Patent attributes
A liquid reducing agent of a necessary amount according to an engine operating condition is injection-supplied from a nozzle having an injection hole opening to inside an exhaust gas passage, positioned on an exhaust gas upstream side of a nitrogen oxide reduction catalyst. On the other hand, when an injection flow rate of the liquid reducing agent becomes zero, compressed air stored in a purge air tank is supplied into the nozzle for a predetermined time. Thus, the liquid reducing agent remaining in the nozzle is forcibly discharged by the compressed air, and hence even if the nozzle is at a high temperature due to the exhaust heat, a situation where reducing agent constituents are deposited does not occur, and clogging of the inside of the nozzle can be prevented.
