JPH11117791A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

(57) [PROBLEMS] To provide an air-fuel ratio control device that performs lean combustion and temporarily performs rich combustion in order to restore the purifying ability of a NOx catalyst, and performs rich combustion at an optimal time to reduce fuel consumption. To improve torque and suppress torque fluctuations. An engine exhaust pipe (3) stores NOx under a lean air-fuel ratio state and reduces and releases the stored NOx with CO and HC under a rich air-fuel ratio state.
An Ox catalyst 19 is provided. CPU in ECU 30
31 sets the target air-fuel ratio of the air-fuel mixture supplied to the engine 1 leaner than the stoichiometric air-fuel ratio, and performs lean combustion based on the target air-fuel ratio. Further, the CPU 31 sets a rich time for rich combustion according to the engine operating state and the NOx purification rate by the NOx catalyst 19. At this time, the shortest rich time is set within a range in which a desired NOx purification rate by the NOx catalyst 19 can be obtained.

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 purifies nitrogen oxides (NOx) in exhaust gas generated during lean combustion. For controlling the air-fuel ratio of an internal combustion engine having a NOx storage-reduction 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.
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, in the above prior art, when switching from lean combustion to rich combustion, the air-fuel ratio near the catalyst does not immediately switch to 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 continued, 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 time is prolonged, the fluctuation of the rotation becomes large, causing a problem that the drivability is deteriorated.

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. In the air-fuel ratio control device,
An object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine that can perform rich combustion at an optimum time and improve fuel efficiency and suppress torque fluctuation.

[0006]

The air-fuel ratio control system for an internal combustion engine according to the present invention is based on the premise that the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set to a side leaner than the stoichiometric air-fuel ratio and the target air-fuel ratio is set. Lean combustion is performed based on the fuel ratio. Also, NOx in exhaust gas discharged during lean combustion
Is stored in a lean NOx catalyst, and the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst.

The first aspect of the present invention is characterized in that the operating state of the engine and the NOx
There is provided rich time setting means for setting a rich time for rich combustion according to the purification rate.

In short, in the conventional apparatus, when the rich combustion is temporarily performed during the lean combustion, the rich time is set to be longer in consideration of a margin. For this reason, there is a possibility that fuel consumption may be deteriorated or torque may be changed. On the other hand, the present invention aims to eliminate the problems of the conventional device by shortening the rich time. That is, in order to shorten the rich time, the rich time and the engine operating state have a certain relationship, and to perform rich combustion without torque fluctuation, the engine operating state such as the engine speed and the intake pressure must be changed. The rich time may be set accordingly. For example, the rich time is increased in a high rotation or high load region of the internal combustion engine, and the rich time is reduced in a low rotation or low load region (see FIG. 4).
On the other hand, rich time and NOx in lean NOx catalyst
The purification rate has a relationship shown in FIG. 5, for example.

In view of the above, the engine operating state and the NO
x If the rich time is set according to the purification rate, it is possible to always perform appropriate rich combustion even when the engine operating state changes. As a result, it is possible to perform rich combustion at an optimal time, thereby improving fuel efficiency and suppressing torque fluctuation.

[0010] In the invention described in claim 2, the rich time setting means is configured to control a desired NO by a lean NOx catalyst.
x Set the shortest rich time within the range where the purification rate can be obtained. For example, in FIG. 5, when the allowable level of the NOx purification rate is set to 95% or more, the optimal rich time at each point of A1, A2, and A3 depends on the engine operation state (engine speed Ne, intake pressure PM). Can be set. in this case,
The NOx purification performance of the NOx catalyst can be maintained.

In the air-fuel ratio control device according to the third aspect of the invention, there are provided: a catalyst state detecting means for detecting a state of NOx purification by a lean NOx catalyst; and a shortening of a rich time for rich combustion at a predetermined time period. Rich time updating means for updating the rich time to the limit value when the rich time at that time is determined to be the limit value from the detected NOx purification state of the catalyst. It is characterized by.

In such a case, the NO by the lean NOx catalyst
x The rich time is gradually reduced while monitoring the purification state. When the limit value is reached in the process of shortening the rich time, the update of the rich time is stopped at that point. This makes it possible to shorten the rich time while ensuring the NOx purification performance of the NOx catalyst. As a result, it is possible to perform rich combustion at an optimal time, thereby improving fuel efficiency and suppressing torque fluctuation.

