JP4075643B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an engine exhaust purification device.
[0002]
[Prior art]
When the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich, the air-fuel ratio of the exhaust gas flowing out from the NOx absorbent is slightly lean while the NOx releasing action from the NOx absorbent is performed, Since it has been found that when the NOx releasing action from the NOx absorbent is completed, the air-fuel ratio of the exhaust flowing out of the NOx absorbent becomes rich, the air-fuel ratio of the exhaust is rich in the exhaust passage downstream of the NOx absorbent. An air-fuel ratio sensor capable of detecting whether the fuel is lean or lean is arranged, and the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is switched from lean to rich to release NOx from the NOx absorbent, and then the NOx absorption is performed by the air-fuel ratio sensor. When it is detected that the air-fuel ratio of the exhaust gas flowing out from the agent has become rich, it is determined that the NOx releasing action from the NOx absorbent has been completed and flows into the NOx absorbent. There is an engine which is adapted to return the air-fuel ratio of the gas to the lean (see Patent Document 1).
[0003]
[Patent Document 1]
JP-A-8-232646
[0004]
[Problems to be solved by the invention]
By the way, when the air-fuel ratio of the inflowing exhaust gas is lean, NOx in the exhaust gas is trapped. When the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or a rich air-fuel ratio, the trapped NOx is desorbed and removed. There is a catalyst that reduces the separated NOx using HC or CO in the exhaust gas as a reducing agent, and has a function as a three-way catalyst. Since this catalyst also functions as a three-way catalyst, it naturally has a so-called oxygen storage function that absorbs and releases oxygen. It is known that this oxygen storage characteristic is actually divided into a characteristic that is absorbed / released at a high speed by a precious metal of the catalyst and a characteristic that is absorbed / released at a low speed by an oxygen storage material such as ceria of the catalyst. (See JP 2001-227331 A). In other words, there is a high-speed component oxygen and a low-speed component oxygen. By the presence of these two types of oxygen, the conventional device is applied as it is, and the air-fuel ratio sensor provided downstream of the catalyst is inverted from lean to rich. In this case, it is determined that the NOx desorption action from the catalyst is completed. In particular, when the catalyst is at a high temperature, the reducing agent necessary for reducing NOx is insufficient, and the regeneration of the catalyst may be insufficient. all right.
[0005]
This will be described. When the air-fuel ratio enrichment process is started to switch the air-fuel ratio of the exhaust gas flowing into the catalyst from lean to rich in order to desorb NOx from the catalyst, the oxygen stored in the catalyst begins to be released, but the oxygen release of the high-speed component The higher the catalyst temperature, the faster the speed. Therefore, when the catalyst temperature increases, the timing at which the air-fuel ratio sensor downstream of the catalyst reverses richly becomes earlier.
[0006]
For this reason, when the air-fuel ratio sensor downstream of the catalyst is reversed to rich, it is determined that the NOx desorption action has been completed and the air-fuel ratio enrichment process is terminated. For NOx, which has been absorbed by the catalyst, the reducing agent necessary for reducing NOx is insufficient on the high temperature side even though the reducing agent necessary for NOx reduction can be supplied on the low temperature side.
[0007]
On the other hand, as the catalyst temperature increases, the absolute amount of oxygen stored in the catalyst as a low-speed component increases. Since the oxygen stored as the low-speed component remains in the catalyst even when the air-fuel ratio sensor downstream of the catalyst is richly inverted, most of the reducing agent supplied by the rich air-fuel ratio is stored as the low-speed component. When the reducing agent does not reach the NOx to be converted because it reacts with the oxygen that is being converted and the air-fuel ratio sensor downstream of the catalyst is richly inverted, it is judged that the NOx desorption action has been completed, and the air-fuel ratio enrichment process When the process is finished, the reducing agent necessary for reducing NOx becomes insufficient as the temperature of the catalyst increases.
[0008]
Thus, when the reducing agent for reducing NOx is insufficient and the catalyst is not sufficiently regenerated, the NOx trapping ability of the catalyst gradually decreases, and finally NOx cannot be trapped.
[0009]
Therefore, an object of the present invention is to sufficiently regenerate the catalyst regardless of the catalyst temperature even when the NOx trap catalyst has a function as a three-way catalyst having an oxygen storage function.
