JP3829422B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine provided with a NOx absorption catalyst.
[0002]
[Prior art]
Conventionally, as disclosed in, for example, Japanese Patent Application Laid-Open No. 6-117221, a technology for adsorbing NOx generated during lean operation by a NOx absorption catalyst is known as an exhaust purification device for an internal combustion engine that is operated with a lean air-fuel mixture.
[0003]
This is a NOx absorption catalyst capable of adsorbing NOx in the exhaust passage of the internal combustion engine. When this catalyst is operated with a rich air-fuel mixture, NOx adsorbed during lean operation is desorbed and exhausted. NOx desorbed by HC and CO contained in is reduced. Therefore, when switching from lean operation to operation with stoichiometric air-fuel mixture (theoretical air-fuel ratio), the air-fuel mixture is temporarily made richer than the stoichiometric air-fuel ratio (rich spike) to desorb adsorbed NOx and be harmless. It is reduced (converted) to a stable state and released into the atmosphere.
[0004]
[Problems to be solved by the invention]
By the way, in a system in which two catalysts, that is, a manifold catalyst of the exhaust manifold and an underfloor catalyst disposed under the floor of the vehicle downstream of the exhaust pipe are arranged in series as NOx absorption catalysts, they are adsorbed during lean operation. Even if a rich spike is performed to desorb and reduce the NOx, the NOx adsorbed on both catalysts is surely desorbed due to the temperature difference between the manifold catalyst and the underfloor catalyst, the degree of deterioration of the catalyst, etc. It cannot be reduced.
[0005]
For example, as shown in FIG. 15, when the air-fuel mixture is switched from the lean operation to stoichiometric, NOx adsorbed on the manifold catalyst and the underfloor catalyst during the lean operation is desorbed. However, in the stoichiometric mixture, HC and CO necessary for reducing the desorbed NOx are insufficient, so that most of the NOx is discharged to the outside as it is.
[0006]
On the other hand, as shown in FIG. 16, when the rich spike is performed only once at the time of switching from the lean operation to the stoichiometric operation, HC and CO in the exhaust gas increase, and the desorbed NOx is reduced and left outside as it is. The amount of NOx discharged is reduced.
[0007]
However, HC and CO generated by one rich spike are used exclusively for the reduction of NOx of the upstream high-temperature manifold catalyst and do not contribute much to the reduction of NOx desorbed by the downstream underfloor catalyst.
[0008]
The NOx desorption characteristics are different between the high temperature manifold catalyst and the lower temperature underfloor catalyst as shown in FIG. 15, and NOx is desorbed in a short time on the high temperature side. Separation takes a long time.
[0009]
Therefore, even if a rich spike is inserted, even if the upstream catalyst reacts with good response, the downstream catalyst does not desorb sufficiently because the reaction is slow in the downstream catalyst. In addition, if one rich spike is increased so that NOx is sufficiently desorbed and reduced by the downstream catalyst, HC and CO that pass through the downstream catalyst before the downstream side is desorbed also come out. , They are discharged outside without burning.
[0010]
As described above, in the method of inserting one rich spike, the NOx desorption time and timing differ depending on the temperature of the manifold catalyst and the underfloor catalyst, the degree of deterioration, etc. It was difficult to satisfy the reduction.
[0011]
The present invention has been proposed in order to solve such problems, and performs rich spikes corresponding to the number of catalysts to achieve efficient NOx desorption and reduction for each catalyst. With the goal.
[0012]
[Means for Solving the Problems]
The first invention is arranged in series in the exhaust passage, and adsorbs NOx when the inflowing air-fuel mixture is lean, and desorbs and reduces NOx adsorbed when it is rich, and a plurality of NOx absorption catalysts, In an exhaust gas purification apparatus for an internal combustion engine, comprising an air-fuel ratio control means that temporarily enriches an air-fuel mixture when switching from lean air-fuel mixture operation to stoichiometric air-fuel mixture operation and performs rich spike to promote NOx desorption and reduction, Dividing means for dividing the rich spike into a plurality of times corresponding to the number of the catalysts is provided.
[0013]
In the second invention, the NOx absorption catalyst is composed of a manifold catalyst and an underfloor catalyst, and the dividing means performs two rich spikes.
[0014]
According to a third aspect of the present invention, there is provided means for estimating the temperature of the catalyst from the operating state, and means for correcting the cycle or size of the rich spike or the cycle and size according to the estimated temperature of the catalyst.
