JP2852589B2 - 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]

The engine exhaust passage for purifying NO _x is branched into a pair of exhaust branch passages BACKGROUND ART In a diesel engine, the exhaust gas by a switching action of the switching valve by placing the switching valve in the branch portion of these exhaust branch passages which is a known one of the exhaust branch passages alternately guide yoke in the diesel engine is arranged a catalyst capable of oxidizing absorb respective NO _x in the exhaust branch passages (see JP-a-62-106826) . In this diesel engine, NO _x in the exhaust gas led into one exhaust branch passage is oxidized and absorbed by a catalyst disposed in the exhaust branch passage. During this time, the flow of exhaust gas into the other exhaust branch passage is stopped, and a gaseous reducing agent is supplied into the exhaust branch passage, and the reducing agent accumulates in the catalyst disposed in the exhaust branch passage. It has been that NO _x is made to reduction. Then, after a while, the switching action of the switching valve stops the introduction of the exhaust gas into the exhaust branch passage from which the exhaust gas has been guided, and exhausts the exhaust branch passage into which the introduction of the exhaust gas has been stopped. Gas introduction is resumed.

[0003]

The object of the invention is to be Solved However, such
Alternately stop introducing exhaust gas into a pair of exhaust branch passages
Then, in the exhaust branch passage where the introduction of exhaust gas was stopped,
The temperature of the catalyst gradually increases while the introduction of exhaust gas is stopped.
Low, significantly lower by the time exhaust gas introduction resumes
It will drop to the temperature. However, the temperature of the catalyst
When the temperature decreases, the catalytic function of the catalyst decreases, so NO _x
Is not sufficiently absorbed, and thus the exhaust gas
From the start of introduction until the catalyst temperature rises, N
Have the O _x from being released into the atmosphere without being absorbed in the catalyst
Problems arise. Also, in terms of absorbing NO _x
If you look at it, one of the catalysts is always idle, so
Effective use of the entire catalyst provided for NO _x absorption
There is also a problem that they do not. Also, a problem that a large amount of the NO _x in a short time when allowed to oxidation absorb all NO _x discharged from the way the engine to the catalyst must be increased frequency of supply of the reducing agent to be accumulated in the catalyst Is generated.

[0004]

According to the present invention, in order to solve the above-mentioned problems, NO _x is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and the oxygen concentration in the inflowing exhaust gas is reduced. Releases absorbed NO _x when reduced
_x Place the absorbent in the engine exhaust passage and
Allowed to flow through the exhaust gas to the medium at all times _the NO _x absorbent, NO in the presence of a hydrocarbon when the air-fuel ratio of the exhaust gas is lean
The the NO _x catalyst that selectively reduces _x arranged in the _the NO _x absorbent in the engine exhaust passage upstream of, when the air-fuel ratio of the exhaust gas flowing into the NO _x catalyst and _the NO _x absorbent is lean N
In O _x catalyst _the NO _x absorbent causes reduction of NO _x by absorbing the NO _x, releasing the NO _x from the NO _x absorbent
Acid in the exhaust gas flowing into _the NO _x absorbent when to be
The element concentration is reduced .

[0005]

[Action] NO_xCatalyst and NO_xExhaust flowing into the absorbent
NO when gas air-fuel ratio is lean_xN in the catalyst
O_xIs reduced, then NO_xNot reduced by catalyst
NO_xOnly NO_xAbsorbed by absorbent. Therefore N
O_xLarge amount of NO in absorbent _xFor a long time to be absorbed
It costs.

[0006]

FIG. 1 shows a case where the present invention is applied to a gasoline engine. Referring to FIG. 1, 1 indicates an engine body, 2 indicates a piston, 3 indicates a combustion chamber, 4 indicates a spark plug, 5 indicates an intake valve, 6 indicates an intake port, 7 indicates an exhaust valve, and 8 indicates an exhaust port.
The intake port 6 is connected to the surge tank 1 via the corresponding branch pipe 9.
The fuel injection valves 11 are connected to the branch pipes 0 and inject fuel into the intake ports 6 respectively. The surge tank 10 is connected to an air cleaner 14 via an intake duct 12 and an air flow meter 13, and a throttle valve 15 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 16 and an exhaust pipe 17 to a catalytic converter 19 having a built-in NO _x catalyst 18.
It is connected to a casing 22 containing an O _x absorbent 21.

