JPH05312026A - Exhaust control device for engine - Google Patents

Abstract

PURPOSE:To keep high purification efficiency of NOx by suppressing bad influence to a lean operation and increase of an external load, and preventing deterioration of a lean NOx catalyst which may be caused by oxygen adsorption. CONSTITUTION:During lean operation, it is judged whether a catalyst resisting temperature T0 is exceeded by a value which is obtained by a presumed temperature increasing rate T2 due to temperature-increasing operation to a catalyst temperature T1 11 periodically by a CPU 12 or not (S13). When the resisting temperature T is exceeded, a temperature-increasing operation is executed by catalyst heating 19 (S14). Other than the resultant periodical reduction of adsorbed oxygen, it is reduced every time when a multiplied value of an oxygen rate passing through a lean NOx catalyst exceeds a specified value. Operation can be carried out with an air-fuel ratio being temperarily shifted to rich instead of heating of the catalyst as a reduction means for the adsorption oxygen.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine exhaust control device having an exhaust system provided with a so-called lean NOx catalyst.

[0002]

2. Description of the Related Art As described in Japanese Patent Laid-Open No. 3-225013, a transition metal is used as a catalyst for purifying NOx in engine exhaust gas in a lean air-fuel ratio region where the air-fuel ratio is larger than the theoretical air-fuel ratio. Alternatively, an engine is known in which an exhaust system is provided with a so-called lean NOx catalyst which is made of zeolite supporting a noble metal and which reduces NOx in exhaust gas in the presence of HC in an oxidizing atmosphere.

Further, as described in, for example, Japanese Utility Model Publication No. 2-94316, an engine having a catalyst in its exhaust system is provided with a catalyst heating means for maintaining the catalyst temperature above a predetermined temperature. is there.

[0004]

The lean NOx catalyst adsorbs HC and NOx in the exhaust gas component to reduce and purify NOx, but if oxygen is adsorbed, it deteriorates and the purifying performance remarkably deteriorates. However, this deterioration due to oxygen adsorption is only temporary deterioration and can be recovered by means such as heating the catalyst or sending a rich air-fuel mixture. However, if the operation for recovering the catalyst performance is executed after the deterioration of the lean NOx catalyst due to such oxygen adsorption becomes remarkable, the time required for the recovery becomes long and the lean operation is adversely affected. There is a problem that the external load due to heating increases.

The present invention has been made in view of the above problems, and can prevent deterioration of the lean NOx catalyst due to oxygen adsorption and maintain a high NOx purification rate while suppressing adverse effects on lean operation and an increase in external load. The purpose is to do so.

[0006]

The invention of this application relates to an exhaust control system for an engine. The first invention is
Lean NOx for reducing and purifying NOx in exhaust gas in a lean air-fuel ratio region in which at least the air-fuel ratio is larger than the theoretical air-fuel ratio
In an engine equipped with a catalyst, an adsorbed oxygen reducing means for reducing oxygen adsorbed on the lean NOx catalyst, a lean operation detecting means for detecting an operation of the engine in a lean air-fuel ratio region, and an output of the lean operation detecting means are received. During the lean operation, the reduction means for reducing the adsorbed oxygen by the adsorbed oxygen reduction means is provided in a predetermined cycle. Here, lean NOx is used as the adsorbed oxygen reducing means.
A catalyst heating means for heating the catalyst can be used, in which case the catalyst temperature detecting means for detecting the temperature of the lean NOx catalyst and the output of this catalyst temperature detecting means are used to receive the lean NOx.
It is advisable to provide execution limiting means for operating the reduction executing means on condition that the temperature of the x catalyst does not exceed a predetermined temperature.

