JP3186394B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.

[0002]

Absorbs NO _x when the air-fuel ratio is lean of the Related Art inflowing exhaust gas, the engine of _the NO _x absorbent air-fuel ratio of the exhaust gas to release the NO _x absorbed when the stoichiometric air-fuel ratio or rich flowing It was disposed in the exhaust passage, estimating the amount of NO _x is absorbed from the engine load and engine speed in _the NO _x absorbent when the lean air-fuel mixture is burned, and is absorbed in _the NO _x absorbent NO _x estimated
Temporarily to rich NO _x internal combustion engine so as to release the NO _x from the absorbent by the present applicant the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent when the amount exceeds the permissible amount predetermined (International application PC
T / J reference No. P93 / 00778).

[0003] Generally, NO _x emitted from an engine
Quantity increases as the higher the engine load, since increases as the engine speed increases _the amount of NO _x discharged from the engine can be roughly estimated from the engine load and engine speed, therefore the engine load and the internal combustion engine the amount of NO _x is absorbed from the engine speed in _the NO _x absorbent is to be estimated.

[0004]

However, to be precise, the amount of NO _x discharged from the engine depends on the air-fuel ratio of the air-fuel mixture. Therefore it is impossible to accurately estimate the amount of NO _x is absorbed in the NO _x absorbent unless take into account the air-fuel ratio of the mixture. The actual air between the target air-fuel ratio to _the amount of NO _x when to be the transient operation particularly estimated that absorbed in the NO _x absorbent taking into account the target air-fuel ratio when the target air-fuel ratio is predetermined Large deviation from the fuel ratio, so that even if the target air-fuel ratio is taken into account, N
O _x absorbent can not be accurately estimated amount of NO _x absorbed in the.

[0005]

According to the present invention, when the air-fuel ratio of the inflowing exhaust gas is lean, NO _x is absorbed, and the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric. when the fuel ratio or rich is arranged to _the NO _x absorbent to release the absorbed NO _x in the engine exhaust passage, NO
_x An air-fuel ratio sensor capable of detecting the actual air-fuel ratio is disposed in the engine exhaust passage upstream of the absorbent, and is provided with intake air amount detection means for detecting the amount of intake air supplied to the engine, which is detected by the air-fuel ratio sensor. NO from the actual air-fuel ratio and the intake air amount
Absorbed NO for estimating the amount of NO _x absorbed in _x absorbent
_{An x} amount estimating means is provided.

[0006]

[Action] amount of NO _x is absorbed in the NO _x absorbent based on the actual air-fuel ratio detected by the air-fuel ratio sensor is estimated.

[0007]

Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is an exhaust port. Show. The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel into the intake port 6 is attached to each branch pipe 9. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 17 containing a NO _x absorbent 18.

The electronic control unit 30 is composed of a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and an input port 35 interconnected by a bidirectional bus 31. And an output port 36. A pressure sensor 19 that generates an output voltage proportional to the absolute pressure in the surge tank 10 is disposed in the surge tank 10, and the output voltage of the pressure sensor 19 is input to an input port 35 via an AD converter 37. . An air-fuel ratio sensor 20 is disposed in the exhaust manifold 15, and an output voltage of the air-fuel ratio sensor 20 is input to an input port 35 via an AD converter 38. The input port 35 has a rotation speed sensor 2 for generating an output pulse representing the engine rotation speed.
1 is connected. On the other hand, the output port 36 is connected to the ignition plug 4 and the fuel injection valve 11 via a corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = f · TP · K Here, f is a constant, TP is a basic fuel injection time, and K is a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. The basic fuel injection time TP is obtained by an experiment in advance, and is stored in advance in the ROM 32 as a function of the absolute pressure PM in the surge tank 10 and the engine speed N in the form of a map as shown in FIG. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-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 larger than the stoichiometric air-fuel ratio, that is, the air-fuel ratio becomes lean. The air-fuel ratio of the air-fuel mixture is smaller than the stoichiometric air-fuel ratio, that is, rich.

