Exhaust emission control system of an internal combustion engine

Abstract

An exhaust emission control system of an internal combustion engine desorbs SO_x by reversing a flow of an exhaust gas through an NO_x storage-reduction catalyst, of which a structure is simplified as follows. A first exhaust pipe connected to an engine is connected to a first port of an emission switching valve having four ports. A second exhaust pipe 10, through which the exhaust gas is discharged into the atmospheric air, is connected to a second port, a third exhaust pipe connected to an inlet of a catalytic converter is connected to a third port. A fourth exhaust pipe connected to an outlet of the catalytic converter 30 is connected to a fourth port. When the emission switching valve is set in a forward flow position, the first exhaust pipe is connected to the third exhaust pipe, and the second exhaust pipe is connected to the fourth exhaust pipe, whereby the exhaust gas flows toward the outlet from the inlet within the catalytic converter. When the emission switching valve is set in a backward flow position, the first exhaust pipe is connected to the fourth exhaust pipe, and the second exhaust pipe is connected to the third exhaust pipe, whereby the exhaust gas flows toward the inlet from the outlet within the catalytic converter.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an exhaust emission control system of an internal combustion engine and, more particularly, to an exhaust emission control system of an internal combustion engine which is capable of switching over a flow direction of an exhaust gas flowing through an exhaust gas purifying element according to the necessity.

2. Related Background Art

In general, an exhaust emission control system for purifying an exhaust gas discharged from an internal combustion engine is provided at an exhaust passageway of the internal combustion engine. When the exhaust gas from the internal combustion engine flows through this exhaust emission control system, a deposit is gradually adhered from an upstream side in the exhaust emission control system. A classification of what this deposit is all about might differ depending upon a composition of the exhaust gas, or a construction of the exhaust emission control system or an exhaust gas purifying mechanism, and what can be exemplified as the deposit may be, e.g., an oxide, a sulfide, nitrate and sulfate. This deposit might cause a decline of a purging performance of the exhaust emission control system and also an increase in an exhaust resistance, and therefore needs to be removed at a predetermined timing.

For example, the exhaust emission control system for purging the exhaust gas of NO_x which is discharged from the internal combustion engine for performing a combustion at a lean air/fuel ratio, may involve the use of an NO_x storage-reduction catalyst. This NO_x storage-reduction catalyst absorbs NO_x when the air/fuel ratio of the inflow exhaust gas is lean, and desorbs NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases, thus effecting reduction to N₂. The NO_x storage-reduction catalyst is disposed in an exhaust passageway, and absorbs a nitrogen oxide (NO_x) contained in the exhaust gas exhibiting the lean air/fuel ratio. After absorbing NO_x, the air/fuel ratio of the exhaust gas flowing into the NO_x storage-reduction catalyst is made rich by increasing a quantity of the fuel supplied to the internal combustion engine, thereby desorbing NO_x absorbed thereto from the NO_x storage-reduction catalyst. Desorbed NO_x is reduced to N₂ with a reducing component such as unburned HC, CO etc contained in the exhaust gas.

By the way, generally speaking, the fuel of the internal combustion engine contains a sulfur content, and, when the fuel is burned in the internal combustion engine, the sulfur content is burned, resulting in a production of sulfur oxide (SO_x). The NO_x storage-reduction catalyst absorbs SOX contained in the exhaust gas with the same mechanism as absorbing NO_x. Therefore, if the NO_x storage-reduction catalyst is disposed in the exhaust passageway of the internal combustion engine, the NO_x storage-reduction catalyst absorbs SO_x as well as NO_x.

SO_x absorbed to the NO_x storage-reduction catalyst, however, generates stable sulfate with a passage of time. SO_x is therefore dissolved and desorbed with a difficulty and has a tendency of being easily accumulated within the NO_x storage-reduction catalyst under a condition of executing desorption, reduction and purging of NO_x out of the NO_x storage-reduction catalyst (which is hereinafter termed a NO_x desorbing/reducing process). If there augments a SO_x accumulation quantity within the NO_x storage-reduction catalyst, a NO_x absorption quantity of the NO_x storage-reduction catalyst decreases, and it is therefore unfeasible to sufficiently purge the exhaust gas of NO_x, with the result that so-called SO_x poisoning occurs, wherein a NO_x, purging rate declines. Such being the case, it is required that SO_x absorbed to the catalyst be desorbed therefrom at a proper timing in order to keep high the NO_x purging rate of the NO_x storage-reduction catalyst for a long period of time.