The second invention of the present application is an engine equipped with a lean NOx catalyst for reducing and purifying NOx in exhaust gas in a lean air-fuel ratio region where the air-fuel ratio is larger than the stoichiometric air-fuel ratio, and is adsorbed on the lean NOx catalyst. Adsorbed oxygen reducing means for reducing the amount of oxygen stored, oxygen amount detecting means for detecting the amount of oxygen passing through the lean NOx catalyst, and integrated oxygen amount value for calculating an integrated value of the oxygen amount detected by the oxygen amount detecting means. Receiving the output of the calculation means and the oxygen amount integrated value calculation means,
It is characterized by further comprising reduction executing means for executing the reduction of the adsorbed oxygen by the adsorbed oxygen reducing means when the integrated value exceeds a predetermined value. Here, as the adsorbed oxygen reducing means, a catalyst heating means for heating the lean NOx catalyst can also be used. At that time, a catalyst temperature detecting means for detecting the temperature of the lean NOx catalyst and an output of the catalyst temperature detecting means. In response to this, it is preferable to provide execution limiting means for operating the reduction executing means on the condition that the temperature of the lean NOx catalyst does not rise above a predetermined temperature.

Further, in the above-mentioned first and second inventions, the adsorbed oxygen reducing means may be a temporary rich operating means for temporarily operating the engine at a rich air-fuel ratio. FIG. 1A is an overall configuration diagram of the first invention,
(B) is a whole block diagram of the said 2nd invention.

[0009]

According to the first aspect of the present invention, the adsorbed oxygen reducing means such as the heating means and the temporary rich operating means operate at a predetermined cycle during the lean operation continuing, whereby the oxygen adsorbed on the lean NOx catalyst is catalyzed. Desorption is reduced and catalyst performance is restored before deterioration becomes significant. At that time, in the case where the catalyst heating means for heating the lean NOx catalyst is provided as the adsorbed oxygen reducing means, the heating is executed on condition that the temperature of the lean NOx catalyst does not reach the predetermined temperature or higher. By performing the recovery operation periodically without waiting for the deterioration in this manner, high purification performance can be maintained with a short recovery operation, so that the adverse effect on lean operation is reduced and the increase in external load is suppressed. In addition, the restriction of catalyst heating prevents the catalyst temperature from exceeding the heat resistance limit.

According to the second aspect of the invention, the lean N
Each time the integrated value of the amount of oxygen passing through the Ox catalyst becomes equal to or greater than a predetermined value, the adsorbed oxygen reducing means such as the heating means and the temporary rich operating means are activated, whereby the oxygen adsorbed on the lean NOx catalyst is deteriorated. Detachment is reduced before it becomes noticeable, and the catalyst performance is restored. Also in this case, in the case where the catalyst heating means for heating the lean NOx catalyst is provided as the adsorbed oxygen reducing means, the heating is executed only when the temperature of the lean NOx catalyst does not reach the predetermined temperature or higher. Also, as described above, since the recovery operation is periodically performed without waiting for the deterioration as described above, the high purification performance can be maintained by the short recovery operation, so that the adverse effect on the lean operation is reduced and the catalyst heating is restricted. When added, the catalyst temperature is prevented from exceeding the heat resistance limit.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a system diagram of an embodiment of the present invention. In the figure, reference numeral 1 denotes an engine, a surge tank 3 is formed in an intake passage 2 of the engine 1, a throttle valve 4 is provided upstream of the surge tank 3, and a fuel injection valve 5 is provided downstream thereof.
Is provided. Further, the inlet of the intake passage 2 is connected to the air cleaner 6, and an air flow meter 7 is installed immediately downstream of the air cleaner 6.

On the other hand, the exhaust passage 8 of the engine 1 is provided with a lean NOx catalyst 9 for reducing and purifying NOx during lean operation in which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. And
The lean NOx catalyst 9 is provided with a heater 10 for heating the catalyst, and a temperature sensor 11 for detecting the catalyst temperature.

The control of the fuel injection valve 5 is performed by the control unit 12 which is composed of a microcomputer. Therefore, the engine speed signal is input to the control unit 12 from the rotation sensor attached to the distributor 13, and the intake air amount signal is input from the air flow meter 7. In addition, the lean NOx catalyst 9
An oxygen concentration signal is input to the control unit 12 from an oxygen concentration sensor 17 installed on the upstream side of the
A catalyst temperature signal is input from a temperature sensor 11 installed on the Ox catalyst 9.

