Exhaust gas purifying device

A device for purifying exhaust gas containing nitrogen oxides in an exhaust passage of a combustion device, comprising:

• nitrogen oxide absorbing means for absorbing nitrogen oxides when a reduction component concentration of the exhaust gas is lower than a predetermined concentration, and discharging nitrogen oxides when the reduction component concentration of the exhaust gas is higher than the predetermined concentration,
• catalyst means for promoting reduction of discharged nitrogen oxides by a reduction component in the exhaust gas,
• means for decreasing the reduction component concentration of the exhaust gas,
• means for increasing the reduction component concentration of the exhaust gas,
• means for determining whether the nitrogen oxide should be discharged from the nitrogen oxide absorbing material or the catalyst,
• means for controlling the decreasing means so that the reduction component concentration of the exhaust gas is less than a first predetermined concentration of 100 ppm for a first predetermined time period when the determining means determines that the nitrogen oxide should be discharged from the nitrogen oxide absorbing material or the catalyst,
• means for controlling the increasing means so that the reduction component concentration of the exhaust gas is higher than a second predetermined concentration for a second predetermined time period after the first predetermined time period has elapsed, and
• means for maintaining the concentration of oxygen in the exhaust gas at or above 4.5%.

A device for purifying exhaust gas containing nitrogen oxides in an exhaust passage of a combustion device, comprising:

• a nitrogen oxide absorbing material which absorbs nitrogen oxides when a reduction component concentration of the exhaust gas is lower than a predetermined concentration, and discharges nitrogen oxides when the reduction component concentration of the exhaust gas is higher than the predetermined concentration,
• a catalyst which promotes reduction of discharged nitrogen oxides by a reduction component in the exhaust gas,
• means for decreasing the reduction component concentration of the exhaust gas,
• means for increasing the reduction component concentration of the exhaust gas, and
• a microprocessor programmed to:
• determine whether the nitrogen oxide should be discharged from the nitrogen oxide absorbing material or the catalyst,
• control the decreasing means for a first predetermined time period when it is determined that the nitrogen oxide should be discharged from the nitrogen oxide absorbing material or the catalyst, so that the reduction component concentration of the exhaust gas is less than that when it is not determined that the nitrogen oxide should be discharged and is less than 100 ppm,
• control the increasing means so that the reduction component concentration of the exhaust gas is higher than that when it is not determined that the nitrogen oxide should be discharged, for a second predetermined time period after the first predetermined time period has elapsed, and
• maintain the amount of oxygen in the exhaust gas at a level of 4.5% or greater.

A device for purifying exhaust gas containing nitrogen oxides in an exhaust passage of a combustion device, comprising:

• a nitrogen oxide absorbing material which absorbs nitrogen oxide when a reduction component concentration of the exhaust gas is lower than a predetermined concentration, and discharges nitrogen oxide when the reduction component concentration of the exhaust gas is higher than the predetermined concentration,
• a catalyst which promotes reduction of discharged nitrogen oxide by a reduction component in the exhaust gas,
• means for decreasing the reduction component concentration of the exhaust gas,
• means for increasing the reduction component concentration of the exhaust gas, and
• a microprocessor programmed to:
• control the decreasing means so that the reduction component concentration of the exhaust gas is less than a first predetermined concentration, and
• control the increasing means so that the reduction component concentration of the exhaust gas is higher than a second predetermined concentration after controlling the decreasing means,
• wherein the microprocessor is programmed to control the decreasing means to maintain 4.5% or more oxygen in the exhaust gas,
• wherein the microprocessor is programmed to control the increasing means to maintain 4.5% or more oxygen in the exhaust gas, and
• wherein the first predetermined concentration is 100 ppm.