JP2605579C - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [0001]     [Industrial applications]   The present invention relates to an exhaust gas purification device for an internal combustion engine, and more particularly, to NOx in exhaust gas._xEfficiently
The present invention relates to a removable exhaust gas purification device for an internal combustion engine. [0002]     [Prior art]   When the air-fuel ratio of the inflowing exhaust gas is
NO when_xAbsorbed when the oxygen concentration in the inflowing exhaust gas decreased_x
Releases NO_xNO in exhaust by placing absorbent_xTo absorb NO_xMachine after absorption
Increase the amount of fuel supplied to the_xRich exhaust air-fuel ratio flowing into the absorbent
By doing so, the NO_xNO absorbed from absorbent_xReleased and released
NO_xInternal combustion engine that purifies air by reducing components such as unburned HC and CO in exhaust gas
Of the exhaust gas purifying device has already been proposed by the present applicant (Japanese Patent Application No. 4-24311).
No. 3). [0003]     [Problems to be solved by the invention]   As described above, NO_xNOx in the exhaust gas when the exhaust air-fuel ratio is lean_xSuck
Take it. However, as described later,_xAbsorbent is NO_xTo absorb the exhaust during
Is required to be present in the exhaust air-fuel ratio is smaller than the stoichiometric air-fuel ratio.
If the oxygen concentration in the exhaust gas is low, the above NO_xWith absorbent
Is NO in exhaust_xCan not be absorbed. [0004]   For this reason, for engines that perform combustion with a lean air-fuel ratio in most operating ranges,
NO_xNO in exhaust gas using absorbent_xCan be almost completely removed
Periodically repeats combustion at rich and lean air-fuel ratios around the stoichiometric air-fuel ratio
The above-mentioned NO_xWhen using an absorbent, the air-fuel ratio is near the stoichiometric air-fuel ratio
NO during period_xAbsorbent is NO_xDoes not absorb NO_xProblem of insufficient purification
There is. [0005]   On the other hand, near the stoichiometric air-fuel ratio, combustion at the rich-side and lean-side air-fuel ratios is repeated periodically.
NO in the exhaust when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the engine is the stoichiometric air-fuel ratio
_xA three-way catalyst that simultaneously performs the reduction of CO and the oxidation of HC and CO
An exhaust gas purification device for an internal combustion engine that purifies three components at the same time
It has been. [0006]   However, the three-way catalyst removes the above three components when the exhaust air-fuel ratio is near the stoichiometric air-fuel ratio.
Although purification can be performed with high efficiency, the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio.
NO_xBecause of the characteristic that the purification rate of the fuel rapidly decreases, it is rich near the stoichiometric air-fuel ratio.
In an engine that alternately repeats combustion at the air-fuel ratio on the lean side and lean side, purification on the lean side
NO as a whole due to lower rate_xPurification tends to be insufficient. [0007]   On the other hand, NO on the lean side of the three-way catalyst_xExhaust passages to compensate for the lower purification rate
NO on the lean side as a whole by arranging a plurality of three-way catalysts in series_xImprove purification rate
Although it is conceivable to make such a configuration, the overall NO
_xThe purification rate hardly improves. This is NO_xNO if concentration is low_xMolecule returned on catalyst
Is reduced, the three-way catalyst NO_xPurification rate is NO in inflow exhaust_xconcentration
This is because if the value is low, it will be greatly reduced. [0008]   Therefore, when a plurality of three-way catalysts are arranged in series in the exhaust passage, the first three-way catalyst
Medium NO_xIs purified and NO in exhaust_xThe concentration decreases, but the second and subsequent
NO in three-way catalyst_xPurification rate has declined significantly, and overall purification rate is the first three yuan
The value becomes close to the purification rate of the catalyst. For example, using the same three-way catalyst in the exhaust passage
When the exhaust gas is flown by arranging two catalysts, the first three-way catalyst has about 95% of NO.
_xAlthough the purification rate can be obtained, the purification rate of the two-way three-way catalyst is significantly reduced.
As a result, the overall purification rate of both three-way catalysts is about 96%.
I can only do that. Therefore, even if a plurality of three-way catalysts are arranged in series in the exhaust passage, NO_x
It is not possible to obtain a significant improvement in the purification rate. [0009]   The present invention has been made in view of the above problems, and has been described in connection with the relationship between the rich side and the lean side with respect to the stoichiometric air-fuel ratio.
NO in exhaust gas of an internal combustion engine that alternately repeats combustion at the air-fuel ratio_xTo effectively purify
It is an object of the present invention to provide an exhaust gas purification device that can perform the following. [0010]     [Means for Solving the Problems]   According to the present invention, the combustion air-fuel ratio of the engine is set to the rich air-fuel ratio side around the stoichiometric air-fuel ratio. Internal combustion engine equipped with an air-fuel ratio control device that alternately changes between the air-fuel ratio and the lean air-fuel ratio periodically
NO in the exhaust when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the Seki is the stoichiometric air-fuel ratio_xof
A three-way catalyst for simultaneously performing reduction and oxidation of HC and CO is provided, and the three-way catalyst is
When the air-fuel ratio of the exhaust gas flowing into the exhaust passage downstream of the medium is lean, the NO
_xNO is absorbed when the oxygen concentration of the exhaust gas that flows in decreases._xReleases NO_x
There is provided an exhaust gas purification device for an internal combustion engine, wherein an exhaust agent is disposed. [0011]   Further, according to the present invention, the air-fuel ratio control device_xDetects the temperature of the absorbent
And the detected NO_xThe stoichiometric air-fuel ratio of the combustion air-fuel ratio of the engine according to the absorbent temperature
Means for changing a period for maintaining the air-fuel ratio at a richer air-fuel ratio side.
An apparatus is provided.   Further, according to the present invention, the air-fuel ratio control device allows a part of the engine exhaust to pass through the engine intake.
Means for controlling the flow of the recirculated exhaust gas, and means for controlling the flow rate of the recirculated exhaust gas.
The engine air-fuel ratio is set to the rich air-fuel ratio side and the lean air-fuel ratio side by periodically controlling the amount.
The present invention provides an exhaust gas purification device for an internal combustion engine, which periodically and alternately changes. [0012]   Further, according to the present invention, the NO_xDetects air-fuel ratio of exhaust gas flowing into absorbent
Means and the NO_xMeans for introducing secondary air into the absorbent, wherein the NO_xSucking
When the air-fuel ratio of the exhaust gas flowing into the absorbent is lean, the NO_xSecondary air in absorbent
The exhaust gas purifying apparatus according to claim 1, wherein the exhaust gas purifying apparatus is introduced. [0013]     [Action]   When the combustion air-fuel ratio of the engine is controlled leaner than the stoichiometric air-fuel ratio, the H
C and CO components were purified by the upstream three-way catalyst and were not purified by the three-way catalyst
NO_xThe component is NO on the downstream side_xAbsorbed by absorbent. In addition, the combustion air-fuel ratio of the engine
When the air-fuel ratio is controlled to the rich side, the NO in the exhaust_xIngredients are upstream ternary contact
NO on the downstream side without being purified by the three-way catalyst_xHC flowing into the absorbent,
NO due to CO component_xNO absorbed from absorbent_xIs released and reduced and purified
It is. For this reason, the exhaust air-fuel ratio alternates between the stoichiometric air-fuel ratio on the rich side and the lean side. NO overall_xPurification rate is maintained. [0014]   In addition, the above NO_xNO from absorbent_xThe time required for release and reduction purification is NO_x
Since the exhaust gas air-fuel ratio varies depending on the temperature of the absorbent, the period for controlling the exhaust air-fuel ratio to the rich side is uniform.
