JP2002235585A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce and discharge NOx stored by catalyst having an oxygen storage capacity so that a torque variation does not occur by rapidly consuming the oxygen in the exhaust emission control device of an internal combustion engine reducing and discharging the NOx stored in NOx catalyst by rich combustion. SOLUTION: In step 111, it is judged that an air/fuel ratio detected by an A/F sensor 26 is rich or not. When the air/fuel ratio is judged to be rich, the oxygen in a catalytic converter rhodium 13 is started to be consumed and, therefore, a target air/fuel ratio is set so that the oxygen can be consumed rapidly in step 116. When the judgment is such that all oxygen stored in the catalytic converter rhodium 13 is consumed, the air/fuel ratio is controlled so as to be gradually followed to the target air/fuel ratio. Since the target air/fuel ratio is set like this, the oxygen stored in the catalytic converter rhodium 13 can be consumed rapidly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that performs lean combustion in an air-fuel ratio lean region, and stores NOx for purifying nitrogen oxides (NOx) generated during lean combustion. TECHNICAL FIELD The present invention relates to an air-fuel ratio control device for an internal combustion engine having a type catalyst.

[0002]

2. Description of the Related Art In recent years, in order to reduce fuel consumption, so-called lean burn control for burning an internal combustion engine in an air-fuel ratio region where the fuel ratio is lower than the stoichiometric ratio has been diversified.

When combustion is performed in a region where the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, there is a problem that a large amount of NOx is generated. In the lean burn control, a NOx catalyst having a function of storing and adsorbing NOx is provided as a catalyst for preventing the emission of NOx. But N
When the amount of NOx stored in the Ox catalyst increases, the NOx purification rate decreases.
When the x amount exceeds a predetermined value, combustion is performed at a rich air-fuel ratio to reduce and release NOx stored and adsorbed on the NOx catalyst.

As a prior art of such lean burn control, we have disclosed in Japanese Patent Application Laid-Open No. 12-018062.
By switching the air-fuel ratio by combustion between a lean region and a rich region, NOx stored in the NOx catalyst is reduced and released.

[0005]

However, in this method, the exhaust gas component supplied to the NOx catalyst by the rich combustion changes depending on the state of the lean combustion performed immediately before. More specifically, the amount of oxygen stored in the three-way catalyst (hereinafter referred to as oxygen storage) changes depending on the degree of leanness of the lean combustion performed immediately before, and the NOx stored and adsorbed on the NOx catalyst changes. Rich combustion components for purifying the quantity are absorbed by the oxygen storage. Therefore, N
No rich combustion component is supplied to the Ox catalyst, and NO
The NOx stored and adsorbed on the x catalyst cannot be reduced and released, and the rich control time may be prolonged.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an exhaust gas purifying apparatus for an internal combustion engine which can quickly consume stored oxygen and can execute rich control quickly. And

[0007]

According to the first aspect of the present invention,
In addition to performing lean combustion in the air-fuel ratio lean region, NOx in exhaust gas discharged during lean combustion is reduced to lean NO.
The stored NOx is stored in lean NOx by rich combustion in which the air-fuel ratio is temporarily controlled to be rich.
In an exhaust gas purifying apparatus for an internal combustion engine configured to release from a catalyst, a catalyst having an oxygen storage ability disposed upstream of the lean NOx catalyst, and a catalyst having an oxygen storage ability disposed upstream of the lean NOx catalyst, An air-fuel ratio sensor for detecting an air-fuel ratio in the exhaust passage; and NOx stored in the NOx catalyst.
Air-fuel ratio change amount setting means for changing an air-fuel ratio change amount per unit time at the time of switching between the rich combustion and the lean combustion based on the air-fuel ratio sensor. The change amount of the air-fuel ratio per unit time is changed based on the air-fuel ratio detected by

Accordingly, the amount of change in the control air-fuel ratio for rich combustion can be changed based on the air-fuel ratio of the internal combustion engine detected by the air-fuel ratio sensor, so that the stored oxygen can be quickly consumed. And the NOx catalyst can be quickly supplied with rich combustion components. Therefore, the stored NOx can be quickly consumed, and the rich control time can be shortened.

According to a second aspect of the present invention, in the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect, an oxygen for estimating an amount of oxygen stored in the catalyst having the oxygen storage capacity and / or the NOx catalyst. An air-fuel ratio change amount setting unit changes the air-fuel ratio change amount per unit time based on the oxygen amount estimated by the oxygen amount estimation unit.

[0010] This makes it possible to estimate the amount of oxygen stored in the catalyst having the oxygen storage capacity at the time of lean combustion, so that the air-fuel ratio detected by the air-fuel ratio sensor indicates the amount of air required to consume the oxygen of the catalyst. The amount of change in the air-fuel ratio from when the fuel ratio is detected to when the air-fuel ratio is consumed can be set large. That is, since the oxygen stored in the catalyst can be quickly consumed, the NOx stored in the NOx catalyst can be quickly consumed. Therefore, the rich control time can be shortened.

The structure of the NOx catalyst is, for example, a metal (Pt / Rh / Pd + CeO2) used in a three-way catalyst.
Na or B as a metal for adsorbing NOx
If a is affirmed, the NOx catalyst also has the ability to occlude oxygen, so the amount of oxygen stored in the NOx catalyst is also estimated, and the above-described change control of the air-fuel ratio change amount is performed. You may go.

