JP2884798B2 - Exhaust gas purification device for internal combustion engine - Google Patents

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 provided with a zeolite NOx reducing catalyst in an exhaust system, and more particularly, to a high NOx purifying rate even when the NOx purifying characteristics of the catalyst change due to deterioration of durability. The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine which can obtain the following.

[0002]

2. Description of the Related Art In order to protect the environment, it is desired to reduce CO ₂ in exhaust gas from an automobile internal combustion engine, and lean burn is one effective solution. However, in lean burn, NOx in exhaust gas cannot be reduced even by using a conventional three-way catalyst, so there is a problem in how to reduce NOx. NOx in lean air-fuel ratio exhaust
(HC) as a catalyst that can reduce
There is known a zeolite catalyst supporting a transition metal, which reduces NOx in the presence of a catalyst (JP-A-1-130735, JP-A-1-135541).

[0003]

However, zeolite-based NOx reduction catalysts are susceptible to thermal deterioration, and there has been a problem that if the catalyst deteriorates, the purification performance of the catalyst cannot be used effectively.

[0004] It is an object of the present invention to provide an internal combustion engine equipped with a zeolite NOx reduction catalyst in an exhaust system, which can exhibit considerably high purification performance even if the catalyst deteriorates. .

[0005]

The above object is achieved by providing an exhaust gas purifying apparatus for an internal combustion engine according to the present invention comprising the following means. A lean-burn internal combustion engine capable, the provided in an exhaust system of an internal combustion engine, becomes a transition metal or precious metal from carrying allowed zeolite, in an oxidizing atmosphere, reducing
Under agents present, and catalyst for reducing NOx, and a catalyst deterioration determining means for determining deterioration of the catalyst, to the catalyst when the catalyst deterioration determining means determines that said catalyst has deteriorated reduced
An exhaust purification system of an internal combustion engine consisting of a reducing agent amount increasing means for increasing the dosage amount.

[0006]

The mechanism of NOx reduction by the lean NOx catalyst is based on the relationship between active species generated by partial oxidation of the reducing agent and NO.
It is presumed to be a reaction with x. As the amount of the reducing agent in the exhaust gas flowing through the lean NOx catalyst increases, the amount of the active species also increases accordingly, so that the NOx purification rate of the lean NOx catalyst increases. In the present invention, the catalyst deterioration determining means determines the degree of progress of the lean NOx catalyst deterioration, and the reducing agent amount increasing means increases the reducing agent supply amount according to the deterioration. Therefore, even if the lean NOx catalyst gradually deteriorates due to use and the reaction rate between NOx and active species decreases , the supply amount of the reducing agent increases in accordance with the degree of deterioration, so that the amount of active species increases. , The reduction of the NOx purification rate is suppressed or the NOx purification rate is increased. In this case, since the amount of the reducing agent is increased in accordance with the degree of deterioration of the catalyst, saving of the reducing agent consumption and improvement of the purification rate are compatible with each other. As the catalyst deteriorates, the temperature at which the NOx purification rate peaks shifts to the high temperature side. Therefore, if the catalyst bed temperature is shifted to the high temperature side simultaneously with the increase in the supply amount of the reducing agent , a more favorable improvement in the NOx purification rate is obtained. Can be

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Three embodiments of the present invention will be described. Example 1
Fig. 1 to Fig. 7 show the case where the degree of catalyst deterioration is determined from the accumulated traveling distance and HC is supplied from the HC amount supply device. The second embodiment is a case where learning control of the degree of catalyst deterioration is performed based on the accumulated traveling distance, and is shown in FIGS.
Example 3 is a case in which the degree of catalyst deterioration is determined from the difference between the catalyst inlet and outlet temperatures, and is shown in FIGS.

