JP3624815B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus capable of purifying nitrogen oxide (NOx) from exhaust gas discharged from a lean burnable internal combustion engine, and more particularly to an apparatus capable of detecting thermal deterioration of a NOx catalyst.
[0002]
[Prior art]
An NOx storage reduction catalyst is an exhaust purification device that purifies NOx from exhaust gas discharged from a lean burnable internal combustion engine. The NOx storage reduction catalyst (hereinafter sometimes simply referred to as catalyst or NOx catalyst) absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (that is, in an oxygen-excess atmosphere), and the oxygen concentration of the inflowing exhaust gas. NOx absorbed when NO decreases and N₂It is a catalyst that reduces to
[0003]
The structure of the NOx catalyst and the NOx purification mechanism will be briefly described. The NOx catalyst carries a catalyst substance such as platinum (Pt) and a NOx storage agent such as barium (Ba) on a support such as alumina. (In the following description, an example of a NOx catalyst in which Pt and Ba are supported) will be described. When a lean air-fuel ratio exhaust gas flows into the NOx catalyst, NOx in the exhaust gas is oxidized on the surface of Pt. NO₂And this NO₂Is absorbed in Ba and combined with BaO, and nitrate ion NO.₃It is absorbed in the form of This NOx absorption is performed at the interface between Ba and Pt. On the other hand, when the stoichiometric (theoretical air / fuel ratio) or rich air / fuel ratio exhaust gas flows into the NOx catalyst and the oxygen concentration decreases, the NOx absorbed in the NOx catalyst₃-NO₂Alternatively, it is released in the form of NO, and further reacts with HC and CO in the exhaust gas to produce N₂To be reduced.
[0004]
Therefore, if this NOx catalyst is disposed in the exhaust passage of an internal combustion engine capable of lean combustion, and the lean air-fuel ratio exhaust gas and the stoichiometric or rich air-fuel ratio exhaust gas flow alternately, the NOx in the exhaust gas is purified. Can do.
[0005]
It has been known that when this NOx catalyst is used for exhaust gas purification for a long time, the NOx catalyst deteriorates with time and the NOx purification ability decreases as shown in FIG. This deterioration is considered to be caused by (a) sulfur in the fuel and (b) heat. The deterioration caused by sulfur in the fuel is called sulfur poisoning deterioration, and the deterioration caused by heat is called heat deterioration. Sometimes they are distinguished. In addition, although sulfur poisoning deterioration may be expressed as S poisoning deterioration or SOx poisoning deterioration, in the following description, it unifies with the expression SOx poisoning deterioration.
[0006]
Explaining the SOx poisoning deterioration, the fuel of the internal combustion engine generally contains a sulfur content. When the fuel is burned in the internal combustion engine, the sulfur content in the fuel is combusted and the SO₂Or SO₃Sulfur oxide (SOx) is generated. The NOx storage reduction catalyst absorbs SOx in the exhaust gas by the same mechanism that absorbs NOx. Therefore, if a NOx catalyst is disposed in the exhaust passage of the internal combustion engine, the NOx catalyst can contain only NOx. SOx is also absorbed.
[0007]
However, SOx absorbed by the NOx catalyst forms a stable sulfate with the passage of time, so that it is difficult to be decomposed and released and tends to accumulate in the catalyst. The accumulation of sulfate on the NOx catalyst is called SOx poisoning. When SOx poisoning progresses and the amount of SOx accumulated in the NOx catalyst increases, the NOx absorption capacity of the catalyst decreases and the NOx purification rate decreases. . This is SOx poisoning deterioration.
[0008]
Explaining thermal degradation, as described above, NOx absorption in the NOx catalyst is carried out at the interface between Pt (catalyst substance) and Ba (NOx absorbent), but Pt causes sintering and growth. It is known that the particle size increases. In the purification of exhaust gas discharged from the vehicle internal combustion engine, the heat load applied to the NOx catalyst is large, and Pt sintering cannot be avoided. Thus, when Pt causes sintering, the contact area between Pt and Ba decreases, that is, the interface between Pt and Ba decreases. As a result, the NOx absorption capability of the NOx catalyst is reduced, and the NOx purification capability is reduced. This is thermal degradation.
[0009]
FIG. 12 shows the influence of the sintering of the catalyst material (catalyst material particle size growth) on the NOx purification performance, and when the catalyst material sintering is not advanced (that is, when the particle size is small). The NOx purification performance is high, and the NOx purification performance decreases as the sintering progresses (that is, as the particle size increases).
[0010]
[Problems to be solved by the invention]
By the way, as for the SOx poisoning deterioration, as disclosed in the patent publication No. 2745985, etc., SOx absorbed in the NOx catalyst is released by performing a predetermined recovery process, and SO₂Therefore, the NOx catalyst can be recovered from the SOx poisoning.
[0011]
On the other hand, it is impossible to return Pt once sintered to the state before sintering, and therefore it is impossible to recover the NOx catalyst from thermal deterioration. Therefore, in managing the NOx catalyst, it is important to detect the degree of thermal degradation (thermal degradation level) of the NOx catalyst.
[0012]
However, at present, a technique for detecting the degree of thermal deterioration has not been established, and development of a technique for detecting the degree of thermal deterioration is eagerly desired.
The present invention has been made in view of such problems of the conventional technology, and the problem to be solved by the present invention is to establish an internal combustion engine by establishing a technique for detecting the degree of thermal degradation of the NOx storage reduction catalyst. The purpose is to improve engine exhaust gas purification technology.
[0013]
[Means for Solving the Problems]
The present invention employs the following means in order to solve the above problems.
An exhaust gas purification apparatus for an internal combustion engine according to a first aspect of the present invention includes (a) an exhaust gas that is provided in an exhaust passage of a lean burnable internal combustion engine and absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean. NOx absorbed is released when the oxygen concentration of N is low.₂(B) poisoning recovery execution time determination means for determining whether it is time to execute SOx poisoning recovery processing for releasing SOx from the NOx storage reduction catalyst; ) When it is determined by the poisoning recovery execution time determination means that the execution time is reached, almost the entire amount of SOx deposited on the NOx storage reduction catalyst is released, and the NOx storage reduction catalyst is SOx poisoned. Poison recovery means for recovering from NO, (d) storage capacity detection means for detecting the NOx storage capacity of the NOx storage reduction catalyst, and (e) the SOx poison recovery process after the SOx poison recovery process is executed by the poison recovery means. The NOx storage capacity of the NOx storage reduction catalyst detected by the storage capacity detection means is compared with the reference NOx storage capacity, and based on the comparison value, the NOx storage capacity of the NOx storage reduction catalyst is compared. And determining heat deterioration determination means for deterioration degree, characterized in that it comprises a.
[0014]
In this exhaust gas purification apparatus for an internal combustion engine, when the poisoning recovery execution time determining means determines that it is time to execute the SOx poisoning recovery process for the NOx storage reduction catalyst, the poisoning recovery means performs the storage reduction. The SOx poisoning recovery process for the type NOx catalyst is executed, and the SOx stored in the NOx storage reduction catalyst is almost completely released. Immediately after the completion of the SOx poisoning recovery process, the storage capacity detecting means detects the NOx storage capacity of the NOx storage reduction catalyst. Thereafter, the thermal deterioration determination means compares the NOx storage capacity detected by the storage capacity detection means with the reference NOx storage capacity, and determines the thermal deterioration degree of the NOx storage reduction catalyst based on the comparison value. .
[0015]
The principle for determining the degree of thermal degradation is as follows. The deterioration of the NOx storage reduction catalyst includes SOx poisoning deterioration caused by sulfur in the fuel and heat deterioration caused by heat. The SOx poisoning deterioration is recovered by executing a predetermined poisoning recovery process. In contrast, once thermal degradation occurs, it cannot be recovered. Therefore, even if the SOx poisoning recovery process is executed, the portion where the performance cannot be recovered can be regarded as being due to thermal deterioration. This is the principle of determining the degree of thermal deterioration in the present invention.
[0016]
In the exhaust gas purification apparatus for an internal combustion engine according to the first aspect of the present invention, the NOx occlusion ability used as a reference by the thermal deterioration determining means is NOx of a new NOx storage reduction catalyst that has not undergone both SOx poisoning deterioration and heat deterioration. It can be the storage capacity.
[0017]
The comparison value obtained by comparing the NOx occlusion capacity of the NOx storage reduction catalyst detected by the occlusion reduction capacity detection means with the reference NOx occlusion capacity may be a difference between the two. The quotient may be obtained by dividing one by the other.