According to a third aspect of the present invention, in the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect, an oxygen concentration sensor is provided upstream of the catalyst and detects an oxygen concentration in exhaust gas. The switching means detects a state of oxygen stored in the catalyst and / or the NOx catalyst by the air-fuel ratio sensor and the oxygen concentration sensor, and performs a unit time based on the estimated oxygen amount. Change the air-fuel ratio change per hit.

Thus, the oxygen concentration sensor disposed downstream of the three-way catalyst can accurately detect the state of oxygen stored in the catalyst having the oxygen storage capacity. In the invention of claim 1 or claim 2, for example, the amount of oxygen stored based on the degree of lean during lean combustion is estimated, but according to the present invention, the stored oxygen starts to be consumed. Can be detected, and that the stored oxygen has been consumed can be detected. At this time, by changing the amount of change in the air-fuel ratio per unit time, the stored oxygen can be quickly consumed.

For example, the fact that the oxygen stored in the catalyst has started to be consumed may be determined by detecting that the air-fuel ratio detected by the oxygen concentration sensor is maintained near the stoichiometric air-fuel ratio. This is because the catalyst is filled with H by rich combustion.
This is based on the fact that even if components such as C and CO are provided to the catalyst, the air-fuel ratio after the catalyst is kept close to the stoichiometric air-fuel ratio by reacting with oxygen stored in the catalyst. Also, when the stored oxygen is consumed,
Since components such as HC and CO in the rich combustion are discharged after the catalyst, the air-fuel ratio detected by the oxygen concentration sensor becomes rich. By detecting this, it is possible to detect that the stored oxygen has been consumed.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> A first embodiment of the present invention will be described below with reference to the drawings.
In the air-fuel ratio control system according to the present embodiment, the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set leaner than the stoichiometric air-fuel ratio, and lean combustion is performed based on the target air-fuel ratio. Perform control. As a main configuration of the system, a three-way catalyst and a NOx storage reduction type catalyst (hereinafter referred to as NO
x catalyst), and a limiting current type air-fuel ratio sensor (A / F sensor) is disposed upstream of the three-way catalyst. An electronic control unit (hereinafter, referred to as an ECU) mainly composed of a microcomputer takes in the detection result of the A / F sensor and performs feedback control of the air-fuel ratio based on the detection result. Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an air-fuel ratio control system according to the present embodiment. As shown in FIG.
The internal combustion engine is configured as a four-cylinder, four-cycle spark ignition engine (hereinafter, referred to as engine 1). The intake air is supplied from upstream to an air cleaner 2, an intake pipe 3, a throttle valve 4, a surge tank 5, and an intake manifold 6.
And is mixed with fuel injected from the fuel injection valve 7 for each cylinder in the intake manifold 6. Then, the mixture is supplied to each cylinder as a mixture having a predetermined air-fuel ratio.

A high voltage supplied from an ignition circuit 9 is distributed and supplied to a spark plug 8 provided in each cylinder of the engine 1 through a distributor 10, and the ignition plug 8 controls the mixture of each cylinder at a predetermined timing. To ignite. Exhaust gas discharged from each cylinder after the combustion passes through an exhaust manifold 11 and an exhaust pipe 12 to reach HC,
Three-way catalyst 13 for purifying three components of CO and NOx
And a NOx catalyst 14 for purifying NOx in exhaust gas
After passing through, it is discharged to the atmosphere.

Here, the NOx catalyst 14 mainly stores NOx during combustion at a lean air-fuel ratio, and reduces the stored NOx with rich components (CO, HC, etc.) during combustion at a rich air-fuel ratio. discharge. Further, the three-way catalyst 13 has a smaller capacity than the NOx catalyst 14, and is activated early after the low temperature start of the engine 1 to purify harmful gases, and has a role as a so-called start catalyst.

The intake pipe 3 is provided with an intake air temperature sensor 21 and an intake air pressure sensor 22. The intake air temperature sensor 21 indicates the temperature of the intake air (intake air temperature Tam), and the intake air pressure sensor 22 is located downstream of the throttle valve 4. Each of the intake pipe negative pressures (intake pressure PM) is detected. The throttle valve 4 is provided with a throttle sensor 23 for detecting the opening of the valve 4 (throttle opening TH). The throttle sensor 23 outputs an analog signal corresponding to the throttle opening TH. The throttle sensor 23 also has a built-in idle switch, and outputs a detection signal indicating that the throttle valve 4 is substantially fully closed.

A water temperature sensor 24 is provided on a cylinder block of the engine 1.
The temperature of the cooling water circulating in the inside (cooling water temperature Thw) is detected. The distributor 10 is provided with a rotation speed sensor 25 for detecting the rotation speed of the engine 1 (engine rotation speed Ne). The rotation speed sensor 25 is provided at equal intervals every two rotations of the engine 1, that is, every 720 ° CA. Outputs 24 pulse signals.

