JP2000352308A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in fuel consumption by effectively discharging and reducing NOx and desorbing SOx by an occlusion reduction NOx catalyst. SOLUTION: An exhaust emission control system for an internal combustion engine is provided with an occlusion reduction NOx catalyst 17 in an exhaust pipe 16 of the engine capable of lean combustion. When the amount of the NOx absorbed in the occlusion reduction NOx catalyst 17 reaches a predetermined amount, air-fuel ratio of the engine is initially controlled to obtain the optimized air-fuel ratio of exhaust gas for NOx emission and reduction. Subsequent to the completion of the NOx emission and reduction, air-fuel ratio of the engine is controlled to obtain the optimized air-fuel ratio of exhaust gas for SOx desorption. Consequently, NOx emission and reduction and SOx desorption are carried out simultaneously and continuously.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing nitrogen oxides (NO
The present invention relates to an exhaust gas purification device capable of purifying x).

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for purifying NOx from exhaust gas discharged from an internal combustion engine capable of lean combustion, there is a NOx absorbent represented by a NOx storage reduction catalyst. N
The Ox absorbent has an air-fuel ratio of the inflowing exhaust gas that is lean (ie,
NOx is absorbed during an oxygen-excess atmosphere, and the absorbed NOx is released when the oxygen concentration of the inflowing exhaust gas decreases. The NOx storage reduction catalyst, which is a type of NOx absorber, air-fuel ratio of the exhaust gas is lean (i.e., under oxygen-rich atmosphere) to absorb NOx when the oxygen concentration of the inflowing exhaust gas is the catalyst for reducing the absorbed NOx release and N ₂ when dropped.

When this storage-reduction type NOx catalyst (hereinafter sometimes simply referred to as a catalyst or NOx catalyst) is disposed in an exhaust passage of an internal combustion engine capable of lean combustion, when exhaust gas having a lean air-fuel ratio flows, the exhaust gas contains NOx is absorbed by the catalyst, and NO is absorbed by the catalyst when exhaust gas having a stoichiometric (stoichiometric air-fuel ratio) or rich air-fuel ratio flows.
x is released as NO ₂ , and HC and C
It is reduced to N ₂ by a reducing component such as O, that is, NOx is purified.

However, if exhaust gas having a lean air-fuel ratio continues to flow through the NOx storage reduction catalyst, the NOx catalyst
And NOx can no longer be absorbed. Therefore, before the NOx catalyst is saturated with NOx, by flowing the exhaust gas of the stoichiometric air-fuel ratio or rich air-fuel ratio at a predetermined timing, together with the release of NOx from the NOx catalyst, need to be reduced and released the NOx into N ₂ There is.

Conventionally, the amount of NOx absorbed by the NOx catalyst is estimated from the operating state of the internal combustion engine, and when the NOx absorption amount reaches a predetermined amount, exhaust gas having a stoichiometric air-fuel ratio or a rich air-fuel ratio is supplied to the NOx catalyst. NOx release / reduction treatment
NOx absorption capacity has been restored. FIG. 12 shows an example of air-fuel ratio control for NOx release / reduction processing.

[0006] In general, the fuel of an internal combustion engine contains sulfur. When the fuel is burned in the internal combustion engine, the sulfur in the fuel is burned and sulfur oxides such as SO ₂ and SO ₃ (SOx ) Occurs. The storage reduction type NOx catalyst includes N 2
Since SOx in the exhaust gas is absorbed by the same mechanism as that for absorbing Ox, NOx is contained in the exhaust passage of the internal combustion engine.
When a catalyst is arranged, this NOx catalyst absorbs not only NOx but also SOx.

However, the sulfur absorbed by the NOx catalyst
Ox forms stable sulfate over time,
It is difficult to be decomposed and released, and tends to accumulate in the catalyst. When the accumulated amount of SOx in the NOx catalyst increases, the N
Since the Ox absorption capacity decreases, the NOx purification rate decreases. This is so-called SOx poisoning. NO of storage reduction type NOx catalyst
In order to maintain high x purification performance over a long period of time, it is necessary to desorb SOx absorbed by the NOx catalyst at an appropriate timing.

Therefore, conventionally, Patent No. 274598
As disclosed in Patent Publication No. 5, etc., a time when SOx should be desorbed from the NOx catalyst (execution time of the SOx desorption process) is determined from the traveling distance of the vehicle and the like. The exhaust gas flows through the NOx catalyst to release SOx. FIG. 13 shows an example of air-fuel ratio control for SOx desorption processing.

[0009]

As disclosed in the above-mentioned publication, conventionally, NOx release /
The execution time of the reduction process and the execution time of the SOx desorption process were independently controlled. When the execution timings are independently controlled in this way, as shown in FIG. 13, the exhaust gas having a lean air-fuel ratio often flows to the NOx catalyst until before the execution of the SOx desorption process, and the NOx catalyst In addition, NOx and oxygen are adsorbed.

