JPH0693844A - Exhaust emission control device for engine - Google Patents

Abstract

PURPOSE:To prevent abrupt desorption of unburnt gas so as to obtain stable exhaust emission purifying performance by making a specified amount of exhaust flow into an unburnt gas adsorbent at the time of lean operation and increasing the amount of exhaust which flows into the unburnt gas adsorbent gradually at the time of change from rich to lean operation. CONSTITUTION:An exhaust emission purifying catalyst 22 is provided in an exhaust passage 13 of an engine 11. Also, an unburnt gas adsorbent 24 is provided in one of branch passage section 16 in the exhaust passage 13. Further, a flow passage change-over valve 18 is provided at a branch point 13a of each branch passage section 15, 16 and a flow rate control valve 51 is provided in one of the branch passage section 16. Each valve 18, 51 is controlled by a control unit 30 based on the detected signals from an air-fuel ratio sensor 36. In this case, the specified amount of exhaust flows into the unburnt gas adsorbent 24 to start desorption in the condition of lean operation. After this, exhaust which flows into the unburnt gas adsorbent 24 is increased gradually at the time when operation is changed from rich operation to lean operation and lean operation is continued.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention efficiently purifies unburned gas components in exhaust gas by switching exhaust passages for adsorbing and not adsorbing unburned gas according to the air-fuel ratio of the engine. The present invention relates to an exhaust emission control device for an engine.

[0002]

2. Description of the Related Art In order to purify engine exhaust gas, a catalyst is often arranged in the exhaust passage of the engine, but the operating temperature or activation temperature of this catalyst is relatively high, for example, 300 ° C. or higher. It is high, and exhaust gas cannot be sufficiently purified below this activation temperature. For this reason, when the engine temperature is low when the exhaust gas temperature, that is, the temperature of the catalyst is low, an adsorbent for adsorbing unburned gas that works at a low temperature is provided in the exhaust passage upstream of the catalyst in order to sufficiently purify the exhaust gas, and the exhaust gas from the engine is exhausted. There has been proposed one in which the flow is switched according to the exhaust gas temperature (see Japanese Patent Laid-Open No. 63-68713, etc.).

The above adsorbent can adsorb unburned gas at low temperatures, but its adsorbing ability gradually decreases as the exhaust temperature rises, and the adsorbed gas component begins to be released from around 100 ° C., for example. is doing. In the purification device in which the unburned gas adsorbent and the exhaust purification catalyst are arranged in the exhaust passage as described above, the unburned gas component in the exhaust is efficiently adsorbed by the adsorbent immediately after the cold start. It will be. However, the temperature from the start of desorption from the adsorbent (eg 100 ℃) to the temperature at which the catalyst starts the reaction (eg 300 ℃)
In the temperature range up to, even though a large amount of unburned gas is desorbed from the adsorbent, most of the desorbed unburned gas is released to the atmosphere without being completely purified by the catalyst. There is a fear.

Therefore, after trapping HC at a low temperature and before the catalyst is sufficiently warmed up, the introduction of exhaust gas to the adsorbent is stopped so that desorption does not occur as described above. There is a device for switching the exhaust flow path as shown in FIG.

[0005]

By the way, a catalyst for purification of exhaust gas exhibits high conversion efficiency when the exhaust gas passing through the catalyst is in the vicinity of the stoichiometric air-fuel ratio. When the HC of the unburned gas that has been retained is rapidly desorbed from the adsorbent in a large amount, the air-fuel ratio of the exhaust gas on the downstream side of the adsorbent becomes excessively rich, and the purification capacity of the catalyst for exhaust gas purification is increased. There is a fear that it will fall.

The present invention has been made in view of such a conventional problem, and an exhaust gas purification catalyst is provided in an exhaust passage downstream of an exhaust passage in which an adsorbent for adsorbing unburned gas is arranged. It is an object of the present invention to provide an exhaust emission control system for an engine in which a stable exhaust emission control performance is obtained under all operating conditions by preventing a rapid desorption of unburned gas in the engine exhaust emission control device. And

[0007]

Therefore, the exhaust emission control system for an engine according to the present invention is, as a first technical means,
An unburned gas adsorbent for adsorbing unburned gas in the exhaust gas is configured to include first and second branch passages that branch again into the exhaust passage of the engine and then join again. An exhaust gas purification catalyst in the exhaust passage downstream of the confluence of the first and second branch passage portions, and an operating state detecting means for detecting an operating state of the engine; An air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust gas on the downstream side of the fuel gas adsorbent, and an exhaust flow rate control means for controlling the exhaust gas flow rate passing through the unburned gas adsorbent are provided, and the operating state detecting means is further provided. When the operating state detected by the above is an operating state in which the combustion is on the lean side of the stoichiometric air-fuel ratio, the exhaust flow rate control means causes a predetermined amount of exhaust gas to flow to the unburned gas adsorbent to remove it from the unburned gas adsorbent. Air release after starting desorption. A configuration in which the exhaust flow rate control means gradually increases the exhaust flow rate flowing to the unburned gas adsorbent whenever a change from rich to lean is detected by the detection means or lean is detected for a predetermined time or longer. And

As a second technical means, the exhaust flow rate control means is provided in the branch portion so that the flow path through which the exhaust gas flows is only the first branch passage portion or only the second branch passage portion. Alternatively, a flow path switching valve that selectively switches between both the first and second branch passage portions, and an exhaust flow rate that is provided on the downstream side of the unburned gas adsorbent and that passes through the second branch passage portion. And a flow rate control valve capable of continuously increasing and decreasing, and the exhaust flow rate control means causes the predetermined amount of exhaust gas to flow to the unburned gas adsorbent to remove it from the unburned gas adsorbent. When starting, the flow control valve may be closed for a predetermined time after the flow passage switching valve switches the exhaust gas to flow through the second branch passage.

