At the customer depletion point the combustion engine fires up and operates at one of several optimized fixed RPMs. The engine turns the generator, producing electricity.
and the first 40 miles may be less then 40 miles, it include power from regenerative braking.
For the first 40 miles, the fully charged battery (80% state of charge) powers the electric motor. Regenerative braking can help recharge the battery to a certain extent.
The generator in the serial hybrid is more efficient because it only has to run at a single, tuned RPM for maximum efficiency and minimum emissions.
Discrete points...not a continuous range.
An ICE car must produce smooth power from idle to redline.
These generators have intakes which are tuned to specific frequencies, and that of course, includes automatically any discrete harmonics.
If you tune something for 1000Hz, it will also resonate some at 2000Hz.
"This I know, This I know, Doc-tor Helmholtz told me so!"
=)