JP2743751B2 - 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]

In the internal combustion engine which is adapted allowed to combust a lean air-fuel mixture, the air-fuel ratio of the inflowing exhaust gas is absorbed NO _x when the lean, the NO _x concentration of oxygen absorbed to decrease in the inflowing exhaust gas emission the _the NO _x absorbent arranged in the engine exhaust passage, the NO _x generated when burned lean mixture is absorbed by _the NO _x absorbent before the absorption of NO _x capacity of the NO _x absorbent is saturated to temporarily already proposed by the internal combustion engine is the applicant which is adapted to reduce the released NO _x with the release of NO _x in the rich from _the NO _x absorbent the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent Have been.

However, fuel and engine lubricating oil contain
Since the exhaust gas contains SO, SO_XContains
Therefore, in this internal combustion engine, this SO_XNO_xWhen
NO together_xAbsorbed by absorbent. However, this SO
_XIs NO_xRicher air-fuel ratio of exhaust gas flowing into the absorbent
NO_xNot released from the absorbent and therefore NO_xabsorption
SO in the agent_XWill gradually increase. Place
Is NO_xSO in absorbent_XNO when the amount of_xabsorption
NO that the agent can absorb_xAmount gradually decreases until finally N
O_xAbsorbent is NO_xCan hardly absorb
U. So NO _xSulfur in the engine exhaust passage upstream of the absorbent
A capture device is provided, and the exhaust gas is
SO contained in_XThe internal combustion engine designed to capture
It has already been proposed by the applicant (Japanese Patent Application No. Hei 4-2080).
No. 90). In this internal combustion engine, S
O_XIs captured by the sulfur capture device, so NO_xabsorption
NO in the agent_xOnly will be absorbed.

[0004]

However, in this internal combustion engine, SO _X trapped by the sulfur trap is not released from the sulfur trap even if the air-fuel ratio of the exhaust gas flowing into the sulfur trap is made rich. It continues to be captured in the capture device. Therefore, S
O _X trapped amount gradually increases, SO _X to the SO _X trap ability of the sulfur capture device is saturated SO _X ends up passed through the sulfur capture device is absorbed in _the NO _x absorbent NO
_x There is the problem of progressive accumulation in the absorbent.

[0005]

In order to solve the above problems, according to the present invention According to an aspect of the oxygen concentration in the exhaust gas air-fuel ratio of the exhaust gas flowing absorbs NO _x when a lean, flows Releases absorbed NO _x when reduced
with placing the _x absorbent engine exhaust passage, the air-fuel ratio of the inflowing exhaust gas absorbs SO _X when a lean air-fuel ratio of the inflowing exhaust gas to release SO _X absorbed and becomes rich When the lean mixture is burned by disposing the SO _X absorbent in the engine exhaust passage upstream of the NO _x absorbent, SO _X in the exhaust gas discharged into the engine exhaust passage is absorbed by the SO _X absorbent. the NO _x in the exhaust gas as well as absorbed in _the NO _x absorbent, S from SO _X absorbent
SO _X absorbing the degree of rich at this time with the O _X from _the NO _x absorbent with releasing the when releasing the NO _x is an air-fuel ratio of the exhaust gas flowing into the SO _X absorbent and _the NO _x absorbent rich The higher the temperature of the agent, the smaller it is.

[0006]

[Function] Exhaust when lean mixture is burned
SO in gas_XIs SO_XSO is absorbed by the absorbent_X
NO located downstream of the absorbent_xNO in the absorbent_xonly
Is absorbed. On the other hand, SO_XAbsorbent and NO_xAbsorbent
When the air-fuel ratio of the exhaust gas flowing into the_X
SO absorbed in the absorbent_XIs decomposed into SO _XRelease
Issued, NO_xNO from absorbent_xIs released. this
In case, SO_XSO absorbed in the absorbent_XDecomposition rate
Is SO_XThe higher the temperature of the absorbent, the faster it will
SO_XThe higher the temperature of the absorbent, the lower the degree of richness
Even SO _XAbsorbent to SO_XIs released. Therefore
The degree of the switch is SO_XThe smaller the temperature of the absorbent, the smaller
Is done.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG.
Ton, 3 is combustion chamber, 4 is spark plug, 5 is intake valve, 6 is intake
A port, 7 indicates an exhaust valve, and 8 indicates an exhaust port. Intake
Port 6 is connected to surge tank 10 via corresponding branch pipe 9
Each branch pipe 9 is connected to the fuel port toward the intake port 6.
A fuel injection valve 11 for injecting fuel is attached. Surger
The link 10 includes an intake duct 12 and an air flow meter 13.
Is connected to the air cleaner 14 through the
Inside, a throttle valve 15 is arranged. On the other hand, the exhaust port
8 through the exhaust manifold 16 and the exhaust pipe 17
O_XAbsorbent 18 and NO_xCase with built-in absorbent 19
Connected to Thing 20. SO_XAbsorbent 18 is NO_xSucking
It is located upstream of the sorbent 19 and in the embodiment shown in FIG.
Is SO _XAbsorbent 18 and NO_xWhen the absorbent 19 is, for example,
One piece of monolithic carrier made of Lumina
Is formed.

