JPH0693845A - Exhaust emission control device for engine - Google Patents

Abstract

PURPOSE:To provide stable exhaust air purifying performance under all the operation conditions by changing over an exhaust flow passage in such a way that exhaust is circulated in both of a plural number of exhaust branch passage sections provided with an unburnt gas adsorption material and an exhaust emission purifying catalyst at the time of lean combustion operation of engine. CONSTITUTION:An exhaust emission purifying catalyst 22 is provided in an exhaust passage 13 of an engine 11. Also, an unburnt gas adsorption material 24 is provided in one of branch passage section 16 in the exhaust passage 13. Further, a flow passage change-over valve 16 is provided at a branch point 13a of each branch passage section 15, 16. The flow passage change-over valve 18 is driven and controlled by a control unit 30 via an actuator 19 based on each detected signal from an air-fuel ratio sensor 31 and a catalyst outlet temperature sensor 32. In this case, the flow passage change-over valve 18 is driven and controlled in such a way that exhaust is circulated in both of each branch passage section 15, 16 in the operation condition which the engine 11 burns on the more lean side than the theoretical air-fuel ratio. Thus, stable NOx purifying performance is ensured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust emission control system for an engine, and more particularly to an exhaust emission control system for an engine in an operating state where combustion is leaner than a stoichiometric air-fuel ratio.

[0002]

2. Description of the Related Art Conventionally, in order to improve fuel economy as much as possible,
There is an engine that burns an air-fuel mixture set leaner than the theoretical air-fuel ratio (hereinafter referred to as lean burn control). By the way, in a lean burn control engine, since the operation is performed in the air-fuel mixture set leaner than the theoretical air-fuel ratio, the NO _x conversion efficiency decreases on the leaner side than the theoretical air-fuel ratio as an exhaust purification device. It is not appropriate to use the three-way catalyst of.

[0003] Therefore, if the air-fuel ratio even when the lean side from the stoichiometric air-fuel ratio, (hereinafter referred to as the lean NO _X catalyst) catalyst having a function of conversion efficiency of the NO _X discharged from the engine is not lowered is used There is also. Here, there is a catalyst containing zeolite as the lean NO _x catalyst, and the lean NO _x catalyst is provided in the exhaust passage,
There is one that purifies the NO _X using the lean NO _X catalyst.

That is, the NO _x is reduced by reducing the NO _x by the HC adsorbed on the zeolite in the lean NO _x catalyst.
The O _X is purification.

[0005]

[SUMMARY OF THE INVENTION However, the apparatus having to purify the NO _X using constituted lean NO _X catalyst contains zeolite, the HC adsorbed on the zeolite in the lean NO _X catalyst Since NO _X is reduced, the purification performance will be affected by the operating conditions. That is, when lean burn control is performed, NO
_Since the HC emission amount is smaller than the _X emission amount and the HC concentration is low, if lean burn control is performed for a long time, the lean NO _x catalyst, which originally has a purifying ability,
There is a fear that the NO _x purification performance may be reduced. Further, when the operating condition is high load and high speed, the exhaust gas flow rate discharged from the engine also increases, so that the NO _x purification performance may be further deteriorated.

On the other hand, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-135126, as an automobile exhaust emission control device, on the upstream side of a catalytic converter arranged in the exhaust system of an engine for purifying harmful components in exhaust gas, Although a trapper containing an adsorbent for adsorbing unburned fuel has been proposed, it does not improve the NO _x purification performance.
In the exhaust gas purification device, HC is trapped in the adsorbent at low temperatures, and HC is desorbed from the adsorbent at high temperatures and purified by a catalyst. As the adsorbent, for example, a monolith carrier coated with zeolite has been proposed because zeolite has excellent adsorbability.

The present invention has been made by paying attention to such a conventional problem, and all operating conditions are maintained even when the engine is controlled for a long period of time so that the engine burns leaner than the stoichiometric air-fuel ratio. It is an object of the present invention to provide an exhaust gas purification device for an engine, which can obtain stable NO _x purification performance at high temperature.