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. 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. Also, input port 3
5 is connected to a rotation speed sensor 23 for generating 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 38, 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, a ROM
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 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 stored in casing 22_x
The absorbent 21 is made of, for example, alumina as a 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 21
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 21, this NO_x
The absorbent 21 is used when the air-fuel ratio of the inflowing exhaust gas is lean.
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 21
(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 21 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-mentioned NO _x absorbent 21 is disposed in the engine exhaust passage, the NO _x absorbent 21 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.
As shown in (A), these oxygens O ₂ adhere to the surface of platinum Pt in the form of O ₂ ⁻ . On the other hand, N
O is O ₂ on the surface of the platinum Pt ^- reacted with, and _{NO 2 (2NO + O 2 →} 2NO 2). NO ₂ generated
Some of the nitrate ions NO ₃ as shown in FIG. 5 (A) while bonding with the barium oxide BaO is absorbed into the absorbent while being oxidized on the platinum Pt ^- diffused in the absorbent in the form of. In this way, NO _x is absorbed in the NO _x absorbent 21.

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 21_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 21
From NO _xWill be released.

On the other hand, when the air-fuel mixture supplied into the combustion chamber 3 is made rich at this time and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG.
C and CO are discharged, and these unburned HC and CO are converted to platinum Pt.
It reacts with the above oxygen O ₂ ^- and is 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 21 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, unburned HC and CO are converted to O on platinum Pt._Two ^-When
Immediately reacted and oxidized, then on platinum Pt
O_Two ^-If unburned HC and CO still remain even if
N released from the absorbent by unburned HC and CO
O_xAnd NO emitted from the engine_xIs reduced
You. Therefore, if the air-fuel ratio of the inflow exhaust gas is made rich,
NO in between_xNO absorbed by absorbent 21_xBut
Released, and this released NO_xWas reduced
NO in the atmosphere_xCan be prevented from being discharged
Will be able to. NO_xAbsorbent 21 is used for reducing catalyst.
Stoichiometric air-fuel ratio of inflow exhaust gas
NO_xNO released from absorbent 21_xIs returned
I can be rejected. However, the air-fuel ratio of the incoming exhaust gas
NO if stoichiometric air-fuel ratio_xNO from absorbent 21_x
Is released only gradually_xAbsorbed by absorbent 21
All collected NO_xIt takes a little longer to release
It costs.

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 21_x
Is released. Therefore NO_xNO from absorbent 21_xRelease
To get it out, lower the oxygen concentration in the incoming exhaust gas.
It will be good. However, NO_xNO from absorbent 21 _x
If the air-fuel ratio of the inflow exhaust gas is lean even if
NO_xNO in absorbent 21_xIs not reduced and therefore
NO in this case_xNO downstream of the absorbent 21_xTo reduce
A catalyst or NO_xDownstream of the absorbent 21
It is necessary to supply a reducing agent. Of course NO like this_x
NO downstream of the absorbent 21_xIt is possible to reduce
But rather NO_xN in the absorbent 21
O_xIs preferably reduced. Therefore, the implementation according to the invention
NO in the example_xNO from absorbent 21 _xWhen to release
Is the stoichiometric or rich air-fuel ratio of the inflow exhaust gas
And thereby NO_xNO released from absorbent 21
_xNO_xReduced in absorbent 21
You.

