JP3256670B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus for an internal combustion engine provided with an exhaust gas purifying means having a built-in nitrogen oxide absorbent in an exhaust system.

[0002]

2. Description of the Related Art When the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set leaner than the stoichiometric air-fuel ratio (so-called lean burn control is executed), the emission amount of nitrogen oxides (hereinafter referred to as "NOx") is reduced. Since the exhaust gas tends to increase, a technique for purifying exhaust gas by providing an exhaust gas purifying unit having a built-in NOx absorbent for absorbing NOx in an exhaust system of an engine has been conventionally known. This NOx absorbent absorbs NOx when the air-fuel ratio is set leaner than the stoichiometric air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high (NOx is large) (hereinafter referred to as "exhaust gas lean state"). On the other hand, when the air-fuel ratio is set to be richer than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas is low, and the HC and CO components are large (hereinafter, referred to as “exhaust gas rich state”), the absorbed NOx is reduced. Has release properties. In the exhaust gas rich state, the exhaust gas purifying means including the NOx absorbent converts NOx released from the NOx absorbent into HC, C
Reduced by O, discharged as nitrogen gas, and
C and CO are oxidized and discharged as water vapor and carbon dioxide.

Since the amount of NOx that can be absorbed by the above-mentioned NOx absorbent naturally has a limit, if lean burn control is continued for a long time, NOx will not be absorbed and will be released to the atmosphere as it is. In its This, temporarily enriching the air-fuel ratio in order to recover the absorption capacity of the NOx, NO from the NOx absorbent
An air-fuel ratio control method for releasing x and reducing the released NOx has been conventionally known (Japanese Patent Laid-Open No. 6-294319). Hereinafter, this temporary enrichment is referred to as “reduction enrichment”. In addition, by this reduction enrichment, all the NOx absorbed by the NOx absorbent
After the release of NOx, the HC and CO components cannot be oxidized. Therefore, it is desirable to terminate the reduction enrichment simultaneously with the completion of the release of NOx from the NOx absorbent.

Therefore, an air-fuel ratio sensor is disposed downstream of the exhaust gas purifying means, and the point at which the output value of this sensor suddenly changes is detected, at which point the reduction enrichment is terminated. Is conventionally known (JP-A-8-232646).

[0005]

However, the method for detecting the point at which the output value of the air-fuel ratio sensor provided downstream of the exhaust gas purifying means changes suddenly has the following problems.

When the reduction enrichment is started, the output of the air-fuel ratio sensor suddenly changes to a value corresponding to the stoichiometric air-fuel ratio (first sudden change point), and thereafter, the state where the value is substantially maintained continues (hereinafter, referred to as the following). This period is referred to as a “stoichiometric maintenance period”), after which the value suddenly changes from a value corresponding to the stoichiometric air-fuel ratio to a value indicating a richer air-fuel ratio (second sudden change time point). The conventional device detects the second sudden change point. However, the output value of the air-fuel ratio sensor does not actually have a waveform as shown in the above-mentioned publication, but rather involves a small fluctuation, so that a sudden change point of the output may be erroneously detected during the stoichiometric maintenance period.

In order to detect a change in the output value of the air-fuel ratio sensor, the difference between the output value (t) at a certain time t and the output value (t + ΔT) after a lapse of a minute time ΔT is detected. In order to eliminate the influence of a small fluctuation,
Need to be longer. For this reason, a problem occurs that the detection timing at the point of sudden change is delayed.

The present invention has been made by paying attention to this point, and it is possible to accurately and quickly determine the completion time of the release of NOx from the NOx absorbent, and to maintain the excellent exhaust gas characteristics. It is intended to provide a purification device.

[0009]

According to a first aspect of the present invention, there is provided an exhaust system of an internal combustion engine, wherein the nitrogen oxide absorbs nitrogen oxide in exhaust gas in an exhaust gas lean state. Exhaust gas purifying means containing an absorbent; first and second oxygen concentration sensors provided upstream and downstream of the exhaust gas purifying device for detecting oxygen concentration in exhaust gas; Reducing means for reducing the nitrogen oxides absorbed by the nitrogen oxide absorbent by enriching the air-fuel ratio of the air-fuel mixture to be performed. after the start, the output value of the second oxygen concentration sensor, before
Engine operating conditions that control the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio
Output value of the second oxygen concentration sensor obtained in
Is corrected according to the output value of the first oxygen concentration sensor.
The air-fuel ratio enrichment is terminated when the air-fuel ratio becomes richer than the predetermined reference value calculated .

