Exhaust system of multi-cylinder internal combustion engine

An exhaust system of an in-line four cylinder internal combustion engine, comprising:

• first, second, third and fourth upstream main exhaust passages extending from first, second, third and fourth cylinders of the engine, the first and fourth cylinders being those whose firing order is not successive and the second and third cylinders being those whose firing order is not successive;
• a first intermediate main exhaust passage that is provided by joining downstream ends of the first and fourth upstream main exhaust passages;
• a second intermediate main exhaust passage that is provided by joining downstream ends of the second and third upstream main exhaust passages;
• a downstream main exhaust passage that is provided by joining downstream ends of the first and second intermediate main exhaust passages;
• a main catalytic converter mounted in the downstream main exhaust passage;
• first and second upstream bypass exhaust passages respectively extending from upstream portions of the first and second upstream main exhaust passages;
• third and fourth upstream bypass exhaust passages respectively extending from upstream portions of the third and fourth upstream main exhaust passages;
• a first intermediate bypass exhaust passage that is provided by joining downstream ends of the first and second upstream bypass exhaust passages;
• a second intermediate bypass exhaust passage that is provided by joining downstream ends of the third and fourth upstream bypass exhaust passages;
• a downstream bypass exhaust passage that is provided by joining downstream ends of the first and second intermediate bypass exhaust passages, the downstream bypass exhaust passage having a downstream end connected to the downstream main exhaust passage at a position upstream of the main catalytic converter;
• an auxiliary catalytic converter mounted in the downstream bypass exhaust passage;
• a first switch valve that selectively opens and closes a fluid communication between each of the first and fourth upstream main exhaust passages and the first intermediate main exhaust passage;
• and
• a second switch valve that selectively opens and closes a fluid communication between each of the second and third upstream main exhaust passages and the second intermediate exhaust passage,
• in which the first and second switch valves are integrally mounted in a valve unit,
• in which an inlet opening of an exhaust gas recirculation system is exposed to an interior of the auxiliary catalytic converter to introduce thereinto the gas flowing in the auxiliary catalytic converter.