The third aspect of the present invention can be implemented in the following fourth or fifth aspect. In the invention described in claim 4, the catalyst state detecting means includes an air-fuel ratio sensor or a NOx sensor provided on the downstream side of the lean NOx catalyst, and NOx by the NOx catalyst based on an output value of the sensor.
And a determination means for determining the degree of purification. That is, the degree of NOx purification by the lean NOx catalyst is determined based on the sensor output, and from the determination result, shortening (learning) of the rich time is permitted or prohibited. In such a case, the learning of the rich time can be appropriately performed.

[0014] The invention according to claim 5 further comprises a storage means for storing the updated rich time for each operating region of the internal combustion engine. In this case, the rich time according to the engine operating state can be set each time, and it is possible to cope with a change in the engine operating state.

The air-fuel ratio control device according to claim 6, further comprising: control command value setting means for setting a control command value for a rich time for rich combustion; and Actual rich time estimating means for estimating the actual time when the exhaust gas during rich combustion becomes rich according to the time command value and supplied to the lean NOx catalyst; and from the estimated actual rich time, the lean time for lean combustion is calculated. Lean time setting means for setting.

That is, a predetermined rich time (control command value)
Even when switching between lean combustion and rich combustion based on the above, the air-fuel ratio of the air-fuel mixture flowing into the engine combustion chamber is not changed due to the effect of wet, etc., and furthermore, the air-fuel ratio of exhaust gas when reaching the lean NOx catalyst. Is further reduced due to mixing with exhaust gas of other cylinders and delay in transport in the exhaust pipe.
Under such circumstances, according to the configuration of claim 6, the lean time can be set without excess or shortage, and even if the actual rich time is set to be short, the NOx is inadvertently discharged due to insufficient lean combustion. There is no. As a result, it is possible to perform rich combustion at an optimal time, thereby improving fuel efficiency and suppressing torque fluctuation.

However, in the invention of claim 6, the lean NOx
As described in claim 7, it is desirable to guard the control command value of the rich time with the lower limit value according to the engine operating state at that time so that the exhaust gas supplied to the catalyst is switched richly.

In the invention described in claim 8, the actual rich time estimating means estimates that the actual rich time with respect to the rich time command value becomes shorter as the internal combustion engine operates at a lower load. In this case, the setting of the rich time and the lean time can be properly performed even under conditions such as a low load where the lean / rich switching of the exhaust gas air-fuel ratio is delayed.

[0019]

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 which reciprocates 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) is provided to the exhaust pipe 3. 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.

An intake port 17 of the engine 1 is provided with an electromagnetically driven injector 18, and fuel (gasoline) is supplied to the injector 18 from a fuel tank (not shown). In the present embodiment, a multipoint injection (MPI) system having one injector 18 for each branch pipe of the intake manifold is configured. In this case, the fresh air supplied from the upstream of the intake pipe and the fuel injected by the injector 18 are connected to the intake port 1.
The mixture is mixed at 7, and the mixture flows into the combustion chamber 13 (in the cylinder 9) with the opening operation of the intake valve 14.

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 a throttle opening TH, an intake pressure PM, an 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. 2 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. 2 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 measured 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), 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, the lean time TL and the rich time TR are set to a predetermined time ratio.
Are set, and lean combustion and rich combustion are performed alternately according to the respective times TL and TR.

In FIG. 3, the CPU 31 first determines in step 201 whether or not the period counter at that time is "0". On condition that the period counter is 0 (step 201: YES), step 202 is executed. Sets the lean time TL and the rich time TR based on the engine speed Ne and the intake pressure PM. If step 201 is NO (if the cycle counter is $ 0), the CPU 31 skips the processing of step 202.

Here, the lean time TL and the rich time TR
Are equivalent to the number of fuel injections at a lean air-fuel ratio and the number of fuel injections at a rich air-fuel ratio, respectively, and are basically set to larger values as the engine speed Ne is higher or the intake pressure PM is higher. Is done. In the present embodiment, FIG.
The rich time TR is obtained by a map search based on the relationship. At this time, the relationship in FIG. 4 is set such that the rich time is the shortest within a range in which the desired NOx purification rate by the NOx catalyst 19 can be obtained.