[0015]
According to a fourth aspect of the invention, there is provided means for detecting the actual temperature of the catalyst and means for correcting the cycle or magnitude of the rich spike or the period and size in accordance with the actual temperature of the catalyst.
[0016]
The fifth invention comprises means for diagnosing the degree of deterioration of the catalyst and means for correcting the cycle or size of the rich spike or the period and size according to the degree of catalyst deterioration.
[0017]
According to a sixth aspect of the invention, a NOx sensor provided near the outlet of the downstream catalyst, a means for learning and updating the response characteristics of the catalyst in accordance with the output of the NOx sensor, and a rich spike cycle based on the learned value. Means for correcting are provided.
[0018]
[Operation and effect of the invention]
In the first aspect of the present invention, NOx is desorbed and reduced by introducing a rich spike at the time of switching from the lean gas mixture operation to the stoichiometric operation. This rich spike is divided into a plurality of times corresponding to the number of NOx absorption catalysts that are interposed in series in the exhaust passage, and accordingly, an optimum period (timing) corresponding to the situation of each catalyst from upstream to downstream. NOx adsorbed on each catalyst can be desorbed and reduced reliably and efficiently, and it prevents unnecessary enrichment of the air-fuel mixture, resulting in deterioration of fuel consumption and emission of HC and CO. Increase in volume can be avoided.
[0019]
In the second invention, NOx can be reliably desorbed from both catalysts and reduced by two rich spikes corresponding to the NOx desorption characteristics of the upstream manifold catalyst and the downstream underfloor catalyst.
[0020]
In the third invention, the desorption / reduction characteristics of the adsorbed NOx change according to the temperature of the catalyst, but the period and the size of the rich spike are corrected by estimating the temperature of the catalyst. Can be eliminated and reduced.
[0021]
In the fourth invention, the actual temperature of the catalyst is detected and the cycle and size of the rich spike are corrected, so that it is possible to desorb and reduce NOx even more accurately.
[0022]
In the fifth aspect of the invention, the NOx adsorption performance, desorption, and reduction characteristics also change according to the degree of deterioration of the catalyst. Accordingly, the deterioration of the catalyst is diagnosed, and the cycle and size of the rich spike are determined in accordance with the deterioration state. By correcting, NOx adsorption, desorption and reduction can be stably performed even if the catalyst is deteriorated.
[0023]
In the sixth aspect of the invention, the actual catalyst response characteristics are learned and updated based on the output of the NOx sensor, and the rich spike cycle is corrected according to the learned value, so that the catalyst performance can always be maintained in the best state. .
[0024]
Embodiment
The best mode for carrying out the present invention will be described below with reference to the drawings.
[0025]
In FIG. 1, reference numeral 15 denotes an engine body, 16 denotes an intake passage, and 17 denotes an exhaust passage. The engine body 15 is provided with a fuel injection valve 3 that directly injects fuel into the combustion chamber. The fuel injection valve 3 injects fuel in the latter half of the compression stroke, such as in the engine low load region, and forms a combustible air-fuel mixture only in the vicinity of the spark plug 4 near the compression top dead center. / F = 40 Super-lean air-fuel mixture is stably stratified, and fuel is injected in the intake stroke in high engine load ranges, and fuel and air are premixed throughout the combustion chamber. Homogeneous combustion is performed near the air-fuel ratio.
[0026]
In the exhaust passage 17, two catalysts, an exhaust manifold catalyst 7 and an underfloor catalyst 8 arranged on the lower surface of the vehicle floor, are provided in series as NOx absorption catalysts, and adsorb NOx generated during operation with a lean air-fuel mixture. To do. These catalysts 7 and 8 desorb NOx adsorbed when the rich air-fuel mixture is supplied, and at the same time reduce NOx by the action of HC and CO contained in the exhaust gas. The enrichment control of the fuel injection amount of the injection valve 3 is performed.
[0027]
The controller 14 serving as the air-fuel ratio control means includes an intake air amount signal detected by the air flow meter 1, a throttle opening signal detected by the throttle valve opening sensor 2, a crank angle signal detected by the crank angle sensor 5, and an engine coolant temperature. The cooling water temperature signal detected by the sensor 6, the catalyst temperature signal of the temperature sensors 9, 10 for detecting the inlet exhaust temperature of the catalysts 7, 8, and the NOx concentration of the NOx sensor 11 for detecting the NOx concentration at the outlet of the downstream catalyst 8 A fuel pressure signal from a fuel pressure sensor 12 that detects a signal, a fuel pressure injected from the fuel injection valve 3, and the like, and based on these, a fuel injection amount so as to perform stratified combustion and homogeneous combustion according to the operating state The injection timing is controlled, and the rich spike divided into two times is performed at the time of switching from the lean operation to the stoichiometric operation, Elimination of wear have been NOx, perform the reduction.
