JP2658756B2 - 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]

2. Description of the Related Art In an internal combustion engine in which a lean air-fuel mixture is burned, a NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the inflowing exhaust gas becomes rich is used. The NOx generated when the lean mixture is burned is disposed in the engine exhaust passage and absorbed by the NOx absorbent.
The present invention relates to an internal combustion engine in which the air-fuel ratio of exhaust gas flowing into a NOx absorbent is temporarily made rich before the Ox absorption capacity is saturated to release NOx from the NOx absorbent and reduce the released NOx. It has already been proposed by humans (see Japanese Patent Application No. 3-284095).

[0003]

When the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich, for example, when the air-fuel mixture supplied into the engine cylinder is made rich, a large amount of unburned HC, CO And the like, and the NOx absorbent releases the absorbed NOx because the oxygen concentration in the inflowing exhaust gas decreases. At this time, a part of the unburned HC and CO discharged from the engine is used to reduce NOx discharged from the engine, and the remaining unburned H
C, CO, etc. are used to reduce NOx released from the NOx absorbent. Therefore, in this case, in order to suppress the emission of NOx into the atmosphere, an unburned HC, CO, etc., which can reduce both the NOx discharged from the engine and the NOx released from the NOx absorbent, is supplied from the engine. Must be discharged.

[0004] However, it is difficult to discharge the minimum amount of unburned HC, CO, and the like necessary to reduce all NOx from the engine.
CO etc. will be less or more than required to be able to reduce all the NOx. In this case, unburned HC,
If the amount of unburned components such as CO is smaller than the amount required to reduce all NOx, NOx is discharged from the NOx absorbent without being reduced, and the amount of unburned components is reduced to all N2.
If the amount exceeds the amount necessary to reduce Ox, unburned components are discharged from the NOx absorbent without being oxidized.

[0005]

According to the present invention, when the air-fuel ratio of the inflowing exhaust gas is lean, NOx is absorbed, and when the air-fuel ratio of the inflowing exhaust gas becomes rich, it is absorbed. A NOx absorbent that releases NOx is disposed in the engine exhaust passage, and a catalyst having an O ₂ storage function is disposed in the engine exhaust passage downstream of the NOx absorbent. When NOx is to be released from the NOx absorbent, the inflowing exhaust gas is discharged. The air-fuel ratio of the exhaust gas is switched from lean to rich, and at this time, the degree of enrichment is increased so that the amount of unburned components in the inflowing exhaust gas becomes an excess amount more than that required for NOx reduction. When the air-fuel ratio is switched from lean to rich, excess unburned components discharged from the NOx absorbent are oxidized by oxygen adsorbed and held on the catalyst.

[0006]

When the air-fuel ratio of the inflowing exhaust gas is switched from lean to rich, the degree of richness is increased so that the unburned component becomes an excess amount more than that required for NOx reduction. It can be reduced. However, at this time, excess unburned components are discharged from the NOx absorbent, but the excess unburned components are oxidized by oxygen adsorbed and held on the catalyst.

[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 includes an intake duct 12 and an air flow meter 13.
Is connected to the air cleaner 14 through the
Inside, a throttle valve 15 is arranged. On the other hand, the exhaust port 8 is connected to the N through the exhaust manifold 16 and the exhaust pipe 17.
Connected to a casing 19 containing an Ox absorbent 18;
The casing 19 is connected to the catalytic converter 2 via an exhaust pipe 20.
Connected to 1.

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. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and the output voltage is input to the input port 35 via the AD converter 37. The engine body 1 has a water temperature sensor 23 for generating an output voltage proportional to the engine cooling water temperature.
The output voltage of the water temperature sensor 23 is input to the input port 35 via the AD converter 38. Also,
The input port 35 is connected to a rotation speed sensor 24 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 is connected to the ignition plug 4 and the fuel injection valve 11 via corresponding drive circuits 39 and 40, 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 = TP · K Here, TP indicates a basic fuel injection time, and K indicates 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. This basic fuel injection time TP is obtained in advance by an experiment, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
2 is stored. 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, if 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, lean, and K> 1.0
, The air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the internal combustion engine shown in FIG.
= 0.6, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is maintained lean, and therefore, in the internal combustion engine shown in FIG. 1, the lean air-fuel mixture is normally burned. Become. FIG. 3 schematically shows the concentrations of representative components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 3, the amounts of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increase as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 increases, and the amount of the exhaust gas discharged from the combustion chamber 3 increases. The amount 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.