As shown in FIG. 3, in the embodiment according to the present invention, the mixture supplied to the combustion chamber 3 is made rich during the full load operation, and the mixture is made the stoichiometric air-fuel ratio during the acceleration operation. NO _x is to be released from _the NO _x absorbent 21 at the time and acceleration operation full load.
However while lean mixture is combusted as such full load operation or acceleration operation is NO _x from the absorbent 21 NO _x only during full load operation and acceleration operation the less frequently performed release NO absorption capacity _{of x} by absorbent 21 NO _x is saturated, no longer able to absorb NO _x by _the NO _x absorbent 21 and thus the. Accordingly, in the embodiment according to the present invention, when the lean air-fuel mixture is continuously combusted, the air-fuel ratio of the inflowing exhaust gas is periodically made rich as shown in FIG. The air-fuel ratio of the inflowing exhaust gas is periodically set to the stoichiometric air-fuel ratio as shown in FIG. In this case, as shown in FIG. 6C, the degree of lean may be periodically reduced, but in this case, NO
must be reduced to NO _x downstream of the NO _x absorbent 21 as described above to NO _x is not reduced in the _x absorbent 21.

As described above, in the embodiment according to the present invention, NO _x
The mixture supplied to the combustion chamber 3 is periodically made to have a stoichiometric air-fuel ratio or rich in order to release NO from the NO _x absorbent 21. In this case, the larger the amount of NO _x absorbed into the NO _x absorbent 21 per unit time, the larger the NO _x
Absorption of NO _x capacity of the absorbent 21 becomes the time until saturation shorter, be shorter the period for the stoichiometric air-fuel ratio or rich air-fuel mixture in order to thus to emit _the NO _x absorbent 21 from the NO _x Must be done. However, the shorter the cycle for making the air-fuel mixture stoichiometric or rich, the worse the fuel consumption rate. Therefore, in the present invention as the air-fuel ratio of the exhaust gas flowing, as shown in FIG. 1 placing the NO _x catalyst 18 capable of reducing the NO _x in _the NO _x absorbent 21 upstream of the exhaust passage when the lean I have.

[0020] The the NO _x catalyst 18 in the embodiment shown in FIG. 1 comprises zeolitic the NO _x catalyst which was allowed supported on zeolite by ion exchange transition metals such as copper Cu, the the NO _x catalyst 18 an exhaust air-fuel ratio of the gas has a function to selectively reduce NO _x in the presence of hydrocarbons HC at the lean. That is, when the air-fuel mixture supplied into the combustion chamber 3 is made lean, the flame does not sufficiently propagate to the peripheral portion of the combustion chamber 3. Therefore, as shown in FIG. The amount of HC increases. The HC reacts with oxygen O ₂ on the NO _x catalyst 18 to generate an active species (HC + O ₂ → active species), the active species NO _x is made to reduction by reaction with NO _x (N
_Ox + active species → N ₂ + CO + CO ₂ ).

[0021] _the NO _x purification rate by this the NO _x catalyst 18 is up to reach the order of 60%, thus nearly half of of the NO _x exhausted from the engine will be able purified by the NO _x catalyst 18. Thus _the amount of NO _x absorbed by _the NO _x absorbent 21 is almost half of of the NO _x discharged from the NO amount of NO _x could not have clarified by _x catalyst 18, i.e. the engine, thus to absorption of NO _x The amount of NO _x absorbed by the agent 21 per unit time is greatly reduced. Improved as a result, NO _x absorbent 21 NO _x can the mixture fed into the combustion chamber 3 in order to release a longer period for the stoichiometric air-fuel ratio or rich from the thus to fuel consumption rate You can do it.

By the way, NO_xNO from absorbent 21_xof
The release effect is a certain amount of NO_xIs NO_xAbsorbed by absorbent 21
When it is, for example, NO_x50% of absorption capacity of absorbent 21
NO _xThis is done when absorbing water. NO_xFor absorbent 21
NO absorbed_xOf the exhaust gas emitted from the engine
Amount and NO in exhaust gas_xIs proportional to the concentration, in this case
The amount of exhaust gas is proportional to the amount of intake air._x
NO because concentration is proportional to engine load_xAbsorbed by absorbent 21
NO_xThe amount is exactly proportional to the intake air amount and the engine load
Will do. Therefore NO_xAbsorbed by the absorbent 21
NO_xIs the cumulative value of the product of the intake air amount and the engine load?
However, in the embodiment according to the present invention,
From the accumulated value of the engine speed_xAbsorbed by absorbent 21
NO_xI try to estimate the amount.