The electronic control unit 30 is composed of a digital computer, and is connected to a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and a power supply connected to each other by a bidirectional bus 31. Backup RAM connected
33a, an input port 35 and an output port 36 are provided. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and the output voltage is input to the input port 35 via the AD converter 37. A temperature sensor 21 for generating an output voltage proportional to the exhaust gas temperature is disposed in the exhaust pipe 17 upstream of the SO _X absorbent 18, and the output voltage of the temperature sensor 21 is supplied to an input port 35 via an AD converter 38. Is entered. 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 = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
2 is stored. 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, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and K> 1.0
, The air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The correction coefficient K is controlled according to the operating state of the engine. FIG. 3 shows an embodiment of the control of the correction coefficient K. In the embodiment shown in FIG. 3, during the warm-up operation, the correction coefficient K is gradually decreased as the engine cooling water temperature increases, and when the warm-up is completed, the correction coefficient K becomes a constant value smaller than 1.0, that is, the engine The air-fuel ratio of the air-fuel mixture supplied into the cylinder is maintained lean. Next, if an acceleration operation is performed, the correction coefficient K is set to, for example, 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio,
When the full load operation is performed, the correction coefficient K is made larger than 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich. As can be seen from FIG. 3, in the embodiment shown in FIG. 3, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is maintained at a constant lean air-fuel ratio except during warm-up operation, acceleration operation and full load operation. Therefore, the lean mixture is burned in most of the engine operating range.

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 concentrations of HC and CO in the exhaust gas discharged from the combustion chamber 3 increase as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer, and the exhaust gas is discharged from the combustion chamber 3. 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.

The NO _x contained in the casing 20
The absorbent 19 uses, for example, alumina as a carrier, and on the carrier, for example, an alkali metal such as potassium K, sodium Na, or cesium Cs, an alkaline earth such as barium Ba or calcium Ca, a rare earth such as lanthanum La or yttrium Y. And at least one noble metal such as platinum Pt. Preferably, lithium Li is added to the NO _x absorbent 19. NO of Toko to refer to the ratio of the engine intake passage and _the NO _x absorbent 19 the exhaust passage upstream of the supply air and fuel into the (hydrocarbon) and the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent 19
_x absorbent 19 absorbs the NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, perform absorption and release action of the NO _x concentration of oxygen in the inflowing exhaust gas to release NO _x absorbed to decrease. The air-fuel ratio of the inflowing exhaust gas when the _the NO _x absorbent 19 upstream of the exhaust passage fuel (hydrocarbons) or air is not supplied coincides with the air-fuel ratio of the mixture supplied into the combustion chamber 3, Thus absorbs NO _x when the air-fuel ratio of the mixture is _the NO _x absorbent 19 to be supplied into the combustion chamber 3 in this case is lean, lowering the oxygen concentration in the mixture fed into the combustion chamber 3 Then, the absorbed NO _x is released.

If the above-mentioned NO _x absorbent 19 is arranged in the engine exhaust passage, the NO _x absorbent 19 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, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
These oxygen O ₂ as shown in (A) is O ₂ ^- or O
It adheres to the surface of platinum Pt in the form of ^2- . On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is absorbed in the absorbent while being oxidized on the platinum Pt, and is absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. Diffuses into agent. In this way, NO _x is absorbed in the NO _x absorbent 19.

As long as the oxygen concentration in the incoming 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 19_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 19
From NO _xWill be released.

On the other hand, at this time, if 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 discharged, and these HC and CO are converted to 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 extremely decreases.
O ₂ is released, and this NO ₂ is reduced by reacting with HC and CO as shown in FIG. 5 (B). 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 19 in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich, that is released.

That is, the air-fuel ratio of the inflowing exhaust gas is made rich.
First of all, HC and CO first become O on platinum Pt._Two ^-or
Is O^2-And immediately oxidized, then platinum
O on Pt_Two ^-Or O^2-HC is consumed,
If CO remains, the absorbent will be absorbed by HC and CO.
NO released from_xAnd NO emitted from the engine _x
Is reduced. Therefore, the air-fuel ratio of the inflow exhaust gas is reset.
NO in a short time_xAbsorbed by absorbent 19
NO_xIs released, and the released NO
_xNO in the atmosphere due to reduction_xIs discharged
It can be blocked. NO_xAbsorbent
19 has the function of a reduction catalyst, so that
NO even if the air-fuel ratio is stoichiometric_xReleased from absorbent 19
NO_xIs reduced. However, inflow and exhaust
NO if the air-fuel ratio of the gas is the stoichiometric air-fuel ratio_xabsorption
NO from agent 19_xIs released only gradually
_xTotal NO absorbed in absorbent 19_xTo release
Takes a little longer.

By the way, as described above, the empty exhaust gas
If the degree of lean fuel ratio is reduced,
NO even if the air-fuel ratio is lean_xNO from absorbent 19_x
Is released. Therefore NO_xNO from absorbent 19_xRelease
To get it out, lower the oxygen concentration in the incoming exhaust gas.
It will be good. However, NO_xNO from absorbent 19 _x
If the air-fuel ratio of the inflow exhaust gas is lean even if
NO_xNO in absorbent 19_xIs not reduced and therefore
NO in this case_xNO downstream of the absorbent 19_xTo reduce
A catalyst or NO_xDownstream of the absorbent 19
It is necessary to supply a reducing agent. Of course NO like this_x
NO at the downstream of the absorbent 19_xIt is possible to reduce
But rather NO_xN in the absorbent 19
O_xIs preferably reduced. Therefore, the implementation according to the invention
NO in the example_xNO from absorbent 19 _xWhen to release
Indicates that the air-fuel ratio of the incoming exhaust gas is stoichiometric or rich.
And thereby NO_xNO released from absorbent 19
_xNO_xReduced in absorbent 19
You.

