JP2004301127A - Exhaust purification device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimally control the execution timing of an SOx desorption process of a storage reduction type NOx catalyst.
SOLUTION: In an exhaust gas purifying apparatus for an internal combustion engine provided with a storage reduction type NOx catalyst 17 in an exhaust pipe 16 of the internal combustion engine capable of lean burn, whether or not the outgassing SOx concentration is increasing, and whether the inflowing gas SOx concentration and the outgoing gas SOx concentration are increasing. The SOx desorption process is performed based on the comparison value with the gas SOx concentration, and based on the comparison value between the output gas SOx concentration and the output gas SOx concentration, whether or not the output gas SOx concentration is falling, and the SOx desorption process. To end.
[Selection diagram] FIG.

Description

  The present invention relates to an exhaust gas purification device capable of purifying nitrogen oxides (NOx) from exhaust gas discharged from an internal combustion engine capable of lean combustion.

  As an exhaust purification device for purifying NOx from exhaust gas discharged from an internal combustion engine capable of lean combustion, there is a NOx absorbent represented by a storage reduction type NOx catalyst. The NOx absorbent absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (that is, in an oxygen-excess atmosphere), and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases. The NOx storage reduction catalyst, which is a type of NOx absorbent, absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (that is, under an atmosphere of excess oxygen), and when the oxygen concentration of the inflowing exhaust gas decreases. The catalyst releases the absorbed NOx and reduces it to N2.

  When this storage-reduction type NOx catalyst (hereinafter sometimes simply referred to as a catalyst or a NOx catalyst) is disposed in an exhaust passage of an internal combustion engine capable of lean combustion, when exhaust gas having a lean air-fuel ratio flows, NOx in the exhaust gas becomes a catalyst. When exhaust gas having a stoichiometric (stoichiometric air-fuel ratio) or rich air-fuel ratio flows, NOx absorbed in the catalyst is released as NO2, and further reduced to N2 by reducing components such as HC and CO in the exhaust gas. It is reduced, that is, NOx is purified.

  By the way, in general, the fuel of the internal combustion engine contains sulfur, and when the fuel is burned in the internal combustion engine, the sulfur in the fuel is burned to generate sulfur oxides (SOx) such as SO2 and SO3. Since the NOx storage reduction catalyst absorbs SOx in exhaust gas by the same mechanism as that of absorbing NOx, if a NOx catalyst is disposed in the exhaust passage of the internal combustion engine, if NOx is the only NOx catalyst, SOx is also absorbed.

  However, the SOx absorbed by the NOx catalyst forms a stable sulfate with the passage of time, and thus tends to be hardly decomposed and released, and easily accumulated in the catalyst. When the SOx accumulation amount in the NOx catalyst increases, the NOx absorption capacity of the catalyst decreases, so that the NOx purification rate decreases. This is so-called SOx poisoning. In order to maintain the NOx purification performance of the NOx storage reduction catalyst at a high level over a long period of time, it is necessary to perform a SOx desorption process on the NOx catalyst to desorb the absorbed SOx. The timing of execution of processing becomes very important.

  Here, as one of the methods of determining the execution time of the SOx desorption process, there is an idea that a predetermined amount of SOx is accumulated in the NOx catalyst. In this case, conventionally, the amount of SOx accumulated in the NOx catalyst is calculated based on the travel distance of the vehicle, the difference in NOx concentration between the inlet and the outlet of the NOx catalyst, or the temperature difference between the inlet and the outlet of the NOx catalyst. (For example, see Patent Document 1). That is, in the related art, the execution time of the SOx desorption process has not been determined based on direct data on the SOx.

For this reason, the amount of SOx accumulated in the NOx catalyst is insufficiently grasped, and there is a possibility that the execution time of the SOx desorption process becomes inappropriate.
Japanese Patent No. 2745985 JP-A-10-31235

  The present invention has been made in view of the problems of the prior art, and an object of the present invention is to solve the problem of SOx desorption based on the change tendency of the SOx concentration in the exhaust gas at the outlet of the NOx storage reduction catalyst. The object of the present invention is to maintain the NOx purifying ability of the NOx storage reduction catalyst at a high level over a long period of time by managing the processing.

  The present invention employs the following means in order to solve the above problems. The exhaust gas purifying apparatus for an internal combustion engine according to the present invention is provided in the exhaust passage of the internal combustion engine capable of lean combustion, absorbs NOx when the air-fuel ratio of the exhaust gas is lean, and oxygen of the exhaust gas flowing therethrough. (B) controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when desorbing the SOx absorbed by the NOx absorbent when releasing the NOx absorbed when the concentration is low; Exhaust air-fuel ratio control means, wherein the exhaust air-fuel ratio control means is operated based on a change tendency of the SOx concentration downstream of the NOx absorbent.

  Here, while SOx in the exhaust gas is being absorbed by the NOx absorbent, the SOx concentration of the exhaust gas downstream of the NOx absorbent is lower than the SOx concentration of the exhaust gas upstream of the NOx absorbent. . However, when the SOx storage capacity of the NOx absorbent increases and the SOx absorption capacity decreases, the SOx concentration downstream of the NOx absorbent gradually approaches the SOx concentration upstream of the NOx absorbent. Therefore, the degree of SOx accumulation in the NOx catalyst 17 can be determined based on how close the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent.

