JP3377404B2 - 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 which contains a nitrogen oxide absorbent in its 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 there is a tendency to increase, there has been conventionally known a technology for purifying exhaust gas by providing an exhaust gas purifying means having a NOx absorbent that absorbs NOx in an exhaust system of an engine. 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, conversely, when the air-fuel ratio is set 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 It has the property of releasing. In the exhaust gas purifying means containing the NOx absorbent, the NOx released from the NOx absorbent is HC, C in the exhaust gas rich state.
Reduced by O, discharged as nitrogen gas, and H
C and CO are configured to be oxidized and discharged as water vapor and carbon dioxide.

There is naturally a limit to the amount of NOx that the NOx absorbent can absorb, and this limit value tends to decrease as the NOx absorbent deteriorates. Therefore, an air-fuel ratio sensor is arranged on the downstream side of the exhaust gas purifying means, air-fuel ratio enrichment for releasing NOx absorbed in the NOx absorbent is executed, and the air-fuel ratio enrichment is started from the air-fuel ratio enrichment start time. A method of determining the degree of deterioration of the NOx absorbent based on the time until the output of the sensor changes to a value indicating the rich air-fuel ratio has been conventionally known (JP-A-8-232644).
Issue).

[0004]

However, in the above-mentioned conventional method, the output of the downstream side air-fuel ratio sensor of the exhaust gas purification means changes until the air-fuel ratio of the air-fuel mixture supplied to the engine is made rich. To measure the time of
There is a problem that the measurement time varies depending on the delay time until the enriched air-fuel mixture is burned and discharged to the exhaust system, and the deterioration cannot be accurately determined.

The present invention has been made in view of this point, and an object thereof is to provide an exhaust gas purifying apparatus capable of accurately determining deterioration of the NOx absorbent.

[0006]

To achieve the above object, the invention according to claim 1 is provided in an exhaust system of an internal combustion engine, and absorbs nitrogen oxide in exhaust gas in a lean exhaust gas state. Exhaust gas purifying means containing an absorbent, first and second oxygen concentration sensors provided upstream and downstream of the exhaust gas purifying means for detecting oxygen concentration in exhaust gas, and supplied to the engine The air-fuel ratio of the air-fuel mixture to be reduced rich depending on the operating state of the internal combustion engine.
In the exhaust gas purifying device comprising a reducing means for reducing the nitrogen oxides absorbed in the nitrogen oxide absorbent by enriching using the target conversion equivalence ratio, after starting the enrichment by the reducing means, the The time from the time when the output value of the first oxygen concentration sensor changes to the value indicating the rich air-fuel ratio to the time when the output value of the second oxygen concentration sensor reaches the value indicating the rich air-fuel ratio is the reduction rich Target equivalence ratio
Is shorter than a predetermined determination time that is set to a smaller value as the value increases. The deterioration determination unit determines that the nitrogen oxide absorbent of the exhaust gas purification unit is deteriorated.

According to this structure, after the enrichment by the reducing means is started, the output value of the second oxygen concentration sensor becomes rich from the time when the output value of the first oxygen concentration sensor changes to a value indicating the rich air-fuel ratio. When the time to reach the value indicating the air-fuel ratio is shorter than the predetermined determination time, it is determined that the nitrogen oxide absorbent of the exhaust gas purification device is deteriorated.

Here, the "predetermined determination time" is the reduction return.
As the target equivalence ratio increases, it is set to a smaller value.
It

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 reducing means has a predetermined lean air-fuel ratio control for leaning the air-fuel mixture from a stoichiometric air-fuel ratio. Activated when control time continues.

