JPH09317447A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately enrich a mixture while no excessive enrichment by correcting the total amount of SOx when the air-fuel ratio in exhaust gas is lean based on the permeability of SOx in an NOx absorbent for the enrichment process depending on the corrected total amount of SOx. SOLUTION: On the basis of the output from an engine running condition detection means 51, a lean/rich decision means 52 calculates a correction coefficient K based on its map. If K<1.0 where the decision means has judged a lean mixture combusted, a total NOx amount estimation means 53 calculates the total amount of NOx that an NOx absorbent absorbs. If the total absorbed NOx amount is of a predetermined value or above, an NOx emission means 58 enriches the air-fuel ratio in exhaust gas and emits the NOx. In the above judgment, a total SOx amount estimation means 63 controls the amount of SOx based on the amount of SOx that the NOx absorbent absorbs such that the SOx absorbed by the NOx absorbent is emitted for the enrichment process.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.

[0002]

2. Description of the Related Art As a conventional exhaust gas countermeasure, in order to prevent air pollution caused by nitrogen oxides (NOx) contained in the exhaust gas, an exhaust gas purification device in which an NOx absorbent is arranged in an exhaust pipe of an internal combustion engine is used. Being developed.

This NOx absorbent absorbs NOx when the air-fuel ratio of the exhaust gas discharged from the combustion chamber is lean, and when the air-fuel ratio of the exhaust gas becomes rich and the oxygen concentration drops, the absorbed NOx. To release.

By the way, since sulfur is contained in the fuel and the lubricating oil of the engine, not only NOx but also SOx is contained in the exhaust gas, and this SOx is absorbed by the NOx absorbent together with NOx. To be done.

In order to prevent deterioration of the NOx absorbent, the absorbed SOx also needs to be released from the absorbent. Therefore, as with the release of NOx, the air-fuel ratio of the exhaust gas flowing into the absorbent is made rich for a certain period of time, but SOx is NO.
Since SOx is less likely to be activated than SOx, SOx release control needs to be performed separately from NOx release control.

For example, in Japanese Patent Laid-Open No. 6-229232,
In addition to the NOx absorbent, the SOx absorbent is arranged on the upstream side of the exhaust pipe, and in the rich process, the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent is first set to the first rich degree set in advance, and then , The rich degree is smaller than that rich degree.

This rich process is performed by the S absorbed by the absorbent.
Since it is performed according to the amount of Ox, it is necessary to estimate the total amount of SOx absorbed in advance. Total SO absorbed by the absorbent
As a method of estimating the x amount, a method of estimating from the fuel injection amount at lean and the engine operating state is known. In JP-A-6-229232, attention is paid to the fact that the SOx amount generated increases as the intake air amount Q increases, and the product of the intake intake amount and a predetermined constant is calculated as the total SOx already absorbed by the absorbent.
A method is presented in which the value obtained by adding the amount (initial value is 0) is regarded as the total SOx amount.

[0008]

However, not all SOx emitted from the internal combustion engine is absorbed by the absorbent, but some of the SOx passes through the absorbent without being absorbed.

Therefore, in the amount estimation method as described above, the amount of SOx that has passed through the absorbent is not included, and the estimated amount of SOx absorbed is estimated to be larger than the actual amount of absorption.

In this case, the rich process is performed more than necessary, which causes a problem that fuel consumption is deteriorated. Also, as SOx is absorbed by the absorbent, it eventually becomes saturated and its absorption capacity reaches a peak.
In the conventional method, even after the saturated state is reached, the absorbed amount is estimated to be larger than the saturated state. Therefore, useless rich processing corresponding to the SOx amount equal to or larger than the saturated amount is performed.

The present invention has been made in view of the above points, and it is a technical object to perform an accurate rich process while omitting a wasteful rich process.

[0012]

In order to solve the above problems, the present invention has the following means. That is, NOx which is arranged in the exhaust passage of the internal combustion engine and absorbs SOx together with NOx contained in the exhaust gas discharged from the internal combustion engine.
In order to release SOx from the absorbent, the total SOx amount estimating means for estimating the total SOx amount absorbed by the NOx absorbent from the operating state of the internal combustion engine and the total SOx amount when the air-fuel ratio of the exhaust gas is lean. Total SO estimated by quantity estimation means
When the x amount becomes equal to or more than a predetermined value, the total SOx amount estimating means includes an SOx releasing means for enriching the air-fuel ratio of the exhaust gas to release SOx. When the air-fuel ratio of the gas is lean, the total SOx amount is corrected to exclude the SOx passage amount in the NOx absorbent by using the SOx passage rate in the NOx absorbent estimated corresponding to the exhaust temperature. The discharging means is characterized by performing a rich process according to the total SOx amount corrected by the total SOx amount estimating means.

When correcting the total SOx amount using the SOx passage rate, when the air-fuel ratio of the exhaust gas is lean, internal or external to the total SOx amount estimating means,
SOx in NOx absorbent estimated according to exhaust temperature
Using the passage rate, the total SOx should be excluded so as to exclude the SOx passage portion.
The total SOx absorption amount correction means for correcting the total SOx amount estimated by the amount estimation means can be considered.

