JP2616262B2 - 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 having a zeolite-based NOx reducing catalyst in an exhaust system. 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, and thus there is a problem how to reduce NOx. NOx in exhaust with lean air-fuel ratio
(HC) as a catalyst that can reduce
A transition metal-supported zeolite catalyst (JP-A-1-130735 and JP-A-1-135541) for reducing NOx in the presence of NO and a noble metal-based catalyst capable of decomposing NOx without HC Catalysts are known.

[0003]

However, the zeolite-based NOx reduction catalyst is susceptible to thermal deterioration, and there is a problem that if the catalyst is deteriorated, the purification performance of the catalyst cannot be used effectively.

[0004] An object of the present invention is to make it possible to exhibit a considerably high purification performance even when 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, a catalyst provided in the exhaust system of the internal combustion engine and made of zeolite carrying a transition metal or a noble metal, and reducing NOx in an oxidizing atmosphere in the presence of HC (hereinafter referred to as a lean NOx catalyst). ), Catalyst deterioration determining means for determining deterioration of the catalyst, and catalyst bed temperature changing means for changing the catalyst bed temperature to a higher temperature side when the catalyst deterioration determining means determines that the catalyst has deteriorated.

[0006]

According to experiments and studies by the inventors, the NOx purification reaction by the lean NOx catalyst has a temperature region where the purification rate has a peak, and the temperature region where the NOx purification rate has a peak is the deterioration of the lean NOx catalyst. Gradually shifts to the high temperature side as the process proceeds. In the present invention, the catalyst deterioration judging means judges the degree of progress of the lean NOx catalyst deterioration, and the catalyst bed temperature changing means shifts the used catalyst bed temperature range to a higher temperature side according to the deterioration. Therefore, the lean NOx catalyst is used in the temperature range where the purification rate reaches a peak at the beginning,
Even if the temperature range where the purification rate peaks due to catalyst deterioration shifts to the high temperature side, the catalyst bed temperature range used shifts correspondingly to the high temperature side, so in the temperature range where the purification rate peaks even after endurance deterioration of the catalyst. As a result, the reduction of the NOx purification rate is suppressed as much as possible. As a result, the NOx purification rate of the lean NOx catalyst is kept relatively high over a long period of time.

[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 the exhaust gas temperature, that is, the catalyst bed temperature is controlled by the engine cooling water and the like. Second Embodiment A second embodiment is a case in which catalyst deterioration is determined from the accumulated traveling distance and learning control of the catalyst bed temperature is performed by ignition timing control, which is shown in FIGS. Example 3 is a case where the catalyst bed temperature is controlled by cooling air, 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 (combustion at an air-fuel ratio leaner than the stoichiometric ratio).
An Ox catalyst 6 is provided. An exhaust temperature control device 8 is provided in the exhaust system 4 on the upstream side of the lean NOx catalyst 6. If the exhaust temperature changes, lean NO
The catalyst bed temperature of x catalyst 6 changes. 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 gas temperature control device 8 introduces secondary air (exhaust gas temperature decreases when secondary air is introduced) and air-fuel ratio control (changes the air-fuel ratio to the rich side on the lean side from the stoichiometric side) in addition to the water cooling as described above. And the exhaust temperature rises), 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, boost pressure control (supercharger) The exhaust gas temperature may increase when the supply pressure is increased), or the intake throttle valve control (the exhaust temperature may increase when the opening degree of the throttle valve is increased). The exhaust temperature control device 8 is an electronic control device (ECU) 10
The operation is controlled by

On the upstream side of the lean NOx catalyst 6, an introduction portion 14 for HC from the HC source 12 is provided. 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
To 0, the output from the exhaust gas temperature sensor 20 provided downstream of the lean NOx catalyst 6 is input, and the travel distance signal S is input.

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, and if it is ON, the process proceeds to step 104 to read the integrated traveling distance S. Next, at step 106, T ₁ when the initial lower limit value T ₁ and the upper limit value T ₂ (FIG. 7 of the temperature range of the purification rate peak of the lean NOx catalyst corresponding to the integrated travel distance S from the map 1 of FIG. 2, T ₂ is illustrated a) is calculated. In this case, as shown in FIG. 2, as the catalyst deterioration progresses, that is, as the integrated traveling distance S increases, T ₁ and T ₂ increase. This is shown in FIG. 7, where the NOx of the lean NOx catalyst 6
The peak temperature region of the purification rate 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 used exhaust gas temperature is set to T1, and the upper limit value TH is set to T1.
Put 2.