According to this configuration, after the air-fuel ratio enrichment is started by the reducing means, the output value of the second oxygen concentration sensor becomes richer than the predetermined reference value set according to the output value of the first oxygen concentration sensor. At this point, it is determined that the release of the nitrogen oxides absorbed by the nitrogen oxide absorbent has been completed, and the air-fuel ratio enrichment ends. The predetermined reference value
Is an engine operation for controlling the air-fuel ratio of the air-fuel mixture to a stoichiometric air-fuel ratio.
Output of the second oxygen concentration sensor obtained in the rotation state.
The force value is supplemented according to the output value of the first oxygen concentration sensor.
It is calculated by correcting.

According to a second aspect of the present invention, in the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect, the predetermined reference value is set in an engine operating state where the air-fuel ratio of the air-fuel mixture is controlled to a stoichiometric air-fuel ratio. A supplementary value calculated according to the output value of the second oxygen concentration sensor is added to a value calculated by performing a smoothing process on the obtained output value of the second oxygen concentration sensor.
It is a value obtained by adding a positive value .

According to this configuration, the predetermined reference value is an output value of the second oxygen concentration sensor obtained in an engine operating state for controlling the air-fuel ratio of the air-fuel mixture to a stoichiometric air-fuel ratio.
A correction value calculated according to the output value of the second oxygen concentration sensor is added to the value calculated by performing the averaging process.
It is a value obtained by calculation.

According to a third aspect of the present invention, in the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect, the reducing means performs lean air-fuel ratio control for leaning the air-fuel mixture from a stoichiometric air-fuel ratio. It operates when the control time is continued.

According to this configuration, when the lean air-fuel ratio control for leaning the air-fuel mixture supplied to the engine from the stoichiometric air-fuel ratio continues for a predetermined lean control time, the air-fuel ratio is enriched by the reducing means.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter, referred to as an “engine”) and an air-fuel ratio control device thereof according to an embodiment of the present invention, for example, in the middle of an intake pipe 2 of a four-cylinder engine 1. Is provided with a throttle valve 3. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3, and outputs an electric signal corresponding to the opening of the throttle valve 3 to output an electronic control unit for engine control (hereinafter referred to as “ECU”) 5. To supply.

A fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of an intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). The ECU 5 is electrically connected to the ECU 5 and controls a valve opening time of the fuel injection valve 6 based on a signal from the ECU 5.

On the other hand, an intake pipe absolute pressure (PBA) sensor 7 is provided immediately downstream of the throttle valve 3, and the absolute pressure signal converted into an electric signal by the absolute pressure sensor 7 is supplied to the ECU 5. . Further, an intake air temperature (TA) sensor 8 is mounted downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies the electric signal to the ECU 5.

An engine water temperature (TW) sensor 9 mounted on the body of the engine 1 is composed of a thermistor or the like, detects an engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5.

An engine speed (NE) sensor 10 is provided around a camshaft or a crankshaft (not shown) of the engine 1.
And a cylinder discrimination (CYL) sensor 11. The engine speed sensor 10 outputs a TDC signal pulse at a crank angle position before a predetermined crank angle with respect to the top dead center (TDC) at the start of the intake stroke of each cylinder of the engine 1 (every 180 ° crank angle in a four-cylinder engine). The cylinder discriminating sensor 11 outputs a cylinder discriminating signal pulse at a predetermined crank angle position of a specific cylinder. These signal pulses are supplied to the ECU 5.

The exhaust pipe 12 is provided with exhaust gas purifying means 16 for purifying exhaust gas.
It incorporates a NOx absorbent that absorbs NOx and a catalyst that has an oxidizing and reducing action. The NOx absorbent is set such that the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is set leaner than the stoichiometric air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high (NOx is large) (exhaust gas lean state). , NOx
On the other hand, when the air-fuel ratio supplied to the engine 1 is set to be richer than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas is low, and the HC and CO components are large (exhaust gas rich state), It has the property of releasing absorbed NOx. In the exhaust gas lean state, the exhaust gas purifying means 16 causes the NOx absorbent to absorb NOx,
In the exhaust gas rich state, NOx released from the NOx absorbent is reduced by HC and CO to be discharged as nitrogen gas, and HC and CO are oxidized and discharged as water vapor and carbon dioxide. I have.
As the NOx absorbent, for example, barium oxide (Ba0)
Is used, and for example, platinum (Pt) is used as the catalyst. This NOx absorbent generally has a characteristic that the higher the temperature, the easier it is to release the absorbed NOx. It should be noted that the NOx absorbent releases NOx when the oxygen concentration decreases and the NOx generation amount decreases even in the exhaust gas lean state.

