Exhaust gas purifying device

Abstract

A nitrogen oxide absorbing material ( ) which absorbs nitrogen oxide when the reduction component concentration of exhaust gas in the exhaust passage ( ) of an engine ( ) is lower than a predetermined concentration, and discharges nitrogen oxide when the reduction component concentration of exhaust gas in the exhaust passage ( ) of the engine ( ) is higher than the predetermined concentration, and a catalyst ( ) which promotes reduction of the discharged nitrogen oxides by a reduction component are provided. A controller ( ) first reduces the reduction component concentration of the exhaust gas over a first predetermined time period by, for example, advancing the fuel injection timing. The controller ( ) then increases the reduction component concentration of the exhaust gas over a second predetermined time period by, for example, retarding the fuel injection timing.