[0008]

Therefore, the present invention provides the first aspect of the present invention.
As a technical means of the present invention, the exhaust passage of the engine is configured to include first and second branch passages that branch into two and then join again, and the unburned gas in the exhaust gas is adsorbed to the second branch passage portion. An unburned gas adsorbent is disposed, an exhaust gas purification catalyst is disposed in the exhaust passage downstream of the confluence of the first and second branch passage portions, and the exhaust gas is exhausted from the first branch passage portion. Flow path switching means for switching the flow paths so that they selectively flow in both the first portion and the second and second branch passage portions, and operating state detecting means for detecting the operating state of the engine. When the operating state detected by the state detecting means is an operating state in which combustion is on the lean side of the stoichiometric air-fuel ratio, the flow passage switching means forms a flow passage so that the exhaust gas flows to both the first and second branch passage portions. I tried to switch.

As a second technical means, the first
Also, even in an operating state in which the exhaust gas purification catalyst arranged in the exhaust passage downstream of the confluence of the second branch passage portion burns leaner than the stoichiometric air-fuel ratio, the NO _X emission amount tends to decrease. The catalyst may be As a third technical means, when the exhaust gas flow rate is large in the operating state in which the operating state detected by the operating state detecting means burns on the lean side of the stoichiometric air-fuel ratio, the flow path switching means causes the exhaust gas to flow. First
Alternatively, the flow path may be switched so as to flow to both the second branch passage portion and the second branch passage portion.

[0010]

According to the above construction, the unburned gas adsorbent for adsorbing the unburned gas in the exhaust is arranged in the second branch passage portion. When the catalyst is cold, the exhaust gas is circulated through the second branch passage portion and guided to the adsorbent. Therefore, at low temperature, HC is trapped in the adsorbent. When the temperature of the catalyst rises and is activated, the HC trapped in the adsorbent is desorbed from the adsorbent.

Here, the operating condition of the engine is such that the engine burns on the lean side of the stoichiometric air-fuel ratio (lean burn control).
Case of, HC emissions in the exhaust is reduced, NO _X emissions increases. As a result, as a function of the first technical means, when the operating state detecting means detects that the operating state is the lean burn control state, the flow path switching means causes the exhaust gas to exhaust the first and second branches. Since the flow path is switched so as to flow to both of the passage portions, the exhaust gas also flows to the second branch passage portion where the adsorbent is arranged.

Since the lean burn control is performed after the engine is warmed up, the adsorbent has reached the desorption temperature of HC during the lean burn control, and the exhaust gas purification catalyst has a sufficient activation temperature. Has reached. Therefore, when the exhaust gas flows through the second branch passage portion, the HC trapped in the adsorbent is desorbed and disposed in the exhaust passage downstream of the confluence portion of the first and second branch passage portions. It is supplied to a catalyst for purifying exhaust gas. Here, the supplied HC acts on NO _X as a reducing agent, and the purification performance of the exhaust purification catalyst arranged in the exhaust passage is improved.

Further, as an operation according to the second technical means, the exhaust gas is trapped in the adsorbent by the flow passage switching means also flowing into the second branch passage portion in which the adsorbent is arranged. The HC is desorbed and supplied to an exhaust gas purification catalyst arranged in an exhaust passage downstream of the confluence of the first and second branch passage portions. Here, the supplied HC acts on NO _X as a reducing agent, and the purification performance of the exhaust purification catalyst arranged in the exhaust passage is improved.
Since the catalyst is a catalyst in which the NO _X emission amount tends to decrease even in an operating state in which the combustion is on the lean side of the stoichiometric air-fuel ratio, according to the second technical means, under all operating conditions. Stable NO _X purification performance can be obtained.

In the third technical means, the flow path switching means is used when the exhaust gas flow rate is large in the operating state in which the operating state detected by the operating state detecting means burns on the lean side of the stoichiometric air-fuel ratio. Switch the flow paths so that the exhaust gas flows into both the first and second branch passage portions, so that the NO _x emission amount in the exhaust gas resulting from the combustion of the engine in the lean side of the theoretical air-fuel ratio In addition to the increase, it is possible to suppress the deterioration of the NO _X purification performance due to the increase of the exhaust gas flow rate, and further it is possible to obtain the stable NO _X purification performance under all operating conditions.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1 showing a system configuration of a first embodiment according to claims 1 and 3 of the present invention, an exhaust passage 13 connected to an exhaust manifold 12 of an engine 11 has an exhaust purification pre-catalyst 21 and a main catalyst. A catalyst 22 is provided.
The main catalyst after branching bifurcated from the exhaust passage on the downstream side of the pre-catalyst 21 (the branch point is 13a)
It joins again on the upstream side of 22 (the joining point is 13b)
A first branch passage portion 15 and a second branch passage portion 16 are provided, and an unburned gas adsorbent (hereinafter simply referred to as an adsorbent) 24 is arranged in the second branch passage portion 16. .

