Chevy Volt: A Pinch of Parallel?

By Bill Moore

Posted: 30 Jun 2010

When it comes to engineering a hybrid electric vehicle, you have two choices. You develop a complex transmission system that allows the vehicle to use blends of power from the electric motor and internal combustion engine, which is referred to as a "parallel" hybrid, or you keep the two separate in what is called a series or serial hybrid architecture. Conceptually, the latter is the easiest to configure. The internal combustion engine spins an onboard electrical generator that provides power to the vehicle's traction motor; no complicated transmission to blend the two is required.

The trouble with series hybrids is that they aren't quite as efficient in some driving cycles as the parallel hybrid. Examples of a parallel hybrid are the ground breaking Toyota Prius (1997), the Honda Insight (1999), and the Ford Escape Hybrid (2003). Series hybrids have tended to be confined to heavier vehicle applications: diesel-electric locomotives and transit buses.

Then along came the Chevrolet Volt: a four-passenger sedan with an onboard 53kW gasoline engine generator and a huge 16kWh lithium ion battery pack giving it up to 40 miles of electric-first driving range, after which the 1.4L Ecotec gasoline engine would power up the electrical generator churning out electrons to spin the electric traction motor, allowing the car to drive another 300 miles at an estimated 50 mpg. For all intents and purposes, the Voltec electric drive system employs a series hybrid architecture. Or so we thought.

From the very outset, GM has insisted that the drive system in the Volt -- and presumably its European cousin, the Ampera -- is different. They even created a new classification for it: Extended-Range Electric Vehicle or EREV, arguing that it's more an electric car with a range extender than a hybrid. The distinct is a subtle one from the consumer's perspective. While it offers features found in an all-electric car: no emissions, excellent accelerating, the ability to 'refuel' it at home from local electric power grid; it also acts like a hybrid: very efficiently consuming gasoline to provide the owner with anxiety-free driving range beyond its battery capacity.

The trouble with series hybrids is that they aren't very good in more demanding driving conditions, especially when climbing long grades. Because the engine-generator set is running flat out producing all the electric power it can to keep the vehicle up to speed, fuel economy -- and emissions -- suffer. In these situations, it makes sense to use the engine torque directly to spin the wheels, but again, that necessitates a complex transmission; and it just so happens that GM has one: theTwo-Mode hybrid transmission used in its Tahoe/Yukon Hybrids.

For months now, it's been rumored that the Voltec drive isn't a purely series/serial hybrid; that there's a touch of parallel architecture in it, incorporated to handle just those long uphill grades in the Appalachians and mountain West. GM has not officially confirmed this and likely won't until early next year. It still remains just a rumor, but one that has a thread of credence, especially if you look at what program engineers have said about the car's EREV fuel economy. It would be very hard to average 50 mpg in a pure series hybrid. Plug-In Conversions Corporation president Kim Adelman has long contended that if the Volt is a series/serial hybrid, it won't do better in fuel economy than a Toyota Camry Hybrid or Ford Fusion; certainly nothing approaching that of Prius, for which his conversion kits are designed. The difference between the 50 mpg average of the Prius and the 38-41 mpg of the Camry and Fusion suggests there is some parallel blending going on in the Voltec powertrain.

While GM's Robert Peterson is reported neither confirming nor denying that the Volt may employ some parallel blending -- and is that really news? -- practical engineering realities would suggest that its powertrain people have adapted aspects of the Two-Mode hybrid transmission -- avoiding potential patent conflicts withToyota -- for use in the car. Of course, we won't know for certain until six months from now, but is it really all that important what they've done under the skin? What owners are going to care about is will it live up to its promised performance? Can we really drive for weeks, neigh months, without ever having to visit a gasoline station?

Here's a preview. My wife has now gone more than 45 days of driving nearly every day to and from work, beside shopping trips on the weekend and still hasn't had to refuel our PICC-converted Prius. We're now estimating she'll be able to drive two months on a single tank of fuel. Previously, she had to refuel her Honda Accord every two weeks.

It's so nice having a 'filling station' in your garage, as Volt owners are about to discover, blended or not.

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