Powertrain Torsional Impact Load Causes, Effects and Mitigation Measures in a Parallel Mild Hybrid Powertrain

Hybrid powertrains generally involve adding an electric propulsion system to an existing internal combustion engine powertrain. Due to their reduced emissions, no reliance on public infrastructure and acceptable cost of ownership, hybrids are seen as a feasible intermediate step to deliver clean and affordable transportation for the masses. Such systems are immensely complex due to the number of interplaying systems and advanced control strategies used to deliver optimum performance under widely varying loads. Resonant torsional impacts arise out of the interactions due to rotational speed variations providing impulses at specific frequencies to the spinning inertias connected by members of finite stiffness. The effects, depending on the magnitude and duration of the impacts range from unacceptably harsh vibrations to catastrophic component failure. A large portion of these impact loads stem from low speed, near-stall driving maneuvers that are quite common on Indian urban roads and hence merits attention. This paper shall detail the powertrain torsional impact loads experienced in the development of a mild hybrid electric powertrain for the Indian market, the analysis of contributing causes and the mitigation measures used to overcome them along with test results validating their effectiveness.