JP2000018062A - Air-fuel ratio controller of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To supply a rich component necessary for the reduction and emission of occluded nitrogen oxide (NOx) in a lean NOx catalyst, neither too much nor too less. SOLUTION: A three way catalyst 13 and a nitrogen oxide occluded reduction type catalyst (a nitrogen oxide catalyst) 14 are installed in an engine exhaust pipe 12 in series, while an air-fuel sensor 27 for measuring the extent of oxygen concentration (rich degree) of exhaust gases just before the nitrogen oxide catalyst is installed in a space between both these catalysts 13 and 14. A central processing unit 31 in an electronic control unit 3O makes it perform a lean combustion at an air-fuel ratio lean area, and occludes nitrogen oxides in the exhaust gas discharged in time of this lean combustion with the nitrogen oxide catalyst 14, and further this central processing unit 31 temporarily controls an air-fuel ratio into being rich, thereby having the occluded nitrogen oxide emitted out of the nitrogen oxide catalyst 14. Likewise, this central processing unit 31 calculates nitrogen oxide integrated quantity to be occluded in the nitrogen oxide catalyst 14 in time of air-fuel ratio lean control, whereby a reference rich quantity necessary for the rich control is calculated according to the calculated value. Then, on the basis of an output of the air-fuel sensor 27, the rich control is continuously performed as far as only the reference rich quantity.

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 device for an internal combustion engine that performs lean combustion in an air-fuel ratio lean region, and for purifying nitrogen oxides (NOx) in exhaust gas generated during lean combustion. The present invention relates to an air-fuel ratio control device for an internal combustion engine having a NOx storage reduction type catalyst.

[0002]

2. Description of the Related Art In recent years, in an air-fuel ratio control apparatus for an internal combustion engine, a technique of performing so-called lean burn control, in which fuel is burned on a lean side from a stoichiometric air-fuel ratio, in order to improve fuel efficiency, is being used frequently. When performing such lean combustion, NOx is contained in exhaust gas discharged from the internal combustion engine.
NOx is contained in the atmosphere from the viewpoint of environmental protection.
Emissions need to be reduced. Therefore, conventionally, for example, a NOx storage reduction catalyst (lean NOx catalyst) is provided in an engine exhaust pipe to purify exhaust NOx, and a NOx sensor for detecting a NOx concentration is provided in an engine exhaust pipe, and NOx in exhaust gas is provided. There were techniques for monitoring concentrations.

On the other hand, NOx generated during lean combustion
In the system where NOx is absorbed by the NOx catalyst, the NOx purification capacity reaches a limit when NOx is saturated by the NOx catalyst. Therefore, the purification ability of the NOx catalyst is restored and NO
There is known a technique in which rich combustion is temporarily performed to suppress the emission of x.

[0004]

However, in the above-mentioned prior art, when the control air-fuel ratio is temporarily changed from lean to rich, some allowance is taken into consideration in consideration of the delay in exhaust transportation in the exhaust pipe. We had to set a rich time. In such a case, there is a concern that the fuel increase will be excessive due to the setting of the rich time longer, leading to deterioration of fuel efficiency. Further, at the time of the rich combustion, the engine generated torque increases as compared with the time of the lean combustion. Therefore, if the rich combustion is prolonged, the fluctuation of the rotation becomes large, and there is a problem that the drivability deteriorates. Conversely, if the rich time is too short, the stored NO in the NOx catalyst
x may be insufficiently reduced, and the NOx gas may be released to the atmosphere without being purified.

[0005] Incidentally, as a prior art related to the present invention,
For example, in an "exhaust gas purifying apparatus for an internal combustion engine" disclosed in JP-A-7-189660, a NOx absorbent (NOx
A catalyst is provided, and an air-fuel ratio sensor is provided upstream of the NOx absorbent, and the NOx absorption amount of the NOx absorbent is estimated based on the detection result of the air-fuel ratio sensor. However, although the device of the above publication describes monitoring the NOx absorption amount, it does not disclose a control method for performing the optimal rich control, and does not disclose the control method of the NOx absorbed by the NOx absorbent. It was not possible to supply the rich components necessary for reduction and release without excess or shortage.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to supply a rich component necessary for reducing and releasing stored NOx in a lean NOx catalyst without excess or shortage. It is to provide an air-fuel ratio control device for an internal combustion engine that can be used.

[0007]

SUMMARY OF THE INVENTION An air-fuel ratio control apparatus for an internal combustion engine according to the present invention performs lean combustion in an air-fuel ratio lean region, and emits NO in exhaust gas discharged during lean combustion.
x is stored in a lean NOx catalyst, and the air-fuel ratio is temporarily controlled to be rich to convert the stored NOx into lean NOx.
It is assumed that the catalyst is released.