As a third technical means, the exhaust passage of the engine is constructed to include first and second branch passages that branch into two and then join again, and exhaust into the second branch passage portion. An unburned gas adsorbent for adsorbing unburned gas therein is disposed, and an exhaust gas purification catalyst is disposed in the exhaust passage downstream of the confluence of the first and second branch passage portions. An exhaust temperature detecting means for detecting an inlet exhaust temperature and an outlet exhaust temperature of the unburned gas adsorbent, and an exhaust flow rate control means for controlling an exhaust flow rate passing through the unburned gas adsorbent are provided. The exhaust flow rate control means controls the exhaust flow rate so that the difference between the inlet exhaust temperature and the outlet exhaust temperature of the unburned gas adsorbent detected by the above is less than a predetermined value.

As a fourth technical means, the exhaust passage of the engine is constructed to include first and second branch passages that branch into two and then join again, and exhaust into the second branch passage portion. An unburned gas adsorbent for adsorbing unburned gas therein is disposed, and an exhaust gas purification catalyst is disposed in the exhaust passage downstream of the confluence of the first and second branch passage portions. An inlet exhaust gas temperature detecting means for detecting the inlet exhaust gas temperature of the unburned gas adsorbent, and an exhaust flow rate control means for controlling the exhaust gas flow rate passing through the unburned gas adsorbent are provided and detected by the inlet exhaust gas temperature detecting means. When the inlet exhaust gas temperature of the unburned gas adsorbent is within a predetermined range, prohibiting means for prohibiting the control of the exhaust flow rate by the exhaust flow rate control means is provided for a predetermined time set based on the inlet exhaust gas temperature. It was configured.

[0011]

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, so that the HC is trapped in the adsorbent when the temperature is low. Then, when the temperature of the catalyst rises and is activated, the HC trapped in the adsorbent is desorbed from the adsorbent.

Here, since the catalyst for purifying the exhaust gas exhibits a high conversion efficiency when the exhaust gas passing through the catalyst is in the vicinity of the stoichiometric air-fuel ratio, it becomes necessary to purify the exhaust gas when the air-fuel ratio becomes rich. The conversion efficiency of the catalyst is reduced.
Therefore, in the first technical means according to the present invention, when 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, the exhaust flow rate control means controls Desorption from the unburned gas adsorbent is started by flowing a fixed amount of exhaust gas to the unburned gas adsorbent.
Here, when the HC trapped from the adsorbent is desorbed,
Since the HC acts as a reducing material, the reducing component in the exhaust gas increases, and the air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust gas downstream of the unburned gas adsorbent material makes the air-fuel ratio rich. Is detected.

Therefore, the HC desorbed according to the predetermined amount
As a result, the air-fuel ratio changes from lean to rich. Here, only a predetermined amount of exhaust gas flows into the unburned gas adsorbent, and the temperature of the adsorbent is a balance between the heat received from the predetermined amount of exhaust gas and the heat released from the adsorbent to the outside. Set to. Then, when only the HC that can be released at this balanced temperature is completely desorbed, the supply amount of the desorbed HC is reduced again.

Here, the exhaust flow rate control means causes the exhaust gas to flow into the unburned gas adsorbent by a predetermined amount only in an operating state in which combustion is performed leaner than the stoichiometric air-fuel ratio.
When only the HC that can be released at the lance temperature is desorbed, the air-fuel ratio changes from rich to lean. Then, the exhaust flow rate control means causes the exhaust gas gradually increased from the predetermined amount to flow to the unburned gas adsorbent,
Therefore, desorption of the adsorbent is gradually performed. That is, since the adsorbent is not desorbed rapidly, the purification performance of the exhaust purification catalyst disposed in the exhaust passage on the downstream side of the unburned gas adsorbent is not reduced, and Stable exhaust purification performance can be obtained under operating conditions.

On the other hand, in the operating state in which the combustion is on the lean side of the stoichiometric air-fuel ratio, the exhaust flow rate control means causes the predetermined amount of exhaust gas to flow to the unburned gas adsorbent to remove it from the unburned gas adsorbent. Even if the separation is started, if the predetermined amount is small, HC that can be released at the balanced temperature
Since only the desorption is performed, the air-fuel ratio does not become rich even when the desorption ends, and the lean state continues for a predetermined time or longer.

At this time, since the lean state continues, the oxygen concentration in the exhaust gas is high, and the HC is desorbed again and the HC is removed.
Even if the oxygen concentration in the exhaust gas is reduced by the reducing action of the above, it can be determined that the exhaust gas purification catalyst still has a sufficient ability to purify the exhaust gas. Therefore, also in this case, the exhaust flow rate control means causes the amount of exhaust gas gradually increased from the predetermined amount to flow to the unburned gas adsorbent, and thus desorption of the adsorbent is gradually performed. . That is,
Since the adsorbent is not desorbed rapidly, it does not reduce the purification performance of the exhaust purification catalyst disposed in the exhaust passage on the downstream side of the unburned gas adsorbent, and all operating conditions are maintained. Thus, stable exhaust gas purification performance can be obtained.

Further, as a second technical means according to the present invention, the exhaust flow rate control means includes a flow path switching valve for switching a flow path through which the exhaust gas flows, and a flow rate for increasing / decreasing the flow rate passing through the second branch passage portion. A control valve is included, and when starting desorption from the unburned gas adsorbent, the flow control valve is closed for a predetermined time after the exhaust gas is switched to flow through the second branch passage. In such a case, the exhaust gas that has entered the second branch passage due to the switching of the flow path switching valve is blocked by the flow control valve for a predetermined time. At this time, the unburned gas adsorbent disposed in the second branch passage is exposed to the exhaust gas, and the temperature of the adsorbent rises.

Therefore, the unburned HC is easily desorbed, and the desorption operation is performed in accordance with the flow rate control by the flow rate control valve. Therefore, the desorption control by the first technical means can be facilitated. Become. Further, as a third technical means according to the present invention, an exhaust temperature detecting means for detecting an inlet exhaust temperature and an outlet exhaust temperature of the unburned gas adsorbent, and an exhaust flow rate passing through the unburned gas adsorbent are provided. In the exhaust gas purification device of the engine including the exhaust flow rate control means for controlling, the difference between the inlet exhaust temperature and the outlet exhaust temperature of the unburned gas adsorbent detected by the exhaust temperature detecting means. The exhaust flow rate control means controls the exhaust flow rate so that is equal to or less than a predetermined value.