According to this structure, when the lean air-fuel ratio control for making the air-fuel mixture supplied to the engine leaner than the stoichiometric air-fuel ratio continues for the predetermined lean control time, the reducing means enriches the air-fuel ratio.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION 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 "engine") and an air-fuel ratio control apparatus therefor according to an embodiment of the present invention. For example, an intake pipe 2 of a 4-cylinder engine 1 is provided midway. Is provided with a throttle valve 3. A throttle valve opening degree (θTH) sensor 4 is connected to the throttle valve 3 and outputs an electric signal according to the opening degree of the throttle valve 3 to output an electronic control unit for engine control (hereinafter referred to as “ECU”) 5 Supply to.

The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). In addition to being electrically connected to the ECU 5, the valve opening time of the fuel injection valve 6 is controlled by 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 attached downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies it to the ECU 5.

The engine water temperature (TW) sensor 9 mounted on the main body of the engine 1 is composed of a thermistor or the like, detects the 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 cam shaft or crank shaft (not shown) of the engine 1.
A cylinder discrimination (CYL) sensor 11 is attached. The engine speed sensor 10 outputs a TDC signal pulse at a crank angle position that is a predetermined crank angle before the top dead center (TDC) at the start of the intake stroke of each cylinder of the engine 1 (every 180 ° of crank angle in a 4-cylinder engine). The cylinder discrimination sensor 11 outputs a cylinder discrimination signal pulse at a predetermined crank angle position of a specific cylinder, and each of these signal pulses is supplied to the ECU 5.

The exhaust pipe 12 is provided with an exhaust gas purifying means 16 for purifying the exhaust gas.
It contains a NOx absorbent that absorbs NOx and a catalyst that has an oxidizing and reducing action. In the NOx absorbent, the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is set to be 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, in the state where the air-fuel ratio supplied to the engine 1 is set to a richer side 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 purification 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 and discharged as nitrogen gas, and HC and CO are oxidized and discharged as water vapor and carbon dioxide. There is.
Examples of the NOx absorbent include barium oxide (Ba0)
Is used, and platinum (Pt), for example, is used as the catalyst. This NOx absorbent generally has a characteristic that the absorbed NOx is more easily released as the temperature thereof increases. The NOx absorbent releases NOx even when the exhaust gas is lean, when the oxygen concentration decreases and the amount of NOx produced decreases.

As explained in the prior art, NO
Limit of NOx absorption capacity of x absorbent, that is, maximum NOx
When NOx is absorbed up to the absorption amount, NOx can no longer be absorbed, and therefore, the air-fuel ratio is richly reduced to release and reduce NOx. If the degree of enrichment is too small, the reduction-enriched NOx is released.
If the degree of enrichment is too large while the degree of reduction is insufficient, HC and CO emissions increase. Therefore, by appropriately controlling the degree of enrichment in reduction enrichment, good exhaust gas characteristics can be obtained. It is possible to maintain.

A proportional air-fuel ratio sensor 14 as a first oxygen concentration sensor is provided at a position upstream of the exhaust gas purifying means 16.
(Hereinafter referred to as "LAF sensor 14") is attached, and this LAF sensor 14 outputs an electric signal substantially proportional to the oxygen concentration (air-fuel ratio) in the exhaust gas and supplies it 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 the downstream position of the exhaust gas purification means 16,
An oxygen concentration sensor (second oxygen concentration sensor) 15 (hereinafter referred to as “O2 sensor 15”) is attached, and a detection signal thereof is supplied to the ECU 5. This O2 sensor 15
Has a characteristic that its output VO2 rapidly changes before and after the stoichiometric air-fuel ratio, and its output VO2 becomes high level on the rich side and low level on the lean side of the stoichiometric air-fuel ratio.

The engine 1 is capable of switching the valve timings of the intake valve and the exhaust valve in two stages, 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. This switching of the valve timing includes switching of the valve lift amount. Further, when the low speed valve timing is selected, one of the two intake valves is deactivated to stabilize the air-fuel ratio leaner than the stoichiometric air-fuel ratio. I try to ensure the burning.

