JP2001227333A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine, capable of easily forming the rich atmosphere on the surface of a NOx catalyst and efficiently performing poisoning recovering treatment of the NOx catalyst. SOLUTION: The exhaust emission control device for the internal combustion engine is provided with a storage reduction type NOx catalyst 18 containing no oxygen storing and releasing agent and provided on an exhaust passage 14 of an internal combustion engine 1 capable of lean-burn; an oxidation catalyst 20 containing an oxygen storing and releasing agent and provided in the downstream side of the NOx storage reduction catalyst; and an air-fuel ratio adjusting means for adjusting the air-fuel ratio of the exhaust passage. When sulfur poisoning amount of the NOx catalyst 18 is larger than a specified amount, the air-fuel ratio of exhaust gas is enriched.

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, and more particularly to a purifying apparatus capable of efficiently performing sulfur poisoning regeneration.

[0002]

2. Description of the Related Art When the air-fuel ratio of exhaust gas is lean, N
A NOx catalyst that absorbs Ox and releases the absorbed NOx when the oxygen concentration in the exhaust gas decreases is disposed in the exhaust passage of the engine, and the NOx catalyst absorbs NOx in the exhaust gas, and absorbs the NOx catalyst. When the efficiency decreases, the flow of exhaust gas is shut off, a reducing agent is supplied to the NOx catalyst, and the NOx absorbed from the NOx catalyst is released.
There is an exhaust gas purification device for an internal combustion engine that performs reduction purification of released NOx (Japanese Patent Application Laid-Open No. 62-106826).

[0003] In such an apparatus, sulfur contained in exhaust gas is absorbed by the NOx catalyst together with NOx when NOx is absorbed, and this sulfur is not released from the NOx catalyst even if the air-fuel ratio of the inflowing exhaust gas is made rich. Therefore, there is a problem that the NOx absorbing power of the NOx catalyst decreases due to an increase in the amount of sulfur stored.

Therefore, as described in Japanese Patent No. 2,745,985, the sulfur absorption estimating means for estimating the sulfur absorption amount of the NOx catalyst and the sulfur absorption amount estimated by the sulfur absorption amount estimating means are predetermined. A sulfur releasing means for releasing sulfur from the NOx catalyst when the amount exceeds the set amount, and when the amount of sulfur absorbed to recover the absorption capacity of the NOx catalyst exceeds the set amount, sulfur is released from the NOx catalyst. There is an exhaust gas purifying device that emits the gas.

More specifically, this apparatus raises the temperature of the inflowing exhaust gas, sets the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or rich, and raises the temperature of the purification device from 500 ° C. during sulfur poisoning regeneration. About 600 ° C., this is supplied to the NOx catalyst of the purification device, and the sulfur present in the NOx catalyst is thermally decomposed and released as sulfate BaSO ₄ ,
Further, the released sulfur is immediately reduced by the unburned HC and CO.

[0006] On the other hand, the N
The Ox catalyst usually contains an oxygen storage / release agent such as ceria. One of the reasons is that when the air-fuel ratio of the exhaust gas is made rich, a large amount of unburned HC and CO is generated from the internal combustion engine, and is oxidized and purified. The oxygen storage and release agent absorbs oxygen when the air-fuel ratio of the exhaust gas is lean and releases oxygen when the air-fuel ratio is rich, and this oxygen reacts with unburned HC and CO to oxidize them.

[0007]

As described above, in the conventional exhaust gas purifying apparatus, it is necessary to make the air-fuel ratio of the exhaust gas rich to form a rich atmosphere on the surface of the catalyst in order to sufficiently regenerate the sulfur. However, at this time, if an oxygen storage agent is contained, a large amount of oxygen is released from the storage agent, and it takes a long time to form a rich atmosphere. But for a long time,
When the exhaust flow rate is continuously increased to form a rich atmosphere, the amount of the supplied reducing agent increases, the temperature of the catalyst including the NOx catalyst increases, and the temperature of the catalyst is maintained at 650 ° C. or lower, which is the heat resistant temperature. It becomes difficult to do.

Therefore, it is necessary to carry out the sulfur poisoning regeneration while suppressing the rise in the catalyst temperature. In this case, however, the supply time of the reducing agent per time, that is, the rich time may be shortened. In this case, there is a problem that the oxygen release from the oxygen storage agent is overcome, and a sufficient rich atmosphere cannot be formed on the surface of the catalyst, so that the poisoning regeneration cannot be realized.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described circumstances, and it is an object of the present invention to easily form a rich atmosphere on the surface of a NOx catalyst and to purify the exhaust gas of an internal combustion engine capable of efficiently poisoning and regenerating the NOx catalyst. It is an object to provide a device.

