JP2000274232A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrict generation of off-flavor due to the sulfur (S) compound included in exhaust gas by fluctuating the exhaust air-fuel ratio of an internal combustion engine around the rich air-fuel ratio as a reference, when sulfur poisoning of the NOx catalyst is detected by a S-poisoning detecting means. SOLUTION: When storage quantity of NOx of the NOx catalyst, namely, the quantity of S-poisoning is detected (S1), a discrimination as to whether or not the NOx catalyst is being regenerated is done (S2), and if it is 'YES', a fuel injector performs two-stage injection of the fuel so as to raise the temperature of the exhaust gas, namely, the temperature of the NOx catalyst (S5). Thereafter, a discrimination whether or not temperature rise of the NOx catalyst is concluded or not, namely, temperature of the NOx catalyst achieves the SOx activating temperature is done, and if it is 'YES', fluctuation control of the exhaust air-fuel ratio is carried out (S7). The fluctuation control is performed by alternately switching the exhaust air-fuel ratio between the theoretical air- fuel ratio as a lean side of the reference air-fuel ratio and the predetermined rich air-fuel ratio as a richer air-fuel ratio. At the time of switching the air-fuel ratio, detecting signal of an O2 sensor is used.

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 particularly suitable for deodorizing exhaust gas.

[0002]

2. Related Background Art In recent years, lean (lean) combustion type internal combustion engines have been increasingly employed in vehicles for the purpose of improving fuel efficiency. In this type of internal combustion engine, since a large amount of nitrogen oxide (NOx) is contained in the exhaust gas, a so-called NOx catalyst is disposed in the exhaust passage. This NOx catalyst stores NOx in the exhaust gas when the internal combustion engine is operating at a lean air-fuel ratio, and stores the NOx when the internal combustion engine is operated at a rich air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio. It has the function of releasing and reducing NOx.

[0003] By the way, this type of NOx catalyst occludes not only NOx in exhaust gas but also sulfur components.
This leads to a decrease in NOx purification capacity. That is, the NOx catalyst has a property of being poisoned by the sulfur component. In order to eliminate such S poisoning, for example, Japanese Unexamined Patent Publication No. 6-66129 discloses that when S poisoning of the NOx catalyst exceeds an allowable level, NOx
A technique is known in which the temperature of the catalyst is raised to a predetermined temperature or higher, and the surroundings are reduced to a reducing atmosphere, that is, the exhaust air-fuel ratio is set to a rich air-fuel ratio. It has been disclosed.

[0004] However, in the above-mentioned exhaust gas purifying apparatus disclosed in JP-A-6-66129, the sulfur component desorbed from the NOx catalyst reacts with the hydrocarbon (HC) in the exhaust gas to form a sulfur (S) compound ( Hydrogen sulfide: H ₂ S) is produced temporarily in large quantities. If such an S compound, that is, hydrogen sulfide is released in a large amount into the atmosphere, it causes an unpleasant odor and is not preferable.

[0005] Under such circumstances, Japanese Patent Application Laid-Open No. 8-294618 discloses that a hydrogen sulfide trap and a catalytic converter having an oxidizing function are arranged downstream of the NOx catalyst in order to suppress the release of hydrogen sulfide into the atmosphere. On the other hand, a technique is disclosed in which the exhaust air-fuel ratio fluctuates, ie, perturbates, between a lean air-fuel ratio and a rich air-fuel ratio around a stoichiometric air-fuel ratio.

[0006]

SUMMARY OF THE INVENTION
In the case of the exhaust purification system of No. 8-294618, the above-mentioned air-fuel ratio perturbation is indispensable for oxidizing hydrogen sulfide trapped in the catalytic converter, that is, for supplying oxygen to the catalytic converter. However, such an air-fuel ratio perturbation causes the sulfur component to be further stored in the NOx catalyst when the exhaust air-fuel ratio is at a lean air-fuel ratio, which increases the time required for regeneration of the NOx catalyst. become.