When NOx is absorbed up to the limit of the NOx absorbing capacity of the NOx absorbent, that is, up to the maximum NOx absorption amount, NOx can no longer be absorbed. Therefore, the air-fuel ratio is reduced and enriched to release and reduce NOx. Execute.
This reduction enrichment, if the degree of enrichment is too small,
If the degree of the enrichment is too great while the released NOx is insufficiently reduced, the amount of HC and CO emissions increases, so by appropriately controlling the degree of the enrichment of the reduction enrichment, Exhaust gas characteristics can be maintained.

A proportional air-fuel ratio sensor 14 serving as a first oxygen concentration sensor is provided upstream of the exhaust gas purifying means 16.
(Hereinafter referred to as “LAF sensor 14”). The LAF sensor 14 outputs an electric signal substantially proportional to the oxygen concentration (air-fuel ratio) in the exhaust gas and supplies the electric signal to the ECU 5. In this embodiment, the output VLAF of the LAF sensor 14 is set to increase when the air-fuel ratio becomes rich. Further, at a position downstream of the exhaust gas purifying means 16,
An oxygen concentration sensor (second oxygen concentration sensor) 15 (hereinafter referred to as “O2 sensor 15”) is mounted, and a detection signal thereof is supplied to the ECU 5. This O2 sensor 15
Has a characteristic that its output VO2 changes abruptly before and after the stoichiometric air-fuel ratio, and its output VO2 becomes higher on the rich side and lower on the lean side than the stoichiometric air-fuel ratio.

In the engine 1, the valve timing of the intake valve and the exhaust valve can be switched between a high-speed valve timing suitable for a high-speed rotation region of the engine and a low-speed valve timing suitable for a low-speed rotation region of the engine. It has a mechanism 30. The switching of the valve timing includes the switching of the valve lift amount. Further, when the low-speed valve timing is selected, one of the two intake valves is stopped to stabilize the air-fuel ratio even when the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio. We have tried to ensure the combustion that we did.

The valve timing switching mechanism 30 switches the valve timing via hydraulic pressure, and an electromagnetic valve and a hydraulic pressure sensor for switching the hydraulic pressure are connected to the ECU 5. The detection signal of the oil pressure sensor is supplied to the ECU 5, and the ECU 5 controls the solenoid valve to control the switching of the valve timing according to the operating state of the engine 1.

The ECU 5 shapes input signal waveforms from various sensors, corrects a voltage level to a predetermined level, and converts an analog signal value into a digital signal value. The CPU 5b includes a storage unit 5c for storing various calculation programs executed by the CPU 5b, calculation results, and the like, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, and the like.

The CPU 5b determines various engine operating states in the air-fuel ratio feedback control region and a plurality of specific operation regions in which the air-fuel ratio feedback control is not performed, as described later, based on the various engine parameter signals. According to the determined engine operating state,
Based on the following equation (1), a fuel injection time TOUT of the fuel injection valve 6 synchronized with the TDC signal pulse is calculated.

TOUT = TI × KCMDM × KLAF × K1 + K2 (1) Here, TI is a basic fuel injection time of the fuel injector 5 and is determined according to the engine speed NE and the intake pipe absolute pressure PBA.

KCMDM is a final target air-fuel ratio coefficient,
As described later, the engine speed NE and the intake pipe absolute pressure P
The target air-fuel ratio coefficient KCMD, which is set according to the engine operating parameters such as BA and engine water temperature TW, is calculated by performing fuel cooling correction. Target air-fuel ratio coefficient KCMD
Is proportional to the reciprocal of the air-fuel ratio A / F, that is, the fuel-air ratio F / A, and takes a value of 1.0 at the stoichiometric air-fuel ratio.

The detected equivalent ratio KACT calculated from the detected value of the LAF sensor 14 is equal to the target equivalent ratio KCMD.
Is an air-fuel ratio correction coefficient calculated by PID control so as to coincide with

K1 and K2 are other correction coefficients and correction variables calculated according to various engine parameter signals, respectively, so that various characteristics such as fuel consumption characteristics and engine acceleration characteristics can be optimized according to the engine operating state. Is determined to be a predetermined value.