At the branch point 13a between the first branch passage portion 15 and the second branch passage portion 16, the exhaust gas is the first branch passage portion 15 only or the second branch passage portion 16 is exhausted. only,
Alternatively, a flow path switching valve 18 as a flow path switching means for switching the flow path to selectively flow through both the first branch passage portion 15 and the second branch passage portion 16 is provided. The flow path switching valve 18 is an actuator 19 controlled by a control unit 30 having a microcomputer described later.
And selectively switches the flow path.

The adsorbent 24 is a monolithic ceramic carrier coated with γ-alumina having a large specific surface area. The adsorbent 24 is not limited to the monolith shape, and may be foam, pellet, mesh, or the like, and the adsorbent itself is not limited to the γ-alumina and a substance having an adsorbing performance such as activated carbon. If

Here, the main catalyst 22 for purifying exhaust gas is an ordinary three-way catalyst which has been conventionally used. For example, a ceramic carrier carrying a precious metal such as platinum, palladium or rhodium is used to chemically exhaust the exhaust gas. Is to be oxidized. Further, a main catalyst outlet temperature sensor 32 for determining whether or not each of the pre-catalyst 21 and the main catalyst 22 is activated is provided in the exhaust passage 13 downstream of the main catalyst 22.

An air flow meter 43 for detecting an intake air flow rate controlled by a throttle valve 42 is provided in an intake passage 41 of the engine 11, and an intake manifold 44 has an electromagnetic type for each cylinder. A fuel injection valve 45 is provided, which is driven by an injection pulse signal from the control unit 30 to open the valve, and a fuel regulated to a predetermined pressure by a pressure regulator fed from a fuel pump (not shown) into the intake manifold 44. Supply by injection.

Further, depending on the oxygen concentration in the exhaust gas in the exhaust passage 13, it is turned on / off whether the air-fuel ratio of the intake air-fuel mixture corresponding thereto is richer or leaner than the stoichiometric air-fuel ratio.
An air-fuel ratio sensor 31 is provided for positive detection. Further, a crank angle sensor 33 that outputs a unit angle signal for each minute unit crank angle in synchronization with the engine rotation and outputs a reference signal for each stroke crank angle period of each cylinder is provided in a distributor or the like. .

An ignition switch for detecting engine start based on the cranking operation of the engine 11
34 is provided. And the control unit 30
Computes the fuel injection amount from the fuel injection valve 45 according to the detection signals from the various sensors, and outputs an injection pulse signal having a pulse width corresponding to it to control the air-fuel ratio, and By controlling the actuator 19,
Purify exhaust gas.

Next, the control performed by the control unit 30 in the exhaust emission control device having the above structure will be described with reference to the flow chart shown in FIG. In step 21 (denoted as S21 in the figure; the same applies hereinafter), an on / off signal of the engine from the ignition switch 34 is input. In step 22, it is determined whether or not the ignition signal is input. If the ignition signal is input, it is determined that the engine 11 is rotating, and the process proceeds to step 23 and the following steps.

If it is determined in step 22 that the ignition signal is not input, it is determined that the engine 11 is not rotating, and the process directly returns. Step 23
Then, the exhaust gas temperature T detected by the main catalyst outlet temperature sensor 32 is input. In step 24, it is judged whether the exhaust temperature T is equal to or higher than a predetermined value T ₀ . If it is determined that the exhaust temperature T is not equal to or higher than the predetermined value T ₀ , the step
Go to 25.

