| abstract |
A nitrogen oxide absorbing material (38) which absorbs nitrogen oxide when the reduction component concentration of exhaust gas in the exhaust passage (31) of an engine (1) is lower than a predetermined concentration, and discharges nitrogen oxide when the reduction component concentration of exhaust gas in the exhaust passage (31) of the engine (1) is higher than the predetermined concentration, and a catalyst (38) which promotes reduction of the discharged nitrogen oxides by a reduction component are provided. A controller (40) 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 (40) then increases the reduction component concentration of the exhaust gas over a second predetermined time period by, for example, retarding the fuel injection timing. |