On the effect of nonlinear vibration absorbers in palliation of the torsional response of automotive drivetrains

Torsional vibrations generated by the internal combustion engines are transmitted to the drivetrain inducing a plethora of noise, vibration and harshness (NVH) concerns such as transmission gear rattle, clutch in-cycle vibration, judder, to name but a few. The main elements of these oscillations are variations in the inertial imbalance and the constituents of combustion power torque, collectively referred to as engine order vibrations. To attenuate the effect of these transmitted vibrations and their oscillatory effects in the drivetrain system, a host of palliative measures are employed in practice, such as clutch pre-dampers, dual mass flywheel, centrifugal pendulum absorbers and others, all of which operate effectively over a narrow range of frequencies and have various unintended repercussions. These included increased powertrain inertia, installation package and cost.

This thesis focuses on the systematic study of incorporating nonlinear energy sink (NES) in the vehicle drivetrain with the aim to attenuate torsional vibrations over a broad range of frequencies and under transient vehicle manoeuvres. The nonlinear energy sink operate on the principle of Targeted Energy Transfer (TET) whereby the mechanical vibration energy from a primary system is transferred in a nearly irreversible way to the secondary system where it is either absorbed, re-distributed or dissipated locally through structural damping. 

The numerical model of a reduced order drivetrain was developed in MATLAB/Simulink and was validated in both time and frequency domain against the experimental measurements obtained from a test vehicle. Once the validity of the model is established and correlated, the numerical model is modified to incorporate a nonlinear energy sink characterised by smooth and non-smooth nonlinearities. A metric was defined to ascertain the effectiveness of the NES, whereby the difference between the acceleration amplitudes of the 1.5 EO of the transmission input shaft for locked and active system are computed. Through parametric study of the drivetrain with NES, it was conjectured that the NES would lead to significant vibration attenuation over a broad range of 1.5 EO frequencies for various transient manoeuvres, thus being robust to input excitation changes. 

Following the parametric study of the drivetrain with the NES, a concept absorber was manufactured and tested on an engine-powertrain rig and also on a small scale rig where an electric motor along with universal joint is used to induce the harmonic oscillations in the NES. The conclusion from the engine-powertrain testing are that the NES could not induce into TET due to the active control of the torsional vibrations by the dynamometer, thus limiting the torsional oscillations in the powertrain. In order to resolve this drawback, an electric motor with universal joint is used to induce vibrations in the Clutch-NES system. The NES did engage in Targeted Energy Transfer with the primary system (electric motor) demonstrating vibration attenuation, with the experimental measurements correlating the numerical model observations of the phenomenon.