In step 25, the flow path switching valve 18 is switched to a position where the exhaust gas flows only through the second branch passage portion 16. In this state, since the exhaust gas flows only through the second branch passage portion 16, the exhaust gas passes through the adsorbent 24, and the unburned gas in the exhaust gas is trapped in the adsorbent 24. In this case, since the exhaust temperature T is not higher than the predetermined value T ₀ , the catalyst 22 has not reached the catalyst activation temperature and is in an inactive state, but the unburned gas in the exhaust is trapped in the adsorbent 24. Therefore, the unburned gas is not released to the outside. Further, in this manner, when the unburned gas in the exhaust gas is guided to the adsorbent 24 when the catalyst 22 is inactive and trapped in the adsorbent 24, the adsorbent 24 also does not reach the desorption temperature of the unburned gas. The unburned gas is not desorbed from the adsorbent 24 as well.

After the step is executed, the process returns to step 23 and the exhaust temperature T is judged. On the other hand, if it is determined in step 24 that the exhaust temperature T is equal to or higher than the predetermined value T ₀ , the process proceeds to step 26. In step 26, the flow path switching valve 18 is switched to a position where the exhaust gas flows only through the first branch passage portion 15. In this state, exhaust is the first
Will flow to the catalyst 22 through the branch passage portion 15 of
The unburned gas in the exhaust gas is not trapped in the adsorbent 24.

However, in this case, the exhaust temperature T is the predetermined value T _0.
As described above, the catalyst 22 has reached the catalyst activation temperature,
It is in an active state. Therefore, the unburned gas in the exhaust gas is also reduced by the catalyst and purified by the catalyst 22. still,
The steps described above are the flow relating to the adsorption of the unburned gas by the adsorbent 24.

Further, in step 31, it is determined by a separately executed routine whether or not the engine 11 is operating in an operating state in which it burns leaner than the stoichiometric air-fuel ratio. Here, a routine for determining the operating state of the engine 11 will be described with reference to FIGS. 3 and 4. Figure 3
Indicates a routine for setting the fuel injection amount, which is executed, for example, every 10 msec.

In step 41, the engine speed N obtained by calculating the cycle of the unit angle signal is input from the crank angle sensor 33. In step 42, the intake air flow rate Q signal from the air flow meter 43 provided in the intake passage 41 of the engine 11 is input. In step 43, the basic fuel injection amount Tp equivalent to the theoretical air-fuel ratio is calculated by the following equation based on the engine speed N and the intake air flow rate Q signal.

Tp = KQ / N (where K is a constant) FIG. 4 shows an air-fuel ratio setting routine, which is executed when it is judged that the engine 11 is started by the ignition signal, for example. In step 51, the exhaust temperature T detected by the main catalyst outlet temperature sensor 32, the engine speed N and the basic fuel injection amount Tp are input.

In step 52, whether the exhaust gas temperature T is within a predetermined range ( _TL <T < _TH ) and whether the engine speed N is within a predetermined range (N _L <N <N). _H ) and whether the basic fuel injection amount Tp is within a predetermined range (Tp _L <Tp
<Tp _H ). When it is determined that the exhaust temperature T, the engine speed N and the basic fuel injection amount Tp are within the predetermined ranges, it is possible to operate in a leaner state than the stoichiometric air-fuel ratio, that is, lean burn control. It is determined that the operating condition for performing is satisfied, and the routine proceeds to step 53, where the target air-fuel ratio is set based on the lean burn control map.

On the other hand, when it is judged that the exhaust temperature T, the engine speed N and the basic fuel injection amount Tp are not within the predetermined ranges, it is judged that the stoichiometric air-fuel ratio should be operated, and the routine proceeds to step 54. , A normal stoichiometric air-fuel ratio is set as the target air-fuel ratio in order to perform stoichiometric operation. Then, in step 55, various correction coefficients C according to the cooling water temperature of the engine 11 and the like.
Air-fuel ratio feedback control is performed to set the final fuel injection amount from the fuel injection valve 45 based on the OEF, the correction amount of the correction amount Ts corresponding to the battery voltage, and the air-fuel ratio feedback correction coefficient LAMBDA. .

