Patent ID: RE42156

Claim:
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.