JP3287083B2 - 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.

[0002]

2. Description of the Related Art When the air-fuel ratio of exhaust gas flowing in is lean.
NO_xAnd the air-fuel ratio of the exhaust gas
NO absorbed at stoichiometric air-fuel ratio or rich_xEmit
NO _xThe absorbent is placed in the engine exhaust passage, and NO_xabsorption
NO from agent_xWhen it should be released into the engine cylinder.
Change the air-fuel ratio of the supplied mixture from lean to stoichiometric or
Switch for a predetermined period of time, and then
An internal combustion engine that returns the air-fuel ratio to lean again
Has already been proposed by humans (International Publication WO93 / 07)
No. 363). In this internal combustion engine, NO_xAbsorbent to N
O_xAir-fuel ratio was adjusted to stoichiometric to release air
Sometimes the output of the air-fuel ratio sensor provided in the engine exhaust passage is
Force the air-fuel ratio to reach the stoichiometric air-fuel ratio based on the force signal.
Feedback control, NO_xNO from absorbent_xRelease
The air-fuel ratio is open
Controlled.

[0003]

SUMMARY OF THE INVENTION
When the ratio is made rich, NO
_xNO from absorbent_xThe release action is completed but the mixture is empty
NO when the fuel ratio is set to the stoichiometric air-fuel ratio_xFrom the absorbent
NO_xIt takes time to complete the release action. Therefore
NO when mainly 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 emission function
Then NO_xNO from absorbent_xUntil the release action is completed
It takes time.

[0004] In this case, greatly so it is sufficient to rich the air-fuel mixture in order to shorten the _the NO _x releasing time from _the NO _x absorbent. However, if the air-fuel mixture is made significantly rich, a large torque change occurs when the air-fuel mixture is switched from lean to rich. Therefore it would be preferable to just slightly rich than the stoichiometric air-fuel ratio in order to suppress the occurrence of short-hidden while the torque change _the NO _x releasing time from _the NO _x absorbent. However, in the above-described internal combustion engine, open-loop control of the air-fuel ratio must be performed to control the air-fuel ratio slightly richer than the stoichiometric air-fuel ratio. However, as a practical matter, it is difficult to maintain the air-fuel ratio slightly richer than the stoichiometric air-fuel ratio by open-loop control.

[0005]

According to the present invention, when the air-fuel ratio of the inflowing exhaust gas is lean, NO _x is absorbed, and the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric. when the fuel ratio or rich is arranged to _the NO _x absorbent to release the absorbed NO _x in the engine exhaust passage, when the lean air-fuel mixture is burned NO _x in the exhaust gas is absorbed in _the NO _x absorbent, 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 of the theoretical air-fuel ratio has been burned,
The air-fuel ratio sensor disposed in the engine exhaust passage, a lean mixture
When the air-fuel ratio is changed from the engine operating state to the engine operating state in which the stoichiometric air-fuel mixture is to 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 control means for feedback-controlling the air-fuel ratio of the air-fuel mixture so that the air-fuel ratio of the air-fuel mixture becomes stoichiometric.
NO _x from _the NO _x absorbent after becoming the engine operating condition to
When the release action is completed or presumed to be completed
The average value of the air-fuel ratio slightly
From the air-fuel ratio feedback control on the
Switch to feedback control where the value becomes the stoichiometric air-fuel ratio
I'm trying.

[0006]

[Action] In the first invention, a mixture of stoichiometric air-fuel ratio is burned
When the engine is in the required operating state, the air-fuel ratio sensor
The average value of the air-fuel ratio is different from the stoichiometric air-fuel ratio based on the force signal.
The air-fuel ratio of the air-fuel mixture is
Feedback control, and NO _x NO from absorbent _x But
Released. Next, the average value of the air-fuel ratio is
Air-fuel ratio feedback control on the slightly rich side
NO after is started _x NO from absorbent _x Complete release action
Of the air-fuel ratio when
Switch to feedback control so that the average value becomes the stoichiometric air-fuel ratio.
It is.

