JP3885331B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an engine, and more particularly, to an engine for lean operation.
[0002]
[Prior art]
When the air-fuel ratio is leaner than the stoichiometric air-fuel ratio (hereinafter abbreviated as stoichiometric), it absorbs NOx, and when the air-fuel ratio becomes richer than stoichiometric, it not only desorbs the absorbed NOx, but also desorbed NOx, There is a catalyst (hereinafter referred to as a NOx storage catalyst) that can be purified using HC and CO that are exhausted in a rich operation as a reducing agent, and this NOx storage catalyst is provided in the exhaust passage to store NOx. If it is determined that the NOx occlusion amount in the type catalyst has reached its limit, there is one in which the air-fuel ratio is controlled to be richer than stoichiometric (riching process) for a very short time (for example, JP-A-7-139340). reference).
[0003]
[Problems to be solved by the invention]
By the way, in the conventional apparatus in which only one NOx occlusion type catalyst is provided in the exhaust passage, there is room for improvement in terms of fuel consumption and drivability.
[0004]
For example, in order to ensure the activity of the catalyst, it is desirable to mount on the upstream side (for example, the manifold position) of the exhaust passage. However, it is difficult to mount a large NOx occlusion type catalyst at the manifold position due to vehicle mounting requirements.
[0005]
Therefore, the NOx occlusion type catalyst mounted at the manifold position is small, and a second NOx occlusion type catalyst is installed downstream of the exhaust passage (for example, under the floor) in order to secure a catalyst capacity that is insufficient with this catalyst alone. It is possible to do.
[0006]
In this case, the exhaust temperature (catalyst temperature) differs between the manifold position and the underfloor position. Therefore, the optimum enrichment degree for each catalyst (for example, the air-fuel ratio is stepped at the start of the enrichment process and then subtracted at a predetermined speed). In this case, the rich amount at the start of the enrichment process and the subsequent subtraction speed) are greatly different from each other. Therefore, the conventional NOx occlusion type catalyst is provided at a plurality of positions where the exhaust temperatures are greatly different. The enrichment process cannot handle this.
[0007]
Even if a large NOx occlusion type catalyst can be installed at the manifold position, the richness of the air-fuel ratio must be increased according to the NOx occluded to the full (torque shock if the degree of enrichment is large). Drivability), drivability deteriorates.
[0008]
Therefore, the present invention prepares a plurality of NOx storage-type catalysts and arranges them in series in the exhaust passage, while the NOx storage amount during the lean operation of the upstream catalyst and the lean operation of the upstream catalyst are also measured. When the NOx occlusion amount reaches the occlusion limit, the NOx occlusion amount during lean operation of the downstream catalyst is calculated, and when the predetermined enrichment processing timing is reached, only the upstream catalyst occludes NOx. If the NOx is stored in the downstream catalyst in addition to the upstream catalyst, the stored NOx amount in the upstream catalyst and the stored NOx amount in the downstream catalyst respectively. By setting the enrichment degree according to the enrichment process characteristics and performing the enrichment process of the air-fuel ratio using this enrichment degree, only one NOx occlusion type catalyst is provided in the exhaust passage. And an object thereof is to improve both fuel economy and drivability of than are.
[0009]
[Means for Solving the Problems]
As shown in FIG. 14, the first invention arranges a plurality of NOx occlusion-type catalysts in series in the exhaust passage 31, while the NOx occlusion amount of the upstream catalyst 32 during the lean operation and the upstream during the lean operation. When the NOx occlusion amount of the side catalyst 32 reaches the occlusion limit, the upstream side catalyst 33 calculates the NOx occlusion amount during the lean operation of the downstream side catalyst 33, and when the predetermined enrichment processing timing is reached. When only 32 stores NOx, the amount of stored NOx And catalyst temperature of upstream catalyst When the NOx is occluded in the downstream side catalyst 33 in addition to the upstream side catalyst 32, the degree of enrichment (for example, the proportion of the enrichment process start and the subsequent rich reduction rate) is occluded. NOx amount And catalyst temperature of upstream catalyst When , NOx storage amount of downstream catalyst 33 And the catalyst temperature of the downstream catalyst Accordingly, a means 35 for setting the degree of enrichment and a means 36 for performing an air-fuel ratio enrichment process using the degree of enrichment are provided.