Further, an A / F sensor 26 of a limiting current type is disposed upstream of the three-way catalyst 13 in the exhaust pipe 12, and the A / F sensor 26 detects the oxygen concentration of the exhaust gas discharged from the engine 1. (Or CO concentration in unburned gas)
Outputs a wide-area and linear air-fuel ratio signal (AF) in proportion to. The A / F sensor 26 includes a heater 47 for activating the element section (solid electrolyte and diffusion resistance layer). As the A / F sensor 26, a cup-type sensor having an element portion formed in a cup-shaped cross section, or a stacked sensor in which a plate-shaped element portion and a heater 47 are stacked can be applied.

The ECU 30 includes a CPU 31, a ROM 32,
The RAM 33, the backup RAM 34, and the like are configured as a logical operation circuit, and are connected via a bus 37 to an input port 35 for inputting a detection signal of each sensor and an output port 36 for outputting a control signal to each actuator and the like. I have. The ECU 30 detects the detection signals (intake temperature Tam, intake pressure PM, throttle opening T
H, cooling water temperature Thw, engine speed Ne, air-fuel ratio signal, etc.) are input through the input port 35.

Then, control signals such as the fuel injection amount TAU and the ignition timing Ig are calculated based on these values, and the control signals are further transmitted through the output port 36 to the fuel injection valve 7.
And to the ignition circuit 9 and the like. CPU3
Reference numeral 1 denotes duty control of the amount of power supplied to the heater of the A / F sensor 26 to maintain the sensor 26 in an active state. In the present embodiment, a necessary amount of electric power is supplied to the heater 47 of the A / F sensor 26, and the element temperature of the sensor 26 is maintained in the active temperature range.

Next, the operation of the air-fuel ratio control system configured as described above will be described. 2 to 6 show the CPU 31
Is a flow chart showing an air-fuel ratio control routine executed by the routine shown in FIG. 1, and is executed every fuel injection of each cylinder (in this embodiment, every 180 ° CA). In the routine of FIG. 2, based on the detection result of the A / F sensor 26, feedback control of the air-fuel ratio is performed in an air-fuel ratio region leaner than the stoichiometric air-fuel ratio, and the air-fuel ratio is temporarily controlled during the air-fuel ratio lean control. Fuel ratio rich control is performed.

When the air-fuel ratio control routine starts, C
The PU 31 first sets a rich control flag XR indicating that the air-fuel ratio rich control is being performed in step 101 of FIG.
It is determined whether EX is “0”. Where XRE
X = 0 indicates that rich control is not being performed, that is, lean control is being performed, and XREX = 1 indicates that rich control is being performed. When the IG key is turned on (when the power is turned on), the flag XREX is cleared to "0" by the initialization processing.

If XREX = 0, the CPU 31 proceeds to step 102, where the NOx amount NOMO contained in the exhaust gas
Estimate L (mol). In estimating the NOMOL value, for example, a NOx basic amount according to the engine speed Ne and the intake pressure PM at each time is obtained using the map in FIG. 10A, and the NOx basic amount is obtained using the relationship in FIG. An A / F correction value corresponding to the air-fuel ratio at each time is obtained. Then, the NOx basic amount is multiplied by the A / F correction value, and the product is calculated as the NOx amount NOM.
OL (NOMOL = NOx basic amount / A / F correction value).

In FIG. 10 (a), the higher the engine speed Ne or the higher the intake pressure PM, the higher the NOx
The base amount is set to a large value. Further, in FIG. 10B, the A / F correction value = 1.0 is set at the stoichiometric air-fuel ratio (λ = 1), and the A / F value of “1.0” or more is set on the lean side.
The correction value is set. However, when the air-fuel ratio is leaner than a certain level (for example, A / F> 16), the combustion temperature drops, so that further correction on the increasing side becomes unnecessary, and the A / F correction value converges to a predetermined value.

Thereafter, the CPU 31 calculates the NOx integrated amount NOMOLAD in step 103. At this time, the NOMOL value calculated in step 102 is
The AD value is added to the previous value, and the sum is set as the current value of the NOMOLAD value (NOMOLAD = NOMOLAD + NO
MOL).

Further, the CPU 31 determines whether or not the NOx integrated value NOMOLAD calculated in step 104 exceeds a predetermined determination value NOMOLDD. Judgment value N
The OMOLSD may be a fixed value, or may be variably set according to the NOx storage capacity of the NOx catalyst 14 using, for example, the relationship shown in FIG. The NOx storage capacity corresponds to the degree of deterioration of the NOx catalyst 14, and means that the higher the NOx storage capacity, the smaller the degree of deterioration of the NOx catalyst 14.

If NOMOLAD ≦ NOMOLSD (NO in step 104), the CPU 31 proceeds to step 10
Go to 5. The CPU 31 performs a target air-fuel ratio AFTG setting process in step 105, and calculates a fuel injection amount TAU based on the AFTG value in a subsequent step 106. In this case, the basic injection amount is corrected by the air-fuel ratio correction coefficient according to the deviation between the target air-fuel ratio AFTG and the actual air-fuel ratio AF (the detection value of the A / F sensor 27), and other various correction coefficients. An injection amount TAU is calculated. Then, the driving of the fuel injection valve 7 is controlled based on the fuel injection amount TAU. That is, when the answer to step 104 is NO, the air-fuel ratio lean control up to that point is continuously performed.