That is, the characteristic of the NOx purification rate with respect to the rich time has a relationship shown in FIG. 5, and according to FIG. 5, the N.sub.x depends on the engine operating state (engine speed Ne, intake pressure PM).
The characteristics of the Ox purification rate change. In general, as Ne and PM are larger, the characteristic of the NOx purification rate shifts to the right side of the figure, and N
As e and PM are smaller, the characteristics of the NOx purification rate shift to the left in the figure. Therefore, in order to shorten the rich time while keeping the NOx purification rate at a predetermined level (for example, 95% or more in FIG. 5), for example, A in FIG.
The optimal rich time is determined from 1, A2, and A3 (however, A1 <A2 <A3).

On the other hand, the lean time TL is obtained from the rich time TR and a predetermined coefficient α as TL = TR · α. The coefficient α may be a fixed value of about “100”, but may be variably set according to the engine operating state such as the engine speed Ne and the intake pressure PM.

Thereafter, the CPU 31 increments the cycle counter by "1" at step 203. Thereafter, the CPU 31 determines in step 204 whether the value of the cycle counter has reached a value corresponding to the set lean time TL. If the cycle counter <TL and the negative determination is made in step 204, the CPU 31 proceeds to step 2
In step 05, the target air-fuel ratio λTG is set as a lean control value based on the engine speed Ne and the intake pressure PM at that time. After setting the λTG value, the CPU 31 returns to the original routine of FIG.

At this time, the λTG value is obtained, for example, by searching a target air-fuel ratio map shown in FIG. 6, and a value corresponding to, for example, A / F = 20 to 23 is set as the λTG value. If the condition for performing the lean combustion is not satisfied, for example, the λTG value is set near the stoichiometric condition).
In such a case, the λTG value set in step 205 described above is used in the calculation of the FAF value in step 105 of FIG. 2 described above, and the air-fuel ratio is lean-controlled by the FAF value.

On the other hand, if the cycle counter ≧ TL and the affirmative determination is made in step 204, the CPU 31 proceeds to step 206 and sets the target air-fuel ratio λTG 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 207 whether or not the value of the cycle counter has reached a value corresponding to the total value "TL + TR" of the set lean time TL and rich time TR. If TL + TR and step 207 is negative, the original FIG.
Return to the routine. In such a case, the above-mentioned step 206
The λTG value set at step 105 is the FA value at step 105 in FIG.
It is used for calculating the F value, and the air-fuel ratio is richly controlled by the FAF value.

On the other hand, if the cycle counter ≧ TL + TR and the affirmative determination is made in step 207, the CPU 31
In step 208, the cycle counter is cleared to "0", and thereafter, the process returns to the original routine of FIG. At the time of the next process following the clearing of the cycle counter, step 201 is affirmatively determined, and the lean time TL and the rich time TR are newly set. Then, the lean time TL and the rich time TR
, The lean control and the rich control of the air-fuel ratio are performed again.

FIG. 7 is a time chart for explaining the control operation according to the routines of FIGS. 2 and 3. In FIG. 7, during the period of the cycle counter = 0 to TL, the air-fuel ratio is controlled lean. At this time, NOx in the exhaust gas is stored in the NOx catalyst 19. Also, cycle counter = T
During the period from L to TL + TR, the air-fuel ratio is richly controlled. At this time, unburned gas components (HC, CO) in the exhaust gas
Thus, the stored NOx of the catalyst 19 is reduced and released.
Thus, the lean control and the rich control of the air-fuel ratio are repeatedly performed according to the lean time TL and the rich time TR.

In this embodiment, step 202 in FIG. 3 (the map in FIG. 4) corresponds to a rich time setting means. According to the present embodiment described in detail above, the following effects can be obtained.

(A) Engine operating state and NOx catalyst 19
The rich time for rich combustion is set in accordance with the NOx purification rate by the above. In short, in the conventional device, the rich time is set to be longer in anticipation of a margin, so that there is a possibility that fuel consumption may deteriorate or torque fluctuation may occur.
In the present embodiment, the problem of the conventional device can be solved by setting the rich time according to the relationship between FIGS. 4 and 5 and shortening the rich time. Therefore, it is possible to always perform appropriate rich combustion even if the engine operating state changes. As a result, it is possible to perform rich combustion at an optimal time, thereby improving fuel efficiency and suppressing torque fluctuation.

FIG. 8 is experimental data showing the relationship between the rich time per cycle 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) Desired NOx by NOx catalyst 19
The shortest rich time is set within the range where the purification rate can be obtained. In this case, the optimal rich time can be set, and the NOx purification performance of the NOx catalyst 19 can be maintained.

