JP2881262B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purification device for an internal combustion engine.

BACKGROUND ART In order to purify NOx in a diesel engine, an engine exhaust passage is branched into a pair of exhaust branch passages, and a switching valve is disposed at a branch portion of these exhaust branch passages, and either one of the exhaust gases is switched by the switching action of the switching valve. A diesel engine is known which alternately guides the catalyst into the exhaust branch passages and arranges a catalyst capable of oxidizing and absorbing NOx in each exhaust branch passage (see JP-A-62-106826). In this diesel engine, NOx in the exhaust gas guided into one exhaust branch passage is oxidized and absorbed by a catalyst arranged 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. NOx that has been reduced is reduced. 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 the exhaust gas is introduced into the exhaust branch passage from which the introduction of the exhaust gas has been stopped. Installation is resumed.

By the way, in order to favorably absorb and oxidize NOx in such a catalyst, it is necessary to make the ratio of the alkali metal, alkaline earth or rare earth supported on the catalyst a certain amount or more, but these alkali metals, alkaline earths or rare earths are required. When the ratio of is increased, the basicity becomes stronger, and as a result, the reducing power of the catalyst becomes weaker. Therefore it is absorbed by the catalyst
When NOx is to be reduced, NOx cannot be reduced sufficiently, thus causing a problem that a large amount of NOx is released to the atmosphere without being reduced.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide an exhaust gas purification device for an internal combustion engine that prevents NOx discharged from the internal combustion engine from being released to the atmosphere.

According to the present invention, a NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas is reduced is placed in the engine exhaust passage. Alkali metal contained in the downstream NOx absorbent, the proportion of alkaline earth or rare earth contained in the upstream NOx absorbent contained alkali metal, alkaline earth or rare earth Exhaust that flows below the NOx absorbent into the NOx absorbent by lowering the NOx absorbent so that the reducing power of the NOx absorbent located downstream is stronger than the reducing power of the NOx absorbent located upstream. Provided is an exhaust gas purification device for an internal combustion engine, which is configured to release from an NOx absorbent when the oxygen concentration in gas is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall view of an internal combustion engine, FIG. 2 is an enlarged side sectional view showing a first embodiment of a NOx absorbent, and FIG. 3 is an enlarged side view showing a second embodiment of a NOx absorbent. FIG. 4 is a view showing a map of the basic fuel injection time, FIG. 5 is a view showing a change in the correction coefficient K, and FIG.
FIG. 7 is a diagram schematically showing the concentrations of HC, CO and oxygen, and FIG.
FIG. 8 is a diagram for explaining the absorption and release action of Ox, FIG. 8 is a diagram showing the timing of NOx release control, FIG. 9 is a flowchart showing an interruption routine, FIG. 10 is a flowchart for calculating a fuel injection time TAU, FIG. 11 is an overall view showing another embodiment of the internal combustion engine, and FIG. 12 is a flowchart for executing the NOx releasing process.

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1 shows a case where the present invention is applied to a gasoline engine.

Referring to FIG. 1, 1 is an engine body, 2 is a piston,
Reference numeral 3 denotes a combustion chamber, 4 denotes a spark plug, 5 denotes an intake valve, 6 denotes an intake port, 7 denotes an exhaust valve, and 8 denotes an exhaust port. 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 toward the intake port 6 is attached to each branch pipe 9. 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 is provided in the intake duct 12.
15 is arranged. On the other hand, the exhaust port 8 is an exhaust manifold.
It is connected to a casing 19 containing a NOx absorbent 18 via an exhaust pipe 16 and an exhaust pipe 17.

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, 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. Further, the input port 35 is connected to a rotation speed sensor 20 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 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. The basic fuel injection time TP is obtained in advance by an experiment, and is previously stored in the ROM 32 in the form of a map as shown in FIG. 4 as a function of the engine load Q / N (intake air amount Q / engine speed N) and the engine speed N. Is stored within. 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 has the stoichiometric air-fuel ratio. On the other hand, when K <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 supplied to the engine cylinder becomes Is smaller than the stoichiometric air-fuel ratio, that is, rich.