[0010]
In the second invention, in the first invention, the predetermined enrichment processing timing is a time when switching from lean operation to stoichiometric operation or a condition (for example, during acceleration) that is considered to have little rebound in drivability.
[0011]
In the third invention, the catalyst having a relatively small catalyst capacity in the first or second invention is defined as an upstream catalyst, and the catalyst having a relatively large catalyst capacity is defined as a downstream catalyst.
[0012]
In the fourth invention, in any one of the first to third inventions, the NOx storage limit of the upstream catalyst is set according to the catalyst temperature.
[0013]
In the fifth invention, in any one of the first to fourth inventions, HC required for the conversion of NOx stored in the NOx storage-type catalyst according to the NOx storage amount and the intake air flow rate at that time, The amount of CO is calculated, and the degree of enrichment is set according to the required amount of HC and CO.
[0014]
In the sixth invention, the NOx occlusion amount is calculated in any one of the first to fifth inventions, and at a predetermined speed after the air-fuel ratio is step-changed at the start of the enrichment process by setting the enrichment degree. When subtracting, there is a delay from the step change to the start of subtraction.
[0015]
【The invention's effect】
In each of the first and second inventions, even when a plurality of NOx occlusion-type catalysts are provided at positions away from the exhaust passage, it is possible to calculate the NOx occlusion amount and set the degree of enrichment, as well as the enrichment processing timing. If the NOx storage amount of the upstream catalyst happens to be at the storage limit as in the conventional case, the conventional device having only one NOx storage type catalyst and a large catalyst capacity is provided. In comparison, the first and second inventions have a smaller degree of enrichment as the catalyst capacity is smaller, and as a result, torque fluctuations associated with the enrichment process can be suppressed compared to the prior art (the drivability rebound is less).
[0016]
In the third aspect of the present invention, the degree of enrichment when the enrichment processing timing is reached in a state where the amount of NOx stored in the upstream catalyst does not exceed the storage limit can be further reduced.
[0017]
If the NOx limit storage amount is a constant value regardless of the catalyst temperature, the NOx storage capacity cannot be utilized effectively. However, in the fourth aspect of the invention, the NOx storage limit is set according to the catalyst temperature. The NOx storage capacity can be utilized to the maximum.
[0018]
In the fifth aspect of the invention, the map search is not required when obtaining the degree of enrichment, so that the memory capacity can be reduced.
[0019]
When calculating the NOx occlusion amount, it cannot be said that the actual NOx occlusion amount may not be larger than the calculated value. At this time, the amount of HC and CO supplied to the NOx occlusion type catalyst by the enrichment process is insufficient. It is conceivable that the amount of desorbed NOx corresponding to the insufficient HC and CO amounts is discharged as it is. In the sixth invention, there is a delay from the step change to the start of subtraction so that a slightly larger amount of HC and CO may be supplied in anticipation of such a situation where the amount of HC and CO is insufficient. Therefore, the amount of HC and CO supplied by the enrichment process increases correspondingly, and desorbed NOx can be reliably converted.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a control system for an in-cylinder direct injection spark ignition engine.
[0021]
In the figure, 1 is an engine body, 2 is a combustion chamber, 3 is a piston, 4 is a spark plug, 5 is an injector, 8 is a swirl control valve, and 9 is a throttle device driven by a throttle actuator 9A comprising a DC motor or the like. . The in-cylinder direct injection spark ignition engine is provided with the injector 5 facing the combustion chamber, and the shape of the combustion chamber 2, the intake port and the piston top surface are devised. Since it does not relate to itself, FIG.
