Vibration comfort and interior noise of a vehicle chassis are critical design features. Depending on travelling and engine speed, the engine is an important source of excitation. The vibrations are transmitted by engine mounts to the vehicle body. Especially at low frequencies, like idle speed, the engine can be represented as an assembly of rigid bodies. The chassis, however, has to be treated as an elastic structure. Component mode synthesis will be used to combine the Multi Body System (MBS) of the engine with the Finite Element structure of the vehicle chassis.
First, the Multi Body System of the engine is modeled. This contains the engine block, the gearbox and all important movable parts. Besides the inertia effects, the excitation by combustion forces has to be taken into consideration. The engine assembly is suspended by engine mounts. The Finite Element model of the chassis may be coarse as it has to represent just those modes, which yield significant response contributions at desired locations. The FE and MBS models are combined to a hybrid system by utilizing specific sets of FE modes which are imported and assembled by the MBS software. Vibrations at comfort locations of the chassis and engine mount forces are computed.
The approach is shown for fitting and vibration optimization of a 3-cylinder direct injection diesel engine (STEYR M13 ENGINE) into a middle class passenger vehicle (FIAT BRAVA). The analysis is performed by the software tools ADAMS, NASTRAN and LMS CADA-X. With the combination of theoretical analysis and measurements, a better noise level is obtained for the 3-cylinder engine than for the original 4-cylinder engine.