According to the first aspect of the present invention, an oxygen concentration sensor disposed upstream of the lean NOx catalyst for detecting an oxygen concentration in exhaust gas and the oxygen concentration sensor when the air-fuel ratio is richly controlled are provided. Rich control means for performing rich control of the air-fuel ratio while monitoring the degree of richness of the exhaust gas sent to the NOx catalyst based on the output of the oxygen concentration sensor.

In short, the stored NOx of the lean NOx catalyst
It is necessary to supply a rich component in an amount commensurate with the stored NOx in order to reliably reduce and release NO. In this case, if the rich component is supplied in excess (rich time is too long),
Problems such as deterioration of fuel efficiency, torque fluctuation, and emission of HC and CO components occur. If the rich component is insufficient (rich time is too short), a problem such as emission of unpurified NOx occurs. It is considered that such a problem is caused by performing the air-fuel ratio rich control to supply a certain amount of the rich component to the lean NOx catalyst.

On the other hand, according to the present invention, since the rich degree of the exhaust gas is monitored based on the output of the oxygen concentration sensor, and the rich control of the air-fuel ratio is performed, it is necessary to reduce and release the stored NOx. Only the rich component can be appropriately supplied. In the rich control, the stored NOx of the lean NOx catalyst is reduced by reacting with the rich component, and the NOx storage amount is controlled to be always less than or equal to a certain amount. As a result, the stored N in the lean NOx catalyst
The rich components necessary for the reduction and release of Ox can be supplied without excess and deficiency, and the conventional problems can be solved.

Incidentally, in the oxygen concentration sensor, a rich component and a lean component react at the electrode, and after the reaction, a signal corresponding to an excess component is output. In this case, the component of the exhaust gas sent to the NOx catalyst downstream of the sensor is not the exhaust gas itself discharged from the internal combustion engine, but the exhaust gas after the reaction by the oxygen concentration sensor. Therefore, detecting the excess component amount after the reaction by the sensor and performing the air-fuel ratio rich control according to the excess component amount is effective in efficiently reducing and releasing the stored NOx by the lean NOx catalyst. It can be said that it is convenient.

In particular, when an upstream catalyst having an oxygen storage function is provided upstream of the oxygen concentration sensor, the effect of the present invention is further enhanced. That is, in the upstream side catalyst located upstream of the lean NOx catalyst, oxygen is stored during lean combustion.
Is done. In this case, even if the mode is switched from the lean combustion to the rich combustion, immediately after that, the rich component in the exhaust gas reacts with the oxygen stored in the upstream catalyst, and the exhaust gas actually supplied to the NOx catalyst does not become rich. Then, after the reaction between the rich component and the stored oxygen of the upstream catalyst ends, the air-fuel ratio of the exhaust gas shifts to rich.

According to the present invention, by monitoring the degree of richness of the exhaust gas based on the output of the oxygen concentration sensor, it is possible to:-react the rich component with the stored oxygen of the upstream catalyst; The end of the reaction of the catalyst with the stored oxygen can be recognized from time to time, and the air-fuel ratio rich control corresponding to the amount of the reaction with the stored oxygen can be performed. As a result, reduction of stored NOx in the lean NOx catalyst
The rich components required for release can be supplied without excess and deficiency.

If the oxygen storage capacity of the upstream side catalyst is constant, the rich time may be extended by that much, but the oxygen storage capacity changes according to the degree of catalyst deterioration. Therefore, even if the air-fuel ratio rich control in consideration of the oxygen storage capacity is performed, the output of the oxygen concentration sensor uses the output of the oxygen concentration sensor for the oxygen storage amount that changes over time instead of increasing the rich amount uniquely. By performing the control, appropriate rich control can be realized.

According to the third aspect of the present invention, the lean N
An upstream catalyst having a small oxygen storage capacity is disposed upstream of the Ox catalyst, and the oxygen concentration sensor is further disposed upstream of the upstream catalyst. In this case, since the oxygen storage capacity of the upstream catalyst is small, the air-fuel ratio behavior before and after the upstream catalyst substantially matches. That is, the exhaust gas component actually supplied to the NOx catalyst follows the change of the control air-fuel ratio at each time. For example, when switching from lean combustion to rich combustion, the rich component passes through the upstream catalyst and is fed to the NOx catalyst as it is. Therefore, according to the oxygen concentration sensor, the exhaust gas component to the NOx catalyst can be monitored sequentially.

By disposing the oxygen concentration sensor upstream of the upstream catalyst as described above, the distance between the engine body and the sensor is shortened, and the response time from when the air-fuel ratio changes to when the sensor output changes. Is shortened. Therefore, the detection accuracy at the time of the transient operation is also improved. In this case, it is not necessary to provide another sensor immediately before the NOx catalyst (between the upstream catalyst and the NOx catalyst) in order to monitor the exhaust gas component to the lean NOx catalyst.

