JPH11107742A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten rich time, improve fuel consumption and control torque fluctuation in an exhaust emission control device that executes lean combustion and also temporarily performs rich combustion to recover the purification capability of a NOx catalyst. SOLUTION: An ECU sets the target air/fuel ratio of the mixture to be supplied to an engine 1 to a lean side more than a theoretical air/fuel ratio and makes engine conduct lean combustion based on the target air/fuel ratio. An engine exhaust pipe 3 is provided with a NOx catalyst 19 for purifying the NOx generated during lean combustion. In particular, the branch pipe 25a of the exhaust manifold 25 connected to each cylinder of the engine 1 is extended directly before the NOx catalyst 19, and an air/fuel ratio (A/F) sensor 16 is arranged in the collecting section 25b. In such a case, the exhaust emission gas discharged from each cylinder reaches the NOx catalyst 19 left separated without being mixed with exhaust emission gas form other cylinders in the exhaust pipe 3. Therefore, the exhaust emission gas in the exhaust pipe 3 is quickly switched to a rich atmosphere when switched to a rich combustion mode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to an air-fuel ratio control device for performing lean combustion in an air-fuel ratio lean region,
Using a lean NOx catalyst, nitrogen oxides (NO
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine that purifies x).

[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.
And the lean NO for purifying this NOx
x Catalyst is required. For example, Patent No. 2600492
The exhaust gas purifying apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open No. H10-197706 absorbs NOx when the air-fuel ratio of the exhaust gas is lean, and absorbs the absorbed NOx when the oxygen concentration of the exhaust gas is reduced, that is, when the exhaust gas is enriched. NOx absorbent (N
Ox storage reduction catalysts).

On the other hand, NOx generated during lean combustion
In a 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, when switching from the lean combustion to the rich combustion is considered, since the exhaust gas of the lean atmosphere remains in the exhaust passage, the control value of the air-fuel ratio (for example, Even if the target air-fuel ratio is changed to the rich side, the air-fuel ratio near the catalyst is not immediately switched to the rich. Therefore, it is necessary to set the rich time to be longer, and to continue rich combustion in a time that allows for the time when the atmosphere in the exhaust passage shifts from lean to rich. In such a case, if the rich combustion is prolonged, the injection amount is excessively increased, and there is a concern that fuel consumption may deteriorate. 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.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to perform lean combustion and temporarily perform rich combustion in order to restore the purifying ability of the NOx catalyst. It is an object of the present invention to provide an exhaust gas purifying apparatus for an internal combustion engine which can shorten a rich time to a necessary minimum and improve fuel efficiency and suppress torque fluctuation.

[0006]

In order to achieve the above object, the exhaust gas purifying apparatus of the present invention presupposes that 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. The present invention is applied to an air-fuel ratio control device for an internal combustion engine that performs lean combustion based on the target air-fuel ratio. Then, NOx in the exhaust gas discharged during lean combustion is occluded by the lean NOx catalyst, and the air-fuel ratio is temporarily controlled to be rich to release the occluded NOx from the lean NOx catalyst.

According to the first aspect of the invention, there is provided an exhaust gas separating means for separating each exhaust gas in the exhaust passage so that the exhaust gas during the lean combustion and the exhaust gas during the rich combustion do not mix.

[0008] In short, when exhaust gas is mixed in the exhaust passage as in the conventional device, the exhaust passage is not immediately switched to the rich atmosphere when switching from lean combustion to rich combustion. This is considered to be due to the fact that exhaust gas from the immediately preceding lean combustion remains in the exhaust passage when switching to the rich combustion. On the other hand, in the configuration of the present invention, the exhaust gas at the time of lean combustion and the exhaust gas at the time of rich combustion are separated and fed to the NOx catalyst. Therefore, when switching to rich combustion, the exhaust gas in the exhaust passage is quickly switched to a rich atmosphere (a desired air-fuel ratio), and the rich time can be reduced. As a result, the rich time can be shortened to a necessary minimum, and the fuel efficiency can be improved and the torque fluctuation can be suppressed.

