Active Transmission: Notes
The active gearbox of the E72 was developed in a co-operative effort between General Motors, Daimler Chrysler (currently Daimler) and BMW. Like a conventional automatic transmission, it makes different ratios between the gearbox input and gearbox output available.
From the driver's perspective, there are seven forward gears. Within the transmission, these seven forward gears are implemented by means of four fixed basic gears and two modes with variable gear ratio. In the four fixed basic gears, the speed of the combustion engine and transmission output shaft are in a fixed relationship to each other. This is not true of the modes with variable gear ratio: in these modes, the ratio between the speed of the combustion engine and the transmission output speed can be adjusted continuously, which is why the abbreviation "CVT" (for "continuously variable transmission") is used for these modes.
Because the active gearbox in the E72 provides two CVT modes, it is frequently also referred to as "two-mode active transmission" in the literature. The gear ratio is adjusted electrically using two electric machines integrated into the active gearbox. Therefore, these two modes are also referred to as "ECVT", where the "E" stands for electrical. As an integral part of the hybrid drive, the electric machines also serve to support the combustion engine and to recover the brake energy. The four fixed basic gears and the two ECVT modes are implemented and switched using three planetary gear trains and four multidisc clutches.
Therefore, the active gearbox in the narrower sense consists of:
- Two electric machines
- Three planetary gear sets
- Four multidisc clutches
| Index | Explanation |
|---|---|
| 1 | Planetary gear set 1 |
| 2 | Planetary gear set 2 |
| 3 | Electric machine B |
| 4 | Planetary gear set 3 |
| 5 | Multidisc clutch 2 |
| 6 | Multidisc clutch 1 |
| 7 | Multidisc clutch 3 |
| 8 | Multidisc clutch 4 |
| 9 | Electric machine A |
A dual-mass flywheel is used as the torsional vibration damper. It is located between the combustion engine and active gearbox. It has a similar design to those in vehicles with a manual transmission. The combustion engine of the E72 is not started via a conventional starter motor. Despite this, the toothed ring gear on the flywheel is present only for measuring the crankshaft speed.
Despite the fact that the active gearbox does not have a hydrodynamic converter, the gearbox components naturally have to be lubricated. Because of this, and to actuate the multidisc clutches, there is an oil pump on the gearbox input, which can be driven either by the combustion engine or by the electric motor installed specifically for this purpose. The oil circuit is also used to cool the gearbox components. The cooling circuit for the transmission oil in the E72 is the same as in the E71 xDrive 50i.
The electronic hybrid gearbox control unit - as in other current automatic transmissions - is part of the electro-hydraulic control module and is housed in the transmission oil sump. The electronic hybrid gearbox control unit in E72 is referred to using the abbreviation "TCM", derived from the "Transmission Control Module" designation used in the development co-operation.
The active gearbox also does not include an automatically actuated clutch, as in a sequential manual gearbox (SMG). The electric machines can compensate for this by creating a speed differential between the input and output shafts. When driving off with the combustion engine, the combustion engine initially powers only one of the two electric machines. This generates electrical energy with which the second electric machine is operated and generates a torque at the transmission output shaft. This ultimately sets the vehicle in motion.
The electric machines are also used when changing gears, for example to support the torque of the combustion engine and provide a comfortable shifting sequence while the multidisc clutches are opened and closed.
| Index | Explanation |
|---|---|
| 1 | Transmission fluid-to-coolant heat exchanger |
| 2 | Transmission oil lines |
| 3 | Dual-mass flywheel |
| 4 | High-voltage cables |
| 5 | Housing of the active gearbox |
| 6 | Hybrid parking lock (direct shift module) |
| 7 | Electro-hydraulic control module |
| 8 | Electrically/mechanically driven transmission oil pump |