JP2004100476A - Engine exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To assure a sufficient amount of reducing agent for richening even under a low load condition with low exhaust flow rate is naturally low. <P>SOLUTION: This engine exhaust emission control device comprises a NOx trap catalyst (10) provided at an exhaust passage; main injection implementation means (7, 21) capable of selectively performing homogeneous combustion or stratified charge combustion at lean operation by changing the timings of main injections of a fuel injection means (7) for injecting fuel directly into a combustion chamber; additional injection implementation means (7, 21) for performing additional injection in an expansion stroke or exhaust stroke in a stratified charge combustion condition when catalyst temperature reaches activation temperature when richening is required in the stratified charge combustion condition; and richening operation means (7, 21) which presets an air-fuel ratio determined by a total of fuel amount and suction air amount of the additional injection and the main injection on richer side than a theoretical air-fuel ratio in performing the additional injection, thus performing the operation at an air-fuel ratio preset on the rich side. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an engine, and more particularly to an apparatus for operating an engine lean (lean mixture).
[0002]
[Prior art]
When the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, NOx is trapped. When the air-fuel ratio of the exhaust gas becomes richer than the stoichiometric air-fuel ratio or the stoichiometric air-fuel ratio, NOx trapped in the catalyst is desorbed, A catalyst having a function of reducing and purifying the desorbed NOx using unburned components (unburned HC and CO) in the exhaust gas as a reducing agent is provided in the exhaust passage, and operation at a lean air-fuel ratio (this operation If it is determined that the NOx trap amount has reached the limit during the “lean operation”, the air-fuel ratio of the exhaust gas is enriched to the stoichiometric air-fuel ratio (or an air-fuel ratio richer than the stoichiometric air-fuel ratio). Various processes for enriching the fuel ratio (hereinafter simply referred to as "enrichment process") have been proposed (see JP-A-6-10725 and JP-A-6-294319).
[0003]
[Problems to be solved by the invention]
By the way, when the enrichment process is performed to desorb and reduce NOx trapped in the catalyst, it is necessary to introduce a necessary amount of reducing component to the catalyst in accordance with the amount of desorbed NOx. Since the exhaust gas flow rate is originally small at the time of low load during operation, even if the enrichment process is performed during the lean operation at this low load, the amount of the reducing component may be insufficient and the NOx purification rate may deteriorate. Therefore, in order to obtain a sufficient NOx purification rate, conventionally, it has been general to perform the enrichment processing at the start of acceleration with a relatively large exhaust flow rate. Therefore, the enrichment process could not be performed until the exhaust gas flow rate was large and the required reduction component was able to reach the catalyst by the enrichment process and the operation state was such that a sufficient NOx purification rate was obtained. Conversely, if the NOx trapping amount is large before the enrichment process is performed, the NOx trapping rate decreases, and the NOx amount released to the atmosphere without being trapped due to the decrease in the NOx trapping rate. Was supposed to increase.
[0004]
On the other hand, in an engine having a fuel injection means for directly injecting fuel into the combustion chamber, a stratified combustion region is provided on the low load side in the lean operation region (the rest is a homogeneous combustion region), and the stratified combustion region on the low load side is provided. As a result, a reduction in pump loss and a reduction in cooling loss due to a decrease in combustion temperature have been realized, thereby further improving fuel efficiency.
[0005]
The inventor of the present invention has obtained the following knowledge when considering a combination of an engine capable of selectively performing such stratified combustion and homogeneous combustion in a lean operation range and desorption / reduction of trap NOx. . That is, in the stratified combustion, the main injection is performed in the latter half of the compression stroke, and at the time of spark ignition by the spark plug, a stoichiometric air-fuel ratio or an air-fuel ratio air-fuel mixture in the vicinity thereof is formed in the vicinity of the spark plug. The mixture is ignited by a spark plug and burned. On the other hand, in the homogeneous combustion, the main injection is performed in the intake stroke to uniformly distribute the air-fuel mixture over the entire combustion chamber, and the air-fuel mixture distributed over the entire combustion chamber is ignited by a spark plug and burned. . Due to such a difference in the combustion state, when compared under the same operating conditions, in the stratified combustion state, a large amount of gas containing almost no fuel is present around the mixed gas mass.
[0006]
This means that even in the same lean operation, the stratified combustion state can contain more fuel later, that is, even in a low load range (stratified lean combustion is performed), Therefore, if additional injection is performed in the expansion stroke or the exhaust stroke in the stratified combustion state, the fuel from the additional injection diffuses into the gas in the stratified combustion state to become a good unburned component, and a sufficient reduction component It means that we can secure the quantity.
[0007]
The present invention has been made based on such findings of the inventor. When the catalyst temperature reaches the activation temperature in the case where the enrichment treatment is required in the stratified combustion state, the expansion process and the exhaust gas in the stratified combustion state are performed. In addition to performing additional injection during the stroke, the air-fuel ratio determined by the fuel amount combined with the main injection and the intake air amount at that time is set to be richer than the stoichiometric air-fuel ratio, and operation at the air-fuel ratio on the rich side The purpose of the present invention is to secure a sufficient amount of reducing agent (unburned components such as HC and CO) for the enrichment process even when the exhaust gas flow rate is low and the load is low.
[0008]
[Means for Solving the Problems]
The invention described in claim 1 traps NOx when the air-fuel ratio of the exhaust gas is lean in the active state, and desorbs the trapped NOx when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or rich. A catalyst having a function of reducing and purifying the separated NOx by using a reducing component in the exhaust gas is provided in the exhaust passage, while a fuel injection means for directly injecting fuel into the combustion chamber and a main injection of the fuel injection means are provided. Main injection execution means capable of selectively performing homogeneous combustion or stratified combustion during lean operation by making the timing different, catalyst temperature detection means for detecting catalyst temperature, and enrichment processing in a stratified combustion state are required. In the case, when the catalyst temperature reaches the activation temperature, additional injection performing means for performing additional injection in an expansion stroke or an exhaust stroke in a stratified combustion state, and when performing this additional injection, A rich operation means for setting an air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount to a richer side than the stoichiometric air-fuel ratio and performing an operation at the air-fuel ratio set to the rich side, An additional injection prohibiting unit for prohibiting the additional injection when the catalyst temperature is higher than the complete warm-up temperature when the enrichment processing is required in the stratified combustion state.
[0009]
【The invention's effect】
The invention described in claim 1 is based on the findings by the inventor of the present invention as described above. To reiterate, stratified combustion generally performs main injection in the second half of the compression stroke, and at the time of spark ignition by the spark plug, a lump of stoichiometric air-fuel ratio or a mixture of air-fuel ratios near the stoichiometric air-fuel ratio near the spark plug Is formed, and the mixture is ignited by a spark plug to be burned. On the other hand, in the homogeneous combustion, the main injection is performed in the intake stroke to uniformly distribute the air-fuel mixture over the entire combustion chamber, and the air-fuel mixture distributed over the entire combustion chamber is ignited by a spark plug and burned. . Due to such a difference in the combustion state, when compared under the same operating conditions, in the stratified combustion state, a large amount of gas containing almost no fuel is present around the mixed gas mass.
[0010]
This means that even in the same lean operation, the stratified combustion state can contain more fuel later, that is, if stratified combustion is performed in the low load region, even in the low load region For the enrichment process, if additional injection is performed in the expansion stroke or exhaust stroke in the stratified combustion state, the fuel from the additional injection diffuses into the gas in the stratified combustion state and becomes a good unburned component, That is, a sufficient amount of the reducing component can be secured.
[0011]
According to the first aspect of the present invention, when the enrichment process is required in the stratified combustion state, additional injection is performed when the catalyst temperature reaches the activation temperature, and the fuel amount and the intake air amount combined with the main injection amount The air-fuel ratio determined by the formula is set to a richer side than the stoichiometric air-fuel ratio, and the operation is performed at the air-fuel ratio set to the rich side. As long as the stratified charge combustion is performed in the operation range, the unburned component in the exhaust gas increases while the exhaust gas flow rate is increased by the additional injection. As a result, a sufficient amount of reducing agent (unburned components such as HC and CO) can be introduced into the catalyst, and NOx trapped in the catalyst can be sufficiently desorbed and reduced.