Next, an embodiment of control of the absorption and release of the NO _x absorbent 21 according to the present invention will be described with reference to FIG. 7 and FIG. FIG. 7 shows an interrupt routine executed at regular intervals. Referring to FIG. 7 , first, step 1
00, the correction coefficient K for the basic fuel injection time TP
Is smaller than 1.0, that is, whether the lean air-fuel mixture is combusted. If K <1.0, that is, if the lean air-fuel mixture is being burned, the routine proceeds to step 101, where the current engine speed NE is set to ΣNE
Is set to ΣNE. Therefore, ΣNE indicates the cumulative value of the engine speed NE. Next, at step 102, it is determined whether or not the cumulative rotation speed ΣNE is larger than a fixed value SNE. This constant value SNE shows a cumulative engine speed, for example, 50% of the amount of NO _x that absorption of NO _x capacity _the NO _x absorbent 21 is estimated to be absorbed. When ΣNE ≦ SNE, the processing cycle is completed,
When [sigma] NE> SNE, that is, _the NO _x absorbent 21 that N
O _x absorption capacity of 50% of the NO _x releasing flag proceeds to step 103 when the amount of NO _x is estimated to be absorbed in is set. Mixture fed into the engine cylinder, as will be described later is set to _the NO _x releasing flag is made to rich.

Next, at step 104, the count value C is incremented by one. Then whether step 105 the count value C is larger than the predetermined value C _0, whether elapsed i.e. for example 5 seconds is determined. C
When ≦ C ₀ , the processing routine is completed. When C> C ₀ , the routine proceeds to step 106, where the NO _x release flag is reset. Mixture of _the NO _x releasing flag is supplied into the engine cylinder, as will be described later is reset is switched from rich to lean, thus to the air-fuel mixture fed into the engine cylinder being in 5 seconds rich become. Next, at step 107, the cumulative rotation speed ΣNE and the count value C are made zero.

On the other hand, in step 100, K ≧ 1.0
When it is determined, that is, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is the stoichiometric air-fuel ratio or rich, the process proceeds to step 108 to determine whether the condition of K ≧ 1.0 has been maintained for a predetermined time, for example, 10 seconds. It is determined whether or not it is. K ≧ 1.0
If the state has not continued for a certain period of time, the processing cycle is completed. If the state of K ≧ 1.0 has continued for a certain period of time, the routine proceeds to step 109, where the cumulative rotation speed ΣNE is made zero. That, NO _x most of the air-fuel mixture fed into the engine cylinder is absorbed in _the NO _x absorbent 21 be continued stoichiometric or time being rich about 10 seconds considered to have released Therefore, in this case, the accumulated rotational speed ΣNE is made zero in step 109.

FIG. 8 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed. FIG.
First, at step 200, 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 operating state is such that the lean air-fuel mixture should be burned. When the operating state is not such that the lean mixture is to be burned, that is, during the warm-up operation, or during the acceleration operation or the full load operation, step 202 is executed.
Then, the correction coefficient K is calculated. When the engine is warmed up, the correction coefficient K is a function of the engine cooling water temperature, and K ≧ 1.
In the range of 0, the temperature decreases as the engine cooling water temperature increases. Further, during the acceleration operation, the correction coefficient K is set to 1.0, and at the time of full load operation, the correction coefficient K is set to a value larger than 1.0. Next, at step 203, the correction coefficient K is set to Kt, and then at step 204, the fuel injection time TAU
(= TP · Kt) is calculated. At this time, the air-fuel mixture supplied to the engine cylinder is set to the stoichiometric air-fuel ratio or rich.

On the other hand, whether or not _the NO _x releasing flag proceeds to step 205 when it is determined that the operating state to perform combustion of lean air-fuel mixture has been set or not in step 201. When the NO _x release flag is not set, the routine proceeds to step 206, where the correction coefficient K is set to, for example, 0.6. Then, after the correction coefficient K is set to Kt in step 207, the routine proceeds to step 204. Therefore, at this time, a lean mixture is supplied into the engine cylinder. On the other hand, when it is determined in step 205 that the NO _x release flag has been set, the routine proceeds to step 208, where a predetermined value KK is set to Kt, and then the routine proceeds to step 204. This value KK is such that the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is from 12.0 to 13.5.
The value is about 1.1 to 1.2. Therefore, at this time, a rich air-fuel mixture is supplied into the engine cylinder,
Thereby it is absorbed in the NO _x absorbent 21 NO _x
Will be released. In the case of the air-fuel mixture when _the NO _x releasing the stoichiometric air-fuel ratio is the value of KK is 1.0.

