As road transportation contributes 17% of global CO₂ emissions, the shift toward electrified powertrains has accelerated. While electric vehicles still face challenges like limited infrastructure, range anxiety, and battery-related concerns, hybrid electric vehicles (HEVs) and plug-in HEVs, which offer a balance between lower emissions, range, and cost, are projected to account for 33% of the global market by 2030 (Statista).

The Toyota Prius, an eCVT-based HEV, is a benchmark for fuel-efficient hybrids but exhibits poor acceleration performance due to limited torque amplification and the inability to use both motor/generators (MGs) for traction. To overcome this limitation, this project introduces two novel multi-speed hybrid transmissions that combine the efficient input-split mode of the Toyota Prius with additional fixed-gear parallel modes to achieve both superior fuel economy and acceleration performance in a compact and manufacturable form.

The innovation lies in a clutch-gear pairing technique that enables strategically adding fixed-gear modes to the baseline input-split architecture. Pairing clutches with speed-altering gears creates functionally distinct fixed-gear modes. This design technique unlocks the ability to tune torque paths and define high-performance parallel modes, enabling the merging of the power-split and parallel design spaces from the outset.

Both transmissions use a single planetary gear (PG) with the engine connected to the carrier, MG1 to the sun, and MG2 (the larger MG) and the output shaft to the ring. MG2 is connected to the output shaft through a speed-altering gear. Each system uses two clutches (CL1 and CL2), and a one-way clutch is added to the engine node. CL1 connects the engine and MG2 in both transmissions, while CL2 placement differs—connecting either MG2 and the engine or MG2 and MG1 through different gear paths. This configuration supports five distinct operating modes: one-motor EV, two-motor EV, input-split hybrid, and two fixed-gear hybrid modes. The one-way clutch assists launch and regenerative braking and helps form the EV modes.

Both systems were generated through a systematic design space exploration, evaluating thousands of candidate layouts for fuel economy (FE), acceleration performance (AP), gradeability, and implementation feasibility. Compared to the Prius benchmark, both systems achieve up to 44% faster 0–160 km/h acceleration and a 5% improvement in city fuel economy.

These systems maintain compact packaging, making them ideal for mass-market HEVs and PHEVs in the compact and midsize segments. They deliver premium hybrid-like performance without requiring large motors or sacrificing cost or efficiency. Ultimately, they offer a compact, manufacturable, and high-impact solution to advance hybrids derived from the Toyota Prius system.