[0012]
In addition, due to the exhaust gas re-combustion accompanying the additional injection, the exhaust gas temperature becomes high even when the exhaust gas temperature is relatively low such as in a low load region, thereby preventing the catalyst temperature from decreasing and maintaining the activity of the catalyst. The effect is also obtained.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0014]
In FIG. 1, reference numeral 1 denotes an engine body, 2 denotes an intake passage, 3 denotes a throttle device in which a throttle valve 3a is driven by a throttle actuator composed of a DC motor or the like, and 7 denotes a predetermined value in accordance with operating conditions based on an injection signal from a controller 21. A fuel injection valve (fuel injection means) 8 for directly injecting fuel into the combustion chamber 5 so as to have an air-fuel ratio, and 8 is a spark plug.
[0015]
The controller 21 has a reference position signal and a unit angle signal from the crank angle sensor 22, an intake air amount signal from the air flow meter 23, an accelerator opening signal from the accelerator sensor 24, an engine cooling water temperature signal from the water temperature sensor 25, A gear position signal from a gear position sensor (not shown) of the transmission, a vehicle speed signal from a vehicle speed sensor (not shown), and the like are input, and a driving state is determined based on these. , A lean operation is performed, and in the other operation range, an operation is performed at the stoichiometric air-fuel ratio (this operation is referred to as “stoichiometric operation”). At this time, the O provided in the exhaust passage 9 near the engine outlet ₂ Based on the output of the sensor 27, feedback control of the air-fuel ratio is performed so that the air-fuel ratio of the exhaust gas matches the stoichiometric air-fuel ratio. In a region near the full load, an operation based on the rich air-fuel ratio (this operation is referred to as “rich operation”) is performed.
[0016]
The lean operation region is further divided into a stratified combustion region and a homogeneous combustion region. The controller 21 performs stratified combustion in a low load region where a large torque is not required, and switches to stratified combustion when the rotation speed or load increases outside the stratified combustion region.
[0017]
Here, the intake control valve (not shown) located upstream of the intake port 4 is closed to restrict the intake air, and the intake air flowing into the combustion chamber 5 from the opening of the intake port 4 to the combustion chamber 5 is tumbled (in the vertical direction). Swirl), and the fuel injected from the fuel injection valve 7 in the latter half of the compression stroke rides on the tumble and is transported to the vicinity of the ignition plug 8 located substantially at the center of the ceiling of the combustion chamber 5 so that the intake air is sucked. The stratified combustion is realized by determining the intake flow angle of the port 4 into the combustion chamber 5 and the shape of the cavity provided in the crown surface 6a of the piston 6. That is, in the stratified combustion, the main injection is performed in the latter half of the compression stroke, and at the time of spark ignition by the spark plug 8, a stoichiometric air-fuel ratio or an air-fuel ratio mixture in the vicinity thereof is formed near the spark plug 8. The mixture is ignited by the spark plug 8 and burned.
[0018]
On the other hand, if the rotation speed or the load increases outside the stratified combustion region, the flow of intake air is large in this operation region (homogeneous combustion region), so that tumble is generated in the combustion chamber 5 even when the intake control valve is open. Therefore, in the homogeneous combustion region, the intake control valve is opened, and fuel is injected in the intake stroke while creating a tumble in the combustion chamber 5, whereby a homogeneous air-fuel mixture is generated in the entire combustion chamber. That is, in the homogeneous combustion, the main injection is performed in the intake stroke to uniformly distribute the air-fuel mixture throughout the combustion chamber 5, and the air-fuel mixture distributed throughout the combustion chamber 5 is ignited by the ignition plug 8 to perform combustion. It is to let.
[0019]
The exhaust passage 9 is provided with a NOx trap catalyst (hereinafter simply referred to as “catalyst”) 10. The catalyst 10 traps NOx in the inflow exhaust gas when the air-fuel ratio of the inflow exhaust gas is leaner than the stoichiometric air-fuel ratio, and when the air-fuel ratio of the inflow exhaust gas becomes richer than the stoichiometric air-fuel ratio or the stoichiometric air-fuel ratio. Has a function of desorbing NOx trapped in the catalyst 10 and reducing and purifying the desorbed NOx by using HC, CO, or the like in an atmosphere having a stoichiometric air-fuel ratio or a rich air-fuel ratio as a reducing agent. . For example, based on a catalyst in which a noble metal such as platinum Pt, palladium Pd, and rhodium Rh is supported on a honeycomb carrier coated with alumina, at least one selected from alkaline earths represented by barium Ba and alkali metals represented by Cs It is configured to carry one component.
[0020]
Since the catalyst 10 has a property that the NOx trap rate decreases as the amount of NOx trapped in the catalyst increases, the lean operation is continued for a while, and when NOx is trapped to a certain extent by the catalyst 10, the enrichment process is forcibly performed. It is necessary to make the exhaust gas rich in reducing components flow into the catalyst 10 to desorb and reduce NOx trapped in the catalyst 10 (regeneration of the catalyst 10). For this reason, the controller 21 repeats the operation of performing the enrichment process and regenerating the catalyst 10 every time the enrichment process is required during the lean operation. This makes it possible to purify NOx in exhaust gas with relatively high efficiency.
[0021]
However, at the time of the enrichment process, it is necessary to introduce a required amount of the reducing component according to the NOx desorption amount into the catalyst 10. However, especially at the time of a low load during which the lean operation is performed, the exhaust gas flow rate is originally small. Even if the enrichment process is performed during the lean operation with the load, the amount of the reducing component may be insufficient and the NOx purification rate may deteriorate. Therefore, the controller 21 is in the stratified combustion state even during the lean operation, and the enrichment process is not performed. When necessary, when the catalyst temperature reaches the activation temperature, additional injection is performed in the expansion stroke and exhaust stroke in the stratified combustion state, and the fuel amount obtained by combining the main injection and the additional injection and the intake air amount at that time Is set on the rich side of the stoichiometric air-fuel ratio, and the operation at the air-fuel ratio on the rich side is performed (the invention according to claim 1).
[0022]
In general, desorption and reduction of NOx trapped in the catalyst 10 are more actively performed in an atmosphere where there is no oxygen and only a reducing component exists than in an atmosphere where oxygen and a reducing component coexist. For this purpose, a volume portion 11 having a predetermined space volume is provided in an exhaust passage (exhaust manifold) 9 upstream of the catalyst 10, and the exhaust gas is retained and agitated in the volume portion 11, additional fuel remaining unburned and oxygen in the exhaust gas are removed. The reaction (combustion) with is accelerated (the invention according to claim 5).
[0023]
Further, since the catalyst 10 exhibits its performance only after reaching the activation temperature, it is necessary to immediately raise the catalyst 10 to the activation temperature after the cold start. For this reason, the controller 21 determines whether or not the catalyst 10 has reached the activation temperature based on the catalyst temperature detected by the temperature sensor 28, and determines whether or not the catalyst 10 has reached the activation temperature only in the stratified combustion state. During the expansion stroke and the exhaust stroke in the stratified combustion state, additional injection is performed (the invention according to claim 10). The combustion by the additional injection does not contribute to the output of the engine, but mainly burns in the exhaust passage 9 exiting the combustion chamber 5 from the combustion chamber 5 (afterburning), and the high-temperature exhaust gas flows into the catalyst 10. To accelerate the temperature rise of the catalyst 10.
[0024]
However, since the catalyst 10 is in an inactive state during the additional injection for raising the temperature of the catalyst, if the fuel due to the additional injection remains unburned in the exhaust passage 9, the fuel is discharged to the atmosphere without being purified by the catalyst 10. It leads to worse exhaust. When the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is a rich air-fuel ratio, it is necessary to completely burn the additional fuel no matter how much the reburning of the exhaust is promoted. Since the amount of oxygen is insufficient, unburned components are discharged to the outside, and the exhaust gas deteriorates.
[0025]
For this reason, when performing additional injection for raising the temperature of the catalyst, the controller 21 sets the air-fuel ratio of the exhaust including the additional injected fuel to be slightly leaner than the stoichiometric air-fuel ratio. The operation is performed with the air-fuel ratio set on the side (the invention according to claim 10).