[0010] Therefore, if the SOx desorption process is performed in this state, HC in the exhaust gas is consumed in the reaction with oxygen and the release and reduction of NOx in the initial stage of the SOx desorption process. Will not be used effectively. As a result, the consumption of the reducing agent is increased, and the fuel efficiency is deteriorated.

It is to be noted that the temperature characteristic of the NOx storage reduction catalyst is a major factor that independently controls the execution time of the NOx release / reduction process and the execution time of the SOx desorption process of the NOx catalyst. FIG. 5 shows temperature characteristics of NOx purification of the NOx storage reduction catalyst. As you can see from this figure,
The temperature range in which NOx purification (absorption / desorption / reduction) is effectively performed is about 250 ° C to 500 ° C.
The Ox release / reduction treatment needs to be performed in this temperature range.

On the other hand, the temperature characteristic of SOx desorption of the conventional NOx storage reduction catalyst is shown by a broken line in FIG. 6, and the temperature range in which SOx desorption is effectively performed is about 500 ° C. C or more. Therefore, the SOx desorption process needs to be performed at 500 ° C. or higher. As described above, since the optimum temperature conditions for each processing are different, the simultaneous processing is not effective, and there is no necessity of performing the continuous processing.

The present invention has been made in view of such problems of the prior art, and the problem to be solved by the present invention is to adjust the timing of NOx release / reduction in accordance with the execution timing of NOx.
By executing the SOx desorption process simultaneously and continuously with the Ox release process, the effective use of the reducing agent can be achieved, and the fuel consumption can be improved.

[0014]

The present invention has the following features to attain the object mentioned above. The exhaust gas purifying apparatus for an internal combustion engine according to the present invention is provided in the exhaust passage of the internal combustion engine capable of lean combustion, absorbs NOx when the air-fuel ratio of the exhaust gas is lean, and oxygen of the exhaust gas which flows therein. A NOx absorbent that releases NOx absorbed when the concentration is low, and (b) NOx absorption amount determination means that determines whether the amount of NOx absorbed by the NOx absorbent has reached a predetermined amount.
(C) When the NOx absorption amount determination means determines that the NOx amount has reached a predetermined amount, the NOx absorption material
NOx release processing for releasing Ox and S from the NOx absorbent
Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas so as to continuously execute the SOx releasing process for releasing Ox.

In this exhaust gas purification apparatus, when the air-fuel ratio of the exhaust gas is lean, NOx and SOx in the exhaust gas are reduced to NOx.
Absorbed by the absorbent. When the NOx absorption amount determining means determines that the NOx amount absorbed by the NOx absorbent has reached a predetermined amount, the exhaust air-fuel ratio control means changes the air-fuel ratio of the exhaust gas to the optimum air-fuel ratio for NOx release. The air-fuel ratio is controlled to be optimal for desorption of SOx and SOx, and NOx release processing and SOx
Desorption processes are performed simultaneously and sequentially. By executing the NOx releasing process and the SOx desorbing process simultaneously and continuously, the reducing agent can be efficiently used without waste. Note that the air-fuel ratio of the exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx absorbent.

As the internal combustion engine capable of lean combustion in the present invention, a direct-injection lean-burn gasoline engine or a diesel engine can be exemplified. In the case of a lean burn gasoline engine, air-fuel ratio control of exhaust gas can be executed by air-fuel ratio control of air-fuel mixture supplied to a combustion chamber. Regarding the air-fuel ratio control of exhaust gas in the case of a diesel engine, so-called sub-injection for injecting fuel in an intake stroke, an expansion stroke, or an exhaust stroke is performed, or a reducing agent is injected into an exhaust passage upstream of a NOx absorbent. It can be performed by supplying.

An example of the NOx absorbent is a storage reduction type NOx catalyst. The storage reduction NOx catalyst is
Air-fuel ratio of the inflowing exhaust gas absorbs NOx when the lean, the oxygen concentration in the exhaust gas flowing to release NOx absorbed to decrease a catalyst for reducing the N _2. This storage-reduction NOx catalyst uses, for example, alumina as a carrier, and on the carrier, for example, alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earths such as barium Ba, calcium Ca, and lanthanum La. And at least one selected from rare earths such as yttrium Y and a noble metal such as platinum Pt.

The amount of NOx absorbed by the NOx absorbent can be estimated from the engine speed and the engine load. Alternatively, it is also possible to provide a NOx sensor upstream of the NOx absorbent and to estimate from the NOx concentration detected by the NOx sensor and the exhaust gas flow rate.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the exhaust air-fuel ratio control means sets the air-fuel ratio of the exhaust gas in the first first period to a second air-fuel ratio following the first period.
Is preferably set smaller than the air-fuel ratio of the exhaust gas during the period.