  Further, if the air-fuel ratio of the exhaust gas is controlled to a stoichiometric air-fuel ratio or a rich air-fuel ratio in order to desorb SOx from the NOx absorbent, the SOx desorbed from the NOx absorbent flows downstream of the NOx absorbent. The SOx concentration downstream of the absorbent becomes higher than the SOx concentration upstream of the NOx absorbent, and the accumulated amount of SOx in the NOx absorbent decreases with time. The SOx concentration downstream of the NOx absorbent reaches a peak at a predetermined time after the start of the air-fuel ratio control described above, and thereafter gradually decreases.

  Then, when SOx is not desorbed from the NOx absorbent, the SOx concentration downstream of the NOx absorbent and the SOx concentration upstream of the NOx absorbent become equal, and at this time, the SOx accumulation amount of the NOx absorbent becomes minimum. Therefore, the degree of SOx desorption of the NOx absorbent can be grasped based on how close the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent.

  Therefore, in the present invention, the exhaust air-fuel ratio control means is operated based on the change tendency of the SOx concentration downstream of the NOx absorbent as described above. Then, the degree of SOx accumulation in the NOx absorbent can be accurately grasped without calculating the SOx accumulation amount. As a result, the NOx purifying ability of the NOx absorbent can be maintained high for a long period of time.

  In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is increasing, and the SOx concentration downstream of the NOx absorbent is SOx upstream of the NOx absorbent. When the concentration approaches a predetermined value, the air-fuel ratio control by the exhaust air-fuel ratio control means can be started. This is because the SOx concentration of the NOx absorbent downstream approaches the SOx concentration of the NOx absorbent upstream as the SOx accumulation amount of the NOx absorbent approaches the saturation state, and the SOx accumulation amount of the NOx absorbent does not need to be calculated. Also, the progress of SOx poisoning can be grasped.

In this case, the NOx absorbed by the NOx absorbent is based on the concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means.
It is preferable that a SOx accumulation amount calculating means for calculating the Ox amount is provided, and when the SOx accumulation amount calculated by the SOx accumulation amount calculating means is equal to or less than a predetermined amount, the air-fuel ratio control by the exhaust air-fuel ratio control means is preferably prohibited. This is because even if the exhaust air-fuel ratio control means is operated in a state where the SOx accumulation amount of the NOx absorbent is small, the SOx cannot be efficiently desorbed from the NOx absorbent and the reducing agent is wasted.

  In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the air-fuel ratio control by the exhaust air-fuel ratio control means is started when the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value. In this case, the amount of SOx absorbed in the NOx absorbent is calculated based on the concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means. The air-fuel ratio control condition of the exhaust air-fuel ratio control means may be corrected according to the amount of SOx accumulation calculated by the SOx accumulation amount calculation means. This is because there is an optimum SOx desorption condition depending on the magnitude of the SOx accumulation amount. Here, the air-fuel ratio control condition is a rich degree of the air-fuel ratio, a rich air-fuel ratio continuation time, or the like.

  In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means is decreasing, and the SOx concentration downstream of the NOx absorbent is SOx upstream of the NOx absorbent. The air-fuel ratio control by the exhaust air-fuel ratio control means can be terminated when the concentration approaches a predetermined value. When SOx is desorbed from the NOx absorbent, even if the exhaust air-fuel ratio is switched from rich to lean before the SOx desorption is completely completed, SOx will be released from the NOx absorbent for a while after switching to lean. This is because they are desorbed. Thereby, the amount of the reducing agent used for desorbing SOx can be reduced.

  In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the SOx concentration upstream of the NOx absorbent can be detected by SOx concentration detecting means provided in an exhaust passage upstream of the NOx absorbent, and the operating state of the internal combustion engine can be detected. It is also possible to estimate from By considering the SOx concentration upstream of the NOx absorbent, the progress of SOx poisoning can be grasped more accurately.

  The exhaust gas purifying apparatus for an internal combustion engine according to the present invention is (a) provided in an exhaust passage of an internal combustion engine capable of lean combustion, and absorbs NOx when the inflowing exhaust gas has a lean air-fuel ratio to exhaust NOx. (B) a NOx absorbent that releases NOx absorbed when the oxygen concentration is low, and (b) SOx concentration detecting means that is provided in an exhaust passage downstream of the NOx absorbent and detects the SOx concentration of exhaust gas. Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when desorbing the SOx absorbed by the NOx absorbent, and detecting the SOx concentration by the SOx concentration detecting means. The exhaust air-fuel ratio control means may be operated based on the SOx concentration downstream of the NOx absorbent.

  With this configuration, since the SOx concentration downstream of the NOx absorbent is detected by the SOx concentration detecting means, the progress of SOx poisoning of the NOx absorbent can be accurately grasped. Then, since the exhaust air-fuel ratio control means is operated based on the SOx concentration detected by the SOx concentration detection means, it is possible to execute the optimum SOx desorption processing for the NOx absorbent.

  Examples of the internal combustion engine capable of lean burn in the present invention include a direct-injection-type lean-burn gasoline engine and a diesel engine. The air-fuel ratio of the exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx absorbent.

When the internal combustion engine is a lean burn gasoline engine, the exhaust air-fuel ratio control means can be executed by means for 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 exhaust air-fuel ratio control means controls so-called sub-injection for injecting fuel in an intake stroke, an expansion stroke, or an exhaust stroke, or an exhaust gas upstream of the NOx absorbent. This can be realized by means for controlling the supply of the reducing agent into the passage.

  As the NOx absorbent, a storage reduction type NOx catalyst can be exemplified. The storage reduction type NOx catalyst is a catalyst that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases to reduce it to N2. This storage-reduction NOx catalyst uses, for example, alumina as a carrier, and on the carrier, for example, alkali metals such as potassium K, sodium Na, lithium Li, and cesium Cs, alkaline earths such as barium Ba and calcium Ca, and lanthanum La. And at least one selected from rare earths such as yttrium Y and a noble metal such as platinum Pt.