The target air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder, that is, the value of the correction coefficient K is changed according to the operating state of the engine, and is basically shown in FIG. 3 in the embodiment according to the present invention. As described above, it is predetermined as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. That is, as shown in FIG. 3, K <1.0 in the low-load operation region on the lower load side than the solid line R, that is, the air-fuel mixture is lean, and K in the high-load operation region between the solid line R and the solid line S. = 1.
0, that is, the air-fuel ratio of the air-fuel mixture is the stoichiometric air-fuel ratio, and K> 1.0, that is, the air-fuel mixture is rich in the full load operation region on the higher load side than the solid line S.

FIG. 4 schematically shows the concentration of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 4, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 increases, and the concentration of the exhaust gas from the combustion chamber 3 increases. The concentration of oxygen O ₂ in the exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NO contained in casing 17_x
The absorbent 18 uses, for example, alumina as a carrier, and on this carrier,
For example, potassium K, sodium Na, lithium Li,
Alkali metals such as Cs, barium Ba, cal
Alkaline earths such as calcium Ca, lanthanum La,
At least one selected from rare earths such as thorium Y
And a noble metal such as platinum Pt. organ
Intake passage and NO_xProvided in the exhaust passage upstream of the absorbent 18
The ratio of supplied air and fuel (hydrocarbon) is set to NO _xabsorption
When this is referred to as the air-fuel ratio of the exhaust gas flowing into the agent 18, this NO_x
When the air-fuel ratio of the inflowing exhaust gas is lean, the absorbent 18
NO_xAnd reduce the oxygen concentration in the incoming exhaust gas
And absorbed NO_xReleases NO_xPerform the absorption and release action of
U. Note that NO_xFuel in the exhaust passage upstream of the absorbent 18
(Hydrocarbons) or inflow and exhaust when air is not supplied
The air-fuel ratio of the gas-gas is the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3.
Ratio, and in this case NO_xAbsorbent 18 burns
When the air-fuel ratio of the mixture supplied to the firing chamber 3 is lean
Is NO_xIn the air-fuel mixture supplied into the combustion chamber 3
NO absorbed when oxygen concentration decreases_xTo release
Become.

If the above-mentioned NO _x absorbent 18 is arranged in the engine exhaust passage, the NO _x absorbent 18 actually performs the absorption and release of NO _x , but the detailed mechanism of the absorption and release is not clear. There is also. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
These oxygen O ₂ as shown in (A) is O ₂ ^- or O
It adheres to the surface of platinum Pt in the form of ^2- . On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the produced NO ₂ is absorbed in the absorbent while being oxidized on the platinum Pt, and is absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. Diffuses into agent. In this way, NO _x is absorbed in the NO _x absorbent 18.

As long as the oxygen concentration in the inflowing exhaust gas is high, platinum
NO on Pt surface_TwoIs generated, and the NO_xAbsorption capacity
NO unless power is saturated_TwoIs absorbed in the absorbent and nitric acid
Ion NO_Three ^-Is generated. In contrast, the inflow exhaust gas
NO in the oxygen concentration in the_TwoWhen the amount of
The reaction is reversed (NO_Three ^-→ NO_Two) And thus suck
Nitrate ion NO in the collector_Three ^-Is NO_TwoFrom the absorbent in the form of
Released. That is, the oxygen concentration in the inflow exhaust gas decreases.
NO_xNO from absorbent 18_xWill be released
You. As shown in FIG. 4, the degree of lean of the incoming exhaust gas
Lower the oxygen concentration in the incoming exhaust gas
If the degree of leanness of the inflow exhaust gas is reduced,
NO even if the air-fuel ratio of the exhaust gas is lean_xAbsorbent 18
From NO _xWill be released.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG.
C and CO are discharged, and these unburned HC and CO are converted to platinum Pt.
It reacts with the above oxygen O ₂ ^- or O ^2- to be oxidized.
Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas extremely decreases.
O ₂ is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 5 (B). In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. Therefore NO _x from the NO _x absorbent 18 in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich, that is released.

That is, the air-fuel ratio of the inflowing exhaust gas is made rich.
First, unburned HC and CO are converted to O on platinum Pt._Two ^-or
Is O^2-And immediately oxidized, then platinum
O on Pt_Two ^-Or O^2-Is consumed but unburned HC,
If CO remains, this unburned HC and CO will absorb
NO released from_xAnd NO emitted from the engine _x
Is reduced. Therefore, the air-fuel ratio of the inflow exhaust gas is reset.
NO in a short time_xAbsorbed by absorbent 18
NO_xIs released, and the released NO
_xNO in the atmosphere due to reduction_xIs discharged
It can be blocked. NO_xAbsorbent
18 has a function of a reduction catalyst, so that
NO even if the air-fuel ratio is stoichiometric_xReleased from absorbent 18
NO_xIs reduced. However, inflow and exhaust
NO if the air-fuel ratio of the gas is the stoichiometric air-fuel ratio_xabsorption
NO from agent 18_xIs released only gradually
_xTotal NO absorbed in absorbent 18_xTo release
Takes a little longer.