The present invention is directed to the total S absorbed by the NOx absorbent.
When estimating the Ox amount by the total SOx amount estimating means, the Ox amount is corrected in consideration of the SOx passage rate in the NOx absorbent, which changes according to the exhaust temperature. For example, if the passage rate is 20%, 80% will be absorbed, so the total SOx amount initially estimated by the total SOx amount estimating means is multiplied by 0.8 to obtain the total SOx that would have actually been adsorbed. Estimate the quantity. The SOx releasing means performs a necessary rich process according to the obtained corrected total SOx amount.

Further, here, the larger the total amount of SOx absorbed by the NOx absorbent (which may be before or after correction by the SOx passage rate), the greater the NOx.
It is also possible to provide auxiliary correction means for correcting the SOx passage rate in the absorbent to a high value and correcting the total SOx amount to be estimated by the total SOx amount estimating means.

That is, in the case of NOx absorbent, SO
The closer the x absorption amount is to the saturated state, the higher the SOx passage rate, and accordingly, the smaller the SOx amount that is absorbed.
Therefore, in this case, the total SOx amount can be corrected in consideration of the SOx passage rate according to the SOx absorption amount, and the appropriate rich processing is performed.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. <Description of Device Configuration> First, as shown in FIG. 1, an engine that is an internal combustion engine includes an engine body 1, a piston 2, a combustion chamber 3, an ignition plug 4, an intake valve 5, an intake port 6, an exhaust valve 7,
It is constituted by the exhaust port 8. Intake port 6
Is connected to a surge tank 10 through a branch pipe 9, and a fuel injection valve 11 that injects fuel into the intake port 6 is attached to the branch pipe 9. The surge tank 10
The intake duct 12 with the throttle valve 15 installed inside
An air cleaner 14 is connected to the intake duct 12 via an air flow meter 13.

On the other hand, an exhaust manifold 16 is connected to the exhaust port 8 and the exhaust pipe 1 is connected to the exhaust manifold 16.
A casing 20 is connected via 7. A NOx absorbent 19 is arranged inside the casing 20.

Next, an electronic control unit 30 for controlling the internal combustion engine is provided. This electronic control unit 30
ROM (Read Only Memory) which is composed of a digital computer and mutually connected by a bidirectional bus 31.
32, RAM (random access memory) 33, CPU
It has a (microprocessor) 34, a backup RAM 33a constantly connected to a power supply, an input port 35, and an output port 36.

The air flow meter 13 generates an output voltage proportional to the amount of intake air, and this output voltage is input to the input port 35 via the AD converter 37. A temperature sensor 21 that generates an output voltage proportional to the exhaust temperature is arranged in the exhaust pipe 17 in front of the casing 20, and the output voltage of the temperature sensor 21 is input to an input port 35 via an AD converter 38. To be done. The exhaust temperature T can be directly detected by the temperature sensor 21, but can also be estimated from the intake air amount Q and the engine speed N. In this case, the relationship between the exhaust temperature T, the intake air amount Q, and the engine speed N is obtained in advance by experiments, this relationship is stored in the ROM 32 in the form of a map, and the exhaust temperature T is calculated from this map.

Further, the input port 35 is connected with a rotation speed sensor 22 for generating an output pulse representing the engine rotation speed. On the other hand, the spark plug 4 and the fuel injection valve 11 are connected to the output port 36 via a drive circuit 39, respectively.

The fuel injection by the fuel injection valve 11 is controlled as follows. That is, the CPU is
First, based on the following formula, fuel injection time (fuel injection amount) TA
Calculate U.

TAU = TP · K Here, TP represents a basic fuel injection time (basic fuel injection amount), and K represents a correction coefficient.

The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. The basic fuel injection time TP is calculated in advance by an experiment, and the engine load Q / N
It is expressed as a function of (intake air amount Q / engine speed N) and engine speed N.

The basic fuel injection time TP is stored in advance in the ROM 32 in the form of a map as shown in FIG. Next, the correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder is the theoretical air-fuel ratio. It becomes the fuel ratio.

On the other hand, when K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes a value larger than the stoichiometric air-fuel ratio and becomes lean, and K> 1.0. Then, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes a value smaller than the stoichiometric air-fuel ratio and becomes rich.

The correction coefficient K is controlled according to the operating state of the engine, and FIG. 3 shows an embodiment of the control of the correction coefficient K. That is, during the warm-up operation, as shown in FIG. 3, the correction coefficient K gradually decreases as the engine cooling water temperature increases, and when the warm-up is completed, the correction coefficient K becomes a constant value smaller than 1.0. Thus, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is maintained lean.

Next, during acceleration operation, the correction coefficient K is set to 1.0, for example, and the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes the stoichiometric air-fuel ratio. Incidentally, during full load operation, the correction coefficient K becomes larger than 1.0, and the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes rich.

In the internal combustion engine according to the present embodiment, as shown in FIG. 3, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is kept constant except during warm-up operation, acceleration operation and full load operation. The lean air-fuel ratio is maintained at, and the lean air-fuel mixture is burning in most engine operating regions.

