JP2001289094A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten time required for the regeneration of an NOx absorbent and to improve fuel consumption by keeping oxygen concentration relatively high while maintaining an air-fuel ratio to a rich air-fuel ratio. SOLUTION: This exhaust emission control device for an internal combustion engine is provided with an upstream catalyst 3 provided with a function of trapping and holding oxygen in exhaust gas when the air-fuel ratio of inflow exhaust gas is a lean air-fuel ratio, and changed in the holdable quantity of oxygen according to oxygen concentration in the atmosphere; a downstream catalyst 4 provided with a function of trapping and holding NOx in the exhaust gas when the air-fuel ratio of the inflow exhaust gas is a lean air-fuel ratio, and reducing the held NOx by reducing agent components in the exhaust gas when the air-fuel ratio of the inflow exhaust gas is a rich air-fuel ratio; and a control means for controlling the exhaust gas flowing into the upstream catalyst 3, into the state of the air-fuel ratio being the rich air-fuel ratio and the oxygen concentration being higher than specified oxygen concentration when the downstream catalyst 4 is to reduce the held NOx.

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 mounted on an automobile or the like.

[0002]

2. Description of the Related Art Purification of reducing agent components (HC, CO) in exhaust gas discharged immediately after the start of an engine and N in exhaust gas discharged when the engine is operated at a lean air-fuel ratio.
In order to achieve both purification of Ox and exhaust gas, a starting catalyst that reaches an activation temperature immediately after the engine is started is disposed upstream, and a NOx absorbent that absorbs NOx in exhaust gas having a lean air-fuel ratio is provided downstream of the starting catalyst. An exhaust purification device provided on the side is known (for example, JP-A-11-62563).

[0003]

As a starting catalyst used in such an apparatus, a catalyst in which a platinum-based noble metal is supported on a honeycomb-shaped carrier coated with alumina is generally used.

Such a catalyst has a function of trapping and holding oxygen in the exhaust gas when the inflowing exhaust gas has an air-fuel ratio or a lean air-fuel ratio. Works very effectively to stabilize the oxygen, but this oxygen-retaining function hinders the supply of the reducing agent component to the NOx absorbent disposed downstream.

[0005] That is, NO is determined by the continuation of the lean operation.
When a certain amount of NOx is absorbed by the x absorbent, a reducing agent component is supplied to the NOx absorbent to reduce the absorbed NOx, and
At this time, a large amount of oxygen is retained in the starting catalyst, and the starting catalyst oxidizes the reducing agent component to be supplied to the NOx absorbent using the retained oxygen. Therefore, the reducing agent component does not reach the NOx absorbent until the amount of oxygen retained in the starting catalyst becomes zero. Therefore, there is a problem that the time required for the regeneration of the NOx absorbent becomes longer, and the fuel efficiency is deteriorated by the amount of time during which the lean air-fuel ratio operation can be performed.

[0006]

In order to solve the above-mentioned problems, according to the present invention, oxygen in an exhaust gas is trapped in an exhaust passage of an engine when the air-fuel ratio of the exhaust gas flowing into the engine is a lean air-fuel ratio. An upstream catalyst, which has a function of holding, and the amount of oxygen that can be held changes according to the oxygen concentration of the atmosphere, and the air-fuel ratio of exhaust gas flowing into an exhaust passage downstream of the upstream catalyst is a lean air-fuel ratio. Sometimes NOx in exhaust gas
A downstream catalyst having a function of reducing the NOx retained by the reducing agent component in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is a rich air-fuel ratio.
When NOx held by the downstream catalyst is to be reduced, control is performed to control the exhaust gas flowing into the upstream catalyst to a state where the air-fuel ratio is a rich air-fuel ratio and the oxygen concentration is higher than a predetermined oxygen concentration. Means. The predetermined oxygen concentration may be an oxygen concentration (point A in FIG. 1) at which the amount of oxygen that can be held by the upstream catalyst with respect to the oxygen concentration reaches saturation.

[0007]

When the air-fuel ratio of the normal exhaust gas is set to the rich air-fuel ratio, the oxygen concentration of the exhaust gas becomes extremely low. All the oxygen held in the catalyst is released, and the above-described problem occurs.

On the other hand, if the fuel injection when the air-fuel ratio of the exhaust gas is set to the rich air-fuel ratio is devised, the oxygen concentration can be kept relatively high while the air-fuel ratio is set to the rich air-fuel ratio.
If such exhaust gas flows into the upstream catalyst, oxygen release from the upstream catalyst can be suppressed, and as a result, the time required for regeneration of the downstream catalyst can be shortened.