NO if set to_xNO absorbed from absorbent_xMay not be released in full,
NO over time_xAbsorbent NO_xAbsorption capacity may be reduced. Therefore, the machine
No period for controlling the combustion air-fuel ratio to the rich side_xChange according to the temperature of the absorbent
NO regardless of temperature change_xNO from absorbent_xRelease and reduction purification
Is completely done and NO_xThe absorption capacity of the absorbent is maintained. [0015]   NO_xAbsorbent NO_xIn order to make the most of the absorption capacity, the engine combustion air
It is desirable to set a relatively long switching cycle between the rich side and the lean side of the fuel ratio.
However, when the air-fuel ratio is switched by controlling the fuel supply amount, the fluctuation of the output torque is relatively large.
Therefore, if the switching cycle is set long, fluctuations in engine output torque during air-fuel ratio switching
Drivability may worsen. On the other hand, changing the amount of exhaust gas recirculation
By controlling the engine combustion air-fuel ratio to rich and lean, the fuel supply to the engine can be reduced.
Since the combustion air-fuel ratio of the engine is controlled without changing, the output torque at the time of switching the air-fuel ratio
Is reduced. Therefore, changing the amount of exhaust gas recirculated to the engine
By controlling the ratio between the rich side and the lean side, the switching cycle of the engine combustion air-fuel ratio
NO_xLow deterioration in drivability due to output torque fluctuation when optimally set for purification
Is reduced. [0016]   Further, the engine combustion air-fuel ratio is controlled lean and NO_xExhaust air flowing into the absorbent
NO when the fuel ratio becomes lean_xBy supplying secondary air to the absorbent, N
O_xThe air-fuel ratio of the exhaust gas flowing into the absorbent is controlled to be larger (thinner). For this reason
NO when the engine air-fuel ratio is controlled to lean_xThe sorbent has enough oxygen
NO in the presence_xWill be absorbed and NO_xAbsorbent NO_xAbsorption capacity is sufficient
Be demonstrated. [0017]     【Example】   FIG. 1 is an overall schematic diagram showing an embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
You. In FIG. 1, 1 is an internal combustion engine main body, 2 is an intake passage of the engine 1, and 4 is a throttle.
Valve 3 indicates an air flow meter arranged in the intake passage 2. Air flow meter 3
Outputs a voltage signal proportional to the amount of intake air, and this output signal is output from the A / D converter 101.
Input / output interface 102 of an electronic control circuit (ECU) 10
Has been entered. In addition, the distributor 4 of the institution 1
Crank that generates a pulse signal for reference position detection every 720 ° in terms of the crank angle CA
The pulse signal for detecting the reference position is detected every 30 ° in terms of the angle sensor 5 and the crank angle CA.
A crank angle sensor 6 for generating a signal is provided.
, 6 are supplied to the input / output interface 102 of the control circuit 10.
I have. [0018]   Further, pressurized fuel is supplied from the fuel supply system to the intake port 2 to the intake port for each cylinder.
A fuel injection valve 7 for supplying is provided.   A voltage signal corresponding to the oxygen component concentration in the exhaust gas is supplied from the upstream side to the exhaust passage 14 of the engine 1.
O which generates a signal_TwoSensor 13, NO in exhaust at stoichiometric air-fuel ratio_xReduction and HC and CO
NOx in exhaust gas at lean air-fuel ratio_xAbsorb and exhaust
NO absorbed when the oxygen concentration decreases_xReleases NO_xThe absorbent 16 is in this order
Are located. [0019]   O_TwoThe sensor 13 detects the partial pressure of oxygen in the exhaust gas.
Since the oxygen concentration changes abruptly from the stoichiometric air-fuel ratio, O_TwoSen
As shown in FIG. 8, the output voltage VOM of the power supply 13 is richer and leaner than the stoichiometric air-fuel ratio.
And shows a characteristic that changes rapidly. For this reason, O_TwoAir fuel from output voltage of sensor 13
It can be accurately determined whether the ratio is richer or leaner than the stoichiometric air-fuel ratio. Book
In the embodiment, O_TwoThe output voltage of the sensor 13 is supplied to the ECU 1 via the A / D converter 101.
0 to the input / output interface 102 for controlling the engine combustion air-fuel ratio described later.
in use. [0020]   The electronic control circuit (ECU) 10 is configured as a microcomputer, for example.
In addition to the input / output interface 102, the AD converter, and the CPU 103, a ROM
104, a RAM 105, a clock generation circuit 107, and the like.   In the present embodiment, the ECU 10 controls the fuel injection valve 7 of the engine 1 to supply fuel to the engine.
Adjust the amount of ingredients. For this purpose, the input / output interface 102 of the ECU 10
In addition to the above, the engine cooling water temperature THW, NO_xThe exhaust temperature TEX at the outlet of the absorbent 16 is
From the cooling water temperature sensor 8 and the exhaust temperature sensor 9 via the A / D converter 101, respectively.
Has been entered. [0021]   Further, the ECU 10 controls the amount of fuel injection from the fuel injection valve 7 by controlling the amount of fuel injection.
A counter 108, a flip-flop 109, and a driving circuit 110.
You. That is, in a routine described later, the fuel injection amount (injection time) TAU is calculated.
When the fuel injection time TAU is preset in the down counter 108,
Is also set to the flip-flop 109. As a result, the drive circuit 110
The firing of the firing valve 7 is started. On the other hand, the down counter 108 outputs a clock signal (not shown).
) Is counted and the output terminal becomes “1” level at the end.
The drive circuit 110 stops the energization of the fuel injection valve 7 when the lop 109 is set.
That is, the fuel injection valve 7 is energized for the above-described fuel injection time TAU, and
An amount of fuel corresponding to the injection time TAU is sent to the fuel chamber of the engine body 1.
You. [0022]   Intake air amount data Q and cooling water temperature data THW of the air flow meter 3, exhaust gas
The temperature data TEX is subjected to A / D conversion performed for a predetermined time or at predetermined crank angles.
It is taken in by a routine and stored in a predetermined area of the RAM 105. That is, R
The data Q and THW in the AM 105 are updated every predetermined time. Also
, Rotation speed data N_eIs performed by the interrupt of the crank angle sensor 6 every 30 ° CA.
The calculated value is stored in a predetermined area of the RAM 105. [0023]   As the three-way catalyst 12, for example, platinum Pt, rhodium Rh on a carrier such as alumina is used.
There is used a ternary catalyst supporting a noble metal such as palladium Pd. Ternary
As shown in FIG. 2, when the air-fuel ratio of the inflowing exhaust gas is equal to the stoichiometric air-fuel ratio, as shown in FIG.
O_xReduction and HC and CO oxidation simultaneously to purify these three components simultaneously.
Can be. However, when the exhaust air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, NO_xNo purification
It has the characteristic that the purification rate of HC and CO components decreases when the conversion rate decreases and it becomes lean.
You. On the lean side, NO_xThe purification rate of the water drops sharply. Because of this
The engine combustion air-fuel ratio fluctuates to the lean side by the fuel injection routine described above and flows into the three-way catalyst.