[0010]
[Means for Solving the Problems]
  The present invention provides a NOx trap catalyst,An oxygen storage function that absorbs and releases oxygen, and the oxygen that is absorbed and released is composed of high-speed oxygen and low-speed oxygen, and the higher the catalyst temperature, the higher the oxygen release rate. And the characteristic that the absolute amount of oxygen absorbed as low-speed component oxygen increases as the catalyst temperature increases.It has a function as a three-way catalyst having an oxygen storage function, and an air-fuel ratio sensor is disposed downstream of this catalyst, and an air-fuel ratio enrichment process for switching the exhaust air-fuel ratio from lean to rich in order to desorb NOx from the catalyst. A detector for detecting a catalyst temperature of the catalyst in an engine exhaust gas purification apparatus including an air-fuel ratio enrichment processing unit that terminates the air-fuel ratio enrichment processing when the NOx desorption action from the catalyst is completed after starting; When the NOx desorption action from the catalyst is completed is when the output of the air-fuel ratio sensor crosses the slice level,Corresponding to an increase in the absolute value of oxygen absorbed as oxygen of the low-speed component and an increase in the release rate of oxygen of the high-speed component as the catalyst temperature increases,Setting means for setting the slice level so as to increase as the catalyst temperature detected by the detection means increases.
[0011]
  According to the present invention, in the exhaust gas purification apparatus, the detection means for detecting the catalyst temperature of the catalyst and the NOx desorption action from the catalyst are completed after the output of the air-fuel ratio sensor crosses the slice level. When the rich side value is set to the initial value, the air-fuel ratio is gradually returned to the lean side from this initial value, and when the air-fuel ratio reaches the stoichiometric air-fuel ratio,Corresponding to an increase in the absolute value of oxygen absorbed as oxygen of the low-speed component and an increase in the release rate of oxygen of the high-speed component as the catalyst temperature increases,The initial value is set so as to become a richer value as the catalyst temperature detected by the detecting means becomes higher, or the catalyst temperature detected by the detecting means determines the speed at which the air-fuel ratio is returned to the lean side. Setting means for setting so as to be gentler as the height increases.
[0012]
【The invention's effect】
  When the NOx desorption action from the catalyst is completed when the output of the air-fuel ratio sensor crosses the slice level, this slice level is reduced at the low temperature of the catalyst to reduce all NOx trapped in the catalyst. If the catalyst is matched so that the amount of reduction required for the catalyst can be supplied, the NOx is detected because the timing at which the output of the air-fuel ratio sensor crosses the slice level is earlier when the catalyst is hot than when the catalyst is cold. However, according to the present invention,Corresponding to an increase in the absolute value of oxygen absorbed as oxygen of the low-speed component and an increase in the release rate of oxygen of the high-speed component as the catalyst temperature increases,Since the slice level is set to a larger value when the catalyst is hot than when the catalyst is cold, the timing at which it is determined that the output of the air-fuel ratio sensor has crossed the slice level is delayed. Also, the amount of reduction of NOx is supplied without shortage.
[0013]
  Also, when the NOx desorption action from the catalyst is completed, after the output of the air-fuel ratio sensor crosses the slice level, the rich side value is taken as the initial value, and the air-fuel ratio is gradually returned to the lean side from this initial value. When the air-fuel ratio has reached the stoichiometric air-fuel ratio, the initial value and the speed at which the air-fuel ratio is returned to the lean side are used to reduce all NOx trapped in the catalyst at a low temperature of the catalyst. When matching is performed so that the reduction amount can be supplied, when the catalyst is at a high temperature, the timing at which the air-fuel ratio reaches the stoichiometric air-fuel ratio after the output of the air-fuel ratio sensor crosses the slice level is However, according to the present invention, the amount of NOx reduction is insufficient.Corresponding to an increase in the absolute value of oxygen absorbed as oxygen of the low-speed component and an increase in the release rate of oxygen of the high-speed component as the catalyst temperature increases,Since the initial value of the richer side and the leaner return speed that becomes more gradual than the low temperature of the catalyst are set when the catalyst is hot, the air-fuel ratio is output after the output of the air-fuel ratio sensor crosses the slice level. The timing at which it is determined that has reached the stoichiometric air-fuel ratio is delayed, so that the reduction amount of NOx is supplied without shortage even when the catalyst is at a high temperature.
[0014]
Thus, according to the present invention, even when the catalyst is at a high temperature, if the reducing agent for reducing NOx can be supplied without excess or deficiency, the catalyst can be completely regenerated, and the NOx trapping ability of the catalyst can be maintained without decreasing.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram showing the configuration of the engine control system of the first embodiment.
[0016]
In FIG. 1, air is measured by a throttle valve 4 and then sucked into a combustion chamber of the engine 1 and mixed with fuel injected from a fuel injection valve 5 to form a predetermined air-fuel ratio mixture in the combustion chamber. Although the fuel injection valve 5 directly injects fuel into the combustion chamber, the fuel injection valve 5 may inject fuel into the intake port. The air-fuel mixture is ignited and burned by spark ignition by the spark plug 6, and the combustion gas is discharged to the exhaust passage 3 as exhaust gas.