[0028]
The rich spike will be described in detail with reference to the flowchart of FIG.
[0029]
In step 1, temperatures Temp1 and Temp2 at the upstream and downstream catalyst inlets are detected. In step 2, the proportional amounts P1 and P2 of the air-fuel ratio control for each catalyst corresponding to the depth of the rich spike as shown in FIG. 3 are based on, for example, temperatures Temp1 and Temp2 detected from the map shown in FIG. Set each.
[0030]
As can be seen from FIG. 15, the NOx desorption characteristics differ between the high temperature upstream catalyst and the low temperature downstream catalyst. When the catalyst inlet temperature is high, the NOx desorption time is short and the desorption amount is low. The peak tends to be higher. Therefore, P1 and P2 take larger values as the temperature is higher.
[0031]
In step 3, integrals I1 and 12 corresponding to the slope of the rich spike for each catalyst are set based on the temperatures Temp1 and Temp2 of each catalyst from the map shown in FIG. 5, for example. Since the NOx desorption time becomes shorter as the catalyst temperature is higher, the slope becomes steeper.
[0032]
Next, in step 4, the interval (cycle) T1 between the first rich spike and the second rich spike is set based on the downstream catalyst temperature Temp2 from the map shown in FIG.
[0033]
Further, in step 5, the proportional part P3 which is the depth of the second rich spike is calculated as P3 = P2- (P1-T1 · I1).
[0034]
Based on these, two rich spikes are performed when switching from lean operation to stoichiometric operation, which will be described with reference to FIG.
[0035]
NOx generated during lean operation is adsorbed by the upstream and downstream catalysts 7 and 8, and when switching from lean operation to stoichiometric operation, a rich spike that makes the air-fuel ratio temporarily richer than stoichiometric is produced. By inserting, it is desorbed from the catalysts 7 and 8 and is reduced by reacting with HC and CO in the exhaust.
[0036]
The desorption characteristics of NOx from the catalysts 7 and 8 differ depending on the temperature of the catalysts 7 and 8, and the higher the catalyst temperature, the shorter the desorption time and the higher the desorption amount peak. Therefore, the desorption of the catalyst 7 having a high upstream temperature is usually abrupt and the desorption of the catalyst 8 having a low downstream temperature is slow.
[0037]
Depending on the temperature of each of these catalysts 7 and 8, the depth and slope of the rich spike performed in two steps are set respectively, and the first rich spike targeting the upstream catalyst 7 is deep and the slope becomes steep. On the other hand, the second rich spike targeting the downstream catalyst 8 has a shallower depth and a gentler inclination than the first rich spike.
[0038]
Further, since the NOx desorption time is earlier as the temperature of the catalyst 8 is higher, the interval (cycle) between the first rich spike and the second rich spike is set to be shorter accordingly.
[0039]
As a result, the first and second rich spikes correspond to the NOx desorption characteristics of the upstream and downstream catalysts 7 and 8, and the supply of HC and CO is performed in accordance with the desorption of NOx. NOx is reliably desorbed from 7 and 8, and the reduction is efficiently performed, and the amount of NOx released to the outside can be reduced.
[0040]
In addition, as a result of the rich spike being accurately performed without excess or deficiency, unnecessary fuel is not supplied unnecessarily, fuel efficiency is improved, and HC and CO are discharged outside without being burned. Disappear.
[0041]
In this embodiment, the inlet temperatures of the catalysts 7 and 8 are detected by the temperature sensors 9 and 10, but the catalyst temperature is estimated from the operating state such as the intake air amount and the engine speed, and based on this. In this case, the temperature sensors 9 and 10 are unnecessary, and the system can be simplified accordingly.
[0042]
Next, a second embodiment shown in FIG. 7 will be described.
[0043]
As the NOx absorption catalyst deteriorates, the amount of NOx absorbed decreases, and the desorption time also becomes faster. Therefore, unless the size and cycle of the rich spike are corrected according to the degree of deterioration, proper desorption and reduction can be achieved. become unable.
[0044]
For this reason, as shown also in the flowchart of FIG. 7, the catalyst deterioration is judged, and thereby the rich spike is corrected.