[0018]
An exhaust gas purification apparatus for an internal combustion engine according to the second invention
(B) provided in the exhaust passage of a lean burnable internal combustion engine, which absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low, and N₂A NOx storage reduction catalyst that reduces to
(B) poisoning recovery execution time determination means for determining whether it is time to execute SOx poisoning recovery processing for releasing SOx from the NOx storage reduction catalyst;
(C) When it is determined by the poisoning recovery execution time determination means that it is the execution time, almost all of the SOx deposited on the NOx storage reduction catalyst is released to make the NOx storage reduction catalyst SOx Poisoning recovery means to recover from poisoning,
(D) a purification rate measuring means for measuring a NOx purification rate of the NOx storage reduction catalyst;
(E) After performing the SOx poisoning recovery process by the poisoning recovery means, the NOx purification rate that is the NOx purification rate standard of the NOx storage reduction catalyst measured by the purification rate measurement is compared, and the comparison value is obtained Thermal degradation determination means for determining the thermal degradation degree of the NOx storage reduction catalyst based on
Characterized by comprising
[0019]
In the second invention, the NOx purification rate measuring means is based on the NOx amount in the exhaust gas flowing into the NOx catalyst and the NOx amount in the exhaust gas by the NOx detecting means installed downstream of the NOx catalyst. Can be measured.
In the exhaust gas purification apparatus for an internal combustion engine according to the second aspect of the invention, when the poisoning recovery execution time determination means determines that it is time to execute the SOx poisoning recovery processing for the NOx storage reduction catalyst, the poison recovery means However, the SOx poisoning recovery process for the NOx storage reduction catalyst is executed, and the SOx stored in the NOx storage reduction catalyst is almost completely released. Immediately after the completion of the SOx poisoning recovery process, the purification rate measuring means measures the NOx purification rate of the NOx storage reduction catalyst. Thereafter, the thermal deterioration determining means compares the NOx purification rate measured by the purification rate measuring means with the reference NOx purification rate, and determines the degree of thermal degradation of the NOx storage reduction catalyst based on the comparison value. .
[0020]
The principle for determining the degree of thermal degradation is as follows. The deterioration of the NOx storage reduction catalyst includes SOx poisoning deterioration caused by sulfur in the fuel and heat deterioration caused by heat. The SOx poisoning deterioration is recovered by executing a predetermined poisoning recovery process. In contrast, once thermal degradation occurs, it cannot be recovered. Therefore, even if the SOx poisoning recovery process is executed, the portion where the performance cannot be recovered can be regarded as being due to thermal deterioration. This is the principle of determining the degree of thermal deterioration in the present invention.
[0021]
In the exhaust gas purification apparatus for an internal combustion engine according to the second aspect of the present invention, the NOx purification rate based on the thermal degradation determination means is the NOx of a new NOx storage reduction catalyst that has not undergone SOx poisoning degradation or thermal degradation. It can be a purification rate.
[0022]
The comparison value obtained by the thermal degradation determination means by comparing the NOx purification rate of the NOx storage reduction catalyst measured by the purification rate measurement means with the reference NOx purification rate may be the difference between the two. The quotient may be obtained by dividing one by the other.
[0023]
As the lean burnable internal combustion engine in the first and second inventions (hereinafter referred to as the present invention), an in-cylinder direct injection type lean burn gasoline engine or diesel engine can be exemplified.
The air-fuel ratio of exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the engine intake passage and the NOx storage reduction catalyst.
[0024]
When the internal combustion engine is a lean burn gasoline engine, the exhaust air / fuel ratio control means can control the air / fuel ratio of the exhaust gas by controlling the air / fuel ratio of the air-fuel mixture supplied to the combustion chamber. Further, when the internal combustion engine is a diesel engine, the reducing agent is supplied by so-called sub-injection in which fuel is injected in the intake stroke, expansion stroke or exhaust stroke, or in the exhaust passage upstream of the NOx storage reduction catalyst. As a result, the air-fuel ratio of the exhaust gas can be controlled.
[0025]
The NOx storage reduction catalyst 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 decreases, and N₂It is a catalyst that reduces to This NOx storage reduction catalyst uses, for example, alumina as a carrier, and on this carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, etc. And at least one selected from rare earths such as yttrium Y and a noble metal such as platinum Pt are supported.
[0026]
Further, the storage capacity detection means switches the lean air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst from lean to rich after the NOx storage state of the NOx storage reduction catalyst is saturated, and maintains rich, The time required from when the air-fuel ratio is switched to when the air-fuel ratio of the exhaust gas at the outlet of the NOx storage reduction catalyst becomes rich can be detected as a barometer of NOx storage capacity.
[0027]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the poisoning recovery execution timing determination means is configured to detect when the NOx storage capacity of the NOx storage reduction catalyst detected by the storage capacity detection means falls below a predetermined lower limit value. It can be determined that it is time to execute the SOx poisoning recovery process. In this case, for example, it is preferable to detect the NOx storage capacity of the NOx storage reduction catalyst by the storage capacity detection means at every fixed fuel consumption or every fixed travel distance.
[0028]
In addition, it includes an input gas SOx concentration detection means for detecting the SOx concentration of the exhaust gas flowing into the NOx storage reduction catalyst, and the poisoning recovery execution timing determination means is the SOx concentration detected by the input gas SOx concentration detection means. When the amount of SOx flowing into the NOx storage reduction catalyst calculated based on the intake air amount of the internal combustion engine exceeds a predetermined amount, it can be determined that it is time to execute the SOx poisoning recovery process.
[0029]
Further, it includes an input gas SOx concentration detection means for detecting the SOx concentration of the exhaust gas flowing into the NOx storage reduction catalyst, and the poisoning recovery execution timing determination means is the SOx concentration detected by the input gas SOx concentration detection means. The SOx poisoning recovery process is executed when the amount of SOx flowing into the NOx storage reduction catalyst calculated based on the sulfur concentration of the fuel of the internal combustion engine and the fuel consumption or the travel distance estimated from the above exceeds a predetermined amount It can be determined that it is time. In this way, the degree of thermal degradation can be determined even when fuels having different sulfur concentrations are used.
[0030]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the input gas SOx concentration detecting means for detecting the SOx concentration of the exhaust gas flowing into the NOx storage reduction catalyst, the exhaust gas flowing out from the NOx storage reduction catalyst, and An outgas SOx concentration detection means for detecting the SOx concentration, and the poisoning recovery execution timing determination means is detected by the SOx concentration detected by the input gas SOx concentration detection means and the output gas SOx concentration detection means. When the SOx occlusion amount estimated based on the concentration difference from the SOx concentration and the intake air amount or exhaust gas amount of the internal combustion engine exceeds a predetermined amount, it is determined that it is time to execute the SOx poisoning recovery process. Can do. This is because when the SOx concentration upstream of the NOx storage reduction catalyst is larger than the SOx concentration downstream of the NOx storage reduction catalyst, it can be considered that the concentration difference is absorbed by the NOx storage reduction catalyst.
[0031]
Input gas SOx concentration detection means for detecting the SOx concentration of the exhaust gas flowing into the NOx storage reduction catalyst, and output gas SOx concentration detection means for detecting the SOx concentration of the exhaust gas flowing out of the NOx storage reduction catalyst And the poisoning recovery execution timing determination means, when the SOx concentration detected by the outgas SOx concentration detection means approaches a predetermined level with respect to the SOx concentration detected by the input gas SOx concentration detection means, It can be determined that it is time to execute the SOx poisoning recovery process. This is because the SOx concentration downstream of the NOx storage reduction catalyst approaches the SOx concentration upstream of the NOx storage reduction catalyst as the SOx accumulation amount of the NOx storage reduction catalyst approaches the saturated state.
[0032]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, when the input gas SOx concentration detection means and the output gas SOx concentration detection means are provided, the SOx storage amount estimated to be stored in the NOx storage reduction catalyst is estimated. From the above, the concentration of the SOx concentration detected by the outgas SOx concentration detection means and the SOx concentration detected by the input gas SOx concentration detection means when performing the SOx poisoning recovery processing by the poisoning recovery means The amount of released SOx estimated based on the difference and the intake air amount or exhaust gas amount of the internal combustion engine is subtracted to estimate the SOx amount remaining in the NOx storage reduction catalyst, and this SOx remaining amount is reduced to a predetermined amount. In this case, the SOx poisoning recovery process by the poisoning recovery means can be terminated. When the SOx concentration downstream of the NOx storage reduction catalyst is larger than the SOx concentration upstream of the NOx storage reduction catalyst, the difference in concentration can be considered as SOx released from the NOx storage reduction catalyst, and the intake air This is because the released SOx amount can be calculated and estimated based on the amount or the exhaust gas amount.