The invention of claim 1 can be realized in the following mode. In the invention described in claim 2, the exhaust gas separating means is configured such that exhaust passages extending from the respective cylinders of the multi-cylinder internal combustion engine are separated from each other immediately before the lean NOx catalyst.

In this case, at the time of switching to rich combustion, the inside of the exhaust passage connected to each cylinder is switched to rich atmosphere one cylinder at a time. That is, even when the immediately preceding combustion cylinder is switched to the rich combustion, immediately after the switching to the rich combustion, the mixing of the exhaust gas during the lean combustion and the exhaust gas during the rich combustion is suppressed, and the inside of the exhaust passage quickly becomes a rich atmosphere. Then, the exhaust gas in the rich atmosphere is supplied to the lean NOx catalyst, whereby the NOx stored in the NOx catalyst is released. With such a configuration, the reduction of the rich time can be realized.

According to the third aspect of the present invention, the exhaust gas separation means includes a first passage and a second passage that branch off an exhaust passage extending from the internal combustion engine, and the first passage is used during lean combustion. The exhaust gas is supplied to the lean NOx catalyst, and at the time of rich combustion, the exhaust gas is supplied to the lean NOx catalyst using the second passage.

By selectively using the exhaust passage (first passage) for lean combustion and the exhaust passage (second passage) for rich combustion, the atmosphere in the exhaust passage changes quickly from lean to rich, thereby shortening the rich time. Can be realized.

In order to embody the third aspect of the present invention, the following configuration of the fourth aspect may be adopted. That is, in the invention described in claim 4, the exhaust gas separation means further includes a switching valve for closing one of the first passage and the second passage and releasing the other, and the first passage is provided during lean combustion. The switching valve is operated so that the release and the second passage are closed, and the first passage is closed and the second passage is closed during the rich combustion.
The switching valve is operated so that the passage is released.

According to the fifth aspect of the invention, when switching between the lean combustion and the rich combustion, the switching between the first passage side and the second passage side is performed with a delay corresponding to a delay corresponding to the exhaust gas transport delay. In this case, it is possible to appropriately and quickly perform the lean-to-rich switching of the exhaust gas in the vicinity of the NOx catalyst by considering the delay in exhaust gas transport in the exhaust passage.

[0015] In the invention according to claim 6, the present invention is applied to an air-fuel ratio control device that performs lean combustion and rich combustion at a predetermined time ratio. In such a case, the rich time is shortened as described above, so that the lean time is also shortened. Therefore, even when the lean storage capacity of the lean NOx catalyst is reduced, such as when the catalyst is deteriorated or when the catalyst temperature is reduced or increased, the effect of reducing the emission can be continued while ensuring the fuel efficiency.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) Hereinafter, a first embodiment of the present invention will be described. 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 NOx storage reduction catalyst (hereinafter, referred to as a NOx catalyst) is provided in the exhaust passage of the internal combustion engine.
x A limiting current type air-fuel ratio sensor (A / F sensor) is disposed upstream of the catalyst. An electronic control unit (hereinafter, referred to as an ECU) mainly composed of a microcomputer
Captures the detection result of the air-fuel ratio sensor and performs feedback control with a lean air-fuel ratio based on the sensor detection result. Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system according to the present embodiment. In FIG. 1, the internal combustion engine is configured as a four-cylinder four-cycle spark ignition engine (hereinafter simply referred to as engine 1), and an intake pipe 2 and an exhaust pipe 3 are connected to the engine 1. The intake pipe 2 is provided with a throttle valve 5 linked to an accelerator pedal 4, and the opening of the throttle valve 5 is detected by a throttle opening sensor 6. Further, an intake pressure sensor 8 is provided in the surge tank 7 of the intake pipe 2.

A piston 10 reciprocating in the vertical direction in the figure is disposed in a cylinder 9 constituting a cylinder of the engine 1, and the piston 10 is connected to a crankshaft (not shown) via a connecting rod 11. . A combustion chamber 13 defined by a cylinder 9 and a cylinder head 12 is formed above the piston 10. The combustion chamber 13 is connected to the intake pipe 2 and the exhaust pipe 3 via an intake valve 14 and an exhaust valve 15. Communicating.