[0045]
First, in step 1, the degree of catalyst deterioration is determined. The deterioration of the catalyst is judged by installing O ₂ sensors upstream and downstream of the catalyst and comparing the outputs of the respective O ₂ sensors, or by judging the elapsed time, total distance, etc. after installing the catalyst. To do. In the case where _two O ₂ sensors are provided, it can be judged from the fact that, as the catalyst progresses, the storage capacity of O ₂ decreases and the change characteristics of the upstream and downstream O ₂ concentrations match.
[0046]
In step 2, the correction amount α for the interval (cycle) T1 between the first rich spike and the second rich spike is retrieved from the map shown in FIG. 8, for example, according to the degree of deterioration.
[0047]
Similarly, in step 3, the correction amount β for Pn, which is the proportional portion of the rich spike, is obtained from a map as shown in FIG. 9, and in step 4, the correction amount γ for the integral portion In is shown in FIG. Obtain from the map as shown.
[0048]
Each correction amount is set so that the amount of NOx adsorbed decreases as the catalyst progresses and the desorption time of adsorbed NOx increases, so that the rich spike is small and the mutual interval is shortened accordingly. That is, each correction amount increases as the degree of deterioration increases.
[0049]
When the correction amounts α, β, γ according to the deterioration state of the catalyst are obtained in this way, in step 5,
T1 ′ = T1-α
Pn ′ = Pn−β
In ′ = In−γ
As described above, the period Ti ′, the proportional component Pn ′, and the integral component In ′ of each rich spike are calculated (where n represents the number of rich spikes).
[0050]
As a result, when the characteristics of the catalyst change over time, that is, deteriorate, the amount of NOx adsorbed correspondingly decreases with the size of the rich spike and the interval becoming shorter. As a result, an appropriate rich spike can be performed to shorten the desorption time, and NOx can be desorbed and reduced reliably and appropriately without unnecessarily supplying excess fuel.
[0051]
Next, a third embodiment shown in FIG. 11 will be described.
[0052]
This is to detect the actual NOx concentration at the outlet of the downstream underfloor catalyst, update the learning value of the cycle T1 set corresponding to the catalyst temperature, and correct the shift of the rich spike cycle (interval). It is.
[0053]
First, when the first rich spike P1 is inserted in step 1, the timer Timer starts counting in step 2. In step 3, the temperature Temp2 of the underfloor catalyst is detected.
[0054]
In step 4, it is determined whether or not a difference ΔNOx (= NOx _n−1 −NOx _n ) between the detected value NOx _n of the NOx sensor and the previous value is greater than zero. And when the ΔNOx ≧ 0, but it returns to return, otherwise, that is, when the current value NOx _n than the previous value NOxN _n-1 becomes greater, the minimum value of NOx emissions proceeds to step 5 NOxnim As a result, the previous detection value NOx _n-1 is set.
[0055]
Next, in step 6, it is judged again whether ΔNOx ≧ 0, and waits until ΔNOx becomes larger than 0. If it becomes larger, the routine proceeds to step 7 where the maximum value NOxmax of NOx emission amount is set as the previous NOx emission amount. Set the value NOx _n-1 . The timer count value Timer = Tα at that time is set.
[0056]
As shown in FIG. 12, this Tα represents the delay time from when the rich spike P1 is inserted until the actual amount of NOx discharged from the downstream catalyst reaches its peak. However, in this delay time Tα, in addition to the response delay time of the catalyst, there is a delay time TM until the exhaust actually reaches the downstream catalyst. As shown in FIG. 13, the delay time TM is proportional to the distance from the engine to the underfloor catalyst, and changes according to the engine speed and the intake air amount at that time.
[0057]
Next, in step 8, it is determined whether NOxmax−NOxmin ≧ A. This A is a determination value of the NOx increase amount, and when the difference between NOxmax and NOxmin is larger than the predetermined value A, it is determined that the NOx desorption peak of the downstream side catalyst has come, and the routine proceeds to Step 9. The actual peak actual delay time TB is obtained as TB = Tα−TM. TM is obtained from the map 14 based on the engine speed rpm and the intake air amount (fuel injection pulse width) TP.
[0058]
When the peak delay time TB resulting from the actual catalyst responsiveness is calculated in this way, in step 10, the cycle (interval) between the first rich spike and the second rich spike is calculated from the map shown in FIG. ) ΔT is calculated as a correction value of T1. In step 11, ΔTNEW = ΔTOLD + ΔT is set as a learning value of ΔT.
