JP3489410B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine that performs lean air-fuel ratio operation leaner than the stoichiometric air-fuel ratio under required operating conditions.

[0002]

2. Description of the Related Art Conventionally, as an exhaust gas purifying apparatus for an internal combustion engine operated with a lean air-fuel ratio, a NOx storage catalyst is arranged in an exhaust pipe, and the NOx storage catalyst discharges the engine from the engine during operation with a lean air-fuel ratio. Absorbs NOx and NO
A technique is known in which, after absorbing x, the NOx storage catalyst is operated at a rich air-fuel ratio to desorb NOx absorbed by the NOx storage catalyst, and the NOx is reduced by HC and CO contained in the exhaust gas.

The maximum amount of NOx absorbed by the NOx storage catalyst decreases as the NOx storage catalyst deteriorates.

Therefore, a determination means for determining the degree of deterioration of the NOx storage catalyst is provided, and the allowable absorption level of NOx of the NOx storage catalyst is changed according to the determination, and the lean air-fuel ratio is changed to the rich air-fuel ratio. What controls the switching timing etc. is proposed.

As the determination means, when the air-fuel ratio is switched from lean to rich, the exhaust gas downstream of the NOx storage catalyst temporarily becomes the stoichiometric air-fuel ratio and then becomes rich, and NOx
Since the state of the stoichiometric air-fuel ratio changes according to the deterioration of the storage catalyst, an air-fuel ratio sensor that generates an output proportional to the air-fuel ratio in the exhaust gas is provided downstream of the NOx storage catalyst, and when switching from lean to rich. , N based on the difference in the output peak generated by the air-fuel ratio sensor or the changing process of the output.
The degree of deterioration of the Ox storage catalyst is determined.

Therefore, during operation with a lean air-fuel ratio,
The NOx emission amount of the engine is calculated from the engine load and engine speed, etc. to estimate the NOx absorption amount absorbed by the NOx storage catalyst. When the estimated NOx absorption amount reaches the allowable absorption level, the air-fuel ratio is temporarily changed. Control so that the NOx storage catalyst becomes rich, and, in accordance with the determination of the degree of deterioration of the NOx storage catalyst, NO
By changing the allowable absorption level of x, NOx is purified according to the amount of NOx absorbed by the NOx storage catalyst.

When the degree of deterioration of the NOx storage catalyst becomes large, the operating time is lengthened while the rich degree is being reduced during operation at a rich air-fuel ratio, and is thereby regenerated (JP-A-8-014030). , 232644, etc.).

[0008]

However, in such a conventional device, the NOx emission amount of the engine varies depending on the atmospheric pressure, the dry and wet temperature, the EGR amount (exhaust gas recirculation amount), the warm-up state, and the like. NO from engine load, engine speed, etc.
Even if the NOx absorption amount of the NOx storage catalyst is estimated by calculating the x emission amount, it is difficult to obtain an accurate value.

Therefore, even if the amount of NOx discharged, that is, the amount of NOx to be absorbed by the NOx storage catalyst, exceeds the allowable absorption level, lean operation may be continued and NOx may be released to the atmosphere. is there.

In this case, even if the allowable absorption level is changed according to the degree of deterioration of the NOx storage catalyst, if rich operation is performed based on the estimated NOx absorption amount, NOx will still occur.
May be released to the atmosphere.

Further, if the permissible absorption level is lowered to advance the rich operation time, the operation period at the lean air-fuel ratio is shortened and the fuel consumption effect is impaired.

An object of the present invention is to provide an exhaust gas purification device which can accurately grasp the NOx absorption level of a NOx storage catalyst and can appropriately control the switching of operation from a lean air-fuel ratio to a rich air-fuel ratio.