When the air-fuel ratio of the exhaust gas becomes lean, the NO_xPurification becomes insufficient. [0024]   In the three-way catalyst, the NO in the exhaust_xWhen the concentration decreases, NO on the catalyst_xReduction
Opportunity for reaction decreases, NO_xPurification rate decreases. For this reason, as described above, the exhaust passage
When a plurality of three-way catalysts are arranged in series at the
_xHas been purified to some extent, and NO in the exhaust gas flowing into the three-way catalyst on the downstream side_xConcentration decreases
As a result, the purification rate of the downstream three-way catalyst decreases, and the exhaust air-fuel ratio becomes lean.
NO_xThe purification rate hardly improves. [0025]   In the present embodiment, the NO._xArrange absorbent 16
As a result, when the exhaust air-fuel ratio is lean,_xTo reduce the purification rate
High NO as a whole_xPurification rate is achieved.   NO used in this embodiment_xThe absorbent 16 is made of, for example, alumina as a carrier.
For example, potassium K, sodium Na, lithium Li, cesium Cs, etc.
Alkaline earths such as rukari metal, barium Ba, calcium Ca, lanthanum La
, At least one selected from rare earths such as yttrium Y and platinum Pt.
Such a noble metal is supported. This NO_xAbsorbent 16 is the air-fuel ratio of the inflowing exhaust
NO if is lean_xNO when the oxygen concentration decreases_xRelease
NO_xPerforms the absorption and release action. [0026]   Here, the above-mentioned exhaust air-fuel ratio is NO here._xExhaust passage upstream of absorbent 16 Of the amount of air supplied to the engine, engine combustion chamber, intake passage, etc., and fuel and reducing agent
Means the ratio of the sum to the amount of Therefore, NO_xDischarge upstream of absorbent 16
When no fuel or air is supplied to the air passage or intake passage, the exhaust air-fuel ratio
It becomes equal to the air-fuel ratio (air-fuel ratio in combustion in the engine combustion chamber). [0027]   In this embodiment, the engine combustion air-fuel ratio is controlled to the lean side and the NO_xFlow into absorbent 16
When the exhaust air-fuel ratio becomes lean, NO_xIs absorbent 16 purified by three-way catalyst?
NO in exhaust_xPerform absorption. Also, the engine combustion air-fuel ratio is controlled to the rich side.
, NO_xWhen the oxygen concentration of the exhaust gas flowing into the absorbent 16 decreases, NO_xAbsorbent 16 absorbs
NO collected_xRelease. [0028]   The detailed mechanism of this absorption / release action is not clear in some parts. Only
However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG.
It is. Next, regarding this mechanism, platinum Pt and barium Ba are supported on the carrier.
I will explain taking the case of letting it be an example, but other noble metals, alkali metals, alkaline earth,
The same mechanism is obtained even when rare earth elements are used. [0029]   That is, when the inflow exhaust gas becomes leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the inflow exhaust gas becomes
The oxygen O greatly increases as shown in FIG._TwoIs O_Two ^-Or O^2-In the form of
It adheres to the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas
O_Two ^-Or O^2-Reacts with NO_Two(2NO + O_Two→ 2NO_Two). Then generate
NO_TwoIs partially absorbed into the absorbent while being oxidized on the platinum Pt, resulting in oxidation burrs.
As shown in FIG. 3A, nitrate ions NO_Three ^-Sucking in the form of
Diffuses into absorbent. NO in this way_xIs NO_xIt is absorbed in the absorbent 16. [0030]   As described above, NO_xIn the absorbent 16, the air-fuel ratio of the inflow exhaust gas is lean and the exhaust gas
NO on the surface of platinum Pt as long as the oxygen concentration inside is high_TwoIs generated, and the NO_xabsorption
NO unless capacity is saturated_TwoIs absorbed in the absorbent and nitrate ion NO_Three ^-Is raw Is done. Therefore, NO_xAs long as the oxygen concentration in the inflowing exhaust gas is high,
NO in exhaust_xHigh NO regardless of concentration_xDemonstrates absorption capacity. [0031]   On the other hand, the oxygen concentration in the inflow exhaust gas decreases and NO_TwoNO decreases when the amount of_xSucking
In the absorbent 16, the reaction is in the opposite direction (NO)_Three ^-→ NO_TwoGo to), nitric acid in the absorbent
Ion NO_Three ^-Is NO_TwoReleased from the absorbent in the form of That is, the acid in the inflow exhaust
NO when the element concentration decreases_xNO from absorbent 16_xWill be released. [0032]   At this time, if reducing components such as HC and CO are present in the inflow exhaust gas, these components
Is oxygen O on platinum Pt_Two ^-Or O^2-To oxidize and eliminate oxygen on platinum Pt.
Spend. Also, NOx decreases due to a decrease in the oxygen concentration in the exhaust_xN released from absorbent 16
O_TwoIs reduced by reacting with HC and CO as shown in FIG. White in this way
NO on the surface of gold Pt_TwoWhen no longer exists, NO will change from absorbent to next_TwoIs released
Is done. Therefore, if HC and CO components are present in the inflow exhaust gas, NO_x
NO from absorbent 16_xIs released and reduced. [0033]   That is, HC and CO in the inflow exhaust gas are first converted to O 2 on platinum Pt._Two ^-Or O^2-Directly
Oxide on platinum Pt_Two ^-Or O^2-Is still consumed
If HC and CO remain, NO released from the absorbent by the HC and CO_x
And NO emitted from the engine_xIs reduced. Thus, NO_xAbsorbent
Low concentration of NO when the air-fuel ratio is lean_xWhen the exhaust oxygen concentration drops
NO absorbed_xReleases NO in a concentrated form_xTo release the N
O_xIs almost completely reduced and low concentrations of NO_xVery high purification rate
Can be. [0034]   Next, the above NO_xThe operation when the absorbent 16 is disposed on the downstream side of the three-way catalyst 12 will be described.
Will be described.   In this embodiment, the engine combustion air-fuel ratio is periodically changed to the stoichiometric air-fuel ratio by a routine described later. On the other hand, the rich side and the lean side are alternately controlled, and the exhaust air-fuel ratio is also controlled accordingly.
The air-fuel ratio alternates between the air-fuel ratio and the rich air-fuel ratio. [0035]   When the engine combustion air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, the oxygen concentration of the exhaust
The degree increases rapidly. In this state, the exhaust gas flowing into the three-way catalyst 12 in the exhaust passage 14
Lean, high oxygen concentration, relatively large amount of NO_xContains. On the other hand, exhaust air
When the fuel ratio is controlled to the lean side with respect to the stoichiometric air-fuel ratio, as shown in FIG.
12, the purification rates of HC and CO are maintained at high values, but NO_xPurification rate greatly
descend. Therefore, in the three-way catalyst 12, most of HC and CO in the exhaust gas are exhausted.
Purified but NO_xIs passed through the catalyst without being purified by the three-way catalyst 12. Follow
And the NO arranged on the downstream side_xThe exhaust flowing into the absorbent 16 has a lean air-fuel ratio,
Relatively low concentration NO not purified by the upstream three-way catalyst 12_xContains.
Therefore, NO_xAbsorbent 16 is NO in exhaust_xAbsorbs almost all of NO_xAbsorbent
NO from exhaust gas passing through_xIs almost completely removed. [0036]   Next, when the engine combustion air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, the exhaust gas
The oxygen concentration drops sharply. In this state, NO_xRelatively low
The amount of generation of unburned HC and CO increases. Therefore, the exhaust gas flowing into the three-way catalyst 12 is
It has a high oxygen concentration and contains relatively large amounts of HC and CO. As shown in Figure 2
When the exhaust air-fuel ratio becomes rich with respect to the stoichiometric air-fuel ratio, the three-way catalyst 12_x
The purification rate of HC is kept high, but the purification rate of HC and CO is reduced.