Here, the process of setting the target air-fuel ratio AFTG will be described with reference to the flowchart of FIG. 6 where AFTG = AFlean. In this flowchart, when switching from rich control to lean control, it is necessary to prevent a sudden change in torque caused by a sudden change in the air-fuel ratio,
It is intended to prevent the generation of a large amount of NOx by promptly passing through an air-fuel ratio region where a large amount of NOx is generated. Therefore, the target air-fuel ratio AFTG is controlled so as to gradually follow the final target air-fuel ratio so as to suppress a large amount of NOx and suppress torque fluctuation.

First, at step 501, a final target air-fuel ratio is set in accordance with the operation state. In this flowchart, the final target air-fuel ratio may be set to a lean air-fuel ratio according to the operating state, or may be set to a fixed value of the air-fuel ratio “1.5”. Then, in step 502, a predetermined value κ is added to the previous target air-fuel ratio AFTG, and step 503 is performed.
Proceed to. The predetermined value κ is set to a value that can suppress a sudden change in torque and can suppress generation of a large amount of NOx. The predetermined value κ may be set variably in accordance with the air-fuel ratio range. When the air-fuel ratio is lean, torque fluctuation is likely to occur, so the predetermined value κ is set to a small value, and the air-fuel ratio in which a large amount of NOx is generated is set. At the fuel ratio of 16 to 18, the predetermined value κ may be set to be large so as to quickly pass through the air-fuel ratio region.

Next, the guard processing when the target air-fuel ratio AFTG reaches the final target air-fuel ratio will be described with reference to steps 503 and 504. In step 504, it is determined whether or not the current target air-fuel ratio AFTG has reached the final target air-fuel ratio. Here, if it is determined that it has not reached, this routine is terminated as it is. If it is determined in step 503 that the target air-fuel ratio AFTG has reached the final target air-fuel ratio, the process proceeds to step 504, where the final target air-fuel ratio is set as the target air-fuel ratio AFTG, and the routine ends. By controlling in this way, when switching from rich control to lean control, it is possible to suppress the deterioration of drivability due to torque fluctuation and to suppress the generation of a large amount of NOx.

As the air-fuel ratio control, if the combustion is performed at a lean air-fuel ratio, the NO stored in the NOx catalyst 14
The x integrated amount NOMOLAD gradually increases. The purification rate of the NOx catalyst decreases as the NOx storage amount increases. Therefore, when the NOx storage amount before the purification rate is reduced is reached, the control is switched to the rich control in order to reduce and release NOx. More specifically, NOMOLAD> NOMOLSD
(YES in step 104), the CPU 31 sets “1” in the rich control flag XREX in step 107. Further, the CPU 31 calculates a reference rich area DRAFND corresponding to the NOx integrated amount NOMOLAD in the subsequent step 108. Here, the reference rich area DR
AFND is equivalent to a rich control amount required to reduce and release all NOx stored in the NOx catalyst 14, and is obtained, for example, according to the NOx integrated amount NOMOLAD and the intake pressure PM using the relationship of FIG. . In FIG. 12, the intake pressure P
The reference rich area DRAFND is set to a larger value as the NOx integrated amount NOMOLAD is larger in a state where M is small.

However, in the relationship of FIG. 12, the intake pressure PM is used as a parameter, but this parameter (PM) is removed or this parameter (PM) is changed to a parameter such as the engine speed Ne or the intake air amount. May be.

Then, in step 109, the three-way catalyst 1
3 is calculated, and a reference rich area DRAFO2 for releasing the stored oxygen amount is calculated. After that, the CPU 31 sets the target air-fuel ratio AFTG in step 109, calculates the fuel injection time TAU according to the target air-fuel ratio AFTG in step 106, and ends this routine. Step 110: target air-fuel ratio AF
In the TG setting process, A / A
This processing is repeated until the actual air-fuel ratio AF detected by the F sensor 26 becomes rich or reaches a predetermined value. As the predetermined value, it is preferable to detect the air-fuel ratio immediately before the oxygen stored in the three-way catalyst 14 is consumed by the rich component. As an element consuming oxygen, the air-fuel ratio immediately before the HC and CO concentrations increase becomes large. Good fuel ratio.

The setting process of the target air-fuel ratio AFTG will be described with reference to the flowchart of FIG. 3 in which AFTG = AFrch1. This processing is performed after switching to rich control.
The process of step 110 in FIG. 2 is repeatedly performed until it is detected that the air-fuel ratio detected by the F sensor 26 is the air-fuel ratio that consumes the oxygen stored in the three-way catalyst 13.

First, at step 201, a rich air-fuel ratio is set as a final target air-fuel ratio in accordance with the operation state. The final target air-fuel ratio may be set to a stoichiometric air-fuel ratio as a fixed value. Then, at step 202, a predetermined value α is subtracted from the previous target air-fuel ratio AFTG to set the current target air-fuel ratio AFTG. The predetermined value α is set to such a value that the torque does not suddenly change and a large amount of NOx is not generated. The predetermined value α may be variably set in accordance with the air-fuel ratio range. When the air-fuel ratio is lean, torque fluctuation is likely to occur, so the predetermined value α is set to a small value, and the air-fuel ratio in which a large amount of NOx is generated is set. For the fuel ratios 16 to 18, the predetermined value α may be set to be large so as to quickly pass through the air-fuel ratio region.

