Exhaust gas purification device for an engine

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust gas purification device for an engine. More specifically, the present invention relates to a device that is capable of effectively removing an NO

x

component from exhaust gas.

2. Description of the Related Art

An exhaust gas purification device capable of removing an NO

x

component from engine exhaust gas is, for example, disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 62-117620. The device disclosed in JPP '620 is used for removing NO

x

components from the exhaust gas of a lean burn engine, i.e., an engine operated at an air-fuel ratio higher than a stoichiometric air-fuel ratio. In the device disclosed in JPP '620, an exhaust passage of the engine is diverged into two branch exhaust passages, and a NO

x

absorbent is disposed in each of the branch exhaust passages, namely, two NO

x

absorbents are disposed in the exhaust gas passage in parallel. The NO

x

absorbent is capable of absorbing an NO

x

component in the exhaust gas in the presence of oxygen. Since the engine is operated on a lean air-fuel ratio, the exhaust gas from the engine contains relatively high concentration of oxygen, therefore, NO

x

components contained in the exhaust gas are absorbed by the NO

x

absorbents.

When the NO

x

absorbent absorbs a certain amount of NO

x

, the ability of the NO

x

absorbent to absorb NO

x

in the exhaust gas is reduced. To regain the absorption ability, the absorbed NO

x

in the absorbent must be discharged and reduced periodically. For example, by supplying a reducing agent to the NO

x

absorbent, the NO

x

absorbents are regenerated to regain the initial NO

x

absorption ability. In the device in JPP '620, two NO

x

absorbents are disposed in parallel in the exhaust passage in order to enable regeneration of the NO

x

absorbent without stopping the purification of the exhaust gas. In JPP '620, two NO

x

absorbents are used alternately during normal engine operation, i.e., only one of the NO

x

absorbents is used for absorbing the NO

x

in the exhaust gas. Namely, in the normal operation, one of the branch exhaust passage is closed, and the exhaust gas from the engine is directed to the other branch exhaust passage where the NO

x

in the exhaust gas is absorbed by the NO

x

absorbent disposed in that exhaust branch passage. While absorbing NO

x

in the exhaust gas by one NO

x

absorbent, the other NO

x

absorbent disposed on the closed exhaust branch passage is regenerated by supplying a reducing agent, such as hydrogen gas.

According to the device disclosed in JPP '620, by using two NO

x

absorbents disposed in the exhaust passage in parallel, the NO

x

absorbents can be regenerated alternately while continuing to remove the NO

x

component from the exhaust gas.

However, a disadvantage of the device disclosed in JPP '620 has is that the reducing agent must be supplied to both exhaust branch passages. This requires two injection nozzles, one in each of the exhaust branch passages, and associated piping connecting the nozzles to the supply source of the reducing agent, and a three-way flow control valve for switching the supply of the reducing agent between the two nozzles. Further, elaborate control of the three-way flow control valve is required to enable the appropriate regeneration of the NO

x

absorbents.

SUMMARY OF THE INVENTION

In view of the problems in the prior art set forth above, the object of the present invention is to provide an exhaust gas purification device for an internal combustion engine which is capable of removing NO

x

components from the exhaust gas without requiring complicated devices and controls.

The above object is achieved by the present invention, an exhaust gas purification device, for an internal combustion engine. The engine is operated with an air-fuel ratio higher than a stoichiometric air-fuel ratio. The device comprises (a) first and second NO

x

absorbents disposed in the exhaust passage of the engine and connected in series by an exhaust connecting passage, each of the NO

x

absorbents absorbs NO

x

components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NO

x

absorbents is lean, and releases the absorbed NO

x

when the oxygen concentration in the exhaust gas becomes lower; (b) a reducing agent supply means for supplying a reducing agent to the exhaust gas in the exhaust connecting passage between the first and the second NO

x

absorbents; (c) a switching means for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NO

x

absorbent, the exhaust connecting passage and the second NO

x

absorbent in this order, and a second direction in which the exhaust gas flows through the second NO

x

absorbent, the exhaust connecting passage and the first NO

x

absorbent in this order; (d) and a control means for controlling the switching means so that the direction of the exhaust gas flow is switched between the first and the second direction each time when a predetermined condition is satisfied.