, CO passes through the catalyst without being purified by the three-way catalyst 12. [0037]   Therefore, the downstream NO_xThe absorbent 16 has a low oxygen concentration and contains HC and CO.
Exhaust gas flows in. Therefore, NO_xAbsorbent 16 reduces exhaust oxygen concentration
NO absorbed during exhaust lean_xIs released and the released NO_xAre HC and C in the exhaust
It is reduced and purified while consuming the O component. This exhaust gas is fed to the three-way catalyst 12 on the upstream side.
Trace NO not purified_xNO in the exhaust gas_xIs the same as above
NO_xSince the catalyst is reduced and purified by the absorbent 16, NO_xPass through the absorbent 16 Exhaust is NO_xAlmost no. [0038]   Thus, when the air-fuel ratio of the exhaust gas flowing into the downstream side of the three-way catalyst 12 is lean,
NO in exhaust_xAbsorbed when the exhaust oxygen concentration decreased_xEmit
NO_xBy disposing the absorbent 16, NO as a whole_xPurification rate significantly improved
Be improved.   Hereinafter, the above-described engine combustion air-fuel ratio control operation by the ECU 10 of FIG. 1 will be described. Book
In the embodiment, the fuel injection amount from the fuel injection valve 7 is_TwoSensor 13
By performing feedback control according to the output, the engine combustion air-fuel ratio
Controlling so that it alternates periodically between the rich side and the lean side as the center
. [0039]   FIG. 4 shows a routine for calculating the amount of fuel injection from the fuel injection valve 7 and includes a predetermined crank angle.
, For example, every 360 °. In step 401, the RAM 105
Inlet air amount data Q and rotation speed data N_eIs read and the basic injection amount TAUP (TA
UP calculates an injection time for obtaining a stoichiometric air-fuel ratio). TAUP is, for example, TAUP
← α ・ Q / N_e(Α is a constant). In step 402, the final injection
The quantity TAU is calculated by TAU ← TAUP · FAF · β + γ. Where FA
F is an air-fuel ratio correction coefficient for periodically changing the air-fuel ratio between the rich side and the lean side
And is set by a routine described later. Β and γ are other operating state parameters.
The correction amount is determined by the data. Next, at step 403, the injection amount TAU is
Set to the down counter 108 and set the flip-flop 109
Start fuel injection. Then, in step 404, this routine ends. [0040]   As described above, when the time corresponding to the injection amount TAU elapses, the down cowl
The flip-flop 109 is reset by the output signal of the
The shooting ends.   FIG. 5 and FIG._TwoCalculates the air-fuel ratio correction coefficient FAF based on the output of the sensor 13
This is an air-fuel ratio feedback control routine, which is executed every predetermined time, for example, every 4 ms. Is performed. [0041]   When the routine starts in FIG._TwoBy sensor 13
It is determined whether a closed loop (feedback) condition for the air-fuel ratio is satisfied. Was
For example, when the cooling water temperature is equal to or lower than a predetermined value, during engine start, during increase after start-up, during warm-up increase,
During power increase, during fuel injection amount increase to prevent catalyst overheating, O_TwoOutput of sensor 13
When the signal has never been inverted or during fuel cut, the closed loop condition is not established
Otherwise, the closed loop condition is satisfied. Closed loop condition not established
If it is, the process proceeds to step 525 (FIG. 6), and the air-fuel ratio correction coefficient FAF is set to 1.0.
And terminate the routine. Closed loop condition is satisfied in step 501
In this case, step 502 and subsequent steps are executed. [0042]   That is, in step 502, O_TwoA / D converting the output VOM of the sensor 13
In step 503, VOM is set to the comparison voltage V_R1Depending on whether the air-fuel ratio
Determine whether it is rich or lean. Comparison voltage V_R1Is O_TwoAmplitude center of sensor 13 output
(See FIG. 8). In step 503, when the air-fuel ratio is lean (VOM ≦ V_R1)
If so, it is determined in step 504 whether or not the delay counter CDLY is positive.
If DLY> 0, CDLY is set to 0 in step 505, and the process proceeds to step 506.
. In step 506, the count value of the delay counter CDLY is decremented by one.
, 508 guard the delay counter CDLY with the minimum value TDL. in this case,
When the delay counter CDLY reaches the minimum value TDL, at step 509
The air-fuel ratio flag F1 is set to "0" (lean). Note that the minimum value TDL is O_TwoSensor
Even if the output of 13 changes from rich to lean, the judgment that the state is rich is maintained.
And is a negative constant value in this embodiment. [0043]   In step 503, the air-fuel ratio is rich (VOM> V_R1),
Proceeding to step 510, it is determined whether or not the delay counter CDLY is negative.
If Y <0, CDLY is set to 0 in step 511, and the process proceeds to step 512. S
In step 512, the delay counter CDLY is incremented by one, and steps 513 and 51 are performed. In step 4, the delay counter CDLY is guarded by the maximum value TDR. In this case,
When the counter LY reaches the maximum value TDR, the air-fuel ratio is determined in step 515.
The flag F1 is set to "1" (rich). Note that the maximum value TDR is O_TwoSensor 13
Even if the output changes from lean to rich, the
This is a rich delay time for holding, and is a positive constant value in this embodiment. [0044]   Next, in step 516 (FIG. 6), the sign of the air-fuel ratio flag F1 is inverted.
That is, it is determined whether or not the air-fuel ratio after the delay processing has been inverted. Air-fuel ratio
If it is inverted, at step 517, the air-fuel ratio flag F1 is changed from rich to
It is determined whether the inversion is lean or lean to rich. Rich to Lean
If it is inverted (“1” → “0”), in step 518, FAF ← FAF + R
SR and increase in a skip manner, and conversely, inversion from lean to rich (“0” → “1”).
"), Skip in step 519 to FAF ← FAF-RSL
Perform skip processing. [0045]   If the sign of the air-fuel ratio flag F1 is not inverted at step 516, step 5
The integration process is performed at 20, 521, and 522. That is, in step 520, F1 = “
1 is determined, and if F1 = "0" (lean), FAF is determined in step 521.
← FAF + KIR, and if F1 = "1" (rich), step 522
And FAF ← FAF-KIL. Here, the integration constants KIR and KIL are skipped.
It is set sufficiently smaller than the amounts RSR and RSL, and KIR (KIL) <RSR
(RSL). Therefore, step 521 is performed in the lean state (F1 = "0").
In step 522, the fuel injection amount is increased in a rich state (F1 = “1”).
The injection amount is gradually reduced. [0046]   Next, in step 523, calculation is performed in steps 523, 524.526, and 527.
The corrected air-fuel ratio correction coefficient FAF is guarded by a minimum value, for example, 0.8, and
Guarded at a value, for example, 1.2. As a result, the air-fuel ratio correction coefficient
If the FAF becomes too large or too small, that value will Controlling the fuel ratio prevents over-rich and over-lean. [0047]   In step 524, the FAF calculated as described above is stored in the RAM 105.
This loop ends in step 526.   FIG. 7 is a timing chart for supplementarily explaining the operation according to the flowcharts of FIGS.
is there. O_TwoAs shown in FIG.
When the air-fuel ratio signal A / F for determining whether or not the air-fuel ratio has been obtained is obtained, the delay counter CDLY returns to FIG.
As shown in B), the count is incremented in the rich state and counted in the lean state.