When the regeneration time of the NOx catalyst becomes longer, the period during which the exhaust air-fuel ratio becomes a rich air-fuel ratio during the air-fuel ratio perturbation is increased, thereby deteriorating the fuel efficiency. Further, since the above-mentioned catalytic converter is special, the cost of the exhaust gas purification device increases. The present invention has been made in view of the above circumstances, and an object of the present invention is to suppress an unpleasant odor caused by an S compound in exhaust gas, and furthermore, to prevent an internal combustion engine from deteriorating fuel consumption and increasing cost. It is an object of the present invention to provide an exhaust gas purification device.

[0008]

The above object is achieved by the exhaust gas purifying apparatus for an internal combustion engine according to the present invention. This exhaust gas purifying apparatus detects S poisoning of a NOx catalyst by sulfur component in exhaust gas. Detecting means; and air-fuel ratio changing means for changing the exhaust air-fuel ratio of the internal combustion engine around a reference rich air-fuel ratio when the S-poisoning detecting means detects S poisoning of the NOx catalyst. .

According to the above-mentioned exhaust gas purifying apparatus, when the sulfur component is desorbed from the NOx catalyst, if the exhaust air-fuel ratio fluctuates around the reference rich air-fuel ratio, that is, the exhaust air-fuel ratio becomes the reference rich air-fuel ratio. When the air-fuel ratio fluctuates between the air-fuel ratio leaner than the fuel ratio and the mower-rich air-fuel ratio richer than the reference rich air-fuel ratio, the sulfur component is gradually desorbed from the NOx catalyst, and S The compound is not produced temporarily and in large quantities.

[0010] It is preferable that the fluctuation region of the exhaust air-fuel ratio is set to be equal to or lower than the stoichiometric air-fuel ratio.
It is preferable to detect or estimate the level of S poisoning of the x catalyst. Then, the NOx is detected by the S poison detection means.
When the S poisoning level of the catalyst is detected, the higher the S poisoning level, the larger the rich air-fuel ratio, which is a reference of the air-fuel ratio changing means, is set to a larger value, or
For a predetermined period from the start of the exhaust air-fuel ratio fluctuation control, 1
Either shorten the time of the moor-rich air-fuel ratio relative to the time of the lean-side air-fuel ratio during the cycle, or further reduce the frequency of transition to the moor-rich air-fuel ratio relative to the frequency of transition to the lean-side air-fuel ratio. Is preferred.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The internal combustion engine schematically shown in FIG.
For example, it is an in-cylinder injection type in-line four-cylinder gasoline engine. This type of internal combustion engine includes a fuel injector 2 that can directly inject fuel into a combustion chamber, and is capable of injecting fuel in various fuel injection modes and exhaust air-fuel ratios according to the operating conditions. Specifically, the fuel injection mode mainly includes an intake stroke injection mode in which fuel is injected in an intake stroke to perform uniform combustion, and a compression stroke injection mode in which fuel is injected in a compression stroke to perform stratified combustion. In particular, in the compression stroke injection mode, the air-fuel ratio (air-fuel ratio of about 12 to 23) in the intake stroke injection mode is compared with the super-lean air-fuel ratio (air-fuel ratio of 25).
Above) is possible.

An exhaust pipe 6 extends from an exhaust manifold 4 of the internal combustion engine, and a small three-way catalyst 8 is inserted into the exhaust pipe 6 at an upstream end thereof. Further, a catalytic converter 10 is inserted downstream of the exhaust pipe 6. The catalytic converter 10 is a combination of a storage type NOx catalyst 12 and a three-way catalyst 14, and the NOx catalyst 12 is positioned upstream of the three-way catalyst 14. The NOx catalyst stores NOx in an oxidizing atmosphere (exhaust air-fuel ratio is lean air-fuel ratio), while it stores NOx in a reducing atmosphere (exhaust air-fuel ratio is rich air-fuel ratio) to nitrogen (N ₂ ) or the like. It has the function of reducing. More specifically, the NOx catalyst 12 has a catalyst such as platinum (Pt) or rhodium (Rh) and a storage material made of an alkali metal or alkaline earth metal such as barium (Ba). As described above, the NOx catalyst 12 includes not only NOx in the exhaust gas but also a sulfur component, that is, SO2.
x also has the property of occluding, and the stability of SOx in the occluding material of the NOx catalyst 12 is high. Therefore, in order to release and reduce SOx from the NOx catalyst 12, it is necessary to raise the temperature of the NOx catalyst 12 to a predetermined SOx activation temperature (for example, 650 ° C.) or more and to make the surrounding area a reducing atmosphere.

