The power-split hybrid is a transmission system with an internal combustion engine and two electric motors. The first motor (M1) is in a special connection with the internal combustion engine: they are the two input shafts of a simple planetary gearset. Consequently, the speed of the engine and the M1 motor together determine the speed of the third (output) shaft of the planetary gear. The M1 motor is therefore often called „the speeder”.

The output of the planetary gearset is connected to the driven axle and to the second motor (M2). The M2 motor is adding torque to the drive shaft, therefore it is often called „the torquer”.

The engine is in mechanical connection with the driven wheels, but thanks to the planetary gearset, the engine speed at a given vehicle speed can be flexible selected by controlling the speed of the M1 motor.

This is a major benefit of a power-split hybrid compared to a parallel hybrid. Because in a parallel hybrid – no matter which topology – the electric motor is used only to shift the required engine torque to a more efficient operating point. In a power-split hybrid however, both the engine speed and the engine torque can be shifted, therefore the engine can mostly operate very close to its most efficient operating point.

The name „power-split” reflects the basic character of the transmission system. The engine power can be split between the two motors, so that the engine can drive the wheels and charge the battery at the same time.

The flexibility of the system allows a high number of operating modes, which can be shifted dynamically, depending on the actual driving situation.

You might have noticed that – unlike a traditional driveline or a parallel hybrid – the power-split hybrid system has no clutch. The particular kinematics of the planetary gearset enables an unusual feature, the so-called geared neutral, which makes the clutch unnecessary. If the speed of the engine and the M1 motor exactly matches a specific ratio, the output shaft of the planetary gearset does not rotate. This allows the engine to run and charge the battery at vehicle standstill, even without the clutch. If the speed ratio of the engine and the M1 motor is slightly changed, the output shaft of the planetary gearset slowly starts to rotate, allowing a smooth vehicle start without clutch slipping.

The planetary gear with the engine and the M1 motor as inputs is generally referenced as Power-split Device (PSD). You will find some more details on the function of the planetary gearset including the geared neutral on the Power-split Device (PSD) page.

For the same benefits as described for the electric axles, it is getting more common to apply a two-speed gearbox between the M2 motor and the driven axle.

This way the operation of the M2 motor is optimized for both low and high vehicles speed: high torque multiplication is achieved in the high ratio at low vehicle speed, and the motor is prevented from overspeeding at high vehicle speed in the low ratio.

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