Next, the routine proceeds to step 110, where the target HC concentration H1 is read from the map 2 shown in FIG. 3 with respect to the integrated traveling distance S, and at step 112, the value of H1 is substituted for the HC concentration HT, and the routine returns. . Step 11
In the case of 0, since the exhaust gas temperature rises as the cumulative traveling distance S increases, the direct oxidation of HC to H ₂ O and CO ₂ progresses, so that the amount of active species generated by the partial oxidation of HC decreases and the NOx reduction reaction proceeds. Is a step for increasing the HC supply amount in order to compensate for the HC amount.

When the catalyst deterioration is determined on the basis of the accumulated traveling distance in the routine of FIG. 1, and various values TC, TH, HT are determined, the control of the exhaust gas temperature, that is, the catalyst bed temperature, and the control of FIG. To control the amount of HC. In the routine of FIG. 4, in step 202, the current exhaust gas temperature TE (output of the exhaust gas temperature sensor 20) is read. Then, step 20
In 4, it is determined whether the exhaust gas temperature TE is lower than the lower limit value TC of the operating temperature range obtained in FIG. 1. If lower, the exhaust gas temperature needs to be raised. The circulation to the control device 8 is stopped. Step 204
If the exhaust temperature TE is equal to or higher than the lower limit value TC, 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 operating temperature range obtained in FIG. Therefore, the process proceeds to step 210, where the cooling water is circulated to the exhaust gas temperature control device 8. TE in step 208
Is equal to or less than TH, the exhaust gas temperature TE is between TC and TH and does not need to be controlled.
And return as it is. In the routine of FIG. 4, the catalyst bed temperature (correlated to the exhaust gas temperature TE) is changed to a high temperature side when the catalyst is deteriorated (when the accumulated traveling distance S becomes large). Configure the change means.

FIG. 5 shows an HC amount control routine. This routine is interrupted at regular intervals, for example, at every 50 milliseconds. In step 302, the intake air amount Q
Read. Next, at step 304, the intake air amount Q and the HC
The target opening VA of the HC amount control valve 16 is calculated from the concentration HT using a predetermined map or the like, and this VA is output in step 306 to make the control valve opening the target opening VA. Controls the HC amount. In this HC control, when the lean NOx catalyst 6 deteriorates, HC is increased to prevent the NOx purification rate of the lean NOx catalyst 6 from decreasing. That is, HC and NOx
Even if the reaction rate of the NOx decreases, control is performed to increase the NOx purification rate 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 catalyst deterioration determination 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 distance is equal to or less than the predetermined traveling distance, it is determined that the catalyst has not deteriorated, and the process proceeds to step 404, where the deterioration factor Gi is learned using the output of the NOx sensor 22. Is determined to be in progress and step 41
Proceeding to 4, a temperature correction coefficient K for controlling the exhaust gas temperature is determined based on the degree of deterioration.

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. In step 406, the operating range is determined from the engine load Q / N and the engine speed NE. In step 408, the learning value Gi is obtained from the corresponding operating range using the Q / N vs. NE map in FIG.
Is read. Next, at step 410, (N + 9Gi) / 10 is calculated (averaged by 1/10 for each routine interruption) based on the output N of the NOx sensor 22, and this is newly set as Gi, and Gi is gradually corrected. And learn.
Next, at step 412, the temperature correction coefficient K used in the routine of FIG.

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, the process returns. Otherwise, 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, from the corresponding operation area, 0-200
In the case of 0 km, the learning value Gi, which has been momentarily learned and controlled, is read. Next, at step 420, the NOx purification rate reduction degree D is calculated as D = N−Gi from the output N of the NOx sensor 22 and the read learning value Gi, and further at step 422, based on the K-to-D map of FIG. Then, the temperature correction coefficient K is obtained from the degree D of purification rate decrease, and the process returns.

FIG. 11 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.

[0020] The routine of FIG.
For example, it is interrupted at every 30 ° crank angle. In step 502, a basic ignition timing θBASE is calculated from the engine load Q / N and the engine speed NE. Then, Step 50
At 4, the temperature correction coefficient K obtained by the routine of FIG.
The advance correction amount θA is obtained from θA = K × _MN . Here, _MN is a coefficient relating the change in the catalyst temperature to the change in the ignition timing. Next, α is guarded in steps 506 and 508 so that θA does not become larger than the fixed value α, and the routine proceeds to step 510, where the basic ignition timing θBASE
And the advance correction amount θA, the ignition timing θ is calculated as θ = θBAS
It is determined as E-θA. However, this is the case where the ignition timing is set in advance to the point MBT where the temperature becomes the lowest or to the retard side thereof. Then proceed to step 512,
A process for outputting θ to set the ignition timing to θ is executed, and the process returns.

