System design and energetic characterization of a four-wheel-driven series-parallel hybrid electric powertrain for heavy-duty applications

Powertrain topology design is vital for system performance of a hybrid electric vehicle. In this paper, a novel four-wheel-driven series-parallel hybrid electric powertrain is proposed. A motor is connected to the differential of the rear axle. An auxiliary power unit is linked to the differential of the front axle via a clutch. First, a mathematical model was established to evaluate the fuel-saving potential. A rule-based energy management algorithm was subsequently designed, and its working parameters were optimized. The hybrid powertrain system was applied to a transit bus, and the system characteristics were analyzed. Compared to an existing coaxial power-split hybrid powertrain, the fuel economy of the four-wheel-driven series-parallel hybrid powertrain can be at the same level under normal road conditions. However, the proposed four-wheel-driven series-parallel hybrid powertrain can recover braking energy more efficiently under road conditions with a low adhesive coefficient and can alleviate the torsional oscillation occurring at the existing coaxial power-split hybrid powertrain. Therefore, the four-wheel-driven series-parallel hybrid powertrain is a good solution for transit buses toward more robust performance.