Exhaust system for an internal combustion engine

An exhaust system for a lean-burn internal combustion engine comprises a first exhaust pipe connected to a first group of cylinders and fitted with a first emission control unit (4) and a second exhaust pipe connected to a second group of cylinders and fitted with a second emission control unit (5), where the emission control units can be connected together by a valve. The first unit is located near the engine and the second unit is a nitrogen oxide emission control unit located far from the engine. An exhaust system for a lean-burn internal combustion engine comprises a first exhaust pipe (3a) connected to a first group of cylinders (2a) and fitted with a first emission control unit (4) and a second exhaust pipe (3b) connected to a second group of cylinders (2b) and fitted with a second emission control unit (5), where the emission control units can be connected together by a valve (6). The exhaust pipes are separate until after the units and the first unit is located near the engine and the second unit is a nitrogen oxide emission control unit located far from the engine. An independent claim is also included for operating an internal combustion engine with an exhaust system as above, where during operation under lean conditions the first exhaust pipe is closed by the valve and the exhaust gas is passed through the second exhaust pipe, and during operation under stoichiometric or rich conditions the two exhaust pipes are connected together by the valve and the exhaust gas is passed through both exhaust pipes.