[0017]
The throttle valve 4 is driven by an actuator (not shown), and an engine controller 11 to which signals from a water temperature sensor 16 for detecting the engine cooling water temperature, an accelerator sensor 17 for detecting the depression amount of the accelerator pedal, and the like are input. Thus, the opening degree of the throttle valve 4 is variably controlled based on these.
[0018]
The exhaust passage 3 includes a three-way catalyst 7 having an oxygen storage function. The exhaust contains harmful three components of HC, CO, and NOx, and these three components are efficiently purified simultaneously by the three-way catalyst 7 when operating at the stoichiometric air-fuel ratio. That is, in order to maintain the maximum conversion efficiency of HC, CO, and NOx in the three-way catalyst, it is necessary to make the catalyst atmosphere the stoichiometric air-fuel ratio, which is achieved by keeping the oxygen storage amount of the catalyst constant. It has been done. For example, when the exhaust gas flowing into the catalyst is shifted to the lean side, the oxygen in the exhaust gas is absorbed by the catalyst, and when it is shifted to the rich side, the oxygen absorbed by the catalyst is released, thereby substantially reducing the catalyst atmosphere. The stoichiometric air-fuel ratio is maintained.
[0019]
On the other hand, when operating at a lean air-fuel ratio, a large amount of NOx is generated, and this NOx cannot be efficiently purified by a three-way catalyst. Therefore, NOx in the exhaust is trapped when the air-fuel ratio of the exhaust is lean, and when the air-fuel ratio of the exhaust is at the theoretical air-fuel ratio or the rich side, the trapped NOx is desorbed and the desorbed NOx is A NOx trap catalyst that performs reduction treatment using CO and HC that are contained in a large amount of exhaust gas as a reducing agent is integrated with the three-way catalyst 7.
[0020]
Thus, the three-way catalyst 7 also has a function of a NOx trap catalyst. Alternatively, it can be said that the NOx trap catalyst has a function as a three-way catalyst having an oxygen storage function. Hereinafter, it is simply referred to as “catalyst”.
[0021]
The target air-fuel ratio of the air-fuel mixture is determined in advance by the engine controller 11 in accordance with the operation region. In the low-load operation region where a large engine output is not required, operation is performed with the lean air-fuel ratio as the target value in order to improve fuel efficiency. On the other hand, when the operating range is on the high load side where a large engine output is required, the operation is performed with the theoretical air-fuel ratio as a target. That is, the rotational speed from the crank angle sensor 12 and the intake air flow rate signal from the air flow meter 13 are input to the engine controller 11 and the O 7 installed upstream of the catalyst 7 is input.₂Output from sensor 14, O installed downstream of catalyst 7₂The output from the sensor 15 is input, and a target equivalence ratio TFBYA having a value smaller than 1.0 is determined in the low load side operation region in accordance with the engine speed and load, and the basic air-fuel ratio (≈ The fuel injection pulse width Ti is determined by correcting the basic injection pulse width Tp from which the theoretical air / fuel ratio is obtained. When the operating range is on the high load side, the target equivalence ratio TFBYA = 1.0 (corresponding to the theoretical air-fuel ratio) and when the air-fuel ratio feedback control condition is satisfied, the upstream side O₂An air-fuel ratio feedback correction coefficient α is calculated based on the output of the sensor 14, and the fuel injection pulse width Ti is determined by correcting the basic injection pulse width Tp with this α.
[0022]
In addition, when the NOx trap amount of the catalyst 7 reaches a predetermined value during the lean operation, the catalyst 7 is regenerated, so that an air-fuel ratio enrichment process for temporarily enriching the exhaust air-fuel ratio is performed.
[0023]
In this case, while the NOx desorption action from the catalyst 7 is performed, the air-fuel ratio of the exhaust gas flowing out from the catalyst 7 is slightly lean, and when the NOx desorption action from the catalyst 7 is completed, the catalyst 7 Since the air-fuel ratio of the exhaust gas flowing out from the exhaust gas becomes rich, after the air-fuel ratio enrichment process is started, the downstream side O₂Compare the output of sensor 15 and slice level,₂When the output of the sensor 15 crosses this slice level, it is determined that the NOx desorption action from the catalyst 7 is completed. At this time, this slice level is detected by the catalyst temperature sensor 18 (catalyst temperature detection means). The catalyst temperature is set so as to increase as the catalyst temperature increases.
[0024]
This will be further explained with reference to FIG. 2. FIG. 2 shows a model of how the air-fuel ratio enrichment process is performed when the catalyst temperature slowly decreases during steady operation in the lean operation region.