  In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the NOx absorbent is absorbed by the NOx absorbent based on a concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent detected by the SOx concentration detecting means. An SOx accumulation amount calculating means for calculating the SOx amount of the exhaust gas, wherein the air-fuel ratio control by the exhaust air-fuel ratio control means is started when the SOx accumulation amount calculated by the SOx accumulation amount calculating means reaches a predetermined amount. Can be

  When the SOx concentration upstream of the NOx absorbent is higher than the SOx concentration downstream of the NOx absorbent, it can be considered that the difference in the concentration is absorbed by the NOx absorbent. Therefore, the SOx amount (SOx accumulation amount) absorbed by the NOx absorbent can be calculated by multiplying the concentration difference by the exhaust gas amount.

  According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, (a) a NOx absorbent provided in an exhaust passage of an internal combustion engine capable of lean combustion, and (b) SOx absorbed by the NOx absorbent are desorbed. Exhaust air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when the exhaust air-fuel ratio is controlled, based on the change tendency of the SOx concentration downstream of the NOx absorbent. By operating, the progress of SOx poisoning of the NOx absorbent can be accurately grasped, and the optimal SOx desorption process can be performed on the NOx absorbent.

  When the SOx concentration downstream of the NOx absorbent is falling and the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value, the air-fuel ratio control by the exhaust air-fuel ratio control means is ended. In such a case, the amount of the reducing agent used for SOx desorption can be reduced, and as a result, fuel economy deterioration due to the SOx desorption process can be reduced.

  Hereinafter, an embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described with reference to the drawings of FIGS.

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

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

  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 storage-reduction type NOx catalyst (NOx absorbent) 17, and the casing 18 is connected via an exhaust pipe 19 to a muffler (not shown). Have been. In the following description, the NOx storage reduction catalyst 17 is abbreviated as the NOx catalyst 17.

  An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and a CPU (Central Processor Unit) 34, which are interconnected by a bidirectional bus 31. , 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.

  The exhaust pipe 16 upstream of the casing 18 is provided with an incoming gas SOx sensor (SOx concentration detecting means upstream of the NOx absorbent) 23 that generates an output voltage proportional to the SOx concentration of the exhaust gas flowing into the NOx catalyst 17. The exhaust pipe 19 downstream of the casing 18 is provided with an output gas SOx sensor (SOx concentration detecting means downstream of the NOx absorbent) 24 that generates an output voltage proportional to the SOx concentration of the exhaust gas flowing out of the NOx catalyst 17. I have. The output voltages of these SOx sensors 23 and 24 are input to the input port 35 via the corresponding AD converters 38, respectively.

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

In this gasoline engine, the fuel injection time TAU is calculated based on, for example, the following equation.
TAU = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. The basic fuel injection time TP is previously obtained by an experiment, 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 stored in The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder has the stoichiometric air-fuel ratio. On the other hand, when K <1.0, the air-fuel ratio of the air-fuel mixture supplied 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, rich.

  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-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 stoichiometric control is performed. In the engine full load operation region, the value of the correction coefficient K is 1.0. The value is set so as to be larger than the value and the rich air-fuel ratio control is performed.

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

FIG. 3 schematically shows the concentrations of representative 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 increases, and The concentration of oxygen O2 in the discharged exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

  The NOx catalyst (storage-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. And at least one selected from alkaline earths such as calcium Ca, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt.

  When the NOx catalyst 17 is disposed in the exhaust passage of the engine, the NOx catalyst 17 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter also referred to as the exhaust air-fuel ratio) is lean, and the NOx catalyst 17 in the inflowing exhaust gas When the oxygen concentration decreases, the absorption and release of NOx that releases the absorbed NOx is performed. 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.

  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, and therefore, in this case, The NOx catalyst 17 absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and absorbs NOx when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases. Will be released.

  It is considered that the NOx absorption / release operation of the NOx catalyst 17 is performed by a mechanism as shown in FIG. Hereinafter, this mechanism will be described with reference to an example in which platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.

First, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases, and as shown in FIG. 4 (A), the oxygen O ₂ becomes platinum Pt in the form of O ₂ ⁻ or O ^2−. Adheres to the surface of On the other hand, NO contained in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, a part of the generated NO ₂ is absorbed in the NOx catalyst 17 while being oxidized on the platinum Pt and combined with the barium oxide BaO, and as shown in FIG. 4A, the nitrate ion NO ₃ ⁻ In the NOx catalyst 17. In this way, NOx is absorbed in the NOx catalyst 17.

The oxygen concentration in the inflowing exhaust gas is NO ₂ on the surface of as high as platinum Pt is generated, as long as the NOx absorption capability of the NOx catalyst 17 is not saturated, NO ₂ is absorbed in the NOx catalyst 17 nitrate ions NO ₃ ^- is Generated.

On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the generation amount of NO ₂ decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and the nitrate ions NO ₃ ⁻ in the NOx catalyst 17 are converted. It is released from the NOx catalyst 17 in the form of NO ₂ or 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 degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, if the degree of leanness of the inflowing exhaust gas decreases, NOx is released from the NOx catalyst 17. The Rukoto.

On the other hand, at this time, if the air-fuel mixture supplied into the combustion chamber 3 has a stoichiometric or rich air-fuel ratio, a large amount of unburned HC and CO is discharged from the engine as shown in FIG. CO is oxygen _{O 2} on the platinum Pt ^- and are oxidized reacting or ^{O 2-} and.

