JPH06200747A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To obtain high exhaust purifying performance even in a condition where the conversion efficiency of a lean NOx catalyst is low. CONSTITUTION:The operating state of an engine 10 is read (S1) to judge whether lean operation is performed (S2), and the exhaust flow V is computed (S3). The NOx conversion efficiency eta1 of a lean NOx catalyst is read or computed (S4), and the optimum value V of exhaust flow passing a catalyst part 12 is obtained (S5). The exhaust flow V1-V is obtained (S6), and the target opening Tv of a flow control valve 15 corresponding to the exhaust flow V1-V is obtained (S7). After reading he present opening T of the flow control valve 15 (S8), a judgment is made on a whether or not the valve body position of a flow control valve 15 can be changed to the target opening Tv (S9). The change to the target opening Tv is performed (S10). A judgment is then made on whether (V'.eta'-V1.eta1) is 0 or more (S11). The opening of the flow control valve 15 is then changed to total closure or full opening (S12).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification apparatus for an internal combustion engine (hereinafter referred to as a lean burn engine) capable of lean combustion, and particularly to an exhaust passage in an oxidizing atmosphere in the presence of unburned hydrocarbons (HC). in an exhaust purifying apparatus for an internal combustion engine which is interposed a catalyst for reducing (lean NO _X catalyst) nitrogen oxides in the exhaust gas (NO _X).

[0002]

2. Description of the Related Art In recent years, a lean burn engine has been proposed which burns an air-fuel ratio in a lean region in an automobile for the purpose of improving fuel efficiency. The lean air-fuel ratio range, can not purify NO _X in the conventional three-way catalyst, NO _X reduction has become a problem of lean-burn engine, purifying catalyst capable of NO _X in the lean air-fuel ratio range, i.e., an oxidizing atmosphere During,
In the presence of unburned hydrocarbons (HC), nitrogen oxides (N
O _X) catalyst for reducing, so-called lean NO _X catalyst has been proposed.

Known lean NO _x catalysts are, for example, zeolites carrying a transition metal, which are disclosed in JP-A-1-130735. As an exhaust purification apparatus of a conventional lean burn engine, as shown in FIG. 10, an exhaust passage 1 has a three-way catalyst 2 for exhaust purification when the air-fuel ratio during engine operation is in a stoichiometric air-fuel ratio range, and a lean air-fuel ratio. In some cases, the lean NO _x catalyst 3 for purifying the exhaust gas is installed in series.

Further, since the lean NO _x catalyst deteriorates in a reducing atmosphere, that is, in an air-fuel ratio rich atmosphere, various techniques for preventing the deterioration of the lean NO _x catalyst have been conventionally known. (See Japanese Utility Model Laid-Open No. 3-83314 and Japanese Patent Laid-Open No. 3-225013). As a technique to prevent the deterioration of such lean NO _X catalyst, FIG. 1
As shown in FIG. 1, the exhaust passage 1 is provided with a bypass passage 4 that bypasses the lean NO _x catalyst 3, and the exhaust passage 1
Of the branch portion of the bypass passage 4, the exhaust is interposed path switching valve 5 for switching the passage and a state passing through the state the bypass passage 4 passing through the lean NO _X catalyst 3, the exhaust gas temperature by the exhaust temperature sensor 6 There is a valve in which the passage switching valve 5 is switched to the side of the bypass passage 4 when the temperature is high so that the exhaust gas flows through the bypass passage 4.

[0005]

The conventional exhaust gas purification apparatus for a lean burn engine as described above is shown in FIG.
In order to prevent degradation of the lean NO _X catalyst 3 as in the technique shown in other than if no flow exhaust in lean NO _X catalyst 3 is configured to pass the entire amount of the basic exhaust lean NO _X catalyst 3, an internal combustion although nO _X conversion performance of the lean nO _X catalyst is required 80% or more of high performance to accommodate a wide operating region of the engine, the lean nO _X catalyst such high performance has not been realized at present Nevertheless, even if the conversion efficiency of the lean NO _x catalyst is low, lean NO
_{Since the X} catalyst had to be used, it was difficult to obtain high exhaust purification performance.

In view of the above conventional problems, it is an object of the present invention to obtain a high exhaust gas purification performance even under conditions where the lean NO _x catalyst conversion efficiency is low.