Even if exhaust gas having a rich air-fuel ratio and a high oxygen concentration flows into the upstream catalyst, oxygen in the exhaust gas reacts with the reducing agent component in the catalyst. Will decrease the oxygen concentration. Therefore,
On the downstream side of the catalyst, release of retained oxygen cannot be avoided. That is, even with the configuration of the present invention, it is impossible to prevent the oxygen retained in the upstream catalyst from being released at all, and a time period during which the reducing agent component does not reach the downstream catalyst immediately after the start of the regeneration control. Occurs. However,
Such time can be shortened compared to the case where the downstream side catalyst is regenerated with the exhaust gas having the rich air-fuel ratio and the oxygen concentration of 0.

"Air-fuel ratio of exhaust gas" used in this specification
Means the ratio of the amount of intake air to the total amount of fuel supplied upstream from the upstream catalyst. For example, when additional fuel is supplied to the combustion chamber in the expansion to exhaust strokes of the engine or additional fuel is supplied to the exhaust passage in addition to the normal fuel supply, the amount of intake air / (normal fuel supply amount + (Fuel supply amount).

[0011]

Embodiments of the present invention will be described below with reference to the drawings. Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows the configuration of the embodiment of the present invention. An upstream catalyst 3 is provided in an exhaust pipe 2 of the engine 1, and a downstream catalyst 4 is provided downstream thereof.
Is installed. A fuel injection valve 6 is provided in the intake pipe 5 and forms a mixture with the intake air and burns in the engine 1. This fuel injection is performed based on the output of an air flow meter 7 that detects an intake air amount Qa installed upstream of the intake pipe 5 and a crank angle sensor 8 that detects the rotational speed Ne of the engine 1. In 9), the basic fuel injection pulse width is calculated, and the fuel is injected based on the result of various corrections such as the target air-fuel ratio. The accelerator opening is also input to the ECU 9, and the engine load L is calculated from the accelerator opening.
The electronically controlled throttle chamber 10 is driven.

The upstream-side catalyst 3 is a catalyst in which a noble metal such as platinum Pt, palladium Pd, and rhodium Rb and cerium Ce are supported on an alumina-coated honeycomb carrier, for example. In the case of, a function (oxygen holding function) of trapping and holding oxygen in the exhaust gas and joining with the oxygen holding the reducing agent component in the inflowing exhaust gas when the air-fuel ratio is rich is provided. .

The amount of oxygen that can be held varies according to the oxygen concentration in the atmosphere. The higher the concentration, the more the amount of oxygen that can be held increases, but reaches saturation to some extent (point A) or more.

The downstream catalyst 4 is, for example, a catalyst in which a noble metal such as platinum Pt, palladium Pd, rhodium Rh or the like is supported on a honeycomb carrier coated with alumina, and an alkaline earth represented by barium Ba, cesium Cs The downstream catalyst 4 traps NOx in the exhaust gas when the air-fuel efficiency of the exhaust gas is lean air-fuel efficiency. And when the air-fuel ratio is rich, reducing components (HC, CO, H
It has a function of reducing and purifying NOx retained in 2).

This operation will be described with reference to the flowcharts of FIGS. FIG. 3 illustrates the operation of setting the target air-fuel efficiency and the process of regenerating the downstream catalyst 4. This routine is for example 10 m
This is executed every second.

At S1, it is determined whether or not the rich spike flag FRS is "1". The FRS is set to FRS = 1 when NOx of a predetermined value or more is held in the downstream side catalyst 4 and it is determined that the reduction and purification of NOx is required by providing rich air-fuel efficiency, and the target air-fuel ratio is set during the period of FRS = 1. To make it rich. Therefore, when FRS ≠ 1 (FRS = 0), it indicates that the engine may be operated at a lean air-fuel ratio. If it is determined in S1 that FRS ≠ 1 (FRS = 0), the process proceeds to S2, where the engine load L and the engine speed Ne are determined.
From the target air-fuel ratio TFBYA. (See Figure 4 for map)

In S3, it is determined whether the operation is a lean operation. That is, it is determined whether TFBYA obtained in S2 is less than 1.0.

When the engine is operated lean, NOx exhausted from the engine is trapped in the downstream side catalyst 4. Therefore, it is necessary to estimate the NOx holding amount and reduce NOx by rich spike in S4-7. Judge the gender.