As described above, when the lean air-fuel mixture is burned, NO _x is absorbed by the NO _x absorbent 18. However there is a limit to the absorption of NO _x capacity of the NO _x absorbent 18, _the NO _x absorbent 18 when saturation absorption of NO _x capacity of the NO _x absorbent 18 is not E longer absorb NO _x.
Therefore NO absorption _of NO _x capacity of the _x absorbent 18 must be released the NO _x from the NO _x absorbent 18 before the saturation, how much to _the NO _x absorbent 18 to the NO _x
Needs to be estimated. Then this N
A method for estimating the O _x absorption amount will be described.

If the amount of intake air supplied to the engine increases,
The amount of exhaust gas discharged from the engine increases proportionally
If the amount of exhaust gas increases, exhaust from the engine proportionally
NO issued_xThe amount increases. Therefore NO_xAbsorbent 18
NO absorbed per unit time _xThe absorption amount NOXA is shown in FIG.
(B) is proportional to the intake air amount Q.
Become. In this case, the intake air amount Q may be actually detected.
It is possible, but the intake air amount Q is the absolute pressure P in the surge tank 10.
Example according to the present invention as a function of M and engine speed N
FIG. 7A shows the relationship between the intake air amount Q and PM, N.
This is obtained in advance in the form of a map such as ROM3.
2 and store the intake air amount Q based on this map.
I want to ask.

Meanwhile, _the amount of NO _x discharged from the engine becomes maximum slightly lean side from the stoichiometric air-fuel ratio, the more _the amount of NO _x if leaner than this maximum point is decreased. Therefore NO _x absorbed per unit time absorbent 18 NO _x absorption amount NOXA air-fuel ratio as shown in FIG. 8 (A) A /
It becomes a function of F. In the embodiment according to the present invention, the air-fuel ratio A
/ F is actually detected by the air-fuel ratio sensor 20 . That is, the air-fuel ratio sensor 20 generates an output voltage E that changes according to the actual air-fuel ratio A / F as shown in FIG.
Therefore, the actual air-fuel ratio A / F can be known from the output voltage E of the air-fuel ratio sensor 20.

After all, basically, NO_xNOXA absorption
It becomes a function of the intake air amount Q and the air-fuel ratio A / F._xSucking
The relationship between the yield NOXA, the intake air amount Q, and the air-fuel ratio A / F
In the form of a map as shown in FIG.
Is stored in Also, NO emitted from the engine_xamount
Also has an effect on the ignition timing, and therefore more accurately NO
_xIn order to estimate the absorption amount NOXA, the ignition timing is also taken into account.
Preferably. In this case, the NO discharged from the engine
_xThe amount decreases as the amount of ignition advance increases.
In the embodiment according to the present invention, the correction value f (θ) shown in FIG.
NO _xBy multiplying the absorption amount NOXA, the final N
O_xThe absorption amount NOXA is calculated. In addition,
The ignition timing θ depends on the absolute pressure PM in the surge tank 10 and the
A map as shown in FIG. 9A as a function of the rotational speed N
Is stored in the ROM 32 in advance.

[0022] In the internal combustion engines, which inject fuel directly into the combustion chamber 3 NO _x amount exhausted from the engine as the injection timing is advanced to increase. Therefore, in such an internal combustion engine, the correction value f (CA) as shown in FIG.
Further it is preferable to multiply the absorption of NO _x amount NOXA. On the other hand, the air-fuel ratio of the mixture fed into the engine cylinder NO _x from the NO _x absorbent 18 becomes the stoichiometric air-fuel ratio or rich is released _the NO _x releasing amount is mainly an exhaust gas amount of this time and the sky Affected by fuel ratio. That, NO _x amount amount of exhaust gas is discharged from the higher per unit time _the NO _x absorbent 18 increases increases, NO _x amount the air-fuel ratio is emitted from more per unit time _the NO _x absorbent 18 becomes rich Increase. In this case, since the exhaust gas amount is proportional to the intake air amount Q, as shown in FIG.
_the amount of NO _x NOXD emitted from _x absorbent 18 increases as the intake air amount Q becomes larger. Further, NO _x amount NOXD released from per unit time _the NO _x absorbent 18 is increased as the air-fuel ratio A / F decreases. _The amount of NO _x NOXD released from the unit time per _the NO _x absorbent 18 is stored in advance in the ROM32 in the form of a map shown in FIG. 10 (C) as a function of Q and A / F.