Next, second and third embodiments of the present invention will be described. However, in the configuration of each of the second and third 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) The details of the air-fuel ratio control system according to the second embodiment will be described with reference to FIGS.

This embodiment is characterized in that the rich time is successively learned while monitoring the NOx purification state by the NOx catalyst 19 in order to optimally shorten the rich time. Briefly, in this embodiment,
As shown in FIG. 9, a NOx sensor 41 is provided downstream of the NOx catalyst 19 as a catalyst state detecting means.
1 is taken into the ECU 30. The ECU 30
x While monitoring the sensor output, learn the rich time gradually to the shorter side. In the process of reducing the rich time, NO
When the x sensor output (NOx concentration) becomes equal to or more than a predetermined value, the rich time at that time is regarded as the minimum limit, and the rich time is stored in the backup RAM 34 in the ECU 30.

The details of the NOx sensor 41 are disclosed in, for example, JP-A-8-271476 and Japanese Patent Application No. 9-171015 by the present applicant.
Outputs a current signal corresponding to the NOx concentration using an oxygen ion conductive solid electrolyte substrate such as stabilized zirconia.

FIG. 10 is a flowchart showing a rich time learning routine, which is executed by the CPU 31 at a cycle of, for example, one second. When the routine shown in FIG. 10 starts, the CPU 31 first proceeds to step 301.
And the operating state of the engine is "i region (where i = 1, 2, 3
.. N) is determined as to whether or not the learning completion flag Fi is “0”. Here, the engine operation regions 1 to n are set according to the engine speed Ne and the intake pressure PM, and a learning completion flag Fi is provided for each of these operation regions. Note that Fi = 0 indicates that the learning of the rich time in the i-region has not been completed, and Fi = 1 indicates that the learning of the rich time in the i-region has been completed. Are initialized to "0" at the beginning of the routine.

In step 302, the CPU 31
Determines whether or not the predetermined engine operating state has been continued for 10 seconds or more.
It is determined whether or not the rich switching is being performed, that is, whether or not the stoichiometric operation is being performed at the time of starting the engine 1 at a low temperature or at the time of high load operation.

If any of steps 301 to 303 is NO, the CPU 31 proceeds to step 304. If all of steps 301 to 303 are YES, the process proceeds to step 305. When proceeding to step 304, the CPU 31
Clears the rich time learning counter for measuring the time interval of the rich time learning to "0", and terminates this routine once.

When the processing proceeds to step 305, the CPU 3
1 increments the rich time learning counter by “1”, and determines whether or not the value of the rich time learning counter at that time has reached a value corresponding to a predetermined time (60 seconds in the present embodiment) at the next step 306. I do. If the rich time learning counter <60 seconds, the CPU 31 terminates the present routine as it is. If the rich time learning counter ≧ 60 seconds, the process proceeds to the next step 307. Incidentally, the time “60 seconds” corresponds to the time required for one rich time learning (learning period).

Then, the CPU 31 determines in step 307 that the output value of the NOx sensor 41 is equal to or less than a predetermined judgment value (in the present embodiment, a value corresponding to NOx concentration = 20 ppm) for securing a desired NOx purification rate. It is determined whether or not there is. At this time, the NOx sensor output may be smoothed and calculated within one learning period, and the calculated smoothed value may be compared with a predetermined determination value (20 ppm).

When the NOx sensor output ≦ 20 ppm, C
The PU 31 considers that the rich time can be further reduced, and in step 308, reduces the rich time (the number of rich injections) by one injection. Incidentally, the initial value of the rich time is, for example, about 10 injections. Further, the CPU 31 clears the rich time learning counter to “0” in the following step 309, and ends this routine. Step 3
In a state where the determination result of 07 is YES, the rich time is gradually reduced.

On the other hand, if the NOx sensor output is greater than 20 ppm, the CPU 31 determines the desired NO in the current rich time.
x Assuming that the purification rate cannot be secured, the rich time (the number of rich injections) is increased by one injection in step 310.
Further, the CPU 31 stores the rich time at that time in the backup RAM 34 in the subsequent step 311. At this time, the rich time learned as described above is stored for each engine operating state (each area of 1 to n). The learning value of the rich time stored in the backup RAM 34 is stored and held even when the power is turned off.

Thereafter, the CPU 31 sets "1" to the learning completion flag Fi corresponding to the operation area i (= 1 to n) at that time in step 312, and proceeds to step 3
In step 13, the rich time learning counter is cleared to "0" and the routine ends.