When the flow direction of exhaust gas is switched to the first direction, the lean air-fuel ratio exhaust gas from the engine first flows into the first NO

x

absorbent and NO

x

components in the exhaust gas are absorbed by the first NO

x

absorbent. After passing through the first NO

x

absorbent, a reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the second NO

x

absorbent. Therefore, the air-fuel ratio of the exhaust gas flowing into the second NO

x

absorbent becomes rich due to the reducing agent. Thus, in the second NO

x

absorbent, the absorbed NO

x

is released and reduced to nitrogen by the reducing agent. Namely, when the exhaust gas flow is switched to the first direction, NO

x

components in the exhaust gas are absorbed by the first NO

x

absorbent and removed from the exhaust gas, and the NO

x

in the second NO

x

absorbent is released and reduced to nitrogen.

On the other hand, when the exhaust gas flow is switched to the second direction, the lean exhaust gas from the engine first flows into the second NO

x

absorbent and NO

x

components in the exhaust gas are absorbed by the second NO

x

absorbent. Then, the reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the first NO

x

absorbent. Therefore, the rich exhaust gas flows into the first NO

x

absorbent, and absorbed NO

x

is released from the first NO

x

absorbent and reduced to nitrogen by the reducing agent. Therefore, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, for example, each time an amount of NO

x

absorbed in the NO

x

absorbent reaches a predetermined value, absorption of NO

x

in the exhaust gas and releasing and reducing of the absorbed NO

x

is performed in each NO

x

absorbent alternatively without stopping the absorption of NO

x

in the exhaust gas as a whole.

According to another aspect of the present invention, there is provided an exhaust gas purification device for an internal combustion engine having an exhaust passage. The engine is operated at an air-fuel ratio equal to, or lower than, a stoichiometric air-fuel ratio. The device comprises (a), first and second NO

x

absorbents disposed in the exhaust passage of the engine and connected each other in series by an exhaust connecting passage, each of the NO

x

absorbents absorbs NO

x

components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NO

x

absorbents is lean, and releases the absorbed NO

x

when the oxygen concentration in the exhaust gas becomes; (b) a secondary air supply means for supplying secondary air to the exhaust gas in the exhaust connecting passage between the first and the second NO

x

absorbents; (c) a switching means for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NO

x

absorbents, the exhaust connecting passage and the second NO

x

absorbent in this order, and a second direction in which the exhaust gas flows through the second NO

x

absorbent, the exhaust connecting passage and the first NO

x

absorbent in this order; and (d) a control means for controlling the switching means so that the direction of the exhaust gas flow is switched between the first and the second direction each time when a predetermined condition is satisfied.

In this aspect of the invention, the air-fuel ratio of the exhaust gas from the engine is rich, i.e., lower than, or equal to, the stoichiometric air-fuel ratio, and relatively low concentrations of oxygen and a relatively high concentrations of HC and CO components are contained in the exhaust gas. Therefore, when the direction of exhaust gas flow is switched to the first direction, the exhaust gas with a rich air-fuel ratio flows into the first NO

x

absorbent, and the absorbed NO

x

is released from the first NO

x

absorbent and reduced to nitrogen by HC and CO components in the exhaust gas. After passing through the first NO

x

absorbent, the secondary air is supplied to the exhaust gas in the exhaust connecting passage before it flows into the second NO

x

absorbent, and the air-fuel ratio of the exhaust gas flowing into the second NO

x

absorbent becomes lean. Therefore, a small amount of NO

x

which passes through the first NO

x

absorbent is absorbed by the second NO

x

absorbent and removed completely from the exhaust gas.