[0026]
Further, if the engine is stopped in a state where a large amount of NOx trapped in the catalyst 10 remains, even if the catalyst 10 is activated after the next engine restart, the NOx remaining in the catalyst 10 is once removed. Unless it is separated and reduced, good NOx trap performance cannot be obtained. For this reason, the controller 21 forcibly issues a request for the enrichment process at the time of starting the engine irrespective of the amount of NOx trapping (the invention according to claim 6). Then, after the cold start of the engine, the enrichment process is performed at the timing when the catalyst reaches the activation temperature by the additional injection for raising the temperature of the catalyst.
[0027]
As described above, the controller 21 controls the fuel injection amount and the fuel injection timing of the main injection and the ignition timing that is the timing of the spark ignition by the spark plug 8 so that the optimum combustion state such as the stratified combustion or the homogeneous combustion is obtained during the lean operation. In addition, the fuel injection amount and the fuel injection timing of the additional injection are controlled in order to perform the temperature increase and the enrichment process on the catalyst 10 in the stratified combustion state.
[0028]
The enrichment process executed by the controller 21, the setting of the combustion state, and the calculation of the target equivalence ratio for determining each fuel amount of the main injection and the additional injection will be described in detail according to the following flowchart.
[0029]
The flowchart in FIG. 2 is for setting the enrichment request flag, and is executed at regular intervals (for example, every 10 msec).
[0030]
In step 1, a signal from a starter switch (abbreviated as "ST SW" in the figure) 29 (see FIG. 1) is checked. If the signal from the starter switch 29 is ON (when the engine is started), the process proceeds to step 2, where the NOx trap amount Vn is read from the non-volatile memory (backup RAM) in the controller 21. This value is the amount of NOx trap remaining in the catalyst 10 at the time of the previous engine stop, and this value is held while the engine is stopped (the invention according to claim 9).
[0031]
For simplicity, Vn corresponding to the saturation trap amount may be stored in the ROM in the controller 21, and this value may be read when the engine is started.
[0032]
In step 3, the enrichment request flag fR is forcibly set to 1 (the invention according to claim 6).
[0033]
Here, the enrichment request flag fR = 1 indicates that enrichment processing is required, whereas fR = 0 indicates that enrichment processing is unnecessary.
[0034]
The reason why the enrichment request flag fR = 1 (enrichment is necessary) when the engine is started is that enrichment is performed at the earliest possible time after the engine is started (when the catalyst temperature Tc reaches an activation temperature TcL described later). In order to eliminate NOx trapped in the catalyst 10 and to satisfactorily trap NOx during the subsequent lean operation.
[0035]
If the signal from the starter switch 29 is not ON (after starting), the processing of steps 2 and 3 is skipped (skipped).
[0036]
In steps 4 to 12, the NOx trap amount Vn is calculated according to the operating state from the start (when NOx in the exhaust gas is trapped by the catalyst 10, the trap amount per unit time is accumulated, and conversely, the trap amount is trapped. When the NOx desorbed from the catalyst 10, the desorbed amount per unit time is subtracted.) Based on the NOx trap amount Vn calculated in this way, it is determined whether or not the enrichment process is necessary. This is the part to do.
[0037]
In step 4, the current total target equivalent ratio tFBYA is compared with 1.0 corresponding to the stoichiometric air-fuel ratio.
[0038]
The total target equivalent ratio tFBYA is calculated according to the flowcharts of FIGS. 6 and 7 described later, and the value calculated according to the flowcharts is used in step 4 of FIG. The target equivalence ratio (the value obtained by dividing the stoichiometric air-fuel ratio by the target air-fuel ratio) corresponds to the stoichiometric air-fuel ratio when this value is 1.0, and when this value is less than 1.0, the target air-fuel ratio is leaner than the stoichiometric air-fuel ratio. When the air-fuel ratio exceeds 1.0, on the contrary, it corresponds to an air-fuel ratio richer than the stoichiometric air-fuel ratio.
[0039]
The reason why “total” is added before the target equivalence ratio is to distinguish the target equivalence ratio tFBYA1 of the main injection and the target equivalence ratio tFBYA2 of the additional injection in some cases where the additional injection is performed after the main injection. That is, the total target equivalent ratio tFBYA is a value obtained by adding the target equivalent ratio tFBYA1 of the main injection and the target equivalent ratio tFBYA2 of the additional injection.
[0040]
In the present embodiment, the total target equivalence ratio tFBYA is used as a parameter representing the air-fuel ratio of the exhaust gas flowing into the catalyst 10, specifically, to determine whether or not the operation is at a lean air-fuel ratio (lean operation). are doing. That is, when the total target equivalent ratio tFBYA is less than 1.0, the operation is a lean air-fuel ratio operation (lean operation). On the other hand, when the total target equivalent ratio tFBYA is 1.0 or more, the operation is a rich air-fuel ratio (lean operation). (Rich operation) or operation at the stoichiometric air-fuel ratio (stoichiometric operation).
[0041]
The parameter representing the air-fuel ratio of the exhaust gas flowing into the catalyst 10 is not limited to this. For example, O ₂ When a wide area air-fuel ratio sensor is provided in the exhaust passage 9 instead of the sensor 27, the detection value of the wide area air-fuel ratio sensor may be used.
[0042]
If the total target equivalent ratio tFBYA is smaller than 1.0 (lean operation), NOx in the exhaust gas is trapped by the catalyst 10, so the process proceeds to step 5, where the NOx emission amount dVn per unit time is calculated. The NOx concentration in the exhaust gas can be estimated from parameters such as engine load (or target torque, which will be described later), air-fuel ratio (or total target equivalent ratio), cooling water temperature, and the like. , DVn can be calculated.
[0043]
FIG. 3 shows the characteristics of the NOx concentration with respect to each parameter. As shown in the upper part of FIG. 3, the engine load and the rotational speed become larger as the engine load becomes larger and the rotational speed becomes larger as the load becomes constant. Becomes higher, the NOx concentration becomes higher. Further, the NOx concentration becomes maximum at an air-fuel ratio (abbreviated as “A / F” in the figure; the same at the top of FIG. 4) as shown in the middle part of FIG. However, even on the rich side, the NOx concentration decreases. As for the water temperature, as shown in the lower part of FIG. 3, the higher the water temperature, the higher the NOx concentration.
[0044]
In step 6, using the NOx emission amount dVn per unit time thus calculated,
Vn = Vnz + C1 × εT × dVn (1)
Where εT: NOx trap rate of the catalyst,
C1: conversion coefficient from per unit time to per operation cycle,
Vnz: previous Vn,
The NOx trap amount Vn is calculated by the following equation.
[0045]
Equation (1) calculates the NOx trap amount per unit time by multiplying the NOx emission amount dVn per unit time by the NOx trap rate εT, and converts this to the NOx trap amount per operation cycle using the conversion coefficient C1. The latest NOx trap amount Vn is calculated by integrating the NOx trap amount per operation cycle with the previous NOx trap amount Vnz.
[0046]
The NOx trap rate εT in the equation (1) can be estimated from parameters such as the air-fuel ratio (or the total target equivalent ratio), the catalyst temperature, the exhaust flow rate, and the NOx trap amount at that time. FIG. 4 shows the characteristic of the NOx trap rate with respect to each parameter. As for the air-fuel ratio, as shown in the uppermost row of FIG. It becomes zero on the rich side. The catalyst temperature reaches its maximum at a certain temperature as shown in the second stage of FIG. 4, and the NOx trapping rate decreases even if the temperature is higher or lower than this temperature. Regarding the exhaust flow rate and the NOx trap amount, as shown in the third stage and the fourth stage of FIG. The trap rate decreases.
[0047]
As a method of detecting the exhaust flow rate, a map of the exhaust flow rate using the basic injection pulse width Tp (corresponding to the engine load) and the rotation speed as parameters may be created in advance, and the map search may be performed.
[0048]
In step 7, the latest NOx trap amount Vn calculated in step 6 is compared with the NOx trap upper limit amount VnH, and when the NOx trap amount Vn is larger than the NOx trap upper limit amount VnH, it is determined that the enrichment process needs to be performed. Then, the process proceeds to step 8 to set the switch request flag fR = 1 (the invention according to claim 7). As the NOx trap upper limit amount VnH, a NOx trap amount at which a good NOx trap rate cannot be maintained may be set (the invention according to claim 8).
[0049]
On the other hand, if the NOx trap amount Vn is equal to or smaller than the NOx trap upper limit amount VnH in step 7, the operation in step 8 is skipped, and the value of the enrichment request flag fR up to immediately before is maintained.
