JPH04255521A - Exhaust gas purifier for internal combustion engine - Google Patents

Abstract

PURPOSE:To keep the NOx purification efficiency of a lean NOx catalyst high even if the lean NOx catalyst is deteriorated. CONSTITUTION:An internal combustion engine 2 includes a lean NOx catalyst 6 provided in an exhaust system 4 thereof, a catalyst deterioration judging means and an HC quantity increasing means for increasing the HC quantity in the exhaust gas when the catalyst deterioration judging means judges the deterioration of the catalyst 6. When the lean NOx catalyst 6 is deteriorated, the NOx purification efficiency is lowered, and the temperature range of the NOx purification efficiency peak is shifted to the high temperature side. According to the above shifting, the HC quantity increasing means increases the HC quantity, so that the NOx purification efficiency of the lean NOx catalyst 6 can be always kept high. If the catalyst floor temperature is shifted to the high temperature side at the same time, a large effect of improving the NOx purification efficiency is produced.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an exhaust gas purification device for an internal combustion engine equipped with a zeolite-based NOx reduction catalyst in the exhaust system. The present invention relates to an exhaust gas purification device for an internal combustion engine that provides the following.

0002

2. Description of the Related Art In order to protect the environment, it is desired to reduce CO2 in the exhaust gas of automobile internal combustion engines, and lean burn is one effective solution. However, in lean burn, NOx in exhaust gas cannot be reduced even with the use of conventional three-way catalysts, so the problem is how to reduce NOx. As a catalyst capable of reducing NOx in exhaust gas at a lean air-fuel ratio, a zeolite-based catalyst supporting a transition metal that reduces NOx in the presence of HC (hydrocarbons) (Japanese Patent Application Laid-Open No. 1-1307
No. 35, Japanese Unexamined Patent Publication No. 1-135541) are known.

0003

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

SUMMARY OF THE INVENTION An object of the present invention is to provide an internal combustion engine equipped with a zeolite-based NOx reduction catalyst in the exhaust system, which can still exhibit considerably high purification performance even if the catalyst deteriorates. .

[0005]

[Means for Solving the Problems] The above object is achieved by providing an exhaust purification device for an internal combustion engine according to the present invention with the following means. A catalyst (hereinafter referred to as "lean NOx") is installed in the exhaust system of lean-burn internal combustion engines, and is made of zeolite supported with transition metals or noble metals, and reduces NOx in the presence of HC in an oxidizing atmosphere.
catalyst deterioration determining means for determining deterioration of the catalyst; and HC increasing the amount of HC to the catalyst when the catalyst deterioration determining means determines that the catalyst has deteriorated.
Means of increasing quantity.

[0006]

[Action] Lean NOx The NOx reduction mechanism by the catalyst is based on active species generated by partial oxidation of HC and N
It is presumed to be a reaction with Ox. As the amount of HC in the exhaust gas flowing through the lean NOx catalyst increases, the amount of active species also increases, so the NOx purification rate of the lean NOx catalyst increases. In the present invention, the catalyst deterioration determining means determines the degree of deterioration of the lean NOx catalyst, and the HC amount increasing means increases the HC supply amount in accordance with the deterioration. Therefore, even if the lean NOx catalyst gradually deteriorates with use and the reaction rate between NOx and active species decreases, the amount of HC supplied is increased according to the degree of deterioration, so the amount of active species increases. A decrease in the NOx purification rate is suppressed or the NOx purification rate is increased. In this case, H
Since the amount of C is increased according to the degree of deterioration of the catalyst, saving in HC consumption and improving the purification rate are compatible with each other. In addition,
As the catalyst deteriorates, the temperature at which the NOx purification rate peaks shifts to a higher temperature side, so shifting the catalyst bed temperature to a higher temperature side at the same time as increasing the HC supply amount will improve NOx even better.
Improved purification rate can be obtained.

[0007]

[Example] Three examples of the present invention will be explained. Embodiment 1 is a case in which the degree of catalyst deterioration is determined from the cumulative travel distance and HC is supplied from an HC amount supply device, as shown in FIGS. 1 to 7. Embodiment 2 is a case where the degree of catalyst deterioration is controlled by learning based on the cumulative travel distance, and is shown in FIGS. 8 to 13. Embodiment 3 is a case where the degree of catalyst deterioration is determined from the catalyst inlet-outlet temperature difference, and is shown in FIGS. 14 to 18.