[0007]

Therefore, as shown in FIG. 1, an exhaust gas purification apparatus for an internal combustion engine according to the present invention has a nitrogen oxide in the exhaust passage in an oxidizing atmosphere in the presence of unburned hydrocarbons. A bypass passage for bypassing the catalyst is provided in the exhaust passage, and a flow rate control means for controlling the flow rate of the exhaust gas flowing through the bypass passage is provided in the exhaust passage while the exhaust passage is provided. An exhaust state detecting means for forming a state of, and a calculating means for calculating an exhaust flow rate to the catalyst that minimizes exhaust emission at the exhaust passage outlet under the exhaust state detected by the exhaust state detecting means,
And a control device that controls the flow rate control means based on the calculated exhaust flow rate.

[0008]

In such a structure, the flow rate control means is controlled so that the exhaust flow rate to the catalyst is such that the exhaust emission at the outlet of the exhaust passage becomes the minimum under the detected exhaust gas condition, and the nitrogen oxides in the lean burn region are controlled. Even if a catalyst with low conversion performance is used, the total nitrogen oxide conversion performance can be optimally demonstrated.
High exhaust purification performance can be obtained.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. FIG. 2 shows the system configuration of an embodiment of the present invention. In this figure, in an exhaust passage 11 of an internal combustion engine (hereinafter referred to as an engine) 10, a catalyst for purifying exhaust gas in a lean combustion region of the engine 10, that is, in an oxidizing atmosphere,
A lean NO _X catalyst 12 for reducing nitrogen oxides in the exhaust under the presence unburned hydrocarbons, positioned downstream of the catalyst 12, a three-way catalyst 13 for exhaust gas purification at the stoichiometric air-fuel ratio range of the engine 10 Are inserted in series.

In the exhaust passage 11, the lean NO _{x is provided.}
The catalyst 12 branches from the upstream side to the downstream side and the three-way catalyst 1
A bypass passage 14 that merges with the upstream side of 3 is provided. A flow rate control valve 15 as a flow rate control means for controlling the flow rate of the exhaust gas flowing through the passage 14 is interposed in the bypass passage 14. Also, the lean N of the exhaust passage 11
O _X on the upstream side of the catalyst 12, the air-fuel ratio sensor 16 for detecting an air-fuel ratio, the exhaust gas temperature sensor 17 for detecting the temperature of the exhaust gas flowing into the catalyst 12 is disposed respectively upstream position and a downstream position.

Furthermore, on the downstream side of the three-way catalyst 13 in the exhaust passage 11, exhaust components (in particular NO _X concentration) at the outlet of catalyst 13
An exhaust sensor 18 for detecting the is provided. On the other hand, a throttle valve 20 is provided in the intake passage 19 of the engine 10, and the opening of the throttle valve 20 is detected by a throttle valve opening sensor 21. A fuel injection valve 22 is provided at an intake port connected to each cylinder of the engine 10. Also, a water temperature sensor 23 for detecting the cooling water temperature of the engine
Also, an engine rotation speed sensor and a boost sensor (not shown) are provided.

The engine 10 is controlled by an engine control unit 24. The flow control valve 15 is controlled by a flow control valve control unit 25. That is, the outputs from the various sensors are
It is input to the engine control unit 24, various calculations are performed by the CPU in the control unit 24, and the output thereof controls various devices on the engine side such as the fuel injection valve 22. Also, the engine control unit 24
From this, the output of a specific sensor among various sensors is input to the flow rate control valve control unit 25, various calculations are performed in the CPU in the control unit 25, and the output controls the flow rate control valve 15.

Next, the operation of this structure will be described. As shown in FIG. 3, the NO _X conversion efficiency of the lean NO _X catalyst 12 tends to decrease as the exhaust gas flow rate increases. Further, the NO _x conversion efficiency also changes with the exhaust temperature, as shown in FIG. Therefore, the portion with high NO _x conversion efficiency is limited to the low engine speed operation region where the displacement is small and there is time to convert NO _x .

Therefore, in a region where the engine displacement is large, such as in a high engine speed region, lean NO _{X is produced under the} condition of low NO _X conversion efficiency.
You are using a catalyst. In this configuration, on the basis of the NO _X conversion efficiency of the lean NO _X catalyst for exhaust gas flow rate and exhaust temperature given in advance based on the condition of the exhaust gas flowing into the operating state and the catalyst 12 of the engine 10, the lean NO _X catalyst A flow rate control valve 15 adjusts and controls the flow rate of exhaust gas flowing through a bypass passage 14 that bypasses the valve 12.