It has already proved that the air/fuel ratio of the inflow exhaust gas needs to be rich and the NO_x storage-reduction catalyst is required to be set at a higher temperature than in the NO_x desorbing/reducing process for desorbing SO_x absorbed to the NO_x storage-reduction catalyst.

Incidentally, a distribution of a SO_x absorption quantity in the NO_x storage-reduction catalyst exhibits a higher concentration in the closer proximity to the inlet of the exhaust gas in the NO_x storage-reduction catalyst. Hence, when desorbing SO_x absorbed to the NO_x storage-reduction catalyst, in the case of flowing the exhaust gas having the rich air/fuel ratio in the same direction as a flow direction of the exhaust gas when absorbing NO_x, though SO_x absorbed is desorbed on the inlet side in the NO_x storage-reduction catalyst, SO_x desorbed therefrom merely migrates to the outlet side of the exhaust gas through the NO_x storage-reduction catalyst and is reabsorbed to the NO_x storage-reduction catalyst. The problem is therefore such that SO_x can not be discharged from the NO_x storage-reduction catalyst.

Under such circumstances, a technology disclosed in Japanese Patent Application Laid-Open Publication No.7-259542, is that when desorbing SO_x absorbed to the NO_x storage-reduction catalyst, the exhaust gas having the rich air/fuel ratio flows through the NO_x storage-reduction catalyst in a direction opposite to the direction when absorbing NO_x. In the case of incorporating a backward flow function of desorbing SO_x by reversing the flow of the exhaust gas as described above, SO_x desorbed from the NO_x storage-reduction catalyst has a shorter migration distance within the NO_x storage-reduction catalyst, and is immediately discharged out of the NO_x storage-reduction catalyst. It is therefore feasible to prevent desorbed SO_x from being reabsorbed to the NO_x storage-reduction catalyst.

The following is an explanation of a construction of the exhaust mission control system of the internal combustion engine which incorporates the backward flow function disclosed in the above Publication. An upstream-side exhaust passageway connected to an inlet of the NO_x storage-reduction catalyst is connected to a downstream-side exhaust gas passageway connected to an outlet of the NO_x storage-reduction catalyst via a bypass passageway for bypassing the NO_x storage-reduction catalyst. A first emission flow switching valve is provided at a confluent portion between the upstream-side exhaust passageway and the bypass passageway. A second emission flow switching valve is provided at a confluent portion between the downstream-side exhaust passageway and the bypass passageway. The first emission flow switching valve is capable of making a changeover to let the exhaust gas flowing from upstream flow through the NO_x storage-reduction catalyst or let the exhaust gas flow into the bypass passageway. The second emission flow switching valve is capable of making a changeover to let the exhaust gas flowing through the NO_x storage-reduction catalyst flow out toward the downstream-side exhaust passageway disposed more downstream than the second emission flow switching valve or let the exhaust gas flowing though the bypass passageway flow out toward the downstream-side exhaust passageway disposed more downstream than the second emission flow switching valve. Further, an exhaust passageway for suction is bypassed from the upstream-side exhaust passageway between the NO_x storage-reduction catalyst and the first emission flow switching valve, and is connected to an intake port of an exhaust pump, and a discharge port of the exhaust pump is connected to the above bypass passageway. Moreover, a reducing agent supply device for supplying a reducing agent is provided at the downstream-side exhaust passageway between the NO_x storage-reduction catalyst and the second emission flow switching valve.

Then, when in the NO_x absorbing process, the first and second emission flow switching valves are switched over to close the bypass passageway so that an entire quantity of exhaust gas from the internal combustion engine flows toward the outlet from the inlet of the NO_x storage-reduction catalyst. On the other hand, when desorbing SO_x from the NO_x storage-reduction catalyst, the first and second emission flow switching valves are switched over to close the bypass passageway so that substantially the entire quantity of exhaust gas from the internal combustion engine flows to the bypass passageway. Simultaneously, the exhaust gas in the upstream-side exhaust passageway between the NO_x storage-reduction catalyst and the first emission flow switching valve is sucked and discharged to the bypass passageway by operating the exhaust pump, thereby causing a flow of the exhaust gas flowing backward from the outlet toward the inlet through the NO_x storage-reduction catalyst. Besides, the reducing agent is supplied to the downstream-side exhaust passageway by operating a reducing agent supply device. The exhaust gas exhibiting the rich air/fuel ratio is thereby flowed backward through the NO_x storage-reduction catalyst, thus desorbing SO_x from the NO_x storage-reduction catalyst.