[0007]

[0008]

[0009]

[0010]

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 a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel into the intake port 6 is attached to each branch pipe 9. 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 is composed of a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and an input port 35 interconnected 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 disposed in the surge tank 10, and the output voltage of the pressure sensor 19 is input to an input port 35 via an 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 disposed in the exhaust manifold 15, and an output voltage of the air-fuel ratio sensor 21 is input to an input port 35 via an AD converter 38. The input port 35 is connected to a 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 ignition plug 4 and the fuel injection valve 11 via a corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = f · TP · K · FAF where f is a constant, TP is a basic fuel injection time, K is a correction coefficient, and FAF is a feedback correction coefficient. The basic fuel injection time TP indicates a fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder a stoichiometric air-fuel ratio. The basic fuel injection time TP is obtained by an experiment in advance, and is stored in advance in the ROM 32 as a function of the absolute pressure PM in the surge tank 10 and the engine speed N in the form of a map as shown in FIG. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, K <
At 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, 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 equal to the 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 to the engine cylinder should be the stoichiometric air-fuel ratio. It is a coefficient for making the air-fuel ratio accurately match the stoichiometric air-fuel ratio based on the above. The 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. In addition, K <1.0 or K>
When it is 1.0, the 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, and is basically shown in FIG. 3 in the embodiment according to the present invention. As described above, 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 in the low-load operation region on the lower load side than the solid line R, that is, the air-fuel mixture is lean, 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 concentration 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 increases, and the concentration of the exhaust gas from the combustion chamber 3 increases. The concentration of oxygen O ₂ in the exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NO contained in casing 17_x
The absorbent 18 uses, for example, alumina as a carrier, and on this carrier,
For example, potassium K, sodium Na, lithium Li,
Alkali metals such as Cs, barium Ba, cal
Alkaline earths such as calcium 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 supplied air and fuel (hydrocarbon) is set to NO _xabsorption
When this is referred to as the air-fuel ratio of the exhaust gas flowing into the agent 18, this NO_x
When the air-fuel ratio of the inflowing exhaust gas is lean, the absorbent 18
NO_xAnd reduce the oxygen concentration in the incoming exhaust gas
And absorbed NO_xReleases NO_xPerform the absorption and release action of
U. Note that NO_xFuel in the exhaust passage upstream of the absorbent 18
(Hydrocarbons) or inflow and exhaust when air is not supplied
The air-fuel ratio of the gas-gas is the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3.
Ratio, and in this case NO_xAbsorbent 18 burns
When the air-fuel ratio of the mixture supplied to the firing chamber 3 is lean
Is NO_xIn the air-fuel mixture supplied into the combustion chamber 3
NO absorbed when oxygen concentration decreases_xTo release
Become.

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

That is, the exhaust gas flowing in becomes considerably lean.
And the oxygen concentration in the inflowing exhaust gas greatly increased, and FIG.
As shown in FIG._TwoIs O_Two ^-Or O
^2-On the surface of platinum Pt. On the other hand, inflow exhaust
NO in the gas becomes O 2 on the surface of platinum Pt._Two ^-Or O^2-When
React, NO_Two(2NO + O_Two→ 2NO_Two). Next
NO generated_TwoIs partially oxidized on platinum Pt
Is absorbed in the absorbent and does not bind to barium oxide BaO.
As shown in FIG. 5A, nitrate ions NO_Three ^-of
It diffuses into the absorbent in form. NO in this way_xIs NO
_xIt is absorbed in the absorbent 18.

As long as the oxygen concentration in the inflowing exhaust gas is high, platinum
NO on Pt surface_TwoIs generated, and the NO_xAbsorption capacity
NO unless power is saturated_TwoIs absorbed in the absorbent and nitric acid
Ion NO_Three ^-Is generated. In contrast, the inflow exhaust gas
NO in the oxygen concentration in the_TwoWhen the amount of
Reaction is reversed (NO_Three ^-→ NO_Two) And thus suck
Nitrate ion NO in the collector_Three ^-Is NO_TwoFrom the absorbent in the form of
Released. That is, the oxygen concentration in the inflow exhaust gas decreases.
NO_xNO from absorbent 18_xWill be released
You. As shown in FIG. 4, the degree of lean of the incoming exhaust gas
Lower the oxygen concentration in the incoming exhaust gas
If the degree of leanness of the inflow exhaust gas is reduced,
NO even if the air-fuel ratio of the exhaust gas is lean_xAbsorbent 18
From NO _xWill be released.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG.
C and CO are exhausted, and the unburned HC and CO are platinum Pt.
It reacts with the above oxygen O ₂ ^- or O ^2- to be oxidized. Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely reduced, so that NO ₂ is released from the absorbent, and this NO ₂ is unreacted as shown in FIG. It is reduced by reacting with the fuel HC and CO.
In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. 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.

