JP3899775B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine mounted on an automobile or the like.
[0002]
[Prior art]
In order to achieve both purification of reducing agent components (HC, CO) in exhaust gas discharged immediately after engine startup and purification of NOx in exhaust gas discharged when the engine is operated at a lean air-fuel ratio, 2. Description of the Related Art An exhaust emission control device is known in which a startup catalyst that quickly reaches an activation temperature after startup is arranged on the upstream side, and a NOx absorbent that absorbs NOx in the exhaust gas with lean air-fuel ratio is arranged on the downstream side of the startup catalyst. (For example, JP-A-11-62563).
[0003]
[Problems to be solved by the invention]
As a starting catalyst used in such an apparatus, a catalyst in which a platinum-based noble metal is supported on a honeycomb-like support coated with alumina is generally used.
[0004]
Such a catalyst has a function of trapping and holding oxygen in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is the lean air-fuel ratio, and this oxygen holding function stabilizes the oxidation function of the catalyst. However, this oxygen retention function is an obstacle when trying to supply the reducing agent component to the NOx absorbent disposed on the downstream side.
[0005]
That is, when a certain amount of NOx is absorbed by the NOx absorbent by continuing the lean operation, the reducing agent component is supplied to the NOx absorbent to reduce the absorbed NOx and regenerate the NOx absorbent. The starting catalyst holds a large amount of oxygen, and the starting catalyst uses this retained oxygen to oxidize the reducing agent component to be supplied to the NOx absorbent. The reducing agent component does not reach the NOx absorbent until. Therefore, there is a problem that the time required for regeneration of the NOx absorbent becomes longer, and the fuel consumption is deteriorated by the amount of time for which the lean air-fuel ratio operation can be performed.
[0006]
[Means for Solving the Problems]
In order to solve this problem, in the present invention,
It is arranged in the exhaust passage of the engine and has a function of trapping and holding oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is a lean air-fuel ratio, and the amount of oxygen that can be held becomes the oxygen concentration of the atmosphere An upstream catalyst that changes accordingly,
This is disposed in the exhaust passage downstream of the upstream catalyst, traps and holds NOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio, and the air-fuel ratio of the inflowing exhaust gas is rich. A downstream catalyst having a function of reducing NOx retained when the air-fuel ratio is maintained 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 is injected into the combustion chamber of the engine with a fuel equivalent to the stoichiometric air-fuel ratio as the entire combustion chamber in the latter half of the compression stroke. Then, an air-fuel mixture layer richer than the stoichiometric air-fuel ratio is formed in the vicinity of the spark plug, a lean air-fuel mixture layer with almost no fuel is formed around it, and combustion is performed in the form of the air-fuel mixture. In addition to leaving the oxygen in the bed unreacted and additionally injecting fuel during the expansion stroke to the exhaust stroke, the amount of oxygen that can be held by the upstream catalyst with respect to the oxygen concentration is that the air-fuel ratio is rich and the oxygen concentration is the oxygen concentration. Control means for controlling to a state higher than a predetermined oxygen concentration reaching saturation;
It was set as the structure provided with.
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]
Operation and effect of the invention
When trying to make the air-fuel ratio of ordinary exhaust gas rich, the oxygen concentration of the exhaust gas becomes extremely low. Therefore, when such exhaust gas flows into the upstream catalyst, all that was held in the upstream catalyst As a result, the above-described problems occur.
[0008]
On the other hand, by devising fuel injection when the air-fuel ratio of the exhaust gas is made rich, it is possible to keep the oxygen concentration relatively high while making the air-fuel ratio rich air-fuel ratio. If it flows into the 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.