A limiting current type air-fuel ratio, which outputs a wide-area and linear air-fuel ratio signal in proportion to the oxygen concentration in the exhaust gas (or the concentration of carbon monoxide or the like in the unburned gas), An A / F sensor 16 composed of a sensor is provided. Further, a NOx catalyst 19 having a NOx purifying function is disposed downstream of the A / F sensor 16 in the exhaust pipe 3. This NOx catalyst 19 is known as a NOx storage reduction type catalyst,
At the rich air-fuel ratio.
Is reduced and released with CO and HC.

In this embodiment, the exhaust pipe 3 is branched in a section from each cylinder of the engine 1 to immediately before the NOx catalyst 19. That is, as shown in FIG. 2, the branch pipe 25a of the exhaust manifold 25 extends to just before the NOx catalyst 19, and the A / F sensor 16 is disposed in the collecting portion 25b. According to such a configuration, the exhaust gas discharged from each cylinder does not mix with the exhaust gas of the other cylinders in the exhaust pipe 3 and reaches the NOx catalyst 19 while being separated. In the present embodiment, the exhaust manifold 25 corresponds to an exhaust gas separation unit described in the claims.

In FIG. 1, an electromagnetically driven injector 18 is provided at an intake port 17 of the engine 1, and fuel (gasoline) is supplied to the injector 18 from a fuel tank (not shown). In the present embodiment, a multipoint injection (M) having one injector 18 for each branch pipe of the intake manifold
PI) system is configured. In this case, fresh air supplied from the upstream of the intake pipe and fuel injected by the injector 18 are mixed at the intake port 17, and the air-fuel mixture is mixed in the combustion chamber 13 (in the cylinder 9) with the opening operation of the intake valve 14.
In).

The ignition plug 27 disposed on the cylinder head 12 is ignited by the ignition high voltage from the igniter 28. The igniter 28 has a distributor 2 for distributing the high ignition voltage to the ignition plug 27 of each cylinder.
0 is connected to the distributor 20, a reference position sensor 21 that outputs a pulse signal at every 720 ° CA according to the rotation state of the crankshaft, and a pulse at each finer crank angle (eg, every 30 ° CA). A rotation angle sensor 22 for outputting a signal is provided.

The cylinder 9 (water jacket)
Is provided with a water temperature sensor 23 for detecting a cooling water temperature. The ECU 30 mainly includes a known microcomputer system, and includes a CPU 31, a ROM 3
2, a RAM 33, a backup RAM 34, an A / D converter 35, an input / output interface (I / O) 36, and the like. The throttle opening sensor 6, the intake pressure sensor 8,
Each detection signal of the A / F sensor 16 and the water temperature sensor 23 is
The signal is input to the A / D converter 35, A / D converted, and taken into the CPU 31 via the bus 37. The pulse signals from the reference position sensor 21 and the rotation angle sensor 22 are taken into the CPU 31 via the input / output interface 36 and the bus 37.

The CPU 31 determines the throttle opening TH, the intake pressure PM, and the air-fuel ratio (A
/ F), a coolant temperature Tw, a reference crank position (G signal), and an engine operation state such as an engine speed Ne are detected. Further, the CPU 31 calculates a control signal such as a fuel injection amount and an ignition timing based on an engine operating state, and outputs the control signal to the injector 18 and the igniter 28.

Next, the operation of the air-fuel ratio control system configured as described above will be described. FIG. 3 is a flowchart showing a fuel injection control routine executed by the CPU 31. This routine is executed every fuel injection of each cylinder (in this embodiment, every 180 ° CA).