FIG. 4 shows the outline of the concentrations of typical components contained in the exhaust gas discharged from the combustion chamber 3. The concentrations of HC and CO contained in the discharged exhaust gas increase as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer as shown in FIG. 4, and the oxygen O contained in the discharged exhaust gas O The concentration of ₂ increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

The NOx absorbent 19 disposed in the casing 20 is, for example, a monolith type carrier made of alumina or the like and potassium K, sodium Na, and
At least one selected from alkali metals such as cesium Cs, alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanum La and yttrium Y, and platinum Pt.
It is configured to carry a precious metal such as.

It is preferable that lithium Li be added to the NOx absorbent 19. This NOx absorbent 19
Absorbs NOx when the air-fuel ratio of the exhaust gas is lean and absorbs NOx when the oxygen concentration in the exhaust gas decreases.
Has the action of releasing.

Here, the air-fuel ratio of the exhaust gas is expressed as a ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage in front of the NOx absorbent 19. When fuel (hydrocarbon) or air is not supplied into the exhaust passage in front of the NOx absorbent 19, the air-fuel ratio of the exhaust gas is lean if the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes lean. Becomes

In this case, the NOx absorbent 19 is used in the combustion chamber 3
NOx when the air-fuel ratio of the mixture supplied to the inside is lean
When the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 decreases, the absorbed NOx is released. <Absorption / release of NOx> It is considered that NOx absorption is carried out by the mechanism shown in FIG. This mechanism is a case where platinum Pt and barium Ba are supported on the carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

First, when the exhaust gas becomes considerably lean, it is exhausted.
Since the oxygen concentration in the gas is greatly increased, Fig. 5 (A)
Oxygen as shown in_Two Is O_Two ^-Or O^2-In the form of platinum Pt
Adhere to the surface. Next, the NO contained in the exhaust gas is white
O on the surface of gold Pt_Two ^-Or O ^2-Reacts with NO_Two Becomes
(2NO + O_Two → 2 NO_Two ).

Thereafter, the produced NO ₂ is absorbed in the absorbent while being oxidized on the platinum Pt as long as the NOx absorbent capacity of the NOx absorbent is not saturated, and the barium oxide BaO.
As shown in FIG. 5 (A), nitrate ions NO
₃ ^- diffuses in the NOx absorbent 19 in the form of. In this way, NOx is absorbed in the NOx absorbent 19.

[0037] In contrast, if the oxygen concentration in the exhaust gas decreases, and decreases the amount of NO _2, the reverse of the reaction and the reaction ^{_{(NO 3 - -O 2 - →}} NO 2) cause. Therefore, N
The nitrate ion NO ₃ ⁻ in the Ox absorbent 19 is released from the NOx absorbent 19 in the form of NO ₂ .

That is, NOx is released from the NOx absorbent 19 when the oxygen concentration in the exhaust gas decreases. As shown in FIG. 4, when the lean degree of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, when the lean degree of the inflowing exhaust gas decreases, even if the inflowing exhaust gas becomes empty, Even if the fuel ratio is lean, N
NOx will be released from the Ox absorbent.

On the other hand, at this time, when the air-fuel mixture supplied to the combustion chamber 3 is made rich and the air-fuel ratio of the exhaust gas becomes rich, a large amount of unburned HC and CO are discharged from the engine as shown in FIG. To be done. These unburned HC and CO are made of platinum Pt.
It is immediately oxidized by reacting with the above oxygen O ₂ ^- or O ^2- .

Further, when the air-fuel ratio of the exhaust gas becomes rich, the oxygen concentration in the exhaust gas drops extremely, so NO
₃ ^- → NOx absorbent 19 reacts in NO ₂ are, NO ₂
To release. This NO ₂ is, as shown in FIG.
It is reduced by reacting with unburned HC and CO. When the NO ₂ on the platinum Pt disappears in this way, NO ₂ is released from the absorbent one after another. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbent 1
NOx is released from 9. O ₂ ⁻ or O ²⁻ on platinum Pt
If the unburned HC and CO remain even after the exhaust gas is consumed, both the NOx released from the NOx absorbent 19 and the NOx discharged from the engine are reduced by the unburned HC and CO.

Next, during warm-up operation and full-load operation, FIG.
As shown in, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich, and the air-fuel mixture becomes the stoichiometric air-fuel ratio during acceleration operation. In most other operating regions, the lean air-fuel mixture is burning in the combustion chamber 3.

However, even if the NOx absorbent 19 releases NOx during full load operation and acceleration operation, if the frequency of full load operation or acceleration operation is low, while the lean air-fuel mixture is burning. NOx gradually accumulates, and the absorption capacity of NOx is saturated.

Then, after a while, the NOx absorbent 1
No. 9 cannot absorb NOx. Therefore, in order to make the NOx absorbent 19 continuously absorb NOx, even when the lean air-fuel mixture is continuously burning,
It is necessary to periodically make the air-fuel ratio of the exhaust gas rich, or periodically make the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio to release NOx from the NOx absorbent 19 periodically. <SOx absorption / release> Next, in the exhaust gas, SOx
Therefore, the NOx absorbent 19 absorbs not only NOx but also SOx. The absorption / emission principle is N
It is similar to the case of Ox. This is similar to the case of NOx
The case where platinum Pt and barium Ba are supported on the carrier will be described as an example.

