JPH07139388A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To discharge NOx from an NOx absorbent in a short time as an air- fuel ratio is maintained at a value approximately similar to a theoretical air-fuel ratio. CONSTITUTION:An NOx absorbent to absorb NOx when an air-fuel ratio of inflow exhaust gas is lean and discharge absorbed NOx when an air-fuel ratio of inflow exhaust gas is a theoretical air-fuel ratio or rich is arranged in an engine exhaust passage. When an engine operation state where fuel-air mixture having a theoretical air-fuel ratio is burnt is produced, feedback control is effected so that an average value of an air-fuel ratio is slightly rich and feedback control is effected so that a following air-fuel ratio is adjusted to a theoretical air-fuel ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine.

[0002]

2. Description of the Related Art When the air-fuel ratio of the exhaust gas that flows in is lean,
No when_xAnd the air-fuel ratio of the inflowing exhaust gas is
The absorbed NO when the air-fuel ratio or rich_xEmit
NO _xThe absorbent is placed in the engine exhaust passage and NO_xabsorption
Agent to NO_xIs to be discharged into the engine cylinder when
Change the air-fuel ratio of the supplied mixture from lean to the theoretical air-fuel ratio or
Switch for a certain period of time, and then
This application is for an internal combustion engine that returns the air-fuel ratio to lean again.
It has already been proposed by people (International Publication WO93 / 07
363). NO in this internal combustion engine_xAbsorbent from N
O_xThe air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio
Sometimes the output of an air-fuel ratio sensor installed in the engine exhaust passage
Based on the force signal, adjust the air-fuel ratio to the stoichiometric air-fuel ratio.
The feedback is controlled and NO_xAbsorbent to NO_xShould release
When the mixture is rich, the air-fuel ratio is open
Controlled.

[0003]

[Problems to be Solved by the Invention] By the way, air-fuel mixture
When the ratio is made rich, NO is reached within a relatively short time.
_xNO from absorbent_xThe release action is complete but the mixture is empty
NO when the fuel ratio is set to the stoichiometric air-fuel ratio_xFrom absorbent
NO_xIt takes time to complete the release action. Therefore
NO when the air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio_x
NO from absorbent _xInternal combustion engine with release action
Then NO_xNO from absorbent_xUntil the release action is complete
It will take time.

In this case, in order to shorten the NO _x release time from the NO _x absorbent, the air-fuel mixture should be made extremely rich. However, if the air-fuel mixture is made extremely rich, a large torque change occurs when the air-fuel mixture is switched from lean to rich. Therefore, it is preferable to make the air-fuel ratio slightly richer than the stoichiometric air-fuel ratio in order to suppress the occurrence of torque change while shortening the NO _x release time from the NO _x absorbent. However, in the above-mentioned internal combustion engine, in order to control the air-fuel ratio to be slightly richer than the stoichiometric air-fuel ratio, the air-fuel ratio must be open-loop controlled. However, as a practical matter, it is difficult to maintain the air-fuel ratio slightly richer than the theoretical air-fuel ratio by open loop control.

[0005]

In order to solve the above problems, according to the present invention, NO _x is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and the air-fuel ratio of the inflowing exhaust gas is the theoretical air-fuel ratio. or when the rich place _the NO _x absorbent to release the absorbed NO _x in the engine exhaust passage, N in the exhaust gas when the lean air-fuel mixture is burned
O _x is absorbed in _the NO _x absorbent, the air-fuel ratio sensor in an internal combustion engine which NO _x is made to release from _the NO _x absorbent when the air-fuel mixture or a rich air-fuel mixture is combusted in the theoretical air-fuel ratio, the engine exhaust passage Is installed, and when the engine is in an operating state in which it should burn a stoichiometric air-fuel mixture, the air-fuel ratio is mixed based on the output signal of the sensor so that the average value of the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio. An air-fuel ratio control means for feedback-controlling the air-fuel ratio of air is provided.

Further, according to the present invention, the above-mentioned air-fuel ratio control means determines the average value of the air-fuel ratio when a predetermined period of time elapses after the engine operating state in which the stoichiometric air-fuel mixture should be burned. The air-fuel ratio feedback control, which is slightly richer than the theoretical air-fuel ratio, is switched to the feedback control in which the average value of the air-fuel ratio becomes the theoretical air-fuel ratio.

Furthermore, the air-fuel ratio control means described above, according to the present invention when _the NO _x releasing action from _the NO _x absorbent after reaching the engine operating state to combust a mixture of the stoichiometric air-fuel ratio is completed or completed When it is estimated that the average value of the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio, the air-fuel ratio feedback control is switched to the feedback control in which the average value of the air-fuel ratio becomes the theoretical air-fuel ratio.

[0008]

In the first aspect of the invention, when the engine is in an operating state in which the stoichiometric air-fuel ratio mixture should be burned, the average value of the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio based on the output signal of the air-fuel ratio sensor. air-fuel ratio of the mixture so that is feedback-controlled, NO _x is released from the NO _x absorbent during this time.

In the second aspect of the invention, the average value of the air-fuel ratio becomes the stoichiometric air-fuel ratio when a certain time has elapsed after the start of the air-fuel ratio feedback control in which the average value of the air-fuel ratio is slightly richer than the theoretical air-fuel ratio. Is switched to feedback control. The third in the invention, NO _x from _the NO _x absorbent after the air-fuel ratio feedback control the average value of the air-fuel ratio becomes just slightly rich side with respect to the stoichiometric air-fuel ratio is started
When the release action is completed or when it is estimated that it is completed, the feedback control is switched to the average air-fuel ratio becomes the stoichiometric air-fuel ratio.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is an exhaust port. Show. The intake port 6 is connected to the surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 17 containing a NO _x absorbent 18.