[0022]
The ECM (electronic control module) 11 includes an air flow sensor 12 that detects an air flow rate upstream of the throttle device 9, a crank angle sensor (a crank angle sensor 13A that outputs a position signal and a crank angle sensor 13B that outputs a REF signal), an accelerator. Signals from a sensor 14 that detects the pedal opening and a sensor (not shown) that detects the coolant temperature of the engine are O ₂ Along with signals from the sensor 15 and the like, and based on these signals, the air-fuel mixture is unevenly distributed only in the vicinity of the spark plug 4 at the time of low engine load, and combustion (stratified combustion) is performed. However, when the engine is operated at an air-fuel ratio of 30 to 40, for example, when the engine is out of low load, a homogeneous air-fuel mixture is formed in the combustion chamber and the average air-fuel ratio in the combustion chamber is controlled stoichiometrically.
[0023]
Now, a NOx occlusion type catalyst is provided in the exhaust passage 21, and NOx generated in the stratified combustion zone is occluded in the NOx occlusion type catalyst. A conventional apparatus that is desorbed and purifies the desorbed NOx using HC and CO that are exhausted in a rich operation as a reducing agent is known. However, the conventional apparatus has only one NOx occlusion catalyst. However, there is still room for improvement in terms of fuel economy and driving performance.
[0024]
For example, in order to ensure the activity of the catalyst, it is desirable to mount it at the manifold position upstream of the exhaust passage. However, it is difficult to mount a large NOx storage type catalyst at the manifold position due to vehicle mounting requirements.
[0025]
Therefore, the NOx occlusion type catalyst mounted at the manifold position is small, and the second NOx occlusion type catalyst is installed at the underfloor position downstream of the exhaust passage in order to secure a catalyst capacity that is insufficient with this catalyst alone. It is possible.
[0026]
In this case, the exhaust temperature (catalyst temperature) differs between the manifold position and the underfloor position, and therefore the optimum enrichment degree for each catalyst differs greatly. Therefore, the NOx occlusion-type catalyst is located at a plurality of positions where the exhaust temperature differs greatly. However, conventional enrichment processing cannot cope with this.
[0027]
Even if a large NOx occlusion type catalyst can be installed at the manifold position, the richness of the air-fuel ratio must be increased according to the NOx occluded to the full (torque shock if the degree of enrichment is large). Drivability), drivability deteriorates.
[0028]
Therefore, in the first embodiment of the present invention, two NOx occlusion type catalysts having relatively different catalyst capacities are prepared, and a relatively small one of the catalyst capacities is disposed upstream of the exhaust passage 21, and The bigger one is placed downstream. Specifically, two NOx storage-type catalysts having a three-way catalyst function are prepared, and as shown in FIG. 1, the upstream NOx storage-type three-way catalyst 22 is placed near the exhaust manifold. A downstream NOx occlusion type three-way catalyst 23 is disposed under the floor of the vehicle. Hereinafter, when distinguishing both, the upstream NOx occlusion type three-way catalyst 22 is abbreviated as a manifold catalyst, and the downstream NOx occlusion type three way catalyst 23 is abbreviated as an underfloor catalyst.
[0029]
Then, the calculation of the NOx occlusion amount and the enrichment process are performed for these two catalysts 22 and 23 as follows.
[0030]
[1] Since the NOx amount during stratified combustion varies depending on the operating conditions, the NOx amount generated per predetermined time is estimated from the rotational speed and engine target torque at that time by a map search or the like. As what is stored in the catalyst 22, the amount of NOx stored in the manifold catalyst 22 is obtained by integrating the above estimated values.
[0031]
[2] As a result, if the integrated value of the NOx storage amount of the manifold catalyst 22 reaches the storage limit, it is not possible to store any more in the manifold catalyst, and the generated NOx is stored in the underfloor catalyst 23. Therefore, at this time, the accumulated value of the NOx occlusion amount to the underfloor catalyst 23 is obtained by integrating the estimated values.
[0032]
[3] The timing of the enrichment process is
(1) When a predetermined enrichment processing condition is satisfied,
(2) When switching from stratified charge combustion (during lean operation) to stoichiometric operation
And
[0033]
[4] At the timing of the enrichment process,
(1) When only the manifold catalyst 22 occludes NOx, a proportional amount at the enrichment start timing and the subsequent rich reduction speed are set according to the occluded NOx amount,
(2) When NOx is occluded up to the underfloor catalyst 23, the proportional amount at the start of the enrichment process and the subsequent rich decrease are matched to the sum of the occluded NOx amount of the manifold catalyst 22 and the occluded NOx amount of the underfloor catalyst 23. Set the speed. The rich reduction rate corresponds to the NOx desorption rate described later in [5].
