal to the predetermined speed SNE (for example, 180
0 rpm) is determined (step S3).
2) When NE> SNE, the absolute pressure PB in the intake pipe
It is determined whether or not A is higher than a predetermined pressure SPBA (for example, 360 mmHg) (step S33). If any one of the steps S31 to S33 is negative (NO), the specific operation state flag FHL and the SOx removal flag F
HLSOx are both set to “0” (step S3
9), end this processing.

On the other hand, if all the answers in steps S31 to S33 are affirmative (YES), the setting is made in the processing of FIG.
SOx absorbed by the NOx absorbent of the NOx purification device 16
If the amount exceeds the allowable value, that is, the NOx purifying device 16
Flag FDSO indicating that the battery has deteriorated by "1"
It is determined whether or not x is “1” (step S34),
When FDSOx = 0 and the SOx amount does not exceed the allowable value, the catalyst temperature TCAT becomes the predetermined temperature STCAT1.
It is determined whether the temperature is higher than (for example, 500 ° C.) (Step S35). Then, TCAT> STCAT1, the exhaust gas flow rate of the engine 1 is large, and the catalyst temperature TCA
When T is in the specific operation state where T is high, the specific operation state flag FHL is set to “1”, and the SOx removal flag FHLS is set.
Ox is set to “0” (step S38), and the process ends. On the other hand, when TCAT ≦ STCAT1, the process proceeds to step S39.

If FDSOx = 1 in step S34 and the NOx purification device 16 has deteriorated, the catalyst temperature T
It is determined whether the CAT is higher than the degradation reproduction temperature STCAT2 (for example, 600 ° C.) higher than the predetermined temperature STCAT1 (step S36). This degradation regeneration temperature STCA
T2 is the temperature at which the NOx absorbent starts to release the SOx absorbed, that is, the lowest temperature at which the NOx absorbent can release SOx. TCAT ≦ STC in step S36
If it is AT2, the process proceeds to step S38 in order to quickly raise the catalyst temperature TCAT. Step S3
8, the specific operation state flag FHL is set to "1", thereby controlling the short-period air-fuel ratio fluctuation control (step S2 in FIG. 2).
3) is executed, and the temperature rise of the catalyst temperature TCAT is promoted. If TCAT> STCAT2 in step S36, the operation state is such that SOx can be removed, and thus the specific operation state flag FHL and the SOx removal flag FHLSO
Both x are set to “1” (step S37), and this processing ends.

FIG. 4 is a flowchart of a process for estimating the amount of SOx absorbed by the NOx absorbent of the NOx purification device 16, and this process is executed by the CPU 5b at regular intervals. First, the engine speed NE and the absolute pressure PBA in the intake pipe
Value ADDS used in the next step S53 according to
Ox is determined (step S52). Increment value ADDSO
x represents SO discharged per unit time during lean operation.
x is a parameter corresponding to the amount of engine revolutions NE
Is set so as to increase as the pressure increases and as the absolute pressure PBA in the intake pipe increases. Since the SOx emission amount per unit time is much smaller than the NOx emission amount, the increment value ADDSOx is smaller than the increment value ADDNOx corresponding to the NOx emission amount.

In step S53, step S
The increment value ADDSOx determined in 52 is applied, and the SOx amount counter CSOx is incremented. This gives S
Ox emission, that is, SOx absorbed by NOx absorbent
A count value corresponding to the quantity is obtained. CSOx = CSOx + ADDSOx

In the following step S54, it is determined whether or not the value of the SOx amount counter CSOx has exceeded the allowable value CSOxREF. If CSOx ≦ CSOxREF, the deterioration flag FDSOx is set to “0” (step S5).
5) When CSOx> CSOxREF, the deterioration flag FDSOx is set to “1” (step S5).
6). The allowable value CSOxREF is set to a value corresponding to, for example, a state where the absorption capacity of the NOx absorbent is reduced to about half that of a new product.

The amount of SOx absorbed by the NOx absorbent by the processing of FIG. 4 is estimated, and when the amount of SOx exceeds an allowable value, the deterioration flag FDSOx is set to "1". As a result, the process proceeds from step S34 to step S36 in FIG. 3 and when the catalyst temperature TCAT is low (TCAT
If ≦ STCAT2, the temperature rise of the NOx absorbent is promoted by the short-period air-fuel ratio fluctuation control (step S38, FIG. 2).
Steps S13 and S23), TCAT> STCAT2
Is reached, the SOx removal processing is executed (step S3).
7, Steps S12 and S24 in FIG. 2).

FIG. 5 is a flowchart of the short-period air-fuel ratio fluctuation control executed in step S22 of FIG. In step S41, it is determined whether or not the specific operation state flag FHL was "1" at the time of the previous execution of the processing of FIG.
If FHL = 0 last time, the lean time TLEAN (for example, 1 second) is set in a down count timer tmLEAN for measuring the lean operation continuation time and started (step S42), and the down time for measuring the rich operation continuation time is performed. A rich time TRICH (for example, 0.2 seconds) is set in the count timer tmRICH and started (step S43). Next, the target air-fuel ratio coefficient K
Lean predetermined value KC corresponding to CMD of about 22 in air-fuel ratio
The MDL is set (step S44), and the process ends.