Now, when the routine of FIG. 3 starts,
First, in step 101, the CPU 31 detects the sensor detection result indicating the engine operating state (the engine speed Ne, the intake pressure P
M, cooling water temperature Tw, etc.) and the following step 102
Calculates the basic injection amount Tp according to the engine speed Ne and the intake pressure PM at that time using the basic injection map stored in the ROM 32 in advance. Also, the CPU 31
Determines whether or not the well-known air-fuel ratio F / B condition is satisfied in step 103. Here, the air-fuel ratio F / B condition means that the cooling water temperature Tw is equal to or higher than a predetermined temperature, that the high-
This includes not being in a high load state, and that the A / F sensor 16 is in an active state.

If the determination in step 103 is negative, (F /
If the condition B is not satisfied), the CPU 31 proceeds to step 104
To set the air-fuel ratio correction coefficient FAF to "1.0". F
Setting AF = 1.0 means that the air-fuel ratio is open-controlled. If an affirmative determination is made in step 103 (if the F / B condition is satisfied), the CPU 31 proceeds to step 200 and performs a process of setting the target air-fuel ratio λTG. The process of setting the target air-fuel ratio λTG is performed according to the routine of FIG.

Thereafter, in step 105, the CPU 31 sets an air-fuel ratio correction coefficient FAF based on the deviation between the actual air-fuel ratio λ (sensor measurement value) at that time and the target air-fuel ratio λTG. In the present embodiment, the air-fuel ratio F / B control based on the modern control theory is performed. In the F / B control, the air-fuel ratio for matching the detection result of the A / F sensor 16 to the target air-fuel ratio is set. The correction coefficient FAF is calculated by the following (1),
It is calculated using equation (2). The setting procedure of the FAF value is disclosed in detail in Japanese Patent Application Laid-Open No. 1-110853.

FAF = K1 · λ + K2 · FAF1 +... + Kn + 1 FAFn + ZI (1) ZI = ZI1 + Ka · (λTG−λ) (2) In the above equations (1) and (2), λ Is the A / F sensor 16
The converted value of the air-fuel ratio of the limiting current by K1 to Kn + 1 is F /
B represents a constant, ZI represents an integral term, and Ka represents an integral constant. The subscripts 1 to n + 1 are variables indicating the number of times of control since the start of sampling.

After setting the FAF value, the CPU 31 uses the following equation (3) in step 106 to calculate the basic injection amount Tp, the air-fuel ratio correction coefficient FAF, and other correction coefficients FALL (variable correction coefficients such as water temperature and air conditioner load). )) To calculate the final fuel injection amount TAU.

TAU = Tp · FAF · FALL (3) After calculating the fuel injection amount TAU, the CPU 31 calculates the TAU.
A control signal corresponding to the value is output to the injector 18, and this routine is once ended.

Next, a λTG setting routine corresponding to the processing in step 200 will be described with reference to FIG. In this routine, the target air-fuel ratio λTG is appropriately set so that rich combustion is temporarily performed during execution of lean combustion. That is, in the present embodiment,
Based on the value of the cycle counter counted for each fuel injection, "1
The lean time TL and the rich time TR are set so as to have a time ratio of about 00: 1, and the respective time TL, TR
, The lean combustion and the rich combustion are performed alternately.

In FIG. 4, the CPU 31 first determines in step 201 whether or not the value of the cycle counter has reached a value corresponding to a predetermined lean time TL. Period counter <
In the case of TL (NO in step 201), the CPU 3
1 proceeds to step 202, and sets the target air-fuel ratio λTG as a lean control value based on the engine speed Ne and the intake pressure PM at that time. At this time, the λTG value is obtained by searching a target air-fuel ratio map shown in FIG.
A value corresponding to, for example, A / F = 20 to 23 is set as the G value (however, when the condition for performing the lean combustion is not satisfied, such as when the engine is not in a steady operation, the λTG value is set near the stoichiometric value).

Thereafter, the CPU 31 increments the cycle counter by "1" at step 203, and returns to the original routine of FIG. In such a case, the λTG value set in step 202 described above is used in the calculation of the FAF value in step 105 of FIG. 3 described above, and the air-fuel ratio is lean-controlled by the FAF value.