The valve timing switching mechanism 30 switches the valve timing via hydraulic pressure, and the solenoid valve and hydraulic sensor for switching the hydraulic pressure are connected to the ECU 5. The detection signal of the hydraulic 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 the input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and the like. Hereinafter, referred to as "CPU" 5b, a storage means 5c for storing various calculation programs executed by the CPU 5b and calculation results, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, and the like.

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

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

KCMDM is the final target air-fuel ratio coefficient,
As will be described later, the engine speed NE and the intake pipe absolute pressure P
It is calculated by performing fuel cooling correction on the target air-fuel ratio coefficient KCMD set according to the engine operating parameters such as BA and engine water temperature TW. 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, so is also called a target equivalence ratio.

In KLAF, the detected equivalence ratio KACT calculated from the detection value of the LAF sensor 14 is the target equivalence ratio KCMD.
Is an air-fuel ratio correction coefficient calculated by PID control so as to match 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 engine operating conditions. To a predetermined value.

The CPU 5b supplies a drive signal for opening the fuel injection valve 6 to the fuel injection valve 6 through 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 reducing means, and the ECU 5 further
Deterioration determination means is configured.

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 so that the detected equivalent ratio KACT matches the target equivalent ratio KCMD. This process is performed by, for example, TD
It 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 water temperature TW or a predetermined high load operating state, a value corresponding to those operating states. Is changed to.

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

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 injection increases as the KCMD value increases and the fuel injection amount increases.

In step S3, reduction enrichment control processing shown in FIGS. 3 and 4 which will be described later is executed, and in step S4, L
The detection equivalent value KACT is calculated by converting the detection value of the AF sensor 14 into an equivalent ratio. 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 equivalence ratio KACT becomes the target equivalence 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 the feedback control flag FLAFFB, which indicates by "1" that the engine 1 is in the operating state in which the feedback control according to the detection value of the LAF sensor 14 is executed, is "1". When it is determined that FLAFFB = 1 and the feedback control is in the operating state, the operating state in which the lean burn control is performed to set the air-fuel ratio to the lean side of the stoichiometric air-fuel ratio is indicated by "0". Lean burn control flag F
It is determined whether KBSMJG is "0" (step S1).
2) When FKBSMJG = 0 and the operating state is to execute lean burn control, the target equivalent ratio KCMD
Is equal to or less than a predetermined equivalence ratio KCMDLB (for example, 0.98) set to a value slightly leaner than the stoichiometric air-fuel ratio (step S13).

If the answer to any of the steps S11 to S13 is negative (NO), the reduction enrichment flag FR indicating "1" that the reduction enrichment is being executed.
ROK is set to "0" and the counter CTRR
Is set to the first predetermined value CTRRINT1 (see FIG. 6C) (step S14), and the present process is terminated without executing reduction enrichment.

When all the answers in steps S11 to S13 are affirmative (YES), that is, when the execution condition of 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 value 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 increases.
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 changed to the first predetermined value CTRRIN.
It is determined whether or not it is equal to or larger than a predetermined threshold value CTRRACT smaller than T1 (see FIG. 6C) (step S17). Immediately after the lean burn control execution condition is met, the counter CTR
Since R is set to the first predetermined value CTRRINT1 (step S14), CTRR ≧ CTRRACT, and the routine proceeds to step S18.

In step S18, it is determined whether the reduction enrichment flag FRROK is "1". First, FRR
Since OK = 0, this is set to "1" (step S19), the process proceeds to step S21, and the engine speed NE is set.
Is the first predetermined rotation speed NKCMDRRL (for example, 1000
rpm), it is determined whether NE> NKCMDR.
When it is RL, the second engine speed NE is higher than the first engine speed NKCMDRRL.
It is determined whether it is higher than MDRRH (for example, 2000 rpm) (step S22). And NE ≦ NKC
When MDRRL is in the low rotation speed region, the down count timer timer tmRR is set to the low rotation speed predetermined time TMR.
Set to RL (for example, 300 msec) (step S2
5) When NKCMDRRL <NE ≦ NKCMDRRH and in the middle rotation range, the timer tmRR is set to the predetermined middle rotation time TM longer than the low rotation predetermined time TMRRL.
Set to RRM (eg 500 msec) (step S
24), when NE> NKCMDRRH and in the high rotation speed region, the timer tmRR is set to the predetermined rotation time TMR for medium rotation.
Predetermined time TMRRH for high rotation longer than RM (eg 80
0 msec) (step S23), and the process proceeds to step S26.