After the next time, since the answer to step S41 is affirmative (YES), the process proceeds to step S45, and it is determined whether or not the value of the timer tmLEAN is "0". At first t
Since mLEAN> 0, the process proceeds to step S43, and the lean operation is continued. TmLE in step S45
When AN = 0, the timer tmRIC is set in step S46.
It is determined whether or not the value of H is “0”. At first tmR
Since ICH> 0, the target air-fuel ratio coefficient KCMD is set to a predetermined enrichment value KCMDR2 corresponding to an air-fuel ratio of about 11 (step S47), and this processing ends. tmR
Until ICH = 0, steps S41 to S45,
Steps S46 and S47 are executed, and the rich operation is continued. When tmRICH = 0, the process proceeds from step S46 to step S42. By the process of FIG. 5, as shown in FIG. 6, the short-period air-fuel ratio fluctuation control that repeats the lean operation for the lean time TLEAN and the rich operation for the rich time TRICH is executed.

FIG. 7 is a flowchart of the SOx removal process executed in step S24 of FIG. Step S
At 61, it is determined whether or not the SOx removal flag FHLSOx was “1” at the time of the previous execution of the processing of FIG.
When FHLSOx = 0, that is, FHLSOx
= 0 to FHLSOx = 1, the down-count timer tmRSOx for measuring the rich operation continuation time for SOx removal has the deterioration regeneration time TR.
SOx (for example, 8 minutes) is set (step S62),
Proceed to step S63. From the next time, the process immediately proceeds from step S61 to step S63.

In step S63, the target air-fuel ratio coefficient KCM
D is set to, for example, a SOx reduction enrichment predetermined value KCMDR3 corresponding to an air-fuel ratio of about 11, and then a timer tmRS
It is determined whether or not the value of Ox is “0” (step S
64). When tmRSOx> 0, this process is immediately terminated. When tmRSOx = 0, the SOx amount counter CSOx is reset to "0", and the deterioration flag FDSOx is set to "0" (step S65). This processing ends. By executing step S65, the SOx removal flag FHLSOx is returned to “0” (FIG. 3, steps S34, S35, S38), and the SOx removal process ends.

FIG. 8 is a diagram showing an example of a temperature rise characteristic of the catalyst temperature TCAT when the air-fuel ratio fluctuation control is started from the time t0. A line L6 in FIG. 7A corresponds to a case where the short-period air-fuel ratio fluctuation control (lean time 1 second, rich time 0.26 second) is executed, and a line L7 corresponds to a long period air-fuel ratio fluctuation control (lean time). 10 seconds and a rich time of 2 seconds). As is clear from this figure, the catalyst temperature TCAT can be increased in a shorter time by executing the short-period air-fuel ratio fluctuation control than in the long-period air-fuel ratio fluctuation control. S if
It is effective in removing Ox. That is, since the higher the temperature of the NOx absorbent, the more easily the SOx absorbed by the NOx absorbent is released, the short-period air-fuel ratio fluctuation control is executed to easily remove the SOx absorbed by the NOx absorbent. It is possible to do.

FIG. 6B shows the temperature rise characteristics when the air-fuel ratio during the rich operation in the short-cycle air-fuel ratio fluctuation control is changed. The line L6 is the same as the line L6 in FIG. 7A, and shows the characteristics when the rich air-fuel ratio is 11, and the lines L8, L9, L10 respectively have the rich air-fuel ratios 12, 13, 14,. 5 shows the characteristics. Due to this characteristic, the lower the rich air-fuel ratio (the richer the air-fuel ratio), the faster the temperature rise of the NOx absorbent and the higher the steady-state temperature. If the rich air-fuel ratio is 13, the catalyst temperature TCAT becomes SO
x is released, the degradation regeneration temperature STCAT2 (about 600
° C), the rich air-fuel ratio needs to be 13 or less, and it is desirable to enrich it to about 11 preferably.

As described above, in the present embodiment, S corresponding to the SOx amount absorbed by the NOx absorbent by the processing of FIG.
When the value of the Ox amount counter CSOx exceeds the allowable value CSOxREF, the deterioration flag FDSOx is set to “1”, and the short-period air-fuel ratio fluctuation control shown in FIG. 5 is executed to accelerate the temperature rise of the NOx absorbent. . When the catalyst temperature TCAT, that is, the temperature of the NOx absorbent exceeds the degradation regeneration temperature STCAT2 in the specific operation state, the SOx in FIG.
The removal process is executed, and the air-fuel ratio is maintained richer than the stoichiometric air-fuel ratio over the degradation regeneration time TRSOx (KCMD
= KCMDR3), SOx is removed from the NOx absorbent. As described above, in the present embodiment, by introducing the short-period air-fuel ratio fluctuation control, the temperature of the NOx absorbent can be quickly reduced to SO.
x can be raised to a temperature at which it can be released, and SOx can be reliably and efficiently removed. As a result, good exhaust gas characteristics can be maintained for a long period of time.