[0033]
The exhaust gas purification apparatus for an internal combustion engine according to the present invention includes a heat deterioration suppressing means for suppressing heat deterioration of the NOx storage reduction catalyst, and the heat deterioration suppressing means is the heat deterioration determined by the heat deterioration determining means. It is not activated when the degree is below a predetermined level, but can be activated when the degree exceeds the predetermined level. Thereby, it can suppress that thermal deterioration progresses more than that, and can suppress the fall of the NOx occlusion capability of a NOx storage reduction catalyst.
[0034]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the thermal deterioration suppressing means may be a fuel cut prohibition control that prohibits a fuel cut in a high temperature operation region of the internal combustion engine. When fuel cut is performed in a high temperature operation region, exhaust gas with high oxygen concentration flows when the high temperature storage reduction type NOx catalyst is in a high temperature state, which promotes sintering of the catalyst material that causes thermal degradation. End up. This can be prevented by prohibiting fuel cut in the high temperature operation region.
[0035]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the thermal deterioration suppressing means may be a lean operation region reduction control for reducing a lean operation region in a high temperature operation region of the internal combustion engine. By reducing the lean operation region in the high temperature operation region, it is possible to reduce the flow of exhaust gas having a high oxygen concentration in the high temperature operation region to the NOx storage reduction catalyst, and as a result, thermal degradation of the NOx storage reduction catalyst. Can be suppressed.
Further, the thermal degradation suppressing means increases the amount of rich or stoichiometric exhaust gas flowing into the NOx catalyst according to the degree of thermal degradation, and / or shortens the period during which the rich or stoichiometric exhaust gas flows into the NOx catalyst. Rich spike control can be performed. In this way, it is possible to increase the amount of the reducing agent for releasing and reducing NOx according to the degree of thermal degradation, and to suppress a decrease in action due to thermal degradation of the NOx storage reduction catalyst.
[0036]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings of FIGS.
[0037]
[First Embodiment]
FIG. 1 is a diagram showing a schematic configuration when the present invention is applied to a vehicle gasoline engine capable of lean combustion. In this figure, reference numeral 1 is an engine body, reference numeral 2 is a piston, reference numeral 3 is a combustion chamber, reference numeral 4 is a spark plug, reference numeral 5 is an intake valve, reference numeral 6 is an intake port, reference numeral 7 is an exhaust valve, and reference numeral 8 is an exhaust port. Respectively.
[0038]
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 into the intake port 6 is attached to each branch pipe 9. 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 disposed in the intake duct 12.
[0039]
On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 18 containing a NOx storage reduction catalyst 17 (in the following description, sometimes abbreviated as NOx catalyst). A muffler (not shown) is connected via 19.
[0040]
An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and a CPU (Central Processor Unit) 34 are connected to each other. An input port 35 and an output port 36. The air flow meter 21 generates an output voltage proportional to the amount of intake air, and this output voltage is input to the input port 35 via the AD converter 38.
[0041]
The exhaust pipe 16 upstream of the casing 18 has an input gas SOx sensor (input gas SOx concentration detection means) that generates an output voltage proportional to the SOx concentration of exhaust gas flowing into the NOx catalyst 17 (hereinafter referred to as input gas). ) 23 is provided.
On the other hand, in the exhaust pipe 19 downstream of the casing 18, an output gas SOx sensor (output gas SOx concentration) that generates an output voltage proportional to the SOx concentration of exhaust gas flowing out from the NOx catalyst 17 (hereinafter referred to as output gas). Detection means) 24, a temperature sensor 25 for generating an output voltage proportional to the temperature of the output gas, and an air-fuel ratio sensor 27 for generating an output voltage representing the air-fuel ratio of the output gas are attached. The output voltages of the SOx sensors 23 and 24, the temperature sensor 25, and the air-fuel ratio sensor 27 are input to the input port 35 via corresponding AD converters 38, respectively.
[0042]
The input port 35 is connected to a rotational speed sensor 26 that generates an output pulse representing the engine rotational speed. The output port 36 is connected to the spark plug 4 and the fuel injection valve 11 via a corresponding drive circuit 39, respectively.
[0043]
In this gasoline engine, the fuel injection time TAU is calculated based on the following equation, for example.
TAU = TP ・ K
Here, TP indicates the basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by experiments, and is previously stored in the ROM 32 in the form of a map as shown in FIG. 2 as a function of the engine load Q / N (intake air amount Q / engine speed N) and the engine speed N. Is remembered. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder. If K = 1.0, the air-fuel mixture supplied into the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, when 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 when K> 1.0, the air-fuel ratio is supplied into the engine cylinder. The air-fuel ratio of the air-fuel mixture is smaller than the stoichiometric air-fuel ratio, that is, becomes rich.
[0044]
In the gasoline engine of this embodiment, the value of the correction coefficient K is set to a value smaller than 1.0 in the engine low and medium load operation region, and the lean air-fuel ratio control is performed. During warm-up operation, acceleration, and high-speed constant speed operation, the value of the correction coefficient K is set to 1.0 and the stoichiometric control is performed. In the engine full load operation region, the value of the correction coefficient K is 1.0. It is set so that the rich air-fuel ratio control is performed with a larger value.
[0045]
The internal combustion engine is usually most frequently operated at low and medium loads. Therefore, the value of the correction coefficient K is made smaller than 1.0 during most of the operation period, and the lean air-fuel mixture is combusted.
[0046]
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. Oxygen in exhaust gas exhausted₂The concentration of increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.
[0047]
The NOx catalyst (occlusion reduction type NOx catalyst) 17 accommodated in the casing 18 uses, for example, alumina as a carrier, and an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, barium Ba on the carrier. At least one selected from alkaline earths such as calcium Ca, lanthanum La, and rare earths such as yttrium Y, and a noble metal such as platinum Pt are supported.
[0048]
When this NOx catalyst 17 is arranged in the exhaust passage of the engine, the NOx catalyst 17 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter sometimes referred to as the exhaust air-fuel ratio) is lean, and the NOx catalyst 17 When the oxygen concentration is lowered, NOx is absorbed and released to release the absorbed NOx. Here, the exhaust air-fuel ratio refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx catalyst 17.
[0049]
When fuel (hydrocarbon) or air is not 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 17 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is lean, and absorbs NOx when the oxygen concentration in the air-fuel mixture supplied into the combustion chamber 3 decreases. Will be released.
[0050]
It is considered that the NOx absorption / release action by the NOx catalyst 17 is performed by the mechanism shown in FIG. Hereinafter, this mechanism will be described by taking as an example the case where platinum Pt and barium Ba are supported on the support, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0051]
First, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases, and as shown in FIG.₂  Is O₂ ⁻Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO contained in the inflowing exhaust gas is O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂  (2NO + O₂  → 2NO₂  ).
[0052]
Then the generated NO₂Is oxidized on platinum Pt while being absorbed into Ba and combined with barium oxide BaO, as shown in FIG.₃ ⁻In the form of Ba. In this way, NOx is absorbed into the NOx catalyst 17.
[0053]
As long as the oxygen concentration in the inflowing exhaust gas is high, NO on the surface of platinum Pt₂And NOx absorption capacity of the NOx catalyst 17 is not saturated.₂Is absorbed in the NOx catalyst 17 and nitrate ions NO.₃ ⁻Is generated.
[0054]
On the other hand, the oxygen concentration in the inflowing exhaust gas decreases and NO₂When the production amount of NO decreases, the reaction reverses (NO₃ ⁻→ NO₂) And nitrate ion NO in Ba₃ ⁻Is NO₂Or released from Ba in the form of NO. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx catalyst 17. As shown in FIG. 3, if the lean degree of the inflowing exhaust gas is lowered, the oxygen concentration in the inflowing exhaust gas is lowered. Therefore, if the lean degree of the inflowing exhaust gas is lowered, NOx is released from the NOx catalyst 17. The Rukoto.
[0055]
On the other hand, when the air-fuel mixture supplied into the combustion chamber 3 at this time becomes a stoichiometric or rich air-fuel ratio, a large amount of unburned HC and CO are discharged from the engine as shown in FIG. CO is oxygen O on platinum Pt.₂ ⁻Or O^2-It reacts with and is oxidized.
