Exhaust gas purifying system for engine

摘要

An exhaust gas purifying system, which has a NOx purifying catalyst (22) disposed in an exhaust line (20) operative to increase a NOx conversion efficiency with a rise in catalyst temperature to a specific temperature at which the NOx purifying catalyst (22) shows a peak NOx conversion efficiency and to decrease the NOx conversion efficiency with a further rise in catalyst temperature beyond the specific temperature, is comprised so as to increase an amount of reducing substance in the exhaust gas that is supplied to the NOx purifying catalyst (22), to estimate a change in a value relating to NOx conversive power of the NOx purifying catalyst at an occurrence of an increase in the amount of reducing substance that is provided by the reducing substance increasing means while the NOx purifying catalyst is at a temperature beyond the specific temperature, and to determines an increasing rate of reducing substance supply on the basis of an estimated change in the value relating to NOx conversive power. The exhaust gas purifying system can provide an increasing rate of reducing substance supply favorable for NOx purification in expectation of a drop in NOx conversion power due to a rise in catalyst temperature.