[0010]

Means for Solving the Problems In order to achieve the above object, the present invention has the following constitution, that is, a storage not containing an oxygen storage / release agent provided in an exhaust passage of an internal combustion engine capable of lean combustion. A reduction type NOx catalyst, an oxidation catalyst provided with an oxygen storage / release agent provided downstream of the catalyst, and air-fuel ratio adjusting means for adjusting an air-fuel ratio of an exhaust gas passage are provided.

In this device, the air-fuel ratio of the exhaust gas can be set to be rich when the sulfur poisoning amount of the NOx storage reduction catalyst is larger than a predetermined amount. NOx storage reduction type
The measurement of the sulfur poisoning amount of the catalyst is, for example, the mileage of the vehicle,
Alternatively, it can be estimated from the fuel consumption. Further, the NOx concentration in the exhaust passage before and after the NOx storage reduction catalyst can be measured, and the NOx concentration can be estimated from a change in these differences.

Here, the oxygen storage / release agent absorbs oxygen when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed oxygen when the oxygen concentration in the inflowing exhaust gas decreases. (Cerium oxide CeO ₂ ).

The storage-reduction type NOx catalyst absorbs NOx when the air-fuel ratio of the inflow exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration of the inflow exhaust gas decreases.
An Ox absorbent is disposed in an exhaust passage to absorb NOx in the exhaust gas, and then a reducing agent is supplied to the NOx absorbent to release the absorbed NOx from the NOx absorbent and to reduce and purify the released NOx. Exhaust purification catalyst. For example, an alumina is used as a carrier, and an alkali metal such as potassium K, sodium Na, lithium Li and cesium Cs, an alkaline earth such as barium Ba and calcium Ca, a rare earth such as lanthanum La and yttrium Y are formed on the carrier. And at least one noble metal such as platinum Pt.

The oxidation catalyst is, for example, platinum (Pt) or palladium (P) which catalyzes the surface of alumina.
d) etc., and the CO in the exhaust gas
And HC are oxidized into harmless CO ₂ and H ₂ O. Here, a material containing an oxygen storage / release agent is used.
As a material containing an oxygen storage / release agent, for example, a material obtained by adding ceria (cerium oxide CeO ₂ ) to alumina as a base material and attaching a noble metal such as platinum (Pt) to the alumina is used.

Further, the adjusting means for adjusting the air-fuel ratio includes an air-fuel ratio sensor installed in an exhaust pipe and an engine control electronic control unit (ECU) for controlling a fuel supply amount. The fuel supply amount is determined on the basis of the output signal of (1), and the air-fuel ratio of the exhaust gas is adjusted to the stoichiometric air-fuel ratio, lean or rich.

The amount of unburned HC and CO in the exhaust gas discharged from the combustion chamber increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber increases, and the amount of exhaust gas discharged from the combustion chamber increases. The amount of oxygen O ₂ in the mixture increases as the air-fuel ratio of the mixture supplied to the combustion chamber becomes leaner. According to the exhaust gas purifying apparatus of the present invention, the storage-reduction NOx catalyst that does not contain the oxygen storage-release agent and is located upstream of the exhaust passage is disposed. No oxygen is released. Therefore, even in the case of a rich spike for a short time, a situation in which the exhaust gas on the surface of the NOx catalyst is not easily made rich by the released oxygen is eliminated, so that a rich atmosphere can be easily formed. Unburned HC in the rich exhaust gas generated at this time,
Combustible materials such as CO are oxidized by oxygen released from an oxidation catalyst provided with an oxygen storage / release agent disposed downstream thereof. The oxygen storage / release agent can store oxygen when the air-fuel ratio is lean.

As described above, in the exhaust gas purifying apparatus of the present invention, the functions of the above-described NOx absorption / release reduction and HC / CO oxidation are shared by the two catalysts installed in the exhaust passage. This will be accomplished.

At the time of sulfur poisoning regeneration, however, the catalyst temperature must be equal to or higher than a predetermined temperature. Therefore, while maintaining the temperature, it is necessary to adjust the temperature so as not to exceed the heat resistant temperature of the NOx storage reduction catalyst. Occurs. Therefore, the sulfur poisoning regeneration is started after the catalyst temperature reaches a predetermined temperature.At this time, if the exhaust gas whose air-fuel ratio is enriched continuously flows into the catalyst, the temperature of the catalyst rises excessively and the heat-resistant temperature is lowered. May exceed. Therefore, it is preferable to suppress the temperature rise of the catalyst while performing the sulfur poisoning regeneration for a predetermined time by cyclically adjusting the air-fuel ratio so as to alternately repeat rich and lean.