Actually, as described in claim 4, the amount of NOx stored in the lean NOx catalyst is calculated (NOx amount calculating means), and according to the calculated NOx storage amount of the lean NOx catalyst, A reference rich amount required for rich control is calculated (reference rich amount calculating means). Then, rich control is continuously performed by the calculated reference rich amount (rich control means).

In such a case, by performing the air-fuel ratio rich control so that the stored NOx amount matches the controlled reference rich amount, an appropriate amount of the rich component can be supplied to the lean NOx catalyst. .

The rich control means may include the following:
It is preferable to perform the air-fuel ratio rich control as in claim 7.
In any of these, the excellent effects as described above can be obtained. In the invention described in claim 5, the rich amount integrated value at the time of the air-fuel ratio rich control is obtained from the output of the oxygen concentration sensor, and the rich control is continued until the obtained rich amount integrated value becomes equivalent to the reference rich amount. . In the invention described in claim 6, the time during which the air-fuel ratio changes to the rich side from the stoichiometric air-fuel ratio during the air-fuel ratio rich control is obtained from the output of the oxygen concentration sensor, and the obtained time is equivalent to the reference rich amount. Continue rich control until. According to the seventh aspect of the invention, the time when the air-fuel ratio shifts to the rich side from the stoichiometric air-fuel ratio during the air-fuel ratio rich control and the rich peak value at that time are obtained from the output of the oxygen concentration sensor. The rich control is continued until the control amount based on the time and the rich peak value becomes equivalent to the reference rich amount.

[0020]

DETAILED 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 to be leaner than the stoichiometric air-fuel ratio, and so-called lean burn control for performing lean combustion based on the target air-fuel ratio is performed. As a main configuration of the system, a three-way catalyst and a NOx storage reduction type catalyst (hereinafter, referred to as NOx catalyst) are provided in the middle of the exhaust system passage of the internal combustion engine.
A limiting current type air-fuel ratio sensor (A / F sensor) is provided between the x-catalyst and the x-catalyst. An electronic control device (hereinafter, referred to as an ECU) mainly composed of a microcomputer is provided with an A
The detection result of the / F sensor is taken in, and the air-fuel ratio is feedback-controlled 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 via 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 detects 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).
, Ie, 24 pulse signals are output at equal intervals every 720 ° CA.

Further, a limit current type A / F is provided between the three-way catalyst 13 and the NOx catalyst 14 in the exhaust pipe 12.
A sensor 27 is provided, and the sensor 27 outputs a wide-area and linear air-fuel ratio signal in proportion to the oxygen concentration of the exhaust gas discharged from the engine 1 (or the CO concentration in the unburned gas). The A / F sensor 27 includes a heater 47 for activating the element section (solid electrolyte and diffusion resistance layer). As the A / F sensor 27, 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 output to the fuel injection valve 7 and the ignition circuit 9 via the output port 36, respectively.

Further, the CPU 31 performs duty control on the amount of current supplied to the heater of the A / F sensor 27 to maintain the sensor 27 in an active state. In the present embodiment, the A / F sensor 27
A necessary amount of electric power is supplied to the heater 47 to maintain the element temperature of the sensor 27 in an active temperature range.

Next, the operation of the air-fuel ratio control system configured as described above will be described. FIG. 2 and FIG.
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 FIGS. 2 and 3, feedback control of the air-fuel ratio is performed in the air-fuel ratio region leaner than the stoichiometric air-fuel ratio based on the detection result of the A / F sensor 27, and the air-fuel ratio lean control is temporarily performed during the lean control. The air-fuel ratio rich control is specifically implemented.

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. 4A, 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 NOMOL.
(NOMOL = NOx basic amount · A / F correction value).

Incidentally, in FIG. 4A, the engine speed N
The higher the e is, or the higher the intake pressure PM, the larger the NOx basic amount is set. In FIG. 4B, the A / F correction value is set to 1.0 at the stoichiometric air-fuel ratio (λ = 1), and on the lean side, the A / F correction value of “1.0” or more is set. Is done. 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 NOMOLSD. 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 the higher the NOx storage capacity, the higher the NOx storage capacity.
This means that the degree of deterioration of the NOx catalyst 14 is small.

If NOMOLAD ≦ NOMOLSD (NO in step 104), the CPU 31 proceeds to step 10
Go to 5. The CPU 31 sets the target air-fuel ratio AFTG to “1.5” in step 105, and calculates the fuel injection amount TAU based on the AFTG value in 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.

