Schaeffler has been testing and demonstrating the performance capabilities of 48 volt hybridization in a range of concept vehicles including the“Gasoline Technology Car” (GTC), co-developed with Ford and Continental and the “Schaeffler Efficient Future Mobility North America” vehicle, as launched at the 2015 Detroit Motor Show (NAIAS) (earlier post). Schaeffler is now applying 48V hybridization to an Audi TT.
The drive for the “Schaeffler System 48 V” is based around an electrified rear axle which complements the front-wheel drive internal combustion engine. An additional belt-driven starter generator is connected to the engine which also operates at 48 V. The 48 V on-board electric subsystem uses a lithium-ion battery as its energy store. It is connected via a voltage transformer to the 12 V on-board electric system which powers all the different electrical components in the vehicle, from headlights to seat adjusters.
The Schaeffler PROtroniC prototype control unit controls and regulates the interaction between the components. The way in which the drive elements work together depends on which of two driving modes—“Eco” and “Sport”—the driver selects.
In Eco mode, which optimizes fuel consumption, the engine is switched off as often as possible. Driving at lower speeds, in stop/start traffic or when maneuvering into a parking space is all electric. When braking, the electric machines act as a generator and recharge the lithium-ion battery.
“Active sailing” is also possible with the electric rear axle: If, for example, when driving in town at a constant speed, very little driving power is required, the engine remains switched off and the electric machine takes over. If the engine has to be used, it is quickly and smoothly started by the belt-driven starter generator. Under acceleration, the engine and electric motor work together, increasing the power in the system. Also, when the engine is supported by the electric machine, it can run at operating points which improve fuel consumption.
Overall, Schaeffler is anticipating a noticeable reduction in fuel consumption—precise measurements will be made available once the concept vehicle has been fully completed. Selecting the right drive configuration also provides increased driving stability. The electric rear axle can provide all-wheel drive for short periods, for example, when setting off on snow or ice.
The characteristics of the whole drive train change if the driver selects Sport mode. In this mode, the engine is used all the time. It sends a permanent charge to the battery via the belt-driven starter generator so that sufficient energy is always available to extract maximum performance from the system. In Sport mode, the torque produced by the electric motor is distributed between the two rear wheels via an integral torque vectoring gear stage.