[0021] That is, when the air-fuel ratio of the inflowing exhaust gas is rich First the unburned HC, CO are O ₂ on the platinum Pt ^- or O ^{2 -} immediately be reacted with oxidized and then O on the platinum Pt Unburned H even if ₂ ^- or O ^2- is consumed
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 _x can be prevented from being discharged. Furthermore, _the NO _x absorbent 18 is NO _x because the has function even if the air-fuel ratio of the inflowing exhaust gas the stoichiometric air-fuel ratio emitted from _the NO _x absorbent 18 of the reduction catalyst is made to reduction. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NO _x
Since NO _x is gradually released from the absorbent 18, it takes a slightly longer time to release all the NO _x absorbed in the NO _x absorbent 18.

As described above, when the lean air-fuel mixture is burned, NO _x is absorbed by the NO _x absorbent 18, and when the air-fuel mixture or the rich air-fuel mixture of the stoichiometric air-fuel ratio is burned, N NO is absorbed.
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
O _x absorbent 18 is at risk of being not E absorbs NO _x. In order to avoid such a risk must be released as quickly as possible NO _x from the NO _x absorbent 18, a engine operating condition to be an air-fuel ratio to the stoichiometric air-fuel ratio in this embodiment of the present invention to its In such a 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 stoichiometric air-fuel ratio.

First, feedback control of the air-fuel ratio will be described with reference to FIGS. 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.
Occurs. FIG. 7 shows the feedback control of the air-fuel ratio performed based on the output signal of the air-fuel ratio sensor 21. The routine shown in FIG. 7 is performed by interruption every predetermined time.

Referring to FIG. 7, first of all, step 50 is executed.
It is determined whether 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, at this time, the feedback control is not performed. On the other hand, when the flag F is set, the routine proceeds to step 51, where it is determined whether or not the output voltage V of the air-fuel ratio sensor 21 is lower than a 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 one. 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 thereafter, the routine proceeds to step 55. Therefore, as shown in FIG.
When it reaches 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 determined in 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 one. Next, at step 57, it is determined whether or not the delay count value CDL has become larger than the maximum value TDL. When CDL> TDL, the routine proceeds to step 58, where CDL is set to TDL, and then at step 5
Go to 5. Therefore, as shown in FIG. 6, when V> Vr, the delay count value CDL is gradually increased, and
Then, the CDL is maintained at the maximum value TDL.

In step 55, it is determined whether or not 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 process proceeds to step 59 to determine whether the inversion is from positive to negative.
That is, it is determined whether or not the inversion is 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, and thus the FAF is rapidly increased by the rich skip value RSR as shown in FIG. On the other hand, at the time of inversion from lean to rich, the routine proceeds to step 61, where the lean skip value RSL is subtracted from FAF, and thus, as shown in FIG.
F is rapidly decreased by the lean skip value RSL.

On the other hand, when it is determined in step 55 that the sign of the delay count value CDL has not been inverted, the routine proceeds to step 62, where it is determined whether or not 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 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 the FAF, and thus the FAF is gradually reduced as shown in FIG.

When such a feedback control method is employed, as shown in FIG. 6, even if the air-fuel ratio temporarily becomes lean, for example, the FAF can be prevented from being affected by this. FIG. 8A shows a case where the air-fuel ratio is maintained at the stoichiometric air-fuel ratio. At this time, the actual air-fuel ratio moves up and down around the stoichiometric air-fuel ratio of 14.6. Therefore, at this time, the average value of the actual air-fuel ratio becomes the stoichiometric air-fuel ratio of 14.6. On the other hand, FIG. 8B shows a case where the rich integral value KIR is made larger than the lean integral value KIL. In this case, the actual air-fuel ratio fluctuates while leaning toward 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 stoichiometric air-fuel ratio.