Namely, when the direction of the exhaust gas flow is switched to the first direction, the absorbed NO

x

is released from the first NO

x

absorbent and reduced to nitrogen, and at the same time, NO

x

in the exhaust gas is absorbed by the second NO

x

absorbent.

Similarly, when the direction of the exhaust gas flow is switched to the second direction, the absorbed NO

x

is released from the second NO

x

absorbent and reduced to nitrogen, and NO

x

in the exhaust gas is absorbed by the first NO

x

absorbent at the same time.

Therefore, in this aspect of the present invention, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, absorption of NO

x

in the exhaust gas and releasing and reducing of the absorbed NO

x

is performed in each NO

x

absorbent alternatively, without stopping absorption of NO

x

in the exhaust gas as a whole.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the description as set forth hereinafter, with reference to the accompanying drawings, in which:

FIG. 1

is a drawing schematically illustrating an embodiment of the exhaust gas purification device according the present invention when applied to an automobile engine;

FIG. 2

is a sectional view of the exhaust passage in

FIG. 1

taken along a line II—II in

FIG. 1

;

FIGS. 3A and 3B

are views for explaining the absorbing and releasing operation of NO

x

by the NO

x

absorbent;

FIG. 4

is a drawing schematically illustrating another embodiment of the exhaust gas purification device according to the present invention;

FIGS. 5A and 5B

are drawings illustrating another embodiment of the exhaust gas purification device according to the present invention; and

FIG. 6

is a drawing illustrating another embodiment of the exhaust gas purification device according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1

schematically illustrates an embodiment of an exhaust gas purification device, according to the present invention, as applied to an automobile engine.

In

FIG. 1

, reference numeral

1

designates an internal combustion engine such as a diesel engine or a lean-burn gasoline engine. The engine

1

is operated at an air-fuel ratio higher than the stoichiometric air-fuel ratio some paint in time during the normal operating range. Numeral

3

designates an exhaust gas passage of the engine

1

. The exhaust gas passage

3

diverges into two branch exhaust passages

3

a

and

3

b

, and a NO

x

absorbent (designated by

7

a

and

7

b

) is connected to each branch exhaust passage. The NO

x

absorbents

7

a

and

7

b

are connected to each other at opposite ends of to the exhaust branch passage

3

a

and

3

b

by an exhaust connecting passage

4

.

A switching valve

5

is disposed on the exhaust gas passage

3

at the point where the branch exhaust passages

3

a

and

3

b

diverge. The switching valve

5

is actuated by a control signal from an engine control circuit (ECU)

20

which is explained later. The switching valve

5

directs the exhaust gas flow to either of the branch exhaust passages

3

a

and

3

b

by closing the inlet of one of the branch exhaust passages

3

a

and

3

b

. On the respective branch exhaust passages

3

a

and

3

b

, a port

6

a

,

6

b

and a shut-off valve

8

a

,

8

b

, which closes the port

6

a

,

6

b

, respectively, are disposed at the portion between the switching valve

5

and the NO

x

absorbents

7

a

and

7

b.

FIG. 2

schematically illustrates the section of the branch exhaust passages

3

a

and

3

b

taken along the line II—II in FIG.

1

. As shown in

FIG. 2

, an exhaust outlet passage

9

is attached to the branch exhaust passages

3

a

and

3

b

. The exhaust outlet passage

9

communicates the branch exhaust passages

3

a

and

3

b

through the ports

6

a

and

6

b

, respectively. Namely, when the shut-off valve

8

a

is opened, the branch exhaust passage

3

a

is connected to the exhaust outlet passage

9

via the port

6

a

, and when the shut-off valve

8

b

is opened, the branch exhaust passage

3

b

is connected to the exhaust outlet passage

9

via the port

6

b

. The exhaust outlet passage

9

opens to the atmosphere downstream of a muffler (not shown) disposed therein.