When the NOx integrated amount NOMOLAD gradually increases and NOMOLAD> NOMOLSD (step 104: YES), the CPU 31 proceeds to step 107.
Sets the rich control flag XREX to "1". Further, the CPU 31 determines in a subsequent step 108 that the reference rich area DRAFND corresponding to the NOx integration amount NOMOLAD is set.
Is calculated. Here, the reference rich area DRAFND is
This corresponds to a rich control amount required to reduce and release all NOx stored in the NOx catalyst 14, and is obtained according to the NOx integrated amount NOMOLAD and the intake pressure PM using, for example, the relationship of FIG. In FIG. 6, when the intake pressure PM is small, N
The larger the Ox integrated amount NOMOLAD is, the larger the reference rich area DRAFND is set.

In the relationship shown in FIG. 6, the intake pressure PM is used as a parameter. However, the parameter (PM) is removed or the parameter (PM) is changed to a parameter such as the engine speed Ne or the intake air amount. May be.

Thereafter, the CPU 31 sets the target air-fuel ratio AFTG to "0.75" in step 109, and calculates the fuel injection amount TAU based on the AFTG value in step 106. As a result, if step 104 is YES, the air-fuel ratio lean control up to that point is switched to rich control.

When the air-fuel ratio control is switched from the lean control to the rich control, the CPU 31 returns to step 101 from the next time.
And proceeds to step 110 in FIG. 3 to calculate the actual air-fuel ratio AF from the air-fuel ratio reference value AFSD (for example, the stoichiometric air-fuel ratio).
And the difference is defined as the rich deviation DRAF (DRA
F = AFSD-AF). Further, the CPU 31 determines whether or not the calculated rich deviation DRAF is a positive value (DRAF> 0) in the subsequent step 111, that is, whether or not the actual air-fuel ratio AF at that time is richer than the air-fuel ratio reference value AFSD. Is determined.

When DRAF ≦ 0 (Step 111 is N
O), the CPU 31 jumps to step 109 in FIG.
By the processing of steps 109 and 106, the air-fuel ratio rich control up to that point is continued.

On the other hand, when DRAF> 0 (step 11)
(1 is YES), the CPU 31 proceeds to step 112 and calculates the actual rich area DRAFAD. At this time, the DRAF value calculated in step 110 is added to the previous value of the DRAFAD value, and the sum is set as the current value of the DRAFAD value (DRAFAD = DRAFAD + DRAF).

Thereafter, the CPU 31 determines whether or not the actual rich area DRAFAD calculated in step 113 exceeds the reference rich area DRAFND (the value calculated in step 108). If DRAFAD ≦ DRAFND (NO in step 113), the CPU 31 jumps to step 109 in FIG. 2 and continues the air-fuel ratio rich control by the processing in steps 109 and 106.

If DRAFAD> DRAFND (YES in step 113), the CPU 31 executes step 1
The program proceeds to 14, where the rich control flag XREX, the NOx integrated amount NOMOLAD and the actual rich area DRAFAD are all cleared to "0". Thereafter, the CPU 31 proceeds to step 105 in FIG. Thus, step 1
If the answer to 13 is YES, the air-fuel ratio rich control up to that point is terminated, and the air-fuel ratio lean control is restarted.

FIG. 7 is a time chart showing the above control operation more specifically. Here, FIG. 7A shows the behavior of the air-fuel ratio and the exhaust gas component in the present embodiment.
(B) shows the behavior of the air-fuel ratio and the exhaust gas component in the prior art for comparison.

In FIGS. 7A and 7B, at time t1 to time t1.
t4, t11 to t13 indicate a period during which the air-fuel ratio rich control is temporarily performed during the air-fuel ratio lean control, and during the same period, the control air-fuel ratio is controlled to the rich side, so that the three-way catalyst 13 before and after the three-way catalyst 13 are controlled. All the air-fuel ratios have shifted to the rich side.
However, in actuality, the air-fuel ratio behind the three-way catalyst changes with a delay of the exhaust gas transport from the front air-fuel ratio, but in FIG. 7, for convenience, the air-fuel ratio changes in synchronization. It is described as one.

In FIG. 7A, before the time t1, the air-fuel ratio lean control is being performed.
NOx amount NOMOL in exhaust gas and its integrated amount NOM
OLAD is calculated from time to time (step 10 in FIG. 2).
2, 103). Then, at time t1 when the NOx integrated amount NOMOLAD reaches the predetermined determination value NOMOLDD, the control air-fuel ratio is switched from lean to rich (step 104 in FIG. 2 becomes YES). Also, at this time t1
Then, the reference rich area DRAFND is calculated based on the NOx integrated amount NOMOLAD (step 10 in FIG. 2).
8).

At time t2, the air-fuel ratio in front of and behind the three-way catalyst 13 reaches the stoichiometric air-fuel ratio (λ = 1). At this time,
Although the air-fuel ratio in front of the three-way catalyst 13 immediately changes to a richer side than the stoichiometric air-fuel ratio, since the three-way catalyst 13 stores the oxygen during the lean control, the stored oxygen and the rich component in the exhaust gas ( HC, CO, etc.), and the air-fuel ratio behind the three-way catalyst 13 is temporarily maintained at the stoichiometric air-fuel ratio. Then, when the reaction between the stored oxygen and the rich component ends, the air-fuel ratio behind the three-way catalyst 13 shifts to the rich side (time t).
3). After time t3, the rich component is supplied to the NOx catalyst 14, so that the NOx stored in the catalyst 14 is reduced and released.