When releasing NOx from the NOx absorbent, it is more efficient to increase the richness of the air-fuel ratio of the exhaust gas. When desorbing SOx from the NOx absorbent, the air-fuel ratio of the exhaust gas is set higher than the stoichiometric air-fuel ratio. Is slightly more rich (ie, slightly rich), it is more efficient.
Desorption takes longer than NOx desorption. Therefore,
In the first period, the air-fuel ratio of the exhaust gas is set to be small (that is, the degree of richness is large) mainly for the purpose of releasing NOx, and in the second period, the air-fuel ratio of the exhaust gas is set to the first for the purpose of desorbing SOx. If the period is set to be longer (the degree of richness is made smaller) than in the period, the release of NOx and the desorption of SOx are performed extremely effectively.

Further, it is preferable that the exhaust air-fuel ratio control means periodically changes the air-fuel ratio of the exhaust gas at the stoichiometric air-fuel ratio or at a richer side than the stoichiometric air-fuel ratio during the second period. This makes it possible to more efficiently desorb SOx from the NOx absorbent.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS.

[First Embodiment] FIG. 1 is a diagram showing a schematic configuration when the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 denotes an engine body, reference numeral 2 denotes a piston, reference numeral 3 denotes a combustion chamber, reference numeral 4 denotes a spark plug, reference numeral 5 denotes an intake valve, reference numeral 6 denotes an intake port, reference numeral 7 denotes an exhaust valve, reference numeral 8 denotes an exhaust port. Are shown respectively.

The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12 and an air flow meter 21, and a throttle valve 14 is arranged in the intake duct 12.

On the other hand, the exhaust port 8 is connected to the exhaust manifold 15.
And a NOx storage reduction catalyst (NOx
The casing 18 is connected to a muffler (not shown) via an exhaust pipe 19. In the following description, the storage reduction type N
The Ox catalyst 17 is abbreviated as the NOx catalyst 17.

An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and a CPU (Central Processor) by a bidirectional bus 31. Unit) 3
4, an input port 35 and an output port 36 are provided. The air flow meter 21 generates an output voltage proportional to the amount of intake air, and this output voltage is input to an input port 35 via an AD converter 37.

On the other hand, the exhaust pipe 19 downstream of the casing 18
A temperature sensor 25 for generating an output voltage proportional to the temperature of the exhaust gas is mounted in the inside.
5 is input to the input port 35 via the AD converter 38.
Is input to The input port 35 is connected to a rotation speed sensor 26 that generates an output pulse representing the engine rotation speed. The output ports 36 are connected to the ignition plug 4 and the fuel injection valve 11 via corresponding drive circuits 39, respectively.

In this gasoline engine, 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.
When it becomes 0, the air-fuel ratio of the mixture supplied to the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the gasoline engine according to this embodiment, the lean air-fuel ratio control is performed in the engine low-medium load operation region with the value of the correction coefficient K being smaller than 1.0, and the engine high-load operation region During warm-up operation, acceleration, and high-speed constant-speed operation at the time of engine start, the value of the correction coefficient K is set to 1.0 and stoichiometric control is performed. The value is set to a value larger than 1.0 to perform the rich air-fuel ratio control.

In the internal combustion engine, low-medium load operation is usually performed most frequently, and therefore, during most of the operation period, the value of the correction coefficient K is made smaller than 1.0, so that the lean mixture is fueled. become.

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

NOx contained in the casing 18
The catalyst (storage-reduction type NOx catalyst) 17 uses, for example, alumina as a carrier, and on the carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, and an alkaline earth such as barium Ba and calcium Ca. , Lanthanum La, at least one selected from rare earths such as yttrium Y, and a noble metal such as platinum Pt.

When this NOx catalyst 17 is disposed in the exhaust passage of the engine, the NOx catalyst 17 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter, also referred to as the exhaust air-fuel ratio) is lean, and the NOx catalyst 17 When the oxygen concentration in the gas decreases, the NOx absorption / release operation is performed to release the absorbed NOx. Here, the exhaust air-fuel ratio refers to the engine intake passage and N
The ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the Ox catalyst 17.

When no fuel (hydrocarbon) or air is supplied into the exhaust passage upstream of the NOx catalyst 17, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, the NOx catalyst 1
Numeral 7 absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases. .

It is considered that the NOx absorbing and releasing action of the NOx catalyst 17 is performed by a mechanism as shown in FIG. In the following, this mechanism will be described by using platinum P
The case where t and barium Ba are supported will be described as an example, but other noble metals, alkali metals, alkaline earths,
The same mechanism is obtained even when rare earth elements are used.

First, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), oxygen O ₂ adheres to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ . On the other hand, NO contained 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 NOx catalyst 17 while being oxidized on the platinum Pt and combined with the barium oxide BaO, and as shown in FIG. NO ₃ ^- diffuses into the NOx catalyst 17 in the form of. In this way, NOx is absorbed in the NOx catalyst 17.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt,
As long as the absorption capacity is not saturated, NO ₂ is absorbed in the NOx catalyst 17 and nitrate ions NO ₃ ⁻ are generated.