[0050]
On the other hand, if the total target equivalent ratio tFBYA is equal to or greater than 1.0 (rich operation or stoichiometric operation) in step 4, NOx trapped in the catalyst 10 is desorbed. Of the NOx desorption amount dVnr is calculated.
[0051]
The NOx desorption rate from the catalyst 10 can be estimated from parameters such as the NOx trap amount, the exhaust flow rate, and the catalyst temperature at that time. If the estimated NOx desorption rate is multiplied by the NOx trap amount Vn, dVnr Can be calculated. FIG. 5 shows the characteristics of the NOx desorption rate with respect to each parameter. The NOx desorption rate increases as the NOx trapping quantity and the exhaust flow rate increase as shown in the upper and middle stages of FIG. . As for the catalyst temperature, the NOx desorption rate becomes higher at a certain temperature or higher as shown in the lower part of FIG. 5, and at a certain temperature or lower, the NOx desorption rate is zero.
[0052]
In step 10, using the NOx desorption amount dVnr per unit time calculated in this way,
Vn = Vnz−C2 × dVnr (2)
Where C2 is a conversion coefficient from per unit time to per operation cycle,
Vnz: previous Vn,
The NOx trap amount Vn is calculated by the following equation.
[0053]
Equation (2) converts the NOx desorption amount per unit time dVnr into a NOx desorption amount per operation cycle using the conversion coefficient C2, and the converted NOx desorption amount is a NOx trap amount immediately before. The latest NOx trap amount Vn is calculated by subtracting from Vnz.
[0054]
In step 11, the latest NOx trap amount Vn calculated in step 10 is compared with the NOx trap lower limit amount VnL. It is determined that the release / reduction has been completed, and the routine proceeds to step 12, where the enrichment request flag fR = 0 is set. The NOx trap lower limit amount VnL is set to zero or a value close to zero.
[0055]
On the other hand, if the NOx trap amount Vn is equal to or larger than the NOx trap lower limit amount VnL in step 11, the operation in step 12 is skipped, and the value of the enrichment request flag fR up to immediately before is maintained.
[0056]
The flowcharts of FIGS. 6 and 7 are for setting the combustion state in accordance with the operating conditions and calculating the target equivalent ratio of the main injection, the target equivalent ratio of the additional injection, and the total target equivalent ratio which is the sum of these. Then, the process is executed every predetermined time (for example, every 10 msec).
[0057]
Here, the target equivalent ratio of the main injection is referred to as “tFBYA1”, the target equivalent ratio of the additional injection is referred to as “tFBYA2”, and the total target equivalent ratio which is the sum of these is referred to as “tFBYA”.
[0058]
In step 21, a combustion flag fCmb and a lean operation permission flag fL are set based on the target torque tTC and the engine speed Ne.
[0059]
For example, the combustion flag fCmb is set to 0 or 1 by searching a map having the contents shown in FIG. 8 from the target torque tTC and the engine rotation speed Ne. Here, fCmb = 0 represents stratified combustion, and fCmb = 1 represents homogeneous combustion. That is, assuming that the operating region is roughly divided into three as shown in FIG. 8, the combustion state is determined by the combustion flag fCmb as follows.
[0060]
(A) Low load / low speed range: fCmb = 0 (stratified combustion)
(B) Medium load / medium rotation speed range: fCmb = 1 (homogeneous combustion),
(C) High load / high rotation speed range: fCmb = 1 (homogeneous combustion)
Further, a lean operation permission flag fL is set to 0 or 1 by searching a map having the contents shown in FIG. 9 from the target torque tTC and the engine rotation speed Ne. Here, fL = 1 indicates that lean operation is permitted, and fL = 0 indicates that lean operation is not permitted. That is, as shown in FIG. 9, assuming that the operation area is roughly divided into three as in FIG. 8, whether or not to allow the lean operation is determined by the lean operation permission flag fL as follows.
[0061]
(D) Low load / low speed range: fL = 1 (lean operation permitted),
(E) Medium load / medium rotation speed range: fL = 1 (lean operation permitted),
(F) High load / high speed range: fL = 0 (lean operation prohibited),
When these two flags fCmb and fL are combined, optimal combustion is performed in each of the three operating ranges as follows.
[0062]
<1> Low load / low speed range:
fCmb = 0 and fL = 1 (lean operation / stratified combustion),
<2> Medium load / medium rotation speed range:
fCmb = 1 and fL = 1 (lean operation / homogeneous combustion),
<3> High load / high speed range:
fCmb = 1 and fL = 0 (stoichiometric operation, homogeneous combustion),
The target torque tTC is calculated in a routine (not shown) based on the accelerator opening Aps detected by the accelerator sensor 24 and the engine speed detected by the crank angle sensor 22. FIG. 10 briefly shows the characteristics with respect to the accelerator opening Aps under the condition that the engine rotation speed is constant.
[0063]
In step 22, the engine coolant temperature Tw detected by the coolant temperature sensor 25 is compared with the lean operation permission lower limit coolant temperature TwL. As the lean operation permission lower limit water temperature TwL, a lower limit water temperature at which sufficient combustion stability can be ensured in the lean operation may be set.
[0064]
If the engine water temperature Tw is lower than the lean operation permission lower limit water temperature TwL, the routine proceeds to step 23, where the combustion flag fCmb = 1 and the lean operation permission flag fL = 0 are set. This is mandatory. That is, even in the low-load / low-speed range in step 21 and fCmb = 0 and fL = 1 (lean operation / stratified combustion), only when the engine water temperature Tw is lower than the lean operation permission lower limit water temperature TwL. Forcibly, stoichiometric operation and homogeneous combustion are performed.
[0065]
On the other hand, if it is determined in step 22 that the engine coolant temperature Tw is equal to or higher than the lean operation permission lower limit coolant temperature TwL, the lean operation can be performed. Therefore, the process proceeds to step 24 and the enrichment request flag fR is checked.
[0066]
If the enrichment request flag fR = 1 (enrichment processing is necessary), the routine proceeds to step 25, where the catalyst temperature Tc and the complete warm-up temperature TcH are compared. Here, the complete warm-up temperature TcH is a temperature at which the catalyst 10 is completely warmed up.
[0067]
If the catalyst temperature Tc is higher than the complete warm-up temperature TcH, the routine proceeds to step 26, where the combustion flag fCmb = 1 (homogeneous combustion) is set (the invention according to claim 1). At this time, the lean operation permission flag fL remains set in step 21.
[0068]
By setting the combustion flag fCmb = 1 (homogeneous combustion), tFBYA1 = tFBYA and tFBYA2 = 0 as described later (see steps 41 and 43 in FIG. 7). Therefore, even if the enrichment processing is requested, additional injection is not performed. Not done. In the case where the enrichment process is required in the stratified combustion state, if the additional injection is performed even if the catalyst temperature Tc becomes higher than TcH, the performance of the catalyst 10 may decrease because the catalyst temperature further increases. However, when the catalyst temperature Tc is higher than the complete warm-up temperature TcH, the additional injection is prohibited as the homogeneous combustion, so that the performance of the catalyst can be prevented from being deteriorated due to an increase in the catalyst temperature due to the exhaust combustion. .
[0069]
If the enrichment request flag fR = 0 in steps 24 and 25, or if the catalyst temperature Tc is equal to or lower than the complete warm-up temperature TcH even if the enrichment request flag fR = 1, the operation in step 26 is skipped, and step 21 is executed. The value of the combustion flag fCmb and the value of the lean operation permission flag fL set in the above are maintained as they are.
[0070]
Steps 27 to 31 are for calculating the target equivalent ratio tFBYA1 of the main injection based on the combination of the two set flags fCmb and fL. The fuel amount from the main injection is determined by the target equivalent ratio tFBYA1.
[0071]
That is, in steps 27 and 28, the combustion flag fCmb and the lean operation permission flag fL are checked. If the combustion flag is fCmb = 0, it is determined that the engine is in the lean operation / stratified combustion region of <1>, and the routine proceeds to step 29, where the main injection is performed based on the target torque tTC and the engine speed Ne at that time. The target equivalent ratio tFBYA1 is calculated.