Now, let us return to the description of the flowchart of FIG. 2 again. When it is determined in step 31 that the lean burn control is being performed, the process proceeds to step 32. Step
At 32, by judging the engine speed N and the basic fuel injection amount Tp, the unburned gas adsorbed by the adsorbent 24 is discharged by flowing the exhaust gas through the adsorbent 24 based on the map as shown in FIG. It is determined whether or not it can be removed. Here, the basic fuel injection amount Tp is a parameter representing the load of the engine 11, and in FIG. 5, OK is an operating region in which control is performed so as to desorb the unburned gas adsorbed by the adsorbent 24, NG
Is an operating region where control is performed so as not to desorb the unburned gas adsorbed by the adsorbent 24.

That is, when the lean burn control is being performed,
Since the HC emission amount is smaller than the NO _X emission amount and the HC concentration is low, the NO _x purification performance of the main catalyst 22 may decrease if the lean burn control is performed for a long time. Therefore, it becomes necessary to desorb the unburned gas adsorbed on the adsorbent 24 and reduce the NO _x by the unburned gas. Furthermore, during lean burn control, when the operating condition is high load and high rotation,
Since the amount of exhaust gas discharged from 11 also increases, there is a risk that the NO _x purification performance of the main catalyst 22 will further deteriorate, so it is necessary to desorb the unburned gas adsorbed by the adsorbent 24.

When it is determined in step 32 that the operation region is such that control is performed so as to be detached, the process proceeds to step 33.
That is, the air-fuel ratio sensor 31, the main catalyst outlet temperature sensor 32,
The crank angle sensor 33, the air flow meter 43, and the like constitute an operating state detecting means. In step 33, the actuator 19 switches the flow passage switching valve 18 as the flow passage switching means so that the exhaust gas flows through both the first branch passage portion 15 and the second branch passage portion 16.

In step 34, the elapsed time from when the adsorbent 24 begins to desorb the adsorbed unburned gas is
It is determined whether 24 has passed the time for completely desorbing unburned gas. The time when the adsorbent 24 completely desorbs the unburned gas can be obtained in advance by experiments or the like. When it is determined that the adsorbent 24 has completely desorbed the adsorbed unburned gas, the process proceeds to step 35, and the flow path is set by the actuator 19 so that the exhaust gas flows only to the first branch passage portion 15. The switching valve 18 is switched. At this time, the exhaust gas is purified only by the main catalyst 22.

On the other hand, if it is determined that the unburned gas adsorbed by the adsorbent 24 has not been completely desorbed, a step is performed.
Return to 31 and judge the operating conditions again. Also, step 31
In, if it is determined that the lean burn control is not being performed, or if it is determined that the lean burn control is being performed in step 31 but is not in the operation region in which the control is performed so as to be detached in step 32, , Go to step 36.

In step 36, it is judged whether or not the flow path switching valve 18 is switched by the actuator 19 so as to desorb the unburned gas which the adsorbent 24 has currently adsorbed. Then, when the adsorbent 24 is not being desorbed, it is assumed that the flow path switching valve 18 has been switched so that the exhaust gas flows only to the first branch passage portion 15, the process returns to step 31, and the operating condition is again set. To judge.

In step 36, when it is determined that the adsorbent 24 is being desorbed, the operating conditions have changed, but the flow path switching valve 18 has the first branch passage portion 15 and the second branch passage portion 15. Assuming that the exhaust gas is switched to flow in both of the branch passage portions 16, the flow path switching valve 18 is switched in step 37 so that the exhaust gas flows only in the first branch passage portion 15, and then the process returns to step 31. , Determine the operating conditions again.

Therefore, as described above, according to the first embodiment, the main catalyst outlet temperature sensor 32, the crank angle sensor 33, the air flow meter 43 and the like detect that the operating condition is the lean burn control condition. Further, when it is detected that the operating state is under high load and high rotation, the HC trapped in the adsorbent 24 is desorbed, and the exhaust passage
Since it is supplied to the exhaust purification catalyst 22 arranged in 13,
The supplied HC acts on NO _X as a reducing agent, and the purification performance of the catalyst 22 is improved regardless of operating conditions.

Next, a second embodiment according to claim 2 of the present invention will be described. Regarding the system configuration, FIG.
The description is omitted because it is the same as the first embodiment shown in FIG. In the second embodiment, the first branch passage portion 15
And the catalyst for exhaust gas purification disposed in the exhaust passage 13 on the downstream side of the merging portion 13b of the second branch passage 16, even in the operating conditions of combustion than the stoichiometric air-fuel ratio in the lean side, emissions of the NO _X There (hereinafter referred to as the lean NO _X catalyst) catalyst tends to decrease 28 is used.