[0059]
As is apparent from FIG. 14, ΔT is a negative value if TB is a positive value. Therefore, as peak delay time TB increases on the positive side, correction value ΔT increases on the negative side. Then, the second rich spike interval T1 is corrected to be shorter.
[0060]
In step 12, the learning value ΔTNEW is updated with a grid assigned in accordance with the current temperature of the underfloor catalyst, and the interval between the first and second rich spikes is corrected, that is, learned and updated.
[0061]
In this way, in the present embodiment, the timing of inserting the second rich spike is corrected corresponding to the actual response delay of the downstream underfloor catalyst, so that the rich spike can always be inserted in the best state. This makes it possible to improve the exhaust composition while preventing excessive fuel supply.
[0062]
In each of the above embodiments, as an NOx absorption catalyst, an example in which two catalysts, a manifold catalyst and an underfloor catalyst, are arranged in series has been shown. In that case, the number of rich spikes is divided and set to match the number of catalysts.
[Brief description of the drawings]
FIG. 1 is an overall schematic configuration diagram showing an embodiment of the present invention.
FIG. 2 is a flowchart showing the same control content.
FIG. 3 is a characteristic explanatory diagram showing NOx emission characteristics when a rich spike is performed.
FIG. 4 is an explanatory diagram showing a proportional portion of a rich spike.
FIG. 5 is an explanatory diagram showing an integral part of a rich spike.
FIG. 6 is an explanatory diagram showing a cycle of rich spikes.
FIG. 7 is a flowchart showing the control content of another embodiment.
FIG. 8 is an explanatory diagram showing a correction amount of a rich spike cycle.
FIG. 9 is an explanatory diagram showing a correction amount corresponding to the proportion of rich spikes.
FIG. 10 is an explanatory diagram illustrating a correction amount corresponding to rich spike integration.
FIG. 11 is a flowchart showing the contents of control in still another embodiment.
FIG. 12 is a characteristic explanatory diagram showing a relationship between rich spikes and NOx emission characteristics;
FIG. 13 is a characteristic explanatory diagram showing a delay time.
FIG. 14 is an explanatory diagram showing a period correction amount of a rich spike.
FIG. 15 is an explanatory diagram showing NOx emission characteristics when the air-fuel ratio is switched from lean to stoichiometric.
FIG. 16 is an explanatory diagram showing NOx emission characteristics when a conventional rich spike is performed.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Air flow meter 2 Throttle valve opening 3 Fuel injection valve 4 Spark plug 7 Exhaust manifold catalyst 8 Underfloor catalyst 11 NOx sensor 14 Controller 15 Engine main body 16 Intake passage 17 Exhaust passage

Claims (6)

A plurality of NOx absorption catalysts that are arranged in series in the exhaust passage and adsorb NOx when the inflowing air-fuel mixture is lean and desorb and reduce the adsorbed NOx when it is rich, and the stoichiometric operation from the lean air-fuel mixture operation In an exhaust gas purification apparatus for an internal combustion engine equipped with an air-fuel ratio control means for facilitating a rich spike by temporarily enriching the air-fuel mixture when switching to the air-fuel mixture operation and promoting NOx desorption and reduction, corresponding to the number of catalysts An exhaust gas purification apparatus for an internal combustion engine, comprising a dividing means for dividing the rich spike into a plurality of times. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the NOx absorption catalyst includes a manifold catalyst and an underfloor catalyst, and the dividing means performs two rich spikes. 3. The internal combustion engine according to claim 1, further comprising: means for estimating the temperature of the catalyst from an operating state; and means for correcting a cycle or size of the rich spike or a cycle and size according to the estimated temperature of the catalyst. Exhaust purification equipment. 3. The internal combustion engine according to claim 1, comprising means for detecting an actual temperature of the catalyst, and means for correcting the cycle or the magnitude of the rich spike or the cycle and the magnitude according to the actual temperature of the catalyst. Exhaust purification equipment. The exhaust purification of an internal combustion engine according to claim 1 or 2, comprising means for diagnosing the degree of deterioration of the catalyst, and means for correcting the period or magnitude of the rich spike or the period and magnitude according to the degree of catalyst deterioration. apparatus. A NOx sensor provided near the outlet of the downstream catalyst, means for learning and updating the response characteristics of the catalyst according to the output of the NOx sensor, and means for correcting the cycle of the rich spike based on the learned value. Item 6. An exhaust emission control device for an internal combustion engine according to any one of Items 1 to 5.

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