JP4569050B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to the technical field of an engine exhaust gas purification device, particularly an engine exhaust gas purification device provided with an NOx absorbent in an exhaust passage.
[0002]
[Prior art]
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. The NOx absorbent absorbs NOx in exhaust gas when the air-fuel ratio is lean (oxygen-excess atmosphere), and reduces and purifies the absorbed NOx when the air-fuel ratio becomes rich (oxygen-deficient atmosphere). In the well-known general NOx release control, the NOx absorption amount to the NOx absorbent is estimated based on the engine speed, the engine load, etc. during lean operation, and the estimated amount is determined based on the NOx purification rate of the NOx catalyst. When the amount falls below the regulation value, the air-fuel ratio is enriched to release NOx from the NOx absorbent.
[0003]
On the other hand, NOx absorbents have a problem of sulfur poisoning. That is, sulfur components such as SOx contained in the fuel adhere to the NOx absorbent and interfere with the NOx absorption of the absorbent, thereby reducing the NOx purification rate. Therefore, sulfur release control is performed as disclosed in JP-A-9-317447. Similar to NOx release control, the well-known general sulfur release control estimates the amount of sulfur adhering to the NOx absorbent based on the fuel flow rate, exhaust gas temperature, etc., and the estimated amount exceeds a predetermined amount. In order to enrich the air-fuel ratio and raise the temperature of the catalyst, the exhaust gas is heated to release sulfur from the NOx absorbent.
[0004]
[Problems to be solved by the invention]
The enrichment of the air-fuel ratio that is performed for sulfur release causes a problem of a decrease in fuel efficiency and deteriorates the characteristics of a lean burn engine that improves the fuel efficiency. Similarly, the temperature rise of the exhaust gas for sulfur release is realized by, for example, the operation of the heater or the retard of the ignition timing, which also involves problems of large energy use and torque reduction. Therefore, even if 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.
[0005]
Therefore, the present invention has an object to suppress the problem of fuel efficiency deterioration, large energy use, or torque reduction by suppressing the degree and degree of air-fuel ratio enrichment and exhaust gas temperature increase for sulfur release. To do.
[0006]
[Means for Solving the Problems]
That is, in order to solve the above-mentioned problem, the invention according to claim 1 of the present application includes a NOx absorbent in the exhaust passage that absorbs NOx in the exhaust gas in an oxygen-excess atmosphere and releases the NOx when the oxygen concentration decreases. NOx absorption amount estimation means for estimating the NOx absorption amount to the NOx absorbent, and when the NOx absorption amount estimated by the estimation means exceeds a predetermined amount, the air-fuel ratio is enriched to reduce NOx from the absorbent NOx releasing means for releasing NOx, sulfur adhering amount estimating means for estimating the amount of sulfur adhering to the NOx absorbent, and rich air-fuel ratio when the sulfur adhering amount estimated by the estimating means exceeds a predetermined amount An exhaust purification device for an engine having a sulfur release means for releasing the sulfur from the absorbent by increasing the temperature of the exhaust gas and determining the detachability of the sulfur adhering to the NOx absorbent Sulfur releasing operation for adjusting at least one of enrichment of air-fuel ratio and temperature rise of exhaust gas performed by the sulfur release unit for sulfur release according to the determination result of the determination unit Adjustment means The release operation adjusting means strengthens the suppression of the sulfur release operation as the sulfur release property increases when performing the sulfur release. It is characterized by that.
[0007]
According to the present invention, the detachability of sulfur adhering to the NOx absorbent is determined, and in accordance with the result, the sulfur release is performed (to recover the NOx purification function of the NOx absorbent). Adjust enrichment and / or temperature rise of exhaust gas. Therefore, for example, if the sulfur release property is high, it is easy to release the sulfur from the NOx absorbent. Therefore, the degree and degree of the above-described sulfur release operation may be suppressed. Or the problem of a torque fall can be suppressed.
[0008]
Conventionally, the sulfur release control is always performed by the sulfur release operation having the same content without determining the detachability of the sulfur adhering to the NOx absorbent. As a result, the air-fuel ratio was enriched more than necessary or the exhaust gas was heated. In addition, until the total amount of sulfur estimated to have adhered became zero, in other words, sulfur release control was performed including the amount of sulfur that could not be desorbed. It is. On the other hand, in the present invention, if, for example, sulfur that cannot be desorbed is detected as a result of the determination of sulfur desorption, the amount is excluded from the release target, and the sulfur release operation is further rationally reduced. It is also possible.
[0009]
Next, the invention according to claim 2 is that, in the invention according to claim 1, the sulfur detachment determining means has a higher detachability as the elapsed time from the previous sulfur release by the sulfur release means is shorter. In this case, the sulfur release operation adjusting means suppresses the sulfur release operation.
[0010]
In the invention described in claim 3, in the invention described in claim 1, the sulfur desorption determination means determines that the lower the exhaust gas temperature at the time of sulfur adhesion to the NOx absorbent, the higher the desorption property. In this case, the sulfur release operation adjusting means suppresses the sulfur release operation.
[0011]
Further, in the invention described in claim 4, in the invention described in claim 1, the sulfur desorption determination unit determines that the smaller the amount of exhaust gas at the time of sulfur adhesion to the NOx absorbent, the higher the desorption. In this case, the sulfur release operation adjusting means suppresses the sulfur release operation.