The NOx contained in the casing 19
The absorbent 18 is made of, for example, alumina as a carrier. On this carrier, for example, alkali metals such as potassium K, sodium Na, lithium Li, and cesium Cs, alkaline earths such as barium Ba and calcium Ca, lanthanum La, and yttrium Y are used. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. When the ratio of air and fuel supplied to the engine intake passage and the exhaust passage upstream of the NOx absorbent 18 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18, the NOx absorbent 18 is the air-fuel ratio of the inflow exhaust gas. Is lean, NOx is absorbed, and when the oxygen concentration in the inflowing exhaust gas decreases, the absorbed N
It acts to absorb and release NOx, which releases Ox. Note that NO
When no fuel or air is supplied into the exhaust passage upstream of the absorbent 18, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3, and in this case, NOx absorption The agent 18 absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases. Become.

If the above-described NOx absorbent 18 is disposed in the exhaust passage of the engine, the NOx absorbent 18 actually performs the absorption / release operation of NOx, but there is a portion where the detailed mechanism of the absorption / release operation is not clear. 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 carried 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 ^- to form a deposition on the surface of the platinum Pt. On the other hand, N
O is O ₂ on the surface of the platinum Pt ^- reacted with, and _{NO 2 (2NO + O 2 →} 2NO 2). NO ₂ generated
Some of bonds with the barium oxide BaO is absorbed into the absorbent while being oxidized on the platinum Pt, FIG. 4 (A) to nitrate ion NO ₃ as shown ^- is diffused in the absorbent in the form. In this way, NOx is absorbed in the NOx absorbent 18.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of platinum Pt, and as long as the NOx absorption capacity of the absorbent is not saturated, NO ₂ is absorbed in the absorbent and nitrate ions NO ₃ ^- are formed. Generated. On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), thus the nitrate ion NO ₃ ⁻ in the absorbent. There are released from the absorbent in the form of NO _2. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx absorbent 18. As shown in FIG. 3, when the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, when the degree of leanness of the inflowing exhaust gas decreases, NOx is released from the NOx absorbent 18. Will be.

On the other hand, at this time, the air-fuel ratio of the inflowing exhaust gas is reset.
As shown in FIG. 3, a large amount of
Fuel HC and CO are discharged, and these unburned HC and CO are converted to platinum.
Oxygen O on Pt_Two ^-And oxidize. Ma
When the air-fuel ratio of the inflow exhaust gas is made rich, the inflow exhaust gas
NOx from the absorbent because the oxygen concentration in the
_TwoIs released and this NO_TwoIs as shown in FIG.
And reacts with unburned HC and CO to be reduced. This
Thus, NO on the surface of platinum Pt_TwoIs no longer present
NO from next to next from absorbent_TwoIs released. Therefore the flow
If the air-fuel ratio of the incoming and exhaust gas is made rich, N
NOx will be released from the Ox absorbent 18.

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 ^-When
Immediately reacted and oxidized, then on platinum Pt
O_Two ^-If unburned HC and CO still remain even if
N released from the absorbent by unburned HC and CO
Ox and NOx emitted from the engine are reduced
You. Therefore, when the air-fuel ratio of the inflow exhaust gas is made rich,
Total NOx released from the absorbent and exhaust from the engine
In order to reduce the total NOx, at least on the platinum Pt
O_Two ^-Of unburned HC and CO required to consume
The amount of unburned HC and CO required to reduce NOx is reduced to N
Resetting the air-fuel ratio of the inflowing gas so that it flows into the Ox absorbent 18
It is necessary to control the degree of the switch.

