Generally, I think of the HCH2 as a "three-quarters" hybrid (Prius=full, HCH1=mild) since it does possess a limited EV-only mode but cannot launch the car on batteries alone. I've seen Honda's animation on how IMA2 works and it seems to be a pretty clever, cost-effective design.

My observation is this: What stops Honda from using a 40HP motor istead of the current 20HP unit? I would think that a more powerful IMA would extend the range of the EV mode. I can understand why the HCH2 can't launch on batteries, but a more powerful IMA could make the HCH act more like a Prius in slow traffic.

If you understand the IMA/IMA2 system correctly, you'll know you can't increase the power by replacing the motor alone.
You also have to replace the HV battery to supply 40HP power from it.

Ken@Japan

Actually this is where the Prius is strongest ( highway driving ) from my own experience. It's where the deviation from the EPA figures is the least signigicant. 60 City / 51 Highway EPA

While the HSD is very very good in city driving there are many 'types' of city driving. Thus one might very well get 60-70 mpg while cruising on a Sunday afternoon drive in your local neighborhood whereas one might only get 40-45 mpg in a different 'type' of city driving; for example going from stop sign to stop sign to stop light to stop sign on the way home from work.

Cruising on the highway on flat terrain in warm weather at 60-65 mph is essentially the same everywhere. One can expect 48-51 mpg in this situation in a Prius.

In those conditions, you can easily get 55MPG.

This is true, however, there is an added side-benefit of having a larger-capacity motor -- the fact that running motors at a lower percentage of their peak load will be more efficient. In the case of the Prius, there are two motors, each of which well more powerful than what the batteries can supply (as they need to be able to handle current generated directly off the engine). When the battery delivers its peak output though in EV mode, this means the motor is only running at partial capacity.

Some sort of parallel battery/ultracapacitor arrangement for IMA, with a larger motor, has struck me as an interesting solution. One problem with batteries is that they are difficult to charge rapidly, and can only charge a fraction of the rate they can discharge at. With ultracpacitors however, those could accept "surges" of charging (from short, hard regenerative braking), and supply "surges" of high power output. The batteries have the advantage of having greater energy desnity. The ultracapacitors though have better power desnity, so it seems like a good match.