Instantaneous speed fluctuations of thermal engines. Linear modeling of drivetrain

Manufacturers are making constant progress in reducing noise and vibration in motor vehicles. But this is not without its difficulties, particularly where the powertrain is concerned. Increasing engine performance generally increases the torsional excitations they generate. Lighter powertrains and drivetrains also make these components more sensitive to the excitations applied to them. These concepts of torsional excitations and torsional responses of powertrains naturally lead us to model the links between excitations and responses. A kinematic chain can easily be reduced to a juxtaposition of torsion springs, damping and rotary inertia. It is therefore possible not only to characterize an existing kinematic chain, but also to modify its behavior in a predictive way, by computationally optimizing elementary components (inertias or stiffnesses, for example) in order to minimize quantities consistent with noise generation (torsional accelerations in particular). This article looks at the main phenomena behind powertrain noise in a vehicle, using the example of an existing powertrain fitted to a mid-range European vehicle. In a way, it raises awareness of the main physical phenomena that control the torsional behavior of the powertrain. Armed with these basic theoretical foundations, a more in-depth study of acyclic driveline noise will be presented in a separate article. Finally, it should be noted that in many of the problems related to kinematic dynamics that engineers and designers have to deal with, an approach based on linear behavior is more than sufficient. This is the approach adopted in this article. However, for the correct analysis of certain phenomena, such as noise problems caused by shocks or slope changes in the torque-deflection characteristic of a clutch, the linear approach is no longer sufficient, and may even lead to erroneous conclusions. In such cases, a non-linear approach is required, which is of course much trickier to handle.