First, a first embodiment will be explained. In FIG. 6, 2 is an internal combustion engine capable of lean combustion, and its exhaust system 4 includes:
A lean NOx catalyst 6 is provided. Lean NOx
An exhaust temperature control device 8 is installed in the exhaust system 4 upstream of the catalyst 6.
is provided. When the exhaust gas temperature changes, the catalyst bed temperature of the lean NOx catalyst 6 changes correspondingly. The exhaust temperature control device 8 circulates a portion of the engine cooling water to the exhaust system, and controls the amount of circulation using a control valve. In addition to water cooling as described above, the exhaust temperature control device 8 is configured to introduce secondary air (introducing secondary air lowers the exhaust temperature),
Air-fuel ratio control (when changing the air-fuel ratio from stoichiometric to lean side to rich side, the exhaust temperature rises), and ignition timing control (when changing the air-fuel ratio to the advanced side, the exhaust temperature decreases up to a certain point, and increases beyond that point). and in the case of diesel,
It may be supercharging pressure control (increasing the supercharging pressure lowers the exhaust gas temperature) or intake throttle valve control (increasing the opening of the throttle valve lowers the exhaust gas temperature). The operation of the exhaust temperature control device 8 is controlled by an electronic control unit (ECU) 10.

[0009] On the upstream side of the lean NOx catalyst 6, HC
An introduction section 14 for HC from a source 12 is provided, in which the HC
The amount of HC introduced can be controlled by controlling the opening and closing of a control valve 16 provided in the middle of a pipe connecting the source 12 and the introduction section 14 with a valve drive device 18. The valve drive device 18 is controlled by the ECU 10. ECU
10 receives outputs from an exhaust temperature sensor 20 provided downstream of the lean NOx catalyst 6 and an exhaust temperature sensor 24 provided upstream thereof, and also receives a mileage signal.

The ECU 10 consists of a microcomputer, including an input interface, an output interface, an analog/digital converter that converts analog signals into digital signals and inputs them to the input interface, a read-only memory (ROM), and a temporary memory. Random access memory for storage (R
AM), consisting of a central processor unit (CPU) that performs calculations. The ROM stores the routines and maps shown in FIGS. 1-5, and the CPU reads them and executes operations.

FIG. 1 shows a routine constituting a catalyst deterioration determining means for determining deterioration of a lean NOx catalyst. This routine is interrupted at regular intervals, for example every 50 milliseconds. In step 102, the ignition switch is turned to O.
It is determined whether the switch has been turned to N, and if it is OFF, there is no need to judge deterioration, so the process returns. If it is ON, the process proceeds to step 104, where the cumulative mileage S is read. Next, in step 106, the lower limit value T1 and upper limit value T2 of the temperature range of the purification rate peak of the lean NOx catalyst corresponding to the cumulative traveling distance S are determined from the map in FIG. 2 (T1 and T2 in the initial case are illustrated in FIG. 7). ) is calculated. In this case, as shown in FIG. 2, as the catalyst deterioration progresses, that is, as the cumulative traveling distance S increases, T1, T
2 will increase. This is shown in Figure 7. Lean NOx NOx of catalyst 6
The peak temperature region of the purification rate shifts toward the high temperature side from a to b and from b to c. Next, the process proceeds to step 108, where the lower limit value TC of the target exhaust temperature is set to T1, and the upper limit value TH is set to T1.
Set it to 2.

Next, the process proceeds to step 110, where the target HC concentration H is determined for the cumulative travel distance S from the map shown in FIG.
1 is calculated, and in step 112, the HC concentration HT is set to H1, and the process returns. In step 110, the exhaust temperature is increased as the cumulative mileage S increases, so H
The direct oxidation of C to H2O, CO2 progresses, and the amount of H
The amount of active species produced due to partial oxidation of C is reduced and NO
x Since the reduction reaction will decrease, this is a step to find the target HC amount when increasing the HC supply amount to compensate for the HC amount.