This correction coefficient K is controlled according to the operating state of the engine,
FIG. 5 shows an embodiment of the control of the correction coefficient K. In the embodiment shown in FIG. 5, 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 cylinder. The air-fuel ratio of the air-fuel mixture supplied to the inside is maintained lean. Next, if the 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, and if the full-load operation is performed, the correction coefficient K becomes 1.0. Be enlarged. 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. 5, in the embodiment shown in FIG. 5, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is constant 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. 6 schematically shows the concentrations of representative components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 6, unburned HC and CO in the exhaust gas discharged from the combustion chamber 3
Is increased as the air-fuel ratio of the mixture supplied to the combustion chamber 3 becomes richer, and the concentration of oxygen O ₂ in the exhaust gas discharged from the combustion chamber 3 is increased. Increases as the air-fuel ratio of the engine becomes leaner.

The NOx absorbent 18 accommodated in the casing 19 has, for example, alumina as a carrier, and on the carrier, for example, potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, barium Ba, magnesium Mg, calcium
At least one selected from alkaline earths such as Ca, rare earths such as lanthanum La, cerium Ce, neodymium Nd, and yttrium Y, and a noble metal such as platinum Pt are supported. The ratio of air and fuel (hydrocarbon) supplied to the engine intake passage and the exhaust passage upstream of the NOx absorbent 18
The air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18 is called N
Ox absorbent 18 is used when the air-fuel ratio of the inflowing exhaust gas is lean
When the concentration of oxygen in the inflowing exhaust gas is reduced, NOx is absorbed and released, which releases the absorbed NOx. In addition,
When the secondary fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx absorbent 18, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, the NOx absorbent 18 absorbs NOx, and when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases, It will release the absorbed NOx.

If the above-described NOx absorbent 18 is arranged in the engine exhaust passage, the NOx absorbent 18 actually performs the NOx absorbing / releasing action, but there is a portion where the detailed mechanism of the absorbing / releasing action is not clear. 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, and as shown in FIG. 7 (A), the oxygen O ₂ is converted into O ₂ ⁻ or O ²⁻ . Attaches to the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of 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 combined with barium oxide BaO to form nitrate ion NO ₃ ⁻ as shown in FIG. 7 (A). Diffuses into absorbent. In this way, NOx becomes NOx absorbent
Absorbed in 18.

As long as the oxygen concentration in the incoming exhaust gas is high,
As long as NO ₂ is produced and the NOx absorption capacity of the absorbent is not saturated
NO ₂ is absorbed in the absorbent and nitrate ions NO ₃ ^- are produced. On the other hand, the oxygen concentration in the
When the production amount of NO ₂ decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrate ion NO ₃ ⁻ in the absorbent is released from the absorbent in the form of NO ₂ . That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx absorbent 18. As shown in FIG. 6, when the degree of lean of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases,
Therefore, if the degree of lean of the inflow exhaust gas is reduced, even if the air-fuel ratio of the inflow exhaust gas is lean, the NOx absorbent 18
Will release NOx.

On the other hand, at this time, if the mixture supplied to the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, the sixth
As shown in the figure, a large amount of unburned HC and CO is emitted from the engine, and the unburned HC and CO react with oxygen O ₂ ^- or O ^2- on platinum Pt to be oxidized. Further, the air-fuel ratio of the inflowing exhaust gas is NO ₂ is released from the absorbent in the oxygen concentration in the inflowing exhaust gas becomes rich is extremely lowered, so that the NO ₂ is shown in Figure No. 7 (B) Unburned HC, C on platinum Pt
Reacts with O to be reduced. In this way, platinum Pt
When NO ₂ is no longer present on the surface of NO, the NO ₂ is released one after another from the absorbent. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 18 in a short time.

That is, first of all unburned HC and air-fuel ratio to the rich inflowing exhaust gas, O on CO platinum Pt ₂ ^- or O ^2- immediately be reacted with oxidized and then on the platinum Pt O ₂ ^- or If unburned HC and CO still remain even if O ^2- is consumed, this unburned HC and CO
NOx released from the absorbent and NOx exhausted from the engine are reduced. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed by the NOx absorbent 18 is released in a short time, and the released Nx
Ox will be reduced. Further, since the NOx absorbent 18 also has a function of a reduction catalyst, the NOx released from the NOx absorbent 18 is reduced even if the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, since NOx is released only gradually from the NOx absorbent 18, it takes a little longer time to release all the NOx absorbed by the NOx absorbent 18. It costs.

