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A thermo-acoustic engine is a device converting thermal energy into high amplitude acoustic waves that can be harvested, for example, to obtain electricity. The core of the device is a stack/regenerator along which a temperature gradient is created using one hot and one cold heat exchanger. Correctly designed, the thermal interaction between the working fluid and the regenerator assists in amplifying incident acoustic waves. Previous studies have indicated good efficiency obtained with a system of low geometrical complexity. However, for the practical application of this technique it is vital to understand and identify critical design parameters and operating conditions. This is of special interest in automotive applications where the operating conditions vary significantly over a drive cycle. This works aims at providing a framework for studying the net power generation over a drive cycle.

Noise encapsulations are widely used in automotive industry to enclose noise sources, such as e.g. the engine or the gearbox, to reduce externally radiated noise. The sound absorption factor of the material on the inside of the noise encapsulation is obviously vital for the sound attenuation. This parameter is in most cases determined experimentally for which there are several methods. The results received from the various methods may vary as different acoustic states are examined and thus influence the choice of method. The absorption factor is crucial since it is used in specifications to material manufacturers as well as being an input parameter in modeling the performance of the noise shield e.g. during a pass-by noise test. In this paper, two standardized measurement methods along with a third, non-standardized method, are applied to determine the properties of an absorbing material used in a commercial noise encapsulation.

The inclusion of flow-acoustic interaction effects in linear acoustic multiport models has been studied. It is shown, using a T-junction as illustration example, that as long the acoustic system is linear the required information is included in a scattering matrix obtained by experimental or numerical studies. Assuming small Mach numbers and low frequencies-as in most automotive silencer applications-the scattering matrix for the T-junction can be approximated using quasi-steady models. Models are derived that holds for all possible configurations of grazing and bias flow in the T-junctions. The derived models are then used to predict the performance of a novel silencer concept, where a resonator is formed by acoustically short-circuiting the inlet and outlet ducts of a flow reversal chamber. The agreement between experiments and simulations is excellent, justifying the use of the quasi-steady modeling approach.

Flow reversal chambers are common design elements in mufflers. Here an idealized flow reversal chamber with large cross-section but small depth has been studied. The inlet and outlet ducts as well as the cross-sectional area are fixed while the depth of the chamber can be varied. The resulting systems are then characterized experimentally using the two-microphone wave decomposition method and compared with results from both finite element modeling and various approaches using two-port elements. The finite element modeling results are in excellent agreement with the measurements over the whole frequency range studied, while two-port modeling can be used with engineering precision in the low frequency range. The influence of mean flow was studied experimentally and was shown to have relatively small influence, mainly adding some additional losses at low frequencies.

Simulation using basic acoustic 2-port elements is a time effective method for prediction of the attenuation of single components as well as of complete exhaust aftertreatment and silencer systems. However, with the complexity of current systems, the transformation from design geometries to networks of basic elements is not straightforward. In this paper a practical example of the modelling of a modern exhaust aftertreatment system is presented. A silencer aimed at the Euro 6 heavy duty emissions legislation containing complex flow turnings, parallel branches, DOC (Diesel Oxidation Catalyst), DPF (Diesel Particulate Filter) and SCR (Selective Catalytic Reduction) catalysts was modelled. Evaluation against measurements in order to understand the influence of the different acoustic elements upon overall attenuation and to improve the model with respect to near field and higher order mode effects was done.

The flow reversal chamber is a commonly used element in practical silencer design. To lower its fundamental eigenfrequency, it is suggested to acoustically short circuit the inlet and outlet duct. In the low frequency limit such a configuration will correspond to a Helmholtz resonator, but with a choked flow through the short circuit, the main flow will be forced through the expansion volume. For the proposed concept, the flow reversal resonator, a theoretical model is derived and presented together with transfer matrix simulations. The possible extension to a semi active device as well as the influence of mean flow on the system is investigated experimentally. Finally the concept is implemented on a truck silencer. The results indicate that the flow reversal resonator would prove an interesting complement to traditional side branch resonators. The attenuation bandwidth is broader and it can be packaged very efficiently.

A parameterization for combustion chamber cylinder pressures is tuned to measurements on an inline 6 cylinder Diesel engine at different operating conditions. Both measured and modeled signals are filtered in the frequency domain and according to the so called Lucas attenuation. Two different estimates of engine radiated noise are obtained and compared in this fashion. The influence of physically meaningful parameters on the frequency contents of the final noise level is analyzed, as well as the contribution of the pressure trace of the motored engine. The scattering of data in the pressure signal is calculated as a function of the crank angle allowing for statistical considerations.

Both linear (frequency domain) and non-linear (time domain) prediction codes are used for the simulation of duct acoustics in exhaust systems. Each approach has its own set of advantages and disadvantages. One disadvantage of the linear method is that information about the engine as an acoustic source is needed in order to calculate the insertion loss of mufflers or the level of radiated sound. The source model used in the low frequency plane wave range is the linear time invariant 1-port model. This source characterization data is usually obtained from experimental tests where multi-load methods and especially the two-load method are most commonly used. These measurements are time consuming and expensive. However, this data can also be extracted from an existing 1-D non-linear CFD code describing the engine gas exchange process.