When catalyst deterioration is determined based on the cumulative travel distance in the routine shown in FIG. 1 and various control values TC, TH, and HT are determined, the exhaust temperature and therefore the catalyst bed temperature are controlled in the routine shown in FIG. In the routine of FIG. 4, step 202
Then, the current exhaust temperature TE (output of the exhaust temperature sensor 20)
Load. Next, in step 204, it is determined whether the exhaust gas temperature TE is lower than the lower limit value TC of the target temperature range determined in FIG. The circulation to the exhaust gas temperature control device 8 is stopped. If the exhaust gas temperature TE is equal to or higher than the lower limit value TC in step 204, the process proceeds to step 208, where it is determined whether the exhaust gas temperature TE is higher than the upper limit value TH of the target temperature range determined in FIG. 1, and if it is higher, the exhaust temperature is lowered. Since it is necessary, the process proceeds to step 210 to circulate cooling water to the exhaust temperature control device 8. If it is determined in step 208 that TE is less than or equal to TH, the exhaust temperature TE is between TC and TH and does not need to be controlled, so the routine returns directly through step 212. The routine in Figure 4 is performed when the catalyst deteriorates (
(When the cumulative travel distance S becomes large), a catalyst bed temperature changing means is configured to change the catalyst bed temperature (correlated with the exhaust temperature TE) to a higher temperature side.

FIG. 5 shows a routine constituting the HC amount increasing means. This routine is interrupted at regular intervals, for example 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 temperature upper limit HT using a predetermined map, etc., and in step 306, this VA is output and the control valve is By setting the opening degree to the target opening degree VA, the HC amount is increased. This H.C.
In the control, when the lean NOx catalyst 6 deteriorates,
In step 112 of FIG. 1, HT is increased, and in the routine of FIG. 5, HC is increased to prevent the NOx purification rate of the lean NOx catalyst 6 from decreasing or to improve the NOx purification rate. In other words, due to deterioration, HC and NO
Even if the reaction rate with x decreases, control is performed to increase the NOx purification rate as indicated by the broken line in FIG. 7 by increasing the amount of HC.

Next, a second embodiment will be explained. The equipment configuration in FIG. 6 is also applied to the second embodiment. However, lean NOx
Downstream of the catalyst 6, there is an NOx sensor that detects the amount of NOx in the exhaust gas.
An x sensor 22 is provided, and its output is sent to the ECU 10.

FIG. 8 shows a catalyst deterioration determination routine including learning control by the NOx sensor 22, which constitutes the catalyst deterioration determination means in the second embodiment. This routine is interrupted at regular intervals, for example every 50 milliseconds. In step 402, it is determined whether the cumulative travel distance S exceeds a certain distance, for example 2000 km. If the mileage is less than a certain distance, it is assumed that the catalyst has not deteriorated, and the process proceeds to step 404, where the learned value is set so that NOx, which is the output of the NOx sensor 22, is lowered by advancing/retarding the ignition timing, which will be described later. Continue to make corrections, and if the mileage exceeds a certain distance, it is determined that the catalyst is deteriorating, and step 4 is performed.
Step 14, temperature correction coefficient K for controlling exhaust temperature;
and a target HC amount HT for controlling the HC amount.

More specifically, in step 404, it is determined whether the learning conditions are in place, for example, whether or not the vehicle is in a steady running state after warm-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, and in step 408, the learned value Gi up to now is read from the corresponding operating range using the Q/N vs. NE map in FIG. put out. Next, in step 410, (N+9Gi)/10 is calculated based on the output N of the NOx sensor 22, and this is newly set as Gi, and Gi is gradually corrected. This Gi is the NOx in each operating range when there is no deterioration.
Represents the sensor output. Then, in step 412,
The temperature correction coefficient K used in the routine of FIG. 12 described later is set to 0, and the HC concentration HT used in the routine of FIG. 5 is set as shown in FIG.
The basic HC concentration H10 is determined by

