JP3055300B2 - 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 an exhaust system provided with a so-called lean NOx catalyst capable of reducing NOx (nitrogen oxide) in lean exhaust at an air-fuel ratio.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. 1-130735 discloses a zeolite in which a transition metal is supported by ion exchange or a noble metal is supported.
A catalyst for purifying NOx in the presence of (hydrocarbon) (hereinafter referred to as a lean NOx catalyst) is disclosed.

[0003] Since it has been found that a lean NOx catalyst exhibits a higher NOx purification rate when the temperature is in a rising state than in a steady or falling state, the present applicant has
In Japanese Patent Application No. 2-317664 (prior to publication), the same type of lean NOx catalyst is arranged in parallel in the exhaust system of an internal combustion engine,
An invention has been proposed in which the exhaust gas flow is alternately switched at the same time interval to forcibly and repeatedly create a catalyst temperature rising process to improve the NOx purification rate.

[0004]

However, as in the prior application, in the case of switching the exhaust flow between the same type of lean NOx catalyst at the same time interval, the type of lean NOx
Only in the temperature range where the catalyst has a high purification rate,
There is a problem that the temperature range showing a high purification rate cannot be expanded only by improving the NOx purification rate by repeatedly forming the temperature raising process. Therefore, it has been difficult to cope with a decrease in the NOx purification rate when the exhaust gas temperature is out of the temperature range showing the high purification rate in a temperature change over a relatively wide temperature range of the exhaust gas temperature.

SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust purification system for an internal combustion engine having a lean NOx catalyst in an exhaust system.
An object of the present invention is to widen a temperature range showing a high NOx purification rate while maintaining the x purification rate improving effect.

[0006]

According to the present invention, the above object is achieved by the following apparatus for purifying exhaust gas of an internal combustion engine. That is, the lean-burn internal combustion engine capable and an exhaust passage, is provided in parallel with each other in the exhaust passage, the high
High temperature lean NO that shows high NOx purification rate in a wide temperature range
x catalyst and low temperature type showing high NOx purification rate in low temperature range
A plurality of lean NOx catalysts , including a lean NOx catalyst;
The exhaust flow provided in the exhaust passage is directed to the high-temperature type
Switching means for switching between the NOx catalyst and the low-temperature lean NOx catalyst, exhaust temperature detecting means for detecting the exhaust gas temperature, and exhaust gas for the high-temperature lean NOx catalyst and the low-temperature lean NOx catalyst.
So that it alternately and repeatedly flows through the NOx catalyst
High exhaust gas temperature detected by the exhaust gas temperature detection means
Occasionally, exhaust gas is passed to the high-temperature lean NOx catalyst
Time during which the time ratio is flowed through the low-temperature lean NOx catalyst
Ratio and is detected by the exhaust gas temperature detecting means.
When the exhaust temperature is low and the exhaust gas temperature is low, the exhaust
The time ratio of the high-temperature lean NO
x so that it is greater than the time fraction flowing through the catalyst.
An exhaust emission control device for an internal combustion engine, comprising: switching means control means for controlling switching means.

[0007]

According to the exhaust gas purifying apparatus for an internal combustion engine of the present invention, a high N
A plurality of lean N catalysts having different catalyst temperature ranges indicating Ox purification rates
An Ox catalyst is provided. Among them, high N in a high temperature range
A high-temperature lean NOx catalyst showing an Ox purification rate
x catalyst, a lean NOx catalyst exhibiting a high NOx purification rate in a low temperature range is called a low-temperature lean NOx catalyst. The time ratio of the high-temperature lean NOx catalyst is set to be greater than the time ratio of the low-temperature lean NOx catalyst.
It is set to be larger than the ratio of the time passed to the Ox catalyst.

As a result, the exhaust gas is alternately flown between the high-temperature lean NOx catalyst and the low-temperature lean NOx catalyst, so that the temperature increasing effect is repeatedly exhibited in each catalyst, and the NOx purification rate by the repeated formation of the temperature increasing effect is increased. The improvement is maintained. Further, since the time of the exhaust gas flow is changed according to the exhaust gas temperature, when the exhaust gas temperature is high, the time during which the high-temperature lean NOx catalyst is working becomes longer, and the NO.
When the exhaust gas temperature is low, the operating time of the low-temperature lean NOx catalyst is prolonged, and the low-temperature lean NOx catalyst has a strong NOx purification property. As a result, the temperature range in which the high NOx purification rate is high is a wide temperature range that is the sum of the temperature range in which the high-temperature lean NOx catalyst exhibits a high NOx purification rate and the temperature range in which the low-temperature lean NOx catalyst exhibits a high NOx purification rate. Can be expanded.