[0009]
Even if a rich air-fuel ratio and high oxygen concentration exhaust gas flows into the upstream side catalyst, oxygen in the exhaust gas reacts with the reducing agent component in the catalyst, so the oxygen concentration of the exhaust gas becomes closer to the downstream side of the catalyst. Will drop. Therefore, the release of retained oxygen cannot be avoided on the downstream side of the catalyst. That is, even with the configuration of the present invention, it is impossible not to release oxygen held in the upstream catalyst at all, and immediately after the start of regeneration control, the reducing agent component does not reach the downstream catalyst. Will occur. However, such time can be shortened as compared with the regeneration of the downstream catalyst by the exhaust gas having a rich air-fuel ratio and an oxygen concentration of 0.
[0010]
As used herein, “air-fuel ratio of exhaust gas” means the ratio of the amount of intake air and 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 stroke of the engine in addition to normal fuel supply, or additional fuel is supplied to the exhaust passage, intake air amount / (normal fuel supply amount + It is the air-fuel ratio determined by the additional fuel supply amount).
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
(Embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 2 shows the configuration of the embodiment of the present invention. An upstream catalyst 3 is provided in the exhaust pipe 2 of the engine 1, and a downstream catalyst 4 is installed on the downstream side thereof. A fuel injection valve 6 is installed in the intake pipe 5, and forms an air-fuel mixture with intake air and burns in the engine 1. The 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. 9) The basic fuel injection pulse width is calculated at 9, and fuel is injected based on the result of various corrections such as the target air-fuel ratio. The ECU 9 also receives the accelerator opening, calculates the engine load L therefrom, and drives the electric throttle chamber 10.
[0012]
As the upstream catalyst 3, for example, a honeycomb carrier coated with alumina is supported with a noble metal such as platinum Pt, palladium Pd, rhodium Rb and cerium Ce, and when the air-fuel ratio of the exhaust gas is a lean air-fuel ratio. In addition, it has a function of trapping and holding oxygen in the exhaust gas and oxidizing it with oxygen that has held the reducing agent component in the exhaust gas flowing in when the air-fuel ratio is rich (oxygen holding function).
[0013]
The amount of oxygen that can be maintained varies depending on the oxygen concentration of the atmosphere. The higher the concentration, the greater the amount that can be retained, but saturation reaches a certain level (A point) or more.
[0014]
The downstream side catalyst 4 is typically represented by cesium Cs, an alkaline earth represented by barium Ba, based on a catalyst in which a noble metal such as platinum Pt, palladium Pd and rhodium Rh is supported on a honeycomb carrier coated with alumina. that it is intended to be configured by loading at least one component selected from alkali metals, the downstream catalyst 4 is held by traps NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is lean In addition, it has a function of reducing and purifying NOx retained by the reducing components (HC, CO, H2) in the exhaust gas when the air-fuel ratio is rich.
[0015]
This operation will be described with reference to the flowcharts of FIGS. FIG. 3 illustrates the setting of the target air-fuel ratio and the operation of the regeneration process for the downstream catalyst 4. This routine is executed every 10 msec, for example.
[0016]
In S1, it is determined whether or not the rich spike flag FRS is 1. FRS is set to FRS = 1 when it is determined that NOx of a predetermined amount or more is held in the downstream catalyst 4 and a rich air-fuel ratio is required to reduce and purify NOx, and the target air-fuel ratio is set during the period of FRS = 1. It is what makes it rich. Therefore, when FRS ≠ 1 (FRS = 0), this 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, and the target air-fuel ratio TFBYA is obtained from the engine load L and the engine speed Ne. (See Figure 4 for the map)
[0017]
In S3, it is determined whether the operation is lean. That is, it is determined whether TFBYA obtained in S2 is less than 1.0.
[0018]
When the engine is operated lean, the NOx discharged from the engine is trapped by the downstream side catalyst 4. Therefore , in S4 to S7 , the NOx retention amount is estimated and the necessity of NOx reduction processing by rich spike is determined. To do.