First, when the air-fuel ratio of exhaust gas becomes lean, oxygen O ₂ adheres to the surface of platinum Pt in the form of O ₂ ⁻ or O ^{2 −} . Next, SO contained in the exhaust gas reacts with O ₂ ⁻ or O ^{2 −} on the surface of platinum Pt to become SO ₂ (2SO
+ O ₂ → 2SO ₂ ). After that, SO ₂ further reacts with O ₂₋ or O ^{2− on} the surface of platinum Pt to become SO ₃ . A part of the generated SO ₃ is further oxidized on platinum Pt (SO ₃ + O ² → → SO ₄ ²⁻ ) while being bonded to barium oxide BaO, and in the absorbent in the form of sulfate ion SO ₄ ²⁻ . In the unstable state of sulfate BaSO ₄ (Ba
^2+, SO ₄ ^- spread with), or stable sulfate Ba
It is absorbed in the form of SO ₄ . In this way SOx is NO
x Absorbed in the absorbent 19.

Stable sulfate BaSO ₄ is difficult to decompose, so it must be activated during the rich treatment, and it is difficult to release it. Therefore, the NOx absorbent 19
So that the absorbed SOx exists in the form of sulfate ion SO ₄ ²⁻ or the sulfate BaSO ₄ remains in the unstable state in the form of Ba ²⁺ and SO ₄ ^2−. It is desirable to do. Therefore, as the NOx absorbent, at least one selected from iron Fe, manganese Mn, nickel Ni, tin Sn, transition metals such as titanium Ti, and lithium Li is supported on a carrier made of alumina. Preferred adsorbents are.

The reason for supporting at least one selected from the transition metal and lithium Li is that when the sulfate BaSO ₄ is converted to Ba ²⁺ and SO ₄ ²⁻ which are unstable states, the transition metal and lithium Li are converted. It is because the conversion energy can be reduced by carrying such substances, so that Ba ²⁺ and SO ₄ ⁻ can be smoothly converted.

The reason for supporting platinum Pt is SO.
₂ is easily attached to the surface of the platinum Pt in the form of SO ₃ ^2,
This is because after a while, SO ₂ is easily absorbed by the NOx absorbent 19 in the form of sulfate ion SO ₄ ²⁻ .

SOx release is based on the sulfate BaSO_Four Exhaust
Ba which is unstable due to heating of gas²⁺And SO_Four ^2-
Must be converted to. In this state, exhaust
When the oxygen concentration in the gas decreases (rich processing), SO_Four
^2-But (SO_Four ^2--O^2-→ SO_Three) Reaction, and
And SO_TwoThe production amount of is reduced, which is the reverse of the absorption reaction.
(SO_Three-O^2-→ SO_TwoAwaken). Therefore, NOx absorption
SOx in the agent 19 is SO _Two NOx absorbent 19 in the form of
Is released from. <NOx and SOx release rate> NOx and SOx release is
Both are due to the decrease in oxygen concentration, so NOx absorbent
When the air-fuel ratio of the exhaust gas flowing into 19 becomes rich, N
NOx and SOx are released from the Ox absorbent 19,
Since NOx and SOx have different properties, there is a difference in release.
Occurs.

[0049] In the NOx absorbent 19, the reaction immediately NO ₂ on the platinum Pt surface is not present (NO ₃ ^- → NO
₂ ) and NOx is rapidly released from the absorbent.
Further, NO ₂ on the surface of platinum Pt will be reduced to H when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 19 is made rich.
It is immediately reduced by C and CO.

Therefore, NO ₂ on the platinum Pt surface is
Disappears rapidly. After a while, NOx is released from the NOx absorbent 19 within a short time as shown in FIG. That is, the NOx release rate of the NOx absorbent 19 is considerably high.

On the other hand, SOx is more stable than NOx and is difficult to decompose. Therefore SOx is N
If the air-fuel ratio of the exhaust gas flowing into the Ox absorbent 19 is made rich for a considerable time, it will not decompose. <Relationship between Air-Fuel Ratio in SOx Release and Temperature of NOx Absorber> FIG. 7 shows a relationship between the air-fuel ratio A / F of the inflowing exhaust gas capable of releasing SOx by the NOx absorbent 19 and the temperature T of the NOx absorbent 19. Is shown.

As can be seen from FIG. 7, in order to release SOx, the lower the temperature T of the NOx absorbent 19, the more N
The air-fuel ratio of the exhaust gas flowing into the Ox absorbent 19 must be made rich.

The reason is that the decomposition action of SOx absorbed in the NOx absorbent 19 is more likely to be decomposed as the temperature of the NOx absorbent 19 is higher, and is less likely to be decomposed as the temperature of the NOx absorbent 19 is lower. Because.