According to the present invention, the ratio of the exhaust gas supply time is changed in accordance with the exhaust gas temperature. Exhaust gas is continuously supplied only to the high-temperature lean NOx catalyst at a high exhaust gas temperature and continuously exhausted only to the lean NOx catalyst at a low exhaust gas temperature. It does not flow.
If the exhaust gas flows continuously, the temperature of the lean NOx catalyst in which the exhaust gas does not flow decreases due to natural heat radiation, and the temperature of the exhaust gas changes, and the lean NOx catalyst in which the exhaust gas has not flown has been used.
When exhaust gas is flowed through the x catalyst, a good NOx purification rate cannot be exhibited until the catalyst reaches the activation temperature, so that the NOx purification rate immediately after the exhaust flow switching is significantly reduced. However, in the present invention, since the exhaust gas is constantly switched repeatedly and is intermittently flown, a decrease in the NOx purification rate at the time of exhaust gas flow switching can be avoided.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings. FIG.
Internal combustion engines (including diesel engines) capable of lean burn (air-fuel ratio leaner than stoichiometric air-fuel ratio)
The two exhaust passages 4 are provided with side-by-side passage portions in which a plurality of passages 4a and 4b are provided side by side. Each passage 4a, 4
The lean NOx catalysts 6a and 6b are provided in parallel with each other by providing the lean NOx catalysts 6a and 6b in b.

Here, the lean NOx catalyst is a catalyst capable of reducing NOx in the exhaust gas of the air-fuel ratio lean combustion. Exchanged transition metal supported on zeolite /
There are zeolite catalysts and Pt / alumina catalysts in which a noble metal such as Pt (platinum) is supported on alumina.

The lean NOx catalyst has a temperature range (temperature window) showing a high NOx purification rate. The temperature window of the transition metal / zeolite catalyst is in a relatively high temperature range, approximately 350-550 ° C. In contrast, Pt
The temperature window of the / alumina catalyst is in the relatively low temperature range, 250-400 ° C. Hereinafter, a lean NOx catalyst having a temperature window in a relatively high temperature range, such as a transition metal / zeolite catalyst, is referred to as a high-temperature lean NOx catalyst.
A lean NOx catalyst having a temperature window in a relatively low temperature range, such as a t / alumina catalyst, is called a low temperature lean NOx catalyst.

The lean N provided in parallel with the exhaust passage 4
As the Ox catalysts 6a and 6b, catalysts having different temperature ranges that show a high purification rate for NOx are selected. For example, in the illustrated example, 6a is a high-temperature lean NOx catalyst, and 6b is a low-temperature lean NOx catalyst.

A switching valve 8 as a switching means for switching the exhaust flow between the lean NOx catalysts 6a and 6b is provided at the branch portion of the plurality of passages 4a and 4b. Switching valve 8
The switching is controlled by a command from an electronic control unit (ECU) 10 described later.

An exhaust temperature sensor 12 as exhaust temperature detecting means for detecting the exhaust temperature is provided in the exhaust passage 4 and preferably upstream of the switching valve 8. The output of the exhaust gas temperature sensor 12 is input to the ECU 10. Instead of detecting the exhaust gas temperature by the exhaust gas temperature sensor 12, the engine load and the engine rotational speed are detected and input to the ECU 10, and the exhaust gas temperature is estimated and obtained from the load and the rotational speed using a map provided in advance. You may. Such components are included in the exhaust gas temperature detecting means of the present invention.

The ECU 10 comprises a microcomputer, an input / output unit having an input interface and an output interface, a read-only memory (ROM) as a read-only memory element, a random access memory (RAM) for temporary storage, and executes operations. It has a central processor unit (CPU). An analog / digital converter for converting an analog signal into a digital signal is provided for the input / output unit. The analog signal from the exhaust gas temperature sensor 12 is converted into a digital signal and supplied to the input interface.

The ROM stores the control routine shown in FIG. 2 and the map shown in FIG. 3, and is called by the CPU to execute an operation. The routine of FIG. 2 is interrupted at regular intervals (for example, every three minutes). First, in step 102,
The exhaust temperature TE corresponding to the output of the exhaust temperature sensor 12 is read.

Subsequently, the routine proceeds to step 104, where the time a for which the exhaust gas should flow to the high-temperature lean NOx catalyst and the time b for which the exhaust gas should flow to the low-temperature lean NOx catalyst, which are optimal for NOx purification, at the current exhaust gas temperature TE. Is calculated by using a predetermined map as shown in FIG.