As described above, when the rich time is learned and its value is updated, in step 202 in FIG. 3, the rich time corresponding to the current operation area i (= 1 to n) is read from the backup RAM 34. It is. At this time, the lean time is calculated as the lean time = α · rich time. However, the coefficient α may be a fixed value of about “100” or may be variably set according to the engine operating state such as the engine speed Ne and the intake pressure PM.

Next, the operation according to FIG. 10 will be described more specifically with reference to the time chart of FIG. In FIG. 11, each period divided from time t <b> 1 to t <b> 4 indicates a rich period learning period (60 seconds in the present embodiment). At this time, at times t1, t2, and t3, the NOx sensor output (averaged value during the learning period) is set to a predetermined value (20p).
pm). Therefore, the rich time is reduced by one injection (step 308 in FIG. 10).

On the other hand, at time t4, the NOx sensor output (the smoothed value from time t3 to t4) becomes a predetermined value (20p).
pm). Therefore, the rich time is added by one injection, and the rich time at that time is stored in the memory as a learning value (steps 310 and 311 in FIG. 10).
At time t4, "1" is set to the learning completion flag Fi (step 312 in FIG. 10).

In this embodiment, step 307 in FIG. 10 corresponds to the catalyst state detecting means (determining means), and step 308 corresponds to the rich time updating means. Step 310 corresponds to an update canceling unit, and step 311 corresponds to a storage unit.

According to the second embodiment described in detail above,
The following effects can be obtained. (A ') While monitoring the NOx purification state of the NOx catalyst 19, the rich time is gradually updated to the shorter side, and when the rich time at that time is determined to be the limit value from the NOx purification state of the catalyst 19, the rich time is reduced. Stop updating to the side. This makes it possible to shorten the rich time while ensuring the NOx purification performance of the NOx catalyst 19. In such a case as well, rich combustion can be performed at an optimal time, thereby improving fuel efficiency and suppressing torque fluctuation.

(B ′) NOx downstream of the NOx catalyst 19
A sensor 41 is provided to determine the degree of NOx purification by the NOx catalyst 19 based on the sensor output. Therefore, the shortening of the rich time is allowed or prohibited based on the NOx sensor output (NOx concentration), and the learning of the rich time can be properly performed.

(C ') The learning value of the rich time is stored in the engine 1
Is stored for each of the operation areas. As a result, the rich time according to the engine operating state can be set each time, and a change in the operating state can be appropriately dealt with.

(D ') When it is determined based on the NOx sensor output that the rich time has reached the limit value on the shortening side, the rich time is updated to the opposite side (one injection is added).
In this case, even if the rich time is set too short, the rich time can be corrected. Further, even when it is necessary to prolong the rich time due to a change with time such as deterioration of the NOx catalyst 19, the optimal rich time can always be set.

(Third Embodiment) Next, the details of the air-fuel ratio control system in the third embodiment will be described with reference to FIGS.
5 will be described.

In the third embodiment, during the lean / rich control, the actual rich time (actual rich time) is estimated from the control command value of the rich time for rich combustion and the engine operating state at each time. The lean time is set based on the actual rich time.

FIG. 12 is a flowchart showing a part of the λTG setting routine in the present embodiment. However,
This flowchart is implemented by replacing a part of the flowchart of FIG. 3 (steps 201 and 202), and the description of the processing according to FIG. 3 will be omitted.

In the routine of FIG. 12, on condition that the period counter at that time is "0" (YES in step 401), the CPU 31 performs a rich operation in step 402 based on the current engine speed Ne and intake pressure PM. Set the time (control command value). Here, the rich time (control command value) is set to a larger value as the engine speed Ne is higher or the intake pressure PM is higher (see FIG. 4). However, at this time, the rich time is guarded by the lower limit value according to the engine operating state at that time so that the exhaust gas supplied to the NOx catalyst 19 is reliably switched to rich. This is because if the rich time is too short, even if the air-fuel ratio is switched from lean to rich, the exhaust gas air-fuel ratio at the catalyst inlet will not be rich, and it will not be possible to substantially reduce NOx.

The CPU 31 proceeds to the next step 403
To calculate the actual rich time. The actual rich time is the time when the exhaust gas air-fuel ratio at the catalyst inlet actually becomes rich,
For example, the actual rich time is calculated as β = rich time (control command value). Here, the coefficient β is set according to the engine load such as the intake pressure PM and the throttle opening as shown in FIG. According to the figure, a smaller value is set for the coefficient β on the assumption that the mixing of the exhaust gas is delayed as the engine load is smaller.

