JPH07139340A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To accurately estimate an NOX absorption rate of an NOX absorber. CONSTITUTION:An NOX absorber 18 is arranged on an exhaust passage 12 of an internal combustion engine 1 for absorbing NOX when an exhaust air-fuel ratio is in a lean condition, and discharging the absorbed NOX when oxygen density in the exhaust is reduced. An NOX absorbing rate counter is provided for indicating an NOX rate absorbed by the NOX absorber. When the engine is operated with a lean air-fuel ratio, a specified rate is added to the counter every specified time according to the operation condition of the engine. When the engine is operated with a rich or a theoretical air-fuel ratio, a rate is subtracted from the counter every specified time, which rate is set according to a temperature of the absorber and a rate of excessive fuel supplied to the engine exceeding the fuel rate required to obtain the theoretical engine air-fuel ratio.

Description

Detailed Description of the Invention

[0001]

The present invention relates to relates to an exhaust purifying apparatus for an internal combustion engine, to effectively removable exhaust purification system the NO _X contained in exhaust gas of an internal combustion engine causing combustion of lean air-fuel ratio in detail .

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for an internal combustion engine of this type, the applicant of the present invention has filed an international application number PCT / JP9310.
There is one proposed in 0778. In the exhaust gas purification apparatus proposed above, NO that absorbs NO _X in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NO _X when the oxygen concentration in the inflowing exhaust gas decreases. _{The X} absorbent is arranged in the exhaust passage of the internal combustion engine, and usually the internal combustion engine is operated at a lean air-fuel ratio, and the NO _X absorbent is exhausted from the NO in the exhaust gas.
Absorb _X. Also, the lean air-fuel ratio operation continues and NO
_{When the} amount of NO _X absorbed by the _X absorbent exceeds a predetermined amount, the operating air-fuel ratio of the internal combustion engine is switched from the lean air-fuel ratio to the rich or stoichiometric air-fuel ratio to reduce the oxygen concentration in the exhaust gas and reduce the NO _X absorption. together to release the absorbed NO _X from the agent, unburned HC, CO in the exhaust gas with this released NO _X
And so as to reduce and purify by components of equal (in this specification, the reduction and release of the NO _X from the _the NO _X absorbent, it referred to the operation of cleaning the "regenerating operation of the NO _X absorbent").

[0003] Further, in the above apparatus, NO in _the NO _X absorbent to _the NO _X absorbent estimates the NO _X absorption amount absorbed
The NO _X absorption amount counter representing the _X absorption provided engine when it is operated at a lean air-fuel ratio, a predetermined addition amount determined in accordance with the engine operating condition in the NO _X absorption counter at predetermined time intervals In addition, when the engine is operating at a rich or stoichiometric air-fuel ratio, the value of the NO _x absorption amount counter is decreased according to the operating time.

That is, during engine operation, the load from the engine,
Amount of NO depending on engine operating conditions such as engine speed_XOccurs
However, if the engine is operating at a lean air-fuel ratio (i.e.
Chi, NO_XThe air-fuel ratio of the exhaust gas flowing into the absorbent is lean.
If), the generated NO _XA certain percentage of the quantity is N
O_XNO is absorbed by the absorbent_XAbsorbed in absorbent
NO_XThe amount of NO_XIncrease according to the amount generated
It Also, the engine is operating at rich or stoichiometric ratio.
If yes, NO_XAbsorbent to NO_XWas released
No_XNO absorbed in the absorbent_XAmount of institution
Is operating at rich or stoichiometric ratio
Decrease. In the above device, the engine operating air-fuel ratio is lean air-fuel ratio.
When the ratio is reached, a predetermined amount is added at regular intervals, and the engine is operated.
Depending on the operating time when the air-fuel ratio is rich or stoichiometric
NO by which a predetermined amount is subtracted_XProvide an absorption amount counter
NO due to_XAbsorbent NO_XThe amount of absorption is estimated.

[0005]

However, when the engine is operated at a rich or stoichiometric air-fuel ratio as in the above-described device, the value of the NO _x absorbent absorption counter is decreased only based on the operating time. If it has, there is a problem of causing an error between the actual value of the amount of NO _X and NO _X absorption counter that is absorbed in the NO _X absorbent.

[0006] sand, the amount of the NO _X released from _the NO _X absorbent per unit time (NO _X release rate) is not always constant, as will be described later, in the temperature or the exhaust of the NO _X absorbent It changes according to the amount of unburned HC, CO, etc. contained. Therefore, based on only the operation time of the rich or stoichiometric air-fuel ratio as described above of the device (i.e., assuming the NO _X release rate from _the NO _X absorbent is constant) decreases the value of the NO _X absorption counter The engine operating condition and N
Depending on the O _X absorbent temperature, the counter decrement and actual N
A large error may occur with the amount of NO _X released from the O _X absorbent.

[0007] In the above described exhaust gas purifying device determines the amount of the NO _X absorption counter value NO _X absorbed in the NO _X absorbent with a predetermined NO _X absorption during the lean air-fuel ratio operation is increased When the value exceeds the value, the air-fuel ratio of the engine is forcibly switched to the rich air-fuel ratio to perform the above-mentioned regeneration operation of the NO _x absorbent. Therefore, NO in reality
When causing an error between the value of the NO _X amount and the NO _X absorbing amount counter which is absorbed into the _X absorbent, there is no need to perform actually NO _X absorption is small regenerating operation of the NO _X absorbent despite deteriorates fuel economy switching is performed for the values of the NO _X absorption counter to the rich air-fuel ratio operation to exceeds a predetermined value, conversely, actually the NO _X absorption of the NO _X absorbent However, since the value of the NO _X absorption amount counter does not exceed the predetermined value, the regeneration operation is not performed and the absorption capacity of the NO _X absorbent decreases and the exhaust gas is being discharged. which may cause a problem such as can not absorb NO _X.