In the map of FIG. 3, as the exhaust gas temperature TE increases, the ratio a of the exhaust gas supply time to the high-temperature lean NOx catalyst 6a increases, and the ratio b of the exhaust gas supply time to the low-temperature lean NOx catalyst 6b decreases. It is defined as follows.
For example, in the illustrated map, when the exhaust gas temperature is 300 ° C., the time ratio flowing through the high-temperature lean NOx catalyst 6a is about 15%.
However, when the exhaust gas temperature rises to 430 ° C., the time ratio flowing through the high-temperature lean NOx catalyst 6a becomes about 85%.
It becomes big.

Subsequently, the routine proceeds to step 106, where the switching valve 8
At the time ratio a: b determined in step 104,
As shown in a time chart of FIG. 4, at least one cycle is performed in which the exhaust gas flows through one of the high-temperature lean NOx catalyst 6a and the low-temperature lean NOx catalyst 6b, and then the exhaust gas flows through the other. Repeat several times. Subsequently, the process proceeds to the return step and returns. At the next interrupt timing, the routine is again interrupted and the same control is executed.
In the above description, the steps 104 and 106 and the map in FIG. 3 constitute the switching means control means.

However, according to the map shown in FIG.
When the temperature rises to 0 ° C. or higher, the time a flowing through the high-temperature lean NOx catalyst is set to 0, and the entire amount of exhaust gas always flows through the low-temperature lean NOx catalyst 6b. The reason is that the zeolite catalyst of the high-temperature type lean NOx catalyst 6a undergoes severe thermal degradation at about 600 ° C. or higher, and the Pt / alumina catalyst of the low-temperature type lean NOx catalyst 6b becomes 600 ° C.
This is because almost no thermal deterioration occurs even at a temperature of not less than 600 ° C., so that at 600 ° C. or more, the entire amount of exhaust gas is caused to flow toward the Pt / alumina catalyst 6b in order to prevent thermal deterioration of the zeolite catalyst 6a.

Next, the operation of the embodiment of the present invention will be described.
By controlling the switching of the exhaust flow of the switching valve 8 in accordance with the routine of FIG. 2, the switching of the exhaust flow is as shown in FIG. That is, when the temperature is high as shown in FIGS. 3 and 4A, the time ratio a during which the exhaust gas flows through the high-temperature lean NOx catalyst 6a is large, and the exhaust gas flows through the low-temperature lean NOx catalyst 6b. The time ratio b is small, and a and b are 1
It is repeated as a cycle. In addition, when the temperature is low as in FIGS. 3 and 4B, the time ratio a during which the exhaust gas flows through the high-temperature lean NOx catalyst 6a is small, and the low-temperature lean NOx is low.
The time ratio b during which the exhaust gas is flowing through the catalyst 6b is large,
This is repeated with a and b as one cycle. The length of one cycle is the same at the time of high temperature and low temperature, and is about 3 minutes (may be set to the interrupt time interval in FIG. 2).

By alternately switching the exhaust gas between the high-temperature lean NOx catalyst 6a and the low-temperature lean NOx catalyst 6b, the NOx purification rate can be improved as shown in FIG. 6B. That is, the lean NOx catalyst exhibits a higher NOx purification rate when the catalyst bed temperature (correlated with the exhaust gas temperature) rises B than when the temperature is in a steady state or when it falls B '. When the exhaust gas is not flowing, the temperature falls due to natural heat radiation, and when the exhaust gas flows again, the temperature rises, so by repeatedly switching,
A high NOx purification rate as shown by B in FIG. 6 is obtained.

A plurality of lean Ns provided in parallel in the exhaust passage 4
Ox catalysts are used as catalysts having different temperature windows showing a high purification rate for NOx, such as high-temperature lean NO.
As shown in FIG. 5, the x-type catalyst 6a and the low-temperature type lean NOx catalyst 6b have a wide temperature range (for example, as shown in FIG. 5) as compared with a catalyst having the same type of lean NOx catalyst as disclosed in Japanese Patent Application No. 2-317664. , 250-550 ° C.). Japanese Patent Application No. 2-317
No. 664 increases the NOx purification rate of one type of lean NOx catalyst, and the temperature window does not expand,
In the present invention, the temperature window corresponds to the temperature window of the high-temperature lean NOx catalyst 6a and the low-temperature lean NOx catalyst 6b.
Temperature window.