[0013]

According to a first aspect of the present invention, an NOx storage catalyst is arranged in an exhaust pipe, and a lean air-fuel ratio operation that is leaner than a theoretical air-fuel ratio is performed under required operating conditions. Air-fuel ratio sensors that generate outputs proportional to the oxygen concentration in the exhaust gas are installed in the exhaust pipes before and after the NOx storage catalyst, and the NOx storage catalyst is output based on the outputs of these air-fuel ratio sensors during lean air-fuel ratio operation. NO
A NOx absorption level determination means for determining the absorption level of x is provided , and at the time of determining the absorption level of NOx
The output of the air-fuel ratio sensor in front of the NOx storage catalyst.
The average value of the current output value and the previous output value
To use .

A second aspect of the present invention is an internal combustion engine in which a NOx storage catalyst is arranged in an exhaust pipe and a lean air-fuel ratio operation leaner than a stoichiometric air-fuel ratio is performed under required operating conditions.
When an air-fuel ratio sensor that produces an output proportional to the oxygen concentration in the exhaust gas is installed in each of the exhaust pipes before and after the NOx storage catalyst, and when the outputs of these air-fuel ratio sensors become substantially equal during lean air-fuel ratio operation In addition to the provision of air-fuel ratio switching means for temporarily switching the air-fuel ratio to a rich air-fuel ratio equal to or higher than the stoichiometric air-fuel ratio, when comparing the outputs of this air-fuel ratio sensor,
The output of the air-fuel ratio sensor in front of the NOx storage catalyst
The average value of the current output value and the previous output value.
There is .

A third aspect of the present invention is the fuel cell system according to the first or second aspect, further comprising output level adjusting means for adjusting the output levels of the air-fuel ratio sensors before and after the NOx storage catalyst.

In a fourth aspect based on the third aspect, the output level adjusting means adjusts the output level of the air-fuel ratio sensor based on the output of the air-fuel ratio sensor during the fuel cut operation and the stoichiometric air-fuel ratio operation.

[0017]

[Effects of the Invention] During lean air-fuel ratio operation, while the NOx storage catalyst is absorbing NOx, the oxygen concentration in the exhaust gas before and after the NOx storage catalyst is different, and NOx of the NOx storage catalyst is different.
When the absorption amount of is maximum (saturated state), the oxygen concentrations in the exhaust gas before and after the NOx storage catalyst become substantially equal.

Therefore, according to the first invention, NOx
Based on the output of the air-fuel ratio sensor provided before and after the storage catalyst, the NOx absorption level of the NOx storage catalyst, that is, the engine operating conditions, the saturation state of the NOx storage catalyst at that time including the deterioration of the NOx storage catalyst, etc. You can judge.

According to the second aspect of the present invention, during the lean air-fuel ratio operation, the operating conditions of the engine, the deterioration of the NOx storage catalyst, etc. are included.
When the NOx storage catalyst approaches the saturation state at that time, the rich air-fuel ratio operation above the stoichiometric air-fuel ratio is performed to
Since the release of x to the outside can be sufficiently reduced and the lean air-fuel ratio operation can be performed near the saturation state of the NOx storage catalyst, fuel efficiency can be sufficiently improved.

According to the third aspect of the present invention, it is possible to eliminate the individual difference of the air-fuel ratio sensor and to accurately control the switching from the lean air-fuel ratio operation to the rich operation.

According to the fourth invention, the output level of the air-fuel ratio sensor can be adjusted appropriately.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, 10 is an engine main body, and 11
Indicates an intake passage (intake pipe), 12 indicates an exhaust passage (exhaust pipe), and the engine body 10 is provided with a fuel injection valve 13 for injecting fuel directly into the combustion chamber.

From the fuel injection valve 13,
The fuel is injected in the latter half of the compression stroke to form a combustible air-fuel mixture layer only near the spark plug 14 at the compression top dead center, so that the overall air-fuel ratio exceeds A / F = 40. Stratified combustion is performed, and fuel is injected in the intake stroke in the medium load region and high load region of the engine, and the fuel and air are premixed in the entire combustion chamber, and the stoichiometric air-fuel ratio and the air-fuel mixture richer than the stoichiometric air-fuel ratio are mixed. It is designed to perform homogeneous combustion in.

