This paper describes the results of a study carried out in Renault about the effects of variation of flywheel bending stiffness on powertrain vibrations in the range of acceleration noise (250 Hz and 500 Hz octave bands). The flywheels used covered a wide range of stiffness, from a standard rigid flywheel to a very flexible one.
On the one hand, NVH tests have been carried out on a Diesel common rail injection engine with 4 flywheels of different bending stiffness, from a rigid flywheel to a flexible one. On the other hand, simulations have been carried out by Excite on a whole Diesel powertrain model. Both test and calculation results have shown that the vibrations at powertrain mounting points in the 250 Hz octave band decrease continuously with the reduction of flywheel bending stiffness, and there is a nearly linear relationship between the vibration amplitudes and the first bending frequency of the cranktrain. It has been found that the decrease of engine vibrations in the 250 Hz octave band is mainly due to the reduction of vibration amplification by the cranktrain bending mode. When reducing the flywheel stiffness, the first bending frequency of the cranktrain decreases, and creates less amplification of the main engine excitations in the 250 Hz octave band. The most important vibration reductions are observed in the odd and half engine orders (3, 3.5, 4.5 and 5), known to be relevant for rough noise. From this study, it is recommended to tune the first bending mode of the cranktrain as low as possible, which means to decrease the flywheel stiffness as much as possible, provided that there is no problem of mechanical resistance of the flywheel.