The higher the exhaust gas temperature, the higher the lean NO.
Since the exhaust gas flow time ratio to the x-catalyst 6a is increased to alternately and repeatedly flow to the high-temperature lean NOx catalyst 6a and the low-temperature lean NOx catalyst 6b, a high NOx purification rate as shown in FIG. can get. If the exhaust gas temperature is high, the exhaust gas is continuously flown only to the high-temperature lean NOx catalyst 6a, and if the exhaust gas temperature is low, switching is performed, and the exhaust gas is continuously flown only to the low-temperature lean NOx catalyst 6b. As shown in FIG. 5C which considers the purification rate characteristics A and B of the catalyst,
Inferior to D of the present invention. In the case of C, at the time of switching,
Since it takes time for the lean NOx catalyst, which has not flowed so far, to be at a low temperature and reach the activation temperature, immediately after the switching, the temperature further drops below C, as shown in FIG. 5E. It becomes a purification rate characteristic. However, in the present invention, since the catalyst always flows alternately in the two catalysts 6a and 6b and is in the active temperature range,
A high NOx purification rate like D can be obtained transiently.

[0026]

According to the present invention, an internal combustion engine capable of lean combustion is provided.
And its exhaust passage and parallel to each other in said exhaust passage
High NOx purification rate in a high temperature range
High temperature lean NOx catalyst and high NOx purification in low temperature range
And a low-temperature lean NOx catalyst exhibiting a conversion rate.
NOx catalyst and exhaust gas flow provided in the exhaust passage.
Between the high-temperature lean NOx catalyst and the low-temperature lean NOx.
switching means for switching between the x catalyst and detecting the exhaust gas temperature
Exhaust temperature detecting means, and the exhaust gas is the high-temperature type lean NOx catalyst.
And the low-temperature lean NOx catalyst flows alternately and repeatedly.
In addition, the exhaust gas temperature detected by the exhaust gas temperature detecting means
When the temperature is high and the temperature is high, the exhaust gas is high-temperature lean NOx.
The low-temperature lean NOx catalyst has a time ratio of flowing to the catalyst.
The exhaust gas temperature detection means
When the exhaust temperature detected by the
The ratio of time flowing through the low-temperature lean NOx catalyst is high.
It is set to be larger than the ratio of time flowing through the warm lean NOx catalyst.
Switching means control means for controlling the switching means
If, because with a, while keeping to obtain a NOx purification rate of the lean NOx catalyst by actively repeatedly forming a heating process, it is possible to expand the temperature range in which it is possible to obtain a high NOx purification rate .

[Brief description of the drawings]

FIG. 1 is a system diagram of an exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention.

FIG. 2 is a flowchart of a control routine of a switching means control means for performing switching control of the switching means of the apparatus of FIG. 1;

FIG. 3 is a map showing an example of a relationship between an exhaust gas supply time ratio to a high-temperature lean NOx catalyst and an exhaust gas temperature used in the control routine of FIG. 2;

FIG. 4 shows high-temperature lean N at high and low temperatures.
5 is a time chart showing steps flowing through an Ox catalyst and a low-temperature lean NOx catalyst in a stepped manner.

FIG. 5 is a comparison diagram of NOx purification rate versus exhaust temperature characteristics in the present invention and various devices.

FIG. 6 is a NOx purification rate-exhaust gas temperature characteristic diagram showing a change in the NOx purification rate when the temperature of the lean NOx catalyst rises, falls, and is steady.

[Explanation of symbols]

 Reference Signs List 2 internal combustion engine 4 exhaust passage 6a (high temperature type) lean NOx catalyst 6b (low temperature type) lean NOx catalyst 8 switching valve (switching means) 10 ECU 12 exhaust temperature sensor (exhaust temperature detecting means)

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F01N 3/28 301 F01N 3/24

Claims (1)

(57) [Claims] 1. An internal combustion engine capable of lean burn and an exhaust passage thereof, and a high temperature range provided in parallel with each other in the exhaust passage.
High temperature lean NOx catalyst showing high NOx purification rate
Low temperature lean N showing high NOx purification rate in low temperature range
A plurality of lean NOx catalyst comprising the Ox catalyst, provided in the exhaust passage, the high-temperature type Li exhaust flow
A switching <br/> replaced switching means between the emissions NOx catalyst and the low temperature lean NOx catalyst, the exhaust temperature detecting means for detecting the exhaust temperature, the low temperature lean exhaust gas and the high-temperature type lean NOx catalyst
The exhaust gas is made to alternately and repeatedly flow through the NOx catalyst, and
When the exhaust gas temperature detected by the air temperature detector is high
Is the time during which exhaust gas is flowed through the high-temperature lean NOx catalyst.
Time ratio in which the ratio is flowed to the low-temperature lean NOx catalyst
And detected by the exhaust gas temperature detecting means.
When the temperature of the exhaust gas is low and the temperature is low, the exhaust gas is exhausted by the low-temperature lean NO.
x time flowing through the catalyst is equal to the high temperature type lean NOx catalyst.
The switching is performed so as to be greater than the time ratio of the medium flowing.
Exhaust purification system of an internal combustion engine, characterized by comprising a switching means control means for controlling the means.