[0019]
When it is determined in S3 that the engine is in the lean operation, the process proceeds to S4, and the NOx emission concentration CNOx is searched from the map from the engine load L and the engine speed Ne. (See Figure 5 for the map)
[0020]
This map is obtained in advance by experiments. In S5, the NOx retention amount TNOx in the downstream side catalyst 4 is calculated. This is obtained by multiplying the previous TNOx by the NOx exhaust concentration CNOx and the intake air amount Qa, and further adding a value obtained by multiplying by a constant K1. .
K1 is a constant for unit conversion.
[0021]
In S6, it is determined whether or not the NOx holding amount TNOx is larger than the predetermined amount TNOx0. This predetermined amount TNOx0 is a ratio (for example, about 1/2) of the saturated NOx retention amount of the downstream side catalyst 4 obtained experimentally in advance, and it is also effective to reduce it as the catalyst deteriorates. . When TNOx> TNOx0, the routine proceeds to S7, where the rich spike flag FRS is set to 1, and the process proceeds to regeneration processing of the downstream catalyst 4 by rich spike.
Since FRS = 1, the process proceeds to S8 by the determination of S1.
[0022]
In S8, the estimated value of the catalyst temperature Tcat of the downstream side catalyst 4 is searched from the map from the engine load L and the engine speed Ne. (See FIG. 6 for the map) The NOx emission concentration RNOx in S9 is obtained. Since the NOx release concentration RNOx during the rich spike changes depending on the catalyst temperature Tcat and the NOx retention amount TNOx at that time, the NOx release concentration RNOx is searched from a map obtained in advance through experiments. (See Figure 7 for the map)
[0023]
In S10, the air-fuel ratio TFBYARNOx necessary for this 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 air-fuel ratio required for reduction purification of released NOx. K3 is a constant for converting the NOx concentration into an air-fuel ratio. In S11, in consideration of the efficiency used for NOx reduction purification of the supplied reducing agent, the ratio of the required air-fuel ratio calculated in S10 to supply an excessive reducing component (1.2 in the flowchart) The air-fuel ratio TFBYA multiplied by the description) is calculated. Next, in FIG. S12, the change in the NOx retention amount TNOx due to the release of NOx is calculated. The TNOx is obtained by subtracting a value obtained by multiplying the released NOx concentration RNOx obtained in S9 by the intake air amount Qa and further multiplying by a constant K1 from the previous TNOx. In S13, it is determined whether or not the calculated TNOx is lower than 0. If it is lower, the rich spike is terminated. In this case, by setting TNOx = 0 in S14, the value of TNOx is limited so as not to be lower than zero. In S15, the rich spike flag is returned to 0 and the rich spike is terminated.
[0024]
In the routine described above, TFBYA during the rich spike and the lean operation is obtained. 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.
[0025]
In S16, the required torque TTC is calculated from the accelerator position Aps from the map, and the intake air amount Q for generating the required torque TTC is calculated in consideration of the target air-fuel ratio TFBYA and the engine efficiency ITA. Since the engine efficiency ITA and the fuel injection timing are different during the rich spike, it is determined whether or not the rich spike flag FRS is 1 in S17. If it is determined that FRS ≠ 1 (FSR = 0), the process proceeds to S18, and TFBYA1 converted to combustion torque and TFBYA2 not converted to torque are obtained from TFBYA obtained in the flowchart of FIG. For operation, TFBYA1 = TFBYA and TFBYA2 = 0. In accordance with TFBYA1 and the engine speed Ne, the engine efficiency ITA is calculated from the map in S19, and the fuel injection timing IT1 is calculated from the map in S20.
[0026]
When the rich spike is executed with FRS = 1 in S17, the process proceeds to S21, and from TFBYA obtained in the flowchart of FIG. 3, TFBYA1 for the fuel injected in the latter half of the compression stroke and the fuel for the injection injected in the expansion to exhaust stroke TFBYA2 is calculated.
