Influence of the crankshaft dynamic phenomena on the fatigue behaviour of a transmission chain in a hybrid power unit

This paper is part of an activity related to a specific hybridization process for an existing and commercialized motorcycle engine. The electric motor is mechanically connected to the engine using the original valvetrain transmission chain leading to a P0 hybrid architecture. The aim of this work is to analyse the structural behaviour of the chain now used as torque transmission device. Although on one side the maximum torque of the electric motor determines a stress state on the chain below its fatigue limit, on the other side, the dynamic behaviour of the system strongly influences the maximum stress on the chain. In fact, the main source of load on the chain derives from the torsional oscillation of the system. Engines rotational speed irregularity can be easily estimated through common analytical approaches which consider the contribution of combustion and inertial forces on the instantaneous torque produced by the crank mechanism and the overall inertia of the moving parts. However, more detailed lumped-parameters and Multibody dynamic models are here developed in order to estimate the actual instantaneous engine speed taking into account the arising of possible high order vibration phenomena. Specifically, the influence of the torsional vibration behaviour of the internal combustion engine is investigated on the resulting stress on the chain. Finally, the structural chain integrity is assessed by performing a fatigue analysis considering the actual operating conditions.