[0025]
In the figure, at the time of lean operation, the target equivalence ratio TFBYA is set to a small value less than 1.0, and the air-fuel ratio feedback correction coefficient α is fixed to the central value of 100%. At this time, downstream O₂The output of the sensor 15 is at a level of approximately 0 mV.
[0026]
During the lean operation, the NOx catalyst remaining capacity decreases (the NOx trap amount of the catalyst 7 increases), and the target equivalent ratio TFBYA reaches the limit value NOXRLS (see the one-dot chain line in the second stage in FIG. 2). 1.0 (corresponding to the theoretical air-fuel ratio) and the air-fuel ratio feedback correction coefficient α is stepwise switched to a predetermined value A% (value on the rich side), which is a value exceeding 100%, thereby enriching the air-fuel ratio. Processing begins.
[0027]
By starting the air-fuel ratio enrichment process, the downstream side O₂It is determined that the NOx desorption action from the catalyst 7 is completed at the timing when the output of the sensor 15 increases and crosses the slice level, and the target equivalent ratio TFBYA is the value during lean operation in order to end the air-fuel ratio enrichment process. Step back to a value less than 1.0.
[0028]
The air-fuel ratio feedback correction coefficient α is the downstream O₂Rather than stepwise returning to 100% at the timing when the output of the sensor 15 crosses the slice level, the initial value is a predetermined value B% (value on the rich side) that exceeds 100% from this timing. It is gradually smaller than the value and returned to 100%. However, the predetermined value B is a value smaller than the predetermined value A.
[0029]
In this case, the difference (A-100) in the waveform of α determines the degree of air-fuel ratio enrichment, and the area surrounded by the α waveform and the horizontal line where α is 100% is supplied to the catalyst 7 of HC and CO. Represents the amount of reduction. That is, the larger the area, the greater the amount of NOx reduction, and the smaller the area, the lower the NOx reduction amount.
[0030]
Now, in this embodiment, downstream O₂The slice level for comparison with the output of the sensor 15 is set so as to increase as the catalyst temperature increases according to the catalyst temperature. Since the figure shows a case where the catalyst temperature is lowered, the slice level is gradually reduced (see the one-dot chain line in the third row in FIG. 2). For this reason, in the high temperature state of the catalyst 7, the downstream O₂The timing at which the output of the sensor 15 crosses the slice level becomes later than when the catalyst 7 is in a low temperature state, and the above-mentioned area representing the amount of NOx reduction increases. That is, in the present embodiment, in order to prevent a reduction amount of NOx from being insufficient even when the catalyst 7 is at a high temperature, the slice level is increased and the downstream O₂The timing at which the output of the sensor 15 crosses the slice level is delayed, thereby increasing the amount of NOx reduction.
[0031]
The details of such control performed by the controller 11 will be described in detail according to the following flowchart.
[0032]
FIG. 3 is for performing the air-fuel ratio enrichment process, and is executed at regular intervals (for example, every 10 msec).
[0033]
At step 1, the rich spike flag FRSPIK is checked. Assuming that the flag FRSPIK = 0 at this time, the routine proceeds to step 2 at this time, whether there is a stoichiometric request (request for operation at the theoretical air-fuel ratio) or NOx catalyst remaining capacity (NOx remaining in the catalyst 7). Trap amount) Check whether or not NOXCAPA is less than the limit value NOXRLS. When there is no stoichiometric request, or when NOXCAPA is not less than the limit value NOXRLS (that is, the catalyst 7 still has room to trap NOx), the current process is terminated.
[0034]
When there is a stoichiometric request or when the NOx catalyst remaining capacity NOXCAPA becomes less than the limit value NOXRLS, the routine proceeds to step 3 to set the rich spike in-progress flag FRSPIK = 1. In step 4, the catalyst temperature detected by the temperature sensor 18 is read and entered into a variable CATTEMP representing the catalyst temperature. In step 5, a slice level corresponding to the value of CATTEMP is set, and the current process is terminated. Here, the slice level is a value that increases as the catalyst temperature (CATTEMP) increases.
[0035]
If rich spike flag FRSPIK = 1 in step 3 above, the process proceeds from step 1 to step 6 onward from the next time. In step 6, downstream O₂The output OSR of the sensor 15 is read and this downstream O₂The output OSR of the sensor 15 is compared with the slice level already set in step 5. Downstream side O₂If the output OSR of the sensor 15 does not exceed the slice level, the current process is terminated. Eventually downstream O₂When the output OSR of the sensor 15 crosses the slice level, it is determined that the NOx desorption action has been completed. Is returned to the maximum value NOXFULL.