In the apparatus of the present invention, sulfur poisoning regeneration can be performed by alternately and repeatedly flowing rich and lean exhaust gas. Therefore, when lean, the storage reduction type NO is used.
The oxygen storage / release agent of the oxidation catalyst downstream of the x catalyst can store a large amount of oxygen. The oxidation catalyst releases the stored oxygen when the air-fuel ratio is rich and exerts a sufficient oxidation action, so that the amounts of HC and CO in the exhaust gas can be extremely reduced.

[0020]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention. In this figure, an internal combustion engine 1 is an in-line six-cylinder diesel engine, and intake air is introduced into a combustion chamber of each cylinder via an intake pipe 3 and an intake manifold 2. An air cleaner 4 is provided at the start end of the intake pipe 3, and an air flow meter 5, a turbocharger compressor 6 a,
An intercooler 7 and a throttle valve 8 are provided.

The air flow meter 5 outputs an output signal corresponding to the amount of fresh air flowing into the intake pipe 3 through the air cleaner 4 to the ECU 9. The ECU 9 determines the amount of intake air based on the output signal of the air flow meter 5. Calculate.

Fuel (light oil) is injected into the combustion chamber of each cylinder of the internal combustion engine 1 from each of the fuel injection valves 10a and 10b. The valve opening timing and the valve opening period of each of the fuel injection valves 10a and 10b are determined by the ECU 9 according to the operating state of the internal combustion engine 1.
Is controlled by

Exhaust gas generated in the combustion chamber of each cylinder of the engine 1 is divided into split-type exhaust manifolds 13a and 13a.
The exhaust gas is exhausted to the exhaust pipe 14 through the exhaust pipe b, and further exhausted to the atmosphere via a muffler (not shown). The exhaust manifold 13a discharges exhaust gas from the first cylinder to the third cylinder, and the exhaust manifold 13b discharges exhaust gas from the fourth cylinder to the sixth cylinder. Exhaust manifold 13
b can be recirculated to the intake manifold 2 through the exhaust gas recirculation pipe 15, and the EGR cooler 16 and the EGR valve 1
7 are provided. The opening of the EGR valve 17 is controlled by the ECU 9 in accordance with the operating state of the internal combustion engine 1, and controls the amount of exhaust gas recirculation.

A turbocharger 6 and a turbine 6b of the turbocharger are arranged in the exhaust pipe 14. The turbine 6b is driven by the exhaust gas, and drives the compressor 6a connected to the turbine 6b to increase the pressure of the intake air.

The fuel injection valves 10a of the first to third cylinders also serve as reducing agent supply injection valves for supplying fuel as a reducing agent. That is, the fuel injection valve 10a supplies the reducing agent by post-injection, and the fuel as the reducing agent flows to the exhaust manifold 13b side via the turbocharger 6, and further to the fourth cylinder via the exhaust recirculation pipe 15. To the sixth cylinder. Therefore, a reducing agent wraparound prevention oxidation catalyst 30 is provided at a branch point on the downstream side of the exhaust manifold 13b branched from the exhaust manifold 13a. Therefore, the reducing agent (HC) spilled to the exhaust manifold 13b side is oxidized by the oxidizing catalyst 30 for preventing the spilling of the reducing agent, so that it can be prevented from flowing into the combustion chamber.

Downstream of the exhaust pipe 14 is a storage-reduction type NOx catalyst (lean NOx catalyst) 1 containing no oxygen storage / release agent.
19, and a casing 2 further containing an oxidation catalyst 20 containing an oxygen storage agent downstream thereof.
9 are provided. The NOx catalyst 18 purifies NOx in the exhaust gas and a part of HC and CO. The downstream oxidation catalyst 20 contains cerium oxide CeO ₂ (hereinafter, ceria), which is an oxygen storage / release agent, and contains platinum P
and an oxidation catalyst such as t.
It purifies C and CO.

In the exhaust pipe 14, a NOx catalyst 18
The first exhaust temperature sensor 22 and the first NO
An x sensor 23 is provided for each. The first air-fuel ratio sensor 2 is located between the NOx catalyst 18 and the oxidation catalyst 20.
4. Second exhaust gas temperature sensor 25 and second NOx sensor 26
Is installed, and furthermore, HC is provided downstream of the oxidation catalyst 20.
A sensor 27 and a second air-fuel ratio sensor 28 are provided. These first exhaust gas temperature sensor 22, first NO
The x concentration sensor 23, the first air-fuel ratio sensor 24, the second exhaust gas temperature sensor 25, the second NOx concentration sensor 26, the HC concentration sensor 27, and the second air-fuel ratio sensor 28 are all connected to the ECU 9, and output detected information to the ECU 9. I do.

The temperature inside the exhaust pipe 3 is measured by the first exhaust gas temperature sensor 22 and the second exhaust gas temperature sensor 25 installed upstream and downstream of the NOx catalyst 18, and the temperature inside the NOx catalyst 18 can be estimated. is there.