If the process proceeds from step 402 to step 414, it is determined in step 414 whether the deterioration determination conditions are met, for example, whether or not the vehicle is running normally after warming up.
If they are not in order, return as is, and if they are in order, proceed to step 416. In step 416, the engine load Q
/N, determine the current operating range from the engine rotation speed NE,
In step 418, the learned value Gi in the case where there is no catalyst deterioration that has been subjected to learning control in the corresponding operating region is read out. Next, in step 420, N is calculated from the output N of the NOx sensor 22 (after catalyst deterioration) and the read learning value Gi (before catalyst deterioration).
The Ox purification rate reduction degree D is calculated as D=N-Gi. Next, in step 422, a temperature correction coefficient K is determined from the degree of reduction in purification rate D based on the K vs. D map in FIG. 10, and in step 424, a target HC concentration is calculated based on the H1 vs. D map in FIG. In step 426, HT is set to H1 and the process returns.

FIG. 12 shows an ignition timing calculation routine constituting the catalyst bed temperature changing means of the second embodiment. There are various means to control the catalyst bed temperature, but in Example 2, in view of the relationship between ignition timing and exhaust temperature as shown in FIG. 13, by controlling ignition timing, exhaust temperature and 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 step 02, the basic ignition timing θBASE is calculated from the engine load Q/N and the engine rotational speed NE. Then step 504
Then, from the temperature correction coefficient K obtained in the routine of FIG. 8, the advance angle correction amount θA is obtained from θA=K×MN. Next, step 5 is performed so that θA does not become larger than the constant value α.
At step 06, 508, α is guarded, and the process proceeds to step 510, where the ignition timing θ is determined from the basic ignition timing θBASE and the advance angle correction amount θA as θ=θBASE+θA. Next, the process proceeds to step 512, where θ is output to set the ignition timing to θ, and the process returns. The HC increasing means in the second embodiment is the same as in the first embodiment, and follows the routine shown in FIG.

FIG. 14 shows a routine constituting the catalyst deterioration determining means of the third embodiment. This routine is interrupted at regular intervals, for example every 50 milliseconds. In step 602, it is determined whether the engine operating conditions are such that it is meaningful to judge catalyst deterioration, and if the operating conditions are meaningless (before warm-up, etc.), the process returns as is, and When the operating condition is met, the process proceeds to step 604. Step 6
At 04, read the engine load Q/N and engine rotation speed NE,
The current engine operating range is calculated, and then in step 606, using the map in FIG. 15, the catalyst inlet-outlet temperature difference ΔTi in the corresponding predetermined operating range before catalyst deterioration is read out.

Next, in step 608, the lean NO.
x Temperature sensors 24 and 20 provided at the inlet and outlet of the catalyst 6
From the outputs t1 and t2 (see FIG. 6), the current catalyst inlet-outlet temperature difference Δt=t2-t1 is calculated. Next, in step 610, a catalyst deterioration function D (D=ΔTi−Δ
t). Then, in step 612, the degree of catalyst deterioration DR is calculated from the catalyst deterioration function D using the map shown in FIG.

Next, in step 614, using the map shown in FIG. 17, a temperature range T1 corresponding to the catalyst deterioration degree DR is determined.
, T2 are determined, and in step 616, TC is set as T1 and TH is set as T2. TC and TH are parameters used for catalyst bed temperature control according to FIG. 4, as in Example 1. Next, the process proceeds to step 618, and a target HC concentration H1 corresponding to the catalyst deterioration degree DR is calculated using the map of FIG. 18, and in step 620, HT is set as H1. This HT is a parameter used for HC concentration control according to FIG. 5, as in Example 1. In the third embodiment as well, catalyst bed temperature control is performed according to the routine shown in FIG. 4, and HC amount control is performed according to the routine shown in FIG. Since the routines shown in FIGS. 4 and 5 have been explained in the first embodiment, their explanation will be omitted.

Next, the functions of the present invention common to each embodiment will be explained. When it is determined that the lean NOx catalyst 6 has deteriorated based on the cumulative mileage, the catalyst inlet-outlet temperature difference, or learning control using the NOx sensor, etc.
The HC amount is increased according to the degree of deterioration by the routine constituting the HC amount increasing means in FIG. Lean NOx
The NOx purification rate of the catalyst 6 decreases as the catalyst deteriorates, and increases as the amount of HC increases. Therefore, lean NOx
Even if the deterioration of the catalyst 6 progresses and the NOx purification rate decreases from a to b and from b to c in Figure 7, the NOx purification rate improves as shown by the broken line in Figure 7 by increasing the HC amount. be done.