[0012]
According to these inventions, several specific modes of determination of sulfur detachability and adjustment of sulfur release operation are shown. The second aspect 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 structure proceeds and it is difficult to remove sulfur. The third aspect focuses on the fact that when the temperature at the time of sulfur adhesion is high, the reaction rate between the sulfur and the NOx absorbent structure is increased and the sulfur is strongly bonded. In the fourth aspect, similarly, when the pressure at the time of sulfur adhesion is high, the binding reaction between the sulfur and the NOx absorbent structure is promoted and the sulfur is strongly bonded. In addition, when the pressure is high, the sulfur penetrates deeply into the NOx absorbent structure, so that it is difficult to remove the sulfur. In claims 3 and 4, not only the temperature and pressure at the time of sulfur deposition but also the thermal history and pressure history to which the deposited sulfur is exposed thereafter may be further taken into consideration.
[0013]
Next, the invention described in claim 5 is characterized in that, in the invention described in claim 1, the sulfur releasing means raises the temperature of the exhaust gas by retarding the ignition timing.
[0014]
According to the present invention, since 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 retard of the ignition timing, particularly the problem of torque reduction is suppressed as much as possible. Hereinafter, the present invention will be described in more detail through embodiments of the invention.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
[System configuration]
As shown in FIG. 1, an engine 1 according to the present embodiment includes a main body 2 having a plurality of pistons 3... 3 (only one of which is shown), a combustion chamber 4 defined by each piston 3, and each combustion. An ignition plug 5 disposed in the upper center of the chamber 4, an injector 6 disposed on the side of each combustion chamber 4, and directly injecting fuel into the combustion chamber 4. An air cleaner 11, an air flow sensor 12, a throttle valve 13, a surge tank 14, and the like are arranged in the intake passage 9 connected to the combustion chamber 4 via the intake valve 7 from the upstream side. 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 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 passage 9b.
[0016]
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 simultaneously removes CO, HC and NOx in the exhaust gas near the theoretical air-fuel ratio (A / F = 14.7). The NOx catalyst 17 is composed of a barium-based NOx absorbent 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. Decorate. When the air-fuel ratio is lean, the NOx catalyst 17 absorbs NOx flowing in without being purified by the three-way catalyst 16 to suppress discharge to the atmosphere, and when the air-fuel ratio is rich, the absorbed NOx is exhausted. Reducing, purifying, and releasing by reacting with CO and HC inside.
[0017]
An exhaust gas recirculation passage 18 is provided 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 to return a part of the exhaust gas to the intake passage 9. The exhaust gas recirculation passage 18 is provided with an exhaust gas recirculation amount adjusting valve 19 for adjusting the recirculation amount of the exhaust gas.
[0018]
The control unit 20 of the engine 1 determines the signal from the air flow sensor 12 that detects the intake air amount, the signal from the throttle opening sensor 21 that detects the opening of the throttle valve 13, and the opening of the exhaust gas recirculation control valve 19. A signal from the recirculation amount sensor 22 to detect, a signal from the boost sensor 23 to detect the negative intake pressure in the surge tank 14, a signal from the fuel pressure sensor 24 to detect the pressure of the fuel supplied to the injector 6, the engine body 2 A signal supplied from the water temperature sensor 25 for detecting the temperature of the cooling water in the interior, provided upstream of the three-way catalyst 16, and supplied to the combustion chamber 4 from the residual oxygen concentration in the exhaust gas discharged from the combustion chamber 4 O to detect whether the air-fuel ratio of the gas is richer or leaner than the stoichiometric air-fuel ratio ₂ A signal from a first air-fuel ratio sensor 26 comprising a sensor, a signal from an exhaust temperature sensor 27 provided between the three-way catalyst 16 and the NOx catalyst 17 and detecting the exhaust gas temperature immediately before flowing into the NOx catalyst 17, NOx An O is provided downstream of the catalyst 17 and detects the residual oxygen concentration in the exhaust gas that has passed through the NOx catalyst 17. ₂ A signal from the second air-fuel ratio sensor 28, which is a sensor, a signal from the engine rotation sensor 29 that detects the number of revolutions of the engine 1, and a signal from the accelerator opening sensor 30 that detects the amount of depression of an accelerator pedal (not shown). A signal from the intake air temperature sensor 31 that detects the temperature of the intake air, a signal from the atmospheric pressure sensor 32 that detects the atmospheric pressure, and the like are input.
[0019]
The control unit 20 includes an actuator 33 that drives the throttle valve 13, an exhaust gas recirculation amount adjustment valve 19, an injector 6, and an actuator 34 that drives the swirl generation valve 15, according to the operating state of the engine 1 indicated by the signal. By outputting a control signal to the ignition circuit 35 for igniting the spark plug 5, throttle opening control, exhaust gas recirculation control, fuel injection amount control, fuel injection timing control, swirl generation control, ignition timing control, and the like are performed. Further, the control unit 20 controls NOx release (NOx purge control) to release NOx absorbed from the NOx absorbent of the NOx catalyst 17 and sulfur release control (sulfur release) to release sulfur adhering from the NOx absorbent of the NOx catalyst 17. Perform poisoning elimination control).
[0020]
[Air-fuel ratio map]
FIG. 2 is an air-fuel ratio map of the engine 1. In this map, the engine operating range whose parameters are engine speed and engine load (represented by throttle opening, intake air amount, etc.) are a lean operating range A, a rich operating range B1, and a stoichiometric air-fuel ratio. It is divided into an operation area B2 and a fuel cut area C.