FIG. 9 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. 9, in this embodiment, the first exhaust passage 2 is provided at the outlet of the exhaust manifold 16.
4 and a second exhaust passage 25, and the first exhaust passage 2
Catalytic converter 19 having a built-zeolitic the NO _x catalyst 18 is disposed within 4. These exhaust passages 24, 25
Joins again downstream of the NO _x catalyst 18 and is connected to a casing 22 containing a NO _x absorbent 21. First
A first exhaust control valve 26 and a second exhaust control valve 27 are disposed at the inlets of the exhaust passage 24 and the second exhaust passage 25, respectively.
Are simultaneously controlled to open and close. This actuator 2
8 is controlled by an output signal of the electronic control unit 30.

In this embodiment, when the mixture supplied to the combustion chamber 3 is lean, the first exhaust control valve 26 is opened and the second exhaust control valve 27 is closed as shown in FIG. Can be Therefore, at this time, the exhaust gas is sent into the first exhaust passage 24. At this time, N in the exhaust gas
O _x is NO _x is made to reduction by catalytic 18, NO _x, which have not been reduced by the NO _x catalyst 18 is _the NO _x absorbent 2
Absorbed in 1. On the other hand, the first exhaust control valve 26 is made to closed when the air-fuel mixture fed into the combustion chamber 3 in order to release the NO _x from the NO _x absorbent 21 is set to the stoichiometric air-fuel ratio or rich, the second exhaust The control valve 27 is opened. Therefore, at this time, the exhaust gas is sent to the NO _x absorbent 21 through the second exhaust passage 25.

As described above, in the NO _x catalyst 18 , NO _x
HC is consumed in order to reduce Hb. Similarly, in the NO _x absorbent 21 , HC is consumed to quickly release NO _x from the absorbent, and the N _x released from the absorbent is also consumed.
HC is consumed to reduce O _x . Therefore NO _x
When NO _x is to be released from the absorbent 21, the exhaust gas is
If the HC is sent to the NO _x absorbent 21 after being sent to the O _x catalyst 18, the amount of HC sent to the NO _x absorbent 21 decreases because HC is consumed in the NO _x catalyst 18, and thus the NO _x in order to reduce from the absorbent 21 to rapidly release the NO _x will have to increase the generation amount of HC and air-fuel mixture fed into the combustion chamber 3 to the rich side. However, when the air-fuel mixture is made rich, the fuel consumption rate deteriorates. Therefore the second exhaust passage 2 on when releasing the NO _x from the NO _x absorbent 21 in this embodiment
Exhaust gas is sent to the NO _x absorbent 21 through the NO.

FIG. 10 shows the fuel injection time of the embodiment shown in FIG.
4 shows a TAU calculation routine. This routine repeats
The first exhaust control valve is executed within this routine.
Opening / closing control of 26 and the second exhaust control valve 27 is performed. What
In this embodiment, NO _xControl emission flags
For this purpose, an interrupt routine shown in FIG. 7 is used. Figure
Referring to FIG. 10, first, at step 300,
The basic fuel injection time TP is calculated from the map shown in FIG.
You. Next, at step 301, the lean mixture is burned.
It is determined whether or not the operating state is to be performed. Lean mixing
When it is not the operating state to burn air, that is, warm-up operation
During operation, or during acceleration operation or full load operation.
Proceeding to step 302, the correction coefficient K is calculated. Engine warm-up
When turning, this correction coefficient K is a function of the engine cooling water temperature,
It becomes smaller as the engine cooling water temperature becomes higher in the range of K ≧ 1.0.
Become. During acceleration operation, the correction coefficient K is 1.0.
In full load operation, the correction coefficient K is larger than 1.0.
Value. Next, at step 303, the correction coefficient K becomes K
t. Next, at step 304, the first exhaust control valve
26 is opened and the second exhaust control valve 27 is closed.
Can be Next, at step 305, the fuel injection time TA
U (= TP · Kt) is calculated. At this time,
If the mixture supplied to the cylinder is stoichiometric or rich
Is done.