[0034]
[5] Since the NOx occlusion limit of the manifold catalyst and the NOx desorption rate from the catalysts 22 and 23 differ depending on the catalyst temperature, each catalyst temperature is estimated in the same manner as the prior application device (Japanese Patent Application No. 9-1648) The NOx occlusion limit and each NOx desorption rate are determined according to the estimated temperature.
[0035]
This will be further explained with reference to FIG. 2. This is the case of the above [3] (2) and [4] (2). The reduction rate IRS1 is given to the underfloor catalyst 23, and the richening start proportional proportion PRS2 and the rich reduction rate IRS2 are given.
[0036]
As a result, the air-fuel ratio feedback correction coefficient α (O at the time of stoichiometric operation is switched on) at the switching timing from stratified combustion to stoichiometric operation. ₂ Calculated based on the sensor output and clamped to 1.0 except for stoichiometric operation) increases stepwise by the amount of PRS1 + PRS2, and thereafter at a speed of IRS1 + IRS2 (however, after IRS1 becomes 0) α becomes smaller. And O ₂ The process is terminated at a timing when the sensor output matches the slice level, and the routine proceeds to normal air-fuel ratio feedback control.
[0037]
The control contents [1] to [5] executed in the ECM 11 will be described according to the following flowchart.
[0038]
FIG. 3 and FIG. 4 are for performing the calculation of the NOx occlusion amount integrated value and the enrichment process, which are executed at regular time intervals (for example, every 10 ms).
[0039]
In step 1, the ignition key switch (IGN SW) is viewed. If the ignition key switch is ON, the process proceeds to step 2, and the rich catalyst start proportional component PRS1 of the manifold catalyst 22 and the rich catalyst start proportional component PRS2 of the underfloor catalyst 23 are detected. The initial value 0 is set to the rich reduction rate IRS1 of the two catalysts 22, the rich reduction rate IRS2 of the underfloor catalyst 23, the rich amount DALP1 for the manifold catalyst, and the rich amount DALP2 for the underfloor catalyst.
[0040]
In step 3, it is determined whether the stratified combustion condition is satisfied. If it is the stratified combustion condition, the process proceeds to step 4, and a map having the contents shown in FIG. 5 is searched from the engine speed NRPM and the engine target torque TTC at that time. NOx emission amount DNOX (per 10 msec) is obtained. Here, the engine target torque TTC is basically determined from the accelerator opening and the rotational speed. Since the engine target torque TTC is equivalent to a load, another load equivalent value may be used.
[0041]
In step 5, a table having the contents shown in FIG. 6 is retrieved from the estimated manifold catalyst temperature TCAT1, and the NOx storage limit value NOXLIM1 of the manifold catalyst 22 is obtained. As shown in the figure, this limit value NOXLIM1 increases as the estimated manifold catalyst temperature TCAT1 increases and reaches a peak, and further decreases when the estimated manifold catalyst temperature increases.
[0042]
In another flow (not shown), the temperatures of the manifold catalyst 22 and the underfloor catalyst 23 are estimated (refer to Japanese Patent Application No. 9-1648 for details).
[0043]
In step 6, the value obtained by adding the above DNOX to the NOx occlusion amount counter value VNOX1 (initially set to 0 at the start) of the manifold catalyst 22 is compared with the limit value NOXLIM1. If VNOX1 + DNOX <NOXLIM1, it is assumed that all the generated NOx is stored in the manifold catalyst 22, and the value of VNOX1 + DNOX is changed to VNOX1 again in step 7, whereby the NOx storage amount of the manifold catalyst 22 is integrated. When VNOX1 + DNOX ≧ NOXLIM1, NOx is not stored in the manifold catalyst any more, and the generated NOx is stored in the underfloor catalyst 23. In step 8, the NOx occlusion amount counter value VNOX2 of the underfloor catalyst 23 is initialized to 0 at the time of starting. The value obtained by adding DNOX to the setting) is again set to VNOX2, and the NOx occlusion amount of the underfloor catalyst 23 is integrated.