That is, as shown in FIG. 7, a large amount of HC is discharged by vigorous desorption, whereas the inlet exhaust temperature of the unburned gas adsorbent is high, but the outlet exhaust temperature is still high. It's when it's barely rising. That is, it can be seen that when the introduction of the exhaust gas into the adsorbent is rapid and the temperature distribution inside the adsorbent is wide, desorption is intense. Therefore, in order to gradually perform desorption, the exhaust gas should be gradually introduced so that the temperature distribution inside the adsorbent becomes small. Here, since the temperature distribution inside the adsorbent exists between the inlet exhaust temperature and the outlet exhaust temperature of the adsorbent,
Therefore, if the temperature difference between the inlet and outlet of the adsorbent is reduced, the temperature distribution inside the adsorbent is reduced.

Accordingly, the adsorbent is desorbed by controlling the exhaust flow rate so that the difference between the inlet exhaust temperature and the outlet exhaust temperature of the unburned gas adsorbent detected by the exhaust temperature detecting means is below a predetermined value. The separation is performed gradually, and the purification performance of the exhaust purification catalyst disposed in the exhaust passage on the downstream side of the unburned gas adsorbent is not reduced, and the exhaust purification performance is stable under all operating conditions. Will be obtained.

Further, as a fourth technical means according to the present invention, when the inlet exhaust gas temperature detecting means for detecting the inlet exhaust gas temperature of the unburned gas adsorbent is included, it is detected by the inlet exhaust gas temperature detecting means. When the inlet exhaust gas temperature of the unburned gas adsorbent is within a predetermined range, the control means controls the exhaust flow rate by the exhaust flow rate control means for a predetermined time. Here, the predetermined time is set based on the inlet exhaust gas temperature of the unburned gas adsorbent.

That is, as explained in the operation of the third technical means, if the introduction of the exhaust gas into the adsorbent is rapid and the temperature distribution inside the adsorbent is wide, the desorption is performed violently. Therefore, in order to gradually perform desorption, exhaust gas should be gradually introduced so that the temperature distribution inside the adsorbent becomes small. That is, if the temperature difference between the inlet and outlet of the adsorbent is reduced, the temperature distribution inside the adsorbent can be reduced, and desorption can be gradually performed.

Since the rise and fall of the outlet exhaust gas temperature of the unburned gas adsorbent has a delay with respect to the rise and fall of the inlet exhaust gas temperature, when the inlet exhaust gas temperature is within a predetermined range, By prohibiting the control of the exhaust flow rate for a predetermined time set based on the inlet exhaust gas temperature of the unburned gas adsorbent, the inlet / outlet temperature of the adsorbent is adjusted by the balance between the amount of introduced exhaust gas and the heat capacity of the unburned gas adsorbent during this period. The difference does not increase.

Therefore, also by the fourth technical means, the adsorbent is gradually desorbed, and the catalyst for purifying the exhaust gas disposed in the exhaust passage downstream of the unburned gas adsorbent is purified. Stable exhaust gas purification performance can be obtained under all operating conditions without lowering the performance.

[0025]

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 2 of the present invention, an exhaust passage 13 connected to an exhaust manifold 12 of an engine 11 has an exhaust passage pre-catalyst 21 and a main catalyst 22. Is provided. Then, after bifurcating 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 the main catalyst 22 (the merge point is 13b).
Is provided with a branch passage portion 15 and a second branch passage portion 16 of the unburned gas adsorbent (hereinafter,
24 is simply provided as an adsorbent.

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 alone or the second branch passage portion 16 is exhausted. only,
Alternatively, a flow passage switching valve 18 as an exhaust flow rate control means for switching the flow passage to selectively flow through both the first branch passage portion 15 and the second branch passage portion 16 is provided. still,
The flow path switching valve 18 is operated by an actuator 19 controlled by a control unit 30 having a microcomputer, which will be described later.

Further, in the second branch passage portion 16, a flow rate control valve 51 such as a butterfly valve as exhaust flow rate control means capable of continuously increasing or decreasing the exhaust flow rate passing through the second branch passage portion 16. Is installed. The flow control valve
51 is also operated by an actuator 52 controlled by a control unit 30 incorporating a microcomputer described later.

The adsorbent 24 is a monolithic ceramic carrier coated with zeolite. The adsorbent 24 is not limited to the monolith shape, and may be, for example, a foam shape, a pellet shape, a mesh shape, and the adsorbent itself is not limited to the zeolite, and is a substance having an adsorbing performance such as activated carbon. I wish I had it. Here, the main catalyst 22 for purifying exhaust gas is an ordinary three-way catalyst that has been conventionally used. For example, a ceramic carrier carrying a precious metal such as platinum, palladium or rhodium is used to chemically oxidize the exhaust gas. To do.

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.
In addition, a throttle valve is installed in the intake passage 41 of the engine 11.
An air flow meter 43 for detecting an intake air flow rate controlled by 42 is provided, and an intake manifold 44 is provided with an electromagnetic fuel injection valve 45 for each cylinder. The valve is driven to open by an injection pulse signal, and the fuel regulated to a predetermined pressure by a pressure regulator pressure-fed from a fuel pump (not shown) is injected and supplied into the intake manifold 44.

Further, in the exhaust passage 13 between the confluence 13b and the main catalyst 22, the oxygen concentration of the exhaust gas flowing in the exhaust passage 13 is the exhaust gas when the air-fuel ratio of the intake air-fuel mixture is the theoretical air-fuel ratio. An oxygen sensor 36 is provided as an air-fuel ratio detecting means for detecting ON or OFF as to whether it is on the rich side or the lean side, compared with the oxygen concentration. 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. A water temperature sensor 35 for detecting the engine cooling water temperature Tw is provided in the engine 11. Then, the control unit 30 calculates 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, thereby producing an air-fuel ratio. Is controlled and the actuators 19 and 52 are controlled to purify the exhaust gas.

A control unit 30 according to a first embodiment of the present invention relating to the exhaust emission control device having the above-described structure will be described below.
The control performed by will be described with reference to the flowchart shown in FIG. 2 and the time chart shown in FIG. In step 21 (denoted as S21 in the figure, the same applies hereinafter),
Input the engine on / off signal from the ignition switch 34.