After switching to the air-fuel ratio rich control, in the state where the air-fuel ratio behind the three-way catalyst 13, that is, the value detected by the A / F sensor 27 becomes richer than the stoichiometric air-fuel ratio (after time t3), the actual rich area DRAFAD is calculated (step 112 in FIG. 3). And the actual rich area DRAFAD
Reaches the reference rich area DRAFND at time t4,
The control air-fuel ratio is returned to the original lean state (step 1 in FIG. 3).
13 is YES).

Thereafter, the air-fuel ratio behind the three-way catalyst 13 is determined by the lean component in the exhaust gas fed from the upstream side and the same
Is maintained at the stoichiometric air-fuel ratio for a predetermined period (time t5 to t6) during which the rich component reacts with the rich component, and then returns to the lean control value.

By the way, in FIG. 7A, at times t3 to t
4, the actual rich area DRAFAD is the area (at time t3 to t3) at which the air-fuel ratio behind the three-way catalyst becomes actually rich.
5 (rich area at 5). In this case, the actual rich area DRAFAD (t3 to t5) from time t3 to t4
Of the reference rich area DRAFN
D may be set. Alternatively, the reference rich area DRAFND is set to correspond to the rich area from time t3 to t5.
Is set, the actual rich area D between times t3 and t4 is set.
The value obtained by doubling RAFAD and the reference rich area DRA
FND may be compared.

On the other hand, in FIG. 7B, at time t11, the control air-fuel ratio is switched from lean to rich, and accordingly, the air-fuel ratio before and after the three-way catalyst 13 starts to change to the rich side. At this time, at time t12, the air-fuel ratio behind the three-way catalyst is once held at the stoichiometric air-fuel ratio by the stored oxygen of the three-way catalyst as described above. The fuel ratio rich control ends (time t13). For example, when the rich time (time t11 to t13) is uniquely determined, such a situation occurs.
Therefore, the amount of NOx stored and reduced in the NOx catalyst 14 is reduced and released, and the amount of NOx discharged to the atmosphere without purification is gradually increased.

When comparing FIG. 7A and FIG. 7B, FIG.
In FIG. 7B, the amount of unpurified NOx increases because the substantial rich time is insufficient, whereas in FIG. 7A, the amount of unpurified NOx can be reduced.

In this embodiment, steps 107 to 113 in FIG. 2 correspond to the rich control means described in claims. Steps 102 and 103 correspond to NOx amount calculating means, and step 108 corresponds to reference rich amount calculating means.

According to the embodiment described above, the following effects can be obtained. In the present embodiment, a three-way catalyst 13 and a NOx catalyst 14 are disposed upstream and downstream of the engine exhaust pipe 12, respectively.
/ F sensor 27 is provided. Then, the rich control of the air-fuel ratio is performed while monitoring the richness of the exhaust gas sent to the NOx catalyst 14 when the air-fuel ratio is richly controlled based on the output of the A / F sensor 27. According to the above configuration, it is possible to appropriately supply a rich component that is actually necessary for the reduction and release of the stored NOx.
The amount of occlusion always falls below a certain amount.

In particular, the three-way catalyst 13 located upstream of the NOx catalyst 14 stores oxygen during lean combustion. By monitoring the richness of exhaust gas based on the output of the A / F sensor 27, The fact that the rich component is reacting with the stored oxygen of the three-way catalyst 13; and the fact that the reaction of the rich component with the stored oxygen of the three-way catalyst 13 has been completed. It is possible to execute the air-fuel ratio rich control corresponding to the minute. As a result, the rich components necessary for the reduction and release of the stored NOx in the NOx catalyst 14 can be supplied without excess and shortage, and the conventional problems are solved. More specifically, problems occur when the rich component is excessively supplied, such as deterioration of fuel efficiency, torque fluctuation, and increase in the emission of HC and CO components, and when the rich component is insufficient, such as emission of unpurified NOx. The problem is solved.

Incidentally, in the A / F sensor 27, the rich component and the lean component react at the electrode, and after the reaction, a signal corresponding to the excess component is output. In this case, the sensor 2
The component of the exhaust gas sent to the NOx catalyst 14 downstream of the exhaust gas 7 is not the exhaust gas itself discharged from the engine 1 but the exhaust gas after the reaction by the A / F sensor 27. Therefore, detecting the excess component amount after the reaction by the sensor 27 and performing the air-fuel ratio rich control according to the excess component amount effectively reduces and releases the stored NOx in the NOx catalyst 14. Above is convenient.