[0036]
FIG. 4 is for calculating the target equivalent ratio [nameless number] and the air-fuel ratio feedback correction coefficient α [%], and this is also executed at regular intervals (for example, every 10 msec).
[0037]
At step 11, the rich spike flag FRSPIK is checked. When the flag FRSPIK = 0, the operations in steps 12 to 15 are repeated. That is, in step 12, the target equivalent ratio TFBYA = 1.0, the air-fuel ratio feedback correction coefficient α = predetermined value A [%], and the two rich spike end flags FRSPIKEND1, FRSPIKEND2 are both set to zero. Here, the predetermined value A is a value (a constant value exceeding 100%) that determines the degree of enrichment of the air-fuel ratio.
[0038]
When the rich spike flag FRSPIK = 1 in step 3 in FIG. 3, the process proceeds from step 11 to step 16 in FIG. 4 to see the first rich spike end flag FRSPIKEND1. At this time, since the first rich spike end flag FRSPIKEND1 = 0, the process proceeds to steps 17 and 18, and after the predetermined value B [%] is set to α, the first rich spike end flag is set.FRSPIKEND1 = 1This processing is finished. Here, the predetermined value B is a value smaller than the predetermined value A, but is a value exceeding 100% (a constant value).
[0039]
First rich spike end flag at step 18FRSPIKEND1 = 1Thus, the process proceeds from step 16 to step 19 next time, and the second rich spike end flag FRSPIKEND2 is checked. At this time, since the second rich spike end flag FRSPIKEND2 = 0, the routine proceeds to step 20, where the air-fuel ratio feedback correction coefficient α is compared with 100%. In the previous time, a predetermined value B is included in α, and since this B exceeds 100%, the process proceeds to step 21, where α is gradually decreased by the following equation, and the current process is terminated.
[0040]
α = α (OLD) −αDEC (1)
Where α (OLD): the previous value of α,
αDEC: α reduction ratio (positive constant value),
If there is not much change in the operating conditions, the next and subsequent steps will follow steps 11, 16, 19, 20, and 21, and the gradual decrease of α is repeated according to the above equation (1). Eventually, when α becomes less than 100%, the routine proceeds from step 20 to step 22 where the second rich spike end flag FRSPIKEND2 = 1. In step 23, it is checked whether or not there is a stoichiometric request. If there is no stoichiometric request, the process proceeds to steps 24 and 25 to return to the operation at the lean air-fuel ratio, and a map value corresponding to the operating condition (engine speed, load) at that time is retrieved and entered into the target equivalent ratio TFBYA , Α = 100%, and the current process is terminated. The map search value is a positive value less than 1.0.
[0041]
On the other hand, when there is a stoichiometric request, in order to respond to this, the routine proceeds to step 26 where TFBYA = 1.0, and at step 27 it is checked whether there is an air-fuel ratio feedback request. If there is no air-fuel ratio feedback request, the current process is terminated. When there is an air-fuel ratio feedback request, the routine proceeds from step 27 to step 28, where the upstream O₂An air-fuel ratio feedback correction coefficient α is calculated based on the output of the sensor 14.
[0042]
In a fuel injection pulse width calculation routine (not shown), the fuel injection pulse width Ti at the time of sequential injection is calculated by the following equation using the target equivalent ratio TFBYA calculated in this way and the air-fuel ratio feedback correction coefficient α, and ignition is performed. The fuel injection valve 6 is opened for a period of Ti at a predetermined timing according to the order.
[0043]
Ti = Tp × TFBYA × (α / 100) × 2 + Ts (2)
Where Tp: basic injection pulse width,
Ts: Invalid injection pulse width,
Here, the operation of the present embodiment will be described.
[0044]
In the present embodiment, when the NOx desorption action from the catalyst 7 is completed, the downstream O₂It is assumed that the output of the sensor 15 is when it crosses the slice level. In this case, if the slice level is matched so that the amount of reduction necessary to reduce all of the NOx trapped in the catalyst 7 can be supplied when the catalyst 7 is at a low temperature, Downstream side O₂Since the timing at which the output of the sensor 15 is determined to cross the slice level is earlier than when the catalyst 7 is at a low temperature, the amount of NOx reduction is insufficient.
[0045]
On the other hand, according to the present embodiment (the invention described in claim 1), the slice level is set to a larger value when the catalyst 7 is at a higher temperature than when the catalyst 7 is at a lower temperature.₂The timing at which it is determined that the output of the sensor 15 has crossed the slice level is delayed, so that even when the catalyst 7 is at a high temperature, the reduction amount of NOx is supplied without shortage. Thus, even when the catalyst 7 is at a high temperature, if the reducing agent for reducing NOx can be sufficiently supplied, the catalyst 7 can be completely regenerated, and the NOx trapping ability of the catalyst 7 can be maintained without decreasing.