[0044]
In step 9 of FIG. 4, it is determined from the enrichment process flag whether or not the enrichment process is being performed. When the enrichment process is not performed (the enrichment process flag = 0), the process proceeds to step 10 to check whether the enrichment process condition is satisfied. The determination of the enrichment process condition is as follows:
<1> The previous time was stratified combustion conditions,
<2> NOx is stored in the manifold catalyst 22;
<3> Conditions that are considered to have little rebound in drivability, such as during acceleration
Are checked one by one, and when all the conditions are satisfied, it is determined that the enrichment processing condition is satisfied, and the process proceeds to step 11.
[0045]
On the other hand, when it is not stratified combustion condition, it progresses from step 3 of FIG. 3 to step 30 of FIG. 4, and it is checked whether it was the stratified combustion condition last time. When the last time was the stratified combustion condition (that is, at the time of switching from the stratified combustion to the stoichiometric operation), the process proceeds to step 11 in the same manner as when the enrichment processing condition is satisfied.
[0046]
In step 11, the NOx occlusion amount counter value VNOX2 of the underfloor catalyst 23 is compared with 0. When VNOX2 ≠ 0, the process proceeds to Steps 12 and 13, and a map containing FIG. 7 is retrieved from the VNOX2 and the underfloor catalyst estimated temperature TCAT2 to determine the richness start proportional component PRS2 of the underfloor catalyst 23 and the underfloor catalyst estimated temperature. A table having the contents shown in FIG. 8 is retrieved from TCAT 2 to obtain the rich reduction ratio IRS 2 of the underfloor catalyst 23. When VNOX2 = 0, there is no need to consider the underfloor catalyst 23. Therefore, the process proceeds to Steps 14 and 15, and both PRS2 and IRS2 are set to 0.
[0047]
In steps 16 and 17, as in steps 12 and 13, a map containing FIG. 9 is retrieved from the NOx occlusion amount counter value VNOX1 of the manifold catalyst 22 and the estimated manifold catalyst temperature TCAT1, and the manifold catalyst enrichment process start proportionality is retrieved. A table containing the contents shown in FIG. 10 is retrieved from the minute PRS1 and the estimated manifold catalyst temperature TCAT1, and the rich reduction ratio IRS1 of the manifold catalyst 22 is obtained.
[0048]
The proportional amounts PRS1 and PRS2 thus obtained are transferred to the corresponding manifold catalyst rich amount DALP1 and underfloor catalyst rich amount DALP2 in step 18, respectively, and in step 19, the sum of these rich amounts DALP1 and DALP2 is added to α. Update α by changing the value to α. Subsequently, in order to perform the enrichment process, the enrichment process flag is set to 1 in step 20. Note that in step 18, 0 is set in VNOX1 and VNOX2 in preparation for the next enrichment process.
[0049]
By setting the rich processing flag to “1”, as long as the stoichiometric operation continues (or as long as the rich processing conditions are not deviated), the process proceeds from step 9 to step 21 and the manifold rich amount DALP1 and the manifold catalyst. 22 rich reduction ratio IRS1 is compared. DALP1 at this timing is the previous value, and when the process proceeds to step 21 at the next timing when the enrichment process flag = 1, DALP1> IRS1, so the process proceeds to step 22, and DALP1 (this is the previous value) The value further reduced by IRS1 is again set to DALP1 (this is the current value).
[0050]
Steps 24 and 25 are the same as steps 21 and 22. That is, for the underfloor catalyst 23, the rich amount DALP2 is compared with the rich reduction ratio IRS2, and when DALP2> IRS2, the process proceeds to step 25, and the value that is decreased by IRS2 from DALP2 (this is the previous value) is changed to DALP2 ( This is the value this time).
[0051]
In step 28, α is updated by substituting α for the value obtained by subtracting the sum of the rich reduction ratios IRS2 and IRS1 from α.