Next, the NOx purification rate achieved by executing the short-period air-fuel ratio fluctuation control will be described with reference to FIGS. FIG. 9 shows that the catalyst temperature TCAT is 570
° C and a high-load operation state of the engine 1,
That is, it is a diagram showing the NOx purification rate in the specific operation state, and the horizontal axis is the enrichment predetermined value KCMDR1 or KCMDR1.
This is the air-fuel ratio AFR during rich operation corresponding to CMDR2. Line L1 shows the characteristics when the short-period air-fuel ratio fluctuation control is executed, and line L2 shows the lean operation time
Characteristics of long-period air-fuel ratio fluctuation control (control corresponding to repetition of conventional lean operation and reduction enrichment) in which the NOx absorbent is not sulfur-poisoned, repeating 2 seconds and rich operation time of 2 seconds Is shown. As is clear from this figure, in the specific operation state, by executing the short-period air-fuel ratio fluctuation control, a higher NOx purification rate can be achieved as compared with the conventional long-period air-fuel ratio fluctuation control. In addition, when the NOx absorbent is poisoned with sulfur, the characteristics of the line L2 are remarkably deteriorated, whereas the characteristics of the line L1 are obtained without using the NOx absorbing and releasing action of the NOx absorbent. Even if the absorbent is poisoned with sulfur, it is not affected at all and a high NOx purification rate can be maintained.

FIG. 10 shows N when the lean time TLEAN is fixed at 1 second and the rich time TRICH is changed.
It shows the Ox purification rate. Here, the lines L3, L4 and L5 are respectively the intake pipe absolute pressure PBA = 660 mm
Hg, 460 mmHg and 310 mmHg. That is, when the short-cycle air-fuel ratio fluctuation control is executed, the NOx purification rate increases as the engine load increases (as the exhaust gas flow rate increases), and the short-cycle air-fuel ratio fluctuation control is particularly effective in a high-load operation state. It can be seen that it is.

Therefore, the short-period air-fuel ratio fluctuation control is executed in the high-load operation state where the exhaust gas flow rate is high, and the conventional reduction enrichment, that is, the long-period air-fuel ratio fluctuation control is performed in the low-load operation state where the exhaust gas flow rate is low. By executing this, a high NOx purification rate can be achieved in a wide range of the engine operating state. Further, in a high load operation state, the influence of sulfur poisoning can be eliminated.

Next, when executing the short-period air-fuel ratio fluctuation control, the lean time TLEAN and the rich time TRICH
Consider the setting range of.

FIG. 11 shows that the rich time TRICH is set to 0.2.
The graph shows the NOx purification rate when the lean time is fixed at 6 seconds and the lean time TLEAN is changed. As is clear from this figure, the shorter the lean time TLEAN, the higher the NOx purification rate is obtained. Here, in order to secure the NOx purification rate of about 50% or more, the lean time TLEAN needs to be 2 seconds or less.

The lower limit value of the lean time TLEAN is shown in FIG.
Although it is not clear from the above, it is appropriate to set it to about 0.5 seconds. When the lean time TLEAN is reduced, the rich time TRICH must be reduced in order to achieve good fuel efficiency. However, when the lean time TLEAN is shorter than 0.5 seconds, the rich time TRICH must be reduced. This makes it difficult to secure good fuel economy. Therefore, it is desirable to set the lean time TLEAN in the range of 0.5 seconds to 2 seconds. Rich time TR
It is desirable that the ICH be at most １ ／ or less of the lean time TLEAN. 1/2 of lean time TLEAN
If the length is longer, the improvement in fuel efficiency due to the lean operation becomes small, and there is no much difference from the case of performing the stoichiometric operation in which the air-fuel ratio is set near the stoichiometric air-fuel ratio using the three-way catalyst.

On the other hand, assuming that the lean time TLEAN is constant,
When the rich time TRICH is shortened, the NOx purification rate decreases as shown in FIG. In the example shown in this figure,
In order to ensure a NOx purification rate of 50%, PBA = 46
At 0 mmHg (line L4), the rich time TRICH needs to be 0.15 seconds (= TLEAN (1 second) /6.7) or more. That is, the lower limit value of the rich time TRICH is determined by the range in which the specific operation state is set and the target value of the NOx purification rate, and it is difficult to determine uniquely.