[0021]
The lean operation region A is set to a low / medium rotation / low / medium load region with the highest operation frequency. In this region A, the air-fuel ratio is made larger than the theoretical air-fuel ratio (λ> 1). During the lean operation, fuel is injected during the compression stroke (late injection), and the fuel is unevenly distributed in the vicinity of the spark plug 5 for stratified combustion. During lean operation, NOx in the exhaust gas is absorbed by the NOx catalyst 17, and both fuel efficiency and exhaust performance are improved.
[0022]
The rich operation region B1 is set to a high rotation / high load region that is an operation region during 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, fuel is injected during the intake stroke (pre-injection), and the fuel is sufficiently vaporized and atomized in the combustion chamber 4. During the rich operation, the NOx absorbed in the NOx catalyst 17 and the CO and HC in the exhaust gas undergo an oxidation-reduction reaction to obtain a good torque and improve the exhaust performance.
[0023]
The stoichiometric air-fuel ratio operation region B2 is set to a region between the lean operation region A and the rich operation region B1. In this region B2, the air-fuel ratio is made the stoichiometric air-fuel ratio (λ = 1). During the stoichiometric air-fuel ratio operation, fuel is injected during the intake stroke (the injection) as in the rich operation, and the fuel is sufficiently vaporized and atomized in the combustion chamber 4. During the theoretical air-fuel ratio operation, CO, HC and NOx in the exhaust gas are simultaneously purified by the three-way catalyst 16.
[0024]
The fuel cut region C is set to a medium / high rotation / low load region. In this region C, fuel injection into the combustion chamber 4 is stopped.
[0025]
The lean operation region A is reduced to a low rotation / low load side as indicated by a chain line A ′ when, for example, sulfur adheres to the NOx absorbent and NOx absorption by the absorbent cannot be expected. The As a result, the rich region B (the rich operation region B1 and the theoretical air-fuel ratio operation region B2 are collectively referred to below) is expanded. Thereby, the lean operation itself with a large amount of NOx generated is restricted, and the NOx release to the atmosphere can be suppressed when the NOx absorbing ability of the NOx absorbent is reduced.
[0026]
However, in this case, as indicated by the dotted line D, the NOx absorbent selective reduction purification region D set in the low rotation / low load region is excluded from the target of the lean operation restriction. 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 and 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 sulfur adheres to the NOx absorbent and the absorption of NOx by the absorbent cannot be expected, the selective reduction and purification action in the selective reduction and purification region D remains regardless. Even when the lean operation is limited, the lean operation is allowed for the selective reduction purification region D so that the NOx absorbent exhibits the selective reduction purification action of NOx. Thereby, the malfunction of the fuel consumption performance fall by reduction of the lean driving | operation area | region A can be reduced, suppressing discharge | release of NOx to air | atmosphere. That is, the lean operation region at the time of lean operation restriction is a hatched portion in the figure.
[0027]
[NOx release control]
As shown in FIG. 3, when the NOx absorption amount of the NOx absorbent 17a increases to a predetermined amount or more as the lean operation continues in the lean operation region A, the NOx release control at least reduces the air-fuel ratio of the exhaust gas. By making it richer than the air-fuel ratio at the time of operation (for example, making it the stoichiometric air-fuel ratio or richer), the NOx absorbed by the NOx catalyst 17 is recovered by decomposing and releasing the absorbed NOx into oxygen and nitrogen. It is something to be made. However, since the fuel efficiency decreases due to the enrichment of the air-fuel ratio and the characteristics of the lean burn engine that improves the fuel efficiency are impaired, the enrichment is made as rich as possible even for NOx release from the NOx absorbent 17a. It is desirable to suppress the degree and degree of the above.
[0028]
<Various parameters used for NOx absorption estimation>
As shown in FIG. 4, during the lean operation, the NOx absorbable amount (instantaneous value) Qnc that can be stored per unit time by the NOx catalyst 17 decreases with the passage of time, and on the other hand, it can be absorbed. NOx passage amount Qnx that is allowed to pass through increases. When the NOx amount in the exhaust gas first discharged from the combustion chambers 4... 4 into the exhaust passage 10 is NOx initial 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 an absorption target is a value obtained by subtracting the NOx selective reduction purification amount Qχ from the NOx initial discharge amount Qna (Qnb = Qna−Qχ). The remaining value obtained by subtracting the NOx absorbable amount Qnc from the NOx supply amount Qnb is the NOx passage amount Qnx.
[0029]
Curves indicated by reference symbols a and b in FIG. 4 represent time variations of the NOx passage amount Qnx. Of these lines, the solid line a represents the low exhaust gas temperature (and therefore the temperature of the NOx catalyst 17), and the dotted line b represents the high temperature. The NOx absorption amount (integrated value) Qn is represented by the area of the portion sandwiched between the curve a or b up to that point and the NOx supply amount Qnb. In FIG. 4, when the exhaust gas temperature is low, a is taken as an example and hatched. Compared with b when the exhaust gas temperature is low, 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 capacity of the NOx catalyst 17 is maintained at a high level.
[0030]
As shown in FIG. 5, the NOx absorbable amount Qnc decreases as the NOx absorbable amount Qn increases. That is, the NOx absorption amount Qn decreases as the value of itself increases. However, although the NOx 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 indicated by the symbol a, when the NOx supply amount Qnb is smaller than the NOx absorbable amount Qnc, the NOx supply amount Qnb is completely absorbed by the NOx catalyst 17, and therefore the NOx supply amount Qnb is obtained as the NOx absorption amount instantaneous value dQn. Adopted. On the other hand, as indicated by symbol (a), when the NOx absorbable amount Qnc is smaller than the NOx supply amount Qnb, the NOx supply amount Qnb is not completely absorbed by the NOx catalyst 17 and a part thereof passes. Therefore, the NOx absorbable amount Qnc is adopted as the NOx absorbed amount instantaneous value dQn.
