JP2008202414A - Exhaust emission control device of engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an engine in which an S poisoning recovery performance is increased. <P>SOLUTION: This exhaust emission control device is used selectively between a first cycle W1 in which oxygen is supplied to an NOx catalyst 17 beyond the oxygen storage capacity of a catalytic converter rhodium 16, and a second cycle which is shorter than the first cycle W1 and in which oxygen is not supplied to the NOx catalyst 17 according to the oxygen storage capacity of the catalytic converter rhodium 16 as the cycle of fluctuation of the air-fuel ratio of the gas, when sulfur contents are removed from the front stage part 17a and the rear stage part 17b of the NOx catalyst 17. Consequently, a reduction atmosphere can be positively formed based on the second cycle W2 by promoting the oxidation reaction based on the first cycle W1 in the NOx catalyst 17. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an engine.

  In a lean burn engine for improving fuel efficiency, a so-called NOx catalyst is provided in the exhaust passage. This NOx catalyst has a NOx absorbent mainly composed of barium, absorbs NOx in exhaust gas when the air-fuel ratio is lean (oxygen-excess state), and occludes when the air-fuel ratio becomes rich (oxygen-deficient state). NOx is reduced and released to purify NOx.

  By the way, this NOx catalyst has a problem of sulfur (S) poisoning. In other words, if sulfur oxide (SOx) is contained in the lean air-fuel ratio exhaust gas based on the S component in the fuel, the SOx in the exhaust gas is stored and accumulated in the NOx catalyst by the same mechanism as NOx. NOx occlusion is to be inhibited based on the SOx that has been made. For this reason, as one of the countermeasure techniques, as shown in Patent Document 1, the NOx catalyst is divided into a catalyst front part and a catalyst rear part so that SOx can be released from the NOx catalyst. During the poisoning recovery process, the lean air-fuel ratio exhaust gas and the rich air-fuel ratio exhaust gas are alternately supplied with a relatively short cycle. During the S poisoning recovery process at the rear stage of the catalyst, the lean air-fuel ratio exhaust gas is supplied at a cycle longer than the cycle. One that alternately supplies rich air-fuel ratio exhaust has been proposed. According to this, by supplying the lean air-fuel ratio exhaust gas and the rich air-fuel ratio exhaust gas to the NOx catalyst with a relatively short switching cycle, it is possible to mainly heat and raise the temperature of the front stage of the catalyst, and the NOx catalyst has the lean air fuel ratio. By supplying the fuel ratio exhaust gas and the rich air fuel ratio exhaust gas at a switching cycle longer than the above cycle, it is possible to mainly heat and raise the temperature of the catalyst rear stage, and the S poisoning of the catalyst front stage and the catalyst rear stage. The temperature conditions during the recovery process can be made preferable.

JP 2005-325693 A

  However, in the above exhaust purification device, the switching between the lean air-fuel ratio exhaust and the rich air-fuel ratio exhaust is only performed from the viewpoint of the temperature condition at the time of the S poison recovery process, and the S coverage for the NOx catalyst. It is still not enough as a poison recovery process.

  This invention is made | formed in view of the said situation, The technical subject is to provide the exhaust gas purification apparatus of the engine which can improve sulfur poisoning recovery performance.

In order to achieve the technical problem, the present invention provides:
A three-way catalyst, a NOx catalyst that is disposed downstream of the three-way catalyst, stores NOx when the air-fuel ratio is lean, and reduces and releases NOx stored when the air-fuel ratio is rich, and attached to the NOx catalyst When removing the sulfur content, by changing the air-fuel ratio for each cylinder, the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are alternately supplied to the three-way catalyst side to periodically vary the exhaust air-fuel ratio. In an engine exhaust purification device comprising an air-fuel ratio fluctuation means,
When the air-fuel ratio fluctuation means removes sulfur from the front stage portion that occupies the upstream region of the NOx catalyst and the rear stage portion downstream of the front stage portion, A first period in which oxygen is supplied to the NOx catalyst beyond the oxygen storage capacity of the three-way catalyst, and an oxygen is supplied to the NOx catalyst based on the oxygen storage capacity of the three-way catalyst that is shorter than the first period. Is set so as to be used by switching between the second cycle in which no power is supplied.