For example, assuming that the target value of the NOx purification rate is 40% or more and the predetermined pressure SPBA for defining the specific operation state is 460 mmHg, the lower limit value of the rich time TRICH is 0.1 second, that is, TLEAN / 10. Become. This embodiment corresponds to the exhaust gas purifying apparatus described in claim 1, and the processing for setting the deterioration flag FDSOx in FIG. 4 corresponds to the deterioration detecting means, and steps S34 and S36 in FIG.
And S38, step S13 in FIG. 2 and the processing in FIG. 5 for performing the short-period air-fuel ratio fluctuation control correspond to the air-fuel ratio fluctuation means,
Steps S36 and S37 in FIG. 3 and Step S12 in FIG.
In addition, the SOx removal processing in FIG. 7 corresponds to the deterioration reproducing means.

(Second Embodiment) In this embodiment, even before the deterioration flag FDSOx is set to "1", when the catalyst temperature TCAT becomes higher than the deterioration regeneration temperature STCAT2, the deterioration of the NOx purification device 16 is reduced. Degree, ie SOx
Deterioration reproduction time TRSO according to the value of the quantity counter CSOx
x is set, and the air-fuel ratio enrichment for reducing SOx is executed over the deterioration regeneration time TRSOx.

In the present embodiment, the operation state determination process of FIG. 12 is executed instead of the operation state determination process of FIG. 3, and the SOx removal process of FIG.
This is executed in U5b. This embodiment is the same as the first embodiment except that the processes in FIGS. 12 and 13 are executed instead of the processes in FIGS. 3 and 7.

The operation state determination process of FIG. 12 is obtained by deleting step S34 of FIG. 3, moving step S36 to that position, and adding step S40 between steps S36 and S37. Otherwise, the process is the same as the process of FIG. All the answers from steps S31 to S33 are affirmative (Y
ES), it is determined whether the catalyst temperature TCAT is higher than the degradation regeneration temperature STCAT2 (step S3).
6) If TCAT> STCAT2, proceed to step S40; if TCAT ≦ STCAT2,
Proceed to step S35.

In step S40, the SOx amount counter C
It is determined whether or not the value of SOx is greater than a lower threshold value CSOxL, and if CSOx ≦ CSOxL, step S
Then, the process proceeds to 38, where the SOx removal flag FHLSOx is set to “0” so that the SOx removal processing is not executed. On the other hand, when CSOx> CSOxL, step S3
In step 7, the SOx removal flag FHLSOx is set to “1”, and the SOx removal process is executed.

In step S40, when the SOx removal processing is completed, the catalyst temperature TCAT is reduced to the deterioration regeneration temperature STC.
Since it is higher than AT2, it is provided to prevent the SOx removal process from starting again, and is provided with a lower threshold CSO
xL is set to, for example, about 1/10 of the allowable value CSOxREF.

Therefore, according to the processing of FIG. 12, all the answers in steps S31 to S33 are normally affirmative (YES).
When the catalyst temperature TCAT exceeds the deterioration regeneration temperature STCAT2, the SOx
The removal flag FHLSOx is set to “1” and the SOx removal processing is executed.
Even if all the answers to 36 are affirmative (YES), the SOx removal process is not executed while the value of the SOx amount counter CSOx is equal to or less than the lower threshold value CSOxL.

The SOx removing process in FIG. 13 is the same as the process in FIG. 7 except that step S66 is added between steps S61 and S62 in FIG. Immediately after the SOx removal flag FHLSOx shifts from “0” to “1”, first, the deterioration reproduction time TRSOx is set according to the value of the SOx amount counter CSOx (step S66). Here, the deterioration reproduction time TRSOx is determined by the SOx amount counter CS.
The longer the value of Ox, the longer it is set. Subsequent step S
At 62, the degradation reproduction time TR set at step S66
SOx is set in a down-count timer tmRSOx and started.

According to the process of FIG. 13, the NOx purifying device 1
6, the rich air-fuel ratio is maintained over the deterioration regeneration time TRSOx corresponding to the value of the SOx amount counter CSOx corresponding to the SOx amount absorbed by the NOx purification device 16, and SOx removal is performed. You.

As described above, in the present embodiment, the engine 1
In the specific operation state, the short-period air-fuel ratio fluctuation control is executed, and the temperature rise of the NOx absorbent is promoted. And
The exhaust gas flow rate is large (steps S31 to S3 in FIG. 12).
3 is affirmative (YES) and the catalyst temperature TCAT becomes higher than the degradation regeneration temperature STCAT2, NOx
Deterioration degree of purification device 16, that is, SOx amount counter C
The deterioration regeneration time TRSOx is set according to the value of SOx, and the air-fuel ratio enrichment for reducing SOx is executed over the deterioration regeneration time TRSOx. As a result, NOx
SOx absorbed by the absorbent can be efficiently removed, and good exhaust gas characteristics can be maintained for a long period of time.