It is done. As a result, as shown in FIG. 7C, the delayed air-fuel ratio signal A
/ F '(corresponding to the flag F1). [0048]   For example, time t₁Even if the air-fuel ratio signal A / F 'changes from lean to rich,
The delayed air-fuel ratio signal A / F 'is held lean for the rich delay time TDR.
Time t after_TwoChanges richly. Time t_ThreeAnd the air-fuel ratio signal A / F is rich
Even if the air-fuel ratio signal changes to lean, the air-fuel ratio signal A / F 'that has been subjected to the delay processing has a lean delay time (-
TDL) time t_FourChanges to lean. [0049]   Therefore, the time during which the air-fuel ratio is controlled to the rich side is the rich delay time TDR.
The air-fuel ratio is determined by the lean skip amount RSL and the lean integration constant KIL.
Is controlled to the lean side by the lean delay time TDL and the rich skip amount.
RSR is determined by the rich integration constant KIR.
The change cycle of the air-fuel ratio can be set arbitrarily. [0050]   Next, another embodiment of setting the air-fuel ratio correction coefficient FAF will be described.   In the above-described embodiment, the calculation routine (FIGS. 5 and 6) of the air-fuel ratio correction coefficient FAF is delayed.
Delay time TDR, TDL, skip amounts RSR, RSL, and integration constants KIR, KI
L is a constant value, and the engine combustion air-fuel ratio (that is, the air-fuel ratio correction coefficient)
FAF) is controlled to fluctuate between a rich side and a lean side at a constant cycle.
. [0051]   By the way, as mentioned above,_xThe air-fuel ratio of the absorbent 16 is controlled to the lean side.
When exhausting_xWhen the air-fuel ratio is controlled to the rich side.
NO collected_xRelease. Therefore, the period during which the air-fuel ratio is controlled to the rich side is NO_xabsorption
NO absorbed by the agent_xNeed to be long enough to release the full amount of rich
NO if air-fuel ratio period is short_xAbsorbent is absorbed NO_xWhile the remains, the next N
O_xWill start to absorb and over time NO_xAbsorbent NO_xIncreased absorption
NO_xAbsorption capacity may be reduced. On the other hand, the period of rich air-fuel ratio is long
And NO_xNO from absorbent_xExhaust gas with a rich air-fuel ratio is supplied even after all
That is, the HC and CO components in the exhaust gas are NO_xNO as it is not consumed for reduction_x
It may pass through the absorbent. [0052]   However, actually, NO_xNO from absorbent_xRelease rate is not constant and NO_x
Varies with the temperature of the absorbent. That is, NO_xNO from absorbent_xThe release rate is
NO below a certain temperature_xThe higher the temperature of the absorbent, the higher the temperature
However, above that, the temperature becomes smaller as the temperature increases (see FIG. 9). Follow
Therefore, the time during which the air-fuel ratio is controlled to the rich side as in the above-described embodiment is always set to a constant value.
And depending on the operating conditions, NO_xNO from absorbent_xRelease is not complete
No, no_xThere may be a period when the air-fuel ratio is maintained rich even after the release is completed
There is a risk. [0053]   In this embodiment, in order to prevent the above problem from occurring, NO_xOf absorbent 16
When the air-fuel ratio (air-fuel ratio correction coefficient FAF) of the engine is kept rich according to the temperature
NO by changing the interval_xPrevention of decrease in absorption capacity of absorbent and unburned HC and CO
The aim is to reduce emissions.   FIG. 9 is NO_xOne example of setting the air-fuel ratio rich side holding time with respect to the temperature of the absorbent 16
It is a figure showing an example. NO as described above_xNO from absorbent 16_xRelease rate is constant temperature
Degrees or less (300 degrees C or less in the example of FIG. 9), it increases as the temperature rises,
Then it decreases with temperature. Therefore, NO_xNO absorbed from absorbent_xTotal amount of As shown in FIG. 9, the time required for releasing the gas is increased at a temperature of 300 ° C. or less.
And at 300 ° C. or more, it is set to become longer with time. What
NO, above a certain temperature_xThe release rate decreases with temperature is NO_xFrom the absorbent
NO_xThe temperature required to consume oxygen on platinum Pt as described in the release action
Above, it is considered that it becomes longer as the temperature rises. [0054]   Further, in the present embodiment, in order to change the time during which the air-fuel ratio is maintained on the rich side,
5 without making the rich delay time TDR_xIt changes according to the temperature of the absorbent 16.
I'm changing. As can be seen from FIG. 7, the lean delay time TDL and the skip amount RSR,
When the RSL and the integration constants KIR and KIL are fixed, the air-fuel ratio correction coefficient FAF is reset.
Since the time held on the switch side is determined only by the rich delay time TDR,
NO rich delay time TDR_xBy changing according to the temperature of the absorbent 16, FIG.
9 can be obtained. [0055]   FIG. 10 shows a routine for setting the rich delay time TDR according to the present embodiment. This routine
Is executed by the ECU 10 at regular intervals (for example, at every 512 ms).   When the routine starts in FIG.
The Seki exhaust temperature TEX is read. NO_xThe temperature of the absorbent 16 is approximately equal to the exhaust temperature.
Therefore, in this embodiment, NO_xApproximating the temperature of the absorbent 16 with the exhaust gas temperature TEX,
, The delay time TDR is calculated. [0056]   In step 1003, the rich holding time RT of the air-fuel ratio is calculated from FIG. Book
In the embodiment, the function of FIG. 9 is stored in the ROM 104 in the form of a numerical value table in advance.
In step 1003, the rich temperature is held from the numerical value table using the exhaust gas temperature TEX.
RT is read during. Note that the holding time shown in FIG._xType and volume of absorbent
Determine the required rich retention time in advance by experiments, etc.
Is preferred. [0057]   Next, in step 1005, the rich holding time obtained as described above is obtained. Delay time TDR is calculated. In this embodiment, the rich holding time RT is
Rich delay time TDR, lean skip amount RSL, rich and lean integration constants K
Using IR and KIL,     RT = △ T (TDR + (TDR × KIR−RSL) / KIL) It is represented by Here, ΔT is the execution interval of the routine in FIGS. 5 and 6 (4 ms in this embodiment)
), (TDR × KIR-RSL) / KIL term on the right side is during rich delay time TDR
FAF increased by rich integration processing (KIR) is lean skip (RSL)
And the time required to reach zero by the lean integration process (KIL).
You. [0058]   Therefore, from the above equation, TDR is   TDR = ((RT / ΔT) + (RSL / KIL)) / (1 + KIR / KIL) Becomes In this embodiment, ΔT, KIR, RSL, and KIL are set to constant values.
Therefore, the above equation is         TDR = (RT + K1) / K2 (K1 and K2 are constants) In the form of In this embodiment, it is determined in advance from the values of ΔT, KIR, RSL, and KIL.
The numbers K1 and K2 are obtained, and in step 1005, the rich delay time is calculated using the above equation.
Is calculated. In step 1007, the TDR obtained as described above is stored in the RAM 105.
And exit the routine. [0059]   NO as above_xBy changing the rich delay time according to the temperature of the absorbent
5 and 6, the rich-side holding time of the air-fuel ratio is changed.   In this embodiment, the exhaust gas temperature TEX directly detected by the exhaust gas temperature sensor 9 is N
O_xAlthough the air-fuel ratio rich retention time is calculated using the temperature instead of the absorbent temperature,
The air temperature TEX may be obtained indirectly from operating conditions such as the engine load. Also, the exhaust temperature
Degree TEX and NO_xThe correlation between the actual temperature of the absorbent and the actual temperature is determined in advance by experiments, etc.
