Patent ID: 6868669

Claim:
An exhaust-emission purifying method comprising the steps of: determining a threshold value used to determine whether an NOx purifying catalyst has been poisoned with a specific substance contained in an exhaust emission from the internal combustion engine, the specific substance being a substance other than NOx, the threshold value being selected from a plurality of pre-stored threshold values; comparing an NOx purifying ability of the NOx purifying catalyst to purify NOx contained in the exhaust emission with the threshold value, to determine whether the NOx purifying catalyst has been poisoned with the specific substance; and releasing the specific substance from the NOx purifying catalyst when it is determined that the NOx purifying catalyst has been poisoned with the specific substance, wherein the threshold value decreases with an increase in load on the internal combustion engine or an increase in an operating speed of the internal combustion engine, and increases with a decrease in the load on the internal combustion engine or a decrease in the operating speed of the internal combustion engine.