First, a first embodiment will be described. In FIG. 6, reference numeral 2 denotes an internal combustion engine capable of lean combustion, and an exhaust system 4 thereof is provided with a lean NOx catalyst 6. Lean NO
x An exhaust temperature control device 8 is provided in the exhaust system 4 on the upstream side of the catalyst 6.
Is provided. If the exhaust gas temperature changes, the catalyst bed temperature of the lean NOx catalyst 6 changes accordingly. The exhaust gas temperature control device 8 circulates a part of the engine cooling water to the exhaust system, and controls the amount of circulation by a control valve. The exhaust temperature control device 8 introduces secondary air (exhaust gas temperature decreases when secondary air is introduced), in addition to water cooling as described above,
Air-fuel ratio control (exhaust gas temperature rises when the air-fuel ratio is changed to the rich side on the lean side from stoichiometric), and ignition timing control (exhaust gas temperature goes down to a certain point when it is advanced, and rises when it exceeds it) And in the case of diesel,
Supercharging pressure control (exhaust gas temperature decreases as the supercharging pressure increases) or intake throttle valve control (exhaust gas temperature decreases as the throttle valve opening increases). The operation of the exhaust gas temperature control device 8 is controlled by an electronic control unit (ECU) 10.

On the upstream side of the lean NOx catalyst 6, an introduction portion 14 for HC from the HC source 12 is provided, and a control valve 16 provided in the middle of a pipe connecting the HC source 12 and the introduction portion 14 is provided with a valve. By controlling the opening and closing by the driving device 18,
The introduction amount of HC can be controlled. The valve driving device 18 is controlled by the ECU 10. ECU1
0, the exhaust temperature sensor 20 provided downstream of the lean NOx catalyst 6 and the exhaust temperature sensor 2 provided upstream
4 and the traveling distance signal.

The ECU 10 comprises a microcomputer, an input interface, an output interface, an analog / digital converter for converting an analog signal into a digital signal and inputting the digital signal to the input interface, a read-only memory (ROM) of a read-only memory, a temporary memory, Random access memory (R
AM), and a central processor unit (CPU) for executing operations. The ROM stores the routines and maps shown in FIGS. 1 to 5, and the CPU reads them out and executes calculations.

FIG. 1 shows a routine constituting a catalyst deterioration judging means for judging the deterioration of the lean NOx catalyst. This routine is interrupted every fixed time, for example, every 50 milliseconds. The ignition switch is turned on in step 102
It is determined whether the operation has been performed. If it is OFF, the process returns as it is because there is no need to make a deterioration determination. Next, at step 106, the integrated travel distance S is calculated from the map shown in FIG.
The lower limit value T ₁ and the upper limit value T ₂ of the temperature range of the purification rate peak of the lean NOx catalyst corresponding to
_1, the T ₂ is illustrated) is calculated. In this case, FIG.
As shown in ( ₂₎ , T ₁ and T ₂ increase as the catalyst deterioration progresses, that is, as the accumulated traveling distance S increases.
As shown in FIG. 7, the peak temperature region of the NOx purification rate of the lean NOx catalyst 6 shifts from a to b to b to c toward the high temperature side. Next, the routine proceeds to step 108, where the lower limit value TC of the target exhaust gas temperature is set to T1, and the upper limit value TH is set to T2.

Next, the routine proceeds to step 110, where the target HC concentration H is calculated with respect to the integrated traveling distance S from the map shown in FIG.
In step 112, the HC concentration HT is set to H1, and the routine returns. In step 110, since the exhaust gas temperature is increased as the accumulated traveling distance S is increased, H
Since the direct oxidation of C to H ₂ O and CO ₂ progresses, the amount of active species produced by the partial oxidation of HC will decrease and the NOx reduction reaction will decrease, so the amount of HC supplied to compensate for the amount of HC will be reduced. This is a step for obtaining a target HC amount when increasing the amount.