[0052]
Each time the operations in steps 22 and 25 are continued, the manifold catalyst rich amount DALP1 and the underfloor catalyst rich amount DALP2 gradually decrease. When DALP1 ≦ IRS1 or DALP2 ≦ IRS2 is eventually reached, the process proceeds to steps 23 and 26. , DALP1, IRS1, DALP2, and IRS2 are all set to 0.
[0053]
However, when the operation in step 26 is performed, it is checked in step 27 whether DALP1 is also 0. If DALP1 is also 0 in addition to DALP2, the process proceeds to step 29 and the enrichment processing flag = 0. And (end the enrichment process).
[0054]
Note that air-fuel ratio feedback control is performed in a flow (not shown) after the end of the enrichment process.
[0055]
Thus, in the present embodiment, the manifold catalyst 22 having a relatively small catalyst capacity and the underfloor catalyst 23 having a relatively large catalyst capacity are arranged in this order from the upstream side, and the NOx occlusion amount of the manifold catalyst 32 during stratified combustion. If the accumulated NOx storage amount of the manifold catalyst 22 reaches the storage limit, the integrated NOx storage amount of the underfloor catalyst 23 is calculated, and only the manifold catalyst 22 stores NOx at a predetermined enrichment processing timing. Is set in proportion to the amount of NOx stored, and the rich reduction rate IRS1 thereafter is set, and when the NOx cannot be stored in the manifold catalyst 22 and is stored in the underfloor catalyst 23, The enrichment start proportional proportion and the subsequent rich reduction rate are set according to the sum of the stored NOx amount of the manifold catalyst 22 and the stored NOx amount of the underfloor catalyst 23. If it is the state that the stored NOx amount of the manifold catalyst has reached the storage limit as before, the conventional device with only one NOx storage type catalyst and a large catalyst capacity is provided. In comparison with the conventional example, the amount of proportional increase in the start of the enrichment process is smaller in the present embodiment as the catalyst capacity is smaller, and thus torque fluctuations associated with the enrichment process can be suppressed as compared with the prior art (the rebound of drivability is less).
[0056]
Further, in the present embodiment, the amount of stored NOx is smaller than that of a conventional device that needs to perform enrichment every time the storage limit is reached when only one NOx storage type catalyst is provided and the catalyst capacity is small. Since it is not necessary to perform the enrichment process every time the storage limit is reached, the fuel efficiency is not deteriorated.
[0057]
In this embodiment, a catalyst having a considerably large capacity is employed as the underfloor catalyst 23 so that the integrated amount of NOx occlusion in the underfloor catalyst 23 does not reach the storage limit. However, the NOx to the underfloor catalyst 23 is used. When it is necessary to consider the case where the storage amount integrated value reaches the storage limit, as in the case of the conventional device, when the NOx storage amount integrated value to the underfloor catalyst 23 reaches the storage limit, the enrichment process is performed. Good.
[0058]
The flowchart of FIG. 11 is a second embodiment and corresponds to FIG. 4 of the first embodiment. The same steps as those in FIG. 4 are given the same step numbers.
[0059]
In the second embodiment, the amount of HC and CO required for conversion of NOx stored in each of the catalysts 22 and 23 is calculated according to the NOx storage amount counter value and the intake air flow rate Qa at that time. , The proportion of start of the enrichment process is calculated according to the amount of CO. In other words, the second embodiment eliminates the need for a map search when determining the proportion of the enrichment processing start, thereby reducing the memory capacity.
[0060]
More specifically, when VNOX2 ≠ 0 in Step 11, the process proceeds to Steps 41 and 42, from the NOx occlusion amount counter value VNOX2 of the underfloor catalyst 23 and the intake air flow rate Qa at that time.
VHCCO2 = VNOX2 / (KCO × Qa) (1)
KCO: CO specific gravity coefficient (constant value)
The required HC and CO amount VHCCO2 of the underfloor catalyst 23 is obtained by the following equation, and from this VHCCO2,
PRS2 = VHCCO2 × KK (2)
KK: Conversion efficiency factor
The proportion PRS2 for starting enrichment of the underfloor catalyst 23 is calculated by the following equation.