The electronic control unit 30 comprises a digital computer, and a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35 which are mutually connected by a bidirectional bus 31. And an output port 36. A pressure sensor 19 that generates an output voltage proportional to the absolute pressure in the surge tank 10 is arranged in the surge tank 10, and the output voltage of the pressure sensor 19 is input to the input port 35 via the AD converter 37. . A throttle switch 20 that is turned on when the throttle opening reaches an idling opening is attached to the throttle valve 14, and an output signal of the throttle switch 20 is input to an input port 35. An air-fuel ratio sensor 21 is arranged in the exhaust manifold 15, and the output voltage of the air-fuel ratio sensor 21 is input to the input port 35 via the AD converter 38. Further, the input port 35 is connected to the rotation speed sensor 22 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 is connected to the spark plug 4 and the fuel injection valve 11 via the corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on the following equation, for example. TAU = f · TP · K · FAF where f is a constant, TP is the basic fuel injection time, K is a correction coefficient, and FAF is a feedback correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by experiments and is stored in advance in the ROM 32 in the form of a map as shown in FIG. 2 as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, K <
When 1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and when K> 1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes empty. The fuel ratio becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The feedback correction coefficient FAF is basically an output signal of the air-fuel ratio sensor 21 when K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder should be the stoichiometric air-fuel ratio. This is a coefficient for accurately matching the air-fuel ratio with the theoretical air-fuel ratio. This feedback correction coefficient FAF moves up and down about 1.0.
This FAF decreases when the mixture becomes rich, and increases when the mixture becomes lean. Note that K <1.0 or K>
When 1.0, FAF is fixed at 1.0.

The target air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder, that is, the value of the correction coefficient K is changed according to the operating state of the engine. In the embodiment of the present invention, it is basically shown in FIG. Thus, it is predetermined as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. That is, as shown in FIG. 3, K <1.0, that is, the air-fuel mixture is lean in the low load operation region on the load lower side than the solid line R, and K in the high load operation region between the solid line R and the solid line S. = 1.
0, that is, the air-fuel ratio of the air-fuel mixture is the stoichiometric air-fuel ratio, and K> 1.0, that is, the air-fuel mixture is rich in the full-load operation region on the higher load side than the solid line S. Further, during idling operation, K = 1.0, that is, the stoichiometric air-fuel ratio.

FIG. 4 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 4, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer, and is discharged from the combustion chamber 3. The concentration of oxygen O _{2 in} the generated exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NO stored in the casing 17_x
The absorbent 18 uses, for example, alumina as a carrier, and
For example, potassium K, sodium Na, lithium Li, ce
Alkali metals such as sium Cs, barium Ba, cal
Alkaline earths such as Ca, lanthanum La,
At least one selected from rare earths such as thorium Y
And a noble metal such as platinum Pt. organ
Intake passage and NO_xProvided in the exhaust passage upstream of the absorbent 18.
The ratio of air and fuel (hydrocarbons) fed is NO _xabsorption
When the air-fuel ratio of the exhaust gas flowing into the agent 18 is called NO_x
The absorbent 18 is used when the air-fuel ratio of the inflowing exhaust gas is lean.
NO_xIs absorbed, and the oxygen concentration in the exhaust gas is reduced.
NO absorbed by_xReleases NO_xTo absorb and release
U Note that NO_xFuel in the exhaust passage upstream of the absorbent 18
(Hydrocarbon) or inflow / outflow when air is not supplied
The air-fuel ratio of the air-gas is the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3.
The ratio and therefore NO in this case_xAbsorbent 18 burns
When the air-fuel ratio of the air-fuel mixture supplied to the firing chamber 3 is lean
Is NO_xOf the air-fuel mixture in the combustion chamber 3
NO absorbed when oxygen concentration decreases_xTo release
Become.

If the above-mentioned NO _x absorbent 18 is arranged in the exhaust passage of the engine, the NO _x absorbent 18 actually acts to absorb and release NO _x , but the detailed mechanism of this absorbing and releasing effect is not clear. There is also. However, it is considered that this absorbing / releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking the case where platinum Pt and barium Ba are supported on the carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, the inflowing exhaust gas becomes considerably lean.
And the concentration of oxygen in the inflowing exhaust gas increased drastically.
As shown in (A), these oxygen O₂Is O₂ ^-Or O
^2-It adheres to the surface of platinum Pt in the form of. On the other hand, inflow exhaust
NO in the gas is O on the surface of platinum Pt.₂ ^-Or O^2-When
Reacts, NO₂Becomes (2NO + O₂→ 2 NO₂). Next
NO generated by₂Is partially oxidized on platinum Pt.
Is absorbed in the absorbent and does not combine with barium oxide BaO
As shown in FIG. 5 (A), nitrate ion NO₃ ^-of
Diffuses into the absorbent in the form. NO in this way_xIs NO
_xIt is absorbed in the absorbent 18.

Platinum as long as the oxygen concentration in the inflowing exhaust gas is high
NO on the surface of Pt₂Is generated and the absorbent NO_xAbsorption capacity
NO unless power is saturated₂Is absorbed in the absorbent and nitric acid
Ionic NO₃ ^-Is generated. On the other hand, the inflow exhaust gas
NO in the oxygen₂When the production of
Reaction is in the opposite direction (NO₃ ^-→ NO₂), And thus suck
Nitrate ion NO in the sorbent₃ ^-Is NO₂From the absorbent in the form of
Is released. That is, the oxygen concentration in the inflowing exhaust gas decreases
And NO_xAbsorbent 18 to NO_xWill be released
It As shown in Fig. 4, the degree of leanness of the inflowing exhaust gas
The lower the value, the lower the oxygen concentration in the inflowing exhaust gas.
Even if the lean degree of the inflowing exhaust gas is lowered by
NO even if the air-fuel ratio of the exhaust gas is lean_xAbsorbent 18
To NO _xWill be released.