In the above, when the catalyst bed temperature is shifted to the high temperature side by the routine of FIG. 4 as the HC amount increases, the NOx purification rate of the lean NOx catalyst 6 is further improved. In other words, even if the lean NOx catalyst 6 deteriorates and the temperature range in which the NOx purification rate peaks shifts from a to b to c in FIG. Since it is shifted to the high temperature side by ignition timing control etc., the lean NOx catalyst 6 always has high N.
It is used in a temperature range that shows the Ox purification rate, and the purification performance of the catalyst is effectively utilized over a long period of time.

[0026]

Effects of the Invention According to the present invention, the catalyst deterioration determining means and the
Since the C amount increasing means is provided, when the catalyst deterioration determining means determines that the deterioration of the lean NOx catalyst has progressed, the H
The C amount increasing means increases the HC amount and reduces the lean NOx catalyst N.
Minimize the decrease in Ox purification rate or reduce NOx
Purification rate can be improved. In this case, since the HC amount increasing means increases HC only when catalyst deterioration progresses, it is possible to achieve both HC economy and improvement in NOx purification rate.

[Brief explanation of the drawing]

FIG. 1 is a flowchart of a routine constituting a catalyst deterioration determining means of a first embodiment of an exhaust gas purification device for an internal combustion engine according to the present invention.

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

FIG. 3 is a target HC concentration versus cumulative travel distance map used in the routine of FIG. 1;

FIG. 4 is a flowchart of a routine constituting a catalyst bed temperature changing means in Embodiment 1 of the exhaust gas purification device for an internal combustion engine of the present invention.

FIG. 5 is a flowchart of a routine constituting the HC amount increasing means of the exhaust purification device for an internal combustion engine according to the present invention.

FIG. 6 is an equipment system diagram of an exhaust purification device for an internal combustion engine according to the present invention.

[Figure 7] NOx purification rate vs. lean NOx in the present invention
It is a catalyst bed temperature characteristic diagram.

FIG. 8 is a flowchart of a routine constituting a catalyst deterioration determining means of a second embodiment of the exhaust gas purification device for an internal combustion engine according to the present invention.

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

[Figure 10] Temperature correction coefficient K used in the routine of Figure 8
This is a D map of the degree of decrease in purification rate.

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

FIG. 12 Embodiment 2 of the exhaust purification device for an internal combustion engine of the present invention
2 is a flowchart of a routine constituting a catalyst bed temperature changing means.

13 is an exhaust temperature versus ignition timing characteristic diagram related to control according to the routine of FIG. 12; FIG.

FIG. 14 Embodiment 3 of the exhaust purification device for an internal combustion engine of the present invention
3 is a flowchart of a routine constituting a catalyst deterioration determining means.

[Fig. 15] Engine load Q/ used in the routine of Fig. 14
This is a map of N versus engine rotational speed NE.

[Figure 16] Catalyst deterioration degree D used in the routine of Figure 14
FIG. 3 is a map of R vs. catalyst deterioration function D.

FIG. 17 is an exhaust temperature versus catalyst deterioration level map used in the routine of FIG. 14;

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

[Explanation of symbols]

2 Internal combustion engine 4 Exhaust system 6 Lean NOx catalyst 8 Exhaust temperature control device 10　ECU 12 HC source 14 HC introduction part 16 Control valve 18 Valve drive device 20 Exhaust temperature sensor 22 NOx sensor 24 Exhaust temperature sensor

Claims (1)

[Claims] Claims: 1. An internal combustion engine capable of lean combustion, and a catalyst which is provided in the exhaust system of the internal combustion engine and is made of zeolite supported with a transition metal or a noble metal and reduces NOx in an oxidizing atmosphere in the presence of HC. , a catalyst deterioration determining means for determining deterioration of the catalyst, and an HC amount increasing means for increasing the amount of HC to the catalyst when the catalyst deterioration determining means determines that the catalyst has deteriorated. Purification device.