Turbocharger Noise - Generation and Control

An important part of modern engine design is the concept of downsizing where a key role is carried by the charging devices. These devices are effective aero-acoustic sources forming a coupled acoustic system with the connected flow-channel components. At KTH a unique test facility for determination of the complete acoustic Two-port for turbochargers has been built. Using this facility both the passive (transmission & reflection) as well as the active (sound generation) data for turbochargers can be measured at a given operating point. One important issue which has been studied in detail using this data is the coupling between the aerodynamic and acoustic fields close to “surge”. In addition, the control of compressor noise is an increasing concern. For instance heavy duty diesels and light duty engines with screw (roots) compressors can create strong charging harmonics well below 10 kHz. The standard noise control solution for these cases is to build a series of resonators. As an alternative KTH has developed a novel compact and very efficient silencer in the form of an expansion chamber with locally reacting cavities. The cavities consists of a micro-perforated plate in front of a closed air volume. The micro-perforate and volume are then chosen so that the cavity impedance equals the so called Cremer impedance at a target frequency. This ensures a very high damping at one frequency (hundreds of dB/m) and using this concept compact silencers with a damping higher than 30 dB in octave around the target frequency can be designed.