Torsional vibration characteristics analysis and mode-switching process optimization of a multi-mode power-split hybrid electric vehicle powertrain system

In a compound power-split hybrid system, engaging and braking the power-split device and power components can improve performance and fuel efficiency. However, the resulting mode transitions introduce abrupt changes in torque path, equivalent inertia, and equivalent stiffness. These changes may intensify driveline torsional vibration during engine start/stop and clutch engagement, leading to degraded NVH performance and potential damage to key components. In this study, an 18-degree-of-freedom (18-DOF) nonlinear powertrain model was established to investigate the torsional vibration behavior of a compound power-split hybrid commercial-vehicle powertrain during mode transitions. The results show that the natural frequencies associated with the power-split device vary significantly with operating mode. During the EV1-to-HEV1 transition, the natural frequencies of torsional damper1 mode, torsional damper2 mode, and input shaft mode decrease by 23.8%, 36.2%, and 43.8%, respectively. To address the torsional vibration issue under wide-open-throttle (WOT) HEV1 operation, a variance-based Sobol global sensitivity analysis was further conducted. The Saltelli sampling results show that flywheel inertia dominates both the peak and mean second-order speed oscillations; EM2 inertia mainly affects the peak response, whereas the third-stage damper stiffness and hysteresis torque mainly affect the mean response. Based on these sensitivity results, a parameter optimization scheme was proposed and validated through numerical simulations and bench tests. The optimized design reduced the second-order peak amplitude of the input shaft speed oscillation by 50.8% and the root mean square (RMS) value of the output shaft angular acceleration by 30.6%, preventing spacer pins impact and improving mode switching comfort and system stability.