[0072]
Similarly, if the combustion flag is fCmb = 1 and fL = 1, it is determined that the engine is in the lean operation / homogeneous combustion range of <2> and the routine proceeds to step 30, where the target torque tTC and the engine rotation The target equivalence ratio tFBYA1 of the main injection is calculated based on the speed Ne.
[0073]
Since these two operation ranges (lean operation / stratified combustion region and lean operation / homogeneous combustion region) are both lean operation regions, only one map may be used to determine the target equivalent ratio tFBYA1 of the main injection. The target equivalent ratio tFBYA1 of the main injection may be calculated by searching a map having the contents shown in FIG. 11 from the torque tTC and the engine rotation speed Ne. In FIG. 11, the values of tFBYA1 are all less than 1.0. Although the figure shows that the value of tFBYA1 tends to be present, the actual value is determined from engine specifications and the like.
[0074]
On the other hand, if the combustion flag fCmb = 1 and the lean operation permission flag fL = 0, it is determined that the engine is in the stoichiometric operation / homogeneous combustion region of <3>, and the routine proceeds to step 31, where the target equivalence ratio tFBYA1 = 1. 0 (equivalent to the stoichiometric air-fuel ratio).
[0075]
Referring to FIG. 7, steps 32 to 40 calculate (set) the total target equivalent ratio in accordance with the engine coolant temperature Tw, the catalyst temperature Tc, the combustion flag fCmb, and the enrichment request flag fR.
[0076]
In step 32, the engine coolant temperature Tw is compared with the lean operation permission lower limit coolant temperature TwL. If the engine water temperature Tw is lower than the greenhouse permission lower limit water temperature TwL, the routine proceeds to step 33, where the total target equivalent ratio tFBYA = 1.0 (corresponding to the stoichiometric air-fuel ratio). This is because fCmb = 1 and fL = 0 (stoichiometric operation / homogenous combustion) in Step 23 of FIG. 6 when Tw <TwL, and this is received.
[0077]
When the engine coolant temperature Tw is equal to or higher than the lean operation permission lower limit coolant temperature TwL, the routine proceeds to step 34, where the catalyst temperature Tc and the activation temperature TcL are compared.
[0078]
Here, as the activation temperature TcL, a catalyst temperature at which NOx trapped in the catalyst 10 can be desorbed and reduced to some extent, if not completely, is employed (the invention according to claim 2). The activation temperature TcL is also a catalyst temperature at which the catalyst 10 can exhibit a certain NOx trapping ability. Of course, the activation temperature TcL is lower than the above-mentioned complete warm-up temperature TcH.
[0079]
If the catalyst temperature Tc is lower than the activation temperature TcL, it is necessary to raise the temperature of the catalyst 10 at an early stage, and the routine proceeds to step 35, where the combustion flag fCmb is checked.
[0080]
Only when the combustion flag fCmb is 0 (stratified combustion), the process proceeds to step 36, where the total target equivalent ratio tFBYA is slightly leaner than the stoichiometric air-fuel ratio in order to perform additional injection for raising the temperature of the catalyst 10. The equivalent ratio TFL is set (the invention according to claim 10).
[0081]
Here, the temperature rise equivalent ratio TFL is set to a value larger than the maximum value of the target equivalent ratio tFBYA1 of the main injection stored in the control map for the lean operation (FIG. 11) (to 1.0 corresponding to the stoichiometric air-fuel ratio). (Close value). By setting the total target equivalence ratio tFBYA to the temperature raising equivalence ratio TFL, additional injection for burning additional fuel in the volume portion 11 of the exhaust passage 9 is performed while performing lean operation and stratified combustion by main injection. .
[0082]
On the other hand, if the combustion flag fCmb = 1 (homogeneous combustion), the routine proceeds to step 37, where the total target equivalence ratio tFBYA is made to match the target equivalence ratio tFBYA1 of the main injection (no additional injection is performed).
[0083]
On the other hand, if the catalyst temperature Tc is equal to or higher than the activation temperature TcL, the NOx trapped in the catalyst 10 can be desorbed and reduced. Therefore, the process proceeds from step 34 to step 38 to check the enrichment request flag fR.
[0084]
If the enrichment request flag fR = 1 (enrichment processing is necessary), the routine proceeds to step 39, where the target reducing agent supply amount tdVr per unit time is calculated. As described in step 9 of FIG. 2, the NOx desorption rate from the catalyst 10 can be estimated from parameters such as the NOx trap amount, the exhaust flow rate, and the catalyst temperature at that time. Is multiplied by the NOx trap amount Vn, the NOx desorption amount dVnr per unit time can be calculated. Therefore, an amount of (equivalent) reducing agent corresponding to this dVnr is set as a target reducing agent supply amount tdVr per unit time.
[0085]
In step 40, based on the target reducing agent supply amount tdVr per unit time thus calculated and the intake air flow rate Qa detected by the air flow meter 23,
tFBYA = C3 × (tdVr / Qa) +1 (3)
However, C3: conversion coefficient to equivalent ratio,
Is used to calculate the total target equivalent ratio tFBYA. The equation (3) is obtained by dividing the target reducing agent supply amount tdVr per unit time by the intake air flow rate (intake air amount per unit time) Qa, and converting the value to the equivalent ratio using the conversion coefficient C3. By performing the calculation, an equivalent ratio capable of realizing the target reducing agent supply amount tdVr per unit time at the current intake air flow rate Qa is calculated, and a stoichiometric air-fuel ratio is added to the calculated equivalent ratio to obtain a total target equivalent in the enrichment processing. The ratio tFBYA is calculated.
[0086]
On the other hand, if the enrichment request flag fR is not 1 (enrichment processing is unnecessary), the process proceeds from step 38 to step 37 to make the total target equivalent ratio tFBYA coincide with the target equivalent ratio tFBYA1 of the main injection (additional injection is performed). Absent).
[0087]
Steps 41 to 43 in FIG. 7 are portions for calculating the target equivalent ratio tFBYA2 of the additional injection.
[0088]
Since the additional injection is performed only in the case of stratified combustion, in step 41, the combustion flag fCmb is checked. If the combustion flag fCmb = 0 (stratified combustion), the routine proceeds to step 42, where a value obtained by subtracting the target equivalent ratio tFBYA1 of the main injection from the total target equivalent ratio tFBYA is calculated as the target equivalent ratio tFBYA2 of the additional injection.
[0089]
Here, the case where the process proceeds to the present step (step 42) is any of the following three cases.
[0090]
(A) When proceeding to step 42 via steps 34, 35 and 36,
(B) Where the process proceeds to step 42 after steps 34, 38, 39 and 40
If
(C) When proceeding to step 42 via steps 34, 38 and 37,
In the case of the above (a), that is, when the catalyst is in a stratified combustion state before the activation of the catalyst, an additional injection for raising the temperature of the catalyst 10 (total air-fuel ratio is slightly lean from the stoichiometric air-fuel ratio) is performed. On the other hand, in the case of the above (b), that is, when the catalyst temperature has reached the activation temperature and there is a request for the enrichment processing, the temperature of the catalyst 10 is increased and additional injection for the enrichment processing is performed.
[0091]
On the other hand, in the case of the above (c), that is, when the catalyst temperature has reached the activation temperature but there is no request for the enrichment processing, since tFBYA and tFBYA1 are made equal in step 37, the target equivalent of the additional injection is set. The ratio tFBYA2 = 0, and only the main injection for realizing the combustion state determined in step 21 of FIG. 6 is performed.
[0092]
If the combustion flag fCmb = 1 (homogeneous combustion), the process proceeds to step 43, where the target equivalence ratio tFBYA1 of the main injection is made equal to the total target equivalence ratio tFBYA, and the target equivalence ratio tFBYA2 of the additional injection is set to 0. That is, no additional injection is performed in the homogeneous combustion state.
[0093]
Here, the case where the process proceeds to the present step (step 43) is any of the following three cases.
[0094]
(D) When proceeding to step 43 via steps 32 and 33,
(E) When the process proceeds to step 43 after steps 34, 38, 39 and 40
If
(F) When proceeding to step 43 via steps 34, 38 and 37,
In the case of the above (e), that is, when the catalyst temperature has reached the activation temperature and there is a request for enrichment processing, rich operation and homogeneous combustion are performed by main injection. In this case, in the enrichment process when the catalyst temperature is higher than the complete warm-up temperature TcH, the rich operation and the homogeneous combustion are always performed.