[0056]
Further, when the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the oxygen concentration in the inflowing exhaust gas extremely decreases, so that the NOx catalyst 17₂Or NO is released and this NO₂Alternatively, NO is reduced by reacting with unburned HC and CO as shown in FIG.₂It becomes.
[0057]
That is, HC and CO in the inflowing exhaust gas are first oxygen O on platinum Pt.₂ ⁻Or O^2-Immediately reacts with and oxidizes, then oxygen O on platinum Pt.₂ ⁻Or O^2-If HC and CO still remain after the consumption of NOx, NOx released from the NOx catalyst 17 by the HC and CO and NOx in the inflowing exhaust gas are N₂To be reduced.
[0058]
In this way, NO on the surface of platinum Pt.₂Or, when NO is no longer present, NOx catalyst 17 continues to NO.₂Or NO is released and N₂To be reduced. Therefore, when the exhaust air-fuel ratio is made the stoichiometric air-fuel ratio or rich, NOx is released from the NOx catalyst 17 within a short time.
[0059]
Thus, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx catalyst 17, and when the exhaust air-fuel ratio is made rich or rich, NOx is released from the NOx catalyst 17 in a short time, and N₂Reduced to Accordingly, NOx emission into the atmosphere can be prevented.
[0060]
By the way, in this embodiment, as described above, the air-fuel mixture supplied into the combustion chamber 3 is made rich during full load operation, and the air-fuel mixture is made the stoichiometric air-fuel ratio during high load operation, etc. Since the air-fuel mixture is made lean during load operation, NOx in the exhaust gas is absorbed by the NOx catalyst 17 during low and medium load operation, and NOx is released from the NOx catalyst 17 during full load operation and high load operation, etc. Will be. However, if the frequency of full load operation or high load operation is low, the frequency of low and medium load operations is high and the operation time is long, NOx release / reduction will not be in time, and the NOx absorption capacity of the NOx catalyst 17 will be saturated. This makes it impossible to absorb NOx.
[0061]
Therefore, in this embodiment, when the lean air-fuel mixture is burned, that is, when the medium / low load operation is being performed, the stoichiometric or rich air-fuel mixture is spiked (short time) in a relatively short cycle. The air-fuel ratio of the air-fuel mixture is controlled so that combustion is performed, and NOx is released and reduced in a short cycle. As described above, in order to absorb and release NOx, the exhaust air-fuel ratio (the air-fuel ratio of the air-fuel mixture in this embodiment) is “lean” and “spike-like stoichiometric air-fuel ratio (rich air-fuel ratio) in a relatively short cycle. The control to repeat “spike)” alternately is called lean / rich spike control. In this application, the lean / rich spike control is included in the lean air-fuel ratio control.
[0062]
On the other hand, the fuel contains sulfur (S). When sulfur in the fuel burns, sulfur oxides (SOx) such as SO2 and SO3 are generated, and the NOx catalyst 17 also absorbs these SOx in the exhaust gas. . The SOx absorption mechanism of the NOx catalyst 17 is considered to be the same as the NOx absorption mechanism. That is, the case where platinum Pt and barium Ba are supported on the carrier as in the case of explaining the NOx absorption mechanism will be described as an example. As described above, when the exhaust air-fuel ratio is lean, oxygen O 2₂Is O₂ ⁻Or O^2-Is attached to the surface of the platinum Pt of the NOx catalyst 17 in the form of SOx (for example, SO₂) Is oxidized on the surface of platinum Pt to form SO.₃It becomes.
[0063]
Then the generated SO₃Is absorbed in Ba while being further oxidized on the surface of platinum Pt, and is combined with barium oxide BaO to form sulfate ion SO.₄ ^2-And diffused into Ba in the form of sulfate BaSO₄Form. This BaSO₄Since crystals tend to coarsen and are relatively stable, once produced, they are difficult to be decomposed and released. And BaSO in the NOx catalyst 17₄When the production amount of NO 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, in order to maintain the NOx absorption capacity of the NOx catalyst 17 high, it is necessary to execute SOx poisoning recovery processing for releasing SOx absorbed by the NOx catalyst 17 at an appropriate timing.
[0064]
In order to release SOx from the NOx catalyst 17, it is necessary to set the air-fuel ratio of the inflowing exhaust gas to the stoichiometric air-fuel ratio or rich air-fuel ratio, and it is found that the higher the catalyst temperature of the NOx catalyst 17, the easier it is to release. ing. Therefore, in the SOx poisoning recovery process, the catalyst temperature of the NOx catalyst 17 is set to a predetermined high temperature at which SOx is easily released (hereinafter referred to as the SOx release temperature), and the exhaust gas having the stoichiometric or rich air-fuel ratio is converted to the NOx catalyst 17. Execute by flowing to.
[0065]
In this embodiment, even when the air-fuel ratio of the exhaust gas is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio for the SOx poisoning recovery process, the amount of fuel injected from the fuel injection valve 11 is controlled by the ECU 30. This is performed by controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 to the theoretical air-fuel ratio or the rich air-fuel ratio. Therefore, the ECU 30 and the fuel injection valve 11 constitute part of the poisoning recovery means.
[0066]
Next, a method for detecting the degree of thermal deterioration of the NOx catalyst 17 of the exhaust purification apparatus in this embodiment will be described.
FIG. 5 shows that every time the SOx poisoning of the NOx catalyst 17 reaches a predetermined level, the SOx poisoning recovery process is executed until the SOx occluded in the NOx catalyst 17 is completely released, and immediately after the SOx poisoning recovery. 3 is a graph showing a result of an experiment in which the NOx purification performance of the NOx catalyst 17 is measured. The NOx purification performance of the NOx catalyst 17 decreases every time the SOx poisoning recovery process is executed.
[0067]
As described above, the SOx poisoning deterioration of the NOx catalyst 17 can be recovered by executing the SOx poisoning recovery process. However, since the NOx catalyst 17 cannot be recovered from the heat deterioration, the SOx poisoning recovery is impossible. The performance recovery portion immediately after the poisoning recovery process can be said to be a performance decrease due to SOx poisoning, and the portion where the performance recovery cannot be performed can be said to be a performance decrease due to thermal deterioration.
[0068]
Therefore, if the NOx storage capacity of the NOx catalyst 17 when it is new and the NOx storage capacity of the NOx catalyst 17 immediately after the SOx poisoning recovery process can be detected, the degree of thermal deterioration is detected from the difference between these NOx storage capacities. be able to.
[0069]
In this embodiment, the NOx storage capacity is detected as follows. First, a lean air-fuel ratio exhaust gas is continuously supplied to the NOx catalyst 17 to saturate the NOx storage capacity of the NOx catalyst 17 so that no more NOx can be stored. Next, the exhaust gas flowing into the NOx catalyst 17, that is, the air-fuel ratio of the input gas is switched from lean to rich, and the rich air-fuel ratio exhaust gas continues to flow through the NOx catalyst 17, so that the NOx stored in the NOx catalyst 17 is stored. Release and reduce the entire amount. At this time, even if the air-fuel ratio of the input gas is switched from lean to rich, the exhaust gas at the outlet of the NOx catalyst 17, that is, the air-fuel ratio of the output gas, does not immediately switch to rich, but remains near the stoichiometric air-fuel ratio for a while. . This is because HC and CO in the input gas are consumed as a reducing agent for the reduction of NOx stored in the NOx catalyst 17. When all of the NOx stored in the NOx catalyst 17 is released and reduced and HC and CO in the input gas are no longer consumed as a reducing agent, the air-fuel ratio of the output gas changes to rich.
[0070]
Therefore, after saturating the NOx storage capacity of the NOx catalyst 17 in this way, NOx release / reduction processing (hereinafter referred to as saturated NOx release processing) is executed, and the air-fuel ratio of the output gas at that time is determined. The time that the air-fuel ratio of the output gas is maintained at the stoichiometric air-fuel ratio after the saturation NOx release process is detected by the air-fuel ratio sensor 27, in other words, the time until the air-fuel ratio of the output gas switches from the stoichiometric air-fuel ratio to rich. , The required time (hereinafter referred to as “rich switching time”) is a barometer representing the NOx storage capacity of the NOx catalyst 17, that is, a purification performance barometer of the NOx catalyst 17.
[0071]
FIG. 6 shows an example of the temporal change in the air-fuel ratio of the incoming gas and the air-fuel ratio of the outgoing gas during the saturated NOx releasing process. In the new NOx catalyst 17, the rich switching time is t1, The rich switching time of the NOx catalyst 17 in which the thermal deterioration has progressed to some extent is t2, which is shorter than t1.