Thereafter, the CPU 31 sets a lean time based on the actual rich time calculated in step 404. At this time, the lean time is calculated as lean time = α1 · actual rich time. However, the coefficient α1 is obtained based on, for example, the relationship shown in FIG.
A large value is set for the coefficient α1.

Thereafter, the CPU 31 alternately performs the above-described lean control and rich control of the air-fuel ratio in accordance with steps 203 to 208 in FIG. FIG. 15 is a time chart for supplementarily explaining the operation in the present embodiment.

In FIG. 15, when the target air-fuel ratio λTG is switched from lean to rich for a predetermined rich time (control command value), the air-fuel ratio (combustion A / F) of the air-fuel mixture flowing into the engine combustion chamber becomes wet or the like. The change is reduced due to the influence of. Furthermore, the air-fuel ratio of the exhaust gas when reaching the NOx catalyst 19 (exhaust gas A / F) further increases due to mixing with the exhaust gas of other cylinders and delay in transport in the exhaust pipe. Therefore, the time when the exhaust gas air-fuel ratio at the catalyst inlet is actually rich (actual rich time) is somewhat shorter than the control command value. In such a case, the rich control of the air-fuel ratio is performed based on a predetermined rich time (control command value), and the lean control of the air-fuel ratio is performed based on the actual rich time.

In the present embodiment, step 402 in FIG. 12 corresponds to a control command value setting means, and step 403 corresponds to an actual rich time estimating means. Step 404 corresponds to a lean time setting unit.

According to the third embodiment described in detail above,
The following effects can be obtained. (A ″) The actual rich time relative to the rich time (control command value) is estimated based on the engine operating state, and the lean time is set from the estimated actual rich time. Even if the actual rich time is set to be short, NOx is not inadvertently discharged due to insufficient lean combustion, and as a result, rich combustion is performed at an optimal time and fuel consumption is improved. And fluctuation of torque can be suppressed.

(B ″) As the engine 1 operates at a lower load,
The actual rich time for the rich time command value is estimated to be short. In this case, the lean exhaust gas air-fuel ratio
The setting of the rich time and the lean time can be appropriately performed even under conditions such as a low load where the rich switching is delayed.

The embodiment of the present invention can be realized in the following modes other than the above. In the first embodiment,
In setting the rich time, the engine speed Ne and the intake pressure PM are used as parameters for knowing the engine operation state, but these are changed. For example, a throttle opening and an accelerator opening may be used as parameters for knowing the engine operating state.

In the second embodiment, the NOx catalyst 1
The NOx sensor 41 is disposed downstream of the catalyst as means for detecting the NOx purification state by the NO. 9 and learning of the rich time is performed based on the output of the NOx sensor, but this configuration is changed. For example, an air-fuel ratio sensor is provided downstream of the catalyst, and learning of the rich time is performed based on the output value of the air-fuel ratio sensor. In this case, the catalyst state is determined from the response (response time) before and after the catalyst at the time of the lean / rich switching of the air-fuel ratio, and the learning of the rich time is permitted or prohibited based on the determination result. Incidentally, the air-fuel ratio sensor used here is a well-known A / F sensor (limit current type air-fuel ratio sensor) that outputs a linear current signal according to the oxygen concentration.
Alternatively, a known O2 sensor that outputs different voltage signals depending on the lean side or the rich side with respect to the stoichiometric air-fuel ratio can be applied.

Further, in the second embodiment, when the rich time is learned, the update width of one rich time is set to one.
Although the injection amount is set, two or more injection amounts may be updated at one time. In this case, for example, the update width may be variably set based on the output of the NOx sensor, taking into account the margin to the limit value.

Further, in the second embodiment, the learning value of the rich time is stored in the backup RAM 34 each time, and the internal information is stored and held even when the power is turned off. The rich time may be re-learned from an initial value (for example, a time equivalent to 10 injections).

In the third embodiment, the rich time (control command value) is set based on the engine speed Ne and the intake pressure PM. However, for example, the rich time learning described in the second embodiment is performed. The control command value for the same time may be set using the value.

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 air-fuel ratio is feedback-controlled according to the deviation between the target air-fuel ratio and the actually detected air-fuel ratio (actual air-fuel ratio) using modern control theory. 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 flowchart showing a fuel injection control routine.