As described above, the NOx absorbent 18 absorbs NOx when the air-fuel ratio of the inflow exhaust gas is lean, and releases NOx and reduces the released NOx when the air-fuel ratio of the inflow exhaust gas becomes stoichiometric or rich. It has the function to do. However, the ability to absorb NOx and reduce the released NOx varies depending on the proportion of the alkali metal, alkaline earth or rare earth supported on the NOx absorbent 18.

That is, as described above, the carrier of the NOx absorbent 18 is, for example, potassium, sodium, lithium, alkali metal composed of cesium, barium, alkaline earth composed of calcium, lanthanum, at least one selected from rare earth composed of yttrium, Platinum is supported. Then, as described above, NOx is absorbed in the absorbent by diffusing into the absorbent while binding to these alkali metal, alkaline earth or rare earth oxides. Therefore, in order to absorb NOx into the absorbent well, it is necessary that at least one selected from the group consisting of alkali metals, alkaline earths and rare earths is supported on a carrier in a certain ratio or more. In this regard, when a monolithic carrier made of alumina is used as the carrier, NOx is contained if at least one selected from the above-mentioned alkali metals, alkaline earths and rare earths is supported in an amount of 0.03 mol or more per liter of the volume of the monolithic carrier. It is known that it can be absorbed well,
In order to ensure a better NOx absorption effect, at least one selected from the above-mentioned alkali metals, alkaline earths and rare earths is used per liter of volume of the monolithic carrier.
Preferably, 0.1 to 0.3 mol is supported.

However, when the proportion of these alkali metals, alkaline earths or rare earths is increased, the basicity becomes stronger, so NO
The reducing power of NOx at the time of reducing NOx released from the x absorbent 18 becomes weak. Therefore, as described above, if at least one ratio selected from the above-mentioned alkali metals, alkaline earths and rare earths is set to 0.03 mol or more per liter of the monolithic carrier in order to secure good absorption of NOx, NOx is released when released. The reduced NOx cannot be satisfactorily reduced.

In this case, if the ratio of at least one selected from the above-mentioned alkali metals, alkaline earths and rare earths is reduced to 0.01 mol or less per liter of the volume of the monolithic carrier, the NOx absorption capacity is reduced, but the NOx absorbent is reduced. 18 is greatly enhanced in reducing power. Further, the reducing power of the NOx absorbent 18 is strengthened even when these alkali metals, alkaline earths and rare earths are not contained at all. Therefore, in the embodiment according to the present invention, as shown in FIG. 2, the NOx absorbent 18 is formed from one monolithic carrier, and the monolithic carrier is directed from the upstream side to the downstream side in a plurality of regions a, b, c. , d, and the proportion of at least one selected from the group consisting of alkali metals, alkaline earths, and rare earths is reduced from region a to region d. That is, at least one ratio selected from the group consisting of alkali metals, alkaline earths and rare earths is 0.1 to 0.3 mol per liter of the volume of the monolithic carrier in the region a.
In the region d, the amount is 0.01 mol or less per liter of the volume of the monolithic carrier, and in the regions b and c, the ratio is intermediate therebetween. In the embodiment shown in FIG. 3, the NOx absorbent 18 is a pair of monolithic carriers A and B spaced from each other.
In the monolithic carrier A on the upstream side, at least one selected from alkali metals, alkaline earths and rare earths has a concentration of 0.03 mol per liter of the monolithic carrier.
l or less, and in the monolithic carrier B on the downstream side, it is 0.01 mol or less per liter of the volume of the monolithic carrier.

When the NOx absorbent 18 is formed as shown in FIGS. 2 and 3, the NOx absorbent 18 located on the upstream side has a high NOx absorption capacity, and the NOx absorbent 18 located on the downstream side has a strong reducing power. Therefore, in the embodiment shown in FIGS. 2 and 3, when the lean air-fuel mixture is combusted, NOx is mainly absorbed by the NOx absorbent 18 located on the upstream side. Then
When the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18 is made stoichiometric or rich to release NOx from the NOx absorbent 18, mainly NOx is released from the NOx absorbent 18 located on the upstream side,
The released NOx is mainly reduced by the NOx absorbent 18 located on the downstream side. Thus, when NOx is released from the NOx absorbent 18, NOx can be prevented from being released to the atmosphere.

