Superior NVH performance is a key focus in the development of new powertrains. In recent years, computer simulations have gained an increasing role in the design, development, and optimization of powertrain NVH at component and system levels. This paper presents the results of a study carried out on a 4-cylinder in-line spark-ignition engine with focus on growl noise. Growl is a low frequency noise (300-700 Hz) which is primarily perceived at moderate engine speeds (2000-3000 rpm) and light to moderate throttle tip-ins. For this purpose, a coupled and fully flexible multi-body dynamics model of the powertrain was developed. Structural components were reduced using component mode synthesis and used to determine dynamics loads at various engine speeds and loading conditions. A comparative NVH assessment of various crankshaft designs, engine configurations, and in-cylinder gas pressures was carried out. The main results include the crankshaft front-end and rear-end vibrations, bearing caps accelerations spectrum, and structure surface velocity levels in octave and 3rd octave bands. The correlation with experimental data was used to validate the analytical model. The analysis shows that a stiffer crankshaft results in a reduction of forced excitation transmitted to the bottom-end structure. The bearing beam stiffener also reduces bearing cap accelerations significantly. Both structural enhancements result in dramatically reduced growl noise.
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