Similarly, the first NOx concentration sensor 23 and the second NOx concentration sensor 26 provided upstream and downstream of the NOx catalyst 18 can measure the NOx reduction rate by the NOx catalyst 18.

The first air-fuel ratio sensor 24 downstream of the NOx catalyst 18 determines the air-fuel ratio in the NOx catalyst 18, and the second air-fuel ratio sensor 28 downstream of the oxidation catalyst 20 determines the oxidation catalyst 20.
Are measured respectively. HC concentration sensor 2
7 detects HC from the oxidation catalyst 20 and determines the deterioration of the catalyst. When the HC emission from the oxidation catalyst 20 detected by the HC concentration sensor 27 is large, the degree of richness of the rich spike or the rich time is reduced to adjust the amount of HC emission.

The ECU 9 is composed of a digital computer and includes a ROM (Read Only Memory), a RAM (Random Access Memory), a CPU (Central Processor Unit), an input port, and an output port interconnected by a bidirectional bus. In addition to performing basic control such as fuel injection amount control of the engine 1, in this embodiment, NO
x Regeneration of the absorbent, adjustment of the supply of the reducing agent, etc. are performed.

For these controls, an input signal from the accelerator opening sensor 61 and an input signal from the crank angle sensor 62 are input to input ports of the ECU 9. The accelerator opening sensor 61 outputs an output voltage proportional to the accelerator opening to the ECU 9.
The engine load is calculated on the basis of the output signal of No. The crank angle sensor 62 outputs an output pulse to the ECU 9 every time the crankshaft rotates by a certain angle, and the ECU 9 calculates the engine speed based on the output pulse. The engine operating state is determined based on the engine load and the engine speed, and the ECU 9 calculates a fuel injection amount according to the engine operating state with reference to an injection amount map (not shown), and corresponds to the calculated fuel injection amount. The operation of the fuel injection valve 10 is controlled by calculating the valve opening period of the fuel injection valve 10.

FIG. 2 shows another embodiment of the exhaust gas purifying apparatus of the present invention. Here, portions different from the embodiment of FIG. 1 will be described, and the same portions will be denoted by the same reference numerals and description thereof will be omitted. In the embodiment shown in FIG.
The fuel injection valve 10a of the cylinder also serves as a reducing agent supply injection valve for supplying fuel as a reducing agent, but here, a reducing agent supply injection valve 21 is separately provided, and this is provided in the exhaust port. The agent is to be sprayed. The reducing agent using fuel is sent by a pressure pump 31 provided in the reducing agent supply passage, and the supply amount is adjusted by a reducing agent supply metering valve 32.

The fuel as a reducing agent supplied from the reducing agent supply injection valve 21 flows around the exhaust manifold 13b through the turbocharger 6, and further flows from the fourth cylinder through the exhaust recirculation pipe 15 from the fourth cylinder. It is necessary not to flow into the six cylinders. Therefore, the exhaust manifold 13
At the branch point between b and the exhaust recirculation pipe 15, an oxidation catalyst 30 for preventing the wraparound of the reducing agent is provided.

Further, downstream of the exhaust pipe 14, NOx carrying a storage-reduction type NOx catalyst containing no oxygen storage / release agent is used.
A casing 19 is provided in which a catalyst 18 and an oxidation catalyst 20 carrying an oxidation catalyst containing an oxygen storage agent are integrally housed. The NOx catalyst 18 is located on the upstream side of the exhaust passage, and the oxidation catalyst 20 is located on the downstream side. However, the difference from the one shown in FIG. 1 is that these are integrated to constitute one catalyst. It is a point.

Next, the storage-reduction type NOx catalyst carried on the NOx catalyst 18 will be described. The storage reduction type NOx catalyst uses, for example, alumina (Al ₂ O ₃ ) as a carrier and, for example, potassium K, sodium N
a, at least one selected from alkali metals such as lithium Li and cesium Cs, alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt. It is carried.

This NOx catalyst absorbs NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter referred to as the exhaust air-fuel ratio) is leaner than the stoichiometric air-fuel ratio, and the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio. performing absorption and release action of NOx which releases NOx concentration of oxygen in the inflowing exhaust gas is absorbed and reduced become as NO ₂ or NO. And NOx
NOx (NO ₂ or NO) released from the catalyst immediately reacts with unburned HC or CO in the exhaust gas and is reduced to N ₂ .

Here, the exhaust air-fuel ratio means the ratio of the total amount of air supplied to the exhaust passage, the engine combustion chamber, the intake passage, and the like upstream of the NOx catalyst to the total amount of fuel (hydrocarbon). It shall be. Therefore, when no fuel, reducing agent, or air is supplied into the exhaust passage upstream of the NOx catalyst, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the engine combustion chamber.