The operating state of the engine 1 is a predetermined lean F /
When in zone B, the basic injection amount of fuel is set based on the engine speed and the intake air amount, and various corrections such as water temperature and feedback correction based on the oxygen concentration signal are added to calculate the final injection amount. It Then, a control pulse corresponding to this final injection amount is applied to the fuel injection valve 5, whereby the air-fuel ratio of the engine 1 is controlled to the lean set target value.

When the engine 1 enters the lean steady operation, the timer operates to measure the duration of the lean operation. Then, while the lean operation is continued, the heater 10 is energized at predetermined time intervals, whereby the lean NOx catalyst 9 is temporarily heated. Further, this catalyst heating is executed within a range in which the catalyst temperature does not exceed the heat resistance limit.

The purification performance of the lean NOx catalyst 9 has the characteristics shown in FIG.
The purification rate changes in mountains. In addition, when the catalyst deteriorates due to oxygen adsorption, the purification performance cannot be exhibited at the low temperature side compared to the characteristic curve in the fresh state of the catalyst, and the characteristic curve changes to a curved shape on the left side in the figure, further deterioration. As it progresses, the low temperature range where purification performance cannot be exhibited will further expand. On the other hand, the chemical reaction inside the lean NOx catalyst is shown in Fig. 4.
The temperature is higher than the temperature at which the NOx reduction reaction begins to decrease, at least within the heat resistance limit range of the catalyst (the side indicated by the arrow AA in the figure).
The oxidation reaction of HC and CO is activated. The shaded region in FIG. 4 is the region that exceeds the heat resistance limit of the catalyst. Therefore, the lean NOx catalyst 9 is always used in a temperature range where the NOx purification rate reaches its peak, but by temporarily raising the catalyst temperature to a high temperature range shown by the arrow AA in FIG. Adsorbed oxygen is released by the activated oxidation reaction of HC and CO. As a result, the characteristics of the NOx purification rate are sufficiently high, for example, the state in which deterioration has progressed as shown by the broken line in FIG. 5 returns as shown by the solid line, and does not return to the fresh state shown by the dashed line. Purification performance can be maintained.

Although the lean NOx catalyst 9 is simply arranged in the exhaust passage 8 in the system diagram of FIG. 2, the lean NOx catalyst is deteriorated by heat. To prevent this, the lean NOx catalyst is added to the exhaust system. In addition to providing a bypass passage for bypassing the bypass passage, a bypass valve is provided in the bypass passage to switch to operation at the stoichiometric air-fuel ratio at the time of high rotation and high load where exhaust gas temperature is high. Exhaust gas is allowed to flow directly to the downstream three-way catalyst via this. In that case, V
In the engine, the three-way catalysts 14a and 14b are arranged in the exhaust passages 8a and 8b of each bank as shown in FIG. 8, the lean NOx catalyst 9 is shared by both banks, and the bypass passages 15a and 15b are provided for each bank. And the bypass valves 16a and 16b are respectively arranged.

FIG. 6 is a flow chart for executing the deterioration recovery process in the above embodiment. S11 to S15 indicate the respective steps.

In the flowchart of FIG. 6, after starting, it is determined in S11 whether or not the lean operation is performed. And
If it is not lean operation, it returns as it is, and if it is lean operation, it is determined whether or not the increment of the elapsed time t ₀ by the lean time t _{1 in} S12 exceeds the reference time t ₂ of the cycle determination. until it exceeds the reference time t ₂ is repeated S11~S12 returns to S11, the process proceeds to S13 exceeds the reference time t _2.

[0021] S13, that the catalyst temperature T ₁ of addition of predicted temperature increase T ₂ by the Atsushi Nobori operation the catalyst heat resistance temperature T ₀
If it becomes lower than the heat resistant temperature T ₀ , it returns to S11 and repeats S11 to S13. When the temperature becomes lower than the heat resistant temperature T ₀ , S
14, the temperature raising operation by heating the catalyst is executed, and S1
At 5, the elapsed time t ₀ is returned to zero and the process returns.

