MUFFLER DESIGN BY SCALING USING ENGINE CHARACTERISTICS
The purpose of this study is to improve the design of exhaust systems for internal combustion engines. Special attention is given to the case of the "small" internal combustion engines, such as those used on tractors and other motorized garden tools. From the one-dimensional linear acoustic theory a design procedure is proposed that includes several new features. The approach chosen is an evaluation of the sound spectrum of the exhaust noise using two graphs, namely, one of the spectrum of the engine exhaust flow at the exhaust port, the other of the exhaust system (muffler plus manifold) transfer impedance (sound pressure of the exhaust noise/input flow). The resulting sound spectrum is obtained by simple addition of the ordinates of the two graphs. The main new feature is the use of the so-called "scaling properties." They were derived in this study in order to be able to use the results of the analysis of one particular case to other cases at a different scale but with geometrical similarities. They apply to both the engine exhaust flow spectrum and the muffler transfer impedance. Hence, it is possible to adapt the method easily to any engine size, and it is also possible to keep a relatively small catalogue of muffler transfer impedance plots, knowing how to extend their use to other cases. The catalogue of mufflers is generated by computer programs that were developed to also serve to verify the scaling properties. The other topics discussed are (1) the use of a new muffler design criterion for better engine performances with lower back pressure, (2) the consideration of multicylinder engine manifolds, and (3) a method attempting to take into account the engine impedance.
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