Next, the guard process when the target air-fuel ratio AFTG reaches the final target air-fuel ratio will be described with reference to steps 203 and 204. In step 204, it is determined whether or not the current target air-fuel ratio AFTG has reached the final target air-fuel ratio. Here, if it is determined that it has not reached, this routine is terminated as it is. If it is determined in step 203 that the target air-fuel ratio AFTG has reached the final target air-fuel ratio, the process proceeds to step 204. The final target air-fuel ratio is set to the target air-fuel ratio AFTG, and this routine ends. The target air-fuel ratio AFT set in this way
When the setting of G is performed, in the subsequent step 106, the AF
The fuel injection amount TAU is calculated based on the TG value.

The above processing is executed by the CPU 31 at step 10.
1 and the process proceeds to step 111 in FIG.
It is determined whether or not the actual air-fuel ratio AF detected by the sensor 26 is rich, and the process is repeatedly performed until it is determined that the actual air-fuel ratio AF is rich.

On the other hand, if it is determined that the actual air-fuel ratio AF detected by the A / F sensor 26 has become rich, the routine proceeds to step 112. In step 112, the air-fuel ratio reference value A
The actual air-fuel ratio AF detected by the A / F sensor 26 is subtracted from the FSD (for example, the stoichiometric air-fuel ratio), and the difference is defined as a rich deviation DRAF (DRAF = AFSD-AF). Then, the CPU 31 proceeds to step 112 and calculates the actual rich area DRAFAD. At this time, step 1
The DRAF value calculated in step 10 is added to the previous value of the DRAFAD value, and the sum is added to the current value of the DRAFAD value and (DRAFAD value).
AD = DRAFAD + DRAF), and step 114
Proceed to.

In step 114, the actual rich area DRA
It is determined whether the FAD is larger than a reference rich area DRAFO2 required to consume the amount of oxygen stored in the three-way catalyst 14. Here, if it is determined that the actual rich area DRAFAD is larger than the actual rich area DRAFO2, the process proceeds to step 115, performs a target air-fuel ratio setting process, and proceeds to step 117. In step 114, the actual rich area DRAFAD is changed to the reference rich area DRAFO2.
If it is determined that the value is smaller than
A target air-fuel ratio setting process is performed, and the routine proceeds to step 117.

When the actual rich area DRAFAD is smaller than the reference rich area DRAFO2, the rich component supplied to the three-way catalyst 14 is used to consume the oxygen stored in the three-way catalyst 14, so that NOx Catalyst 15
Is not supplied with a rich component. Therefore, in order to quickly consume the oxygen stored in the three-way catalyst 14, a target air-fuel ratio setting process in step 105 is performed. In the process of step 105, the oxygen stored in the three-way catalyst 14 is quickly consumed by rapidly switching the air-fuel ratio to rich.

The details will be described with reference to the flow chart of AFTG = AFrchfst in FIG.
The final target air-fuel ratio is set in accordance with the operation state, and the routine proceeds to step 302. The final target air-fuel ratio may be a fixed value, for example, may be an air-fuel ratio “0.75”. In step 302, the current target air-fuel ratio is calculated by subtracting a predetermined value β from the previous target air-fuel ratio AFTG.
The predetermined value β is a value set for quickly consuming the oxygen stored in the three-way catalyst 14, and therefore, is larger than the predetermined value α, the predetermined value κ, and the predetermined value γ described later. Is set.

Thereafter, it is determined whether or not the target air-fuel ratio AFTG has reached the final target air-fuel ratio. Target air-fuel ratio AFT
If G has not reached the final target air-fuel ratio, this routine ends. On the other hand, if the final target air-fuel ratio has been reached, the routine proceeds to step 304, where the final target air-fuel ratio is set to the target air-fuel ratio AFTG, and this routine ends. Thus, in step 115 of FIG. 2, the target air-fuel ratio A
By setting the FTG, the oxygen stored in the three-way catalyst 14 can be quickly consumed.

At step 114, if the actual rich area DRAFAD is larger than the reference rich area DRAFO2 with respect to the amount of oxygen stored in the three-way catalyst 14, that is, all the oxygen stored in the three-way catalyst 14 is consumed. If so, the process proceeds to step 116. In step 116, since all the oxygen stored in the three-way catalyst 14 has been consumed, the rich component obtained by subtracting the reference air-fuel ratio AFSD from the actual air-fuel ratio AF detected by the A / F sensor 26 is stored in the NOx catalyst. It is used to reduce and release the NOx that has been used. Therefore, as a subroutine of step 116, in the flowchart of FIG. 5, the target air-fuel ratio AFTG is set so as to suppress the torque fluctuation and reduce the generation of NOx. When the air-fuel ratio is rich, torque fluctuation does not occur as compared to when the air-fuel ratio is lean. Therefore, the predetermined value γ added to the previous target air-fuel ratio AFTG can be set to a value larger than the above-mentioned predetermined value β.

The flowchart of FIG. 5 for AFTG = AFrch2 will be described below. First, in step 401, a final target air-fuel ratio is set. A rich air-fuel ratio is set as the final target air-fuel ratio in accordance with the operation state. This final target air-fuel ratio may be set to “0.75” as a fixed value. Then, in step 502, a predetermined value γ is subtracted from the previous target air-fuel ratio AFTG to set the current target air-fuel ratio AFTG. The predetermined value γ is set to such a value that the torque does not suddenly change and NOx is not generated in a large amount.

