Hyundai Motor Company’s TMED-II hybrid system adopts a P1–P2 parallel motor layout, which improves power distribution flexibility but increases reliance on electric drive components. Failures in motors, inverters, or other power electronics can critically affect drivability and safety, making robust Fail-Safe strategies essential. This study proposes a three-stage, sequential Limp-Home strategy for P1–P2 HEVs under P2 motor system failure. Unlike conventional methods that open the main relay and rely solely on the engine, the proposed approach keeps the high-voltage (HV) system active whenever possible to maintain performance, safety, and comfort.
Stage 1 – P1 motor-based State of Charge (SOC) control:
Keeps the main relay closed and uses the P1 motor to maintain SOC within set limits. Overcharge is mitigated by operating the motor in discharge mode, and overdischarge is mitigated through regenerative operation. Engine torque is adjusted to match motor torque demand, preserving launch performance and gradeability.
Stage 2 – Engine speed-limiting control:
At higher speeds, Stage 1 alone may be insufficient to manage SOC. This stage limits engine speed (and consequently P1 motor speed as they are mechanically coupled) to suppress overcharge caused by back electromotive force (back-EMF). Coordinated control applies gearshift intervention and fuel cut to prevent rapid engine speed rises, reducing overcharge risk at the source.
Stage 3 – HV Battery main relay-off mode:
If overcharge or overdischarge risks remain after Stage 2, the HV system is cut off as the final safeguard. After cutoff, the P1 motor’s back-EMF powers essential loads such as HVAC, electric oil pump, and the low-voltage DC–DC converter, and the vehicle transitions to engine-only Limp-Home driving to avoid shutdown.
Production-vehicle tests confirmed the stepwise, software-only strategy maintains SOC within safe limits, preserves key loads, sustains drivability, and is patented in production models.