It's a mere two months before GM is scheduled to start delivering its Chevy Volt to retail customers, yet there are still unanswered questions about how the would-be revolutionary vehicle works. GM has been coy about the Volt's fuel economy when its gasoline engine is operating, and a recent statement concerning the vehicle's electric range sure looks like GM is backpedalling from previous claims of 40 miles on battery power alone. (The Volt is now said to have an electric range of 25-50 miles.) But even more curious to some onlookers are the technical details that haven't been made public. Most importantly, GM is still mum on the design of the Volt's transmission – and whether this configuration allows for the gasoline engine to drive the wheels of the car.

There are potential benefits to allowing the gasoline engine to mechanically drive the wheels of the car, especially at higher speeds, where it would likely be more efficient than using electric drive alone. Yet this would contradict GM's longstanding assertion that the Volt is an electric vehicle, rather than a hybrid like the Toyota Prius.

GM routinely describes the Volt as an electric vehicle with extend range, thanks to its gasoline engine "generator," and has insisted that the car is always driven electrically. But this narrative leaves the details unclear. On first glance, the Volt drivetrain seems simple: An electric drive motor powers the front wheels; that motor gets its electricity from a battery pack until the pack's charge gets low enough, when the gasoline engine starts up to spin another electric motor generator to supply the electricity. But all those components have to be packaged together for a variety of reasons, including having an efficient regenerative braking system. So except for the battery pack, they all live in the same place: The transmission, or to be more accurate, the transaxle.

The Volt is fairly conventional, in that it has a transverse four-cylinder engine mounted in the front of the car, bolted to a transaxle that drives the front wheels, just like in a Malibu or a Cruze. It's what's in that transaxle – and what that configuration of parts might be capable of – that has engineering types engaged in rampant speculation. The consensus on fan site is that the Volt is using a variant of GM's two-mode hybrid transmission, which features a "power split" mode, in which torque from the engine is blended with torque from the electric drive motor through a planetary gear set.

A recent post of a GM patent application at has reignited debate over whether a mechanical connection between the gasoline engine and the wheels is possible in the Volt. The patent document appears to describe a simplified variation on the two-mode hybrid design, which would include two electric motors, a planetary gearset, and two or three clutches to link the motors and the gasoline engine. If this is the design of the Volt's transaxle, it seems that this configuration would allow for the torque of the gasoline engine to be blended with that of the electric drive motor in a "power split" mode to drive the wheels of the car.

However, there is no guarantee that even if GM is using the transmission described in the patent document that the Volt's electronic controls would allow linking the gasoline engine to the wheels by closing the appropriate sequence of clutches. And even if GM does not have the electronics programmed to allow mechanical drive of the wheels, the system could potentially be hacked or retrofitted to allow for a "power split" mode.

Unfortunately, this is all merely speculation, as GM will not reveal any details in advance of a media event it has scheduled to begin on October 10. We will be in attendance and will hopefully be able to put this issue to rest once and for all.

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