[0031]
Therefore, the NOx supply amount Qnb and the NOx absorbable amount Qnc are compared, and the smaller value is set as the NOx absorption amount instantaneous value dQn. When the NOx absorbable 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 also corrected by the exhaust gas temperature (the temperature of the NOx catalyst 17), the NOx supply amount Qnb, and the like. The estimation accuracy is improved by estimating the NOx absorption amount Qn taking these tendencies into consideration.
[0032]
<Control operation example>
An example of a specific operation of NOx release control is shown in the flowcharts of FIGS.
This program is repeatedly executed in a predetermined cycle while the engine 1 is in a lean operation, that is, in an environment where NOx is absorbed by the NOx absorbent. During the rich operation period or the theoretical air-fuel ratio operation period, NOx is naturally released from the NOx absorbent. In particular, the NOx absorption amount Qn estimation operation performed in this NOx release control is stopped, and instead the NOx release amount. The remaining amount of NOx is estimated separately based on the above. In addition, since NOx is also released by the sulfur release operation (the enrichment of the air-fuel ratio and the temperature rise of the exhaust gas) performed as a result of the sulfur release control described later, the NOx absorption amount Qn is also estimated during the sulfur release operation. Instead, the NOx remaining amount is estimated.
[0033]
First, in step S1, the NOx absorption amount Qn to the NOx absorbent 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 exhaust concentration. The selective reduction purification rate χ is calculated as a larger value as the exhaust gas temperature is lower and as the NOx initial exhaust concentration is higher.
[0034]
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 calculated by subtracting the selective reduction purification amount (Qχ = Qna × χ) from the NOx initial discharge amount Qna (Qnb = Qna × (1-χ)). Next, in step S14, the NOx absorption amount Qnc is calculated from the NOx absorption amount Qn (the previous value of the NOx absorption amount, that is, the integrated value of NOx already absorbed by the NOx absorbent), the exhaust gas temperature, and the NOx supply amount Qnb. The NOx absorbable amount Qnc is calculated as a smaller value as the NOx absorbed amount Qn is larger, the exhaust gas temperature is higher, and the NOx supply amount Qnb is smaller.
[0035]
Next, in step S15, the smaller value of the NOx supply amount Qnb and the NOx absorbable amount Qnc is set as the NOx absorption amount instantaneous value dQn. In step S16, the NOx absorption amount Qn is estimated by adding the instantaneous NOx absorption amount dQn to the NOx absorption amount Qn, that is, the previous value.
[0036]
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 set in advance. As a result, when it is larger than the predetermined amount Qnx, the process proceeds to step S3, and when 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 of the NOx catalyst 17 from being kept above a predetermined regulation value (limit value).
[0037]
In step S3, whether or not the amount of sulfur adhering to the NOx absorbent (total amount) Qs estimated by sulfur release control described later is smaller than a predetermined amount (NOx release execution valid judgment value) X1 (see FIG. 14). Determine. 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.
[0038]
In step S4, a NOx releasing operation is executed. That is, the air-fuel ratio is enriched to purify and release 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 remaining amount (the previous value of the NOx remaining amount, that is, the NOx remaining amount excluding NOx already released from the NOx absorbent) Qnz to the NOx released amount instantaneous value dQnz. Is calculated. The instantaneous NOx emission amount dQnz is calculated as a larger value as the exhaust gas temperature is higher, the exhaust gas flow rate is larger, the air-fuel ratio is richer, and the NOx remaining amount Qnz is larger. In step S6, the NOx remaining amount Qnz is estimated by subtracting the NOx released amount instantaneous value dQnz from the NOx remaining amount Qnz, that is, the previous value.
[0039]
Next, in step S7, it is determined whether or not the NOx remaining amount Qnz is zero. As a result, when it is zero, the NOx releasing operation is terminated in step S8. That is, the enrichment of the air-fuel ratio is finished, and the lean operation is restored. On the other hand, when the NOx remaining amount Qnz is not zero, the process returns to step S4 until the NOx remaining amount Qnz becomes zero, and the NOx releasing operation is continued.
[0040]
<Other control operation examples>
In the above description, the NOx releasing operation is started when the NOx absorption amount Qn is equal to or larger than the predetermined amount Qnx (step S2), but instead of this, or together with this, the NOx absorption possible amount Qnc is The NOx releasing operation may be started also when it becomes less than a predetermined amount (when the NOx passage amount Qnx becomes larger than a predetermined amount).
[0041]
In other words, as described above, for example, when the exhaust gas temperature is low, a is higher than b when the exhaust gas temperature is high. As a result, the NOx absorbable amount Qnc is large over a long period of time. The NOx absorption amount Qn increases early. Therefore, if the start of the NOx release operation is determined only by the NOx absorption amount Qn, the NOx release operation is started immediately for the catalyst 17 having a high purification ability (the catalyst 17 having a large NOx absorption amount Qnc). It is unreasonable that the NOx releasing operation is not easily started for the catalyst 17 whose purification capacity has already been lowered (the catalyst 17 whose NOx absorbable amount Qnc is reduced). Therefore, not only when the NOx absorption amount Qn becomes equal to or greater than the predetermined amount Qnx, but regardless of such a determination condition, the NOx absorbable amount Qnc decreases below the predetermined amount, and the NOx passage amount Qnx exceeds the predetermined amount. It is preferable to start the NOx releasing operation even when the amount increases. Thereby, it is difficult to reliably increase the amount of NOx that passes through the NOx catalyst 17 and is released to the atmosphere.