[0095]
In the case of (d), that is, when the engine water temperature is lower than the lean operation permission lower limit water temperature TwL, the stoichiometric operation / homogeneous combustion is performed by the main injection in the case of (f), and the stoichiometric operation / homogeneous combustion is performed by the main injection. Alternatively, lean operation and homogeneous combustion are realized.
[0096]
In a fuel injection control routine (not shown), the fuel injection pulse width Ti1 of the main injection is added based on the target equivalent ratio tFBYA1 of the main injection, the target equivalent ratio tFBYA2 of the additional injection calculated in this way, and the intake air flow rate Qa. The fuel pulse width Ti2 of the injection is calculated, and the injection timing of the main injection is determined based on the combustion flag fCmb. For example, in the case of the sequential fuel injection system, the arithmetic expressions of Ti1 and Ti2 are as follows.
[0097]
Ti1 = Tp × tFBYA1 × (α + αm−1) × 2 + Ts (4)
Ti2 = Tp × tFBYA2 × (α + αm−1) × 2 + Ts (5)
Here, Tp: basic injection pulse width,
α: air-fuel ratio feedback correction coefficient,
αm: Air-fuel ratio learning value,
Ts: invalid pulse width,
Further, the injection timing of the main injection when the combustion flag fCmb = 0 (stratified combustion) is substantially in the latter half of the compression stroke, and the injection timing of the main injection when the combustion flag fCmb = 1 (homogeneous combustion) is the intake stroke. It is. The injection timing of the additional injection is set after the middle of the expansion stroke or in the exhaust stroke.
[0098]
Further, in an intake air amount control routine (not shown), a target intake air flow rate is calculated based on the target torque tTC and the target equivalent ratio tFBYA1 of the main injection, and the throttle valve of the throttle device 3 is set so as to obtain the target intake air flow rate. The opening of 3a is controlled.
[0099]
Here, the operation of the present embodiment will be described with reference to FIG. 12. FIG. 12 shows that when the engine is cold started and the operating condition determined from the target torque and the rotational speed is in the lean operation / stratified combustion region for a while, The target equivalent ratio tFBYA1 of the main injection, the target equivalent ratio tFBYA2 of the additional injection, the total target equivalent ratio tFBYA as the sum thereof, the catalyst temperature Tc, the lean operation permission flag fL, the combustion flag fCmb, the enrichment request flag fR, and the NOx trap amount Vn Is a model showing how changes occur.
[1] From t1 to immediately before t2:
In the figure, at the time of starting at t1, the enrichment request flag fR = 1 is forcibly set regardless of the NOx trap amount at that time (steps 1, 3 in FIG. 2).
[0100]
Further, according to this embodiment (the ninth aspect of the present invention), since the NOx trap amount at the end of the previous engine operation is stored even after the operation of the engine is stopped until the restart, the stored NOx is stored. The trap amount is set as the initial value of the NOx trap amount during the current operation (Steps 1 and 2 in FIG. 2). For this reason, even if NOx remains trapped in the catalyst 10 at the time of restart, it can be desorbed and reduced immediately after the catalyst temperature reaches the activation temperature, including NOx remaining in the catalyst.
[0101]
Since the operating condition determined from the target torque and the rotation speed is in the lean operation / stratified combustion region, even if the combustion flag fCmb = 0 and fL = 1 should be set (step 21 in FIG. 6), the engine water temperature Tw is lean. Only during the period lower than the operation permission lower limit water temperature TwL, fCmb = 1 and fL = 0 (stoichiometric operation / homogeneous combustion) are forcibly set (steps 22 and 23 in FIG. 6). At this time, the total target equivalent ratio tFBYA = 1.0 (steps 27, 28, and 31 in FIG. 7), and by setting the total target equivalent ratio tFBYA = 1.0, the NOx trap amount Vn becomes gradually smaller than the value at the time of starting. (Steps 4, 9, and 10 in FIG. 2). At this time, since tFBYA1 = tFBYA and fTBYA2 = 0 (steps 32, 33, 41, and 43 in FIG. 7), no additional injection is performed.
[2] From t2 to just before t3:
At the timing t2 when the engine water temperature Tw rises due to the stoichiometric operation and homogeneous combustion from the start and reaches the lean operation permission lower limit water temperature TwL, the combustion flag fCmb and the lean operation permission flag fL are set in step 21 in FIG. It is switched to the original value. That is, the combustion flag is switched to the combustion flag fCmb = 0 and the lean operation permission flag fL = 1 (lean operation / stratified combustion). At this time, the fTBA1 having a value less than 1.0 according to the target torque and the rotation speed is obtained as shown in FIG. The calculation is performed as shown by the one-dot chain line at the top (steps 27 and 29 in FIG. 7).
[0102]
Further, the total target equivalence ratio tFBYA is set to the temperature raising equivalence ratio TFL since the operation is in the lean operation / stratified combustion region and the catalyst temperature Tc has not reached the activation temperature TcL (steps 32, 34 and 35 in FIG. 7). , 36), since the target equivalent ratio tFBYA2 of the additional injection is calculated by tFBYA2 = tFBYA-tFBYA1 (steps 41 and 42 in FIG. 7), the additional injection for increasing the temperature of the catalyst is performed.
[0103]
The temperature rise equivalent ratio TFL is slightly smaller than 1.0 (corresponding to the stoichiometric air-fuel ratio) as shown by the solid line when viewed at the top of FIG. The difference between the total target equivalence ratio tFBYA shown by the solid line and the target equivalence ratio tFBYA1 of the main injection shown by the one-dot chain line is the target equivalence ratio tFBYA2 of the additional injection, and when the additional injection is performed by the difference, As shown in the second stage of FIG. 12, the catalyst temperature Tc rises sharply.
[0104]
Since the catalyst 10 is in an inactive state during the additional injection, if the fuel due to the additional injection remains in the exhaust passage 9, the fuel is not purified by the catalyst 10 but is discharged to the atmosphere and deteriorates the exhaust. When the air-fuel ratio of the exhaust gas is rich, no matter how much the reburning of the exhaust gas is promoted, the amount of oxygen required to completely burn the fuel for the additional injection is insufficient. It is discharged and deteriorates exhaust gas.
[0105]
On the other hand, according to the present embodiment (the invention according to claim 10), the temperature rise equivalent ratio TFL includes a value slightly smaller than 1.0 (corresponding to the stoichiometric air-fuel ratio), that is, the fuel amount of the additional injection is included. The total target equivalent ratio tFBYA is determined such that the air-fuel ratio of the exhaust gas is slightly leaner than the stoichiometric air-fuel ratio, and the fuel amount of the additional injection is adjusted so as to realize the total target equivalent ratio tFBYA. Therefore, when the catalyst temperature Tc has not reached the activation temperature TcL as in the case after the cold start, the unburned components discharged without being used for desorption and reduction of NOx trapped in the catalyst 10 are discharged. While suppressing the temperature, the catalyst 10 can be rapidly heated by exhaust gas reburning.
[0106]
At this time, since the total target equivalent ratio tFBYA (= TFL) is a value less than 1.0, the NOx trap amount Vn increases as shown in the lowermost part of FIG. 12 (steps 4, 5, and 5 in FIG. 2). 6).
[3] From t3 to t4:
When the catalyst temperature Tc rises and reaches the activation temperature TcL at the timing of t3, NOx trapped in the catalyst 10 can be desorbed and reduced even if not completely.
[0107]
In this case, when the engine is started, the enrichment request flag fR = 1 is forcibly set as described above, and since the stratified combustion state (fCmb = 0) continues from t3, the enrichment process is performed immediately from the timing of t3. to go into.
[0108]
In the enrichment process in the stratified combustion state, the NOx desorption amount per unit time dVnr is calculated, and the amount of the reducing agent corresponding to the NOx desorption amount per unit time dVnr is equal to the target reducing agent supply per unit time. The current intake air flow rate is calculated by dividing the target reducing agent supply amount tdVr per unit time by the intake air flow rate Qa, and performing a unit conversion to an equivalent ratio using a conversion coefficient C3. The equivalent ratio that can realize the target reducing agent supply amount tdVr per unit time is calculated by Qa, and 1.0 of the stoichiometric air-fuel ratio is added to the equivalent ratio to calculate the total target equivalent ratio tFBYA in the enrichment process. (Steps 32, 34, 37, 38, 39, 40 in FIG. 7).
