Tempest in a Teapot
By Bill Moore
Posted: 13 Oct 2010
"GM lied to us!"
"You can never trust what they tell you."
"GM's Chevy Volt Bait and Switch."
"It's Not a Real Electric Car."
That's the wisp of the steam rising from the tempest in a teapot that is GM's revelation that its Volt powertrain system isn't purely a series hybrid. It is, in fact, a very sophisticated hybrid of an electric car. It is what GM has always said it is: an extended-range electric vehicle or EREV. In effect, boys and girls, it is a class of EV unto itself; and presumably GM has a boat-load of patents covering it now.
The media made-up maelstrom (likely generated to spike traffic on a handful of web sites) reached its boiling point this week when General Motors revealed to the first wave of journalists attending its official briefing and test drive, how its range-extension system works. Using a very complex system of planetary ring and sun gears, plus three clutches and two electric motors, GM engineers were able to extract additional efficiency and range out of the system; up to "two miles," states GM's technical paper .
As the paper describes it, the Volt has four driving modes: single motor EV, two motor EV, single motor extended-range, two motor extended range combined. The single e-motor (EM1) is used to launch the vehicle and operate at it lower speeds. It can develop 111kW of power, giving the car brisk acceleration. For high-speed EV-only travel, the electric generator that is attached to the IC engine, converts into an electric motor (EM2), providing additional torque to propel the vehicle using battery power only.
It is only when the battery is depleted below 45% state-of-charge (SOC), that the IC engine fires up and the second electric motor (EM2) reverts back into a generator, producing enough electric power to propel the car another 300+ miles. State's GM's technical brief:
Regardless of which drive mode is used, the Volt is always propelled with electric power delivered by the traction motor. The engine cannot propel the Volt unless the traction motor is also running. In order for a planetary gear-set to transmit torque, at least one of the three main elements (ring gear, sun gear, or planet carrier) must be able to be locked or held. Since there is no clutch to lock the sun gear, the traction motor must be used to provide the reaction torque needed for propulsion.
According to GM, "this unique propulsion architecture allows the Volt to achieve 10-15 percent better efficiency at highway speeds than would have been possible by using only the single traction motor."
Good for them. I am all for better efficiency and performance. All along, the concern among knowledgeable observers has been that while a pure series hybrid works well for vehicles like buses and refuge trucks, but really sucks when used in an automobile. Had GM's system been just a series hybrid -- and GM never said it was -- the Volt's performance would have suffered. According to all the reports coming in from the first wave of journalists, the car is an outstanding performer.
There has been speculation that GM had to re-engineer the system after test drives in the mountains revealed that the original set up wasn't working; the car simply wasn't delivering the kind experience American drivers would expect. Frankly, I doubt that this was a recent fix (within the last six months, some have speculated). My guess is, GM powertrain engineers figured this out way back at the start of the program. You don't come up with a sophisticated gear system like this in six months and have all manufacturing bits and pieces -- not to mention testing -- ready for commercial production in half a year.
Whatever the backstory is -- and I'd love to hear it -- the Volt is, in my book an electric car. Sure it's got a range-extender, but so did AC Propulsion's t-Zero at one point. Would you question whether or not it is an electric car?
I would argue with GM's assertion that theirs in an industry first. I think TM4 in Canada can lay claim to that accolade. Also, I should point out that Lotus has also developed a range-extender system for electric cars, and so has Quantum, whose drive will power the Fisker Karma.
So, for all the cry babies and rabblerousers out there who are all flummoxed by GM's revelations… grow up!
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