Further, in the above embodiment, the means for heating the catalyst by the heater is used as the deterioration recovery means for reducing the desorption of adsorbed oxygen. However, as the deterioration recovery means, the engine may be temporarily air-fuel ratio rich. It is also possible to use a means for operating in such a state. The deterioration recovery by the rich operation is performed during the idle operation, for example. This is because the amount of NOx generated is extremely small during idle operation, and therefore the emission is less deteriorated by making the air-fuel ratio rich.

FIG. 7 is a flow chart for executing the control for temporarily removing the adsorbed oxygen by operating the engine in the air-fuel ratio rich state as described above. S21-
S25 indicates each step.

In the flow chart of FIG. 7, after starting, it is determined in S21 whether lean operation is being performed. And
If it is not lean operation, it returns as it is, and if it is lean operation, it is determined whether or not the increment of the elapsed time t ₀ by the lean time t _{1 in} S22 exceeds the reference time t ₂ of the cycle determination. until it exceeds the reference time t ₂ is repeated S21~S22 returns to S21, the process proceeds to S23 exceeds the criteria time t _2.

At S23, the catalyst heat resistance temperature T ₀ is obtained by adding the expected temperature rise due to the rich operation to the catalyst temperature T _1.
If it becomes lower than the heat resistant temperature T ₀ , it returns to S21 and repeats S21 to S24. And when it becomes lower than the heat resistant temperature T ₀ ,
The routine proceeds to S24, where the air-fuel ratio (A / F) rich operation is executed, and at S25, the elapsed time t ₀ is reset to zero and the routine returns.

In the above embodiment, the adsorbed oxygen is periodically reduced under the condition that a predetermined time has elapsed while the lean operation is continued, but in addition, the integrated value of the amount of oxygen passing through the lean NOx catalyst is predetermined. It is also possible to carry out the reduction of adsorbed oxygen each time the value is exceeded.

FIG. 9 is a flow chart showing a deterioration recovery process when adsorbed oxygen reduction is performed by heating the catalyst every time the integrated value of the amount of oxygen passing through the lean NOx catalyst becomes a predetermined value or more. S31 to S35 show the respective steps.

In the flowchart of FIG. 9, when starting, it is determined whether or not the value obtained by increasing the oxygen concentration integrated value O ₀ by the detected integrated amount O ₁ in S31 becomes the reference integrated value O ₂ or more. _It returns as it is until it becomes ₂ or more, and when it becomes the reference integrated value O ₂ or more, it proceeds to S32.

In S32, it is judged whether or not a _value obtained by adding the expected temperature increase T ₂ due to the heating operation to the catalyst temperature T ₁ is lower than the catalyst heat resistant temperature T ₀ , and if it is higher than the heat resistant temperature T ₀ , the process proceeds to S31. It returns and repeats S31-S32.
When T ₁ + T ₂ becomes lower than T ₀ , the process proceeds to S33, the temperature is raised by heating the catalyst, and then S34
From the detected integrated amount O ₁ to the expected desorbed oxygen amount O ₄ due to the temperature rise effect.
It determines whether the value obtained by subtracting smaller than the end reference integrated value O ₃ warm to, the term does not become smaller than the end reference integrated value O ₃ heating repeat the S33~S34 back to S33. When the temperature rise end reference integrated value O ₃ is smaller than the temperature rise end reference integrated value O ₃ , the process proceeds to S35, and the temperature increase end reference integrated value O ₃ is returned as a new oxygen concentration integrated value O ₀ .

Also, when the adsorbed oxygen reduction is executed every time the integrated value of the amount of oxygen passing through the lean NOx catalyst becomes a predetermined value or more, the adsorbed oxygen reducing means serves as a heater to raise the temperature of the catalyst. In addition to this, it is also possible to use a means for operating while temporarily making the air-fuel ratio rich.