[0072]
In this embodiment, the rich switching time is also used to determine when to perform the SOx poisoning recovery process on the NOx catalyst 17. More specifically, in the exhaust emission control device of this embodiment, the saturated NOx release process is performed on the NOx catalyst 17 at an appropriate timing, for example, at every constant travel distance or every constant fuel consumption. Measure rich switching time. Then, when the measured rich switching time becomes shorter than the predetermined time t3 shown in FIG. 6, it can be considered that the NOx purification performance of the NOx catalyst 17 has decreased to a predetermined level. It is determined that it is time to execute the poison recovery process.
[0073]
Further, in the exhaust purification apparatus of this embodiment, after the SOx poisoning recovery process is completed, the saturated NOx release process is executed to measure the rich switching time, and the measured rich switching time t2 and the richness of the new NOx catalyst 17 are measured. When the difference from the switching time t1 is calculated and the difference becomes longer than the predetermined time t4, it is determined that the thermal deterioration of the NOx catalyst 17 has progressed to a predetermined degree, and the thermal deterioration is suppressed to suppress the thermal deterioration. Activating means. Note that the rich switching time t1 of the new NOx catalyst 17 is experimentally obtained in advance and stored in the ROM 32 of the ECU 30, and a threshold t4 for determining that the thermal deterioration suppressing means should be operated is also set in advance. It memorize | stores in ROM of ECU30.
[0074]
Next, a method for suppressing thermal deterioration will be described. For example, in general, when the engine is decelerated, fuel cut is executed, but when the thermal deterioration has progressed to a predetermined degree, the engine is in a high temperature operating range. When the vehicle is decelerated, it is possible to suppress thermal degradation by prohibiting fuel cut. This is because when the engine is in a high temperature operation region, the NOx catalyst 17 is also in a high temperature state, and if fuel cut is performed at this time, exhaust gas having a high oxygen concentration flows into the NOx catalyst 17. It is known that the sintering of the catalyst material (Pt) in the NOx catalyst 17 is faster as the oxygen concentration is higher when the catalyst temperature is the same. Therefore, if fuel cut is performed during deceleration in the high temperature operation region, exhaust gas with a high oxygen concentration flows when the catalyst temperature is high, which promotes sintering. Therefore, if the fuel cut is prohibited when the deceleration operation is performed in the high temperature operation region, sintering of the NOx catalyst 17 can be suppressed and thermal degradation can be suppressed. In this case, the fuel cut prohibition control that prohibits the fuel cut in the high temperature operation region constitutes the thermal deterioration suppressing means.
[0075]
Further, as another method for suppressing thermal degradation, when the thermal degradation has progressed to a predetermined degree, the leanness in the high temperature operation region of the engine is higher than before that (that is, when the thermal degradation has not progressed to the predetermined degree). There is a way to reduce the operating range. This is also the same reason as described above, and when the catalyst temperature of the NOx catalyst 17 is high, the thermal deterioration of the NOx catalyst 17 can be suppressed by lowering the oxygen concentration of the input gas. As a specific control method, a part or all of the high temperature operation region and the lean operation region before the heat deterioration has progressed to a predetermined degree may be stoichiometric ( Change to the stoichiometric air-fuel ratio) operating range. In this case, the lean operation region reduction control for reducing the lean operation region in the high temperature operation region constitutes the thermal deterioration suppressing means.
[0076]
Next, with reference to FIG. 7, the routine for determining the thermal deterioration of the NOx catalyst 17 in this embodiment will be described. A flowchart including the steps constituting the control routine is stored in the ROM 32 of the ECU 30, and the control routine is executed by the CPU 34 at regular intervals.
[0077]
<Step 101>
First, in step 101, the ECU 100 determines whether it is time to execute the SOx poisoning recovery process on the NOx catalyst 17. As described above, in this embodiment, whether or not it is the SOx poisoning recovery process execution time is determined by performing a saturated NOx release process every predetermined period, and the rich switching time measured at that time is smaller than the threshold value t3. It is determined that it is time to execute the SOx poisoning recovery process. If a negative determination is made in step 101, the ECU 30 once ends the execution of this routine.
[0078]
<Step 102>
If the determination in step 101 is affirmative, the ECU 30 proceeds to step 102 and executes SOx poisoning recovery processing. In the SOx poisoning recovery process, when the NOx catalyst 17 has not reached the SOx release temperature, a predetermined temperature increase process is executed to set the catalyst temperature of the NOx catalyst 17 to be equal to or higher than the SOx release temperature. This is done by making the air-fuel ratio of the input gas slightly richer than stoichiometric.
[0079]
<Step 103>
Next, the ECU 30 proceeds to step 103 and determines whether or not the SOx poisoning recovery process is completed. Here, the SOx poisoning recovery process is executed until the SOx absorbed in the NOx catalyst 17 is almost completely released. Determination of the completion of the SOx poisoning recovery process will be described later. If a negative determination is made in step 103, the ECU 30 returns to step 102 and continues execution of the SOx poisoning recovery process.
[0080]
<Steps 104 and 105>
If the determination in step 103 is affirmative, the ECU 30 proceeds to step 104 to execute a saturated NOx releasing process, and further proceeds to step 105 to measure the rich switching time t.
[0081]
<Step 106>
Next, the ECU 30 proceeds to step 106 and calculates the difference between the rich switching time t1 of the new NOx catalyst 17 stored in advance in the ROM 32 of the ECU 30 and the rich switching time t measured in step 105. That is, the current degree of thermal deterioration of the NOx catalyst 17 is calculated.
[0082]
<Step 107>
Next, the ECU 30 proceeds to step 107 and determines whether or not the rich switching time difference (degree of thermal deterioration) calculated in step 106 is greater than or equal to a preset threshold value t4. If a negative determination is made in step 107, the ECU 30 once ends the execution of this routine.
[0083]
<Step 108>
If an affirmative determination is made in step 107, it is determined that the degree of thermal deterioration of the NOx catalyst 17 has reached a predetermined level, and the ECU 30 proceeds to step 108 and operates the thermal deterioration suppressing means to temporarily execute this routine. finish. By the operation of the thermal deterioration suppression means, that is, the execution of the fuel cut prohibition control in the high temperature operation region or the lean operation region decrease control in the high temperature operation region, the subsequent progress of the thermal deterioration of the NOx catalyst 17 is suppressed. Is done.
[0084]
In this embodiment, the ECU 30 executes step 101 in the thermal deterioration determination processing routine, thereby realizing the poisoning recovery execution time determining means in the present invention. Further, the ECU 30 executes step 102. The poisoning recovery means in the present invention is realized, and the storage capacity detecting means in the present invention is realized by the ECU 30 executing the steps 104 and 105, and the ECU 30 executes the steps 106 and 107. The thermal deterioration determination means in the present invention is realized.
[0085]
Next, the SOx poisoning recovery process in this embodiment will be described with reference to FIG.
FIG. 8 shows the air-fuel ratio of the input gas of the NOx catalyst 17 during the SOx poisoning recovery process in this embodiment, the SOx concentration of the input gas detected by the input gas SOx sensor 23, and the detection by the output gas SOx sensor 24. It is the figure which showed the time-dependent change of SOx density | concentration of the made outgas. It is assumed that the catalyst temperature of the NOx catalyst 17 is equal to or higher than the SOx release temperature during the execution period of the SOx poisoning recovery process.
[0086]
First, the air-fuel ratio of the input gas is changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio by the SOx poisoning recovery processing execution command, and the richness of the air-fuel ratio of the input gas is gradually increased. When the output gas SOx concentration detected by the output gas SOx sensor 24 matches the input gas SOx concentration detected by the input gas SOx sensor 23, the air-fuel ratio of the input gas becomes constant at the rich air-fuel ratio. Hold. After the output gas SOx concentration matches the input gas SOx concentration, SOx is substantially released from the NOx catalyst 17, so the output gas SOx concentration becomes larger than the input gas SOx concentration, and the output gas is discharged at a predetermined concentration. The SOx concentration is almost constant. After that, if the release of SOx from the NOx catalyst 17 continues, the amount of SOx released from the NOx catalyst 17 decreases. Therefore, the concentration of the output gas SOx decreases, and when the SOx remaining in the NOx catalyst 17 disappears, The gas SOx concentration becomes lower than the input gas SOx concentration. Therefore, if the SOx poisoning recovery process is continued until the output gas SOx concentration is lower than the input gas SOx concentration by a predetermined value, it can be considered that the NOx catalyst 17 has been almost completely recovered from the SOx poisoning. In this embodiment, the SOx poisoning recovery process is continued until the outgas SOx concentration becomes equal to or lower than the poisoning recovery completion level in FIG.