The present invention is NO in order to solve the above problems.
It is an object of the present invention to provide a means capable of accurately estimating the amount of NO _X absorbed in the _X absorbent.

[0009]

According to the Summary of the Invention The present invention as set forth in claim 1, disposed in an exhaust passage of an internal combustion engine, the air-fuel ratio of the exhaust gas absorbs NO _X in the exhaust gas when the lean exhaust A NO _x absorbent that releases absorbed NO _x when the oxygen concentration in the inside decreases, and a predetermined addition amount that is determined according to the engine operating state when the internal combustion engine is operating at a lean air-fuel ratio. It said added to the NO _X absorption counter indicating the NO _X absorption of _the NO _X absorbent at predetermined intervals, when the internal combustion engine is operated at a rich or stoichiometric air-fuel ratio, a certain time a predetermined subtraction amount by the subtracting from the NO _X absorption counter for each, and the absorption amount estimation means for estimating the NO _X absorption of the _the NO _X absorbent, when the engine is operated at a rich air-fuel ratio, the engine air-fuel ratio Required to make the stoichiometric air-fuel ratio And an excess fuel quantity detecting means for detecting the amount of fuel supplied to the over-engine beyond the amount, when the engine is operated in a rich air-fuel ratio, of the NO _X absorption counter on the basis of the excessive fuel amount There is provided an exhaust gas purification device for an internal combustion engine, comprising: a subtraction amount setting means for determining the subtraction amount.

According to the present invention as set forth in claim 2,
Disposed in an exhaust passage of an internal combustion engine, the air-fuel ratio of the exhaust gas absorbs NO _X in the exhaust gas when the lean and _the NO _X absorbent to the oxygen concentration in the exhaust gas to release NO _X absorbed when reduced the when the internal combustion engine is operated at a lean air-fuel ratio, N representing the NO _X absorption of the _the NO _X absorbent at regular time intervals a predetermined addition amount determined in accordance with the engine operating condition
O _X is added to the absorption amount counter, when the internal combustion engine is operated at a rich or stoichiometric air-fuel ratio, by subtracting from the NO _X absorption counter a predetermined subtraction amount for each predetermined time, the NO _X absorption amount estimation means for estimating the NO _X absorption amount of the absorbent, a temperature detecting means for detecting a temperature of the _the NO _X absorbent when the engine is operated in a rich air-fuel ratio, the _the NO _X absorbent temperature An exhaust gas purification apparatus for an internal combustion engine is provided, which comprises a subtraction amount setting means for determining the subtraction amount of the NO _X absorption amount counter based on the above.

According to the present invention as defined in claim 3,
Disposed in an exhaust passage of an internal combustion engine, the air-fuel ratio of the exhaust gas absorbs NO _X in the exhaust gas when the lean and _the NO _X absorbent to the oxygen concentration in the exhaust gas to release NO _X absorbed when reduced the when the internal combustion engine is operated at a lean air-fuel ratio, N representing the NO _X absorption of the _the NO _X absorbent at regular time intervals a predetermined addition amount determined in accordance with the engine operating condition
O _X is added to the absorption amount counter, when the internal combustion engine is operated at a rich or stoichiometric air-fuel ratio, by subtracting from the NO _X absorption counter a predetermined subtraction amount for each predetermined time, the NO _X absorption amount estimation means for estimating the NO _X absorption amount of the absorbent, the means for detecting the engine intake air quantity, means for detecting the operating air-fuel ratio of the engine, the NO
A means for detecting the temperature of the _X absorbent, and the engine intake air amount when the engine is operating at a rich air-fuel ratio,
There is provided an exhaust gas purification device for an internal combustion engine, comprising: a subtraction amount setting means for determining the subtraction amount of the NO _X absorption amount counter based on the engine operating air-fuel ratio and the NO _X absorbent temperature.

[0012]

NO _X release rate from _the NO _X absorbent when the [action] engine is operated at a rich or stoichiometric air-fuel ratio, as the temperature of the NO _X absorbent is high, as explained later, also in _the NO _X absorbent The larger the amount of reducing components such as CO and unburned HC that are supplied, the larger the amount. Therefore, when the engine is operating at a rich or stoichiometric air-fuel ratio, the amount of reduction of the NO _X amount absorbed in the NO _X absorbent per unit time (that is, the NO _X release amount per unit time) is Unburned HC inside,
The larger the CO content and the higher the temperature of the NO _X absorbent, the larger the CO content. On the other hand, the concentration of unburned HC and CO in the exhaust increases as the operating air-fuel ratio of the engine decreases (that is, as the rich air-fuel ratio increases). That is, the fuel excessively supplied to the engine in excess of the amount required for operating the engine at the stoichiometric air-fuel ratio is discharged as unburned HC and CO from the engine together with the exhaust gas. As the amount of fuel increases, the amount of unburned HC and CO supplied to the NO _X absorbent increases.

[0013] In the present invention according to claim 1, the amount of fuel that has been excessively supplied to the engine, namely unburned HC supplied to _the NO _X absorbent, as the amount of CO is large, NO _X absorption counter The subtraction amount of is set to a large value. Therefore, NO
The subtracted amount of the _X absorption counter is the actual NO of the NO _X absorbent.
Would correspond to the _X release rate, a large error does not occur between the amount of NO _X in the actual of the NO _X absorbent and the value of the NO _X absorption counter.

In the present invention according to claim 2, NO
_The higher the temperature of the _X absorbent, the larger the subtraction amount of the NO _X absorption counter is set. For this reason, NO _x
In fact of the NO _X subtraction amount of absorption counter of the NO _X absorbent
Would correspond to the absorbent release rate, a large error does not occur between the amount of NO _X in the actual of the NO _X absorbent and the value of the NO _X absorption counter.

