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.
looks to complicated. 3 clutches for the system?
Look at the Toyota HSD driveline, it is much simpler and is 100% reliable.
Ummm, really? Did I just read that someone thinks the Toyota HSD driveline is better and 100% reliable?
Have they not heard of the number of recalls and NOW as of this week another one for steering?
GM needs to spread this technology out more to all brands and just think of how epic a sports car would be if it had the fast take off power of the electric and then once up to speed switched to petro.
Think Lotus Elise combined with a Tesla….:)
The sun and planet gearbox in my view is the best gear box possible. However you need three clutches and 2 electric motors to accurately control the ratios so that the gearbox can act as a CVT when the Gasoline engine is engaged which is much more efficient than the Toyota driveline and can handle more power. Thats correct isn’t it Uwe?
@ Tom Barron: The three clutches are necessary to handle the different drive modes, so its not a necessity to modulate a CVT kind of transmission behavior. Remember, we like to have the large E-Motor being at its optimum as well, so in the pure electrical mode we are also using the clutches C1 and C2 to sum up the two E-Motor speeds to one wheel speed. This gives you a comparable CVT feeling (no shift points), but as you know we don’t variate the ratio of the transmission.
The C3 clutch is always necessary to engage and disengage the gas engine from the generator. With the functions above we are also able to operate the gas engine in certain load levels where the engine does have a very good specific fuel consumption.
However you are right in your observation that with the three clutches, the two E-Motors and the gas engine we have many more engineering opportunities than an automotive engineer normally has, so we used the entire hardware to optimize the energy consumption and driving comfort. So it’s this extra on the powertrain side, that makes the Ampera such a great car and gives it such a smooth driving performance.
Thanks Uwe
If I understand correctly it sounds like the rotational speed of the primary E motor remains the same abouve 100km/h?
I’m looking forward to reading how the gasoline motor interacts with the drivetrain. I read somewhere that a direct drive was added between the gasoline engine and the wheels for high speed efficiency. Looking at the simplified diagram this doesn’t seem to be the case unless there is an extra clutch or brake you haven’t shown.
For Tom: It seems to me that the C2 and C3 clutch closed together, will offer direct thermic drive to the weel.Probably a very good point for the car yield.