[0087]
[Second Embodiment]
In this embodiment, a case will be described in which the degree of thermal deterioration of the NOx catalyst 17 of the exhaust purification device is detected based on a change in the NOx purification rate of the NOx catalyst.
FIG. 13 is a schematic diagram of the exhaust emission control device in this embodiment. The engine body 40 is provided with four cylinders 41, and a fuel injection valve 42 is attached to an intake port connected to each cylinder 41. Yes. The intake manifold 43 is connected to an air cleaner 45 via an intake duct 44. A throttle valve 46 is provided in the intake duct 44.
[0088]
On the other hand, the exhaust gas is connected to a casing 49 containing the NOx storage reduction catalyst 50 via an exhaust manifold 47 and an exhaust pipe 48, and the casing 49 is connected to a muffler (not shown) via an exhaust pipe 51.
The other components such as the ECU 30 and the NOx catalyst 17 are the same as those in the exhaust emission control device according to the first embodiment, and thus the same reference numerals are given and description thereof is omitted.
[0089]
An output gas NOx sensor 50 that generates an output voltage proportional to the NOx concentration of the output gas flowing out from the NOx catalyst 17 is provided downstream of the casing 49, and this output voltage is input to the input port 35 of the ECU 30.
FIG. 14 is a diagram illustrating an example of the NOx purification rate by the NOx catalyst 17. Here, the input NOx indicates the NOx concentration per unit time in the exhaust gas flowing into the NOx catalyst 17, and the output NOx indicates the NOx concentration per unit time in the exhaust gas flowing out from the NOx catalyst 17. Here, when the NOx concentration per unit time in the exhaust gas discharged from the NOx catalyst 17 reaches the threshold value X, the exhaust gas is stoichiometric or rich to release and reduce the NOx stored in the NOx catalyst 17 to reduce the NOx. The NOx occlusion capacity of the catalyst 17 is recovered. However, when the operation with the exhaust gas lean is continued and NOx in the exhaust gas is occluded in the NOx catalyst 17, the saturation state is reached again, and the concentration of the output NOx reaches the threshold value X. Therefore, NOx is released and reduced by executing rich spike again.
[0090]
However, if this is repeated, as described in detail in the first embodiment, the SOx poisoning of the NOx catalyst 17 reaches a predetermined level, so that the SOx poisoning recovery process is executed.
The SOx poisoning of the NOx catalyst 17 can be recovered by executing the SOx poisoning recovery process. However, since the thermal deterioration of the NOx catalyst 17 cannot be recovered, the performance unrecoverable part immediately after the SOx poisoning recovery process. May be a performance degradation due to thermal degradation.
[0091]
Therefore, if the NOx purification rate of the NOx catalyst 17 when it is new and the NOx purification rate immediately after the SOx poisoning recovery process are measured and compared, the degree of thermal deterioration can be known from the difference in the NOx purification performance. .
Here, the NOx purification rate is based on the amount of NOx in the exhaust gas flowing into the NOx catalyst (incoming NOx amount) and the amount of NOx in the exhaust gas by the NOx detection means installed downstream of the NOx catalyst (outgoing NOx amount). Is required.
At this time, the amount of the input NOx can be calculated from the integrated value of the NOx concentration per unit time and the exhaust gas amount from the output value by attaching a NOx sensor to the exhaust passage upstream of the NOx catalyst 17. The amount of input NOx can also be obtained by the following method.
[0092]
That is, when the lean air-fuel mixture is burning, the amount of NOx discharged from the engine per unit time increases as the engine load increases, and the amount of NOx discharged from the engine per unit time increases as the engine speed increases. Increase. Therefore, the NOx amount per unit time is a function of the engine load and the engine speed. In this case, since the engine load can be represented by the absolute pressure in the surge tank, the amount NOXA of the output NOx per unit time is a function of the absolute pressure PM in the surge tank and the engine speed N. Therefore, in this embodiment, the amount of NOx per unit time is obtained beforehand by experiment as a function of the absolute pressure PM and the engine speed N, and this is stored in the RAM of the ECU 30 in the form of a map as shown in FIG. deep.
[0093]
On the other hand, by attaching a NOx sensor to the exhaust passage downstream of the NOx catalyst 17, the NOx concentration per unit time is detected from the output value, so the amount of output NOx is the integrated value of this NOx concentration and the exhaust gas amount. Can be calculated based on
In this way, the NOx purification rate of the NOx catalyst 17 is measured, and this is compared with the purification rate or the necessary purification rate in the case of an initial new article, so that the timing of SOx poisoning regeneration can be determined.
[0094]
If the NOx purification rate of the NOx catalyst 17 when it is new and the NOx purification rate of the NOx catalyst 17 immediately after the SOx poisoning recovery process can be detected and compared, the NOx catalyst 17 can be calculated from the difference between the purification rates. The degree of thermal degradation of can be detected.
Note that the NOx purification rate and the necessary purification rate of the new NOx catalyst 17 are experimentally obtained in advance and stored in the ROM 32 of the ECU 30, and the necessary purification rate when it is determined that the thermal deterioration suppressing means should be operated. Are preset and stored in the ROM of the ECU 30.
[0095]
In this embodiment, the NOx purification performance is detected as follows. First, a lean air-fuel ratio exhaust gas is continuously supplied to the NOx catalyst 17 to saturate the NOx storage capacity of the NOx catalyst 17 so that no more NOx can be stored. At this time, the purification performance of the NOx catalyst is measured in consideration of the temperature window.
[0096]
Next, the exhaust gas flowing into the NOx catalyst 17, that is, the air-fuel ratio of the input gas is switched from lean to rich, and the rich air-fuel ratio exhaust gas continues to flow through the NOx catalyst 17, so that the NOx stored in the NOx catalyst 17 is stored. Release and reduce the entire amount. At this time, even if the air-fuel ratio of the input gas is switched from lean to rich, the exhaust gas at the outlet of the NOx catalyst 17, that is, the air-fuel ratio of the output gas, does not immediately switch to rich, but remains near the stoichiometric air-fuel ratio for a while. . This is because HC and CO in the input gas are consumed as a reducing agent for the reduction of NOx stored in the NOx catalyst 17. When all of the NOx stored in the NOx catalyst 17 is released and reduced and HC and CO in the input gas are no longer consumed as a reducing agent, the air-fuel ratio of the output gas changes to rich.
[0097]
Further, in this embodiment, the determination of the timing for executing the SOx poisoning recovery process for the NOx catalyst 17 is made based on whether or not the NOx purification rate has become a predetermined value or less. More specifically, in the exhaust emission control device of this embodiment, the saturated NOx release process is performed on the NOx catalyst 17 at an appropriate timing, for example, at every constant travel distance or every constant fuel consumption. Measure the NOx purification rate. In this measurement, the output NOx sensor 50 grasps the purification performance of the NOx catalyst under each load and temperature condition.
[0098]
And when the measured NOx purification rate falls below the threshold value x shown in FIG. 14, it is considered that the NOx purification performance of the NOx catalyst 17 has fallen to a predetermined level.
Further, this time is determined as the time when the SOx poisoning recovery process should be executed.
[0099]
When the SOx poisoning recovery process is completed, the NOx purification rate is measured to determine whether the performance of the NOx catalyst has been recovered. As shown in FIG. 15, this is determined by whether or not the NOx purification rate exceeds the necessary purification rate within a predetermined temperature range (for example, about 300 ° C. to 420 ° C.). In FIG. 15, (1) indicates the NOx purification rate when the NOx catalyst is new, and (2) indicates the NOx purification rate when SOx is poisoned. In the state (2), SOx poisoning recovery is required, and (3) indicates the purification rate after SOx poisoning recovery.
[0100]
If the required purification rate is not secured, next, a rich spike according to the degree of thermal deterioration of the NOx catalyst is executed. This is because when the lean mixture is burned in the internal combustion engine, that is, when the medium / low load operation is being performed, the combustion of the stoichiometric or rich mixture is performed in a spike-like (short time) with a relatively short cycle. As described above, the air-fuel ratio of the air-fuel mixture is controlled to release and reduce NOx in a short cycle, but if the recovery of the performance deterioration is insufficient, the rich spike is increased. This includes both increasing the amount of rich spikes, or increasing the number of rich spikes, i.e. shortening the cycle of rich spikes, and either or both are performed in proportion to the degree of thermal degradation. .