By the way, in the case of a diesel engine,
Since the combustion is performed in a lean region much more than the stoichiometric ratio (the stoichiometric air-fuel ratio, A / F = 14.6), the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is very lean under a normal engine operating condition. NOx in the exhaust gas is absorbed by the NOx catalyst, and the amount of NOx released from the NOx catalyst is extremely small.

Therefore, in a diesel engine, N
At a predetermined timing before the NOx absorption capacity of the Ox catalyst is saturated, it is necessary to supply a reducing agent to the exhaust gas to reduce the oxygen concentration in the exhaust gas and release and reduce the NOx absorbed by the NOx catalyst. There is. Therefore, in this embodiment, the amount of NOx absorbed by the NOx catalyst is estimated from the history of the operating state of the engine 1 by the ECU 9, and the estimated N
When the Ox amount reaches a predetermined value set in advance, the control valve 23 is opened for a predetermined time and a predetermined amount of fuel is supplied to the casing 1.
Supplied upstream of 9, to reduce the oxygen concentration in the exhaust gas flowing into the NOx catalyst, NOx absorbed in the NOx catalyst is released, so that reduced to N _2.

As described above, when the air-fuel ratio of the inflowing exhaust gas becomes lean, NOx is absorbed by the NOx catalyst, and when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx catalyst in a short time. However, the inflowing exhaust gas contains sulfur, and the NOx catalyst absorbs not only NOx but also sulfur. The mechanism of sulfur absorption into the NOx catalyst is considered to be the same as the mechanism of NOx absorption. That is, the case where platinum Pt and barium Ba are carried on the carrier will be described as an example. When the air-fuel ratio of the inflowing exhaust gas is lean, oxygen O ₂ adheres to the surface of platinum Pt in the form of O ^2-. And S in the incoming exhaust gas
O ₂ reacts with O ^2-on the surface of platinum Pt to form SO ₃ . Next, the generated SO ₃ is further oxidized on the platinum Pt, and is absorbed in the NOx catalyst to form barium oxide Ba.
While bonding with O, it diffuses into the absorbent in the form of sulfate ions SO ₄ ^2- . Next, this sulfate ion SO ₄ ^2- is combined with barium ion Ba ²⁺ to form sulfate Ba SO ₄ .

The sulfate Ba SO ₄ is hardly decomposed, and even if the air-fuel ratio of the inflowing exhaust gas is enriched for a short time, the sulfate Ba SO ₄ remains without being decomposed. Therefore N
In the Ox catalyst, sulfate BaSO ₄
Increases, and as the time elapses, the amount of NOx that can be absorbed by the NOx catalyst decreases.

Sulfate generated in these NOx catalysts
BaSO_Four Means that the temperature of the NOx catalyst is high and
Decomposes in lively or rich atmosphere and sulfate ion
SO _Four ^2- Is SO_Three As released from the absorbent. There
Therefore, in the exhaust gas purifying apparatus of the present invention, sulfur is contained in the NOx catalyst.
Raise the catalyst temperature when absorption exceeds a certain amount, and
Make the air-fuel ratio of the inflow exhaust gas rich over a certain period of time
Sulfur was released from the NOx catalyst.

FIG. 3 is a view showing a state in which the NOx catalyst 18 and the oxidation catalyst 20 of the exhaust gas purifying apparatus according to the present embodiment are arranged on the upstream side and the downstream side of the exhaust passage, respectively.
The Ox catalyst 18 does not contain an oxygen storage / release agent, while the oxidation catalyst 20 contains an oxygen storage / release agent. The conditions under which sulfur poisoning can be regenerated in this apparatus will be described with reference to FIGS.

FIG. 4 is a graph showing the relationship between the change in temperature of the NOx catalyst 18 and the sulfur poisoning regeneration rate.
In the case where the air-fuel ratio of the exhaust gas on the surface of No. 8 is 14 and the sulfur poisoning regeneration time is 60 seconds, the catalyst temperature is 550 ° C. or more,
Desirably, 600
It can be seen that it is necessary to maintain the temperature around ° C.

FIG. 5 is a graph showing the relationship between the change in the exhaust air-fuel ratio on the surface of the NOx catalyst 18 and the sulfur poisoning regeneration rate. The sulfur poisoning regeneration time is 60 seconds when the temperature of the NOx catalyst 18 is 600 ° C. In this case, it is understood that the exhaust air-fuel ratio on the catalyst surface needs to be the stoichiometric air-fuel ratio or rich.

FIG. 6 is a graph showing the relationship between the change in the sulfur poisoning regeneration time (rich exhaust gas passage time) and the sulfur poisoning regeneration rate. The air-fuel ratio of the exhaust gas on the catalyst surface is 14, and the catalyst temperature is reduced. When the temperature is set to 600 ° C., it takes about 60 seconds to complete the sulfur poisoning regeneration.