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 NO ₂ or NO is released from the NOx catalyst 17, and this NO ₂ or NO is unburned HC as shown (B), the is caused to reduction by reaction with CO becomes _{N 2.}

That, HC in the inflowing exhaust gas, CO, first platinum Pt on the oxygen _{O 2} in ^- or ^{O 2-} immediately be reacted with oxidized and then the oxygen _{O 2} on the platinum Pt ^- is or ^{O 2-} is consumed even yet HC, any remaining CO is the HC, NOx of NOx and the inflow exhaust gas discharged from the NOx catalyst 17 by the CO is made to reduction to _{N 2.}

When NO ₂ or NO no longer exists on the surface of the platinum Pt in this manner, NO ₂ or NO is released from the NOx catalyst 17 one after another, and further reduced to N ₂ . Therefore, when the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio or rich, NOx is released from the NOx catalyst 17 within a short time.

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

  By the way, in this embodiment, as described above, the mixture supplied to the combustion chamber 3 is made rich during the full load operation, and the stoichiometric mixture is made the stoichiometric air-fuel ratio during the high load operation and the like. Since the air-fuel mixture is lean during the load operation, NOx in the exhaust gas is absorbed by the NOx catalyst 17 during low and medium load operation, and NOx is released and reduced from the NOx catalyst 17 during full load operation and high load operation. Will be done. However, if the frequency of full-load operation or high-load operation is low, and the frequency of low-medium load operation is high and the operation time is long, the release and reduction of NOx cannot be made in time, and the NOx absorption capacity of the NOx catalyst 17 becomes saturated. NOx can no longer be absorbed.

  Therefore, in this embodiment, when the lean air-fuel mixture is being 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, due to the absorption and release of NOx, the exhaust air-fuel ratio (the air-fuel ratio of the air-fuel mixture in this embodiment) is set to a “lean” and “spike-like stoichiometric air-fuel ratio or rich air-fuel ratio (rich Spikes) "is called lean-rich spike control. In this application, the lean-rich spike control is included in the lean air-fuel ratio control.

On the other hand, the fuel contains sulfur (S), and when the sulfur in the fuel burns, sulfur oxides (SOx) such as SO ₂ and SO ₃ are generated, and the NOx catalyst 17 also removes these SOx in the exhaust gas. Absorb. It is considered that the SOx absorption mechanism of the NOx catalyst 17 is the same as the NOx absorption mechanism. That is, a case where platinum Pt and barium Ba are supported on a carrier in the same manner as when the NOx absorption mechanism is described will be described as an example. As described above, when the exhaust air-fuel ratio is lean, oxygen O ₂ is reduced. The NOx catalyst 17 adheres to the surface of platinum Pt in the form of O ₂ − or O ² −, and SOx (eg, SO ₂ ) in the inflowing exhaust gas is oxidized on the surface of platinum Pt to become SO ₃ .

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

  In order to desorb SOx from the NOx catalyst 17, it is necessary to set the air-fuel ratio of the inflowing exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio, and the higher the catalyst bed temperature of the NOx catalyst 17, the easier it is to desorb. I know.

  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 desorption processing, the ECU 30 controls the amount of fuel injected from the fuel injection valve 11 to perform combustion. This is performed by controlling the air-fuel ratio of the air-fuel mixture supplied to the chamber 3 to a stoichiometric air-fuel ratio or a rich air-fuel ratio. Therefore, the ECU 30 and the fuel injection valve 11 constitute exhaust air-fuel ratio control means.

  FIG. 5 shows the exhaust air-fuel ratio when the lean / rich spike control is being performed for the NOx absorption / release / reduction process and when the stoichiometric or rich air-fuel ratio control is being performed for the SOx desorption process. 3 shows an example of a change with time of the SOx accumulation amount of the NOx catalyst 17 and the SOx concentrations upstream and downstream of the NOx catalyst 17. Note that, in this figure, the exhaust air-fuel ratio during the NOx absorption / release / reduction processing is shown in a lean manner without the rich spike, and the rich display in the exhaust air-fuel ratio during the SOx desorption processing includes the stoichiometric air-fuel ratio. It is a concept. Hereinafter, the change over time of the SOx accumulation amount and the SOx concentration will be described with reference to FIG.

(1) t1 to t2
If the exhaust gas with a lean air-fuel ratio flows through the NOx catalyst 17 when the SOx accumulation amount of the NOx catalyst 17 is small, the SOx in the exhaust gas is absorbed by the NOx catalyst 17, so that the SOx accumulation amount of the NOx catalyst 17 changes with time. To increase. Also, while SOx in the exhaust gas is being absorbed by the NOx catalyst 17, the SOx concentration of the exhaust gas downstream of the NOx catalyst 17 (hereinafter, referred to as catalyst output gas) becomes higher than that of the exhaust gas upstream of the NOx catalyst 17 (hereinafter, referred to as catalyst). SOx concentration).

(2) t2 to t3
When the SOx accumulation amount of the NOx catalyst 17 increases and the SOx absorption capacity decreases, the SOx concentration of the catalyst output gas gradually approaches the SOx concentration of the catalyst input gas. This means that the amount of SOx that passes through without being absorbed by the NOx catalyst 17 gradually increases, and as a result, the degree of increase in the SOx accumulation amount of the NOx catalyst 17 decreases.

(3) t3 to t4
At time t3, when the high-temperature / rich air-fuel ratio control (constant air-fuel ratio) is started in order to desorb SOx from the NOx catalyst 17, the SOx desorbed from the NOx catalyst 17 flows downstream of the NOx catalyst 17, so that the catalyst output gas Becomes higher than the SOx concentration of the gas entering the catalyst, and the SOx accumulation amount of the NOx catalyst 17 decreases with time. The SOx concentration of the catalyst output gas reaches a peak in a predetermined time after starting the high temperature / rich air-fuel ratio control, and thereafter gradually decreases.