  With the above-described configuration, when the sulfur content is removed from the front stage and the rear stage in the NOx catalyst, the first cycle and the second cycle are used as the cycle of the fluctuation of the exhaust gas. In any removal of the sulfur component in the rear stage, the oxidation reaction (oxidation reaction between flammable components such as HC and oxygen) is promoted based on the first cycle of the fluctuation of the exhaust air-fuel ratio, and the NOx catalyst The temperature can be increased to a desired temperature. Further, in the second period, the NOx catalyst is actively made a reducing atmosphere, and the sulfur content (SOx) is reduced based on the actively created reducing atmosphere. Can be removed.

According to a preferred aspect of the present invention, the air-fuel ratio varying means varies the exhaust air-fuel ratio between the first period and the second period when removing sulfur from the front stage of the NOx catalyst. It is possible to adopt a configuration in which the cycle is set to be executed before the second cycle (corresponding to claim 2). With this configuration, when removing the sulfur content from the front part of the NOx catalyst, the temperature of the front part can be raised and the front part can be made into a reducing atmosphere, and the sulfur content can be effectively removed from the front part of the NOx catalyst. it can.
Further, the first stage of the NOx catalyst is positioned upstream of the exhaust passage, and the fact that the temperature tends to be higher than the temperature of the rear stage is used to increase the temperature of the front stage of the NOx catalyst. One cycle can be made only once, and the fluctuation cycle of the exhaust air-fuel ratio can be switched only once from the first cycle to the second cycle. For this reason, switching control can be simplified.

  As a preferred aspect of claim 1, the air-fuel ratio changing means is set so as to change the exhaust air-fuel ratio by repeating the first period and the second period when removing sulfur from the rear stage of the NOx catalyst. It is possible to adopt the configuration described above (corresponding to claim 3). With this configuration, by changing the exhaust air-fuel ratio by repeating the first cycle and the second cycle, the temperature of the rear stage of the NOx catalyst can be maintained at a desired high temperature state, and a reducing atmosphere can be created there. Sulfur can be effectively removed from the rear stage of the NOx catalyst.

  According to a preferred aspect of the present invention, the air-fuel ratio changing means executes the first period of the exhaust air-fuel ratio that is executed when the sulfur content is removed from the rear stage portion of the NOx catalyst when the sulfur content is removed from the front stage portion of the NOx catalyst. It is possible to adopt a configuration in which the exhaust air-fuel ratio is set to be longer than the first cycle. With this configuration, mixing of combustible components such as HC and oxygen can be delayed so that their oxidation reaction can be performed in the latter part of the NOx catalyst, and the sulfur content can be effectively removed from the latter part of the NOx catalyst. In order to remove, the temperature in the latter stage part of the NOx catalyst can be increased.

  As a preferred aspect of claim 1, the lean period of the first cycle executed when the air-fuel ratio changing means removes the sulfur content from the rear stage of the NOx catalyst is made longer than the rich period of the first cycle. It is possible to adopt a configuration set to (corresponding to claim 5). With this configuration, it is possible to supply a sufficient amount of oxygen in consideration of the oxygen storage capacity of the NOx catalyst while reducing CO and the like. In order to effectively remove sulfur from the rear stage of the NOx catalyst, the NOx catalyst The temperature in the rear stage can be increased to a desired temperature.

  According to a preferred aspect of the present invention, a temperature suppression unit that suppresses an increase in the catalyst temperature of the NOx catalyst is provided, and the temperature suppression unit includes at least the air-fuel ratio fluctuation unit that changes the fluctuation cycle from the second cycle to the first cycle. It is possible to adopt a configuration that is set to operate on the condition that it is changed to (corresponding to claim 6). With this configuration, when the air-fuel ratio changing means changes the change period from the second period to the first period, it is possible to suppress the NOx catalyst from rapidly rising in temperature in an oxidizing atmosphere, and the heat resistance of the NOx catalyst is deteriorated. Can be suppressed.

  As a preferred aspect of claim 1, it is possible to adopt a configuration in which the temperature suppressing means changes in a direction to reduce the amplitude of fluctuation of the exhaust air-fuel ratio (corresponding to claim 7). With this configuration, temperature suppression means can be specifically provided.

  As a preferred aspect of claim 1, a configuration in which the temperature suppression means is changed in a direction to shorten the first period (corresponding to claim 8). With this configuration, temperature suppression means can be specifically provided.

  According to a preferred aspect of the present invention, there is provided an estimation means for estimating a location where sulfur content adheres to the NOx catalyst, and the air-fuel ratio fluctuation means is configured to change the exhaust air-fuel ratio according to the location estimated by the estimation means. It is possible to adopt a configuration in which the cycle is changed (corresponding to claim 9). With this configuration, it is possible to accurately set the period according to the sulfur content adhesion location in the NOx catalyst, and to perform sulfur content removal in a short period of time.

  ADVANTAGE OF THE INVENTION According to this invention (invention which concerns on Claim 1), the exhaust purification apparatus of the engine which can improve S poison recovery performance can be provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[System configuration]
As shown in FIG. 1, an engine (Otto cycle engine) 1 according to this embodiment includes a main body 2, a plurality of pistons 3... 3 (only one of which is shown), and a combustion chamber defined by each piston 3. 4 (multi-cylinder), a spark plug 5 disposed in the upper center of each combustion chamber 4, an injector 6 disposed on the side of each combustion chamber 4 and directly injecting fuel into the combustion chamber 4. An intake passage 9 is connected to the combustion chamber 4 via an intake valve 7, and an air cleaner 11, an air flow sensor 12, a throttle valve 13, a surge tank 14, and the like are connected to the intake passage 9 from the upstream side to the downstream side. They are arranged sequentially. The downstream passage of the surge tank 14 is branched for each cylinder, and the downstream portion of each branch passage 9a is divided into two passages 9b and 9c. When the swirl generating valve 15 provided in one passage 9c is closed, swirl is generated in the combustion chamber 4 by the intake air introduced from the other 9b.

  An exhaust passage 10 is connected to the combustion chamber 4 via an exhaust valve 8. A three-way catalyst 16 and a NOx catalyst 17 are arranged in series in the exhaust passage 10 from the upstream side to the downstream side. . The three-way catalyst 16 has a function of simultaneously removing CO, HC and NOx in the exhaust gas in the vicinity of the theoretical air-fuel ratio (A / F = 14.7). The NOx catalyst 17 is mainly composed of barium, and an NOx occlusion material in which an alkali metal such as potassium, magnesium, strontium, or lanthanum, an alkaline earth metal, or a rare earth, and a noble metal having a chemical reaction catalytic action such as platinum is supported. In this embodiment, it is divided into a front stage part 17a and a rear stage part 17b arranged on the downstream side of the front stage part 17a from the upstream side toward the downstream side. This NOx catalyst 17 occludes NOx flowing in without being purified by the three-way catalyst 16 when the air-fuel ratio is lean, and suppresses discharge to the outside, and stores the NOx that was occluded when the air-fuel ratio is rich. Has a function of purifying by reacting with CO and HC in the exhaust gas.

  In FIG. 1, reference numeral 20 denotes a control unit (ECU) of the engine 1. The ECU 20 includes a signal from a boost sensor 23 that detects intake negative pressure in the surge tank 14, a signal from a fuel pressure sensor 24 that detects the pressure of fuel supplied to the injector 6, and an upstream side of the three-way catalyst 16. A first air-fuel ratio provided by an O2 sensor that is provided and detects whether the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 4 is richer or leaner than the stoichiometric air-fuel ratio from the residual oxygen concentration in the exhaust gas discharged from the combustion chamber 4 A signal from the sensor 26, a signal from the exhaust temperature sensor 41 that is disposed between the front stage 17a and the rear stage 17b of the NOx catalyst 17 and detects the temperature of the front stage 17a, and downstream of the rear stage 17b of the NOx catalyst 17 A signal from the exhaust temperature sensor 42 that detects the temperature of the rear stage portion 17b and the concentration of residual oxygen in the exhaust gas that is provided downstream of the NOx catalyst 17 and passes through the NOx catalyst 17 A signal from the second air-fuel ratio sensor 28, which is an O2 sensor for detecting engine speed, a signal from the engine rotation sensor 29 for detecting the rotational speed of the engine 1, and an accelerator opening sensor for detecting the amount of depression of an accelerator pedal (not shown) A signal from 30 is input. On the other hand, the ECU 20 outputs a control signal to the injector 6 to control the fuel injection amount and the fuel injection timing, as well as a process for releasing the NOx being stored from the NOx catalyst 17 (NOx purge process), and a NOx catalyst. Performs 17 sulfur poisoning recovery treatments comprehensively. In this case, the ECU 20 calculates the intake air amount based on the intake negative pressure detected by the boost sensor 23 and the engine rotational speed detected by the rotational speed sensor 29, and based on the intake air amount during the theoretical air-fuel ratio operation or The fuel injection amount during the rich air-fuel ratio operation is controlled, and the fuel injection amount during the lean air-fuel ratio operation is determined based on the accelerator opening detected by the accelerator opening sensor 30 and the engine rotational speed detected by the rotational speed sensor 29. Control.

Next, the outline of the control contents by the ECU 20 regarding the NOx purge process and the S poison recovery process will be described.
[NOx purge process]
As the lean operation continues, the amount of NOx absorbed by the NOx catalyst 17 increases. If this is left as it is, it becomes saturated and the purification capacity of the NOx catalyst 17 decreases. Therefore, the air-fuel ratio is enriched at a predetermined timing, whereby the stored NOx is decomposed and released into oxygen and nitrogen to restore the absorption capacity of the catalyst 17. The frequency of this NOx purge process is generally greater than the frequency of the sulfur poisoning recovery process.

[Sulfur poisoning recovery process]
As the sulfur poisoning recovery process for the NOx catalyst 17, it is preferable that the NOx catalyst 17 is maintained in a desired high-temperature state while reducing atmosphere. For this reason, the ECU 20 basically exhausts the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas alternately as shown in FIG. 2 when removing the sulfur content from the front stage 17a and the rear stage 17b in the NOx catalyst 17. The exhaust gas (exhaust air / fuel ratio) is periodically changed by being supplied to the passage 10 and, as shown in FIG. 3, the exhaust air / fuel ratio is changed between a first period W1 and a second period W2 as shown in FIG. Used.
In this case, the variation in the exhaust air-fuel ratio is obtained by alternately discharging the rich air-fuel ratio exhaust based on the rich air-fuel ratio operation and the lean air-fuel ratio exhaust based on the lean air-fuel ratio operation, and the cycle thereof is Of these cylinders, rich air-fuel ratio operation is performed in some cylinders, lean air-fuel ratio operation is performed in other cylinders, rich air-fuel ratio operation and lean air-fuel ratio operation are performed in order for every fixed period in all cylinders, etc. And adjusted as appropriate. The first cycle W1 and the second cycle W2 are set by such adjustment, and the first cycle W1 exceeds the oxygen storage capacity of the three-way catalyst 16 (passes through the three-way catalyst 16) and the NOx catalyst. The second cycle W2 is shorter than the first cycle W1 and oxygen cannot be supplied to the NOx catalyst 17 based on the oxygen storage capacity of the three-way catalyst 16. It is set to the one of the cycle.
Therefore, the oxidation reaction (oxidation reaction of flammable components such as HC and oxygen) is promoted based on the change cycle of the exhaust air-fuel ratio being the first cycle W1, and the temperature of the NOx catalyst 17 is increased to a desired temperature. By using the second period W2, the sulfur content (SOx) can be removed from the NOx catalyst under a state in which the NOx catalyst 17 is raised to a desired temperature with the NOx catalyst 17 as a reducing atmosphere.

In the present embodiment, the above content is set in a more preferable aspect.
When the sulfur content is removed from the front stage portion 17a of the NOx catalyst 17, as shown in FIG. 3, the exhaust air-fuel ratio is varied between the first cycle W1 and the second cycle W2, and the first cycle W1 is changed to the second cycle W2. It is executed before. When removing the sulfur content from the NOx catalyst front stage portion 17a, it is preferable to raise the temperature of the front stage portion 17a to a desired temperature and then bring the front stage portion 17a into a reducing atmosphere from the viewpoint of effectively removing the sulfur content. Because. At this time, utilizing the fact that the front stage portion 17a of the NOx catalyst 17 is positioned upstream of the rear stage portion 17b and its temperature tends to be higher than the temperature of the rear stage portion 17b originally, the NOx catalyst front stage portion is used. The first cycle W1 for increasing the temperature of 17a is not repeated for the first time. As a result, the changeover cycle of the exhaust air-fuel ratio can be switched only once from the first cycle W1 to the second cycle W2, and the switching control of the first cycle W1 and the second cycle W2 can be simplified. it can.

On the other hand, when the sulfur content is removed from the NOx catalyst rear stage 17b, as shown in FIG. 3, the exhaust air-fuel ratio is changed by repeating the first cycle W1 and the second cycle W2. This is because the temperature of the front stage portion 17a of the NOx catalyst 17 generally tends to be higher, but this cannot be used in the rear stage portion 17b. Therefore, the exhaust air-fuel ratio is set to the first cycle W1 and the second cycle W2. Thus, the temperature of the NOx catalyst rear stage portion 17b is maintained at a desired high temperature state and a reducing atmosphere is created.
In addition, at this time, in the first period W1-2 (W1) executed when the sulfur content is removed from the NOx catalyst rear stage portion 17b, as shown in FIG. 2, it is executed when the sulfur content is removed from the front stage portion 17a of the NOx catalyst. A longer cycle than the first cycle W1-1 (W1) is to be used. This is because the mixing of combustible components such as HC passing through the three-way catalyst 16 and oxygen is delayed so that their oxidation reaction is accurately performed in the rear stage portion 17b of the NOx catalyst. This makes it possible to raise the temperature of the NOx catalyst rear stage portion 17b to a desired temperature without excessively raising the temperature of the front stage portion 17a (while preventing thermal degradation). As shown in FIG. 2, the first cycle W1-2 (W1) is appropriately used depending on whether the NOx catalyst 17 capacity is large or the NOx catalyst 17 capacity is small.

  In this case, as shown in FIG. 4, the lean period L2 of the first cycle W1-2 executed when removing the sulfur content from the NOx catalyst rear stage portion 17b is longer than the rich period L1 of the first cycle W1-2. It is preferable to make it longer. A sufficient amount of oxygen in consideration of the oxygen storage capacity of the NOx catalyst 17 can be supplied to the NOx catalyst rear stage 17b, and the sulfur content can be effectively removed from the NOx catalyst 17 rear stage 17b. This is because the temperature at the NOx catalyst rear stage 17b can be increased to a desired temperature.

  Further, in the present embodiment, a temperature suppression unit that suppresses an increase in the catalyst temperature of the NOx catalyst 17 is provided, and the temperature suppression unit changes at least the fluctuation cycle from the second cycle W2 to the first cycle W1. It is set to operate on the condition. This is because when the fluctuation cycle of the exhaust air-fuel ratio is changed from the second cycle W2 to the first cycle W1, the temperature of the NOx catalyst 17 is rapidly increased due to an oxidizing atmosphere. As this temperature suppression means, one that changes in the direction of decreasing the amplitude of the exhaust air-fuel ratio fluctuation, or one that changes in the direction of shortening the first period W1 can be used.

  Further, in the present embodiment, an estimation means for estimating the location where sulfur adheres to the NOx catalyst 17 is provided, and is set so as to change the cycle of fluctuation of the exhaust air-fuel ratio in accordance with the location estimated by the estimation means. ing. This is because the period corresponding to the sulfur adhering location in the NOx catalyst 17 is accurately set so that the sulfur content can be removed in a short period of time. As this estimation means, specifically, the accumulated operation time of the lean operation can be used. This is because SOx in the exhaust gas is gradually stored from the front end side (inflow end side) of the NOx catalyst 17 toward the rear end side.

Next, a control example of the S poisoning recovery process will be specifically described based on the flowcharts shown in FIGS.
First, in Q1 to Q3, it is determined whether or not to start the S poison recovery process. Specifically, the sulfur poisoning of the NOx catalyst 17 during lean operation is mainly due to the sulfur content in the fuel, so the fuel amount during the lean operation time is determined in Q1, and based on the fuel amount, In Q2, the S poisoning amount is calculated. Then, in the next Q3, it is determined whether or not the S poisoning amount of Q2 is larger than the S poisoning equivalent amount FS0 (set value) of the NOx catalyst front stage 17a. If Q3 is NO, the S poisoning recovery process is performed. On the other hand, when Q3 is YES, it is determined that the S poison recovery process needs to be started, and the process proceeds to the next Q4.

  In Q4, the S poisoning recovery processing time t is set to t = 0. Subsequently, in Q5, the rich air-fuel ratio and lean air-fuel ratio for each cylinder are adjusted in the exhaust passage by rich-lean adjustment of the air-fuel ratio. The exhaust gas is supplied alternately with the state of the first cycle W1-1. This is to increase the temperature of the front stage portion 17a to a desired temperature based on the oxidation reaction in order to promote the S poison recovery process at the NOx catalyst front stage portion 17a. For this reason, when the exhaust gas that fluctuates in the first period W1-1 passes through the three-way catalyst 16, as described above, even if oxygen is occluded based on its oxygen occlusion capacity, oxygen can still be supplied to the NOx catalyst. In addition, based on the first cycle W1-1, the combustible component such as HC and oxygen are set to be accurately mixed in the front stage portion 17a.

  In the next Q6, it is determined whether or not the temperature Tf (detected by the exhaust temperature sensor 41) of the NOx catalyst front stage 17a has become equal to or higher than a set temperature T1 (for example, 700 ° C. to 750 ° C. (preferably 700 ° C.)). This determination is made to determine whether or not the temperature Tf of the NOx catalyst front stage portion 17a has risen to a desired temperature. When this determination is NO, fluctuations in the exhaust gas under the first period W1-1 occur. On the other hand, while the NOx catalyst front stage 17a continues to be heated, when Q6 is YES, it is assumed that the temperature has reached a temperature at which the S poison recovery process can be promoted. The second period W2 is shorter than the one period W1-1. This is because the NOx catalyst front stage portion 17a is brought into a reducing atmosphere at a temperature that can promote the S poisoning recovery process, and SOx is released based on the reducing atmosphere. For this reason, the second period W2 of the exhaust gas is set so that when the exhaust gas passes through the three-way catalyst 16, oxygen is occluded based on the oxygen storage capacity and oxygen is not supplied to the NOx catalyst 17.

  Next, in Q8, it is determined whether or not the S poisoning amount is larger than the S poisoning equivalent amount BS0 (set value). This is to determine whether or not the S poison amount exceeds the front stage portion 17a and also reaches the rear stage portion 17b in the NOx catalyst 17. For this reason, the S poison equivalent amount BS0 is equal to the aforementioned S poison amount. The poison equivalent amount is larger than FS0. When the determination of Q8 is YES, it is assumed that the S poisoning amount exceeds the front stage 17a in the NOx catalyst 17 and reaches the rear stage 17b. First, in Q9, the S poison recovery process in the NOx catalyst front stage 17a. In order to ensure the time, it is determined whether or not the elapsed time (S poisoning recovery processing time) t from the start of the S poisoning recovery processing is equal to or longer than the set time t1. When the determination of Q9 is NO, the exhaust gas that fluctuates with the second cycle W2 continues to be supplied to the NOx catalyst 17, while when the determination of Q9 is YES, the process proceeds to Q10.

  In Q10, it is determined whether or not the temperature Tb (exhaust temperature sensor detection) of the NOx catalyst rear stage 17b has become equal to or lower than a set temperature T2 (for example, 600 ° C.). This is to determine whether or not the temperature in the NOx catalyst rear stage portion 17b has decreased too much in order to perform the S poison recovery process in the NOx catalyst rear stage portion 17b. When Q10 is NO, the exhaust gas continues to be supplied to the NOx catalyst 17 with a fluctuation cycle of the second cycle W2, while when Q10 is YES, the first cycle W1-2 whose exhaust fluctuation cycle is longer than the second cycle W2. Changed to In order to increase the temperature of the rear stage portion 17b to the desired temperature by promoting the oxidation action in the rear stage portion 17b of the NOx catalyst so that the temperature of the rear stage portion 17b of the NOx catalyst becomes a desired temperature that can promote the S poison recovery process. It is. In this case, the first cycle W1-2 of Q11 is longer than the first cycle W1-1 of Q5 (see FIG. 2). In consideration of the fact that the rear stage part 17b of the NOx catalyst 17 is separated from the front stage part 17a in the flow direction, the mixing of the flammable components such as HC and oxygen is delayed so that their oxidation reaction is performed in the rear stage of the NOx catalyst. This is for the purpose of accurately performing in the portion 17b. In particular, at this time, from the viewpoint of increasing the amount of oxygen to be supplied, it is preferable to make the lean period L2 of the first period W1-2 longer than the rich period L1 of the first period W1-2 (see FIG. 4).

  When changing the fluctuation cycle of the exhaust gas from the second cycle W2 to the first cycle W1-2 (Q11), in Q12, whether or not the temperature Tb of the NOx catalyst rear stage portion 17b has become equal to or higher than a set temperature T3 (for example, 680 ° C.). Is determined. This determination is for determining whether or not the NOx catalyst rear stage 17b suddenly rises in temperature in the oxidizing atmosphere when the exhaust fluctuation period is changed from the second period W2 to the first period W1. For this reason, when Q12 is YES, it is assumed that the temperature of the NOx catalyst rear stage portion 17b is rapidly increased. In Q13, the temperature suppression process (the amplitude of the first cycle W1-2 is reduced) of the NOx catalyst rear stage portion 17b. Change to the direction to be changed, change to the direction to shorten the first cycle W1-2, etc.) are executed, and after that, the process proceeds to the next Q13. On the other hand, when Q12 is NO, the process bypasses Q13 and proceeds to Q14.

  In Q14, it is determined whether or not the temperature Tb of the NOx catalyst rear stage 17b is equal to or higher than the set temperature T1 that is the same as the set temperature T1 of Q6. This is to determine whether the NOx catalyst rear stage 17b has reached a desired temperature for the S poison recovery process. When the determination of Q14 is NO, the exhaust continues to fluctuate under the first cycle W1-2, while when the determination of Q14 is YES, the exhaust gas is exhausted at Q15 to create a reducing atmosphere in the NOx catalyst rear stage 17b. Is changed to the second period W2. As a result, a reducing atmosphere is created at a desired temperature in the NOx catalyst rear stage 17b, and the S poisoning recovery process is accurately performed in the rear stage 17b.

  Next, in Q16, whether or not the elapsed time t from the start of the S poison recovery process is equal to or longer than the set time t2 (t2> t1) in order to ensure the S poison recovery process time in the NOx catalyst rear stage 17b. Is determined. When the determination of Q16 is NO, the process returns to Q10 and the processing so far is repeated (see FIG. 3), while when the determination of Q15 is YES, the S poison recovery process is terminated (Q17). . Thus, in the S poison recovery process in the NOx catalyst rear stage 17b, the exhaust gas is fluctuated with repetition of the first period W1-2 and the second period W2 until the S poison recovery process time t2. .

  On the other hand, when Q8 is NO, it is determined whether or not the elapsed time t from the start of the S poison recovery process is equal to or longer than the set time t1 in Q18, assuming that the S poison remains in the NOx catalyst front stage 17a. Is done. This is to ensure the S poisoning recovery processing time in the NOx catalyst front stage 17a. When the determination of Q18 is NO, exhaust gas that fluctuates with the second period W2 is continuously supplied to the NOx catalyst 17 to create a reducing atmosphere (S poisoning recovery process), while when the determination of Q18 is YES, NOx Assuming that the S poison recovery process is performed for the S poison recovery process time required for the catalyst front stage 17a, the S poison recovery process is terminated (Q17).

The engine control system block diagram which concerns on embodiment. Explanatory drawing explaining the difference in the fluctuation cycle of exhaust (exhaust machine air fuel ratio). Explanatory drawing explaining the content of the S poison recovery process with respect to a NOx catalyst. The figure which shows the modification in case an exhaust air fuel ratio fluctuates with 1st period W1-2. The flowchart which shows the example of control which concerns on S poison recovery process. 6 is a flowchart showing a continuation of FIG.

Explanation of symbols

1 Engine 4 Combustion chamber (cylinder)
6 Injector (Air-fuel ratio fluctuation means)
16 Three-way catalyst 17 NOx catalyst 17a Front stage part 17b Rear stage part 20 Control unit (air-fuel ratio fluctuation means)
W1-1 1st period W1-2 1st period W2 2nd period

Claims (9)

A three-way catalyst, a NOx catalyst that is disposed downstream of the three-way catalyst, stores NOx when the air-fuel ratio is lean, and reduces and releases NOx stored when the air-fuel ratio is rich, and attached to the NOx catalyst When removing the sulfur content, by changing the air-fuel ratio for each cylinder, the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are alternately supplied to the three-way catalyst side to periodically vary the exhaust air-fuel ratio. In an engine exhaust purification device comprising an air-fuel ratio fluctuation means,
When the air-fuel ratio fluctuation means removes sulfur from the front stage portion that occupies the upstream region of the NOx catalyst and the rear stage portion downstream of the front stage portion, A first period in which oxygen is supplied to the NOx catalyst beyond the oxygen storage capacity of the three-way catalyst, and an oxygen is supplied to the NOx catalyst based on the oxygen storage capacity of the three-way catalyst that is shorter than the first period. Is set to be used by switching to the second cycle that will not supply
An exhaust emission control device for an engine. In claim 1,
The air-fuel ratio changing means changes the exhaust air-fuel ratio between the first period and the second period and removes the first period from the second period when removing sulfur from the front stage of the NOx catalyst. Set to run first,
An exhaust emission control device for an engine. In claim 1,
The air-fuel ratio changing means is set so as to change the exhaust air-fuel ratio by repeating the first cycle and the second cycle when removing sulfur from the subsequent stage of the NOx catalyst.
An exhaust emission control device for an engine. In claim 1,
The first period of the exhaust air-fuel ratio, which is executed when the sulfur content is removed from the rear stage portion of the NOx catalyst, is changed by the air-fuel ratio changing means. Set to be longer than one cycle,
An exhaust emission control device for an engine. In claim 3,
The air-fuel ratio changing means is set to make the lean period of the first cycle executed when removing the sulfur content from the rear stage portion of the NOx catalyst longer than the rich period of the first cycle.
An exhaust emission control device for an engine. In claim 1,
Temperature suppression means for suppressing an increase in the catalyst temperature of the NOx catalyst is provided,
The temperature suppression means is set to operate on condition that at least the air-fuel ratio changing means changes the changing period from the second period to the first period;
An exhaust emission control device for an engine. In claim 6,
The temperature suppression means changes in a direction to decrease the amplitude of fluctuation of the exhaust air-fuel ratio.
An exhaust emission control device for an engine. In claim 6,
The temperature suppression means changes in a direction to shorten the first period.
An exhaust emission control device for an engine. In claim 1,
Estimating means for estimating the location where sulfur content adheres to the NOx catalyst is provided,
The air-fuel ratio changing means is set to change the cycle of fluctuation of the exhaust air-fuel ratio according to the location estimated by the estimating means;
An exhaust emission control device for an engine.

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