On the other hand, whether _the NO _x releasing flag proceeds to step 306 is set is determined when it is determined that the operating state to perform combustion of lean air-fuel mixture in step 301. When the NO _x release flag is not set, the routine proceeds to step 307, where the correction coefficient K is set to, for example, 0.6. Then, after the correction coefficient K is set to Kt in step 308, the routine proceeds to step 309. In step 309, the first exhaust control valve 26 is opened, and the second exhaust control valve 27 is closed. Next, the routine proceeds to step 305. At this time, a lean mixture is supplied into the engine cylinder.

On the other hand, in step 306, NO_xrelease
If it is determined that the flag has been set, step 3
Proceeding to 10, the predetermined value KK is set as Kt. Next
In step 311, the first exhaust control valve 26 is closed.
Then, the second exhaust control valve 27 is opened. Next
Then, the process proceeds to step 305. Value K in step 310
K indicates that the air-fuel ratio of the mixture supplied to the engine cylinder is 1
2.0 to 13.5, which is about 1.1 to 1.2
Value. Therefore, at this time, the rich
An air-fuel mixture is supplied, whereby NO_xAbsorbed by absorbent 21
NO contained _xWill be released. Note that N
O_xWhen the air-fuel ratio is set to the stoichiometric air-fuel ratio during release, KK
The value is set to 1.0.

FIG. 11 shows a case where the present invention is applied to a diesel engine. In FIG. 11, the same components as those in FIG. 1 are indicated by the same reference numerals. Diesel engines usually have an excess air ratio of 1.0 or more in all operating conditions,
That is, the air-fuel mixture in the combustion chamber 3 is burned with the average air-fuel ratio being lean. Therefore, a part of the NO _x discharged at this time is reduced by the zeolite-based NO _x catalyst 18, and the remaining NO _x is absorbed by the NO _x absorbent 21. On the other hand, N
From O _x absorbent 21 when releasing the NO _x is, NO
The air-fuel ratio of the exhaust gas flowing into the _x absorbent 21 is made rich. In this case, absorption of NO _x by supplying the average air-fuel ratio of the hydrocarbon to leave this lean NO _x absorbent 21 in the engine exhaust passage upstream of the air-fuel mixture in the combustion chamber 3 in the embodiment shown in FIG. 11 The air-fuel ratio of the exhaust gas flowing into the agent 21 is made rich.

Referring to FIG. 11, in this embodiment, a load sensor 41 for generating an output voltage proportional to the amount of depression of an accelerator pedal 40 is provided, and the output voltage of the load sensor 41 is supplied via an AD converter 42 to an input port. 35 is input. Further, in this embodiment, a reducing agent supply valve 43 is arranged in an exhaust pipe 20 connecting the catalytic converter 19 and the casing 22, and the reducing agent supply valve 43 is connected to the supply pump 4
4 is connected to the reducing agent tank 45. The output ports 36 of the electronic control unit 30 are connected to a reducing agent supply valve 43 and a supply pump 44 via a drive circuit 38, respectively. Gasoline, isooctane,
It is filled with hydrocarbons such as hexane, heptane, light oil and kerosene, or hydrocarbons such as butane and propane which can be stored in a liquid state.

In this embodiment, the mixture in the normal combustion chamber 3 is
Under excessive air, that is, when the average air-fuel ratio is lean,
NO that has been burned and discharged from the engine at this time
_xIs partly NO as described above._xReturned by catalyst 18
And the rest is NO_xIs NO _xAbsorbed by the absorbent 21
You. NO_xNO from absorbent 21_xWhen to release
The supply pump 44 is driven and the reducing agent supply valve 43 is
The valve is opened, thereby filling the reductant tank 45.
The charged hydrocarbon is supplied from the reducing agent supply valve 43 to the exhaust pipe 2.
0 for a certain period of time, for example, 5 to 20 seconds
You. At this time, the supply amount of hydrocarbon is NO_xFor absorbent 21
The air-fuel ratio of the inflowing exhaust gas is set to be rich.
And therefore NO at this time_xFrom absorbent 21
NO_xWill be released.

