Method for exhaust gas purification and system for exhaust gas

Abstract

A method for exhaust gas purification using an exhaust gas purification system including (a) an adsorbent for adsorbing hydrocarbons in internal combustion engine exhaust gas provided in the engine exhaust pipe, (b) at least one catalyst for reducing harmful substances present in the exhaust gas, provided in the exhaust pipe upstream of the adsorbent in the flow direction of the exhaust gas, (c) at least one catalyst for reducing harmful substances present in the exhaust gas, provided in the exhaust pipe down-stream of the adsorbent in the flow direction of the exhaust gas, (d) a first inlet for introduction of secondary air into exhaust gas, formed in the exhaust pipe upstream of the most downstream catalyst of the catalyst (b), and (e) a second inlet for introduction of secondary air into exhaust gas, formed in the exhaust pipe between the catalyst (b) and the most downstream catalyst of the catalyst (c). Secondary air is introduced from the first inlet at a time from engine start to activation of catalyst (b); when catalyst (b) has been activated, stopping secondary air introduction from the first inlet; meanwhile, introducing secondary air from the second inlet while the hydrocarbons of exhaust gas adsorbed by adsorbent (a) are desorbed therefrom as its temperature rises by the heat of the exhaust gas. This method effectively purifies the hydrocarbons discharged from an internal combustion engine in a large amount during cold start, without adversely affecting the purification of also-discharged NOx.