On the other hand, at this time, if the air-fuel mixture supplied to the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG. 4, a large amount of unburned H
C and CO are discharged, which is unburned HC and CO is platinum Pt.
It is oxidized by reacting with the oxygen O ₂ ⁻ or O ^{2 −} above. Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, so NO ₂ is released from the absorbent, and this NO ₂ is unrecovered as shown in FIG. 5 (B). It is reduced by reacting with fuel HC and CO.
When NO ₂ is no longer present on the surface of platinum Pt in this way, NO ₂ is released one after another from the absorbent. Therefore NO _x from the NO _x absorbent 18 in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich, that is released.

That is, when the air-fuel ratio of the inflowing exhaust gas is made rich, first, unburned HC and CO immediately react with O ₂ ⁻ or O ^{2 −} on the platinum Pt to be oxidized, and then O on the platinum Pt is oxidized. ₂ ^- or O ^{2 -} yet be consumed unburned H
C, CO is any remaining This unburned HC, discharged from the NO _x and engine released from the absorbent by CO N
O _x is reduced. Thus _the NO _x absorbed in _the NO _x absorbent 18 in a short period of time if the air-fuel ratio of the inflowing exhaust gas is made rich is released, yet NO to the atmosphere for the released NO _x is reduced It will be possible to prevent _{x from} being ejected. Further, since the NO _x absorbent 18 has the function of a reduction catalyst, the NO _x released from the NO _x absorbent 18 can be reduced even if the air-fuel ratio of the inflowing exhaust gas is changed to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NO _x
To release all NO _x absorbed in _the NO _x absorbent 18 to the absorbent 18 NO _x is not only released gradually take some long time.

As described above, when the lean air-fuel mixture is burned, NO _x is absorbed by the NO _x absorbent 18, and when the stoichiometric air-fuel ratio air-fuel mixture or rich air-fuel mixture is burned N.
NO _x is released from the O _x absorbent 18. However it takes time until the releasing action of the NO _x from _the NO _x absorbent 18 to the Ki that the mixture of the theoretical air-fuel ratio is combusted as described above is completed, therefore _the NO _x absorbent 1
Sufficient NO _x from 8 shifts to the operating state (K <1.0) to combust a lean air-fuel mixture as shown in FIG. 3 before it is released when _the NO _x absorbent 18 of the NO _x absorbing capacity is saturated N
There is a risk that the O _x absorbent 18 cannot absorb NO _x . In order to avoid such a risk, it is necessary to release NO _x from the NO _x absorbent 18 as quickly as possible, and therefore, in the embodiment according to the present invention, the engine operating state where the air-fuel ratio should be the stoichiometric air-fuel ratio is achieved. In this case, the air-fuel ratio of the air-fuel mixture is feedback-controlled so that the average value of the air-fuel ratio becomes slightly richer than the theoretical air-fuel ratio.

Therefore, first, the feedback control of the air-fuel ratio will be described with reference to FIGS. 6 and 7. As shown in FIG. 6, the air-fuel ratio sensor 21 generates an output voltage V of about 0.9 (V) when the air-fuel mixture is rich, and outputs an output voltage V of about 0.1 (V) when the air-fuel mixture is lean.
To occur. FIG. 7 shows the feedback control of the air-fuel ratio performed based on the output signal of the air-fuel ratio sensor 21, and the routine shown in FIG. 7 is performed by interruption at regular time intervals.

Referring to FIG. 7, first, step 50.
At, it is determined whether or not the flag F indicating that the feedback control should be executed is set. When the flag F is not set, the processing cycle is completed, and therefore the feedback control is not performed at this time. On the other hand, when the flag F is set, the routine proceeds to step 51, where it is judged if the output voltage V of the air-fuel ratio sensor 21 is smaller than the reference voltage Vr of about 0.45 (V). When V ≦ Vr, that is, when the air-fuel ratio is lean, the routine proceeds to step 52, where the delay count value CD
L is decremented by 1. Then step 53
Then, it is determined whether or not the delay count value CDL has become smaller than the minimum value TDR. When CDL <TDR, the routine proceeds to step 54, where CDL is set to TDR, and then the routine proceeds to step 55. Therefore, as shown in FIG.
When reaching Vr, the delay count value CDL is gradually decreased, and then the CDL is maintained at the minimum value TDR.

On the other hand, when it is judged at step 51 that V> Vr, that is, when the air-fuel ratio is rich, the routine proceeds to step 56, where the delay count value CDL is incremented by 1. Next, at step 57, it is judged if the delay count value CDL has become larger than the maximum value TDL. If CDL> TDL, the routine proceeds to step 58, where CDL is made TDL, and then step 5
Go to 5. Therefore, as shown in FIG. 6, when V> Vr, the delay count value CDL is gradually increased,
The CDL is then maintained at the maximum value TDL.