A NOx storage catalyst 15 is arranged in the exhaust passage 12. The NOx storage catalyst 15 is made of alumina or the like on a carrier, a noble metal such as platinum Pt, an alkali metal such as potassium K, sodium Na, lithium Li, or cesium Cs, an alkaline earth such as barium Ba or calcium Ca, At least one of rare earth elements such as lanthanum La and yttrium Y is carried.

As the NOx storage catalyst 15, platinum P
For example, when using t and barium Ba, NOx discharged from the engine during operation at a lean air-fuel ratio is bound to oxygen O ₂ on platinum Pt and is absorbed by barium Ba in the form of barium nitrate. . In this state,
When operating at a rich air-fuel ratio equal to or higher than the stoichiometric air-fuel ratio, the absorbed NOx is desorbed and reduced by the action of HC and CO contained in the exhaust gas.

On the upstream side and the downstream side of the exhaust passage 12 of the NOx storage catalyst 15, first and second air-fuel ratio sensors 16 and 1 which generate outputs proportional to the oxygen concentration in the exhaust gas, respectively.
7 are installed and these outputs are control unit 1
8 is input.

As means for detecting the operating conditions of the engine, a rotation speed sensor (crank angle sensor) 20 for detecting the rotation speed and crank angle of the engine, and an intake air amount (load) of the engine.
An intake sensor 21 for detecting the intake air, an accelerator opening sensor 22 for detecting the accelerator opening, a water temperature sensor 23 for detecting the cooling water temperature of the engine, etc. are provided, and these signals are also input to the control unit 18.

Based on these sensor outputs and signals, the control unit 18 controls the fuel injection amount and injection timing so that stratified charge combustion and homogeneous combustion are performed in accordance with the operating conditions, and the air-fuel ratio sensors 16 and 17 are controlled. From output to N
The NOx absorption level of the Ox storage catalyst 15 is determined, and during lean operation (during stratified charge combustion operation), control is performed so as to temporarily switch to rich operation at or above the stoichiometric air-fuel ratio according to the determination.

Next, the control contents will be described based on the flowcharts of FIGS. It should be noted that these flows are executed at a predetermined control cycle.

As shown in FIG. 2, in step 1, the basic fuel injection amount (or injection time) Tp is calculated from the engine speed, the intake air amount, and the like.

In step 2, the target air-fuel ratio KMAP is obtained from the target air-fuel ratio map determined based on the engine speed and the target torque (basic fuel injection amount Tp, etc.). KMAP <1.0 (lean than theoretical air-fuel ratio) in the low speed and low load range of the engine, KMAP = 1.0 (theoretical air-fuel ratio) in the medium and medium speed load range, and KMAP> 1.0 (theoretical) in the high speed and high load range. Richer than the air-fuel ratio).

At step 3, the target air-fuel ratio KMAP ≧
Determine whether it is 1.0.

When the target air-fuel ratio KMAP ≧ 1.0, the value of KMAP is set to the air-fuel ratio correction coefficient K in step 4, and the λ control convergence (operation of KMAP = 1.0 is executed in steps 5 to 7). When the actual air-fuel ratio converges within a predetermined range around the theoretical air-fuel ratio by feedback control of the air-fuel ratio based on the output of the air-fuel ratio sensor 16, λ control convergence flag FLGLMD is set to 1 and λ control is performed. If not, or if the λ control is not converged, the λ control convergence flag FLGLMD is cleared.

When the target air-fuel ratio KMAP <1.0, that is, in the lean air-fuel ratio range, the λ control convergence flag FLGLMD is cleared in steps 8 and 9 and whether the rich request flag FLGRICH is present (= 1). To confirm.

When the rich request flag FLGRICH is not present, the value of KMAP is set to the air-fuel ratio correction coefficient K in steps 10 and 11, and the first and second air-fuel ratio sensors 16 before and after the NOx storage catalyst 15 are set. Difference of outputs of 17 DLTA
F (delta A / F) is compared with the set value RICH1.