[0041] That is, as shown in FIG. 6, the lean NO _X catalyst 28 is compared with the first main catalyst (three-way catalyst) for purifying exhaust gas used in Example 22, the air-fuel ratio becomes lean However, the NO _x emission amount does not increase, and the NO _x conversion efficiency is good. Therefore, as in the first embodiment, the main catalyst outlet temperature sensor 32, the crank angle sensor 33, the air flow meter 43, etc., detect that the operating condition is the lean burn control condition, and the operating condition is high load. When it is detected that the engine speed is high, the HC trapped in the adsorbent 24 is desorbed and supplied to the lean NO _x catalyst 28 for exhaust gas purification arranged in the exhaust passage 13. The HC acts on NO _X as a reducing agent to improve the purification performance of the lean NO _X catalyst 28 irrespective of operating conditions, and the lean NO _X catalyst 28 itself is in a lean burn control state. However, since it has a good NO _X conversion efficiency, stable NO _X purification performance can be obtained under all operating conditions.

[0042]

As described above, according to the present invention, the exhaust passage of the engine is configured to include the first branch passage and the second branch passage in which the unburned gas adsorbent is disposed, and the theoretical empty space is provided. When the exhaust flow rate increases due to high load and high speed operation in an operating state in which combustion is on the lean side of the fuel ratio, and further in an operating state in which the combustion is on the lean side, the exhaust is in both the first and second branch passage portions. Since the catalyst for purifying exhaust gas disposed in the exhaust passages on the downstream side of the first and second branch passage portions can obtain stable NO _x purification performance under all operating conditions. There is an effect that.

[Brief description of drawings]

FIG. 1 is a system configuration diagram showing an overall configuration of an engine exhaust emission control device according to an embodiment of the present invention.

FIG. 2 is a flowchart for explaining the operation according to the embodiment.

FIG. 3 is a flowchart for explaining the operation according to the embodiment.

FIG. 4 is a flowchart for explaining the operation according to the embodiment.

FIG. 5 is a map for determining desorption of the adsorbent 24 according to the above embodiment.

FIG. 6 is a characteristic diagram showing conversion efficiency of the lean NO _X catalyst according to the second embodiment of the present invention.

[Explanation of symbols]

 11 Engine 13 Exhaust passage 15 First branch passage 16 Second branch passage 18 Flow path switching valve 22 Main catalyst 24 Adsorbent 30 Control unit 31 Air-fuel ratio sensor 32 Main catalyst outlet temperature sensor

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location F01N 3/18 F F02D 45/00 301 G 7536-3G

Claims (3)

[Claims] 1. An exhaust passage of an engine is configured to include first and second branch passages that branch into two branches and then join again.
An unburned gas adsorbent for adsorbing unburned gas in the exhaust gas is disposed in the second branch passage portion, and exhaust gas is purified in the exhaust passage downstream of the confluence of the first and second branch passage portions. And a flow path switching means for switching the flow path so that exhaust gas selectively flows into only the first branch passage portion and both the first and second branch passage portions, and An exhaust gas purifying device for an engine provided with an operating state detecting means for detecting an operating state, wherein the operating state detected by the operating state detecting means is an operating state in which combustion is leaner than the stoichiometric air-fuel ratio, An exhaust gas purifying apparatus for an engine, wherein the path switching means switches a flow path so that the exhaust gas flows through both the first and second branch passage portions. 2. An exhaust gas purifying catalyst disposed in an exhaust passage downstream of a merging portion of the first and second branch passage portions is N even in an operating state in which the catalyst burns leaner than a theoretical air-fuel ratio.
Engine exhaust purification device according to claim 1, wherein the discharge amount of O _X is the catalyst which tends to decrease. 3. An operating state in which the operating state detected by the operating state detecting means burns leaner than the stoichiometric air-fuel ratio,
3. The exhaust gas purification system for an engine according to claim 1, wherein the flow passage switching means switches the flow passage so that the exhaust gas flows through both the first and second branch passage portions when the flow rate of the exhaust gas is large. apparatus.