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An exhaust system comprises at least one muffler, the back pressure generated by the muffler exponentially grows as the engine speed increases. Accordingly, fuel consumption and direct CO2 emissions are penalized due to the back pressure generated by the muffling body in order to reduce noise emissions. To obviate this, it has been suggested to construct an exhaust system with two differentiated paths according to the engine speed, so that at low speeds the exhaust gases follow a first high acoustic attenuation (high back pressure) path, while at high speeds (high exhaust gas pressure), the exhaust gases follow a second low acoustic attenuation (low back pressure) path. Simulation and experimental analysis will be presented. A control valve is provided to alternatively direct the exhaust gases along the desired path according to the engine speed. These control valves usually include an electric or electro-pneumatic actuator, but are heavy, large in size and expensive.

Through a case history, we report a procedure to age exhaust flexibles on a Road Simulation Bench, in order to reproduce the acoustic failures occurred during vehicle ageing. The bench ageing has been the key instrument to solve an acoustic problem (a ring) observed on the vehicle after ageing. It is shown that it was mainly due to the partial loosening of the external outer braid (which caused the flapping of the outer braid against the bellows).