At step 55, it is judged if the sign of the delay count value CDL has been inverted from positive to negative or from negative to positive between the previous processing cycle and the current processing cycle. When the sign of the delay count value CDL is inverted, the routine proceeds to step 59, where it is determined whether the inversion is from positive to negative.
That is, it is determined whether or not the reversal from rich to lean. Step 60 when reversing from rich to lean
Then, the rich skip value RSR is added to the feedback correction coefficient FAF, so that the FAF is rapidly increased by the rich skip value RSR as shown in FIG. On the other hand, when the lean-to-rich inversion is performed, the routine proceeds to step 61, where the lean skip value RSL is subtracted from FAF, and as a result, as shown in FIG.
F is sharply reduced by the lean skip value RSL.

On the other hand, when it is judged at step 55 that the sign of the delay count value CDL is not inverted, the routine proceeds to step 62, where it is judged if the delay count value CDL is negative. When CDL ≦ 0, the routine proceeds to step 63, where the rich integral value KIR (KIR <RSR) is added to the feedback correction coefficient FAF, and thus the FAF is gradually increased as shown in FIG. On the other hand, when CDL> 0, the routine proceeds to step 64, where the lean integral value KIL is subtracted from FAF, and thus FAF is gradually decreased as shown in FIG.

By adopting such a feedback control method, as can be seen from FIG. 6, even if the air-fuel ratio becomes lean temporarily, the FAF can be prevented from being affected by this. FIG. 8A shows the case where the air-fuel ratio is maintained at the stoichiometric air-fuel ratio. At this time, the actual air-fuel ratio fluctuates up and down around the theoretical air-fuel ratio 14.6, so that at this time, the average value of the actual air-fuel ratio becomes the theoretical air-fuel ratio 14.6. On the other hand, FIG. 8B shows the case where the rich integration value KIR is made larger than the lean integration value KIL. In this case, the actual air-fuel ratio fluctuates while deviating to the rich side as a whole, and the rich time and the rich degree during this time are larger than the lean time and the lean degree during this time. Therefore, at this time, the average value of the air-fuel ratio is slightly richer than the theoretical air-fuel ratio.

In order to make the average value of the air-fuel ratio slightly richer than the theoretical air-fuel ratio, the rich skip value RSR shown in FIG. 6 may be made larger than the lean skip value RSL, and also shown in FIG. The absolute value of the minimum value TDR may be larger than the maximum value TDL. 9 to 11
Shows a first embodiment in which the rich integrated value KIR is made larger than the lean integrated value KIL so that the average value of the air-fuel ratio is slightly shifted to the rich side with respect to the theoretical air-fuel ratio. In this embodiment, K =
When it becomes 1.0, that is, the operating state in which the air-fuel ratio should be the stoichiometric air-fuel ratio (during idling operation and K in FIG. 3 =
When the operating range is 1.0), the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio.

In the first embodiment, the count-up operation of the count value C is started when K = 1.0, and when the count value C reaches the set value C ₀ , the average value of the air-fuel ratio becomes the theoretical air-fuel ratio. The air-fuel ratio is feedback-controlled so that 10 and 11 show a routine for executing the air-fuel ratio control shown in FIG. 9, and this routine is executed by interruption at regular time intervals.

Referring to FIGS. 10 and 11, first, at step 100, the basic fuel injection time TP is calculated from the map shown in FIG. Next, at step 101, it is judged if the fuel supply is stopped during the deceleration operation. When the fuel supply is not stopped, the routine proceeds to step 102, where it is judged if the idle switch 20 is on, that is, if the throttle valve 14 is at the idling opening degree. When the idle switch 20 is not on, the routine proceeds to step 103, where the correction coefficient K is calculated from the operating state of the engine based on the relationship shown in FIG. Next, at step 104, it is judged if K = 1.0. When K is not 1.0, that is, when the lean air-fuel mixture or the rich air-fuel mixture is to be burned, the routine proceeds to step 105, where the count value C is made zero, and then the routine proceeds to step 110.

On the other hand, when the idle switch 20 is turned on at step 102, the routine proceeds to step 106, where K = 1.0. Next, at step 107, the count value C is incremented by 1, and the routine proceeds to step 110. In step 104, K = 1.
If it is determined to be 0, the routine proceeds to step 107, where the count value C is incremented by 1, and then the routine proceeds to step 110. Therefore, the count-up action of the count value C is started when K = 1, that is, during the idling operation and in the region of K = 1 shown in FIG.

On the other hand, when it is judged in step 101 that the fuel supply is stopped, step 108 is executed.
Then, K = 0 is set. Next, at step 109, the count value C is made zero, and the routine proceeds to step 110. At step 110, it is judged if K = 1.0. When K is not 1.0, that is, when the lean air-fuel mixture or the rich air-fuel mixture is to be burned, the routine proceeds to step 111, where the feedback correction coefficient FAF is set to 1.0. Next, at step 112, the flag F is reset, then at step 119, the fuel injection time TAU is calculated based on the following equation.

TAU = f.TP.K.FAF Therefore, at this time, the air-fuel ratio is open-loop controlled to a predetermined lean air-fuel ratio or rich air-fuel ratio. On the other hand, when it is determined in step 110 that K = 1.0, that is, when the air-fuel ratio should be the stoichiometric air-fuel ratio, the routine proceeds to step 113, where the count value C becomes larger than the set value C ₀ (FIG. 9). Whether or not it is determined. C ≦ C ₀
If it is, the routine proceeds to step 114, where the reference rich integral value K
The sum of IR ₀ and the constant value k ₁ is set as the rich integral value KIR, and then in step 115, the reference lean integral value KIL.
_The result of subtraction of the constant value k ₂ from ₀ is the lean integral value K
It is called IL. Next, at step 118, flag F
Is set, and the process proceeds to step 119. When the flag F is set, the feedback correction coefficient FAF is calculated in the feedback control routine shown in FIG. 7, and the air-fuel ratio feedback control is started.