If it is determined in S3 that the operation is lean,
4 to the engine load L and the engine speed Ne to N
The Ox emission concentration CNOx is retrieved from the map. (See Figure 5 for map)

This map is obtained in advance by experiments. In S5, the NOx holding amount TNOx in the downstream catalyst 4
Is calculated by multiplying the conventional TNOx by the NOx emission concentration CNOx and the intake air amount Qa, and furthermore, a constant K
It is obtained by adding values multiplied by 1. K1 is a constant for unit conversion.

In S6, the NOx holding amount TNOx is reduced to a predetermined amount T.
It is determined whether or not NOx0 has been exceeded. The predetermined amount TNOx0 is a certain ratio (for example, about ２) of the saturated NOx holding amount of the downstream side catalyst 4, which is experimentally obtained in advance.
It is also effective to decrease the value with deterioration of the catalyst. When TNOx> TNOx0, the process proceeds to S7, where the rich spike flag FRS is set to 1, and the process proceeds to the rich spike regeneration process of the downstream catalyst 4.
Since FRS = 1, the process proceeds to S8 according to the determination of S1.

In S8, an estimated value of the catalyst temperature Tcat of the downstream side catalyst 4 is retrieved from the map from the engine load L and the engine speed Ne. (See FIG. 6 for map) NO in S9
The x release concentration RNOx is determined. NO during rich spike
The x release concentration RNOx is determined by the catalyst temperature Tcat and the NO at that time.
Since it changes depending on the x holding amount TNOx, a search is made from a map obtained in advance through experiments. (See Figure 7 for map)

In S10, the air-fuel ratio TFBYARNOx required for the RNOx reduction purification is calculated by the following equation. TFBYARNOx = 1 + K3.RNOx Here, 1 is the amount of the stoichiometric air-fuel ratio, and K3.RNOx is the amount of the air-fuel ratio required for the reduction and purification of the released NOx. K3
Is a constant for converting the NOx concentration into an air-fuel ratio. S
In step 11, in consideration of the efficiency of the supplied reducing agent used for NOx reduction and purification, S
The air-fuel ratio TFBYA is calculated by multiplying the required air-fuel ratio calculated in 10 by a certain ratio (described as 1.2 in the flowchart). Next, in FIG. S12, NOx due to release of NOx
The change in the holding amount TNOx is calculated. TNOx is determined by subtracting the value obtained by multiplying the NOx concentration RNOx released in S9 by the intake air amount Qa and further multiplying by a constant K1 from TNOx up to now. In S13, it is determined whether or not the calculated TNOx is lower than 0, and if it is lower, the rich spike is terminated. In this case, TNOx = 0 in S14.
Thus, the value of TNOx is limited so as not to be lower than 0. At S15, the rich spike flag is set to 0.
To end the rich spike.

The TFBYA during the rich spike and during the lean operation is determined by the above-described routine. The control of the actual fuel injection amount Ti, the injection timing IT, and the intake air amount Q will be described with reference to the flowchart of FIG.

In S16, the required torque TTC is calculated from the accelerator position Aps from the map, and the required torque TTC is calculated.
Intake air amount Q for generating the target air-fuel ratio TFBY
The calculation is performed in consideration of A and the engine efficiency ITA. At the time of the rich spike, the setting of the engine efficiency ITA and the fuel injection timing are different, so it is determined at S17 whether or not the rich spike flag FRS is 1. When it is determined that FRS ≠ 1 (FSR = 0), the process proceeds to S18, in which the TF converted to the combustion torque is selected from the TFBYA obtained in the flowchart of FIG.
BBY1 and TFBYA2 which is not converted to torque are obtained. In S18, however, TFBYA1 = TFB for normal operation.
YA, TFBYA2 = 0. The engine efficiency ITA is calculated from the map in S19 and the fuel injection timing IT1 is calculated in S20 according to TFBYA1 and the engine speed Ne.

When the rich spike is executed with FRS = 1 in S17, the process proceeds to S21, where TFBYA1 for the fuel injected in the latter half of the compression stroke and TFBYA1 for the fuel injected in the latter half of the compression stroke are injected from the TFBYA obtained in the flowchart of FIG. Calculate TFBYA2 for the fuel.