Therefore, in the exhaust gas purification apparatus according to the present embodiment, in order to suppress the rich process of the air-fuel ratio of the inflowing exhaust gas to the minimum, the NOx absorbent 19 is heated by the heater to raise the temperature of the NOx absorbent 19. It is also conceivable to perform a heating treatment.

It is also possible to delay the ignition timing and open the exhaust valve while the combustion temperature in the combustion chamber is high to keep the temperature of the exhaust gas flowing in high and raise the temperature of the NOx absorbent 19. These are performed by the SOx releasing means according to the present invention. <NOx and SOx Release Timing> The NOx or SOx release means according to the present embodiment, together with the above method, periodically enriches the air-fuel mixture supplied to the combustion chamber 3 to release NOx and SOx. To do.

FIG. 8 shows the timing of making the mixture rich in this way. The symbol P in FIG. 8 is NO.
The timing of releasing NOx from the x absorbent 19 is shown, and the symbol Q shows the timing of releasing SOx from the NOx absorbent 19.

As can be seen from FIG. 8, in order to release NOx from the NOx absorbent 19, it is necessary to make the air-fuel mixture rich at a rate of once every several minutes and to make the period for making it rich considerably short. Such rich processing is performed with NOx
Is sufficient for the emission of SOx, but insufficient for the emission of SOx.

On the other hand, the amount of SOx contained in the exhaust gas is much smaller than the amount of NOx. Therefore,
It takes a considerable time for the NOx absorbent 19 to be saturated with SOx. In other words, in order to release SOx from the NOx absorbent 19, it is necessary to considerably lengthen the cycle of enriching the air-fuel mixture, such as enriching the air-fuel mixture once every several hours. Further, since SOx is hard to decompose, it is necessary to heat and activate it at a higher temperature than in the case of NOx.
Therefore, the SOx release control is performed separately from the NOx release.

As shown in FIG. 6, when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is made rich, NOx is released from the NOx absorbent 19 within a short time, but SOx is
It takes a considerable amount of time to be released from the NOx absorbent 19.

Therefore, the time period during which the air-fuel mixture is kept rich for releasing SOx is considerably longer than the time during which the air-fuel mixture is kept rich for releasing NOx. For example, when NOx is released, the air-fuel mixture is made rich for several seconds, whereas when SOx is released, the air-fuel mixture is made rich for several minutes.

As described above, when SOx is released, it is necessary to make the air-fuel mixture rich for a long time. However, since the period for making the air-fuel mixture rich in order to release SOx is long, the fuel consumption is large. It does not increase in width. <Means for rich control realized on CPU> The above-mentioned release control of NOx / SOx from the NOx absorbent is
It is performed by the following means realized by software on the CPU.

The NOx release control is performed by total NOx amount estimating means for estimating the total NOx amount absorbed in the NOx absorbent from the operating state of the internal combustion engine, and when the air-fuel ratio of the exhaust gas is lean, the total NOx amount estimation is performed. When the total NOx amount estimated by the means becomes equal to or more than a predetermined value, the NOx releasing means for releasing the NOx by increasing the air-fuel ratio of the exhaust gas is performed.

The SOx release control corresponds to the exhaust temperature when the exhaust gas air-fuel ratio is lean and the total SOx amount estimating means for estimating the total SOx amount absorbed in the NOx absorbent from the operating state of the internal combustion engine. Estimated S in NOx absorbent
Using the Ox passage rate, the total SOx estimated by the total SOx amount estimating means so as to exclude the SOx passage amount in the NOx absorbent.
If the total SOx absorption correction means for correcting the amount and the exhaust gas air-fuel ratio are lean and the total SOx estimated amount corrected by the total SOx amount correction means becomes equal to or greater than the specified value, the exhaust gas air-fuel ratio And SOx releasing means for releasing SOx. Further, as the total amount of SOx absorbed by the NOx absorbent (which may be before or after being corrected by the SOx passage rate) is increased, the SOx passage rate at the NOx absorbent is corrected to a higher value so that the total SOx An auxiliary correction means for correcting the total SOx amount to be estimated by the amount estimation means is provided as SOx passage rate correction means. More specifically, as shown in FIG. 11, by the following means implemented on the CPU. (1) An operating state detecting means 51 for detecting the operating state of the engine. Referring to the map of FIG. 3 from the detected driving state,
It is possible to obtain the correction coefficient K. (2) Lean / rich determination means 52 for determining lean combustion or rich combustion by determining whether the correction coefficient K is smaller than 1.0. (3) When this lean rich determination means satisfies K <1.0 and it is determined that the lean air-fuel mixture is burning,
Total NOx amount estimating means 53 for calculating the total NOx amount Wn absorbed in the NOx absorbent 19 by the following formula.

Wn = Wn + k1QQ / N k1 is a constant, Q is the air intake amount Q / N is the engine load (N is the engine speed) Wn is the initial value 0, and the total NOx amount estimating means is repeatedly executed. Each time, k1 · Q · Q / N is added to obtain the total NOx amount.