[0042]
[Sulfur release control]
Similarly to the NOx release control, the sulfur release control enriches the air-fuel ratio of the exhaust gas and exhaust gas (NOx catalyst 17) when the sulfur adhesion amount of the NOx absorbent 17a increases to a predetermined amount or more as shown in FIG. By raising the temperature, the adhering sulfur is released, and the NOx absorption capacity of the NOx catalyst 17 is recovered. However, the interval of the sulfur release operation performed as a result of this sulfur release control is much longer than the interval of the NOx release operation performed as a result of NOx release control. Here, the temperature of the exhaust gas is increased by retarding the ignition timing. In this case, as with the NOx release control, the fuel efficiency decreases due to the enrichment of the air-fuel ratio, and the characteristics of the lean burn engine that improves the fuel efficiency are impaired. For this reason, the sulfur is released from the NOx absorbent 17a. Even so, it is desirable to suppress the degree and degree of enrichment as much as possible. In addition, since the adverse effect of torque reduction is caused by the retard of the ignition timing for raising the temperature of the exhaust gas, the degree and degree of the retard are suppressed as much as possible even for the release of sulfur from the NOx absorbent 17a. It is desirable to do.
[0043]
<Control operation example>
An example of the specific operation of sulfur release control is shown in the flowcharts of FIGS. This program is continuously executed at a predetermined cycle while the sulfur is attached to the NOx absorbent. When the exhaust gas temperature is high during the rich operation period or the stoichiometric air-fuel ratio operation period, sulfur is naturally released from the NOx absorbent. In particular, the estimation operation of the sulfur adhesion amount Qs performed by this sulfur release control is as follows. Instead, the remaining amount of sulfur is estimated separately based on the amount of released sulfur. However, since the NOx releasing operation performed as a result of the above-described NOx releasing control only enriches the air-fuel ratio and does not raise the temperature of the exhaust gas, sulfur is not released only by the NOx releasing operation, and thus the NOx releasing operation. During operation, the estimation of the sulfur adhesion amount Qs is continued.
[0044]
(Estimation of sulfur adhesion)
First, in step S21, the sulfur adhesion amount Qs to the NOx absorbent is estimated. That is, in step S41, the basic value dQso of the sulfur deposition amount instantaneous value dQs is calculated from the flow rate of the fuel that is the source of sulfur. This basic value dQso is calculated as a larger value as the fuel flow rate increases. Next, in steps S42 to S44, the first to third correction factors K1 to K1 are calculated from the lean duration, the exhaust gas temperature, and the sulfur adhesion amount Qs (the previous value of the sulfur adhesion amount, that is, the integrated value of sulfur already adhered to the NOx absorbent). K3 is calculated.
[0045]
The first correction coefficient K1 takes a maximum value for a predetermined lean continuation time, and is calculated as a smaller value as the lean continuation time becomes shorter or 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. The third correction coefficient K3 is calculated to be smaller as the sulfur adhesion amount Qs is larger than the predetermined adhesion amount.
[0046]
Next, in step S45, the basic value dQso is corrected with these correction coefficients K1 to K3, thereby calculating the instantaneous sulfur adhesion amount dQs. In step S46, the sulfur deposition amount Qs is estimated by adding the sulfur deposition amount instantaneous value dQs to the sulfur deposition amount Qs, that is, the previous value.
[0047]
(Determination of sulfur desorption)
The amount Qs estimated as described above is the total amount of sulfur adhering to the NOx catalyst 17. However, sulfur adhering to the NOx absorbent has various detachability from the NOx absorbent depending on the surrounding environment, and those that are easily desorbed, those that are difficult to desorb, and those that cannot be desorbed are mixed. The ratio and ratio are not constant. Therefore, in the present embodiment, sulfur desorption is analyzed and determined in step S47, and the sulfur release operation is adjusted according to the result, thereby suppressing useless sulfur release operation.
[0048]
That is, in steps S51 to S53, sulfur desorption is determined based on the sulfur adhesion elapsed time (elapsed time since the previous sulfur release operation was completed) tim, the exhaust gas temperature tmp, and the exhaust gas amount vlm. That is, among the total amount Qs of sulfur adhering to the NOx catalyst 17, the sulfur ratios α1 to α3 that are easily desorbed, the sulfur ratios β1 to β3 that are difficult to desorb (but can be desorbed), and the sulfur that cannot be desorbed The ratios γ1 to γ3 are calculated.
[0049]
As shown in FIG. 13, as the sulfur adhesion elapsed time tim, the exhaust gas temperature tmp, and the exhaust gas amount vlm increase, the sulfur ratio α that is easily desorbed decreases, the sulfur ratio β that is difficult to desorb and the desorption is impossible. The sulfur ratio γ increases. That is, the easily desorbable sulfur region Ra is decreased, and the difficultly desorbed sulfur region Rb and the non-detachable sulfur region Rc are increased.
