The Ampera’s control technology automatically finds the optimal setting of the drive unit for every driving situation. The driver does not need to actively switch between the operating modes. Assume I want to drive to France on holiday in a pure battery-powered electric car. Starting at home in Mainz, I would not even reach Saarbrücken – battery empty, end of trip. This is where the Ampera’s range extender comes into play. When the battery reaches its minimum state of charge somewhere on the Autobahn A 63, the driver just sees a symbol on the display. Almost imperceptibly the gasoline auxiliary power unit has already kicked in.

Single Motor Extended-Range Driving

From now on, the Ampera produces its electricity on its own, and the second e-motor works as a generator. The electricity produced flows into the power electronics that then decides if the energy is fed into the battery or used to power the primary traction motor. Clutch C1 is engaged and locks the ring gear, the disengaged clutch C2 prevents torque between the generator and the ring gear. The electric primary traction motor drives the front wheels, while the gasoline engine operates within so-called stationary maps. The speed at which the engine runs is not directly related to vehicle speed and does not vary with the wheel speed. This enables the gasoline engine to always run at optimal operating points, which benefits both the environment and customers’ wallets.

Two Motor Extended-Range Combined Driving

We developed an additional trick to keep the Ampera as efficient as possible at higher speeds. Now clutch C2 is also engaged, the ring gear is driven by some of the mechanical power of the gasoline engine. As already mentioned, the main electric motor should not exceed a pre-defined maximum rpm. So now we need the second motor to reach maximum speed. The Ampera is still being driven electrically! In contrast to a hybrid, we vary performance and speed as described above in this mode over the e-motor, not over the gasoline engine. Through the clever integration of the planetary gearbox in the generator axle we can also use the gasoline motor’s speed for propulsion at high speeds. With this we achieve around 10 percent more efficiency in these driving conditions than a design without this technical trick.

Because “self-produced” electricity is more expensive than that from the socket, we hardly charge the battery during driving at all – that should happen at the destination over a power outlet. This means the battery is only charged to the point necessary for comfortable electrically-powered driving. As soon as this state of charge is reached – recuperative braking also contributes to this – the internal combustion engine turns itself off. Then if the minimum state of charge is again reached during the drive, the whole cycle begins again.