Therefore, the catalytic converter 10 has NOx
A temperature sensor 16 for detecting the temperature of the exhaust gas flowing into the catalyst 12 and, if necessary, a concentration sensor 18 for detecting the NOx concentration in the exhaust gas are provided between the NOx catalyst 12 and the three-way catalyst 14. The sensors 16 and 18 are connected to an electronic control unit (ECU) 20. Further, the ECU 20 includes the fuel injector 2 described above.
In addition to, O ₂ sensor 2 for detecting the oxygen concentration in the exhaust gas
2. The spark plug 24, the throttle opening sensor 26, and the crank angle sensor 28 are also electrically connected.

The ECU 20 is a one-board type microcomputer including a microprocessor, and controls the switching of the fuel injection mode and the drive control of the fuel injector 2 and the ignition coil 24 based on the detection signal from the sensor. Meanwhile, SOx regeneration control of the catalytic converter 10 is performed. FIG. 2 shows the procedure of the SOx regeneration control. The regeneration control will be described below with reference to FIG.

First, the ECU 20 determines whether the NOx catalyst 12
Estimate the Ox storage amount, that is, the S poisoning amount (step S
1). Specifically, the S poisoning amount Qs is calculated by executing the following equation at each execution cycle of the fuel injection control routine executed by the ECU 20. Qs (n) = Qs (n-1) + ΔQf · K
-Rs Here, Qs (n) indicates the current calculated value, and Qs (n-1) indicates the previous calculated value. ΔQf, Rs are the integrated value of the injected fuel per execution cycle, the SOx release amount, and K is the correction coefficient.

The correction coefficient K is a correction coefficient K1 corresponding to the exhaust air-fuel ratio (A / F), a correction coefficient K2 corresponding to the content of the sulfur component in the fuel, and a correction corresponding to the catalyst temperature of the NOx catalyst 12. It is represented by the product of the coefficients K3, that is, K1, K2, K2. The catalyst temperature is obtained based on the detection signal from the temperature sensor 16 described above, but the detection signal from the temperature sensor 16 does not directly indicate the temperature of the NOx catalyst 12.
Therefore, the ECU 20 estimates the temperature of the NOx catalyst 12 by correcting the detection signal of the temperature sensor 16 based on the map determined from the target average effective pressure of the internal combustion engine and the engine speed. Note that the target average effective pressure and the engine speed can be obtained based on detection signals from a throttle opening sensor and a crank angle sensor.

The SOx release amount Rs is calculated from the following equation. Rs = α · R1 · R2 · dT where α is the SOx emission rate per unit time (set value), dT is the execution cycle of the fuel injection control routine,
R1 and R2 indicate a SOx release capacity coefficient corresponding to the catalyst temperature and an SOx release capacity coefficient corresponding to the exhaust air-fuel ratio.

In step S1, when the amount of S poisoning of the NOx catalyst 12 is estimated, that is, detected, the ECU 20 sets NO.
It is determined whether or not the x catalyst 12 is being regenerated (S purge is being performed), that is, whether or not a regeneration flag described later is set (step S2). Here, since the regeneration flag has not yet been set, the determination result is false (No), and the ECU 20 determines whether the S poisoning amount of the NOx catalyst 12 is equal to or lower than an allowable level (step S3). . If the determination result is true (Yes), the ECU 20 repeats steps S1 and S2. Here, the allowable level of the S poisoning amount is a set value obtained from the capacity of the NOx catalyst 21.