The CPU 5b supplies a drive signal for opening the fuel injection valve 6 to the fuel injection valve 6 via the output circuit 5d based on the fuel injection time TOUT obtained as described above.

In this embodiment, the ECU 5 and the fuel injection valve 6 constitute a reduction means.

FIG. 2 is a flowchart of a process for calculating the target equivalent ratio KCMD and calculating the air-fuel ratio correction coefficient KLAF by PID control such that the detected equivalent ratio KACT matches the target equivalent ratio KCMD. This processing is performed by, for example, TD
This is executed in synchronization with the generation of the C signal pulse.

First, in step S1, the target equivalent ratio KCM
Calculate D. The target equivalence ratio KCMD is basically calculated according to the engine speed NE and the intake pipe absolute pressure PBA, and in a low temperature state of the engine coolant temperature TW or a predetermined high load operation state, a value corresponding to those operation states. Is changed to

In step S2, fuel cooling correction of the target equivalent ratio KCMD is performed by the following equation to calculate a final target air-fuel ratio coefficient KCMDM.

KCMDM = KCMD × KETC KETC is a fuel cooling correction coefficient, and is set so as to increase as the KCMD value increases. The fuel cooling correction is performed in consideration of the fact that the fuel cooling effect by the injection increases as the KCMD value increases and the fuel injection amount increases.

In step S3, a reduction enrichment control process shown in FIGS. 3 and 4 to be described later is executed.
The detection value of the AF sensor 14 is converted into an equivalent ratio to calculate a detected equivalent ratio KACT. In the following step S5, PI based on the deviation between the detected equivalent ratio KACT and the target equivalent ratio KCMD
By the D control, the detected equivalent ratio KACT is changed to the target equivalent ratio KCM.
The air-fuel ratio correction coefficient KLAF is calculated so as to match D.

FIGS. 3 and 4 are flowcharts of the reduction enrichment control process executed in step S3 of FIG.

In step S11 of FIG. 3, it is determined whether or not a feedback control flag FLAFFB indicating "1" indicates that the engine 1 is in an operation state in which feedback control is performed in accordance with a value detected by the LAF sensor 14. If FLAFFB = 1 and the operation state in which the feedback control is executed is determined, "0" indicates that the operation state is the operation state in which the lean burn control for setting the air-fuel ratio to the lean side of the stoichiometric air-fuel ratio is executed. Lean burn control flag F
It is determined whether or not KBSMJG is "0" (step S1).
2) When FKBSMJG = 0 and the operation state in which the lean burn control is executed, the target equivalent ratio KCMD
Is equal to or less than a predetermined equivalent ratio KCMDLB (for example, 0.98) set to a value slightly leaner than the stoichiometric air-fuel ratio (step S13).

If any one of the steps S11 to S13 is negative (NO), the reduction enrichment flag FR indicating "1" indicates that the reduction enrichment is being executed.
ROK is set to “0” and the counter CTRR
Is set to the first predetermined value CTRINT1 (see FIG. 6B) (step S14), and the process is terminated without executing the reduction enrichment.

When all of the answers of steps S11 to S13 are affirmative (YES), that is, when the condition for executing the lean burn control is satisfied, the process proceeds to step S15.
The CTSV map shown in FIG. 5A is searched to calculate the increment CTSV of the counter CTRR. The CTSV map is a map in which the increment value CTSV is set according to the engine speed NE and the intake pipe absolute pressure PBA. As the engine speed NE increases, the intake pipe absolute pressure P
The CTSV value is set to increase as the BA increases. In the following step S16, the counter CTR
The value of R is incremented by the increment value CTSV, and then the value of the counter CTRR is increased to the first predetermined value CTRRIN.
It is determined whether it is equal to or more than a predetermined threshold value CTRACT (see FIG. 6B ) smaller than T1 (step S17). Immediately after the lean burn control execution condition is satisfied, the counter CTR
Since R is set to the first predetermined value CTRINT1 (step S14), CTRR ≧ CTRACT, and the process proceeds to step S18.

In step S18, it is determined whether or not the reduction enrichment flag FRROK is "1". At first, FRR
Since OK = 0, this is set to "1" (step S19), and the process proceeds to step S24 to search the KCMDRR map shown in FIG. 5B to calculate the reduction enrichment target equivalent ratio KCMDRR. Next, the final target air-fuel ratio coefficient K
The CMDM is set to the reduction enrichment target equivalent ratio KCMDRR (step S25), and the process ends.