[0109]
When the total target equivalent ratio tFBYA at the time of this enrichment processing is viewed at the top of FIG. 12, it is a value greatly exceeding 1.0 (corresponding to the stoichiometric air-fuel ratio) as indicated by the solid line. The difference between the total target equivalence ratio tFBYA shown by the solid line and the target equivalence ratio tFBYA1 of the main injection shown by the dashed line is the target equivalence ratio tFBYA2 of the additional injection. By introducing the fuel component into the catalyst 10, NOx trapped in the catalyst 10 is desorbed into the exhaust passage 9, and the desorbed NOx is reduced and purified using the unburned component as a reducing agent. The trap amount Vn decreases as shown in the lowermost part of FIG. 12 (steps 4, 9, and 10 in FIG. 2).
[0110]
When the NOx trap amount Vn becomes less than the NOx trap lower limit amount VnL at the timing of t4 due to the enrichment process in the stratified combustion state, the desorption / reduction of NOx trapped in the catalyst 10 is completed. The flag fR = 0 (steps 11 and 12 in FIG. 2), and the additional injection ends (steps 32, 34, 38, 37, 41 and 43 in FIG. 7).
[0111]
As described above, according to the present embodiment (the invention of claim 1), when the enrichment process is required in the stratified combustion state, the additional injection is performed when the catalyst temperature Tc reaches the activation temperature TcL, and the main injection is performed. The air-fuel ratio, which is determined by the fuel amount and the intake air amount together with the injection amount, is set to be richer than the stoichiometric air-fuel ratio, and the operation is performed at the air-fuel ratio set to the rich side. Even in an operating region such as the region, as long as stratified combustion is performed in the operating region, the unburned components in the exhaust gas increase while the exhaust gas flow rate is increased by the additional injection. As a result, a sufficient amount of reducing agent (unburned components such as HC and CO) can be introduced into the catalyst, and NOx trapped in the catalyst 10 can be sufficiently desorbed and reduced.
[0112]
In addition, the exhaust gas re-combustion accompanying the additional injection increases the exhaust gas temperature even when the exhaust gas temperature is relatively low, such as in a low load region, thereby preventing the temperature of the catalyst 10 from decreasing and reducing the activity of the catalyst 10. The effect of being able to maintain is also obtained.
[0113]
In general, desorption and reduction of NOx trapped in the catalyst 10 are more actively performed in an atmosphere where there is no oxygen and only a reducing component exists than in an atmosphere where oxygen and a reducing component coexist. According to this embodiment (invention of claim 4), the unburned component combustion promoting means is provided upstream of the catalyst 10, and the remaining oxygen in the exhaust gas is consumed by the combustion of the unburned component upstream of the catalyst 10 by the means. Therefore, the exhaust gas flowing into the catalyst 10 contains almost no oxygen and only a reducing component, so that NOx trapped in the catalyst 10 can be efficiently desorbed and reduced.
[0114]
In this case, the unburned component combustion promoting means is the volume portion 11 in the exhaust passage 9 upstream of the catalyst 10 and capable of retaining and stirring the exhaust gas (the invention of claim 5). Can be obtained with an inexpensive and simple hardware configuration.
[0115]
Further, when the catalyst temperature Tc has not reached the activation temperature TcL in the case where the enrichment process is required in the stratified combustion state, the catalyst is operated at the air-fuel ratio set to the lean side with additional injection, thereby performing the catalyst. 10, when the catalyst temperature Tc reaches the activation temperature TcL, the additional injection is continued and the operation is performed at the air-fuel ratio set on the rich side accompanied by the additional injection. Invention), it is possible to prevent a decrease in the temperature of the catalyst 10 which is concerned when the additional injection is temporarily stopped after the catalyst temperature Tc reaches the activation temperature TcL.
[0116]
Further, it is desirable that the exhaust gas reburning by additional injection for desorbing and reducing NOx trapped in the catalyst 10 be performed until almost no residual oxygen is present in the exhaust gas. It is advantageous. According to the present embodiment (the invention according to claim 12), the enrichment process is continuously performed after the catalyst temperature Tc reaches the activation temperature TcL. Therefore, when performing the enrichment process, the residual oxygen in the exhaust gas is substantially reduced. It is possible to realize a state where there is no fuel, thereby improving the efficiency of desorption / reduction of NOx trapped in the catalyst 10 and preventing a decrease in fuel efficiency of the additional fuel consumed for raising the temperature again. The effect is also obtained.
[0117]
Further, in the stratified combustion state, the operation from the air-fuel ratio set on the lean side with additional injection (catalyst temperature increase control) to the operation on the air-fuel ratio set on the rich side with additional injection (enrichment processing). At the time of the shift, the target equivalent ratio tFBYA1 of the main injection is not changed before and after the shift, and only the target equivalent ratio tFBYA2 of the additional injection is changed (the empty amount of the main injection determined by the fuel amount of the main injection and the intake air amount). Since the fuel amount of the additional injection is increased without changing the fuel ratio (the invention according to claim 13), there is no torque step at the time of transition from the catalyst temperature increase control to the enrichment process, and therefore, the catalyst temperature increase No torque shock occurs at the time of transition from the control to the enrichment process.
[0118]
When the engine is stopped in a state where a large amount of NOx trapped in the catalyst 10 remains, even if the catalyst 10 is activated after the next engine start, the NOx remaining in the catalyst 10 is once desorbed and reduced. However, according to the present embodiment (the invention of claim 6), when the engine is started, regardless of the amount of NOx trapping, forced NOx trapping performance cannot be obtained. Since the enrichment request flag fR = 1 (that is, the enrichment process is required), the enrichment process is performed every time the catalyst temperature Tc reaches the activation temperature TcL in the stratified combustion state after the engine is started. As a result, there is no NOx remaining in the catalyst 10. As a result, NOx in the exhaust gas can be efficiently trapped in the catalyst 10 during the lean operation at the time t4.
[4] After t4:
After t4, the combustion state is controlled in accordance with the two flags fCmb and fL set in step 21 in FIG. The figure shows a case where the engine is still in the lean operation / stratified combustion region after t4 (fL = 1 and fCmb = 0), while changing the accelerator opening and the rotation speed. At this time, the total target equivalent ratio tFBYA (= the target equivalent ratio tFBYA1 of the main injection) changes according to the target torque and the rotation speed, and the catalyst temperature Tc and the NOx trap amount Vn change accordingly.
[0119]
That is, the NOx trap amount Vn increases due to the continuation of the lean operation and the stratified combustion (steps 4, 5, and 6 in FIG. 2). If the NOx trap amount eventually becomes larger than the NOx trap upper limit amount VnH at the timing of t5, the enrichment request flag is set. fR = 1 (steps 7 and 8 in FIG. 2). At this time, since the stratified combustion state is established, the enrichment process by the additional injection is performed while performing the stratified combustion by the main injection (steps 32, 34, and 38 in FIG. 7). , 39, 40, 41, 42).
[0120]
In this case, according to the present embodiment (the invention of claim 8), if the NOx trap amount at which a good NOx trap rate cannot be maintained is set as the NOx trap upper limit amount VnH (predetermined value), NOx in the stratified combustion state is set. Before the trap amount increases and a good NOx trap rate cannot be maintained, the NOx trapped in the catalyst 10 is desorbed / reduced, thereby suppressing the deterioration of the fuel consumption due to the enrichment process while suppressing the NOx. The trap rate can be kept high.
[0121]
In the drawing, the time scale after t4 is different from the time scale up to t4, and it goes without saying that the time scale after t4 is much larger than the time scale up to t4. The movement of Vn is also indicated by a simple straight line.
[0122]
In the embodiment, the equivalence ratio is partially described instead of the air-fuel ratio. However, the portion calculated based on the equivalence ratio may be calculated based on the air-fuel ratio. For example,
1) Calculate the NOx desorption amount dVnr per unit time,
2) A reducing agent amount corresponding to the NOx desorption amount dVnr per unit time is set as a target reducing agent supply amount tdVr per unit time,
3) The target reducing agent supply amount tdVr per unit time is divided by the intake air flow rate Qa, and the value is converted into an equivalent ratio by using a conversion coefficient C3 to obtain a unit time at the current intake air flow rate Qa. Calculate the equivalent ratio that can achieve the target reducing agent supply amount tdVr per
4) By adding 1 / stoichiometric air-fuel ratio to this, the total target equivalent ratio tFBYA at the time of the enrichment processing was calculated.