In step 22, it is judged 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 thereafter. If it is determined in step 22 that the ignition signal has not been input, it is determined that the engine 11 is not rotating, and the process directly returns.

In step 23, the engine cooling water temperature Tw detected by the water temperature sensor 35 is input. Step
At 24, it is determined whether the cooling water temperature Tw is equal to or higher than a predetermined temperature Tw ₀ . The cooling water temperature Tw is the predetermined temperature Tw.
If it is determined that the value is not ₀ or more, the process proceeds to step 25.

If it is determined in step 24 that the cooling water temperature Tw is equal to or higher than the predetermined temperature Tw ₀ , the temperature is high even if the unburned gas in the exhaust gas is trapped in the adsorbent 24. The unburned gas desorbed from the adsorbent 24 will increase. Therefore, in this case, the process proceeds to step 27. Step
In 25, the actuator 19 is operated and 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 step 26, the actuator 52 is operated, the flow rate control valve 51 is fully opened, and the exhaust gas introduced into the second branch passage portion 16 by the flow passage switching valve 18 is the second branch passage portion. Let all 16 flow out,
The unburned gas in the exhaust is trapped in the adsorbent 24. Then, after the step is executed, the process returns to step 24 and the cooling water temperature Tw is determined.

In step 27, the exhaust gas temperature T detected by the main catalyst outlet temperature sensor 32 is input. Step
At 28, it is determined whether the exhaust temperature T is equal to or higher than a predetermined value T ₀ . When it is determined that the exhaust temperature T is not equal to or higher than the predetermined value T ₀ , the process proceeds to step 29. In step 29,
Since the exhaust gas temperature T is not higher than the predetermined value T ₀ , the catalyst 22 has not reached the catalyst activation temperature and is in the inactive state. On the other hand, since the adsorbent 24 has a high cooling water temperature Tw, the adsorbent 24
It is conceivable that the amount of unburned gas desorbed from the fuel cell is also increasing. Therefore, the actuator 19 is operated, and 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 step 30, the actuator 52
Is operated to fully close the flow control valve 51. That is, at steps 29 and 30, the flow control valve 51 is fully closed to prevent the adsorbent 24 from reaching the desorption temperature of the unburned gas. Also, the exhaust is the first
In this case, the unburned gas contained in the exhaust gas is discharged to the atmosphere as it is, but the amount of unburned gas contained in the exhaust gas is small in this temperature range.

After the step is executed, the process returns to step 28 and the exhaust temperature T is judged. On the other hand, if it is determined in step 28 that the exhaust temperature T is equal to or higher than the predetermined value T ₀ , the process proceeds to step 31. In step 31, the actuator 19 is operated to switch the flow path switching valve 18 to the intermediate position so that the exhaust gas flows to the first branch passage portion 15 and the second branch passage portion 16.

In this state, since the flow rate control valve 51 is initially held in the fully closed state in step 30, the exhaust gas passes through the first branch passage portion 15 and the catalyst 22. If the exhaust temperature T is equal to or higher than a predetermined value T ₀ ,
22 has reached the catalyst activation temperature and is in an active state. Therefore, the unburned gas in the exhaust gas is also oxidized by the catalyst and purified by the catalyst 22.

Further, in step 30, the flow control valve
Since 51 is held in the closed state, the flow path switching valve 18 is switched to the intermediate position, so that the second branch passage portion is formed.
The exhaust gas flowing into 16 is blocked by the flow control valve 51. Here, the adsorbent 24 is exposed to high temperature exhaust gas, and the temperature of the adsorbent 24 rises.

Then, in step 32, the step
At 31, the elapsed time t after the passage switching valve 18 is switched to the intermediate position is counted. In step 33, it is determined whether or not the elapsed time t has passed a predetermined time t ₀ or more, and if it is determined that it has passed, the process proceeds to step 61 and subsequent steps shown below. That is, prior to the desorbing operation of the adsorbent 24, the temperature of the adsorbent 24 is raised in advance by exposing the adsorbent 24 to high temperature exhaust gas to facilitate desorption of unburned HC. As a result, the time lag during the desorption operation can be reduced, and the desorption control can be facilitated, so that the unburned gas can be purified more reliably.

The steps described above are the flow relating to the adsorption of the unburned gas by the adsorbent 24. Further, in step 61, it is determined by a separately executed routine whether or not the engine 11 is operating in an operating state in which the engine 11 burns leaner than the stoichiometric air-fuel ratio. Here, FIG. 3 and FIG.
A routine for determining the operating state of the engine 11 will be described with reference to FIG.

FIG. 3 shows 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. Step
At 42, an intake air flow rate Q signal from an 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 corresponding 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 determined that the engine 11 is started by the ignition signal, for example.

In step 51, the exhaust gas temperature T detected by the main catalyst outlet temperature sensor 32, the engine speed N and the basic fuel injection amount Tp are input. At step 52, whether the exhaust temperature T is within a predetermined range ( _TL <T <
T _H ), 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 ) To judge.

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, When it is determined that the operating condition for performing lean burn control is satisfied, 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 determined that the exhaust temperature T, the engine speed N and the basic fuel injection amount Tp are not within the predetermined ranges, it is determined that the stoichiometric air-fuel ratio should be operated, and the routine proceeds to step 54, where the stoichiometric operation is performed. In order to perform the above, a normal stoichiometric air-fuel ratio is set as the target air-fuel ratio.

Then, in step 55, based on the various correction coefficients COEF according to the cooling water temperature of the engine 11, the correction amount of the correction amount Ts according to the battery voltage, and the air-fuel ratio feedback correction coefficient LAMBDA, Fuel injection valve
Air-fuel ratio feedback control is performed to set the fuel injection amount from 45. Here, the description returns to the flowchart of FIG. 2 again.

In step 61, it is judged whether or not the operating state is an operating state in which combustion is on the lean side of the stoichiometric air-fuel ratio. If it is determined that the lean operation is being performed, the routine proceeds to step 62. In step 62, by determining the engine speed N and the basic fuel injection amount Tp, the exhaust gas is caused to flow through the adsorbent 24 based on the map as shown in FIG.
It is determined whether or not the unburned gas adsorbed on 24 can be desorbed. 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 operation region in which control is performed so as not to desorb the unburned gas adsorbed on the adsorbent 24.