When C> C ₀ , the routine proceeds to step 116, where the rich integral value KIR is made the reference rich integral value KIR _0, and then proceeds to step 117, the lean integral value KIL.
Is set as the reference lean integration value KIL ₀ . Then, the process proceeds to step 119 via step 118. Therefore K = 1.0
After that, until the condition of C> C ₀ is reached, the rich integral value KI
Since R is increased and the lean integral value KIL is decreased, the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes slightly richer than the stoichiometric air-fuel ratio.
When C> C ₀ , the rich integrated value KIR and the lean integrated value KIL are returned to the reference values KIR ₀ and KIL ₀ , respectively, so that the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes the stoichiometric air-fuel ratio.

12 to 16 show the rich skip value RS
A second embodiment is shown in which the average value of the air-fuel ratio is slightly shifted to the rich side with respect to the theoretical air-fuel ratio by making R larger than the lean skip value RSL.
In this embodiment, as shown in FIG. 12, when K = 1.0, that is, in the operating state where the air-fuel ratio should be the stoichiometric air-fuel ratio (during idling and the operating region of K = 1.0 in FIG. 3). When it becomes, the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio is slightly richer than the theoretical air-fuel ratio.

Further, in this second embodiment, the NO _x absorbent 1
The NO _x amount ΣNOX estimated to be absorbed in 8 is calculated, and when the NO _x amount ΣNOX becomes zero, the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes the stoichiometric air-fuel ratio. Therefore, in this second embodiment, NO _x absorbent 1
When it is estimated that all NO _x has been released from 8, the average value of the air-fuel ratio is slightly changed from the rich side to the stoichiometric air-fuel ratio. As described above, in the second embodiment, the air-fuel ratio is switched based on the NO _x amount ΣNOx estimated to be absorbed by the NO _x absorbent 18, and therefore the method for calculating the NO _x amount ΣNOx is first of all calculated. Will be described.

[0038] is _the amount of NO _x absorbed per unit time per _the NO _x absorbent 18 to _the amount of NO _x discharged from the higher unit time per engine becomes higher the engine load increases when the lean air-fuel mixture is burned As the engine speed increases and the engine speed increases, NO emitted from the engine per unit time
NO _{x to x} amount is absorbed per unit time _the NO _x absorbent 18 in order to increase is increased. Therefore, NO per unit time
_The amount of NO _x absorbed by the _x absorbent 18 is a function of the engine load and the engine speed. In this case, the engine load is absolute pressure PM and the engine speed N of the absolute amount of NO _x that have at pressure representative absorbed in unit time per _the NO _x absorbent 18 since it is the surge tank 10 in the surge tank 10 Is a function of. Therefore, in the embodiment according to the present invention, NO _x per unit time
Absolute pressure PM of the amount of NO _x NOXA that is absorbed by the absorbent 18
And as a function of the engine speed N, previously obtained by experiments,
1 The amount of NO _x NOXA as a function of PM and N
3 (A) is stored in the ROM 32 in advance in the form of a map.

On the other hand, the air-fuel mixture supplied into the engine cylinder
NO when the air-fuel ratio of becomes the stoichiometric air-fuel ratio or rich_xAbsorbent
18 to NO_xIs released, but NO at this time_xRelease amount
Is mainly affected by the amount of exhaust gas and the air-fuel ratio. That is, exhaust
NO per unit time as the amount of gas increases_xAbsorbent 18?
NO released from_xAmount increases and the air-fuel ratio becomes rich
NO per unit time_xNO released from absorbent 18
_xThe amount increases. In this case, the amount of exhaust gas, that is, the intake air
The amount depends on the engine speed N and the absolute pressure PM in the surge tank 10.
Can be represented by the product of
NO per unit time as shown in_xRelease from absorbent 18
NO issued_xThe amount NOXD increases as N ・ PM increases
Increase. The air-fuel ratio corresponds to the value of the correction coefficient K.
Therefore, as shown in FIG. 13 (C), NO per unit time
_xNO released from absorbent 18 _xThe quantity NOXD is the value of K
Increases as is larger. NO per unit time_xSucking
NO released from sorbent 18_xThe amount NOXD is N · PM
R as a function of K in the form of the map shown in FIG.
It is stored in the OM32.

Further, _the NO _x absorbent 18 nitrate ions NO ₃ and the absorbent temperature is high in ^- that _the NO _x releasing rate from _the NO _x absorbent 18 is increased so easily decomposed. In this case, since the temperature of the NO _x absorbent 18 is almost proportional to the exhaust gas, the NO _x release rate K as shown in FIG.
f increases as the exhaust gas temperature T increases. Therefore N
O _x release rate per unit time when the taking into account Kf N
The amount of NO _x released from the O _x absorbent 18 is shown in FIG.
It will be expressed by the product of the NOXD and _the NO _x releasing factor Kf shown in. In the embodiment according to the present invention, the exhaust gas temperature T is stored in advance in the ROM 32 in the form of a map shown in FIG. 14C as a function of the absolute pressure PM in the surge tank 10 and the engine speed N.

As described above, the lean air-fuel mixture is burned.
NO per unit time when given _xAbsorption amount is NOXA
The air-fuel mixture with the stoichiometric air-fuel ratio or the rich air-fuel mixture is burned.
NO per unit time when burnt_xThe amount released
NO as it is represented by Kf NOXD_xSoak up in absorbent 18
NO estimated to have been collected_xThe amount ΣNOX is represented by the following formula
Will be forgotten.