From the exhaust temperature TEX to the actual NO_xAccuracy can be obtained by determining the absorbent temperature
Good control can be performed. Further, instead of using the exhaust temperature TEX, N
O_xNO detected directly by providing a temperature sensor on the absorbent itself_xI will use the absorbent temperature It can also be done. [0060]   Further, in the above embodiment, the rich delay time TDR is made variable so that the air-fuel ratio becomes richer.
The retention time is changed, but the rich integration constant KIR or lean skip
NO for RSL or TDR, KIR, RSL_xDepending on the temperature of the absorbent 16
It is also possible to change the air-fuel ratio rich side holding time by changing
Noh. [0061]   Next, still another embodiment of the present invention will be described with reference to FIG.   FIG. 11 shows an overall view of an exhaust gas purifying apparatus for an internal combustion engine similar to that of FIG. 1, and FIG.
The same reference numerals as those in FIG. 1 denote elements having the same functions. In the embodiment shown in FIG.
And an EGR device 50 that recirculates part of the exhaust gas of the engine to the intake side is provided.
This is different from the embodiment of FIG. The EGR device 50 is located upstream of the three-way catalyst 12 in the exhaust passage 14.
The recirculated exhaust gas is injected into the exhaust inlet port 51 that takes out exhaust from the side and the engine intake port.
E which connects the exhaust port 53 with the inlet port 51 and the injection port 53.
A GR passage 55 is provided, and the EGR passage 55 has an exhaust gas recirculation amount (hereinafter referred to as an “EGR amount”).
) Is provided. [0062]   The EGR valve 57 is applied, for example, from the ECU 10 via a drive circuit 510.
A flow control valve in the form of a solenoid valve operated by a drive pulse.
Return to the engine intake port through the EGR passage 55
It controls the flow rate of exhaust gas flowing. In this embodiment, the intake passage 2 is
A throttle opening sensor 62 for detecting the throttle opening near the throttle valve 4;
An intake pressure sensor 6 for detecting an engine intake negative pressure in the intake manifold downstream of the rotary valve 4
3 are provided. The throttle opening sensor 62 responds to the opening of the throttle valve 4.
In addition, the intake pressure sensor 63 generates the same voltage signal.
A negative pressure (differential pressure between atmospheric pressure and absolute pressure in the intake manifold)
Live. Output signals of these sensors 62 and 63 are transmitted through an A / D converter 101 to E
The data is input to the input / output interface 102 of the CU 10 and periodically stored in the RAM 105. Is stored. [0063]   In the above-described embodiment, the fuel injection amount from the fuel injection valve 7 can be periodically increased or decreased.
Changes the engine combustion air-fuel ratio alternately between rich and lean with respect to the stoichiometric air-fuel ratio.
However, in this embodiment, the fuel injection amount from the fuel injection valve 7 depends on the operating state.
The EGR amount returned from the EGR passage 55 to the intake port is maintained at a constant value.
Is that the air-fuel ratio alternates between rich and lean
This is different from the above-described embodiment. [0064]   As mentioned above, NO_xNO of absorbent 16_xTo prevent a decrease in absorption capacity,
NO_xNO absorbed from absorbent 16_xRich air-fuel ratio for just enough time to release
Need to be maintained. For this reason, in the above embodiment, when the air-fuel ratio is kept rich,
It is necessary to increase the interval to some extent, and the switching cycle between the rich side and the lean side of the air-fuel ratio
Needs to be set relatively long. However, as in the previous embodiment,
Change the engine combustion air-fuel ratio between rich and lean by increasing or decreasing the fuel supply
In this case, the engine output torque fluctuates with the change in the fuel supply amount. This out
When the air-fuel ratio switching cycle is short, fluctuations in force torque are difficult for the driver to feel and drive.
The influence on the characteristics is small, but when the air-fuel ratio switching cycle becomes longer,
The degree of sensation is increased, which adversely affects the drivability. [0065]   On the other hand, to change the engine combustion air-fuel ratio, the fuel supply amount to the engine must be changed.
It is also possible to change the intake air amount of the engine. In this case the institution
The engine output torque does not change even if the air-fuel ratio changes
Relatively small, even if the air-fuel ratio switching cycle is set long, the drivability is greatly affected.
Absent. This embodiment pays attention to this point, and passes through the EGR passage 55 to the engine intake port.
By controlling the flow rate of the recirculating exhaust gas, the amount of engine intake air is changed.
It is a thing. [0066]   That is, in this embodiment, during steady operation, the fuel injection amount from the fuel Pressure (intake manifold negative pressure) Pm and engine speed N_eControl to a value determined according to
However, the fuel injection amount is not periodically increased or decreased. On the other hand, exhaust gas returning to the engine exhaust port
Is the throttle opening TH and the engine speed N_eCentered on the amount determined according to
The air-fuel ratio is periodically increased or decreased by the EGR valve 57, and the air-fuel ratio changes according to the change in the EGR amount.
I do. For example, when the flow rate of exhaust gas recirculated to the intake port is increased, the throttle valve 4 opens.
Even if the degree is kept constant, the amount of fresh air flowing through the throttle valve recirculates
It decreases by an amount corresponding to the exhaust flow rate. Also, the oxygen concentration of the recirculating exhaust gas is
It does not contribute to combustion because it is low. Therefore, by increasing the EGR amount,
The engine intake air amount is reduced while maintaining the opening of the throttle valve 4, and the engine combustion air-fuel ratio is reduced.
Moves to the rich side. In this embodiment, the fuel injection amount from the fuel injection valve 7 is EGR
Is set so that the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio without performing
It is possible to periodically change the flow rate of exhaust gas recirculated from the GR passage 55 to the engine intake port.
Thus, the air-fuel ratio is switched between the lean side and the rich side. [0067]   Next, the air-fuel ratio control operation of this embodiment will be described with reference to FIGS.   FIG. 12 shows the calculation of the fuel injection amount from the fuel injection valve 7 and the EGR passage 5 in this embodiment.
5 is a flowchart showing an exhaust gas recirculation amount calculation routine from FIG. This routine is also E
It is executed by the CU 10 at a predetermined crank angle, for example, every 360 °.   When the routine is started in FIG.
From 05, intake manifold negative pressure Pm, throttle opening TH, rotation speed data N_eBut
Is read. Next, at step 1203, the EGR execution flag XEGR is set.
(XEGR = “1”) is determined. Here, XEGR is different
Set when the EGR execution condition is satisfied in a routine (not shown)
Is the flag to be used. In this embodiment, the engine cooling water temperature is equal to or higher than a predetermined value,
That the engine operating conditions are not in a transient state (the engine load does not fluctuate rapidly);
The EGR execution condition is that the engine is not in full load operation or no load operation.
The flag XEGR is set only when all of the above conditions are satisfied (= "1").
In other cases, the flag XEGR is reset (= "0"). [0068]   If the flag XEGR is set in step 1203, step 1
Step 205 is executed to calculate the basic fuel injection time TAUO at the time of EGR execution. This
Here, the basic fuel injection time TAUO is controlled by the EGR valve 57 to provide a basic EGR amount described later.
The fuel injection amount is such that the stoichiometric air-fuel ratio is obtained when the opening degree is set to SEGR.
Yes, intake manifold negative pressure Pm and rotation speed data read in step 1201
N_eIs calculated using [0069]   FIG. 13 shows TAUO, Pm, N_eFIG. 4 is a diagram showing an example of the relationship of FIG.