In the present embodiment, the amount of NOx stored in the NOx catalyst 14 during the air-fuel ratio lean control (the integrated NOx amount NOMOLAD) is calculated, and the reference rich amount (reference value) required for the rich control is calculated according to the NOMOLAD value. Rich area DRAFND) was calculated. Then, the rich control is continuously performed by the reference rich area DRAFND. Specifically, the rich amount integral value (actual rich area DRAFAD immediately before the NOx catalyst) during the air-fuel ratio rich control is represented by A
/ F sensor 27 and the obtained DRAFA
The rich control is continued until the D value reaches the reference rich area DRAFND. This makes it possible to supply an appropriate amount of the rich component to the NOx catalyst 14.

Further, since the rich control is performed while monitoring the output of the A / F sensor 27 by the reference rich area DRAFND substantially required for the rich control, the operating temperature and the deterioration state of the three-way catalyst 13 change. Even if the oxygen storage capacity of the catalyst 13 fluctuates, it is possible to always supply an appropriate amount of the rich component to the NOx catalyst 14.

Next, second and third embodiments of the present invention will be described. However, in the configurations of the following embodiments, the same components as those of the above-described first embodiment are denoted by the same reference numerals in the drawings, and the description is simplified. The following description focuses on differences from the first embodiment.

(Second Embodiment) A second embodiment will be described with reference to FIGS. In the first embodiment, the A / F sensor 27 is provided downstream of the three-way catalyst 13, but this is changed to an O2 sensor. That is, as shown in FIG. 8, an O2 sensor 28 that outputs an electromotive force signal VOX2 that differs depending on whether the exhaust gas air-fuel ratio is rich or lean is disposed downstream of the three-way catalyst 13.

FIGS. 9 and 10 are flow charts showing an air-fuel ratio control routine according to the present embodiment. This routine is executed in place of the routine shown in FIGS. 2 and 3. In the following description, only the differences from FIGS. 2 and 3 will be described with different numbers of steps.

Now, the CPU 31 sets the rich control flag X
When REX is “0”, the NOx integrated amount NOMOL
AD is calculated (steps 101 to 103). However, in this embodiment, an O2 sensor 28 is used instead of the A / F sensor. Therefore, when the NOx amount NOMOL is calculated (step 102), the A / F correction is not performed and FIG.
The NOx basic amount based on (a) is set as a NOMOL value.

Step 104 is NO (NOMO
If (LAD ≦ NOMOLSD), the CPU 31 sets the air-fuel ratio correction coefficient FAF to “0.5” in step 201, and executes the air-fuel ratio lean control based on the FAF value.

Step 104 is YES (NOMO
If LAD> NOMOLSD), the CPU 31 sets the rich control flag XREX to “1” (step 10).
7). Further, the CPU 31 determines in step 202 that NOx
Reference rich time DRA corresponding to integrated amount NOMOLAD
Calculate FTM. Here, the reference rich time DRAFT
M corresponds to a rich control amount required to reduce and release all NOx stored in the NOx catalyst 14. in this case,
The reference rich time DRAFTM is obtained according to the NOx integrated amount NOMOLAD and the intake pressure PM using, for example, the relationship in FIG. In FIG. 11, the reference rich time DRAFTM is set to a larger value as the NOx integrated amount NOMOLAD is larger in a state where the intake pressure PM is smaller.

In the relationship shown in FIG. 11, the intake pressure PM is used as a parameter. However, this parameter (PM) is removed or the parameter (PM) is changed to a parameter such as the engine speed Ne or the intake air amount. May be.

Thereafter, the CPU 31 sets the air-fuel ratio correction coefficient FAF to “1.25” in step 203,
The air-fuel ratio rich control is performed based on the value. This allows
The air-fuel ratio control is switched from the lean control to the rich control.

After switching to the air-fuel ratio rich control, C
The PU 31 proceeds to step 204 in FIG. 10, and determines whether or not the output (electromotive force signal VOX2) of the O2 sensor 28 is a rich signal. Then, Step 204 is YES
On the condition that the actual rich time C
Calculate RAFAD. At this time, the actual rich time CRA
The previous value of FAD is incremented by “1” to obtain the same value this time.

Thereafter, the CPU 31 determines whether or not the actual rich time CRAFAD calculated in step 206 exceeds the reference rich time DRAFTM (the value calculated in step 202). If CRAFAD ≦ DRAFTM (NO in step 206), the CPU 31 jumps to step 203 in FIG. 9 and continues the air-fuel ratio rich control up to that point.

If CRAFAD> DRAFTM (YES in step 206), CPU 31 proceeds to step 2
In step 07, the rich control flag XREX, the NOx integrated amount NOMOLAD, and the actual rich time CRAFAD are all cleared to "0". Thereafter, the CPU 31 proceeds to step 201 in FIG. Thus, step 2
If 06 is YES, the air-fuel ratio rich control up to that point is terminated, and the air-fuel ratio lean control is restarted.

