Patent ID: 7891172

Claim:
An exhaust emission control system of an internal combustion engine, comprising: an exhaust passage having one end connected to the internal combustion engine, through which exhaust gas discharged from the internal combustion engine flows; an exhaust gas flow control unit that controls a flow rate of the exhaust gas that flows through the exhaust passage; a NOx catalyst provided in the exhaust passage for purifying NOx contained in the exhaust gas; and a reducing agent supply device that supplies a reducing agent to the exhaust gas that flows through the exhaust passage by at least one discharging operation of the reducing agent under a rich spike control in which an air/fuel ratio of the exhaust gas is temporarily brought into a rich state, wherein the exhaust gas flow control unit changes the flow rate of the exhaust gas that flows through the exhaust passage when the reducing agent is to be supplied to the NOx catalyst so as to reduce one of the NOx and SOx on the NOx catalyst, wherein the reducing agent supply device supplies the reducing agent to the exhaust passage at a plurality of timings at which the exhaust gas flows at different flow rates during a time period from a timing at which a fully closing command for a valve in the exhaust passage is issued, to a timing at which the flow rate of the exhaust gas changes to substantially zero in response to the command, and wherein each of the timings at which the reducing agent is supplied occurs at a different exhaust gas flow rate.