FIG. 3 shows an embodiment according to the present invention.
During warm-up operation and full load operation,
The supplied air-fuel mixture is enriched.
Aiki is assumed to be the stoichiometric air-fuel ratio, but most other operating conditions
In the region, the lean mixture burns in the combustion chamber 3
It is. In this case, it is burned in the combustion chamber 3
The air-fuel ratio of the mixture is approximately 18.0 or more and is shown in FIG.
In some embodiments, lean mixing with an air / fuel ratio of about 20 to 24
The chi is burned. When the air-fuel ratio exceeds 18.0
If the three-way catalyst is reducing at lean air-fuel ratios
NO _xCannot be fully reduced,
NO under such a lean air-fuel ratio_xTo reduce
Cannot use a three-way catalyst. The air-fuel ratio is 1
NO even if it is 8.0 or more_xC as a catalyst capable of reducing
There is a u-zeolite catalyst, but this Cu-zeolite catalyst
Uses this Cu-zeolite catalyst due to lack of heat resistance
It is not desirable as a matter of fact. So in the end,
NO when the air-fuel ratio is 18.0 or more_xBook to purify
NO used in the invention_xUse absorbent 19
There is no other way.

In the embodiment according to the present invention, as described above, the mixture supplied to the combustion chamber 3 is made rich during the full load operation, and the mixture becomes the stoichiometric air-fuel ratio during the acceleration operation. NO _x at the time and the acceleration operation
NO _x will be released from the absorbent 19. However while lean mixture is combusted as such full load operation or acceleration operation is NO _x from the NO _x absorbent 19 only at the time and acceleration operation full load operation the less frequently performed release _the NO _x absorbent 19 to
NO _x absorption capacity is saturated, and thus N
NO _x cannot be absorbed by the O _x absorbent 19. Thus when the lean air-fuel mixture is burned continuously air-fuel ratio periodically or rich of the inflowing exhaust gas or the air-fuel ratio of the inflowing exhaust gas is periodically to the stoichiometric air-fuel ratio _the NO _x absorbent 19 it is necessary to periodically release the NO _x from.

[0021] Incidentally, the exhaust gas contains SO _X, the _the NO _x absorbent 19 well NO _x SO _X
Is also absorbed. The mechanism of absorption of SO _X into the _the NO _x absorbent 19 is considered to be the same as the mechanism of absorption of NO _x. That is, the case where platinum Pt and barium Ba are carried on the carrier as in the case of the NO _x absorption mechanism will be described as an example. As described above, when the air-fuel ratio of the inflowing exhaust gas is lean, oxygen O ₂ Is O ₂
^- or O is deposited on the surface of the platinum Pt in ^2-form, SO ₂ in the inflowing exhaust gas on the surface of the platinum Pt O ₂ ^- or O ^2-
And SO ₃ . Next, a part of the generated SO ₃ is further oxidized on the platinum Pt, is absorbed in the absorbent and combines with the barium oxide BaO, and the sulfate ion SO 3
₄ Diffusion into the absorbent in the form of ^2- , stable sulfate BaSO
Generate ₄ .

However, the sulfate BaSO ₄ is stable and hard to be decomposed, and even if the air-fuel ratio of the inflowing exhaust gas is made rich, the sulfate BaSO ₄ remains without being decomposed. Thus will be sulfates BaSO ₄ increases as NO _x time to absorbent 19. elapses, that _the amount of NO _{x the} NO _x absorbent 19 can absorb as thus to time has elapsed is reduced Become.

[0023] Therefore, in this embodiment of the present invention absorbs SO _X when the air-fuel ratio of the exhaust gas flowing to not flow into the SO _X into _the NO _x absorbent 19 is lean, the air-fuel ratio of the exhaust gas flowing The SO _X absorbent 18 that releases the absorbed SO _X when it becomes rich is disposed upstream of the NO _x absorbent 19. This SO _X absorbent 18 is a SO _X absorbent 18
When the air-fuel ratio of the exhaust gas flowing into the air is lean, SO _X
At the same time, NO _x is absorbed, but when the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent 18 is made rich, not only the absorbed NO _x but also the absorbed SO _X is released.

As described above, the NO _x absorbent 19 contains SO
_{When X} is absorbed, stable sulfate BaSO ₄ is formed. As a result, even if the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 19 is made rich, SO _X is not released from the NO _x absorbent 19. Therefore, when the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent 18 is made rich, the SO _X absorbent 18
In order for SO _{X to} be released from
As present in the absorbent in the state where no stable sulfates BaSO ₄ as it is generated either, or sulfate BaSO ₄ to be present in the absorbent in the form of ^- O _X is sulfate ion SO ₄ ² It is necessary to As the SO _X absorbent 18 that enables this, iron Fe, manganese Mn, nickel Ni, tin Sn, titanium T
An absorbent carrying at least one selected from a transition metal such as i and lithium Li can be used. In this case, it has been found that an absorbent in which lithium Li is supported on a carrier made of alumina is most preferable.