In this embodiment, the switching valve

5

and the shut-off valves

8

a

and

8

b

are functionally connected to each other by a mechanical link

10

and driven by a common actuator

11

. The actuator

11

may be a solenoid actuator, a vacuum actuator, or any other appropriate type of actuator that can drive the switching valve

5

and shut-off valves

8

a

and

8

b

synchronously in accordance with the control signal from the ECU

20

. In this embodiment, the shut-off valves

8

a

and

8

b

are operated through the mechanical link

10

in such a manner that the shut-off valve

8

b

opens the port

6

b

and the shut-off valve

8

a

closes the port

6

a

when the switching valve

5

is at a position closing the inlet of the branch exhaust passage

3

b

(i.e., the position shown in FIG.

1

), and in such a manner that the shut-off valve

8

a

opens the port

6

a

and the shut-off valve

8

b

closes the port

6

b

when the switching valve

5

is at a position closing the inlet of the branch exhaust passage

3

a.

Therefore, when the switching valve

5

is set at the position closing the inlet of the branch exhaust passage

3

b

, the exhaust gas from the engine flows into the branch exhaust passage

3

a

and flows through the NO

x

absorbent

7

a

, the exhaust connecting passage

4

and the NO

x

absorbent

7

b

in this order, and, after leaving the NO

x

absorbent

7

b

, flows into the exhaust outlet passage

9

through the port

6

b

. This direction of the exhaust gas flow is called a first direction hereinafter.

On the other hand, when the switching valve

5

is set at the position closing the inlet of the branch exhaust passage

3

a

, the exhaust gas from the engine flows into the branch exhaust passage

3

b

and flows through the NO

x

absorbent

7

b

, the exhaust connecting passage

4

and the NO

x

absorbent

7

a

in this order, and, after leaving the NO

x

absorbent

7

a

, flows into the exhaust outlet passage

9

through the port

6

a

. This direction of the exhaust gas flow is called a second direction hereinafter. It will be understood from the above description, that the direction of the exhaust gas flow in the NO

x

absorbents

7

a

,

7

b

and the exhaust connecting passage

4

is reversed by switching the position of the switching valve

5

.

In

FIG. 1

, numeral

12

designates a reducing agent supply unit which supplies a reducing agent to the exhaust connecting passage

4

. The reducing agent supply unit

12

includes a supply source

12

c

of a pressurized reducing agent, such as a pressurized container of the reducing agent or a reducing agent supply pump, and a nozzle

12

a

for injecting the reducing agent into the exhaust connecting passage

4

. The reducing agent supply unit is provided with a flow control valve

12

b

which controls the injection rate of the reducing agent in accordance with a control signal from the ECU

20

.

In this embodiment, substances which generate components such as hydrogen H

2

, hydrocarbon HC, and carbon monoxide CO can be used as the reducing agent. Therefore, reducing gases such as hydrogen gas or carbon monoxide gas, gaseous or liquid hydrocarbons such as propane, propylene or butane, and liquid fuels such as gasoline, gas oil or kerosene, etc., can be used as the reducing agent in the present invention.

The engine control unit

20

in

FIG. 1

comprises, for example, a digital computer including a ROM (read only memory), a RAM (random access memory), a CPU (central processing unit), and I/O (input and output) interface, all connected each other by a bi-directional bus. The ECU

20

performs the basic control of the engine

1

such as fuel injection control. In addition, the ECU

20

controls the absorption and releasing operation of NO

x

absorbents

7

a

and

7

b

as explained later in detail. For this purpose, signals representing engine operating conditions, such as an engine speed signal, an accelerator signal which represents the depression of the accelerator pedal, and an inlet air flow amount signal, are fed to the I/O interface from the respective sensors (not shown). Further, the I/O interface of the ECU

20

is connected to the actuator

11

of the link

10

and the flow control valve

12

b

of the reducing agent supply unit

12

in order to supply the control signals.