When the deterioration of the catalyst is determined on the basis of the accumulated traveling distance and the various control values TC, TH and HT are determined in the routine of FIG. 1, the exhaust gas temperature, that is, the catalyst bed temperature is controlled in the routine of FIG. In the routine of FIG.
And the current exhaust temperature TE (the output of the exhaust temperature sensor 20)
Read. Next, in step 204, it is determined whether or not the exhaust gas temperature TE is lower than the lower limit value TC of the target temperature range obtained in FIG. 1, and when it is low, it is necessary to raise the exhaust gas temperature. Of the exhaust gas to the exhaust gas temperature control device 8 is stopped. If the exhaust temperature TE is equal to or higher than the lower limit value TC in step 204, the process proceeds to step 208, and it is determined whether or not the exhaust temperature TE is higher than the upper limit value TH of the target temperature range obtained in FIG. Since it is necessary, the process proceeds to step 210 and the cooling water is circulated to the exhaust gas temperature control device 8. If it is determined in step 208 that TE is equal to or lower than TH, the exhaust gas temperature TE is between TC and TH and does not need to be controlled. The routine in FIG. 4 constitutes a catalyst bed temperature changing means for changing the catalyst bed temperature (correlated to the exhaust gas temperature TE) to a higher temperature side when the catalyst is deteriorated (when the accumulated traveling distance S becomes larger).

FIG. 5 shows a routine constituting the HC amount increasing means. This routine is interrupted at regular intervals, for example, at every 50 milliseconds. In step 302, the intake air amount Q is read. Next, in step 304, the target opening VA of the HC amount control valve 16 is calculated from the intake air amount Q and the exhaust gas upper limit value HT using a predetermined map or the like. The HC amount is increased by setting the opening to the target opening VA. This HC
In the control, when the lean NOx catalyst 6 deteriorates, the HT is increased in step 112 of FIG. 1 and the HC is increased in the routine of FIG. 5 to prevent the NOx purification rate of the lean NOx catalyst 6 from decreasing or to reduce NOx. Improve the purification rate. That is, even if the reaction rate between HC and NOx decreases due to deterioration, control is performed to increase the NOx purification rate as indicated by the broken line in FIG. 7 by increasing the HC amount.

Next, a second embodiment will be described. 6 is also applied to the second embodiment. However, lean NOx
Downstream of the catalyst 6, a NOx sensor 22 for detecting the amount of NOx in the exhaust gas is provided, and the output is sent to the ECU 10.

FIG. 8 includes learning control by the NOx sensor 22, which constitutes the catalyst deterioration determining means in the second embodiment.
4 shows a catalyst deterioration determination routine. This routine is interrupted at regular intervals, for example, at every 50 milliseconds. In step 402, it is determined whether or not the accumulated traveling distance S has exceeded a certain distance, for example, 2000 km. If the travel distance is equal to or less than the predetermined travel distance, the process proceeds to step 404 on the assumption that the catalyst has not deteriorated, and the learning value is set so that NOx, which is the output of the NOx sensor 22, becomes low due to advance / retardation of the ignition timing described later. The correction is continued, and if the distance exceeds a certain mileage, it is determined that the catalyst is deteriorating, and step 414 is performed.
To obtain a temperature correction coefficient K for controlling the exhaust gas temperature and a target HC amount HT for controlling the HC amount.

More specifically, at step 404, it is determined whether the learning conditions are satisfied, for example, whether the vehicle is in a steady running state after warming up. If so, proceed to step 406. At step 406, the operating range is determined from the engine load Q / N and the engine speed NE. At step 408, the learning value Gi is read from the corresponding operating range using the Q / N vs. NE map of FIG. put out. Next, at step 410, (N + 9Gi) / 10 is calculated from the output N of the NOx sensor 22, this is newly set as Gi, and Gi is gradually corrected. This Gi is the NOx of each operation region when not deteriorated.
Represents the output of the sensor. Next, in step 412, the temperature correction coefficient K used in the routine of FIG. 12 described below is set to 0, and the HC concentration HT used in the routine of FIG.
The basic HC concentration H10 determined by

When the process proceeds from step 402 to step 414, it is determined in step 414 whether or not the conditions for determining deterioration are satisfied, for example, whether or not the vehicle is traveling normally after warm-up.
If not, return is made. If it is, the process proceeds to step 416. In step 416, the engine load Q
/ N, the current operating range is determined from the engine speed NE,
In step 418, the learning value Gi for the case where there is no catalyst deterioration that has been subjected to learning control of the corresponding operation region is read. Next, at step 420, NO is determined from the output N of the NOx sensor 22 (after catalyst deterioration) and the read learning value Gi (before catalyst deterioration).
x The purification rate reduction degree D is calculated as D = N-Gi.
Next, in step 422, a temperature correction coefficient K is obtained from the purification rate decrease degree D based on the K-to-D map in FIG. 10, and in step 424, a target HC concentration is calculated based on the H1-to-D map in FIG. In step 426, HT is set to H1, and the routine returns.