During operation of the lean air-fuel ratio (except during fuel cut), while the NOx storage catalyst 15 is absorbing NOx, the oxygen concentration in the exhaust gas before and after the NOx storage catalyst 15 is different, and the NOx storage catalyst 15 is different. When the NOx absorption amount of 15 becomes maximum (saturated state), the oxygen concentrations in the exhaust gas before and after the NOx storage catalyst 15 become substantially equal. That is, during operation of the lean air-fuel ratio, when the NOx storage catalyst 15 absorbs NOx, oxygen in the exhaust gas is consumed (as described above, NO
x and oxygen O ₂ are bound to each other on platinum Pt of the catalyst 15 and barium Ba absorbs NOx in the form of barium nitrate. On the other hand, when the NOx storage catalyst 15 stops absorbing NOx, oxygen in the exhaust gas is consumed. Therefore, if the outputs of the first and second air-fuel ratio sensors 16 and 17 before and after that are different, the NOx absorption amount of the NOx storage catalyst 15 represents a non-saturation level, and the first and second air-fuel ratio sensors. 1
If the outputs of 6 and 17 are almost the same, the NOx storage catalyst 1
The NOx absorption amount of 5 represents the saturation level.

Therefore, in step 11, DLTAF
Is the set value RICH1 or more, the NOx storage catalyst 15
It can be determined that the NOx absorption amount of the
If LTAF is within the set value RICH1, it can be determined that the NOx absorption amount of the NOx storage catalyst 15 is close to the saturation level.

If it is at the unsaturated level, the lean operation is continued.

When it is close to the saturation level, the NOx storage catalyst 1
In order to desorb and reduce NOx in step 5, steps 12, 1
At 3, the rich request flag FLGRICH is set to 1, and the rich operation time CHARGE is set to the predetermined time data MAXNO.

The time data MAXNO is set to a time for desorbing the maximum amount of NOx that can be absorbed by the NOx storage catalyst 15 without deterioration. Since the desorption rate of NOx changes depending on the temperature state of the NOx storage catalyst 15 (the higher the temperature, the faster), the temperature of the NOx storage catalyst 15 or the temperature of the exhaust gas before and after that is detected and the temperature is detected according to the temperature. The time data MAXNO may be changed.

In steps 14 to 16, if there is a fuel cut request (for example, during deceleration), the fuel cut flag FLGF.
If CT is set to 1 and there is no fuel cut request, the fuel cut flag FLGFCT is cleared.

On the other hand, in step 9, the rich request flag F
If LGRICH is present (= 1), a predetermined air-fuel ratio value KRICH is set to the air-fuel ratio correction coefficient K in step 17, and rich operating time CHARG is set in steps 18 and 19.
At the same time as counting E, the fuel cut flag FLGFCT is cleared in step 16.

The air-fuel ratio value KRICH is set to a rich air-fuel ratio which is equal to or higher than the stoichiometric air-fuel ratio. Since the desorption rate of NOx changes depending on the temperature state of the NOx storage catalyst 15, the temperature of the NOx storage catalyst 15 or the temperature of exhaust gas before and after that is detected, and the air-fuel ratio value KR is detected according to the temperature.
The ICH may be changed.

When the rich operation time CHARGE has elapsed, the rich request flag FLGRICH is determined in step 20.
To clear.

In step 21, the set air-fuel ratio correction coefficient K is multiplied by the basic fuel injection amount Tp to determine the fuel injection amount (or injection time) TMINJ. During operation of the target air-fuel ratio KMAP = 1.0, the air-fuel ratio sensor 16
The feedback coefficient of the air-fuel ratio based on the output of is set.

FIG. 3 shows the first and second air-fuel ratio sensors 16, 1.
7, a flow for calculating the difference DLTAF of the outputs of FIG.
Is a flow for adjusting the output levels of the first and second air-fuel ratio sensors 16, 17.

In FIG. 3, in step 31, it is confirmed whether or not the rich request flag FLGRICH is present (= 1).

When the rich request flag FLGRICH is present, the lean operation is not being performed, and therefore the following processing is not performed,
If there is no rich request flag FLGRICH, execute.

