The spark ignition engine is a prime source of vibration energy. NVH disturbances generated by the engine ultimately reach the customer in the form of objectionable noise or NVH. Exhaust Manifolds are one of the many sources of noise contributors among the engine components. Often, the exhaust manifold is identified as a source of objectionable NVH late in the design and development process.
Due to the lack of an upfront NVH analysis tool, a new CAE NVH methodology for evaluating new exhaust manifold designs has been investigated and developed by the Ford Motor Company's V-Engine CAE and Exhaust Manifold Design Sections. This new CAE methodology has been developed to compare the NVH performance of current production exhaust manifolds to new design levels.
Mechanical induced radiated shell noise is the predominate cause of objectionable NVH in exhaust manifolds. However, flow induced noise caused by the engine exhaust pulsation and wave dynamics also contributes to the radiated shell noise and should therefore be included in the analysis.
This paper will describe the two analytical approaches that were investigated.
The structural-mechanical approach is considered by including base engine excitation of crank train and valve train mechanical loads.
The mechanism of flow-induced noise is studied by applying engine pulsation and flow pressure fluctuation as acoustic excitations in coupled structure-acoustic model. The overall radiated noise levels are determined by superposition of both base and flow-induced excitations.