Here, it is assumed that TFBYA1 is 1, which is a value corresponding to the stoichiometric air-fuel ratio, and TFBYA-1 is TFBYA2. When fuel equivalent to the stoichiometric air-fuel ratio is injected in the intake stroke,
A homogeneous mixture is formed in the combustion chamber, and the oxygen concentration of the exhaust gas after the mixture has burned becomes extremely low. On the other hand, when fuel equivalent to the stoichiometric air-fuel ratio is injected in the latter half of the compression stroke, a mixture layer richer than the stoichiometric air-fuel ratio is formed near the ignition plug, and a layer containing almost no fuel can be formed around it. is there. When combustion is performed in such an air-fuel mixture form, oxygen in the lean air-fuel mixture layer can be left unreacted, so that the oxygen concentration of the exhaust gas is kept relatively high while keeping the air-fuel ratio of the exhaust gas at the stoichiometric air-fuel ratio. be able to.
Furthermore, if additional fuel is injected during the expansion-exhaust stroke after combustion in such a mixture mode, the oxygen concentration of the exhaust gas is compared while the air-fuel ratio of the exhaust gas is made richer than the stoichiometric air-fuel ratio. Can be kept high.

In step S22, the engine efficiency ITA is calculated in consideration of the fact that not all the fuel corresponding to the stoichiometric air-fuel ratio injected in the latter half of the compression stroke burns to generate engine torque. Of the fuel injection timing IT1 and the fuel injection timing IT2 of the expansion-exhaust stroke are calculated.

Next, at S24, the intake air amount Q required to generate the necessary torque TTC is obtained by the equation (1), and the electronically controlled throttle is driven so as to reach the intake air amount Q. Q = TTC / TFBYA1 / ITA (1)

In S25, the fuel amount Ti converted into torque is
1 and Ti2 that is not converted to torque is determined by the intake air amount Qa /
An injection amount is calculated by multiplying the basic fuel injection pulse width Tp calculated based on the rotation speed Ne by TFBYA. That is, the fuel injection pulse width Ti is calculated by the following equation. Tin = Tp · TFBYAn + Ts where Ts is an invalid injection pulse width. (N is 1 and 2)

Although the preferred embodiment of the present invention has been described in detail with reference to the drawings, the specific configuration is not limited to this preferred embodiment, and a design change or the like may be made without departing from the gist of the present invention. Even if present, it is included in the present invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing an oxygen storage amount of a catalyst.

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

FIG. 3 is a diagram showing a first flowchart of the present invention.

FIG. 4 is a diagram showing a target air-fuel ratio map.

FIG. 5 is a diagram showing a NOx emission concentration map.

FIG. 6 is a diagram showing a catalyst temperature prediction map.

FIG. 7 is a view showing a NOx purification degree map.

FIG. 8 is a diagram showing a second flowchart of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust pipe 3 Upstream catalyst 4 Downstream catalyst 5 Intake pipe 6 Fuel injection valve 7 Air flow meter 8 Crank angle sensor 10 Electric throttle chamber

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/24 F01N 3/24 R 3/28 301 3/28 301C F02D 43/00 301 F02D 43/00 301E 301T (72) Inventor Hirofumi Tsuchida 2 Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd. In-house F-term (reference) CB02 CB03 CB07 DA01 DA02 DA10 DB06 DB08 DB09 DB10 EA01 EA03 EA05 EA07 EA34 FB10 FB11 FB12 GA06 GB01X GB03W GB04W GB05W GB06W GB07W GB10X GB16X HA18 HA20 3G301 JA25 LA03 MA01 MA12 MA18 MA08 NE13 PE01 PDZ

Claims (2)

[Claims] An exhaust gas passage provided in an exhaust passage of an engine has a function of trapping and retaining oxygen in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio. An upstream catalyst that changes in accordance with the oxygen concentration in the atmosphere; and an NOx in the exhaust gas that is provided in an exhaust passage downstream of the upstream catalyst and that has a lean air-fuel ratio when the inflowing exhaust gas is lean. NOx that was held when the air-fuel ratio of the inflowing exhaust gas was a rich air-fuel ratio
And a downstream catalyst having a function of reducing the exhaust gas with a reducing agent component in the exhaust gas. When the NOx held by the downstream catalyst is to be reduced, the exhaust gas flowing into the upstream catalyst has an air-fuel ratio of Control means for controlling the air-fuel ratio to be rich and the oxygen concentration to be higher than a predetermined oxygen concentration. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the predetermined oxygen concentration is an oxygen concentration at which the amount of oxygen that can be held by the upstream catalyst with respect to the oxygen concentration reaches saturation.