In order to satisfy the above conditions, the reducing agent is supplied as shown in FIG.
The air-fuel ratio at the point A at the inlet of the x catalyst 18 and the NOx catalyst 1
The air-fuel ratio at point B at the exit of exit 8 is rich 50,5
Must be 1. In this case, the NOx catalyst 18
It is recognized that a rich atmosphere was formed on the surface of the. When the rich exhaust gas passes through the inside of the oxidation catalyst 20, oxygen is released from the oxygen storage / release agent, and the oxygen is oxidized. Therefore, the air-fuel ratio at the point C at the outlet of the oxidation catalyst 20 becomes the stoichiometric air-fuel ratio state or the lean state 53. Become.

However, when the temperature of the NOx catalyst 18 is set at about 600 ° C., and the air-fuel ratio of the exhaust gas is made rich for about 60 seconds, and this is passed through the NOx catalyst 18,
A situation occurs in which the temperature of the Ox catalyst 18 exceeds its heat-resistant temperature. Therefore, for example, the operation of supplying rich exhaust gas for 20 seconds and repeating the flow of lean exhaust gas for the next 40 seconds is repeated three times, that is, cyclically rich and lean are repeated, and this is passed through the NOx catalyst 18. Thus, it is possible to make the air-fuel ratio of the exhaust gas rich for a total of 60 seconds.

The enrichment of the air-fuel ratio of the inflowing exhaust gas is performed when sulfur is absorbed in the NOx catalyst 18 by a certain amount or more. It is difficult to detect the amount, so the amount of sulfur absorbed in the NOx catalyst 18 must be estimated. Therefore, paying attention to the fact that the amount of sulfur absorbed in the NOx catalyst 18 depends on the traveling distance of the vehicle, the amount of sulfur absorbed in the NOx catalyst 18 is estimated from the traveling distance of the vehicle.

FIG. 7 shows the relationship between the amount of sulfur absorbed and the traveling distance of the vehicle obtained by experiments. In FIG. 7, So on the horizontal axis indicates the cumulative traveling distance of the vehicle, and S on the vertical axis indicates N
The distance traveled by the vehicle after sulfur was released from the Ox catalyst. Curve f (So) represents the absorption capacity of the NOx catalyst at 80.
The case where sulfur occupies% is shown. When the new vehicle is driven, the cumulative traveling distance So and the traveling distance S are equal to each other, and in the embodiment of the present invention, when the traveling distances So and S increase as indicated by the dashed line and reach the curve f (So). First, sulfur is released from the NOx catalyst 18 for the first time. When the sulfur is released, the traveling distance S becomes 0 as indicated by the dashed line.
(Zero), and then when the point determined by the accumulated traveling distance So and the traveling distance S reaches the curve f (So), the second sulfur release is performed, and then the sulfur release is performed in the same manner. Note that the curve f (So) shown in FIG.
The relationship between S and S is stored in a ROM (not shown) in the ECU 9 in advance.

Next, the sulfur release control according to the present invention will be described with reference to FIG. FIG. 8 shows a time interruption routine. First, in step 100, the traveling distance ΔS from the time of the previous interruption to the time of this interruption is calculated from the output pulse of the speed sensor, and this traveling distance ΔS becomes the accumulated traveling distance So. Is added.

Next, at step 101, the running distance S is set to Δ
S is added. The travel distances So and S calculated in steps 100 and 101 are stored in the backup RAM in the ECU 9.

Next, at step 102, each traveling distance S
It is determined whether the point determined by o and S has exceeded the curve f (So) in FIG. 7, and if not, the routine proceeds to step 103.

In step 103, the amount W of NOx absorbed in the NOx catalyst is calculated. That is, the NOx amount discharged from the combustion chamber increases as the intake air amount Q increases, and increases as the engine load Q / N increases.
The NOx amount W absorbed by the catalyst is W and k · Q · Q / N
(K is a constant).

Next, at step 104, it is determined whether or not the NOx amount W absorbed by the NOx catalyst is larger than a predetermined set amount Wo. This set amount Wo is, for example, about 30% of the maximum NOx amount that can be absorbed by the NOx catalyst. If W ≦ Wo, the processing cycle is completed, and W> W
If it is o, the routine proceeds to step 105, where the NOx release flag is set.

At step 106, it is determined whether or not a predetermined time, for example, 3 seconds, has elapsed since the execution of the NOx releasing process. When 3 seconds have elapsed, the routine proceeds to step 107, where the NOx releasing process ends. During the execution of the NOx release process, the exhaust gas is made rich, so that the exhaust gas is made rich for 0.5 seconds to 1 second. During this time, NOx absorbed by the NOx catalyst is released. If NOx is not sufficiently released in one rich process, the rich process is repeated several times.