UO, the horizontal axis is the absolute pressure of the intake manifold (that is, the atmospheric pressure P0-the intake manifold).
Negative pressure Pm). FIG. 13A shows the rotation speed N._e= 2000rpm
FIG. 13B shows the rotation speed N._e= 4000 rpm
. In this embodiment, TAUO is obtained in advance for each engine speed by an experiment or the like, and Pm
, N_eIs stored in the ROM 104 of the ECU 10 in the form of a numerical table using
In step 1205, Pm, N read in step 1201_eFrom the value of
UO is read. [0070]   As shown in an example in FIG. 13, the basic fuel injection time TAUO is the same rotation speed.
In this case, the pressure increases along a downwardly convex curve with respect to the intake manifold negative pressure Pm, and Pm becomes one.
If constant, rotation speed N_eShows a tendency to increase as increases. This is the basic
As shown in FIG. 14, the EGR amount is changed to the throttle opening TH and the rotation speed N._eAnd depending on the key
The stoichiometric air-fuel ratio is obtained when the basic EGR amount of exhaust gas is recirculated.
This is for setting the TAUO to be set. [0071]   Next, at step 1207, the basic opening degree SEGR of the EGR valve 57 (basic EGR
Amount) is the throttle opening TH and the rotation speed N_eIs calculated from FIG. 14 shows the basic opening S
FIG. 14A is a diagram showing an example of setting of EGR, and FIG._eA place where
14B shows the relationship between the SEGR and the throttle opening TH, and FIG.
SEGR and rotation speed N when opening degree is fixed_eThe relationship is shown. [0072]   As shown in FIG. 14A, the basic opening degree SEGR of the EGR valve 57 is determined by the rotation speed N._eBut
If it is constant, it increases with the throttle opening TH (engine load).
When the throttle opening is equal to or larger than the throttle opening, the throttle opening is reduced as the throttle opening increases. This is an institution
On the high load side, the EGR amount is reduced to increase the output. In addition, FIG.
), The SEGR is equal to the engine speed N if the throttle opening TH is constant._e
Is reduced as is increased. This is because the combustion speed decreases when EGR is performed.
This is to prevent combustion from deteriorating on the high engine speed side. [0073]   In this embodiment, each throttle opening TH and the engine speed N_eAbout experiments in advance
Thus, the optimal basic opening degree SEGR of the EGR valve 57 is set.
N_eAre stored in the form of a numerical table using
TH, N read in step 1201_eIf the numerical value table is used, the value of SEGR is
Is read. [0074]   Next, in step 1209, from the basic opening degree SEGR calculated as described above,
The actual set opening degree (actual EGR amount) TEGR of the EGR valve 57 is             TEGR ← SEGR / FEGR In step 1211, TAUO and TEGR set as described above are calculated.
Is stored in the RAM 105, and the routine ends in step 1217. [0075]   Fuel injection separately executed when TAUO is set in RAM 105 as described above.
The fuel injection valve 7 is opened for a period of TAUO by a routine, and the amount corresponding to TAUO is
Of fuel is injected. Similarly, when TEGR is set in the RAM 105,
The EGR valve 57 is controlled so that the opening degree TEGR is obtained by a separately executed routine.
The duty ratio of the drive pulse is controlled. Here, the FEGR is similar to the coefficient FAF in FIG.
It is an EGR correction coefficient for changing between the side and the lean side. FEGR is separately E
The routine shown in FIG. 15 and FIG.
O disposed in the exhaust passage 14_TwoThe setting is made based on the output of the sensor 13. In addition, The routines in FIGS. 15 and 16 are substantially the same as the routines described in FIGS.
Here, the invention is omitted. [0076]   On the other hand, in step 1203, the EGR flag XEGR is reset (= "0").
If so, steps 1213 and 1215 are executed. In this case,
The fuel injection time TAUO is a fuel value at which the stoichiometric air-fuel ratio can be obtained without performing EGR.
The fuel injection amount is determined only by the intake manifold negative pressure Pm.
It is given in the form O ← α · Pm. In step 1215, the EGR valve 57 is opened.
The degree TEGR is set to zero, and in steps 1209 and 1211 TAUO, TEG
R is set in the RAM 105, and the routine ends in step 1217. to this
Accordingly, the engine air-fuel ratio is maintained at the stoichiometric air-fuel ratio, and the exhaust gas recirculation is stopped. [0077]   As described above, when the routines of FIGS. 12, 15, and 16 are executed, the EGR execution
When the condition is satisfied, the EGR amount is periodically increased or decreased around the basic EGR amount, and
The fuel ratio alternately fluctuates between the rich side and the lean side around the stoichiometric air-fuel ratio. In addition,
In the present embodiment, the fluctuation cycle of the air-fuel ratio is relatively long, for example, in the range of about 2 to 10 seconds.
The delay times TDR and TDL and the integration constants KIR and KI shown in FIGS.
L, skip amounts RSR and RSL are set. [0078]   Next, FIG. 17 shows still another embodiment of the exhaust gas purification apparatus of the present invention.   FIG. 17 shows an overall view of an exhaust gas purifying apparatus for an internal combustion engine similar to that shown in FIG.
The same reference numerals as those in FIG. 1 denote elements having the same functions. In the embodiment shown in FIG.
And another O on the downstream side of the three-way catalyst 12 in the exhaust passage 17._TwoA sensor 61 is provided, and
This O_TwoSensor 61 and NO_xSecondary air for supplying secondary air to the exhaust passage between
The point that a secondary air supply device 66 is provided is different from the embodiment of FIG. [0079]   The secondary air supply device 66 includes a pressurized air supply source 65 such as an air pump and a tank.
The pressurized air from the supply source 65 is_TwoSensor 61 and NO_xExhaust between absorbent 16
An air pipe 67 leading to a passage, and controlling supply of secondary air to the air pipe 67. A control valve 69 is provided. In this embodiment, the control valve 69 is connected via the drive circuit 610.
And is a shut-off valve that is turned on / off by a control signal from the ECU 10.
As described later, the secondary air flow rate can be continuously controlled by a signal from the ECU 10.
A flow control valve may be used. [0080]   As described with reference to FIG._xAbsorbent 16 is NO_xTo absorb
Aerial oxygen is needed. Therefore, NO_xAbsorbent has high oxygen concentration in exhaust gas
NO, that is, the leaner the exhaust_xImproves the absorption capacity. Therefore, FIG.
The engine alternates between a rich air-fuel ratio and a lean air-fuel ratio around the stoichiometric air-fuel ratio
When changing the air-fuel ratio, the change width of the air-fuel ratio, especially the change width on the lean air-fuel ratio side
It is preferable to take as large as possible. However, if the variation range of the air-fuel ratio is large,
The output torque fluctuation of the Seki becomes excessive, and the influence on the drivability becomes large.
It is difficult to set the air-fuel ratio at the time of
You. Further, as in the case of the engine shown in FIG.
Also, if a large amount of exhaust gas is recirculated, the combustion state of the engine will deteriorate, and the air-fuel ratio will also change.
It is difficult to increase the width of the area. [0081]   For this reason, the rich air-fuel ratio and the lean air-fuel ratio alternately around the stoichiometric air-fuel ratio.
In the engine that returns, the air-fuel ratio at the time of lean cannot be made sufficiently large (thin).