[0046]
FIGS. 5 and 6 are model waveform diagrams of the second and third embodiments, respectively, which replace FIG. 2 of the first embodiment.
[0047]
  In the first embodiment, when the NOx desorption action from the catalyst 7 is completed, the downstream O₂Although it was based on the assumption that the output of the sensor 15 crosses the slice level, the second and third embodiments indicate that when the NOx desorption action from the catalyst 7 is completed, the downstream O₂After the output of the sensor 15 crosses the slice level, when the predetermined value B is set as an initial value, the air-fuel ratio feedback correction coefficient α is reduced at a constant speed from the initial value, and α reaches 100% (that is, the rich side value). Is assumed to be the initial value when the air-fuel ratio is gradually returned to the lean side from the initial value and the air-fuel ratio reaches the stoichiometric air-fuel ratio). That is, in the second embodiment, the predetermined value B is changed according to the catalyst temperature, and is increased as the catalyst temperature is increased. In the third embodiment, the reduction ratio αDEC of α that determines the return speed of α is set. Change each according to the catalyst temperature, the higher the catalyst temperatureMake smaller. 5 and 6 also show a case in which the catalyst temperature decreases as in FIG. 2, the predetermined value B in FIG. 5 gradually decreases (Fig. 5 4th rowThe speed of α gradually returning to 100% in FIG.It is faster (see the 4th row in Fig. 6). For this reason, in the high temperature state of the catalyst 7, the downstream side O₂The timing at which α reaches 100% after the output of the sensor 15 crosses the slice level becomes later than when the catalyst 7 is in a low temperature state, and the above-mentioned area representing the amount of NOx reduction increases. That is, in each of the second and third embodiments, the predetermined value B is set to a richer value or the rate of decrease of α is set so that the reduction amount of NOx is not insufficient when the catalyst 7 is in a high temperature state. Make it more gradual, downstream O₂The timing at which α reaches 100% after the output of the sensor 15 crosses the slice level is delayed, thereby increasing the amount of NOx reduction. In the second and third embodiments, the slice level is a constant value regardless of the catalyst temperature (FIG. 5, FIG. 6 third stage).Dotted line reference).
[0048]
More specifically, FIGS. 7 and 8 are flowcharts of the second embodiment, which are replaced with FIGS. 3 and 4 of the first embodiment, respectively. However, the same step number is attached | subjected to the step same as FIG. 3, FIG. The difference from the first embodiment is in step 10 in FIG. 7 and step 31 in FIG. That is, in the second embodiment, the slice level set in step 10 of FIG. 7 is a constant value regardless of the catalyst temperature, and the predetermined value B is set to be higher as the catalyst temperature is higher in step 31 of FIG. Yes.
[0049]
7 and 9 are flowcharts of the third embodiment, which are replaced with FIGS. 3 and 4 of the first embodiment, respectively. However, the same step number is attached | subjected to the step same as FIG. 3, FIG. The difference from the first embodiment is in step 10 in FIG. 7 and step 41 in FIG. That is, also in the third embodiment, the slice level set in step 10 in FIG. 7 is a constant value regardless of the catalyst temperature, and in step 41 in FIG. 9, the α reduction ratio αDEC is set to be smaller as the catalyst temperature is higher. is doing.
[0050]
When the NOx desorption action from the catalyst 7 is completed, the downstream side O₂After the output of the sensor 15 crosses the slice level, the predetermined value B, which is a rich value, is set as an initial value, and the air-fuel ratio feedback correction coefficient α is gradually returned to the lean side from this initial value, and α reaches 100%. In this case, the return speeds of the predetermined values B and α are matched so that the amount of reduction necessary for reducing all of the NOx trapped in the catalyst 7 can be supplied at a low temperature of the catalyst. When the temperature of the catalyst 7 is high, the downstream side O₂Although the timing at which α reaches 100% after the output of the sensor 15 crosses the slice level is earlier than when the catalyst 7 is at a low temperature, the amount of NOx reduction is insufficient. According to each of the third embodiments (inventions according to claims 2 and 3), when the catalyst 7 is at a high temperature, the richer side of the predetermined value B or a return of α that is more gradual than when the catalyst 7 is at a low temperature. Since the speed is set, downstream O₂The timing at which it is determined that α has reached 100% after the output of the sensor 15 crosses the slice level is delayed, so that even when the catalyst 7 is at a high temperature, the reduction amount of NOx is supplied without shortage.
[0051]
In the embodiment, the air-fuel ratio sensor is O₂Although described in the case of the sensor, it may be a sensor that can detect the exhaust air-fuel ratio in a wide area, such as a wide-area air-fuel ratio sensor.