As described above, the engine operating air-fuel ratio is low.
Indeed, the concentration of unburned HC and CO in the exhaust increases. here
Therefore, the exhaust flow rate increases in proportion to the intake air amount.
NO if the customs operation air-fuel ratio and intake air amount are determined_XFor absorbent
The amount of unburned HC and CO supplied will be determined. Contract
In the present invention described in claim 3, the engine operating air-fuel ratio and the intake air
The amount of unburned HC and CO can be obtained by detecting
Therefore, the greater the amount of unburned HC and CO, the more NO_XSucking
The higher the temperature of the sorbent, the more NO_XThe subtraction amount of the absorption counter is
Largely set. As a result, NO_XAbsorption amount counter
Is NO_XAbsorbent actual NO _XCorresponding to the release rate
It will be the amount.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is an overall view of an internal combustion engine to which the exhaust emission control device of the present invention is applied. In FIG. 1, reference numeral 1 is an internal combustion engine such as a gasoline engine that burns at a lean air-fuel ratio, 2 is a piston of the engine 1, 3 is a combustion chamber, and 4 is an ignition plug.
Further, 6 is an intake port of the engine, 5 is an intake valve, 8 is an exhaust port, 7 is an exhaust valve, and each intake port 6 is an intake branch pipe 9
A fuel injection valve 11 for injecting fuel into each intake port 6 is arranged in each branch pipe 9 while being connected to the surge tank 10 via the.

The surge tank 10 is connected to an air cleaner 13 via an intake passage 12, and a throttle valve 14 having an opening degree in accordance with the driver's operation of an accelerator pedal (not shown) is provided in the intake passage 12. It is arranged. Also,
The surge tank 10 is provided with an intake pressure sensor 15 that generates an output voltage proportional to the absolute pressure in the surge tank 10.

On the other hand, the exhaust port 8 of the engine 1 is connected to an exhaust passage 17 via an exhaust manifold 16, and the exhaust passage 17 is connected to a casing 19 containing a NO _x absorbent 18 described later. Reference numeral 30 in FIG. 1 indicates that
It is an electronic control circuit of the engine 1. The electronic control circuit 30 is RO
M (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 3
4, a input port 35, and an output port 36 are each connected by a bidirectional bus 31 and are composed of a digital computer of a known structure, which performs basic control such as fuel injection amount control and ignition timing control of the engine 1 functions as a subtraction amount setting means for determining an absorption amount estimation means and the counter subtraction amount that is according to claims 1 to 3 for estimating the NO _X absorption of _the NO _X absorbent 17 performs an operation of the NO _X absorption counter in the example Plays.

For the above purpose, the input port 35 of the control circuit 30 has a voltage signal from the intake pressure sensor 15 and NO.
A voltage signal corresponding to the exhaust temperature is supplied from the exhaust temperature sensor 20 provided in the exhaust passage on the downstream side of the _X absorbent 18 respectively.
In addition to being input via the converter 37, pulse signals representing the engine speed are also input from the engine speed sensor 21 provided in the distributor (not shown) of the engine.

The output port 36 of the control circuit 30 is
The fuel injection valve 11 is driven through the corresponding drive circuit 38.
Is connected to the ignition plug 4 to control the fuel injection from the fuel injection valve 11 and the ignition timing of the ignition plug 4. The NO _x absorbent 18 contained in the casing 19 uses, for example, a carrier such as alumina, on which potassium K, sodium Na, lithium Li, cesium Cs, an alkali metal such as barium Ba and calcium Ca are contained. At least one selected from such alkaline earths, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt are supported. This NO _X
The absorbent 18 absorbs NO _X when the air-fuel ratio of the inflowing exhaust gas is lean, and performs the NO _X absorption-release action of releasing NO _X when the oxygen concentration decreases.

The above-mentioned exhaust air-fuel ratio is N in this case.
It means the ratio of the total amount of air and the total amount of fuel supplied to the exhaust passage on the upstream side of the O _X absorbent 18, the engine combustion chamber, the intake passage, and the like. Therefore, NO _X absorbent 1
When fuel or air is not supplied to the upstream exhaust passage 8 of No. 8, the exhaust air-fuel ratio becomes equal to the air-fuel ratio of the engine (air-fuel ratio in combustion in the engine combustion chamber).

In this embodiment, since an engine that burns lean air-fuel ratio is used, the exhaust air-fuel ratio during normal operation is lean, and the NO _X absorbent 18 absorbs NO _{X in} the exhaust gas. Further, when the air-fuel ratio of the engine is switched from the lean air-fuel ratio to the rich or stoichiometric air-fuel ratio and the oxygen concentration in the exhaust gas decreases, the NO _X absorbent 18 releases the absorbed reducing agent.

[0023] There are some points where the detailed mechanism of this absorption / release action is not clear. However, it is considered that this absorbing / releasing action is performed by the mechanism shown in FIG. Next, regarding this mechanism, platinum P on the carrier
A case of supporting t and barium Ba will be described as an example, but other precious metals, alkali metals, alkaline earth metals,
The same mechanism can be achieved by using rare earths.

That is, the inflow exhaust becomes considerably lean.
And the oxygen concentration in the exhaust gas increased significantly, as shown in Fig. 2 (A).
As these oxygen O₂Is O₂ ^-Or O^2-In the form of
It adheres to the surface of platinum Pt. On the other hand, the NO in the exhaust gas is
This O on the surface of platinum Pt₂ ^-Or O^2-Reacts with N
O₂Becomes (2NO + O₂→ 2 NO₂ ). Then generated
NO₂Part of the inside of the absorbent while being oxidized on platinum Pt
While being absorbed by and bound to barium oxide BaO,
As shown in (A), nitrate ion NO₃ ^-Absorbent in the form of
Diffuse in. NO in this way_XIs NO_XAbsorbent 1
Absorbed in 8.