Next, at step 108, the NOx amount W absorbed by the NOx catalyst is set to 0 (zero). This NO
The release action of x occurs at relatively short intervals. On the other hand, if it is determined in step 102 that the point determined by the traveling distances So, S exceeds the curve f (So) in FIG. 7, that is, if it is determined that 80% of the NOx catalyst absorption capacity is occupied by sulfur, the process proceeds to step 102. Proceeding to 109, the execution of the sulfur treatment is started, and then, in step 110, the air-fuel ratio of the exhaust gas is made rich and the rich exhaust gas flows into the NOx catalyst. Next, at step 111, the rich exhaust passage time (processing time) T of the NOx catalyst is updated.
In step 2, it is determined whether the total of the rich exhaust passage times T has reached a predetermined set time To. If the passage time of the rich exhaust has already reached the set time To, for example, 2 minutes, the routine proceeds to step 113, where the re-sulfur treatment is completed and the supply of the rich exhaust gas is stopped.

When the total inflow time T of the rich exhaust does not reach the predetermined set time To in step 112, the flow of the rich exhaust is continued and step 116 is performed.
Then, the temperature of the NOx catalyst 18 is measured. If the catalyst temperature exceeds a predetermined temperature, for example, 650 ° C. in step 116, the supply of the rich exhaust gas is stopped, and the lean exhaust gas flows into the NOx catalyst 18. Next, the routine proceeds to step 119, where the catalyst temperature is measured. If the catalyst temperature is equal to or lower than 500 ° C. in step 120, the process returns to step 110, and the rich exhaust gas flows into the NOx catalyst 18 again. Catalyst temperature 500
If the temperature exceeds ℃, lean exhaust gas flows into the catalyst to lower the catalyst temperature. Next, proceeding to step 120, if the catalyst temperature is 500 ° C. or lower, proceeding to step 110, proceeding again to step 110, and the rich exhaust passes through the inside of the catalyst. In step 111, the rich exhaust passage time (processing time) T of the NOx catalyst is updated, and in step 112, it is determined whether or not the total rich exhaust passage time T has reached a predetermined set time To. If the rich exhaust passage time has already reached the set time To, for example, 2 minutes, step 113
Then, the re-sulfur treatment is completed, and the supply of the rich exhaust gas is stopped.

Next, at step 114, the running distance S is 0
(Zero), and the NOx amount W absorbed by the NOx catalyst.
Is set to 0 (zero). At this time, HC and CO not oxidized by the NOx catalyst 18 flow into the oxidation catalyst 20 provided downstream thereof, and are sufficiently oxidized by releasing oxygen stored in the oxidation catalyst.

In this embodiment, a first NOx concentration sensor 23 is provided at the inlet of the NOx catalyst 18, and a second NOx concentration sensor 26 is provided at the outlet of the NOx catalyst 18.
Each of these NOx concentration sensors 23 and 26 generates an output voltage proportional to the NOx concentration, and these output voltages are input to the ECU 9 via the corresponding AD converters. N
If most of the absorption capacity of the Ox catalyst is occupied by sulfur, the NOx absorption capacity of the NOx catalyst decreases, and a part of the NOx flowing into the NOx catalyst is discharged from the NOx catalyst without being absorbed by the NOx catalyst.

Therefore, in this embodiment, the first NOx
The NOx concentration C1 at the inlet of the NOx catalyst detected by the concentration sensor 23 and the NOx concentration C at the outlet of the NOx catalyst detected by the second NOx concentration sensor 26
A difference ΔC (= C1−C2) is calculated, and an average value ΔCm (= 1 / n 例 え ば Δ) of the difference ΔC for one minute, for example, is calculated.
C) is calculated, and when this average value ΔCm is lower than the set value C0, for example, 50% of the NOx concentration C1 at the inlet,
It is determined that sulfur occupies most of the absorption capacity of the NOx catalyst, and the exhaust gas is set to the stoichiometric air-fuel ratio.

In this embodiment, the NOx concentration C1 at the inlet of the NOx catalyst is detected by the first NOx concentration sensor 23. However, the NOx concentration C1 at the inlet of the NOx catalyst depends on the engine load Q / N. It becomes a function of the engine speed N. Therefore, N at the inlet of the NOx catalyst
The Ox concentration C1 is previously obtained by an experiment as a function of the engine load Q / N and the engine speed N, and the relationship between these is stored in advance in the ROM of the ECU 9 as a map, and the NOx concentration C1 is obtained from this map. You can also do so. In this case, it is not necessary to provide the first NOx concentration sensor 23.