The KCMDRR map indicates the engine speed N
Is a map in which the reduction-enrichment target equivalent ratio KCMDRR is set in accordance with E and the intake pipe absolute pressure PBA. As the engine speed NE increases, the intake pipe absolute pressure PB
It is set so that the KCMDRR value increases as A increases. Note that all the set values are values larger than 1.0.

When the reduction enrichment flag FRROK is set to "1" in step S19 and the reduction enrichment is started, the answer in step S18 becomes affirmative (YES) thereafter, and the process proceeds to step S21, where the LAF sensor 14 The correction term DVO2REF is calculated according to the output VLAF. The correction term DVO2REF is set to a smaller value as the VLAF value increases (the degree of reduction enrichment increases ) .

In the following step S22, a predetermined reference value VO2REF is calculated by the following equation (2).

VO2REF = VO2REF0 + DVO2REF (2) Here, VO2REF0 is the target equivalent ratio KCMD = 1.
The output value VO of the O2 sensor 15 in the operating state of 0
2 is a basic reference value obtained by performing an averaging process according to the following equation (3).

VO2REF0 = a × VO2 + (1−a) × VO2REF0 (3) Here, a is a smoothing coefficient set to a value between 0 and 1, and VO2REF0 on the right side is the value of the previous calculation. Value.

Next, it is determined whether or not the output value VO2 of the O2 sensor 15 is higher than a predetermined reference value VO2REF. Since VO2 <VO2REF at the beginning of the reduction enrichment, the process proceeds to step S24, where VO2> VO2REF.
Then, it is determined that the release of NOx from the NOx absorbent is completed, the reduction enrichment flag FRROK is set to “0” (step S26), and the counter CTRR is set.
To a second predetermined value CT smaller than the predetermined threshold value CTRACT
RRINT2 (for example, 0) (step S2
7), the reduction enrichment ends. Step S26, S2
7 is performed, the final target air-fuel ratio coefficient KCMDM holds the value calculated in step S2 of FIG.
Lean burn control is started.

Thereafter, steps S16 and S17 are repeatedly executed, that is, lean burn control is executed, and
When the value of the counter CTRR reaches the predetermined threshold value CTRACT (FIG. 6, time t3), the process proceeds to step S18 and thereafter to execute the reduction enrichment.

FIG. 6 is a time chart for explaining the processing of FIGS. 3 and 4. FIGS. 6A and 6B show the final target air-fuel ratio coefficient KCMDM and the counter CTRR, respectively.
KCMDM0 in FIG. 9A is a value (1.0) corresponding to the stoichiometric air-fuel ratio, and KCMDML is a value equivalent to A / F = 22, for example. FIG. 6 shows an operation example in the case where the lean burn control execution condition shifts from the unsatisfied state to the satisfied state at time t1. When the lean burn control execution condition is satisfied, first, the reduction enrichment process is executed from time t1 to t2, and then lean burn control is started. At this time, the counter CTRR is set to a second predetermined value CTRINT2. Here, the reduction enrichment target equivalent ratio KCMDRR is set to a larger value as the engine speed NE is higher and the intake pipe absolute pressure PBA is higher. Therefore, the degree of enrichment is higher as the engine speed NE is higher. , And is controlled to increase as the intake pipe absolute pressure PBA increases. As the engine speed NE increases and the intake pipe absolute pressure PBA increases, the exhaust gas flow rate (volume / hour) or the exhaust gas flow rate (volume / (time / cross-sectional area)) increases. The control is performed so that the degree of the reduction enrichment increases as the exhaust gas flow rate or the exhaust gas flow rate increases.

Therefore, when the engine shifts from the engine operating state in which the stoichiometric air-fuel ratio or the air-fuel ratio is controlled to a richer side than the stoichiometric air-fuel ratio to the engine operating state in which the lean burn control is executed, first, the reduction enrichment is executed, and then the lean operation is performed. Burn control is performed. In addition, the degree of the reduction enrichment is controlled so as to increase as the engine speed NE increases and as the absolute pressure PBA in the intake pipe increases, so that appropriate reduction enrichment according to the engine operating state is performed. And good exhaust gas characteristics can be maintained without increasing the amount of NOx or HC and CO emissions.

