NVH Analysis of Powertrain Start/Stop Transient Phenomenon by using Wavelet Analysis and Time Domain Transfer Path Analysis 2015-01-2293

Tactile vibration during vehicle key on/off is one of the critical factors contributing to the customer perceived quality of the vehicle. Minimization of the powertrain transient vibration in operating conditions such as key on/off, tip in/out and engagement/disengagement of engine in hybrid vehicles must be addressed carefully in the vehicle refinement stage. Source of start/stop vibration depends on many factors like engine cranking, engine rpm at which the combustion process starts and rate of engine rpm rise etc. The transfer path consists of elastomeric mounts of powertrain and the part of vehicle structure from mounts to tactile response location.

In this paper, the contribution of rigid body motion of powertrain of a front wheel drive vehicle during key on/off is analyzed in both frequency and time domain. The signal is analyzed in frequency domain by using fast fourier transform, short time fourier transform and wavelet analysis. The merits and demerits of each method are illustrated. Wavelet analysis is used to analyze the transient event in frequency domain with small time steps. Also, the operational deflection shape analysis is used to visualize the modal behavior of powertrain at each time step. From the results of wavelet analysis, the contribution of each rigid body mode of the powertrain to the tactile vibration is ascertained by conducting frequency domain transfer path analysis for each time step. Design modifications at the powertrain mount level are suggested to reduce the intensity of tactile vibration presuming that the best is achieved in the parameters related to engine cranking and combustion initiation. Also, time domain transfer path analysis is conducted to estimate the mount forces and path contributions in time domain. The mount forces are estimated using relative displacements and mount stiffness data in time domain. The path contribution in time domain are ascertained by calculating the spectral inverse of product of force spectrums with corresponding vibration transfer function to target locations. The results of path contribution analysis of frequency and time domain methods are compared. The time domain TPA analysis is very useful in understanding the nonlinear behavior of the powertrain mounts. The elastomeric mount design is optimized by considering the vibration comfort and the manufacturability.