Therefore, NO ₂ is produced on the surface of platinum Pt as long as the oxygen concentration in the inflowing exhaust gas is high, and NO ₂ is absorbed in the absorbent and nitrate ion NO ₃ unless the NO _X absorbing capacity of the absorbent is saturated. ^- is generated. Organization 1
When the air-fuel ratio of is switched to rich or stoichiometric air-fuel ratio,
The oxygen concentration in the inflowing exhaust gas decreases and the amount of NO ₂ produced decreases. Thus the reaction is reverse (NO ₃ ^- → NO ₂₎ proceeds, the nitrate ions NO in absorbent ₃ ^- are released from the absorbent in the form of NO _2. On the other hand, unburned HC,
When components such as CO are present, these components are oxidized by reacting with oxygen O ₂ ⁻ or O ^{2 −} on platinum Pt, and platinum Pt
It consumes oxygen above. Further, the NO ₂ released from the NO _X absorbent 18 is reduced by reacting with HC and CO as shown in FIG. 2 (B). In this way, N on the surface of platinum Pt
When O ₂ is no longer present, NO ₂
Is released.

That is, HC and CO in the inflowing exhaust gas are
O on the platinum Pt₂ ^-Or O^2-Reacts immediately with the acid
And then O on the platinum Pt₂ ^-Or O^2-Consumed
If HC and CO still remain after this,
Therefore NO released from the absorbent_X, And with exhaust
NO flowing into_XIs reduced. Therefore, use it for platinum Pt.
If the supplied amount of HC and CO is large, on the platinum Pt surface
NO₂Consumption increases and the absorbent releases accordingly.
NO issued_XAlso increases. That is, NO_Xabsorption
The greater the amount of HC and CO components supplied to the agent, the NO_XSucking
NO from the collecting agent _XThe amount of release will increase.

The amount of HC and CO components flowing into the NO _x absorbent together with the exhaust gas is determined from the amount of fuel supplied to the engine by the amount of fuel required to operate the engine at the stoichiometric air-fuel ratio. It increases in proportion to the subtracted fuel amount, that is, the excess fuel amount. Therefore, NO _X amount discharged from _the NO _X absorbent per unit time (NO _X release rate) increases with the amount of excess fuel supplied to the engine unit time.

FIG. 3 shows the amount of excess fuel supplied to the engine per unit time and the NO arranged in the engine exhaust passage.
An example of the relationship with the NO _X release amount (NO _X release rate) from the _X absorbent is shown. As shown in FIG. 3, NO _X release rate from _the NO _X absorbent and substantially proportional to the excess fuel amount increases.
Therefore, it is possible to know the NO _X release rate from _the NO _X absorbent by detecting the amount of excess fuel supplied per unit time to the engine.

On the other hand, NO _X release rate from _the NO _X absorbent, other than the amount of excess fuel supplied to _the NO _X absorbent,
Influenced by the temperature of _the NO _X absorbent, the maximum value of the NO _X release rate by _the NO _X absorbent temperature substantially determined. Figure 4
Is NO when the amount of excess fuel in the engine is sufficiently high
Is a graph showing the relationship between the temperature and the NO _X release rate of the _X absorbent. As shown in FIG. 4, in the presence of sufficient excess fuel, N
O _X release rate tends to increase as the temperature of the NO _X absorbent becomes higher. Thus, the NO _X release rate increases in both temperature rise, bound nitrate ions is considered to become easy to separate the temperature of the NO _X absorbent becomes high as BaO in the absorbent.

[0030] Therefore, the lower the temperature of the NO _X absorbent at most the amount of excess fuel supplied to the engine, NO _X emission rate of _the NO _X absorbent becomes smaller, the temperature of the NO _X absorbent conversely be higher, NO _X release rate of the NO _X absorbent the less amount of excess fuel supplied to the engine will be reduced. Therefore, in order to know the NO _X release rate from _the NO _X absorbent in the rich or stoichiometric air-fuel ratio during operation in precisely, NO
_Both the temperature of the _X- absorbent and the amount of excess fuel delivered to the engine need to be detected. In the present embodiment, as will be described later, the NO _x absorbent temperature is detected by using the exhaust temperature sensor 20 provided in the exhaust passage on the downstream side of the NO _x absorbent 18, and the amount of excess fuel supplied to the engine is calculated as follows. It is detected using the engine operating air-fuel ratio and the engine intake air amount.

Next, the air-fuel ratio control of the engine of this embodiment will be described. In this embodiment, the above-mentioned international application number PC
The same air-fuel ratio control as T / JP93100778 is performed.
The air-fuel ratio control will be briefly described below. In this embodiment, the fuel injection amount from the fuel injection valve 11, that is, the valve opening time of the fuel injection valve 11 at the time of fuel injection (fuel injection time)
The TAU is controlled by the control circuit 30, for example, TAU = TP ×
Calculated as K.

Here, TP represents the fuel injection time required to bring the air-fuel ratio of the air-fuel mixture supplied into the engine combustion chamber to the stoichiometric air-fuel ratio, that is, the basic fuel injection time, which is detected by the intake pressure sensor 15. As a function of the absolute pressure PM in the surge tank 10 and the engine speed N obtained, it is obtained in advance by experiments or the like and stored in the ROM 32 of the control circuit 30 in the form of a numerical table as shown in FIG.

Further, K is a correction coefficient for controlling the engine air-fuel ratio, and when K = 1.0 is set, the engine air-fuel ratio becomes the theoretical air-fuel ratio. If K> 1.0 is set, the engine air-fuel ratio is smaller than the theoretical air-fuel ratio (that is, rich air-fuel ratio).
When K <1.0 is set, the engine air-fuel ratio becomes larger than the theoretical air-fuel ratio (that is, lean air-fuel ratio). The value of the correction coefficient K is given as a function of the absolute pressure PM in the surge tank 10 and the engine speed N, for example, as shown in FIG. That is, as shown in FIG. 6, in the present embodiment, a region where PM is relatively low (engine low / medium load operating region)
Then, the correction coefficient K is set to be smaller than 1.0, and the engine is operated at a lean air-fuel ratio. Further, in a region where PM is relatively high (engine high load operating region), the value of the correction coefficient K is 1.0, and the engine is operated at the stoichiometric air-fuel ratio. Moreover, PM
In a high range (engine full load operation range), the value of the correction coefficient K is set to be larger than 1.0, and the engine is operated at the rich air-fuel ratio. In particular, a vehicle engine or the like usually has the highest frequency of low-to-medium-load operation. Therefore, these engines are operated at a lean air-fuel ratio for most of the period during operation.