FIG. 12 shows an ignition timing calculation routine constituting the catalyst bed temperature changing means of the second embodiment. Although there are various means for controlling the catalyst bed temperature, in the second embodiment, in consideration of the relationship between the ignition timing and the exhaust temperature as shown in FIG. Therefore, the catalyst bed temperature is controlled.

The routine of FIG. 12 is interrupted at every constant crank angle, for example, every 30 ° crank angle. Step 5
At 02, the basic ignition timing θBASE is calculated from the engine load Q / N and the engine speed NE. Next, step 504
Then, from the temperature correction coefficient K obtained in the routine of FIG. 8, the advance correction amount θA is obtained from θA = K × _MN . Then
Step 50 so that θA does not become larger than the constant value α.
In step 510, the ignition timing θ is determined as θ = θBASE + θA from the basic ignition timing θBASE and the advance correction amount θA. Then, the process proceeds to a step 512, wherein a process of outputting θ to set the ignition timing to θ is executed, and the process returns. The HC increasing means in the second embodiment is the same as in the first embodiment, and follows the routine of FIG.

FIG. 14 shows a routine constituting the catalyst deterioration judging means of the third embodiment. This routine is interrupted at regular intervals, for example, every 50 milliseconds. In step 602, it is determined whether or not the engine operating condition is an operating condition that is meaningful even when catalyst deterioration is determined. If the operating condition is meaningless (such as before warm-up), the process returns and is meaningful. The process proceeds to step 604 under the operating condition. Step 6
At 04, the engine load Q / N and the engine speed NE are read,
Calculate the current engine operating area, then step 606
Then, using the map of FIG. 15, the catalyst inlet-outlet temperature difference ΔTi before catalyst deterioration in the corresponding operating region determined in advance.
Is read.

Next, at step 608, the lean NO
x Temperature sensors 24, 20 provided at the inlet and outlet of the catalyst 6
From the outputs t ₁ and t _{2 of} FIG.
The outlet temperature difference Δt = t ₂ −t ₁ is calculated. Next, at step 610, the current temperature difference Δt and the temperature difference ΔTi
Then, a catalyst deterioration function D (D = ΔTi−Δt) is calculated from the comparison with the above. Then, in step 612, the degree of catalyst deterioration DR is calculated from the catalyst deterioration function D using the map of FIG.

Next, at step 614, the temperature range T corresponding to the degree of catalyst deterioration DR is determined using the map shown in FIG.
1, T2 is determined, and in step 616, TC is set to T1 and TH is set to T2. TC and TH are parameters used in the catalyst bed temperature control according to FIG. Next, the routine proceeds to step 618, where the target HC concentration H1 corresponding to the catalyst deterioration degree DR is calculated using the map of FIG. 18, and HT is set to H1 at step 620. This H
T is a parameter used for the HC concentration control according to FIG. 5 as in the first embodiment. Also in the third embodiment, the catalyst bed temperature control is performed according to the routine of FIG. 4, and the HC amount control is performed according to the routine of FIG. Since the routines of FIGS. 4 and 5 have been described in the first embodiment, description thereof will be omitted.

Next, the operation of the present invention common to the embodiments will be described. When it is determined that the lean NOx catalyst 6 has deteriorated by the accumulated traveling distance, the catalyst inlet-outlet temperature difference, or the learning control by the NOx sensor, etc., the degree of the deterioration is determined by the routine constituting the HC amount increasing means in FIG. , The amount of HC is increased. The NOx purification rate of the lean NOx catalyst 6 decreases as the catalyst deteriorates,
It increases as the amount increases. Therefore, the lean NOx catalyst 6
Even if the NOx purification rate decreases from a to b in FIG. 7 and from b to c in FIG. 7, the HC amount is increased, so that the NOx purification rate is improved as indicated by the broken line in FIG. .