That is, when the lean operation is being performed, NO _X
The amount of HC emission is smaller than that of
Since the C concentration becomes low, the NO _x purification performance of the main catalyst 22 may be deteriorated, so that the unburned gas adsorbed by the adsorbent 24 is desorbed and the NO _{x is} removed by the unburned gas.
Will need to be reduced. Further, when the operating condition is high load and high rotation speed, the exhaust flow rate discharged from the engine 11 also increases, so that the NO in the main catalyst 22 is further increased.
_{Since the X} purification performance may be deteriorated, the exhaust gas can be efficiently purified by desorbing the unburned gas adsorbed by the adsorbent 24.

If it is determined in step 62 that the operation region is such that control is performed so as to be detached, step 65 to be described later is performed.
Continue below. Further, when it is determined in step 62 whether or not it is in the operation region in which control is performed so as to be detached, the elapsed time after engine start, engine water temperature, etc. may be taken into consideration. Further, when it is determined in step 61 that the lean operation is not being performed, or even when it is determined that the lean operation is being performed in step 61, it is determined in step 62 that it is not in the operation range in which the control is performed so as to be detached. If so, go to step 63.

In step 63, 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. If the adsorbent 24 is not being desorbed, the first
Assuming that the flow path switching valve 18 is switched so that the exhaust gas flows only in the branch passage portion 15, the process returns to step 61 and the operating condition is judged again.

If it is determined in step 63 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. In step 64, it is assumed that the exhaust gas is switched so that the exhaust gas flows to both of the branch passage portions 16, and the first branch passage portion
After switching the flow path switching valve 18 so that the exhaust gas flows only to 15, the process returns to step 61 and the operating condition is judged again.

That is, the crank angle sensor 33, the air flow meter 43 and the like constitute an operating condition detecting means.
In step 65, the flow control valve 51 that is closed is
Open only alpha ° as the first predetermined angle, the exhaust of a predetermined amount q ₁ is to flow into the second branch passage 16, exhaust predetermined amount q ₁ is to pass through the adsorbent 24. here,
When the unburned HC trapped in the adsorbent 24 is desorbed,
Since the HC acts as a reducing material, the reducing component in the exhaust increases and becomes rich.

Then, in step 66, the confluence 13b
Oxygen sensor 36 provided in the exhaust passage 13 between the main catalyst 22 and the main catalyst 22, the oxygen concentration of the exhaust gas flowing in the exhaust passage 13, the oxygen concentration of the exhaust gas when the air-fuel ratio of the intake mixture is the stoichiometric air-fuel ratio Compared to, it detects whether it is on the rich side or the lean side. Here, when it is detected that it is on the richer side, the unburned HC trapped in the adsorbent 24 is desorbed by the predetermined amount q ₁ and the oxygen concentration in the exhaust gas is reduced by the reducing action of the HC. Is detected to be on the richer side, the process proceeds to step 67. That is, the oxygen concentration has changed from lean to rich due to HC desorbed according to the predetermined amount q ₁ corresponding to the case where the flow control valve 51 is opened by α °.

In step 67, the oxygen sensor 36 again measures the oxygen concentration of the exhaust gas flowing in the exhaust passage 13 more than the oxygen concentration of the exhaust gas when the air-fuel ratio of the intake mixture is the stoichiometric air-fuel ratio. Detects whether it is on the rich side or the lean side. Here, since the exhaust gas has flowed into the adsorbent 24 by a predetermined amount q ₁ corresponding to the case where the flow rate control valve 51 is opened by α °, the adsorbent 24 has a heat absorption amount from the exhaust gas temperature and the adsorbent 24. The temperature is balanced with the amount of heat exhausted to the outside. Then, from the adsorbent 24, set the flow control valve 51 to α °.
When only the amount of HC commensurate with the predetermined amount q ₁ corresponding to the amount of desorption at the temperature when only opened is completely desorbed, the supply of unburned HC is reduced again (see FIGS. 6c and 6d).

That is, when it is judged in step 67 that the oxygen concentration of the exhaust gas is on the lean side, the supply of unburned HC is reduced, and the oxygen concentration detected by the oxygen sensor 36 changes from rich to lean. Steps that changed
Continue to 69. On the other hand, when it is detected in step 66 that the oxygen concentration of the exhaust gas flowing in the exhaust passage 13 is on the lean side as compared with the oxygen concentration of the exhaust gas when the air-fuel ratio of the intake mixture is the stoichiometric air-fuel ratio. Proceeds to step 68.

In step 68, it is judged whether or not the lean state has continued for a predetermined time or longer. If the lean state has not continued for a predetermined time or longer, in step 66 again,
The oxygen concentration of the exhaust gas flowing through the exhaust passage 13 is determined. Here, the predetermined time in step 68 means the flow control valve
This is the time from when the valve opening / closing control of 51 is performed until the change in oxygen concentration can be read by the oxygen sensor 36. That is, the valve operation of the flow control valve 51, the adsorbent 24
Change in the amount of exhaust gas flowing into the tank, rise in temperature of adsorbent 24, desorption of unburned HC from adsorbent, outflow of HC excess gas and passage of oxygen sensor 36, detection by oxygen sensor 36, rich by control unit 30 It is the time required for the determination, and is basically obtained as a constant in advance by experiments or the like.

If it is determined in step 68 that the lean state has continued for a predetermined time or more, the α ° as the first predetermined angle set in step 65 is small, so that the unburned fuel to be supplied is supplied. Since the amount of HC is small and the amount of reducing material is small, it is judged that the oxygen concentration of the exhaust gas is on the lean side, and the routine proceeds to step 69 (see FIG. 6e). In step 69, it is determined whether or not the valve opening degree of the flow control valve 51 is less than or equal to a predetermined angle (for example, the fully open value).

That is, in the operating state in which combustion is on the lean side of the stoichiometric air-fuel ratio, the oxygen concentration detected by the oxygen sensor 36 once becomes rich and then becomes lean or it is detected for a predetermined time. Whether or not it is detected that the lean state has continued, or in any case, and when the flow control valve 51 still has a margin in the opening direction,
You have advanced to step 70.

