Series Hybrid Transmissions
The series hybrid layout includes an internal combustion engine and two electric motors: the one is used as generator, the other as drive motor. There is no mechanical connection between the engine and the wheels, the engine is used solely to charge the battery through the generator.
The naming “series hybrid” reflects that the generation of drive power happens sequentially through both means of energy storage and energy conversion: first the chemical energy from the fuel tank to electricity, then the electricity from the battery to drive power. This series of conversion comes with increased losses, but in turn, the engine can be optimized for a given speed and torque, resulting in a much higher efficiency compared to other drivelines, where the engine operates under dynamically changing conditions.
The battery can not only be charged by the engine, the drive motor is capable of recuperating during braking. It depends on the capacity of the battery, how often and how long the engine shall operate to charge. If the vehicle is a plug-in hybrid (PHEV), engine operation is significantly reduced. With sufficiently high battery capacity, a series hybrid is considered as an electric vehicle (BEV), where the engine is just a comfort function: a range extender that operates only if the battery is empty.
For the same benefits as described for the electric axles, it is getting more common to apply a two-speed gearbox between the motor and the driven axle. This way the operation of the 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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