In the SO _X absorbent 18, the SO _X absorbent 1
8 air-fuel ratio of the inflowing exhaust gas is absorbed in the SO ₂ contained in the exhaust gas in the absorbent in the surface is oxidized while sulfate ion SO ₄ ^2-in the form of the absorbent during the lean, then the absorbent Spread inside. In this case, SO ₂ when allowed to carry platinum Pt on the support of the SO _X absorbent 18 is liable to stick onto the platinum Pt in SO ₃ ^2-form, SO ₂ and thus
Is easily absorbed into the absorbent in the form of sulfate ions SO ₄ ^2- . Therefore, in order to promote the absorption of SO ₂ , it is preferable to carry platinum Pt on the carrier of the SO _X absorbent 18. As described above, when the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent 18 becomes lean, the SO _X becomes SO _X absorbent 18
Therefore, the NO _x absorbent 19 provided downstream of the SO _x absorbent 18 absorbs only NO _x .

On the other hand, as described above, SO_XFor absorbent 18
SO absorbed_XIs sulfate ion SO _Four ^2-In the form of an absorbent
Sulfate BaS is diffused into or unstable
O_FourIt has become. Therefore SO_XFlows into the absorbent 18
When the air-fuel ratio of the exhaust gas becomes rich, SO_XFor absorbent 18
SO absorbed_XIs SO_XReleased from absorbent 18
Will be. At this time, NO_xAbsorbent 19 to N
O_xIs released.

As described above, the NO _x absorbent 19
When NO ₂ on the platinum Pt surface is no longer present, the reaction immediately proceeds in the direction of (NO ₃ ⁻ → NO ₂ ), and NO _x is immediately released from the absorbent. When the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 19 is made rich, NO ₂ on the platinum Pt surface is immediately reduced by HC and CO, so that NO ₂ on the platinum Pt surface disappears immediately. As shown in FIG. 6, NO _x is released from the NO _x absorbent 19 in a short time. That is, the NO _x releasing speed of the NO _x absorbent 19 is considerably high.

[0028] In contrast SO _X SO _X absorbed in the absorbent 18 is absorbed in _the NO _x absorbent 19 NO
This SO is difficult to decompose because it is stable compared to _x.
Decomposition of _X does not occur unless the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent 18 rich. That is, if the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent 18 is made rich,
SO _X in the _X absorbent 18 is decomposed and released from the absorbent. The degradation rate is rather slow, a long time in comparison with the NO _x to be thus to release even SO _X by the air-fuel ratio to rich exhaust gas flowing into SO _X absorbent 18 as shown in FIG. 6 is completed Will be required. That is, the release rate of SO _X is considerably lower than the release rate of NO _x .

On the other hand, decomposition of SO _X absorbed in the inside SO _X absorbent 18 is dependent on the temperature of the SO _X absorbent 18, the temperature of the SO _X absorbent 18 becomes hard to break down as low. Therefore, unless the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent 18 becomes richer as the temperature of the SO _X absorbent 18 becomes lower, the SO _X will not be decomposed, and thus the SO _X absorbent 18 will not release SO _X. Will be. FIG. 7 shows SO
₂ shows the relationship between the air-fuel ratio A / F of the inflow exhaust gas from which the _X absorbent 18 can release SO _X and the temperature T of the SO _X absorbent 18. To release SO _X from FIG 7 it can be seen that should the air-fuel ratio of the exhaust gas flowing into the more SO _X absorbent 18 temperature T of the SO _X absorbent 18 is lowered rich.

In an embodiment according to the invention, NO _x and SO
_The mixture supplied to the combustion chamber 3 to release _X is periodically made rich, and FIG. 8 shows the timing at which the mixture is made rich. Note that in FIG. 8 P shows the timing of releasing the NO _x from the NO _x absorbent 19, Q is a timing for releasing SO _X from SO _X absorbent 18. Cycle of the air-fuel mixture rich quite short in order to release the NO _x from the NO _x absorbent 19 as can be seen from FIG. 8, the air-fuel mixture at a rate of once every few minutes is made rich. On the other hand, since the amount of SO _X contained in the exhaust gas is much smaller than the amount of NO _x , it takes a considerable time before the SO _X absorbent 18 is saturated with SO _X. Accordingly, the period for enriching the air-fuel mixture to release SO _X from the SO _X absorbent 18 is considerably long, for example, the air-fuel mixture is enriched once every several hours.

By the way, as shown in FIG.
NO when the air-fuel ratio of the air-fuel mixture supplied to the
_xIs NO in a short time_xReleased from absorbent 19
SO _XAbsorbent 18 to SO_XBy the time it is released
It takes time. Therefore SO_XMixed to release
No time to keep my mind enriched_xTo release
It is much longer than the time the mixture remains enriched. An example
For example, NO_xThe mixture is rich for several seconds when releasing
SO_XWhen the mixture is released
It is enriched for several minutes. Thus SO_XEmit
Sometimes the mixture is enriched for a long time,
SO_XThe mixture is enriched for release of
The fuel cycle is greatly increased due to the long cycle
I will not do it.

[0032] Figure 9 shows the rich control (P in FIG. 8) mixture when to release NO _x. Kt represents a correction coefficient for the basic fuel injection time TP. As shown in FIG. 9, when NO _x is to be released from the NO _x absorbent 19, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is increased by increasing the correction coefficient Kt to KK (> 1.0). made rich, then only C ₁ hour is maintained in the rich state. Next, the correction coefficient Kt is gradually decreased, and then the correction coefficient Kt is maintained at 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is maintained at the stoichiometric air-fuel ratio. Then again the correction coefficient Kt when passed C ₂ hours after the rich control is started 1.0 combustion of the lean air-fuel mixture is initiated again is smaller than.

