The noise of passenger cars has to be further decreased according to customer and legislative requirements. Both, a lower sound level and a better sound quality are the aims to achieve a noise more accepted by the passengers. Taking into account the transmission of structure-borne and air-borne noise primary measures on the engine as the dominant noise excitation source of a motor vehicle are most promising and effective to solve these problems.
In the early design stage of an engine the fundamentals for an advantageous acoustical behavior have to be laid down. In this connection, the length of crankcase-skirts and the design in the main bearing area, i.e. the bottom end design, are important degrees of freedom concerning the acoustical optimization process of the engine block.
In this report the influence of different bottom end designs on the acoustical behavior of two high-performance spark-ignition engines -' one with short and one with deep skirt crankcase - is described. The two baseline engines - short and deep skirt with individual bearing caps - were modified with bearing beam, ladder frame, ladder frame girdle, bedplate and bearing cap cross-bolting. The investigations include analyses of the crankshaft dynamics and of the vibration transmission from the main bearings to the noise radiating surfaces of the engine. In addition, the vibration excitation at the powertrain mounts - input to the vehicle body - is assessed. For these analyses the different engine versions were investigated under fired conditions. Additional structural investigations were performed to gain knowledge of the dynamic behavior.