[0101]
Next, in the exhaust purification system of this embodiment, after performing the rich spike control, the NOx purification rate is measured, and when the purification rate is below a predetermined level, the thermal deterioration of the NOx catalyst 17 recovers the performance. Judging that it has progressed to an impossible level, the thermal deterioration suppressing means is operated to suppress the thermal deterioration. That is, even when such control is performed, a sufficient NOx purification rate cannot be ensured.
[0102]
First, lean operation is prohibited in a temperature range where performance is insufficient. This is to suppress the thermal deterioration of the NOx catalyst 17 by reducing the lean operation region and lowering the incoming gas oxygen concentration of the NOx catalyst 17 than when the thermal deterioration has not progressed to a predetermined degree. Specifically, as shown in FIG. 15, the lean operation is prohibited and the stoichiometric operation region is changed in the temperature region where the NOx catalyst performance falls below the required purification rate (the region that does not return even after recovery). Is.
[0103]
Next, as described in the first embodiment, as another thermal degradation suppression method or a thermal degradation suppression method executed in conjunction with the above, when the engine is in a high temperature operating range Even when the operation is decelerated, thermal degradation is suppressed by prohibiting fuel cut.
Further, as a method for suppressing the thermal deterioration, a method is adopted in which the lean operation region in the high temperature operation region of the engine is reduced as compared with the case where the heat deterioration does not progress to a predetermined degree. This overlaps the above-described range of prohibition of lean operation in a temperature range where performance is insufficient, but prohibits lean operation in all high temperature operation regions above a predetermined temperature.
[0104]
Hereinafter, with reference to FIG. 17, the routine for determining the thermal deterioration of the NOx catalyst 17 in this embodiment will be described. A flowchart including the steps constituting the control routine is stored in the ROM 32 of the ECU 30, and the control routine is executed by the CPU 34 at regular intervals.
[0105]
<Step 201>
First, in step 201, the ECU 100 determines whether it is time to execute the SOx poisoning recovery process for the NOx catalyst 17. To determine whether it is time to execute the SOx poisoning recovery process, a saturated NOx release process is performed every predetermined period, and the NOx purification rate is measured after the release process is completed. This is calculated based on the output value of the outgoing NOx sensor 50.
When this value becomes lower than the necessary purification rate, it is determined that it is time to execute the SOx poisoning recovery process. If a negative determination is made in step 201, the ECU 30 once terminates execution of this routine.
[0106]
<Step 202>
If the determination in step 201 is affirmative, the ECU 30 proceeds to step 202 and executes SOx poisoning recovery processing. In the SOx poisoning recovery process, when the NOx catalyst 17 has not reached the SOx release temperature, a predetermined temperature increase process is executed to set the catalyst temperature of the NOx catalyst 17 to be equal to or higher than the SOx release temperature. This is done by making the air-fuel ratio of the input gas slightly richer than stoichiometric.
[0107]
<Step 203>
Next, the ECU 30 proceeds to step 203 and determines whether or not the SOx poisoning recovery process is completed. Here, the SOx poisoning recovery process is executed until the SOx absorbed in the NOx catalyst 17 is almost completely released. If a negative determination is made in step 203, the ECU 30 returns to step 202 and continues execution of the SOx poisoning recovery process.
[0108]
<Step 204>
If the determination in step 203 is affirmative, the ECU 30 proceeds to step 204 to execute a saturated NOx release process, and further proceeds to step 205 to measure the NOx purification rate.
[0109]
<Step 205>
Next, the ECU 30 proceeds to step 205 and measures the NOx purification rate of the NOx catalyst 17.
[0110]
<Step 206>
Proceeding to step 206, the ECU 30 determines whether or not the NOx purification rate measured in step 206 is equal to or greater than a preset necessary purification rate and the NOx catalyst function has been restored. If the determination in step 206 is affirmative, the ECU 30 once ends the execution of this routine.
[0111]
<Step 207>
If a negative determination is made in step 206, rich spike control is executed to try the performance of the NOx catalyst 17.
[0112]
<Step 208>
It is determined whether or not the NOx purification rate of the NOx catalyst has been recovered by the rich spike control executed in step 207. This is determined by whether or not the NOx purification rate in a predetermined temperature range is lower than the required purification rate. If the purification rate is equal to or higher than the necessary purification rate, the ECU 30 once terminates execution of this routine.
[0113]
<Step 209>
If the NOx purification rate is equal to or less than the required purification rate in step 208, lean operation in a temperature range where performance is insufficient is prohibited. When the NOx purification rate is less than or equal to the required purification rate, performance recovery is impossible and it is determined that thermal degradation is large.
[0114]
<Step 210>
Furthermore, other thermal deterioration suppression control is implemented. This is a control for reducing the lean operation region in the high temperature operation region and prohibiting the fail cut in the high temperature operation region. As a result, the subsequent thermal deterioration of the NOx catalyst 17 is suppressed.
[0115]
In this embodiment, the poisoning recovery execution time determining means in the second invention is realized by the ECU 30 executing step 201 in the thermal deterioration determination processing routine, and the ECU 30 executes step 202. Thus, the poisoning recovery means is realized, and when the ECU 30 executes Step 204 and Step 205, the NOx purification rate measuring means is realized.
[0116]
Further, in this embodiment, when the NOx purification rate is not ensured by the rich spike control, it can be determined that the thermal deterioration of the NOx catalyst 17 is large, and the replacement of the catalyst can be promoted.
[0117]
[Other Embodiments]
The completion of the SOx poisoning recovery process execution can be determined based on whether or not the SOx poisoning recovery process execution time exceeds a predetermined time set in advance. In this case, the poisoning recovery processing time required to almost completely release the SOx stored in the NOx catalyst 17 is experimentally obtained in advance, and this is stored in the ROM 32 of the ECU 30.
[0118]
Whether or not it is the SOx poisoning recovery processing execution time is determined that the SOx poisoning recovery processing execution time is when the amount of sulfur passing through the NOx catalyst 17 exceeds a predetermined amount (hereinafter referred to as allowable sulfur amount). May be.
[0119]
In that case, the sulfur concentration in the fuel is estimated from the SOx concentration of the input gas detected by the input gas SOx sensor 23, and the fuel consumption or travel distance corresponding to the allowable sulfur amount based on the estimated sulfur concentration in the fuel. And the time when the fuel consumption or travel distance is reached can be determined to be the SOx poisoning recovery process execution timing. Further, in this case, the estimated sulfur concentration in the fuel may be displayed on a panel or the like in front of the driver's seat so that the frequency of the SOx poisoning recovery process can be roughly determined.
[0120]
Alternatively, the amount of SOx flowing into the NOx catalyst 17 is calculated based on the SOx concentration of the input gas detected by the input gas SOx sensor 23 and the intake air amount of the engine, and when the integrated value exceeds a predetermined amount, It can be determined that it is time to execute the SOx poisoning recovery process.
[0121]
Thus, when the amount of sulfur passing through the NOx catalyst 17 exceeds the allowable amount of sulfur, it is determined that the SOx poisoning recovery processing execution timing is reached, and the completion of the SOx poisoning recovery processing execution is determined as described above. In the case of performing the poison recovery processing execution time, the outgas SOx sensor 24 can be omitted.
[0122]
Whether or not it is the SOx poisoning recovery process execution time is estimated by estimating the SOx amount occluded in the NOx catalyst 17, and when this estimated stored SOx amount exceeds a predetermined amount, the SOx poisoning recovery process execution time is reached. You may determine that there is. As shown in FIG. 9, the amount of occluded SOx is estimated by calculating the difference in concentration between the SOx concentration of the inlet gas detected by the inlet gas SOx sensor 23 and the SOx concentration of the outlet gas detected by the outlet gas SOx sensor 24 as NOx. As what is occluded by the catalyst 17, it can be calculated based on this SOx concentration difference (incoming gas SOx concentration−outgoing gas SOx concentration) and the intake air amount of the engine. Then, the storage SOx amount calculated and estimated in this way is integrated by the SOx deposition counter, and when the count value of the SOx deposition counter exceeds a predetermined value, it can be determined that the SOx poisoning recovery processing execution timing is reached. .
[0123]
Further, when the SOx accumulation counter is provided as described above, the completion of the SOx poisoning recovery process can be determined as follows. In the period during which the SOx poisoning recovery process is being executed and the output gas SOx concentration is greater than the input gas SOx concentration, the concentration difference between the output gas SOx concentration and the input gas SOx concentration is SOx released from the NOx catalyst 17. Since it can be estimated, the SOx amount released from the NOx catalyst 17 is calculated based on this SOx concentration difference (outgoing gas SOx concentration−inlet gas SOx concentration) and the intake air amount of the engine, and the calculated estimated SOx emission amount Is subtracted by the SOx deposition counter, and when the count value of the SOx deposition counter is reduced to a predetermined value (poisoning recovery completion level) as shown in FIG. Execution of the recovery process can be terminated.