On the other hand, when the determination result of step S3 is false, the ECU 20 sets a regeneration flag (step S3).
4). Thereafter, the determination result of step S2 becomes true, and E
The CU 20 raises the temperature of the NOx catalyst 12 (Step S5). In step S5, the ECU 20 causes the fuel injector 2 to perform two-stage fuel injection to increase the temperature of the exhaust gas. More specifically, the fuel injector 2 performs a sub-injection of fuel in an expansion stroke in addition to a main injection of fuel in a compression stroke or an intake stroke, and the fuel of the sub-injection is burned in the exhaust pipe 6. By doing so, the temperature of the exhaust gas, that is, the temperature of the NOx catalyst 12 is raised. Here, the sub-injection amount of the fuel is adjusted according to the current catalyst temperature of the NOx catalyst 12, and even when the above-described two-stage injection is executed, the overall exhaust air-fuel ratio is controlled by the operation. Needless to say, it is controlled according to the situation. When the internal combustion engine is in a high speed range and the temperature of the NOx catalyst 12 has already reached the above-mentioned SOx activation temperature, the sub injection amount of the fuel becomes zero. Is not substantially executed.

Thereafter, at step S6, it is determined whether or not the temperature rise of the NOx catalyst 12 has been completed.
It is determined whether or not the temperature of No. 2 has reached or exceeded the SOx activation temperature. If the determination here is false, step S5 is repeatedly executed. When the result of the determination in step S6 becomes true, the ECU 20 executes the exhaust air-fuel ratio (A / F) variation control (step S7), the details of which are as follows.

In step S7, the exhaust air-fuel ratio is set to the rich side.
Centering on the reference air-fuel ratio X (for example, 14.35)
For a predetermined period. Specifically, the exhaust air-fuel ratio is
The stoichiometric air-fuel ratio as the air-fuel ratio leaner than the quasi-air-fuel ratio X
(14.7) and a predetermined rich as a mower rich air-fuel ratio
A predetermined time (eg, 14.0) between the air-fuel ratio (eg, 14.0)
5 seconds). Note that the exhaust air-fuel ratio
In exchange, the O _TwoThe detection signal from the sensor 22 is used.
Needless to say, and in this case,
The air-fuel ratio is O_TwoThe average value obtained from the detection signal of the sensor
is there.

When the exhaust air-fuel ratio variation control (S purge) is executed as described above, the exhaust air-fuel ratio is set in a region richer than the stoichiometric air-fuel ratio (stoichio) as shown in FIG. , Around the reference rich air-fuel ratio X, and fluctuates up and down. Therefore, since the SOx stored in the NOx catalyst 12 is released and reduced in a larger amount when the exhaust air-fuel ratio is changed to a richer side, the concentration of the S compound in the exhaust pipe 6 periodically increases and decreases. And the average concentration per hour can be reduced. Further, as is apparent from FIG. 3, with respect to the periodic release of the S compound, the concentration level of the S compound at the time of the release gradually decreases with the passage of time.
This is because the amount of stored Ox gradually decreases.

Therefore, the above-mentioned fluctuation control, that is, NOx
Even if the regeneration control of the catalyst 12 is executed, the S compound is not temporarily and largely released into the exhaust pipe 6. This means that hydrogen sulfide (H ₂ S) obtained by a chemical reaction between the S compound and a reducing agent such as H ₂ in the exhaust pipe 6 is not generated temporarily and in large amounts. Offensive odor due to hydrogen can be effectively suppressed.

FIG. 3 also shows a change in the concentration of the S compound in the rear area of the vehicle, and the two-dot chain line in FIG. 3 indicates that the exhaust air-fuel ratio continues to be maintained at the richer air-fuel ratio. 5 shows changes in the concentration of the S compound in the exhaust pipe and in the rear area of the vehicle in the case of the above. As is clear from the comparison between the solid line and the two-dot chain line in FIG. 3 regarding the change in the concentration of the S compound, in the case of this embodiment, immediately after the start of the fluctuation control, a large amount of hydrogen sulfide is discharged to the rear of the vehicle. There is no such thing, and the occupants in the own vehicle and the following vehicles do not feel uncomfortable due to the strange smell.