The air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is
NO when rich (Kt = KK) _xAbsorbed by absorbent 19
Most of the NO_xIs released rapidly. Compensator
Value of number KK and time C₁At this time is O on platinum Pt_Two
^-Or O^2-Consumption and all NO_xNecessary to reduce
It is set to generate a large amount of HC and CO beforehand.
You. In this case, the exhaust gas temperature becomes high and NO_xAbsorbent 1
The higher the temperature of No. 9, the more NO_xReleased from absorbent 19
NO_xIncreases. Therefore, as shown in FIG.
Thus, the value of the correction coefficient KK increases as the exhaust gas temperature T increases.
The time C is increased as shown in FIG.₁
Is shortened as the exhaust gas temperature T increases. The figure
Relationship between correction coefficient KK shown in FIG. 10 (A) and exhaust gas temperature T
And time C shown in FIG.₁And the exhaust gas temperature T
The relationship is stored in the ROM 32 in advance.

On the other hand, when the air-fuel ratio of the mixture supplied to the combustion chamber 3 becomes rich (Kt = KK), N
Most of the NO _x absorbed by the O _x absorbent 19 is rapidly released, and thereafter, even if the air-fuel ratio is made rich, the N
NO _x is released little by little from the O _x absorbent 19. Therefore, if the air-fuel ratio is kept rich, HC, CO
Will be released to the atmosphere. Therefore, as shown in FIG. 9, after the air-fuel ratio is made rich (Kt = KK), the degree of richness is gradually reduced, and then the air-fuel ratio is maintained at the stoichiometric air-fuel ratio (Kt = 1.0) and NO _x so that allowed to successively reduce the NO _x released little by little from the absorbent 18.

When the air-fuel ratio is made rich, NO _x
The amount of the NO _x amount of the NO _x released from the absorbent 19 is released from the more then _the NO _x absorbent 19 often less, therefore _the NO _x absorbent 19 is shorter the time until you release the NO _x It becomes scary. The amount of the NO _x released to the air-fuel ratio as the exhaust gas temperature T becomes higher as described above from _the NO _x absorbent 19 when the rich is increased, thus figure 1
0 time C ₂ of the air-fuel ratio as shown in (C) from the rich to again return to lean is hidden short higher the exhaust gas temperature T becomes higher. Note that the time C shown in FIG.
The relationship between ₂ and the exhaust gas temperature T is stored in the ROM 32 in advance.

As described above, KK, C ₁ and C ₂ are controlled according to the exhaust gas temperature T. When the exhaust gas temperature T is high, the correction coefficient Kt changes in a pattern shown by a solid line in FIG. Is low, it changes in the pattern shown by the broken line in FIG. In this case, the air-fuel mixture hardly SO _X is released from the SO _X absorbent 18 to buy time for which a rich short, only _the NO _x releasing action from substantially _the NO _x absorbent 19 Done.

FIG. 11 shows the state when SO _X is released (FIG. 8).
Q) shows rich control of the air-fuel mixture. As shown in FIG. 11, even when SO _X is to be released from the SO _X absorbent 18, the correction coefficient Kt is increased to KK (> 1.0) so that the air-fuel mixture supplied to the combustion chamber 3 becomes empty. ratio is made rich, then only C ₁ hour is maintained in the rich state. Next, the correction coefficient Kt is gradually decreased, and then the correction coefficient Kt is maintained at Ko (> 1.0), that is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is kept rich. That is, when SO _X is to be released, first, the air-fuel mixture is greatly enriched up to the first rich degree (Kt = KK), and thereafter, the first rich degree (Kt = KK).
KK) is maintained at a second rich degree (Kt = Ko) smaller than KK). Then again the correction coefficient Kt when passed C ₂ hours after the rich control is started 1.0 combustion of the lean air-fuel mixture is initiated again is smaller than. Note that SO
Time C ₂ at the time of emitting _X is much longer than the time C ₂ when _the NO _x releasing shown in FIG. 9, for example, about 3 minutes to 10 minutes.

As described above, S from the SO _X absorbent 18
O _X release rate is rather slow, but not SO _X release rate becomes faster in proportion to it even if the air-fuel ratio of the mixture rich in greatly. In other words, making the air-fuel ratio of the air-fuel mixture very rich only unnecessarily increases the fuel consumption. Therefore, when SO _X is to be released, the air-fuel ratio of the air-fuel mixture is maintained at the lowest rich degree at which NO _x can be well released, and this lowest rich degree is indicated by Ko in FIG. Therefore, if the correction coefficient Kt is maintained at Ko, SO _X can be satisfactorily released from the SO _X absorbent 18. Nevertheless, when SO _X is to be released, the mixture is first made very rich (Kt = KK). Next, the reason will be described.