[0050]
When the next sulfur releasing operation is performed from the end of the sulfur releasing operation until the time tim1 elapses, all sulfur is easily desorbed (easy desorption ratio α1 = 100%). On the other hand, when the next sulfur releasing operation is performed during the time tim1 to tim2 after the sulfur releasing operation is completed, it is difficult to desorb some percent (desorption difficulty ratio β1> 0%). . Furthermore, when the next sulfur release operation is performed after the elapse of time tim2, a certain percentage of the sulfur cannot be desorbed (desorption impossible ratio γ1> 0%). Such a tendency is derived from the fact that as time passes, the binding reaction between sulfur and the NOx absorbent structure proceeds and it becomes difficult to remove sulfur.
[0051]
In accordance with this, all of the sulfur adhering under the condition of the exhaust gas temperature of tmp1 or less is easily desorbed (easy desorption ratio α2 = 100%). However, some of the sulfur adhering under the conditions where the exhaust gas temperature is tmp1 to tmp2 becomes difficult to be desorbed (desorption difficulty ratio β2> 0%). Then, some of the sulfur adhering under the condition where the exhaust gas temperature is tmp2 or more cannot be desorbed (desorption impossible ratio γ2> 0%). Such a tendency is derived from the fact that the higher the reaction temperature at the time of sulfur deposition, the faster the binding reaction rate between sulfur and the NOx absorbent structure, and the stronger the sulfur is bound. Instead of or at the same time as the exhaust gas temperature at the time of sulfur adhesion, the thermal history received by the adhered sulfur may be taken into consideration until the next sulfur release operation is performed.
[0052]
In accordance with this, all of the sulfur adhering under the condition that the amount of exhaust gas is vlm1 or less is easily desorbed (easy desorption ratio α3 = 100%). However, some of the sulfur adhering under the conditions where the amount of exhaust gas is vlm1 to vlm2 is difficult to desorb (desorption difficulty ratio β3> 0%). And some 10% of the sulfur adhering under the condition that the amount of exhaust gas is vlm2 or more cannot be desorbed (desorption impossible ratio γ3> 0%). Such a tendency is derived from the fact that the amount of exhaust gas at the time of sulfur adhesion is large, and thus the higher the reaction pressure, the more the sulfur and NOx absorbent structure are promoted, and the sulfur is strongly bonded. In addition, the higher the reaction pressure at the time of sulfur deposition, 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) at the time of sulfur adhesion, the pressure history received by the adhering sulfur during the next sulfur release operation may be taken into consideration.
[0053]
In step S54, using these ratios α1 to α3, β1 to β3, and γ1 to γ3, the total sulfur amount Qs adhering to the NOx catalyst 17 is converted into the sulfur amount Qsa that is easily desorbed and the sulfur amount that is difficult to desorb. Analysis is made into Qsb and a non-desorbable sulfur amount Qsc. In addition, in step S54 of FIG. 12, although the arithmetic mean of each ratio (alpha) 1- (alpha) 3, (beta) 1- (beta) 3, (gamma) 1- (gamma) 3 is taken, not only this but each ratio (alpha) 1- (alpha) 3, (beta) 1- (beta) 3, (gamma) 1- Weighting or the like may be performed between γ3.
[0054]
After the above sulfur desorption analysis / determination, it is determined in step S22 whether or not the total sulfur adhesion amount Qs is larger than a predetermined amount (sulfur release execution determination value) Qsx (see FIG. 14). As a result, when it is larger than the predetermined amount Qsx, 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 kept above a predetermined regulation value (limit value).
[0055]
(Regulation of sulfur release operation)
The next determination in steps S23 and S25 is a determination of whether to allow or restrict the sulfur release operation. That is, in step S23, among the total sulfur amount Qs, the sum of the easily desorbable sulfur amount Qsa and the difficultly desorbable sulfur amount Qsb, that is, the desorbable sulfur amount is a predetermined amount (a sulfur release execution valid determination value). It is determined whether it is larger than X2 (see FIG. 14). That is, it is determined in advance that the non-desorbable sulfur amount Qsc is small, the sulfur release operation is effective, and the sulfur release operation is not wasted.
[0056]
As a result, only when the desorbable sulfur amount (Qsa + Qsb) is larger than the predetermined amount X2, the process proceeds to step S24, and the sulfur release flag F for releasing desorbable sulfur (Qsa + Qsb) is turned on. On the other hand, when the non-desorbable sulfur amount Qsc is large, the process returns as it is. In other words, the sulfur release operation is postponed (regulated). As a result, the sulfur Qsc, which cannot be released, is not released, the wasteful air-fuel ratio enrichment and the exhaust gas temperature increase are avoided, the fuel consumption performance is lowered, and the ignition timing is delayed. The problem of torque reduction due to corners is suppressed.
[0057]
Since it is unlikely that the non-desorbable sulfur Qsc changes to desorbable sulfur Qsa or Qsb, in the present embodiment, once it is determined NO in step S23, the process proceeds to step S24 and thereafter in the future. No, the sulfur release operation will not be performed for a long time.
[0058]
In step S25, it is determined whether or not the vehicle speed V is higher than the sulfur release execution vehicle speed Vx. In other words, the vehicle speed is low, and therefore the exhaust gas temperature is low in the first place, and no matter how many attempts are made to retard the ignition timing in order to raise the exhaust gas temperature, the exhaust gas temperature does not rise to the sulfur release effective temperature (for example, 550 to 600 ° C.). Such a situation is judged in advance.
[0059]
As a result, only when the vehicle speed V is higher than the sulfur release execution vehicle speed Vx, the routine proceeds to step S26 and the subsequent steps, and the sulfur release operation is executed. On the other hand, when the exhaust gas temperature is low, the process returns as it is. In other words, the sulfur release operation is postponed (regulated). This also avoids wasteful enrichment of the air-fuel ratio and temperature rise of the exhaust gas, and suppresses the problem of fuel efficiency degradation and the problem of torque reduction due to the retard of the ignition timing.