In step 32, the first air-fuel ratio sensor 1
The output AF1 of 6 is read, and the average value of the current value AF1 _NEW and the previously read value AF1 _OLD is set as the output value AF1 of the first air-fuel ratio sensor 16. This averaging process is
This is performed because the air-fuel ratio of the exhaust gas upstream of the NOx storage catalyst 15 changes every moment in the operating state of the engine. Note that the above-mentioned NO
Due to the oxygen consumption characteristic of the x storage catalyst 15, the air-fuel ratio of the exhaust gas downstream of the NOx storage catalyst 15 does not change as much as upstream.

In step 33, the output value AF1 of the first air-fuel ratio sensor 16 and the corrected output value RRA of the second air-fuel ratio sensor 17 whose output level has been corrected by the processing of FIG.
DLTAF is calculated from the difference from F2.

In FIG. 4, in steps 41 and 42, the λ control convergence flag FLGLMD is 1, that is, the first and the second when the operation is performed at the stoichiometric air-fuel ratio (within a predetermined range around the stoichiometric air-fuel ratio). The outputs AF1 and AF2 of the two air-fuel ratio sensors 16 and 17 are read, the output AF1 of the first air-fuel ratio sensor 16 is stored in the memory Y2, and the output AF2 of the second air-fuel ratio sensor 17 is stored in the memory X2. .

In steps 43 and 44, the outputs AF1 and AF of the first and second air-fuel ratio sensors 16 and 17 are set when the fuel cut flag FLGFCT is 1, that is, when the fuel injection amount is 0.
2 is read, and the output AF of the first air-fuel ratio sensor 16
1 in the memory Y1 and the output AF of the second air-fuel ratio sensor 17
2 is stored in the memory X1.

Since the fuel cut may not be performed, in this case, in steps 45 and 46, the memories Y1 and X
A predetermined value AFFCT is set to 1.

In step 47, memories Y2, X2, Y
The values of 1, X1 and the second air-fuel ratio sensor 17 currently read
From the output AF2 of RRAF2 = (Y2-Y1) /
(X2-X1) x (AF2-X1) + Y1 gives the second
The corrected output value RRAF2 of the air-fuel ratio sensor 17 is calculated.

That is, the first and second air-fuel ratio sensors 16 and 17 at the time of fuel cut when the oxygen concentration is maximum and at the stoichiometric air-fuel ratio.
Since the output of the second air-fuel ratio sensor 17 is corrected on the basis of the output of, the output levels of the first and second air-fuel ratio sensors 16 and 17 can be appropriately adjusted regardless of the individual difference of the sensors. . Therefore, the first and second air-fuel ratio sensors 16,
Based on the output of 17, it is possible to accurately detect the difference DLTAF between the air-fuel ratios before and after the NOx storage catalyst 15.

With this configuration, when NOx exhausted from the engine during lean operation is absorbed by the NOx storage catalyst 15, and the amount of NOx absorbed by the NOx storage catalyst 15 approaches the saturation level (allowable absorption level). , The air-fuel ratio (oxygen concentration) of the exhaust gas before and after the NOx storage catalyst 15 becomes substantially equal, and this is the first and second air-fuel ratio sensors 16,
17, the rich operation above the stoichiometric air-fuel ratio is performed for a predetermined time, the NOx absorbed by the NOx storage catalyst 15 is desorbed by the rich operation, and is reduced by HC and CO in the exhaust gas. .

In this case, in order to estimate the NOx absorption amount of the NOx storage catalyst by obtaining the NOx emission amount of the engine from the engine load, the rotational speed, etc., the NOx emission amount of the engine is at atmospheric pressure, dry and wet temperature, The saturation level of the NOx storage catalyst cannot be accurately grasped because it varies depending on the EGR amount (exhaust gas recirculation amount), the warm-up state, and the like, but it is possible that the air-fuel ratios of the exhaust gas before and after the NOx storage catalyst 15 become substantially equal. Use, N
When the Ox storage catalyst 15 approaches the saturation level, the first and second
The air-fuel ratio sensors 16 and 17 of FIG.