In order to make the average value of the air-fuel ratio slightly richer than the stoichiometric air-fuel ratio, the rich skip value RSR shown in FIG. 6 may be made larger than the lean skip value RSL. 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 stoichiometric air-fuel ratio. In this embodiment, as shown in FIG.
1.0, that is, an operation state in which the air-fuel ratio should be set to the stoichiometric air-fuel ratio (during idling operation and K =
(Operating range of 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, when K = 1.0, the count-up operation of the count value C starts, and when the count value C reaches the set value C ₀ , the average value of the air-fuel ratio becomes equal to the stoichiometric air-fuel ratio. The air-fuel ratio is feedback-controlled so that FIGS. 10 and 11 show a routine for executing the air-fuel ratio control shown in FIG. 9, and this routine is executed by interruption every predetermined time.

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 determined whether or not the fuel supply is stopped during the deceleration operation. If the fuel supply is not stopped, the routine proceeds to step 102, where it is determined whether or not the idle switch 20 is on, that is, whether or not the throttle valve 14 is at the idling opening. If 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 determined whether or not K = 1.0. When K is not 1.0, that is, when the operating state is such that 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 set to zero, and then 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 one, and the routine proceeds to step 110. In step 104, K = 1.
When it is determined to be 0, the process proceeds to step 107, where the count value C is incremented by 1, and then proceeds to step 110. Therefore, the count-up operation of the count value C is started when K = 1, that is, at the time of idling operation and in the region of K = 1 shown in FIG.

On the other hand, if it is determined in step 101 that the supply of fuel has been stopped, step 108
And K = 0. Next, at step 109, the count value C is set to zero, and the routine proceeds to step 110. In step 110, it is determined whether or not K = 1.0. When K is not 1.0, that is, when a lean air-fuel mixture or a 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, and 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 controlled in an open loop 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). Is determined. C ≦ C ₀
In step 114, the routine proceeds to step 114, where the reference rich integration value K
The sum of IR ₀ and the constant value k ₁ is set as the rich integral value KIR, and then at step 115, the reference lean integral value KIL
₀ by subtracting the constant value k ₂ from the subtraction result is a lean integration value K
IL. Next, at step 118, the flag F
Is set, and the routine 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 set to the reference rich integral value KIR _0, and then, to step 117, the lean integral value KIL
Is the reference lean integral value KIL ₀ . Next, the routine proceeds to step 119 via step 118. Therefore, K = 1.0
And then C> C ₀ until the rich integrated 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 rich with respect to the stoichiometric air-fuel ratio.
When C> C ₀ , the rich integral value KIR and the lean integral 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.

FIGS. 12 to 16 show the rich skip value RS.
The second embodiment shows that the average value of the air-fuel ratio is slightly shifted to the rich side with respect to the stoichiometric air-fuel ratio by making R larger than the lean skip value RSL.
In this embodiment, when K = 1.0 as shown in FIG. 12, that is, in an operating state where the air-fuel ratio should be the stoichiometric air-fuel ratio (during idling operation and an operating region where K = 1.0 in FIG. 3). When this happens, 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 second embodiment, the NO _x absorbent 1
8 _the amount of NO _x ΣNOX is estimated to be absorbed is calculated, the air-fuel ratio so that the average value of the air-fuel ratio when the amount of NO _x ΣNOX becomes zero is the stoichiometric air-fuel ratio is feedback controlled. Thus _the NO _x absorbent in the second embodiment 1
All NO _x from 8 is switched to the stoichiometric air-fuel ratio from the rich side average of the air-fuel ratio has just slightly when it is estimated to have been released. Thus switching of the second embodiment in an air-fuel ratio based on the amount of NO _x ΣNOX is estimated to be absorbed in _the NO _x absorbent 18 is performed, thus First method of calculating the amount of NO _x ΣNOX 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 Increases and the higher the engine speed, the more NO is discharged 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
amount of NO _x absorbed in the _x absorbent 18 becomes 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 obtained by experiments in advance,
1 The amount of NO _x NOXA as a function of PM and N
It is stored in the ROM 32 in advance in the form of a map shown in FIG.