On the other hand, the oxygen concentration in the inflowing exhaust gas
Decreases and NO_TwoThe reaction goes in the reverse direction when the amount of
(NO_Three ^-→ NO_Two), And nitrate ion in the NOx catalyst 17
NO _Three ^-Is NO_TwoAlternatively, the NOx catalyst 17 is discharged in the form of NO.
Will be issued. That is, the oxygen concentration in the inflow exhaust gas decreases.
Then, NOx is released from the NOx catalyst 17.
As shown in FIG. 3, the degree of lean of the incoming exhaust gas
Lower the oxygen concentration in the incoming exhaust gas
Therefore, if the degree of leanness of the inflow exhaust gas is reduced, NO
NOx is released from the x catalyst 17.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 reaches a stoichiometric or rich air-fuel ratio, a large amount of unburned HC and CO is discharged from the engine as shown in FIG. retardant HC, CO is oxygen on the platinum Pt O ₂ ^-
Or, it is oxidized by reacting with O ²⁻ .

When the exhaust air-fuel ratio reaches the stoichiometric air-fuel ratio or the rich air-fuel ratio, the oxygen concentration in the inflowing exhaust gas is extremely reduced, so that NO ₂ or NO is released from the NOx catalyst 17, and this NO ₂ or NO As shown in FIG. 4 (B), it reacts with unburned HC and CO to be reduced to N ₂
Becomes

[0042] That is, HC in the inflowing exhaust gas, CO, first oxygen O ₂ on the platinum Pt ^- or O ^2- immediately be reacted with oxidized, then the platinum Pt on the oxygen O ₂ ^- or O ^2- If HC and CO still remain after consumption, this H
C, NOx in the released NOx and the inflow exhaust gas is made to reduction to N ₂ from the NOx catalyst 17 by the CO.

In this way, NO _{2 is} deposited on the surface of platinum Pt.
Alternatively, when NO is no longer present, NO ₂ or NO is released from the NOx catalyst 17 one after another, and is further reduced to N ₂ . Therefore, when the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio or rich, NOx is released from the NOx catalyst 17 within a short time.

[0044] Thus, NOx when the exhaust air-fuel ratio becomes lean is absorbed in the NOx catalyst 17, NOx when the exhaust air-fuel ratio to the stoichiometric air-fuel ratio or rich is released in a short time from the NOx catalyst 17, N ₂ Is reduced to Therefore, emission of NOx into the atmosphere can be prevented.

In this embodiment, as described above, the mixture supplied to the combustion chamber 3 is made rich during full load operation, and the stoichiometric mixture is made stoichiometric during high load operation. During low-medium load operation, the air-fuel mixture becomes lean, so that NOx in the exhaust gas becomes NOx during low-medium load operation.
The NOx is absorbed by the catalyst 17, and is released and reduced from the NOx catalyst 17 during full load operation and high load operation. However, if the frequency of full load operation or high load operation is low, and the frequency of low / medium load operation is high and the operation time is long, the release and reduction of NOx cannot be made in time, and the NOx absorption capacity of the NOx catalyst 17 becomes saturated. NOx
Can not be absorbed. Therefore, when the amount of NOx absorbed in the NOx catalyst 17 has reached the predetermined amount is reduced to N ₂ by releasing NOx from the NOx catalyst 17 N
It is necessary to execute Ox release / reduction processing.

On the other hand, the fuel contains sulfur (S), and when the sulfur in the fuel burns, sulfur oxides (SOx) such as SO ₂ and SO ₃ are generated, and the NOx catalyst 17 These SOx also absorb. It is considered that the SOx absorption mechanism of the NOx catalyst 17 is the same as the NOx absorption mechanism. That is, assuming that platinum Pt and barium Ba are supported on the carrier in the same manner as when the NOx absorption mechanism is described, as described above, when the exhaust air-fuel ratio is lean, oxygen O ₂ is reduced. O ₂ ^- or O ^2-
The SOx (for example, SO ₂ ) in the inflowing exhaust gas becomes platinum Pt in the form of
Is oxidized to SO ₃ on the surface.

Thereafter, the generated SO ₃ is further oxidized on the surface of the platinum Pt, is absorbed in the NOx catalyst 17 and combines with the barium oxide BaO, and enters the NOx catalyst 17 in the form of sulfate ion SO ₄ ^2−. diffused to form the sulfate BaSO _4. Since this BaSO ₄ crystal tends to be coarse and relatively stable, it is difficult to decompose and desorb once formed.
When the amount of BaSO ₄ generated in the NOx catalyst 17 increases, the amount of BaO that can participate in the absorption of the NOx catalyst 17 decreases, and the NOx absorption capacity decreases. This is SOx poisoning. Therefore, the NOx of the NOx catalyst 17
In order to maintain a high absorption capacity, it is necessary to execute a SOx desorption process for desorbing the SOx absorbed by the NOx catalyst 17 at an appropriate timing.

