As I already mentioned in my first blog post, I find the Ampera’s silent electric propulsion extremely appealing. The heart of the propulsion system is the electric motor with 111 kW/150 hp and maximum torque of 370 Nm. Also on board is a 16 kWh battery and a 54 kW/72 hp generator unit. This starts automatically when the battery reaches its minimum state-of-charge. Power is transmitted to the front wheels through a planetary gearbox. With fully automatic and imperceptible operation, the three clutches (C1, C2, C3) handle the switch between two purely electric modes and two operating modes we call extended range. Today I’d like to talk about the two electric modes.
Single motor battery powered driving
The first electric driving mode is designed for “low” speeds up to 100 km/h. This is the mode we start out with, meaning the battery is fully charged, the stored energy flows into the power electronics and provides electricity to the primary traction motor. It transforms electrical power into mechanical energy and transfers this as torque to the front wheels via the differential. In this mode the inoperative gasoline engine and the generator are disengaged by the clutches C2 and C3 (diagram). C1 is engaged and locks the ring gear of the planetary gearbox to the housing.
Two motor battery powered driving at higher speed
At higher speeds the electrical energy stored in the battery is fed to the power electronics. In contrast to the first mode, the electronics now also deliver power to the generator. This reverses the generator’s operating principle, now also making it an electric motor and transforming electrical energy into mechanical energy in the form of additional torque. Clutch C2 is engaged and thus connects the ring gear with the generator. At the same time clutch C1 is opened. The planetary gearbox is now decisive. With the two motors and the planetary gearbox we can synchronize the two engine speeds to one vehicle speed. The primary traction motor thus works very efficiently at lower engine rpm.
Now, you might ask why we are doing this in such a complicated fashion. I need to explain that with some electro-technology: electric motors lose efficiency and performance at higher rpm. Because we have the generator on board, we can downsize the primary traction motor to a level where we can utilize it best at the – frequently occurring – low speed conditions. At higher speeds we achieve best possible energy efficiency and maximum speed with both motors mechanically synchronized over the differential.
So to sum up, we can say that thanks to cleverly combining both electric motors, we can drive electrically very efficiently with the Ampera while reaching maximum speed for emission-free driving fun!
In my next post I’ll talk about the role the gasoline engine plays.