In the above, when the catalyst bed temperature is shifted to the high temperature side by the routine of FIG. 4 with the increase of the HC amount, the NOx purification rate of the lean NOx catalyst 6 is further improved. That is, even if the lean NOx catalyst 6 deteriorates and the temperature range where the NOx purification rate peaks from a to b and c in FIG. Since the lean NOx catalyst 6 is shifted to a high temperature side by the ignition timing control or the like, the lean NOx catalyst 6 is always used in a temperature range showing a high NOx purification rate, and the purification performance of the catalyst is effectively used for a long period of time.

[0026]

According to the present invention, instead of the catalyst deterioration determining means
When the catalyst deterioration determining means determines that the deterioration of the lean NOx catalyst has progressed, the return amount is increased.
The main agent amount increasing means increases the amount of the reducing agent to minimize the decrease in the NOx purification rate of the lean NOx catalyst, or
The purification rate can be improved. In this case, since the reducing agent amount increasing means increases the reducing agent only when the catalyst deterioration proceeds, it is possible to achieve both the economy of the reducing agent and the improvement of the NOx purification rate.

[Brief description of the drawings]

FIG. 1 is a flowchart of a routine that constitutes catalyst deterioration determination means of Embodiment 1 of the exhaust gas purification apparatus for an internal combustion engine of the present invention.

FIG. 2 is an exhaust temperature vs. integrated travel distance map used in the routine of FIG. 1;

FIG. 3 is a map of a target HC concentration versus an accumulated traveling distance used in the routine of FIG. 1;

FIG. 4 is a flowchart of a routine that constitutes a catalyst bed temperature changing unit of the first embodiment of the exhaust gas purifying apparatus for an internal combustion engine of the present invention.

FIG. 5 is a flowchart of a routine that constitutes an HC amount increasing means of the exhaust gas purification device for an internal combustion engine of the present invention.

FIG. 6 is a system diagram of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention.

FIG. 7 is a characteristic diagram of NOx purification rate versus lean NOx catalyst bed temperature in the present invention.

FIG. 8 is a flowchart illustrating a routine that constitutes a catalyst deterioration determination unit according to a second embodiment of the exhaust gas purification apparatus for an internal combustion engine of the present invention.

9 is an engine load Q / N vs. engine speed NE map used in the routine of FIG. 8;

FIG. 10 is a temperature correction coefficient K used in the routine of FIG. 8;
It is a purification rate fall degree D map.

FIG. 11 is a map of the HC concentration H1 versus the purification rate decrease degree D used in the routine of FIG. 8;

FIG. 12 is a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
7 is a flowchart of a routine that constitutes a catalyst bed temperature changing means of the present invention.

FIG. 13 is a characteristic diagram of the exhaust gas temperature versus the ignition timing according to the control by the routine of FIG. 12;

FIG. 14 is a third embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
5 is a flowchart of a routine that constitutes catalyst deterioration determining means of the present invention.

FIG. 15 shows an engine load Q / used in the routine of FIG.
It is an N vs. engine speed NE map.

16 is a catalyst deterioration degree D used in the routine of FIG.
It is an R vs. catalyst deterioration function D map.

FIG. 17 is an exhaust gas temperature-catalyst deterioration degree map used in the routine of FIG. 14;

FIG. 18 is a target HC concentration-catalyst deterioration degree map used in the routine of FIG. 14;

[Explanation of symbols]

 Reference Signs List 2 internal combustion engine 4 exhaust system 6 lean NOx catalyst 8 exhaust temperature control device 10 ECU 12 HC source 14 HC introduction unit 16 control valve 18 valve drive device 20 exhaust temperature sensor 22 NOx sensor 24 exhaust temperature sensor

Claims (1)

(57) [Claims] 1. A catalyst for reducing NOx in an oxidizing atmosphere in the presence of a reducing agent , comprising: an internal combustion engine capable of lean combustion; and a zeolite provided in an exhaust system of the internal combustion engine and carrying a transition metal or a noble metal. An internal combustion engine comprising: catalyst deterioration determining means for determining deterioration of the catalyst; and reducing agent amount increasing means for increasing the amount of reducing agent to the catalyst when the catalyst deterioration determining means determines that the catalyst has deteriorated. Engine exhaust purification device.