On the other hand, the air-fuel mixture supplied into the engine cylinder
NO when the air-fuel ratio becomes rich or stoichiometric_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 gas amount increases_xAbsorbent 18
NO released from_xThe amount increases and the air-fuel ratio becomes rich
NO per unit time_xNO released from absorbent 18
_xThe amount increases. In this case, the exhaust gas amount, that is, the intake air
The quantity is 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 NPM increases
Increase. The air-fuel ratio corresponds to the value of the correction coefficient K.
Therefore, as shown in FIG.
_xNO released from absorbent 18 _xNOXD is the value of K
Increases as the value increases. NO per unit time_xSucking
NO released from sorbent 18_xNOXD is NPM
R as a function of K in the form of a 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 substantially proportional to the exhaust gas temperature , the NO _x release rate Kf increases as the exhaust gas temperature T increases, as shown in FIG. Thus _the amount of NO _x released from per unit time _the NO _x absorbent 18 when taking into account _the NO _x releasing factor Kf is 14
Will be expressed by the product of the NOXD and _the NO _x releasing factor Kf shown in (A). In the embodiment according to the present invention, the exhaust gas temperature T is stored in the ROM 32 in advance 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 mixture burns.
NO per unit time _xNOXA absorption
And a mixture of stoichiometric air-fuel ratio or rich mixture
NO per unit time when burned_xThe amount released
NO because it is expressed by Kf · NOXD_xAbsorbed by absorbent 18
NO presumed to be collected_xThe quantity ΣNOX is expressed by the following equation.
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 predetermined time. FIG. 15 and FIG.
Referring to FIG. 6, first in step 200, FIG.
The basic fuel injection time TP is calculated from the map shown in FIG.
Next, at step 201, it is determined whether or not the fuel supply is stopped during the deceleration operation. If the fuel supply has not been stopped, the routine proceeds to step 202, where it is determined whether or not the idle switch 20 is on, that is, whether or not the throttle valve 14 is at the idling opening.
Step 2 when the idle switch 20 is not on
In step 03, the correction coefficient K is calculated from the operating state of the engine based on the relationship shown in FIG.

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 obtained during the idling operation or in the region where K = 1.0 shown in FIG. In step 205, it is determined whether or not K <1.0, that is, whether or not the operating state is such that the lean air-fuel mixture is to be burned. When K <1.0, i.e. when the operating state to combust a lean air-fuel mixture absorption of NO _x amount NOXA per unit time from the map shown in FIG. 13 (A) proceeds to step 206 is calculated. Then _the NO _x releasing amount NOXD step 207 is made zero, then the feedback correction coefficient FAF in step 211 is fixed to 1.0. Then step 21
2 the flag F is reset, 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 case, the routine proceeds to step 208, where K
It is determined whether or not> 1.0, that is, whether or not the operation state is such that the rich air-fuel mixture should be burned. K> 1.0 time, i.e. released amount of NO _x NOXD per unit time from the map shown in FIG. 14 (A) proceeds to step 209 when the operating state to combust a rich air-fuel mixture is calculated, Figure 1
From the relationship shown in FIG. 4 (B) and the map shown in FIG.
O _x emission rate Kf is calculated. Then absorption of NO _x amount NOXA per unit in step 210 the time is made zero.
Next, at step 211, the correction coefficient K is fixed at 1.0, and 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 Accordingly, at this time, the air-fuel ratio is controlled in an open loop to a predetermined lean air-fuel ratio or a rich air-fuel ratio. Then _the amount of NO _x ΣNOX being absorbed in the NO _x absorbent 18 on the basis of the following equation at step 223 is calculated. .SIGMA.NOX = .SIGMA.NOX + whether NOXA-Kf · NOXD then _the amount of NO _x .SIGMA.NOX step 224 becomes negative is discriminated, are .SIGMA.NOX is zero proceeds to step 225 when it is .SIGMA.NOX <0. On the other hand, if K> 1.0 is not satisfied in step 208, that is, K =
When a 1.0 to be the stoichiometric air-fuel ratio whether _the amount of NO _x ΣNOX proceeds to step 213 it is zero or not. 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. Next, in step 216, FIG.
_The NO _x releasing amount NOXD is calculated from the map shown in, _the NO _x releasing factor Kf is calculated from the map shown in relation to FIG. 14 (C) shown in FIG. 14 (B). Then step 220
In absorption of NO _x amount NOXA it is made zero. Next, at step 221, the flag F is set, and at step 22
Proceed 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 changed to the reference lip skip value R.
Is a SR _0, then the lean skip value RSL is the reference lean skip value RSL ₀ proceeds to step 218. Next, at step 219 NO _x emissions NOXD is made zero. Next, the routine proceeds to step 222 via steps 220 and 221. Therefore, after K = 1.0, ΣNO
Until X = 0, the rich skip value RSR is increased and the lean skip value RSL is decreased, so that the air-fuel ratio is controlled so that the average value of the air-fuel ratio becomes slightly richer than the stoichiometric 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 that the air-fuel ratio is feedback-controlled so that the average value of the air-fuel ratio becomes the stoichiometric air-fuel ratio.

