The Influence of Eigentones in Intake and Exhaust Manifolds on Intake and Exhaust Noise Radiation

Natural oscillation modes in intake and exhaust manifolds of multi-cylinder internal combustion engines (mainly car four-cylinder engines) are considered. The engine with manifolds is considered as the source of the sound waves in the intake and the exhaust systems. The basic rules governing the spectrum formation of the manifold eigenmodes are investigated.

It is shown that some of the natural modes do not depend very much on the intake and exhaust systems after the manifolds. The sound increase (particularly for an asymmetrical excitation) caused by these modes depends on the degree of the asymmetry of the manifold and on the energy dissipation in the manifold. Under some conditions this increase creates a resonance sound radiation.

The tendency for resonance excitation at the branch of intake (exhaust) pipes with mufflers, with unfavourable and arbitrary combinations of their characteristics, is considered as one of the criteria for the quality of manifold construction. Special attention is paid to the intake manifold of modern four cylinder engines equipped with airbox. It is shown that a good design of the manifold of these engines allows decreasing their intake noise by 4–6 dB(A) in a wide range of crankshaft RPM.