If the cycle counter ≧ TL (YES in step 201), the CPU 31 proceeds to step 20
Proceeding to 4, the target air-fuel ratio λTG is set as a rich control value. At this time, the λTG value may be a fixed value in a rich region, or may be variably set by searching a map based on the engine speed Ne and the intake pressure PM. When performing a map search, the λTG value is set such that the richer the engine speed Ne or the intake pressure PM, the higher the richness.

Thereafter, the CPU 31 determines in step 205 whether or not the value of the cycle counter has reached a value corresponding to the total time "TL + TR" of the predetermined lean time TL and the rich time TR. If cycle counter <TL + TR (NO in step 205), the CPU 31 increments the cycle counter by “1” in step 203, and then returns to the original routine of FIG. In such a case, the λTG value set in step 204 described above is used in the calculation of the FAF value in step 105 in FIG.

On the other hand, if the cycle counter ≧ TL + TR (YES in step 205), the CPU 31 clears the cycle counter to “0” in step 206, and thereafter returns to the original routine of FIG. At the next processing, CP
U31 makes a negative determination in step 201 (cycle counter <
TL), the lean control is restarted.

FIG. 6 is a time chart for explaining the control operation according to the routines of FIGS. 3 and 4. In FIG. 6, during the period of the period counter = 0 to TL, the air-fuel ratio is lean-controlled. Also, the cycle counter = TL to T
During the period of L + TR, the air-fuel ratio is richly controlled. Thus, the lean control and the rich control are repeatedly performed at a predetermined cycle. In such a case, as shown in the exhaust pipe configuration diagram of FIG.
x Extends to just before catalyst 19. Therefore, the exhaust gas discharged from each cylinder is less likely to be mixed with the exhaust gas of the other cylinders in the exhaust pipe 3, and when switching between the lean control and the rich control, the exhaust gas in each branch pipe 25a quickly becomes a rich atmosphere. Switch to lean atmosphere.

That is, considering the switching to the rich combustion, the inside of the branch pipe 25a connected to each cylinder is switched to the rich atmosphere one cylinder at a time. At this time, even when the immediately preceding combustion cylinder is switched to the rich combustion, immediately after the switching to the rich combustion, the mixing of the exhaust gas during the lean combustion and the exhaust gas during the rich combustion is suppressed, and the inside of the exhaust pipe 3 quickly becomes rich atmosphere. Becomes Then, the exhaust gas in the rich atmosphere is fed to the NOx catalyst 19, whereby the NOx stored in the NOx catalyst 19 is stored.
Ox is released. For this reason, in the present embodiment, the rich time can be reduced as compared with the conventional device.

According to the embodiment described above, the following effects can be obtained. (A) In the present embodiment, each exhaust gas is separated in the exhaust pipe 3 so that the exhaust gas during the lean combustion and the exhaust gas during the rich combustion do not mix. Specifically, the exhaust passages (branch pipes 25a of the exhaust manifold 25) extending from the respective cylinders of the engine 1 are separated from each other up to immediately before the NOx catalyst 19.

In short, when exhaust gases of all cylinders are mixed in the exhaust passage as in a conventional apparatus, the exhaust passage is not immediately switched to the rich atmosphere when switching from lean combustion to rich combustion. On the other hand, in the configuration of the present embodiment, the exhaust gas at the time of lean combustion and the exhaust gas at the time of rich combustion are separated and fed to the NOx catalyst 19. Therefore, when switching to rich combustion, the exhaust gas in the exhaust pipe 3 is quickly switched to a rich atmosphere (a desired air-fuel ratio),
The rich time can be reduced. As a result, the rich time can be shortened to a necessary minimum, and the fuel efficiency can be improved and the torque fluctuation can be suppressed.

FIG. 7 is experimental data showing the relationship between the rich time per operation and the torque fluctuation at each time. According to the figure, it can be seen that the shorter the rich time, the more the fluctuation of the value is suppressed.

