The advent of turbochargers and the Eco-Boost technology at Ford in gasoline engines creates new challenges that need to be addressed with innovative designs. One of them is flow induced noise caused by airflow entering the turbocharger during off design operation. At certain vehicle operation conditions, the mass flow rate and pressure ratio are such that compressor wheel can generate a wide range of acoustic frequencies. Characterization of ‘whistles’ or pure tonal noises, ‘whoosh’ or broad band frequency noise caused by flow separation from the blade surfaces, and chirps, where the frequency increases or decreases with time are a few of the common error states.
Understanding the fundamental mechanisms of such noise generation is necessary for developing effective countermeasures for the noise source generation. Computational Aero-Acoustic (CAA) analyses are performed to study the effects of inlet and outlet conditions to find the source of the noise. These analyses are carried out with three-dimensional (3D) Computational Fluid Dynamics (CFD) models including the rotating compressor wheel, inlet duct geometry, volute, diffuser, and exit duct geometry. Geometric changes to the turbocharger inlet are developed and their effects on both the performance and acoustic signature of the turbocharger are studied.
After CFD and CAA studies, the turbocharger inlet geometry modifications are tested on a turbocharger gas stand. The test stand is instrumented to acquire time resolved pressure fluctuations that are post processed into acoustic data. The acoustic data is overlaid on a standard compressor map. The development of the new acoustic compressor map identifies the areas in the performance region where noise sources of various frequencies are prevalent.