[0034] As described above, NO NO _X absorbent engine absorbs NO _X in the exhaust gas when being operated at a lean air-fuel ratio, the engine is absorbed when being operated at a rich or stoichiometric air-fuel ratio _X To release. Therefore, if the rich or stoichiometric air-fuel ratio operation is appropriately performed, the NO _x absorbent NO
_{The X} absorption does not increase above a certain level and no special regeneration operation is required. However, when the engine operating conditions it may as operation at a lean air-fuel ratio continues for a long time, NO _X absorption of the NO _X absorbent increases, NO
_X absorption capacity may be saturated.

[0035] In this embodiment, when the absorbed amount of NO _X by integrating, NO _X absorption of the NO _X absorption counter by _the NO _X absorbent 18 to be described is equal to or greater than a predetermined amount below,
Regardless of the region of FIG. 6, the value of the correction coefficient K for a predetermined time is set to 1.0 or more (for example, K = 1.2), the engine is operated at the rich air-fuel ratio, and the regeneration operation of the NO _X absorbent is performed. NO _X absorption of the NO _X absorbent is prevented from being saturated by performing.

Next, the estimation of the NO _X absorption amount using the NO _X absorption amount counter of this embodiment will be described. As mentioned above, when the engine is operated at a lean air-fuel ratio, N
N of O _X absorbent absorbs NO _X in the exhaust gas _the NO _X absorbent
O _X absorption amount is increased. At this time, N per unit time
_The amount of NO _X O _X absorbent absorbs, i.e. amount of increase per unit time of the NO _X absorbed in _the NO _X absorbent, the amount of NO _X contained in the exhaust, i.e. engine generated per unit time It is considered to be proportional to the amount of NO _{x that} is generated. Therefore, when the engine is operating at a lean air-fuel ratio,
Engine is absorbed _the NO _X absorbent by integrating the amount obtained by multiplying a constant factor in _the amount of NO _X generated every predetermined time N
The total amount of O _X can be calculated.

On the contrary, when the engine is operating at a rich or stoichiometric air-fuel ratio, the NO _x absorbent absorbs the absorbed NO.
_It releases _X and the NO _X absorption of the NO _X absorbent decreases.
Here, the amount of NO _X released from the NO _X absorbent per unit time is an amount determined by the air-fuel ratio of the engine, the temperature of the NO _X absorbent, and the like. Therefore, when the engine is operated at a rich or stoichiometric air-fuel ratio, released from _the NO _X absorbent by integrating the amount corresponding to the NO _X emissions from _the NO _X absorbent at predetermined time intervals NO The total amount of _X can be calculated.

[0038] Therefore, NO _X and NO _X absorption amount counter representing the NO _X absorption amount of the absorbent is provided, when the engine is operated at a lean air-fuel ratio, a certain time an addition amount which is proportional to the NO _X generation amount of the engine Each time it is added to the NO _x absorption counter,
Conversely, when the engine is operating at a rich or stoichiometric air-fuel ratio, the subtraction amount corresponding to the NO _x release amount from the NO _x absorbent per unit time is subtracted from the NO _x absorption amount counter at regular intervals. , it is possible to accurately estimate the amount of the NO _X absorbed in the current _the NO _X absorbent.

By the way, engine NO during lean air-fuel ratio operation
_{The X} generation amount increases as the engine load (that is, intake pressure PM) increases and as the engine speed N increases. Further, the engine NO _x generation amount also changes depending on the engine operating air-fuel ratio.
In the present embodiment, the engine operating air-fuel ratio is determined as a function of the intake pressure PM and the engine speed N (see FIG. 6). Therefore, the amount of NO _x produced by the engine is ultimately the intake pressure PM and the engine speed N.
Will only be a function. Therefore, in this embodiment, the engine NO _X
The generated amount is obtained in advance as a function of the intake pressure PM and the engine speed N by experiments, and this NO _x generated amount is multiplied by a constant coefficient, and the addition amount of the NO _x absorbed amount counter at fixed time intervals. The (NO _x absorption amount) α is stored in the ROM 32 in the form of a numerical table as shown in FIG. 7. Then, when the engine is operated with a lean air-fuel ratio, the value of the addition amount α is read from this numerical table using the values of the intake pressure PM and the engine speed N, and the value of the NO _X absorption amount counter of the NO _x absorption amount counter is read at regular intervals. The addition amount α is added to the value.

Next, a description will be given estimates of the NO _X release rate from _the NO _X absorbent in the rich or stoichiometric air-fuel ratio during operation in this embodiment. As described above, in order to accurately determine the NO _X release rate from _the NO _X absorbent, it is necessary to know the temperature of the amount and _the NO _X absorbent 18 of the excess fuel supplied to the engine, this embodiment In the example, the NO _X absorbent temperature T is set to NO _X
Exhaust temperature sensor 2 provided in the exhaust passage on the downstream side of the absorbent 18
0 to detect the amount of excess fuel supplied to the engine,
It is detected using the engine operating air-fuel ratio and the engine intake air amount. That is, the exhaust gas temperature detected by the exhaust temperature sensor 20 is a temperature of the exhaust gas that has passed through the _the NO _X absorbent 18, it is considered to be equal to approximately _the NO _X absorbent 18 temperature itself, NO _X The temperature of the NO _X absorbent 18 can be detected by detecting the exhaust temperature with the exhaust temperature sensor 20 on the downstream side of the absorbent 18.