[0027]
Here, TFBYA1 is set to 1, which is a theoretical air-fuel ratio equivalent value, and TFBYA-1 is set to TFBYA2. When fuel corresponding to the stoichiometric air-fuel ratio is injected in the intake stroke, a homogeneous air-fuel mixture is formed in the combustion chamber, and the oxygen concentration of the exhaust gas after the air-fuel mixture burns 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, it is possible to form an air-fuel mixture layer that is richer than the stoichiometric air-fuel ratio in the vicinity of the spark plug, and to form a layer with almost no fuel around it. is there. When combustion is performed in such an air-fuel mixture form, oxygen in the lean air-fuel mixture can be left unreacted, so that the oxygen concentration of the exhaust gas is kept relatively high while the air-fuel ratio of the exhaust gas is made the stoichiometric air-fuel ratio. be able to. Furthermore, if additional fuel is injected during the expansion to exhaust stroke after combustion is performed in such a mixture, the oxygen concentration of the exhaust gas is compared while making the air-fuel ratio of the exhaust gas richer than the stoichiometric air-fuel ratio. Can be kept high.
[0028]
In S22, the engine efficiency ITA is calculated in consideration of the fact that not all of the fuel corresponding to the theoretical air-fuel ratio injected in the latter half of the compression stroke is burned and becomes the engine torque. In S23, the fuel injection in the latter half of the compression stroke is calculated. The timing IT1 and the fuel injection timing IT2 of the expansion to exhaust stroke are calculated.
[0029]
Next, in S24, the intake air amount Q required to generate the required torque TTC is obtained by the equation (1), and the electric throttle is driven so that the intake air amount Q is obtained.
Q = TTC / TFBYA1 / ITA (1)
[0030]
In S25, the fuel amount Ti 1 converted into torque and Ti 2 not converted into torque are calculated by multiplying the basic fuel injection pulse width Tp calculated based on the intake air amount Qa / rotational speed Ne by TFBYA, and the injection. To do. That is, the fuel injection pulse width Ti is calculated by the following equation.
Tin = Tp · TFBYAn + Ts
Here, Ts is an invalid injection pulse width. (N is 1 and 2)
[0031]
The embodiment of the present invention has been described in detail with reference to the drawings, but the specific configuration is not limited to this embodiment, and even if there is a design change without departing from the gist of the present invention. It is included in the present invention.
[Brief description of the drawings]
FIG. 1 is a graph showing the oxygen storage amount of a catalyst.
FIG. 2 is a diagram showing an embodiment of the present invention.
FIG. 3 is a first flowchart of the present invention.
FIG. 4 is a view 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 diagram showing a NOx purification degree map.
FIG. 8 is a second flowchart of the present invention.
[Explanation 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

Claims (1)

It is arranged in the exhaust passage of the engine and has a function of trapping and holding oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is a lean air-fuel ratio, and the amount of oxygen that can be held becomes the oxygen concentration of the atmosphere An upstream catalyst that changes accordingly,
This is disposed in the exhaust passage downstream of the upstream catalyst, traps and holds NOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio, and the air-fuel ratio of the inflowing exhaust gas is rich. A downstream catalyst having a function of reducing NOx retained when the air-fuel ratio is maintained 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 is injected into the combustion chamber of the engine with a fuel equivalent to the stoichiometric air-fuel ratio as the entire combustion chamber in the latter half of the compression stroke. Then, an air-fuel mixture layer richer than the stoichiometric air-fuel ratio is formed in the vicinity of the spark plug, a lean air-fuel mixture layer with almost no fuel is formed around it, and combustion is performed in the form of the air-fuel mixture. In addition to leaving the oxygen in the bed unreacted and additionally injecting fuel during the expansion stroke to the exhaust stroke, the amount of oxygen that can be held by the upstream catalyst with respect to the oxygen concentration is that the air-fuel ratio is rich and the oxygen concentration is the oxygen concentration. Control means for controlling to a state higher than a predetermined oxygen concentration reaching saturation;
An exhaust emission control device for an internal combustion engine, comprising:

Images