Here, the NOx amount discharged from the combustion chamber 3 increases as the intake air amount Q increases, and the engine load Q /
It increases as N increases. (4) NOx releasing means 58 which releases the NOx by increasing the air-fuel ratio of the exhaust gas when the total NOx amount Wn obtained by the total NOx amount estimating means 53 becomes equal to or more than a predetermined value. (5) When it is determined by the lean / rich determination means that K <1.0 is satisfied and the lean air-fuel mixture is burning,
A total SOx amount estimating means 63 for calculating the total SOx amount Ws1 absorbed by the NOx absorbent 19 by the following formula.

Ws1 = Ws1 + k2Q k2 is a constant, Q is an air intake amount Ws1 is an initial value 0, and k2Q is added every time the total SOx amount estimating means is repeatedly executed to obtain the total SOx amount.

The SOx amount discharged from the combustion chamber 3 increases as the intake air amount Q increases. (6) SO calculated from the map (FIG. 9) stored in the ROM for the SOx passage rate in the NOx absorbent corresponding to the exhaust temperature
x passage rate calculating means 64. (7) A map stored in advance in the ROM 32 and defining the relationship between changes in the total SOx absorption amount and the passage rate Ps (see FIG. 1).
0), the SOx passage rate correction means 65 for correcting the SOx passage rate Ps obtained by the SOx passage rate calculation means to Ps0.
(Auxiliary correction means). (8) The Sx passage rate Ps with respect to the exhaust gas temperature calculated by the SOx passage rate calculation means is corrected by the passage rate correction means S
Based on the Ox passing rate Ps0, the total passing Sx amount Ws0 passing through the NOx absorbent is calculated by the following formula.
Ox amount calculation means 66.

Ws0 = Ws0 + K1.Q.Ps0 / 100 Ws0 has an initial value of 0, and K1.Q.Ps0 / 100 is added every time the total passing SOx amount calculating means is repeatedly executed to calculate the total passing SOx amount. To be done. (9) Total passing SOx amount total passing SOx calculated by the calculation means
Total SOx amount W calculated by total SOx amount estimating means Ws0
Total SOx amount correction means 67 for subtracting from S1 to calculate the true total SOx amount Ws2 absorbed in the NOx absorbent by the following formula.

Ws2 = Ws1-Ws0 (10) Total SOx amount Ws2 which is the true total SOx amount obtained by the correction means
SOx release means 68 which releases the SOx by increasing the air-fuel ratio of the exhaust gas when the value becomes equal to or more than a predetermined value.

The SOx releasing means is the S of the NOx absorbent 19
To activate Ox, heat the NOx absorbent with a heater, or open the exhaust valve while the ignition timing of the spark plug is delayed and the combustion temperature in the combustion chamber is high to raise the temperature of the exhaust gas flowing in. Control is performed to raise the temperature of the maintained NOx absorbent 19. In the present embodiment, the total S
Ws1 is calculated by the Ox amount estimating means, and then the total passing SOx is calculated.
The amount calculation means calculates Ws0, and Ws1-Ws0 is calculated as total SOx.
Although Ws2 is obtained by performing the amount correction means, the total SOx amount may be calculated in advance in consideration of the NOx passing rate in the NOx absorbent by the following formula.

Ws2 = Ws2 + K1.Q. (100-Ps0) / 100 In this sense, it can be said that the total SOx absorption amount correction means for correcting the total SOx amount is inherent in the SOx amount estimation means.

Since each of the above means is realized by a program, it cannot be physically recognized.
It exists as a mixture in the program. <Rich Control> Next, the rich control by the means will be described.

FIG. 12 shows a rich control routine for releasing NOx. This routine is executed by interruption at regular time intervals. First, the basic flow of the rich process for releasing NOx or SOx will be described with reference to this figure. First, the operating state is detected (step 10
1) Then, it is determined whether the air-fuel ratio (A / F) is in the lean state (step 102). In the lean state, the amount of NOx (SOx) flowing into the absorbent during lean operation is calculated (step 103). If NOx is released, step 1
At 08, the value is set as the NOx absorption amount, and when the total NOx absorption amount is not less than the predetermined value, that is, when it exceeds the predetermined value, NOx release control is performed (step 109).

In the case of SOx release control, after step 103, the SOx passage rate with respect to the exhaust temperature and the passage rate with respect to the SOx absorption amount are read out from the map (step 1
04), the passage rate for the exhaust temperature is corrected by the passage rate for the absorption amount (step 105), and the original total SOx
The absorption amount is calculated (step 106). The calculation result is stored in the RAM (step 107). When the total SOx absorption amount is not less than or equal to the predetermined value, that is, when it exceeds the predetermined value (step 108), SOx release control is performed (step 109).

Hereinafter, this flow will be described in more detail.
First, the NOx release control will be described with reference to FIG. First, in step 150, the operating condition is read, and in step 151,
The correction coefficient K is 1.0 by the lean / rich determination means.
Is less than or equal to.

When K> 1.0, it is determined that the rich air-fuel mixture is burning, and Wn is initialized to 0 in step 152. When K <1.0, it is determined that the lean air-fuel mixture is burning.

When it is judged that the lean air-fuel mixture is burning, the routine proceeds to step 153, where the total NOx amount estimating means calculates the total NOx amount Wn absorbed by the NOx absorbent 19. .