[0124]
Further, the determination as to whether or not it is the SOx poisoning recovery process execution time may be determined as the SOx poisoning recovery process execution time when the SOx concentration of the output gas approaches the SOx concentration of the input gas to a predetermined level. . As shown in FIG. 11, the SOx concentration of the output gas of the NOx catalyst 17 approaches the SOx concentration of the input gas as the SOx poisoning proceeds. Therefore, it can be said that the degree of approach of the SOx concentration of the output gas to the SOx concentration of the input gas corresponds to the degree of SOx poisoning. Therefore, it is set in advance to what extent the SOx concentration of the output gas approaches the SOx concentration of the input gas, and the SOx poisoning recovery process is executed, and the SOx of the output gas detected by the output gas SOx sensor 24 is set. When the concentration exceeds the set SOx concentration, it can be determined that the SOx poisoning recovery processing execution time is reached.
[0125]
In each of the above-described embodiments, the input gas SOx sensor 23 is provided upstream of the NOx catalyst 17, and the SOx concentration of the exhaust gas flowing into the NOx catalyst 17 is detected by the input gas SOx sensor 23. However, the NOx catalyst 17 Since the SOx concentration of the exhaust gas flowing into the engine depends on the sulfur concentration in the fuel, the fuel amount, and the exhaust gas amount, if the sulfur concentration in the fuel is known, the engine operating state (fuel injection amount, air-fuel ratio, It can be estimated from the intake air amount, engine speed, etc.). Therefore, in that case, instead of providing the input gas SOx sensor 23, the SOx concentration of the catalyst input gas may be calculated and estimated by the ECU 30 from the engine operating state.
[0126]
In the above-described embodiment, the example in which the present invention is applied to a gasoline engine has been described. However, it is needless to say that the present invention can be applied to a diesel engine. In the case of a diesel engine, combustion in the combustion chamber is performed in a much leaner region than stoichiometric, so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 17 is very lean under normal engine operation conditions, and NOx and Although SOx is absorbed, NOx and SOx are hardly released.
[0127]
In the case of a gasoline engine, as described above, the air-fuel ratio supplied to the combustion chamber 3 is stoichiometric or rich to make the exhaust air-fuel ratio stoichiometric or rich, and the oxygen concentration in the exhaust gas is reduced to reduce NOx. NOx and SOx absorbed by the catalyst 17 can be released. However, in the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is stoichiometric or rich, there is a problem that soot is generated during combustion. Yes, it cannot be adopted.
[0128]
Therefore, when the present invention is applied to a diesel engine, in order to make the exhaust air-fuel ratio stoichiometric or rich, a reducing agent (for example, light oil as a fuel) is used in addition to burning the fuel to obtain engine output. It is necessary to supply the exhaust gas. The reducing agent can be supplied to the exhaust gas by sub-injecting fuel into the cylinder in 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 to supply.
[0129]
In addition, even if it is a diesel engine, when it has an exhaust gas recirculation device (so-called EGR device), a large amount of exhaust gas recirculation gas is introduced into the combustion chamber, so that the air fuel ratio of the exhaust gas is the stoichiometric air fuel ratio. Or it is possible to make it a rich air-fuel ratio.
[0130]
【The invention's effect】
According to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, it is possible to determine the degree of thermal deterioration of the NOx storage reduction catalyst by including the thermal deterioration determination means for determining the degree of thermal deterioration of the NOx storage reduction catalyst. . Therefore, it can be known that the NOx purification performance is deteriorated and cannot be recovered, and a situation in which NOx in the exhaust gas is not purified by replacement of the catalyst or the like can be avoided.
[0131]
The exhaust gas purification apparatus for an internal combustion engine according to the present invention further includes a thermal deterioration suppression unit that suppresses thermal degradation of the NOx storage reduction catalyst, and the thermal degradation suppression unit includes the thermal degradation determined by the thermal degradation determination unit. When the degree of thermal degradation of the NOx storage reduction catalyst reaches the predetermined level, the thermal degradation progresses after the degree of thermal degradation of the NOx storage reduction catalyst reaches the predetermined level. Can be suppressed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 2 is a diagram showing an example of a map of basic fuel injection time.
FIG. 3 is a diagram schematically showing unburned HC, CO and oxygen concentrations in exhaust gas discharged from the engine.
FIG. 4 is a diagram for explaining the NOx absorption / release action of the NOx storage reduction catalyst.
FIG. 5 is a diagram showing the progress of deterioration of the NOx storage reduction catalyst.
FIG. 6 is a diagram showing a change with time of the air-fuel ratio of the output gas during the saturated NOx releasing process in the first embodiment.
FIG. 7 is a flowchart showing a thermal deterioration determination processing routine in the first embodiment.
FIG. 8 is a graph showing temporal changes in the air-fuel ratio of the input gas and the SOx concentration of the output gas during the SOx poisoning recovery process in the first embodiment.
FIG. 9 is a diagram illustrating a method for determining the execution timing of SOx poisoning recovery processing in the exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 10 is a diagram for explaining a method of determining completion of SOx poisoning recovery processing in the exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 11 is a diagram illustrating a method for determining the execution timing of SOx poisoning recovery processing in the exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 12 is a diagram showing the relationship between the sintering of the catalyst substance and the NOx purification performance of the NOx storage reduction catalyst.
FIG. 13 is a schematic view of an exhaust emission control device for an internal combustion engine according to a second embodiment.
FIG. 14 is a graph showing changes in the NOx purification rate in the exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 15 is a view for explaining SOx poisoning recovery processing of the NOx storage reduction catalyst in the exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 16 is a diagram showing a numerical value table used for determining the input NOx amount.
FIG. 17 is a flowchart showing a thermal deterioration determination processing routine in the first embodiment.
[Explanation of symbols]
1, 40 Engine body (internal combustion engine)
3 Combustion chamber
4 Spark plugs
11, 42 Fuel injection valve (poisoning recovery means)
16, 48 Exhaust pipe (exhaust passage)
17 NOx storage reduction catalyst
18, 49 Casing
19, 51 Exhaust pipe (exhaust passage)
21, 45 Air flow meter
23 Input gas SOx sensor (input gas SOx concentration detection means)
24 Outgas SOx sensor (Outgas SOx concentration detection means)
25 Temperature sensor
27 Air-fuel ratio sensor
30 ECU
50 Outgas NOx sensor (Outgas NOx concentration detection means)

Claims (4)

(A) provided in the exhaust passage of a lean burnable internal combustion engine, which absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low, and N a NOx storage reduction catalyst for reducing the _2, and (ii) the storage-reduction type NOx catalyst timing to perform the SOx poisoning recovery process for releasing SOx from whether poisoning recovery execution timing determining determination means, ( C) When it is determined by the poisoning recovery execution time determining means that the execution time is reached, almost the entire amount of SOx deposited on the NOx storage reduction catalyst is released, and the NOx storage reduction catalyst is made to be SOx covered. A poisoning recovery means for recovering from the poison; (d) a storage capacity detection means for detecting the NOx storage capacity of the NOx storage reduction catalyst; and (e) after executing the SOx poisoning recovery process by the poisoning recovery means. The storage capacity Thermal degradation determination for comparing the NOx storage capability of the NOx storage reduction catalyst detected by the detection means with the reference NOx storage capability and determining the thermal degradation degree of the NOx storage reduction catalyst based on the comparison value and means, and a suppressing heat deterioration prevention means the thermal degradation of the occlusion reduction type the NO x catalyst, the thermal deterioration suppressing means, when thermal degradation degree which is determined by the thermal degradation determination means following a predetermined level An exhaust gas purification apparatus for an internal combustion engine, which is operated when the predetermined level is not exceeded and is exceeded . 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 , wherein the thermal deterioration suppressing means is fuel cut prohibition control that prohibits fuel cut in a high temperature operation region of the internal combustion engine. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 , wherein the thermal deterioration suppressing means is a lean operation region reduction control for reducing a lean operation region in a high temperature operation region of the internal combustion engine. The thermal degradation suppressing means increases the amount of rich or stoichiometric exhaust gas flowing into the NOx catalyst according to the degree of thermal degradation, and / or shortens the period during which the rich or stoichiometric exhaust gas flows into the NOx catalyst. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 .

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