By the way, as described above, if the degree of leanness of the air-fuel ratio of the inflowing exhaust gas is reduced, NOx is released from the NOx absorption 18 even if the air-fuel ratio of the inflowing exhaust gas is lean. Therefore
In order to release NOx from the NOx absorbent 18, the oxygen concentration in the inflowing exhaust gas needs to be reduced. However, NOx
Even if NOx is released from the absorbent 18, if the air-fuel ratio of the inflowing exhaust gas is lean, NOx is not reduced in the NOx absorbent 18, and in this case, a catalyst capable of reducing NOx downstream of the NOx absorbent 18 Or a reducing agent needs to be supplied downstream of the NOx absorbent 18. Of course, it is possible to reduce NOx downstream of the NOx absorbent 18, but it is more preferable to reduce NOx in the NOx absorbent 18. Therefore, in the embodiment according to the present invention, the NOx absorbent 1
When NOx is to be released from 8, the air-fuel ratio of the inflowing exhaust gas is made stoichiometric or rich, whereby NOx released from the NOx absorbent 18 is reduced in the NOx absorbent 18.

By the 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. NOx absorbent 18 to N during operation
Ox will be released. However, if such a full-load operation or an acceleration operation is performed less frequently, the NOx absorbent 18 only releases the NOx during the full-load operation and during the acceleration operation.
Even if the NOx is released, the NOx absorption capacity of the NOx absorbent 18 is saturated while the lean air-fuel mixture is being burned, so that the NOx absorbent 18 cannot absorb NOx. 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. As shown in (B), the air-fuel ratio of the inflowing exhaust gas is periodically set to the stoichiometric air-fuel ratio.

By the way, the release effect of NOx from the NOx absorbent 18 is a certain amount.
When NOx is absorbed by the NOx absorbent 18, for example, NOx absorbent 1
It is performed when 50% NOx of the absorption capacity of 8 is absorbed. NOx
The amount of NOx absorbed by the absorbent 18 is proportional to the amount of exhaust gas discharged from the engine and the NOx concentration in the exhaust gas.In this case, the amount of exhaust gas is proportional to the amount of intake air, and
Since the NOx concentration is proportional to the engine load, the NOx amount absorbed by the NOx absorbent 18 is accurately proportional to the intake air amount and the engine load. Therefore, the NOx absorbed in the NOx absorbent 18
Can be estimated from the cumulative value of the product of the intake air amount and the engine load, but in the embodiment according to the present invention, the amount of NOx absorbed in the NOx absorbent 18 is simply estimated from the cumulative value of the engine speed. I am trying to do it.

Next, with reference to FIG. 9 and FIG.
An embodiment of controlling the absorption and release of the absorbent 18 will be described.

FIG. 9 shows an interrupt routine executed at regular intervals.

Referring to FIG. 9, first, at step 100, it is determined whether or not the correction coefficient K for the basic fuel injection time TP is smaller than 1.0, that is, whether or not the lean mixture is burned. When K <1.0, that is, when the lean air-fuel mixture is being burned, the routine proceeds to step 101, where the result of adding ΣNE to the current engine speed NE is set as ΣNE. Therefore, ΔNE indicates the cumulative value of the engine speed NE. Next, at step 102, it is determined whether or not the cumulative rotational speed ΣNE is larger than a constant value SNE. This constant value SNE
Indicates the cumulative rotational speed at which it is estimated that the NOx absorbent 18 has absorbed, for example, an NOx amount of 50% of its NOx absorption capacity. When ΣNE ≦ SNE, the processing cycle is completed, and ΣNE> S
In the case of NE, that is, the NOx absorbent 18 has 50% of its NOx absorption capacity
When it is estimated that the NOx amount is absorbed, the routine proceeds to step 103, where the NOx release flag is set. When the NOx release flag is set, the mixture supplied to the engine cylinder is made rich as described later.

Next, at step 104, the count value C is incremented by one. Next, at step 105, the count value C
Is larger than a constant value Co, that is, for example, whether 5 seconds have elapsed. When C ≦ Co, the processing routine is completed, and when C> Co, the routine proceeds to step 106, where the NOx release flag is relatched. When the NOx release flag is reset, the air-fuel mixture supplied into the engine cylinder is switched from rich to lean as described later, and thus the air-fuel mixture supplied into the engine cylinder is made rich for 5 seconds. Become. Next, at step 107, the cumulative rotation speed ΣNE and the count value C are made zero.