The NO

x

absorbents

7

a

and

7

b

use alumina as a carrier, and on this carrier, precious metals such as platinum, and at least one substance selected from alkali metals such as potassium K, sodium Na, lithium Li and cesium Cs; alkali-earth metals such as barium Ba and calcium Ca; and rare-earth metals such as lanthanum La and yttrium Y are carried. The NO

x

absorbents

7

a

and

7

b

absorb NO

x

in the exhaust gas when the air-fuel ratio of the exhaust gas flowing through the NO

x

absorbents is lean, and release the absorbed NO

x

when the oxygen concentration in the exhaust gas becomes low. In this specification, the term “air-fuel ratio of the exhaust gas” means a ratio of the air and the fuel (or reducing agent and other hydrocarbons) which are supplied to the engine or exhaust passages upstream of the NO

x

absorbents. Therefore, when no air and fuel (or reducing agent) is supplied in the exhaust passage upstream of the NO

x

absorbents

7

a

and

7

b

, the air-fuel ratio of the exhaust gas becomes the same as the operating air-fuel ratio of the engine (i.e., the air-fuel ratio of the air-fuel mixture supplied to combustion chambers of the engine).

Though the mechanism of this absorption and releasing operation of the NO

x

absorbent is not clear, it is thought that the absorption and releasing operation is conducted by the mechanism shown in

FIGS. 3A and 3B

.

FIGS. 3A and 3B

explain the mechanism of the absorption and the releasing operation in the case where platinum Pt and barium Ba are carried on the carrier, as an example, but it is thought that a similar mechanism also applies even if other precious metals, alkali metals, alkali earth metals, or rare earth metals are used.

Namely, when the air-fuel ratio of the exhaust gas flowing into the NO

x

absorbent is lean, the concentration of oxygen in the exhaust gas is relatively high. In this case, as shown in

FIG. 3A

, the oxygen O

2

is deposited on the surface of platinum Pt in the form of O

2

−

or O

2−

. The NO in the exhaust gas reacts with O

2

−

or O

2−

on the surface of the platinum Pt and becomes NO

2

(2NO+O

2

→2NO

2

). Then, a part of the produced NO

2

is oxidized on the platinum Pt and absorbed into the NO

x

absorbent. While bonding with the barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO

3

−

as shown in FIG.

3

A. In this way, NO

x

is absorbed in the NO

x

absorbent. As long as the oxygen concentration in the exhaust gas is high, the NO

x

is produced on the surface of the platinum Pt, and as long as the NO

x

is produced on the surface of the platinum Pt, and as long as the NO

x

absorption capacity of the absorbent is not saturated by the absorbed NO

x

, the NO

x

is absorbed into the NO

x

absorbent and nitric acid ions NO

3

−

are produced.

On the other hand, when the oxygen concentration in the exhaust gas flowing into the NO

x

absorbent is lowered, the production of NO

2

is lowered and the reaction proceeds in an inverse direction (NO

3

−

→NO

2

), and thus nitric acid ions NO

3

−

in the absorbent are released in the form of NO

2

from the NO

x

absorbent.

In this case, if components such as HC, CO exist in the exhaust gas, these components react with the oxygen O

2

in the exhaust gas and oxygen O

2

−

or O

2

−

on the platinum Pt and are oxidized. By this oxidation of the HC, CO components, the oxygen component existing near the NO

x

absorbent is consumed, and the concentration of oxygen in the atmosphere around the NO

x

absorbent is lowered. Also, the NO

2

released from the NO

x

absorbent reacts with the HC and CO components in the exhaust gas as shown in FIG.

3

B and is reduced to N

2

. In this way, when the NO

2

on the surface of the platinum Pt reacts with HC and CO in the exhaust gas, and when the NO

2

no longer exist on the surface of the platinum Pt, the NO

2

is successively released from the absorbent.