Further, _the NO _x absorbent 18 nitrate ions NO ₃ and the absorbent temperature is high in ^- that _the NO _x releasing rate from _the NO _x absorbent 18 is increased so easily decomposed. In this case, NO since the temperature of the _x absorbent 18 is proportional to substantially exhaust gas as shown in FIG. 11 (A) NO _x release rate K
f increases as the exhaust gas temperature T increases. Therefore N
When the O _x release rate Kf is taken into consideration, N
The NO _x amount released from the O _x absorbent 18 is shown in FIG.
It will be expressed by the product of the NOXD and _the NO _x releasing factor Kf shown in. The exhaust gas temperature T in the embodiment according to the present invention is stored in advance in the ROM32 in FIG 11 (B) are shown the form of a map as a function of the absolute pressure PM and the engine speed N in the surge tank 10.

As described above, the lean mixture burns.
NO per unit time _xNOXA absorption
And a mixture of stoichiometric air-fuel ratio or rich mixture
NO per unit time when burned_xThe amount released
NO because it is expressed by Kf · NOXD_xAbsorbed by absorbent 18
NO presumed to be collected_xThe quantity ΣNOX is expressed by the following equation.
Will be forgotten.

ΣNOX = ΣNOX + NOXA−Kf · NOXD As described above, in the embodiment according to the present invention, FIG.
In, the air-fuel ratio is controlled according to the value of the correction coefficient K divided by the solid lines R and S. Therefore, in the region on the lower load side than the solid line R in FIG. 3, the lean mixture (K <1.0) is burned, so that NO _x is absorbed by the NO _x absorbent 18 and the load is higher than the solid line R in FIG. In the region on the side, a mixture having a stoichiometric air-fuel ratio (K = 1.0) or a rich mixture (K> 1.
0) is burned, so that the NO _x absorbent 18
_x will be released. Therefore, if the low load operation and the high load operation are alternately repeated with the solid line R in FIG.
without absorption _of NO _x capacity of the _x absorbent 18 becomes saturated,
NO when the lean mixture is burned in this way
NO _x is absorbed well by the _x absorbent 18.

[0026] Actually, however is often low load operation is performed, i.e. many opportunities for combustion of lean air-fuel mixture takes place, thus the absorption of NO _x capacity of the NO _x absorbent 18 during this time is saturated Become. So when exceeding the allowable value MAX, _the amount of NO _x ΣNOX is estimated to be absorbed in _the NO _x absorbent 18 is predetermined as shown in FIG. 12, in the embodiment according to the present invention, for example _the amount of NO _x ΣNOX is The mixture is temporarily rich until zero (K =
To KK), and so as to release the NO _x from the NO _x absorbent 18 during this time.

FIGS. 13 and 14 show a routine for controlling the air-fuel ratio, and this routine is executed by interruption every predetermined time. Referring to FIGS. 13 and 14, first, at step 100, the basic fuel injection time TP is calculated from the map shown in FIG. Then whether _the amount of NO _x ΣNOX is estimated to be absorbed in _the NO _x absorbent 18 at step 101 is greater than the allowable amount MAX is determined. When the .SIGMA.NOX ≦ MAX whether _the NO _x releasing flag showing that should be released NO _x proceeds to step 103 has been set or not. If the NO _x release flag has not been set, the routine proceeds to step 104, where the correction coefficient K is calculated from the relationship shown in FIG.