The calculation method of the total NOx amount Wn is that the NOx absorbed in the NOx absorbent 19 at the time of combustion of the lean air-fuel mixture.
Quantity Wn (initial value 0) and k1 · Q · Q / N (k1 is a constant)
It is represented by the sum of and. Here, the amount of NOx discharged from the combustion chamber 3 increases as the intake air amount Q increases, and increases as the engine load Q / N increases.

Next, the routine proceeds to step 158, where the total NOx is
The amount is stored in the backup RAM 33a. Next, it is judged whether or not the total NOx amount Wn is larger than a predetermined value (step 159), and if it is less than a predetermined value, for example, a saturation value, this processing routine is repeated again from the beginning. When the true total NOx amount Wn exceeds a predetermined value, the processing is N
Delivered to Ox emission means. The NOx releasing means obtains the rich processing amount corresponding to the total NOx amount from the map which stores the relationship between the total NOx amount and the rich processing amount stored in the ROM in advance and performs the rich processing (step 160).

The rich processing amount is determined by the richness of the air-fuel ratio and the processing time. Therefore, the processing amount is controlled by increasing / decreasing the richness and controlling the processing time. The rich process changes the correction coefficient K in TAU = TP · K described above,
It is performed by changing the fuel injection time (fuel injection amount) TAU.

In the rich process, the current air-fuel ratio is detected based on the output value of the O ₂ sensor, and the correction coefficient K is corrected so that the air-fuel ratio becomes the target rich degree. The above processing is performed at a predetermined cycle, as indicated by P in FIG. Next, rich control for SOx release will be described with reference to FIG. This routine is executed by interruption at regular time intervals.

Referring to FIG. 13, after the operating state is detected (step 150), the lean / rich determining means determines whether or not the correction coefficient K is smaller than 1.0 (step 151).

When K> 1.0, it is determined that the rich air-fuel mixture is burning, and in step 152, Ws1 and Ws0
Is initialized to 0. When K <1.0, it is determined that the lean air-fuel mixture is burning.

When it is determined that the lean air-fuel mixture is burning, the routine proceeds to step 153, where the total SOx amount estimating means calculates the SOx amount Ws1 absorbed by the NOx absorbent 19.

The calculation method of the SOx amount Ws1 is such that the SOx absorbed in the NOx absorbent 19 at the time of combustion of the lean air-fuel mixture.
It is represented by the sum of the quantity Ws1 (initial value 0) and k2 · Q (k2 is a constant). Here, the amount of SOx discharged from the combustion chamber 3 increases as the intake air amount Q increases.

Then, the SOx passage rate calculation means RO calculates the SOx passage rate in the NOx absorbent corresponding to the exhaust temperature.
It is calculated from the map stored in M (step 154).
Further, from the map stored in advance in the ROM 32 and defining the relationship between the change in the total SOx absorption amount and the passage rate Pn,
The SOx passage rate Pn obtained by the SOx passage rate calculation means is set to P
It is corrected to n0 by the passage rate correction means (step 155).

Next, in step 156, the total passing SOx amount Ws0 passing through the SOx absorbent is calculated by the total passing SOx amount calculating means based on the SOx passing ratio Pn0. After that, the total passing SOx calculated by the total passing SOx amount calculation means
Total SOx amount W calculated by total SOx amount estimating means Ws0
The true total SOx amount Ws2 absorbed by the NOx absorbent is subtracted from s1 and calculated by the total SOx amount correcting means (step 157). This total SOx amount Ws2 is stored in the backup RAM 33a (step 158). Next, it is judged whether or not the true total SOx amount Ws2 is larger than a predetermined value (step 159), and when it is less than a predetermined value, for example, a saturation value, this processing routine is repeated again from the beginning. When the true total NOx amount Ws2 exceeds a predetermined value,
The treatment is handed over to the SOx releasing means. The SOx releasing means obtains the rich processing amount corresponding to the total SOx amount from the map which stores the relationship between the total SOx amount and the rich processing amount stored in the ROM in advance, and performs the rich processing (step 16).
0).

Since the rich processing amount is determined by the richness of the air-fuel ratio and the processing time, the processing amount is controlled by increasing / decreasing the richness and controlling the processing time. Further, upon releasing SOx, the sulfate BaSO ₄ is converted into Ba ²⁺ and SO ₄ ²⁻ and activated. Therefore, in order to raise the temperature of the NOx absorbent 19, the NOx absorbent 19 is heated by a heater, or the exhaust valve is opened by opening the exhaust valve while the ignition timing of the engine is delayed and the combustion temperature of the combustion chamber is high. The temperature of NOx absorbent 19 is raised and the temperature of NOx absorbent 19 is raised.

The rich process is performed by the above-described TAU = T.
In P and K, the correction coefficient K is changed and the fuel injection time (fuel injection amount) TAU is changed. In the rich process, the current air-fuel ratio is detected based on the output value of the O ₂ sensor, and the correction coefficient K is corrected so that the air-fuel ratio has the target rich degree.