SO_XCorrection coefficient Kt when
SO when maintained at Ko_XSO 18 gradually from the absorbent 18_TwoBut
Released. At this time, NO_xNO from absorbent 19
_TwoIs released, but because the degree of richness is small, NO_xSucking
NO from collector 19_TwoIs gradually released. But here
NO like_xNO from absorbent 19_TwoIs gradually released
SO when_XSO released from absorbent 18_TwoBut
NO_xNO when flowing into the absorbent 19_TwoAnd SO_TwoAnd anti
Response (SO_Two+ NO_Two→ SO_Three+ NO), thus generated
SO_ThreeIs SO_Four ^-NO in the form of_xAbsorbed by absorbent 19
Will be done. Such a reaction is NO_TwoExists
Does not occur unless it is present and therefore NO_xSO in absorbent 19
_TwoTo prevent SO from being absorbed_XAbsorbent 18
To SO _TwoIs released when NO is released_xAbsorbent 19
From NO_TwoMust not be released. That
As shown in FIG._XWhen to release
First, the air-fuel ratio of the air-fuel mixture is very rich (Kt =
KK).

That is, the mixture is greatly rich (Kt = KK)
NO_xMost of NO from absorbent 19_TwoBut at once
Released, then NO_xAlmost NO from absorbent 19
_TwoIs not released. Therefore, the correction coefficient Kt subsequently becomes Ko
SO when maintained_XReleased from absorbent 18
SO_TwoIs NO_TwoWithout reacting with SO _Two
Is NO_xThe risk of being absorbed by the absorbent 19 is eliminated.

The air-fuel ratio of the mixture as described above is a rich (Kt = KK) to become the _the NO _x absorbent most absorbed in 19 NO _x sharply released, this time high exhaust gas temperature, Thus the amount of the NO _x which the temperature of _the NO _x absorbent 19 is released from the higher _the NO _x absorbent 19 is increased. Therefore, as shown in FIG.
Is increased as the exhaust gas temperature T increases, and the time C
₁ becomes shorter as the exhaust gas temperature T becomes higher. In addition,
The relationship between the correction coefficient KK and the exhaust gas temperature T shown in FIG. 12A and the relationship between the time C ₁ and the exhaust gas temperature T shown in FIG. 12B are stored in the ROM 32 in advance.

On the other hand, the air-fuel ratio of the air-fuel mixture is greatly rich (K
After t = KK, a relatively small rich degree (K
t = Ko), and at this time SO_XFrom absorbent 18
SO _XContinues to be released. At this time, as shown in FIG.
To SO_XThe temperature of the absorbent 18, that is, the exhaust gas temperature T becomes high.
The air-fuel ratio A / F of the air-fuel mixture_XRelease
You can keep it going. Therefore, in the embodiment according to the present invention,
Is that as the exhaust gas temperature T increases, the air-fuel ratio A / F of the air-fuel mixture increases.
Be reduced. That is, when the exhaust gas temperature T is high, FIG.
1, the value of Ko is relatively small as indicated by the solid line.
11 when the exhaust gas temperature T is low.
As shown by the line, the value of Ko is made relatively large. Figure
12 (C) shows the relationship between the value of Ko and the exhaust gas temperature T.
This relationship is stored in the ROM 32 in advance.
You.

When the air-fuel ratio of the air-fuel mixture is rich (Kt = K
The longer the amount of SO _X released from the SO _X absorbent 18 while maintaining the condition (o), the shorter the time until the SO _X absorbent 18 finishes releasing SO _X. As described above, the higher the exhaust gas temperature T, the higher the decomposition rate of SO _{X and} the higher the release rate of SO _X. Therefore, FIG.
As shown in ( _2), the time C2 from when the air-fuel ratio becomes rich to when it returns to lean again becomes shorter as the exhaust gas temperature T becomes higher. Note that the relationship between the time C ₂ and the exhaust gas temperature T shown in FIG. 12D is stored in the ROM 32 in advance. As shown in FIGS. 10 and 12, each value KK, C
_1, Ko, C ₂ is a function of exhaust gas temperature T, exhaust gas temperature T in the embodiment according to the present invention is detected by the temperature sensor 21. As described above, the exhaust gas temperature T can be directly detected, but can also be estimated from the intake air amount Q and the engine speed N. In this case, the relationship between the exhaust gas temperature T, the intake air amount Q, and the engine speed N is determined in advance by an experiment, and this relationship is determined in advance in the form of a map as shown in FIG.
The exhaust gas temperature T may be stored in the OM 32 and calculated from this map.

[0044] Next, an embodiment of the present invention, with reference to absorbing of the NO _x and SO _X by leaving control 16 from FIG. 14 will be described. 14 and 15 show a routine for calculating the correction coefficient KK at the time of the rich control, and this routine is executed by interruption every predetermined time. Referring to FIGS. 14 and 15, first, at step 50, it is determined whether or not the correction coefficient K is smaller than 1.0, that is, whether or not the lean air-fuel mixture is being burned. K <
When 1.0, i.e. _the amount of NO _x Wn that is absorbed in the NO _x absorbent 19 proceeds to step 51 is calculated when the lean air-fuel mixture is burned. That, NO _x amount exhausted from the combustion chamber 3 is the intake air amount Q increases as increases, the engine load Q / N NO _x amount Wn that is absorbed in the NO _x absorbent 19 so increases as increases the Wn
And k ₁ · Q · Q / N (k ₁ is a constant).