[0061]
In steps 43 and 44, as in steps 41 and 42, from the NOx occlusion amount counter value VNOX1 of the manifold catalyst 22 and the intake air flow rate Qa at that time,
VHCCO1 = VNOX1 / (KCO × Qa) (3)
However, KCO: CO specific gravity coefficient
The required HC and CO amount VHCCO1 of the manifold catalyst are obtained by the following formula:
PRS1 = VHCCO1 × KK (4)
KK: Conversion efficiency factor
Based on the following formula, the proportion PRS1 for starting the enrichment process of the manifold catalyst is calculated.
[0062]
Here, as in the formulas (1) and (3), the required HC and CO amounts (VHCCO2, VHCCO1) are obtained in inverse proportion to Qa because the same amount of HC and CO is supplied to the catalyst. This is because as the air flow rate Qa is larger, the proportion of the enrichment process start is smaller.
[0063]
The above KK is obtained by matching or calculation.
[0064]
On the other hand, when it is not the enrichment processing condition, it is not necessary to calculate VHCCO2 and VHCCO1, so the process proceeds from step 10 to steps 45 and 46, where VHCCO2 = VHCCO1 = 0.
[0065]
As described above, in the second embodiment, the map search is not required when obtaining the enrichment process start proportional proportions RPS1 and PRS2, and the memory capacity can be reduced.
[0066]
The flowchart of FIG. 12 is a third embodiment and corresponds to FIG. 4 of the first embodiment. The same steps as those in FIG. 4 are given the same step numbers.
[0067]
In the first embodiment, since the NOx occlusion amount is obtained by calculation, if the actual NOx occlusion amount is larger than the calculated value, the amount of HC and CO supplied to the catalyst by the enrichment process is insufficient. It is conceivable that the amount of desorbed NOx corresponding to the amount of HC and CO that was insufficient is discharged as it is.
[0068]
Therefore, in the third embodiment, a delay time DIRS is provided at the start of the subtraction of the rich amount so that a slightly larger amount of HC and CO is supplied in anticipation that this HC and CO amount may be insufficient. It is a thing. Since the amount of HC and CO supplied by the enrichment process corresponds to the area shown by hatching in FIG. 2, the amount of HC and CO supplied by the enrichment process is increased by giving the delay time DIRS as shown in FIG. Thus, desorbed NOx can be reliably converted.
[0069]
Specifically, when the enrichment process start timing is reached in FIG. 12, the NOx occlusion amount counter value VNOX1 of the manifold catalyst 22 and the NOx occlusion amount counter value VNOX2 of the underfloor catalyst 23 are used in step 51.
DIRS = (VNOX1 × KDLY1 + VNOX2 × KDLY2) / Qa
However, KDLY1: Delay coefficient for the manifold catalyst 22
KDLY2: Delay coefficient for the underfloor catalyst 23
The rich weight loss delay time DIRS is calculated by the following formula, and the current process is terminated.
[0070]
From the next time, the process proceeds from step 9 to step 52. Compare this rich reduction delay time DIRS with 0, and while DIRS> 0, proceed to step 53 to decrement DIRS by ΔD and hold α at step 54 (The previous value is the current value.) When DIRS ≦ 0, the process proceeds to step 21 and subsequent steps.
[0071]
On the other hand, when it is not the enrichment processing condition, the process proceeds from step 10 to step 55 and DIRS = 0.
[0072]
As described above, in the third embodiment, since the amount of HC and CO can be supplied by the delay time DIRS, the lack of conversion of desorbed NOx when the actual NOx storage amount is larger than the calculated value of the NOx storage amount is solved. it can.
[0073]
In the third embodiment, the case where the delay time is given to the start of the subtraction of the rich amount is explained with respect to the first embodiment, but the delay time is given to the start of the subtraction of the rich amount as compared to the second embodiment. You can also Further, the delay period may be used instead of the delay time.
