Does anyone who owns a hybrid have problems with pedestrians not hearing their vehicle? The usual sound of the internal combustion engine is a warning for distracted pedestrians. Maybe an artificially generated sound might be appropriate.

Another reason they're more efficient is because, if they're series hybrids, the gasoline engine runs at a constant speed where it produces peak power, extending the life and fuel economy of the engine. It also reduces or eliminates the need for an expensive-to-produce transmission.

And it will help mature electric motor technology which will aid in the switch to pure EV or hydrogen fuel cell vehicles.

"Maybe an artificially generated sound might be appropriate."

It's called a horn. :)

One way may work is, the gas engine "only" works as an electric generator, which runs at an optimized rpm. This generator will then in turn generates electricity for charging a small flywheel/battery.

The energy storage device will then in turn provide electricities to the electric motors.

This idea is actually much like diesel powered submarine. Instead of running directly on fuel, the fuel is used instead to directly provide electricity.

The advantage is that, by doing so, the engine can be optimized at a single speed for electricity generation which is far more efficient.

stomv:  Horns are excessive.  Something more directional, less intrusive and more constant is in order, the audible equivalent of the daytime running lights.

I'm aware of NO (zero, zilch, nada) production hybrid which runs its engine at a constant speed.  The efficiency cannot be attributed to that; the factors are instead

1. A smaller engine with lower frictional losses, and
2. Operating the engine at lower speed but higher throttle settings (load) for the same power, which cuts throttling losses.