FIG. 5 shows the rich control of the air-fuel ratio of the inflow gas used in the embodiment according to the present invention.
In the embodiment shown in FIG. 5, the NOx absorbent 18
Is to be released, the air-fuel ratio of the mixture supplied to the combustion chamber 3 is made rich by increasing the correction coefficient K used for calculating the fuel injection time TAU to KK (> 1.0). . Next, the correction coefficient K is gradually decreased, and then the correction coefficient K is maintained at 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is maintained at the stoichiometric air-fuel ratio. Next, when the time C ₀ elapses after the rich control is started, the correction coefficient K is made smaller than 1.0 again, and the combustion of the lean mixture is started again.

When the air-fuel ratio of the mixture supplied to the combustion chamber 3 becomes rich (K = KK), most of the NOx absorbed by the NOx absorbent 18 is rapidly released. The value of the correction coefficient KK is O ₂ on this time the platinum Pt ^- are defined as consumed and excess unburned components or amount necessary to reduce all NOx to occur. That is, the dashed line in FIG. 5 indicates that the amount of unburned components discharged from the engine when rich control is started consumes O ₂ ⁻ on platinum Pt and all N
Correction coefficient K when the amount required for reducing Ox is obtained.
K ′, and the correction coefficient KK is the correction coefficient KK ′.
It is determined to be larger than

In this case, as the temperature of the exhaust gas rises and the temperature of the NOx absorbent 18 rises, the NOx absorbent 1
8 increases the amount of NOx released. Therefore, FIG.
As shown by the solid line in (A), the value of the correction coefficient KK increases as the exhaust gas temperature T increases. FIG.
The (A) the amount of unburned components O ₂ on the platinum Pt ^- shown in broken lines for reference correction coefficient KK 'when the amount necessary to reduce the consumed and total NOx.

The relationship between the correction coefficient KK and the exhaust gas temperature T shown in FIG. 6A is stored in the ROM 32 in advance. In this case, the exhaust gas temperature T can be directly detected, but can also be estimated from the intake air amount Q and the engine speed N.
Therefore, in the embodiment according to the present invention, the relationship between the exhaust gas temperature T, the intake air amount Q, and the engine speed N is obtained in advance by an experiment, and this relationship is previously determined in the form of a map as shown in FIG.
M32, and stores the exhaust gas temperature T from this map.
Is calculated.

On the other hand, if the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich (K = KK) as described above, NO
x Most of the NOx absorbed in the absorbent 18 is rapidly released, and thereafter, even if the air-fuel ratio is made rich,
NOx is released little by little from the x absorbent 18. Therefore, if the air-fuel ratio is kept rich, unburned HC, CO
Will be released to the atmosphere. Therefore, as shown in FIG. 5, after the air-fuel ratio is made rich (K = KK), the degree of richness is gradually reduced, and then the air-fuel ratio is maintained at the stoichiometric air-fuel ratio (K = 1.0) to absorb NOx. NOx released little by little from the agent 18 is sequentially reduced.

When the air-fuel ratio is made rich, NOx
The greater the amount of NOx released from the absorbent 18, the smaller the amount of NOx subsequently released from the NOx absorbent 18, and accordingly the shorter the time until the NOx absorbent 18 finishes releasing NOx. As described above, when the air-fuel ratio is made richer as the exhaust gas temperature T becomes higher, the NOx absorbent 1
The amount of NOx released from FIG.
As shown in (B), the time C ₀ from when the air-fuel ratio becomes rich to when it returns to lean again becomes shorter as the exhaust gas temperature T becomes higher. The relationship between the time C ₀ and the exhaust gas temperature T shown in FIG. 6B is stored in the ROM 32 in advance.