[0060]
That is, in this embodiment, when there is a strong tendency to drive at a low vehicle speed, the process does not proceed to step S26 and subsequent steps in the future, and the sulfur release operation is not executed for a long time.
[0061]
(Sulfur release operation)
Before starting the sulfur release operation, in step S26, it is determined whether or not the difficulty desorption sulfur amount Qsb is larger than a predetermined amount (sulfur release operation adjustment determination value) X3 (see FIG. 14). As a result, when the value is large, the process proceeds to step S27 and the sulfur release operation is executed under strong conditions. When the value is small, the process proceeds to step S28 and the sulfur release operation is executed under suppression conditions.
[0062]
That is, in any case, the air-fuel ratio is enriched and the exhaust gas is heated to release sulfur adhering from the NOx absorbent, but among the desorbable sulfur (Qsa + Qsb) to be released, difficult desorption 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, when the proportion of sulfur Qsb that is difficult to desorb is small, the degree / degree of enrichment of the air / fuel ratio is suppressed (for example, the air / fuel ratio in the theoretical air / fuel ratio operation region B2), and the degree / degree of exhaust gas temperature rise is also suppressed. (For example, 550 ° C.). As a result, excessive enrichment of the air-fuel ratio and temperature rise of the exhaust gas are avoided, and problems of fuel consumption performance degradation and torque reduction due to ignition timing retardation are suppressed.
[0063]
(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 Qsaz of easy desorption sulfur to be released and the remaining amount Qsbz of difficult desorption sulfur are estimated. The estimation of the remaining amount is performed based on, for example, the conditions / contents of the sulfur release operation (air-fuel ratio and exhaust gas temperature), the execution time of the sulfur release operation, and the like.
[0064]
Next, in step S30, it is determined whether or not the residual amount of sulfur to be released (Qsaz + Qsbz) is zero. As a result, when the result is zero, the sulfur release operation is terminated in step S31 and the sulfur release flag F is turned off. That is, the enrichment of the air-fuel ratio is finished, and the lean operation is restored. Further, the ignition timing retardation is terminated, and the temperature rise of the exhaust gas is stopped.
[0065]
On the other hand, when the amount of remaining sulfur to be released (Qsaz + Qsbz) is not zero, the process returns to step S27 or S28 until the zero is reached, and the sulfur releasing operation is continued. Confirm that it is not lower than 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.
[0066]
Even when the vehicle speed V is lower than the sulfur release interruption vehicle speed Vy, if the target sulfur remaining amount (Qsaz + Qsbz) is smaller than the predetermined amount (sulfur release flag release determination value) X4 (see FIG. 14) in step S33. When it is determined, 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.
[0067]
However, when the vehicle speed V is lower than the sulfur release interruption vehicle speed Vy and a considerable amount of target sulfur (Qsaz + Qsbz) still remains, the sulfur release operation is temporarily suspended (restricted). When it is determined in step S25 that the vehicle speed V has recovered to the sulfur release execution vehicle speed Vx or higher, the sulfur release operation is resumed.
[0068]
Here, the above determination values relating to the amount of sulfur are set to a magnitude relationship as shown in FIG. 14, for example.
[0069]
[Features of the above control]
(1) When it is determined NO in steps S23 and S25 (including the case of temporary interruption) of sulfur release control, as described above, the sulfur release operation is postponed and restricted in both cases. As a result, as shown in FIG. 15, adhesion of sulfur to the NOx absorbent 17a proceeds, and almost no absorption of NOx to the absorbent 17a can be expected.
The determination in step S3 of NO release control is to determine whether or not the NOx absorbent 17a is in such a state.
[0070]
That is, when it is determined in step S3 that the total sulfur adhesion amount Qs is larger than the NOx release execution validity judgment value X1 (as shown in FIG. 14, the judgment value X1 is set to a considerably large value). The NOx absorption capacity of the NOx absorbent 17a is remarkably reduced, and it is judged that the NOx to be released is actually hardly absorbed in the NOx absorbent 17a, as shown by the symbol F in FIG. . Therefore, even if it is estimated that the NOx absorption amount Qn is larger than the NOx release execution determination value Qnx in the previous step S2, the process does not proceed to step S4. That is, the NOx releasing operation is not executed.
[0071]
Even if the air-fuel ratio is enriched in such a state, since the sulfur is not released only by enriching the air-fuel ratio, the NOx absorbent 17a has a slight amount of NOx as shown by the symbol in FIG. Is only released. After that, the NOx absorbent 17a cannot be expected to satisfy satisfactory NOx absorption, and is a wasteful enrichment. Thus, the determination in step S3 avoids wasteful enrichment of the air-fuel ratio and suppresses the problem of deterioration in fuel efficiency.
[0072]
(2) When it is determined in step S3 that the total sulfur adhesion amount Qs is larger than the NOx release execution validity determination value X1, the process proceeds to step S9 instead, and as described above, the lean operation region A is rotated at a low speed. -Reduce to the area A 'on the low load side. Thereby, the lean operation itself with a large amount of NOx generated is restricted, and NOx release to the atmosphere when the NOx absorbent capacity of the NOx absorbent 17a is reduced is fundamentally suppressed.
[0073]
However, in this case, since the lean operation is allowed for the selective reduction purification region D, the problem of reduction in fuel efficiency due to the reduction of the lean operation region A is reduced while suppressing the release of NOx to the atmosphere.