That is, in step 70, the oxygen sensor
Since the oxygen concentration detected by 36 is in a lean state, the oxygen concentration in the exhaust gas is high, and HC is desorbed again to reduce the oxygen concentration detected by the oxygen sensor 36 by the reducing action of the HC. However, it can be determined that the exhaust purification catalyst 22 still has a sufficient ability to purify the exhaust.

Therefore, in step 70, the flow control valve is
51 of the valve opening and the first by an incremental beta ° from alpha ° is a predetermined angle larger the second predetermined angle (α + β) °, the predetermined amount q ₁ predetermined amount q ₂ which was increased from (= q ₁ +
Exhaust gas of Δq) is caused to flow to the adsorbent 24, so that desorption of unburned HC from the adsorbent 24 is further performed (see FIGS. 6c and 6e).

That is, since the unburned HC is not rapidly desorbed from the adsorbent 24, the oxygen concentration in the exhaust gas flowing through the exhaust passage 13 on the downstream side of the adsorbent 24 is determined to be the empty space of the intake air-fuel mixture. Compared with the oxygen concentration of the exhaust when the fuel ratio is the theoretical air-fuel ratio,
It is possible to prevent the conversion efficiency of the catalyst 22 for purifying the exhaust gas from being lowered (see FIGS. 6d and 6e) without largely shifting to the rich side.

Further, as described above, steps 65-
Step 70 has the function of the exhaust flow rate control means. Therefore, stable exhaust gas purification performance can be obtained under all operating conditions. Here, the α ° and β ° will be described. In the first embodiment, α ° is the initial opening and is set differently from the normal opening increment β °.

Here, if unburned HC starts to be desorbed from the adsorbent for some reason before the start of purging, and if the desorbed HC rapidly increases at the initial stage of purging, the purifying performance of the catalyst in the downstream flow will be improved. Therefore, in order to prevent this, it is desirable to set α ° small. Also, α ° and β ° are not set only as constant values, but
For example, it may be changed according to operating conditions and exhaust temperature.
Further, by increasing β ° gradually according to the number of operations, for example, α, α + β, α + 3β, α + 6β, ...
The valve operation pattern (valve opening) may be changed as shown in.

Therefore, according to the first embodiment, as described above, the unburned HC is rapidly desorbed from the adsorbent 24 by opening the flow control valve 51 by α °. Therefore, it is possible to prevent the conversion efficiency of the catalyst 22 for purifying the exhaust gas from decreasing.
The effect is that stable exhaust gas purification performance can be obtained under all operating conditions.

Further, prior to the desorption operation of the adsorbent 24,
By previously exposing the adsorbent 24 to high-temperature exhaust gas to raise the temperature of the adsorbent 24 and facilitating desorption of unburned HC, the time lag during desorption operation can be reduced,
Therefore, by facilitating the desorption control, there is an effect that the unburned gas can be purified more reliably. Next, a second embodiment according to claim 3 of the present invention will be described with reference to the drawings. In the system configuration shown in FIG. 8, the same components as those in the first embodiment shown in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

In the second embodiment, the adsorbent 24 provided in the second branch passage portion 16 serves as an exhaust temperature detecting means for detecting the inlet exhaust temperature and the outlet exhaust temperature of the unburned gas adsorbent. An inlet exhaust gas temperature sensor 37 and an outlet exhaust gas temperature sensor 38, which are thermocouples, are provided at the inlet and the outlet. Then, the control unit 30 controls the actuators 19 and 52 according to detection signals from various sensors including the inlet exhaust gas temperature sensor 37 and the outlet exhaust gas temperature sensor 38 to purify the exhaust gas.

Next, in the exhaust emission control device having the above structure,
Regarding the control performed by the control unit 30,
This will be described with reference to the flowcharts shown in FIGS. 9 and 10 and the time chart shown in FIG. 9 and 10
In the flowchart shown in FIG. 2, the flow relating to the adsorption of the unburned gas by the adsorbent 24 is the same as the flow shown in FIG.
Descriptions of steps 21 to 33 will be omitted.

Further, I to IV shown in FIG. 11 indicate that the states are in regions I to IV. In step 71, the flow control valve 51 in the closed state is opened by α ° as the first predetermined angle. Where the steps
At 31, the flow path switching valve 18 is switched to the intermediate position so that the exhaust gas flows to the first branch passage portion 15 and the second branch passage portion 16. Therefore, by opening the flow rate control valve 51 by α °, a predetermined amount q ₁ of exhaust gas flows to the second branch passage portion 16 so that a predetermined amount q ₁ of exhaust gas passes through the adsorbent 24. Here, when the unburned HC trapped in the adsorbent 24 is desorbed, the HC acts as a reducing agent, so that the reducing component in the exhaust increases and the exhaust air-fuel ratio becomes rich.

At step 72, the exhaust gas temperature T ₁ at the inlet of the adsorbent 24 detected by the inlet exhaust gas temperature sensor 37 is input. In step 73, the exhaust temperature T ₂ at the outlet of the adsorbent 24 detected by the outlet exhaust temperature sensor 38 is input. In step 74, the temperature difference T _X (= T ₁ −T ₂ ) between the exhaust temperature T ₁ at the inlet of the adsorbent 24 and the exhaust temperature T _{2 at the} outlet is calculated, and the temperature difference T _{X from} the previous value is calculated. incremental T _^'X (= d / d
xT _x ) is calculated.

In step 75, the area relating to the current operating state is set. That is, the region is divided depending on whether the temperature difference T _X between the exhaust gas temperature T ₁ at the inlet of the adsorbent 24 and the exhaust gas temperature T _{2 at the} outlet is too large or too small, and the temperature difference T _X tends to widen. The operating state from area I to area I
Divide by V.

At step 76, the current operating condition is the region I.
It is judged whether or not it is the driving state. Here, the region I is the temperature difference T
_This is a region where _X is very large and the temperature difference T _X is about to widen. In this region, it is necessary to suppress the desorption amount by suppressing the exhaust gas flow rate. Therefore, when it is determined that the operating state is in the region I, the flow proceeds to step 77, and the flow rate control valve 51 is closed by β °.