Next, the guard processing when the target air-fuel ratio AFTG reaches the final target air-fuel ratio will be described with reference to steps 503 and 504. In step 504, it is determined whether or not the current target air-fuel ratio AFTG has reached the final target air-fuel ratio. Here, if it is determined that it has not reached, this routine is terminated as it is. If it is determined in step 503 that the target air-fuel ratio AFTG has reached the final target air-fuel ratio, the process proceeds to step 504. The final target air-fuel ratio is set to the target air-fuel ratio AFTG, and this routine ends. The target air-fuel ratio AFT set in this way
When the setting of G is performed, in the following step 117, FIG.
The actual rich area DRAF calculated in step 113
AD is the reference rich area DRAFND (step 10 described above).
It is determined whether or not the calculated value exceeds the value obtained by adding the reference rich area to the oxygen storage amount (the calculated value in step 109) to the oxygen storage amount. DRAFAD ≦ DRAFND + DRAF
In the case of O2 (NO in step 117), the CPU 31 continues the air-fuel ratio rich control up to that time by the processing in step 106 in FIG.

DRAFAD> DRAFND + DR
In the case of AFO2 (YES in step 117), the CPU 3
1 proceeds to step 118, where the rich control flag XRE
X, NOx integrated amount NOMOLAD and actual rich area DR
Clear all AFAD to “0”. And then C
The PU 31 proceeds to step 106 in FIG. Thus, if step 117 is YES, the air-fuel ratio rich control up to that point is terminated, and the air-fuel ratio lean control is restarted.

FIG. 9 is a time chart showing the above control operation more specifically. Here, FIG. 9A shows the behavior of the air-fuel ratio before the three-way catalyst 14 in the present embodiment,
FIG. 9B shows the behavior of the air-fuel ratio after the three-way catalyst 14. In FIG. 9A, the lean control is performed until time t1. In the combustion with a lean air-fuel ratio, the NOx component contained in the exhaust gas is large as shown in FIG. 9C (NOx−IN), so that NOx discharged to the NOx catalyst 15 is stored. In the lean combustion, the amount of oxygen stored in the three-way catalyst 14 also increases. If it is determined that the amount of NOx stored in the NOx catalyst 15 has exceeded the determination value set based on the storage capacity of the NOx catalyst, the NOx stored in the NOx catalyst 14 is reduced and released in order to reduce and release the NOx stored in the NOx catalyst 14.
At time t1, the control is switched to the rich air-fuel ratio control as shown in FIG.

At time t2 until the air-fuel ratio reaches the stoichiometric air-fuel ratio, torque fluctuation is likely to occur in lean combustion, and NOx in the air-fuel ratio regions 16 to 18 as shown in FIG.
The air-fuel ratio is gradually shifted to the rich side in consideration of the increase in the concentration. In the transition to the rich side, the predetermined value α is set so that torque fluctuation does not easily occur and the vehicle can quickly pass through the air-fuel ratio region where the NOx concentration is large. After time t2, the air-fuel ratio before the three-way catalyst 14 becomes rich and consumes oxygen stored in the catalyst, so that the air-fuel ratio after the three-way catalyst 14 becomes as shown in FIG. 9C. Until time t3, it is maintained near the stoichiometric air-fuel ratio. From time t2 to time t3, the air-fuel ratio is quickly shifted to the rich side as shown in FIG. 9A in order to consume the stored oxygen. In FIG. 9A, the target air-fuel ratio AFT
When G reaches the final target air-fuel ratio, it is guarded by the final target air-fuel ratio. Since the air-fuel ratio is quickly shifted to the rich state as described above, the oxygen stored in the three-way catalyst 14 can be quickly consumed, and based on the lean operation state until time t1, the three-way catalyst 14 Since the amount of oxygen stored in the fuel cell is estimated, the consumption of oxygen can be realized with high accuracy.

When it is determined at time t3 that the oxygen stored in the three-way catalyst 14 has been consumed, the subsequent rich components are used to reduce and release the NOx stored in the NOx catalyst 15. Therefore, the NOx integrated value NO calculated in step 103 of the flowchart of FIG.
NOx can be reduced and released without any excess or shortage based on MOLAD. As shown in FIG. 9A, at time t4, the air-fuel ratio is shifted to the lean side to release the stored NOx, and when the air-fuel ratio reaches time t5,
If the air-fuel ratio before the three-way catalyst 14 is controlled to be close to the stoichiometric air-fuel ratio, NOx can be reduced and released more accurately. In this way, the amount of oxygen stored in the three-way catalyst 14 is controlled at the time t as shown in FIG.
Since it can be consumed between 2 and t3, it is possible to quickly reduce and release NOx when switching from lean control to rich control.

The NOx catalyst uses Na or Ba as the metal (Pt / Rh / Pd + CeO2) used for the three-way catalyst.
For example, the NOx catalyst itself has the ability to store oxygen because it carries a metal capable of storing NOx. Therefore, control may be performed so that the air-fuel ratio is quickly shifted to the rich side so that the oxygen stored in the NOx catalyst is also consumed quickly.