(B) In the air-fuel ratio control system according to the present embodiment, the lean combustion and the rich combustion are performed at a predetermined time ratio (for example, 100: 1).
Therefore, the lean time is shortened by shortening the rich time as described above. Therefore, even when the lean storage capacity of the NOx catalyst 19 is reduced, such as when the catalyst is deteriorated or when the catalyst temperature is decreased or increased, the effect of reducing the emission can be continued while ensuring the fuel efficiency.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.
However, in the configuration of the second embodiment, the first
The same reference numerals are given to the same components in the drawings and the description is simplified. The following description focuses on differences from the first embodiment.

That is, in the first embodiment, the exhaust pipe extending from each cylinder is branched to just before the NOx catalyst 19, thereby preventing the exhaust gas from each cylinder from being mixed (see FIG. 2). In the present embodiment, this configuration is changed as follows. That is, in the present embodiment, the engine exhaust pipe 3 is configured as shown in FIG. In FIG. 8, an exhaust passage 42 extending immediately before the NOx catalyst 19 is connected downstream of the exhaust manifold 41, and the exhaust passage 42 is divided into two by a partition 43. In the present embodiment, one of the exhaust passages 42 partitioned by the partition portion 43 is referred to as an “exhaust first passage 44”, and the other is referred to as an “exhaust second passage 45”. Exhaust first passage 44 and exhaust second
The passage 45 has the same cross-sectional shape and the same shape resistance.

The first exhaust passage 44 and the second exhaust passage 4
An exhaust switching valve 46 is provided upstream of 5, and the passages 44 and 45 are selectively opened and closed by the switching operation of the valve 46. The exhaust switching valve 46 is
When driven by the actuator 47 to the first passage side or the second passage side and the exhaust switching valve 46 is switched to the second passage side, the exhaust first passage 44 is released and the exhaust second passage 45 is closed ( The state of the solid line in FIG. 8). Also,
When the exhaust switching valve 46 is switched to the first passage side,
The exhaust first passage 44 is closed and the exhaust second passage 45 is closed.
Is released (the state shown by the two-dot chain line in FIG. 8).

FIG. 9 is a flowchart showing a valve switching routine for switching the exhaust switching valve 46. This routine is executed by the CPU 31 for example at 4 m.
It is executed in s cycles.

Now, when the routine of FIG. 9 starts,
First, in step 301, the CPU 31 determines whether or not the lean control is currently being performed. During lean control,
The CPU 31 proceeds to step 302 and determines whether or not the previous processing was rich control. If the lean control is not currently being performed, the CPU 31 proceeds to step 303 to determine whether or not the previous process was the lean control.

In this case, if (1) the lean control is continued, step 301 is YES, step 302 is NO, and (2) if rich control is continued, both steps 301 and 303 are NO. (3) If switching from lean control to rich control, step 301 is NO, step 303 is YES, and (4) if switching from rich control to lean control, step 30
1302 are both YES.

In the case of (1), the CPU 31 proceeds to step 305, and decrements the counter by "1". At this time, the counter value is guarded by “0” so that the counter value does not become a negative value. Then, C
The PU 31 determines in step 306 whether the value of the counter is “0”. If the counter is 0, the CPU 31 operates the exhaust switching valve 46 so that the exhaust first passage 44 in FIG.
If the counter is $ 0, the exhaust switching valve 46 is operated in step 308 so that the second exhaust passage 45 is released.
After the operation of the exhaust switching valve 46, the CPU 31 once ends the present routine.

In the case of the above (2), the CPU 31 proceeds to step 309 and decrements the counter by "1". At this time, the counter value is guarded by “0” so that the counter value does not become a negative value. Thereafter, the CPU 31 determines in step 310 whether the value of the counter is “0”. And the CPU 31
If the counter is 0, the exhaust switching valve 46 is operated so that the exhaust second passage 45 is released in step 312, and if the counter is 0, the exhaust first passage 4 is released in step 311.
The exhaust switching valve 46 is operated so that the valve 4 is released. After the operation of the exhaust switching valve 46, the CPU 31 once ends the present routine.