In step 78, it is determined whether or not the current operating state is the operating state in the area II. Here Region II has to do with it the temperature difference T _X is to spread, the temperature difference T _X is small area. In this region, the opening degree of the flow rate control valve 51 is maintained at the current value, the exhaust gas flow rate is maintained at the current value, and the desorption amount is also maintained at the current value. In step 79, it is determined whether or not the current operating state is the operating state in region III. Here, the region III is a region where the temperature difference T _X is small and the temperature difference T _X is about to become smaller. In this region, the exhaust gas temperature T at the inlet of the adsorbent 24
This is a region where ₁ and the exhaust gas temperature T _{2 at the} outlet are about to become the same temperature, and it is considered that desorption related to the exhaust gas flow rate is almost completed. Therefore, when it is determined in step 79 that the operating state is in the region III, the process proceeds to step 80,
The opening of the flow control valve 51 is opened by γ ° to further promote desorption.

Here, if NO in steps 76, 78 and 79, it can be determined that the operating state is in the region IV. Here region IV is large temperature difference T _X, but an area where the temperature difference T _X is about to Sebamaro. In this case, the exhaust temperature T _{2 at} the outlet of the adsorbent 24 is in the region where the exhaust temperature T ₂ is about to become the same temperature, the exhaust flow rate is kept the same as the current state, and the desorption amount is also kept the current state, and the process returns to step 72.

Here, steps 74 to 80 function as the exhaust flow rate control means. Further, in step 81, it is determined whether the opening degree of the flow rate control valve 51 is a predetermined value or more, and if it is determined that the flow rate control valve 51 is opened to a predetermined value or more, the adsorbent 24 is not desorbed. Almost done
The routine is finished. Therefore, as described above, according to the second embodiment as well, the opening degree of the flow rate control valve 51 is controlled based on the exhaust temperature T ₁ at the inlet of the adsorbent 24 and the exhaust temperature T _{2 at the} outlet thereof. As a result, desorption of unburned HC from the adsorbent 24 does not suddenly occur, and it is possible to prevent the conversion efficiency of the catalyst 22 for purifying the exhaust gas from decreasing, and all operations are performed. The effect is that stable exhaust gas purification performance can be obtained under certain conditions.

Further, similar to the first embodiment, there is an effect that the time lag during the desorption operation can be reduced and the unburned gas can be purified more reliably. Next, a third embodiment according to claim 4 of the present invention will be described with reference to the drawings. The system configuration is different from that of the second embodiment shown in FIG. 8 only in that the outlet exhaust gas temperature sensor 38 is not provided, and the other points are the same, so the description thereof will be omitted.

Then, the control unit 30 is
The actuators 19 and 52 are controlled according to detection signals from various sensors including the inlet exhaust gas temperature sensor 37 to purify the exhaust gas. Next, the control performed by the control unit 30 in the exhaust emission control device having such a configuration will be described with reference to the flowcharts shown in FIGS. 12 and 13.

In the flow charts shown in FIGS. 12 and 13, the flow relating to the adsorption of the unburned gas by the adsorbent 24 is the same as the flow shown in FIG. Descriptions of steps 21 to 33 will be omitted. In step 85, the exhaust gas temperature T ₁ at the inlet of the adsorbent 24 detected by the inlet exhaust gas temperature sensor 37 is input.

In step 86, the inlet / outlet temperature difference T _{X of the} adsorbent 24, which is estimated in consideration of the balance between the amount of exhaust gas introduced and the heat capacity of the adsorbent 24 at the exhaust temperature T ₁ , does not become excessive. Lower limit value T _{L of the} temperature change range set to
And the upper limit value T _H are read from a map (not shown). In step 87, the closed flow control valve 51
Open by α ° as the predetermined angle of. Here, in step 31, the flow path switching valve 18 is configured so that the exhaust gas is the first branch passage portion.
A switching operation is performed at an intermediate position so as to flow to 15 and the second branch passage portion 16. Therefore, set the flow control valve 51 to α
By opening only by °, a predetermined amount of exhaust gas flows into the second branch passage portion 16, a predetermined amount of exhaust gas passes through the adsorbent 24, and the unburned HC trapped in the adsorbent 24 is desorbed. .

[0081] At step 88, a predetermined defined time until detecting the exhaust gas temperature T ₁ of the re-entry from the detection of the exhaust temperature T ₁ of the inlet is detected at step 85 the time t
_i is read from the map, detected in step 85,
It is determined whether or not the predetermined time t _{i has} elapsed. The process proceeds to step 89 only when the predetermined time t _{i has} elapsed. In step 89, the adsorbent 24 is again detected by the inlet exhaust temperature sensor 37.
The exhaust gas temperature T ₁ at the inlet of is detected.

At step 90, it is judged if the inlet exhaust gas temperature T ₁ detected at step 89 is higher than the lower limit value T _L. In addition the step 91, the exhaust gas temperature T ₁ of the inlet detected in step 89 it is determined whether larger than the upper limit value T _H. Then, when it is confirmed that the newly detected inlet exhaust gas temperature T ₁ at steps 90 and 91 is between the lower limit value T _L and the upper limit value T _H ( _TL <T ₁ ≦
In (T _H ), the routine proceeds to step 92, where the opening operation of the flow control valve 51 is stopped and the opening is maintained.

In step 93, the map is a predetermined time t ₀ that defines the time from when the opening operation of the flow control valve 51 is stopped in step 92 until the opening operation of the flow control valve 51 is started again. Then, in step 92, it is determined whether or not the predetermined time t _{0 has} elapsed since the opening operation of the flow rate control valve 51 was started. Then, only when the predetermined time t _{0 has} elapsed, the routine returns and the routine is started again.

Here, the predetermined time t ₀ is the predetermined time according to the fourth technical means. Then, steps 92 and 93 perform the function of prohibiting means. On the other hand, if it is determined in step 90 that the newly detected exhaust gas temperature T ₁ at the inlet is lower than the lower limit value T _L, it is determined that there is no temperature change.
Return to step 89. Further, when it is determined that the newly detected exhaust gas temperature T ₁ at the inlet at steps 90 and 91 has changed to the upper limit value T _H or more, the exhaust gas temperature T ₁ at the inlet of the adsorbent 24 is rapidly increased. It is conceivable that the temperature inside the adsorbent 24 is also rapidly rising. That is, since the temperature distribution inside the adsorbent 24 is wide and desorption is performed violently, there is a possibility that the purifying ability of the catalyst on the downstream side of the adsorbent 24 may be reduced. Therefore, in step 94, the flow rate control valve 51, which was opened by α °, is opened by β ° (β ° / se
c) After closing, return to step 89.