When the value of the counter CTRR reaches the predetermined threshold value CTRACT during execution of the lean burn control (t3 in FIG. 6), the reduction enrichment target is set in accordance with the engine speed NE and the intake pipe absolute pressure PBA at that time. Equivalent ratio K
CCMDR is set, and reduction enrichment is started. Thereafter, the output VO2 of the O2 sensor 15 is changed to a predetermined reference value VO2.
When REF is exceeded (t4 in FIGS. 6 and 7), the reduction enrichment is terminated, and the value of the counter CTRL becomes the second predetermined value CT.
RRINT2 is returned. Thereafter, if the condition for executing the lean burn control is satisfied, the time t2 to the time t2 after the time t4 is satisfied.
The same operation as described above up to 4 is repeated.

As described above, the lean burn control is performed by the counter C
When the predetermined lean control time (TL1, TL2, TL3,...) Determined by the value of TRR and the predetermined threshold value CTRACT continues, the reduction enrichment is executed, and the degree of the reduction enrichment increases as the engine speed NE increases. In addition, since the control is performed so as to increase as the intake pipe absolute pressure PBA becomes higher, it is possible to perform appropriate reduction enrichment according to the engine operating state, and without increasing the emission amount of NOx or HC and CO. Good exhaust gas characteristics can be maintained.

Here, the counter CTRR determines that the lean air-fuel ratio control has continued for a predetermined lean control time by a predetermined threshold CT.
Judgment is made by reaching RRACT, and the counter CT
The increment value CTSV of the RR is set to a larger value as the engine speed NE is higher and the absolute pressure PBA in the intake pipe is higher, and the leaner burn control is continued as the value of the increment value CTSV increases. The control time (TL1, TL2, TL3,...) Becomes shorter. Therefore, the time ratio of the reduction enrichment also increases in response to the increase in the flow rate of the exhaust gas, so that the appropriate reduction enrichment can be performed according to the engine operating state.

FIG. 6 is shown for explaining the processing of FIGS. 3 and 4. For the sake of simplicity, the time ratio of the lean burn control (= TL / (TR + T
L)) is smaller than the actual value, in other words, the time ratio (= TR / (TR + TL)) for executing the reduction enrichment is larger than the actual value. Further, since the increment value CTSV of the counter CTRR changes according to the engine operating state, the value of the counter CTRR does not always increase linearly as shown in FIG.

FIG. 7 is a diagram showing transitions of the output VLAF of the LAF sensor 14 and the output VO2 of the O2 sensor 15 when the reduction enrichment is executed from the time t3 in FIG.
At time t3 ', which is slightly later than time t3, the LAF sensor output VLAF changes to a value indicating the rich air-fuel ratio, and further slightly after time t3', the O2 sensor output VO2 starts increasing. Then, at time t4, the O2 sensor output VO
When 2 exceeds the predetermined reference value VO2REF, the reduction enrichment ends and lean burn control is started. Therefore, the LAF sensor output VLAF and the O2 sensor output VO
2 changes to a value indicating the lean air-fuel ratio a little later than the time t4.

As described above, in this embodiment, the predetermined reference voltage VO2REF is set according to the output VLAF of the LAF sensor 14 provided on the upstream side of the exhaust gas purifying means 16 (steps S21 and S22), and the output of the O2 sensor is set. When VO2 exceeds a predetermined reference voltage VO2REF (when it becomes a value indicating an air-fuel ratio richer than the air-fuel ratio corresponding to the predetermined reference voltage VO2REF), the reduction enrichment is terminated. Can be determined with high accuracy, and good exhaust gas characteristics can be maintained. Moreover, the predetermined reference value VO2REF
Is set based on the basic reference value VO2REF0, it is possible to make a highly accurate determination without being affected by variations in the characteristics of the O2 sensor 15 or changes over time.

The present invention is not limited to the embodiment described above, and various modifications are possible. For example, in the above-described embodiment, the basic reference value VO2R applied to the equation (2)
EF0 is a smoothing value corresponding to the stoichiometric air-fuel ratio calculated by the equation (3), but VO2REF0 = VO2R
By the calculation of EF0-ΔV, the basic reference value VO2RE
F is the air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio (for example, A /
F = 15), and may be applied to equation (2). Accordingly, due to the delay until the fuel injected into the intake pipe reaches the exhaust system, the end of the reduction enrichment process is delayed from the optimal timing, and HC, C
An increase in the amount of O discharged can be prevented.