As described above, according to the second embodiment, the first
As in the third embodiment, the storage N in the NOx catalyst 14
It is possible to supply rich components necessary for reduction and release of Ox without excess and deficiency. Further, since the rich control is performed while monitoring the output of the O2 sensor 28 for the reference rich time DRAFTM substantially necessary for the rich control, the operating temperature and the deterioration state of the three-way catalyst 13 change, and It is possible to always supply an appropriate amount of the rich component to the NOx catalyst 14 even if the oxygen storage capacity of the NOx catalyst fluctuates.

(Third Embodiment) Next, a third embodiment will be described. In the present embodiment, as shown in FIG. 12, a three-way catalyst 15 having a small oxygen storage capacity is provided upstream of the NOx catalyst 14, and an A / F is provided further upstream thereof.
A sensor 29 is provided. That is, the three-way catalyst 15 is configured by supporting only the noble metal (platinum Pt) having no oxygen storage capacity on the carrier. Specifically, a carrier made of ceramic such as stainless steel or cordierite is coated with a catalyst layer constituted by supporting only platinum Pt on the surface of porous alumina Al2 O3.

In this case, since the storage of oxygen by the three-way catalyst 15 is suppressed as much as possible, the air-fuel ratio behavior before and after the three-way catalyst 15 substantially matches. That is, the exhaust gas component actually supplied to the NOx catalyst 14 follows the change of the control air-fuel ratio at that time. For example, when switching from lean combustion to rich combustion, the rich component passes through the three-way catalyst 15 and is fed to the NOx catalyst 14 as it is.

According to the A / F sensor 29, the NOx catalyst 1
In this embodiment, the air-fuel ratio control is performed using, for example, the air-fuel ratio control routine shown in FIGS. That is, the NOx storage amount of the NOx catalyst 14 during the air-fuel ratio lean control is calculated, and the rich control is continuously performed by the reference rich amount according to the NOx storage amount. As a result, similarly to the above embodiments, the rich components necessary for the reduction and release of the stored NOx in the NOx catalyst 14 can be supplied without excess and deficiency.

Further, by disposing the A / F sensor 29 on the upstream side of the three-way catalyst 15, the distance between the engine 1 and the sensor 29 is shortened, and from when the air-fuel ratio changes to when the sensor output changes. Response time is reduced. Therefore, the detection accuracy at the time of the transient operation is also improved. Further, in order to monitor the exhaust gas component to the NOx catalyst 14, there is no need to provide another sensor immediately before the NOx catalyst, and the inconvenience of complicating the configuration is avoided.

The embodiment of the present invention can be embodied in the following forms other than the above. In the first embodiment, the feedback control is performed according to the difference between the target air-fuel ratio AFTG and the actual air-fuel ratio AF during the air-fuel ratio lean control or the rich control, but this is changed to the open control. Is also good.

In the second embodiment, the period during which the air-fuel ratio rich control is performed is controlled using the detection value of the O2 sensor 28, but the control is performed using an A / F sensor instead of the O2 sensor 28. In this case, the A / F sensor determines the time during which the air-fuel ratio changes to a richer side than the stoichiometric air-fuel ratio and the rich peak value at that time. Therefore, the actual rich control amount is calculated from the obtained rich time and rich peak value, and the actual rich control amount is calculated as the reference rich amount (NOx
The rich control is continued until the rich amount corresponding to the integrated amount is reached. Even in such a configuration, similar to the above embodiments, excellent effects such as the supply of rich components necessary for the reduction and release of the stored NOx in the NOx catalyst 14 can be obtained.

In the third embodiment, the A / F sensor provided upstream of the three-way catalyst 15 is changed to an O2 sensor. In this case, similar to the second embodiment, the rich control may be performed while monitoring the output of the O2 sensor for a reference rich time substantially required for the rich control.

In the third embodiment, the following configuration can be applied as the three-way catalyst 15 having a small oxygen storage capacity. The three-way catalyst 15 is configured such that a cocatalyst having a large oxygen storage capacity is not supported on a carrier, or the amount of the promoter is reduced.
In this case, ceria CeO2, barium B, lanthanum La and the like are known as cocatalysts having a large oxygen storage capacity. The three-way catalyst 15 is configured by reducing the amount of the noble metal (Rh, Pd) having an oxygen storage capacity. Especially rhodium Rh
In this case, the carrying amount is preferably 0.2 g / liter or less, and in the case of palladium Pd, the carrying amount is preferably 2.5 g / liter or less.