Further, even if the NOx storage catalyst 15 is deteriorated, the NOx storage catalyst 1 is not provided with the deterioration determining means.
When 5 approaches the saturation level in the state at that time, that is, when the NOx amount that the NOx storage catalyst 15 can absorb in the state at that time approaches regardless of the engine operating conditions, the deterioration of the NOx storage catalyst 15, and the like, It is accurately grasped by the first and second air-fuel ratio sensors 16 and 17.

Therefore, the switching to the rich operation can be performed accurately, and the NOx absorption amount of the NOx storage catalyst exceeds the saturation level and the saturation level at that time when the NOx storage catalyst deteriorates as in the conventional example. There is no chance that lean operation will be continued without knowing this.

Therefore, the release of NOx to the outside can be sufficiently reduced. Further, since the lean operation can always be performed up to a saturation level which can be absorbed by the NOx storage catalyst 15, the lean operation period is extended and the fuel consumption can be improved.

As described above, the air-fuel ratio sensor 1 is based on the fuel cut time when the oxygen concentration is maximum and the stoichiometric air fuel ratio.
Since the output levels of 6 and 17 are adjusted, the difference DLTAF between the air-fuel ratios before and after the NOx storage catalyst 15 can be accurately detected, and therefore the switching control from lean operation to rich operation can be performed more accurately.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment.

FIG. 2 is a flowchart showing control contents.

FIG. 3 is a flowchart showing control contents.

FIG. 4 is a flowchart showing control contents.

[Explanation of symbols]

10 engine body 11 Intake pipe 12 Exhaust pipe 13 Fuel injection valve 14 Spark plug 15 NOx storage catalyst 16,17 Air-fuel ratio sensor 18 control unit 20 Revolution sensor (crank angle sensor) 21 Intake sensor 22 Accelerator position sensor 23 Cooling water temperature sensor

─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-8-177571 (JP, A) JP-A-11-93744 (JP, A) JP-A-11-93743 (JP, A) JP-A-11- 107741 (JP, A) JP-A-61-286549 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) F02D 41/00-41/40 F01N 3 / 08,3 / 20

Claims (4)

(57) [Claims] 1. An internal combustion engine in which a NOx storage catalyst is arranged in an exhaust pipe and which performs lean air-fuel ratio operation leaner than a theoretical air-fuel ratio under required operating conditions, in exhaust pipes before and after the NOx storage catalyst. An air-fuel ratio sensor that generates an output that is proportional to the oxygen concentration in the exhaust gas is installed, and the NOx absorption level of the NOx storage catalyst is determined based on the output of these air-fuel ratio sensors during lean air-fuel ratio operation. Rutotomoni provided level determining means, upon determination of the absorption level of the NOx, catalyst NOx storage
Only the output of the air-fuel ratio sensor in front of the medium
An exhaust emission control device for an internal combustion engine, which uses an average value of a force value and a previous output value . 2. In an internal combustion engine in which a NOx storage catalyst is arranged in the exhaust pipe and which performs lean air-fuel ratio operation leaner than the stoichiometric air-fuel ratio under required operating conditions, the exhaust pipe before and after the NOx storage catalyst is provided. An air-fuel ratio sensor that generates an output proportional to the oxygen concentration in the exhaust gas is installed, and the air-fuel ratio is temporarily changed to the theoretical air-fuel ratio when the outputs of these air-fuel ratio sensors become almost equal during lean air-fuel ratio operation. co When Ru is provided an air-fuel ratio switching means for switching to a rich air-fuel ratio of the above
In comparing the output of this air-fuel ratio sensor,
Only the output of the air-fuel ratio sensor in front of the medium
An exhaust emission control device for an internal combustion engine, which uses an average value of a force value and a previous output value . 3. The exhaust emission control device for an internal combustion engine according to claim 1, further comprising output level adjusting means for adjusting an output level of the air-fuel ratio sensor before and after the NOx storage catalyst. 4. The exhaust gas purification of an internal combustion engine according to claim 3, wherein the output level adjusting means adjusts the output level of the air-fuel ratio sensor based on the output of the air-fuel ratio sensor during fuel cut operation and stoichiometric air-fuel ratio operation. apparatus.