In step S26, steps S23 and S
The timer tmRR set in 24 or S25 is started (see FIG. 6 (b), time t1), and then the deterioration monitor flag indicating "1" that the deterioration determination of the NOx absorbent of the exhaust gas purification means 16 is being executed. It is determined whether FNOXMON is "1" (step S41). As a result, when FNOXMON = 0 and the deterioration determination is not executed, the KCDMRR map shown in FIG. 5B is searched to calculate the reduction rich enrichment target equivalent ratio KCMDRR (step S28), and the final target empty space is calculated. The fuel ratio coefficient KCMDM is set to the reduction enrichment target equivalent ratio KCMDRR (step S29), and this processing ends.

The KCMDRR map shows the engine speed N
It is a map in which the reduction rich enrichment target equivalence ratio KCMDRR is set according to E and the intake pipe absolute pressure PBA. As the engine speed NE increases, the intake pipe absolute pressure PB increases.
It is set so that the KCMDRR value increases as A increases. Note that all the set values are values larger than 1.0.

On the other hand, in step S41, FNOXMON = 1
Therefore, when the deterioration determination of the NOx absorbent is being executed,
The reduction enrichment target equivalent ratio KCMDRR is set to a fixed value KCMDRMON for deterioration determination (for example, a value corresponding to A / F = 11) (step S42), and the step S2 is performed.
Proceed to 9.

When the reduction enrichment flag FRROK is set to "1" in step S19 and the reduction enrichment is started, the answer to step S18 is thereafter affirmative (YES), the process proceeds to step S27, and the timer tmRR is set. It is determined whether the value is "0". Initially, tmRR> 0, so the routine proceeds to step S41, and when tmRR = 0 (time t2 in FIG. 6), the reduction enrichment flag FRROK is set.
Is set to "0" (step S30), and the counter CTR is set.
R is a second predetermined value C that is smaller than a predetermined threshold value CTRRACT
Set to TRINT2 (for example, 0) (step S3
1) The reduction enrichment is completed. Steps S30 and S3
When 1 is executed, the final target air-fuel ratio coefficient KCMDM holds the value calculated in step S2 of FIG.
The lean burn control is started.

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

FIG. 6 is a time chart for explaining the processing of FIGS. 3 and 4, and FIGS. 6 (a), (b) and (c) are
Final target air-fuel ratio coefficient KCMDM, timer tmR
The transition of the value of R and the value of the counter CTRR is shown. KCMDM0 in FIG. 4A is a value corresponding to the theoretical air-fuel ratio (1.0)
And KCDMML is, for example, a value corresponding to A / F = 22. FIG. 6 shows an operation example when the lean-burn control execution condition is changed from the non-fulfilled state to the established 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 the lean burn control is started. At this time, the counter CTRR has the second predetermined value CTRRIN.
It is set to T2. Here, the timer tmRR is set to a longer time as the engine speed NE is higher (steps S21 to S25), and the reduction rich enrichment target equivalence ratio KCM is set.
Since the DRR is set to a larger value as the engine speed NE is higher and the intake pipe absolute pressure PBA is higher,
The degree of enrichment is controlled to increase as the engine speed NE increases and as the intake pipe absolute pressure PBA increases. The exhaust gas flow rate (volume / time) or the exhaust gas flow velocity (volume / (time / cross-sectional area)) increases as the engine speed NE is higher and the intake pipe absolute pressure PBA is higher. Therefore, in the present embodiment, The degree of reduction enrichment is controlled to increase as the exhaust gas flow rate or the exhaust gas flow velocity increases.