Here, in the case of the conventional NOx storage reduction catalyst, the temperature range in which NOx purification is effectively performed and the temperature range in which SOx desorption is effectively performed are different as described above. It has been difficult to simultaneously perform the NOx release / reduction treatment and the SOx desorption treatment. However, recent studies on storage-reduction NOx catalysts have led to the development of storage-reduction NOx catalysts that can release SOx at lower temperatures than before, and the simultaneous and continuous NOx release / reduction processing and SOx desorption processing have been performed. Processing is now possible.

For example, as shown by a solid line in FIG.
In the case of the NOx storage reduction catalyst having the temperature characteristic of Ox desorption, SOx desorption is effectively performed at about 300 ° C. or more. Therefore, with this NOx catalyst, a temperature range effective for NOx purification (about 250 to 500 ° C., see FIG. 5)
And the temperature range effective for SOx desorption (about 300 ° C. or more) partially overlaps, so that the NOx release / reduction process and the SOx desorption process can be performed simultaneously and continuously.

Therefore, in the exhaust gas purifying apparatus of this embodiment, a NOx storage-reduction type NOx catalyst having a SOx desorption temperature characteristic shown by a solid line in FIG.
When the amount of NOx absorbed in the catalyst 17 reaches a predetermined amount,
When it is time to execute the Ox release / reduction process, the NOx release / reduction process and the SOx desorption process are performed simultaneously and continuously. Since BaSO ₄ absorbed by the NOx catalyst 17 is not easily decomposed and desorbed, and NOx is more easily released, the NOx releasing / reducing process is performed first.
Thereafter, the SOx desorption process is performed. Less than,
Simultaneously and continuously performing the NOx release / reduction process and the SOx desorption process is referred to as a simultaneous NOx reduction SOx desorption process.

The relationship between the NOx reduction amount of the NOx catalyst 17 and the exhaust air-fuel ratio (A / F) is as shown in FIG.
The smaller the exhaust air-fuel ratio (ie, the larger the rich degree), the larger the NOx reduction amount.

On the other hand, the relationship between the SOx desorption amount of the NOx catalyst 17 and the exhaust air-fuel ratio (A / F) is as shown in FIG. 8, which is slightly richer than the stoichiometric air-fuel ratio (A / F = 14.0). ) Indicates the maximum value, and when the exhaust air-fuel ratio is higher than that (that is, when the air-fuel ratio becomes lean), the SOx desorption amount decreases drastically, but when it becomes small (that is, when the rich degree increases), Although the amount of desorption gradually decreases, the effect of the air-fuel ratio is not so large.

In consideration of this air-fuel ratio characteristic, in this embodiment, the processing period of the simultaneous NOx reduction and SOx desorption processing is divided into two periods before and after, and the first period of the first half is effective in releasing and reducing NOx. The air-fuel ratio in the engine is set to A / F = 10 to 12 in order to obtain a proper exhaust air-fuel ratio, and the air-fuel ratio in the engine is set to A / F in the second half of the second half to obtain an exhaust air-fuel ratio effective for SOx desorption. = 14.0 to 14.5.

FIG. 9 shows the exhaust air-fuel ratio and the NOx emission amount (N
FIG. 3 is a diagram showing changes over time of an Ox concentration) and an SOx emission amount (SOx concentration). With reference to FIG. 9, the operation in the simultaneous processing of desorbing NOx and SOx will be described.

Before the simultaneous execution of the NOx reduction SOx desorption process, the operation is performed at a lean air-fuel ratio.
The catalyst 17 adsorbs or absorbs oxygen, NOx, and SOx. When the simultaneous NOx reduction and SOx desorption process is started, the rich air-fuel ratio (A /
F = 10 to 12) flows to the NOx catalyst 17.

In the first period, oxygen adsorbed on the NOx catalyst 17 is first consumed for oxidizing CO and HC in the exhaust gas. When the oxygen adsorbed on the NOx catalyst 17 runs out, CO and HC in the exhaust gas become
It is consumed for releasing and reducing the NOx absorbed in the NOx catalyst 17. On the other hand, the SOx absorbed by the NOx catalyst 17
Since it is difficult to decompose and desorb, SOx desorption starts after a considerable time has elapsed from the start of NOx release. And
In the latter half of the first period, the release and reduction of NOx and the SOx desorption in the NOx catalyst 17 proceed simultaneously.

When the release and reduction of NOx are completed, the first period is ended, and the simultaneous NOx reduction and SOx desorption process enters the second period. In the second period, a slightly rich air-fuel ratio close to stoichiometric (A / F = 14.0 to 14.5)
Exhaust gas flows to the NOx catalyst 17. This gives N
SOx is efficiently desorbed from the Ox catalyst 17.

As described above, the NOx releasing / reducing process and the SOx
Since the desorption processing is performed simultaneously and continuously, the reducing agent can be efficiently used without waste, and as a result, fuel economy deterioration due to exhaust gas purification can be reduced.

Next, with reference to FIG. 10, a description will be given of a NOx reduction SOx desorption simultaneous processing execution routine in this embodiment. A flowchart comprising the steps constituting this routine is stored in the ROM 32 of the ECU 30.
It is executed by the CPU 34 of the CU 30.

