Exhaust gas purification apparatus

Abstract

Excessive generation NOby oxidation catalyst arrangement upstream of a selective reduction catalyst is suppressed to prevent falling of NOreduction rate. A selective reduction catalystcapable of selectively reacting NOwith ammonia even in the presence of oxygen is incorporated in an exhaust pipefrom an engine. A pair of oxidation catalystsA andB are arranged in parallel with each other and upstream of the selective reduction catalyst. In an operation condition with low exhaust temperature, amounts of the exhaust gasdistributed to the oxidation catalystsA andB are adjusted so as to make NO/NOratio in the exhaust gasto about 1-1.5. In an operation condition with higher exhaust temperature where the NOreduction rate is less affected by the NO/NOratio, as a countermeasure against increasing, leaking ammonia, passage changeover is conducted to pass the exhaust gasin backflow through the selective reduction catalystand the oxidation catalystsA andB, so that ammonia passing through the selective reduction catalystwithout reacted is oxidized in the oxidation catalystsA andB.