Scott over at PetroZero had the opportunity to pose some questions to Bob Lutz the other day and, as always, the responses were interesting. First off is the idea of the E-Flex platform operating entirely free of batteries, with the engine/generator simply running continuously and feeding electricity to motor. While this is certainly technically possible, the current E-Flex system is not architected for that possibility. The generator has no direct connection to the motor, rather it feeds the battery. The engine is designed to run at constant speed and wouldn't achieve anywhere near the efficiency if it had to provide transient operation. Because the motor requires more power on demand for acceleration, the engine would have to speed up. In this usage scenario, the efficiency losses of converting mechanical to electrical and back to mechanical power would come in to play as described by Toyota and others when they criticize the whole concept. A conventional parallel hybrid would actually be preferable in this case. The whole premise of improved efficiency from E-Flex is built on the idea that the vehicle will run off grid energy the majority of the time.
The other possibility that Lutz mentions is eliminating the engine/generator and going battery only. This is actually not a new idea and was discussed as a long-term possibility back in late 2006 when GM held the first background briefings on the Volt prior to its auto show debut. The idea is that if battery technology progresses to the point that a Volt type vehicle could provide a reasonable electric range without a range extender just by using a bigger battery, it could easily be done. The idea is mentioned in the first E-Flex technical description I wrote here. Finally, Lutz indicated that within the next two weeks he will be taking an initial test drive in a real Volt prototype. According to Scott this is an actual Volt prototype not a Malibu-based mule. I find that unlikely at this stage. A mule drive is almost a certainty though.
Update: Rob Peterson at GM confirmed that it is only a mule that Lutz will be driving. Actual Volt prototypes are still many months away.