Therefore, when the engine operating state in which the stoichiometric air-fuel ratio or the air-fuel ratio on the rich side of the stoichiometric air-fuel ratio is controlled is changed to the engine operating state in which the lean burn control is executed, reduction enrichment is first executed and then lean enrichment is executed. Burn control is executed. Moreover, the degree of reduction enrichment is controlled to increase as the engine speed NE increases and the intake pipe absolute pressure PBA increases. Therefore, appropriate reduction enrichment according to the engine operating state should be performed. Therefore, good exhaust gas characteristics can be maintained without increasing the emissions of NOx, HC, and CO.

When the value of the counter CTRR reaches the predetermined threshold value CTRRACT during the lean burn control (t3 in FIG. 6), the timer tmRR is set to the predetermined time TMRRL, TMRRM, according to the engine speed NE at that time.
Alternatively, TMRRH is set, and the reduction rich enrichment target equivalent ratio KCMDRR is set in accordance with the engine speed NE and the intake pipe absolute pressure PBA at that time, and the reduction enrichment is started. After that, when the value of the timer tmRR becomes "0" (t4 in FIG. 6), the reduction enrichment is ended, and the value of the counter CTRR becomes the second predetermined value CTRRIN.
T2 is returned. After that, if the lean burn control execution condition is satisfied, the same operation from time t2 to time t4 is repeated after time t4.

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 CTRRACT is continued, reduction enrichment is executed, and the degree of reduction enrichment is higher as the engine speed NE is higher. Further, the higher the absolute intake pipe pressure PBA is, the larger the control is performed. Therefore, reduction rich enrichment can be appropriately performed according to the engine operating state, and NOx or HC or CO emissions can be increased without increasing. Good exhaust gas characteristics can be maintained.

Here, the counter CTRR indicates that the lean air-fuel ratio control has continued for a predetermined lean control time by a predetermined threshold CT.
The counter CT is judged by the fact that it has reached RRACT.
The increment value CTSV of the RR is set to a larger value as the engine speed NE is higher and the intake pipe absolute pressure PBA is higher, and the lean burn control is continued as the increment value CTSV increases. The control time (TL1, TL2, TL3, ...) Is shortened. Therefore, the time ratio of reduction enrichment also increases in response to the increase in the exhaust gas flow rate, and it is possible to perform appropriate reduction enrichment according to the engine operating state.

Note that FIG. 6 is shown to explain the processing of FIGS. 3 and 4, and for the sake of clarity, the time ratio of the lean burn control (= TL / (TR + T + T
L)) is smaller than the actual value, in other words, the time ratio (= TR / (TR + TL)) of executing the reduction enrichment is shown 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 flow chart of a process for determining the deterioration of the NOx absorbent of the exhaust gas purification means 16. This process is executed every predetermined time (for example, 80 msec).

In step S51, it is determined whether or not the end flag FNOXMEND which indicates by "1" that the deterioration determination is completed is "1", and when FNOXMEND = 1 and the deterioration determination has already been completed. , And proceeds to step S54. When FNOXMEND = 0 and the deterioration determination has not ended, it is determined whether or not a predetermined time TLBCNT has elapsed after the execution condition of the lean burn control is satisfied (step S52). If not, the step S
54. When the time has passed, the reduction rich flag F
It is determined whether RROK is "1" (step S5).
3). If FRROK = 0 and the reduction enrichment is not executed, the process proceeds to step S54, the deterioration monitor flag FNOXMON is set to "0", and then the deterioration determination up-count timer tmMON is set to "0" and the timer is set. The timer operation flag FTMR indicating "1" indicating that the tmMON is in operation is set to "0" (step S57), and this processing ends. In this embodiment,
Immediately after the lean burn control execution condition is satisfied (up to time t2 in FIG. 6), the enrichment control is executed. Therefore, in order to prevent the deterioration determination from being executed during that period, step S5 is performed.
Two are provided.