The conventional backward-flow-function-incorporated exhaust emission control system of the internal combustion engine requires the exhaust pump and the plurality of emission flow switching valves and therefore involves the use of an increased number of parts, resulting in a rise in costs. Further, the increased number of parts leads to a good deal of labors for maintenance and inspection, correspondingly.

Moreover, the SO_x desorbing process involving the backward flow in the NO_x storage-reduction catalyst may be defined as a processing method giving attention to avoidance of the SO_x reabsorption by decreasing the SO_x migration distance when desorbing SO_x. While on the other hand, if this processing method is adopted, there increases a distance till the exhaust gas arrives at the NO_x storage-reduction catalyst, and hence there must be a large drop in temperature of the exhaust gas during a period for which the exhaust gas flows through this long route. The above processing method is not necessarily, as the case may be, considered the best method of desorbing SO_x in terms of a temperature condition when desorbing SO_x.

SUMMARY OF THE INVENTION

It is a primary object of the present invention, which was devised to obviate the above problems inherent in the prior art, to provide an exhaust emission control system of an internal combustion engine, which is capable of reducing costs by simplifying its own structure and decreasing the number of parts thereof.

It is another object of the present invention to provide an exhaust emission control system of an internal combustion engine, which is capable of establishing a SO_x desorption processing technology with respect to an NO_x storage-reduction catalyst NO_x.

To accomplish the above objects, according to a first aspect of the present invention, an exhaust emission control system of an internal combustion engine comprises an exhaust gas purifying element provided in an exhaust passageway of the internal combustion engine, a flow direction switching device including four ports and provided at an exhaust passageway disposed more upstream than the exhaust gas purifying element, a first exhaust passageway connected to the internal combustion engine and further to a first portion of the flow direction switching device, a second exhaust passageway communicating with the atmospheric air and further to a second port of the flow direction switching device, a third exhaust passageway connected to one side of the exhaust gas purifying element and further to a third port of the flow direction switching device, and a fourth exhaust passageway connected to the other side of the exhaust gas purifying element and further to a fourth port of the flow direction switching element. The flow direction switching device can be switched over to a first position for permitting the exhaust gas to flow in a direction than when in the SO_x desorbing process when in the SO_x desorbing process than when absorbing NO_x. In this case, it is feasible to speed up both of a rise in temperature of the NO_x storage-reduction catalyst and the SO_x desorption when in the SO_x desorbing process.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, and the sweeper is provided at the second exhaust passageway. In this case, even when the flow of the exhaust gas is bypassed without flowing through the exhaust gas purifying element in the process of the switching process by the flow direction switching device, the exhaust gas is purified by the sweeper and can be then discharged into the atmospheric air.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, another catalyst is provided at the first exhaust passageway, and the sweeper is provided at the second exhaust passageway. The flow direction switching device is switched over to a third position for connecting the first port to the second port for an initial predetermined time during the SO_x desorbing process, and switches over the flow direction of the exhaust gas flowing though the NO_x storage-reduction catalyst in a direction opposite to that when absorbing NO_x after the predetermined has elapsed. In this case, even when SO_x is desorbed from another catalyst at the initial stage of the SO_x desorbing process, SO_x desorbed therefrom does not flow into the NO_x storage-reduction catalyst, and hence the NO_x strage-reduction catalyst is prevented from SO_x poisoning. Besides, the sweeper is capable of purifying SO_x desorbed from another catalyst.