The above SOx release control is performed according to a predetermined cycle as shown by Q in FIG.
It is longer than the controlled release cycle. This is because SOx absorption is NO
This is because it takes time as compared with x absorption. As described above, the SOx release rate is controlled by considering the SOx passage rate of the NOx absorbent.
The release control of x is frequently performed in a short cycle, and the release control is performed before the difference between the actual absorption amount and the estimated NOx absorption amount becomes a problem. It takes a long time to accumulate and the actual total S
This is because a large error appears between the Ox absorption amount and the estimated total SOx amount. In this sense, it is significant to consider the SOx passage rate in rich control for SOx release.

[0091]

According to the present invention, since the SOx absorption amount of the NOx absorbent is estimated in consideration of the SOx passage rate of the NOx absorbent, it is possible to estimate the SOx absorption amount of a value closer to the actual value.

Since the rich process is performed on the basis of the total amount of SOx absorbed in consideration of the SOx passage rate, the rich process amount can be minimized as necessary and fuel consumption can be saved.

Furthermore, the larger the estimated total SOx amount is, the more S
Focusing on the fact that the Ox passage rate becomes high, the auxiliary correction means for correcting the total SOx estimated amount to a small amount is provided, so that the SOx absorption amount is further estimated after the SOx is absorbed by the absorbent and becomes saturated. There is no need to perform useless rich processing corresponding to the SOx amount that is equal to or higher than the saturation amount.

[Brief description of drawings]

FIG. 1 ... Overall view of internal combustion engine

FIG. 2 is a diagram showing a map of basic fuel injection time.

FIG. 3 is a diagram showing changes in the correction coefficient K.

[Fig. 4] ... H in the exhaust gas discharged from the engine
A diagram schematically showing the concentrations of C, CO and oxygen

FIG. 5: Diagram for explaining the action of absorbing and releasing NOx.

FIG. 6 is a diagram showing SOx and NOx emission rates.

FIG. 7: Diagram showing SOx emission characteristics

FIG. 8: Diagram showing SOx and NOx release timing

FIG. 9: Diagram showing the relationship between exhaust temperature and SOx passage rate

FIG. 10: A diagram showing the relationship between the amount of SOx absorbed by the absorbent and the SOx passage rate.

FIG. 11 is a block diagram of various means realized on a CPU.

FIG. 12: NOx / SOx release control flowchart

[Fig. 13] NOx / SOx release control flowchart (detailed diagram)

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine body 2 ... Piston 3 ... Combustion chamber 4 ... Spark plug 5 ... Intake valve 6 ... Intake port 7 ... Exhaust valve 8 ... Exhaust port 9 ... Branch pipe 10 ... Surge tank 11 ... Fuel injection valve 12 ... Intake duct 13 ... Air flow meter 14 ... Air cleaner 15 ... Throttle valve 16 ... Exhaust manifold 17 ... Exhaust pipe 19 ... NOx absorbent 19 ... NOx absorbent 20 ... Casing 21 ... Temperature sensor 22 ... Rotation speed sensor 30 ... Electronic control unit 31 ... Bidirectional bus 32 ... ROM (read only memory) 33 ... RAM (random access memory) 33a ... Backup RAM 34 ... CPU (microprocessor) 35 ... Input port 36 ... Output port 36 37, 38 ... AD converter 39 ... Drive circuit 51 ... Operation State detection means 52 ... Lean / rich determination means 53 ... Total NOx amount estimation Setting means 58 ... NOx releasing means 63 ... Total SOx amount estimating means 64 ... SOx passage rate calculating means 65 ... SOx passage rate correcting means (auxiliary correcting means) 66 ... Total passing SOx amount calculating means 67 ... Total SOx amount correcting means 68 ... SOx discharge means TAU ... fuel injection time TP ... basic fuel injection time _{K, KK, K 0, K1} , K2 ... correction coefficient C, Cz, C1, C2 ... time T ... exhaust temperature N ... engine speed Q ... intake air amount

Claims (2)

[Claims] 1. An NOx absorbent, which is disposed in an exhaust passage of an internal combustion engine and absorbs SOx together with NOx contained in exhaust gas discharged from the internal combustion engine, is absorbed by the NOx absorbent in order to release SOx. When the exhaust gas air-fuel ratio is lean, the total SOx amount estimated by the total SOx amount estimation means for estimating the total SOx amount from the operating state of the internal combustion engine has exceeded the predetermined value. In this case, in the exhaust gas purification apparatus for an internal combustion engine, comprising: an SOx releasing means for making the air-fuel ratio of the exhaust gas rich and releasing SOx; The SOx passing rate in the NOx absorbent estimated corresponding to the temperature is used to correct the total SOx amount so as to exclude the SOx passing portion in the NOx absorbent, and the SOx releasing means uses the total SOx amount estimating method. An exhaust purification system for an internal combustion engine, characterized in that rich processing is performed according to the total SOx amount corrected in each stage. 2. Total SOx absorbed by the NOx absorbent
The larger the amount, the higher the SOx passage rate in the NOx absorbent should be corrected, and the total SOx amount estimation means should estimate.
The exhaust emission control device for an internal combustion engine according to claim 1, further comprising auxiliary correction means for correcting a small amount.