[0052]
A combination of the second embodiment and the third embodiment is also conceivable.
[0053]
  The method for determining the completion of the NOx desorption action is not limited to that of the embodiment. For example,NOx trap catalyst 7The time when the NOx desorption action from the air-fuel ratio is completed can be the time when a predetermined delay time has elapsed after the output of the air-fuel ratio sensor crosses the slice level. That is, when the predetermined delay time is matched so that the reduction amount necessary for reducing all of the NOx trapped in the catalyst 7 can be supplied when the catalyst 7 is at a low temperature, Since the timing at which the delay time has elapsed after the output of the air-fuel ratio sensor crosses the slice level is earlier than when the catalyst 7 is at a low temperature, the amount of NOx reduction is insufficient. Therefore, in this case, the predetermined delay time may be set to become longer as the catalyst temperature becomes higher (the invention according to claim 4). That is, by setting a longer delay time when the catalyst 7 is at a higher temperature than when the catalyst 7 is at a lower temperature, the timing for determining that the delay time has elapsed after the output of the air-fuel ratio sensor crosses the slice level is delayed. Thus, even when the catalyst 7 is at a high temperature, the reduction amount of NOx is supplied without shortage.
[0054]
  The function of the slice level setting means according to claim 1 is as shown in step 5 of FIG.Has been fulfilled.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a configuration of a first embodiment of the present invention.
FIG. 2 is a waveform diagram for explaining the operation of the first embodiment.
FIG. 3 is a flowchart for explaining air-fuel ratio enrichment processing according to the first embodiment;
FIG. 4 is a flowchart for explaining calculation of a target equivalent ratio and an air-fuel ratio feedback correction coefficient according to the first embodiment.
FIG. 5 is a waveform diagram for explaining the operation of the second embodiment.
FIG. 6 is a waveform diagram for explaining the operation of the third embodiment.
FIG. 7 is a flowchart for explaining air-fuel ratio enrichment processing according to second and third embodiments.
FIG. 8 is a flowchart for explaining calculation of a target equivalent ratio and an air-fuel ratio feedback correction coefficient according to the second embodiment.
FIG. 9 is a flowchart for explaining calculation of a target equivalent ratio and an air-fuel ratio feedback correction coefficient according to the third embodiment.
[Explanation of symbols]
7 Catalyst
11 Engine controller
15 Downstream O₂Sensor (Air-fuel ratio sensor)
18 Catalyst temperature sensor (catalyst temperature detection means)

Claims (4)

When the air-fuel ratio of the inflowing exhaust gas is lean, NOx in the exhaust gas is trapped. When the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich, the trapped NOx is desorbed and the desorbed NOx is exhausted in the exhaust gas. A catalyst for reducing a component as a reducing agent,
This catalyst has an oxygen storage function that absorbs and releases oxygen, and the oxygen that is absorbed and released is composed of oxygen of high-speed components and oxygen of low-speed components. It has a function as a three-way catalyst having an oxygen storage function, which has a characteristic of increasing the oxygen release rate and a characteristic of increasing the absolute amount of oxygen absorbed as low-speed component oxygen as the catalyst temperature increases ,
An air-fuel ratio sensor that can detect whether the air-fuel ratio of the exhaust gas flowing out from the catalyst is rich or lean is disposed downstream of the catalyst,
After starting the air-fuel ratio enrichment process for switching the exhaust air-fuel ratio from lean to rich to desorb NOx from the catalyst, the air-fuel ratio enrichment process is terminated when the NOx desorption action from the catalyst is completed In an exhaust emission control device for an engine provided with a enrichment processing means,
Detecting means for detecting the temperature of the catalyst;
When the NOx desorption action from the catalyst is completed when the output of the air-fuel ratio sensor crosses the slice level, the absolute value of oxygen absorbed as oxygen of the low-speed component as the catalyst temperature increases And a slice level setting means for setting the slice level to be larger as the catalyst temperature detected by the detection means becomes higher, in response to the increase in the rate of release of the high-speed component oxygen. An exhaust emission control device for an engine, comprising: When the air-fuel ratio of the inflowing exhaust gas is lean, NOx in the exhaust gas is trapped. When the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich, the trapped NOx is desorbed and the desorbed NOx is exhausted in the exhaust gas. A catalyst for reducing a component as a reducing agent,
This catalyst has an oxygen storage function that absorbs and releases oxygen, and the oxygen that is absorbed and released is composed of oxygen of high-speed components and oxygen of low-speed components. It has a function as a three-way catalyst having an oxygen storage function, which has a characteristic of increasing the oxygen release rate and a characteristic of increasing the absolute amount of oxygen absorbed as low-speed component oxygen as the catalyst temperature increases ,