But instead of this
1) Calculate the NOx desorption amount dVnr per unit time,
2) Calculate a reducing agent amount corresponding to the NOx desorption amount dVnr per unit time as a target reducing agent supply amount tdVr per unit time,
3 ′) By dividing the intake air flow rate Qa by the target reducing agent supply amount tdVr per unit time, an air-fuel ratio capable of realizing the target reducing agent supply amount tdVr per unit time with the current intake air flow rate Qa is calculated.
4 ') A value obtained by subtracting this from 14.7 which is the stoichiometric air-fuel ratio is set as the rich-side air-fuel ratio at the time of executing the additional injection.
This may be done (the invention according to claim 11).
[0123]
In the embodiment, the case where the unburned component combustion promoting means is the volume portion 11 in the exhaust passage 9 upstream of the catalyst 10 and capable of retaining and stirring exhaust gas is described, but the present invention is not limited to this. Any means may be used as long as the means promotes the combustion of unburned components in the exhaust gas generated by executing the injection upstream of the catalyst 10 (the invention according to claim 4).
[0124]
In the embodiment, when performing the additional injection for raising the catalyst temperature in the stratified combustion state, the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is slightly smaller than the stoichiometric air-fuel ratio. As described in the case of setting on the lean side, this is to give a margin from the stoichiometric air-fuel ratio.Therefore, the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is theoretically determined. The air-fuel ratio may be set.
[Brief description of the drawings]
FIG. 1 is a control system diagram of one embodiment.
FIG. 2 is a flowchart for explaining setting of a rich request flag;
FIG. 3 is a characteristic diagram of NOx concentration.
FIG. 4 is a characteristic diagram of a NOx trap rate.
FIG. 5 is a characteristic diagram of a NOx desorption rate.
FIG. 6 is a flowchart for setting a combustion state and calculating a target equivalent ratio.
FIG. 7 is a flowchart for setting a combustion state and calculating a target equivalent ratio.
FIG. 8 is a characteristic diagram of a combustion flag.
FIG. 9 is a characteristic diagram of a lean operation permission flag.
FIG. 10 is a characteristic diagram of a target torque with respect to an accelerator opening.
FIG. 11 is a characteristic diagram of a target equivalent ratio of main injection during lean operation.
FIG. 12 is a waveform chart for explaining an operation from a cold start in one embodiment.
[Explanation of symbols]
1 Engine body
7 Fuel injection valve (fuel injection means)
10 Catalyst
11 volume part (unburned component combustion promotion means)
21 Controller
28 Catalyst temperature sensor

Claims (13)

In the active state, when the air-fuel ratio of the exhaust gas is lean, NOx is trapped, and when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or rich, the trapped NOx is desorbed. While providing a catalyst having a function of reducing and purifying using exhaust gas in the exhaust passage,
Fuel injection means for directly injecting fuel into the combustion chamber,
Main injection execution means capable of selectively performing homogeneous combustion or stratified combustion during lean operation by making the main injection timing of the fuel injection means different,
Catalyst temperature detecting means for detecting a catalyst temperature;
Additional injection performing means for performing additional injection in the expansion stroke or the exhaust stroke in the stratified combustion state, when the catalyst temperature reaches the activation temperature in the case where the enrichment processing is required in the stratified combustion state,
When performing this additional injection, the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is set to be richer than the stoichiometric air-fuel ratio, and the air-fuel ratio is set to the rich side. Rich driving means for driving the vehicle,
An exhaust gas purifying apparatus for an engine, comprising: additional injection prohibiting means for prohibiting the additional injection when the catalyst temperature is higher than the complete warm-up temperature when the enrichment process is required in the stratified combustion state. 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the activation temperature is a catalyst temperature at which NOx trapped in the catalyst can be desorbed and reduced to some extent, if not completely. 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the operation range in which the stratified combustion is performed is a low load range of the engine. 2. The exhaust gas purifying apparatus for an engine according to claim 1, further comprising an unburned component combustion promoting means for promoting the combustion of unburned components in the exhaust gas generated by performing the additional injection upstream of the catalyst. 5. The exhaust gas purifying apparatus for an engine according to claim 4, wherein the unburned component combustion promoting means is a volume in an exhaust passage upstream of the catalyst and capable of retaining and stirring exhaust gas. 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the enrichment process is forcibly performed when the engine is started. NOx trap amount estimating means for estimating the NOx trap amount trapped in the catalyst is provided. When the NOx trap amount estimated by the NOx trap amount estimating means exceeds a predetermined value VnH in a stratified combustion state, enrichment processing is required. The exhaust gas purifying apparatus for an engine according to claim 1, wherein it is determined that the following condition is satisfied. The exhaust gas purifying apparatus for an engine according to claim 7, wherein the predetermined value is a NOx trap amount at which a good NOx trap rate cannot be maintained. 8. The exhaust gas purifying apparatus for an engine according to claim 7, wherein after stopping the operation of the engine, the NOx trap amount is stored at least until restarting. Additional injection executing means for performing additional injection in an expansion stroke or an exhaust stroke in a stratified combustion state when the catalyst temperature has not reached the activation temperature in a case where the enrichment processing is required in the stratified combustion state; When performing the above, the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is set slightly leaner than the stoichiometric air-fuel ratio, and the air-fuel ratio at the air-fuel ratio set on the lean side is set. The exhaust gas purifying apparatus for an engine according to claim 1, further comprising a lean operating means for performing an operation. When executing the additional injection, the air-fuel ratio setting means for setting the air-fuel ratio determined by the total fuel of the additional injection and the main injection and the amount of intake air to be richer than the stoichiometric air-fuel ratio is provided by NOx removal per unit time. Means for calculating a separation amount; means for calculating a reducing agent amount corresponding to the NOx desorption amount per unit time as a target reducing agent supply amount per unit time; and setting the intake air flow rate to the target per unit time. A means for calculating the air-fuel ratio that can achieve the target reducing agent supply amount per unit time at the current intake air flow rate by dividing by the reducing agent supply amount, and executing the additional injection by subtracting this from the stoichiometric air-fuel ratio 2. The exhaust gas purifying apparatus for an engine according to claim 1, further comprising means for setting the air-fuel ratio on the rich side when the engine is running. In the active state, when the air-fuel ratio of the exhaust gas is lean, NOx is trapped, and when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or rich, the trapped NOx is desorbed. While providing a catalyst having a function of reducing and purifying using exhaust gas in the exhaust passage,
Fuel injection means for directly injecting fuel into the combustion chamber,
Main injection execution means capable of selectively performing homogeneous combustion or stratified combustion during lean operation by making the main injection timing of the fuel injection means different,
Catalyst temperature detecting means for detecting a catalyst temperature;
When the catalyst temperature has not reached the activation temperature when the enrichment process is required in the stratified combustion state, additional injection execution means for performing additional injection in the expansion stroke or the exhaust stroke in the stratified combustion state,
When performing this additional injection, the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is set slightly leaner than the stoichiometric air-fuel ratio, and the air set on the lean side is set. Lean operating means for operating at a fuel ratio;
When the enrichment process becomes necessary in the stratified combustion state, the additional injection is performed by continuously performing the additional injection when the catalyst temperature reaches the activation temperature by the operation at the air-fuel ratio set to the lean side accompanied by the additional injection. Continuation execution means,
When the additional injection is continuously performed, the air-fuel ratio determined by the total fuel amount of the additional injection and the main injection and the intake air amount is set to be richer than the stoichiometric air-fuel ratio, and the air-fuel ratio set to the rich side is set. An exhaust purification device for an engine, comprising: rich operation means for performing the operation in the engine. When shifting from the operation with the air-fuel ratio set on the lean side with additional injection to the operation with the air-fuel ratio set on the rich side with additional injection, the main injection air determined by the fuel amount of the main injection and the intake air amount 13. The exhaust gas purifying apparatus for an engine according to claim 12, wherein the fuel amount of the additional injection is increased without changing the fuel ratio.

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