In the third embodiment, as shown in FIG. 14, the catalyst bed temperature is controlled by controlling the cooling air (cooling air from an air pump, vehicle running wind, etc.). FIG. 14 shows an air pump 2 rotated in conjunction with the engine crankshaft.
The cooling air from the air pump 24 is blown from the air nozzle 26 to the lean NOx catalyst 6. The amount of air ejected from the air nozzle 26 is controlled by the control valve 28
It controls by controlling by U10. Reference numeral 30 denotes a muffler, 32 denotes a cooling water temperature sensor, 34 denotes an incoming gas temperature sensor, and 36 denotes an outgoing gas temperature sensor.

In the third embodiment, the catalyst deterioration determining means and the catalyst bed temperature changing means comprise a cooling air control routine shown in FIG. The routine of FIG. 13 is interrupted at regular intervals, for example, at every 50 milliseconds. In step 602, it is determined from the output of the cooling water temperature sensor 32 whether or not the engine is being warmed up based on the cooling water temperature. For example, if the cooling water temperature is 90 ° C. or lower, it is determined that the engine is being warmed up. If it is determined in step 602 that the engine is being warmed up,
Since it is better to warm up and activate the lean NOx catalyst 6 quickly, step 61 is performed to stop the blowing of the cooling air.
Proceeding to 0, the control valve 28 is closed.

If it is determined in step 602 that the engine is not being warmed up, the process proceeds to step 604, in which it is determined whether or not the current engine operating state is in an area where the catalyst should be cooled. If it is in the area, step 61
Proceeding to 0, the control valve 28 is closed. If it is determined in step 604 that it is in the cooling area, the process proceeds to step 606.
In step 606, it is determined whether or not the temperature difference TI-TE between the output TI of the incoming gas temperature sensor 34 and the output TE of the outgoing gas temperature sensor 36 exceeds a certain temperature TA, and if it is less than TA, the lean NOx catalyst 6 is deteriorated. It is assumed that you are doing. Here, step 606 constitutes the catalyst deterioration determining means in the third embodiment. If it is determined in step 606 that the catalyst has deteriorated, the process proceeds to step 610 to close the control valve 28,
Cooling is prohibited and the catalyst bed temperature is raised to a high temperature side. Therefore,
Step 610 constitutes the catalyst bed temperature changing means for increasing the catalyst bed temperature in the third embodiment. If it is determined in step 606 that catalyst deterioration has not progressed, step 6
In step 08, the control valve 28 is opened, and the lean NOx catalyst 6 is cooled.

Next, the operation of the present invention common to each embodiment 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, the catalyst bed temperature is shifted to a high temperature side by stopping the cooling or controlling the ignition timing. Is done. On the other hand, when the lean NOx catalyst 6 actually deteriorates, the temperature region where the NOx purification rate shows a peak shifts from a in FIG. 7 to b and c in FIG. As described above, the catalyst bed temperature is shifted to a higher temperature side in response to this shift, so that the lean NOx catalyst 6 is always used in a temperature range where a high NOx purification rate can be exhibited, and over a long period of time. The purification performance of the catalyst is effectively used.

[0025]

According to the present invention, since the catalyst deterioration determining means and the catalyst bed temperature changing means are provided, when the catalyst deterioration determining means determines that the deterioration of the lean NOx catalyst has progressed, the catalyst bed temperature changing means is activated. The catalyst bed temperature of the lean NOx catalyst can be shifted to a higher temperature side. Therefore, even if the lean NOx catalyst deteriorates and the temperature indicating the peak purification rate shifts to the high temperature side, the catalyst bed temperature also shifts to the high temperature side, so that the high NOx purification rate region of the catalyst is always used. Therefore, the NOx purification rate of the lean NOx catalyst can be kept high over a long period of time.

[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 an HC amount control routine used in the first embodiment of the exhaust gas purifying apparatus 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 a first embodiment of the present invention.

FIG. 7 is a graph showing the relationship between the NOx purification rate and the 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 shows an exhaust gas purifying apparatus for an internal combustion engine according to a second embodiment of the present invention.
7 is a flowchart of a routine that constitutes a catalyst bed temperature changing means of the present invention.

FIG. 12 is an exhaust temperature-ignition timing characteristic diagram according to control by the routine of FIG. 11;

FIG. 13 shows a third embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
6 is a flowchart of a cooling air control routine of FIG.

FIG. 14 is a device system diagram according to a third embodiment of the present invention.

[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 air pump 28 control valve 34 gas temperature sensor 36 exit Gas temperature sensor

Claims (1)

(57) [Claims] 1. An internal combustion engine capable of lean combustion and a catalyst provided in an exhaust system of the internal combustion engine and made of zeolite carrying a transition metal or a noble metal, and reducing NOx in an oxidizing atmosphere in the presence of HC. An internal combustion engine comprising: catalyst deterioration determining means for determining deterioration of the catalyst; and catalyst bed temperature changing means for changing the catalyst bed temperature to a higher temperature when the catalyst deterioration determining means determines that the catalyst has deteriorated. Exhaust gas purification device.