ΣNOX = ΣNOX + NOXA−Kf · NOXD FIGS. 15 and 16 show a routine for executing the air-fuel ratio control shown in FIG. 12, and this routine is executed by interruption for a fixed time. 15 and 1
Referring first to FIG.
The basic fuel injection time TP is calculated from the map shown in FIG.
Next, at step 201, it is judged if the fuel supply is stopped during the deceleration operation. When the fuel supply is not stopped, the routine proceeds to step 202, where it is judged if the idle switch 20 is on, that is, if the throttle valve 14 is at the idling opening degree.
Step 2 when the idle switch 20 is not on
03, the correction coefficient K is calculated from the operating state of the engine based on the relationship shown in FIG. 3, and then the routine proceeds to step 205.

On the other hand, when the idle switch 20 is turned on in step 202, the routine proceeds to step 204, where K = 1.0, and then to step 205.
Therefore, K = 1.0 is in the idling operation or in the region of K = 1.0 shown in FIG. In step 205, it is determined whether K <1.0, that is, whether the lean air-fuel mixture is in an operating state in which combustion should be performed. When K <1.0, that is, when the lean air-fuel mixture is in an operating state where combustion should be performed, the routine proceeds to step 206, where the NO _x absorption amount NOXA per unit time is calculated from the map shown in FIG. 13 (A). Next, at step 207, the NO _x release amount NOXD is made zero, and then at step 211, the feedback correction coefficient FAF is fixed to 1.0. Then step 21
Flag F is reset at 2, then step 2
Proceed to 22.

On the other hand, in step 205, K <1.0
If it is determined that it is not, the routine proceeds to step 208, where K
It is determined whether or not> 1.0, that is, whether or not the operating state in which the rich air-fuel mixture should be burned. When K> 1.0, that is, in the operating state where the rich air-fuel mixture should be burned, the routine proceeds to step 209, where the released NO _x amount NOXD per unit time is calculated from the map shown in FIG.
From the relationship shown in FIG. 4 (B) and the map shown in FIG. 14 (C), N
The O _x release rate Kf is calculated. Next, at step 210, the NO _x absorption amount NOXA per unit time is made zero.
Next, at step 211, the correction coefficient K is fixed to 1.0, then at step 212 the flag F is reset, and then the routine proceeds to step 222. In step 222, the fuel injection time TAU is calculated based on the following equation.

TAU = f.TP.K.FAF Therefore, at this time, the air-fuel ratio is open-loop controlled to a predetermined lean air-fuel ratio or rich air-fuel ratio. Next, at step 223, the NO _x amount ΣNOx absorbed in the NO _x absorbent 18 is calculated based on the following equation. ΣNOX = ΣNOX + NOXA−Kf · NOXD Next, at step 224, it is judged if the NO _x amount ΣNOX becomes negative. If ΣNOX <0, the routine proceeds to step 225, where ΣNOX is made zero. On the other hand, when K> 1.0 is not satisfied in step 208, that is, K =
When it is 1.0 and the air-fuel ratio should be the theoretical air-fuel ratio, the routine proceeds to step 213, where it is judged if the NO _x amount ΣNOx is zero. The sum of a constant value k ₃ a reference rich skip value RSR ₀ proceeds to step 214 when not .SIGMA.NOX = 0 is set to the rich skip value RSR, then subtracting the predetermined value k ₄ from the reference lean skip value RSL ₀ in step 215 Subtraction result is lean skip value RSL
It is said that Then, in step 216, FIG.
The NO _x release amount NOXD is calculated from the map shown in FIG. 14, and the NO _x release rate Kf is calculated from the relationship shown in FIG. 14 (B) and the map shown in FIG. 14 (C). Then step 220
Then, the NO _x absorption amount NOXA is made zero. Next, at step 221, the flag F is set, and at step 22
Go to 2. When the flag F is set, the feedback correction coefficient FAF is calculated in the feedback control routine shown in FIG. 7, and the air-fuel ratio feedback control is started.

When ΣNOX = 0, the routine proceeds to step 217, where the rich skip value RSR is the reference lip skip value R.
SR ₀ is set, and then the routine proceeds to step 218, where the lean skip value RSL is set to the reference lean skip value RSL ₀ . Next, at step 219, the NO _x release amount NOXD is made zero. Then, the process proceeds to step 222 through steps 220 and 221. Therefore, after K = 1.0, ΣNO
The rich skip value RSR is increased and the lean skip value RSL is decreased until X = 0, so that the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes slightly richer than the theoretical air-fuel ratio. ΣN
When OX = 0, the rich skip value RSR and the lean skip value RSL are returned to the reference values RSR ₀ and RSL ₀ , respectively, so the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes the theoretical air-fuel ratio.

As described above, the average value of the air-fuel ratio is maintained slightly on the rich side with respect to the theoretical air-fuel ratio until ΣNOX = 0. Therefore, at this time, the average value of the air-fuel ratio is the theoretical air-fuel ratio. NO _x absorbent 18
_The NO _x releasing amount NOXD from increases. Therefore, when the routine proceeds to step 216, K = 1.0, but at step 216, it is based on the value of K (K> 1.0) corresponding to the average value of the air-fuel ratio on the rich side as shown in FIG. A)
The NO _x release amount NOXD is calculated from the map shown in FIG.

On the other hand, when it is determined in step 201 that the fuel supply is stopped, step 226
Then, the NO _x absorption amount NOXA is made zero, and then the routine proceeds to step 227, where the NO _x release amount NOXD is made zero. Next, at step 228, K = 0 is set, then at step 229, the flag F is reset, and then the routine proceeds to step 222.