FIG. 12 shows this NO_xPerform the release process
This routine is shown at regular intervals.
Is executed by the interrupt of Referring to FIG.
First, at step 400, the current engine speed NE is increased.
The result of adding ΣNE is set as ΣNE. Therefore this Σ
NE indicates the cumulative value of the engine speed NE. Then
In step 401, the cumulative rotational speed ΣNE is larger than the constant value SNE.
Is also determined. This constant value SNE is NO
_xNO in the absorbent 21_x50% of the absorption capacity N
O_xIndicates the cumulative speed at which
ing. Processing cycle is completed when ＥNE ≦ SNE
When ΣNE> SNE, that is, NO_xAbsorbent 21
NO_xNO at 50% of absorption capacity _xThe amount has been absorbed
If it is estimated, the process proceeds to step 402. Steps
At 402, the supply pump 44 is turned on for a certain time, for example, 5 seconds.
For about 20 seconds. Then in step 403
The reducing agent supply valve 43 is kept for a certain period of time, for example, 5 seconds to 20 seconds.
The valve is opened for a while, and then in step 404
The cumulative rotation speed ΣNE is set to zero.

[0038]

To institutions in operation at all times _the NO _x absorbent, according to the present invention the exhaust gas
The temperature of the NO _x absorbent is
High temperature can be maintained at all times, thus reducing exhaust gas
It is possible to absorb the NO _x in at all times _the NO _x absorbent
You. Further, NO _x amount partially absorbed per unit time in _the NO _x absorbent so is reduced by the NO _x catalyst of the NO _x generated when the lean air-fuel mixture is burned is reduced. As a result, if it is possible to extend the period for a cycle or a mixture supplied to the reducing agent in the stoichiometric air-fuel ratio or rich to release the NO _x from the NO _x absorbent, or not these periods long Since the supply amount of the reducing agent or the time for making the air-fuel mixture stoichiometric or rich can be shortened, the consumption amount of the reducing agent or the fuel consumption amount can be reduced.

[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 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 diagram showing timing for releasing NO _x from a NO _x absorbent.

FIG. 7 is a diagram showing an interrupt routine.

FIG. 8 is a flowchart for calculating a fuel injection time TAU.

FIG. 9 is an overall view showing another embodiment of the internal combustion engine.

FIG. 10 is a flowchart for calculating a fuel injection time TAU.

FIG. 11 is an overall view showing still another embodiment of the internal combustion engine.

FIG. 12 is a flowchart for performing a NO _x release process.

[Explanation of symbols]

16 ... exhaust manifold 18 ... NO _x catalyst 21 ... NO _x absorbent

Continued on the front page (72) Inventor Tetsu Iguchi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Kiyoshi Nakanishi 1 Toyota Town Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Invention Person Hideaki Muraki 41-1, Chuchu-ji, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory, Inc. (56) References JP-A-4-141218 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) F01N 3/08 F01N 3/10 F01N 3/24

Claims (1)

(57) [Claims] 1. A 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 Placed in the passage and always during engine operation
The exhaust gas to _the NO _x absorbent allowed to flow, the exhaust gas
Select the NO _x in the presence of a hydrocarbon when the air-fuel ratio is lean
The NO _x catalyst for reducing the択的 placed on the NO _x absorbent in the engine exhaust passage upstream of, NO _x catalysts and NO when the air-fuel ratio of the inflowing exhaust gas lean in _x absorbent NO _x catalyst in NO _x At the same time as reducing the NO _x
The O _x was absorbed, it should be released NO _x from the NO _x absorbent
Oxygen concentration in the exhaust gas sometimes flows into _the NO _x absorbent
The exhaust gas purifying apparatus for an internal combustion engine is designed to reduce the exhaust gas.