In this embodiment, the amount of oxygen stored in the three-way catalyst 14 is estimated from the operating state at the time of the lean control, whereby the stored oxygen can be quickly consumed, and the subsequent rich control can be performed. , A rich component necessary for accurately reducing and releasing the NOx storage amount can be supplied. In this configuration, by arranging the A / F sensor before the three-way catalyst, the control of the present embodiment can be performed, so that accurate control can be performed without providing another sensor. Become.

In this embodiment, the switching means performs steps 111 to 1 in the flowchart of FIG.
16 and the oxygen amount estimating means corresponds to step 109 in the flowchart of FIG. 2 and functions.

<Second Embodiment> In the first embodiment, the fuel is stored in the three-way catalyst 14 based on the air-fuel ratio detected by the A / F sensor 26 disposed upstream of the three-way catalyst 14. The amount of oxygen stored and the amount of NOx stored in the NOx catalyst 15
Was estimated. Then, based on the estimated amount of oxygen and the amount of stored NOx, by supplying rich components necessary for consuming, reducing and releasing these, the amount of NOx stored is not too much or too little.
Has been released.

In this embodiment, by arranging the oxygen concentration sensor 27 downstream of the three-way catalyst 14, it is possible to detect that the oxygen stored in the three-way catalyst has been consumed.
The amount of NOx stored in the x catalyst can be accurately reduced and released.

FIG. 13 shows a schematic configuration diagram of the present embodiment. An A / F sensor 2 is provided in an exhaust passage from the engine 1.
6 and a three-way catalyst 14 is provided downstream thereof.
An oxygen concentration sensor 27 that detects oxygen concentration and outputs electric power according to the oxygen concentration is provided between the three-way catalyst 14 and the NOx catalyst 15. In this embodiment, the state of oxygen stored in the three-way catalyst 14 can be accurately detected from the two sensors.

This embodiment will be described with reference to the flowchart of FIG. Steps in which the same processing as in the flowchart of FIG. 2 is performed are denoted by the same reference numerals, and description thereof is omitted. First, in step 101, it is determined whether or not a flag XREX indicating whether to perform rich control is 1 or not. If it is determined that the value is not 1, that is, if it is determined that the lean control is to be performed, steps 102 to 1 are performed.
At 03, the amount of NOx stored in the NOx catalyst is calculated based on the lean air-fuel ratio detected by the A / F sensor 26. The calculation of the NOx amount is performed by the oxygen concentration sensor 2
7 may be performed. In this case, since the air-fuel ratio at a position closer to the NOx catalyst can be detected, the amount of NOx stored in the NOx catalyst can be calculated more accurately. When it is determined that the amount of NOx stored in the NOx catalyst 15 exceeds a predetermined determination value due to the continuation of the lean combustion, 1 is input to the flag XREX in step 107, and the switching control to the rich combustion is performed. Be executed.

If it is determined in step 101 that the flag XREX is 1, the processes in and after step 601 are repeatedly performed. In step 601, it is determined whether the air-fuel ratio detected by the A / F sensor 26 has become rich. If it is determined that the air-fuel ratio is not yet rich, the process proceeds to step 110, and the AFTG setting process is executed. In the AFTG setting process of step 110,
AFTG is set so as to suppress the NOx concentration from becoming high and to suppress the occurrence of a large torque shock.

When the air-fuel ratio (AF) before the three-way catalyst becomes rich, the routine proceeds to step 602, where it is determined whether the air-fuel ratio (rear AF) detected by the oxygen concentration sensor 27 is larger than the reference air-fuel ratio AFSD2. Is determined. When the AF is rich and the rear AF is smaller than the reference air-fuel ratio AFSD2, the oxygen stored in the three-way catalyst 14 has been consumed, so that no rich component is supplied to the NOx catalyst. Therefore, in order to promptly consume the oxygen stored in the three-way catalyst 14, the target air-fuel ratio AFTG setting process of step 115 is performed as shown in the first embodiment, and the process proceeds to step 106.

Thereafter, when it is determined that the rear AF detected by the A / F sensor 26 has become richer than the reference air-fuel ratio AFSD2, the oxygen stored in the three-way catalyst 14 is consumed, and the NOx catalyst 15 A rich component is supplied. Therefore, in step 116, the target air-fuel ratio AFTG
Is set as shown in the first embodiment, and based on the rich component set here, N
An actual rich area for reducing and releasing Ox is obtained, and the process proceeds to step 604. In step 604, the NOx integrated value NOMOLA stored in the NOx catalyst by the lean control
The reference rich area DRAFND for D and the actual rich area DRAFAD are compared. At this time, the actual rich area D
When RAFAD exceeds the reference rich area DRAFND,
All the NOx stored in the NOx catalyst 15 is reduced and
Switch to lean control assuming release.

In this embodiment, the provision of the oxygen concentration sensor 27 after the three-way catalyst 14 makes it possible to accurately detect before or after the oxygen stored in the three-way catalyst 14 is consumed. . At this time, by rapidly switching the target air-fuel ratio AFTG to rich, the oxygen stored in the three-way catalyst 14 can be quickly consumed, and the NOx stored in the NOx catalyst 15 can be reduced without excess or shortage.・ Can be released.

