Improvement of engine NVH for the new pass-by noise regulation

The vehicle pass-by noise regulation will change in the near future and the noise limit will be lowered significantly. This evolution will require improvement of engine’s sound radiation. On the other hand, in the current context with high pressure of fuel economy, future engines will be more and more lighten, and this will have negative impact on engine’s sound radiation. 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 should be developed in order to improve noise of engine while keeping engine’s mass reduced.
Firstly, the requirement of sound radiation should be taken into account in the design phase of a new engine. To be able to do that, it is necessary to identify the main engine parts having important impact on engine’s sound radiation, and to know how much these parts affect the radiated noise. Original studies had been conducted in Renault on the crankshaft and engine bottom structure, which enabled to find quantitative relationships between the design parameters and engine’s sound radiation. 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 of them could make a gain of about 1 dB on the global acoustic radiation of the powertrain. 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 screen, we could obtain an acoustic attenuation comparable to that of 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.