FIG. 3 is a flowchart illustrating a λTG setting routine.

FIG. 4 is a map for setting a rich time according to an engine speed and an intake pressure.

FIG. 5 is a graph showing the relationship between the rich time and the NOx purification rate for each engine operating state.

FIG. 6 is a map for setting a lean target air-fuel ratio according to an engine speed and an intake pressure.

FIG. 7 is a time chart for explaining an operation in the first embodiment.

FIG. 8 is a graph showing the relationship between rich time and torque fluctuation at that time.

FIG. 9 is a configuration diagram showing an additional part of a control system in the second embodiment.

FIG. 10 is a flowchart illustrating a rich time learning routine according to the second embodiment;

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

FIG. 12 is a flowchart illustrating a part of a λTG setting routine according to the third embodiment;

FIG. 13 is a graph showing a relationship between an engine load and a coefficient β.

FIG. 14 is a graph showing a relationship between an actual rich time and a coefficient α1.

FIG. 15 is a time chart for explaining the operation in the third embodiment.

[Explanation of symbols]

1: engine (internal combustion engine), 3: exhaust pipe, 19: NOx
Catalyst (NOx storage reduction catalyst), 30 ... ECU (electronic control unit), 31 ... rich time setting means, rich time updating means, catalyst state detecting means (judging means), updating stop means,
CPU constituting storage means, control command value setting means, actual rich time estimating means, lean time setting means, 34 ... backup RAM, 41 ... N constituting catalyst state detecting means
Ox sensor.

Claims (8)

[Claims] A target air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set leaner than a stoichiometric air-fuel ratio, and lean combustion is performed based on the target air-fuel ratio. An air-fuel ratio control device for an internal combustion engine in which NOx is stored with a lean NOx catalyst and the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst. An air-fuel ratio control device for an internal combustion engine, comprising: rich time setting means for setting a rich time for rich combustion according to a NOx purification rate by a catalyst. 2. The method according to claim 1, wherein the rich time setting means includes a lean NOx
2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the shortest rich time is set within a range in which a desired NOx purification rate by the catalyst is obtained. 3. A target air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set to a side leaner than a stoichiometric air-fuel ratio to perform lean combustion based on the target air-fuel ratio. An air-fuel ratio control device for an internal combustion engine in which NOx is occluded by a lean NOx catalyst and the air-fuel ratio is temporarily controlled to be rich to release the occluded NOx from the lean NOx catalyst. A catalyst state detecting means for detecting a purification state; a rich time updating means for updating a rich time for rich combustion to a shorter side in a predetermined time cycle; and a rich time limit from the detected NOx purification state of the catalyst. An air-fuel ratio control device for an internal combustion engine, comprising: update canceling means for canceling the update of the rich time to the shortened side when the value is determined to be a value. 4. An air-fuel ratio control device for an internal combustion engine according to claim 3, wherein said catalyst state detecting means includes an air-fuel ratio sensor or a NOx sensor provided downstream of a lean NOx catalyst, and an output value of said sensor. Based on the NOx catalyst
An air-fuel ratio control device for an internal combustion engine, comprising: determination means for determining the degree of Ox purification. 5. The air-fuel ratio control apparatus for an internal combustion engine according to claim 3, further comprising a storage unit for storing the updated rich time for each operating region of the internal combustion engine. apparatus. 6. A target air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set leaner than a stoichiometric air-fuel ratio so that lean combustion is performed based on the target air-fuel ratio. An air-fuel ratio control device for an internal combustion engine that stores NOx with a lean NOx catalyst, and further temporarily controls the air-fuel ratio to be rich to release the stored NOx from the lean NOx catalyst. Control command value setting means for setting a time control command value; and based on the engine operating state, the exhaust gas during rich combustion based on the set rich time command value becomes rich and lean N
An internal combustion engine comprising: an actual rich time estimating means for estimating an actual time supplied to the Ox catalyst; and a lean time setting means for setting a lean time for lean combustion from the estimated actual rich time. Air-fuel ratio control device. 7. The air-fuel ratio control apparatus according to claim 6, wherein the control command value setting means guards a control command value for a rich time with a lower limit value according to a current engine operation state. Control device. 8. The air-fuel ratio of an internal combustion engine according to claim 6, wherein the actual rich time estimating means estimates that the actual rich time with respect to the rich time command value becomes shorter as the internal combustion engine operates at a lower load. Control device.