(4) t4 to t5
However, at time t4, the desorption of SOx from the NOx catalyst 17 continues for a while after the high temperature / rich air-fuel ratio control is ended and the control is shifted to the lean / rich spike control. Although the reason why SOx desorption continues even after the end of the high temperature / rich air-fuel ratio control is not clear, it is clear from many experimental results that this phenomenon has reproducibility. Then, at time t5, when SOx does not desorb from the NOx catalyst 17, the SOx concentration of the catalyst outgas and the SOx concentration of the catalyst incoming gas become equal, and at this time, the SOx accumulation amount of the NOx catalyst 17 becomes minimum.

(5) t5 to t6
After t5, SOx is again absorbed by the NOx catalyst 17, and the SOx concentration of the catalyst output gas gradually becomes smaller than the SOx concentration of the catalyst input gas. At t6, the SOx concentration of the catalyst output gas becomes Equilibrate.

  That is, the point C at which the SOx concentration of the catalyst exit gas coincides with the SOx concentration of the catalyst entrance gas is a switching point from the SOx desorption state to the SOx absorption state, and the NOx catalyst 17 adsorbs SOx in the exhaust gas. When the SOx concentration is lower than the SOx concentration of the catalyst input gas, the SOx concentration of the catalyst output gas becomes higher than the SOx concentration of the catalyst input gas when SOx is desorbed from the NOx catalyst 17. .

  Then, during a period (t0 to t3) in which the SOx concentration of the catalyst exit gas is smaller than the SOx concentration of the catalyst input gas, the SOx concentration absorbed by the NOx catalyst 17 is calculated by multiplying the SOx concentration difference by the exhaust gas amount, By multiplying the difference in SOx concentration by the amount of exhaust gas during the period (t3 to t5) in which the SOx concentration of the catalyst output gas is larger than the SOx concentration of the catalyst input gas, the amount of SOx desorbed from the NOx catalyst 17 is calculated. Become.

  Therefore, in the first embodiment, the SOx amount absorbed by the NOx catalyst 17 is calculated from t0, and the SOx accumulation amount is calculated by integrating the SOx amount. The calculated SOx accumulation amount is set to a predetermined upper limit value. , The high temperature / rich air-fuel ratio control is started. After the start of the high temperature / rich air-fuel ratio control, the amount of SOx desorbed from the NOx catalyst 17 is calculated, and the SOx amount is sequentially subtracted from the SOx storage amount to calculate the SOx storage amount during the SOx desorption. Then, when the SOx accumulation amount becomes equal to or less than the predetermined lower limit, the high temperature / rich air-fuel ratio control is terminated.

  In this manner, the amount of accumulated SOx in the NOx catalyst 17 can be accurately grasped, the SOx desorption can be started at an optimal time, and the supply of exhaust gas with a rich air-fuel ratio can be ended at an optimal time. . As a result, the NOx purifying ability of the NOx catalyst 17 can be kept high for a long period of time, and the deterioration of fuel efficiency due to the SOx desorption can be reduced.

  Next, an SOx desorption control execution routine according to the first embodiment will be described with reference to FIG. A flowchart comprising the steps constituting this control routine is stored in the ROM 32 of the ECU 30, and this control routine is executed by the CPU 34 at regular intervals.

  <Step 101> First, in step 101, the ECU 30 reads the SOx concentration of the catalyst input gas detected by the input gas SOx sensor 23 (hereinafter, may be abbreviated as the input gas SOx concentration) and outputs the output gas SOx sensor 24. The SOx concentration of the catalyst output gas (hereinafter, may be abbreviated as the output gas SOx concentration) detected in step (1) is read.

  <Step 102> Next, the ECU 30 proceeds to step 102, and determines whether the incoming gas SOx concentration is higher than the outgoing gas SOx concentration. An affirmative determination in step 102 means that the NOx catalyst 17 is absorbing SOx, and a negative determination means that the NOx catalyst 17 is desorbing SOx.

  <Step 103> If an affirmative determination is made in step 102, the ECU 30 proceeds to step 103 and adds the SOx accumulation amount. More specifically, the output gas SOx concentration is subtracted from the input gas SOx concentration read in step 101 to obtain an SOx concentration difference. On the other hand, the current intake air amount detected by the air flow meter 21 is read and this is used as an exhaust gas amount. The amount of SOx absorbed by the NOx catalyst 17 between this execution and the next execution of this routine is calculated, and the absorbed SOx amount is added by a SOx counter to obtain the current SOx accumulation amount.

  <Step 104> Next, the ECU 30 proceeds to step 104, and determines whether the SOx accumulation amount exceeds a preset upper limit value. If a negative determination is made in step 104, it is not yet time to execute the SOx desorption process, and the process proceeds to return.

  <Step 105> If an affirmative determination is made in step 104, the ECU 30 proceeds to step 105, in which the air-fuel ratio of the exhaust gas is controlled to a stoichiometric air-fuel ratio or a rich air-fuel ratio in order to desorb SOx from the NOx catalyst 17. Start air-fuel ratio control. During the execution of the rich air-fuel ratio control, the temperature raising control for the NOx catalyst 17 is executed by appropriate means, and the catalyst bed temperature of the NOx catalyst 17 is controlled to a temperature optimum for SOx desorption.

