Internal combustion engine exhaust emission control device and exhaust emission control method

Abstract

An exhaust emission control device for an internal combustion engine is provided that can suppress the adherence of HC to an EGR path and the like accompanying enrichment of the exhaust air/fuel ratio. The exhaust emission control device includes: a high-pressure EGR paththat recirculates a portion of the exhaust gas upstream of a turbineinto an intake manifold; a high-pressure EGR control portion; a low-pressure EGR paththat recirculates a portion of the exhaust gas downstream of the turbineinto intake plumbing; a low-pressure EGR control portion; a NOx purification catalystthat is provided inside exhaust plumbingdownstream of an exhaust inlet of the low-pressure EGR path, traps NOx under an oxidizing atmosphere, and purifies the trapped NOx under a reducing atmosphere; a NOx purification catalyst reduction control unitthat performs reduction control of the exhaust air/fuel ratio; and an EGR switching portionthat selects recirculation control of exhaust gas by the low-pressure EGR control portionin a case of the exhaust air/fuel ratio being at least a predetermined threshold value, and selects recirculation control of exhaust gas by the high-pressure EGR control portionin a case of being lower than the threshold value.