IC-engine intake noise predictions based on linear acoustics

Abstract: Shorter product development cycles, densely packed engine compartments and intensified noise legislation increase the need for accurate predictions of IC-engine air intake noise at early stages. The urgent focus on the increasing CO2 emissions and the efficiency of IC-engines, as well as new techniques such as homogeneous charge compression ignition (HCCI) might worsen the noise situation. Non-linear onedimensional (1D) computational fluid dynamic (CFD) time domain prediction codes are used within the automotive industry to predict intake and exhaust orifice noise. The inherent limitation of 1D plane wave propagation, however, limits this technique to sufficiently low frequencies where non-plane wave effects are small. Therefore this type of method will first fail in large components such as air cleaners. Further limitations, that might not be important for simulation of engine performance but indeed for acoustics, include difficulties to apply frequency dependent boundary conditions and losses as well as to include effects of vibrating walls. This thesis deals with the use of linear acoustics to improve the accuracy of intake orifice noise predictions.In order to predict intake noise using linear acoustics, knowledge is required about the engine as an acoustic source. The first part of this thesis describes how a linear time invariant one-port source model can be extracted using non-linear 1D CFD. Predicted source data for a six cylinder naturally aspirated engine is validated using experimental data obtained from engine test bench measurements.Acoustic 3D finite elements (FE) or boundary elements (BE) can be used to predict sound transmission through duct systems and include effects of non-plane waves. However, acoustic losses can not be predicted by linear theory. The second and third paper in this thesis include experimental investigations dealing with the influence of mean flow, yielding walls and filter paper on the acoustics of air intake systems.The third paper also describes how a linear source, extracted from 1D CFD, can be coupled to acoustic FE describing the intake system and to BE describing the radiation to the surroundings. These couplings create an entirely virtual methodology to predict intake orifice noise. Simulations and measurements are performed for a large number of engine revolution speeds in order to make the first systematic validation of a complete intake noise model for a wide engine speed range.Charge air coolers (CACs) are used in the intake system on many turbocharged engines to increase the volumetric efficiency. The acoustics of these devices has, however, so far gained interest from very few authors. In the last paper in this thesis a detailed acoustic analysis of a CAC is presented. Models to predict sound transmission in narrow cooling tubes, including losses due to viscous and thermal boundary layers as well as turbulence, are discussed. An efficient matrix formalism based on multi-ports and including 3D effects is proposed. From this, the first linear frequency domain model for CACs, which includes a complete treatment of losses in the narrow tubes and 3D effects in the connecting tanks, located on each side of the bundle of tubes, is extracted in the form of a two-port. The frequency dependent transmission loss is calculated and validated for a passenger car CAC with good accuracy.