This embodiment corresponds to the exhaust gas purifying apparatus according to the second aspect of the present invention.
The process of estimating the Ox amount corresponds to the deterioration degree detecting means,
Steps S31 to S33, S35 and S38 in FIG.
The process in FIG. 5 for performing the short-period air-fuel ratio fluctuation control in step S13 in FIG. 2 corresponds to the air-fuel ratio fluctuation means, and FIG.
Steps S36, S40 and S37, step S12 in FIG. 2, and SOx removal processing in FIG. 13 correspond to the deterioration reproducing means.

(Third Embodiment) This embodiment uses NOx
Sulfur poisoning of the absorbent proceeds, and the SOx amount counter CSOx
When the value reaches the allowable value CSOxREF, the short-period air-fuel ratio fluctuation control is executed over a predetermined temperature rise time TSR, and then the SOx removal processing is executed.
That is, in the present embodiment, it is estimated that the catalyst temperature TCAT has reached the degradation regeneration temperature STCAT2 by executing the short-period air-fuel ratio fluctuation control over the predetermined temperature rise time TSR, and the SOx
Execute the removal process. The predetermined temperature rise time TSR is obtained by actually measuring the time required for the catalyst temperature TCAT to rise to a temperature at which the SOx removal processing can be performed by the short-period air-fuel ratio fluctuation control for a plurality of devices, and according to the measured time (for example, average Value + standard deviation x 3).

In this embodiment, instead of the KCMD calculation processing of FIG. 2, the operation state determination processing of FIG. 3, the short cycle air-fuel ratio fluctuation processing of FIG. 5, and the SOx removal processing of FIG.
The MD calculation processing, the operation state determination processing in FIG. 15, the short cycle air-fuel ratio fluctuation processing in FIG. 16, and the SOx removal processing in FIG. 17 are each executed by the CPU 5b. In the present embodiment, FIG.
It is the same as the first embodiment except that the processing of FIGS. 14 to 17 is executed instead of the processing of FIGS. 3, 5 and 7.

FIG. 14 is a flowchart of the KCMD calculation process according to the present embodiment.
1, S84 to S92 correspond to steps S11 and S14 in FIG.
This is the same process as steps S22 to S22. In step S71, the deterioration regeneration mode flag F, which is set by the process of FIG. 15 and indicates “1” as the NOx absorbent degradation regeneration mode.
It is determined whether or not SRCMODE is “1”, and the FSR
When CMODE = 0, a down-count timer tmSR that measures the time for executing the short-period air-fuel ratio fluctuation process
Then, a predetermined temperature rise time TSR (for example, 60 seconds) is set and started (step S72), and the SOx removal execution flag FSCATOK indicating "1" indicating that the SOx removal processing is being performed is set to "0". (Step S7
3), execute the processing of step S81 and subsequent steps.

When FSRCMODE = 1 in step S71, it is determined whether or not the value of the timer tmSR is "0" (step S74). While tmSR> 0, the process proceeds to step S75 to execute the short-period air-fuel ratio fluctuation control, and the tmS
When R = 0, it is determined that the catalyst temperature TCAT has risen to a temperature at which SOx removal processing can be performed, and the SOx removal execution flag FSCATOK is set to “1” (step S7).
6) Execute the SOx removal processing (step S77).

FIG. 15 shows an engine operation state determination process for setting the deterioration regeneration mode flag FSRCMODE referred to in step S71 of FIG. This processing corresponds to steps S35 to S3 of the engine operating state determination processing of FIG.
9 is deleted and steps S40 and S41 are added. That is, according to this processing, steps S31 to S31 are performed.
When all the answers in S34 are affirmative (YES), that is, when the exhaust gas flow rate is large and the deterioration flag FDSOx is "1", the deterioration regeneration mode flag FSRCM
If ODE is set to "1" and any of the answers in steps S31 to S34 is negative (NO), the deterioration reproduction mode flag FSRCMODE is set to "0". When the deterioration regeneration mode flag FSRCMODE is set to “1”, the processing of step S74 and subsequent steps in the processing of FIG. 2, that is, the short-cycle air-fuel ratio fluctuation processing and the SOx removal processing are executed.
SOx accumulated in the NOx absorbent is removed.

FIG. 16 is a flowchart of the short-period air-fuel ratio variation process in step S75 of FIG. This process is the same as the process of FIG. 5 except that step S41 of FIG. 5 is changed to step S48 and steps S49 and S50 are added.

In this embodiment, the execution condition of the short-period air-fuel ratio fluctuation control is set to the deterioration regeneration mode flag FSRCMODE = 1.
Therefore, step S4 in FIG.
In step S48, the specific operation state flag FHL of 1 is replaced with a deterioration regeneration mode flag FSRCMODE.

That is, in step S48, it is determined whether or not the previous deterioration reproduction mode flag FSRCMODE was "1". When FSRCMODE = 0, that is, immediately after the flag FSRCMODE changed from "0" to "1". When there is, the process proceeds to step S42, and thereafter, the process proceeds from step S48 to step S45.