By the way, as shown in FIG.
There O ₂ on the platinum Pt in the NOx absorbent 18 is made to increase until KK ^- consumed and excess unburned components or amount necessary to reduce all NOx and is supplied, NOx and thus the It can be reduced well. However, in this case, the surplus unburned components are discharged from the NOx absorbent 18, so that it is necessary to oxidize the surplus unburned components. Therefore, in the embodiment according to the present invention, NOx
A catalytic converter 21 having a built-in catalyst 22 having an O ₂ storage function is disposed in an exhaust passage downstream of the absorbent 18,
Excess unburned components are oxidized by the catalyst 22.

That is, the catalyst 22 comprises, for example, alumina as a carrier, a noble metal such as platinum Pt, an alkaline earth such as calcium Ca, and cerium Ce supported on the carrier. When cerium Ce is carried on the catalyst 22, the catalyst 22 adsorbs and holds oxygen contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst 22 is lean, and adsorbs when the air-fuel ratio of the exhaust gas flowing into the catalyst 22 becomes rich. It has an O ₂ storage function in which unburned HC and CO take away the retained oxygen. Therefore, such O ₂
If the catalyst 22 having a storage function is disposed in the exhaust passage downstream of the NOx absorbent 18, a large amount of oxygen is adsorbed and held by the catalyst 22 while the lean mixture is being burned, so that the NOx absorbent 18 The mixture supplied to the combustion chamber 3 to release NOx is enriched and NOx is released.
Even if the unburned HC and CO are discharged from the absorbent 18, the unburned HC and CO deprive the oxygen absorbed by the catalyst 22 and are oxidized. Thus, the unburned HC and CO are released to the atmosphere. Will be prevented.

Next, referring to FIG. 8 to FIG.
Of one embodiment of controlling the absorption and release of NOx absorbent 18
I will tell. FIGS. 8 and 9 show interrupts executed at regular intervals.
Only shows the routine. Referring to FIG. 8 and FIG.
First, in step 60, the correction coefficient K is set to 0.1.
The lean mixture is burned.
Is determined. When K ≧ 1.0, that is,
The mixture supplied to the sintering chamber 3 has a stoichiometric air-fuel ratio or a rich air-fuel ratio.
In the case of, the processing cycle is completed. On the other hand, K <
1.0, that is, the lean mixture is burned
To step 61, the NOx absorbent 18 absorbs
The released NOx amount W is calculated. That is, the combustion chamber 3
The amount of NOx discharged from the tank increases as the intake air amount Q increases.
And the engine load Q / N increases as the engine load Q / N increases.
The NOx amount W absorbed and released by the Ox absorbent 18 is W and k ₁
・ Q ・ Q / N (k₁Is a constant).
And

Next, at step 62, it is determined whether or not the execution flag is set. Execution flag whether NOx amount W absorbed in the NOx absorbent 18 proceeds to step 63 is larger than the set amount W _O predetermined when not been set or not. The set amount W _O is, for example, the maximum NOx that can be absorbed by the NOx absorbent 18.
About 30 percent of the volume. If W ≦ W _O , the processing cycle is completed, and if W> W _O , the routine proceeds to step 64, where the execution flag is set. Therefore, the execution flag is set when W> W _O.

When the execution flag is set, step 65
In FIG. 6, the correction coefficient KK is calculated from the relationship shown in FIG. 6A and the map shown in FIG. Next, at step 66, the final correction coefficient KK is calculated by multiplying KK by k ₂ · W (k ₂ is a constant). That is, the smaller the NOx amount W absorbed by the NOx absorbent 18, the smaller the degree of richness (KK). Next, at step 67, the time C is obtained from the relationship shown in FIG. 6B and the map shown in FIG.
_O is calculated. Next, at step 68, k ₃ · W is added to _CO.
(K ₃ is a constant) to obtain the final time C
_O is calculated. That is, the smaller the amount W of NOx absorbed in the NOx absorbent 18, the shorter the time _CO . Then the processing cycle is completed.

