Objective sound quality evaluation of the electric two-stage centrifugal compressor for fuel cell vehicles-experimental investigations

Sound quality is one of the most challenging studies in the field of automotive NVH. The most important noise source in fuel cell vehicles (FCVs) is the air compressor, and its noise seriously affects the sound quality of FCVs. The centrifugal compressor is now the mainstream choice for commercial FCVs, but there are few studies on its sound quality evaluation using objective psychoacoustic parameters. Therefore, a comprehensive vibration-noise experiment is conducted on an electric two-stage centrifugal compressor (ETCC) for FCVs to evaluate the sound quality accurately, including all operating conditions. The sound quality is evaluated by four objective psychoacoustic parameters: loudness, sharpness, fluctuation strength and roughness, which are the most widely used indicators. The reasons for the variation of these acoustic parameters under dynamic operating conditions are accordingly explained. It can be concluded that the loudness and sharpness show a strong and positive correlation with the rotational speed, while the fluctuation strength and roughness just reflect the sound quality of the ETCC for the low-frequency band and high-frequency band, respectively. So, taking loudness and sharpness as the core indicators under accelerating conditions is recommended. Then, the sound quality of the ETCC under unstable conditions at variable speeds is investigated. It is found that the sound quality of the ETCC deteriorates under surge conditions. When entering into the deep surge, the loudness and sharpness both fluctuate drastically, which is caused by the periodic impulse noise. The fluctuation strength rises sharply and keeps a high level in the process of deep surge because of the obvious low-frequency characteristics of the surge. The above three parameters have a clear correlation with the rotational speed, while roughness is the only parameter that is not significantly connected with the rotational speed, which can be explained by the existence of a high-frequency fluctuation under the entire surge process. Finally, the causes of sound quality deterioration are explored using secondary signal processing. The results show that the outlet pressure pulsations of the ETCC under surge conditions lead to the increases and fluctuations of these acoustic parameters, and compressor surge is the main cause of deterioration in sound quality for FCVs. Based on the results, the objective sound quality evaluation of the ETCC is accomplished, and four psychoacoustic parameters can be modified to improve the sound quality. This study provides a theoretical basis for proposing methods to accurately evaluate and improve the sound quality of FCVs.