In step S49, the predetermined lean value KCMDL is set to the engine speed NE and the absolute pressure P in the intake pipe.
In step S44, the target air-fuel ratio coefficient KCMD is set to the predetermined leaning value KCMDL determined in step S49. In step S50, the predetermined enrichment value KCMDR2 is determined according to the engine speed NE and the intake pipe absolute pressure PBA. In step S47, the target air-fuel ratio coefficient KCMD is set to the predetermined enrichment value KCMDR2 determined in step S50.

The short-period air-fuel ratio fluctuation control is executed by the processing of FIG. 16 in the same manner as the processing of FIG. 5. Further, in the present embodiment, the predetermined leaning value KCMD is set according to the engine operating state.
L and the enrichment predetermined value KCMDR2 are set, and the optimum air-fuel ratio can be set according to the engine operating state.

FIG. 17 is a flowchart of the SOx removal processing in step S77 of FIG. This process replaces step S61 of FIG. 7 with step S101,
Steps S102 and S103 are added,
The other points are the same as the processing of FIG.

In step S101, it is determined whether or not the previous SOx removal execution flag FSCATOK was "1". When FSCATOK = 0, that is, when SOx
If the removal execution flag FSCATOK has just changed from “0” to “1”, the timer tmRSOx is set to the deterioration reproduction time TRSOx and started (step S
62) Then, immediately after step S101, step S
Proceed to 102.

In step S102, the engine speed N
The SOx reduction enrichment predetermined value KCMDR3 is determined according to E and the intake pipe absolute pressure PBA. Next, the target air-fuel ratio coefficient KCMD is set to a predetermined enrichment value KCMDR3 (step S63), and the process ends immediately while the timer tmRSOx> 0, and when tmRSOx = 0, the counter CS
The value of Ox is reset to “0” and the deterioration flag FDSOx
Is reset to “0” (step S65),
The SOx removal execution flag FSCATOK is reset to “0” (step S103), and the process ends.

By this processing, the deterioration reproduction time TRSOx
, The target air-fuel ratio coefficient KCMD is set to the SOx reduction enrichment predetermined value KCMDR3, and the NOx absorbent SOx
Is removed. The enrichment predetermined value KCMDR3 is
Since it is set according to the engine speed NE and the intake pipe absolute pressure PBA, it is possible to complete the removal of SOx within the deterioration regeneration time TRSOx regardless of the engine operating state.

According to the present embodiment, when it is determined that the NOx absorbent has deteriorated due to the absorption of SOx, the temperature of the NOx absorbent is reduced to the deterioration regeneration temperature at which the SOx can be released early by the short-period air-fuel ratio fluctuation control. Therefore, SOx can be reliably and efficiently removed. As a result, good exhaust gas characteristics can be maintained for a long period of time.

Next, as shown in FIG. 18, a three-way catalyst 18 is disposed immediately downstream of the engine 1, and the NOx purifying device 16 is located downstream of the three-way catalyst 18 and slightly away from the engine 1, that is, Lean time TLEAN and rich time TRIC for short-period air-fuel ratio fluctuation control in a case where a configuration in which the engine 1 is mounted under the floor of a vehicle is adopted.
H, the setting of the lean air-fuel ratio AFL corresponding to the predetermined lean value KCMDL and the setting of the rich air-fuel ratio AFR corresponding to the predetermined rich value KCMDR2 will be discussed.

FIG. 19A is a diagram for explaining the result of an experiment in which the combination of the lean time TLEAN and the rich time TRICH is changed, and grid points in which A to D are entered correspond to each combination ( Hereinafter, "combination A",
"Combination B" etc.). When the combination C, that is, the lean time TEAN and the rich time TRICH are both set to 0.2 seconds or less, since the respective times are too short, the feedback control according to the output of the LAF sensor 14 does not operate normally. If the combination D, that is, the lean time TLEAN and the rich time TRICH are unbalanced, NOx
There was a tendency for the temperature rise of the absorbent to be slow (the rate of temperature rise was reduced). Therefore, it is preferable that the variation period is relatively long and the lean time TLEAN is equal to the rich time TRICH as in the combination A or B, but in the combination B, the temperature TC of the NOx absorbent placed under the floor is preferred.
There is a problem that the AT does not reach the temperature (600 ° C.) at which SOx can be removed. Fluctuation cycle (TLEAN + TRIC
The reason why the temperature rise becomes insufficient when H) is lengthened is that the HC and CO components discharged when the rich air-fuel ratio is set are NO
The time difference between the time when the fuel reaches the x purification device 16 and the time when the oxygen that increases when the lean air-fuel ratio is set reaches the NOx purification device 16 increases, and the amounts of HC and CO burned near the NOx purification device 16 decrease. It is supposed to be.

