A P2.5 parallel hybrid is obtained typically as an add-on to a DCT. In contradiction to the P2 topology, the electric motor is located after the dual clutch unit and is driving one of the input shafts. This way, the clutch of that input shaft acts also as a separating clutch, so there is no dedicated C0 clutch needed to separate the internal combustion engine from the electric motor.

This topology makes use of the working principle of the DCT. Even in the case, when the active gear is in the other sub-transmission, the electric motor is also capable of driving the wheels through the pre-selected gear. The power paths from the engine and the electric motor flow through different sub-transmissions and are merged in the final drive. As the engine and the electric motor are driving through different gear ratios, they are driving with different speed.

If the active gear is in the sub-gearbox driven by the electric motor, the power paths from the engine and the electric motor are merged already on the input shaft.

Besides the advantages of the P2.5 topology over the P2 in terms of complexity, there are also some limitations. The electric motor cannot drive the wheels through all gear ratios of the transmission, only through the ratios of the one sub-gearbox (every second gear). The other limitation is that since one sub-transmission in itself is not powershiftable, the electric power path is shortly interrupted when the pre-selected gear is changed in the sub-gearbox driven by the electric motor.

One possible realization of the P2.5 topology is to mount the electric motor directly on the hollow input shaft.

The possible operating modes are shown in the figures:

• only the internal combustion engine is driving (for both odd or even gears)
• only the electric motor is driving (only for odd or only for even gears)
• the electric motor recuperates during braking (for both odd or even gears)
• the engine and the electric motor are both driving (through different input shafts or through the same input shaft )
• the engine is driving and charging the battery at the same time (for both odd or even gears)
• the engine is charging the battery at vehicle standstill (gearbox in neutral)

For transverse engines, where the installation length is critical, a slightly different layout is favored. The electric motor is not mounted directly on the hollow input shaft, but drives it through a gear train. This way, the electric motor can be placed next to the gearbox and requires virtually no additional installation length. 

In order to save an additional gear wheel, the electric motor drives the input shaft through the existing loose gear of one forward gear. This ensures a permanent connection between the electric motor and the input shaft, while the loose gear itself is connected to the output shaft only if that particular gear is engaged.

The operating modes of this layout are the same as of the layout without the gear train:

• only the internal combustion engine is driving (for both odd or even gears)
• only the electric motor is driving (only for odd or only for even gears)
• the electric motor recuperates during braking (for both odd or even gears)
• the engine and the electric motor are both driving (through different input shafts or through the same input shaft )
• the engine is driving and charging the battery at the same time (for both odd or even gears)
• the engine is charging the battery at vehicle standstill (gearbox in neutral)

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