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

On the other hand, when it is determined in step 201 that the supply of fuel has been stopped, the flow proceeds to step 226.
Willing absorption of NO _x amount NOXA to is made zero, then _the NO _x releasing amount NOXD proceeds to step 227 is made zero. Then is the K = 0 in step 228, then the routine proceeds to step 222 after the flag F is reset in step 229.

FIGS. 17 to 20 show a third embodiment. Second air-fuel ratio sensor 40 is provided in _the NO _x absorbent 18 in the downstream of the engine exhaust passage in this third embodiment, as shown in Figure 17. The second air-fuel ratio sensor 40 has the same output characteristics as the air-fuel ratio sensor 21 with respect to the air-fuel ratio, and the output signal of the second air-fuel ratio
Through the input port 35.

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

In the third embodiment, the output signal of the second air-fuel ratio sensor 40 is used to determine the NO _x from the NO _x absorbent 18.
It is determined whether or not the release operation has been completed. When the output voltage V of the second air-fuel ratio sensor 40 becomes higher than the constant value V ₀ as shown in FIG. The air-fuel ratio is feedback-controlled so as to become the stoichiometric air-fuel ratio. Thus, in this third embodiment is to detect the fact that _the NO _x releasing action from _the NO _x absorbent 18 has been completed from the change in output voltage of the second air-fuel ratio sensor 40,
Therefore, this will be described below.

That is, the mixture supplied to the combustion chamber 3 is recycled.
As shown in FIG.
O_TwoAnd exhaust gas containing unburned HC and CO
This oxygen O_TwoReacts almost with unburned HC and CO
And thus this oxygen O_TwoIs NO_xPass through the absorbent 18
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_xNO from absorbent 18 _xIs released. At this time exhaust
Unburned HC and CO contained in the gas are released NO_xTo
NO because it is used to reduce_xAbsorbent 18 to N
O_xIs released while NO is released_xNot yet from absorbent 18
Combustion HC and CO will not be emitted. Therefore NO_x
NO from absorbent 18_xNO while release is continued_x
The exhaust gas discharged from the absorbent 18 contains oxygen O 2_TwoContains
Although unburned HC and CO are not contained at all,
I mean no during this time_xExhaust gas discharged from absorbent 18
Has a slightly lean air-fuel ratio.

Next, when the action of releasing NO _x from the NO _x absorbent 18 is completed, the 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 into 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, the NO _x absorbent 1
When the release action of NO _x from NO. 8 is completed, NO _x absorbent 1
The exhaust gas discharged from 8 changes from lean to rich. Hence the output voltage V of the second air-fuel ratio sensor 40 that rises as _the NO _x releasing action from _the NO _x absorbent 18 is shown in Figure 18 when completed.

FIGS. 19 and 20 show a routine for executing the air-fuel ratio control shown in FIG. 18, and this routine is executed by interruption every predetermined time. Referring to FIG. 19 and FIG.
0, the basic fuel injection time TP from the map shown in FIG.
Is calculated. Next, at step 301, it is determined whether or not the fuel supply is stopped during the deceleration operation. Step 3 when the fuel supply is not stopped
In step 02, it is determined whether or not the idle switch 20 is on, that is, whether or not the throttle valve 14 is at the idling opening. If 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. Next, the routine proceeds to step 306.