If the NO_x storage-reduction catalyst and the sweeper are integrated into one unit so that the exhaust gas can not flow therebetween and the heat can be transmitted therebetween, a temperature of the sweeper can be kept high, and a purging performance can be enhanced.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, and the lengths of the third and fourth exhaust passageways are set so that the distance from the internal combustion engine to the exhaust gas purifying element becomes different by switching over the flow direction switching device to the first or second position. Then, the flow direction switching device is switched over by selecting a flow path having a shorter distance from the internal combustion engine to the NO_x storage-reduction catalyst during the SO_x desorbing process of desorbing SO_x absorbed to the NO_x storage-reduction catalyst out of this NO_x storage-reduction catalyst. In this case, the higher-temperature exhaust gas can be flowed through the NO_x storage-reduction catalyst. As a result, SO_x can be desorbed at a higher efficiency.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, and the lengths of the third and fourth exhaust passageways are set so that the distance from the internal combustion engine to the exhaust gas purifying element becomes different by switching over the flow direction switching device to the first or second position. Then, the flow direction switching device is switched over by selecting a flow path having a longer distance from the internal combustion engine to the NO_x storage-reduction catalyst when the NO_x storage-reduction catalyst absorbs NO_x and when the exhaust gas temperature or the temperature of the NO_x storage-reduction catalyst is over a predetermined temperature, and selecting a flow path having a shorter distance from the internal combustion engine to the NO_x storage-reduction catalyst when the NO_x storage-reduction catalyst absorbs NO_x and when the exhaust gas temperature or the temperature of the NO_x storage-reduction catalyst is lower than the predetermined temperature. In this case, the NO_x storage-reduction catalyst can be kept within a temperature range suited to the NO_x absorption, and the NO_x purging rate can be enhanced.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, and the lengths of the third and fourth exhaust passageways are set so that the distance from the internal combustion engine to the exhaust gas purifying element becomes different by switching over the flow direction switching device to the first or second position. The sweeper is provided at the second exhaust passageway, and the flow direction switching device can be switched over to a third position in which the first port can be connected to the second port. When the exhaust gas temperature or the temperature of the NO_x storage-reduction catalyst is higher than a NO_x absorbable temperature range of the NO_x storage-reduction catalyst, the flow direction switching device is selectively switched over to the third position. In this case, the high-temperature exhaust gas can takes a short path from the first exhaust passageway to the second exhaust passageway without flowing though the NO_x storage-reduction catalyst, thereby preventing the NO_x storage-reduction catalyst from being deteriorated due to the high temperature.

In the exhaust emission control system of the internal combustion engine according to the present invention, a cooling device for cooling the exhaust gas is provided at either the third exhaust passageway or the fourth exhaust passageway, which increases the distance from the internal combustion engine to the NO_x storage-reduction catalyst. In this case, the NO_x storage-reduction catalyst can be held more surely within the temperature range suited to the NO_x absorption by the NO_x storage-reduction catalyst.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, and the first exhaust passageway is provided with the SO_x absorbing agent for absorbing SO_x when the air/fuel ratio of the inflow exhaust gas is lean, and desorbs SO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases. The flow direction switching device can be switched over to the third position in which to connect the first port to the second port, and is selectively switched over to the third position when the internal combustion engine comes to a continuous stoichiometric ratio operation. In this case, even if the exhaust gas exhibiting the stoichiometric ratio flows into the SO_x absorbing agent and SO_x is desorbed from the SO_x absorbing agent, it is possible to prevent SO_x from flowing into the NO_x storage-reduction catalyst and to prevent the NO_x storage-reduction catalyst from suffering the SO_x poisoning.

If the SO_x absorbing agent and the NO_x storage-reduction catalyst are concentrically disposed, the exhaust emission control system can be structured in a compact configuration.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the NO_x storage-reduction catalyst, and the first exhaust passageway is provided with a three-way catalyst exhibiting SO_x absorbing power. The flow direction switching device can be switched over to the third position in which to connect the first port to the second port, and is selectively switched over to the third position when the internal combustion engine comes to a continuous stoichiometric ratio operation. In this case, even if the exhaust gas exhibiting the stoichiometric ratio flows into the three-way catalyst and SO_x is desorbed from the three-way catalyst, it is possible to prevent SO_x from flowing into the NO_x storage-reduction catalyst and to prevent the NO_x storage-reduction catalyst from suffering from the SO_x poisoning.

In the exhaust emission control system of the internal combustion engine according to the present invention, the exhaust gas purifying element is composed of the catalyst, any one of the third and fourth exhaust passageways is provided with an HC adsorbing agent for adsorbing hydro carbon. The flow direction switching device is switched over to select a flow path on which the catalyst is positioned more upstream than the HC adsorbing agent when the temperature of the exhaust gas or of the HC adsorbing agent is in a temperature region where the HC adsorbing agent adsorbs the hydro carbon, and to select a flow path on which the HC adsorbing agent is positioned more upstream than the catalyst when the temperature of the exhaust gas or of the HC adsorbing agent is in a temperature region where the HC adsorbing agent desorbs the hydro carbon. In this case, HC can not be discharged even when the temperature of the exhaust gas is low.

