Parallel Hybrid Transmissions
Parallel hybrid drivelines are obtained by adding an electric motor to a conventional driveline. As the engine is in mechanical connection with the vehicle wheels, a transmission is still necessary with the usual number of gears.
The notation „parallel” comes from the character of those topologies, that the electric and internal combustion power paths are practically independent from the energy storage to the vehicle wheels.
There are 5 typical locations, where an electric motor might be added reasonably, resulting in 4 dedicated parallel hybrid topologies, denoted by P0, P1, P2, P2.5 and P3. The topologies are not equivalent from the functionality point of view.
Any of the conventional driveline layouts might be enhanced to any of the topologies, the figures show the respective locations only for the longitudinal and transverse front engine layouts.
P0 topology
The P0 topology is the so-called Belt-Driven Starter-Generator (BSG) and is basically an upgraded version of the conventional starter-generator, known in every traditional driveline. The configuration is either a 12 V micro-hybrid or a 48 V mild-hybrid system.
The BSG does not allow pure electric drive, the scope of functions is limited to – besides starting the engine during start-stop operation – recuperation during braking and some boosting of the combustion engine during acceleration.
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P1 topology
The P1 topology is the so-called Integrated Starter-Generator (ISG). In this case the motor is directly assembled to the engine crankshaft, between the engine and the gearbox. An ISG is more typical a 48V mild-hybrid system.
An ISG does not allow pure electric drive either, and offers exactly the same scope of functions as a BSG: starting the engine, recuperation during braking and some boosting of the combustion engine during acceleration.
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P2 topology
Hybrid functions are much more evolved with the P2 topology, when the electric motor is placed between the gearbox and the engine. Additionally, the driveline is extended with a so-called C0 clutch to decouple the electric motor from the engine. This way, the motor is bounded by clutches in both directions: the C0 clutch towards the engine, and the usual transmission clutch towards the gearbox.
P2 parallel hybrid vehicles are either 48 V mild-hybrid, or high-voltage full hybrid or plug-in hybrid vehicles.
In order to achieve similar seamless operation as a power-split hybrid, powershift transmissions are applied (either an AT or a DCT). In case of extending a DCT into a P2 hybrid topology, in total a triple-clutch transmission is obtained: the C0 clutch and the dual-clutch unit of the DCT.
The P2 topology allows a wider range of electric functions. The motor is capable of driving the vehicle in full electric mode without the engine, through the transmission with open C0 clutch. When the vehicle is in standstill, the engine is able the charge the battery through the C0 clutch with open transmission clutch. The electric motor can boost the engine in case of high power demand, but can operate in generator mode if temporary not the full engine power is needed. Finally, the electric motor can recuperate during braking.
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P2.5 topology
The P2.5 topology enables virtually the same electric functions as the P2 with some benefits with respect to the packaging.
On one hand, the C0 clutch of the P2 topology is not necessary. The electric motor can be disconnected from the engine through opening the transmission clutch, and can be disconnected from the wheels as well, through putting the transmission into neutral. This way, battery charging during vehicle standstill is also possible.
On the other hand, the installation of the electric motor is optimal for transverse engines, because the electric motor can be placed parallel to the gearbox, keeping the same total length as the traditional transmission.
The P2.5 topology is typically combined with a DCT transmission and the electric motor is driving the input shaft of one sub-transmission. This leads to a disadvantage compared to the P2 topology: every time, when the sub-transmission driven by the electric motor is shifted, the electric drive power is temporary interrupted.
P2.5 parallel hybrid vehicles are either 48 V mild-hybrid, or high-voltage full hybrid or plug-in hybrid vehicles.
The motor is capable of driving the vehicle in full electric mode without the engine, can boost the engine in case of high power demand, or operate in generator mode if temporary not the full engine power is needed. Finally, the electric motor can recuperate during braking.
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→ To the P2.5 topology page.
P3 topology
In the P3 topology, the electric motor is connected to the gearbox output shaft. Compared to the P2 topology, the P3 has some pro’s and con’s.
The P3 has higher energy recuperation potential, because the losses within the gearbox are not affecting the recuperation, and has some advantage in terms of installation space for transverse engines, because the electric motor can be placed parallel to the gearbox, keeping the same total length as the traditional transmission. On the other hand, the P3 topology does not allow all the electric functions of a P2, since the electric motor cannot be used to re-start the engine or to charge the battery during vehicle standstill.
P3 parallel hybrid vehicles are either 48 V mild-hybrid, or high-voltage full hybrid or plug-in hybrid vehicles, and the topology is typically combined with a DCT transmission.
The motor is capable of driving the vehicle in full electric mode without the engine, can boost the engine in case of high power demand, or operate in generator mode if temporary not the full engine power is needed. Finally, the electric motor can recuperate during braking.
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