In the above-described embodiment, the correction term DV
Although O2REF is calculated according to the output VLAF of the LAF sensor 14, it may be calculated according to the detected equivalent ratio KACT.

In the above-described embodiment, a proportional type air-fuel ratio sensor is provided upstream of the exhaust gas purifying means 16, and a binary oxygen concentration sensor whose output changes rapidly before and after the stoichiometric air-fuel ratio is provided downstream. However, conversely, a binary oxygen concentration sensor may be provided on the upstream side and a proportional air-fuel ratio sensor may be provided on the downstream side, or both may be provided as proportional air-fuel ratio sensors, or both may be provided as binary oxygen concentration sensors. It may be a density sensor.

[0062]

As described above in detail, according to the first aspect of the present invention, after the air-fuel ratio enrichment is started by the reducing means, the output value of the second oxygen concentration sensor is changed to the output value of the first oxygen concentration sensor. At the time when the air-fuel ratio becomes richer than a predetermined reference value set in accordance with the output value, it is determined that the release of the nitrogen oxides absorbed by the nitrogen oxide absorbent is completed, and the air-fuel ratio enrichment ends. Therefore, it is possible to accurately and promptly determine the completion point of the release of the nitrogen oxide regardless of the degree of the enrichment in the reduction enrichment, and to maintain good exhaust gas characteristics.

[0063] The predetermined reference value, the output value of the second oxygen concentration sensor obtained in the engine operating condition for controlling the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio, the output value of the first oxygen concentration sensor Therefore, the determination can be made with high accuracy without being affected by variations in the characteristics of the second oxygen concentration sensor and changes with time.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an internal combustion engine and an air-fuel ratio control device thereof according to an embodiment of the present invention.

FIG. 2 is a flowchart of a process for executing air-fuel ratio feedback control according to an output of an air-fuel ratio sensor.

FIG. 3 is a flowchart of a process for executing return enrichment.

FIG. 4 is a flowchart of a process for executing return enrichment.

FIG. 5 is a diagram showing a map used in the processing of FIGS. 3 and 4;

FIG. 6 is a time chart showing transition of parameter values used in the processes of FIGS. 3 and 4;

FIG. 7 is a time chart showing changes in an air-fuel ratio sensor output and an oxygen concentration sensor output.

[Explanation of symbols]

 Reference Signs List 1 internal combustion engine 5 electronic control unit (reducing means) 6 fuel injection valve (reducing means) 12 exhaust pipe 14 air-fuel ratio sensor (first oxygen concentration sensor) 15 oxygen concentration sensor (second oxygen concentration sensor) 16 exhaust gas purification means

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ identification code FI F02D 45/00 368 F02D 45/00 368F (58) Field surveyed (Int. Cl. ⁷ , DB name) F01N 3/24 F01N 3 / 08 F02D 41/04 F02D 45/00

Claims (3)

(57) [Claims] 1. An exhaust gas purifying means provided in an exhaust system of an internal combustion engine and containing a nitrogen oxide absorbent for absorbing nitrogen oxides in exhaust gas in an exhaust gas lean state, and an upstream side of the exhaust gas purifying device. And first and second oxygen concentration sensors provided on the downstream side for detecting the oxygen concentration in the exhaust gas; and enriching the air-fuel ratio of the air-fuel mixture supplied to the engine to the nitrogen oxide absorbent. An exhaust gas purifying apparatus comprising: a reducing unit configured to reduce the absorbed nitrogen oxides.
The output value of the oxygen concentration sensor determines the air-fuel ratio of the air-fuel mixture.
Obtained in the engine operating state in which the air-fuel ratio is controlled.
The output value of the second oxygen concentration sensor is used as the first oxygen concentration
When the air-fuel ratio becomes richer than a predetermined reference value calculated by correcting according to the output value of the sensor, the air-fuel ratio enrichment is terminated. Purification device. 2. A smoothing process for an output value of the second oxygen concentration sensor obtained in an engine operating state for controlling an air-fuel ratio of the air-fuel mixture to a stoichiometric air-fuel ratio.
And a correction value calculated in accordance with the output value of the second oxygen concentration sensor.
The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the value is obtained by the following method . 3. The internal combustion engine according to claim 1, wherein the reducing means operates when a lean air-fuel ratio control for leaning the air-fuel mixture from a stoichiometric air-fuel ratio continues for a predetermined lean control time. Exhaust gas purification equipment.