17 to 20 show a third embodiment. As shown in FIG. 17, in the third embodiment, a second air-fuel ratio sensor 40 is provided inside the engine exhaust passage downstream of the NO _x absorbent 18. The second air-fuel ratio sensor 40 has an output characteristic similar to that of the air-fuel ratio sensor 21 with respect to the air-fuel ratio, and the output signal of the second air-fuel ratio sensor 40 is an AD converter 41.
Is input to the input port 35 via.

Also in the third embodiment, as shown in FIG. 18, when K = 1.0, that is, the operating state in which the air-fuel ratio should be the stoichiometric air-fuel ratio (during idling operation and FIG. 3).
(K = 1.0 operating region), the air-fuel ratio is feedback controlled so that the average value of the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio. At this time, in this third embodiment, by increasing the absolute value of the minimum value TDR (FIG. 6) and decreasing the maximum value TDL (FIG. 6), the average value of the air-fuel ratio is slightly richer than the theoretical air-fuel ratio. I am trying to shift it.

[0051] Further, in the third embodiment NO _x from _the NO _x absorbent 18 from the output signal of the second air-fuel ratio sensor 40
It is determined whether or not the release action is completed, and as shown in FIG. 18, when the output voltage V of the second air-fuel ratio sensor 40 becomes higher than the constant value V ₀ , the average value of the air-fuel ratio becomes The air-fuel ratio is feedback-controlled so that it becomes the stoichiometric air-fuel ratio. As described above, in the third embodiment, the completion of the NO _x releasing action from the NO _x absorbent 18 is detected from the change in the output voltage of the second air-fuel ratio sensor 40.
Therefore, this will be described below.

That is, the air-fuel mixture supplied into the combustion chamber 3 is reheated.
As shown in FIG. 4, oxygen is discharged from the combustion chamber 3
O₂And exhaust gas containing unburned HC and CO is discharged
This oxygen O₂Almost reacts with unburned HC and CO
Therefore, this oxygen O₂Is NO_xPass through absorbent 18
Gite NO_xIt will be discharged from the absorbent 18. one
On the other hand, when the air-fuel mixture supplied into the combustion chamber 3 becomes rich, N
O_xAbsorbent 18 to NO _xIs released. Exhaust at this time
Unburned HC and CO contained in gas are released NO_xTo
NO because it is used to reduce_xAbsorbent 18 to N
O_xNO while is being released_xNo absorption from absorbent 18
The HC and CO will not be discharged. Therefore NO_x
Absorbent 18 to NO_xIs continuously released, NO_x
Oxygen O is contained in the exhaust gas discharged from the absorbent 18.₂Includes
Although it is rare, it does not contain any unburned HC or CO.
I mean NO during this time_xExhaust gas discharged from the absorbent 18
The air-fuel ratio is slightly lean.

Next, when the action of releasing NO _x from the NO _x absorbent 18 is completed, unburned HC contained in the exhaust gas,
CO is discharged from it _the NO _x absorbent 18 without being acting for the reduction of the NO _x in the _the NO _x absorbent 18. Therefore, at this time, when the air-fuel mixture supplied to the combustion chamber 3 is rich, the air-fuel ratio of the exhaust gas discharged from the NO _x absorbent 18 also becomes rich. That is, NO _x absorbent 1
When the action of releasing NO _x from 8 is completed, NO _x absorbent 1
The exhaust gas discharged from 8 changes from lean to rich. Therefore, when the NO _x releasing action from the NO _x absorbent 18 is completed, the output voltage V of the second air-fuel ratio sensor 40 rises as shown in FIG.

19 and 20 show a routine for executing the air-fuel ratio control shown in FIG. 18, and this routine is executed by interruption at regular time intervals. Referring to FIGS. 19 and 20, first, step 30
0 from the map shown in FIG. 2 to the basic fuel injection time TP
Is calculated. Next, at step 301, it is judged if the fuel supply is stopped during the deceleration operation. Step 3 when the fuel supply is not stopped
The routine proceeds to 02, where it is judged if the idle switch 20 is on, that is, if the throttle valve 14 is at the idling opening degree. When the idle switch 20 is not on, the routine proceeds to step 303, where the correction coefficient K is calculated from the operating state of the engine based on the relationship shown in FIG. Then, it proceeds to step 306.

On the other hand, when the idle switch 20 is turned on at step 302, the routine proceeds to step 304, where K = 1.0 is set, and then the routine proceeds to step 306.
If it is determined in step 301 that the fuel supply is stopped, the process proceeds to step 305 and K =
It is set to 0, and then the process proceeds to step 306. Step 30
At 6, it is determined whether K = 1.0. K = 1.
When it is not 0, that is, when the lean air-fuel mixture or the rich air-fuel mixture is to be burned, the routine proceeds to step 307, where the flag X indicating that the process of slightly increasing the average value of the air-fuel ratio is completed is reset. Next, in step 308, the feedback correction coefficient FAF is set to 1.0. Next, at step 309, the flag F is reset, then at step 318, the fuel injection time TAU is calculated based on the following equation.

TAU = f · TP · K · FAF Therefore, at this time, the air-fuel ratio is open-loop controlled to a predetermined lean air-fuel ratio or rich air-fuel ratio. On the other hand, when it is determined in step 306 that K = 1.0, that is, when the air-fuel ratio should be the stoichiometric air-fuel ratio, the routine proceeds to step 310, where it is determined whether the flag X is set. When K = 1.0, flag X
Has been reset, the process proceeds to step 311, and the second
The output voltage V of the air-fuel ratio sensor 40 of the constant value V ₀ (see FIG.
It is determined whether or not it is larger than 8). When K = 1.0 from the state where the lean air-fuel mixture is being burned, V ≦ V ₀ , so the routine proceeds to step 312.