On the other hand, when it is determined in step 100 that K ≧ 1.0, that is, when the air-heat 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 and the state of K ≧ 1.0 is constant. It is determined whether or not the time has continued, for example, 10 seconds. If the state of K ≧ 1.0 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 is, it is considered that most of the NOx absorbed in the NOx absorbent 18 has been released if the time during which the mixture supplied to the engine cylinder is set to the stoichiometric air-fuel ratio or rich continues for about 10 seconds, Therefore, in this case, in step 109, the cumulative rotational speed 零 NE is set to zero.

FIG. 10 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed.

Referring to FIG. 10, 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, the acceleration operation, or the full load operation, the process proceeds to step 202, where the correction coefficient K is calculated. During the warm-up operation of the engine, the correction coefficient K is a function of the temperature of the engine cooling water.
It becomes smaller as the engine cooling water temperature becomes higher in the range of ≧ 1.0. During acceleration operation, the correction coefficient K is set to 1.0, and during 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, when it is determined in step 201 that the operating state is such that the lean air-fuel mixture should be burned, step 20 is executed.
Proceeding to 5, it is determined whether the NOx release flag is set. When the NOx release flag is not set, the routine proceeds to step 206, where the correction coefficient K is set to, for example, 0.6.
Next, after the correction coefficient K is set to Kt in step 207, the process proceeds to step 204. Accordingly, at this time, a lean air-fuel mixture is supplied into the engine cylinder. Step 205
When it is determined that the NOx release flag has been set in step, the routine proceeds to step 208, where the predetermined value KK is set to Kt, and then the routine proceeds to step 204. This value KK is a value of about 1.1 to 1.2 at which the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes about 12.0 to 13.5. Therefore, at this time, the rich air-fuel mixture is supplied into the engine cylinder, whereby the NOx absorbed in the NOx absorbent 18 is released. When the air-fuel mixture is set to the stoichiometric air-fuel ratio at the time of NOx release, the value of KK 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.

Normally, a diesel engine is burned in any operating state with an excess air ratio of 1.0 or more, that is, a lean air-fuel ratio of the air-fuel mixture in the combustion chamber 3. Therefore, the NOx discharged at this time is absorbed by the NOx absorbent 18. On the other hand, NO
When NOx should be released from x absorbent 18, NOx absorbent 18
The air-fuel ratio of the exhaust gas flowing into the air is made rich. In this case, in the embodiment shown in FIG. 11, the average air-fuel ratio of the air-fuel mixture in the combustion chamber 3 is kept lean and hydrocarbons are supplied into the engine exhaust passage upstream of the NOx absorbent 18 so that the NOx absorbent The air-fuel ratio of the exhaust gas flowing into 18 is made rich.

Referring to FIG. 11, in this embodiment, the accelerator pedal 21
Sensor that generates an output voltage proportional to the amount of depression
The output voltage of the load sensor 22 is input to an input port 35 via an AD converter. In this embodiment, a reducing agent supply valve 23 is disposed in the exhaust pipe 17, and the reducing agent supply valve 23 is connected to a reducing agent tank 25 via a supply pump 24. The output ports 36 of the electronic control unit 30 are connected to the reducing agent supply valve 23 and the supply pump 24 via a drive circuit 38, respectively. The reducing agent tank 25 is filled with hydrocarbons such as gasoline, isooctane, 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 air-fuel mixture in the combustion chamber 3 is normally burned under an excess of air, that is, in a state in which the average air-fuel ratio is lean. At this time, the NOx discharged from the engine is reduced by the NOx absorbent.
Absorbed by 18. When NOx is to be released from the NOx absorbent 18, the supply pump 24 is driven and the reducing agent supply valve 23 is opened, whereby the hydrocarbons filled in the reducing agent tank 25 are discharged from the reducing agent supply valve 23. The gas is supplied to the exhaust pipe 17 for a predetermined time, for example, about 5 to 20 seconds. The supply amount of hydrocarbons at this time is determined so that the air-fuel ratio of the inflow exhaust gas flowing into the NOx absorbent 18 becomes rich,
Therefore, at this time, NOx is released from the NOx absorbent 18.

FIG. 12 shows a routine for executing the NOx releasing process, and this routine is executed by interruption every predetermined time.