During the regeneration of the NOx catalyst 12, the exhaust air-fuel ratio is not switched to a lean air-fuel ratio larger than the stoichiometric air-fuel ratio. Therefore, the regeneration of the NOx catalyst 12 can be performed quickly, and the fuel efficiency is improved. . Moreover, in the case of this embodiment, a special catalyst for trapping hydrogen sulfide is not required, and an inexpensive exhaust gas purification device can be provided. After execution of step S7, the ECU 20 sets the NOx catalyst 1
It is determined whether or not the reproduction of No. 2 has been completed (step S8),
Step S7 is repeatedly executed until the result of the determination becomes true. On the other hand, when the determination result of step S8 becomes true, the ECU 20 resets the regeneration flag (step S9), and thereafter, the determination of step S3 is repeatedly executed. Here, the determination in step S8 can be performed based on the elapsed time from the start of the exhaust air-fuel ratio fluctuation control (step S7) or the S storage amount estimated in step S1.

The ECU 20 performs feedback control or open-loop control in varying the exhaust air-fuel ratio between the air-fuel ratio leaner than the reference rich-side air-fuel ratio (theoretical air-fuel ratio) and the air-fuel ratio on the rich side. Can be used. In the present embodiment, the reference air-fuel ratio X is varied between the stoichiometric air-fuel ratio and the moor-rich air-fuel ratio. However, the lean air-fuel ratio with respect to the reference air-fuel ratio X is slightly richer than the stoichiometric air-fuel ratio. May be set to the side air-fuel ratio.

The regeneration routine of the NOx catalyst 12 in FIG. 3 may be executed at predetermined intervals in consideration of the traveling distance of the vehicle. In this case, when executing the fluctuation control of the exhaust air-fuel ratio (step S7),
When the fluctuation range of the air-fuel ratio is constant, the NOx catalyst 12
3, the level of the reference rich air-fuel ratio X may be varied up and down as shown by the arrow Y in FIG. Specifically, the more the S poisoning amount, the more
The reference rich air-fuel ratio X is displaced toward the stoichiometric air-fuel ratio. When the reference rich air-fuel ratio X is displaced in this manner, the fluctuation of the exhaust air-fuel ratio to the rich side is suppressed,
It is possible to effectively prevent the temporary and large amount of the S compound from being generated in the exhaust pipe 6. The air-fuel ratio leaner than the reference rich air-fuel ratio may be leaner than the stoichiometric air-fuel ratio.

On the basis of the same concept, when the reference rich air-fuel ratio X is constant, the exhaust air-fuel ratio is changed to the reference rich air-fuel ratio X during one fluctuation cycle of the air-fuel ratio as shown in FIG. Time A at which the air-fuel ratio on the lean side is maintained
Considering the time B in which the rich air-fuel ratio is maintained on the richer side with respect to the reference rich air-fuel ratio X,
As the S poisoning amount of the NOx catalyst 12 increases, the time B is shortened with respect to the time A, or the exhaust air-fuel ratio is changed to the air-fuel ratio on the lean side instead of the times A and B described above. In view of the leaning frequency at which the shift takes place and the riching frequency at which the transition to the more rich air-fuel ratio takes place, the greater the amount of S poisoning, the smaller the richening frequency with respect to the leaning frequency. As a result, the operation frequency at the moir-rich air-fuel ratio is reduced, and the temporary generation of a large amount of S compounds can be effectively prevented.