In step 312, the subtraction result obtained by subtracting the constant value k ₅ from the reference minimum value TDR ₀ is set as the minimum value TDR. That is, the absolute value of the minimum value TDR is the reference minimum value TDR _0.
The absolute value of is increased by a constant value k ₅ . Next, at step 313, the subtraction result obtained by subtracting the constant value k ₆ from the reference maximum value TDL ₀ is set as the maximum value TDL. Next, at step 317, the flag F is set, and the routine proceeds to step 318. When the flag F is set, the feedback correction coefficient FAF is calculated in the feedback control routine shown in FIG. 7, and the air-fuel ratio feedback control is started.

When it is determined in step 311 that V> V ₀ , the flag X is set in step 314, and then the process proceeds to step 315. When the flag X is set, the process jumps from step 310 to step 315 from the next processing cycle. In step 315, the minimum value TDR is set to the reference minimum value TDR _0, and then in step 316, the maximum value TDL is set to the reference maximum value TDL ₀ . Then, the process proceeds to step 318 via step 317.

Therefore, after K = 1.0, the absolute value of the minimum value TDR is increased and the maximum value TDL is decreased until V> V ₀ , so that the air-fuel ratio is the average value of the air-fuel ratio. Is feedback-controlled so as to be slightly rich with respect to the stoichiometric air-fuel ratio, and when V> V ₀ , the minimum value T
DR and maximum value TDL are reference values TDR ₀ and DTL, respectively.
_Since it is returned to ₀ , the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes the stoichiometric air-fuel ratio.

[0060]

Effect of the Invention of the NO _x in a short time from the _the NO _x absorbent while suppressing the torque change when the air-fuel ratio changes in order to release the NO _x from a lean air-fuel mixture in the embodiment described in claim 1 The release can be completed. According to the second and third aspects of the invention, the average value of the air-fuel ratio is the theoretical value when the NO _x releasing action from the NO _x absorbent is completed or when the NO _x releasing action is completed. Since the fuel ratio is switched to, it is possible to prevent HC and CO from being released into the atmosphere after the completion of NO _x release.

[Brief description of drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a diagram showing a map of a basic fuel injection time.

FIG. 3 is a diagram showing a correction coefficient K.

FIG. 4 Unburned HC and C in exhaust gas discharged from the engine
It is a diagram which shows the concentration of O and oxygen roughly.

FIG. 5 is a diagram for explaining the action of absorbing and releasing NO _x .

FIG. 6 is a time chart showing changes in a feedback correction coefficient FAF.

FIG. 7 is a flowchart for performing feedback control.

FIG. 8 is a diagram showing a change in air-fuel ratio.

FIG. 9 is a time chart showing a first embodiment of air-fuel ratio control.

FIG. 10 is a flowchart for executing the first embodiment of air-fuel ratio control.

FIG. 11 is a flowchart for executing a first embodiment of air-fuel ratio control.

FIG. 12 is a time chart showing a second embodiment of air-fuel ratio control.

FIG. 13 is a diagram showing a NO _x absorption amount NOXA and the like.

FIG. 14 is a diagram showing a NO _x release amount NOXD and the like.

FIG. 15 is a flowchart for executing a second embodiment of air-fuel ratio control.

FIG. 16 is a flowchart for executing a second embodiment of air-fuel ratio control.

FIG. 17 is an overall view showing another embodiment of the internal combustion engine.

FIG. 18 is a time chart showing a third embodiment of air-fuel ratio control.

FIG. 19 is a flowchart for executing a third embodiment of air-fuel ratio control.

FIG. 20 is a flow chart for executing a third embodiment of air-fuel ratio control.

[Explanation of symbols]

16 ... Exhaust pipe 18 ... NO _x absorbent

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location F01N 3/28 ZAB 301 C F02D 41/14 310 C 8011-3G

Claims (3)

[Claims] 1. When the air-fuel ratio of the inflowing exhaust gas is lean
No when_xAnd the air-fuel ratio of the inflowing exhaust gas is
The absorbed NO when the air-fuel ratio or rich _xEmit
NO_xAbsorbent is placed in the engine exhaust passage for lean mixing
NO in the exhaust gas when the air is burned_x
Is NO_xAbsorbed by the absorbent and stoichiometric air-fuel ratio
NO when the air-fuel mixture is burned_xAbsorbent
Et NO _xIn an internal combustion engine in which
An air-fuel ratio sensor is installed in the air passage, and the stoichiometric mixture
When the engine operating condition in which the
The average value of the air-fuel ratio is
The air-fuel ratio of the air-fuel mixture should be slightly richer.
Internal combustion equipped with air-fuel ratio control means for feedback control
Exhaust gas purification device for engines. 2. The average value of the air-fuel ratio with respect to the theoretical air-fuel ratio when the predetermined period of time has elapsed after the engine operating state in which the air-fuel ratio mixture should be burned The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the air-fuel ratio feedback control on the slightly rich side is switched to the feedback control on which the average value of the air-fuel ratio becomes the theoretical air-fuel ratio. Wherein when the air-fuel ratio control means which is estimated to have or completed when _the NO _x releasing action from _the NO _x absorbent after becoming the engine operating state to combust a mixture of the stoichiometric air-fuel ratio is completed 2. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the air-fuel ratio feedback control in which the average value of the air-fuel ratio is slightly richer than the theoretical air-fuel ratio is switched to the feedback control in which the average value of the air-fuel ratio becomes the theoretical air-fuel ratio. .