In the embodiment of the present invention, the NOx catalyst 1
In carrying out the sulfur poisoning regeneration of No. 8, since the NOx catalyst 18 was arranged upstream of the exhaust passage so as not to contain the oxygen storage / release agent, the air-fuel ratio of the exhaust gas was made rich during the sulfur poisoning regeneration of the NOx catalyst 18. As a result, an atmosphere can be easily formed on the catalyst surface, and sulfur poisoning can be regenerated by supplying rich exhaust gas in a short time. Therefore, rich exhaust for a short time,
By performing the supply of the lean exhaust cyclically, the sulfur poisoning regeneration of the NOx catalyst 18 can be performed while controlling the supply of the rich exhaust so as not to exceed the allowable temperature limit of the NOx catalyst 18.

On the other hand, when the air-fuel ratio of the exhaust gas is lean, the oxidation catalyst 2 containing an oxygen storage / release agent such as ceria downstream is used.
0 means that oxygen is stored. Therefore, unburned combustibles such as HC and CO generated when the rich exhaust gas is supplied are oxidized by the oxygen released from the oxidation catalyst 20, so that the emission of HC and CO to the outside is suppressed. .

Since the NOx storage-reduction catalyst does not have an oxygen storage / release agent, it is not necessary to supply a reducing agent necessary for reducing oxygen released from the oxygen storage / release agent during sulfur poisoning regeneration. Fuel economy deterioration during sulfur poisoning regeneration can be reduced.

As described above, the exhaust gas purification apparatus of the present invention
x absorption and release functions and HC and CO oxidation functions
The sharing of the Ox catalyst 18 and the oxidation catalyst 20 realizes easy and efficient sulfur poisoning regeneration.

[0068]

As described above, according to the present invention, it is possible to easily form a rich atmosphere on the surface of the NOx catalyst of the exhaust gas purification apparatus, and therefore it is possible to carry out the poisoning regeneration of the NOx catalyst very efficiently. An exhaust gas purification device for an engine can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an exhaust gas purification device of the present invention.

FIG. 2 is a diagram showing another embodiment of the exhaust gas purification device of the present invention.

FIG. 3 is a diagram showing a state in which the storage reduction type NOx catalyst and the oxidation catalyst containing an oxygen storage and release agent of the exhaust gas purification apparatus of the present invention in FIG. 1 are installed.

FIG. 4 is a graph showing a relationship between a change in the temperature of a NOx catalyst and a sulfur poisoning regeneration rate.

FIG. 5 is a graph showing the relationship between the change in the exhaust air-fuel ratio on the surface of the NOx catalyst and the sulfur poisoning regeneration rate.

FIG. 6 is a graph showing a relationship between a change in sulfur poisoning regeneration time (rich exhaust gas passage time) and a sulfur poisoning regeneration rate.

FIG. 7 is a diagram illustrating a relationship between a sulfur absorption amount and a traveling distance of a vehicle.

FIG. 8 is a flowchart illustrating a routine for performing sulfur poisoning regeneration of a NOx catalyst.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 3 ... Intake body 9 ... ECU 10 ... Fuel injection valve 14 ... Exhaust pipe 18 ... Storage reduction type NOx catalyst 20 ... Oxidation catalyst containing oxygen storage and release agent

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/24 F02D 41/04 355 F02D 41/04 355 45/00 314Z 45/00 314 B01D 53/36 101A F term (reference) 3G084 AA01 BA08 BA09 BA13 BA20 DA10 DA25 EA07 EA11 EB08 EB25 EC01 EC03 FA05 FA10 FA18 FA27 FA28 FA29 FA33 3G091 AA02 AA10 AA11 AA18 AA28 AB02 AB06 AB09 BA04 BA11 BA14 BA15 DA02 DA02 DA02 DA02 DB07 DB08 DB09 DB10 EA01 EA03 EA05 EA07 EA17 EA18 EA30 EA31 EA33 EA38 FB10 FB12 FC02 GB01X GB02W GB03W GB04W GB06W GB10W GB10X GB16X HA09 HA18 HA36 HA37 HA42 HA47 HAB05 HB06 3G301 HA02 NA01 NA13 NA01 NA01 MA01 PD01Z PD03A PD11Z PE01Z PE03Z PF01Z PF03Z 4D048 AA06 AB03 BD02 BD04 EA04

Claims (2)

[Claims] An oxygen storage / release NOx catalyst not provided with an oxygen storage / release agent provided in an exhaust passage of an internal combustion engine capable of lean combustion, and an oxygen storage / release agent provided downstream of the storage / reduction NOx catalyst. An exhaust gas purification device for an internal combustion engine, comprising: an oxidation catalyst; and an air-fuel ratio adjusting means for adjusting an air-fuel ratio of an exhaust passage. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas is made rich when the sulfur poisoning amount of the NOx storage reduction catalyst is larger than a predetermined amount.