| 摘要 |
PROBLEM TO BE SOLVED: To properly reduce noise without impairing engine performance at the engine high load and according to load at the engine low load and middle load. SOLUTION: When engine speed is in a low speed region, an opening/closing valve 40 is controlled so that exhaust gas passes through only a first passage R1. Since exhaust gas flow rate at this time is small, back pressure hardly rises, even if exhaust gas passes though only the first passage R1 of small passing cross sectional area, and a noise reducing effect by restriction can be sufficiently obtained. In the case of a middle speed area, the opening/closing valve 40 is controlled so that exhaust gas passes through only a second passage R2. Since exhaust gas flow rate at this time is a middle degree, the back pressure hardly rises, even if exhaust gas passes through only the second passage R2 with passing cross sectional area larger than the first passage R1, and the noise reducing effect by restriction can be obtained sufficiently. In the case of a high speed region, the rise in back pressure is prevented and engine performance is improved by passing exhaust gas through both of the passages R1, R2, since exhaust gas flow rate is high.
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