Next, at step 52, the SO _X absorbent 18
SO _X amount Ws being absorbed is calculated. That is, since the SO _X amount discharged from the combustion chamber 3 increases as the intake air amount Q increases, the SO _X amount Wn absorbed by the SO _X absorbent 18 is Wn and k ₂ · Q (k ₂ is a constant). And the sum of This SO _X amount Ws is stored in the backup RAM 33a. Then SO _X release flag indicating that should be released in SO _X step 53 whether it is set or not. SO _X release flag is whether _the NO _x releasing flag showing that should be released NO _x proceeds to step 54 is set when not been set or not. When the NO _x release flag has not been set, the routine proceeds to step 55.

The set amount Wso the amount of SO _x Ws absorbed in the SO _x absorbent 18 at step 55 is predetermined
It is determined whether or not it is greater than This set amount Wso is, for example, 3 of the maximum SO _x amount that the SO _x absorbent 18 can absorb.
It is about 0%. When Ws ≦ Wso, the process proceeds to step 61. Amount of NO _x Wn that is absorbed in the NO _x absorbent 19 at step 61 reaches a predetermined set amount Wn
It is determined whether it is greater than o. This set amount Wno
Is, for example, about 30% of the maximum NO _x amount that the NO _x absorbent 19 can absorb. When Wn ≦ Wno, the processing cycle is completed.

On the other hand, in step 55, Ws> Wso
When it is determined that the
_{The X} release flag is set. Next, at step 57, the correction coefficient KK is calculated from the relationship shown in FIG.
Then time C ₁ is calculated from the relationship shown in step 58 FIG. 12 (B). Next, at step 59, FIG.
The correction coefficient Ko is calculated from the relationship shown in (C), and then, in step 60, the time C ₂ is calculated from the relationship shown in FIG.
Is calculated. Then the processing cycle is completed. In addition,
The SO _X release flag is set, and each value KK, C ₁ , K
o, the air-fuel mixture is made rich so as to be described later and C ₂ is calculated.

On the other hand, in step 61, Wn> Wno
When it is determined that is
_{The X} release flag is set. Next, at step 63, the correction coefficient KK is calculated from the relationship shown in FIG.
Then time C ₁ is calculated from the relationship shown in step 64 FIG. 10 (B). Next, at step 65, the correction coefficient Ko is set to 1.0.
Time C ₂ is calculated from the relationship shown in (C). Then the processing cycle is completed. When the NO _X release flag is set and each value KK, C ₁ , Ko, C ₂ is calculated, the air-fuel mixture is made rich as described later.

The SO _X release flag or _the NO _x releasing flag when is set count value C proceeds from step 53 or step 54 to step 67 is incremented by 1. Next, at step 68, the count value C is changed to the time C.
_It is determined whether it is smaller than ₁ . When C <C ₁ to complete the processing cycle, thus during the time C ₁ is the correction coefficient is maintained at the KK. Then whether or not the count value C proceeds to step 69 becomes the C ≧ C ₁ is less than the time C ₂ is determined. C <Step 70 when the C ₂
Then, the constant value α is subtracted from the correction coefficient KK. Therefore, the value of the correction coefficient KK gradually decreases.

Next, at step 71, the correction coefficient KK becomes K
It is determined whether it has become smaller than o. KK> Ko
In the case of (1), the processing cycle is completed, and if KK ≦ Ko, the routine proceeds to step 72, where KK is made Ko. Therefore, KK =
After Ko, the correction coefficient becomes Ko if SO _{X is} released.
(> 1.0), and the correction coefficient is maintained at 1.0 when NO _{x is} released.

Next, when it is determined in step 69 that C ≧ C ₂ , the routine proceeds to step 73, where it is determined whether or not the SO _X release flag is set. SO
_{When the X} release flag is set, step 74
Then, the SO _X release flag is reset. When the SO _X release flag is reset, the combustion of the lean mixture is started again as described later. Next, at step 75, S
O _X absorbed in the absorbent 18 SO _X amount Ws is made zero, then _the amount of NO _x Wn that is absorbed in the NO _x absorbent 19 in step 77 is made zero. Next, the count value C is set to zero.

On the other hand, when it is determined in step 73 that the SO _X release flag has not been set, the routine proceeds to step 76, where the NO _X release flag is reset. N
O _X release flag is burned again lean air-fuel mixture, as will be described later is reset is initiated. Then step 77
Amount of NO _x W that is absorbed in the NO _x absorbent 19 in
n is set to zero. Next, the count value C is set to zero.

On the other hand, when it is determined in step 50 that K ≧ 1.0, that is, when the air-fuel ratio of the mixture supplied to the engine cylinder is the stoichiometric air-fuel ratio or rich, the routine proceeds to step 79, where K ≧ 1. It is determined whether the state of 0 has continued for a predetermined time t ₁ , for example, 10 seconds. K ≧ 1.0
When the state has not continued for a predetermined time t _1, the processing cycle is ended, when the state of K ≧ 1.0 is continued for a predetermined time t ₁ are Wn is zero proceeds to step 80. That is, if the time during which the air-fuel mixture supplied to the engine cylinder is kept at the stoichiometric air-fuel ratio or rich continues for about 10 seconds, NO
Most of the NO _x absorbed in the _x absorbent 19 is considered to have been released, and therefore, in this case, Wn is made zero in step 80.