In each of the above embodiments, the NO stored in the NOx catalyst 14 depends on the engine operating state (Ne, PM).
Although the Ox amount was estimated, this is changed. For example, a NOx sensor for detecting the NOx concentration may be provided immediately before the NOx catalyst 14, and the storage amount of NOx may be obtained from the sensor output. In this case, the oxygen concentration (A / F) and N
A configuration in which a composite sensor that can simultaneously measure the Ox concentration may be applied.

In each of the above embodiments, when the invention is embodied, the three-way catalyst 13 is provided in the engine exhaust pipe 12 as a start catalyst, but this configuration is changed. For example, the three-way catalyst 13 is changed to an oxidation catalyst. Alternatively, in the first and second embodiments, the three-way catalyst 13 is eliminated, and only the NOx catalyst 14 is provided in the exhaust pipe 12. In any of these cases, the rich components necessary for the reduction and release of the stored NOx in the lean NOx catalyst can be supplied without excess and deficiency, as in the above embodiments.

[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 an embodiment of the invention.

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

FIG. 3 is a flowchart showing an air-fuel ratio control routine continued from FIG. 2;

FIG. 4A is a diagram for obtaining a basic amount of NOx,
(B) is a diagram for obtaining a correction value.

FIG. 5 is a determination value NOMOLSD according to NOx storage capacity
The figure for setting.

FIG. 6 is a diagram for setting a reference rich area DRAFND corresponding to the NOx integrated amount NOMOLAD.

FIG. 7A is a time chart showing the behavior of the air-fuel ratio and the exhaust gas component in the present embodiment, and FIG. 7B is a time chart showing the behavior of the air-fuel ratio and the exhaust gas component in the prior art.

FIG. 8 is a configuration diagram showing an outline of a control system in a second embodiment.

FIG. 9 is a flowchart illustrating an air-fuel ratio control routine according to the second embodiment.

FIG. 10 is a flowchart showing an air-fuel ratio control routine continued from FIG. 9;

FIG. 11 is a diagram for setting a reference rich time DRAFTM corresponding to a NOx integrated amount NOMOLAD.

FIG. 12 is a configuration diagram showing an outline of a control system in a third embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine, 12 ... Exhaust pipe, 13, 15 ... Three-way catalyst as upstream catalyst, 14 ... NOx catalyst (NOx storage reduction type catalyst), 27, 29 ... A / F as oxygen concentration sensor
Sensor, 28 ... O2 sensor as oxygen concentration sensor, 3
0: ECU (electronic control device), 31: rich control means,
C constituting the NOx amount calculating means and the reference rich amount calculating means
PU.

Claims (7)

[Claims] 1. A method for performing lean combustion in an air-fuel ratio lean region, and NOx in exhaust gas discharged during the lean combustion.
Is stored in a lean NOx catalyst, and further, the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst. And an oxygen concentration sensor that detects the oxygen concentration in the exhaust gas, while monitoring the richness of the exhaust gas sent to the lean NOx catalyst when the air-fuel ratio is richly controlled based on the output of the oxygen concentration sensor. An air-fuel ratio control device for an internal combustion engine, comprising: rich control means for performing rich control of the air-fuel ratio. 2. An air-fuel ratio control device for an internal combustion engine according to claim 1, wherein an upstream catalyst having an oxygen storage action is disposed upstream of said oxygen concentration sensor. 3. An internal combustion engine according to claim 1, wherein an upstream catalyst having a small oxygen storage capacity is provided upstream of said lean NOx catalyst, and said oxygen concentration sensor is further provided upstream thereof. Fuel ratio control device. 4. NOx stored in the lean NOx catalyst
NOx amount calculating means for calculating the amount, and reference rich amount calculating means for calculating a reference rich amount necessary for rich control in accordance with the calculated NOx storage amount of the lean NOx catalyst. 4. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the rich control is continuously performed by the calculated reference rich amount. 5. The air-fuel ratio control device according to claim 4, wherein the rich control means obtains a rich amount integrated value at the time of air-fuel ratio rich control from an output of the oxygen concentration sensor, and obtains the obtained rich amount integrated value. Is an air-fuel ratio control device for an internal combustion engine that continues rich control until the value becomes equivalent to the reference rich amount. 6. The air-fuel ratio control device according to claim 4, wherein the rich control means determines a time when the air-fuel ratio changes to a richer side than the stoichiometric air-fuel ratio during the air-fuel ratio rich control by an output of the oxygen concentration sensor. An air-fuel ratio control device for an internal combustion engine that determines and continues rich control until the determined time becomes equivalent to the reference rich amount. 7. The air-fuel ratio control device according to claim 4, wherein the rich control means controls a time when the air-fuel ratio changes to a richer side than the stoichiometric air-fuel ratio during the air-fuel ratio rich control, and a rich peak value at that time. Is obtained from the output of the oxygen concentration sensor, and the rich control is continued until the control amount based on the obtained rich time and rich peak value becomes equivalent to the reference rich amount.