Accordingly, if components such as HC, CO exist in the exhaust gas, the NO

x

is released from NO

x

absorbent and reduced to N

2

in a short time.

Namely, the HC and CO components in the reducing agent supplied to NO

x

absorbent immediately react with the O

2

−

or O

2

−

on the platinum Pt and are oxidized, and subsequently if the HC and CO still remain after the O

2

−

or O

2

−

on the platinum Pt are consumed, the NO

x

released from the absorbent is reduced to N

2

.

In the embodiment in

FIG. 1

, the absorbing and releasing operations of the respective NO

x

absorbents

7

a

and

7

b

are controlled by switching the direction of the exhaust gas flow between the first and the second direction using the switching valve

5

as explained above.

Next, the operation of the exhaust gas purification device in

FIG. 1

is explained. First, the case in which the switching valve

5

is set at the position closing the branch exhaust passage

3

b

(i.e., the position shown in

FIG. 1

) is explained. In this case, the exhaust gas flows in the first direction as explained before and flows into the NO

x

absorbent

7

a

first, and then flows into NO

x

absorbent

7

b

after passing through the NO

x

absorbent

7

a

. In this case, the reducing agent injected into the exhaust connecting passage

4

from the nozzle

12

a

of the reducing agent supply unit

12

flows into the NO

x

absorbent

7

a.

Since the engine

1

in the present embodiment is operated at a lean air-fuel ratio, the air-fuel ratio of the exhaust gas from the engine

1

is substantially lean. Therefore, the exhaust gas flowing into the NO

x

absorbent

7

a

contains a relatively high concentration of oxygen, and the NO

x

in the exhaust gas is absorbed by the NO

x

absorbent,

7

a

as explained in

FIG. 3A

, and thus removed from the exhaust gas. Thus, an exhaust gas having very low concentration of NO

x

component is discharged from the NO

x

absorbent

7

a

and flows into the exhaust connecting passage

4

. In the exhaust connecting passage

4

, the reducing agent is supplied to the exhaust gas by the nozzle

12

a

of the reducing agent supply unit

12

, and a mixture of the exhaust gas and the reducing agent flows into the NO

x

absorbent

7

b

. As explained later, the injection rate of the reducing agent is determined in such a manner that the air-fuel ratio of the mixture of the exhaust gas and the reducing agent flowing into the NO

x

absorbent

7

b

becomes lower than, or at least equal to, the stoichiometric air-fuel ratio. Therefore, the oxygen in the exhaust gas reacts with HC and CO components generated by the reducing agent in the NO

x

absorbent

7

b

, and the oxygen concentration in the exhaust gas in the NO

x

absorbent

7

b

becomes lower. Thus, the NO

x

absorbed in the NO

x

absorbent

7

b

is released from the absorbent and reduced by the HC and CO components in the exhaust gas as explained in FIG.

3

B. Further, the NO

x

components, in the exhaust gas, which has passed the NO

x

absorbent, though very little, are also reduced to N

2

in the NO

x

absorbent

7

b

. Therefore, the exhaust gas, which has passed through the NO

x

absorbent

7

b

and which is discharged to atmosphere through the port

6

a

and the exhaust outlet passage

9

, is almost completely free from NO

x

components.

When the switching valve

5

is switched to the position in which the inlet of the branch exhaust passage

3

a

is closed, the exhaust gas flows in the second direction, and flows into the NO

x

absorbent

7

b

first. In this case, contrary to the above, absorption of the NO

x

in the exhaust gas is performed in the NO

x

absorbent

7

b

, and the releasing and reducing of the absorbed NO

x

is performed on the NO

x

absorbent

7

a.

From the above explanation, it will be understood that, if the switching valve

5

is operated so that it switches the direction of the exhaust gas flow at the appropriate time, the NO

x

absorbents

7

a

and

7

b

can absorb and release NO

x

alternately without stopping the exhaust gas flow. Thus, it becomes possible to remove NO

x

components in the exhaust gas continuously while preventing the NO

x

absorbents

7

a

and

7

b

from being saturated by the absorbed NO

x

.