Further, in the above cases (3) and (4), the CP
U31 proceeds to step 304 and sets a predetermined counter threshold value in the counter. This counter threshold is
It is desirable to set in consideration of the exhaust gas transport delay in the exhaust passage. In the present embodiment, the same value is set to a smaller value as the intake air amount to the engine is larger, according to the relationship of FIG. The intake air amount is the engine speed Ne at that time.
And the intake pressure PM. After setting the counter threshold value, the CPU 31 ends this routine once.

Next, the operation of FIG. 9 will be described more specifically with reference to the time chart of FIG. In FIG. 11, the lean control is continued before time t1, and in FIG. 9, the processing is performed in the order of steps 301 → 302 → 305. At this time, the counter is maintained at "0", and the exhaust switching valve 46 is fixed at the position where the first exhaust passage 44 is released (step 307 in FIG. 9). Therefore, the exhaust gas of the lean combustion is supplied to the NOx catalyst 19 through the first exhaust passage 44.

When the control is switched from the lean control to the rich control at time t1, in FIG. 9, the processing is performed in the order of steps 301 → 303 → 304.
A predetermined threshold value is set in the counter (Step 304 in FIG. 9).

Thereafter, the rich control is continued from time t1 to t2. Therefore, steps 301 → 303 → of FIG.
Processing is performed in the order of 309, and the counter is counted down. Then, when the counter becomes 0 at time t2,
The exhaust switching valve 46 is set at a position where the exhaust second passage 45 is released.
Is switched (step 312 in FIG. 9). As a result, the exhaust gas of the rich combustion passes through the second exhaust passage 45 and becomes N
It is fed to the Ox catalyst 19.

When the control is switched from the rich control to the lean control at time t3, as shown in FIG.
Processing is performed in the order of → 302 → 304, and a predetermined threshold value is set in the counter (step 30 in FIG. 9).
4).

Thereafter, from time t3 to t4, the processing is performed in the order of steps 301 → 302 → 305 in FIG. 9 and the counter is counted down. Then, when the counter becomes 0 at time t4, the exhaust switching valve 46 is switched again to the position where the first exhaust passage 44 is released (step 307 in FIG. 9).

In the present embodiment, each of the exhaust first passage 44, the exhaust second passage 45, and the exhaust switching valve 46, and the routine shown in FIG. 9 correspond to an exhaust gas separating means.

As described above, according to the present embodiment, as in the first embodiment, the rich time can be shortened to a necessary minimum to improve fuel economy and suppress torque fluctuation. Aim is achieved. That is, in the present embodiment, the exhaust first passage 44 for lean combustion and the exhaust second passage 45 for rich combustion are selectively used according to the switching operation of the exhaust switching valve 46. As a result, the atmosphere in the exhaust pipe 3 is quickly changed from lean to rich, and the rich time can be reduced.

Further, in this embodiment, when switching between the lean combustion and the rich combustion, the switching between the exhaust first passage 44 side and the exhaust second passage 45 side is delayed by a time corresponding to the exhaust gas transport delay. It was implemented.
In this case, lean to rich switching of the exhaust gas in the vicinity of the NOx catalyst 19 can be appropriately and promptly performed by taking into account the delay in the exhaust gas transport in the exhaust pipe 3.

The embodiment of the present invention can be realized in the following modes other than the above. In the λTG setting routine of FIG. 4, the lean time TL and the rich time TR are variably set according to the engine operating state. For example, the lean time TL and the rich time TR are set to be longer as the engine speed is higher or the load is higher, and conversely, the lean time T is lower as the engine speed is lower or the load is lower.
L and the rich time TR are set shorter.

In the second embodiment, one of the exhaust first passage 44 and the exhaust second passage 45 shown in FIG. 8 is closed depending on whether the combustion is lean or rich.
Release the other, but change this configuration. For example, when stoichiometric combustion is performed, both passages 44 and 45 are released. That is, the exhaust switching valve 46 is adjusted so as to be parallel to the exhaust gas flow direction. Thus, the object of the present invention can be achieved even when the exhaust gas switching valve 46 is switched at three positions.