The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims

1. An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means can be switched over to a first position for permitting the exhaust gas to flow in a direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port.

2. An exhaust emission control system of an internal combustion engine according to claim 1, wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases.

3. An exhaust emission control system of an internal combustion engine according to claim 2, An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means is switched over to a first position for permitting the exhaust gas to flow in a first direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port,

wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases,

wherein when in a SO_x desorbing process of desorbing SO_x absorbed to said NO_x storage-reduction catalyst from said NO_x storage-reduction catalyst, said flow direction switching means is switched over to the first position and the second position, thereby making a flow direction of the exhaust gas flowing through said NO_x storage-reduction catalyst opposite to the direction when absorbing NO_x.

4. An exhaust emission control system of an internal combustion engine according to claim 3, wherein said exhaust gas purifying means composed of said NO_x storage-reduction catalyst is such that, a NO_x storage-reduction catalyst disposed on inlet side in the flow direction of the exhaust gas when absorbing NO_x exhibits higher SO_x absorbing power than that of an NO_x storage-reduction catalyst disposed on an outlet side in the flow direction of the exhaust gas when absorbing NO_x.

5. An exhaust emission control system of an internal combustion engine according to claim 3, further comprising heating means for heating a portion close to an inlet of said NO_x storage-reduction catalyst in the flow direction of the exhaust gas when said NO_x storage-reduction catalyst absorbs NO_x.

6. An exhaust emission control system of an internal combustion engine according to claim 3, wherein the switch-over of said flow direction switching means when in the SO_x desorbing process is executed when an exhaust gas temperature or a catalytic temperature of said NO_x storage-reduction catalyst rises.

7. An exhaust emission control system of an internal combustion engine according to claim 3, wherein lengths of said third exhaust passageway and of said fourth exhaust passageway are set so that a distance from said internal combustion engine to said NO_x storage-reduction catalyst is shorter when in the SO_x desorbing process than when absorbing NO_x than when in the SO_x desorbing process by switching over said flow direction switching means to the first position or the second position.

8. An exhaust emission control system of an internal combustion engine according to claim 2, wherein a sweeper is provided at said second exhaust passageway.

9. An exhaust emission control system of an internal combustion engine according to claim 8, An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means is switched over to a first position for permitting the exhaust gas to flow in a first direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port,

wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases,

wherein a sweeper is provided at said second exhaust passageway, and

wherein said sweeper is a selective reduction type NO_x catalyst for reducing or dissolving NO_x under an existence of hydro carbon in an over-oxygen atmosphere.

10. An exhaust emission control system of an internal combustion engine according to claim 8, An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means is switched over to a first position for permitting the exhaust gas to flow in a first direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port,

wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases,

wherein a sweeper is provided at said second exhaust passageway,

wherein another catalyst is provided in said first exhaust passageway, and

said flow direction switching means is switched over to the third position for connecting the first port to the second port for an initial predetermined time during the SO_x desorbing process and, after the predetermined time has elapsed, switched over to make the flow direction of the exhaust gas flowing though said NO_x storage-reduction catalyst opposite to the direction when absorbing NO_x.

11. An exhaust emission control system of an internal combustion engine according to claim 8, An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means is switched over to a first position for permitting the exhaust gas to flow in a first direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port,

wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases, and

wherein a sweeper is provided at said second exhaust passageway,

wherein said NO_x storage-reduction catalyst and said sweeper are integrated into one unit so that the exhaust gas can not cannot flow therebetween and the heat can be is transmitted therebetween.

12. An exhaust emission control system of an internal combustion engine according to claim 2, An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means is switched over to a first position for permitting the exhaust gas to flow in a first direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port,

wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases,

wherein said first exhaust passageway is provided with a SO_x absorbing agent for absorbing SO_x when the air/fuel ratio of the inflow exhaust gas is lean, and desorbs SO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases, and

said flow direction switching means can be is switched over to the third position in which to connect the first port to the second port, and is selectively switched over to the third position when said internal combustion engine comes to a continuous stoichiometric ratio operation.

13. An exhaust emission control system of an internal combustion engine according to claim 12, wherein said SO_x absorbing agent and said NO_x storage-reduction catalyst are concentrically disposed.