  <Step 106> Since the execution of the rich air-fuel ratio control and the temperature rise control causes SOx to be desorbed from the NOx catalyst 17, and the outgassing SOx concentration becomes higher than the incoming gas SOx concentration, the next time this routine is executed, the step 102 is executed. Is negative, the ECU 30 proceeds to step 106 and subtracts the SOx accumulation amount.

  More specifically, the SOx concentration difference is obtained by subtracting the incoming gas SOx concentration from the outgoing gas SOx concentration read in step 101, while the current intake air amount detected by the air flow meter 21 is read, and this is taken as the exhaust gas amount. The amount of SOx desorbed from the NOx catalyst 17 between this execution and the next execution of this routine is calculated, and the desorbed SOx amount is subtracted by the SOx counter to obtain the current SOx accumulation amount.

  <Step 107> Next, the ECU 30 proceeds to step 107, and determines whether the SOx accumulation amount is equal to or less than a preset lower limit value. If a negative determination is made in step 107, SOx has not yet been sufficiently desorbed from the NOx catalyst 17, so the flow proceeds to return, and the rich air-fuel ratio control and the temperature increase control are continued.

  <Step 108> If an affirmative determination is made in step 107, the SOx has sufficiently desorbed from the NOx catalyst 17, so the ECU 30 proceeds to step 108 and ends the rich air-fuel ratio control and the temperature increase control.

  In the first embodiment, the part that executes step 103 in the series of signal processing by the ECU 30 is based on the difference between the SOx concentration upstream and downstream of the NOx catalyst (NOx absorbent). It can be said that it is an SOx accumulation amount calculating means for calculating the SOx amount absorbed in the fuel cell.

  [Second Embodiment] In the above-described first embodiment, the SOx accumulation amount of the NOx catalyst 17 is calculated from the difference between the SOx concentration upstream and downstream of the NOx catalyst 17, and based on the calculated SOx accumulation amount. Although the start time and the end time of the high temperature / rich air-fuel ratio control for the SOx desorption process are determined, in the second embodiment, the SOx concentration downstream of the NOx catalyst 17 is calculated without calculating the SOx accumulation amount. Is determined based on whether the SOx concentration downstream of the NOx catalyst 17 is increasing or decreasing, and a comparison value of the SOx concentration upstream and downstream of the NOx catalyst 17. The start time and the end time of the air-fuel ratio control are determined.

  As described above, when the accumulated amount of SOx in the NOx catalyst 17 increases and approaches the saturation state, the SOx concentration of the catalyst output gas approaches the SOx concentration of the catalyst input gas (between t2 and t3 in FIG. 5). Therefore, the degree of SOx accumulation in the NOx catalyst 17 can be determined based on how close the outlet gas SOx concentration approaches the inlet gas SOx concentration. Therefore, in the second embodiment, when the ratio between the outgassing SOx concentration and the incoming gas SOx concentration becomes a predetermined ratio (for example, 1: 2) (part A in FIG. 5), the high temperature / rich air Start fuel ratio control.

  Further, when SOx is desorbed from the NOx catalyst 17, even if the exhaust air-fuel ratio is switched from rich to lean before the SOx desorption is completely completed, the NOx catalyst 17 remains in the NOx catalyst 17 for a while after being switched to lean. SOx is desorbed (between t4 and t5 in FIG. 5). Therefore, when the rich air-fuel ratio control is being executed, the high temperature / rich air state is set when the outgoing gas SOx concentration approaches the incoming gas SOx concentration to a predetermined value before the outgoing gas SOx concentration matches the incoming gas SOx concentration. The end time of the fuel ratio control can be reached, and by doing so, the amount of the reducing agent used can be reduced. Therefore, in the second embodiment, when the ratio between the outgassing SOx concentration and the incoming gas SOx concentration reaches a predetermined ratio (for example, 2: 1) (part B in FIG. 5), the high-temperature / rich air The fuel ratio control is ended.

  In this way, the degree of SOx accumulation in the NOx catalyst 17 can be accurately grasped, and SOx desorption can be started at an optimal time. Further, the supply of the exhaust gas having the rich air-fuel ratio can be terminated at an optimum time. As a result, the NOx purifying ability of the NOx catalyst 17 can be kept high for a long period of time, and the deterioration of fuel efficiency due to the SOx desorption can be reduced.

  Next, an SOx desorption control execution routine according to the second embodiment will be described with reference to FIG. A flowchart comprising the steps constituting this control routine is stored in the ROM 32 of the ECU 30, and this control routine is executed by the CPU 34 at regular intervals.

  <Step 201> First, in step 201, the ECU 30 reads the SOx concentration of the catalyst input gas detected by the input gas SOx sensor 23, and reads the SOx concentration of the catalyst output gas detected by the output gas SOx sensor 24.

  <Step 202> Next, the ECU 30 proceeds to step 202 and determines whether or not the incoming gas SOx concentration is higher than the outgoing gas SOx concentration. An affirmative determination in step 202 means that the NOx catalyst 17 is absorbing SOx, and a negative determination means that the NOx catalyst 17 is desorbing SOx.

  <Step 203> If an affirmative determination is made in step 202, the ECU 30 proceeds to step 203 and determines whether or not the outgassing SOx concentration is increasing. As shown in FIG. 5, immediately after the NOx catalyst 17 switches from the SOx desorbing state to the SOx absorbing state, the output gas SOx concentration decreases (t0 to t1), and the SOx accumulation amount of the NOx catalyst 17 approaches saturation. As a result, the outgassing SOx concentration increases (t2 to t3). In step 203, it is determined which state the NOx catalyst 17 is in. If a negative determination is made in step 203, it is not yet time to start the SOx desorption processing, and the ECU 30 proceeds to return.