<Step 101> First, the ECU 30
In step 101, the operating state of the engine is read. <Step 102> Next, the ECU 30 proceeds to step 10
Proceeding to step 2, it is determined based on the engine operating state read in step 101 whether the engine is operating under a lean air-fuel ratio. If a negative determination is made in step 102, the routine proceeds to return, and returns to air-fuel ratio control according to the normal operating state of the engine.

<Step 103> If an affirmative determination is made in step 102, the ECU 30 proceeds to step 103.
Then, the NOx storage amount absorbed by the NOx catalyst 17 is read from the NOx counter, and the SOx amount absorbed by the NOx catalyst 17 is read from the SOx counter.

The amount of NOx discharged from the combustion chamber 3 when the engine is operating at a lean air-fuel ratio is determined by the operating state of the engine (the intake air amount Q and the engine load Q / N). The NOx amount absorbed by the NOx catalyst 17 can be estimated as a function of the intake air amount Q and the engine load Q / N. Incidentally, the NOx amount discharged from the combustion chamber 3 increases as the intake air amount Q increases, and increases as the engine load Q / N increases. On the other hand, when the engine is operated at the stoichiometric air-fuel ratio or the rich air-fuel ratio, the amount of NOx released from the NOx catalyst 17 is also determined according to the operating state of the engine, and can be obtained in advance by experiments. Therefore, these data are
R of the ECU 30 as an absorption map and a NOx emission map
The map is stored in advance in the OM 32 and the NOx absorption amount or the N
By obtaining the Ox release amount and adding or subtracting it in the NOx counter, the NOx amount remaining in the NOx catalyst 17 at the present time (that is, the NOx storage amount absorbed in the NOx catalyst 17) can be obtained.

On the other hand, when the engine is operating at a lean air-fuel ratio, the amount of SOx absorbed by the NOx catalyst 17 can be estimated from the fuel consumption. Therefore, the fuel consumption is calculated from the operating state of the engine, the SOx amount absorbed by the NOx catalyst 17 is calculated based on the fuel consumption, and S
By adding with the Ox counter, N
The amount of SOx absorbed in the Ox catalyst 17 can be obtained.

<Step 104> Next, the ECU 30
Proceeding to step 104, the N read in step 103
It is determined whether or not the Ox storage amount has reached a predetermined amount. In this embodiment, the predetermined amount is the NOx saturation amount in the NOx catalyst 17. If a negative determination is made in step 104, the process proceeds to return, and returns to the air-fuel ratio control according to the normal operating state of the engine.

<Step 105> If an affirmative determination is made in step 104, the ECU 30 proceeds to step 105 and determines whether or not the air-fuel ratio control of the engine is under the rich air-fuel ratio control. In step 105, a negative determination that the rich air-fuel ratio control is not being performed means that the enrichment of the air-fuel ratio is to be started thereafter, and an affirmative determination that the rich air-fuel ratio control is being performed is affirmatively determined that the rich air-fuel ratio is already being controlled. It means that the conversion has started.

<Step 106> If a negative determination is made in step 105, the ECU 30 proceeds to step 10
6 to set the target air-fuel ratio of the engine to the rich air-fuel ratio (A / F) in order to obtain the optimum exhaust air-fuel ratio for NOx emission and reduction.
= 10-12).

<Step 107> Next, the ECU 30
Proceeding to step 107, rich air-fuel ratio control is performed so that the air-fuel ratio of the engine becomes the target air-fuel ratio set in step 106. By executing this rich air-fuel ratio control,
The NOx catalyst 17 has a rich air-fuel ratio (A / F = 10-1).
The exhaust gas of 2) flows. This time is the beginning of the first period in the simultaneous NOx reduction and SOx desorption process. After step 107, the ECU 30 returns to step 105.

<Step 108> When the routine returns from step 107 to step 105, since the rich air-fuel ratio control is already being executed, an affirmative determination is made in step 105, and the ECU 30 proceeds to step 108 to determine NOx.
It is determined whether the release / reduction has been completed. This determination is made by the NOx counter. When the NOx storage amount by the NOx counter becomes zero, it is determined that the NOx release / reduction has been completed.

If a negative determination is made in step 108,
Since the emission and reduction of NOx has not been completed yet, the ECU
30 proceeds to steps 106 and 107 to continue the rich air-fuel ratio control.

<Step 109> And step 10
If an affirmative determination is made in step 8, the ECU 30 proceeds to step 1
In step 09, it is determined whether SOx desorption is completed. This time is the end of the first period and the beginning of the second period in the simultaneous NOx reduction and SOx desorption process.

<Step 110> If a negative determination is made in step 109, the ECU 30 proceeds to step 110 and sets the target air-fuel ratio of the engine to a slight rich (A / A) so that the exhaust air-fuel ratio is optimal for SOx desorption. F = 14.0
１４14.5).