Further, the above-described variation control of the exhaust air-fuel ratio is NO.
Even if it is not carried out over the entire region of the x catalyst regeneration period (see FIG. 3), it is carried out only in a region where SOx is released and reduced by a predetermined level or more from the NOx catalyst 12, and thereafter, the exhaust air-fuel ratio is changed to the stoichiometric air-fuel ratio Alternatively, a predetermined rich air-fuel ratio near the stoichiometric air-fuel ratio may be maintained. In the above-described feedback control of the exhaust air-fuel ratio, the exhaust air-fuel ratio can be switched to a lean air-fuel ratio or a more rich air-fuel ratio by changing the integral gain of the feedback control or the proportional gain. Specifically, to switch the exhaust air-fuel ratio to the more rich air-fuel ratio, at least one of control for increasing the enrichment gain (integral or proportional gain) for the exhaust air-fuel ratio or decreasing the leaning gain is performed.

The exhaust air-fuel ratio can be switched to a lean air-fuel ratio or a more rich air-fuel ratio by changing the upper limit or lower limit of the correction coefficient of the feedback control instead of the integral or proportional gain. In this case, specifically, in order to switch the exhaust air-fuel ratio to the moir-rich air-fuel ratio, at least one of the controls for increasing or decreasing the upper limit value of the correction coefficient is performed.

Further, in the above-described embodiment, the two-stage injection of the fuel injector 2 is carried out in order to raise the temperature of the NOx catalyst 12, but instead of such a two-stage injection, the ignition timing is retarded. Alternatively, the temperature of the NOx catalyst 12 may be raised by a heat source such as an electric heater.

[0032]

As described above, according to the exhaust gas purifying apparatus for an internal combustion engine of the present invention, when the S poisoning of the NOx catalyst exceeds an allowable level, the exhaust air-fuel ratio is centered on the rich air-fuel ratio as a reference. Since it is made to fluctuate up and down, the NOx catalyst can be quickly regenerated without generating an off-flavor in the exhaust gas, and the fuel cost can be improved and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an internal combustion engine including an exhaust gas purification device of one embodiment.

FIG. 2 is a flowchart showing a NOx catalyst regeneration control routine executed by an ECU of FIG. 1;

FIG. 3 is a time chart showing a change in an exhaust air-fuel ratio, a change in the concentration of an S compound in an exhaust pipe, and a change in the concentration of an S compound behind a vehicle during execution of regeneration control.

[Explanation of symbols]

2 Fuel Injector 10 Catalytic Converter 12 NOx Catalyst 14 Three-Way Catalyst 22 O ₂ Sensor 20 ECU

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/20 F02D 41/04 305Z 3/28 301 B01D 53/36 101B F02D 41/04 305 102E (72) Inventor Yuki Tamura 5-33-8 Shiba 5-chome, Minato-ku, Tokyo F-term in Mitsubishi Motors Corporation (reference) 3G091 AA17 AA24 AA28 AB03 AB06 AB11 DB11 DB13 DC03 EA01 EA07 EA08 EA17 EA33 EA34 FB12 GB02Y GB03Y GB05W GB06W HA12 HA19 HA19 HA36 HA38 HA42 HA47 3G301 HA01 HA04 HA06 HA16 JA25 MA01 MA11 MA19 MA26 NA09 NC02 ND01 ND05 NE13 PA11Z PD02Z PD11Z PD12Z PE01Z PE03Z 4D048 AA06 AA13 AA18 AB02 AB05 BA15Y BA30Y BA33Y BD02 BD04 CC32 DA44 DA01 DA02 DA04

Claims (1)

[Claims] 1. An exhaust passage for an internal combustion engine, which stores nitrogen oxides in exhaust gas when the exhaust air-fuel ratio is lean, and stores nitrogen oxides when the exhaust air-fuel ratio is lower than the stoichiometric air-fuel ratio. NOx catalyst for releasing and reducing sulfur, S poisoning detecting means for detecting S poisoning of the NOx catalyst by sulfur components in the exhaust gas, and S poisoning of the NOx catalyst detected by the S poisoning detecting means And an air-fuel ratio varying means for varying the exhaust air-fuel ratio around a reference rich air-fuel ratio to desorb the sulfur component stored in the NOx catalyst. Exhaust gas purification device.