[0074]
Such lean operation restriction ends when it is determined YES in step S3. That is, in the future, the restriction of the sulfur release operation is released, and the sulfur release operation is executed. As a result, the sulfur adhesion amount Qs decreases, and the process ends when the NOx absorbent capacity is restored to the NOx absorbent 17a. 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 the non-desorbable sulfur does not change to desorbable sulfur. That is, there is almost no possibility that the sulfur emission restriction will be lifted in the future. Therefore, in the present embodiment, when NO is determined in step S23, the sulfur emission restriction continues for a long time, and the possibility of determining YES in step S3 in the future is reduced. As a result, the execution of the NOx releasing operation is performed. The possibility of is reduced.
[0075]
(3) Sulfur release is determined in step S47 of sulfur release control, and the degree of enrichment of the air-fuel ratio for sulfur release and the temperature increase of the exhaust gas is increased or decreased according to the determination result (step S26, etc.) -Since it adjusted (step S27, S28), excessive sulfur discharge | release operation | movement is avoided and the sulfur discharge | release operation | movement without waste is implement | achieved, and the problem of a fuel consumption performance fall and the problem of a torque fall are suppressed. In addition, as indicated by the reference symbol in FIG. 16, the non-desorbable sulfur Qsc is excluded from the release target (step S30, etc.), so that the sulfur release operation can be further rationally reduced.
[0076]
In the present embodiment, in adjusting the sulfur release operation, both the air-fuel ratio and the exhaust gas temperature are subject to adjustment, but only one of them may be adjusted, and by enriching their combination pattern, It becomes possible to control sulfur release precisely according to the degree of sulfur desorption.
[0077]
【The invention's effect】
As described above, according to the present invention, since the sulfur release operation is adjusted according to the detachability of sulfur, a reasonable and wasteful sulfur release control is realized. The present invention is widely applicable to a lean burn engine including a NOx absorption reduction 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 is a conceptual diagram of a NOx absorbent in which NOx is released by NOx release control.
FIG. 4 is a time chart of NOx passage amount.
FIG. 5 is a characteristic diagram of the absorbable amount with respect to the NOx absorbed amount.
FIG. 6 is a flowchart showing an example of a specific operation of NOx release control.
FIG. 7 is a flowchart showing an example of an NOx absorption amount estimating operation.
FIG. 8 is a conceptual diagram of a NOx absorbent in which sulfur is released by sulfur release control.
FIG. 9 is a flowchart showing an example of a specific operation of sulfur release control.
FIG. 10 is a flowchart of the same.
FIG. 11 is a flowchart showing an example of an operation for estimating the amount of sulfur adhesion.
FIG. 12 is a flowchart showing an example of a determination operation of sulfur desorption.
FIG. 13 is a characteristic diagram of changes in sulfur desorption coefficients α, β, and γ with respect to various parameters.
FIG. 14 is a correlation diagram of various determination values used in the NOx release control and sulfur release control.
FIG. 15 is a conceptual diagram of a NOx absorbent in which adhesion of 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 in which sulfur adhesion has progressed and a case where sulfur is released.
[Explanation of symbols]
1 engine
10 Exhaust passage
16 Three-way catalyst
17a NOx absorbent
20 Control unit

Claims (5)

NOx absorption amount estimating means for absorbing NOx in exhaust gas in an oxygen-excess atmosphere and providing the exhaust passage with a NOx absorbent that releases the NOx due to a decrease in oxygen concentration, and estimating the NOx absorption amount to the NOx absorbent; NOx releasing means for releasing the NOx from the absorbent by enriching the air-fuel ratio when the NOx absorption amount estimated by the estimating means exceeds a predetermined amount, and estimating the amount of sulfur adhering to the NOx absorbent. Sulfur adhering amount estimating means, and sulfur releasing means for enriching the air-fuel ratio and raising the temperature of the exhaust gas to release sulfur from the absorbent when the sulfur adhering amount estimated by the estimating means exceeds a predetermined amount; An exhaust purification device for an engine having a sulfur desorption determination unit for determining the desorption of sulfur adhering to the NOx absorbent, and the sulfur desorption determination unit according to a determination result of the determination unit Detecting means is a sulfur releasing adjusting means for adjusting at least one of enrichment or exhaust gas Atsushi Nobori of the air-fuel ratio performed for sulfur emission, said discharging operation adjustment means, sulfur when performing the sulfur release An exhaust emission control device for an engine, wherein the higher the detachability, the stronger the suppression of sulfur release operation . The sulfur detachability determining means determines that the detachability is higher as the elapsed time from the previous sulfur release by the sulfur release means is shorter, and the sulfur release operation adjusting means suppresses the sulfur release operation at that time. The exhaust emission control device for an engine according to claim 1. The sulfur desorption determining means determines that the desorption is higher as the exhaust gas temperature at the time of sulfur adhesion to the NOx absorbent is lower, and the sulfur release operation adjusting means suppresses the sulfur release operation at that time. Item 2. An exhaust emission control device for an engine according to Item 1. The sulfur desorption determining means determines that the desorption is higher as the amount of exhaust gas at the time of sulfur adhering to the NOx absorbent is smaller, and at that time, the sulfur release operation adjusting means suppresses the sulfur release operation. Item 2. An exhaust emission control device for an engine according to Item 1. The engine exhaust gas purification apparatus according to claim 1, wherein the sulfur release means raises the temperature of the exhaust gas by retarding the ignition timing.

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