When the execution flag is set, in the next processing cycle, the routine proceeds from step 62 in FIG. 8 to step 69 in FIG. 9, and the NOx release flag is set. Next, at step 70, the count value C is incremented by one. Next, at step 71, the count value C becomes equal to the time C _O.
It is determined whether or not the time has become greater than the threshold value, that is, whether or not the time _CO has elapsed since the start of the rich control. C ≦ C
_{In the case of O,} the routine proceeds to step 72, where a constant value α is subtracted from the correction coefficient KK. Next, at step 73, it is determined whether or not the correction coefficient KK has become smaller than 1.0.
When KK ≦ 1.0, the routine proceeds to step 74, where KK is set to 1.
It is set to 0. Therefore, as shown in FIG. 5, the correction coefficient K gradually decreases, and when K = 1.0, thereafter, the correction coefficient K is held at 1.0.

Next, when C> C _O , the routine proceeds from step 71 to step 75 where the execution flag is reset, and then in step 76 the NOx release flag is reset. Next, at step 77, the NOx amount W absorbed by the NOx absorbent 18 is made zero, and then at step 78
The count value C is set to zero. FIG. 10 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed.

Referring to FIG. 10, first, in step 9
0, the basic fuel injection time TP from the map shown in FIG.
Is calculated. Next, at step 91, it is determined whether or not the NOx release flag is set. If the NOx release flag has not been set, the routine proceeds to step 92, where K = 0.6, for example. Next, at step 94, the fuel injection time TAU is calculated by multiplying the basic fuel injection time TP by the correction coefficient K. Therefore, at this time, the lean air-fuel mixture is burned.

On the other hand, when it is determined in step 91 that the NOx release flag is set, the routine proceeds to step 93, where the correction coefficient KK calculated by the routine of FIGS. 8 and 9 is set to K, and then to step 94. move on. Therefore, at this time, the air-fuel mixture supplied into the combustion chamber 3 is temporarily made rich, and then maintained at the stoichiometric air-fuel ratio for a while.

[0032]

According to the present invention, when NOx is released from the NOx absorbent, NOx can be reduced well and unburned components can be oxidized well.

[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 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. 4 is a diagram for explaining the effect of absorbing and releasing NOx.

FIG. 5 is a diagram illustrating a change in a correction coefficient K during rich control.

FIG. 6 is a diagram showing a relationship among a correction coefficient KK, time C _O, and exhaust gas temperature T.

FIG. 7 is a view showing a map of an exhaust gas temperature T;

FIG. 8 is a flowchart of a time interrupt routine.

FIG. 9 is a flowchart of a time interrupt routine.

FIG. 10 is a flowchart for calculating a fuel injection time TAU.

[Explanation of symbols]

 16 Exhaust manifold 18 NOx absorbent 21 Catalytic converter 22 Catalyst

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location F01N 3/24 F02D 45/00 301G F02D 41/14 310 B01D 53/34 129A 45/00 301 (56 References JP-A-4-90826 (JP, A) JP-A-3-135417 (JP, A) JP-A-1-247710 (JP, A) JP-A-62-117620 (JP, A) JP-A 62-106826 (JP, A) JP-A-62-97630 (JP, A)

Claims (1)

(57) [Claims] When the air-fuel ratio of the inflowing exhaust gas is lean, NOx is absorbed. When the air-fuel ratio of the inflowing exhaust gas becomes rich, a NOx absorbent that releases the absorbed NOx is disposed in the engine exhaust passage. A catalyst having an O ₂ storage function is arranged in the engine exhaust passage downstream of the absorbent, and N 2
When NOx is to be released from the Ox absorbent, the air-fuel ratio of the inflowing exhaust gas is switched from lean to rich, and at this time, the amount of unburned components in the inflowing exhaust gas becomes an excess amount more than that required for NOx reduction. In this manner, the degree of richness is increased, and when the air-fuel ratio of the inflowing exhaust gas is switched from lean to rich, excess unburned components discharged from the NOx absorbent are oxidized by oxygen adsorbed and held on the catalyst. Exhaust purification device for an internal combustion engine.