Next, at step 81, it is determined whether or not the state of K> 1.0 has continued for a fixed time t ₂ (t ₂ > t ₁ ), for example, 10 minutes. The state of K> 1.0 is constant time t
_{2 If} not continued, the processing cycle is completed and K>
When the 1.0 state of constant time t ₂ continues being Ws is made zero proceeds to step 82. In other words, if the time during which the mixture supplied to the engine cylinder is enriched continues for about 10 minutes, it is considered that most of the SO _X absorbed by the SO _X absorbent 18 has been released. In this case, in step 82, Ws is set to zero.

FIG. 16 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed. Referring to FIG. 16, first, at step 90, the correction coefficient K is calculated. The correction coefficient K is set to, for example, 0.6 in an operation state in which the lean air-fuel mixture is to be burned.
Further, the correction coefficient K is a function of the engine cooling water temperature during the engine warm-up operation, and is reduced as the engine cooling water temperature increases in the range of K ≧ 1.0. During the acceleration operation, the correction coefficient is set to 1.0, and during the full load operation, the correction coefficient K is set to a value larger than 1.0.

Next, at step 91, the correction coefficient K is set to 1.
It is determined whether it is smaller than 0 or not. If K ≧ 1.0, the routine proceeds to step 95, where K is set to Kt. On the other hand, when K <1.0, the routine proceeds to step 92, where it is determined whether or not the SO _X release flag is set. SO
Step 9 when the _X release flag is not set
_The NO _x releasing flag proceeds to 3 whether it is set or not. When _the NO _x releasing flag has not been set, the routine proceeds to step 95. Next, at step 96, the basic fuel injection time TP is calculated from the map shown in FIG. 2, and then at step 97, the fuel injection time TAU (= T
P · Kt) is calculated. Therefore, when K ≧ 1.0, or even when K <1.0, the SO _X release flag and the NO _x
When the release flag is not set, the air-fuel ratio of the air-fuel mixture is set to an air-fuel ratio corresponding to the correction coefficient K.

On the other hand, when the SO _X release flag or the NO _x release flag is set, the routine proceeds from step 92 or step 93 to step 94, where Kt is set to KK calculated in the routine shown in FIGS. Next, at step 97 after step 96, the fuel injection time TAU is calculated. Therefore, at this time, the air-fuel ratio of the air-fuel mixture is forcibly made rich.

FIG. 17 shows another embodiment. In this embodiment, the same components as those in the embodiment shown in FIG. 1 are denoted by the same reference numerals. As shown in FIG. 17, in this embodiment, the exhaust manifold 16 is connected to the inlet of the casing 41 containing the SO _X absorbent 40, and the outlet of the casing 40 contains the NO _x absorbent 43 via the exhaust pipe 42. Casing 4
4 is connected to the entrance. Also in this embodiment, when the lean air-fuel mixture is combusted in the combustion chamber 3, SO _X is absorbed by the SO _X absorbent 40 and NO
NO _x is absorbed by the _x absorbent 43. On the other hand, when the mixture supplied to the combustion chamber 3 is enriched, the SO _X absorbent 4
0 SO _X is released from the _the NO _x absorbent 43 NO _x
Is released.

[0059]

According to the present invention be used for a long time _the NO _x absorbent NO _x
The high NO _x absorption capacity of the absorbent can be maintained.
Also, the degree of rich at this time is smaller as the temperature of the SO _X absorbent becomes higher when the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent to be released SO _X is rich. Therefore, the fuel consumption can be reduced by an amount corresponding to a reduction in the degree of richness.

[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 change in a correction coefficient K;

FIG. 4 HC and C in exhaust gas discharged from an 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 diagram showing the release rates of SO _X and NO _x .

FIG. 7 is a graph showing SO _X emission characteristics.

FIG. 8 is a diagram showing release timings of SO _X and NO _x .

FIG. 9 is a diagram illustrating rich control at the time of NO _x release.

FIG. 10 is a diagram showing a relationship between various parameters and exhaust gas temperature.

FIG. 11 is a diagram illustrating rich control during SO _X release.

FIG. 12 is a diagram showing the relationship between various parameters and exhaust gas temperature.

FIG. 13 is a diagram showing a map of exhaust gas temperature.

FIG. 14 is a flowchart for calculating a correction coefficient KK.

FIG. 15 is a flowchart for calculating a correction coefficient.

FIG. 16 is a flowchart for calculating a fuel injection time TAU.

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

[Explanation of symbols]

16 ... exhaust manifold 18, 40 ... SO _X absorbent 19,43 ... NO _x absorbent

Claims (1)

(57) [Claims] [Claim 1] absorbs NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, the engine exhaust to _the NO _x absorbent to release the NO _x absorbed to decrease the oxygen concentration in the exhaust gas flowing while disposed within the passage, to absorb SO _X when the air-fuel ratio of the inflowing exhaust gas is lean, the air-fuel ratio of the inflowing exhaust gas is absorbed and becomes rich SO _X
SO in the exhaust gas discharged into the engine exhaust passage when the lean air-fuel mixture to SO _X absorbent disposed in _the NO _x absorbent in the engine exhaust passage upstream of the are burned releasing
_X is absorbed by the SO _X absorbent and NO _x in the exhaust gas
It was absorbed in _the NO _x absorbent, the air-fuel ratio of the exhaust gas flowing into the SO _X absorbent and _the NO _x absorbent in when releasing the NO _x from the NO _x absorbent with releasing SO _X from SO _X absorbent An exhaust purification device for an internal combustion engine, wherein the device is made rich and the degree of the richness is made smaller as the temperature of the SO _X absorbent increases.