A method of controlling the exhaust gas flow amount bypassing the bypass passage 14 will be described with reference to the flowchart of FIG. That is, in step 1 (abbreviated as S1 in the figure; the same applies hereinafter), various parameters for determining the operating state of the engine 10, that is, the lean operation of the engine 10, such as engine speed, boost, air-fuel ratio, and exhaust temperature, are determined. The parameters and the parameters necessary for calculating the exhaust flow rate V ₁ in step 3 described later are read. In step 2, the engine 10
Determines whether the engine is lean, and the engine 10
If it is determined that the vehicle is running lean, step 3
Then, the exhaust flow rate V ₁ is calculated based on the operating condition of the engine 10 and the engine speed, boost, and exhaust temperature, which are the conditions of the exhaust flowing into the catalyst 12.

This calculation is performed based on the following equation.

[0017]

[Equation 1]

In step 4, the NO _x conversion efficiency η ₁ of the lean NO _x catalyst is read out or calculated from the map shown in FIG. 6 based on the exhaust temperature and the exhaust flow rate. The relationship between the exhaust temperature and the exhaust flow rate with respect to the NO _x conversion efficiency has the tendency shown in FIG. 6, but to be exact, it differs for each catalyst type and engine. In step 5, based on the NO _x conversion efficiency η ₁ obtained in step 4, the condition that the total emission after passing through the catalyst portion is minimized, that is, the optimum value V of the exhaust flow rate passing through the catalyst portion is obtained.

That is, in the flowchart of FIG. 7 showing the routine for obtaining the optimum value V of the exhaust flow rate, step 21
Then, the exhaust flow rate V which is smaller than the current exhaust flow rate V ₁ by α
The conversion efficiency (η) when = (V ₁ -α) is read from the map of FIG. 7. Then, in step 22, this temporary V and η
The following formula is calculated based on V · η-V ₁ · η ₁ where V ₁ · η ₁ is NO when exhaust gas flows through the catalyst
_A constant for the conversion amount of _X (NO _X under the operating conditions at this time)
Value obtained by multiplying the density), are V · eta is a value in which a part of the exhaust gas is multiplied by the same constant conversion amount of the NO _X when the flowing catalyst.

[0020] Then, in step 22, further, takes the value of _{(V · η-V 1 ·} η 1) is determined by changing the value of V when the maximum alpha (see FIG. 8). In step 6,
Obtaining the exhaust flow rate V ₁ -V that bypasses the catalyst 12,
In the next step 7, the target opening degree T _v of the flow rate control valve 15 corresponding to the exhaust flow rate V ₁ -V is obtained.

In this case, the target opening of the flow control valve 15
T_vAnd bypass exhaust flow rate V₁The relationship of -V depends on the operating conditions.
It is affected by the exhaust flow rate and has the characteristics shown in Fig. 9.
It Therefore, from the map of this FIG._vAsk for.
In step 8, the current opening T of the flow control valve 15 is read.
After taking into account, in step 9 the target opening T_vFlow control to
It is determined whether the valve position of the valve 15 can be changed.
U That is, 0 <T_v<T_maxTo determine whether or not
U 0 <T_v<T _maxAnd the target valve opening T_vFlow to
When the valve body position of the quantity control valve 15 can be changed
Goes to step 10 to open the flow control valve 15.
Is the target opening T_vChange to. 0 <T_v<T_maxThen
The target valve opening T_vBody position of the flow control valve 15 to the
If the change of is not possible, go to step 11.

In step 11, the flow control valve 15
The opening was set to a limit value close to the target value (fully closed or fully open)
In case of NO_XIf a conversion is obtained, that is,
Η ′ for exhaust flow rate V ′ when fully closed or fully opened
Calculated from (V · η-V ₁・ Η₁) Is calculated
, (V '· η'-V₁・ Η₁) Is 0 or more
Is determined. V '・ η'-V₁・ Η₁If ≧ 0,
Proceed to step 12 and set the opening of the flow control valve 15 to full.
Change to closed or fully open. V '・ η'-V₁・ Η ₁<0
If so, do not change the current opening of the flow control valve 15.
Yes.