[0074]
In the embodiment, the case has been described in which the air-fuel ratio is largely switched between when the stratified combustion condition is satisfied and when it is not satisfied. For example, the operating range when the stratified combustion condition is not satisfied (that is, when the homogeneous combustion condition is satisfied) is further divided into an operating range centered on stoichiometric and a range operating with a lean air-fuel ratio such as 20-25. The present invention can also be applied. That is, in the present invention, the case of lean operation includes not only stratified combustion but also operation at a lean air-fuel ratio such as 20 to 25 when the homogeneous combustion condition is satisfied.
[Brief description of the drawings]
FIG. 1 is a control system diagram of a first embodiment.
FIG. 2 is a waveform diagram for explaining air-fuel ratio enrichment processing;
FIG. 3 is a flowchart for explaining calculation of NOx occlusion amount and enrichment processing;
FIG. 4 is a flowchart for explaining calculation of NOx occlusion amount and enrichment processing.
FIG. 5 is a characteristic diagram of NOx emission amount DNOX per calculation time.
FIG. 6 is a characteristic diagram of a NOx storage limit value NOXLIMT1 of the manifold catalyst.
FIG. 7 is a characteristic diagram of the proportion PRS2 for starting the underfloor catalyst enrichment process.
FIG. 8 is a characteristic diagram of an underfloor catalyst rich reduction ratio IRS2.
FIG. 9 is a characteristic diagram of a manifold catalyst enrichment start proportional proportion PRS1.
FIG. 10 is a characteristic diagram of a rich reduction ratio IRS1 of the manifold catalyst.
FIG. 11 is a flowchart for explaining calculation and enrichment processing of a NOx occlusion amount according to the second embodiment.
FIG. 12 is a flowchart for explaining calculation and enrichment processing of a NOx occlusion amount according to the third embodiment.
FIG. 13 is a waveform diagram for explaining the air-fuel ratio enrichment process of the third embodiment.
FIG. 14 is a view corresponding to a claim of the first invention.
[Explanation of symbols]
5 Injector
11 ECM
22 Manifold catalyst (upstream catalyst)
23 Underfloor catalyst (downstream catalyst)

Claims (6)

While a plurality of NOx storage-type catalysts are arranged in series in the exhaust passage,
Means for calculating the NOx occlusion amount of the upstream catalyst during the lean operation, and if the NOx occlusion amount of the upstream catalyst during the lean operation reaches the occlusion limit, means for calculating the NOx occlusion amount during the lean operation of the downstream catalyst;
If only the upstream catalyst stores NOx when the predetermined enrichment processing timing is reached, the enrichment level is set according to the stored NOx amount and the catalyst temperature of the upstream catalyst, and the upstream catalyst is also stored. In addition, when NOx is occluded in the downstream catalyst, it is enriched according to the amount of NOx stored in the upstream catalyst and the catalyst temperature of the upstream catalyst, and the amount of NOx stored in the downstream catalyst and the catalyst temperature of the downstream catalyst. Means to set the degree,
An engine exhaust gas purification apparatus comprising means for performing an air-fuel ratio enrichment process using the degree of enrichment. 2. The engine exhaust gas purification apparatus according to claim 1, wherein the predetermined enrichment processing timing is when switching from lean operation to stoichiometric operation or when it is considered that there is little rebound in drivability. 3. The engine exhaust gas purification apparatus according to claim 1, wherein a catalyst having a relatively small catalyst capacity is an upstream catalyst, and a catalyst having a relatively large catalyst capacity is a downstream catalyst. The engine exhaust gas purification apparatus according to any one of claims 1 to 3, wherein the NOx occlusion limit of the upstream catalyst is set according to the catalyst temperature. The amount of HC and CO required for the conversion of NOx stored in the NOx storage type catalyst is calculated according to the NOx storage amount and the intake air flow rate at that time, and the degree of enrichment is calculated according to the required amount of HC and CO. The engine exhaust gas purification apparatus according to any one of claims 1 to 4, wherein the engine exhaust gas purification apparatus is set. When calculating the NOx occlusion amount and subtracting at a predetermined speed after changing the air-fuel ratio at the start of the enrichment process by setting the enrichment degree, there should be a delay from the step change to the start of the subtraction. The engine exhaust gas purification device according to any one of claims 1 to 5.

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