  <Step 204> If an affirmative determination is made in step 203, the ECU 30 proceeds to step 204 and calculates a concentration ratio α between the outgassing SOx concentration and the incoming gas SOx concentration. α = (outgassing SOx concentration) / (incoming gas SOx concentration)

<Step 205> Next, the ECU 30 proceeds to step 205, and determines whether the concentration ratio α calculated in step 204 is larger than an upper limit value (for example, 0.5). Step 2
When a negative determination is made in 05, it is not yet time to execute the SOx desorption process, and the ECU 30 proceeds to return.

  <Step 206> If an affirmative determination is made in step 205, the ECU 30 proceeds to step 206 and controls the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio to desorb SOx from the NOx catalyst 17. Start air-fuel ratio control. During the execution of the rich air-fuel ratio control, the temperature raising control for the NOx catalyst 17 is executed by appropriate means, and the catalyst bed temperature of the NOx catalyst 17 is controlled to a temperature optimum for SOx desorption.

  <Step 207> The execution of the rich air-fuel ratio control and the temperature increase control causes SOx to be desorbed from the NOx catalyst 17, and the outgassing SOx concentration becomes higher than the incoming gas SOx concentration. Is negative, the ECU 30 proceeds to step 207.

  In step 207, the ECU 30 determines whether or not the output gas SOx concentration is falling. As shown in FIG. 5, the output gas SOx concentration increases for a while after the start of the rich air-fuel ratio control, reaches a peak value, and thereafter, the output gas SOx concentration decreases (t3 to t4). In step 207, it is determined which of the states the NOx catalyst 17 is in.

  If a negative determination is made in step 207, the rich air-fuel ratio control should not be terminated yet, so the ECU 30 proceeds to return and continues the rich air-fuel ratio control and the temperature increase control.

  <Step 208> If an affirmative determination is made in step 207, the ECU 30 proceeds to step 208 and calculates the concentration ratio α between the outgassing SOx concentration and the incoming gas SOx concentration. α = (outgassing SOx concentration) / (incoming gas SOx concentration)

  <Step 209> Next, the ECU 30 proceeds to step 209, and determines whether the concentration ratio α calculated in step 208 is smaller than a lower limit (for example, 2). If a negative determination is made in step 209, the rich air-fuel ratio control should not be terminated yet, so the ECU 30 proceeds to return and continues the rich air-fuel ratio control and the temperature increase control.

  <Step 210> If an affirmative determination is made in step 209, the ECU 30 proceeds to step 210 and ends the rich air-fuel ratio control and the temperature increase control for the SOx desorption process.

  [Third Embodiment] In the above-described second embodiment, the start timing and the start of the high temperature / rich air-fuel ratio control are determined based on the concentration ratio of the SOx concentration upstream and downstream of the NOx catalyst 17. However, when the SOx concentration of the exhaust gas flowing into the NOx catalyst 17 is low, the SOx accumulation amount of the NOx catalyst 17 may be small even if the SOx concentration ratio satisfies a predetermined condition. As described above, even if the high-temperature / rich air-fuel ratio control is executed in a state where the SOx accumulation amount of the NOx catalyst 17 is small, the SOx is not efficiently desorbed from the NOx catalyst 17 and the reducing agent is consumed wastefully.

  Therefore, in the third embodiment, when the SOx accumulation amount of the NOx catalyst 17 has not reached the predetermined amount, the execution of the SOx desorption process is prohibited, and the SOx accumulation amount is equal to or more than the predetermined amount, and The SOx desorption process is executed only when the ratio between the SOx concentration upstream and the downstream of the NOx catalyst 17 satisfies a predetermined condition.

Next, a SOx desorption control execution routine according to the third embodiment will be described with reference to FIG. A flowchart comprising the steps constituting this control routine is stored in the ROM 32 of the ECU 30, and this control routine is executed by the CPU 34 at regular intervals.

  <Steps 301 to 302> Steps 301 and 302 are the same as steps 201 and 202 in the second embodiment, respectively, and a description thereof will be omitted.

  If an affirmative determination is made in step 302, the ECU 30 proceeds to step 303 and adds the SOx accumulation amount. More specifically, the output gas SOx concentration is subtracted from the input gas SOx concentration read in step 101 to obtain an SOx concentration difference. On the other hand, the current intake air amount detected by the air flow meter 21 is read and this is used as an exhaust gas amount. The amount of SOx absorbed by the NOx catalyst 17 between this execution and the next execution of this routine is calculated, and the absorbed SOx amount is added by a SOx counter to obtain the current SOx accumulation amount.

  <Step 304> Next, the ECU 30 proceeds to step 304, and determines whether the SOx accumulation amount exceeds a preset SOx desorption execution lower limit value. If a negative determination is made in step 304, it is not yet time to execute the SOx desorption process, and the process proceeds to return.

  <Steps 305 to 311> If an affirmative determination is made in step 304, the ECU 30 proceeds to step 305. Steps 305 to 311 are the same as steps 204 to 210 in the second embodiment, and a description thereof will be omitted.

  It should be noted that the process corresponding to step 203 of the control routine according to the second embodiment is not included in the control routine according to the third embodiment because the SOx accumulation amount is equal to or more than the SOx desorption execution lower limit value in step 304. This is because sometimes the period during which the output gas SOx concentration falls (t0 to t1 in FIG. 5) has already passed.

  <Step 312> After ending the rich air-fuel ratio control and the temperature raising control in step 311, the ECU 30 proceeds to step 312, resets the SOx counter, and ends this routine.