In this embodiment, the ECU

20

estimates the air-fuel ratio of the exhaust gas at the engine outlet based on the amount of the inlet air flow and the amount of the fuel injection. Then, the ECU

20

calculates the amount of the reducing agent required to adjust the air-fuel ratio of the exhaust gas to a value lower than the stoichiometric air-fuel ratio, and controls the flow control valve so that the calculated amount of the reducing agent is injected into the exhaust connecting passage

4

. The required amount of the reducing agent varies in accordance with types of NO

x

absorbent, an amount of NO

x

absorbed in the NO

x

absorbent and the discharge rate of NO

x

(i.e., the temperature of the NO

x

absorbent). Therefore, it is preferable to determine the amount of the injection of the reducing agent by, for example, experimenting with the actual NO

x

absorbent used for the engine.

Though the switching valve may be switched at regular intervals, it is preferable to switch the valve

5

each time when the amount of absorbed NO

x

in the NO

x

absorbent reaches a predetermined value (for example, about 60% of the maximum absorbing capacity of the NO

x

absorbent) so that the releasing and reducing operation of NO

x

absorbent is performed before the NO

x

absorbent is saturated with absorbed NO

x

.

In this case, for example, the amount of NO

x

generated by the engine

1

is measured beforehand in the respective engine load conditions, and stored in the ROM in the ECU

20

as a function of the parameter representing the engine load conditions (such as, the amount of inlet air flow and engine speed, or the accelerator signal and the engine speed). The ECU

20

calculates the amount of the NO

x

generated by the engine at regular intervals based on the engine load conditions. The amount of NO

x

absorbed by the NO

x

absorbent during the intervals is estimated as a value obtained by multiplying the amount of the generated NO

x

by a predetermined constant. The total amount of NO

x

absorbed by the NO

x

absorbent is obtained by accumulating the value obtained by the above calculation. The ECU

20

performs the estimation of the amount of NO

x

absorbed by the NO

x

absorbent during each interval and calculates the accumulated value thereof at regular intervals, and when the accumulated value reaches a predetermined value, the ECU

20

switches the switching valve

5

.

Usually, the time required for the NO

x

absorbent to release and reduce the absorbed NO

x

is substantially shorter than the time required for the amount of absorbed NO

x

to reach the predetermined value. For example, usually it takes a few minutes to more than one hour for the absorbed NO

x

to reach the predetermined value while the releasing and reducing operation of NO

x

requires only less than one second to ten seconds. In actual operation of the engine

1

, the reducing agent is injected only for a short time after the switching valve

5

is switched. Therefore, the amount of consumption of the reducing agent is relatively small in actual operation. Thus, even if the fuel of the engine

1

is used as the reducing agent, the fuel consumption of the engine does not increase significantly. When the releasing and reducing operation of NO

x

absorbent is completed, the supply of the reducing agent is stopped. This causes the air-fuel ratio of the exhaust gas flowing into the downstream side NO

x

absorbent to return to lean air-fuel ratio and the downstream NO

x

absorbent becomes available for absorbing NO

x

in the exhaust gas. Therefore, after the releasing and reducing operation is completed, even very small amount of NO

x

which is not absorbed by the upstream NO

x

absorbent can be absorbed by the downstream NO

x

absorbent, and the NO

x

components in the exhaust gas are almost completely removed even after the supply of the reducing agent is stopped.

Further, the reducing agent is supplied only to the exhaust connecting passage

4

in this embodiment and it is not required to supply the reducing agent to the respective branch exhaust passages

3

a

and

3

b

, i.e., it is not required to dispose the reducing agent injection nozzle and flow control valve on the respective branch exhaust passages

3

a

and

3

b