After treatment devices such as catalysts and diesel particulate filters are used extensively in exhaust systems of modern internal combustion engines. This is primarily to reduce and control the emission levels produced by such engines. However, they also affect the gas dynamics, performance and acoustics. This paper describes work on simulating the acoustic behaviour of these components using both linear and non-linear techniques. Two existing computer simulation codes (one linear, one non-linear) have been extended and combined into a single package. The linear method is carried out in the frequency domain and makes use of plane wave assumptions. This is done with the transfer matrix method, also called the four pole method. Specific models of after treatment components are used to generate transmission loss predictions versus frequency. The non-linear method solves the mass, movement and energy equations in the time domain and makes use of white noise excitation and Fast Fourier Transforms (FFT) to obtain results in the frequency domain. The advantages and disadvantages of both the linear and non-linear techniques are contrasted. The predicted transmission losses have been validated by comparing to measured data from a commercial after treatment device consisting of a combined catalyst monolith and diesel particulate filter section.
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