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Chevy Volt Powertrain Stirs Controversy

There’s a storm brewing in the halls and labs of General Motors (GM) this week, as GM tries to explain why the forthcoming Chevy Volt should still be considered an electric car.

The controversy emerged after Chevy explained the inner workings of the Volt to a group of auto writers. When the vehicle reaches 70 mph, GM explained, it employs an unusual power-split: The Volt’s wheels are driven partially by an electric traction motor and partially by a motor-generator that’s connected to a gas-burning engine.

Therein lies the problem – at least as some auto writers see it. Because the motor-generator is driven by the engine, they’re crying foul. The wheels of a true electric vehicle, they say, should always be driven by an electric motor.

But that’s where the discussion gets dicey. Some publications have inaccurately reported that there’s a direct mechanical connection between the gas-burning engine and the Volt’s planetary gear set. In truth, it’s not a direct mechanical connection. The engine delivers the power to the motor-generator, which relays it to the ring gear of the planetary set. So…it’s not a direct mechanical connection in the manner of a conventional gas-burning car, but the motor-generator arrangement is similar to that of a Prius, which is a true hybrid.

So does this mean the Volt is a hybrid? Or an EV? As would be expected, GM says it’s an EV. The company reports that its engineers devised the power-split arrangement as a means of boosting efficiency. At 70 mph, with the traction motor spinning frantically, efficiency would normally fall. To prevent that, they split the power between two motors.

“You’re taking the rpm’s of the main traction motor down, bringing the rpm’s of the other motor up, and improving the efficiency,” noted Kevin Kelly of GM in a discussion with Design News yesterday. “But the important thing is that if you take the main traction motor and battery out of the equation, the vehicle will not run.” That’s why the Volt should still be considered an EV, GM says.

Members of the automotive press are angrily arguing that GM is not only wrong; it’s being deceitful. “GM lied,” wrote in a headline earlier this week. The New York Times and The Wall Street Journal also weighed in on the controversy. Many members of the press are citing earlier comments by GM saying, in effect, we’re not using the engine to drive the wheels. And the already-bad feeling is being worsened by the fact that buyers of the Volt will qualify for a government-sponsored $7,500 electric car rebate.

The controversy raises some important questions, the main one being: Should the government shell out millions of dollars in electric car rebates for the Volt?

Whatever, the decision, one matter is clear: It never should have gotten to this point. Three years ago, it should have been obvious to everyone that the Volt was a hybrid. It has an internal combustion engine on board. It burns gasoline. It uses two sources of energy. And yet now, we’re embroiled in an argument over…what? Whether or not a connection through a motor-generator constitutes a hybrid powertrain? C’mon.

The truth is, all of us in the automotive press (including Design News) should have called the Volt a plug-in hybrid from the beginning. Why didn’t we? Because GM trotted out an excessively narrow SAE definition in which the car was a hybrid only if it used both power sources (gas and electric) to drive the wheels. Since the Volt didn’t do that (or so we thought), it became an EV. And the moniker stuck.

So now GM has an elegant solution for hybrid propulsion – one that may even be superior to that of the much-praised Prius – and we’re arguing about whether it’s an electric car.

The truth is, GM has created a great, new, innovative kind of vehicle.

It’s called a plug-in hybrid.

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