From the above experimental results, it was found that NO
x From the viewpoint of emphasizing the early temperature rise of the absorbent, both the lean time TLEAN and the rich time TRICH are 0.3
Most preferably, it is set to seconds. The engine operation state in the experiment whose results are shown in FIG. 19 is as follows: the engine speed NE is 2000 rpm, and the absolute pressure PBA in the intake pipe is 66.
0 mmHg.

FIG. 19B is a diagram for explaining the result of an experiment in which the combination of the lean air-fuel ratio AFL and the rich air-fuel ratio AFR is changed, and grid points E to H correspond to each combination. I do. In addition, the blank is a combination in which no experiment was performed. As described above, the rich air-fuel ratio A
As the FR becomes smaller (rich), the temperature of the NOx absorbent rises faster, and the temperature in the steady state can be made higher. However, if it is made too small as in the combination G, the upstream three-way catalyst 18 A problem occurs that the temperature of the substrate is too high. The smaller the lean air-fuel ratio AFL is, the more NOx
Since the rate of temperature rise of the absorbent tends to decrease, the combination F or H has a problem that the temperature rise of the NOx absorbent is slow. In combination H, there is also a problem that the temperature of the three-way catalyst 18 is too high. Therefore, three-way catalyst 1
In order to realize the early temperature rise of the NOx purification device 16 while avoiding the excessive temperature rise of the three-way catalyst 18 in consideration of the temperature rise of both the NOx absorbent 8 and the NOx absorbent of the NOx purification device 16, the combination E, that is, Most preferably, the lean air-fuel ratio AFL = 20 and the rich air-fuel ratio AFR = 11.

FIG. 20 shows the above-mentioned engine operating state (N
E = 2000 rpm, PBA = 660 mmHg), lean time TLEAN = rich time TRICH = 0.3
Second, lean air-fuel ratio AFL = 20, rich air-fuel ratio AFR =
11 is a time chart showing an example of a temperature rise characteristic when the temperature is set to 11; FIG. 7A shows the temperature rise characteristics of the NOx absorbent disposed under the floor when the three-way catalyst 18 is not provided. When the initial temperature is about 390 ° C., the temperature is controlled to be about 390 ° C. from the start of the short-period air-fuel ratio fluctuation control. It reaches 600 ° C. in 12 seconds. TLBCR of the same figure
S is the temperature in the cruise state of the lean operation (about 43
0 ° C.), and from this temperature TLBCRS, 6
Reach 00 ° C.

FIG. 13B shows the temperature rise characteristics (line L11) of the three-way catalyst 18 when the arrangement shown in FIG.
The temperature rise characteristics of the x absorbent (line L12) are shown. As described above, the presence of the three-way catalyst 18 considerably reduces the temperature rising rate of the NOx absorbent, and it takes about 57 seconds from the start of the short-period air-fuel ratio fluctuation control to 600 ° C.

This embodiment corresponds to the exhaust gas purifying apparatus described in claim 1, and corresponds to steps S71 and S7 in FIG.
Steps S2, S74 and S75 correspond to air-fuel ratio changing means, and steps S72, S74, S76 and S77 correspond to deterioration regeneration means.

(Other Embodiments) The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, FIG. 3 that defines a specific operating state of the engine 1
Are determined at least based on the engine speed NE and the intake pipe absolute pressure PBA in steps S32 and S33.
And the determination in steps S31 and S34 may be omitted. Further, the internal combustion engine is not limited to the type in which the fuel is injected into the intake pipe, but may be the type in which the fuel is directly injected into the combustion chamber of each cylinder.

[0092]

As described above in detail, according to the first aspect of the present invention, when the deterioration detecting means detects the deterioration of the nitrogen oxide purifying means, at a cycle set to a predetermined time or less, When the air-fuel ratio of the air-fuel mixture supplied to the engine is controlled to fluctuate between the lean side and the rich side with respect to the stoichiometric air-fuel ratio, and the temperature of the nitrogen oxide purifying means becomes higher than the degradation regeneration temperature during the air-fuel ratio fluctuation control. Since the air-fuel ratio is maintained richer than the stoichiometric air-fuel ratio over the degradation regeneration time, the temperature of the nitrogen oxide purifying means is quickly raised to the degradation regeneration temperature at which SOx can be removed, and the efficiency is improved. SOx absorbed by the nitrogen oxide purifying means can be removed. as a result,
Good exhaust gas characteristics can be maintained over a long period of time.

According to the second aspect of the present invention, in a specific operating state in which the exhaust gas flow rate of the engine is large, the air-fuel ratio of the air-fuel mixture supplied to the engine is less than the stoichiometric air-fuel ratio in a cycle set to a predetermined time or less. When the temperature of the nitrogen oxide purifying means becomes higher than the degradation regeneration temperature as a boundary, the air-fuel ratio becomes stoichiometric over the degradation regeneration time corresponding to the detected degradation degree. Since the fuel ratio is maintained on the rich side from the fuel ratio, the temperature of the nitrogen oxide purifying means is reduced to S while achieving a high nitrogen oxide purifying rate in the specific operation state.
The temperature can be quickly raised to the deterioration regeneration temperature at which Ox can be removed, and SOx absorbed by the nitrogen oxide purifying means can be efficiently removed. As a result, good exhaust gas characteristics can be maintained over a long period of time.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an internal combustion engine and a control device thereof according to a first embodiment of the present invention.