Next, at step 105, it is determined whether or not the correction coefficient K is smaller than 1.0. When K <1.0, i.e. absorption of NO _x amount NOXA of willing per unit time from the map shown in FIG. 8 (B) in step 106 when the operating state to combust a lean air-fuel mixture is calculated. Next, at step 107, the map shown in FIG.
A correction value f (θ) is calculated based on the relationship shown in FIG.
Then the final absorption of NO _x amount NOXA is calculated by multiplying the f (theta) to NOXA step 108. Next, at step 109 NO _x emissions NOXD
Is made zero, and then at step 110, the fuel injection time TAU is calculated based on the following equation.

[0029] TAU = whereas f · TP · K, NO _x emission amount per unit time from the map shown in FIG. 10 (C) proceeds to step 114 when it is judged that K ≧ 1.0 at step 105 NOXD Is calculated. Then _the NO _x releasing factor Kf is calculated from the map shown in relation to FIG. 11 (B) shown in step 115 FIG. 11 (A). Then absorption of NO _x amount NOXA per unit step 116 time is made zero, then the routine proceeds to step 110.

[0030] Based on the step 117 following equation continues to step 110 is absorbed in _the NO _x absorbent 18 NO _x
The quantity ΣNOX is calculated. ΣNOX = ΣNOX + NOXA−Kf · NOXD Next, at step 118, it is determined whether or not the NO _x amount ＸNOX has become negative. When ΣNOX <0, the routine proceeds to step 119, where ΣNOX is made zero.

On the other hand, in step 101, {NOX>
_The NO _x releasing flag proceeds to step 102 when it is determined that MAX is set. Next, the routine proceeds from step 103 to step 111, where the correction coefficient K is set to a constant value KK. This constant value KK is such that the air-fuel ratio of the air-fuel mixture is 11
The value is about 1.1 to 1.3, which results in a rich mixture of about 13 to about 13. Next, at step 112, the NO _x amount ΣN
It is determined whether OX has become zero or negative. ΣNOX
If> 0, the process jumps to step 114. On the other hand, when ΣNOX ≦ 0, the routine proceeds to step 113, where N
The O _x release flag is reset, then step 114
Proceed to. Therefore, if ΣNOX> MAX, ΣNOX ≦ 0
Until the mixture becomes rich (K = KK).

[0032]

The Effect of the Invention amount of NO _x is absorbed in the NO _x absorbent by estimating the amount of NO _x absorbed in _the NO _x absorbent based on the actual air-fuel ratio can be accurately estimated.

[Brief description of the drawings]

FIG. 1 is an overall view of an internal combustion engine.

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

FIG. 3 is a diagram showing a correction coefficient K;

FIG. 4 shows unburned HC and C in exhaust gas discharged from the engine.
FIG. 3 is a diagram schematically showing the concentrations of O and oxygen.

FIG. 5 is a diagram for explaining a NO _x absorption / release effect.

FIG. 6 is a diagram showing an output of an air-fuel ratio sensor.

7 is a diagram showing the absorption of NO _x amount NOXA like.

8 is a diagram showing the absorption of NO _x amount NOXA like.

FIG. 9 is a diagram showing a correction value f (θ) and the like.

10 is a diagram showing the _the NO _x releasing amount NOXD like.

FIG. 11 is a diagram showing a correction value Kf and the like.

FIG. 12 is a time chart of air-fuel ratio control.

FIG. 13 is a flowchart showing air-fuel ratio control.

FIG. 14 is a flowchart showing air-fuel ratio control.

[Explanation of symbols]

16 ... exhaust pipe 18 ... NO _x absorbent

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-171921 (JP, A) JP-A-3-242415 (JP, A) WO 93/25806 (WO, A1) (58) Fields surveyed (Int.Cl. ⁷ , DB name) F01N 3/08 F01N 3/18 F02D 45/00 G01M 15/00

Claims (1)

(57) [Claims] 1. A absorbs NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, the engine and _the NO _x absorbent air-fuel ratio of the exhaust gas to release the NO _x absorbed when the stoichiometric air-fuel ratio or rich flowing was disposed in the exhaust passage, the air-fuel ratio sensor arranged capable of detecting an actual air-fuel ratio in _the NO _x absorbent in the engine exhaust passage upstream of the intake air amount detecting means for detecting an intake air quantity supplied to the engine provided with an exhaust purification system of an internal combustion engine provided with the absorption amount of NO _x estimating means for estimating the amount of NO _x is absorbed from the actual air-fuel ratio and the intake air amount detected by the air-fuel ratio sensor in _the NO _x absorbent.