In each of the above embodiments, the target air-fuel ratio λTG
Is switched between the lean control value and the rich control value to perform the lean combustion and the rich combustion, but this is changed. For example, the air-fuel ratio correction coefficient FAF may be switched between the lean correction side and the rich correction side so that lean combustion and rich combustion are performed.

In the air-fuel ratio control system in each of the above embodiments, the modern control theory is used to feedback-control the air-fuel ratio in accordance with the deviation between the target air-fuel ratio and the actually detected air-fuel ratio (actual air-fuel ratio). Changes this configuration. For example, feedback control of the air-fuel ratio or open control of the air-fuel ratio may be performed by PI control.

[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 diagram schematically showing a configuration of an engine exhaust system.

FIG. 3 is a flowchart showing a fuel injection control routine.

FIG. 4 is a flowchart showing a λTG setting routine.

FIG. 5 is a map for setting a lean target air-fuel ratio according to the engine speed and the intake pressure.

FIG. 6 is a time chart for explaining an operation in the first embodiment.

FIG. 7 is a graph showing a relationship between a rich time and a torque fluctuation at that time.

FIG. 8 is a diagram schematically showing a configuration of an engine exhaust system in a second embodiment.

FIG. 9 is a flowchart illustrating a valve switching routine according to the second embodiment.

FIG. 10 is a graph showing a relationship between an intake air amount and a counter threshold value.

FIG. 11 is a time chart for explaining an operation in the second embodiment.

[Explanation of symbols]

1: engine (internal combustion engine), 3: exhaust pipe, 19: NOx
Catalyst (NOx storage reduction catalyst), 25 ... Exhaust manifold constituting exhaust gas separation means, 25a ... Branch pipe, 30 ...
ECU (electronic control device), 31: CPU constituting exhaust gas separation means, 42: exhaust passage, 44: exhaust first passage constituting exhaust gas separation means, 45 ... exhaust second passage constituting exhaust gas separation means, 46 ... Exhaust switching valve that constitutes exhaust gas separation means.

Claims (6)

[Claims] The present invention is applied to an air-fuel ratio control device for an internal combustion engine that sets a target air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine to a lean side of a stoichiometric air-fuel ratio and performs lean combustion based on the target air-fuel ratio. NOx in exhaust gas discharged during lean combustion is converted to lean N
In an exhaust gas purifying apparatus for an internal combustion engine in which the occluded NOx is stored and the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst, the exhaust gas during lean combustion and the exhaust gas during rich combustion are removed. An exhaust gas purifying apparatus for an internal combustion engine, comprising: exhaust gas separating means for separating each exhaust gas in an exhaust passage so that the exhaust gas is not mixed. 2. The exhaust gas separation means applied to a multi-cylinder internal combustion engine, wherein the exhaust passages extending from each cylinder of the internal combustion engine are separated from each other immediately before the lean NOx catalyst. Item 2. An exhaust gas purifying apparatus for an internal combustion engine according to item 1. 3. The exhaust gas separation means includes a first passage and a second passage branching an exhaust passage extending from the internal combustion engine, and supplies exhaust gas to the lean NOx catalyst using the first passage during lean combustion. Send it,
The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein exhaust gas is supplied to the lean NOx catalyst using the second passage during rich combustion. 4. The exhaust gas separation means further comprises a switching valve for closing one of the first passage and the second passage and releasing the other, and the first passage is opened during lean combustion, and the second passage is opened during lean combustion. The switching valve is operated so that the passage is closed.
The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the switching valve is operated so that the passage is closed and the second passage is released. 5. When switching between lean combustion and rich combustion, switching between the first passage side and the second passage side is performed with a delay corresponding to a time corresponding to a delay in exhaust gas transportation.
An exhaust gas purifying apparatus for an internal combustion engine according to claim 4. 6. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the apparatus is applied to an air-fuel ratio control device that performs lean combustion and rich combustion at a predetermined time ratio.