Since the reciprocal of the engine operating air-fuel ratio represents the amount of fuel supplied per unit amount of intake air, the value obtained by multiplying the reciprocal of the engine operating air-fuel ratio by the intake air amount was supplied to the engine. It becomes the total amount of fuel. Further, the amount of fuel required to bring the engine to the stoichiometric air-fuel ratio also becomes a value obtained by multiplying the reciprocal of the stoichiometric air-fuel ratio by the engine intake air amount. The amount is expressed as {(1 / operating air-fuel ratio)-(1 / theoretical air-fuel ratio)} x intake air amount using the engine operating air-fuel ratio and the intake air amount.

On the other hand, the engine intake air amount Q is determined by the engine intake pressure PM and the engine speed N, and the engine operating air-fuel ratio is determined by the value of the fuel injection amount correction coefficient K described above.
Therefore, in this embodiment, the intake air amount Q of the engine intake is obtained as a function of the intake pressure PM and the rotation speed N by experiments in advance and stored in the ROM 32 in the form of a numerical table as shown in FIG. The engine intake air amount Q is determined from this numerical table using the pressure PM and the rotation speed N.

In addition, {(1 / operating air-fuel ratio)-(1
The value of / theoretical air-fuel ratio)} can be calculated as (K-1.0) × A using the value of the correction coefficient K (A is a constant coefficient). Therefore, the NO _X release rate calculated from the amount of excess fuel will be expressed as {(K-1.0) × A} × Q.

In the present embodiment, the above (K
-1.0) × A is calculated in advance, and the actual N
O_XRelease rate is NO_XIt is converted to the value of the absorption counter.
Figure in ROM32 as a numerical value FADJ multiplied by a constant value for
It is stored in the form of a numerical table as shown in FIG. Obey
In this embodiment, NO calculated from the amount of excess fuel _X
Whether the subtraction amount of the absorption amount counter is Q calculated from FIG.
It is expressed as FADJ × Q, which is the product of FADJ and
It

Further, as described above, the NO _x absorbent NO
_{The X} release rate is also affected by the NO _X absorbent temperature,
A low temperature of the NO _X absorbent even much amount of excess fuel supplied to the engine, NO _X release rate of the NO _X absorbent becomes smaller, even at high temperature of the NO _X absorbent is reversed, the engine the less the amount of excess fuel supplied to the NO _X absorbent N
O _X release rate decreases. That, NO _X release rate of the NO _X absorbent when the amount of excess fuel is sufficiently large N
It is determined by the temperature of the O _X absorbent, NO _X release rate of the NO _X absorbent when the amount of excess fuel is small will be determined by the amount of excess fuel.

Therefore, in this embodiment, N under each temperature condition under the condition that the excess fuel shown in FIG. 4 is sufficiently present.
The O _X release rate is obtained in advance by experiments or the like, and the NO _X absorption amount counter subtraction amount TDEC corresponding to this release rate is obtained.
Is stored in the ROM 32 in the form of a numerical table using the exhaust temperature T as shown in FIG. 10, and is obtained from the value of the counter subtraction amount FADJ × Q obtained from the excess fuel amount and the exhaust temperature. The smaller value of the counter subtraction amount TDEC is adopted as the counter subtraction amount.

FIG. 11 is a flow chart showing the operation of the NO _x absorption amount counter described above. This routine is executed by the control circuit 30 at regular intervals. Figure 1
When the routine is started by placing it in step 1, step 110
At 1, the intake pressure PM, the engine speed N, the NO _X absorbent temperature T, and the above-mentioned correction coefficient K are read from the RAM 33. The values of the intake pressure PM, the engine speed N, and the NO _X absorbent temperature T correspond to the sensor 1 corresponding to each fixed time.
Read from 5, 20, 21 and the latest value is always R
It is stored in AM33. Next, at step 1103, it is judged from the value of the correction coefficient K whether or not the engine is currently operated at the lean air-fuel ratio or the rich air-fuel ratio or the stoichiometric air-fuel ratio. And step 1
If the engine is operating at a lean air-fuel ratio at 103 (K
<1.0), the routine proceeds to step 1105, where the intake air pressure PM and the engine speed N are used to calculate the NO _X absorption amount during lean air-fuel ratio operation from the numerical table of FIG. 7 stored in the ROM 32. Determine the counter addition amount α, and step 1
At 109, the addition amount α obtained above is added to the value of the NO _X absorption amount counter C. In addition, step 110 after α addition
In 9, 1111, the value of the NO _X absorption amount counter C is guarded by the maximum value CF. Here, CF is a counter value corresponding to the maximum amount of NO _X that can be absorbed by the NO _X absorbent.
That, NO _X absorbent to become unable to absorb any more NO _X when having absorbed NO _X to a maximum, the addition of the counter is stopped.

When K ≧ 1.0 in step 1103, that is, when the engine is operating at the rich air-fuel ratio or the stoichiometric air-fuel ratio, steps 1113 to 1123 are executed and the value of the NO _X absorption amount counter C is It is subtracted according to the amount of excess fuel supplied to the engine. That is, step 11
In step S13, the values of the intake pressure PM and the engine speed N read in step 1101 are stored in the ROM 32 based on the values shown in FIG.
The engine intake air amount is read out using the numerical table of
In step 1115, the FAD is calculated from the value of the correction coefficient K using the numerical table of FIG. 9 similarly stored in the ROM 32.
J is read out. Further, in step 1117, N is calculated from the numerical table of FIG. 10 stored in the ROM 32 using the NO _X absorbent temperature T read in step 1101.
The counter subtraction amount TDEC based on the O _X absorbent temperature is read.

Next, at step 1119, the value of the counter subtraction amount Q × FADJ based on the excess fuel amount calculated from the values of Q and FADJ determined above and the NO _X absorbent temperature T obtained above are set. The value of the counter subtraction amount TDEC is compared, and steps 1121 and 11 are executed.
At 23, the smaller value of the value of Q × FADJ and the value of TDEC is adopted as the counter subtraction amount NOXDEC.