An air-fuel ratio sensor that can detect whether the air-fuel ratio of the exhaust gas flowing out from the catalyst is rich or lean is disposed downstream of the catalyst,
After starting the air-fuel ratio enrichment process for switching the exhaust air-fuel ratio from lean to rich to desorb NOx from the catalyst, the air-fuel ratio enrichment process is terminated when the NOx desorption action from the catalyst is completed In an exhaust emission control device for an engine provided with a enrichment processing means,
Detecting means for detecting the temperature of the catalyst;
When the NOx desorption action from the catalyst is completed, after the output of the air-fuel ratio sensor crosses the slice level, the rich side value is taken as the initial value, and the air-fuel ratio is gradually returned to the lean side from this initial value. When the air-fuel ratio has reached the stoichiometric air-fuel ratio, the higher the catalyst temperature, the higher the absolute value of oxygen absorbed as the low-speed component oxygen and the higher the high-speed component oxygen release rate. Corresponding to the initial value setting means for setting the initial value so as to become a richer value as the catalyst temperature detected by the detection means becomes higher. When the air-fuel ratio of the inflowing exhaust gas is lean, NOx in the exhaust gas is trapped. When the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich, the trapped NOx is desorbed and the desorbed NOx is exhausted in the exhaust gas. A catalyst for reducing a component as a reducing agent,
This catalyst is an oxygen storage function or to absorb oxygen or release, absorption or oxygen or released consists oxygen of the oxygen and slow components of the high speed component, the high speed component catalyst temperature is high nearly as It has a function as a three-way catalyst with an oxygen storage function, which has the characteristics of increasing the oxygen release rate and increasing the absolute amount of oxygen absorbed as low-speed component oxygen as the catalyst temperature increases. ,
An air-fuel ratio sensor that can detect whether the air-fuel ratio of the exhaust gas flowing out from the catalyst is rich or lean is disposed downstream of the catalyst,
After starting the air-fuel ratio enrichment process for switching the exhaust air-fuel ratio from lean to rich to desorb NOx from the catalyst, the air-fuel ratio enrichment process is terminated when the NOx desorption action from the catalyst is completed In an exhaust emission control device for an engine provided with a enrichment processing means,
Detecting means for detecting the temperature of the catalyst;
When the NOx desorption action from the catalyst is completed, after the output of the air-fuel ratio sensor crosses the slice level, the rich side value is taken as the initial value, and the air-fuel ratio is gradually returned to the lean side from this initial value. When the air-fuel ratio has reached the stoichiometric air-fuel ratio, the higher the catalyst temperature, the higher the absolute value of oxygen absorbed as the low-speed component oxygen and the higher the high-speed component oxygen release rate. And a lean-side return speed setting means for setting the speed at which the air-fuel ratio is returned to the lean side so that the speed becomes higher as the catalyst temperature detected by the detection means becomes higher. Exhaust purification equipment. When the air-fuel ratio of the inflowing exhaust gas is lean, NOx in the exhaust gas is trapped. When the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich, the trapped NOx is desorbed and the desorbed NOx is exhausted in the exhaust gas. A catalyst for reducing a component as a reducing agent,
This catalyst has an oxygen storage function that absorbs and releases oxygen, and the oxygen that is absorbed and released is composed of oxygen of high-speed components and oxygen of low-speed components. It has a function as a three-way catalyst having an oxygen storage function, which has a characteristic of increasing the oxygen release rate and a characteristic of increasing the absolute amount of oxygen absorbed as low-speed component oxygen as the catalyst temperature increases ,
An air-fuel ratio sensor that can detect whether the air-fuel ratio of the exhaust gas flowing out from the NOx trap catalyst is rich or lean is disposed downstream of the catalyst,
After starting the air-fuel ratio enrichment process for switching the exhaust air-fuel ratio from lean to rich to desorb NOx from the catalyst, the air-fuel ratio enrichment process is terminated when the NOx desorption action from the catalyst is completed In an exhaust emission control device for an engine provided with a enrichment processing means,
Detecting means for detecting the temperature of the catalyst;
When the NOx desorption action from the catalyst is completed when the predetermined delay time has elapsed after the output of the air-fuel ratio sensor crosses the slice level, the oxygen of the low-speed component increases as the catalyst temperature increases. This predetermined delay time becomes longer as the catalyst temperature detected by the detection means becomes higher, corresponding to the increase in the absolute value of oxygen absorbed as the oxygen and the release rate of the high-speed component oxygen. An exhaust purification device for an engine, comprising: a delay time setting means for setting on a side.

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