O_xIn some cases, the absorbing ability of the absorbent cannot be sufficiently exhibited. This implementation
In the example, NO_xWhen the air-fuel ratio of the exhaust gas flowing into the absorbent 16 becomes lean,
O_xBy supplying the secondary air only to the absorbent 16, NO during lean operation_xAbsorbent 1
6 to make the air-fuel ratio of the exhaust flowing into_xNO of absorbent 16_xAbsorption capacity
Improving power. [0082]   That is, in the present embodiment, the engine combustion air-fuel ratio increases or decreases the fuel injection amount (see FIGS. 5, 6, and 9).
10) or the increase / decrease of the exhaust gas recirculation amount (FIG. 13).
The control is alternately performed between the rich side and the lean side near the ratio. On the other hand, the ECU 10 is a three-way catalyst
12 O downstream_TwoThe output of the sensor 61 is monitored, and_TwoOutput of sensor 61 When the force has a lean output (that is, NO_xExhaust gas on the upstream side of the absorbent 16
(When the engine is lean), the control valve 69 is opened and the NO._xabsorption
Secondary air is introduced into the agent 16. This makes NO_xExhaust air-fuel flowing into the absorbent 16
The ratio becomes even leaner, NO_xAbsorbent NO_xAbsorption capacity increases. [0083]   In addition, the ECU 10 controls the downstream O_TwoWhen the output of the sensor 61 is rich,
The control valve 69 is closed to stop the supply of the secondary air. Thereby, NO_xAbsorbent upstream
When the side exhaust is rich, the exhaust can maintain a rich air-fuel ratio, so NO_xAbsorbent
NO from 16_xIs sufficiently released.   FIG. 18 shows NO when the above control is performed._xOf the exhaust air-fuel ratio flowing into the absorbent 16
It is a figure showing a change. Thus, NO_xWhen the exhaust air-fuel ratio is
Supply of secondary air only when the
Worse or NO_xNO from absorbent 16_xNO without affecting release of NO_xSucking
NO of absorbent 16_xIt is possible to improve the absorption capacity. [0084]   In this embodiment, the control valve 69 of the secondary air supply device 66 is an ON / OFF valve.
The amount of secondary air when the control valve 69 is opened is substantially constant. But from the institution
The actual exhaust flow is not necessary because the exhaust flow rate varies depending on the operating conditions (load, rotation speed).
The secondary air flow rate for obtaining the exhaust air-fuel ratio varies depending on the operating condition. others
Therefore, a flow control valve of a type that can continuously control the flow rate is used as the control valve 69.
Therefore, if the secondary air flow rate is controlled in accordance with the above operating state, more efficient
NO_xAbsorbent NO_xIt is possible to improve the absorption capacity. [0085]   In the present embodiment, the O provided on the downstream side of the three-way catalyst 12 is_TwoSecondary empty by sensor 61
Qi supply is controlled, but O_TwoThe O on the upstream side of the three-way catalyst 12 without the sensor 61 is provided._Two
The output of the sensor 13 determines whether the exhaust air-fuel ratio is rich or lean and supplies secondary air.
Feeding may be controlled. [0086]     【The invention's effect】   According to the exhaust gas purification apparatus of the present invention, the engine combustion air-fuel ratio is reduced around the stoichiometric air-fuel ratio.
Three-way in the exhaust passage of the internal combustion engine that alternates periodically between the air-fuel ratio side and the lean side
A catalyst is arranged, and NO_xExhaust due to the placement of the absorbent
NO_xCan significantly improve the purification rate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing one embodiment of an exhaust gas purification apparatus of the present invention. FIG. 2 is a diagram illustrating a relationship between a purification rate of a three-way catalyst and an exhaust air-fuel ratio. FIG. 3 is a diagram illustrating the NO _x absorption / release action of a NO _x absorbent. FIG. 4 is a flowchart illustrating an example of a fuel injection amount calculation routine of the fuel injection valve in FIG. 1; FIG. 5 is a part of a flowchart showing an example of an air-fuel ratio control routine. FIG. 6 is a part of a flowchart showing an example of an air-fuel ratio control routine. FIG. 7 is a timing chart supplementing the control operation of the routines of FIGS. 5 and 6. FIG. 8 is a diagram illustrating output characteristics of an O ₂ sensor. FIG. 9 is a diagram showing a relationship between the NO _x absorbent temperature and the rich air-fuel ratio holding time. FIG. 10 is a flowchart illustrating an example of a routine for setting control constants in the flowcharts of FIGS. 5 and 6; FIG. 11 is a view showing another embodiment of the exhaust gas purification apparatus of the present invention. FIG. 12 is a flowchart illustrating an example of an air-fuel ratio control routine according to the embodiment of FIG. 11; FIG. 13 is a diagram illustrating an example of setting of a fuel injection amount in FIG. 12; FIG. 14 is a diagram showing an example of an EGR valve opening degree setting of FIG. FIG. 15 is a part of a flowchart showing the setting of an EGR amount in the embodiment of FIG. 11; FIG. 16 is a part of a flowchart showing the setting of an EGR amount in the embodiment of FIG. 11; FIG. 17 is a view showing another embodiment of the exhaust gas purification apparatus of the present invention. 18 is a timing chart for supplementarily explaining the operation of the embodiment in FIG. [Reference Numerals] 1 ... internal combustion engine 2 ... intake passage 3 ... air flow meter 4 ... throttle valve 5, 6 ... crank angle sensor 7 ... fuel injection valves 10 ... control circuit (ECU) 12 ... three-way catalyst 13 ... O ₂ sensor 14 ... exhaust passage 16 ... NO _x absorbent 50 ... EGR device 57 ... EGR valve 62 ... throttle opening sensor 66 ... secondary air supply system 69 ... control valve

Claims (1)

Claims 1. An air-fuel ratio control device that periodically and alternately changes a combustion air-fuel ratio between a rich air-fuel ratio side and a lean air-fuel ratio side around a stoichiometric air-fuel ratio during normal operation of an engine. in an exhaust passage of an internal combustion engine having, together with the air-fuel ratio of the exhaust gas flowing to place the three-way catalyst to perform the reduction and HC of the NO _x in the exhaust, and the oxidation of CO at the same time of the stoichiometric air-fuel ratio, the three-way the exhaust passage downstream of the catalyst, placing _the NO _x absorbent the oxygen concentration of the exhaust gas absorbed flowing NO _x in the exhaust to release NO _x absorbed and reduced when the air-fuel ratio of the exhaust gas flowing is lean An exhaust gas purifying apparatus for an internal combustion engine, comprising: Wherein said air-fuel ratio control system includes means for detecting the temperature of the _the NO _x absorbent, the theoretical air-fuel ratio of the combustion air-fuel ratio of the engine according to the detected _the NO _x absorbent temperature to the rich air-fuel ratio side 2. The exhaust gas purifying apparatus according to claim 1, further comprising means for changing a holding period. 3. The air-fuel ratio control device includes means for recirculating a part of the engine exhaust gas to an engine intake passage, and means for controlling the recirculated exhaust gas flow rate. The exhaust gas purifying apparatus according to claim 1, wherein the fuel ratio is periodically and alternately changed to a rich air-fuel ratio side and a lean air-fuel ratio side. 4. A means for detecting an air-fuel ratio of exhaust gas flowing into the NO _x absorbent,
And means for introducing secondary air into the _the NO _x absorbent, the introducing secondary air to _the NO _x absorbent when the air-fuel ratio of the exhaust gas flowing into the _the NO _x absorbent is lean The exhaust gas purifying apparatus according to claim 1, wherein: