JP2002364348A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problems of the reduction of fuel consumption performance and the use of large amount of energy or the reduction of torque by suppressing the enrichment of an air-fuel ratio for sulfur release and the rise of temperature of exhaust gas. SOLUTION: A control unit 20 estimates an absorption amount of NOx into an NOx absorption material 17 in an exhaust gas passage 10 and enriches an air-fuel ratio when the estimated NOx absorption amount exceeds a predetermined amount to release NOx from the absorption material 17. Moreover, it estimates an amount of sulfur adhered to the NOx absorption material 17, enriches an air-fuel ratio when the estimated amount of adhered sulfur exceeds a predetermined value, and raises a temperature of exhaust gas to release sulfur from the absorption material 17. Furthermore, it judges the desorption property of sulfur adhered to the NOx absorbent 17 and adjusts and suppresses at least either of the enrichment of air-fuel ratio and raising of temperature of exhaust gas for sulfur release in accordance with the results of the judgement.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of an exhaust gas purifying apparatus for an engine, and more particularly to an exhaust gas purifying apparatus for an engine provided with an NOx absorbent in an exhaust passage.

[0002]

2. Description of the Related Art In a lean burn engine for improving fuel efficiency, a so-called NOx catalyst is provided in an exhaust passage.
This NOx catalyst has a NOx absorbent mainly composed of barium. The NOx absorbent absorbs NOx in the exhaust gas when the air-fuel ratio is lean (oxygen-rich atmosphere), and reduces and purifies the absorbed NOx when the air-fuel ratio becomes rich (oxygen-deficient atmosphere) and discharges it. In a well-known general NOx release control, the amount of NOx absorbed by a NOx absorbent during lean operation is estimated based on an engine speed, an engine load, and the like. When the amount becomes equal to or less than the regulated value, the air-fuel ratio is enriched to release NOx from the NOx absorbent.

On the other hand, the NOx absorbent has a problem of sulfur poisoning. That is, the sulfur component such as SOx contained in the fuel adheres to the NOx absorbent and hinders the absorption of NOx by the absorbent, thereby lowering the NOx purification rate. Accordingly, sulfur release control is performed as disclosed in Japanese Patent Application Laid-Open No. 9-31747. Similar to the NOx release control, in the well-known general sulfur release control, the amount of sulfur adhering to the NOx absorbent is estimated based on the fuel flow rate, the exhaust gas temperature, and the like, and when the estimated amount is equal to or more than a predetermined amount. In order to enrich the air-fuel ratio and raise the temperature of the exhaust gas to raise the temperature of the catalyst, NO
x Release sulfur from sorbent.

[0004]

The enrichment of the air-fuel ratio for sulfur release involves the problem of reduced fuel economy and impairs the characteristics of lean burn engines that attempt to improve fuel economy. Also, the temperature rise of the exhaust gas for releasing sulfur is realized by, for example, the operation of the heater or the retardation of the ignition timing, and also suffers from a large use of energy and a decrease in torque. Therefore, even when sulfur is released from the NOx absorbent, it is desirable to suppress the degree and degree of enrichment of the air-fuel ratio and the temperature rise of the exhaust gas as much as possible.

Accordingly, the present invention is to suppress the degree of air-fuel ratio enrichment and the degree and degree of temperature rise of exhaust gas for sulfur release, thereby suppressing the problem of reduced fuel consumption performance, large amount of energy use or reduced torque. As an issue.

[0006]

That is, in order to solve the above-mentioned problems, the invention according to claim 1 of the present application absorbs NOx in exhaust gas in an oxygen-excess atmosphere and releases the NOx by lowering the oxygen concentration. NOx absorbent is provided in the exhaust passage to estimate the amount of NOx absorbed by the NOx absorbent.
Absorption amount estimating means, NOx releasing means for enriching the air-fuel ratio and releasing NOx from the absorbent when the NOx absorption amount estimated by the estimating means becomes a predetermined amount or more, A sulfur deposition amount estimating means for estimating the sulfur deposition amount; and, when the sulfur deposition amount estimated by the estimating means exceeds a predetermined amount, enrich the air-fuel ratio and raise the temperature of the exhaust gas to remove sulfur from the absorbent. An exhaust gas purifying apparatus for an engine having a sulfur releasing means for releasing, comprising: a sulfur desorbing property determining means for determining a desorbing property of sulfur attached to the NOx absorbent; and
The sulfur discharging means includes a sulfur discharging operation adjusting means for adjusting at least one of the enrichment of the air-fuel ratio and the temperature rise of the exhaust gas for releasing sulfur.

According to the present invention, the desorbing property of the sulfur adhering to the NOx absorbent is determined, and according to the result, the sulfur is released (performed to recover the NOx purifying function of the NOx absorbent). The air-fuel ratio is enriched and / or the exhaust gas temperature is adjusted. Therefore, for example, if the release property of sulfur is high, the sulfur can be easily released from the NOx absorbent, and thus the degree and degree of the above-mentioned sulfur release operation may be suppressed. Alternatively, the problem of torque reduction can be suppressed.

Conventionally, sulfur release control is always performed by the same sulfur release operation without determining the desorbability of sulfur attached to the NOx absorbent. As a result, the air-fuel ratio was unnecessarily enriched or the exhaust gas temperature was raised. Moreover, since the sulfur release control was performed until the total amount of sulfur, which was estimated to be attached, was reduced to zero, in other words, the amount of sulfur that could not be desorbed, the air-fuel ratio was further wastefully enriched. It is. On the other hand, according to the present invention, if the determination of the desorbing property of sulfur, for example, the detection of non-desorbable sulfur is excluded from the release target, the sulfur release operation is further reduced rationally. It is also possible.

Next, according to a second aspect of the present invention, in the first aspect of the present invention, the sulfur desorption determining means determines that the shorter the elapsed time from the previous sulfur release by the sulfur releasing means, the more desorbable. Is determined to be high, and at that time, the sulfur releasing operation adjusting means suppresses the sulfur releasing operation.

According to a third aspect of the present invention, in the first aspect of the present invention, the sulfur desorption determining means is N.
It is determined that the lower the exhaust gas temperature at the time of sulfur attachment to the Ox absorbent, the higher the desorption property is, and at this time, the sulfur release operation adjusting means suppresses the sulfur release operation.

Further, according to a fourth aspect of the present invention, in the first aspect of the present invention, the sulfur desorption property determining means has a higher desorption property as the amount of exhaust gas when sulfur is attached to the NOx absorbent is smaller. Is determined, and at this time, the sulfur release operation adjusting means suppresses the sulfur release operation.

According to these inventions, several specific modes of judging the sulfur desorption property and adjusting the sulfur releasing operation are described. Claim 2 focuses on the fact that if the sulfur release control is not performed for a long time, the binding reaction between the sulfur and the NOx absorbent tissue proceeds, making it difficult to remove sulfur. Claim 3 focuses on the fact that when the temperature at the time of sulfur attachment is high, the reaction rate between sulfur and the NOx absorbent structure increases, and the sulfur is strongly bonded. In a fourth aspect of the present invention, similarly, when the pressure at the time of sulfur attachment is high, the bonding reaction between the sulfur and the NOx absorbent structure is promoted, and the sulfur is strongly bonded. In addition, attention was paid to the fact that when the pressure was high, sulfur penetrated deeply into the NOx absorbent structure, and this also made it difficult to remove sulfur. In claims 3 and 4, not only the temperature and pressure at the time of sulfur deposition but also the heat history and pressure history to which the deposited sulfur is exposed thereafter may be further considered.

Next, a fifth aspect of the present invention is characterized in that, in the first aspect of the present invention, the sulfur discharging means raises the temperature of the exhaust gas by retarding the ignition timing.

According to the present invention, the temperature of the exhaust gas for raising the temperature of the NOx absorbent is increased not by the operation of the heater but by the retardation of the ignition timing. . Hereinafter, the present invention will be described in more detail through embodiments of the present invention.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [System Configuration] As shown in FIG. 1, an engine 1 according to the present embodiment includes a plurality of pistons 3 (only one of them is shown) , A spark plug 5 disposed at the upper center of each combustion chamber 4, and an injector 6 disposed on a side of each combustion chamber 4 and directly injecting fuel into the combustion chamber 4
Etc. An air cleaner 11, an air flow sensor 12, a throttle valve 13, a surge tank 14, and the like are arranged from an upstream side in an intake passage 9 connected to the combustion chamber 4 via an intake valve 7. The downstream passage 9a of the surge tank 14 branches for each cylinder, and the downstream portion of each branch passage 9a is divided into two passages 9b and 9c. When the swirl generation 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 passage 9b.

A three-way catalyst 16 and a NOx catalyst 17 are arranged in series in an exhaust passage 10 connected to the combustion chamber 4 via an exhaust valve 8. The three-way catalyst 16 has a stoichiometric air-fuel ratio (A / F =
14.7) CO, HC and NOx in exhaust gas are simultaneously removed in the vicinity. The NOx catalyst 17 contains barium as a main component,
A NOx absorbent in which an alkali metal such as potassium, magnesium, strontium, and lanthanum, an alkaline earth metal, or a rare earth, and a noble metal having a chemical reaction catalysis such as platinum is supported. When the air-fuel ratio is lean, the NOx catalyst 17 absorbs NOx that flows without being purified by the three-way catalyst 16 and suppresses emission to the atmosphere. When the air-fuel ratio is rich, the absorbed NOx is exhausted. C inside
Reacts with O and HC to reduce, purify and release.

Note that between the upstream side of the three-way catalyst 16 in the exhaust passage 10 and the upstream side of the surge tank 14 in the intake passage 9,
An exhaust gas recirculation passage 18 for recirculating a part of the exhaust gas to the intake passage 9
Is provided. The exhaust gas recirculation passage 18 is provided with an exhaust gas recirculation amount control valve 19 for adjusting the amount of exhaust gas recirculation.

The control unit 2 of the engine 1
0 is from the air flow sensor 12 which detects the intake air amount.
Signal to detect the opening of the throttle valve 13.
A signal from the tor opening sensor 21 and the exhaust gas recirculation amount control valve 19
Signal from the recirculation amount sensor 22 that detects the opening of the
Boost sensor 23 for detecting intake negative pressure in tank 14
From the fuel pressure supplied to the injector 6
Signal from fuel pressure sensor 24 to be detected, inside engine body 2
A signal from a water temperature sensor 25 for detecting the temperature of the cooling water,
Provided upstream of the three-way catalyst 16 and discharged from the combustion chamber 4
From the residual oxygen concentration in the exhaust gas
Whether the air-fuel ratio of the mixture is richer or leaner than the stoichiometric air-fuel ratio
O to detect₂From the first air-fuel ratio sensor 26
Signal, provided between the three-way catalyst 16 and the NOx catalyst 17.
And detects the temperature of the exhaust gas immediately before flowing into the NOx catalyst 17.
From the exhaust gas temperature sensor 27, which is below the NOx catalyst 17.
Is provided on the flow side, and the exhaust gas passing through the NOx catalyst 17
O to detect residual oxygen concentration ₂The second air-fuel ratio sensor
A signal from the sensor 28 and an error for detecting the rotation speed of the engine 1
The signal from the engine rotation sensor 29, the accelerator pedal (Fig.
Accelerator opening sensor 3 for detecting the amount of depression (not shown)
Signal from 0, intake air temperature sensor 31 for detecting intake air temperature
From the atmospheric pressure sensor 32 that detects the atmospheric pressure
Input a signal, etc.

The control unit 20 includes an actuator 33 for driving the throttle valve 13, an exhaust gas recirculation amount control valve 19, an injector 6, and a swirl generation valve 15 according to the operation state of the engine 1 indicated by the signal.
By outputting control signals to an actuator 34 for driving the ignition, an ignition circuit 35 for igniting the ignition plug 5, and the like.
It performs throttle opening control, exhaust gas recirculation control, fuel injection amount control, fuel injection timing control, swirl generation control, ignition timing control, and the like. Further, the control unit 20
NOx release control (NOx purge control) for releasing NOx absorbed from the NOx absorbent of the NOx catalyst 17 and sulfur release control (sulfur poisoning elimination control) for releasing sulfur attached from the NOx absorbent of the NOx catalyst 17 are also performed. .

[Air-fuel ratio map] FIG. 2 is an air-fuel ratio map of the engine 1. In this map, the operating range of the engine using the engine speed and the engine load (represented by the throttle opening, intake air amount, and the like) as parameters is
It is divided into a lean operation region A, a rich operation region B1, a stoichiometric air-fuel ratio operation region B2, and a fuel cut region C.

The lean operation region A is set in a low-medium rotation / low-medium load region where the operation frequency is the highest. This area A
In this case, the air-fuel ratio is made larger than the stoichiometric air-fuel ratio (λ> 1).
During the lean operation, the fuel is injected during the compression stroke (late injection), and the fuel is unevenly distributed near the ignition plug 5 to perform stratified combustion. During the lean operation, NOx in the exhaust gas is absorbed by the NOx catalyst 17, and both the fuel economy performance and the exhaust performance are improved.

The rich operation area B1 is set in a high rotation / high load area which is an operation area such as at the time of high-speed operation or acceleration. In this region B1, the air-fuel ratio is made smaller than the stoichiometric air-fuel ratio (λ <1). During the rich operation, the fuel is injected during the intake stroke (first injection), and the fuel is sufficiently vaporized and atomized in the combustion chamber 4. During the rich operation, the NOx absorbed by the NOx catalyst 17 and the CO and HC in the exhaust gas undergo an oxidation-reduction reaction, thereby obtaining a good torque and improving the exhaust performance.

The stoichiometric air-fuel ratio operation region B2 is set in a region between the lean operation region A and the rich operation region B1. In this region B2, the air-fuel ratio is set to the stoichiometric air-fuel ratio (λ
= 1). During the stoichiometric air-fuel ratio operation, the fuel is injected during the intake stroke (the above-described injection), and the fuel is injected into the combustion chamber 4 as in the rich operation.
Vaporize and atomize within. During stoichiometric air-fuel ratio operation, CO, HC, and NOx in exhaust gas are simultaneously purified by the three-way catalyst 16.

The fuel cut area C is set in a medium-high rotation / low load area. In this region C, the fuel injection into the combustion chamber 4 is stopped.

In the lean operation region A, for example, when the adhesion of sulfur to the NOx absorbent progresses and the absorption of NOx by the absorbent cannot be expected, the low-speed / low-load side as indicated by a chain line A ' Is reduced to As a result, the rich region B (together with the rich operation region B1 and the stoichiometric air-fuel ratio operation region B2; the same applies hereinafter) is expanded. Thereby, NO
x Lean operation itself with a large amount of generation is limited and NO
It is possible to suppress the release of NOx to the atmosphere when the NOx absorption capacity of the x absorbent is reduced.

However, in this case, as shown by the dotted line D, the selective reduction purification region D of the NOx absorbent set in the low rotation / low load region is excluded from the target of the lean operation. That is, the NOx absorbent can be expected to have a selective reduction purification action in the low rotation / low load region D where the exhaust gas temperature is low. This selective reduction purification action is an action of selectively reducing and purifying a certain amount of NOx during lean operation, and functions as a function different from the above-described function of absorbing, reducing, and releasing NOx. Therefore, even if the attachment of sulfur to the NOx absorbent progresses and the absorption of NOx by the absorbent cannot be expected, the selective reduction purification operation in the selective reduction purification region D remains irrespective of this. Even in the case where the lean operation is restricted, the lean operation is allowed in the selective reduction purification region D, and the NOx absorbent exerts the selective reduction purification operation of NOx. As a result, it is possible to reduce the problem of reduced fuel efficiency due to the reduction in the lean operation region A while suppressing the release of NOx to the atmosphere. In other words, the lean operation area when the lean operation is restricted is the shaded portion in the figure.

[NOx Release Control] As shown in FIG. 3, NOx release control is performed when the NOx absorption amount of the NOx absorbent 17a increases to a predetermined amount or more with the continuation of the lean operation in the lean operation region A. By making the air-fuel ratio of the exhaust gas richer than the air-fuel ratio at least during the lean operation (for example, by making it stoichiometric air-fuel ratio or more rich),
Decompose and release absorbed NOx into oxygen and nitrogen to produce NOx
This is for recovering the NOx absorption capacity of the catalyst 17. However, the fuel efficiency is degraded by the enrichment of the air-fuel ratio, and the characteristics of the lean burn engine for improving the fuel efficiency are impaired. It is desired to suppress the degree of the degree.

<Various Parameters Used for Estimating NOx Absorption Amount> As shown in FIG. 4, during lean operation, the amount of NOx that can be absorbed by the NOx catalyst 17 per unit time with the passage of time is shown. (Instantaneous value) Qnc decreases, and on the other hand, the NOx passing amount Qnx that cannot be absorbed and passes through increases. Assuming that the NOx amount in the exhaust gas initially discharged from the combustion chambers 4... 4 into the exhaust passage 10 is the initial NOx emission amount Qna, the selective reduction purification rate of the NOx catalyst 17 is χ, and the purification amount is Qχ, the NOx catalyst 17 The NOx supply amount Qnb supplied as a target to be absorbed is a value obtained by subtracting the NOx selective reduction purification amount Q # from the NOx initial discharge amount Qna (Qnb = Qna-Q #). And that NO
The remaining value obtained by subtracting the NOx absorbable amount Qnc from the x supply amount Qnb is the NOx passage amount Qnx.

The curves indicated by reference numerals a and b in FIG. 4 represent the time change of the NOx passing amount Qnx. The solid line a indicates that when the exhaust gas temperature (therefore, the temperature of the NOx catalyst 17) is low,
Dotted line b represents a high time. NOx absorption amount (integrated value) Q
n is the curve a or b up to that point and the NOx supply amount Q
It is represented by the area of the portion sandwiched between nb. In FIG. 4, when the exhaust gas temperature is low, a is hatched by taking a as an example. When the exhaust gas temperature is low, a is higher than when it is high,
The NOx absorbable amount Qnc, which is an instantaneous value, is large over a long period of time, and the NOx absorption amount Qn, which is an integrated value, increases early. That is, the purification ability of the NOx catalyst 17 is maintained at a high level.

As shown in FIG. 5, the NOx absorbable amount Qn
c decreases as the NOx absorption amount Qn increases. That is, as the value of the NOx absorption amount Qn increases, the amount of increase per unit time decreases. Where N
Although the Ox absorption amount Qn is an integrated value of the NOx absorption amount instantaneous value dQn, the NOx absorption possible amount Qnc is not always adopted as the NOx absorption amount instantaneous value dQn. As shown by a symbol a, the NOx supply amount Qnb is
When the value is smaller than nc, the entire amount of the NOx supply amount Qnb is absorbed by the NOx catalyst 17, so that the instantaneous NOx absorption amount d
The NOx supply amount Qnb is adopted as Qn. Conversely, when the NOx absorbable amount Qnc is smaller than the NOx supply amount Qnb, as shown by the symbol A, the NOx catalyst 17
The entire amount of the NOx supply amount Qnb is not absorbed, and a part of the NOx supply amount Qnb passes therethrough.
The Ox absorbable amount Qnc is adopted.

Accordingly, the NOx supply amount Qnb and the NOx
Comparing with the absorbable amount Qnc, the smaller value is set to NO.
Let x absorption amount instantaneous value dQn. Then, when the NOx absorption possible amount Qnc is adopted as the NOx absorption amount instantaneous value dQn, as described above, the NOx absorption amount instantaneous value dQn decreases as the NOx absorption amount Qn increases. The NOx absorbable amount Qnc is determined by the exhaust gas temperature (N
It is also corrected by the temperature of the Ox catalyst 17), the NOx supply amount Qnb, and the like. By estimating the NOx absorption amount Qn in consideration of these tendencies, the estimation accuracy is improved.

<Example of Control Operation> FIGS. 6 and 7 are flowcharts showing an example of a specific operation of the NOx release control. This program is executed during the lean operation of the engine 1, that is, NOx
While the environment in which NOx is absorbed by the absorbent is continuously executed repeatedly at a predetermined cycle. During the rich operation period or the stoichiometric air-fuel ratio operation period, NOx is naturally released from the NOx absorbent. Therefore, in particular, the estimation operation of the NOx absorption amount Qn performed in this NOx release control is stopped, and the NOx emission amount is replaced instead. Is estimated separately based on the above. NOx is also released by a sulfur release operation (enrichment of air-fuel ratio and temperature rise of exhaust gas) performed as a result of the sulfur release control described later. Instead, the estimation of the remaining NOx amount is performed.

First, in step S1, NO to the NOx absorbent is
The x absorption amount Qn is estimated. That is, in step S11, the NOx initial emission amount Qna is calculated from the engine speed and the engine load. The NOx initial emission amount Qna is calculated to be larger as the engine speed is higher and the engine load is larger. Next, in step S12, the selective reduction purification rate χ is calculated from the exhaust gas temperature and the NOx initial emission concentration. The selective reduction purification rate χ increases with decreasing exhaust gas temperature,
The higher the Ox initial emission concentration, the larger the calculated value.

Next, in step S13, the NOx supply amount Qnb is calculated from the NOx initial discharge amount Qna and the selective reduction purification rate χ. The NOx supply amount Qnb is the NOx initial emission amount Qn
It is calculated by subtracting the selective reduction purification amount (Qχ = Qna × χ) from “a” (Qnb = Qna × (1−
χ)). Next, in step S14, the NOx absorption amount Qn (N
The NOx absorbable amount Qnc is calculated from the previous value of the Ox absorption amount, that is, the integrated value of NOx already absorbed in the NOx absorbent, the exhaust gas temperature and the NOx supply amount Qnb. NO
The x absorbable amount Qnc is calculated to be smaller as the NOx absorption amount Qn is larger, the exhaust gas temperature is higher, and the NOx supply amount Qnb is smaller.

Next, at step S15, the NOx supply amount Qn
The smaller value of b and the NOx absorption possible amount Qnc is set as the NOx absorption amount instantaneous value dQn. And step S1
In step 6, the NOx absorption amount Qn is estimated by adding the instantaneous NOx absorption amount value dQn to the NOx absorption amount Qn, that is, the previous value.

Next, in step S2, it is determined whether or not the NOx absorption amount Qn is larger than a predetermined amount (NOx release execution determination value) Qnx. As a result, when it is larger than the predetermined amount Qnx, the process proceeds to step S3, and when it is not larger, step S1 is repeated until it becomes larger. Here, the NOx release execution determination value Qnx is an amount that prevents the NOx purification rate by the NOx catalyst 17 from being maintained at or above a predetermined regulation value (limit value).

In step S3, the amount of sulfur adhering to the NOx absorbent (total amount) estimated by the sulfur release control described later.
It is determined whether Qs is smaller than a predetermined amount (NOx release execution validity determination value) X1 (see FIG. 14).
The significance of this determination and step S9 will be described later.
As a result, the process proceeds to step S4 only when the sulfur adhesion amount Qs is smaller than the predetermined amount X1.

In step S4, a NOx releasing operation is performed. That is, the air-fuel ratio is made rich to purify and release the NOx absorbed from the NOx absorbent. Next, in step S5, the exhaust gas temperature, the exhaust gas flow rate, the air-fuel ratio, and the NOx
The remaining amount (the previous value of the remaining NOx amount, that is, the remaining amount of NOx excluding the NOx already released from the NOx absorbent) Qn
An instantaneous NOx release amount dQnz is calculated from z. NOx
The release amount instantaneous value dQnz is calculated to be larger as the exhaust gas temperature is higher, the exhaust gas flow rate is larger, the air-fuel ratio is richer, and the NOx residual amount Qnz is larger. Then, in step S6, the instantaneous NOx release amount dQnz is
The remaining amount Qnz, that is, N
The remaining amount of Ox Qnz is estimated.

Next, at step S7, this NOx remaining amount Q
It is determined whether or not nz is zero. If the result is zero, the NOx releasing operation ends in step S8. That is, the air-fuel ratio enrichment is terminated, and the operation returns to the lean operation. On the other hand, N
When the remaining amount of Ox Qnz is not zero, the process returns to step S4 until the amount becomes zero, and the NOx releasing operation is continued.

<Another Control Operation Example> In the above description, the NOx releasing operation is started when the NOx absorption amount Qn becomes equal to or more than the predetermined amount Qnx (step S2). Alternatively, when the NOx absorbable amount Qnc decreases to a predetermined amount or less (NOx
NO when the passing amount Qnx becomes larger than a predetermined amount).
The x release operation may be started.

That is, as described above, for example, when the exhaust gas temperature is low, the NOx catalyst 17
As a result, the NOx absorption capacity Qnc is large for a long time, and the NOx absorption capacity Qn is large.
Grows early. Therefore, when the start of the NOx releasing operation is determined only by the NOx absorption amount Qn, the NOx releasing operation is started immediately with respect to the catalyst 17 having high purification ability (the catalyst 17 having the large NOx absorbable amount Qnc). However, it is unreasonable that the NOx releasing operation is not easily started for the catalyst 17 whose purification ability has already been lowered (the catalyst 17 whose NOx absorbable amount Qnc has become small). Therefore, not only when the NOx absorption amount Qn becomes equal to or more than the predetermined amount Qnx, but also irrespective of such determination conditions, the NOx absorption possible amount Qnc becomes smaller than or equal to the predetermined amount, and the NOx passage amount Qnx becomes equal to or more than the predetermined amount. It is preferable to start the NOx releasing operation also when the number has increased. As a result, N that is released to the atmosphere through the NOx catalyst 17 is released.
The increase in the Ox amount can be determined to be difficult.

[Sulfur release control]
Similar to the Ox release control, as shown in FIG. 8, when the sulfur adhering amount of the NOx absorbent 17a increases to a predetermined amount or more,
The air-fuel ratio of the exhaust gas is enriched and the exhaust gas (NOx catalyst 1
By raising the temperature of 7), the attached sulfur is released, and the NOx absorption capacity of the NOx catalyst 17 is restored. However, the interval of the sulfur release operation performed as a result of the sulfur release control is equal to N which is performed as a result of the NOx release control.
Much longer than the interval between Ox release operations. Here, the temperature rise of the exhaust gas is performed by retarding the ignition timing. In that case,
As in the case of the NOx emission control, the richness of the air-fuel ratio deteriorates the fuel efficiency, and the characteristics of the lean burn engine for improving the fuel efficiency are impaired.
It is desired that the degree and degree of enrichment be suppressed as much as possible even for sulfur release from a. Also,
Since the retardation of the ignition timing performed to raise the temperature of the exhaust gas causes a negative effect of torque reduction, the degree and degree of the retardation should be suppressed as much as possible even if sulfur is released from the NOx absorbent 17a. Is desired.

<Example of Control Operation> FIGS. 9 to 12 are flowcharts showing an example of a specific operation of the sulfur release control. This program is repeatedly executed at a predetermined cycle continuously while there is an environment where sulfur adheres to the NOx absorbent. During the rich operation period or the stoichiometric air-fuel ratio operation period, and when the exhaust gas temperature is high, the sulfur is naturally released from the NOx absorbent. Suspension is performed, and a separate estimation of the remaining sulfur based on the sulfur release is performed.
However, NOx performed as a result of the NOx release control described above
Since the release operation only enriches the air-fuel ratio and does not raise the temperature of the exhaust gas, no sulfur is released only by the NOx release operation. Therefore, during the NOx release operation, estimation of the sulfur deposition amount Qs continues. Can be

(Estimation of Sulfur Adhesion Amount) First, Step S2
In step 1, the amount Qs of sulfur adhering to the NOx absorbent is estimated. That is, in step S41, the basic value dQso of the instantaneous sulfur deposition amount dQs is calculated from the flow rate of the fuel that is the source of sulfur. The basic value dQso is calculated to be larger as the fuel flow rate is larger. Next, in steps S42 to S44, the first to third correction coefficients K1 to K4 are calculated from the lean continuation time, the exhaust gas temperature, and the sulfur adhesion amount Qs (the previous value of the sulfur adhesion amount, that is, the integrated value of the sulfur already adhering to the NOx absorbent).
K3 is calculated.

The first correction coefficient K1 takes a maximum value for a predetermined lean continuation time, and is calculated to be smaller as the lean continuation time becomes shorter and longer. The second correction coefficient K2 takes a maximum value at a predetermined exhaust gas temperature, and is calculated to be a smaller value as the exhaust gas temperature becomes lower and higher therefrom. The third correction coefficient K3 is calculated to be smaller as the sulfur adhesion amount Qs is larger than a predetermined adhesion amount.

Next, at step S45, the basic value dQs
By correcting o with these correction coefficients K1 to K3, the instantaneous value dQs of the amount of attached sulfur is calculated. Then, in step S46, the sulfur adhesion amount Qs is estimated by adding the sulfur adhesion amount instantaneous value dQs to the sulfur adhesion amount Qs, that is, the previous value.

(Determination of Sulfur Desorption) The quantity Qs estimated as described above is the total quantity of sulfur adhering to the NOx catalyst 17. However, the sulfur adhering to the NOx absorbent has various desorption properties from the NOx absorbent depending on the surrounding environment, and some of the sulfur are easily desorbed, some are hardly desorbed, and some are not desorbed. Also, the ratios and ratios are not constant. Therefore, in the present embodiment, useless sulfur releasing operation is suppressed by analyzing and determining the desorbability of sulfur in step S47 and adjusting the sulfur releasing operation according to the result.

That is, in steps S51 to S53, the elapsed time of sulfur deposition (elapsed time from the end of the previous sulfur release operation) tim, the exhaust gas temperature tmp, and the exhaust gas amount v
Based on lm, the desorption of sulfur is determined. That is, NO
x of the total amount Qs of sulfur adhering to the catalyst 17, the ratio of easily desorbable sulfur α1 to α3, the ratio of hardly desorbable (but desorbable) sulfur β1 to β3, and the ratio of nondesorbable sulfur γ1 Γ3 is calculated.

As shown in FIG. 13, as the elapsed time of sulfur deposition tim, the exhaust gas temperature tmp and the exhaust gas amount vlm increase, the ratio α of sulfur that is easily desorbed decreases, and the ratio β of sulfur that is difficult to desorb and β and The proportion γ of unremovable sulfur increases. That is, the region Ra of easily desorbable sulfur is reduced, and the region Rb of difficult-to-desorb sulfur and the region Rc of non-desorbable sulfur are reduced.
Increase.

Time tim after the end of the sulfur release operation
When the next sulfur release operation is performed before the time 1 has elapsed, all the sulfur is easily desorbed (the desorption easy ratio α
1 = 100%). On the other hand, when the next sulfur releasing operation is performed during the time tim1 to tim2 after the end of the sulfur releasing operation, some of the sulfur is difficult to desorb (the rate of difficult desorption is β1> 0%). . Furthermore, when the next sulfur releasing operation is performed after the lapse of time tim2, some of the sulfur cannot be desorbed (non-desorption ratio γ1> 0%).
Such a tendency is likely that the more time passes, the more sulfur and NO
This is due to the fact that the binding reaction with the x-absorbent tissue proceeds to make it difficult to remove sulfur.

According to this, all the sulfur adhering under the condition that the exhaust gas temperature is tmp1 or less is easily desorbed (easy desorption rate α2 = 100%). However, when the exhaust gas temperature is tm
Some of the sulfur adhered under the conditions of p1 to tmp2 is difficult to desorb (the ratio of difficult desorption is β2> 0%). And
Some of the sulfur adhered under the condition that the exhaust gas temperature is tmp2 or more cannot be desorbed (non-desorption ratio γ2> 0
%). This tendency is derived from the fact that the higher the reaction temperature at the time of sulfur attachment, the faster the binding reaction speed between the sulfur and the NOx absorbent tissue, and the more the sulfur is bound. In addition, in place of or together with the temperature of the exhaust gas at the time of sulfur deposition, until the next sulfur release operation is performed,
The heat history received by the attached sulfur may be considered.

According to this, all the sulfur adhering under the condition that the exhaust gas amount is vlm1 or less is easily desorbed (easy desorption ratio α3 = 100%). However, the amount of exhaust gas is vl
Some of the sulfur adhered under the condition of m1 to vlm2 becomes difficult to desorb (difficulty of desorption β3> 0%). And
Some of the sulfur adhered under the condition that the exhaust gas amount is vlm2 or more cannot be desorbed (non-desorption ratio γ3> 0%).
This tendency is derived from the fact that the larger the amount of exhaust gas at the time of sulfur deposition, and thus the higher the reaction pressure, the faster the bonding reaction rate between the sulfur and the NOx absorbent tissue and the stronger the bonding of sulfur. In addition, the higher the reaction pressure at the time of sulfur attachment, the deeper the sulfur penetrates into the NOx absorbent structure, which also makes it difficult to remove sulfur. Instead of, or in addition to, the amount of exhaust gas (reaction pressure) when sulfur is attached, the pressure history of the attached sulfur may be taken into consideration until the next sulfur release operation is performed.

Then, in step S54, using these ratios α1 to α3, β1 to β3, and γ1 to γ3, NOx
The total amount of sulfur Qs attached to the catalyst 17 is analyzed into a readily desorbable sulfur amount Qsa, a hardly desorbable sulfur amount Qsb, and a non-desorbable sulfur amount Qsc. In step S54 of FIG. 12, each ratio α1 to α3, β1 to β3, γ1
Although the arithmetic mean of γ3 is taken, the present invention is not limited to this. For example, weights may be assigned to the respective ratios α1 to α3, β1 to β3, and γ1 to γ3.

After analyzing and judging the above-mentioned sulfur desorption properties, it is determined in step S22 whether or not the total amount of sulfur deposition Qs is larger than a predetermined amount (sulfur release execution determination value) Qsx (see FIG. 14). judge. As a result, the predetermined amount Q
When it is larger than sx, the process proceeds to step S23, and when it is not larger, step S21 is repeated until it becomes larger. Here, the sulfur release execution determination value Qsx is an amount that prevents the NOx purification rate of the NOx catalyst 17 from being maintained at or above a predetermined regulation value (limit value).

(Regulation of Sulfur Release Operation) Next Step S
The determinations of 23 and S25 are for determining whether to permit or restrict the sulfur release operation. That is, in step S23, the sum of the easily desorbed sulfur amount Qsa and the hardly desorbable sulfur amount Qsb of the total sulfur amount Qs, that is, the desorbable sulfur amount is a predetermined amount set in advance (sulfur release execution validity determination value). It is determined whether it is larger than X2 (see FIG. 14).
That is, it is determined beforehand that the non-desorbable sulfur amount Qsc is small, the sulfur releasing operation is effective, and the sulfur releasing operation is not wasted.

As a result, the amount of desorbable sulfur (Qsa + Q
Only when sb) is larger than the predetermined amount X2, step S
Proceeding to 24, a sulfur release flag F for releasing removable sulfur (Qsa + Qsb) is turned on. On the other hand, when the amount of non-desorbable sulfur Qsc is large, the process returns. That is, the sulfur release operation is postponed (regulated). This makes it impossible to release sulfur Qs
Since the discharging operation is not performed with respect to c, wasteful air-fuel ratio enrichment and exhaust gas temperature rise are avoided, and the problem of reduced fuel efficiency and the problem of reduced torque due to retarded ignition timing are suppressed.

Since there is little possibility that the non-desorbable sulfur Qsc changes to the desorbable sulfur Qsa or Qsb,
In the present embodiment, once NO is determined in step S23, the process does not proceed to step S24 and subsequent steps in the future, and the sulfur releasing operation is not performed for a long time.

In step S25, it is determined whether the vehicle speed V is higher than the sulfur release execution vehicle speed Vx. In other words, even if the vehicle speed is low and the exhaust gas temperature is low in the first place, no matter how much the ignition timing is retarded to raise the exhaust gas temperature,
When the exhaust gas temperature becomes the sulfur release effective temperature (for example, 550 to 60)
A situation in which the temperature does not rise to 0 ° C.) is determined beforehand.

As a result, the vehicle speed V becomes the sulfur release execution vehicle speed V
Only when it is larger than x, the process proceeds to step S26 and thereafter to execute the sulfur releasing operation. On the other hand, when the temperature of the exhaust gas is low, the process returns. That is, the sulfur release operation is postponed (regulated). As a result, wasteful enrichment of the air-fuel ratio and exhaust gas temperature rise are avoided, and the problem of reduced fuel efficiency and the problem of reduced torque due to retarded ignition timing are suppressed.

That is, in the present embodiment, when there is a strong tendency to operate at a low vehicle speed, the process does not proceed to step S26 and subsequent steps in the future, and the sulfur releasing operation is not executed for a long time.

(Sulfur release operation) Before starting the sulfur release operation, in step S26, the sulfur amount Qsb that is difficult to desorb
Is a predetermined amount (sulfur release operation adjustment determination value) set in advance X
3 (see FIG. 14). As a result, when it is larger, the process proceeds to step S27 to execute the sulfur releasing operation under the strong condition, and when it is smaller, the process proceeds to step S28 to execute the sulfur releasing operation under the suppression condition.

That is, in any case, the air-fuel ratio is enriched and the exhaust gas is heated to release the sulfur adhering from the NOx absorbent. Among the desorbable sulfur (Qsa + Qsb) to be released, Difficult sulfur Qsb
When the ratio is large, the air-fuel ratio is made richer (for example, the air-fuel ratio in the rich operation region B1), and the exhaust gas temperature is made higher (for example, 600 ° C.). On the other hand, sulfur Qsb
When the ratio is small, the degree and degree of enrichment of the air-fuel ratio are suppressed (for example, the air-fuel ratio in the stoichiometric air-fuel ratio operation region B2), and the degree and degree of temperature rise of the exhaust gas are also suppressed (for example, 550 ° C.). As a result, an excessive enrichment of the air-fuel ratio and an increase in the temperature of the exhaust gas are avoided, and the problem of reduced fuel efficiency and the problem of reduced torque due to retarded ignition timing are suppressed.

(End of Sulfur Release Operation) During the execution of the sulfur release operation, in step S29, the remaining amount of sulfur, more specifically, the remaining amount Qsa of easily released sulfur to be released is determined.
z and the remaining amount Qsbz of the difficult-to-desorb sulfur are estimated. The estimation of the remaining amount is performed based on, for example, the conditions and contents of the sulfur release operation (air-fuel ratio and exhaust gas temperature), the execution time of the sulfur release operation, and the like.

Next, in step S30, it is determined whether or not the remaining amount (Qsaz + Qsbz) of the sulfur to be released is zero. As a result, when the value is zero, the sulfur release operation is terminated in step S31, and the sulfur release flag F is turned off. That is, the air-fuel ratio enrichment is terminated, and the operation returns to the lean operation. Further, the ignition timing retard is ended, and the temperature rise of the exhaust gas is stopped.

On the other hand, the remaining amount of sulfur to be released (Qsaz +
If Qsbz) is not zero, the process returns to step S27 or S28 to continue the sulfur release operation until it becomes zero. Each time, in step S32, it is confirmed that the vehicle speed V is not lower than the sulfur release suspension vehicle speed Vy. . That is, it is confirmed that the vehicle speed is high and the exhaust gas temperature can be raised to the sulfur release effective temperature.

Even if the vehicle speed V becomes lower than the sulfur release suspended vehicle speed Vy, in step S33, the remaining amount of sulfur to be released (Qsaz + Qsbz) is a predetermined amount (sulfur release flag release determination value) X4 (see FIG. 14). If it is determined to be smaller, the process proceeds to step S34, and the sulfur release operation is ended and the sulfur release flag F is turned off, as in step S31.

However, the vehicle speed V is the vehicle speed Vy at which the sulfur release is interrupted.
Lower and release target sulfur (Qsaz + Qsbz)
If there is still a considerable amount remaining, the sulfur release operation is suspended (restricted). Then, in step S25, when it is determined that the vehicle speed V has recovered to the sulfur release execution vehicle speed Vx or higher, the sulfur release operation is restarted.

Here, the above-mentioned respective judgment values relating to the amount of sulfur are set in a magnitude relationship as shown in FIG. 14, for example.

[Characteristics of the above control] (1) When NO is determined in steps S23 and S25 (including the case of temporary suspension) of the sulfur release control, as described above, the sulfur release operation is postponed in any case. Be regulated. As a result, as shown in FIG.
a progresses, and NO on the absorbent 17a
The absorption of x can hardly be expected. The determination in step S3 of the NO release control is to determine whether the NOx absorbent 17a is in such a state.

That is, when it is determined in step S3 that the total amount of deposited sulfur Qs is larger than the NOx release execution validity determination value X1 (as shown in FIG. 14).
1 is set to a considerably large value), NOx absorbent 17
a, the NOx absorption capacity of the NOx a significantly decreases, and the NOx absorbent 17a actually has
It is determined that NOx to be released is hardly absorbed. Therefore, even if the previous step S2
Even if it is estimated that the NOx absorption amount Qn is larger than the NOx release execution determination value Qnx, the process does not proceed to step S4. That is, the NOx releasing operation is not performed.

Even if the air-fuel ratio is enriched in such a state, sulfur is not released only by enrichment of the air-fuel ratio. Therefore, the NOx absorbent 17a has a slight Only the amount of NOx is released. The NOx absorbent 17a has a satisfactory NOx
Absorption cannot be expected, resulting in wasteful enrichment. Therefore, by the determination in step S3, useless enrichment of the air-fuel ratio is avoided, and the problem of reduced fuel efficiency is suppressed.

(2) In step S3, the total amount Q of sulfur deposited
When it is determined that s is larger than the NOx release execution validity determination value X1, the process instead proceeds to step S9, and the lean operation region A is reduced to the low rotation speed / low load side region A 'as described above. . As a result, the lean operation itself that generates a large amount of NOx is limited, and the emission of NOx to the atmosphere when the NOx absorbing capacity of the NOx absorbent 17a is reduced is fundamentally suppressed.

However, in that case, the selective reduction purification region D
For lean operation, the NOx
While the emission of fuel is suppressed, the problem of reduced fuel consumption performance due to the reduction of the lean operation region A is reduced.

Note that such a restriction on the lean operation ends when the determination in step S3 is YES. That is, in the future, the regulation of the sulfur release operation is released, and the sulfur release operation is executed. As a result, the sulfur deposition amount Qs decreases, and the process ends when the NOx absorbent 17a recovers the NOx absorption capability. However, in the present embodiment, the sulfur release operation is restricted when the non-desorbable sulfur amount Qsc is large (step S23) or when the vehicle speed V is low. Of these, the vehicle speed V may increase in the future, but the desorbability of sulfur decreases with time, and non-desorbable sulfur does not change to desorbable sulfur. In other words, there is little possibility that the sulfur emission regulation will be lifted in the future. Therefore, in the present embodiment, when NO is determined in step S23, the sulfur release regulation continues for a long time,
In the future, the possibility that YES is determined in step S3 will decrease, and as a result, the possibility of executing the NOx releasing operation will also decrease.

(3) In step S47 of the sulfur release control, the desorbability of sulfur is determined, and the determination result (step S2
6), the air-fuel ratio for sulfur release and the degree of temperature rise of the exhaust gas are increased / decreased / adjusted (steps S27, S28), so that excessive sulfur release operation is avoided, and wasteful sulfur is eliminated. The release operation is realized, and the problem of reduced fuel consumption performance and the problem of reduced torque are suppressed. In addition, as shown by the symbol ク in FIG.
Since c is excluded from the target of release (step S30, etc.),
The sulfur release operation can be reduced even more rationally.

In the present embodiment, both the air-fuel ratio and the exhaust gas temperature are adjusted in adjusting the sulfur releasing operation. However, only one of them may be adjusted, and the combination pattern of these may be varied. Thus, precise sulfur release control corresponding to various degrees of sulfur desorption can be achieved.

[0077]

As described above, according to the present invention,
Since the sulfur release operation is adjusted according to the sulfur desorption property, rational and efficient waste release control is realized. INDUSTRIAL APPLICABILITY The present invention is widely and preferably applicable to a lean burn engine having a NOx absorption reduction type catalyst.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an engine control system according to an embodiment of the present invention.

FIG. 2 is an air-fuel ratio map of the engine.

FIG. 3 shows N in which NOx is released by NOx release control
It is a conceptual diagram of an Ox absorbent.

FIG. 4 is a time chart of a NOx passing amount.

FIG. 5 is a characteristic diagram of an absorbable amount with respect to a NOx absorption amount.

FIG. 6 is a flowchart illustrating an example of a specific operation of NOx release control.

FIG. 7 is a flowchart illustrating an example of an operation for estimating the NOx absorption amount.

FIG. 8: N in which sulfur is released by sulfur release control
It is a conceptual diagram of an Ox absorbent.

FIG. 9 is a flowchart illustrating an example of a specific operation of sulfur release control.

FIG. 10 is the same flowchart.

FIG. 11 is a flowchart illustrating an example of an operation of estimating a sulfur adhesion amount.

FIG. 12 is a flowchart illustrating an example of an operation for determining sulfur desorption properties.

FIG. 13 is a characteristic diagram showing changes in sulfur desorption coefficients α, β, and γ with respect to various parameters.

FIG. 14 is a correlation diagram of various judgment values used in the NOx release control and the sulfur release control.

FIG. 15 is a conceptual diagram of a NOx absorbent to which sulfur has progressed due to sulfur release regulation.

FIG. 16 is a conceptual diagram showing a comparison between a case where NOx is released from a NOx absorbent to which sulfur has progressed and a case where sulfur is released.

[Explanation of symbols]

 Reference Signs List 1 engine 10 exhaust passage 16 three-way catalyst 17a NOx absorbent 20 control unit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F02P 5/15 Z F02D 41/04 305 B01D 53/36 101A F02P 5/15 ZAB (72 ) Inventor Yoichi Kuji 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima F-term in Mazda Co., Ltd. (reference) 3G022 AA03 AA10 DA02 GA05 GA06 GA07 GA08 GA09 GA10 3G091 AA02 AA11 AA13 AA17 AA28 AB03 AB09 BA11 BA14 CA26 CB02 CB05 DB02 EA01 EA05 EA07 EA14 EA16 EA17 EA34 EA39 3G301 HA01 HA04 HA13 HA17 JA25 LA01 LA05 LB04 MA01 MA11 MA19 ND02 NE13 NE14 PA01Z PA07Z PA09Z PA10Z PA11Z PB08Z PD02Z PD11Z PD15Z PE01Z PE08Y BA03Y4AB048A PF03Y CC46 DA01 DA02 DA03 DA06 DA08 DA20 EA04

Claims (5)

[Claims] 1. NOx that absorbs NOx in exhaust gas in an oxygen-excess atmosphere and releases the NOx when the oxygen concentration decreases.
NOx absorption amount estimating means for providing an absorbent in the exhaust passage and estimating the amount of NOx absorbed into the NOx absorbent; N to release NOx from the absorbent
Ox releasing means, sulfur adhering amount estimating means for estimating the amount of sulfur adhering to the NOx absorbent, and when the sulfur adhering amount estimated by the estimating means becomes equal to or more than a predetermined amount, the air-fuel ratio is enriched and the exhaust gas is exhausted. A sulfur release means for raising the temperature of the NOx absorbent to release sulfur from the absorbent, comprising: sulfur release determination means for determining the release of sulfur attached to the NOx absorbent; and A sulfur release operation adjusting means for adjusting at least one of the air-fuel ratio enrichment or the exhaust gas temperature increase performed by the sulfur release means for sulfur release, according to the determination result of the determination means. Characteristic engine exhaust purification device. 2. The sulfur desorption determining means determines that the desorption is higher as the elapsed time from the previous sulfur release by the sulfur releasing means is shorter. At that time, the sulfur releasing operation adjusting means suppresses the sulfur releasing operation. The exhaust gas purifying apparatus for an engine according to claim 1, wherein: 3. The sulfur desorption determining means determines that the lower the exhaust gas temperature at the time of sulfur deposition on the NOx absorbent, the higher the desorption property, and the sulfur release operation adjusting means suppresses the sulfur release operation. The exhaust gas purifying apparatus for an engine according to claim 1, wherein: 4. The sulfur desorption determining means determines that the smaller the amount of exhaust gas at the time of sulfur adhering to the NOx absorbent, the higher the desorption property, and the sulfur releasing operation adjusting means suppresses the sulfur releasing operation. The exhaust gas purifying apparatus for an engine according to claim 1, wherein: 5. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the sulfur releasing means raises the temperature of the exhaust gas by retarding the ignition timing.