Referring to FIG. 12, first, in step 300, the result of adding ΣNE to the current engine speed EN is set as ΣNE. Therefore, ΔNE indicates the cumulative value of the engine speed NE. Next, at step 301, the cumulative rotational speed ΣNE is a constant value SNE.
It is determined whether or not it is greater than This constant value SNE is NOx
The figure shows the cumulative number of revolutions at which it is estimated that the NOx amount of, for example, 50% of the NOx absorption capacity of the absorbent 18 is absorbed. ΣNE
When ≦ SNE, the processing cycle is completed, and when ΣNE> SNE, that is, when it is estimated that the NOx absorbent 18 has absorbed 50% of the NOx amount of its NOx absorption capacity, the process proceeds to step 302. In step 302, the supply pump 24 is driven for a predetermined time, for example, about 5 to 20 seconds. Then step 30
In 3, the reducing agent supply valve 23 is kept for a certain period of time,
The valve is opened for about a second, and then at step 304, the cumulative rotation speed ΣNE is made zero.

As described above, according to the present invention, when NOx is to be absorbed by the NOx absorbent, NOx can be satisfactorily absorbed, and when NOx absorbed by the NOx absorbent should be released, the released NOx can be reduced well. it can.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kiyoshi Nakanishi 455-11 Tomizawa, Susono City, Shizuoka Prefecture (72) Inventor Tetsu Iguchi 629-11 Tokukura, Mishima City, Shizuoka Prefecture (72) Inventor Tetsuro Kihara 527 Imazato, Susono City, Shizuoka Prefecture (72) Inventor Hideaki Muraki 3-2303 Umegaoka, Tenpaku-ku, Nagoya City, Aichi Prefecture (56) References JP-A-4-141219 (JP, A) JP-A-3-118820 (JP, A) (58) Fields investigated (Int.Cl. ⁶ , DB name) B01D 53/34 B01D 53/56 F01N 3/28

Claims (28)