14. An exhaust emission control system of an internal combustion engine according to claim 2, An exhaust emission control system of an internal combustion engine, comprising:

exhaust gas purifying means provided in an exhaust passageway of said internal combustion engine;

flow direction switching means including four ports and provided at an exhaust passageway disposed more upstream than said exhaust gas purifying means;

a first exhaust passageway connected to said internal combustion engine and further to a first port of said flow direction switching means;

a second exhaust passageway communicating with the atmospheric air and further to a second port of said flow direction switching means;

a third exhaust passageway connected to one side of said exhaust gas purifying means and further to a third port of said flow direction switching means; and

a fourth exhaust passageway connected to the other side of said exhaust gas purifying means and further to a fourth port of said flow direction switching means,

wherein said flow direction switching means is switched over to a first position for permitting the exhaust gas to flow in a first direction through said exhaust gas purifying means by connecting the first port to the third port and connecting the second port to the fourth port, to a second position for permitting the exhaust gas to flow in a direction opposite to the first direction through said exhaust gas purifying means by connecting the first port to the fourth port and connecting the second port to the third port, and to a third position in which the first port is connectable to the second port,

wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases,

wherein said first exhaust passageway is provided with a three-way catalyst exhibiting SO_x absorbing power, and

said flow direction switching means can be is switched over to the third position in which the first port is connectable to the second port, and is selectively switched over to the third position when said internal combustion engine comes to a continuous stoichiometric ratio operation.

15. An exhaust emission control system of an internal combustion engine according to claim 1, wherein lengths of said third exhaust passageway and of said fourth exhaust passageway are set so that a distance from said internal combustion engine to said exhaust gas purifying means becomes different by switching over said flow direction switching means to the first position or the second position.

16. An exhaust emission control system of an internal combustion engine according to claim 13, wherein the switch-over of said flow direction switching means is executed based on an exhaust gas temperature or a temperature of said exhaust gas purifying means.

17. An exhaust emission control system of an internal combustion engine according to claim 16, wherein said exhaust gas purifying means is an NO_x storage-reduction catalyst for absorbing NO_x when an air/fuel ratio of the inflow exhaust gas is lean, and desorbing NO_x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases.

18. An exhaust emission control system of an internal combustion engine according to claim 17, wherein said flow direction switching means is switched over by selecting a flow path having a shorter distance from said internal combustion engine to said NO_x storage-reduction catalyst when in the SO_x desorbing process of desorbing SO_x desorbed to said NO_x storage-reduction catalyst from said NO_x storage-reduction catalyst.

19. An exhaust emission control system of an internal combustion engine according to claim 17, wherein said flow direction switching means is switched over by selecting a flow path having a longer distance from said internal combustion engine to said NO_x storage-reduction catalyst when said NO_x storage-reduction catalyst absorbs NO_x and when the exhaust gas temperature or the catalytic temperature of said NO_x storage-reduction catalyst is over a predetermined temperature, and selecting a flow path having a shorter distance from said internal combustion engine to said NO_x storage-reduction catalyst when said NO_x storage-reduction catalyst absorbs NO_x and when the exhaust gas temperature or the catalytic temperature is lower than the predetermined temperature.

20. An exhaust emission control system of an internal combustion engine according to claim 17, wherein a cooling device for cooling the exhaust gas is provided at either said third exhaust passageway or said fourth exhaust passageway, which increases the distance from said internal combustion engine to said NO_x storage-reduction catalyst.

21. An exhaust emission control system of an internal combustion engine according to claim 17, wherein a sweeper is provided at said second exhaust passageway, and said flow direction switching means can be is switched over to a third position in which the first port can be connected to the second port,

when the exhaust gas temperature or the catalytic temperature of said NO_x storage-reduction catalyst is higher than a NO_x absorbable temperature range of said NO_x storage-reduction catalyst, said flow direction switching means is selectively switched over to the third position.

22. An exhaust emission control system of an internal combustion engine according to claim 1, wherein said exhaust gas purifying means is a catalyst,

any one of said third exhaust passageway and of said fourth exhaust passageway is provided with an HC adsorbing agent for adsorbing hydro carbon, and

said flow direction switching means is switched over to select a flow path on which said catalyst is positioned more upstream than said HC adsorbing agent when the temperature of the exhaust gas or of said HC adsorbing agent is in a temperature region where said HC adsorbing agent adsorbs the hydro carbon, and to select a flow path on which said HC adsorbing agent is positioned more upstream than said catalyst when the temperature of the exhaust gas or of said HC adsorbing agent is in a temperature region where said HC adsorbing agent desorbs the hydro carbon.