In step 1125, the subtraction amount NOXDEC determined above is subtracted from the NO _X absorption amount counter C, and in step 1127, the value of the NO _X absorption amount counter C is guarded to the minimum value zero. It should be noted that C = zero means that the NO _X absorbent has released all the absorbed NO _X. As described above, when the engine is operated at the rich air-fuel ratio or the stoichiometric air-fuel ratio, the amount of excess fuel supplied to the engine (that is, the amount of unburned HC and CO supplied to the NO _x absorbent) is increased. By setting the subtraction amount of the value of the NO _X absorption amount counter based on this, the value of the NO _X absorption amount counter is accurately reduced according to the engine operating state during rich or stoichiometric air-fuel ratio operation, so the NO _X absorption amount it is possible to estimate the NO _X absorption of exactly _the NO _X absorbent by using the value of the counter.

Next, the execution timing of the regeneration operation of the NO _X absorbent of this embodiment will be described. In this embodiment, NO
_{If the X} absorption amount counter value exceeds the specified value,
By setting the value of the air-fuel ratio correction coefficient K to 1.0 or more (for example, about K = 1.2) for a certain period of time (about several seconds) regardless of the operating region of FIG. 6, and operating the engine at the rich air-fuel ratio, The NO _x absorbent regeneration operation is performed. FIG. 12 is a flowchart showing the setting operation of the control flag FR for executing the above-mentioned reproduction operation. This routine is executed by the control circuit 30 at regular intervals.

In FIG. 12, step 1201
NO calculated by the routine of 11_XAbsorption amount counter
The value of C is read from the RAM 33. Then step
In 1203, the reproduction operation control flag FR is set (= 1)
If FR ≠ 1, it is determined that
If the playback operation is not currently being executed, step 120
NO in 5_XThe value of the absorption counter C is the predetermined value C₀Or more
No, i.e. whether a replay operation needs to be performed
Is determined. Here, the predetermined value C₀Is the above NO _Xabsorption
Maximum NO of agent_XAbout 70% of absorbed CF
Is set.

If C ≧ C ₀ in step 1205, it is necessary to regenerate the NO _x absorbent.
In step 1209, the reproduction operation control flag FR is set (= 1). Here, when the flag FR is set, in the fuel injection control routine that is separately executed, the value of the air-fuel ratio correction coefficient K is set to K> 1.0, the engine air-fuel ratio is switched to rich, and NO _x absorption is performed. Release of NO _X from the agent and reduction purification are started. Also, step 120
If C <C ₀ in 5, the routine ends without changing the value of the flag FR.

When FR = 1 in step 1203, the counter CT representing the elapsed time from the start of the reproduction operation execution is incremented by 1 in step 1209, and in step 1211, the elapsed time C after the start of the reproduction operation C.
It is determined whether or not T has reached the predetermined time CT ₀ , and if the elapsed time has not reached CT ₀ (CT <CT ₀ ), the routine ends as it is. Further, when the predetermined time has elapsed after the start of the reproduction operation in step 1211, (CT ≧ CT ₀ ), the reproduction operation flag FR is reset (= 0) in step 1213. This allows
In the fuel injection control routine that is executed separately, the value of the air-fuel ratio correction coefficient K is determined based on the operating region of FIG.
_{The X-} absorbent regeneration operation ends. Then step 121
In 5, the value of the elapsed time counter CT is cleared (= 0), and this routine ends.

As described above, in this routine once NO _X
When the value of the absorption amount counter regenerating operation of the NO _X absorbent exceeds the predetermined value C ₀ is started, the value of the counter by a routine executed subsequent Figure 11 is a predetermined time CT ₀ even when it becomes smaller than C ₀ During that time, a reproduction operation is executed. Note that N
Since the release rate of NO _X from the O _X absorbent is extremely high, the NO _X absorbent regeneration operation execution time CT ₀ is set to a relatively short time, for example, about 1 second in this embodiment. Further, since the routine of FIG. 11 is executed even during the regeneration operation, the value of the NO _X absorption counter is decreased by performing the regeneration operation, and when the NO _X absorbent is completely regenerated. Causes the value of the counter C to be 0 (see FIG.
1 steps 1127, 1129).

[0056] The exhaust in the present embodiment, the temperature of the NO _X absorbent is detected by the exhaust temperature sensor 20 installed in an exhaust passage of the NO _X absorbent 18 downstream, once the load condition of the engine Since the temperature (NO _x absorbent temperature) is substantially determined, the exhaust temperature is obtained in advance as a function of the engine load condition (intake pressure PM and engine speed N) by experiments, and stored in the ROM 32 in the form of a numerical table. If the exhaust temperature is determined from the intake pressure PM and the engine speed N, the exhaust temperature sensor 20 can be omitted.

Further, in the routine of FIG. 11, the counter subtraction amount Q × FADJ based on the excess fuel amount and the counter subtraction amount TDEC based on the NO _x absorbent temperature are individually calculated, and the smaller one is adopted as the subtraction amount NOXDEC. However, for example, the subtraction amount NO is corrected by correcting Q × FADJ in the form of NOXDEC = (Q × FADJ) −TADJ by the temperature correction amount TADJ set by the NO _x absorbent temperature.
XDEC may be set.

In this embodiment, as described above, the counter subtraction amount NOXDEC is determined based on both the excess fuel amount and the NO _X absorbent temperature, but the counter subtraction amount Q × based on the excess fuel amount is determined. It is possible to simplify the calculation by calculating only one of FADJ and the counter subtraction amount TDEC based on the NO _X absorbent temperature and adopting it as the value of the counter subtraction amount NOXDEC.

Further, in this embodiment, NO_XAbsorbent NO_X
Maximum absorption (equivalent to CF in step 1109 in Fig. 11)
And NO_XNO to execute absorbent regeneration operation_XAbsorption
(FIG. 12, Step 1205 C₀Is a constant value
Yes, but no_XAbsorbent NO_XMaximum absorption is NO_X
Since it also changes depending on the temperature of the absorbent, NO_XAbsorbent
CF and C depending on the temperature₀To change the value of
If so, a more appropriate NO _XIt is possible to carry out the regeneration operation of the absorbent
it can.