Therefore, as described above, also according to the third embodiment, the internal temperature of the adsorbent 24 is estimated based on the exhaust gas temperature T ₁ at the inlet of the adsorbent 24, and the opening degree of the flow control valve 51 is increased. The unburned HC from the adsorbent 24 is controlled by controlling the
It is possible to prevent the conversion efficiency of the catalyst 22 for purifying the exhaust gas from being lowered because the desorption of the exhaust gas is not performed rapidly, and a stable exhaust gas purifying performance is obtained under all operating conditions. There is an effect.

Further, similar to the first embodiment, there is an effect that the time lag during the desorption operation can be reduced and the unburned gas can be purified more reliably.

[0087]

As described above, according to the present invention,
When a change in the air-fuel ratio from rich to lean is detected after the start of desorption, or when lean is detected for a predetermined time or longer, the exhaust flow rate flowing to the unburned gas adsorbent is controlled to be gradually increased, The exhaust flow rate control means controls the exhaust flow rate so that the difference between the inlet exhaust temperature of the fuel gas adsorbent and the outlet exhaust temperature is below a predetermined value, and the inlet exhaust temperature of the unburned gas adsorbent falls within a predetermined range. At certain times, the exhaust flow rate is not controlled for a predetermined period of time, which makes it possible to prevent the rapid release of unburned gas from the unburned gas adsorbent, thus ensuring stable exhaust under all operating conditions. The effect is that purification performance can be obtained.

[Brief description of drawings]

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

FIG. 2 is a flowchart for explaining the control contents performed by the control unit according to the first embodiment.

FIG. 3 is a flowchart (part 1) explaining the control content performed by the control unit according to the first embodiment.

FIG. 4 is a flowchart (part 2) explaining the control content performed by the control unit according to the first embodiment.

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

FIG. 6 is a time chart explaining the operation of the first embodiment.

FIG. 7 is a characteristic diagram of the adsorbent showing the relationship between the amount of HC discharged from the adsorbent and the exhaust temperature.

FIG. 8 is a system configuration diagram showing an overall configuration of an engine exhaust emission control device according to a second embodiment of the present invention.

FIG. 9 is a flowchart for explaining control contents performed by the control unit according to the second embodiment.

FIG. 10 is a flowchart illustrating the control contents performed by the control unit according to the second embodiment.

FIG. 11 is a time chart explaining the operation of the second embodiment.

FIG. 12 is a flowchart (part 1) explaining the control contents performed by the control unit according to the third embodiment.

FIG. 13 is a flowchart (part 2) explaining the control contents performed by the control unit according to the third embodiment.

[Explanation of symbols]

 11 Engine 13 Exhaust passage 15 First branch passage 16 Second branch passage 18 Flow switching valve 22 Main catalyst 24 Adsorbent 30 Control unit 31 Air-fuel ratio sensor 32 Main catalyst outlet temperature sensor 33 Crank angle sensor 35 Water temperature sensor 36 Oxygen sensor 37 Inlet exhaust temperature sensor 38 Outlet exhaust temperature sensor 51 Flow control valve

Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location F02D 45/00 368 G 7536-3G

Claims (4)

[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. The operating condition detecting means for detecting the operating condition of the engine, the air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust gas on the downstream side of the unburned gas adsorbent, and the unburned gas adsorbent. An exhaust gas flow control device for controlling an exhaust gas flow amount passing through the engine, and an engine exhaust gas purification device provided with an operating condition detected by the operating condition detecting means in a combustion state leaner than the theoretical air-fuel ratio. The exhaust flow rate control means starts desorption from the unburned gas adsorbent by flowing a predetermined amount of exhaust gas to the unburned gas adsorbent,
After the start of desorption, whenever the change from rich to lean is detected by the air-fuel ratio detection means or lean is detected for a predetermined time or more, the exhaust flow rate control means causes the exhaust gas to flow to the unburned gas adsorbent. An exhaust emission control device for an engine, which is characterized by gradually increasing the flow rate. 2. The exhaust flow rate control means is provided in a branch part, and a flow path through which exhaust gas flows is provided only in the first branch passage part, only in the second branch passage part, or in the first and second branch passage parts. A flow path switching valve that selectively switches to both of the branch passage portions,
The exhaust flow rate control means is provided on the downstream side of the unburned gas adsorbent, and includes a flow rate control valve capable of continuously increasing and decreasing the exhaust flow rate passing through the second branch passage portion. When the desorption from the unburned gas adsorbent is started by flowing a predetermined amount of exhaust gas into the unburned gas adsorbent, the exhaust gas is switched so as to flow through the second branch passage by the flow path switching valve. The exhaust emission control device for an engine according to claim 1, wherein the flow control valve is closed for a predetermined time from the start of the process. 3. 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. Exhaust gas temperature detecting means for detecting an inlet exhaust temperature and an outlet exhaust temperature of the unburned gas adsorbent, and an exhaust flow rate control means for controlling an exhaust flow rate passing through the unburned gas adsorbent. And an exhaust gas flow rate such that the difference between the inlet exhaust temperature and the outlet exhaust temperature of the unburned gas adsorbent detected by the exhaust temperature detecting means is equal to or less than a predetermined value. An exhaust emission control device for an engine, wherein the control means controls an exhaust flow rate. 4. 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 an exhaust gas flow rate control means for controlling the exhaust gas flow rate passing through the unburned gas adsorbent, and an inlet exhaust gas temperature detection means for detecting the inlet exhaust gas temperature of the unburned gas adsorbent. When the inlet exhaust temperature of the unburned gas adsorbent detected by the inlet exhaust temperature detecting means is within a predetermined range, the predetermined time set based on the inlet exhaust temperature is An exhaust emission control device for an engine, comprising an inhibiting means for inhibiting the control of the exhaust flow rate by the exhaust flow rate control means.