<Step 107> Next, the ECU 30
Proceeding to step 107, rich air-fuel ratio control is performed so that the air-fuel ratio of the engine becomes the target air-fuel ratio set in step 110. By executing this rich air-fuel ratio control,
The NOx catalyst 17 has a slightly rich air-fuel ratio (A / F =
The exhaust gas of 14.0 to 14.5) flows.
After step 107, the ECU 30 returns to step 105, and continues the rich air-fuel ratio control of the slight rich until an affirmative determination is made in step 109.

<Step 111> If an affirmative determination is made in step 109, the ECU 30 proceeds to step 111, ends the enrichment of the air-fuel ratio for the simultaneous processing of NOx reduction and SOx desorption, and terminates the normal engine operating state. Returns to the air-fuel ratio control corresponding to This time is the end of the second period in the simultaneous NOx reduction and SOx desorption process.

The completion of SOx desorption is determined by the second
Is performed based on the elapsed time from the start of the period. That is, the time required for SOx desorption varies depending on the amount of SOx absorbed in the NOx catalyst 17, and the longer the SOx absorption, the longer the time required. Therefore, through experiments in advance,
The relationship between the SOx absorption amount and the desorption time required for almost completely desorbing the SOx is determined, this is stored in the ROM 32 as a desorption time map, and based on the SOx absorption amount read in step 103. The SOx desorption time is set, and in step 109, it is determined whether the elapsed time from the start of the second period has reached the SOx desorption time, and it is determined whether the SOx desorption is completed.

The part that executes step 104 in the series of signal processing by the ECU 30 is a NOx absorption amount determining means for determining whether the amount of NOx absorbed by the NOx catalyst (NOx absorbent) has reached a predetermined amount. It can be said that the part that performs steps 105 to 111 is an exhaust air-fuel ratio control unit that controls the air-fuel ratio of the exhaust gas so that the NOx release processing and the SOx desorption processing are continuously performed. .

The NOx catalyst 17 used in the above embodiment has the SOx desorption temperature characteristic shown by the solid line in FIG. 6 and has a SOx desorption temperature characteristic in the range of about 300 ° C. to about 650 ° C.
Since there is almost no change in the Ox desorption amount, the SOx desorption time was set based on the SOx absorption amount.

However, depending on the NOx catalyst 17 used, as shown by the dashed line in FIG.
In some cases, the SOx has SOx desorption characteristics in which the desorption amount varies greatly depending on the catalyst temperature. In such a case, it is preferable to correct the SOx desorption time according to the catalyst temperature. That is, the correction is performed so that the SOx desorption time is shorter when the catalyst temperature is high than when the catalyst temperature is low. In the case of the exhaust gas purification system shown in FIG. 1, the exhaust gas temperature detected by the temperature sensor 25 is used as the catalyst temperature.

[Second Embodiment] Next, a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described with reference to FIG. The second embodiment is different from the first embodiment in the method of controlling the exhaust air-fuel ratio in the second period in the simultaneous NOx reduction and SOx desorption process, and otherwise the first embodiment. Same as the form.

As described above, the second period is a period in which the exhaust air-fuel ratio is made slightly rich in order to desorb SOx from the NOx catalyst 17, and in the first embodiment, during the second period, The air-fuel ratio was constant.

However, from the experimental results on SOx desorption in recent years, it is more effective to desorb SOx by periodically changing the air-fuel ratio on the stoichiometric side or on the richer side than the stoichiometric side, rather than by keeping the air-fuel ratio constant. It turned out that there was.

Therefore, in the second embodiment, NO
In the second period of the x-reduction SOx desorption simultaneous processing, the exhaust air-fuel ratio is periodically changed on the richer side than the stoichiometric ratio.

It is not clear why periodic changes in the exhaust air-fuel ratio promote SOx desorption. However, until now S
From the experimental results on Ox desorption, SOx was found to be NOx catalyst 1
In the case of desorbing from the NOx catalyst 7, the NOx catalyst 17 moves downstream while repeating desorption, adsorption, desorption and adsorption.
It is estimated that it is discharged from. It is presumed that the periodic change in the exhaust air-fuel ratio has an effect of promoting the repetition of the desorption and adsorption.

[Other Embodiments] In each of the embodiments described above, the present invention is applied to a gasoline engine. However, it goes without saying that the present invention can be applied to a diesel engine. In the case of a diesel engine, the combustion in the combustion chamber is performed in a much leaner region than the stoichiometric region, so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 17 is very lean under a normal engine operating state, and NOx
And SOx are absorbed, but NOx and SOx are hardly released.

In the case of a gasoline engine, the air-fuel ratio of the exhaust gas is made stoichiometric or rich by making the air-fuel mixture supplied to the combustion chamber 3 stoichiometric or rich, as described above, thereby reducing the oxygen concentration in the exhaust gas. And N
Although NOx and SOx absorbed in the Ox catalyst 17 can be released, in the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is made stoichiometric or rich, soot is generated during combustion. And cannot be adopted.