[0031]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, while suppressing adverse effects on lean operation and increase in external load to a minimum, deterioration of the lean NOx catalyst due to oxygen adsorption is prevented and a high NOx purification rate is achieved. Can be maintained.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of the present invention.

FIG. 2 is a system diagram of an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the relationship between the temperature of the lean NOx catalyst and the NOx purification rate in one embodiment of the present invention.

FIG. 4 is a characteristic diagram showing the relationship between the temperature of the lean NOx catalyst and the purification performance of NOx, HC and CO in one embodiment of the present invention.

FIG. 5 is a characteristic diagram showing an effect of deterioration recovery in one embodiment of the present invention.

FIG. 6 is a flowchart for executing deterioration recovery processing according to an embodiment of the present invention.

FIG. 7 is a flowchart for executing deterioration recovery processing according to another embodiment of the present invention.

FIG. 8 is a modified arrangement diagram of an exhaust system including a lean NOx catalyst in each example of the present invention.

FIG. 9 is a flow chart for executing deterioration detection processing according to still another embodiment of the present invention.

[Explanation of symbols]

 1 Engine 5 Fuel Injection Valve 9 Lean NOx Catalyst 10 Heater 11 Temperature Sensor 12 Control Unit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location F02D 45/00 301 G 7536-3G (72) Inventor Suetsugu Gen 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture No. 1 in Mazda Co., Ltd. (72) Inventor Yoshihiko Imamura No. 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (72) No. 3 Shinchi in Fuchu-cho, Aki-gun, Hiroshima Prefecture (72) Inventor, Kazuhiko Hashimoto, 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. (72) In-house, Kazushi Kataoka, 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Corporation

Claims (5)

[Claims] 1. An engine equipped with a lean NOx catalyst for reducing and purifying NOx in exhaust gas in a lean air-fuel ratio region in which at least an air-fuel ratio is larger than a stoichiometric air-fuel ratio, and adsorption for reducing oxygen adsorbed on the lean NOx catalyst. Oxygen reducing means, lean operation detecting means for detecting the operation of the engine in the lean air-fuel ratio region, and output of the lean operation detecting means, and during the lean operation continues, the adsorbed oxygen by the adsorbed oxygen reducing means at a predetermined cycle. An exhaust control device for an engine, comprising: a reduction execution means for performing the reduction. 2. Lean NOx as means for reducing adsorbed oxygen
A catalyst heating means for heating the catalyst, and a catalyst temperature detecting means for detecting the temperature of the lean NOx catalyst;
2. The exhaust control device for an engine according to claim 1, further comprising execution limiting means for operating the reduction executing means on condition that the temperature of the lean NOx catalyst does not exceed a predetermined temperature in response to the output of the catalyst temperature detecting means. 3. A lean N for reducing and purifying NOx in exhaust gas in a lean air-fuel ratio region where the air-fuel ratio is larger than the theoretical air-fuel ratio.
In an engine equipped with an Ox catalyst, the lean NOx
An adsorbed oxygen reducing means for reducing oxygen adsorbed on the catalyst,
Passing oxygen amount detecting means for detecting the amount of oxygen passing through the lean NOx catalyst, integrated value calculating means for calculating an integrated value of the oxygen amount detected by the passing oxygen amount detecting means, and output of the integrated value calculating means In response to the above, the engine exhaust control device is provided with a reduction execution unit that executes the reduction of the adsorbed oxygen by the adsorbed oxygen reduction unit every time the integrated value becomes a predetermined value or more. 4. Lean NOx as means for reducing adsorbed oxygen
A catalyst heating means for heating the catalyst, and a catalyst temperature detecting means for detecting the temperature of the lean NOx catalyst;
The exhaust control device for an engine according to claim 3, further comprising execution limiting means that receives the output of the catalyst temperature detecting means and operates the reduction executing means on condition that the temperature of the lean NOx catalyst does not exceed a predetermined temperature. 5. The engine exhaust control device according to claim 1, wherein the adsorbed oxygen reducing means is a temporary rich operating means for temporarily operating the engine at a rich air-fuel ratio.