On the other hand, when the idle switch 20 is turned on in step 302, the routine proceeds to step 304, where K = 1.0, and then proceeds to step 306.
When it is determined in step 301 that the supply of fuel has been stopped, the routine proceeds to step 305, where K =
It is set to 0, and then the process proceeds to step 306. Step 30
At 6, it is determined whether or not K = 1.0. K = 1.
If it is not 0, that is, if 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 processing for slightly increasing the average value of the air-fuel ratio is completed is reset. Next, the routine proceeds to step 308, where the feedback correction coefficient FAF is set to 1.0. Next, at step 309, the flag F is reset, and 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 controlled in an open loop 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 is to 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, the flag X
Has been reset, the process proceeds to step 311 and the second
Output voltage V is a constant value V ₀ which is the air-fuel ratio sensor 40 (FIG. 1
It is determined whether or not it becomes larger than 8). When K = 1.0 from the state in which the lean air-fuel mixture is being burned, V ≦ V ₀ , and the process proceeds to step 312.

[0057] subtraction result obtained by subtracting the constant value k ₅ from the reference minimum value TDR ₀ at step 312 is the minimum value TDR. That is, the absolute value of the minimum value TDR is equal to the reference minimum value TDR _0.
It is made to increase by a constant value k ₅ with respect to absolute values. Then the subtraction result obtained by subtracting the constant value k ₆ from the reference maximum value TDL ₀ at step 313 is 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.

If it is determined in step 311 that V> V ₀ , the flag X is set in step 314, and then the routine 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 . Next, the routine 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 becomes the average value of the air-fuel ratio. Is controlled 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 the air-fuel ratio 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]

NO while suppressing the torque change when the air-fuel ratio changes in order to release the NO _x from a lean air-fuel mixture, according to the present invention
From _x absorbent in a short period of time can complete the release of NO _x, the average value of the air-fuel ratio is switched to the stoichiometric air-fuel ratio when the addition is NO _x releasing action from the NO _x absorbent has been completed NO It is possible to prevent HC and CO from being released into the atmosphere after the completion of _x release.

[Brief description of the 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 shows unburned HC and C in exhaust gas discharged from the engine.
FIG. 3 is a diagram schematically showing the concentrations of O and oxygen.

FIG. 5 is a diagram for explaining a NO _x absorption / release effect.

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

FIG. 7 is a flowchart for performing feedback control.

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

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

14 is a diagram showing the _the NO _x releasing amount NOXD like.

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

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

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

FIG. 20 is a flowchart for executing a third embodiment of the air-fuel ratio control.

[Explanation of symbols]

16 ... exhaust pipe 18 ... NO _x absorbent

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI F01N 3/28 301 F01N 3/28 301C ZAB ZAB F02D 41/14 310 F02D 41/14 310C (58) Fields surveyed (Int.Cl. . ^7, DB name) F02D 41/04 305 F01N 3/08 F01N 3/18 F01N 3/24 F01N 3/28 F02D 41/14 310

Claims (1)

(57) [Claims] 1. A absorbs NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, the engine and _the NO _x absorbent air-fuel ratio of the exhaust gas to release the NO _x absorbed when the stoichiometric air-fuel ratio or rich flowing It is arranged in the exhaust passage, and when the lean mixture is combusted, NO _x in the exhaust gas
Arranged but is absorbed in _the NO _x absorbent, the air-fuel mixture or a rich air-fuel mixture NO _x is an internal combustion engine is caused to release from _the NO _x absorbent is when combusted in the theoretical air-fuel ratio, the air-fuel ratio sensor in the engine exhaust passage And the lean mixture burns
When the engine operation state in which the stoichiometric air-fuel ratio air-fuel mixture is to be burned from the restricted engine operation state, 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 as to comprise an air-fuel ratio control means for feedback controlling a spatial
The fuel ratio control means operates the engine to burn a stoichiometric mixture.
Is _the NO _x releasing action from _the NO _x absorbent after becoming the rolling state
When the air-fuel ratio is
The average value is slightly richer than the stoichiometric air-fuel ratio
From the air-fuel ratio feedback control, the average value of the air-fuel ratio
An exhaust gas purification device for an internal combustion engine that switches to feedback control that provides a fuel ratio .