When FRROK = 1 in step S53 and reduction enrichment is being executed, the deterioration monitor flag FNOXMON is set to "1" (step S5).
5), the output VLAF of the LAF sensor 14 is the predetermined output value V
It is determined whether it is higher than LAFREF (for example, a value corresponding to the theoretical air-fuel ratio) (indicating an air-fuel ratio rich) (step S).
56). While VLAF≤VLAFREF, the process proceeds to step S57, and when VLAF> VLAFREF, the process proceeds to step S58, and it is determined whether or not the timer operation flag FTMR is "1". Since FTMR = 0 at the beginning, the timer tmMON is started and the timer operation flag FTMR is set to "1" (step S
59) and proceeds to step S60. After that, FTMR =
Since it becomes 1, immediately from step S58 to step S60.
Proceed to.

In step S60, it is determined whether or not the output VO2 of the O2 sensor 15 is higher than a predetermined output value VO2REF which is slightly higher than the value corresponding to the theoretical air-fuel ratio. At first, the influence of the enrichment of the air-fuel ratio does not appear on the downstream side of the exhaust gas purifying means 16, so VO2 ≦ VO2REF, and immediately the routine proceeds to step S62, where the timer operation flag FTMR is “0”.
Or not. While VO2 ≤ VO2REF,
The timer operation flag FTMR = 1, and step S60
Since the answer is negative (NO), this process is immediately terminated. When VO2> VO2REF in step S60, the timer tmMON is stopped and the timer operation flag F is set.
TMR is set to "0" (step S61), and the process proceeds to step S62. At this time, the answer to step S62 is affirmative (YES), so the routine proceeds to step S63, where it is determined whether or not the value of the timer tmMON is smaller than the predetermined determination time TNOXREF. And tmMON> TNO
When it is XREF, it is determined that the NOx absorbent is normal (step S65), and when tmMON≤TOXREF, it is determined that the NOx absorbent is deteriorated (step S64), and the end flag FNOXMEND is "1". Is set (step S66), and this processing ends. The predetermined determination time TNOXREF is, for example, NOx of the NOx absorbent.
It is determined by an experiment so as to correspond to the delay time when the absorption capacity becomes about 50% of the new product.

According to the processing of FIG. 7, the lean burn control continues for the predetermined lean control time TL (for example, TL1), and NO.
When the reduction-enrichment is performed after the NOx absorbent has absorbed NOx to the limit of its absorption capacity, FIG.
As shown in, the output LAF of the LAF sensor 14 provided on the upstream side of the exhaust gas purifying means 16 has a predetermined output value VLA.
From the time t11 when FREF is exceeded, the output VO2 of the O2 sensor 15 provided on the downstream side has a predetermined output value VO2REF.
The delay time TMON until the time t12 is exceeded by the timer t
Measured by mMON. This delay time TMON is
It corresponds to the time required to release all the NOx absorbed in the NOx absorbent up to the limit of its absorption capacity,
The NOx absorption capacity of the NOx absorbent is shown. That is, the shorter the time TMON is, the lower the NOx absorption capacity is. Therefore, when the TMON value is lower than the predetermined determination time TNOXREF, it is determined that the NOx absorbent is deteriorated.

As described above, in this embodiment, from the time when the detection value of the LAF sensor 14 provided on the upstream side of the exhaust gas purifying means 16 changes to the value indicating the rich air-fuel ratio, the O2 sensor 15 provided on the downstream side of the exhaust gas purifying means 16 changes. Since the deterioration of the NOx absorbent is determined based on the time until the detected value changes to the value indicating the rich air-fuel ratio, the delay until the rich air-fuel mixture is burned and discharged to the exhaust system. The accuracy of deterioration determination can be improved without being affected by time.

