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Shocker: Volt's gas engine is indeed mechanically connected to the drivetrain (updated with GM engineer's response)

Tim Stevens

Interesting news from General Motors today that's resulted in some puzzled expressions at Engadget HQ. We've learned that the Volt, which Chevrolet has been making quite a fuss about calling an "extended range electric vehicle," is actually just a traditional hybrid with some... potentially misleading marketing behind it. Since the concept stage the company has been saying how the onboard internal combustion engine was just to charge the batteries, that only the electric motors (there are two) are actually connected to the drivetrain. Indeed that's what we were told in person when we test drove the thing back in March. We're now learning that is not the case, that the Volt's gasoline engine can directly provide power to the wheels in concert with the electric motors.

Is that a problem? In terms of efficiency the answer is "apparently not," as we're guessing the car would not have been designed this way if it weren't the most frugal way to go. So, why all the deception? Why insist this isn't just a hybrid when it apparently is? When the company went looking for a government bailout it was in part awarded one because of the innovation shown in the Volt. Now that we're learning the Volt is basically just a plug-in hybrid with a bigger than average battery pack (Popular Mechanics is finding 30-odd miles of purely electric range), we're left wondering: where's the innovation?

Update: We've added some further details and analysis below, and a confirmation from Chevrolet as well.

So what is this, if it isn't a proper EV with just an onboard internal combustion generator to recharge the batteries? The Motor Trend story we've linked to provides the cleanest explanation differentiating Chevy's technology from that used by Toyota on the Prius. Basically, the Volt's ICE is coupled to the ring gear within the transmission, which at times spins up to provide direct mechanical power when the batteries are near depleted. This is important because this is exactly when electric motors are least efficient. Again, this is also contrary to what we were told before, that the ICE would only spin up on-demand to provide extra juice to the electric motor.

Now, because the ICE is not spinning the ring gear at lower speeds this layout should mean for less drivetrain loss around town than the Prius. However, at highway speeds, once that initial charge you got from plugging it in overnight is depleted this more or less drives like any other hybrid. Is this important? If you commute on the highway it certainly could be, but really what is needed is more real-world mpg figures, and we'll need a lot more Volts to hit the road before we can call any of those conclusive.

Update 2: Further explanation of how the system works has been posted at, confirming the above.

Update 3: Final clarification directly from Chevrolet (and again courtesy of Autoblog), which states that because there is no "fixed gear ratio" between the ICE and the wheels "no direct mechanical connection" exists and, therefore, nobody was ever misled. We'd have to say one gear spinning another gear certainly is a "mechanical connection" even if it only spins it some of the time, but we'll let you choose which semantic sandcastle you favor.

Update 4: Here is what should be the final update, an interview posted at PluginCars with Andrew Farah, the Volt's Chief Engineer explaining the whole process. Put simply, there is a mechanical connection between the ICE and the drivetrain, but it is not being called a "direct" mechanical connection as it only works in concert with torque from the electric motors. There are "situations where we will take mechanical torque from the engine," according to Farah, but there is no arbitrary speed restriction. It's "based on the efficiency map" and is related not to speed, but overall torque in the system. You can listen to the whole thing here:

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