Experimental investigation of the pulsation noise induced by automotive turbocharger compressors

Automotive turbochargers have become common in passenger cars as well as commercial vehicles. They are an excellent mechanism to effectively increase fuel efficiency and power, but they unfortunately cause several noise problems. The noises are classified as structure-borne noises, generated from the vibration of rotating shaft modules (cartridges), and air-borne noises, from air flow inside turbochargers or their coupling ducts. The structure-borne noises are whine and howling, and the air-borne noises are BPF (blade-passing frequency), blow, pulsation, surge, and hissing noises. These noises bother passengers because their levels are higher, or their frequencies are clearly distinctive from other noises, even at low levels. Since people easily recognize them and complain to car manufacturers about them, car manufacturers as well as turbocharger makers have made a concerted effort to satisfy their customers. The number of noises in an automotive turbocharger increases as other vehicle noises are lowered. The noise investigated in this study is the pulsation noise generated from the compressor wheels, whose frequency is the same as the whine noise. Therefore engineers are often confused about whine and pulsation noises. Although they have the same frequency, their noise sources are absolutely different. The whine noise is induced by unbalanced rotating modules, while the pulsation noise is generated by the wheel's dimensional asymmetry. The asymmetry considered in this study was prepared by partly cutting the compressor wheels' edge in order to remove its unbalance in a semi-circular shape. Since the size of the shapes is limited, the number of cuttings may be greater than one. It was realized through testing that the number was directly related to the level of the pulsation noise, and a specific number had to exist to cause the noise problem. Additional tests were done to investigate the characteristics of the noise by changing the numbers, the size, and the arrangements of the cuttings. Finally, this study concluded that the pulsation noise was related to the three factors of cuttings.