[0060]

As described above, according to the exhaust gas purifying apparatus of the present invention, when the engine is operated at the rich or stoichiometric air-fuel ratio, the subtracted amount of the NO _X absorption amount counter is supplied to the engine excessively. by you be set based on the fuel amount and _the NO _X absorbent temperature, it becomes possible to accurately estimate the NO _X absorption of real of the NO _X absorbent, appropriate reproduction of the NO _X absorbent There is an advantage that operations can be performed.

[Brief description of drawings]

FIG. 1 is an overall view of an internal combustion engine showing an embodiment of an exhaust emission control device of the present invention.

FIG. 2 is a diagram for explaining the NO _X absorption / release mechanism of the NO _X absorbent used in the present invention.

FIG. 3 is a diagram showing the relationship between the amount of excess fuel supplied to the engine and the NO _x release rate of the NO _x absorbent.

FIG. 4 is a diagram showing the relationship between the NO _X absorbent temperature and the NO _X release rate.

5 is a diagram showing a form of a numerical table used for determining a basic fuel injection amount of the engine of FIG.

FIG. 6 is a diagram showing a setting example of a correction coefficient K of a fuel injection amount.

FIG. 7 is a diagram showing the form of a numerical table used for determining the NO _X absorption amount counter addition amount during lean air-fuel ratio operation.

FIG. 8 is a diagram showing a form of a numerical table used for determining an engine intake air amount.

FIG. 9 is a diagram showing a form of a numerical table used for determining a coefficient FADJ for calculating an excess fuel amount.

FIG. 10 is a diagram showing a form of a numerical table used for determining a coefficient TDEC for calculating NO _X release rate based on the temperature of NO _X absorbent.

FIG. 11 is a flowchart showing a calculation operation of a NO _X absorption amount counter.

FIG. 12 is a flowchart showing a setting operation of a NO _X absorbent regeneration operation control flag.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 10 ... Surge tank 11 ... Fuel injection valve 15 ... Intake pressure sensor 17 ... Exhaust passage 18 ... NO _X absorbent 20 ... Exhaust temperature sensor 21 ... Rotation speed sensor 30 ... Electronic control circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F01N 3/28 301 C F02D 41/14 310 A 8011-3G 45/00 314 R 366 F

Claims (3)

[Claims] 1. Exhaust gas arranged in an exhaust passage of an internal combustion engine
NO in the exhaust when the air-fuel ratio is lean_XAbsorbs and drains
NO absorbed when the oxygen concentration in the air decreased_XEmit
NO to_XAbsorbent, when the internal combustion engine is operating at a lean air-fuel ratio,
When the predetermined amount of addition determined according to the engine operating condition is constant
NO in every interval_XAbsorbent NO_XNO indicating the amount of absorption_XSucking
It is added to the yield counter and the internal combustion engine is rich or
The predetermined subtraction amount is kept constant when operating at the theoretical air-fuel ratio
NO for each time_XTo subtract from the absorption counter
From the NO_XAbsorbent NO_XAbsorption to estimate absorption
Quantity estimation means, and when the engine is operating at a rich air-fuel ratio, the engine
The amount of fuel required to make the air-fuel ratio the stoichiometric air-fuel ratio
Excessive over detection of excess fuel supplied to the engine
When the fuel amount detection means and the engine are operated at a rich air-fuel ratio, the excess
NO based on fuel quantity _XThe subtracted amount of the absorption amount counter
Exhaust gas purification of internal combustion engine with subtraction amount setting means for determining
apparatus. Wherein disposed in an exhaust passage of an internal combustion engine, the air-fuel ratio of the exhaust gas absorbs NO _X in the exhaust gas when the lean, the oxygen concentration in the exhaust gas to release NO _X absorbed when reduced NO _x absorbent, and when the internal combustion engine is operating at a lean air-fuel ratio,
The added to the NO _X absorption counter indicating the NO _X absorption of _the NO _X absorbent to predetermined addition amount determined at regular intervals in accordance with the engine operating conditions, operating the internal combustion engine is rich or the stoichiometric air-fuel ratio when it is, by subtracting from the NO _X absorption counter a predetermined subtraction amount at predetermined time intervals, and the absorption amount estimation means for estimating the NO _X absorption of the _the NO _X absorbent, the NO _X absorbent The temperature detecting means for detecting the temperature of the agent and the NO when the engine is operated at a rich air-fuel ratio.
An exhaust gas purification device for an internal combustion engine, comprising: a subtraction amount setting means for determining the subtraction amount of the NO _X absorption amount counter based on the _X absorbent temperature. 3. The NO _x in the exhaust gas, which is arranged in the exhaust passage of the internal combustion engine, is absorbed when the air-fuel ratio of the exhaust gas is lean, and the absorbed NO _x is released when the oxygen concentration in the exhaust gas is reduced. NO _x absorbent, and when the internal combustion engine is operating at a lean air-fuel ratio,
The added to the NO _X absorption counter indicating the NO _X absorption of _the NO _X absorbent to predetermined addition amount determined at regular intervals in accordance with the engine operating conditions, operating the internal combustion engine is rich or the stoichiometric air-fuel ratio when it is, by subtracting from the NO _X absorption counter a predetermined subtraction amount at predetermined time intervals, and the absorption amount estimation means for estimating the NO _X absorption of the _the NO _X absorbent, the engine intake air means for detecting the amount, and means for detecting the operating air-fuel ratio of the engine, means for detecting the temperature of the _the NO _X absorbent, when the engine is operated at a rich air-fuel ratio, the engine intake air Amount, the engine operating air-fuel ratio, and the NO _x
An exhaust gas purification device for an internal combustion engine, comprising: subtraction amount setting means for determining the subtraction amount of the NO _X absorption amount counter based on the absorbent temperature.