Therefore, when the present invention is applied to a diesel engine, in order to make the exhaust air-fuel ratio stoichiometric or rich, separately from burning fuel to obtain engine output, a reducing agent (for example, fuel) is used. (Light oil) must be supplied to the exhaust gas. The supply of the reducing agent to the exhaust gas can be performed by sub-injecting the fuel into the cylinder during the intake stroke, the expansion stroke, or the exhaust stroke, or the reducing agent can be supplied into the exhaust passage upstream of the NOx catalyst 17. It is also possible by supplying.

When a diesel engine is provided with an exhaust gas recirculation device (so-called EGR device),
By introducing a large amount of exhaust gas recirculation gas into the combustion chamber, it is possible to make the air-fuel ratio of the exhaust gas a stoichiometric air-fuel ratio or a rich air-fuel ratio.

[0087]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, (a) a NOx absorbent provided in an exhaust passage of an internal combustion engine capable of lean burn, and (b) absorbed by the NOx absorbent. NOx for determining whether or not the NOx amount reached has reached a predetermined amount
And (c) a NOx releasing process for releasing NOx from the NOx absorbent when the NOx amount determining unit determines that the NOx amount has reached a predetermined amount, and a SOx from the NOx absorbent. Exhaust gas air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas in order to continuously execute the SOx release process for releasing SOx, thereby simultaneously and continuously releasing NOx from the NOx absorbent and desorbing SOx. Thus, the reducing agent can be effectively used without waste, and as a result, deterioration of fuel efficiency due to NOx release and SOx desorption can be reduced.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
It is a schematic structure figure of an embodiment.

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

FIG. 3 Unburned HC in exhaust gas discharged from the engine,
FIG. 3 is a diagram schematically showing the concentrations of CO and oxygen.

FIG. 4 is a diagram for explaining the NOx absorbing / releasing action of a NOx storage reduction catalyst.

FIG. 5 is a graph showing the relationship between the NOx purification rate of the NOx storage reduction catalyst and the catalyst temperature.

FIG. 6 is a graph showing the relationship between the SOx desorption amount of a storage reduction type NOx catalyst and the catalyst temperature.

FIG. 7 is a diagram showing the relationship between the NOx reduction amount of the storage reduction type NOx catalyst and the air-fuel ratio.

FIG. 8 is a diagram showing the relationship between the SOx desorption amount of the NOx storage reduction catalyst and the air-fuel ratio.

FIG. 9 is a diagram showing a change over time of an air-fuel ratio and a NOx / SOx emission amount at a catalyst outlet in the first embodiment.

FIG. 10 is a routine for executing simultaneous processing of NOx reduction and SOx desorption in the first embodiment.

FIG. 11 is a diagram illustrating air-fuel ratio control according to a second embodiment of the present invention.

FIG. 12 is a diagram showing air-fuel ratio control in a conventional NOx release / reduction process.

FIG. 13 is a diagram showing air-fuel ratio control in a conventional SOx desorption process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body (internal combustion engine) 3 Combustion chamber 4 Spark plug 11 Fuel injection valve 16 Exhaust pipe (exhaust passage) 17 Storage-reduction type NOx catalyst (NOx absorbent) 18 Casing 19 Exhaust pipe (exhaust passage) 30 ECU

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) F02D 41/04 355 F02D 41/04 355 F term (reference) 3G091 AA11 AA12 AA17 AA18 AA23 AB06 BA11 BA14 CA18 CB02 CB03 DA02 DC01 EA17 EA35 FB10 FB11 FB12 GB02W GB03W GB04W GB05W GB06W GB17X HA37 3G301 HA01 HA02 HA15 JA25 JA33 LB02 MA01 MA11 MA19 ND01 NE13 NE14 PD00Z PD11Z

Claims (3)

[Claims] (1) The exhaust gas is provided in an exhaust passage of an internal combustion engine capable of lean combustion and absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and absorbs when the oxygen concentration of the inflowing exhaust gas is low. A NOx absorbent that releases NOx, and (b)
NOx absorption amount determining means for determining whether or not the NOx amount absorbed by the NOx absorbent has reached a predetermined amount; and (c) the NOx absorption amount determining means determines that the NOx amount has reached the predetermined amount. Exhaust air-fuel ratio control for controlling the air-fuel ratio of the exhaust gas so that the NOx releasing process for releasing NOx from the NOx absorbent and the SOx desorbing process for desorbing SOx from the NOx absorbent are successively performed. Means for purifying exhaust gas of an internal combustion engine. 2. The exhaust air-fuel ratio control means according to claim 1, wherein:
The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas in the period is set smaller than the air-fuel ratio of the exhaust gas in the second period following the first period. . 3. The exhaust air-fuel ratio control means periodically changes the exhaust gas air-fuel ratio on the stoichiometric air-fuel ratio or on the richer side than the stoichiometric air-fuel ratio during the second period. 3. The exhaust gas purifying apparatus for an internal combustion engine according to 2.