FIG. 2 is a flowchart of a process for setting a target air-fuel ratio coefficient (KCMD).

FIG. 3 is a flowchart of a process for determining a specific operation state and an operation state in which SOx can be removed.

FIG. 4 is a flowchart of a process for estimating the amount of SOx absorbed by a NOx absorbent.

FIG. 5 is a flowchart of a process for performing short-period air-fuel ratio fluctuation control.

FIG. 6 is a time chart for explaining the processing of FIG. 5;

FIG. 7 is a flowchart of a process for removing SOx absorbed by a NOx absorbent.

FIG. 8 is a time chart for explaining a temperature rise characteristic of the NOx purification device.

FIG. 9 shows NOx when short-period air-fuel ratio fluctuation control is executed.
It is a figure for explaining a purification rate.

FIG. 10 is a diagram showing a relationship between a rich time (TRICH) and a NOx purification rate.

FIG. 11 is a diagram showing a relationship between a lean time (TLEAN) and a NOx purification rate.

FIG. 12 is a flowchart of a process (second embodiment) for determining a specific operation state and an operation state in which SOx can be removed.

FIG. 13 is a flowchart of a process (second embodiment) for removing SOx absorbed by a NOx absorbent.

FIG. 14 is a flowchart of a process (third embodiment) for setting a target air-fuel ratio coefficient (KCMD).

FIG. 15 is a flowchart of a process (third embodiment) for determining the deterioration regeneration mode of the NOx absorbent.

FIG. 16 is a flowchart of a short-period air-fuel ratio variation process (third embodiment).

FIG. 17 is a flowchart of a process (third embodiment) for removing SOx absorbed by a NOx absorbent.

FIG. 18 is a diagram showing a configuration in which a three-way catalyst is arranged on the upstream side of the NOx purification device.

FIG. 19 is a diagram for describing an experimental result for determining a control parameter value.

FIG. 20 is a time chart showing the temperature rise characteristics of the NOx absorbent and the three-way catalyst.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 5 Electronic control unit (deterioration detection means, deterioration degree detection means, air-fuel ratio fluctuation means, deterioration regeneration means) 6 Fuel injection valve 7 Intake pipe absolute pressure sensor 10 Engine speed sensor 12 Exhaust pipe 16 NOx purification device (Nitrogen Oxide purification means) 17 Catalyst temperature sensor (temperature detection means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F01N 3/28 301C F02D 41/02 301 F02D 41/02 301H 41/04 305 41/04 305D 305E

Claims (2)

[Claims] 1. An exhaust system for an internal combustion engine, comprising: a nitrogen oxide purifying means provided in an exhaust system of the internal combustion engine and absorbing nitrogen oxides in the exhaust gas when the exhaust gas is in a lean state of an exhaust gas having a relatively high oxygen concentration. In the gas purification device, a deterioration detection unit that detects deterioration of the nitrogen oxide purification unit, and a cycle set to a predetermined time or less when the deterioration detection unit detects the deterioration of the nitrogen oxide purification unit. Air-fuel ratio changing means for changing the air-fuel ratio of the air-fuel mixture supplied to the engine between a lean side and a rich side with a stoichiometric air-fuel ratio as a boundary, and the operation of the air-fuel ratio changing means reduces the temperature of the nitrogen oxide purifying means. Exhaust gas purification for an internal combustion engine, comprising: a regeneration unit that maintains the air-fuel ratio on the richer side than the stoichiometric air-fuel ratio over the degradation regeneration time when the temperature becomes higher than the degradation regeneration temperature. apparatus. 2. An exhaust system for an internal combustion engine, comprising: a nitrogen oxide purifying means provided in an exhaust system of the internal combustion engine and absorbing nitrogen oxides in the exhaust gas when the exhaust gas is in a lean state of an exhaust gas having a relatively high oxygen concentration. In the gas purifying apparatus, a deterioration degree detecting means for detecting a degree of deterioration of the nitrogen oxide purifying means, and a supply to the engine at a cycle set to a predetermined time or less in a specific operation state in which the exhaust gas flow rate of the engine is large. Air-fuel ratio varying means for varying the air-fuel ratio of the mixture to be leaned and riched with the stoichiometric air-fuel ratio as a boundary, and when the temperature of the nitrogen oxide purifying means has become higher than the degradation regeneration temperature, the detection is performed. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a deterioration regeneration unit that maintains the air-fuel ratio on the richer side than the stoichiometric air-fuel ratio for a deterioration regeneration time according to the degree of deterioration.