Incidentally, the NOx catalyst includes a metal (Pt / Rh / Pd + CeO ₂ ) used for a three-way catalyst carrying a metal capable of storing NOx such as Na or Ba. Some have oxygen storage capacity. For this reason, even if it is detected by the oxygen concentration sensor that the air-fuel ratio after the three-way catalyst has become rich, it is necessary to quickly make the air-fuel ratio rich in order to consume the oxygen in the NOx catalyst. In the present embodiment, the three-way catalyst 14
The rate of change of the air-fuel ratio was increased only in order to consume the amount of oxygen stored in the NOx catalyst, but when a NOx catalyst having an oxygen storage capacity was provided as described above, the amount of oxygen stored in the NOx catalyst was estimated. Then, the switching control of the air-fuel ratio may be performed.

In this embodiment, the switching means functions and corresponds to step 602 in the flowchart of FIG.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system for an engine according to a first embodiment.

FIG. 2 is a flowchart showing an air-fuel ratio control routine.

FIG. 3 is a flowchart illustrating an air-fuel ratio control routine.

FIG. 4 is a flowchart illustrating an air-fuel ratio control routine.

FIG. 5 is a flowchart showing an air-fuel ratio control routine.

FIG. 6 is a flowchart showing an air-fuel ratio control routine. N
Reference rich area D corresponding to Ox integrated amount NOMOLAD
FIG. 9 is a diagram for setting RAFND.

FIG. 7 is a flowchart illustrating an air-fuel ratio control routine according to the second embodiment.

FIG. 8 is a diagram showing the NOx concentration with respect to the air-fuel ratio.

FIG. 9 is a timing chart when air-fuel ratio control is performed in the first embodiment.

FIG. 10A is a diagram for obtaining a basic NOx amount,
(B) is a diagram for obtaining a correction value.

FIG. 11 is a determination value NOMOLS according to NOx storage capacity;
The figure for setting D.

FIG. 12 is a diagram for setting a reference rich time DRAFND corresponding to the NOx integrated amount NOMOLAD.

FIG. 13 is a schematic configuration diagram of a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Engine, 12 ... Exhaust pipe, 13 ... Three-way catalyst as an upstream side catalyst, 14 ... NOx catalyst (NOx storage reduction type catalyst), 26 ... A / F sensor as an oxygen concentration sensor, 27 ... As an oxygen concentration sensor O2 sensor, 30 ... ECU (electronic control unit), 31 ... CPU as central processing unit.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) F02D 41/04 305 F02D 41/04 330Z 330 43/00 301E 43/00 301 301T 45/00 314Z 45/00 314 368F 368 B01D 53/36 101B 103B F-term (reference) 3G084 AA03 AA04 BA09 BA13 BA24 DA10 DA22 EA11 EB01 EB11 EB12 FA07 FA10 FA11 FA13 FA20 FA26 FA29 FA30 FA33 FA36 3G091 AA12 AB14 AB28 AB28 CB02 CB05 DA01 DA02 DA04 DA10 DB06 DB10 DC01 DC03 EA01 EA06 EA07 EA15 EA16 EA26 EA31 EA34 FA02 FA04 FA12 FA13 FB02 FB10 FB11 FB12 FC02 FC07 GB02Y GB03Y GB04Y GB05W GB06W GB07W HA08 JA10 HA36 HA06 MA03 HA03 HA01 NA08 ND05 NE01 NE06 NE13 NE14 NE15 PA01A PA01B PA01Z PA07A P A07B PA07Z PA10A PA10B PA10Z PA11A PA11B PA11Z PA14A PA14B PA14Z PD02A PD02B PD02Z PD09A PD09B PD09Z PD11A PD11B PD11Z PE01A PE01B PE01Z PE08A PE08B PE08Z PF16A PF16B PF16A BAX BA13A13A14AXA

Claims (3)

[Claims] 1. A lean combustion in an air-fuel ratio lean region is performed, and NO in exhaust gas discharged during the lean combustion is reduced.
x is stored by a lean NOx catalyst, and the stored NOx is stored in rich combustion in which the air-fuel ratio is temporarily controlled to be rich.
An exhaust gas purifying apparatus for an internal combustion engine configured to release oxygen from a lean NOx catalyst, comprising: a catalyst having an oxygen storage capacity disposed upstream of the lean NOx catalyst; and a catalyst having an oxygen storage capacity disposed upstream of the catalyst having the oxygen storage capacity. An air-fuel ratio sensor that detects an air-fuel ratio in the exhaust passage; and an air-fuel ratio change amount per unit time when switching between the rich combustion and the lean combustion based on the amount of NOx stored in the NOx catalyst. Air-fuel ratio change amount setting means for setting the air-fuel ratio change amount per unit time based on the air-fuel ratio detected by the air-fuel ratio sensor. An exhaust gas purification device for an internal combustion engine. 2. An oxygen amount estimating means for estimating the amount of oxygen stored in the catalyst having the oxygen storage capacity and / or the NOx catalyst, wherein the air-fuel ratio change amount setting means is provided by the oxygen amount estimating means. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio change amount per unit time is changed based on the estimated oxygen amount. 3. An oxygen concentration sensor disposed upstream of the catalyst and detecting an oxygen concentration in exhaust gas, and the air-fuel ratio change amount setting means includes: an air-fuel ratio sensor; And / or detecting a state of oxygen stored in the NOx catalyst and changing an air-fuel ratio change amount per unit time based on the estimated oxygen amount. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1.