Technology moves along...
As mentioned in another thread, the Prius engine must operate over a wide range of RPM and load because it provides a combination of propulsion and battery-charging. The electric motor means that the Prius engine will not have to meet the challenge of acceleration from zero speed, thus limiting its range of RPM and load and permitting improved efficiency.
Engines can be made extremely efficient when designed for a constant load and rpm. The Volt, for example, will charge from the grid and then drive for (supposedly) 40 miles, and then the engine kicks in to begin charging the system. That engine will always see the same load, etc. so can be designed to run very fuel-efficiently.
From an engineering viewpoint, the Prius is a great achievement. The Volt and the many cars to follow that model are far simpler - and in engineering, simplicity is always better than complexity. In fact, there's a good argument that the Volt is not a "hybrid" - it's an electric car that carries an onboard charger...
Like most, I hope GM does a good job of executing the Volt, that Toyota soon introduces its plug-in car, and that many other car companies follow suit. And I especially hope that emerging ultracapacitor technologies overtake electrochemical batteries for car applications.
Ike