The present invention is not limited to the above-described embodiment, but various modifications can be made. For example, in the above-described embodiment, the reduction enrichment target equivalent ratio KCMDRR is set to the fixed value KCMDRRMON when the deterioration determination is executed (FIG. 4, steps S41 and S42).
The reduction rich enrichment target equivalent ratio KCMDRR retrieved from the KCMDRR map may be used also when the deterioration determination is executed. However, in that case, the delay time TMON is
Since the KCMDRR value influences, it is desirable to set the predetermined determination time TNOXREF used for the deterioration determination to a smaller value as the KCMDRR value increases.

Further, in the above-described embodiment, the proportional air-fuel ratio sensor is provided on the upstream side of the exhaust gas purification means 16, and the binary oxygen concentration sensor whose output suddenly changes before and after the stoichiometric air-fuel ratio is provided on the downstream side. However, on the contrary, 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 proportional air-fuel ratio sensors, or both may be binary oxygen sensors. It may be a concentration sensor.

[0059]

As described above in detail, according to the present invention, after the enrichment by the reducing means is started, the output from the first oxygen concentration sensor changes to the value indicating the rich air-fuel ratio, and the second The time period until the output value of the oxygen concentration sensor reaches the value that indicates the rich air-fuel ratio, the reduction rich enrichment target equivalence ratio increases
If it is shorter than the predetermined determination time that is set to a smaller value, it is determined that the nitrogen oxide absorbent of the exhaust gas purification means has deteriorated, so the rich mixture burns and is discharged to the exhaust system. It is possible to improve the accuracy of the deterioration determination without being affected by the delay time until the completion.

[Brief description of 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 of 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 performing reduction rich enrichment.

FIG. 4 is a flowchart of a process for performing reduction rich enrichment.

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

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

FIG. 7 is a flowchart of a process for determining deterioration of a NOx absorbent.

FIG. 8 is a time chart for explaining a deterioration determination method by processing in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 5 Electronic control unit (reducing means, deterioration determining 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

Continuation of front page (51) Int.Cl. ⁷ Identification code FI F02D 41/04 ZAB F02D 41/04 ZAB 45/00 368 45/00 368F (56) Reference JP-A-8-232646 (JP, A) Kaihei 8-121216 (JP, A) JP 9-79028 (JP, A) JP 8-144745 (JP, A) International Publication 94/17291 (WO, A1) (58) Fields investigated (Int .Cl. ⁷ , DB name) F01N 3/08-3/24 F02D 41/04 F02D 45/00

Claims (2)

(57) [Claims] 1. An exhaust gas purifying means which is provided in an exhaust system of an internal combustion engine and which contains a nitrogen oxide absorbent that absorbs nitrogen oxides in the exhaust gas in an exhaust gas lean state, and an upstream side of the exhaust gas purifying means. And the first and second oxygen concentration sensors that are provided on the downstream side to detect the oxygen concentration in the exhaust gas, and the air-fuel ratio of the air-fuel mixture supplied to the engine.
Reduction rich enrichment target equivalence ratio determined according to engine operating conditions
In the exhaust gas purifying apparatus that includes a reduction unit for reducing nitrogen oxides absorbed in the nitrogen oxide absorber by enriching with, the rear enrichment start by reduction means, the first oxygen The time from the time when the output value of the concentration sensor changes to the value indicating the rich air-fuel ratio to the time when the output value of the second oxygen concentration sensor reaches the value indicating the rich air-fuel ratio is the reduction rich
When the target conversion equivalence ratio is shorter than a predetermined determination time that is set to a smaller value as the target equivalence ratio increases, a deterioration determination unit that determines that the nitrogen oxide absorbent of the exhaust gas purification unit is deteriorated is provided. Exhaust gas purification device for internal combustion engine. 2. The exhaust gas of an internal combustion engine according to claim 1, wherein the reducing means operates when lean air-fuel ratio control for leaning the air-fuel mixture from a stoichiometric air-fuel ratio continues for a predetermined lean control time. Gas purification device.