In the above-mentioned flowchart, steps 1 and 3 correspond to the exhaust state detecting means for detecting the exhaust state of the present invention, and step 5 is the exhaust passage under the exhaust state detected by the exhaust state detecting means. It corresponds to a calculation unit that calculates the exhaust flow rate to the catalyst that minimizes the exhaust emission at the outlet, and corresponds to a control device that controls the flow rate control unit based on the calculated exhaust flow rate in step 10.

The control of the flow control valve 15 based on the above flow chart is the most basic control. As a method for further improving the control accuracy, the flow control valve calculated in accordance with the engine operating condition of step 1 is used. The opening of valve 15 is learned, and the flow control valve 15 is calculated from the calculation result.
It is possible to control the opening degree of the flow rate control valve 15 using the learning value until the opening degree is determined and the opening degree of the flow rate control valve 15 is controlled.

According to the exhaust emission control device having such a configuration,
The operating conditions of the gin 10 and the conditions of the exhaust gas flowing into the catalyst 12
Based on the given exhaust flow rate and exhaust temperature
Lean NO_XNO of catalyst_XLean based on conversion efficiency
NO_XExhaust gas flowing through the bypass passage 14 that bypasses the catalyst 12
Adjust the flow rate with the flow rate control valve 15 to obtain a lean NO _X
NO that changes depending on the exhaust conditions that flow into the catalyst 12_XTurning
Conversion efficiency minimizes exhaust emissions at the outlet of the catalyst
Therefore, NO in the lean burn region_XLow conversion performance
Total NO even with catalyst_XBest conversion performance
It is possible to obtain high exhaust purification performance.

Although the present invention has been described with reference to a specific embodiment, the present invention is not limited to this, and a person skilled in the art can obtain the claims attached to the present invention. It should be noted that various changes and modifications can be made without departing from the scope.

[0027]

As described above, according to the present invention, the exhaust passage is provided with a catalyst for reducing nitrogen oxide in the exhaust gas in the presence of unburned hydrocarbons in an oxidizing atmosphere, and the bypass bypasses the catalyst. Since the flow rate control means disposed in the passage is configured to control the flow rate of the exhaust gas to the catalyst that minimizes the exhaust emission at the exhaust passage outlet under the detected exhaust gas condition, the conversion efficiency of the catalyst is reduced. It is of great utility because a high exhaust gas purification performance can be obtained even under low conditions.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an exhaust gas purification device for an internal combustion engine according to the present invention.

FIG. 2 is a system diagram showing an embodiment of the same device.

FIG. 3 is a characteristic diagram showing the relationship between exhaust flow rate and NO _X conversion efficiency.

FIG. 4 is a characteristic diagram showing a relationship between exhaust temperature and NO _X conversion efficiency.

FIG. 5 is a flowchart illustrating flow rate control according to the above embodiment.

FIG. 6 is a characteristic diagram showing the relationship between the exhaust flow rate and the exhaust temperature and the NO _x conversion efficiency.

FIG. 7 is a flowchart showing a routine for obtaining an optimum value of exhaust flow rate.

FIG. 8 is a characteristic diagram showing the relationship between exhaust flow rate and NO _X conversion efficiency.

FIG. 9 is a characteristic diagram showing the relationship between valve opening and valve exhaust flow rate.

FIG. 10 is a system diagram showing a conventional exhaust emission control device.

FIG. 11 is a system diagram showing a conventional exhaust emission control device.

[Explanation of symbols]

10 engine 11 exhaust passage 12 lean NO _X catalyst 14 bypass passage 15 flow control valve 16 an air-fuel ratio sensor 17 exhaust gas temperature sensor 18 exhaust sensor 25 flow rate control valve control unit

Claims (1)

[Claims] 1. An exhaust passage is provided with a catalyst for reducing nitrogen oxide in exhaust gas in the presence of unburned hydrocarbon in an oxidizing atmosphere,
A bypass passage for bypassing the catalyst is provided in the exhaust passage, and a flow rate control means for controlling the flow rate of the exhaust gas flowing through the bypass passage is provided, while an exhaust state detecting means for detecting a state of the exhaust gas is provided. Calculating means for calculating an exhaust flow rate to the catalyst that minimizes exhaust emission at the exhaust passage outlet under the exhaust gas state detected by the exhaust gas state detecting means; and the flow rate control based on the calculated exhaust flow rate. An exhaust emission control device for an internal combustion engine, comprising: a control device that controls the means.