Fixed Basic Gears

Unlike the two ECVT modes, the fixed basic gears of the active gearbox are characterized by a fixed gear ratio of the transmission input shaft to the transmission output shaft. Thus a change in speed of the combustion engine results in a proportionate change in driving speed. This fixed gear ratio would be a disadvantage only if the combustion engine were not in the optimum efficiency range. However, it is selected by the operational strategy specifically when a high torque of the combustion engine is required anyway. In that case, its efficiency is already very good. Compared to the ECVT modes, the advantage of the fixed gears is that the double energy conversion in the electric machine is omitted. Losses are also associated with the generation of electrical energy by the one electric machine and the use of electrical energy by the other.

In all fixed basic gears, the electric machines can do the following (with the exception of basic gear four):

• Rotate without load
• Be operated as a motor to support the combustion engine
• Be operated as a generator to charge the high voltage battery

Exception: in fixed basic gear four, electric machine B is at a standstill, so that only electric machine A can be used as flexibly as described.

The generator mode is used particularly in overrun phases or when decelerating the vehicle in order to convert the kinetic energy into electrical energy and store it in the high voltage battery.

If we ignore for a moment the different ratios in the fixed basic gears, the active gearbox acts as if the electric machines and the combustion engine were installed on the same shaft. This arrangement corresponds exactly to that of a parallel hybrid drive.

All fixed basic gears are implemented in the active gearbox by closing two multidisc clutches.

Fig 1: Power Flow In Basic Gears 1 To 4

Courtesy of BMW OF NORTH AMERICA, INC.