(57) [Claims] An NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas is reduced, is placed in the engine exhaust passage. The proportion of the alkali metal, alkaline earth or rare earth contained in the NOx absorbent located on the downstream side is determined by the proportion of the alkali metal, alkaline earth or rare earth contained in the NOx absorbent located on the upstream side. Lowering the reducing power of the NOx absorbent located on the downstream side to N located on the upstream side.
An internal combustion engine that is stronger than the reducing power of the Ox absorbent and releases the NOx absorbed by the NOx absorbent from the NOx absorbent when the oxygen concentration in the exhaust gas flowing into the NOx absorbent is reduced. Exhaust purification equipment. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said alkali metal comprises potassium, sodium, lithium and cesium. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said alkaline earth comprises barium, magnesium and calcium. 4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said rare earth element comprises lanthanum, cerium, neodymium and yttrium. 5. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said NOx absorbent contains platinum. 6. The NOx absorbent carries the alkali metal, alkaline earth or rare earth on a carrier, and contains the alkali metal, alkaline earth or rare earth contained in the upstream NOx absorbent. 2. The carrier according to claim 1, wherein the amount of the alkali metal, the alkaline earth or the rare earth contained in the downstream NOx absorbent is not more than 0.01 mol per liter of the carrier. Exhaust purification device for internal combustion engine. 7. The exhaust gas of an internal combustion engine according to claim 6, wherein the amount of alkali metal, alkaline earth or rare earth contained in the upstream NOx absorbent is 0.1 mol to 0.3 mol per liter of the volume of the carrier. Purification device. 8. The exhaust gas purifying apparatus for an internal combustion engine according to claim 6, wherein the carrier of the NOx absorbent comprises one monolith type carrier. 9. The NOx absorbent located at the most downstream side, wherein the alkali metal, alkaline earth or rare earth contained in the NOx absorbent portion located at the most upstream side is at least 0.03 mol per liter of the volume of the carrier, and the NOx absorbent located at the most downstream side is The alkali metal, alkaline earth or rare earth contained in the portion is 0.01 mol or less per 1 liter of the volume of the carrier, and is directed from the most upstream NOx absorbent portion to the most downstream NOx absorbent portion. N
The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein the proportion of the alkali metal, alkaline earth or rare earth contained in the Ox absorbent is gradually reduced. 10. An exhaust gas purifying apparatus for an internal combustion engine according to claim 6, wherein said NOx absorbent carrier comprises two monolithic carriers, a monolithic carrier located upstream and a monolithic carrier located downstream. . 11. The exhaust gas purifying apparatus for an internal combustion engine according to claim 6, wherein the carrier of the NOx absorbent is contained in one casing. 12. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas constantly flows through the NOx absorbent during operation of the engine. 13. The NOx absorbed in the NOx absorbent is reduced by enriching the exhaust gas flowing into the NOx absorbent.
The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas is released from the x absorbent. 14. The internal combustion engine according to claim 1, wherein the NOx absorbed by the NOx absorbent is released from the NOx absorbent by setting the exhaust gas flowing into the NOx absorbent to substantially the stoichiometric air-fuel ratio. Exhaust gas purification device. 15. An air-fuel ratio control means for controlling an air-fuel ratio of an air-fuel mixture formed in an engine combustion chamber, wherein the air-fuel ratio of the air-fuel mixture formed in the engine combustion chamber is controlled by the air-fuel ratio control means. NOx absorption into NOx absorbent and NOx by
2. The method according to claim 1, wherein the emission of NOx from the absorbent is controlled.
An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. 16. The air-fuel ratio control means makes the air-fuel ratio of the air-fuel mixture formed in the combustion chamber lean when NOx is to be absorbed by the NOx absorbent, and forms the air-fuel ratio in the combustion chamber when NOx is to be released from the NOx absorbent. 16. The exhaust gas purifying apparatus for an internal combustion engine according to claim 15, wherein the air-fuel ratio of the air-fuel mixture is set to a stoichiometric air-fuel ratio or rich. 17. An internal combustion engine comprising a gasoline engine, wherein said air-fuel ratio control means controls the amount of fuel supplied to the engine to obtain a NO.
17. The exhaust gas purification device for an internal combustion engine according to claim 16, wherein the exhaust gas purification device controls the absorption of NOx into the x absorbent and the release of NOx from the NOx absorbent. 18. An air-fuel ratio control means for controlling an air-fuel ratio of exhaust gas discharged from an engine combustion chamber and flowing into a NOx absorbent in an engine exhaust passage.
By controlling the air-fuel ratio of the exhaust gas flowing into the absorbent, the absorption of NOx into and from the NOx absorbent
2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas is controlled. 19. The air-fuel ratio control means sets the air-fuel ratio of exhaust gas flowing into the NOx absorbent to lean when NOx is to be absorbed by the NOx absorbent, and switches to NOx absorbent when NOx is to be released from the NOx absorbent. 19. The exhaust gas purifying apparatus for an internal combustion engine according to claim 18, wherein the air-fuel ratio of the inflowing exhaust gas is set to a stoichiometric air-fuel ratio or rich. 20. The air-fuel ratio control means converts NOx absorbent to NOx
20. The exhaust gas purifying apparatus for an internal combustion engine according to claim 19, wherein a reducing agent is supplied into the engine exhaust passage when the exhaust gas is to be discharged. 21. The method according to claim 20, wherein the reducing agent comprises a hydrocarbon.
An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. 22. The exhaust gas purifying apparatus for an internal combustion engine according to claim 21, wherein the hydrocarbon is at least one selected from gasoline, isooctane, hexane, heptane, butane, propane, light oil, and kerosene. 23. When the period during which the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made lean and NOx is absorbed by the NOx absorbent exceeds a predetermined first set period.
2. The internal combustion engine according to claim 1, further comprising NOx release control means for reducing the oxygen concentration in the exhaust gas flowing into the NOx absorbent for a second predetermined period for releasing NOx from the NOx absorbent. Exhaust gas purification device. 24. The NOx release control means converts NOx absorbent to NOx.
24. The exhaust gas purification device for an internal combustion engine according to claim 23, wherein the air-fuel ratio of the exhaust gas flowing into the NOx absorbent when x is to be released is set to the stoichiometric air-fuel ratio or rich. 25. The NOx release control means includes NOx amount estimating means for estimating the NOx amount absorbed by the NOx absorbent.
24. The exhaust gas purification system according to claim 23, wherein the release control unit determines that the first set period has elapsed when the NOx amount estimated by the NOx amount estimation unit exceeds a predetermined set amount. apparatus. 26. The NOx amount estimating means determines that the NOx amount absorbed by the NOx absorbent exceeds the set amount when the cumulative value of the engine speed exceeds a predetermined set value. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. 27. When the air-fuel ratio of the air-fuel mixture formed in the engine combustion chamber is maintained at a stoichiometric air-fuel ratio or rich for a certain period of time or more, substantially all of the NOx absorbed by the NOx absorbent is reduced. 26. The exhaust gas purification device for an internal combustion engine according to claim 25, wherein the device is determined to have been released. 28. The exhaust gas purifying apparatus for an internal combustion engine according to claim 23, wherein said second set period is approximately 20 seconds or less.