  In the third embodiment, the part that executes step 303 in the series of signal processing by the ECU 30 is based on the difference between the SOx concentration upstream and downstream of the NOx catalyst (NOx absorbent). It can be said that it is an SOx accumulation amount calculating means for calculating the SOx amount absorbed in the fuel cell.

  [Other Embodiments] In the above-described second and third embodiments, the determination of the end time of the rich air-fuel ratio control and the temperature increase control is made by determining that the ratio α between the outgassing SOx concentration and the incoming gas SOx concentration is a predetermined value. The determination is made based on whether the condition (α <2) is satisfied. Alternatively, the end timing is determined based on whether the elapsed time from the start of the rich air-fuel ratio control has reached a predetermined time. May be.

  When the end time of the rich air-fuel ratio control is determined by the elapsed time as described above, the SOx accumulation amount before the start of the SOx desorption process is calculated, and the rich amount is calculated according to the magnitude of the SOx accumulation amount. It is also possible to execute the rich air-fuel ratio control for the SOx desorption process by correcting the SOx desorption process conditions (air-fuel ratio control conditions of the exhaust air-fuel ratio control means) such as the degree and the rich air-fuel ratio continuation time. .

In each of the above-described embodiments, the incoming 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 incoming gas SOx sensor 23. Since the SOx concentration of the exhaust gas flowing into the fuel cell depends on the fuel amount and the exhaust gas amount, it can be estimated from the engine operating state (fuel injection amount, air-fuel ratio, intake air amount, engine speed, etc.). Therefore, instead of providing the incoming gas SOx sensor 23, the ECU 30 may calculate and estimate the SOx concentration of the catalyst incoming gas from the engine operating state.

  In each of the embodiments described above, an 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, the combustion in the combustion chamber is performed in a much leaner region than the stoichiometric region, so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 17 is very lean under a normal engine operating condition, and NOx and Although SOx is absorbed, NOx and SOx are hardly released.

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

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

  When a diesel engine is provided with an exhaust gas recirculation device (a so-called EGR device), a large amount of exhaust gas recirculated gas is introduced into the combustion chamber to reduce the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio. Alternatively, a rich air-fuel ratio can be set.

FIG. 1 is a schematic configuration diagram of a first embodiment of an exhaust gas purification device for an internal combustion engine according to the present invention. It is a figure showing an example of a map of basic fuel injection time. FIG. 3 is a diagram schematically showing the concentrations of unburned HC, CO and oxygen in exhaust gas discharged from an engine. FIG. 5 is a diagram for explaining the NOx absorbing / releasing action of a storage reduction type NOx catalyst. FIG. 4 is a diagram showing a change over time in an exhaust air-fuel ratio, an SOx accumulation amount of a NOx catalyst, and an SOx concentration upstream and downstream of a NOx catalyst. It is a SOx desorption control execution routine in the first embodiment. 9 is an SOx desorption control execution routine in a second embodiment of the exhaust gas purification device for an internal combustion engine according to the present invention. 9 is an SOx desorption control execution routine in a third embodiment of the exhaust gas purification device for an internal combustion engine according to the present invention.

Explanation of reference numerals

1 ... Engine body (internal combustion engine)
3 ... Combustion chamber
4 ... Spark plug
11. Fuel injection valve (exhaust air-fuel ratio control means)
16. Exhaust pipe (exhaust passage)
17 .. NOx storage reduction catalyst (NOx absorbent)
18. Casing
19. Exhaust pipe (exhaust passage)
23 ··· Incoming gas SOx sensor (SOx concentration detection means upstream of the NOx absorbent)
24. Outgassing SOx sensor (SOx concentration detecting means downstream of NOx absorbent)
30 ECU (exhaust air-fuel ratio control means)

Claims (7)

(A) NOx that is provided in an exhaust passage of an internal combustion engine capable of lean combustion and absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low. Absorbent material,
(B) exhaust air-fuel ratio control means for controlling an air-fuel ratio of exhaust gas to a stoichiometric air-fuel ratio or a rich air-fuel ratio when desorbing SOx absorbed by the NOx absorbent,
An exhaust gas purifying apparatus for an internal combustion engine, wherein the exhaust air-fuel ratio control means is operated based on a change tendency of the SOx concentration downstream of the NOx absorbent.   When the SOx concentration downstream of the NOx absorbent is increasing, and the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value, the air-fuel ratio control by the exhaust air-fuel ratio control means is performed. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the apparatus is started.   SOx accumulation amount calculating means for calculating the amount of SOx absorbed in the NOx absorbent based on the concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the air-fuel ratio control by the exhaust air-fuel ratio control means is prohibited when the SOx accumulation amount calculated by the means is equal to or less than a predetermined amount.   SOx accumulation amount calculating means for calculating the amount of SOx absorbed in the NOx absorbent based on the concentration difference between the SOx concentration upstream of the NOx absorbent and the SOx concentration downstream of the NOx absorbent. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the air-fuel ratio control condition of the exhaust air-fuel ratio control unit is corrected according to the magnitude of the SOx accumulation amount calculated by the unit.   When the SOx concentration downstream of the NOx absorbent is falling and the SOx concentration downstream of the NOx absorbent approaches the SOx concentration upstream of the NOx absorbent to a predetermined value, the air-fuel ratio control by the exhaust air-fuel ratio control means is performed. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1 or 2, wherein the operation is terminated.   6. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the SOx concentration upstream of the NOx absorbent is detected by SOx concentration detecting means provided in an exhaust passage upstream of the NOx absorbent. .   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 2 to 5, wherein the concentration of SOx upstream of the NOx absorbent is estimated from an operation state of the internal combustion engine.

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