Improvement of Engine Sound Radiation for the New Pass-By Noise Regulation 2014-01-2074
The vehicle pass-by noise regulation will change in the near future and noise limits will be lowered significantly. This evolution will require improvement of engine's sound radiation. On the other hand, under the current pressure for fuel economy, future engines will be more and more lightened, and this will have negative impact on engine's sound emission. Therefore, the requirements related to the new pass-by noise regulation should be taken into account in the design of new powertrains, and in some cases, innovative solutions must be developed in order to improve the level of noise of the engine while reducing the masse of the engine.
One effective way is to optimize the design of some key engine parts, such as crankshaft and engine bottom structure. Original approaches had been conducted and showed how much these engine parts can affect powertrain radiated noise, and in addition to find a quantitative relationship between crankshaft stiffness and powertrain radiated noise. In practice, by improving crankshaft stiffness, engine radiated noise can be reduced by 1-2 dB. Regarding the engine bottom structure, its impact on radiated noise can potentially reach 3 dB.
Another effective way to improve the radiation engine is adding acoustic covers. 3 types of acoustic covers had been studied: engine top cover, engine bottom cover and cover on exhaust side. Each cover could improve the global acoustic radiation of the powertrain by about 1dB. Regarding the engine top cover, a lightweight version had been developed in Renault with a mass reduction of at least 50% while maintaining a satisfactory acoustic performance. Renault had also developed a new concept of thermo-acoustic shield for engine exhaust face which provided both thermal insulation and acoustic attenuation. This shield was made of a thin steel layer and a thick layer of glass fibers. With this type of shield, we could obtain an acoustic attenuation comparable to an engine top cover. Moreover, compared to a simple heat shield with laminated steel sheets, the new solution introduced no additional mass and could even be lighter.